Inisurfs : surface-active initiators : their synthesis andapplication in emulsion polymerizationCitation for published version (APA):Kusters, J. M. H. (1994). Inisurfs : surface-active initiators : their synthesis and application in emulsionpolymerization Eindhoven: Technische Universiteit Eindhoven DOI: 10.6100/IR410004

DOI:10.6100/IR410004

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INISURFS:

SURFACE-ACTIVE INITIATORS

THEIR SYNTHESIS AND APPLICATION IN

EMULSION POLYMERIZATION

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de

Technische Universiteit Eindhoven, op gezag van

de Rector Magnificus, prof. dr. J .H. van Lint, voor

een commissie aangewezen door het College

van Dekanen in het openbaar te verdedigen op

dinsdag 25 januari 1994 om 16.00 uur

door

JOSEPH MARIA HUBERTUS KUSTERS geboren te Nieuwenhagen

druk: wîbro dissortatiodrukkcrij, he!mond

Dit proefschrift is goedgekeurd door

de promotoren:

en de copromotor:

prof. dr. ir. A.L. Oerman

prof. dr. R.G. Gilbert

dr. J.J.G.S. van Es

------- -----

INISURFS:

SURFACE-ACTIVE INITIATORS

THEIR SYNTHESIS AND APPLICATION IN EMULSION

POLYMERIZATION

CIP-DATA KONINKLIJKE BIBLIOTIIEEK, DEN HAAG

Kusters, Joseph Maria Hubertus

Inisurfs: Surface-Active Initiators Their Synthesis and Application in Emulsion Polymerization I Joseph Maria Hubertus Kusters. Thesis Eindhoven. - With ref. With a summary in Dutch. ISBN 90-386-0153-0 Subject heading: emulsion polymerization.

Aan mijn ouders,

To Jola

Summary VII

Summary

Synthetic latices produced via emulsion polymerization find a broad range

of applications in for example paper, textile, adhesive, and paintlcoating industries.

The basic recipe to make these latices usually contains one or more surfactants of

possibly different types. These surfactants can cause difficulties in the application

of a latex. The stability of the latex can change over time due to desorption and

adsorption of the surfactants. When applying the latex, which quite often means

forming a polymer film on a substrate, the surfactant may migrate to the surface

and or to the interface of the polymer film, which leads to poor mechanica}

properties, low water resistance and also to blooming and blushing. The main

solution to these problems is fixation of the surfactant onto the surface of the

polymer latex particle. There are three different ways to achieve this fixation:

1) use of a surface-active monomer (surfmer), 2) use of a surface-active chain

transfer agent (transsurf), and 3) use of a surface-active initiator (inisurf).

This thesis describes the results of the investigations on the synthesis of

inisurfs and on the application of these inisurfs in the emulsion polymerization of

styrene. Hereby, the capability of the inisurfs to act as initiator as well as

surfactant could be determined. Moreover, such a study yields more insight into

the parameters governing the formation of (surface-active) radicals at the surface of

the polymer particles, and on the ways exit and entry are influenced by these

radicals.

Chapter 1 gives a general introduetion to polymers and polymerization and

it presents a background for this investigation, as well as a bistorical overview of

the obtained results in the field of inisurfs. The aims of the investigation are

stat ed.

VliJ Summary

In Chapter 2 the choice of the initiator and surfactant constituent parts of

the inisurf and the metbod of linking them is corroborated. The initiator moiety

was of the azo-type, with one or two carboxyl groups. The surfactant moiety was

a nonionic poly(ethylene oxide), having one hydroxyl terminal group and

optionally either a methyl or a nonylphenyl ether end-group. The results of the

esterification experiments, as well as the synthesis of an asymmetrically substituted

azo-initiator are described.

In Chapter 3 a brief overview of free radical polymerization is given.

Next, qualitative and quantitative roodels for emulsion polymerization are

discussed. In Chapter 4 the various experimental and analytica! techniques used

in performing an emulsion polymerization and characterizing the formed latex are

given.

A survey of the performance of the various synthesized inisurfs in the ab

initio emulsion polymerization of styrene is given in Chapter 5. The colloidal

stability of the reaction mixtures was positively influenced by lengthening the

ethylene oxide chain and by the presence of a nonylphenyl end group in the inisurf.

The results also indicated that the recombination of newly formed free radicals was

of great influence on the reaction rate and the nucleation of particles. This effect

was more pronounced for symmetrical inisurfs.

From the range of synthesized inisurfs two inisurfs were selected, a

symmetrical one (SC0-880) and an asymmetrical one (AC0-880), and these were

studied in more detail, as described in Chapter 6. The dissociation rates of the

inisurfs were measured (kti (323 K): 6.3 10'6 s· 1 and 9.3 10·7 s·', respectively) and

showed that there was only a minor effect due to the attachment of the surfactant.

The measured CMCs (2.0 10·5 mol dm·3 and 6.3 10·4 mol dm·l, respectively) were

similar to those reported for nonionic surfactants. The reaction rates of these

inisurf systems and the PSDs at the end of the reaction deviated from those

obtained for conventional systems. This was due to differences in stahilizing

Summary IX

ability and to the occurrence of geminate recombination.

The above mentioned inisurfs have also been used in seeded experiments, as

discussed in Chapter 7. Both inisurfs showed similar behaviour. Due to the

presence of a steric stabilizer at the surface of the polymer partiele the exit rate of

a radical from the partiele (k: 7.4 10·5 s·1 and 5.0 104 s·1, respectively) was smaller

than in systems with anionic stabilization. The entry rate of a radical (p: 2 10·5 s·1

and 2 w-s s·1, respectively) was very low as was the efficiency (j: 2 w-4 and 1 10"3

,

respectively). This could be readily explained qualitatively and quantitatively by

comparison of the time scale for geminate recombination of the two free radicals,

formed by inisurf decomposition, with the time scale for escape by diffusion of one

of the two free radicals from the vicinity of the partiele. In the case of the

symmetrical inisurf there was hardly any effect of surface coverage of inisurf on

efficiency, where as in the case of the asymmetrical inisurf, the efficiency

increased with increasing surface coverage. This investigation bas shown that the

efficiency of inisurfs will be low, as long as it is not possible to separate the two

newly formed free radicals at the surface of the polymer particles in an effective

manner.

This thesis concludes with an Epilogue, in which the most important

conclusions are summarized and some incentives are presented for further research.

x Samenvatting

Samenvatting

Synthetische latices gevormd met behulp van emulsiepolymerisatie vinden

een brede toepassing in bijvoorbeeld de papier, textiel, lijm en verf industrie. Het

basisrecept voor deze latices bevat gewoonlijk één of meer emulgatoren van

mogelijkerwijs verschillend type. De aanwezigheid van deze emulgatoren kan tot

problemen leiden bij het gebruik van de latex. De stabiliteit van de latex kan in de

tijd veranderen doordat de emulgatoren adsorberen en desorberen. Bij toepassing

van de latex, wat meestal gebeurt in de vorm van een polymere film op een

substraat, zal de emulgator naar het oppervlak en/of het grensvlak diffunderen,

hetgeen leidt tot slechte mechanische eigenschappen, lage water resistentie en ook

tot blooming en blushing. Een oplossing voor dit probleem is het binden van de

emulgator aan het oppervlak van de polymeerdeeltjes. Deze fixatie kan op drie

manieren bereikt worden: 1) via oppervlakte-actief monomeer (surfmer), 2) via

oppervlakte-actief chain transfer agent (transsurf) en 3) via oppervlakte-actieve

initiator (inisurf).

Dit proefschrift beschrijft de resultaten van een onderzoek naar de synthese

van inisurfs en de toepassing van deze inisurfs in de emulsiepolymerisatie van

styreen, om zo het gedrag van inisurfs als initiator en als emulgator vast te kunnen

stellen. Bovendien geeft zo'n studie meer inzicht in de factoren die de vorming

van (oppervlakte-actieve) radicalen aan het oppervlak van deeltjes beïnvloeden en

de manier waarop exit en entry hierdoor beïnvloed worden.

Hoofdstuk 1 geeft een algemene inleiding in polymeren en polymerisatie.

De achtergrond van dit onderzoek, alsmede een historisch overzicht van de

verkregen resultaten op het gebied van inisurfs worden gegeven en de doelen van

het huidige onderzoek worden geformuleerd.

Samenvatting XI

In Hoofdstuk 2 wordt de keuze van het initiator- en emulgatordeel van de

inisurf en de methode van koppeling beargumenteerd. Het initiatordeel was een

azo initiator met één of twee carboxyl groepen. Het emulgatordeel was een

nonionisch polyetheenoxide met een primaire hydroxyl groep en met als eindgroep

een methyl of een nonylphenyl groep. De resultaten van de esterificatie

experimenten en van de synthese van een asymmetrisch gesubstitueerde azo­

initiator zijn eveneens beschreven.

In Hoofdstuk 3 wordt een overzicht gegeven van vrije radicaal

polymerisatie. Daarnaast worden ook qualitatieve en quantitatieve modellen voor

emulsiepolymerisatie besproken. In Hoofdstuk 4 worden de verschillende

experimentele en analytische technieken besproken welke zijn gebruikt voor het

uitvoeren van de emulsiepolymerisaties en het karakteriseren van de gevormde

latex.

Een overzicht van het gedrag van de verschillende inisurfs in de ab initio

emulsiepolymerisatie van styreen wordt in Hoofdstuk 5 gegeven. Door het

gebruik van een langere polyetheenoxide keten kon de colloïdale stabiliteit van het

reactiemengsel positief beïnvloed worden. De resultaten lieten ook zien dat

recombinatie van nieuw gevormde vrije radicalen van belang is voor de

reactiesnelheid en voor het vormen van nieuwe deeltjes. Dit effect was meer

uitgesproken in het geval van de symmetrische inisurfs.

Van de groep van gesynthetiseerde inisurfs werden er twee gekozen, een

symmetrische (SC0-880) en een asymmetrische (AC0-880), die in meer detail

onderzocht zijn, zoals beschreven in Hoofdstuk 6. De dissociatiesnelheid van de

inisurfs is gemeten (kd (323 K) is respectievelijk 6.3 10-6 s- 1 en 9.3 I0-7 s·1) en liet

zien dat er slecht een gering effect was van de aanhechting van de emulgator. De

gemeten CMC's (respectievelijk, 2.0 w-s mol dm-3 en 6.3 I0-4 mol dm'3) bleken

ongeveer gelijk te zijn aan die welke in de literatuur worden gevonden voor

niet-ionische emulgatoren. De reactiesnelheid in deze inisurfsystemen en de

XII Samenvatting

deeltjesgrootteverdeling aan het einde van de reactie verschilden van die verkregen

met conventionele systemen. Dit verschil werd toegeschreven aan een verschil in

stabiliserende werking en aan het optreden van geminate recombinatie.

De eerder genoemde inisurfs werden ook gebruikt in seeded experimenten,

zoals beschreven in Hoofdstuk 7. Beide inisurfs vertoonden soortgelijk gedrag.

Ten gevolge van de aanwezigheid van een sterische stabilisator op het oppervlak

van de polymeerdeeltjes was de snelheid van uittreden van een radicaal uit deze

deeltjes (respectievelijk, k: 7.4 10"5 s·l en 5.0 10"4 s"1) kleiner dan in systemen met

een anionische stabilisator. De snelheid voor het intreden van een radicaal

(respectievelijk, p: 2 10·5 s·1 en 2 10·5 s·1) was erg klein en dientengevolge ook de

efficiëntie (respectievelijk, f 2 10·4 en 1 10-3). Dit kon qualitatief en quantitatief

verklaard worden door de tijdschaal voor geminate recombinatie van de twee vrije

radicalen gevormd door dissociatie van de inisurf te vergelijken met de tijdschaal

voor ontsnapping door diffusie van één van de twee radicalen. In het geval van de

symmetrisch inisurf had een toename van de bedekkingsgraad met inisurf bijna

geen effect, terwijl in het geval van de symmetrische inisurf wel een toename van

de efficiëntie gezien werd met toenemende bedekkingsgraad. Het uitgevoerde

onderzoek heeft laten zien dat de efficiëntie van de inisurfs laag zal blijven zolang

de twee pasgevormde vrije radicalen aan het oppervlak van de polymeerdeeltjes

niet op een effectieve manier gescheiden worden.

Dit proefschrift wordt afgesloten met een Epiloog. Hierin worden de

belangrijkste conclusies samengevat en worden enige aanzetten gegeven voor

verder onderzoek.

Contents

Contents

Summary

Samenvatting

Contents

Chapter 1 Introduetion

1.1 1.2 1.3 1.4 1.5

General Introduetion Background of this Investigation Ristorical Overview Aim of this Investigation Survey of this Thesis References

Chapter 2 Synthesis

2.1 2.2

2.3

2.4 2.5

Introduetion Choice of Intitiator and Surfactant 2.2.1 Initiator 2.2.2 Surfactant Results and Discussion 2.3.1 Asymmetrical Azo-Initiator 2.3.2 lnisurf Conclusions Experimental Section References

Chapter 3 Theory of Emulsion Polymerization

3.1 3.2 3.3 3.4 3.5

Introduetion Free-Radical Polymerization Model of Harkins The Rate of Polymerization The Smith and Ewart Theory 3.5.1 The Recurrence Relationships

XIII

VII

x

XIII

4 6 8

10 11

13 14 14 17 19 20 21 25 25 29

31 32 35 38 40 40

XIV

3.6 3.7

3.8

3.5.2 Solution to the Recurrence Relationships 3.5.3 Partiele Number Nucleation Models Zero-One System 3.7.1 Population Balance Equations 3.7.2 Slope and Intercept Metbod 3.7.3 Interval lil Kinetics The Fate Parameter References

Chapter 4 Experimental Procedures

4.1 Introduetion 4.2 Emulsion Polymerization Reactions

4.2.1 Reactor Design 4.2.1.1 Batch reactor 4.2.1.2 Dilatometer 4.2.1.3 Densimeter

4.2.2 Ah lnitio Reactions 4.2.3 Seeded Reactions

4.3 Seed Preparation 4.4 Conversion Measurements

4.4.1 Gravimetry 4.4.2 Di1atometry 4.4.3 Densimetry

4.5 Partiele Size Measurements 4.5.1 Dynamic Light Scattering 4.5.2 Transmission Electron Microscopy 4.5.3 Disk Centrifuge Photosedimentometry 4.5.4 Partiele Nurnber Concentration

4.6 Surface Tension Measurements 4.6.1 Du Nouy Ring-Metbod 4.6.2 Maximum Pressure Bubble Tensiometry

4.7 Initiator Decomposition Measurements References

Chapter 5 The Application of Inisurfs

5.1 5.2

Introduetion Syrnmetrica1 Inisurfs 5.2.1 Ah lnitio Reactions with Inisurf SE0-350 (7a) and

SE0-550 (7b) 5.2.2 Ah lnitio Reactions with lnisurf SC0-630 (7c)

Contents

44 46 47 49 50 51 53 55 57

59 60 60 60 60 62 64 64 66 68 68 69 70 71 72 72 73 74 75 75 75 76 77

81 83

83 84

Contents

5.2.3 Ab Initio Reaction with Inisurf SC0-880 (7d) 5.3 Asymmetrical Inisurfs

5.3.1 Ab Initio Reactions with Inisurf AE0-550 (7e) 5.3.2 Ab Initio Reactions with Inisurf AC0-630 (7f) 5.3.3 Ab Initio Reaction with Inisurf AC0-880 (7g)

5.4 Discussion References

Chapter 6

6.1 Introduetion

Inisurfs with Antarox C0-880 as Surfactant Moiety

6.2 Dissociation Rate 6.3 Critical Micelle Concentration 6.4 Ab Initio Reactions with SC0-880 and AC0-880 6.5 Molecular Weight

References

Chapter 7

7.1 Introduetion

Kinetics of Partiele Growtb in Seeded Emulsion Polymerization witb Inisurfs

7.2 Symmetrica1 Inisurf 7.3 Asymmetrical lnisurf 7.4 Discussion

References

Epilogue

Glossary of Symbols

Acknowledgement

Curriculum Vitae

XV

88 88 89 89 92 92 95

97 98

100 104 109 110

114 116 123 126 134

137

141

145

147

XVI

Chapter 1

Introduetion

Summary: In this chapter a general introduetion to polymers and polymerization wilt be given. Elaborating on emulsion polymeriza· tion, the various problems associated with the use of Latices wiJl be discussed, as welt as the solutions of!ered. A historica/ overview including the work done by other groups will he given, focusing on the use of surface-active initiators: "inisurf'. Following this, the aim of the investigation wiLt he given. The chapter closes with a survey of the thesis.

1.1 General Introduetion

A polymer is a macromolecule, which is obtained by permanently

combining a large number of small organic molecules, called monomers, by

covalent bonds. Formation of a polymer, called polymerization, can be carried out

in two ways: step-reaction and chain-reaction polymerization.

Step-reaction or condensation polymerization takes place between small, at

least bifunctional, molecules, with the elimination of a small molecule such as

water. Each polymer chain formed can react further with monomer, oligomerîc or

polymerie chains, which means that each polymer chain grows over the whole

course of the reaction and that the average molecular weight of the polymer

2 Chapter 1

increases during the whole reaction.

Chain-Reaction or addition polymerization involves a growing chain with

one reactive site, which may be an ion or a free radical, and an unsaturated

molecule as monomer. Only these reactive ends can react. A chain-reaction

polymerization consists of three stages: initiation, the formation of reactive sites

and the addition of the first monoroer unit; propagation, the addition of more

monoroer to the growing chain end; termination, the disappearance of reactive sites

and the formation of "non-living" polymer chains: polymer ebains without a

reactive endgroup. However, new polymer ebains are continuously generaled

throughout the course of the polymerization and a polymer molecule is formed

very rapidly on the timescale of the entire polymerization.

The production of polymer can be carried out in different ways. Focusing

on radical chain polymerization, four methods can be distinguished. These

different methods will be outlined briefly, and their main advantages and

disadvantages will be included.

Bulk Polymerization. Only monoroer is charged into the reaction vessel

and, when necessary, an initiator is also charged. Normally the polymer formed is

soluble in the monomer. The advantages of this metbod is the easy processing, the

use of simple equipment, and the formation of a polymer with a minimum of

contamination. The disadvantage is that it is difficult to control the reaction due to

an increase in the viscosity. As a result, the removal of the heat of polymerization

becomes a problem. Locally the temperature can increase, which causes the

production of heterogeneities.

Solution Polymerization. A solvent is used to dissolve the monomer. In

general this process is easier to control since the viscosity does not change much

during the reaction. The problem of the heat removal is therefore reduced to a

significant extent. The disadvantages include: the use of an expensive, toxic and

often inflammable solvent, the separation of the polymer from the solvent at the

end of the reaction, and the possible serious reduction of the molecular weight by

chain transfer to solvent.

Introduetion 3

Suspension Polymerization. Water is used as the continuous phase and acts

as a heat-absorbing and -conducting medium. The monomer is suspended in smal!

droplets. The initiator is soluble in the organic phase. Polymerization takes place

inside the micron size droplets, which behave like little bulk polymerization

vessels. In this system the problerns of heat removal and toxicity of the solvent are

solved. However, attention bas to be paid to the stability of the dropiets in order

to avoid coalescence. The dropiets are kept in suspension by the generation of

sheer, resulting from agitation, in combination with the use of water-soluble

stabilizers. At the end of the reaction, the polymer beads can easily be separated

from the continuous (aqueous) phase. The polymer so obtained is in general less

pure compared to the polymer formed in bulk or solution polymerization.

Emulsion Polymerization. Water acts here also as the continuous phase.

The monomer is dispersed in micron size dropiets in the continuous aqueous phase

and stabilized by an oil-in-water emulsifier. Usually a water-soluble initiator is

added to start the free-radical addition polymerization. In contrast to suspension

polymerization, the loci of polymerization are the smal! micelles. This results in a

reaction medium consisting of submicron polymer particles swollen with the

monomer and dispersed in the aqueous phase. The final product is a stabie latex: a

dispersion of submicron polymer particles in water. In emulsion polymerization

these particles are much smaller compared to suspension polymerization and are

typically 50-500 nm in diameter depending upon recipe and polymerization

conditions 1• Advantages of this system include: it retains at solid contents of less

than 40% its relatively low viscosity. This results in an efficient heat transfer and

therefore good temperature controL Toxic and flammable organic solvents do not

have to be used and the reaction can proceed to high conversion at a relatively high

speed. A major advantage is that it is possible to obtain polymer with a high

molecular weight, which can be controlled easily by a chain transfer agent. A

disadvantage is that the polymer recovered from the latex bas a relatively high

proportion of additives, such as surfactant and chain transfer agent.

In the remainder of this thesis we will focus only on emulsion

polymerization.

4 Chapter 1

1.2 Background of this Investigation

In order to cope with the shortage of natura! rubber prior to and during

World War I, some first attempts to develop an artificial product as a substitute

were undertaken in Germany, by means of emulsion-polymerization-like processes.

Nevertheless, it was not until 1927 when the first reference to a process, which can

be regarcled as a true emulsion polymerization, appeared; a patent was granted to

Dinsmore2 working for the Goodyear Tire & Rubber Company. Luther and

Heuck3 were the first to introduce initiators to facilitate rapid polymerization.

With these successful attempts, the development of a complete industry started.

World War 11 gave a new impulse to this development. In the United States the

Synthetic Rubber Program under the supervision of the Office of Rubber Reserve

led to the successful production of synthetic rubbers for general purpose. Since

then the number of papers that appeared on emulsion polymerization has grown

exponentially, and the field is still growing in industrial importance. A detailed

historica! survey has been given by Blackley4•

The synthetic rubbers produced by emulsion polymerization are obtained in

the form of a latex. These latices are used in a wide variety of applications,

particularly as coating either by themselves or in formulation with pigments and

other additives. Thus, they are used, for example, in paints and floor-polishes, and

in coatings applied to, for example, paper, paperboard, and plastic films. However,

when conventional surfactants are used in effecting emulsion polymerization,

inherent difficulties are encountered. In the latex state, the surfactant freely

desorbs from and adsorbs onto the surface of the polymer particles. This can lead

to a change of stability over time. When the resultant latex is coated and dried on

a substrate such as paper, the surfactant, which is only physically bonded, does not

remain distributed uniformly between the coalescing polymer particles of the

drying latex. Instead, the surfactant molecules exude to the surface or the interface

of the coating. This leads to poor mechanica! properties, low water resistance, and

also to blooming and blushing5.

Introduetion 5

A solution to the problems caused by the non-bonded surfactant could be

the fixation of the surfactant to the surface of the polymer particle, thîs without

Iosing the stahilizing ability of the surfactant molecule. There are three ways to

obtain in situ a chemica! bond between the polymer and the surfactant:

1) A monomer with surfactant properties (surfmer) could be added. In

principle this would mean performing a copolymerization. The incorporation of the

surfmer into the polymer depends on the reactivity of monomer and surfmer, and

also on the way the polymerization is carried out. More than one monomeric

surfactant can be built in into a polymer chain. In the extreme case, this would

lead to a water-soluble polymer.

2) A surfactant with the properties of a chain-transfer agent (transsurf)

could be used. The transsurf and the monomer should be consumed at such rates

as to keep their concentration ratio constant. If the transsurf is consumed more

quickly, a product with a different molecular weight and molecular weight

distribution will be formed at the end of the polymerization due to the fact that

chains are no Jonger stopped by the chain transfer agent If the transsurf is

consumed slower there will be free surfactant left after the polymerization 1s

completed. The formed polymer chains contain one surfactant unit at most.

3) The last possibility is the application of an initiator with surface-active

properties (inisuif *). In this case the initiator dissociates into surface-active free

radicals. Every initiation by such a primary radical leads to a surfactant-polymer

link. The surfactant is gradually linked by a chemica} bond to the polymer

throughout the course of the polymerization. Depending on chain transfer to

monomer and the type of termination, the polymer chain can contain zero, one or

two surfactant units.

Instead of surfactant fixation in the course of the polymerization, post

In this thesis the term "inisurf' is used for substances that contain one initiating moiety and at least one emulsifying moiety. Matcrials with more initiating groups in one molecule, whether or not they produce surface-active rad ie als, are referred to as "poly-initiators".

6 Chapter I

polymerization partiele surface modification could be applied to obtain a surface

with bonded surfactant. This cumhersome method, which involves a comonomer

with an additional, suitably reactive group, ineludes several cleaning and reaction

steps and will nol be discussed any further.

From the aforesaid possibilities of solving the surfactant problem the

application of an inisurf has our major interest. lt appears that this approach has

the least disadvantages. Moreover, little is known about inisurf synthesis and their

application in emulsion polymerization.

Besides the above mentioned practical application of an inisurf, the creation

of radicals at the surface of a polymer partiele is also of scientific interest. Such a

localized production of radicals might influence the formation of new particles, and

might have an effect on the partiele size distribution, as well as on the rate of

polymerization. Thus a study of inisurf in emulsion polymerization could lead to

more insight into the formation of polymer particles.

Possible application areas of inisurfs, other than above mentioned, are the

formation of particles with a specific size, the production of well-grafted polymer

systems and the production of particles with a well-defined core-shell morphology.

1.3 Bistorical Overview

In this historica! overview only the work carried out in the field of inisurfs

or surface-aclive poly-initiators will be considered. Until now little work has been

done on inisurfs or other compounds which conneet the surfactant to the polymer

in the course of the polymerization.

Pavljucenko et al.6 reported in 1978 the modification of non-ionic

surfactant by the introduetion of peroxy groups. The introduetion of the peroxy

Introduetion 7

group on the terminal hydroxyl group had a large influence on the colloid chemica!

properties. This led to the condusion that the formed micelles had a more ordered

structure. It was advised to use these initiating surfactauts in combination with

other surfactants, either ionic or non-ionic.

In 1981 Ivancev et aC publisbed work on inisurf based on a peroxide

initiator and a non-ionic surfactant of the poly(ethylene oxide) type. In this work a

low initiation efficiency was mentioned. Moreover, a high molecular weight of the

formed polymer was observed, as well as a narrow partiele size distribution (PSD),

which was readily explained by the reduced rate of initiation and an extremely low

chain terminalion rate. lt was also found that the polymerization rate was

independent of the concentration of added co-surfactant. This was explained by the

high rates of partiele growth and disappearance of micelles compaired with the rate

of partiele formation. These authors8 also found that the dissociation rate of the

inisurf was higher than the dissociation rate of the corresponding initiator. lt was

suggested that this was due to the localization of inisurf on the surface. It is

important to mention that in all this work besides the surface-active initiators other

surfactanis were used as well, so as to increase the stability of the system.

Tauer et al.9 reported in 1988 on work done on surface-active initiators,

where the initiator moiety is of the azo-type and the surfactani is an a,ro-diol with

either a poly(ethylene oxide) or a poly(propylene oxide) centre part. Depending on

the diollinitiator ratio, it was possible to produce an inisurf or a poly-initiator with

up to 104 initiator groups. A stabie latex could be produced with only monomer,

water and surface-active initiator, and with a solid content of up to 40 %. The

latex produced in this way had a lower level of electrolyte content and a lower

foam formation ability. The final product showed a lower water absorption, an

increase in mechanica! properties, and an increase in heat and light resistance. An

added advantage was a decrease of environmental polJution by lowering the

surfactant content of the waste water.

In 1990, in a second paper, Tauer et al. 10 reported on the increase of the

molecular weight of the formed polymer compared to a conventional system. This

result wil! be discussed in Chapter 6.

8 Chapter 1

Salamone et al. 11 reported on surface-active initiators with an azo-type

initiator moiety and either a poly(ethylene oxide) or a poly(ethylene oxide) methyl

ether surfactant moiety. In addition to the synthesis of the inisurf and the surface­

active poly-initiators, the molecular weight measurements of the formed

components, the determination of the critica! micelle concentration and the micellar

molecular weight were reported as well.

A patent was granted to Tauer et alY in 1990. lt describes the synthesis

of a surface-active initiator with an azo-type initiator moiety and an ionic surfactant

moiety, as well as the production of a highly monodisperse latex by emulsion

polymerization, where the reaction mixture contains only monomer, water and

surface-active initiator. It was suggested that the major application of the so

formed latex will be as a dispersion binder, or in medica! and biomedical systems.

In a series of newsletters of the International Polymer Colloid Group,

Blackley13 reported on the inisurfs of symmetrical azo-initiators and nonionic

surfactants of the alkyl poly(ethylene oxide) type. The synthesis and the

dissociation constants of the formed inisurfs were given.

1.4 Aim of this Investigation

The aim of the study described in this thesis is to synthesize inisurfs, and to

investigate their behaviour in emulsion polymerization. This aim can be translated

into four sub-tasks: I) synthesis, 2) check of the possiblity of application in

emulsion polymerization, 3) physical properties of the inisurf and 4) determination

of kinetic parameters in emulsion polymerization.

Synthesis. A route has to be developed for the preparation of a material

with both surfactant and initiator properties. An azo-type initiator with carboxy

groups and a nonionic surfactant with one hydroxy group will be tested for use as

starting materials. An esterification will be carried out to combine the initiator and

Introduetion 9

surfactani parts. The molecular structure of the resultant inisurf will be verified

with the aid of proton Nuclear Magnetic B.esonance eH NMR).

Check of the Possibility of Application in Emulsion Polymerization. After

the synthesis, the inisurf will be tested in emulsion polymerization. The inisurf

must be able to stabilize the monomer suspension and preferably form micelles. It

should also form free radicals that can initiale the polymerization. At the end of

the polymerization the inisurf must be able to stabilize the latex. In these emulsion

polymerizations styrene is chosen because of the substantial knowledge available

about this monomer. During the polymerization, the possiblr occurrence of phase

separation, as well as the polymerization rate will indicate suitability of the inisurf.

Physical Properties of the lnisurf In order to gain further insight, it is also

necessary to delermine certain physical parameters. Since an inisurf is surface­

aclive it will form micelles and the Çritical Micelle Çoncentration (CMC) wil! be

determined by the DuNouy-ring-method14 or with the Maximum Bubble Pressure

Method15• Surface coverage of an inisurf has to be taken into account as wel!,

and thus the adsorption isotherm onto polystyrene particles will be determined.

Since the actdition of a surfactani to this initiator might influence its dissociation

behaviour, the rate of dissociation wil! be measured by means of UV-measurements

as a function of time and temperature.

