Heterogeneous nucleation in the crystallization of high ...

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Doctoral Dissertations 1896 - February 2014

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Heterogeneous nucleation in the crystallization of high polymers Heterogeneous nucleation in the crystallization of high polymers

from the melt. from the melt.

Ananda Mohan Chatterjee University of Massachusetts Amherst

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s

HETEROGENEOUS NUCLEATION IN THE CRYSTALLIZATION OF

HIGH POLYMERS FROM THE MELT

A Dissertation Presented

by

Ananda Mohan Chatterjee

Submitted to the Graduate School of the

University of Massachusetts in

partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

March 1974

Major Subject: Polymer Science and Engineering

• •

111

HETEROGENEOUS NUCLEATION IN THE CRYSTALLIZATION OF

HIGH POLYMERS FROM THE MELT

A Dissertation Presented

by


Approved as to style and content by:

raser P. Pri^e,^&-Chairman ofCommitte

Seymour [Newman, Co-Chairman of Committee

RichardS. Stein, Member

i

/mRoger S. Porter, HeadPolymer Science and Engineering

March 1974

IV

ACKNOWLEDGMENTi

The author acknowledges his deep gratitude to Professor Fraser

Price, Thesis Co-Director, for his most helpful advice and encouragement

during the course of this work. He also sincerely thanks Professor Seymour

Newman, Thesis Co-Director, for initiating this project and for his valuable

guidance and suggestions as the research progressed. Helpful suggestions

of Professor Richard Stein, member, Thesis Committee, are high appre-

ciated .

I am thankful for the help and friendliness of my fellow students

at the University of Massachusetts. Financial support of this work by the

Ford Motor Company in the form of a fellowship to the author is gratefully

acknowledged.. .

vii

HETEROGENEOUS NUCLEATION IN THE CRYSTALLIZATION OFHIGH POLYMERS FROM THE MELT

(March 1974)

AnandaM. Chatterjee

B . Tech. , University of CalcuttaM. Tech.

, University of CalcuttaM.A.Sc.

, University of Waterloo

Directed by: Dr. Fraser P. PriceDr. Seymour Newman

ABSTRACT

In spite of the large amount of work expended in the study of

crystallization of polymers , the nature of the surfaces responsible for

heterogeneous nucleation and the mechanism as to how foreign surfaces

induce nucleation are poorly understood. The present investigation has

been directed toward a better understanding of the behavior of surfaces

in the nucleation of polymer crystals from the melt.

Isotactic polypropylene (iPP) , poly (ethylene oxide) (PEO),

polycaprolactone (PCL) and poly (butene-1) (PB1) have been crystallized

in contact with various substrates (mostly polymeric) , whose nucleating

abilities have been studied using hot stage polarizing microscopy coupled

with still and motion photography. The substrates have been classified

into three groups, based on their observed nucleating ability. Type I

substrates (very active nucleants) are able to induce transcrystallinity

,

nucleation being strongly favored on the substrate. With Type II sub-

strates (moderately active) , nucleation is favored neither on the substrate

nor in the bulk polymer. With Type III substrates (inactive) ,nucleation

viii

is favored in the bulk. Furthermore , the evolution of nuclei on substrates

and in bulk has been followed by counting the number of spherulites

nucleated as a function of time , at different crystallization temperatures.

Nucleating abilities of surfaces have been characterized (a)

qualitatively by studying the morphologies of the growing polymer induced

by substrates and (b) quantitatively (i) by directly measuring nucleation

densities and nucleation rates on substrates and in the bulk polymer and

(ii) by indirectly evaluating the interfacial energy parameters appearing

in the theory of heterogeneous nucleation. The interfacial energy para-

meters were obtained from the experimentally measured temperature

coefficients of heterogeneous nucleation rates and of spherulitic growth

rates.

A survey of forty three substrate-crystallizing polymer pairs

indicated that similarity in chemical structure and in crystallographic

unit cell type of the substrate and the crystallizing polymer are no cer-

tain criteria for nucleating ability. The survey also indicated that close

match of the lattice parameters of the substrate and the forming polymer

crystal is not a necessary condition for strong nucleating action. It has

been concluded that crystallinity is a necessary but not sufficient condi-

tion for a substrate to be a strong nucleating agent and that the surface

energy of a substrate does not dictate its nucleating ability. Low energy

surfaces of polymers have been found to be able to induce transcrystalline

nucleation of polymer crystals

.

Contents of Ti and Al catalytic residues in iPS samples could

be reduced by fractionating the polymer and chelating Ti and Al with

acetylacetone. Use of these two methods and the use of an iPS sample of

IX

negligible Ti and Al contents showed that the high nucleating ability of

iPS samples in the crystallization of iPP, as evidenced by the generation

of transcrystallinity, is due to the polymer (iPS) itself.

Motion pictures of the development of transcrystallinity in poly-

caprolactone showed that in the early stage of this process, nuclei appear

sporadically on the substrate. The growth rate and melting temperature

of the transcrystalline polymer were found to be the same as those of

bulk-nucleated spherulites.