Kinetic Parameters. Besides the batch reactions, from which reaction rate

and partiele diameter are determined, seeded reaelions will be carried out to

delermine the parameters that govern the growth of the polymer particles. These

seeded reaelions will be carried out in dilatometers, where the drop in volume wil!

be monitored as a function of time 16, to obtain the information on conversion

necessary for calculating the growth parameters. Besides dilatometry,

densimetry 17 will be used in the seeded reactions. In a closed loop the reaction

mixture will be pumped through a density cel! and back into the reactor. The

change in density will be monitored as a function of time.

10 Chapter 1

1.5 Survey of this Thesis

A short outline is now given of the remaining chapters in this thesis.

Chapter 2: The various possible choices for the initiator part and the

surfactant part will be discussed. The synthesis of initiators and inisurfs will be

outlined in detail and the measured 1H NMR spectra will be discussed.

Chapter 3: Qualitative and quantitative models for emulsion

polymerization will be discussed. A kinetic description will be given of the so

called "zero-one" system in Intervals II and III. In addition, a metbod determining

kinetic parameters, such as the entry (p) and exit (k) rate coefficients, will be

descri bed.

Chapter 4: The various experimental and analytica! techniques used in

carrying out the emulsion polymerizations and characterizing the formed latex will

be described. Attention will also be paid to the methods used in determining some

important characteristics of the synthesized inisurfs.

Chapter 5: The first results of applying the various inisurfs in the

emulsion polymerization of styrene will be discussed. Only the results of ab initia

reaction will be given in this chapter. Applicability of the inisurf will be deduced

from the stability of the emulsion polymerization system. The effect of co­

surfactant, applied in some systems, will be discussed as wel!.

Chapter 6: The results of detailed studies of two inisurfs: a symmetrical

and an asymmetrical one, will be discussed. Dissociation rates of these inisurfs

wil! be given as a function of time. The CMC and the adsorption isotherm of

these inisurfs at room temperature, will be discussed. Reaction rates, partiele size

distributions, and molecular weight distributions wil! be evaluated.

Introduetion ll

Chapter 7: The results of seeded emulsion po1ymerization reactions

employing the two inisurfs as described in Chapter 6, will be given. The growth

parameters of a polymer particle, i.e., entry (p) and exit (k), will be discussed.

Efficiency of initiation of the primary surface-active free radicals will be given as a

function of coverage of the seed particles with inisurf and wil! be compared to

conventional systems.

Epilogue: The conclusions drawn from the results mentioned in the

previous chapters, will be discussed and some suggestions for further research will

be given.

Parts of this work have been presented at the IUP AC International

Symposium on Macromolecules (Montréal, Canada, July 1990) and the Gordon

Research Conference on Polymer Colloids (lrsee, FRG, September 1992).

Parts of this thesis have been publisbed or will be published: the synthesis

of symmetrical inisurfs (part of Chapter 2) and the ab initia reactions with these

inisurfs as described in Chapter 518; the seeded study with symmetrical inisurf of

Chapter 719; and the synthesis of asymmetrical inisurf (part of Chapter 2), the ab

initia reactions with these asymmetrical inisurfs as described in Chapter 5 and the

seeded study with asymmetrical inisurfs of Chapter 720•

References:

I. Bovey, F.A.; Kolthoff, I.M.; Medalia, AL; Meehan, E.J. Emu/sion Palymerizatian; Interscience Publishers: New York, 1955.

2. Dinsmore, R.P. U.S. Pat. 1,732,795, 1929; Chem. Abstr. 1930, 24, 266. 3. Luther, M.; Heuck, C. U.S. Pat. 1,860,681, 1932; Chem. Abstr. 1932, 26,

3804. 4. Blackley, D.C. Emulsian Palymerisation; Applied Science Publishers Ltd.:

London, 1975.

12 Chapter 1

5. Dickstein, J. Polym. Prep.-Am. Chem. Soc., Div. Polym. Chem. 1986, 27, 427. 6. Pavljucenko, V.N.; Ivancev, S.S.; Ro~kova, D.A.; Dikaja, N.N.; Domniceva,

N.A.; Budtov, V.P. Kolloid Z. 1978, 40, 64; Eng. Transl.: Colloid J. USSR 1978, 40, 48.

7. Ivancev, S.; Pavljucenko, V.N. Acta Polym. 1981, 32, 407. 8. Ivancev, S.; Pavljucenko, V.N.; Byrdina, N. J. Polym. Sci., Polym. Chem. Ed.,

Part A-1 1987, 25, 47. 9. Tauer, K.; Goebel, K.-H.; Kosmella, S.; Neelsen, J.; Stähler, K. Plaste

Kautsch. 1988, 35, 373. 10. Tauer, K.; Goebel, K.-H.; Kosmella, S.; Stähler, K.; Neelsen, J. Makromol.

Chem., MacromoL Symp. 1990, 31, 107. 11. Salamone, J; Liao, W; Watterson, A. Polym. Prep.-Am. Chem. Soc., Div.

Polym. Chem. 1988, 29, 275. 12. Tauer, K.; Goebel, K.-H.; Neelsen, J.; Kosmella, S.; Stähler, K.; Schirge, H.;

Kaltwasser, H. G.D.R. Pat. 276 877 Al, 1990; Chem. Abstr. 1990, 113, 232262s.

13. (a) Blackley, D. In International Polymer Colloids Group's Newsletter; Napper, D., Ed.; 1989, 20 (1), 4. (b) Blackley, D. In International Polymer Colloids Group's Newsletter; Napper, D., Ed.; 1989, 20 (2), 4. (c) Blackley, D. In International Polymer Colloids Group's Newsletter; Napper, D., Ed.; 1990, 21 (1), 8.

14. DuNouy 1. Gen. Physiol. 1918, 1, 521. 15. Sirnon Ann. Chim. Phys. 1851, 32, 5. 16. Fryling, C.F. Ind. Eng. Chem., Anal. Ed. 1944, 16, 1. 17. Poehlein, G.W.; Dougherty, D.J. Rubber Chem. Techno!. 1977, 50, 601. 18. Kusters, J.M.H.; Leijten, A.M.M.; Van Es, J.J.G.S.; German, AL. in

preparation. 19. Kusters, J.M.H.; Napper, D.H.; Gilbert, R.G.; German, A.L. Macromolecules

1992, 25, 7043. 20. Kusters, J.M.H.; Verweerden, T.M.M.; Van den Enden, M.J.W.A.; German,

AL.; Gilbert, R.G. in preparation.

Chapter 2

Synthesis

Summary: In this chapter the choice of initiator and surfactant, used in the synthesis of inisurf, wil/ be examined. An outfine of the method used for the synthesis of inisurf will be given. The various inisurfs synthesized wilt be mentioned and their 1H NMR spectra will be discussed. This is foliowed by a description of the experimental procedures of the syntheses.

2.1 Introduetion

An inisurf consists of two parts: an emulsifying moiety and an initiating

moiety. The link between the surfactant and the initiator in the inisurfs used in this

investigation, was envisioned to result from an esterification. This type of reaction

can result in a very high yield, while being performed under comparatively mild

conditions. In particular a low reaction temperature is necessary in order to obtain

the inisurf, since the initiators used, being similar to the ones normally used in

emulsion polymerization, would readily dissociate during the synthesis, if carried

out at elevated temperatures.

Since not every initiator is compatible with the conditions of esterification,

the different types of initiators, as well as the selection procedures used for these

14 Chapter 2

initiators, will be discussed. This will be foliowed by an overview of the available

types of surfactants, including the ones selected. Not all the initiators, which were

of interest for this investigation, are commercially available. Therefore, the

discussion will be extended to the synthesis of a non-commercially available

initiator. The choice of the metbod of esterification of initiator with surfactant will

be elucidated. Results of the syntheses of the initiator and of the various inisurfs

will then be discussed, and the chapter will close with the experimental details.

2.2 Choice of Initiator and Surfactant

Different types of initiators and surfactants can be used in emulsion

polymerization. These types of initiators and surfactants will be reviewed, after

which the choice made will be corroborated.

2.2.1 Initiator

An initiator is a chemica! substance or a radiation source, which is capable

of producing free radicals. There are several types of free radical initiators, and

some of these will be discussed now.

Peroxides. These initiators have the general structure: ROOR' or ROOH,

and dissociate thermally into two free radicals by cleavage of the oxygen-oxygen

bond. The dissociation rate of the peroxides is strongly influenced by the groups

neighbouring the peroxide-function. This influence can be divided into three major

effects: (1) the relative stability of the radicals formed, (2) steric effects, and (3)

electronic effects. In addition to these structural effects, some peroxides are also

susceptible to radical-induced dissociation, which leads to a loss of efficiency,

smce the peroxides then dissociate without adding more radicals to the system.

This radical-induced dissociation does not occur in vinyl-monomer

___ " ___________ _

Synthesis 15

polymerization 1• Be si des the structural effects and the radical promoting effects,

the dissociation rate may also be influenced by pH, solvent, and the presence of

transition metals and contaminants.

Redox Systems. These systems consist of an oxidizing and a reducing

agent. A wide variety of compounds can be used in these systems. In the case

where peroxides are used as one of the two components of the redox system, the

dissociation temperature is lower, and the radical efficiency is decreased, when

compared with the peroxide system.

Azo Compounds. These initiators have the general structure: RN=NR'.

This type of initiator dissociates thermally and produces two free radicals and a

nitrogen molecule. As with peroxides, the side groups that are attached to the

azo-function have an effect on the dissociation rate. In contrast to the case of most

peroxides, the dissociation rate of azo compounds shows only a minor solvent

effect and is hardly affected by transition metals, acids, bases, or contaminants.

Radiation Sources. High energy radiation, e.g., X-ray, y-ray, or ~-ray, is

able to fragment molecules into ions, as well a<> into free radicals. Alternatively,

UV -radiation, in combination with a photo-initiator, is capable of producing free

radicals. In some cases a photosensitizer may be used.

In choosing an initiator for this work eertaio requirements have to be met.

Firstly, it is desirabie to have only one component in the initiating system. As was

mentioned in Chapter 1, one of the disadvantages of emulsion polymerization is

that the polymer obtained has a relatively high content of non-polymerie materiaL

Use of an initiator system, which consists of two or more components (e.g., redox

initiators), would increa<>e the number and amount of non-polymerie "contaminants"

and could subsequently lead to a lower performance of the final polymer.

Secondly, the components should be handled with a minimum of safety hazards.

Thirdly, the initiator needs a reactive group, besides the initiating group, that can

be used to establish the link between initiator and surfactant. Formation of this

linkage should not have any adverse influence on the initiating part. Finally, the

initiator should not dissociale during the synthesis, but the initiating moiety of the

16 Chapter 2

inisurf should still dissociate at an appreciable rate at elevated temperatures.

Only peroxides and azo-initiators satisfy the first requirement. The second

requirement eliminales peroxides as a possibility. In genera!, peroxides have to be

handled with extreme care. Thus, an azo-initiator appears to be the only type of

initiator, which satisfies the first two requirements.

The third requirement is met in a commercially available azo-initiator:

4,4'-b,zobis(4-Çyanofentanoic Acid) (la) (ACPA, see Fig. 2.1)

0 CN CN ,, ' c-CH -CH -cN=N-c-CH ·

/ 2 2 ' ' 2 HO CH3 CH3

0 "' CH -c

2 ' OH

Figure 2.1 Structure of 4,4'-Azobis(4-cyanopentanoic Acid) (la).

The two carboxy groups of this initiator can he utilized in an esterification.

Extension of the carbon chain by addition of surfactant has probably only a smal!

effect on the dissociation rate2·3·' The use of this initiator will lead to inisurfs,

which are symmetrical in the azo-function, and these will subsequently he

designated as "symmetrical inisurfs".

In the course of this investigation, an idea was developed to investigate

asymmetrical inisurfs, that will give a surface-active radical and a smal!,

water-soluble radical upon thermolysis. With regard to this, 4-t-!!utylb,zo-

4-Çyanopentanoic Acid4 (lb) (BACA, see Fig. 2.2) was envisaged to meet the

above requirement. This compound is not commercially available. The synthesis

of this initiator wil! be discussed in Section 2.3.1. The inisurf formed with this

initiator, wil! be designated as "asymmetrical inisurf".

The last requirement, suitability, wil! be investigated by carrying out ab

Determination of thc dissociation ratc of the various inisurfs synthcsized will be described in the relevant section of Chapter 6.

Synthesis 17

initio emulsion polymerization and will be discussed in Chapter 5.

0 CN CH ~ ' ' 3 C-CH -CH2-C-N=N-C-CH

/ 2 ' ' 3 HO CH3 CH3

Figure 2.2 Structure of 4-t-Butylazo-4-cyanopentanoic Acid ( lb ).

2.2.2 Surfactant

Surfactauts are substances with a hydrophilic and a hydrophobic part. The

hydrophobic part usually is a linear or branched hydrocarbon-chain, which can be

an unsaturated, or, less frequently, a halogenated or oxygenated hydrocarbon, or a

siloxane chain. The hydrophilic part is an ionic or other highly polar group. In

general, surfactauts are classified by their hydrophilic part:

Anionic Suifactants. This type of surfactant bears a negatively charged

group, e.g., RCOU Na+ and RS03- Na+.

Cationic Suifactants. This type of surfactant bears a positively charged

group, e.g., RNH/ er and RN(CH3) 3+ CL

Zwitterionic Suifactants. This type of surfactant has both positively and

negatively charged groups in the surface-active part, e.g., RNH/CH2COo- and

RN(CH3)z +CHzCH2S03-.

Nonionic Suifactants. This type of surfactant bears no ionic charge, e.g.,

RCOOCH2CHOHCH20H and R(OC2H4)pH.

Mainly anionic and nonionic types of surfactants are used in emulsion

polymerization. By applying an anionic surfactant in the synthesis, an anionic

inisurf will be formed. It was envisioned that the use of such an ionic inisurf in

emulsion polymerization would create a situation comparable with the situation in

emulsifier-free emulsion polymerization. In emulsifier-free emulsion polymeriza-

18 Chapter 2

tion5•6

•7

, mainly sodium peroxydisulphate is used as initiator. The amomc free

radicals formed upon dissociation, will oligomerize in the aqueous phase and will

become surface active. These in situ formed, surface-active free radicals are very

similar to the free radicals which would be formed by dissociation of anionic

inisurfs. Thus, it may be reasoned that the effects of anionic inisurfs on emulsion

polymerization are probably similar to the results obtained by emulsifier-free

emulsion polymerizations.

Also from a synthetic point of view, nonionic surfactants are to be preferred

in the synthesis of inisurf, because the use of an anionic surfactant would require

the selective protectionldeprotection of the anionic group as additional reaction

steps. Due to the presence of the labile azo-group, the synthesis of the inisurf

should be as simple and as mild as possible.

Additionally, nonionic surfactants have some advantages over anionic

surfactants. The degree of surface activity of nonionic surfactants can easily be

adjusted by changing the fudrophile-hipophile-,!lalance8 (HLB) of the surfactant.

Nonionic surfactants are less sensitive to the acidity (pH) or ionic strength of the

medium, since they primarily work by means of the steric stabilization

mechanism9·10

• Moreover, nonionic surfactants with a functional group, which

can be applied in the synthesis of inisurf, are more readily available than suitably

functionalized anionic surfactants.

The nonionic surfactants used, are of the poly(ethylene oxide) type. Two

different types of polyether chains have been used. The first type was not a

surfactant in itself. Water-soluble monomethyl poly(ethylene oxide)s (2) have been

used (see Fig. 2.3).

Figure 2.3

CH ~o~CH ~eH -O~H 3 L 2 2 L n

Structure of Monomethyl Poly(ethylene Oxide)s (2), with 2a: nave :::: 7; 2b: nuve = 12.

We reasoned that the resulting inisurf would be soluble in the aqueous phase like

Synthesis 19

the initiators normally used in emulsion polymerisation. After dissociation the

poly(ethylene oxide)-based radical would initiate polymerization in the aqueous

phase. After adding some monomer units the oligomer would become surface

active, upon which it would adsorb onto the surface of a polymer particle. The

surface-active radical so formed would contribute to the colloidal stability of the

polymer particles and would lead to a continuation of the polymerization in the

particles, as in the case of peroxydisulphate initiatod.

The seeond type of surfactants are "real" surfactants and of the nonylphenyl

poly(ethylene oxide) type (2) (see Fig. 2.4).

Figure 2.4

H

Structure of Nonylphenyl Poly(ethylene Oxide)s (2), with 2c: nav. = 9; 2d: nav• = 30.

We reasoned that with this surfactant, the formed inisurf would already be

surface-active. This implies that the radicals will already be formed at the surface

of the polymer particles.

2.3 Results and Discussion

Before going into the details of the syntheses of the inisurfs via

esterification of the initiator and surfactant, the synthesis of a specific type of

azo-initiator will be discussed.

20 Chapter 2

2.3.1 Asymmetrical Azo-Initiator

The asymmetrical azo-initiator BACA (lb) has been prepared in two steps

from commercially available levulinic acid (3) and t-butylhydrazine (4) (see Eq.

2.1). The first step, carried out in an aqueous acidified environmene·4, involved

condensation of the ketone group in (3) with the hydrazine (4) to give the

hydrazone (5), which reacted in situ with hydrogen cyanide to give (6).

0~ /,/0 /c-GI2-CH:2-C, +

~H3 H2N-NH-~-GI3

HO CH3 (3)

Gl (4) 3

~0

' CH:l 0~ 1'N··NH-C-GI3

/c rn2 c~-c, 013 HO (S) Gl3

0~ ~ ~~ / C-CH:2-CH2-~·-NH- NH-~-GI3

HO GI3 CH3 (6)

Without purification, (6) is oxidized to (lb) (see Eq. 2.2).

0~ ~ ~3 Oxidation

0~ ~

(2.1)

/C CH2 ·CH2-~-NH-NH-~-CH3 / C- CHc Cl-!:2· N= N c-- GI3 HO rn3 GI3

(6)

HO CH CH 3 3 (2.2)

(lb)

Upon precipitation, BACA (lb) was obtained as a white powder in 70%

yield and had a melting point of 353.5 K (lit4: 353 K). The 1H and 13C NMR

spectra (see Section 2.5) are in good agreement with the proposed molecular

Synthesis 21

structure. Experimental details are described in Section 2.5. BACA was used in

the esterification without further purification.

2.3.2 Inisurf

The choice of initiator and surfactani moieties was governed by the fact that

the conceived way of obtaining the link an esterification - could be carried out

under comparatively mild conditions, i.e., at, or preferably below, room

temperature. This requirement is necessary since the azo-initiator, used as one of

the reaction components, dissociates at elevated temperatures. Within the

boundaries of this requirement there are a number of ways of performing an

esterification, the most important being: ( 1) actdition of an excess of one of the

reactants, usually the alcohol, and (2) transformation of the carboxylic acid into an

acid halide or active ester compound, which readily reacts with a stoichiometrie

amount of alcohol.

Method 1: the addition of an excessof the alcohol, which in our case is the

surfactant, is not really feasible. Due to the fact that physical properties, e.g.,

solubility in an organic or aqueous phase, surface activity (foaming), of the formed

inisurf and the used surfactani are comparable, it is almost impossible to carry out

a separation on a preparalive scale. In the case of expensive starting materials, the

addition of an excess of one of the materials is not preferabie either.

Method 2: the transformation of a carboxylic acid into its acid halide was

tried with la (ACPA) (using phosphorus pentachloride in benzene at 273 K), but

only a 40% yield of the dihalide was obtained. The reaction of this bisacylchloride

with an alcohol had, in genera!, a yield of about 70%. This leads to an overall

yield of only 30%.

A better way was found in the use of Q.i_çyclohexylç_arbodiimide (DCC) as

the condensing agent. This is a one pot procedure that can generally be carried out

at, or significantly below, room temperature. By means of some modifications of

22 Chapter 2

the original procedure, yields of up to 95 % can be achieved, using a wide variety

of alcohols and acids as the reaction partners in stoichiometrie amounts (see Eq.

2.3). These modifications and improvements have been described by a number of

groups, e.g., Holroberg et a/. 11, Hassner et al. 12

, and Shinkai et alY. They

reported that the use of DCC by itself does not give satisfactory results. The yield

can be increased by the actdition of a catalytic amount of a strong acid (Jzara­

!oluene.§.ulfonic ~cid, pTSA) to the solution of the carboxylic acid and the alcohol

in pyridine11• Pyridine also acts as a catalyst; it can be replaced by an

aminopyridine (generally .Q.imethyl~mino.t:!Yridine, DMAP) (added in catalytic

amounts) in toluene as solvent12, which is preferabie for health reasons. As a side

product gi.f}'clohexyl!J.rea (DCU) is formed. Further experimental details are

described in Section 2.5.

DCC

pTSA

0 DMAP «o ij (2.3)

R1c, + R20H R1c,

OH DCU o-R2 (1) (2) (7)

For the various inisurfs synthesized, different abbreviations are used. The

abbreviation begins with an S or an A indicating whether it is a .§.Ymmetrical or an

~symmetrical inisurf. The remainder of the abbreviation: two letters and a number,

indicates the type of surfactant and the average length of the ethylene oxide chain

in the surfactant, respectively: EO-# for monomethyl poly(l;;.thylene QXide) and

CO-# for nonylphenyl poly(ethylene oxide) (Antarox® CO#).

The following inisurfs have been synthesized: SE0-350 (7a) and SE0-550

(7b) (see Fig. 2.5), SC0-630 (7c) and SC0-880 (7d) (see Fig. 2.6), AE0-550 (7e)

(see Fig. 2.7), AC0-630 (7f) and AC0-880 (7g) (see Fig. 2.8).

Synthesis 23

Figure 2.5 Structure of Symmetrical Inisurfs SEO-# (7) with a Monomethyl Poly( ethylene Oxide) Chain as Surfactant Moiety, with 7a: nave ::: 7; 7b: nav. = 12.

Ctjll ,, ' C-UI ·····CH -C-N 0 CN l

~))-o-{rn,-rn,-of, 2 2 rn, 2

Figure 2.6 Structure of Symmetrical Inisurfs SCO-# (7) with a Nonylphenyl Poly(ethylene Oxide) Chain as Surfactant Moiety, with 7c: nave = 9; 7d: nave = 30.

Figure 2.7 Structure of Asymmetrical lnisurf AE0-550 (7e) with a Monomethyl Poly(ethylene Oxide) Chain as Surfactant Moiety (nave ::: 12).

Figure 2.8 Structure of Asymmetrical Inisurfs ACO-# (7) with a Nonylphenyl Poly( ethylene Oxide) Chain as Surfactant Moiety, with 7f: n,we = 9; 7g: na•• = 30.

The esterifications were monitored by 1 H NMR. For the surfactant starting

materials, all ethylene oxide units appear in the same region, at o 3.50-3.90 ppm

24 Chapter 2

for monomethyl poly(ethylene oxide) and at a 3.55-3.75, 3.85, 4.11 for

nonylphenyl poly(ethylene oxide). Upon esterification, the methylene group(s) of

the first ethylene oxide unit(s) directly attached to the ester group(s), gave rise to a

distinct resonance at a 4.2-4.3 ppm. The integral of these protons was compared

with the integral of the CH1-CH2-group of the initiator moiety, which was not

significantly shifted upon esterification.

The inisurfs (7) were isolated by flitration of the formed

1,3-.!!i_çyclohexylQrea (DCU) (not soluble in toluene), the dimethylaminopyridine

(DMAP), and the p-toluenesulfonic acid (pTSA) foliowed by evaporation of the

solvent. The yields were determined from both the isolated amounts of inisurf (7)

and 1 ,3-dicyclohexylurea (DCU), and are reported in Tabel 2.1. The resultant

inisurfs (7) were kept at 277 K and were used without further purification.

Table 2.1. lsolated Yields of the Formed lnisurfs (7).

Inisurf Isolated Yield (%)

Inisurf (7) 1,3-Dicyclohexylurea (DCU)

SE0-350 99 97

SE0-550 96 95

SC0-630 99 99

SC0-880 98 98

AE0-550 99 98

AC0-630 98 98

AC0-880 100 99

lnisurfs were mainly characterized by their 1H NMR spectrum as described

above. The 13C NMR spectra were rather complex due to the fact that the used

surfactants were commercial products, containing ethylene oxide units, with a

varying chain length and, in the case of Antarox-based inisurfs, isomerie nonyl

phenoxy units. Nevertheless, these spectra revealed the presents of the ester group

Synthesis 25

and of the nitrile group, and, in case of the asymmetrical inisurf, of the t-butyl

group.

Biemental analyses of the inisurfs were not perforrned since the used

surfactants are mixtures containing poly(ethylene oxide) chains of different lengths.

2.4 Conclusions

From the synthetic work presented in this chapter it can he concluded that

the method for synthesizing an asymmetrical azo-initiator is suitable. It can also he

concluded that the chosen esterification method is suitable for the syntheses of

inisurfs in a very high yield.

2.5 Experimental Section

Reagents 4,4' -Azobis( 4-.Çyano~entanoic Acid) (la) (ACPA, purum, Fluka

AG, Buchs, Switzerland) (see Fig. 2.1) was used as received. 4-t-Butylazo-4-

cyanopentanoic acid (lb) (BACA) (see Fig. 2.2) was obtained from commercially

available levulinic acid (3) and t-butylhydrazine hydrochloride (4) as described

below and was used without forther purification. Levulinic acid (3) (80%, Janssen

Chimica, Geel, Belgium) was distilled once before use. t-Butylhydrazine

hydrochloride (4) (>95%, Janssen Chimica, Geel, Belgium), sodium cyanide (pro

analysi, Merk, Darmstadt, FRG), and chlorine (Hoekloos, Schiedam, The

Netherlands) were used as received. Monomethyl poly(ethylene oxide)s (2a) and

(2b) (Aidrich Chemie, Bornem, Belgium), as well as nonylphenyl poly(ethylene

oxide)s (2c) and (2d) (Antarox® Co, GAF, New York, USA) were dried by means

of azeotropic distillation with toluene. Dicyclohexy lcarbodiimide (DCC,

C6H 11N=C=NC6H11 , 99%, Janssen Chimica, Geel, Belgium) was recrystallized from

26 Chapter 2

diethyl ether. p-Toluenesulfonic acid (pTSA, 99%, lanssen Chimica, Geel,

Belgium) was dried at 313 K in vacuum shortly before use.

Dimethylaminopyridine (DMAP, 97%, lanssen Chimica, Geel, Belgium) was used

as received.

Solvents Dichloromethane (pro analysi, Merk, Darmstadt, FRG) was used

as received. Toluene (pro analysi, Merk, Darmstadt, FRG) was dried over

molecular sieves (pore size: 3 Á).

lnstrumentation The 1H and 13C NMR spectra were recorded at room

temperature with a Bruker AM 400 spectrometer, using trichloromethane-d (CDC13)

as the solvent and tetramethylsilane (TMS) as internal standard, unless stated

otherwise. Infrared spectra were obtained with a Mattson Polaris™ FT-IR

Spectrometer. Melting points were determined on a Büchi melting point apparatus

and are uncorrected.

Synthesis of the Asymmetrical Azo-Initiator4• To a salution of 31.2 g (0.25

mol) of t-butylhydrazine hydrachloride (4) and 12.3 g (0.25 mol) of sodium

cyanide in 80 mL of water in a jacketed 0.5 dm3 reactor equipped with an efficient

mechanica! stirrer, thermometer, condenser and dropping funnel, was added

dropwise 29.0 g (0.25 mol) of freshly distilled levulinic acid (3) over a period of 5

min. The reaction mixture heated up to 313-318 K during this addition period and

was then heated for 2 h at 323 K. The reaction mixture became more viscous

during this period. Subsequently, the reaction mixture was cooled to 273 K, and

50 mL of dichloromethane and 50 mL of water were added. Chlorine was then

passed into the reaction mixture through a gas inlet tube reaching to the bottorn of

the reactor, until approximately 28.4 g (0.40 mol) of chlorine had been absorbed.

During this period the reaction temperature was maintained below 298 K. At the

end of the chlorine addition, the dichloromethane layer was separated. The

unreacted hydragen cyanide in the aqueous layer was treated with a 20 wt%

salution of NaOH in water, foliowed by sodium hypochlorite, until bleaching of pH

Synthesis 27

paper persisted and was then discarded. The dichloromethane layer was stirred into

200 g of a 5 wt% solution of NaOH in water, and after 10 min the

dichloromethane layer was separated and discarded. The aqueous layer was

transferred into a 0.5 dm3 beaker and acidified to a pH of 1-2 with concentrated

hydrochloric acid. The precipitated 4-t-Butylazo-4-cyanopentanoic acid (lb) was

filtered off, washed with 200 mL of water and air dried.

4-t-Butylazo-4-cyanopentanoic acid (lb): white powder; mp 353.5 K (lit4:

353 K); IR (KBR) 3300-2500, 2241, 1742, 1722, 1229 cm·1;

1H NMR (CDC13) ö

1.10-1.40 (br, 9 H, C-(CH3) 3), 1.60 (s, 3 H, CH3), 2.10-2.60 (m, 4 H, CH2CH2);

13C NMR (DMSO-d6) ö 23.3 (C-5), 26.3 ((ÇH3) 3C), 28.8 (C-3), 32.7 (C-2), 68.1

(q-C, C-4), 71.0 (q-C, (CH3)&}, 119.1 (q-C, CN), 172.9 (q-C, COOH).

Synthesis of lnisurf All esterifications have been carried out under the

same conditions. The recipe used in these syntheses can be found in Table 2.2.

For each reaction, surfactant (2), dicyclohexylcarbodiimide, p-toluenesulfonic acid

and dimethylarninopyridine, as well as 60 mL of toluene were charged into a dry

250 mL round-bottom flask, equiped with a stirring bar and a drying tube. The

initiator (1) was crushed in a mortar and added in smal! portions to the solution

over a period of 1 h. Another 60 mL of toluene were added to the solution. The

flask was wrapped in aluminium foil and the contents was stirred at 277 K. After

two days, the 1,3-dicyclohexylurea (DCU) (insoluble in toluene) was removed

together with p-to1uenesulfonic acid and dimethylaminopyridine, by filtration using

a fritted glass disc filter. The toluene in the filtrate was evaporated on a rotary

evaporator at 303 K, to give the inisurfs (7), in yields as specified in Table 2.1.

SE0-350 (7a): pale yellow Iiquid; 1H NMR (CDCI3) ö 1.68-1.80 (m, 6 H,

CH3), 2.20-2.60 (m, 8 H, CH2CH2), 3.4 (s, 6 H, CH,O), 3.50-3.90 (m, CH2CHzO),

4.10-4.30 (m, 4 H, CH20CO).

SE0-550 (7b): pale yellow wax; 1H NMR (CDCI3) ö 1.68-1.80 (m, 6 H,

CH3), 2.20-2.60 (m, 8 H, CH2CH2), 3.4 (s, 6 H, CH,O), 3.50-3.90 (m, CH2CH20),

4.10-4.30 (m, 4 H, CH20CO).

28

Table 2.2. Recipe for the Esterification of Initiator and Surfactant.

Components

Surfactant (2)

Dicyclohexylcarbodiimide

Initiator (1)

p-Toluenesulfonic acid

Dimethy1arninopyridine

Toluene

Amount (mmol)

21.8", 10.9b

21.8", 10.9b

10.9

1.1

1.1

120 mL

a: Recipe for esterification with symmetrical initiator (la)

b: Recipe for esterification with asymmetrical initiator (lb)

Chapter 2

SC0-630 (7c): pale yellow wax; 1H NMR (CDC13) o 0.50-0.90 (br 6 H,

CH2C!!3), 1.20-1.40 (m, 24 H) and 1.50-1.90 (m, 14 H) (C9H19 and CH3), 2.20-2.70

(m, 8 H, CH2CH2), 3.55-3.75 (m, CH2CH20), 3.85 (t, 4 H, J 15 Hz,

ArOCH2C!!2), 4.11 (t, 4 H, J = 12 Hz, Ar0CH 2CH2), 4.20-4.30 (m, 4 H,

CH10CO), 6.90-7.10 (m, 8 H, Ar).

SC0-880 (7d): pale yellow solid; 1H NMR (CDC13) o 0.50-0.90 (m, 6 H,

CH2C!!J), 1.20-1.40 (m, 24 H) and 1.50-1.90 (m, 14 H) (C9H19 and CH3), 2.20-2.70

(m, 8 H, CH2CH2), 3.55-3.75 (m, CH2CH20), 3.85 (t, 4 H, J = 15 Hz,

ArOCH2CH 2), 4.11 (t, 4 H, J = 12 Hz, Ar0C!!2-CH2), 4.20-4.30 (m, 4 H,

CH20CO), 6.90-7.10 (m, 8 H, Ar).

AE0-550 (7e): pa1e yellow wax; 1H NMR (CDC13) o 1.05-1.40 (br, 9 H,

C(CH3h), 1.68-1.80 (br, 3 H, CH3), 2.20-2.60 (m, 4 H, CH2CH2), 3.4 (s, 3 H,

CHp), 3.50-3.90 (m, CH2CH20), 4.10-4.30 (m, 2 H, CHzOCO).

AC0-630 (7f): pa1e yellow liquid; 1H NMR: (CDC13) o 0.60-0.95 (br, 3 H,

CH2CH 3), 1.05-1.45 (m, 21 H) and 1.55-1.80 (m, 7 H) (C(CH3h, C9H19, and CH3),

2.20-2.65 (m, 4 H, CH2CH2), 3.55-3.75 (m, CH2CH20), 3.85 (t, 2 H, J 15 Hz,

ArOCH2C.tl2), 4.11 (t, 2 H, J = 12 Hz, ArOCH 2CH2), 4.20-4.30 (m, 2 H,

C-CHzOCO), 6.75-6.90 (m, 4 H, Ar).

Synthesis 29

AC0-880 (7g): pale yellow wax; 1H NMR: (CDC13) ö 0.60-0.95 (br, 3 H,

CH2CH 3), 1.05-1.45 (m, 21 H) and 1.55-1.80 (m, 7 H) (C(CH3h, C9H19, and CH3),

2.20-2.65 (m, 4 H, CH2CH2), 3.55-3.75 (m, CH2CH20), 3.85 (t, 2 H, J = 15 Hz,

ArOCH2C!!2), 4.11 (t, 2 H, J = 12 Hz, ArOCH 2CH2), 4.20-4.30 (m, 2 H,

CHpCO), 6.75-6.90 (m, 4 H, Ar).

References:

I. Sheppard, C.S.; Kamath, V. In Kirk-Othmer Encyclopedia of Chemica/ Technology; Mark, H.F., Othmer, D.F., Overberger, C.G., Seaborg, G.T., Eds.; Wiley: New York, 3. ed., 1981; Vol. 13; p 355.

2. Overberger, C.G.; Hale, W.F.; Berenbaum, M.B.; Finestone, A.B. J. Am. Chem. Soc. 1954, 76, 6185.

3. Sheppard, C.S. In Encyclopedia of Polymer Science and Engineering; Mark, H.F., Bikales, N.M., Overberger, C.G., Menges, G., Eds.; Wiley: New York, 2. ed., 1985; Vol. 2; p 143.

4. MacLeay, R.E.; Sheppard, C.S. U.S. Pat. 3,931,143, 1976; Chem. Abstr. 1976, 84, 12258lv.

5. Willes, J.M. lnd. Eng. Chem. 1949,41, 2272. 6. Kotera, K.; Furusawa, K.; Takeda, Y. Kolloid Z. Z. Polym. 1970, 239, 677. 7. Goodwin, J.W.; Heam, J.; Ho, C.C.; Ottewill, R.H. Brit. Polym. J. 1973, 5,

347. 8. Griffin, W.C. J. Soc. Cosmet. Chem. 1949, 1, 311. 9. Ottewill, R.H. Ann. Rep. A 1969, 66, 183.

10. Napper, D.H. lnd. Eng. Chem. Prod. Res. Develop. 1970, 9, 467. 11. Holmberg, K.; Hansen, B. Acta Chem. Scand. 1979, B 33, 410. 12. Hassner, A.; Alexanian, V. Tetrahedron Lett. 1978, 4475. 13. Shinkai, S.; Tsuji, H.; Hara, Y.; Manabe, 0. Bull. Chem. Soc. Jpn. 1981, 54,

631.

30

Chapter 3

Theory of Emulsion Polymerization

Summary: After an overview of free-radical polymerizalion, qualilalive and quantitative models developed for emulsion polymerizatlon will be discussed. The "zero-one" system, a special case in emulsion polymerization, will be discussed in greater detail. Some equations wil/ be derived to obtain entry (p) and exit (k) rate coefficients from raw conversion-time data at different stages of the emulsion polymerization reaction.

3.1 Introduetion

In the years following the appearance of the first papers on emulsion

polymerization1·2, the number of contributions to the onderstanding of the

mechanism of emulsion polymerization have increased substantially. Work done

by Fryling and Harringtonl, Hohenstein and co-workers4·5

·6

·7

, Kolthoff and

Dale8, and Filette and Hohenstein9

, made a large contribution to the unravelling of

the mechanism of emulsion polymerization. One of the most important models put

forward in this period is the one by Harkins. In 1945 and 1946 short notes were

published10•11

• Descriptions with greater detail of this model appeared in 194712

and 195013• This model will be outlined briefly in Section 3.3, but first all

32 Chapter 3

relavant reactions occurring in free-radical polymerization will be discussed

(Section 3.2).

The presentation of the model of Harkins will be foliowed in Section 3.4 by

a discussion of the effect of several parameters on the reaction rate, whereafter the

model description of Smith and Ewart will be given. In this description more

recent developments have been incorporated. In Section 3.6 the various nucleation

models will be discussed.

After this more general description of emulsion polymerization, attention

will be drawn, in Section 3.7, to a more specific system: the "zero-one" system.

Inisurfs will have an influence on those kinetic events, which involve the partiele

surface, i.e., entry and exit. Hence, if the effect an inisurf has on the kineties is

studied, this should be done in systems where entry and exit dominate, i.e. in

"zero-one" systems. Within the boundaries of this system, Interval III kineties will

be diseussed as well. This chapter will close with a discussion of the fate of the

exited free radieals (Section 3.8).

3.2 Free-Radical Polymerization

As already outlined in Chapter 1, a free-radieal polymerization is a ehain

reaction polymerization in which the aetive eentres are free radieals, and an

emulsion polymerization is a partieular type of free-radieal polymerization. All

free-radical reaelions involved in polymerization - initiation, propagation,

terminalion and ehain transfer occur simultaneously. These reactions wiJl now be

explained in more detail.

Initiator Decomposition and Initiation. Chain polymerization eannot occur

without an aetive eentre. In free-radieal polymerization these aetive eentres are

free radieals. The possible sourees of free radicals have already been mentioned in

Chapter 2. For ehemieal initiation, initiators undergo homolytic cleavage and form

Theory of Emu/sion Polymerization 33

two primary free radicals, according to:

I 2 R' (3.1)

where I and R' denote the initiator and the formed primary free radical,

respectively, and kd is the first-order rate constant for dissociation (s-1).

Two fates are possible for these free radicals. The fastest of these is the

relatively facile recombination with the other primary free radical derived from the

same initiator molecule (cage-effect)14•15

, or reaction with other radical-bearing

substances, ultimately producing an inert species incapable of initiating

polymerization. The other fate is the actdition of a monomer unit to a primary free

radical, called initiation:

R' + M RM; (3.2)

where M denotes monomer and RM; denotes a radkal composed of a primary free

radical moiety and one single monomerunit now carrying the radical activity. The

second-order rate constant for the addition of the first monomer unit to a primary

free radical is denoted as kP1 (dm3 mor1 s-1).

Propagation. The addition of a monomer unit to a growing chain is called

propagation. A polymer chain with n+ I monomer units is formed from a chain

with n monomer units. The general equation for the propagation reaction is:

M~ + M (3.3)

where M~ and M:+J represent growing polymer chains with n and n+ 1 monomer

units, respectively, and kP is the second-order rate constant for propagation

(dm3 mor1 s- 1), which is presumed to be virtually independent of chain length: the

starting free radical influences the rate of propagation only for a very small number

of consecutive monomer unit additions.

34 Chapter 3

Bimolecular Termination. Two growing polymer chains can react with each

other and form "non-living" polyrner material. This reaction, called termination,

can occur in two distinct ways. The first one, called combination, is the reaction

of the free radical ends of two growing chains, resulting in one saturated

"non-living" polymer chain. The second one takes place by hydrogen abstraction.

A hydrogen atom is transferred from one free radical to another, which results in

the formation of two "non-living" polymer chains, one saturated, the other with a

terminal double bond. This process is called disproportionation. A general

equation for terminalion is:

M~ + M~ ---~ Non-living polymer (3.4)

where k, is the second-order rate constant for termiaation (dm3 mor1 s-1). It was

recognized by Benson and North16, and has been shown conclusively by

Adams et al. 11, and by Russen et a/. 18

•19 that terrnination is a ditfusion

controlled process and depends on the weight fraction of polymer in the polymer

particles and the length of the growing polymer chains.

Chain Transfer. This is the reaction by which an atom is transferred to (or

abstracted from) a growing polymer chain. The growth of the original polymer

chain is stopped. The molecule T, from which the atom is abstracted (or to which

the atom is transferred), is turned into a radical, which can reinitiate a new polymer

chain (if sufficiently reactive):

M~ + T M. + T (3.5)

where k" is the second-order rate constant for transfer (dm3 mol" 1 s- 1). In principle,

polymerie free radicals can undergo chain transfer with just about any other species

present in a polymerizing system. However, in emulsion polymerization, without

any added chain transfer agent and otherwise usual conditions, it is chain transfer

to monomer that is by far the most important and prevalent form of chain transfer

Theory of Emulsion Polymerization 35

(T=M). This produces a monomerk free radkal and a terminally unsaturated

polymer chain.

3.3 Model of Harkins

As already mentioned, a basic recipe for emulsion polymerization contains

monomer, water, surfactant and initiator. At the beginning of the reaction the

monoroer is dispersed in dropiets in the continuous aqueous phase by agitation.

Depending on the amount of added surfactant, micelles of this surfactant will be

present and some of the monoroer may be present in the hydrophobic core of these

micelles. Usually, the initiator will be dissolved in the aqueous phase.

The whole of the emulsion polymerization process in a batch reactor can be

divided into three intervals.

Interval 1: Partiele Formation

Only the case where the overall concentration of the added surfactant is

higher than the Çritical Micelle Çoncentration (CMC) wiJl be considered, i.e, the

case where micelles are present at the start of the reaction. Primary free radicals,

formed by dissociation of the initiator, will enter the monoroer swollen micelles

and will initiate the polymerization. Subsequently, the micelles will be converted

into polymer particles. The entry of a radkal into a monoroer droplet is kinetically

negligible due to the small total surface area of all the monomer droplets, as

compared to the total surface area of all the monoroer swollen micelles. The

formed particles will be stabilized by adsorption of extra surfactant from not as yet

initiated micelles. This model for partiele formation is called the micellar

nucleation model (the various nucleation models will be discussed more extensively

in Section 3.6). This partiele nucleation is thought to be the primary reason for the

increase in reaction rate, which is one of the characteristics of this interval.

36 Chapter 3

When the surfactant concentration eventually drops below the CMC, due to

adsorption of emulsifier molecules onto the monomer-polymer particles, partiele

nucleation can only occur in the aqueous phase11 and in the monomer droplets10•

Since the monomer concentration in the aqueous phase is usually very low, this can

not be an effective souree for polymer particles. As already mentioned, due to the

low ratio of total surface area of the monomer dropiets to the total surface area of

the particles, monomer dropiets do not play a significant role in partiele nucleation

either. Thus, after the disappearance of the micelles, the nucleation rate will drop

dramatically.

Interval 11: Constant Reaction Rate

In this interval all three possible phases are present: there will be an

aqueous phase, polymer particles saturated with monomer, and monomer droplets.

lt is usually characterized by a constant number of polymer particles and a constant

reaction rate. It is presumed that the radical production does not change

significantly during the reaction, as is in the case of long-living initiators. The

formed radicals are absorbed by the growing polymer particles. These polymer

particles are the main loci of polymerization and they grow in size. The monomer

concentration inside the polymer particles is kept constant during this interval by

diffusion of monomer from the monomer dropiets through the aqueous phase into

the polymer particles. In order to keep the monomer concentration inside the

polymer particles constant, not only the monomer which has recently been

converted into polymer, needs to be replaced, but also the monomer which is

needed for the swelling of the newly formed polymer, has to be incorporated. The

monomer dropiets function as monomer reservoirs only. At a certain conversion

these reservoirs are exhausted and disappear, which marks the end of Interval Il.

Interval III: Decreasing Reaction Rate

At the beginning of this final stage of the reaction only the polymer

Theory of Emulsion Polymerization 37

particles and the aqueous phase are present. The monomer is present in both

phases at the saturation concentrations. This interval is characterized by a constant

number of particles and by a decreasing rate of polymerization. The latter results

from the decrease of the monomer concentration in the polymer particles, as

polymerization proceeds. This depletion of monomer in the polymer phase leads to

an increase in the intrapartiele viscosity, which eventually results in a slower

terminalion rate. Consequently, the radical concentration wil! increase and so will

the overall polymerization rate. This is called the Trommsdorff, or gel, effect.

The end product is a latex: a dispersion of polymer particles in an aqueous

medium, stabilized by surfactant.

In this model, as it was first described by Harkins12, it is presumed that the

monomer is essentially insoluble in water and the polymer is miscible with the

monomer in all proportions. This type of system is referred to as an "ideal"

emulsion polymerization system.

Fig. 3.1 shows idealised conversion-time and rate-time curves of an

emulsion polymerization. The three different intervals are indicated in this figure.

The Trommsdorff effect can be observed as an increase in the rate of polymeri­

zation in Interval lil. In virtually any particular system some of the features,

shown in this idealised picture, may be absent, or other may be so dominant that

they mask them.

Figure 3.1 ldealised Plot of Fractional Conversion (x) and Rate (dxldt) against Time (t) in an Emulsion Polymerization System.

38 Chapter 3

3.4 The Rate of Polymerization

The rate of polymerization Rpo~ is in general given by the following

equation:

Rf>Ol (3.6)

where [M] and [R'] are the monomer and free radical concentrations, respectively.

The constituentpartsof Eq. 3.6 will be discussed briefly.

As long as propagation is chernically controlled, it can be presumed that kP

is independent of w/n, the weight fraction polyrner in the solution of polymer in

monomer, in which the propagation is taking place. Purthermore, kP is independent

of chain length, as already mentioned in Section 3.2. Thus, the so-caUed "long

chain" value for kP can be used, except for very high wP.

In emulsion polymerization, as described by Harkins (see Seetion 3.3), the

principle loci of polymerization are the polymer particles. For that reason it bas

become customary to use CM, inslead of [M], to denote the concentration of

monomer in the polymer particles. The value of CM is deterrnined

thermodynarnically by the striving of the system to rninirnize its free energy: an

equilibrium is reached between the opposing effeets of lowering the surface tension

and of reducing the free energy of mixing. It is generally accepted21•22

•23

•24

that in Intervals I and 11 CM depends on the diameter of the partiele for small

particles up to about 30 nm and that CM is constant for particles above this size. In

Interval lil CM will deerease with încreasing conversion and can be trivially

calculated by means of a mass balance·25• It is assumed that CM is the same in

each polymer particle.

The radical concentration [R'] is usually unknown and is difficult to predict.

This straightforward calculation is only applicable with sparingly warer-soluble monomers, where the arnount of monomer dissolved in the aqueous phase, is negligible. Since in these studies maînly styrene, which has a maximum water solubility of 4.3 10~3 mol dm~' at 323 K, is used as monomer, the foregoing assumption holds.

Theory of Emu/sion Polymerization 39

According to the Harkins Model, the rate of polymerization can be described as the

rate of reaction within a single particle, times the number of particles per unit

volume of aqueous phase (NJ. The reaction rate per partiele is the average number

of radicals per partiele (n) multiplied by (kp CM)INAv• where NAv is Avogadro's

constant. The overall polymerization rate in mole per unit of time, per unit volume

of aqueous phase, is then expressed as:

k cMnN p c (3.7) NAv

The parameters now most difficult to predict are n and Ne. lt is most

convenient to express the rate of polymerization as the increase in fractional

conversion of monomer into polymer per unit of time. This is achieved by

dividing Eq. 3.7 by the initia! number of moles of monomer present, per unit

volume of aqueous phase, n~:

dx

dt (3.8)

where dx/dt represents the change in percentage conversion per unit of time. Since

n~ is known from the recipe, this equation can be used if the values of Ne, kP and

CM are known, and Eq. 3.8 can then be simplified to:

dx

dt An (3.9)

where the value of A kP CM Ne I (n~ NA.), this, so-called conversion factor, is

independent of conversion in Interval Il.

40 3

3.5 The Smith and Ewart Theory

Besides the qualitative theories about emulsion polymerization, such as the

Harkins model, quantitative theories have also been developed. With these theories

it should be possible to predict fi and Ne.

One of the earliest theories was publisbed by Corrin26 in 1947. This

theory is concerned only with that part of the process, that occurs after free

micellar soap has disappeared from the aqueous phase. A more general theory was

developed by Smith and Ewart27• This theory is also based on the model of

Harkins.

3.5.1 The Recurrence Relationships

In their theory, Smith and Ewart formulated population balance equations

for the relative number of polymer particles containing i free radicals, N;. The

average number of radicals per partiele is then defined as:

-n (3.10)

For convenience N; is normalized, so that:

(3.11)

The population balance equations are derived by taking the following possible

processes into account:*

More recent additions to the theory of Smith and Ewart have been incorporated in the present description of the various kinetic events.

Theory of Emulsion Polymerization 41

Entry. In emulsion polymerization the initiator used is in general water

soluble and the polymer particles are the main loci of polymerization. This

suggests that free radicals are created in the aqueous phase by dissociation of the

initiator, and that they ultimately migrate into the particles. As has been pointed

out by several authors24'28

•29

•30

•31

•32

, the newly formed free radicals do not

show a tendency to enter a polymer partiele or micelle. Free radicals have to add a

critica!, monomer-depending number of monomer units (z), before they become

surface-active and will adsorb onto the surface of a polymer partiele or micelle,

foliowed by true entry. Entry will increase the number of radicais in a given

partiele by unity and will be an important determinant of the rate of

polymerization. The rate of entry is quantified by a pseudo-first-order rate

coefficient p and represents the average number of free radicals entering a latex

partiele per unit of time (s. 1). The quantity p depends on the initiator type, the

initiator concentration, the partiele number, and the type of monomer. After a

radical enters a particle, it will propagate. Although propagation does not influence

i directly, it will affect events which do influence i. For instance, the propagation

of a free radkal and thus the growth of a polymer chain decreases the likelibood of

termination and exit. Given the definition of p, the following equation for the

change in the population of loci with i radicals can be derived:

(3.12)

enlry

Termination. The presence of two or more radicals in a single polymer

partiele will lead to termination of two of these radicals and to the production of

"non-living" polymer materiaL As already mentioned before, terminatien is a

diffusion-controlled process, and, therefore, the second-order rate constant for

termination (k,) is a function of conversion. It can also be inferred that

macroscopie terminatien rate constants are a function of chain length. From this it

is clear that there is not just one k, for a given system. Nevertheless, it is

presumed for simplicity that termination can be characterized by one single

42 Chapter 3

termination rate constant k,.

It is customary33 to use a pseudo-first-order rate constant, designated c, to

quantify termination in emulsion polymerization: c is the frequency of bimolecular

termination per free radical per partiele (s-1) (c = k,l NA.Vs• where V' is the swollen

volume of a polymer particle). According to the above definition the contribution

of terminatien to dNJ dt must be:

(i+2)(i+l)cN;.2 - i(i-l)cN; (3.13)

termlootion

Exit. Apart from termination, a growing chain can he stopped by transfer of

the active centre toanother molecule. This other molecule may be monomer, but it

may also be any added substance with the ability of chain transfer and even

polymer. Similarly to propagation, transfer does not affect i, but it produces a

"non-living" polymer chain and a reactive site (free radical). In the situation where

no added chain transfer agent is present and transfer to polymer is neglected,

transfer wi11 only be to monomer. This produces a monomeric free radical.

Just as free radicals can enter a polymer particle, it must also be possible

that free radicals exit from a polymer partîcle. Based on the nature of the various

free-radical species, it is, in the case of styrene, argued that only monomeric free

radicals can exit from a polymer particle. Exit is then presumed to follow a three­

step mechanism34: 1) a monomerk free radical must be generated by transfer, 2)

this spedes must diffuse through the interior of the latex partiele to the partiele

surface, and 3) thereupon it must undergo desorption. Exit of a free radical leads

to a decrease of the radical concentration in the polymer partiele and thus to a

decrease in the rate of polymerization.

Normally, the rate of exit is described by an overall first-order rate

coefficient, usua11y denoted as k: this is the freqilency with which desorption occurs

from a single polymer particle, per free radical (s- 1). lt wil! he clear from the

above that the maximum value of k is k,,CM. Here k" is the second-order rate

constant for transfer. The exit-related contri bution to dNJ dt is:

Theory of Emulsion Polymerization 43

dN; - = (i+1)kN I ikN,. dt ' ,.

<XII

(3.14)

The multiplicative factors (i+l) and appear in Eq. 3.14, because k has been

defined as the rate of exit per free radical.

The overall time evo1ution of the N; is thus given by sumrning Eqs. 3.12-

3.14 and will give an infinite set of coupled differential equations. In the steady

state approximation the resulting relationship (Eq. 3.15) is referred to as the Srnith­

Ewart recurrence relationship:

p[N;_1 - N;] + k[(i+l)N;,1 (3.15)

+ c[(i+2)(i+1)N;,2 i(i-l)N;]

Fig. 3.2 gives an overview of the above mentioned kinetic events in

emulsion polymerization (no nucleation). The aqueous-phase terminalion processes

and the process of re-entry mentioned in this figure, will be discussed in Section

3.8.

Limiled aqueous phase propagatton +

en try

/ I- > 2R.

a Exil Aqueousph hetemtennin withentrant free radtcal V

(l= -I

Aqueous phase bomotennination with ar10ther desorbed free radical

(l:O ~· ~

ll>w"--/1 . Terminalion Propagation

or Re-entry M~

Figure 3.2 Kinetic Events in Emulsion Polymerization.

44 Chapter 3

3.5.2 Solutions to the Recurrence Relationships

Smith and Ewart themselves did not present a general solution to their

recurrence relationships. Instead, they considered three limiting cases.

Case 1: The number of free radicals per polymer partiele is small

compared with unity.

This situation occurs when exit of free radicals out of the loci is much faster than

entry of free radicals into such loci, i.e., p « k. Termination can be neglected due

to the Iow value of i. The recurrence relationships then reduce to:

(3.16)

In the steady-state conditions: dN0 I dt = 0. Since Ni « N0, it follows that:

fl = Ni I N0 = pIk. The overall rate of polymerization Rpo1 per unit volume of

aqueous phase can then be written as:

(3.17)

Case 2: The number of free radicals per polymer partiele is approximately

equal to 0.5.

This is the best known solution in the Smith-Ewart theory. The following

conditions have to be satisfied simultaneously: (I) bimolecular termination is

instantaneous when a second radkal enters a partiele containing one radical, and

(2) radical exit is negligible, i.e., k « p « c. Trivially, this leads to N0 = N1•

Since it follows from the above mentioned conditions that only particles with zero

Theory of Emu/sion Polymerization 45

or one radical will be present, N0 = N1 = 0.5 and thus fi = 0.5. The overall rate of

polymerization per unit volume of aqueous phase can then be expressed as:

(3.18)

Case 3: The number of Jree radicals per polymer partiele is large

compared with unity.

This situation occurs when entry is much faster than bimolecular termination,

i.e., c « p. In the treatment of this case Smith and Ewart neglected exit. By

assuming that fi is large, the system can be approximated as one, in which all

particles contain the same number of free radicals and for which the steady-state

condition is: p = 2cfi 2• The overall rate of polymerization per unit volume of

aqueous phase can then be expressed as:

_ ( p )0.5 Ne R 1 -kCM--

po P 2c N Av

(3.19)

In later days new solutions to the Smith-Ewart equations were derived by,

for example, Stockmayer15, O'Toole36

, and Ugelstad et al. 31. Even though

these solutions were more genera!, all of them suffered from the shortcoming of

being applicable to steady states only. The first time-dependent solution of the

Smith-Ewart equations was obtained by Gilbert and Napper38• Due to the

assumptions used in their derivation, the solution obtained was limited to Smith­

Ewart Case 1 kinetics. A short time later, a general time-dependent solution for

Smith-Ewart Cases 1 and 2 was found by the same group39·40

.

46 Chapter 3

3.5.3 Partiele Number

Smith and Ewart also developed an expression for the partiele number under

the lirnitations of their Case 2. Since their theory is based on the model of

Harkins, they presumed that particles are formed by micellar nucleation only.

Other nucleation models will be discussed in the next section.

In order to develop an expression for the partiele number they also assumed

that: I) surfactant is only present in micelles or adsorbed at the surface of polymer

particles. The amount adsorbed at the surface of the monomer dropiets and

dissolved in the water phase is neglected. 2) The surface-coverage by one surfac­

tant molecule is independent of the kind of surface (rnicelle-water, polymer-water),

i.e., the total surface of micelles and polymer particles is constant in Interval L 3)

The reaction rate per partiele is constant in the nucleation stage of the reaction,

since it is assumed that the monomer/polymer ratio is constant as wel!.

Smith and Ewart took two limiting cases into account. The first case, too

many particles, prediets more and the second case, too few particles, prediets less

particles than in the actual situation.

Too many particles. As long as micelles are present, it is assumed that they

capture all available radicals. This leads to a calculation of too many particles

since free radicals can also be captured by already existing particles.

Too few particles. A given interfacial area is assumed to capture the same

number of free radicals regardless of the size (and therefore curvature) of the

particle. It is known from classica! diffusion theory that the flux of diffusing

matter across a unit area of interface is inversely proportional to the radius of

curvature at the interface. This means that more particles will be formed than

predicted, since bigger particles are less effective in capturing free radicals.

Both cases lead to a formula of the same form for calculating the number of

particles formed during the nucleation stage (Interval I) in an emulsion

Theory of Emulsion Polymerization 47

polymerization system:

x [:·r(" s)"" (3.20)

where p, is the total entry-rate coefficient per unit of volume of aqueous phase (s·1

m·3), J1 is the volume growth rate of a partiele (m3 s·1

), ti1 is the interfacial area

occupied by unit mass of surfactant (m2 g-1), and S is the total mass of surfactant

contained in the system per unit volume of aqueous phase (g m-3). Further, X is a

constant of which the true value lies in between 0.37 (too few particles) and 0.53

(too many particles).

3.6 Nucleation Models

Since the pioneering work of Harkins 12, and Smith and Ewart27 on the

partiele formation in emulsion polymerization, some other nucleation models have

been developed. In this section a qualitative description of these models will be

given.

Micellar Nucleation. As already explained, radicals will be absorbed into

monomer-swollen surfactant micelles, which are then transformed into polymer

particles. As long as micelles are present in the system, new particles will be

formed and nucleation ceases with the disappearance of the micelles. This model

is able to predict the partiele number for systems with sparingly water-soluble

monomers (e.g., styrene) and with surfactauts with a Jow CMC41• The

shortcomings of this theory are that I) in some cases particles are formed even

when no micelles are present, 2) partiele numbers estimated with Eq. 3.20 deviate

about a factor of two from what is found experimentally, even for styrene, and

3) more water-soluble monomers do not fit the theory at all.