The substrate-induced morphology of a crystallizing polymer

has been found to be temperature dependent, changing from transcrystal-

line to spherulitic on increasing the crystallization temperature. At inter-

mediate temperatures,mixed surface morphologies are observed (trans-

crystalline plus spherulitic) .

In the crystallization of polymers in contact with substrates,

the existence of a fixed number of nucleating sites on the substrate has

been indicated. Two variations of the shape of the experimental nuclea-

tion density (on substrate and in bulk) vs. time curve have been observed,

namely (i) curve with an initial linear part, and (ii) S-shaped curve.

V

DEDICATION

To

All in my family and to my friends

Ashesh Chatterjee

and

Kenneth Hill

vi

XABLE OF CONTENTS

CHAPTER I . INTRODUCTION1

CHAPTER II. NATURE OF HETEROGENEOUS NUCLEATINGAGENTS AND THE MECHANISM OF NUCLEATINGACTION 4

CHAPTER in. TRANSCRYSTALLINITY 10

CHAPTER IV. THEORETICAL BACKGROUND 17

CHAPTER V . EXPERIMENTAL 40

CHAPTER VI. RESULTS 52

CHAPTER VII. DISCUSSION 69

CHAPTER VIII. SUMMARY AND CONCLUSIONS 83

CHAPTER IX. SUGGESTIONS FOR FURTHER RESEARCH 90

REFERENCES 94

TABLES 103

CAPTIONS FOR FIGURES 151

FIGURES 157

4

CHAPTER I

INTRODUCTION

The study of the crystallization of polymers from the melt andsolution is an area of considerable current interest

1 " 3. Such interest stems

from the fundamental scientific value as well as the technological implications

of the organization of long-chain molecules into ordered structures4 ' 8

. The

crystallization of polymers from the melt, the subject of concern in the pre-

sent study, proceeds through the "birth" enucleation) and growth of spheru-

lites, until mutual impingement of the spherulites occurs and the volume is

filled.The spherulite is the most prominent structural organization in

polymers crystallized from the melt. It is a spherical aggregate of twisted

lamellae and is recognizable by its characteristic appearance in the polarizing

microscope, where it is seen as a circular birefringent area possessing a

dark Maltese cross pattern. Growth and cessation of spherulitic growth is

accompanied and followed by "secondary crystallization" which involves

slow reordering of polymer chains within the spherulites2

.

The term "nucleation" denotes the formation of fragments of a

new phase within a pre-existing mother phase. Nucleation plays the cen-

tral role in a wide range of phenomena, e.g. condensation of vapors, boiling,

devitrification of glasses, crystallization of metals and polymers, atmosphericin 11

cloud formation , etc. .

Two types of nucleation are distinguished, namely "heterogeneous"

and "homogeneous", depending on whether a foreign surface facilitates the

12 13birth of the new phase or not '. Homogeneous nucleation occurs as a

result of random fluctuation of order in the supercooled (or supersaturated)

phase, while heterogeneous nucleation is induced by foreign surfaces, e.g.

the surfaces of the container, particles of impurities, dust or additives pre-

14sent in the specimen

. These foreign nucleating agents decrease the free

energy barrier to the formation of the new phase. Thus, heterogeneous

nucleation normally takes place at supercoolings lower than that required

for homogeneous nucleation to occur °'. Nucleating agents, by introducing

increased number of crystallizing centers into play, consequently enhance

the rate of phase transformation.

The importance of the phenomenon of nucleation in polymer crys-

tallization should be clear from the fact that not only it is responsible for

the initiation of the phase transformation (primary nucleation) , but addi-

tionally for the subsequent growth of the primary polymer crystals16 " 18

.

Thus, after the formation of the primary nuclei, further crystal growth

proceeds through secondary nucleation on the developed crystal faces. The

number of primary nuclei determines the ultimate size of the final morpho-

logical units (spherulites) ; this, together with the internal spherulite order,

plays important roles in determining the physical and mechanical properties

2 19of the resulting bulk polymer '

In spite of the large amount of research effort expended on studies

of polymer crystallization, the nature of the nucleating agents and mechanism

through which foreign surfaces promote nucleation are largely unknown.

The present work has been directed toward a better understanding of the

behavior of foreign surfaces in the nucleation of polymer crystals from the

melt. Detailing the objectives of the present study will be postponed until

the last section of Chapter III.

3

From this point onwards, the phrase "crystallization of polymers-

should be taken ip mean crystallization from the melt, unless crystallization

from solution is specifically mentioned.

CHAPTER II

NATURE OF HETEROGENEOUS NUCLEATING AGENTSAND THE MECHANISM OF NUCLEATING ACTION

From the results of a number of studies, one can ascertain the

efficiency of various additives in promoting nucleation of the crystallization

of polymers20 40

. These additives have been mostly low molecular weight

organic and inorganic substances. Exact correlations between the chemical

composition, crystal structure and physical properties of the nucleating

agents with their nucleating ability have not yet been clearly established,

though attempts have been made in this direction. In this chapter, the

chemical and physical characteristics of nucleating agents in polymer

crystallization, based on reports in the. literature , will be summarized.