48 Chapter 3

Homogeneaus Nucleation. A radical will react with dissolved monomer to

gîve a growîng polymer chaîn dissolved in the aqueous phase. Thîs oligomeric

radical will continue to grow. Upon reaching a critica! chaîn Iength it will

precipitate and form a polymer partiele stabilized against flocculatîon by adsorbed

emulsifier and swollen with absorbed monomer. The critica! chain Iength is

directly related to the type of monomer used. The micelles act only as surfactant

reservoirs, and the number of particles is only determined by the amount of

surfactant and its intrinsic efficiency. This model, first described by Priest42, and

later quantified by Roe43, by Hansen and Ugelstad44

, and by Fitch and Tsai45

has become known as the "HUFT'' (Hansen-Ugelstad-Fitch-Tsai) theory46•

Homogeneous!Coagulative Nucleation. This is an extension of the HUFT

model, introduced by Gilbert and Napper and co-workers47•48

•49

• As in the case

of the homogeneaus nucleation mechanism, radicals will react with monomer and

will precipitate above a critica! chain length. The so formed particles are not

"true" latex particles and are called "precursor" particles. These particles differ

from mature latex particles in at least two important respects: firstly, they are

colloidally unstable, undergoing coagulatîon with other precursor particles or a

mature latex particle; secondly, they polymerize very slowly. This slow

polymerization may arise from the reduced swelling of the particles by

monomer1.47 and/or from the rapid exit of any free radical in the precursor particles

due to their small size23• The precursor particles are presumed to grow mainly by

coagulation, although some growth must occur by polymerization. Mature latex

particles are formed by coagulation of precursor particles, until the coagulated

entities are stabie and clearly segregated hydrophilic and lipophilic regions have

been formed. These mature latex particles grow rapidly. Coagulation of a

precursor partiele with a mature latex partiele wil! contribute to the process of

free-radical en try. When the surfactant is exhausted, its surface coverage per area

on the mature latex particles decreases more rapidly than on the precursor particles,

due to the higher growth rate of the mature particles. This leads to a situation

where precursor partiele-mature partiele coagulation is favoured over coagulation of

Theory of Emulsion Polymerization 49

two precursor particles and the partiele nucleation rate falls rapidly.

Droplet Nucleation. Usually, monomee dropiets are not believed to play

any other role in emulsion polymerization than being a reservoir of monomer. In a

series of papers5051.s2.s3.s4.s5, it has been shown, however, that in cases with

very smal! monomer droplets, these may become important, or even the sole, loci

for partiele nucleation. The system may then be regarded as a microsuspension

polymerization with a water-soluble initiator.

Up till now, experimental data can not refute one of the nucleation models,

and it has been shown by Morrison et al. 56 that none of the above mentioned

mechanisms is solely responsible for the partiele formation above the CMC. These

authors condurled that the nucleation mechanism above the CMC involves both

homogeneouslcoagulative nucleation and growth of captured oligomeric radicals in

micelles with the incorporation of the extended entry model, as developed by

Maxwell et a/. 32• The capture of oligomeric radicals in micelles competes with the

capture of these radicals in (pre-)existing particles (entry).

For systems with low or zero surfactant concentration, nucleation is

dominated by homogeneous nucleation and by coagulation of precursor particles.

3.7 Zero-One System33

As described in Section 3.5, the value of fi is determined by values of the

rate parameters p, k and c. lt is very difficult to determine the values of any of

these parameters independently. Thus, it is very difficult to extract the values of

specific rate parameters from the experimental values of ii. This problem can be

circumvented by not consirlering emulsion polymerizations in genera!, but rather by

restricting oneself to specific types of kinetic systems. One of these specific

systems is the so-called "zero-one" system, in which a latex partiele contains at

50 Chapter 3

most one free radical at any time. Moreover, it has been shown19 recently that the

Smith-Ewart equations, as they have been described in Section 3.5, are invalid for

systems where fi is larger than 0.5. Proper chain-length dependent termination has

to he incorporated for these systems. Therefore, only zero-one systems are of

consideration in this investigation.

In the "zero-one" system it is assumed that re-entry can he neglected as a

first approach. Re-entry is the entry of an exited free radical into a partiele

irrespective of this being the "parent" partiele or another particle. The process of

re-entry will he discussed in greater detail in Section 3.8. The parameter p is then

denoted as the rate of entry of free radicals per particle, regardless of their history,

and k is denoted as the rate of exit of free radicals per particle, regardless of their

ultimate destiny. The assumption that a partiele contains, at all times, either one

single free radical or none, is accomplished when the entry of a second radical

leads to instantaneous termination. Thus, terminalion is so rapid that it is not rate

determining (p, k « c), and only p and k need to he considered in the kinetic

analysis of a "zero-one" system. Hence, this system is very well suited for

studying the characteristics of inisurfs.

3.7.1 Population Balance Equations

According to the above given definition, the populations of the two species

of particles being present, are denoted as N0 and N1, and the total population is

normalized for reasons of convenience: fl = N1• The different species can he

related to each other as follows. A partiele with zero free radicals is formed by

exit of a radical out of a partiele with one free radical or by entry of a free radical

into a partiele with one free radical, foliowed by instantaneous termînation. A

partiele with zero free radicals is converted into a partiele with one free radical by

entry of a free radical. From this, the following population balance equations can

he derived:

Theory of Emu/sion Polymerization 51

(3.21)

Assuming that p and k are constant, Eq. 3.21 can be solved trivially. lt is then

found that:

p P ) exp{ -(2p + k)t}

(2p + k) (3.22)

(2p + k)

where ii0 is the initial (t = 0) value of ii. The steady state value of ii (fl5,,) is found

by taldng the long time limit of Eq. 3.22:

The assumption that p and k are constant, is valid in genera!. In the

experimental situation of a seeded system (see Section 4.2.3), there is usually an

induction period at the beginning of a polymerization, which is associated with the

residual dissolved oxygen acting as an inhibitor. Thus, p increases from zero to a

constant value, but this happens on a time scale much smaller than that of the

kinetic events that govern iï.

Since CM is constant and the surface of a partiele does not change much

during Interval 11 of a polymerization, k will be constant during Interval 11 (Section

3.7.2). Interval 111 will be discussed in Section 3.7.3.

3. 7.2 Slope and Intercept Method23

A theoretica! expression for the variation of conversion with time in a

simple "zero-one" system can be obtained by substituting the population Eq. 3.22

into the conversion Eq. 3.9. During Interval 11 kP, CM, and Ne are constant and so

52 Chapter 3

is the conversion factor from Eq. 3.9. Given this, substitution of Eq. 3.22 into

Eq. 3.9 and integration yields:

x(t) -x = A [pt + (n - p J(l 0 (2p + k) 0 (2p + k) oxp{-(2p • k)t})l

(3.24)

where Xo is the fractional conversion at t = 0. For long reaction times the

exponential part of Eq. 3.24 becomes negligible and this equation reduces to:

A (pt + no (2p + k)

(3.25)

This theoretica! expression shows that at long times the conversion-time

curve can be described by a straight line. It is indeed known from experiment that

the rate of polymerization in this type of systems climbs from zero to a steady state

value, whereafter x(t) is linear: x(t) - x0 = a + bt. Fig 3.3 shows a typical

conversion-time curve for a seeded system, in which intercept a and slope b are

Figure 3.3

Slope b

Typical Plot of Fractional Conversion (x) vs. Time (t) for a Seeded Emulsion Polymerization System.

indicated. The intercept a and the slope b can be obtained from experimental data.

From Eq. 3.25 it follows that:

Theory of Emulsion Polymerization 53

(3.26)

This metbod has the advantage that by rnaicing use of the intercept, the

information contained in the approach to steady state is effectively incorporated as

well. In terms of accuracy this metbod is also preferabie to, for example, a

non-linear least-square fit method. The intercept is not subject to as much

uncertainty as any single early-time value of conversion, and thus the problem of

noise in the approach to the steady state region of the experimental data is, to a

large extent, overcome. Another souree of uncertainty is the error in the value of

the conversion factor A. lt can be easily shown that an uncertainty of 10% in the

value of A can lead to an ultimate uncertainty of more than 50% in the value of p.

Fortunately, for "zero-one" systems A can be determined with a high accuracy from

kinetic data alone, and thus this difficulty can be overcome.

3.7.3 Interval 111 Kinetics

Up till now, it bas been presumed that CM is constant. But during Interval

III CM decreases with conversion, which means that the above derived expressions

do not hold for Interval lil data. However, most of the kinetic experiments have

been carried out in Interval lil for reasons, which will be explained in Chapter 4.

Thus, a closer look at Interval lil is necessary.

As might be clear from all of the above, Ne and kP are constant, at least in

the early stage of this polymerization phase. Presuming that in Interval lil all the

monomer is contained in the particle, i.e., the monomer is not water soluble, and

that the volume contraction due to polymerization can be neglected, it can be

shown that:

54 Chapter 3

(1-x) c.~ (3.27)

where C~ is the value of CM at the beginning of the experiment. Substituting

Eq. 3.27 into Eq. 3.8 leads to:

This can be simplified to:

-dln(I-x)

dt

-dln(l-x)

dt

(3.28)

k -__ r_n A'n (3.29) VsNAv

where V, is strictly speaking the swollen partiele volume at the beginning of the

experiment, but since the volume contraction has been assumed to be negligible, Vs

is just the swollen partiele volume and is constant. The conversion factor for

Interval III is denoted as A'. As can be seen, Eq. 3.28 takes a form simHar to Eq.

3.9, indicating that data from Interval III experiments should be analyzed by using

values of -ln(l-x) instead of the conversion x directly. Since Eqs. 3.9 and 3.28 are

of the same form, the method given above for the calculation of p and k can be

altered trivially to take variation of CM with conversion into account. Although it

seems that p and k can be calculated from Interval lil data, a few problems arise

when applying this altered method.

Firstly, it is presumed that p and k are independent of conversion. Because

of the decrease of CM and, hence, the increase of wl' during Interval lil, this

assumption may no Jonger be valid for this interval. In the case of p, which is

mainly determined by aqueous-phase events32, it might intuitively be anticipated

that it is relatively insensitive to variations in CM, except for low CM. In the case

of k, it is expected that this rate parameter changes throughout Interval lil. Only

when w" changes very little during the course of the experiment can k be presumed

Theory of Emu/sion Polymerization 55

constant. Only so-called y-relaxation experiments (see Chapter 7) meet this

criterion, since relaxation occurs over a smal! conversion range, and these are

therefore recommended for determining Interval lil values of k.

Secondly, with a decrease in CM, the viscosity increases and this leads to a

decrease in the rate of terrnination. If c becomes sufficiently smal!, the

instantaneous terrnination assumption may no Jonger be tenable, and the "zero-one"

approximation may no Jonger be valid. In those cases the method for deterrnining

p and k loses its applicability.

3.8 The Fate Parameter

Up till now, no attention has been paid to the fate of the free radicals which

have exited from a partiele into the aqueous phase. For clarity it has been assumed

that exit is a first-order loss process. However, this is only the case when an

exited free radical homo-terminates (with another exited free radical). There are

actually two other fates possible for the exited free radicals: 1) re-entry into a

latex particle. Most like1y, this wil! be a latex partiele other than the one, from

which this free radical exited. Exit is then not a true radical loss process.

2) Termination in the aqueous phase with an initiator-derived free radical. This

hetero-termination does not only make exit a loss process, but it also reduces the

effective rate of entry. Ugelstad et al. 31 were the first to consicter the effect of re­

entry (read exit) on the overall rate of entry. This work has been reviewed and

expanded by Ugelstad and Hansen44• However, Whang et al. 51 simplified the

model and proposed that p be correctly expressed as:

(3.30)

where PA is defined as being the value of p in the absence of exit and a is known

as the fate parameter, which has a value between -1 and 1. Eq. 3.30 expresses all

the possible fates of the desorbed free radicals. They may either re-enter a latex

56

partiele (a. = 1), undergo aqueous-phase homo-termination (a. = 0), or experience

aqueous-phase hetero-termination (a. = -I). The advantage of applying a fate

parameter is that the complex model of emulsion polymerization can be simplified.

A disadvantage is that a value of a. does not describe one unique physical situation.

Nevertheless, the fate parameter will be applied because of its simplicity.

Substitution of Eq. 3.30 into Eq. 3.21 gives:

(3.31)

Solving this set of equations and substitution into the conversion equation, results

in an equation, which again describes a straight line at long times, comparable with

Eq. 3.24:

with:

F

k

G

A~ (2a.a)

2a.b 2 +A( 1 - a.)b A(A - 2b)

2G + (l - a.) + 4a.ÏÏ0 0.5 + ---=-------..,:-----------::-::

1 +4G(a. + 1)

(3.32)

(3.33)

where a and b are the intercept and slope of the long-time experimental

conversion-time curve, respectively, and tï0 is the initia! value of tï. Eq. 3.32 is not

applicable in the case of a. = 0 and Eq. 3.26 bas to be applied. With the same ease

as already explained in Section 3.7.3, Interval III data can be used by plotting

Theory of Emulsion Polymerization 57

-ln(l-x) versus time and by determining intercept and slope of the long time

straight line.

References

1. Dinsmore, R.P. U.S. Pat. 1,732,795, 1929; Chem. Abstr. 1930, 24, 266. 2. Luther, M.; Heuck, C. U.S. Pat. 1,860,681, 1932; Chem. Abstr. 1932, 26,

3804. 3. Fryling, C.F.; Harrington, E.W. lnd. Eng. Chem. 1944, 36, 114. 4. Hohenstein, W .P.; Vingiello, F.; Mark, H. India Rubber Wld. 1944, 1 JO, 291. 5. Hohenstein, W.P.; Siggia, S.; Mark, H.1ndia Rubber Wld. 1944, 111, 173. 6. Siggia, S.; Hohenstein, W.P.; Mark, H.1ndia Rubber Wld. 1945, 111, 436. 7. Hohenstein, W.P.; Mark, H. J. Polym. Sci. 1946, 1, 549. 8. Kolthoff, I.M.; Dale, W.J. J. Am. Chem. Soc. 1945, 67, 1672. 9. Frilette, V.J.; Hohenstein, W.P. J. Polym. Sci. 1948, 3, 22.

10. Harkins, W.D. J. Chem. Phys. 1945, 13, 381. 11. Harkins, W.D. J. Chem. Phys. 1946, 14, 47. 12. Harkins, W.D. J. Am. Chem. Soc. 1947, 69, 1428. 13. Harkins, W.D. J. Polym. Sci. 1950, 5, 217. 14. Franck, J.; Rabinowitsch, E. Trans. Faraday Soc. 1934, 30, 120. 15. Rabinowitsch, E.; Wood, W.C. Trans. Faraday Soc. 1936, 32, 1381. 16. Benson, W.S.; North, A.M. J. Am. Chem. Soc. 1962, 84, 935. 17. Adams, M.E.; Russell, G.T.; Casey, B.S.; Gilbert, R.G.; Napper, D.H.;

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Polym. Chem. Ed. 1986, 24, 1027. 21. Morton, M.; Kaizermann, S.; Altier, M.W. J. Colloid Sci. 1954, 9, 300. 22. Gardon, J.L. J. Polym. Sci., Polym. Chem. Ed., Part A-11968, 6, 2859. 23. Hawkett, B.S.; Napper, D.H; Gilbert, R.G. J. Chem. Soc., Faraday Trans. 1

1980, 76, 1323. 24. Penboss, LA.; Gilbert, R.G.; Napper, D.H. J. Chem. Soc., Faraday Trans. l

1986, 82, 2247. 25. Lane, W.H. 1nd. Eng. Chem. 1946, 18, 295. 26. Corrin, M.L. J. Polym. Sci. 1947, 2, 257. 27. Smith, W.V.; Ewart, R.H. J. Chem. Phys. 1948, 16, 592. 28. Penboss, LA.; Gilbert, R.G.; Nappper, D.H. J. Chem. Soc., Faraday, Trans. 1

1983, 79, 1257. 29. Alexander, A.E.; Napper, D.H. Prog. Polym. Sci. 1971, 3, 145. 30. Nomura, M.; Harada, M.; Eguchi, W.; Nagata, S. Polym. Prepr.-Am. Chem.

Soc., Div. Polym. Chem. 1975, 16, 217.

58 Chapter 3

31. Barrett, K.E.J. Dispersion Polymerization in Organic Media; Wiley, New York, 1975.

32. Maxwell, I.A.; Morrison, B.R.; Napper, D.H.; Gilbert, R.G. Macromolecules 1991, 24, 1629.

33. Gilbert, R.G.; Napper, D.H. J. MacromoL Sci., Rev. Macromol. Chem. Phys. 1983, C23, 127.

34. Nomura, M.; Harada, M. J. Appl. Polym. Sci. 1981, 26, 17. 35. Stockmayer, W.H. J. Polym. Sci. 1957, 24, 314. 36. O'Toole, J.T. J. Appl. Polym. Sci. 1965, 9, 1291. 37. Ugelstad, J.; M~rk, P.C.; Aasen, J.O. J. Polym. Sci., Polym. Chem. Ed.,

Part A-l 1967, 5, 2281. 38. Gilbert, R.G.; Napper, D.H. J. Chem. Soc., Faraday Trans. /1974, 70, 391. 39. Hawkett, B.S.; Napper, D.H.; Gilbert, R.G. J. Chem. Soc., Faraday Trans. I

1975, 71' 2288. 40. Hawkett, B.S.; Napper, D.H.; Gilbert, R.G. J. Chem. Soc., Faraday Trans. 1

1977, 73, 690. 41. Gerens, H. Ber. Bunsenges. Phys. Chem. 1963, 67, 741. 42. Priest, W.J. J. Phys. Chem. 1952, 56, 1077. 43. Roe, C.P. lnd. Eng. Chem. 1968, 60, 20. 44. Ugelstad, J.; Hansen, F.K. Rubber Chem. Techno[. 1976, 49, 536. 45. Fitch, R.M.; Tsai, C.H. In Polymer Colloids; Fitch, R.M., Ed.; Plenum Press:

New York, 1971; p 73. 46. Hansen, F.K., Ugelstad, J. In Emulsion Polymerization; Piirma, 1., Ed.;

Academie Press: New York, 1982; p 45. 47. Lichti, G.; Gilbert, R.G.; Napper, D.H. J. Polym. Sci., Polym. Chem. Ed.,

Part A-l 1983, 21, 269. 48. Feeney, P.J.; Napper, D.H.; Gilbert, R.G. Macromolecules 1976, 49, 536. 49. Richards, J.R.; Congalidis, J.P.; Gilbert, R.G J. Appl. Polym. Sci. 1989, 37,

2727. 50. Sujkov, A.V.; Grizkova, LA.; Medvedev, S.S. Kolloid Z. 1972, 34, 203; Eng.

Trans!.: Colloid J. USSR 1972, 34, 154. 51. Ugelstad, J.; El-Aasser, M.S.; Vanderhoff, J.W. J. Polym. Sci., Polym. Lett.

Ed. 1973, 11, 503. 52. Ugelstad, J.; Hansen, F.K.; Lange, S. Makromol. Chem. 1974, 175, 507. 53. Hansen, F.K.; Ofstad, E.B.; Ugelstad, J. In Theory and Practice of Emulsion

Technology; Smith, A.L., Ed.; Acadamic Press, New York, 1976; p 13. 54. Azad, A.R.M.; Ugelstad, J; Fitch, R.M.; Hansen, F.K. ACS Symp. Ser. 1976,

24, 1. 55. Hansen, F.K.; Ugelstad, J. J. Polym. Sci., Polym. Chem. Ed., Part A-1 1979,

17, 3069. 56. Morrison, B.R.; Maxwell, LA.; Gilbert, R.G.; Napper, D.H. ACS Symp. Ser.

1992, 492, 28. 57. Whang, B.C.Y.; Napper, D.H.; Ballard, M.J.; Gilbert, R.G.; Lichti, G. J.

Chem. Soc. Faraday Trans. 1 1982, 78, 1117.

Chapter 4

Experimental Procedures

Summary: In this chapter the various experimental and analytica! techniques used in monitoring the emulsion polymerizations wilt be described. The chapter will close with a description of the various methods used in determining important characteristics, e.g., surface tension of an aqueous salution and dissociation rate of the synthesized inisurfs.

4.1 Introduetion

In order to enhance the clarity of the subsequent chapters, the experimental

techniques used in this investigation will be described and explained in this

chapter·. The various reactor designs used and the different polymerization

procedures will be described (Section 4.2), along with the preparation and eleaning

of a seed latex (Section 4.3), and the various ways of monitoring an emulsion

polymerization (Section 4.4). The various ways of measuring partiele size and

partiele size distribution wiJl be explained in Section 4.5. Surface tension

measurements of aqueous solutions of inisurf wiJl be discussed in Section 4.6.

• The techniques used for the syntheses of the various inisurfs have been described in Chapter 2.

60 Chapter4

Finally a description of the measurements used for determining the dissociation rate

of the synthesized initiator and inisurfs will be given in Section 4.7.

4.2 Emulsion Polymerization Reaelions

In the course of this investigation two types of emulsion polymerization

reaelions have been carried out in different set-ups. Besides ab initia reactions,

seeded reaelions have been carried out. Befare going into more detail about these

reactions, a description of the various types of reactors will be given.

4.2.1 Reactor Design

Three different reactors have been used, depending on the reaelions

investigated. The different set-ups wil! now be discussed.

4.2.1.1 Batch reactor

For the ab initia reaelions (see Section 4.2.2 ), a glass-jacketed 1 dm3

stainless steel reactor, equipped with a twelve-blade flat-blade turbine impeller,

thermocouple and sample withdrawing tube, was used. This reactor (see Fig. 4.1)

could also be flushed with an inert gas, and it could be evacuated by a pump.

The reactions were monitored by taking samples to delermine conversion

and/or partiele size.

4.2.1.2 Dilalometer

A dilalometer is one of the two reactor designs used for monitoring seeded

reaelions (see Section 4.2.3). Originally, dilalometers were used for measuring the

Experimental Procedures 61

1. Stainless Steel Reactor 2. Glass Jacket 3. Electric Motor 4. Speed lndicating Controller 5. Twelve-Blade Flat-Blade

Turbine 6. Raffles 7. Chromel-Alumel

Thermocouple 8. Temperature Transmitter to

Thermostalie Bath 9. Filling Tube with Funnel

10. Sample Tube 11. Bottom Tube 12. Gas Tube 13. Evacuation Tube 14. Pressure Indicator 15. Safety Valve

Figure 4.1 Batch Reactor

thermal expansion or contraction of liquids or solids. When it was found that a

contraction in volume occurred during polymerization, the dilatometer was adapted

to study these reactions1• A dilatometer for studying polymerization rates consists

of a reaction vessel, a volume-sensing device, associated equipment for filling,

deoxygenation and stirring, and a temperature-control apparatus. Dilatometers for

emulsion polymerization require provisions for agitation to maintain homogencity

of the oil in water emulsion.

The use of dilatometers in emulsion polymerization was first reported by

Fryling2• He studied successfully the rate of emulsion copolymerization of

butadiene and styrene in end-over-end rotating tubular dilatometers.

Figure 4.2 shows a dilatometer, which was used in our studies. Since most

experiments were carried out in Interval lil, agitation with a magnetic stirrer was

sufficient. This type of dilatometer is equipped with an automatic tracker to follow

62

Figure 4.2 One Piece Glass Vilatometer

Chapter 4

the meniscus of the liquid in the capillary automatically in order

to determine the fractional conversion of the polymerization as a

function of time. The accuracy of this metbod is determined by

1) the precision of the temperature control of the bath in which

the dilatometer was immersed, so that a constant temperature of

the dilatometer could be maintained, and 2) the size and

uniformity of the measuring capillary. The disadvantage of this

highly accurate apparatus is that during the reaction samples can

only be taken or substances can only be added with serious

difficulties and at the cost of losing accuracy.

4.2.1.3 Densimeter

The second reactor design used to monitor seeded

reactions, was the densimeter. Since the density of polymer is

higher than the density of monomer, the density of an emulsion

will increase during polymerization. Thus, emulsion

polymerization can be monitored by the change in density of the

mixture. Poeblein et al. 3 were the first to report on the

application of densimetry: a y-ray densimeter was used to

measure conversion for control purposes in a continuous

emulsion polymerization experiment.

In our studies density was measured on-line by an Anton

Paar DMA digital densimeter (see Fig. 4.3). The measuring

principle of this densimeter is based on the change of the natura!

frequency of a hollow oscillator when filled with different

liquids or gases. The oscillator consists of a hollow elastic U­

shaped glass tube. This is electronically excited, perpendicular

to the plane of the U-shaped glass tube, in an undamped

harmonie oscillation. The density can be measured in six

significant figures.

Experimental Procedures 63

In our set-up, the latex was pumped from a reactor (similar to the one

described in Section 4.2.1.1, but with a volume of 0.5 dm3) to the densimeter and

via a peristaltic pump back into the reactor, as shown in Figure 4.3. The tubing

used in the peristaltic pump, had to be mechanically stabie and inert. 1t must also

be resistant to the dispersion pumped through, i.e., it must not be swollen by

monomer and it must be free from any substances liable to be washed out by the

emulsion. Viton tubing (Watson Marlow Ltd., Falmouth, UK) was found to be

suitable in this investigation.

I I I I I I I I

Densimeter

@ ---------

t I Ho~ngboili Figure 4.3 Densimeter Set-up

Reactor

Since density strongly depends on temperature, the temperature of the latex

had to be kept constant. This was achieved by keeping the length of the tubing

between the reactor and the densimeter as short as possîble, by insulating the

tubing from the reactor to the densimeter and back, and by thermostaling the

densimeter. Moreover, the densimeter had a built-in heat exchanger to maintain

the temperature of the latex at its reaction temperature. The latex flowed first

through the heat exchanger and then through the oscillator.

64 Chapter 4

4.2.2 Ab Initio Reaelions

During an ab initia experiment all three intervals, as described in Section

3.3, will occur. In this study, all the ab initia reactions were carried out in a batch

reactor. Befare filling the reactor, it was flushed with argon and evacuated several

times in order to remave oxygen and any volatile materials. First the inisurf was

dissolved in doubly distilled de-ionized and argon-flushed water. Details of the

recipe will be given in the relevant sections of the following chapters. This

salution was then charged into the reactor along with the balance of the water

needed. Befare charging the reactor with the monomer (styrene, for synthesis,

Merck, Darmstadt, FRG, distilled under nitrogen at reduced pressure, and

subsequently stored at 277 K under nitrogen until use), it was again flushed with

argon and evacuated to remave the last traces of oxygen. After charging the

monomer into the reactor, the reactor content was stirred at a speed of 250 rpm and

heated to the reaction temperature (323 K or 343 K). During the reaction, samples

were taken at regular intervals to determine the conversion by gravimetry (see

Section 4.4.1) or for partiele size analyses (see Section 4.5).

4.2.3 Seeded Reaelions

In a "seeded" study, the reaction is deliberately started in either Interval II

or lil. In those studies a preformed cleaned monodisperse seed latex (for

preparation and cleaning see Section 4.3) is further polymerized by actdition of

more monomer, surfactant and initiator. Since the nucleation stage of the emulsion

polymerization, which is difficult to control, is avoided, kinetic data obtained from

those reaelions give more reliable information about the various kinetic events and

the mechanism of emulsion polymerization4·5

. One of the first groups to use

seeded polymerization reactions was Vanderhoff et al. 6·7

·8

• They examined the

competitive growth of seeded polymerization of styrene under Case 3 conditions.

Theoretica! and experimental studies of seeded polymerization have been published

Experimental Procedures 65

by for example Gilbert and Napper9•10

•11

•

The monomer added to the seed latex will preferentially migrate to the latex

particle. Depending on the amount of monomer added, the monomer concentration

in the particles will reach its equilibrium concentration, Ct,;t. lf suffïcient monomer

is added, monomer dropiets will be formed, and Interval II polymerization will

(initially) take place. If, however, the amount of the added monomer is just

sufficient or insufficient to fully saturate the particles with monomer, the system

will commence under conditions corresponding to Interval lil. In this case, the

concentration of monomer at the beginning of the polymerization, C~. can be

calculated as follows, assuming molar-volume additivity:

0 ms

0

c~ ns mw5

V 0 0 s ms mps

(4.1)

+ --Ps PPs

where n~ is the initia! number of moles of styrene per particle, V, is the swollen

volume of a particle, m~ and m~5 are the initial amounts of styrene and polystyrene

respectively, mw5 is the molecular weight of styrene, and Ps and PPs are the density

of styrene and polystyrene, respectively.

Seed latices were prepared in such a way as to obtain a latex with an

essentially monodisperse partiele size. This avoided complications when analyzing

the kinetic data, since rate coefficients may very well be partiele size

dependene2·13

•

Another advantage of seeded polymerization is that the approach to steady

state can be monitored. In ab initio polymerizations Interval II begins with a

steady state concentration of free radicals. This is in contrast to seeded

polymerizations, where the experiment begins in a non-steady state with regard to

the free radical concentration. As has been seen in Chapter 3, this approach to

steady state provides crudal information in rate and mechanistic analyses.

66 Chapter 4

Seeded experiments were carried out in two different ways. Most of the

experiments were carried out with a chemica) initiator, inisurf in our case. In a

number of experiments the initiation was by means of a y-ray souree (see

Chapter 7). Since the recipes for the latter polymerizations were different, i.e.,

depended on the monitoring technique that was applied, the various recipes and

techniques of carrying out these polymerizations will be discussed in the relevant

sections of Chapter 7.

For all these seeded polymerizations it was important that the preselected

number of polymer particles per unit volume of aqueous phase, Ne, did not change

during the polymerization. This meant that no new partiele nucleation ("secondary

nucleation") should occur and that no coagulation of seed particles should take

place. This was checked by comparing the partiele size distribution as obtained via

electron microscopy (see Section 4.5.3) before and after the polymerization.

4.3 Seed Preparation

As has been mentioned in the previous section a seed latex must be

monodisperse, but it should also be stabie against secondary nucleation and

coagulation. Various recipes for different monomers have been

reportedl2,14,l5,16,17.tB,t9. One condition to favour monodispersity is to carry

out the polymerization at higher temperatures (353 K or 363 K). This leads to a

higher dissociation rate of the initiator, and, thus, to a higher concentration of free

radicals and a shorter nucleation period. Therefore, the final latex, in which most

polymer formation has occurred in the absence of new partiele formation, will be

monodisperse.

Besides that the seed applied in these studies had to be monodispers it had

to be easily cleaned from all surfactant, oligomers and residues of initiator; and

should still be stabie enough to be swollen with monomer and not show secondary

nucleation or coagulation during polymerization. The latex had to be totally

Experimental Procedures 67

cleaned from any physically adsorbed material at the surface of the particles, since

these materials can interfere with the adsorption equilibrium of the inisurfs used.