Where appropriate , mention will also be made of the mechanism of the

nucleating action, as offered by different investigators.

The techniques used to measure nucleating efficiency include

21 28microscopic measurement of spherulite size '

, differential thermal

24 25 28analysis (DTA) ' , depolarization of plane polarized light , etc.

24Beck and Ledbetter used the DTA technique to measure the

effectiveness of additives as nucleants, in the crystallization of polypropy-

lene (PP) . This technique is based upon the ability of an active nucleant

to induce crystallization at a lower supercooling, compared to the base

polymer. The low molecular weight nucleants used by these authors varied

greatly in activity, demonstrating the specificity of heterogeneous nuclea-

tion processes. Among the more efficient nucleants reported were alkali

metal or aluminum salts of aromatic or alicyclic carboxylic acids. Salts of

aliphatic mono- and dibasic acids or arylalkyl acids were found to be "inter-

mediate" in activity. The relative nucleating ability within these classes of

compounds was found to change considerably by varying the organic sub-

stituents, metal cations, etc. Inorganic compounds used by Beck and

Ledbetter (alums, silica, Ti0 2> CaO, MgO, carbon black, clays) were

classified as "poor" nucleating agents for PP crystallization.

25Beck undertook the evaluation of the nucleating ability of a

further series of compounds (mostly organocarboxylic acid salts) in the

crystallization of PP, using the DTA technique. The nucleating efficiencies

were discussed in terms of the chemical structures of the additives. It was

suggested that a model nucleating agent for PP might be envisioned as one

consisting of an organic group (solubilizing portion) and a polar group

(insolubilizing portion) . Organocarboxylic acid salts represent such a

model. In fact, sodium benzoate and basic aluminum dibenzoate were

among the best nucleants for PP.

25Beck reached the following conclusions for the crystallization

ofPP:

(i) Aromatic groups impart higher nucleating ability than ali-

phatic groups.

(ii) Salts of carboxylic acids are more effective nucleants than

the free acids.

(iii) Sodium benzoate is a more effective nucleant than the

benzoates of a large number of other metals; however, no correlation was

found between nucleating ability and the atomic weight, atomic number,

atomic radius or valence of the cations.

(iv) Substituents in the aromatic ring generally reduce the

more

nucleanng ability; however, subshtuents in the para position areeffective than the corresponding ortho or meta substituents.

(v) A tertiary butyl group, especially in the para position

often increases the activity of the intermediate or poor nucleants, but notthat of a good nucleant.

(vi) Among aliphatic mono- and dicarboxylates and alicyclic

carboxylates, those containing four to seven chain or ring carbon atomsare the most effective.

(vii) Among fused-ring aromatic carboxylates, those having the

polar group in the 2 -position are the most effective.

The effect of polymer molecular weight on the response of PP to

heterogeneous nucleation of its crystallization by basic aluminum dibenzoate

non-existent26(an effective nucleant) was found to be small or practically

Binsbergen's study of a large number of additives in polyolefin

crystallization27

' 28led to the conclusion that nucleating agents for poly-

olefins are crystalline solids of melting point higher than that of the polymer,

insoluble in the polymer melt, having no chemical reactivity toward the

crystallizing polymer, and consisting of both hydrocarbon groups (good

"solvents" for the polymer) and either polar groups or a condensed aromatic

structure which renders the nucleant insoluble in the polymer melt. He

further concluded that nucleating ability is strongly dependent on the degree

and sometimes on the method of dispersion of the nucleant in the polymer.

The conclusions of Kargin, et al.29 " 37

, who studied the nucleating

ability of a number of foreign substances in the crystallization of isotactic

polypropylene (iPP) and isotactic polystyrene (IPS), are in agreement with

those of Binsbergen. Their work also shows the role of nucleating agents

in modifying the supermolecular structure of polymers, and the influence

of the latter,in turn

, on the mechanical behavior of the polymers.

In a study of the mechanism of the action of the nucleating agents,

33Kargin, et al.,considered the role of wettability, lattice match and nucle-

ant dimension in heterogeneous nucleation. First, wettability of a foreign

surface by the polymer was found to improve nucleating action (no mention

was made as to whether the wettability by the crystalline embryo or by

the polymer melt was the key factor) . Thus, hydrophilic inorganic oxides

exert nucleating action in hydrophilic polymers, but not, as expected, in

iPS; also, organic substances like indigo, alizarin, quinacridine and 1,5-

anthraquinone are active nucleating agents for iPS . It was also found that

hydrophilic cotton fibers do not function as nucleants for iPS, but become

active in this respect when their surfaces are rendered hydrophobic.

Secondly, a match of lattice type and lattice parameters between

the substrate and crystallizing iPS was not found to be necessary for strong

nucleating action.