Colloidal stability, monodispersity and cleanability were achieved by

carrying out a surfactant-free emulsion polymerization20•21

•22 using a small

amount of an ionic co-monomer (sodium p-styrenesulphonate)23•24 and an ionic

initiator (sodium peroxodisulphatei5•26

• The surfactant-like ionic components,

formed during the polymerization, are either chemically bonded to the polymer or

consist of oligomers physically adsorbed onto the latex particles. In both cases

these components add to the electrostalie stabilization. Applying an ionic co­

monomer increased the electric charge at the surface and, thus, contributed to the

electrostatic stabilization.

Table 4.1 shows the recipe used for the production of the seed latex. These

have been carried out either in a bottie polymerizer or in a stainless steel tank

reactor with a volume of 5 dm3 (the recipe was proportionally increased).

Table 4.1. Basic Recipe fortheSeed Latex.

Styrene

Water

Sodium p-Styrenesulphonate

Sodium Peroxodisulphate

Amount (gram)

29.30

150.00

1.42

0.39

Stirring rate: 40 rpm for bottie polymerizer (end-over-end) and 300 rpm fortank reactor.

Temperature: 358 K. Reaction time: 24 hours.

The cleaning of the latex can be done by dialysis27, ion exchange28.z9

,

ultracentrifugation30•31 andlor serum replacemene2

•33

• Dialysis removes ions,

but faits to remove oligomers. Nevertheless, it is a very useful first step in

cleaning a latex. Although similar in scope to dialysis, ion exchange bas an

additional disadvantage: the resin granules require extensive precleaning to ensure

that all extraetabie contaminants are removed, which makes this a very tedious

method. The centrifugation process, as well as the serum replacement process

remove both oligomers and ionic species.

In our studies, dialysis was used as a first step in cleaning the seed latex.

As a second step either ion exchange or ultracentrifugation was used. Ion

exchange was applied following the procedure of Vanderhoff et al. 2s, in which a

mixed bed of anionic (OH· form) and cationic (W form) ion-exchange resins

removed the unreacted materials and reaction side-products from the latex and

replaced the sodium ions with protons quantitatively. Ultracentrifugation was a

faster method compared to ion exchange, and used less material as welL The

ultracentrifugation was carried out on a Kontron Instruments, eentrikon T-2000, at

a speed of 45,000 rpm for 3 h. The precipitate was redispersed by ultrasonification

in doubly distilled de-ionized water. The seed latex was centrifuged twice before

u se.

4.4 Conversion Measurements

For the determination of the conversion of the polymerization as a function

of time various methods have been applied. The three methods used in these

studies will now be discussed. Besides the experimental procedure of each method,

the method of calculating the conversion of the polymerization from the raw data

will be included.

4.4.1 Gravimetry

The conversion was determined by means of the solid weight of a sample

and calculated according to Eq. 4.2:

Experimental Procedures

x(t) DS(t) - DSmin

DSmax DSmin

69

(4.2)

where x(t) is the fractional conversion at time t, DS(t) is the fraction dry solid

content in the sample at time t, and DSm~n and DS,_ are the dry solid contents at

the start of the reaction (0% conversion) and at the end of the reaction (100%

conversion), respectively. DSmin and DS,_ were calculated from the recipe.

The sample was drawn from the reactor directly into a dry, clean aluminium

cup, where the reaction was shortstopped with added hydroquinone (jor synthesis,

Merck, Darmstadt, FRG). The sample was weighed and dried on a steambath and

in a vacuum oven at 313 K, until constant weight was obtained. With this method

conversion could be determined within 1% accuracy.

4.4.2 Dilatometry

With dilatometry the conversion was determined by means of the change in

volume during the polymerization. The following equation for the calculation of

the fractional conversion, x(t), was used:

1tr21!Jt

x(t) (4.3)

0 I gm d d m p

where r is the bore radius, Ah is the incremental change in the meniscus level, dm

and ~ are the densities of monoroerand polymer, respectively, and g~ is the initial

mass of monomer.

The movements of the meniscus were followed, measured and recorded by

70 Chapter 4

an automated tracker·: the presence of the meniscus is determined by an optical

detector, which was moved by a stepping motor to follow the height of the liquid

in the capillary. The number of steps was monitored by a computer, and, tagether

with the previously determined step height, db could be calculated and substituted

in Eq. 4.3. The resolution obtained with this method depended on the step-heights

of the motor and was in the order of microns. The accuracy in the conversion was

0.01%.

Filling Procedure

The required amount of seed tagether with the monomer were added into

the dilatometer and were stirred slowly overnight, to swell the polymer particles of

the seed with the monomer. A salution of inisurf was made in doubly distilled

de-ionized and degassed water·· and the required arnount was added\ into the

dilatometer. The dilatometer was further filled with doubly distilled de-ionized and

degassed water up to the required level. The whole dilatometer was placed into an

ultrasonic bath for emulsification of the reaction mixture. The dilatometer was

immersed in a water bath and the tracker was aligned. The emulsion level in the

capillary of the dilatometer was sealed with hexane and the tracker was adjusted.

Every fifteen to thirty seconds a reading was taken of the level of the meniscus,

and the fractional conversion was calculated applying Eq. 4.3.

4.4.3 Densimetry

Densimetry was- used in a relative way for the determination of the

conversion of the polymerization. The polymerization was foliowed not only

densimetrically, but also gravimetrically (small number of samples). The

Apparatus designed by Mr. D.F. Sangster and Mr. G. Baxter of CSIRO, Di vision of Chemieals

and Polymers, Lucas Heîghts Research Laboratories, Menai, NSW 2234, Australia.

Degassed water was needed sincc a gas bubble formed in the dilalometer could change thc level in the capîllary and thus innuencc the accuracy of the measurements.

Experimental Procedures 71

gravimetrie data were used for calibration of the densimetric data. Conversion was

calculated from the gravimetrie data and plotted against time-corresponding density.

The resulting curve enabled the translation of density into conversion. Fig. 4.4

shows a typical density versus conversion plot of a seeded styrene emulsion

polymerization, which was used in converting density into conversion.

Filling Procedure

Similar to dilatometry, the required amounts of seed latex and monomer

were first charged into the reactor and stirred slowly, for swelling. A solution of

the required amount of inisurf in doubly distilled de-ionized and argon-flusbed

water was then added, foliowed by the remainder of the water.

Figure 4.4

0.985 0.987 0.989 0.991 0.993 0.995

A Typical Plot of Density versus Conversion of a Seeded Styrene Emulsion Polymerization.

4.5 Partiele Size Measurements

For the determination of the various kinetic parameters it is necessary to

know the partiele diameter in order to calculate the number of particles per unit

volume of aqueous phase, Ne, and the polymerization rate per partiele ( or the

average number of radicals per particle).

Different methods have been used to measure the partiele diameter and the

partiele size distribution. These methods will now be discussed below.

72 Chapter4

4.5.1 Dynamic Light Scattering

Dynamic light scattering is a relatively rapid metbod for determining

partiele sizes. The dynamic light scattering technique is based on the scattering of

a beam of coherent laser light by a number of particles present in a diluted and

filtered latex sample. The intensity of the scattered light beam is measured at a

certain angle to the primary beam as a function of time at room temperature. The

fluctuation of the intensity with time is directly related to the (Brownian) motion of

the particles in the dispersion. This diffusion is determined by the size and sh:.:pe

of the particles. For spherical particles the Stokes-Einstein relation can be applied

for the relation between the diffusion coefficient and the radius of the particles.

In this study a Malvem Autosizer Ilc was used, with a 5 m W He-Ne laser,

which produced a coherent light of 633 nm wave length. A pboton multiplicating

detector is placed at an angle of 90°. Particles with a size ranging from 20 to

2,000 nm can be analysed. A so called z-average diameter is measured, from

which a weight-average diameter is calculated.

4.5.2 Transmission Electron Microscopy

Another technique used to obtain an average partiele size of the sample,

was Iransmission Qlectron Microscopy (TEM). With this technique it was also

possible to obtain a total partiele size distribution. Diluted samples (0.05 wt.%

solids) were dried on 400 Mesh Formvar covered grids, and afterwards covered

with a carbon film to enhance stability and conductivity in the electron microscope

(Jeol 2000 FX). In the case of a polystyrene latex the polymer was stabie enough

in the electron beam and provided enough contrast for taking micrographs.

Typically 750- 1000 particles were counted with a Zeiss TGA-10 partiele analyzer.

It was found that counting extra particles did not change the partiele size

distribution any more.

Experimental Procedures 73

4.5.3 Disk Centrifuge Photosedimentometry

Another metbod to obtain partiele size distributions is Disk Çentrifuge

fhotosedimentometry (DCP). This rapid metbod is used for measuring partiele size

distributions of colloidal systems, e.g., pigments and synthetic latices, of partiele

sizes ranging from 50 nm to 2 J..Ull. DCP is based on the fact that particles witb

different density and diameter migrate at a different speed througb a fluid in a

force field, as described by Stokes law34• The field applied in tbis technique is a

centrifugal force field within a bollow rotating disk. Presuming tbat tbe particles to

be analyzed all have tbe same density, Stokes law can be written as:

18 Tl ln(R4 /Rm)

r. Ap ar (4.4)

where dw is the partiele diameter (nm), Tl is tbe viscosity of the spinfluid

(kg m·1 s-1), R4 is the radius at wbich tbe detector is positioned (m), Rm is tbe radius

at whicb the meniscus of the spinfluid (suspension medium through wbich the

particles migrate) is and at wbicb the sample starts to migrate (m), t, is the time

needed for sedimentation of a partiele from Rm to R4 (s), Ap is the difference in

partiele density and spinfluid density (kg m·3), and ro is tbe rotational speed of tbe

disk (s-1). The tbeory of sedimentation and detection are well described in

literature35•36

•37

•38

• DCP eliminales problems encountered witb microscopy of

deformable partieles, the need for stains or gold coating on the particles, and the

requirement for image analysis of a large number of particles for accurate

distribution averages. It bas been found that tbe partiele size distributions obtained

witb DCP are in good agreement with those obtained with TEM39•40

•41

•

Two different procedures are used for obtaining tbe partiele size

distribution: 1) the Jine-start metbod (LIST)42.43, wbere a small sample is injected

into tbe spinning disk, whicb already contains tbe spinfluid; 2) the homogene­

ous-start metbod (HOST)44'45

.46, where the spinfluid initially contains a uniform

concentration of the colloid, wbose partiele size distribution is to be determined.

74 Chopter 4

The LIST-metbod is to be preferred, since it is able to obtain results with a higher

precision and a better resolution. The main advantage of the HOST-metbod is its

applicability to colloids with partiele densities less than that of the spinfluid47•

To obtain even better results, a modified LIST-metbod has been applied in

this investigation. In this so-called buffered line-start method48•49

•50

, 1 mL of

methanol is added to 15 mL of doubly distilled de-ionized water already present in

the cavity of the spinning disk. The density gradient formed in this way at the

inner boundary eliminates to a great extent the density discontinuity and interfacial

tension between the suspension medium and spinfluid. Hence, stabie sedimentation

and, thus, satisfactory separation was obtained.

The latex sample is prepared by actding 10-15 drops of polystyrene latex to

15 mL of doubly distilled de-ionized water foliowed by actdition of 5 mL of

methanol. The solid weight content in the samples is estimated to be 0.025-0.05%.

A small amount of this sample (0.25 mL) is injected into the cavity of the spinning

disk, where the water and methanol is already present, as described above. The

used instrument was a Brookhaven BI DCP Partiele Sizer.

4.5.4 Partiele Number Concentration

After the partiele size bas been determined, the partiele number per unit

volume of aqueous phase, N,, can be calculated as follows:

6xmsoPw

1td3 mw p PS

(4.5)

where x is the fractional conversion, d is the partiele diameter, mw is the amount of

water, and Pw is the density of water.

Experimental Procedures 75

4.6 Surface Tension Measurements

To determine the Çritical Micelle ,Ç_oncentration (CMC) of the synthesized

inisurfs the surface tension has been measured. This bas been done by two

methods, and both will be described bere. Surface tension measurements have also

been used to determine the adsorption isotherm of inisurf on polystyrene latex

particles.

4.6.1 Du Nouy Ring-Method51

This metbod is based on the measurement of the force required to detach a

frame, usually in the form of a ring, from the surface of a liquid or solution. The

commercially available equipment consists of a torsion balance and a platinum

ring. This metbod is very quick and simple, does not require large volumes of

liquid or solution, and is very suitable for comparative measurements. For absolute

measurements of surface tension, it is essential to apply correction factors, as

described by Harkins et al. 52, and by Fox et a/.53

•

1t is essential to use a clean platinum ring (cleaned by flaming) and to

ensure that the surface of the liquid or solution is clean as well. The ring should

be free of kinks and as flat as possible.

4.6.2 Maximum Pressure Bubble Tensiometry

The determination of surface tension by measuring the pressure required to

liberate bubbles from a vertical capillary tube immersed in a liquid was first

suggested by Simon54• This is a quick method, requiring simple apparatus, and it

is very accurate. Since the surface is renewed quite quickly (almost every second),

surface contamination is minimized. Contact-angle problems as with the capillary­

height method55, do not usually arise, although due care must be taken to ensure

76 Chapter 4

that the bubble forms on either the inside or outside diameter of the capillary. 1t

must also be ensured that the diameter of the capillary does not change during the

measurement, due to contamination on the inside of the capillary.

Some modifications for an even simpler application have been described by

Sugden56•57

• The application of two capillaries with different radii has been

suggested, with both tubes vertically immersed to the same depth in the liquid or

solution. The surface tension, y, can then be calculated from the difference of the

pressures, required to liberate bubbles from each capillary, using the following

simple empirica! equation:

y = l flp (4.6)

where flp is the difference in the pressures needed for releasing bubbles from the

narrow and wide capillary, and l is an apparatus constant, found through

calibration. The radius of the larger capillary is usually in between 1 and 4 mm,

and that of the smaller one below 0.5 mm. The rate of bubble release is

approximately one per second for the small capillary and slightly lower for the

large capillary. The method gives an accuracy of ca. 0.5% and can also be used

for liquid-liquid interfaces58, on remote control, and in continuous flow cells to

obtain a surface tension reading as a function of time. Accuracy can even be

increased by re-orienting the capillaries so that the eentres of the formed bubbles,

rather than the tips of the capillaries, are level. A SensaDyne 6000 Surface

Tensiometer was used in our investigations.

4.7 Initiator Decomposition Measurements

The dissociation rates of the formed initiator and inisurfs were measure

different temperatures by means of UV-analysis applying a Hewlett Packard r d.

Diode Array Spectrophotometer. The disappearance of the clearly-m able

absorption-band of the azo-group (À = 350 nm) was monitored as a fu• .m of

Experimental Procedures 77

time and temperature. Since the rate of dissociation of azo-compounds is not

influenced by concentration and other substances dissolved and hardly changes with

solvent59'60

, the dissociation rate of the initiator and the inisurfs was only

determined in water at one concentration. The solutions of the initiator or inisurfs

were kept in a thermostated bath, and samples were withdrawn at constant time

intervals. The UV spectrum of each sample was obtained and the decrea'ie in the

absorption of the azo-group, at 350 nm, was followed.

Keferences

1. Starkweather, H.W.; Taylor, G.B. J. Am. Chem. Soc. 1930, 52, 4708. 2. Fryling, C.F. Ind. Eng. Chem., Anal. Ed. 1944, 16, 1. 3. Pochlein, G.W.; Dougherty, DJ. Rubber Chem. Techno/. 1977, 50, 601. 4. Pitch, R.M.; Shih, L.-B. Progr. Colloid Polym. Sci. 1975, 56, 1. 5. Gutta, G.; Benetta, G.; Talamini, G.P.; Vianello, G. Adv. Chem. Ser. 1969, 91,

158. 6. Vanderhoff, J.W.; Vitkuske, J.F.; Bradford, E.B.; Alfrey, T., Jr. J. Colloid Sci.

1956, 11' 135. 7. Vanderhoff, J.W.; Bradford, E.B.; Tarkowski, H.L.; Wilkinson, B.W.

J. Polym. Sci. 1961, 25, 265. 8. Poehlein, G.W.; Vanderhoff, J.W. J. Polym. Sci., Polym. Chem. Ed., Part A-1

1973, 11, 447. 9. Hawkett, B.S.; Napper, D.H.; Gilbert, R.G J. Chem. Soc., Faraday Trans. I

1975, 71' 2288. 10. Gilbert, R.G.; Napper, D.H. J. Chem. Soc., Faraday Trans. I 1974, 70, 391. ll. Gilbert, R.G.; Napper, D.H. J. Macromol. Sci., Rev. Macromol. Chem. Phys.

1983, C23, 127. 12. Hawkett, B.S.; Napper, D.H.; Gilbert, R.G. J. Chem. Soc., Faraday Trans. I,

1980, 76, 1323. 13. Morrison, B.R.; Maxwell, I.A.; Gilbert, R.G.; Napper, D.H. ACS Symp. Ser.

1992, 492, 28. 14. Smith, W.V. J. Am. Chem. Soc. 1948, 70, 3695. 15. Verdurmen, E.M.; Albers, J.G.; Dohmen, E.H.; Zirkzee, H.F.; Maxwell, LA.;

German, A.L. to be published. 16. Hjertberg, T.; Sörvik, E.-M. J. Polym. Sci., Polym. Chem. Ed., Part A-1 1978,

16, 645. 17. Lee, C.K.; Forsyth, T.H. ACS Symp. Ser. 1981, 165, 567.

78 Chapter 4

18. Nak:amuro, Y.; Tabata, H.; Suzuki, H.; Iko, K.; Okubo, M.; Matsumoto, T. J. Appl. Polym. Sci. 1986, 32, 4865.

19. Lange, D.M.; Poehlein, G.W.; Hayashi, S.; Komatsu, A.; Hirai, T. J. Polym. Sci., Polym. Chem. Ed., Part A-I 1991, 29, 785.

20. Willes, J.M. Ind. Eng. Chem. 1949, 4I, 2272. 21. Kotera, K.; Furusawa, K.; Tak:eda, Y. Kolloid Z. Z. Polym. 1970, 239, 677. 22. Goodwin, J.W.; Hearn, J.; Ho, C.C.; Ottewill, R.H. Brit. Polym. J. 1973, 5,

347. 23. Liu, L.; Krieger, I.M. In Emulsion, Latices and Dispersions; Becher, P., Ed.;

Marcel Dekker Inc.: New York, 1978; p 26. 24. Juang, M.S.; Krieger, I.M. J. Polym. Sci., Polym. Chem. Ed., Part A-I 1976,

I4, 2089. 25. Hearn, J.; Wilkinson, M.C.; Goodall, AR.; Chainey, M. J. Polym. Sci., Poly:n.

Chem. Ed., Part A-I 1985, 23, 1869. 26. Fitch, R.M.; Tsai, C.H. In Polymer Colloids; Fitch, R.M., Ed.; Plenum Press:

New York, 1971; p 103. 27. Ottewill, R.H.; Shaw, J.N. Kolloid Z. Z. Polym. 1970, 2I8, 34. 28. Van den Hul, H.J.; Vanderhoff, J.W. J. Colloid Interface Sci. 1968, 28, 336. 29. Vanderhoff, J.W.; Van den Huil, H.J.; Tausk, R.J.; Overbeek, J.T. In Clean

Surfaces: Their Preparation and Characterization for lnterfacial Studies; Goldfinger, G., Ed.; Marcel Dekker: New York, 1970; p 15.

30. Voegtli, L.P.; Zukoski C.F. J. Colloid Interface Sci. 1991, I4I, 92. 31. Chonde, Y.; Krieger, I.M. J. Colloid Interface Sci. 1980, 77, 138. 32. Ahmed, S.M. Ph.D. Thesis, Lehigh University, Bethlehem, PE, USA 1979. 33. Ahmed, S.M.; El-Aasser, M.S.; Pauli, G.H.; Poehlein, G.W.; Vanderhoff J.W.

J. Colloid Interface Sci. 1980, 73, 388. 34. Koglin, B.; Leschonski, K.; Alex, W. Chem. Ing. Techn. 1974, 46, 563. 35. Langer, G. Colloid Polym. Sci. 1979, 257, 522. 36. Oppenheimer, L.E. J. Colloid Interface Sci. 1983, 92, 350. 37. Coll, H.; Haseler, S.C. J. Colloid Interface Sci. 1984, 99, 591. 38. Coll, H.; Searles, C.G. J. Colloid Interface Sci. 1987, 115, 121. 39. Zirkzee, H.F.; Maxwell, I.A.; German, AL. to be published. 40. Verdurmen, E.M.; Albers, J.G.; German, A.L. Colloid Polym. Sci. accepted

for publication. 41. Koehler, M.E.; Provder, T. ACS Symp. Ser. 1987, 332, 231. 42. Coll, H.; Oppenheimer, L.E.; Searles, C.G. J. Colloid Interface Sci. 1985, 104,

193. 43. Coll, H.; Searles, C.G. J. Colloid Interface Sci. 1986, I JO, 65. 44. Lloyd, P.J.; Scarlet, B.; Sinclair, I. In Partiele Size Analysis I970; Groves,

M.l., Wyatt, J.L., Eds.; Wiley: New York, 1970; p 60. 45. Groves, M.J.; Yalabik, H.S. Powder Techno!. 1977, 17, 213. 46. Svarovsky, L.; Svarovska, J. J. Phys., Appl. Phys., Sec. D 1975, 8, 181. 47. Groves, M.J.; Yalabik, H.S. Powder Techno!. 1975, I2, 233. 48. Jones, M.H. Proc. Soc. Anal. Chem. 1966, 3, 116. 49. Beresford, J. Oil Col. Chem. Assoc. 1967, 50, 594.

Experimental Procedures 79

50. Faser, 1.; Patton, T.C. In Pigment Handbook; Patton, T.C., Ed.; Wiley: New York, 1973; Vol. 3, p 327.

51. DuNouy 1. Gen. Physiol. 1918, 1, 521. 52. Harkins, W.D.; Jordan, H.F. 1. Am. Chem. Soc. 1930, 52, 1751. 53. Fox, H.W.; Chrisman, C.H. 1. Phys. Chem. 1952, 56, 284. 54. Sirnon P. Ann. Chim. Phys. 1851, 32, 5. 55. Adamson, A.W. Physical Chemistry of Surfaces; Wiley: New York, 1982; p 4. 56. Sugden, S. 1. Chem. Soc. 1922, 121, 858. 57. Sugden, S. 1. Chem. Soc. 1924, 124, 27. 58. Hutchinson, E. Trans. Faraday Soc. 1943, 39, 229. 59. Overberger, C.G.; Hale, W.F.; Berenbaum, M.B.; Finestone, A.B. 1. Am.

Chem. Soc. 1954, 76, 6185. 60. Sheppard, C.S. In Encyclopedia of Polymer Science and Engineering; Mark,

H.F., Bikales, N.M., Overberger, C.G., Menges, G., Eds.; Wiley: New York, 2. Ed., 1985; Vol. 2, p 143.

80

Chapter 5

The Application of Inisurfs A Survey of the Performance of the Various Inisurfs in the

ab Initio Emulsion Polymerization of Styrene

Summary: The results obtained with different inisurfs in ab initia emulsion polymerization experiments of styrene will be presented. ft will be shown that with an increase in the length of the poly( ethylene oxide) part in the surfactant moiety, the colloidal stability during emulsion polymerization increases. Furthermore, systems with an anionic co-surfactant and a nonionic inisurf show similar behaviour as systems with anionic and nonionic surfactant mixtures.

5.1 Introduetion

In this chapter the first results of the application of inisurfs in the emulsion

polymerization of styrene wil! be discussed. A criterion for a "good" inisurf is that

it should be able to perfarm as an emulsifier and as an initiator in an emulsion

polymerization in the absence of any other emulsifying and/or initiating substance.

Only ab initia experiments (see Section 4.2.2) will be of concern in this part of the

investigation.

This chapter has been divided into two major sections. The results obtained

with symmetrical inisurfs (7a) - (7d) are reported in Section 5.2 and those obtained

82

with asymmetrical inisurfs (7e) - (7g) are reported in Section 5.3.

For the ab initio reactions a standard recipe with styrene as monomer has

been used which is given in Table 5.1. The concentration of inisurf was based on

the volume of the aqueous phase. The monomer-water ratio was 0.1 weight by

weight. The reactions were carried out in a stainless steel batch reactor with a

volume of l dm3 (Section 4.2.l.l) at 323 K, with an impeller speed of 300 rpm

and monitored by gravimetry, unless stated otherwise. This temperature was

chosen because a considerable amount of data is available for the emulsion

polymerization of styrene at 323 K, which can be used for comparison. The

applied impeller speed was similar to the one used for systems where surfactant is

present. The type and amount of co-surfactant wil! be mentioned in the relevant

sec ti ons.

Table 5.1 Standard Recipe for ab lnitio Reactions.

Component

Water

Monomer

Inisurf

Co-surfactant

Amount

900 g

90 g

6.3 1 o-3 mol dm-3

variabie

As was already mentioned in Section 3.2, primary free radicals originating

from the same initiator molecule can combine, before they diffuse away from the

vicinity of forrnation, and form an unreactive compound. As can be envisioned,

this so-called cage-effect1•2 wil! be more pronounced in the case of large free

radicals. In the case of the inisurfs it can be anticipated that with an increase in

the length of the surfactant moiety, the probability of geminate recombination

increases as well. Moreover, it is expected that symmetrical inisurfs show more

geminate recombination than asymmetrical ones, since a large surface-active free

radkal diffuses more slowly than the much smaller t-butyl free radicals. This

The Application of lnisurf 83

effect will be discussed to some extent in Section 5.4.

5.2 Symmetrical Inisurfs

The results of ab initia reactions with the symmetrical inisurfs (7a)-(7d)

will be discussed in order to elucidate the effect of increasing length of the

poly(ethylene oxide) chain in the surfactant moieties of these inisurfs.

5.2.1 Ab lnito Reaelions with Inisurfs SE0-350 (7a) and SE0-550 (7b)

With SE0-350 and with SE0-550 it was not possible to start an emulsion

polymerization with inisurf as the only stahilizing agent in the system. We

reasoned that, since these inisurfs probably were hardly surface-active (no well­

defined hydrophobic and hydrophilic moieties in these molecules), the formed

particles could not be colloidally stabilized. The envisioned in situ formation of

the surfactant was probably too slow to form sufficient surface-active material for

stabilization and subsequent normal emulsion polymerization.

Therefore with these inisurfs, Antarox C0-630 was used as a co-surfactant

in order to have only steric stabilization in these systems. The ab initio reactions

of SE0-350 and SE0-550 were carried out with 5.56 g and 7.82 g of inisurf,

respectively, and with 16.63 g of Antarox C0-630 (dried as described in Chapter

2). The conversion-time curves of these experiments are shown in Fig. 5.1.

As can be seen from these curves, the reaelions with SE0-350 and SE0-550

proceeded at a very slow rate (approximately 1 10·5 and 7 w-6 mol dm-3 s-1,

respectively, between 20% and 40% conversion) and stopped altogether at about

70% conversion due to massive coagulation. No region with a constant reaction

rate was observed.

84 Chapter5

From these experiments it could be concluded that inisurfs SE0-350 and

SE0-550 were not suited for ab initio emulsion polymerizations.

100,_----------------------------------~

80

Figure 5.1

-t (min)

Conversion-Time Curves for ab Initio Reactions with Symmetrical Inisuif SEO-#, with e: SE0-350; ~: SE0-550.

5.2.2 Ab lnitio Reaelions with luisurf SC0-630 (7c)

Applying the standard recipe, an ab initio reaction with SC0-630 was

carried out using 8.53 g of this inisurf. Although a co-surfactant was not necessary

to start the emulsion polymerization, the reaction already stopped at a conversion

of approximately 40% (see Fig. 5.2) due to coagulation.

Stabilization problems, originating from the binding of the surfactant moiety

to the initiator moiety, could have been a reason for the cessation of the

polymerization. Alternatively, poor initiator performance resulting from the

altachment of the initiator moiety to the surfactant moiety could also have been a

reason.

The Application of lnisurf 85

80

~160 40

20

60 120 180 240 300 360 420 480

t (min)

Figure 5.2 Conversion-Time Curve for ab lnitio Reaction with Symmetrical lnisurf SC0-630.

As a check on the stahilizing ability of the inisurf SC0-630, several

experiments were performed in the presence of an anionic co-surfactant (§,odium

f!.odecyl§.ulphate, SDS, for synthesis, Merk, Darmstadt, FRG). The influence of the

SDS concentration on polymerization with SC0-630 has been presented in Table

5.2. The conversion-time curves are plotted in Fig. 5.3.

Table 5.2 Inftuence of the Co-Surfactant Concentration on the Polymerization with SC0-630.

[Co-surfactant] Ne R!_l dw .

pol

(10'3 mol dm-3) (1017 dm-3) (10'4 mol dm·3 s· 1

) (nm)

1.88 1.36 0.78 107.6

7.51 2.85 1.65 84.7

30.0 4.12 2.36 75.2

* Determined by light scattering as described in Section 4.5.3.

As would be expected, the increase in surfactant concentration resulted in

an increase in partiele number, which was seen as an increase in reaction rate in

Interval II (see Figure 5.3), and a drop in partiele diameter (see Table 5.2). From

these results it can be concluded that the stahilizing ability of the surface-active

86 Chapter 5

moiety in the inisurf SC0-630 alone is insufficient to stabilize the formed polymer

partides.

Figure 5.3

..-·

~·,________,_·· _,.·~ ...... !... .. ~~ i 60 120 180 240 300 360

Conversion-Time Curves for ah Initio Reactions with SC0-630 and Different Co-Suifactant Concentrations, with •: 1.88 mol dm-1

,- A: 7.51 mol dm'1; •: 30.0 mol dm·1.

As a check on the initiating moiety an emulsion polymerization was carried

out with the initiator and surfactant used in the synthesis of SC0-630. Thus, a

"traditional" emulsion polymerization was carried out, where initiation and

stabilization were performed by two different substances. ACPA (la) was used as

initiator and Antarox C0-630 (2c) as surfactant. An otherwise identical recipe and

identical conditions as reported in Table 5.1 were used (Table 5.3).