Thirdly, the effect of the dimensions of the nucleating agent on

the nucleating action was explored, using the system iPS-indigo. It was

found that there is no upper limit to the dimension of indigo crystals , for

functioning as an active nucleant at any given temperature; but, a lower

limit does exist. The lower critical size of the nucleant depends on the

temperature of polymer crystallization and diminishes as the supercooling

is increased; thus, as the crystallization temperature gets lower, smaller

and smaller particles become capable of functioning as nucleants. According

8

.° Kargin. et a, 33. the^ size q{^ ^^^^

perature in the same way as does the critical size of the pollerAnother study hy Kargin, et I." lndicated that when nucl(jating

agents in the form of sohd particles are incorporated in a polymer, stresses

arise in the layer of polymer surrounding the particles, and a, the sites of

stresses micro-ordered segments are formed, which promote the initiation ofthe crystallizahon of the polymer

. The magnitude of the induced stress

depends on the size of the particle, the nature of the polymer and the

nucleating substance and on their interaction. This concept is consistent

with the observation, made by these authors as well as by Rybnikar41

.

that gas or vapor bubbles can act as efficient nucleating centers in PP.38Rybnikar studied the nucleating efficiency of six additives

(phthalic acid, benzoic acid, aluminum phthalate, sodium salicylate, carbon

black and the pigment Irgalilechtbrilant blau) in the crystallization of eight

commercial types of iPP. The results indicate that most of the nucleating

agents do not exhibit a universal nucleating ability for all the samples of

iPP examined; in other words, a particular additive shows different

nucleating ability for different samples of polypropylenes . It was suggested

that the real heterogeneities might be other substances originally present

in the polymer, possibly remainders of the catalytic system or occasional

impurities. The additives were thought to activate the originally present

heterogeneities (e.g. by improving the wetting), thus enhancing their

nucleating effect.

The above concept is consistent with the observation of

Rybnikar 38and Binsbergen 42

that the number of heterogeneous nuclei

cannot be indefinitely increased by increasing the concentration of additives.

9

in the cases studied, a. about 1% additive concentration, the number of crys-

tallization centers reached a maximum

.

Heterogeneous nucleating agents (e.g. indigotin, anthraquinone)

were necessary to induce crystallization in poly(2,6-dimethyl phenyleneoxide) which by itself has practically no tendency tc crystallize

40. Also,

the organic dyes indigo and alizarin were found to increase the crystalli-

zadon rate of the very slowly crystallizing IPS by acting as centers of

heterogeneous nucleation31

.

Several studies of the kinetics of crystallization have been madeusing added nucleating agents in polymers 19 ' 21

•42 • 43

. Tne nucieants

reduced the induction period for crystallization and enhanced the rate

of crystallization without changing the shape of the crystallinity vs. time

isotherms.

Two interesting aspects of the physical features of nucleants,

namely the influence of the substrate structure, particularly orientation44

,

and the enhancement of the nucleating ability of a substrate by morphologi-45

cal imprinting, observed by Hobbs, come within the scope of this chapter,

but will be mentioned in the following chapter, as they involve transcrystal-

linity.

10

CHAPTER III

TRANSCRYSTALLINITY

Formation

On cooling a polymer melt to a temperature below the melting

point such that it is in contact with a surface of high nucleating ability

toward the polymer, a large number of spherulites are nucleated on the

surface. These spherulites, due to lateral crowding , cannot grow with

spherical symmetry and are forced to grow in the direction perpendicular

to the substrate surface. The resulting columnar morphology, consisting

of elongated spherulites, has been termed "transcrystalline"46

' 47.

The formation of a transcrystalline region is indicative, there-

fore, of a high nucleating ability of the substrate in the crystallization of a

particular polymer. Transcrystallinity has been observed in polyamides 46 ,48

polyurethanes46

, polyethylene49 " 53

, polypropylene52

'54,55 'Present study

^

and in polycaprolactone (present study) . The formation of the transcrystal-

line region also depends upon the conditions of crystallization, e.g. crystal-

lization temperature, cooling rate, etc.54

The effect of internal transcrystal-

linity (within the bulk of a polymer) is observed if sufficient number of

adjacent nuclei are formed in the interior of the polymer, giving rise to a

"row structure"; examples found are polyamides56,57

and polyethylene53

57aand certain fiber-filled systems

It has been suggested that transcrystallinity results from a

temperature gradient between the nucleating surface and the interior of

48 49the polymer under which the polymer is crystallizing ' . Subsequent

experiments with polypropylene showed that a temperature gradient by

, as

11

itself is ineffective for the development of transcrystallinity and that poly-

propylene can transcrystallize in the virtual absence of a temperature54

gradient .

Transcrystallinity was also claimed to be induced only by high

energy surfaces like metals, metal oxides and alkali halides; low energy

surfaces (like polymers) being apparently unable to induce transcrystal-

lization '

58•

59. However

, studies by Fitchmun and Newman 53'54

well as the present study, show that transcrystallinity can be generated

by low energy surfaces as well.