Figure 5.4 shows the conversion-time curve of this "control" reaction as

well as the conversion-time curve of the reaction with inisurf and co-surfactant. As

can be seen, the polymerization rates in Interval 11 per unit volume of aqueous

phase were approximately the same. The partiele diameters were determined at the

end of the reaction using dynamic light scattering. These partiele diameters, as

well as the calculated partiele numbers per unit volume of aqueous phase, the rates

per partiele (R~) and the average number of radicals per partiele (fl) are given in

Table 5.4.

The Application of lnisurf 87

Table 5.3 Recipe for Reaction with Initiator and Surfactant.

Component

Water

Monomer

Initiator (ACPA)

Surfactant (Antarox C0-630)

Co-Surfactant (SDS)

Amount (Concentration)

900 g

90 g

1.59 g (6.3 10"3 mol dm"3

)

6.99 g (12.6 10·3 mol dm-3)

7.88 g (30.0 10"3 mol dm"3)

loo r------···········~······.··-__ --:w:~ ... ~ ... ~ .. ~ ... -=,7'==-=---=.,:.:..::_ .. ;: __ :::.:: .. ~.;:::=~ • .-l

, .. ~..······

60

40

20

0 60 120 180 240

1 (min)

Figure 5.4 Conversion-Time Curves for ah lnitio Reactions with lnisurf SC0-630 ( "') and with Initiator and Surfactants (e).

Since the polymerization rates were comparable and the initiation of

polymerization with SC0-630 was entirely due to the inisurf it can be concluded

from the above results that the performance of the initiating moiety in the inisurf

was not significantly influenced by the link of the initiator moiety to the surfactant

moiety. Only the surfactant moiety of the inisurf should be changed in order to

improve the performance.

88 Chapter 5

Table 5.4 The Effect of Linking Initiator and Suifactant on the Emu/sion Polymerization of Styrene in the Presence of a Co-Suifactant.

Initiator Ne RI/ dw R~ol pol

(1017 dm-3) (104 mol dm·3 s·1) (nm) (10"22 mol s"1

)

SC0-630 4.12 2.36 75.2 5.73

ACPA 6.41 2.81 65.1 6.41

* n is calculated by appling Eq. 3.7, were CM = 5.8 mol dm·' and kP = 258 dm' mol·' s·'

5.2.3 Ab Initio Reaction with SC0-880 (7d)

0.23

0.26

For the ab initio reaction with SC0-880 the standard recipe was applied

with 18.89 g of this inisurf. Figure 5.5 shows the resultant conversion-time curve.

As was the case with SC0-630 (7c), no co-surfactant was necessary to start an

emulsion polymerization, but in this case coagulation did not occur in the course of

the reaction. lt can be concluded that this inisurf, SC0-880, satisfied the

requirements, as set out in Chapter 2. Emulsion polymerizations with SC0-880

will be discussed in more detail in Chapter 6.

5.3 Asymmetrical Inisurfs

In this section the results obtained with the asymmetrical inisurfs (7e) - (7g)

will be discussed. As was the case with the symmetrical inisurfs, the results will

be presented in the sequence of the increasing length of the poly(ethylene oxide)

chain in the surfactant moiety of the inisurfs.

The Application of lnisurf 89

100

80

~1 60

40

20

0 0 120 240 360 480 600

Figure 5.5 Conversion-Time Curve for the ab Initio Reaction with Symmetrical Inisurf SC0-880.

5.3.1 Ab lnitio Reactions with Inisurf AE0-550 (7e)

The reaction with AE0-550 was carried out according to the recipe in Table

5.1 with 4.22 g of this inisurf. As with the symmetrical inisurf SE0-550, a co­

surfactant (Antarox C0-630, 16.63 g) was needed to start an emulsion

polymerization. The resultant conversion-time curve is shown in Fig. 5.6. As was

the case with the symmetrical inisurf SE0-550, an emulsion polymerization

commenced, but after 20% conversion the reaction stopped, probably due to

probieros with the colloidal stability of the particles.

5.3.2 Ab lnitio Reactions with Inisurf AC0-630 (70

The batch reaction with AC0-630 was carried out using the standard recipe

with 4.68 g of this inisurf. The resultant conversion-time curve is shown in

Fig. 5.7. The emulsion polymerization started normally, but after 20% conversion

massive coagulation occurred and the reaction stopped.

90 Chapter 5

100,----------------------------------,

80

160

tf ~ 40

20 ..................•. ~ .... - ...

0 200 400 600 800 1000 1200 1400 1600

t(min)

Figure 5.6 Conversion-Time Curve for the ab Initia Reaction with Asymmetrical Inisuif AE0-550.

80

160

tf ~ 40

20

o~~~=d~~~~~~~~~~~~~~

0 250 500 750 1000 1250 1500 1750

t (min)

Figure 5.7 Conversion-Time Curve for the ab Initia Reaction with Asymmetrical Inisuif AC0-630.

Table 5.5 Influence of the Co-Suifactant Concentration on the Polymerization with AC0-630.

[Co-surfactant] Ne R~ol dw ( 1 0"3 mol dm"3

) (1016 dm.3) (10"5 mol dm"3 s" 1) (nm)

1.88 0.91 0.67 262.6

7.51 3.07 2.40 175.1

30.0 3.82 3.00 162.7

The Application of Inisurf 91

With this inisurf reactions have also been carried out in the presence of an

anionic co-surfactant (SDS). The influence of different co-surfactant concentrations

on the polymerization is shown in Table 5.5. The resultant conversion-time curves

are shown in Fig. 5.8.

As was observed m the case of the symmetrical inisurf SC0-630, an

increase in the co-surfactant concentration resulted in an increase in the partiele

number. Hence, this led to an increase of the overall reaction rate and to a

decrease in the mean partiele diameter, as shown in Table 5.5.

100~--------------------------------------,

80

160

i?: ';' 40

20

0

Figure 5.8

100 200 300 400 500 600 700 800

t (min)

Conversion-Time Curves for ab Initia Reactions with AC0-630 and Different Co-Surfactant Concentrations, with e: 1.88 mol dm-3

; + : 7.51 mol dm-3; .11: 30.0 mol dm-3

•

It can be coneluded that upon addition of co-surfactant normal emulsion

polymerization behaviour was observed. Thus, the steric stahilizing ability of

inisurf AC0-630 was insufficient and, if an emulsion polymerization has to be

carried out where initiation and stabilization stem solely from inisurf, the

stahilizing ability of the surfactant moiety of the inisurf has to be improved. It can

also be concluded that the initiating ability has not been impaired significantly by

attaching the surfactant moiety to the initiator moiety.

92 Chapter 5

5.3.3 Ab Initio Reaction with luisurf AC0-880 (7g)

For the ab initio reaction with AC0-880 the standard recipe was applied

with 9.86 g of this inisurf. Fig. 5.9 shows the resultant conversion-time curve. As

was the case with SC0-880 (7d), a co-surfactant was not necessary to perform an

emulsion polymerization and coagulation did not take place during the reaction. It

can be concluded that this inisurf satisfied the requirements, as set in Chapter 2.

Further results of emulsion polymerizations with AC0-880 will be presented in

Chapter 6.

100

80

~~ 60

40

20

o' 0 200 400 600 800 1000

,. t (min)

Figure 5.9 Conversion-Time Curve for the ab lnitio Reaction with Asymmetrieal/nisurf AC0-880.

5.4 Discussion

In the series, where the co-surfactant concentration was varied, the results

obtained with the symmetrical and the asymmetrical inisurf show no significant

differant behaviour. Fig. 5.10 shows the size of the particles at the end of the

reaction versus the ratio [anionic co-surfactant]/[nonionic inisurf], defined as v.

The partiele sizes obtained with asymmetrical inisurf are larger than those

obtained with symmetrical inisurf. Since it is presumed that partiele formation

occurred mainly via the micellar nucleation mechanism, the larger partiele nurnber

The Application of Inisurf 93

for symmetrical inisurfs can be explained as follows. The symmetrical inisurf has

two surfactant moieties whereas the asymmetrical inisurf has only one; hence, the

concentration of nonionic surfactant for the symmetrical inisurf was twice as large.

Thus in the case of symmetrical inisurfs, more micelles and consequently, more

particles were formed.

300 ,--r----------------

········ ....•.. . .................. .

V

Figure 5.10 Partiele Size ( dw) versus the Ratio ( v) of Anionic Co­Surfactant to Nonionic lnisurf, with +: SC0-630; ,.,,. AC0-630.

Sirnilar to the results obtained by Woods et a/. 3, by Chu et al.\ and by

Kato et a/. 5, it was found that the size of the polystyrene particles decreased with

an increasing amount of the anionic component (co-surfactant) in the surfactant

mixture. It is known that an increasing amount of anionic surfactant in a mixture

of surfactants leads to a decreasing size of the mixed micelles6• The decreasing

size of the polymer particles with increasing SDS concentration was not only the

result of the amount of surfactant itself, but also of the smaller micelle size3•7

•

In the emulsion polymerization of styrene, according to the Smith-Ewart

theory, a logarithmic plot of the number of particles formed and, therefore, the rate

of polymerization, Rpot• versus the total surfactant concentration, [S],01 , should

generate a straight line (substitution of Eq. 3.20 into Eq. 3.18). However,

experimental results of the present study did not give a straight line, as is

94 Chapter 5

illustrated in Fig. S.ll. Similar results have been obtained by Kamath8 when

applying mixed emulsifier systems. Other factors were possibly affecting the rate

of polymerization. It was assumed7·8 that the size of the micelle was a contributing

factor. Additionally, it was shown by Piirma et ae, and this can also be deduced

from results presenled by Medvedev9, that the probability of the micelle to become

a latex partiele depends on the size of the micelle. This problem lies beyond the

scope of the present investigation. However, it bas been shown that systems where

nonionic inisurfs are used in combination with anionic co-surfactant, behave

similarly as systems with a traditional initiator-emulsifier combination.

18 10

-2 10

17 10 A_::--

'"'

1~ -3 o:-e JO

1016 ."

! ~v -4 JO

r:.:.l w's

-5 JO

1014

5 10"3 10-2 Jo·'

[SJ (mol rm

Figure 5.11 Number of Polymer Particles (N) and Rate of Polymerization (R~01 ) versus the Total Surfactant Concentration ([S],0,), with .1,o: SC0-630; -',•: AC0-630.

At last, the results obtained with inisurfs synthesized with Antarox Co

(SC0-630, SC0-880, AC0-630, and AC0-880) are discussed in more detail. Fig.

5.12 shows the conversion-time curves for these inisurfs.

In the case of the asymmetrical inisurfs an increase in the chain length of

the ethylene oxide chain (increasing HLB) resulted in a higher reaction rate 10 and

in a more stabie latex11, as was expected.

For the symmetrical inisurf a lower initia! reaction rate bas been observed

The Application of lnisurf 95

for SC0-880 as compared with SC0-630. As can be anticipated intuitively, with

an increase in length of the inisurf molecule the influence of a cage-effect would

be more pronounced. Thus, a longer ethylene oxide chain in the symmetrical

inisurf leads to a deercase in the efficiency of initiation. A lower initiation rate

results in less particles and a slower initial polymerization rate, and thus masks the

effect of the increased stabilization by the longer ethylene oxide chain. The

constant increase in the reaction rate, as observed in the case with SC0-880, will

be discussed in more detail in Chapter 6. This effect of a longer ethylene oxide

chain is apparently very minor or even absent in the case of asymmetrical inisurfs.

80

20

250 500 750 1000 1250 1500 1750

t (min)

Figure 5.12 Conversion-Time Curves for the Varies lnisurfs Synthesized with Ll: SC0-630; o: SC0-880; "-: AC0-630; •: AC0-880.

Keferences

1. Franck, J.; Rabinowitsch, E. Trans. Faraday Soc. 1934, 30, 120. 2. Rabinowitsch, E.; Wood, W.C. Trans. Faraday Soc. 1936, 32, 1381. 3. Woods, M.; Dodge, J.; Krieger, J.M.; Pierce, R. J. Paint. Tech. 1968, 40, 541. 4. Chu, H.-H.; Piirma, I. Polym. Bull. 1989, 21, 301. 5. Kato, K.; Kondo, H.; Esumi, K.; Meguro, K. Bull. Chem. Soc. Jpn. 1986, 59,

3741.

96 Chapter 5

6. Kuriyama, K.; Inoue, H.; Nakagawa, T. Kolloid Z Z Polym. 1962, 183, 68. 7. Piirma, 1.; Wang, P.-C. ACS Symp. Ser. 1976, 24, 34. 8. Kamath, V. Ph.D. Thesis, University of Akron, Akron, Ohio, USA, 1973. 9. Medvedev, S.S.; Gritskova, LA.; Zuikov, A.V.; Sedak:ova, L.I.; Berejnoi, G.D.

J. Macromol. Sci., Chem. 1973, Al, 715. 10. Chao, T.-C.; Piirma, I. Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem.

1983, 24, 69. 11. Greth, G.G.; Wilson, J.E. 1. Appl. Polym. Sci. 1961, 5, 135.

Chapter 6

Inisurfs with Antarox C0-880 as Surfactant Moiety

Summary: Two inisurfs, one symmetrical and one asymmetrical, both with Antarox Co-880 as surfactani moiety, wil/ be discussed in detail. The dissociation rates and the Critica/ Micelle Concentrations will be given and discussed. The results of ab initio reactions, i.e., reaction rate, partiele size distribution and molecular weight, will be compared with the results obtained for conventional systems with initiator and surfactant similar to the initiator moiety and the surfactant moiety of the inisurfs.

6.1 Introduetion

In this Chapter two of the inisurf systems, described in Chapter 5 and which

were found to be suitable in emulsion polymerization, will be discussed in more

detail. The symmetrical inisurf SC0-880 and the asymmetrical inisurf AC0-880

have been investigated. First, some physical properties, i.e., the dissociation rate

(Section 6.2), and the CMC and surface coverage (Section 6.3), will be discussed.

This is foliowed in Section 6.4 by a eloser look at the rate of polymerization,

partiele size and partiele size distribution. This chapter ends with a discussion of

98 Chapter 6

the obtained molecular weights (Section 6.5).

6.2 Dissociation Rate of the Inisurfs

The rate coefficient of dissociation of the inisurfs was determined in water

at a concentration above the CMC of those inisurfs. The dissociation was

monitored by following the deercase of the UV absorption at À. = 350 nm, which is

caused by the disappearance of the azo-group.

As has been reported by numerous researchers, the dissociation of azo

compounds is a unimolecular, first-order rate process1.2.3.4. The rate constant is

independent of concentration3.5·6 and is virtually unaffected by the reaction

medium1•2

•3•7

•8

•9

•10

. However, the dissociation rate is influenced by steric effects7

and by branching of the molecule4• It can be anticipated that the dissociation rate

of the inisurfs will differ according to their solubilization state, i.e., whether they

are adsorbed at the surface of a partiele or agglomerated in a rnicelle or whether

they are molecularly dissolved in the aqueous phase. Since the behaviour of inisurf

at the surface of polymer particles and in micelles is of interest, we will only

consider the dissociation rate above the CMC. The contribution of

monomolecularly dissolved inisurf to the dissociation rate is assumed to be

negligible due to the low value of the CMC.

It is known from literature that the absorption originating from to the azo­

group obeys Beer's law5•11

• Thus, absorbtion can be translated directly to

concentration. As shown in Figs. 6.1 and 6.2, excellent linear first-order

correlations were obtained by plotting -In (Abs, I Abs 0) versus time (t), where Abs 0

and Abs, denote the absorbtion at zero and at t seconds, respectively. The

absorbtion at t = = is zero (water is used as a reference). The kd values for the

SC0-880 and AC0-880 were determined from the slopes of the plots and are

summarized in Table 6.1. Fig 6.3 shows the plot of In kd versus T'.

lnisurfs from Antarox C0-880 99

1.20

'""'

[f 0.80

~ 0.40

..!?

6 12 18 24 30 36

t (!0 3 s)

Figure 6.1 Logarithm of the UV Absorbtion at 350 nm versus Time for SC0-880 at Various Temperatures: + = 323 K, o = 343 K, ~ = 353 K.

'""' l.20 .-----,~--rl ------,

I c

[ ~- 0.80

~ 0.40 c: ..

l ,' : I

/ I // ; I

i'

I I

I

I 00 200 300 400 500 -t (103 s)

Figure 6.2 Logarithm of the UV Absorbtion at 350 nm versus Time for AC0-880 at Various Temperatures: ~ = 323 K, o 334 K, + = 441 K.

Table 6.1 Dissociation Rate for SC0-880 and AC0-880

Temperature kd

(K) SC0-880 AC0-880 (10-6 s·1) ( 10-6 s·1)

323 6.3 ± 0.6 0.93 ± 0.09

334 5.0 ± 0.5

341 11 ± 1

343 12 ± 1

353 30 ± 3

Eact (kJ mof1) 47 ± 5 1.3 102 ± 0.1 102

k0 (s' 1) 2.3 IOZ ± 0.2 102 7.6 1014 ± 0.8 1014

100 Chapter 6

The activation energies (Eac,) and the pre-exponential factor (k0) were determined

and these are reported in Table 6.1. The values for the dissociation constant at

323 K are of the sarne order of magnitude as those reported for other azo-type

initiators1•2

•5

•7

•8

•10

• In the case of the asymmetrical inisurf an activation energy and a

pre-exponential factor were found simHar to the ones, obtained for other azo-type

compounds. For the symmetrical inisurf a substantially lower activation energy and

pre-exponential factor were found. The decreases in the activation energy and in

the pre-exponential factor were attributed to steric and resonance effects. For a

symmetrical inisurf at the surface of a particle, this implies that there is an increase

in strain within the molecule, when compared to an asymmetrical one.

Figure 6.3

0.00310

The Dissociation Rate Constant versus Temperafure for SC0-880 (---) and AC0-880 r---1.

The values for the dissociation rate constants will be used in Chapter 7 to

determine the initiator efficiency of the inisurfs.

6.3 Critical Micelle Concentration

The critical micelle concentration of the two inisurfs under investigation

have been measured by means of maximum pressure bubble tensiometry (Section

lnisurfs from Antarox C0-880 101

4.6.2). For comparison the CMC of the symmetrical inisurf has also been

measured by means of the Du Nouy-ring metbod (Section 4.6.1 ). Not withstanding

the fact that the CMC is temperature dependent12, the measurements have been

carried out at room temperature due to the thermolability of the inisurf. Fig. 6.4

and 6.5 show the surface tension versus concentration. The CMC can be easily

determined by extrapolation of the two linear regions in these plots. Table 6.2

gives the measured CMC of the two insurfs with the 'different methods. Good

agreement was found between the two methods.

15.----------------------,

0.10 0.20 0.30 0.40 0.50

Figure 6.4 Surface Tension versus Concentration of SC0-880.

so.----------------------.

30L-~~~~~~~~~~~

0.00 0.20 0.40 0.60 0.80 1.00

molr 1)

Figure 6.5 Surface Tension versus Concentration of AC0-880.

Table 6.2 Criticle Micelle Concentration (mol dm'3).

SC0-880

AC0-880

Du Nouy Metbod

6.4 10·4

Maximum Pressure Bubble Metbod

2.0 w-s 6.3 10'4

Since the behaviour of the inisurf on the surface of polymer particles is of

interest, it is also important to know the surface coverage as a function of the

concentration in the continuous phase, i.e., the adsorption isotherm must be known.

The adsorption isotherm and, thus, the surface coverage as reported by Kronberg et

a/. 13 for nonylphenol poly(ethylene oxide) are assumed to be applicable for the

102 Chapter 6

inisurfs14• As a cheque the adsorption isotherm of the asymmetrical inisurf on

polystyrene polymer particles has been measured. The measurements were carried

out by means of maximum pressure bubble tensiometry (see Section 4.6.2) on a

cleaned seed latex (see Section 4.3). The surface tension was measured as function

of the overall inisurf concentration. By using the curve, obtained by measuring the

surface lension as a function of inisurf concentratien in pure water (Fig. 6.5), and

by applying a mass-balance, it was possible to calculate the amount of inisurf

adsorbed at the surface as a function of the inisurf concentratien in the aqueous

phase (Fig 6.6). The adsorption (r) has been expressed as the number of mo!es

per square meter of surface and the concentratien of inisurf (X) as the number of

J.UllOles per mole salution (inisurf + water).

2,50

2.00 n 1.50

1.00

(.... 0.50

0.00 0.0 2.0 4.0 6.0 8.0 10.0 12.0

X (f.llllol mol-I)

Figure 6.6 Adsorption Isothenn for AC0-880 at 293 K.

This adsorption isotherm seems to obey approximately a Langmuir-type

expression:

r r KX Ml+KX

(6.1)

where r M is the maximum amount of inisurf that can be adsorbed per unit surface

area and K is a constant, which can be seen as an equilibrium constant governing

the partilioning of inisurf between the surface layer and the bulk phase15•

The Langmuir equation applied to solutions is derived under the assumption

that the following conditions are met: ( 1) the absorbent is homogeneous, (2) the

lnisurfs from Antarox C0-880 103

adsorption takes place in only one molecular layer, (3) the solvent and the solute

have equal molecular cross-sectional surface areas, and (4) there is no net solute­

solvent interaction at the surface or in the bulk phase, i.e., the denvation is based

on an ideal mixing model. For nonionic surfactants on latex particles16, and also

for inisurfs based on these surfactants, the two first assumptions are reasonable, but

the latter two conditions are clearly not met. Nevertheless, an equation of the

Langmuir-type generally gives a good fit of the adsorption isotherms of surfactants.

By applying the Flory-Huggins theory for polymer solutions, it has been shown17

that deviations from assumptions (3) and (4) cause oppositely directed deviations

from a Langmuir-type adsorption, i.e., the Langmuir-type expression is

approximately obeyed due to a compensation of two counteracting effects.

If llr is plotted versus 1/X (see Fig. 6.7), a straight line is obtained within

experimental error. This straight line is in accordance with the Langmuir equation.

Rewriting Eq. 6.1 gives:

1 1 1 (1) - = _+ __ _ r rM KrM x

(6.2)

The intercept of the line in Fig 6.7 gives llr M• which is equal to the cross-sectional

surface area at complete coverage, aad, of the inisurf on polystyrene latex particles.

K, the equilibrium constant, can be related to a standard molar free energy of

adsorption, AJ.t:

AJ.t = -RT ln(K) (6.3)

The various values determined are summarized in Table 6.3. For comparison a set

of literature data is given as well13. The values obtained are of the same order of

magnitude as those obtained for the adsorption of nonionic surfactants on

hydrophobic surfaces 13•16

• Keeping in mind that the structure of the asymmetrical

inisurf is only slightly different from that of the nonionic surfactants, can explain

the deviation in the values obtained.

The adsorption isotherm data were used for determining the fractional

surface coverage (see Chapter 7).

104 Chapter 6

Table 6.3 Adsorption Data of AC0-880.

Inisurf rM a ad K 11~0

(mol m·2) (Hf m2 mor1) (kJ mol-1

)

AC0-880 2.26 w-6 4.43 3.7 l(f -31.2

NF-E20 . 1.56 w-6 6.41 3.0 106 -37.0

• NF-E,o is a nonyl phenol poly(ethylene oxide) surfactani with 20 ethylene oxide units

sr-----------------------~

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Figure 6.7 1 Ir versus 1 /X for AC0-880 on a Polystyrene Seed.

6.4 Ab Initio Reactions with SC0-880 and AC0-880

The results of ab initia emulsion polymerizations with the inisurfs

containing the nonionic surfactant Antarox Co-880 in the absence of any

co-surfactant at 323 K will be discussed in detail.

Fig. 6.8 shows the conversion-time curve obtained for a system with

SC0-880. For comparison the results, obtained for a system with ACPA (la) and

Antarox Co-880 (2d), have been plotted as wel!. As can be seen the reaction rate

of the "control" system was much higher than that of the inisurf system. It was

assumed that due to the cage-effect and subsequent geminate recombination, the

initiation rate was much smaller for the inisurf system. This led to Jess particles

early in the reaction and, thus, to a smaller initia! rate.

lnisurfs from Antarox C0-880 105

As can be seen in Fig. 6.8, the rate of the inisurf system increased

dramatically as the reaction proceeded and approached the rate of the "control"

system closer. This indicated that partiele nucleation occured up to higher

conversions than for the ACP N Antarox recipe. The partiele size distri bution

(PSD) at the end of the reaction was obtained by means of transmission electron

microscopy (see Section 4.5.2). Fig 6.9 shows the PSD, obtained with the inisurf

system. As can be seen, this distribution had a long tail of very smal! amplitude in

the higher partiele size range, thereby establishing, that at the start of the reaction a

small number of particles was formed and that partiele nucleation continued in the

course of the reaction. This continuous nucleation rate could be due to the cage­

effect: the initiating ability disappeared but the substance formed by geminate

recombination still was a surfactant. This surfactant could stabilize particles, but it

could also form micelles, that functioned as precursers for new particles. This

process would continue until the monomer dropplets disappeared. Fig. 6.10 shows

the PSD of the control reaction. A rather narrow PSD with an average diameter,

which is slightly smaller than the one obtained for the symmetrical inisurf system,

was found. The large tailing in the higher range of the PSD did not occur, which

indicates that a cage effect, if present, was not important in this system.

80

Figure 6.8

480 600

t (min)

Conversion-Time Curves for the ab lnitio Polymerization with Symmetrical Initiator SC0-880 (•) and with ACPA (la) & Antarox C0-880 (2d) ( t:.). ([SC0-880] = {ACPA] = 0.5 [Antarox C0-880])

106 Chapter 6

o.Js,-------;;:::;;:'Flr"""'""""'==----..,~.oo

Partiele Diameter (nm)

Figure 6.9 Partiele Size Distribution at the End of the ab Initio Reaction with SC0-880 (7d).

0.35 1.00

~ 0.30 ~

~ 0.80 c::: c::: 0 0.25 .g ·.:::: i\ :::>

0.60 .0 ·c: 0.20 ·.s

~ "' i5 i5 ~

0.15 0.40

<U ;. . ."

0 3 . ." ~ E Ii: 0.20 :::> u

5 55 105 155 205 255 295 o.oo

Partiele Diameter (nm)

Figure 6.10 Partiele Size Distribution at the End of the ab lnitio Reaction with ACPA (la) and Antarox Co-880 (2d).

Fig 6.11 shows the conversion-time curve obtained for a system with AC0-

880. In this tigure the conversion-time curve of the "control" reaction with BACA

(lb) and the appropriate amount of nonionic surfactant Antarox Co-880 has been

plotted as well. A slightly reversed situation occurred, as compared with the

symmetrical system. The reaction with the asymmetrical inisurf proceeded faster

than the "control" reaction. It was observed that in the control reaction the reaction

rate increased drastically between 30 and 40% conversion. A similar effect was

observed by Piirma et al. 18•19 for similar systems. The explanation given for this

lnisurfs from Antarox C0-880 107

was that phase inversion occurred during the reaction. The system started as a

water-in-oil emulsion and then changed to an oil-in-water emulsion. The first erop

of particles was formed in the discreet aqueous dropiets and the second erop was

formed in the continuous aqueous phase after the phase inversion. This

phenomenon is govemed not only by temperature and the amount of surfactant

present, but also by the chemical structure of the surfactant, for which the HLB

value can be a measure.

. -•· _ ..

, . .. ...• .......

•' ~-

0 ~--~--i_--~------~---L........... o .................. L ............... ~--~

0 200 400 600 800 1000

t (min)

Figure 6.11 Conversion-Time Curves for the ab Initia Polymerization with Asymmetrical Initiator AC0-880 ( •) and with BACA (lb) & Antarox C0-880 (2d) (•). ([AC0-880] = [BACA] = [Antarox C0-880]).

Fig 6.12 and Fig 6.13 show the PSD, obtained for the ab initia

polymerization of the asymmetrical inisurf system and of the control system,

respectively. In both cases a bimodal PSD was observed. The distribution of the

control system indeed indicated that a small partiele population was formed at the

beginning of the reaction and a much larger second partiele population formed at a

later stage. The distribution for the asymmetrical inisurf system showed a bimodal

or an extremly skewed distribution.

I 08 Chapter 6

0.35 1.00

0.30 0.80 ~

.J: -._!.,

c 0.25 g 0 ·g ·a .6 0.60 .0

0.20 "E -~ "' i.5 i.5 " ~ 0.15 ~0.40

,. -~

0 ê •z:j

~ 0.10 8 0.20

0.05

0.00 0.00 5 55 105 155 205 255 295

Partiele Diameter (nm)

Figure 6.12 Partiele Size Distribution at the End of the ab lnitio Reaction with AC0-880 (7g).

5 205 255 295

Partiele Diameter (nm)

Figure 6.13 Partiele Size Distribution at the End of the ab lnitio Reaction with BACA (lb) and Antarox Co-880 (2d).

From the above results it may be derived that the nucleation mechanism

was not only governed by the effectiveness of the initiator system to form radicals

and by the amount of surfactant, but also by the chemical structure of the surfactant.

lnisurfs from Antarox C0-880 109

6.5 Molecular Weight

The molecular weights of the polymers, formed in the system with SC0-

880 and AC0-880, were measured by means of Gel Permcation Chromatography

(GPC) and are given in Table 6.5, where M. and M,. are the number and weight

average molecular weight, respectively.

Table 6.5 Average Molecular Weight.