The degree of penetration of the transcrystalline region into the

bulk of the crystallizing polymer depends on the conditions of crystalliza-

tion, e.g. temperature, and the relative nucleating ability of the substrate

and the adventitious particles present in the polymer sample. Thus, the

presence of bulk-nucleated spherulites in the vicinity of the transcrystal-

line region limits the depth of penetration of the transcrystalline layer to

a depth occupied by spherulites formed in the bulk. Transcrystalline

layers of depth 50-100u are often observed.

Structure

The structure of the transcrystalline region in polyethylene has

been revealed by x-ray crystallographic studies of Keller56

, Eby49

, and

6 0Gieniewski and Moore . The results indicate that the transcrystalline

region consists of symmetrically packed oriented lamellas, with their

longest axes (b) perpendicular to the substrate surface and with the a and

c axes oriented randomly in the plane perpendicular to the b axis.

ers

.

ease

12

Properties

In this section.the important physical and mechanical properties

of the transcrystalline region will be briefly reviewed to show how controlof the morphology of the surface region of polymers through heterogeneous

nucleadon enables one to modify the physical and mechanical behavior of

polymers.

Kwei, et al.51

, reported that the dynamic Young's modulii of

transcrystalline PE and PP are higher than those of the bulk polym

They showed that the modulii of molded films of these polymers deer

with increasing film thickness.

Matsuoka, et al.61

, reported that in the temperature range of

-160 to 120°C, transcrystalline PE exhibits a higher in-phase modulus, a

higher loss modulus and a higher loss tangent (in tensile mode) than bulk

PE.

62Kargin,et al

. ,showed that a transcrystalline surface layer

imparts greater abrasion resistance, greater impact strength and lower

C02permeability to polypropylene films.

Conflicting reports exist in the literature regarding the density,

heat of fusion and crystallinity of the transcrystalline region as compared

to the bulk polymer crystallized under the same conditions. Hara and63

Schonhorn observed higher values of the above three properties for

transcrystalline PE, compared to bulk PE. An opposite conclusion, namely

lower values for the transcrystalline region, was drawn earlier byfil

Matsuoka, et al.

Wettability of the transcrystalline surface has been found to be

different from that of conventional PE; thus, the critical surface tension of

wetting of gold-nucleated PE (transcrystalline) has been observed to be

considerably in excess of the value for ordinary PE 52, and the difference

has been accounted for on the basis of a modified Fowkes-Young equation

for the wettability of polymer surfaces. Transcrystalline PE was found by

Schonhorn and Ryan 64to be amenable to conventional adhesion bonding and

to form a strong adhesive joint, e.g. with aluminum, in contrast to the poor

adhesion with conventional PE. Thus, transcrystallinity offers a route to

improved adhesion of polymers to other surfaces.

Rationalization of the observed physical and mechanical properties

of the transcrystalline region in terms of structural features like lamellar

orientation and packing has not yet appeared in the literature.

Columnar Crystals in Metals

The phenomenon of transcrystallinity is not restricted to polymers

and has been observed in the crystallization of metals65-67

. In fact, the

term "transcrystallinity" was adapted from metallography 46. Casting of I

metals involves a liquid to solid phase transformation and if the nucleation

density is much higher at the mold wall (chilled) than in the bulk of the

metal, long columnar grains extend perpendicular to the mold surface

inward, in the direction of the greatest temperature gradient during freezing

The columnar crystals are not formed if the mold is preheated to a tempera-

ture approaching the freezing point of the metal. The conclusion must be

drawn, therefore, that it is the increased nucleation density at the lower

temperature of crystallization that is responsible for the generation of the

layer of columnar crystals in contact with the mold surface.

14

Northcotfs investigations into the structure of cast metal

ingots68" 70

showed that the columnar crystals are often dendritic and that

they have a preferred orientation; thus, for cubic materials the [100] direc-

tion is parallel to the growth direction.

Substrate Structure and Morphological Imprinting

Two noteworthy features of the action of nucleating agents involvin

transcrystallinity will now be pointed out. First, Hobbs44

observed that the

ability of graphite fibers to nucleate the crystallization of isotactic polypro-

pylene (iPP) is dependent on the substrate structure, particularly the

orientation. Thus, iPP transcrystallizes in contact with Morganite I, but

not in contact with Morganite II. Morganite II is composed of very small

(about 25& graphitic nuclei with a high degree of disorientation, while

Morganite I is made up of much larger (>100#) graphitic planes which are

highly oriented along the fiber axis. According to Hobbs, the observed

difference in nucleating ability between Morganites I arid II is due to the

difference in the size and orientation of their constituent graphite planes.