SC0-880

AC0-880

0.34

1.2

2.7

4.2

D

7.9

3.4

From a comparison of these data with those, obtained for emulsion

polymerization of styrene at 323 K (see for example Piirma et al. 20, and Whang et

a/. 21), it may be concluded that with inisurfs the molecular weights were slightly

higher than those in "conventional" systems. This phenomenon was also reported

by Tauer et al. 22•

As will be discussed in more detail in Chapter 7, the entry rate coefficient

for the systems under study was rather small. This leads to the hypothesis that

most growing ebains were stopped by transfer, and not by terminalion through

combination or disproportionation. Of all chain transfer reactions transfer to

monomer was the most likely one to have occurred. It is known that transfer to

poly(ethylene oxide) can occur. The reported chain transfer constant (6.1 10·2 dm3

mol' s·1)

23 is even significantly larger than the one, reported for transfer to

styrene (l 10·2 dm3 mor' s·1)

24• Assume as a first approximation that transfer to

polyethylene was negligible due to its small concentration as compared to the

monomer concentration in the particles. This appears even more reasonable when

one realizes that most of the poly(ethylene oxide) was located at the surface of the

particles. Transfer to polymer was neglected as well in this approximation, since

the transfer constant (7.5 10'5 dm-3 mol·' s·1)

25 is very small. The average degree

110 Chapter 6

of polymerization P. is than expressed as:

p n

k" [M] + k, [R ·1 (6.4)

Due to the low entry rate this can be simplified to:

k [M] p (6.5)

k,, [M]

The data reported for k,, by Clay et al. 24, Whang et al. 26

, and Tobolsky et al. 27,

are in the order of magnitude of 10·2 dm3 mol" 1 s·1• Substitution of this value,

tagether with kr = 258 dm3 mol·1 s·1 28•29

, in Eq. 6.5 gives a P. of 2.58 104 and

an average molecular weight M. of 2.69 106 g mol'1,

The obtained values for M. in the inisurf systems were large but

nevertheless significantly less than the one estimated above. This leads to the

condusion that transfer to monomer was dominant but not exclusive. Other events,

like transfer to poly(ethylene oxide) or termination (e.g., when the system becomes

non-zero-one or due to some coagulation during partiele formation3<) were of a

bigger importance than was assumed at first. Wether one of these or both events

influenced the MWD, can be answered by investigating the time evolution of the

MWD. Since determination of the time evolution of the MWD lay beyond the

scope of this investigation, no further explanation can be given for the molecular

weight.

References:

I. Lewis, F.M.; Matheson, M.S. J. Am. Chem. Soc. 1949, 71, 747.

lnisurfs from Antarox C0-880 111

2. Overberger, C.G.; O'Shaughnessy, M.T.; Shalit, H. J. Am. Chem. Soc. 1949, 71, 2661.

3. Bawn, C.E.H.; Mellish, S.F. Trans. Faraday Soc. 1951, 47, 1216. 4. Vernekar, S.P.; Ghatge, N.D.; Wadgaoknar, P.P. J. Polym. Sci., Polym. Chem.

Ed. 1988, 26, 953. 5. Van Hook, J.P.; Tobolsky, A.V. J. Am. Chem. Soc. 1958, 80, 779. 6. Talet-Erben, M.; Bywater, S.J. Am. Chem. Soc. 1955, 77, 3712. 7. Overberger, C.G.; Berenbaurn, M.B. J. Am. Chem. Soc. 1951, 73, 2618. 8. Petersen, R.C.; Markgraf, J.H.; Ross, S.D. J. Am. Chem. Soc. 1961, 83, 3819. 9. Funt, B.L.; Pawelchak, G. J. Polym. Sci., Polym. Let. Ed. 1975, 13, 451.

10. Blackley, D.C.; Haynes, A.C. J. Chem. Soc., Faraday Trans. 11919, 75, 935. 11. Seltzer, S.J. Am. Chem. Soc. 1961, 83, 2625. 12. Schick, M.J. J. Phys. Chem. 1963, 67, 1796. 13. Kronberg, B.; Käll, L.; Stenius, P.J. Dispersion Sci. Techno/. 1981, 2, 215. 14. Kusters, J.M.H.; Napper, D.H.; Gilbert, R.G.; Gerrnan, AL. Macromolecules

1992, 25, 7043. 15. Brown, C.E.; Everett, D.H. In Specialist Periodical Reports Colloid Science;

Everett, D.H., Ed.; The Chemica! Society: London, 1975; Vol. 2; p 28. 16. Kronberg, B.; Stenius, P.; Igeborn, G. J. Colloid Interface Sci. 1984, 102, 418. 17. Kronberg, B. J. Colloid Interface Sc i. 1983, 96, 55. 18. Piirrna, 1.; Chang, M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 489. 19. Piirrna, 1.; Maw, T.S. Polym. Bull. 1984, 11, 497. 20. Piirrna, 1.; Kamath, V.R.; Morton, M. J. Polym. Sci., Polym. Chem. Ed. 1975,

13, 2087. 21. Whang, B.C.Y.; Napper, D.H.; Ballard, M.J.; Gilbert, R.G. J. Chem. Soc.,

Faraday Trans. 11982, 78, 1117. 22. Tauer, K.; Goebel, K.-H.; Kosrnella, S.; Stähler, K.; Neelsen, J. Makromol.

Chem., Macromol. Symp. 1990, 31, 107. 23. Okarnura, S.; Katagiri, K.; Motoyarna, T. J. Polym. Sci. 1960, 43, 509. 24. Clay, P.; Napper, D.H.; Gilbert, R.G. To be published. 2S Henrici-Olivé, G.; Olivé, S.; Schulz, G.V. Z. Phys. Chem. [N.F./ 1959, 20,

176. 26. Whang, B.C. Y.; Ballard, M.; Napper, D.H.; Gilbert, R.G. Aust. J. Chem. 1991,

44, 1133. 27. Tobolsky, A.V.; Offenbach, J. J. Polym. Sci. 1955, 16, 311. 28. Hawkett, B.S.; Napper, D.H.; Gilbert, R.G. J. Chem. Soc., Faraday Trans. 1

1980, 76, 1323. 29. Lansdowne, S.W.; Gilbert, R.G.; Napper, D.H.; Sangster, D.F. J. Chem. Soc.,

Faraday Trans. I 1980, 76, 1344. 30. Morrison, B.R.; Maxwell, I.A.; Gilbert, R.G.; Napper, D.H. ACS Symp. Ser.

1992, 492, 28.

112

Chapter 7

Kinetics of Partiele Growth in Seeded Emulsion Polymerization with Inisurfs

Summary: The kinetics of partiele growth in seeded emulsion polymerization of styrene, initiated by inisurf, will be investigated. Two inisutfs, SC0-880 and AC0-880, wilt be used. These systems are characterized by a relatively low fï. In the case of the symmetrical inisutf the rate coefficient for free radical exit is obtained using y-radiolysis relaxation. This value wilt then be used to determine the rate coefficient for radical entry and the initiator efficiency in seeded studies with varying degrees of sutface coverage by symmetrical inisurf. In the case of the asymmetrical inisutf the slope and intercept method is applied to obtain entry and exit rate coefficients at different degrees of surface coverage. These data are then used to calculate the efficiency of this inisurf. The initiator efficiencies in both cases is very low (ca. 0.4 10"3 and 1.2 10"3

,

respectively ). This result can be readily explained qualitatively and quantitatively by comparison of the time scale for gemmate recombination of the two free radicals, formed by inisutf decomposition, with the time scale for escape by dijfusion of one of the two free radicals from the vicinity of the particle.

114 Chapter 7

7.1 Introduetion

In developing a better understanding of the formation and application of

polymer colloids, it is essential to have qualitative and quantitative mechanistic

k.nowledge of the events governing partiele formation and growth. It is now

accepted1.2 that such understanding must start with studies of polymer growth in

seeded systems, i.e., in the presence of preexisting latex particles and without

secondary partiele nucleation (see Sections 4.2.3 and 4.3). Hence, one can

investigate partiele growth without the complexities of concomitant partiele

formation. The k.nowledge so obtained can then be used to unravel the

complexities of partiele formation itself 3•4.s.

This chapter deals with the effects of inisurf on a seeded styrene emulsion

polymerization. This system was chosen, because the growth kinetics - both ab

initio and seeded - of styrene systems with conventional initiators and surfactants

are now comparatively well understood, as has been discussed in Section 3.5.1.

With a surface-active initiator it is expected that the events leading to initiation of

propagation inside a latex partiele (i.e., entry) will be quite different from those in

a corresponding conventional system with an aqueous-phase initiator. In an inisurf

system one may expect that the free radicals will be formed at the partiele/water

interface and, hence, that the rate-determining step for the successful penetration of

a free radical into the interior of the partiele will be quite different. One may also

expect that the presence of polymerie surfactant on the surface of the partiele can

affect the rate of desorption (exit) of free radicals from the particles in an inisurf­

initiated system. These questions will be addressed in the mechanistie studies

presented in this chapter.

Therefore, the objective is to unravel the mechanism for partiele growth of

a well characterized polystyrene latex initiated by inisurf. The questions that have

to be answered by this study are: what is the initiator efficiency? What is the

mechanism for the successful entry of free radicals into the interior of the latex

particles? How large is the contribution of exit (desorption) of free radicals from

the partides, and if significant, what is the mechanism for this Joss of free radical

Partiele Growth 115

activity? What is ii, the average number of free radicals per particle? Does

termination within the latex particles occur as a result of entry of a new free

radical causing instantaneous terminalion with another free radical, i.e., does one

have a zero-one1·2

·6

, or Smith-Ewart cases 1 and 2 combined7, system, with ii s

0.5? Or is terminalion between two growing ebains in a partiele slow enough to be

kinetically significant?

In the past such questions have been investigated for styrene emulsion

pol ymerization systems 1 ·2

·6

·8

·9

•10

• 1

1. 12

• When the emulsion polymerization

methodology is to be applied in the investigation of the mechanism, certain

modifications must be introduced. In a conventional emulsion polymerization

mechanistic information can be gained from studies, in which initiator

concentration is varied; in an inisurf-iniliated system the equivalent is to vary the

surface coverage of the partiele by inisurf. Latex prepared from an inisurf cannot

be used, because it has surfactant moieties partly covalently bonded to the surface.

Therefore, a seed latex was prepared by emulsifier-free emulsion polymerizalion

(as described in Section 4.3) and subsequently stripped of all (presumably ionic)

surfactant arising from the seed-making process. The surface coverage can then be

changed by adding various amounts of inisurf. Apart from cleanability, colloidal

stability and monodispersity, the seed should have a radius of less than 70 nm. It

was established for the styrene system that this results in ii s 0.5 over a wide range

of conditions6•13

• It then becomes possible to analyze the data obtained as a

"zero-one" system (Section 3.7).

In Section 7.2 the study of seeded systems with symmetrical inisurf

SC0-880 will be discussed. Next to a short explanation of the experimental

techniques and procedures involved, the results obtained will be given and

discussed. In Section 7.3 the results obtained with the asymmetrical inisurf AC0-

880, will be presenled and discussed. The results of these seeded studies wil! be

compared and further explained in the last section of this chapter.

116 Chapter 7

7.2 Symmetrical luisurf

In thls section the results obtained for seeded reactions with the symmetrical

inisurf SC0-880 wil! be presented. Kinetic data for these systems in the form of

fractional conversion as a function of time were obtained by automated dilatometry

(Section 4.4.2). To avoid problems with secondary nucleation, the amount of

monomer added to the cleaned seed was such that all runs were commenced at the

lowest possible weight fraction of polymer that still maintained the system in

Interval lil (absence of monomer droplets). This also ensured that the only ph<1se

onto which inisurf could be adsorbed, was the polymer phase. Since the initiating

moiety of the inisurf was an azo initiator, nitrogen was formed during its

dissociation. Therefore, the formation of gas bubbles in the dilatometer had to be

avoided. This was accomplished by keeping the concentration of inisurf in the

dilalometer at 3.2 10-4 mol dm-3 (this is the solubility concentration of nitrogen in

water at 323 K) and by thoroughly degassing the water before use. In almost all

runs the inisurf concentration was kept at the highest possible concentration within

this constraint. Variation in surface coverage of the seed partiele with inisurf was

then achieved by using different amounts of seed latex. Recipes used in the

various runs are collected in Table I.

Table 7.1 Recipe for the Seeded Reactions with Symmetrical Inisurf SC0-880.

Polystyrene Monomer Inisurf Water Coverage inSeed (g) (g) (g) (%)

2.26 4.04 0.06 96.81 7

2 0.97 1.80 0.11 100.89 21

3 0.63 1.13 0.11 101.93 28

4 0.47 0.93 0.11 102.11 32

5 0.32 0.57 0.11 102.10 40

6 0.32 0.55 0.11 102.90 40

7 0.24 0.49 0.11 102.19 45

Partiele Growth 117

As discussed in Section 3.7.3, the optima! means of displaying Interval lil

kinetic data in the case of a sparingly water soluble monomer is as a plot of -In (1-

x) against time (t). The data obtained for different degrees of surface coverage by

symmetrical inisurf SC0-880 have been plotted in Fig. 7 .1. All these data showed

a steady-state region·. Numerical differentiation of the data of Fig. 7.1 and of the

other en tri es of Table 7.1, and calculations using kP = 258 dm3 mol- 1 s· 1 6•9 gave the

steady-state values of fl, which are displayed in Table 7.2. The steady-state fl

values showed that the system was indeed in the zero-one regime, as expected.

The data of Fig. 7.1 also showed an apparent approach to steady state. If

this approach to steady state was not some artifact due to inhibition effects and the

starting time for the reaction was precisely known, then these data could be used to

infer the rate coefficients forentry and exit in these zero-one systems (Section 7.3).

1~

Figure 7.1

1.00

0.80

0.60

0.40

0.20

0.00 0 6 10

t (101

s)

-In (1-x) versus Time (t) for Symmetrical-lniswf-lnitiated Systems with Different Percentages of Suiface Coverage of Monomer-Swollen Polymer Particles; D: 0%, 0: 7%, o: 21%, ... 40%, •. 45%.

The small fluctuations in ra te in the steady-state region seen by close inspeetion of Fig 7 .I are typical for data from the equipment used and rise from small but regular temperature variations arising from the thermostal in the course of the run and from "jitter" inherent in the tracker of the automated dilatometer.

118 Chapter 7

A check for the presence of inhibition effects was carried out using y­

radiolysis6.I4. The aqueous-phase free radicals produced by y-radiolysis are able

to completely "buro out" inhibitors in a short time and, hence, a suitable test for

inhibition effects is to carry out a kinetic run under radiolysis, observe the rate of

approach to steady state on the first insertion into the 60Co source, remove the

system from the souree and let it decay to its background therrnal rate, and then

observe the rate of approach to steady state upon a second insertion. If the rates of

approach to steady state in the first and second insertion are the same, then

inhibitor-induced artifacts are absent.

In our case, occurrence of inhibition effects was checked using a seed

covered for 29% with the nonionic surfactant moiety of the symmetrical inisurf

SC0-880 instead of the inisurf itself. The coverage was calculated from literature

data (as already mentioned in Section 6.3)15 using the surface area per molecule

and the adsorption isotherm of nonylphenyl poly(ethylene oxide) on polystyrene

latex particles. The area of one symmetrical inisurf molecule was presumed to be

twice that of one surfactant moiety. The other assumption made bere was that the

nonionic surfactant behaved sirnilarly as the inisurf, as far as the kinetics of

inhibition were concerned. For the surfactant system, it was found that the

approach to steady state from the second insertion was about the same as that from

the first, as shown in Figs. 7.2 and 7.3.

Given this result that the symmetrical inisurf behaves as an inhibitor and

not a retarder, it was tempting to use the observed rate of approach to steady state

in systems, where only inisurf was used for initiation, in order to obtain values for

the entry and exit rate coefficients1. However, in this case such a treatment was

not possible, because the metbod of preparatien of the system, including adsorption

of the inisurf onto the seed particles, was such that it was impossible to identify

accurately the starting time for the polymerization, since the reaction commenced

while the system was still undergoing thermal equilibration.

Partiele Growth

i I ' ._, .s

Figure 7.2

119

1.2

0.8

0.4

0.0 0 2 4 5

-ln (1-x) versus Time (t) for y-lnitiated Systems at First lnsertion, Relaxation, and Second lnsertion, for a System Stabilized with the Suifactant Moiety of the lnisurf.

0.50 ~-·~-----............. ----------,

0.40 .. · 0.30 . .·· .

r ••• 0,20 ~ • • ••••••

••••••

.. ... ····· ·····

OIOL~ ·······===·· .. ·· •••• 000 ' ' • • .~~~"--'~~~_j

0 250 500 750 1000 1250

t (s)

Figure 7.3 Data for First (•) and Second (•) lnsertions of Fig. 7.2, Superimposed to Show that these Exhibit the Same Kinetics for Approach to Steady State.

While this shows that the approach to steady state observed with inisurf

initiation cannot be used to infer kinetic information, the steady-state rate itself can,

because it is reasonable to assume that inhibitor has been completely removed

when the system attains its steady state. The steady-state value for n in zero-one

systems is given by2·6:

120 Chapter 7

(2p + k) (3.23)

Hence, if a value for k is available from another experiment, the value of p can be

inferred from the experimental fi,,.

A value for k was determined from y-radiolysis relaxation experiments2•9

, as

mentioned above, by watching the rate of relaxation after removal from the souree

(when all inhibitor has been scavenged by the radiolysis process). A complication

in applying this to an inisurf system was that, because the inisurf functions both as

stabilizer and as initiator, it was possible that fi would exceed 0.5 upon insertion

into the source, i.e., that the slow rate of terminalion must be taken into account in

interpreting the kinetics. Termination kinetics have been shown to be quite

complex because of the chain-length dependenee of k/6•17

•18

•19

, and it would be

difficult to produce an unambiguous interpretation of the kinetics in a system, in

which both termination and exit were significant radical-loss events. Because of

this, we adopted the approach of finding k by the y-radiolysis relaxation technique

for a latex covered only with the surfactant moiety of the symmetrical inisurf. The

absence of any souree of free radicals except for y-rays of sufficiently low

intensity, ensured that the system was truly zero-one. The value for k so obtained

could then be used for the symmetrical inisurf system by assuming that the exit

kinetics were unaffected by the absence of the initiating moiety of the inisurf.

The results of the relaxation experiment are shown in Fig. 7.2. These data

were analyzed using the "slope-intercept" metbod (Section 3.7) with the assumption

(found to be appropriate for y-relaxation experiments in normal emulsion

polymerization systems20) that all desorbed free radicals re-entered the particles.

The latter implied that the dimensionless "fa te parameter", o., which takes account

of the aqueous-phase kinetics of desorbed free radicals2, see Section 3.8, had a

value of + l. This yields k = 5.6 10·4 s· 1• The corresponding value for the rate

coefficient of background, or thermal, entry of free radicals2 was Prherm = 3 106 s 1•

Now, for our systems, where there was a layer of polymerie surfactant

Partiele Growth 121

around the latex particles, it could be argued that desorbed free radicals (in the

condition of absence of aqueous-phase free radicals as pertaining to y-radiolysis

relaxation) could well homo-terminate in the aqueous phase instead of (or in

competition with) re-entry, i.e., that a was 0. Interpreting the relaxation data of

Fig. 7.2 with this alternative assumption yielded k = 7.4 w-s s·1 and

Prhenn = 4 10-6 s·l.

These values for k may be compared with those found at 323 K for normal

emulsion polymerizations, using the formula reported by Hawkett et al.6 and

Morrison et al. 3: k (s- 1

) = 4.3/ (rl, where r, is the swollen radius in nm. The

value for k in the present system was found to be an order of magnitude smaller

than that estimated for latex particles in conventional emulsion polymerizations

with a comparable size of the swollen seed particles. This in turn could be

rationalized in terms of the transfer-diffusion mechanism for exie 1•12

, for which

there is now a considerable body of supporting evidence1•10

• The polymerie

surfactant could well have formed a "hairy" layer near the partiele surface, which

could have strongly decreased the rate, at which a monomeric free radical diffused

into the bulk phase, thus resulting in a decrease in the rate of exit.

Table 7.2 Process Parameters and Kinetic Parameters of Symmetrical Inisurf SC0-880.

A' n .. p f (10·4 s·I) (10·6 s·l) (10-3)

a=O a= I a=O (l

6.4 0.03 2.5 1.2 0.23 0.11

2 6.2 0.06 5.4 5.1 0.12 0.11

3 6.3 0.16 18 44 0.25 0.62

4 5.9 0.17 20 53 0.21 0.55

5 6.4 0.16 18 44 0.12 0.31

6 6.5 0.26 41 160 0.29 1.2

7 5.8 0.22 28 91 0.15 0.50

122 Chapter 7

The value of k thus obtained for 29% coverage by the noninitiating

analogue of the inisurf was assumed to be independent of surface coverage and

also to hold for inisurf. While this assumption was certainly not very accurate, it

should certainly be suftkient to estimate p from Eq. 3.23 within an order of

magnitude using the iiss from the inisurf-initiated runs of Fig. 7 .I. The used values

for A' and the values obtained for ii and for p for both a = 0 and a 1 are

reported in Table 7.2. The values for p have been displayed in Fig. 7.4.

6.0 ,------~·········--------------,

0

)

~ 40

s ~

Cl.

0

•

•

Figure 7.4

2.0 • • •

• 0.0 L... ......... _"--~~....L.~~-------~-~.L~~~~-----·'----"

0 2.5 5.0 7.5

[inisurf] (JO 3 molecules particle-1)

First-Order Rate Coefficient for Entry (p) for Different Symmetrical lnisurf Surface Coverage deduced for the Steady-State with a = 0 (•) and a = I (o).

From these, the initiator efficiency f can be calculated as:

f (7.1)

where N1 is the number of inisurf molecules per particle. The results are shown in

Fig 7.5. Values for f, computed for both a = 0 and 1, we re of comparable

magnitude. What is remarkable about these results is that, while p increased with

coverage as one would expect, the efficiency was only very weakly dependent on

coverage at least for a 0 (the most likely case, as already discussed before), and

indeed could be said lo be independent of coverage within the uncertainty of the

Partiele Growth 123

experimental data. Fig 7.5 showed for a = 0 at most only a slight trend with

coverage, with f changing by at most a factor of 2 while the coverage varied by

more than a factor of 6.

~ >,

.~ <,.)

!2 <>

Figure 7.5

0.12

0.09

0.06

O.Q3

0.00

0

•

0 •

~' ~~·-0

0 2.5 5.0 7.5

[inisurf] (1 0 3 molecules partiele-l )

Initiator Efficiency of a Symmetrical lnisuif for Different Amounts of Sulface Coverage, Deduced from Values of p for a = 0 (•) and a = 1 (o).

7.3 Asymmetrical luisurf

In this section the results obtained for seeded reactions with asymmetrical

Inisurf AC0-880 will be given. Kinetic data for these systems, in the form of

fractional conversion as a function of time, were obtained by densimetry (Section

4.4.3). As for the reactions with the symmetrical inisurf (Section 7.2), the

polymerizations with AC0-880 were carried out in such a way that the runs

commenced at the lowest weight fraction of polymer that still maintained the

system in Interval lil. By following the reactions by densimetry, no restrictions

were imposed with respect to the amount of inisurf because the liberated nitrogen

did not interfer with the measurements. As can be seen in Table 7.3 the amount of

seed was kept constant and the amount of inisurf was varied in order to change the

surface coverage.

124

Table 7.3 Recipe for the Seeded Reacrions with Asymmetrical Inisurf AC0-880.

Polystyrene Monomer Inisurf Water Coverage inSeed (g) (g) (g) (%)

(g)

12.(10 21.45 2.03 454.93 52

2 12.04 22.34 3.01 455.04 75

3 12.06 22.32 3.83 455.06 94

The data, obtained for varying degrees of surface coverage by asymmetrical

inisurf AC0-880, are plotted in Fig. 7.6 as -In (1-x) versus time (t). lt has been

shown in the previous section that the nonionic surfactant moiety of the inisurf

behaved as an inhibitor and not as a retarder. Therefore, it would be reasonable to

assume that the asymmetrical inisurf under study also behaved as an inhibitor. In

the runs monitored via densimetry, it was far better possible to determine the start

of the reaction. Thus, the "slope-intercept" metbod (Section 3.7.2) could be applied

to obtain values for the entry and exit rate coefficients. The used value of A' and

the values obtained for fi, p, and k are reported in Table 7.4.

1.00

0.80

1~ 0.60

0.40

0.20

0.00

0 5 10 15 20 --1 (103 s)

Figure 7.6 -ln (1-x) Versus Time (t) for Asymmetrical Inisurf-Initiated Systems with Different Percentages of Surface Coverage of Monomer-Swollen Polymer Particles; o· 52%, 0: 75%, • 93%.

Partiele Growth 125

2

3

Table 7.4 Process Parameters and Kinetic Parameters of Asymmetrical lnisurf AC0-880.

A' flss p k f oo-4 s·l) (10-5 s·l) cw-3 s·l) (%)

a=O a=1 a=O a=1 a=O Cl=1

6.3 0.01 0.56 0.39 0.91 52 0.046 0.032

6.2 0.04 2.0 1.4 0.51 4.9 0.12 0.082

6.2 0.09 4.9 3.6 0.47 2.0 0.22 0.16

5.0

4.0

,......

1~ 3.0

2.0

1.0

0.0

0 5 10 15 20 25 30 ~~~~~~~~-~

[in i surf] (I 0 3 molecules partiele -1 )

Figure 7.7 First-Order Rate Coefficients for Entry (p), for Different Asymmetrical lnisurf Surface Coverages at 323 K, Deduced from the Slope and intereepi with a 0 (•) and a= 1 (o).

55

44

-'"' 33

"' b ..... 22 '-'

..... 11

0

0 5 10 15 20 25 30

[inisurf] (I 0 3 molecules partiele-l )

Figure 7.8 First-Order Rate Coefficient for Exit (k), for Different Asymmetrical lnisurf Surface Coverages at 323 K, Deduced from the Slope and Intereepi with a 0 (•) and a = 1 (o).

126

1~ g Q)

·o lil

Q)

Figure 7.9

Chapter 7

0.25

0.20

0.15

0.10

0.05

0.00

0 5 10 15 20 25 30

[ inisurf] (1 0 3

molecules partiele -1 )

Initiator Efficiency of an Asymmetrical lnisurf for Different Degrees of Surface Coverage at 323 K, Deduced for Values ofpfor a= 0 (•) and a= 1 (o).

Contrary to the results with the symmetrical inisurf the system with the

asymmetrical inisurf showed an increase in entry and in efficiency with coverage.

And for a = 0 (the most likely case as explained in the previous section) exit

showed only a slight variation with coverage, which was an indirect confirmation

of the correctness of the approach applied in Section 7.2. The value for k obtained

in the system with asymmetrical inisurf was of the same order of magnitude as the

one obtained in the y-relaxation experiments, as discussed in the previous section.

7.4 Discussion

A point of consideration is the observation that the initiator efficiency for

the symmetrical inisurf was apparently independent of (or perhaps only weakly

dependent on) surface coverage. This result can be explained as follows. The

value of kJ implies that the rate of decomposition per particle, N1kJ, was low: N1kJ

= 3.2 w-z s·' for NI = 5 103 molecules per partiele (see Table 7.1), i.e., a typical

time of 40 s between inisurf decomposition events in a given particle. A good

starting point is to assume that the free radical that cammeneed propagation inside

Partiele Growth 127

a latex partiele originated from inisurf decomposition on the surface of the same

partiele (rather than from a free radical which desorbed from another partiele and

re-entered from the aqueous phase). This hypothesis implies that initiator

efficiency must be controlled solely by the dynamics of the two surface-active free

radicals, formed upon decomposition of a single symmetrical inisurf molecule, and

that f must be independent of the amount of symmetrical inisurf on the surface of a

given partiele. The results in Fig. 7.5, showing this independence, are therefore

completely consistent with the original hypothesis, viz., that the free radicals which

entered the interior of the partiele and caused formation of macroradicals,

predominantly arise from the decomposition of inisurf molecules on the same

particle. The corollary that re-entry of exited free radicals was kinetically

insignificant, is consistent with the observation that the exit rate coefficient k in

these systems was assumed to be small, for reasons discussed above. Moreover,

these conclusions suggest that the correct values for p and for f are those obtained

fora= 0, rather than fora 1 (the latter implying complete re-entry).

In the case of the asymmetrical inisurf a dependency of entry and of

efficiency on surface coverage was found. In this case the dissociation rate was

slower compared to that of the symmetrical inisurf, i.e., the time between inisurf

decomposition events for a given partiele was even Jonger. Therefore, the

hypothesis that the initiator efficiency was solely controlled by the dynamics of the

two free radicals formed upon the decomposition of a single inisurf molecule,

should hold here as wel!. However, with the asymmetrical inisurf an increase of

inisurf molecules per partiele could lead to a state of orientation of the molecule, in

which the "desorption" of the t-butyl radical was less hindered, a situation which

was not possible in the case of a symmetrical inisurf. The orientation of the

asymmetrical inisurf molecules on the surface of the particles would then lead to

an increase in en try and subsequently an increase in efficiency, as can be seen in

Figs. 7.7 and 7.9.

The next observation from the data is that the initiator efficiency was

extremely low: ca. 10·3. This is in contrast with typical efficiencies of 0.1 - 1 in

128 Chapter 7

emulsion polymerization processes with conventional chemica! initiators1, the

explanation for these chemieal-initiator efficiencies now being well understood8•

The very low efficiency with an inisurf can · be understood qualitatively and

quantitatively by the hypothesis that, because of the close confines of the particle,

very rapid geminate recombination would occur between the two free radicals

formed by inisurf dissociation, unless one of them could have desorbed into the

aqueous phase. This hypothesis can be quantified as fellows.

After an inisurf molecule has dissociated, each of the resulting surface­

active free radicals underwent one of the following fates: propagation, desorpti éln

or geminate recombination (see Fig. 7.10). By our hypothesis, propagation to form

a macroradical could only occur, if one of the free radicals formed by inisurf

decomposition desorbed into the aqueous phase. Now, simple timescale arguments

show that propagation was much slower than geminate recombination. The

timescale for propagation is given by tp = 1/ (kpl cz), where cz is the

concentration of monomer in the surface layer of the particle. A typical value for

kP1 for free radicals resulting from decomposition of initiators of the type in our

inisurf is ca. 105 dm3 mol-1 s·1 21• In the present system a reasonable value for CM

is 3.4 10·3 mol dm-3, which is the saturated aqueous-phase concentration: the radical

would probably be in close proximity of the aqueous phase. This yields lp ""' w-3 s.

Propagation -17. Partiele

~esorption 1-

~nnination

1~ Figure 7. JO Fates of Surface-Active Free Radicals

Partiele Growth 129

If instead the calculation is done for the upper boundary of CM (5 mol dm-3, i.e.,

the saturated polymer-phase concentration), one obtains instead tP "' 10-6. However,

the radicals formed by înisurf decomposition were in the surface layer of inisurf

around the particle. The thickness of this surface layer was at most the stretched

length of the water-soluble part of the surfactant radical The concentration of

monomer in this surface layer would be far less than that in the polymer particle.

For this reason, we presurne that the concentration was close to the saturation

concentration of monomer in water and, thus, that tP "" 10'3 s was a better estimate.