Secondly, in a study of the crystallization of iPP on morphological

45templates

,Hobbs showed that by using a surface replication technique,

one can transform a non-active surface (like amorphous carbon) into a

surface which can readily induce transcrystallization . A 300$ thick layer

of carbon was deposited by evaporation from a carbon electrode under high

vacuum onto iPP films . These iPP films , on subsequent melting and crystal-

lization, showed development of transcrystallinity. The increase in nuclea-

tion density was shown to result directly from the structural information

contained in the carbon replicas and neither from molecular bonding between

15

the polypropylene surface and the carbon replica nor from the unmelted seed

crystals adhering to the carbon overlayer.

Objectives of the Present Study

Surfaces responsible for heterogeneous nucleation are, in general,

poorly characterized. The first objective of the present study was to charac-

terize surfaces (a) qualitatively by examining the morphologies of the growing

polymer induced by substrates and (b) quantitatively by directly measuring

nucleation densities and nucleation rates on substrates, and by indirectly

evaluating the interfacial energy parameters appearing in the theory of

heterogeneous nucleation. The interfacial energy parameters were intended

to be experimentally determined from the temperature coefficients of hetero-

geneous nucleation rates and of spherulitic growth rates.

The second objective of this study was to generate appropriate

experimental data to throw light on the role played by the following factors

in the heterogeneous nucleation of polymer crystals:

(i) similarity in chemical structure of the substrate and the

crystallizing polymer,

(ii) similarity in unit cell type and lattice parameters of the

substrate and the crystallizing polymer,

(iii) crystallinity of the substrate , and

(iv) surface energy of the substrate.

In view of the claim in the literature that low energy substrates

52 58 59are ineffective in inducing transcrystallinity in polymers ' ' and

53 54evidence to the contrary '

, it was felt desirable to investigate the pos-

sibility of finding further examples of transcrystalline nucleation induced

by polymer substrates.

Finally, it was also intended (i) to investigate the temperature

dependence of the morphology of a growing polymer induced by a substrate,

and (ii) to compare the growth rate and melting temperature of the trans-

crystalline region with those of bulk-nucleated spherulites.

17

CHAPTER IV

THEORETICAL BACKGROUND

Much of our present understanding of phase stability and phasetransformation rests on the classical work of Gibbs

71. Theories of nuclea-

te were subsequently developed by Volmer and Weber, Becker and Coring,Turnbull, and others for low molecular weight substances72 ' 73

. Theseconcepts formed the basis for the development of the theories treating

nucleation in polymer crystallization; Gornick and Hoffman 74and, more

1

8

recently, Price have reviewed the field. The airn of this chapter is to

introduce the concepts of the classical nucleation theory and develop the

equations for the kinetics of homogeneous and heterogeneous nucleation in

polymer crystallization from the melt. The theory of the nucleation control

of spherulitic growth will complete the chapter

.

Homogeneous Nucleation - General

Homogeneous nucleation occurs as a result of random fluctuations

of order in the supercooled phase, in the absence of foreign surfaces. These

nuclei are clusters of molecules (or atoms) and appear at random throughout

the mother phase.

Let us consider the formation of a spherical embryo of a daughter

phase p in a surrounding medium of mother phase a , at constant temperatur

and pressure. The Gibbs free energy of formation of the embryo of radiu

r can be written as

e

AG = - (4/3)7tr3AG + 4tt r

2a (1)

where AGy (=G/ - Gya

) is the difference of Gibbs free energy per unit

volume between the a and p phases and o is the interfacial energy between

the embryo and the mother phase. From this point onwards, free energies

should be taken as Gibbs free energies, unless mentioned otherwise.

When p is the more stable phase, the relationship between AGand r is shown in Figure 1

.The maximum value of AG (designated AG*)

occurs at a critical size r* and represents the free energy of formation of

the "critical-sized nucleus." Such a nucleus can grow in size with a

decrease, rather than an increase, in free energy.

Differentiation of Equation 1 yields

dAG „ 2= -4tc r AG

y+ 8tt ra

Setting this derivative equal to zero, one obtains for the critical

nucleus:

-4tc (r*)2 AG

y= 8tt r*a

or

r* = 2o/AGv (2)

19

Substituting this into Equation 1

AG* =16

3 (AG )

2 (3)

The spherical shape of the nucleus is a good assumption in many

cases, but not in the case of polymers, where a cylindrical nucleus is con-

sidered to be more appropriate15

. For a cylindrical nucleus of radius r and

length 1 and characterized by two interfacial energies, namely, c for the

s

side and o&for the end (Figure 2) , we may write

2 9AG = - itr 1AG„ + 2tc r lo + 2tt r ov s e

This equation, on partial differentiation, yields

= - 2tc r 1 AG + 2tt 1 a + 4tt r aor v s e

and'

8AG 2 A „-aT = - n r AG +27iraol V s

20

Setting the partial derivatives equal to zero and solving, one

obtains for the critical nucleus

r = 2 o /AGs v (5)

J * = 2°e/AG

v (6)

Substitution of these two values into Equation 4 gives the free

energy of formation of the critical nucleus:

AG* = 87C os

2ae/(AG

v )

2

(7)

This equation is similar in form to Equation 3 for a spherical nucleus. The

insensitivity of such formulation to embryo geometry is worth noting.

75Volmer and Weber assumed that nuclei form by a series of

bimolecular reactions of the type

V a ai + 1

(8)

21

With specific reference to condensation from the vapor, they further assumedthat the concentration of critical-sued nuclei was that characteristic of

equilibrium and that the back flux of supercritical nuclei could be neglected.

Their equation for the rate of nucleation (I) per unit volume is

I = a A nv (9)

Here a is the collision frequency of single molecules with nuclei per unit

area of the nuclei, A* is the area of the critical nucleus, and ny*

is the

number of critical nuclei per unit volume, given by76

nv* = n

v°exp (-AG*/kT) .

'•

(10)

In this equation ny

is the number of unassociated molecules per unit volume,

k is Boltzmann's constant, and T is the absolute temperature.

The back flux in Equation 8 and the decrease in embryo population

caused by growth of the nuclei were subsequently taken into account by

77Becker and Doring who calculated the steady-state concentration of

critical nuclei. Their equation for the nucleation rate is

22

I = a A n B (11)

Here B is the Becker-Doring non-equilibrium factor, given by

B =(3o/kT)

1/2

2tt (r )

2(12)

where v is the molecular volume of phase p (embryo) and r* and c havebeen defined in the beginning of this section for a spherical nucleus.

For nucleation in condensed phases, the free energy of activation

for motion across the daughter phase-mother phase interface should be

included in the expression for nucleation rate. This was done by Becker78

and later more quantitatively by Turnbull and Fisher79

, who combined the

absolute reaction rate theory 80 with the Becker-Doring approach and derived

the following equation for the steady state nucleation rate:

I = IQexp (-AG /kT) exp (-AG /kT)

(13)

where AGtis the activation energy for transport across the embryo-mother

phase interface. Equation 13 is referred to as the Turnbull-Fisher equation.

23

For most nucleation problems of interest, I may be taken to

order-of-magnitude accuracy as

I s n nO V o

where nQ

is the molecular jump frequency. The term I is dependent on

molecular parameters, but is essentially independent of temperature. At

comparatively small supercoolings , the second exponential in Equation 13

that is the transport term, exerts negligible influence on the nucleation

rate; the term becomes significant at large supercoolings.*

The quantity AG depends on AGv

(Equation 3); AGv

> in turn

is proportional to the supercooling AT. Hence the Turnbull-Fisher equation

predicts that under conditions where the transport term is negligible, the

homogeneous nucleation rate should vary exponentially with the super-

o-i

cooling. Experimentally, this variation is very sharp. Thus, Turnbull

observed the nucleation rate of crystals in liquid mercury droplets to vary

by four orders of magnitude in a temperature interval of 3°C

.

*The term AG in the Turnbull-Fisher equation can be expressed

in terms of the supercooling and the interfacial energies characterizing the

nucleation process. This allows one to determine the dependence of the

nucleation rate on these parameters.

With the above-mentioned aim in mind, let us start with a model

for a small crystal of a low molecular weight substance in contact with the

melt from which it formed (Figure 3) . The crystal dimensions are a, b,

and c, respectively. Let oebe the interfacial energy of the two AB faces

and a be the interfacial energy of all the other faces and AGy

the bulk free

energy of fusion per unit volume. The free energy of formation of the

above-mentioned crystal, relative to the melt, is given by

AG = - abc AGy

+ 2 ab ag

+ 2a (be + ac) (1/

This equation represents a hyper-surface whose positions of extrema are

determined by equating to zero the partial derivatives of AG with respect

to a, b, and c and solving for the critical values, namely a , b' and c

which define the dimensions of the critical nucleus. Thus, Equation 14

yields

9AG-~— = - be AG + 2 b o + 2acoa v e

9AG _

9b- ac AG + 2 a a + 2ac

v e

and

= - ab AG + 2ab + 2aadc v

For the critical nucleus:

25

a* = b* = 4a/AGv

C*=4

°e/AG

v (16)

Substitution of these values in Equation 14 gives the free energy of formation

of the critical nucleus:

* 2 9AG = 32 o oe/(AG

v)

z

(17)

Heterogeneous Nucleation - General

The nucleation of most phase transformations takes place hetero-

geneously on impurity particles, container walls or structural imperfec-

tions14

'

81a'

81b. Intentionally added nucleating agents also catalyze the

nucleation process. A formal description of heterogeneous nucleation will

1 *3

be presented in this section, following Turnbull .

Let us consider the formation of a spherical cap of the daughter

phase p from the mother phase a, on a flat substrate s (Figure 4); the

substrate-nucleus contact angle is 0 . The net free energy of formation in a

of unit area of the p- s interface is given by

Wc= on -a

s ps as (18)

where oqs

and are, respectively, the a-s and p-s interfacial energies.