As shown below, the timescale for termination, t,, was significantly shorter than

this: t, "' 10·6 s. Hence for the purpose of an order-of-magnitude estimate of the

initiator efficiency, the only events we need to consicter are desorption and

termination. Hence the efficiency f can be written as:

f = rate of desorption (7.2)

rate of desorption + rate of geminate recombination

Moreover, as will be seen when quantitative values are obtained for the rates of

geminate recombination and desorption, the latter was significantly slower than the

farmer and hence one has to an excellent approximation:

f rate of desorption (7.3)

rate of geminate re combination 2kJR.]

where kw, is the first-order rate coefficient for desorption of a free radical from a

particle, and k, the second-order rate coefficient for termination of free radicals

within the particles (note that in general k, depends on the length of each

chain17•18

•19

; however, in the case of the symmetrical inisurf we are consictering

geminate recombination of two radicals formed by decomposition of a single

molecule and, hence, this chain-length dependenee does not need to be explicitly

indicated).

130 Chapter 7

We now give estimates of the quantities required in Eq. 7.3. The

mechanism of desorption from a latex partiele of a monomeric free radical

originating from transfer to monomer is well understood 11•12

•10

•22

• However, the

present case is for desorption of a surface-active free radical, and thus the cited

theory cannot be employed. However, an expression for the desorption rate

coefficient of a surface-active free radical from the surface of a partiele can be

readily derived using exactly the same procedure as used to derive that for an

ordinary (non-surface-active) monomeric free radical11, viz., from the microscopie

reversibility relation23 relating the forward (adsorption) and reverse (desorpticn)

rate coefficients. The final result is24:

a ad kd = kad __ _ e As bad

(7.4)

where kad is the rate coefficient for adsorption, As (= 47tr/) is the surface area of

the monomer-swollen partiele, and aad and bad are the Langmuir adsorption

isotherm quantities for the partitioning of the surface-active species between the

continuous phase and the partiele surface, i.e., the quantities in the Langmuir

adsorption isotherm expression25:

a [1 + -=-=."..--1-.."--l ad [S]aq bad

(7.5)

where Aad is the area occupied per surfactant molecule on the partiele surface at an

aqueous-phase concentration of surfactant, [S]aq· The quantity a thus is the elose­

packed area per surfactant molecule (see Section 6.3). The adsorption rate

coefficient can be obtained from the Smoluchowski equation (there is now

experimental evidence3 that this is quantitatively applicable to adsorption of small

species onto latex partieles):

(7.6)

where NA is Avogadro's constant, Dw is the mutual surfactant/partiele ditfusion

coefficient in the medium (in this case, water), r,, is the hydrodynamic radius of the

surfactant, and r, is that of the partiele; implicit here is the assumption that the

Partiele Growth 131

hydrodynamic and swollen radii of the partiele are the same. Since r., « r, we

have kad = 2 7t NA Dw r,. The surfactant/particle mutual diffusion coefficient can be

estimated from the Stokes-Einstein relation:

(7.7) 61tllwr.,

where llw is the viscosity of the medium. The final result is:

(7.8)

An expression for k, is obtained by noting that this process is diffusion­

controlled, and hence the Smoluchowski equation can be employed again. For

terrnination of small species (such as those formed by decomposition of an inisurf)

it is often assumed 16 that the diffusion process involved is segmental. However, in

the present case the species are small enough (mean number of ethylene oxide

units n = 30) that centre-of-mass diffusion of the whole entity is probably rate

determining. In this case, one then has for this diffusive encounter of two identical

species:

(7.9)

where D 5 is the diffusion coefficient in the surface layer. Note that it could be

argued that the encounter radius for termination should be smaller than the size of

the surfactant; however, a more sophisticated description would not change the

predictions of Eq. 7.9 by more than (say) a factor of 5 forthese small entities, and

so Eq. 7.9 is quite adequate for our present purpose of semiquantitative

explanation.

The final step in the evaluation of Eq. 7.3 is to estimate [R"], the

concentration of the free radicals formed by decomposition of inisurf. We assume

that the radicals are randomly distributed throughout a surface layer of thickness d1•

Since the number of free radicals is 2, we have:

132 Chapter 7

[R"] 2

NA (4/3) 1t [(r, + dY - r,3]

(7.10)

the last step following because r, » d1• Using the Stokes-Einstein relation, one

then has:

2 k, [R"]

where TJs is the viscosity in the surface layer. The final result is then:

f NA dl r, a ad 1'1s

8 'sr bad 1'1w

(7.11)

(7 .12)

if we ignore any difference between the viscosity of the water and of the surfactant

layer. Adrnittedly, when we make this last approximation, we are a ware that the

small value of the exit rate coefficient found for this system, and ascribed to slower

diffusion in a surface layer, could make the viscosity in the surface layer somewhat

larger than that in water; however, this would only change the quantitative, but not

the qualitative substance of the present calculation.

We now turn to an evaluation of the efficiency for the present systems. Up

till now no distinction has been made between symmetrical and asymmetrical

inisurfs. It was assumed that the differences in behaviour between surface-active

free radicals and t-butyl free radicals do not vary too much. First the efficiency for

the symmetrical inisurf system will be calculated and secondly the one for the

asymmetrical inisurf.

Symmetrical Inisurf. The swollen radius, r, was estimated as 55 nm; rsr was

0.9 nm, as calculated with the Le Bas additive volume method26. The area aad

was found by interpolation of the literature data for adsorption of nonylphenyl

poly(ethylene oxide) 15 and estimated to 1.32 nm2• The thickness d1 of the

surfactant-containing shell was presumed to be the length of the water-soluble part

Partiele Growth 133

of the inisurf, in a completely stretched conformation; this gave d1 = 6.3 nm. The

adsorption isotherm quantity bad was estimated from studies by Kronberg et al. 15 on

the adsorption of nonylphenyl poly(ethylene oxide) on polystyrene latex particles

(radius: 213 nm) at 298 K. Note that these authors used the same Langmuir

adsorption isotherm as in Eq. 7.6, albeit expressed in a slightly different way; their

equilibrium constant K, governing the surfactant partitioning between the

continuous and surface phases, is related to the quantity b of Eq. 7.6 (see Section

6.3) by:

(7.13)

where M< and de are the molecular weight and the density of the continuous phase

(water), respectively. These authors reported K = 2.3 106• The temperature

dependenee of this quantity is unknown, but use of the literature value at 298 K for

the present case of experiments at 323 K should not make a significant difference

for our purpose of an order-of-magnitude estimate. The same remarks hold for the

relatively weak dependenee of bad on partiele size27 and for the unknown, but

relatively broad, molecular-weight polydispersity of the inisurf, which should not

have any effect on the order-of-magnitude estirnate. While the values of the

quantities aad and bad were derived for entities somewhat different in configuration

and molecular weight distribution than those of our inisurf, the differences between

values for surface-active species of the same generic type are never sufficiently

large as to invalidate an order-of-magnitude estimate for the initiator efficiency27•

These quantities then give the final result that f = 9 104 for the symmetrical inisurf

system.

Asymmetrical lnisurf. The swollen radius r, was estimated 55 nm; r51 was

estimated as 0.3 nm, this is the average for the two radicals formed as calculated

with the Le Bas additive volume method. The area aad was 0.74 nm1 (see Section

6.3), the thickness d1 was 6.3 nm. The adsorption isotherm quantity bad was

134 Chapter 7

estimated from the data reported in Section 6.3 in a similar fashion as mentioned

above for the symmetrical inisurf system. These quantities then gave the final

result that f = 10'3 for the asymmetrical inisurf system.

The calculated values for the efficiency obtained bere have indeed the order

of magnitude of the values inferred from our experiments (Figs. 7.5 and 7.9), viz.,

ca. I 0-4. While one can certainly quibble about the precise values of the quantities

used in this evaluation and the approximations inherent in its derivation, a more

precise treatment would not change the order of magnitude of the results. The

agreement between the calculated and experimental efficiencies thus suggests the

correctness of the basic model used in the derivation. In addition, the condusion

that geminate recombination among initiator moieties was highly probable in the

confined vicinity of a latex partiele is in accord with the finding28 that initiation

with lipophilic initiators such as AIBN occurs more likely via the small number of

radicals generated by aqueous-phase decomposition, for exactly the same reason.

Lastly, we note that the exit rate coefficient is low in these inisurf systems,

but the entry rate coefficient (or, equivalently, the initiator efficiency) is even

lower. Hence, one would expect that zero-one conditions (which were obtained by

design in the present studies) could be attained over a range of conditions wider

than normal in emulsion polymerizations. Precise predictions of the requisite

conditions could be carried out using standard means2, given the quantitative

knowledge of initiator efficiency and exit deduced in this chapter.

References

I. Napper, D.H.; Gilbert, R.G. In Comprehensive Polymer Science; Allen, G.A., Bevington, J.C., Eastwood, G.C., Eds.; Pergamon: Oxford, U.K., 1989; Vol. 4, p 171.

2. Gilbert, R.G.; Napper, R.G. J. MacromoL Sci. Rev. MacromoL Chem. Phys. C 1983, 23, 127.

Partiele Growth 135

3. Morrison, B.R.; Maxwell, LA.; Gilbert, R.G.; Napper, D.H. ACS Symp. Ser. 1992, 492, 28.

4. Lichti, G.; Gilbert, R.G.; Napper, D.H. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 269.

5. Feeney, P.J.; Napper, D.H.; Gilbert, R.G. Macromolecules 1987, 20, 2922. 6. Hawkett, B.S.; Napper, D.H.; Gilbert, R.G. J. Chem Soc., Faraday Trans. I

1980, 76, 1323. 7. Smith, W.V.; Ewart, R.H. J. Chem. Phys. 1948, I6, 592. 8. Maxwell, I.A.; Morrison, B.R.; Napper, D.H.; Gilbert, R.G. Macromolecules

1991, 24, 1629. 9. Lansdowne, S.W.; Gilbert, R.G.; Napper, D.H.; Sangster, D.F. J. Chem. Soc.,

Faraday Trans. 11980, 76, 1344. 10. Adams, M.; Napper. D.H.; Gilbert, R.G.; Sangster, D.F. J. Chem. Soc.,

Faraday Trans. I1986, 82, 1979. 11. Ugelstad, J.; Hansen, F.K. Rubber Chem. Techno[. 1976, 49, 536. 12. Nomura, M.; Harada, M. J. Appl. Polym. Sci. 1981. 26, 17. 13. Hawkett, B.S.; Napper, D.H.; Gilbert, R.G. J. Chem. Soc., Faraday Trans. I

1981, 77, 2395. 14. Ballard, MJ.; Napper, D.H.; Gilbert, R.G. J. Polym. Sci., Polym. Chem. Ed.

1984, 22, 3225. 15. Kronberg, B.; Käll, L.; Stenius, P.J. Dispersion Sci. Techno/. 1981, 2, 215. 16. Benson, W.S.; North, A.M. J. Am. Chem. Soc. 1962, 84, 935. 17. Adams, M.E.; Russell, G.T.; Casey, B.S.; Gilbert, R.G.; Napper, D.H.;

Sangster, D.F. Macromolecules 1990, 23, 4624. 18. Russell, G.T.; Gilbert, R.G.; Napper, D.H. Macromolecules 1992, 25, 2459. 19. Russell, G.T.; Gilbert, R.G.; Napper, D.H. Macromolecules 1993, 26, 3538. 20. Whang, B.C.Y.; Napper, D.H.; Ballard, M.J.; Gilbert, R.G.; Lichti, G. J.

Chem. Soc., Faraday Trans. 11982, 78, 1117. 21. Moad, G.; Solomon, D.H. In Comprehensive Polymer Science; Eastmond,

G.C., Ledwith, A., Russo, S., Sigwa1t, P., Eds,; Pergamon: London, 1989; Vol. 3, p 97.

22. Nomura, M. In Emulsion Polymerization; Piirma, 1., Ed.; Academie Press: New York, 1982; p 191.

23. Gilbert, R.G.; Smith, S.C. Theory of Unimolecu/ar and Recombination Reactions; Blackwell Scientific: Oxford, U.K., 1990.

24. Morrison, B.R.; Gilbert, R.G.; Napper, D.H. to be published. 25. Ahmed, S.M.; El-Aasser, M.S.; Mica1e, F.J.; Poehlein, G.W.;Vanderhoff, J.W.

In Polymer Colloids JJ; Fitch, R.M., Ed.; Plenum: New York, 1980; p 265. 26. Reid, R.C.; Sherwood, T.K. The properties of gases and liquids; McGraw­

Hill: New York, 1966, 2"d ed. 27. Piirrna, 1.; Chen, S.-R. J. Colloid Intelface Sci. 1980, 74, 90. 28. Nomura, M.; Ikoma, J.; Fujita, K. ACS Symp. Ser. 1992, 492, 55.

136

Epilogue

The primary aim of this investigation was to synthesize nonionic surface­

aclive initiators (inisurfs) and to study their behaviour in emulsion polymerization.

The main conclusions that can be drawn as well as some prospects for further

research will now be given.

It has been shown that the route for the synthesis of the various inisurfs is

suitable and that it is possible to obtain sufficiently pure inisurfs in high yields.

Nonionic azo-type inisurfs, containing poly(ethylene oxide) (2a) and (2b),

are not suited when used as the sole stabilizer in ab initio emulsion

polymerizations of styrene. Polymerization does occur, but the system is unstable

and major coagulation occurs. Even addition of small amounts of a nonionic

co-surfactant does not significantly enhance the colloidal stability.

Nonionic azo-type inisurfs, containing Antarox Co-630 (2c), cannot be used

as the sole stabilizer in ab initio emulsion polymerizations of styrene either.

However, addition of an anionic co-surfactant (SDS) shows that these inisurfs are

capable of initiating the polymerization at a normal rate. Variation of the

concentration of this co-surfactant shows a behaviour also observed in conventional

systems with mixed surfactants. Comparison of the performance of inisurf SC0-

630 (7c) with that of a system consisting of ACPA (la) and Antarox Co-630 (2c)

(both in the presence of SDS), shows that the partiele number and the

polymerization rate per partiele are comparable. Therefore, it can be concluded

that the initiating ability of the azo-type initiator is not affected by attaching a

surfactant moiety to these inisurfs.

138 Epilogue

Colloidally stabie latices, not requiring supplementary surfactant, can be

obtained by applying a nonionic surfactant with a longer poly(ethylene oxide) chain

(Antarox C0-880) as the constituent part of the inisurf. In genera!, it can be

condurled that for both symmetrical and asymmetrical inisurfs the colloidal

stability is enhanced by changing the surfactant moiety from a more or less water­

soluble poly(ethylene oxide) to a surface-active nonylphenyl poly(ethylene oxide),

as well as by increasing the length of the ethylene oxide chain in the latter

surfactant moiety.

The increase in colloidal stability does not necessarily lead to the formation

of more particles and, therefore, to an increase in polymerization rate. Larger

surface-active free radicals diffuse slower and are therefore more liable to geminate

recombination, which leads to decreases in both partiele nucleation and partiele

growth. For obvious reasons this so-called cage-effect is more pronounced in the

case of symmetrical inisurfs.

The CMCs of the inisurfs are of a comparable magnitude as that of the

nonylphenyl poly(ethylene oxide) nonionic surfactant. The CMC of the

symmetrical inisurf is smaller than that of the asymmetrical inisurf due to

differences in molecular configuration.

The adsorption isotherm of the asymmetrical inisurf shows a Langmuir type

behaviour. The surface area per molecule at complete surface coverage and the

adsorption-desorption equilibrium constant are of a sirnilar magnitude to those of

the nonylphenyl poly(ethylene oxide) surfactant.

Dissociation rates of the inisurfs at 323 K are similar to those of the

corresponding initiators. Comparison of the activation energy (E.c,) and the pre­

exponential factor (A) of the asymmetrical inisurf with those of the corresponding

initiator does not reveal a significant difference. On the other hand, the

symmetrical inisurf shows a lower activation energy, which is most likely caused

Epilogue 139

by the extra intramolecular strain due to the conformation adopted by the inisurf

molecule in the micelle or at the surface of the particle.

The PSD in an inisurf system is not only govemed by the initiating

efficiency of this inisurf, but also by the continuons formation of a surface-aclive

(geminate) recombination product.

Molecular weight determinations showed that the molecular weight of the

polymer formed in the inisurf systems is higher than that in comparable

conventional systems. The hypothesis that molecular weight is only deterrnined by

transfer to monomer, cannot be supported fully by these measurements. Therefore,

it is concluded that the molecular weight is predominantly, but not exclusively

determined by transfer to monomer. Events like transfer to surfactant or to

polymer and bimolecular terminalion have a measurable effect on molecular

weight.

In systems with a nonionic surfactant and, therefore, also in systems with

nonionic inisurf the exit rate of a free radical from a polymer latex partiele is

found to be an order of magnitude smaller than the one in systems with anionic

surfactant. This is thought to be due to a larger surface harrier of this steric

stabilizer. Therefore, in the case of nonionic surfactant/inisurf the assumption that

a = 0 will describe the system more accurately, when inferring the rate coefficient

for entry and exit from kinetic data from seeded experiments, while tak:ing the

aqueous-phase kinetics into account.

The fact that the entry rate increases with surface coverage in the case of

asymmetrical inisurf, suggests a certain orientation of the molecules on the surface

to occur. Such a more or less oriented arrangement of the inisurf molecules would

subsequently lead to a less hindered escape of the tertiary butyl radical after

dissociation. In the case of the symmetrical inisurf a coverage-induced orientation

probably occurs as well, but since both radicals are adsorbed on the surface, this

140 Epilogue

does not significantly affect the desorption of one of the radicals.

The entry rate coefficient and hence the efficiency of the inisurf is

determined by the rate of desorption of the formed free radicals from the surface of

the particle.

Notwithstanding the fact that some of the synthesized inisurfs were found to

be unsuitable, it has been shown by this investigation that it is indeed possible to

develop inisurfs, i.e., (7d) and (7g), that can function both as initiator anct as

surfactant in emulsion polymerization. Since the level of performance of the

inisurf is governed by the surfactant moiety and by the initiator moiety, future

work in this field should be focused on improving the performance of these

constituent parts.

With respect to the initiator moiety, asymmetrical inisurfs could be

developed, in which one of the free radicals formed upon dissociation is surface­

active, whereas the other one is highly water soluble (charged) or provides via

intramolecular rearrangement an almost instantaneous separation of the radicals.

Hence, less geminate recombination would occur. Additionally, a hydroperoxide

based inisurf could be developed, which gives a catalyzed decomposition in a

redox system and renders only one surface-active radical.

In terms of the surfactant moiety, the development of an inisurf is

advocated, in which the initiator moiety is linked to the hydrophobic part of the

surfactant. This could be advantageous with respect to the entry rate of adsorbed

surface-active free radicals.

On the basis of the present study, Jatices formed with inisurf can be used in

various applications and the effect of inisurfs on the properties of the final products

can be studied.

Glossary 141

Glossary of Symbols

Symbol SI-units

a intercept in the x or -ln(l-x) vs. time plot

A

cross-sectional surface area per molecule at complete coverage

conversion factor

conversion factor for Interval lil

occupied area per surfactant molecule at [S]aq

Abs, absorbtion at t seconds

Abs0 absorbtion at zero seconds

As surface area of a monoroer-swollen partiele m2

b slope in the x or -ln(l-x) vs. time plot s-1

bad Langmuir adsorption isotherm quantity m3 mot1

c frequency of bimolecular terminalion per free radical per partiele s-1

CM concentration of monoroer in the polymer partiele mol m-3

c~ CM at the beginning of the experiment mol m-J

Cff' CM at equilibrium mol m-3

C~ concentration of monoroer in the surface layer of the partiele mol m-3

d average partiele diameter m

d1 thickness of the surface layer m

dm density of monoroer kg m- 1

dP density of polymer kg m- 1

dw partiele diameter in Stokes law m

Ds diffusion coefficient in the surface layer m2 s-1

Dw mutual diffusion coefficient in water m2 s-1

DS(t) fraction dry solid content in the sample at time t

142 Glossary

DSmm fraction dry solid content at the start of the reaction

DSmax fraction dry solid content at the end of the reaction

Eact activation energie J mof 1

f inisurf efficiency

g! initial mass of monomer kg

M incremental change in the meniscus level m

number of radicals per partiele

k first-order rate coefficient for radical exit s-1

kad second-order rate coefficient for adsorption m3 mof 1 s· 1

kB Boltzmann constant JK!

kd first-order rate constant for initiator decomposition s·l

kde first-order rate coefficient for desorption s·l

kp second-order rate constant for propagation m3 mof1 s·1

kpl kP for the addition of first monomer unit to primary radical m3 mof1 s·1

k, second-order rate constant for termination m3 mof1 s-I

ktr second-order rate constant for transfer m3 mol"1 s·1

ko pre-exponential factor s·I

K adsorptionldesorption equilibrium constant

apparatus constant in bubble tensicmeter m

mo s initia! amount of styrene kg

m~s initia! amount of polystyrene kg

mw amount of water kg

mw5 molecular weight of styrene kg mof1

M. n growing polymer ebains with n monomer units

[M] monomer concentratien mol m·3

Mn number average molecular weight kg mof1

Mw weight average molecular weight kg mol"1

ij average number of radicals per partiele

ijo initial (t = 0) value of iï

n .. steady state value of ij (= N 1 ••• )

Glossary 143

no M initia! number of moles of monomer present per unit volume mol m·3

of aqueous phase

no s initia! number of moles of styrene per partiele mol

NAv Avogadro's constant mol' 1

Ne number of particles per unit volume of aqueous phase m·3

N; relative number of polymer particles containing i free radicals

NI number of inisurf molecules per partiele

!lp pressure difference in the bubble tensiometer kgm·' s·2

P. average degree of polymerization

r bore radius in the dilatometer m

rs swollen radius of partiele m

r., hydrodynamic radius of the surfactant m

[{ primary free radical

[l?'] free radical concentration mol m·3

Rd radius at which detector in the DCP is positioned m

R". radius at which the sample is injected in the DCP m

RM" radical composed of a primary free radical and one single

monomer unit

Rpo/ rate of polymerization mol m·3 s·'

g:o/ rate of polymerization in Interval II mol m·3 s·'

R~o! rate of polymerization per partiele mol m·3 s·'

s total mass surfactant in the system per unit volume kg m·3

of aqueous phase

[S]aq aqueous-phase concentration of surfactant mol m·3

[S]tot total surfactant concentration mol m·3

time s

tp timescale for propagation s

ts sedimentation time in the DCP s

t, timescale for terminalion s

T temperature K

144 Glossary

v ratio [anionic co-sudactant]/[nonionic inisud]

swollen partiele volume

wP weight fraction polymer in the partiele

x fractional conversion

x0 fractional conversion at t = 0

x(t) fractional conversion at time = t X concentration in moles solute per mole solution

z degree of polymerizatîon at which oligomers become sudace active

a fate parameter

'Y sudace tension kg s-2

r moles adsorbed per unit surface mol m-2

rM maximum number of moles adsorbed per unit surface mol m·2

TJ viscosity of the spintluid in the DCP kg m·1 s·1

Tls viscosity of the sudace layer kg m-1 s-1

Tlw viscosity of water kg m·1 s·1

J1 volume growth rate of a partiele m3 s-I

Ajlo molar free energy of adsorption J mol· 1

p (pseudo first-order) rate coefficient for radical entry s-l

PA p in the absence of exit s·l

PPs density of polystyrene kg m-3

Ps density of styrene kg m-3

p, total entry-rate coefficient per unit volume of aqueous phase s-l m-3

Pw density of water kg m-3

Ap difference in partiele density and spintluid density kg m-3

x constant in the Smith-Ewart theory (0.37-0.53)

0) rotation speed of the disk in the DCP s·l

(iJ interfacial area occupied by unit mass of sudactant mz kg-l

Acknowledgement 145

Acknowledgement

The work presented in this thesis has been carried out in the department of

Polymer Chemistry at the Eindhoven University of Technology. I wish to express

my gratitude towards the memhers of this group, who have contributed to this

thesis. Some of them I would like to mention in person.

My first promoter Ton German, for his confidence in me and for his

pleasant way of coaching.

My co-promoter Steven van Es, for his contributions to the research and for

the accurate and highly appreciated way, in which he has corrected and commented

upon the manuscript of this thesis.

I am very grateful to all the students, who contributed via experimental

work and useful discussions, for their enthusiasm: Cees de Jongh, Anja Leijten,

Carla Kampman, Theo Verweerden, Phillip Moffatt, Cecile Henssen, and Mark van

den Enden. I also want to thank Mr. H. Ladan (TEM) and Wieb Kingrna (GPC).

Of the people outside the department who have helped me during my Ph.D.

work, I would like to thank my second promoter Bob Gilbert and the Sydney

University Polymer Group for their help during the practical work and for the

fruitful discussions, as well as for their hospitality during my stay Down Under. I

also wish to thank Bob for his comments on the manuscript of this thesis.

The Australian Government, Department of Employment, Education and

Training is gratefully acknowledged for the financial support under the Australian­

European Awards Program. Financial support from the Foundation Emulsion

Polymerization (SEP) is also gratefully acknowledged.

Finally I would like to thank everybody who has shown an interest in my

Ph.D.-work.

146

Curriculum Vitae 147

Curriculum Vitae

Jos Kusters was born in Nieuwenhagen, The Netherlands, December 19,

1961. After graduation from secondary school (Atheneum-B) at Eyckhagencollege

in 1982, he started his academie study in chemica! technology and chemistry at the

Eindhoven University of Technology. During this time he was a research assistant

for six months in the group of Prof. K.C. Frisch at the University of Detroit. He

obtained his Degree in Chemica! Engineering and Chemistry (ir-diploma) in August

1988 in the group of Prof. A.L. German on the project: Emu/sion Polymerization of

Butadiene.

He started his Ph.D. project on Inisuifs in the group of Prof. A.L. German

in the same year. During this period he was granted a scholarship under the

Australian-European Awards Program, and studied for 10 months in the group of

Prof. R.G. Gilbert and Prof. D.H. Napper at the School of Chemistry at the

University of Sydney.

As from September 1993, the author has joined Vinamul Ltd., Carshalton,

United Kingdom as a polymer scientist.

148

Stellingen

behorende bij het proefschrift

INISURFS: Surface-Active Initiators

Their Synthesis and Application in Emulsion Polymerization

van

Joseph Maria Hubertus Kusters

1. Het testen van nieuwe thermische initiatoren voor radicaalpolymerisatie bij een temperatuur boven de 100 oe mag, met het oog op initiatie door nevenreacties, op zijn minst tot enige argwaan en scepsis leiden. Hyson. A.M.; Schreyer, R.C. US Patent 2,n8,818, 1957; Chem. Abstr. 1957, 54, Sn6f.

2. Het algemeen gangbare, maar foutieve gebruik van de engelse term "densitometry" in plaats van het correcte "densimetry", bij het bepalen van de conversie als functie van de tijd, laat veel lezers waarschijnlijk in het duister over wat er precies mee bedoeld wordt.

3. De observatie van Medvedev et aL dat vanaf lage conversie de gemiddelde deeltjesdiameter tijdens emulsiepolymerisatie constant blijft, rechtvaardigt, naast het poneren van een alternatief nucleatiemechanisme, een kritische beschouwing van de meetmethode. Medvedev, S.S.; Gritskova, IA.; Zuikov, A.V.; Sedakova, L.I.; Berejnoi, G.D./. Macromol. Sci., Chem. 1973,A7, 715.

4. Het opnieuw publiceren van een deel van een eerder verschenen boek levert geen wezenlijke bijdrage tot het verspreiden van kennis. Candan. F.; Ottewill, R.H. An Introduetion to Polymer Colloids; Kluwer Academie Publishers, Dordrecht, 1990. Candau, F.; Ottewill, R.H. Scientific Methods for the Study of Polymer Co/loids and Their Applications; Kluwer Academie Publishers, Dordrecht, 1990 (NATO ASI Series C, Volume 303).

5. Een alternatieve verklaring voor de door Ford en Yu gevonden afname in katalytische activiteit als functie van het gehalte vinylbenzylchloride in kationische latices zou ook verklaard kunnen worden door het optreden van additionele crosslinking. Ford, W.T.; Yu, H. Langmuir 1993, 9, 1999.

6. De snelheidsverhoging bij hoge conversies in de emulsiecopolymerisatie van vinylacetaat en butylacrylaat wordt door Kong et aL ten onrechte toegeschreven aan een stijging van de gemiddelde propagatiesnelheids­constante van het systeem. Kong, X.Z.; Pichot, C.; Guillot, J. Eur. Polym. I. 1988, 24, 485.

7. Het blijven gebruiken van statistisch ongeldige technieken om reactiviteitsverhoudingen van copolymerisaties uit meetwaarden te bepalen valt sterk te betreuren, mede gezien de moeite die men zich getroost heeft om deze meetwaarden te verkrijgen. Monthéard. J.-P.; Boinon, B.; Raihane, M. Makromol. Olem. 1992, 193,1213. O'Driscoll, K.F.; Reilly, P.M. Makromo/. Olem., MacromoL Symp. 1987, 10/11, 355.

8. De kennis van eenvoudige organische chemie moet niet vergeten worden bij het werken aan geavanceerde materialen. Van der Made, A.W.; Van Leeuwen, P.W.N.M.; De Wdde, J.C.; Brandes, R.A.C. Adv. Mater. 1993, 5, 466.

9. Dat een Europese eenwording nog ver verwijderd is wordt onderstreept door het (vooralsnog) achterwege blijven van uniforme standaarden op het gebied van electrische apparatuur.

10. Het feit dat stellingen bij proefschriften, tot stand gekomen aan de Technische Universiteit Eindhoven, nooit terug te vinden zijn in de rubriek "Stellingen" van NRC/Handelsblad, zou ten onrechte de indruk kunnen wekken dat eindhovense promovendi niet voldoende maatschappelijk bewust zijn.

11. Stellingen doen geen afbreuk aan een goed proefschrift. Verdurmen, E.M.F J. Paniek Nuclealion and Growth in Butmliene Emulsion Polymerization; Proefschrift, Eindhoven 1993, Stelling 11: "De stellingen doen doorgaans atbreuk aan het proefschrift".

Eindhoven, 25 januari 1994