The magnitude of Wgdetermines whether or not the substrate catalyzes the

nucleation of p. The substrate will be a preferred (relative to the bulk of

a) p nucleation site, provided Wg

is less than oap , the a-p interfacial energy

The greater are the intermolecular attractive forces between p and s, the

less is Wg

.If the substrate interacts more strongly with p than with a,

W will be negative (since on < a )fa ps as"

The relation of Wgwith the contact angle 0 is obtained by a hori-

zontal force balance in Figure 4:

o a cos0 + on = aap ps as

or

cos0 = (a - o„ )/oas °ps J/aap (19)

Using Equation 18, we have

27

cos© = - w /as «P (20)

Volmer 82showed that AG

g*, the free energy of formation of a

critical nucleus heterogeneously, is given by

AG * -167tO

a33f(0)

S3 (AG

y+ E)

2 (21)

= AG* f (0)(22)

where

f (0) = (2 + cos0) (1 - cos0)2

4 (23)

Here E is the elastic strain energy per unit volume and AG* is the thermo-

dynamic barrier for homogeneous nucleation in the bulk of a (see, for

example, Equation 3 for a spherical nucleus)

.

The magnitude of f (0) lies between zero and unity, depending

on Wg

. The limiting values are:

28

f (0) = 0 when - W > as ^ ap

and

f (0) = 1 when W = os ap

Equations 22 and 23 predict that the thermodynamic barrier to

nucleation on a substrate should decrease with decreasing contact angle 6,and approach zero as 0 -> 0.

Following the Turnbull-Fisher 79treatment for homogeneous

nucleation, Turnbull 83 ' 84derived the following equation for the rate of

heterogeneous nucleation (per unit area of substrate)

:

Is= K

sexp(-AG

s"/kT) exp(-AG

t/kT)

where

gfc

Ks= n n

sf (a

ap/kT)1/2

(2V/ 9*)

1/3[f(0)] 1/6

(24)

Here n is the number of molecules in the nucleus in contact with a, V is

the volume per molecule of liquid, ng

is the number of molecules of a in

contact with unit area of the substrate, and f is the fundamental jump fre-

quency. According to the absolute rate theory, f = kT/h, where h is

Planck's constant.

29

It should be noted that Equation 24 is of the same form as the

Turnbull-Fisher equation, i.e. Equation 13.

The general action of a nucleating substrate is to reduce the

free energy barrier for nucleation to occur. When a nucleus forms on a

substrate, in addition to the creation of the nucleus-mother phase interface,

some relatively high energy substrate-mother phase interface is replaced

by a lower energy substrate-nucleus interface.

The subject of retention of embryos in cavities of foreign bodies,

under conditions where such embryos would be unstable on a flat surface

,

was treated in detail by Turnbull83

for conical as well as cylindrical cavities

In the latter case, embryos will be retained in cavities of radii less than r ,

c

which is given by

r = 2a n cos0/AGc ccp v

Here 0 is the angle shown in Figure 5 and AGv

is the motivating free energy

for a •* p transformation.

If a system is heated above the melting temperature (T ) by an

amount AT+

to eliminate embryos in cavities of radii greater than r , andc

then cooled below T , embryos retained in cavities of radii less than rm Jc

will grow to the mouth of the cavities. They will not, however, serve as

nuclei unless the radius at the cavity opening equals or exceeds r , the

30

critical size. Hence a linear relation between AT+and AT~ (undercooling)

is anticipated, until the range of undercooling is reached where normal

heterogeneous nucleation on the substrate is possible.

Nucleation in Polymer Crystallization.

Extension of the concepts discussed above to the nucleation of

polymer crystals will be presented in this section, following the treatment18

of Price.The treatment for crystallization from the melt, which will be

presented, is essentially valid for crystallization from dilute solution also,

excepting for minor corrections like replacing the polymer melting tempera-

ture by the solution temperature, etc. The validity stems from the fact

that the heats of solution are small compared to heats of crystallization and

that the entropies of solution can be easily calculated.

Homogeneous Nucleation in Polymers . For homogeneous

nucleation in polymer crystallization , a model for the nucleus depicted in

Figure 6 will be used. The embryo is built up by the successive addition

of adjacent segments of the polymer chain . The length c is called the

"fold length" or "segment length". The AB surfaces are called "fold

surfaces" and since most folds occur in the AC plane, this plane will be

called the "fold plane" . According to the treatment given, the size and

free energy of formation of the critical nucleus are given by Equations 15

through 17.

To illustrate the insensitivity of the aforementioned formulations

to embryo geometry, it should be pointed out that if a right circular cylinder

had been assumed, the diameter would be given by Equation 15, the length

would be given by Equation 16, and the constant 32 in Equation 17 wouldbe replaced by 8tt .

The temperature dependence of AG* resides in the quantity

AGv .

If AHyand AS

yare, respectively, the enthalpy and entropy of

fusion of the polymer per unit volume

,

AGv

= AHv

- T ASv

At the melting temperature (T ) , we may write

0 = AH - T As flJ{nv m v 126)

or

T = Ah /As , 97 ,m v v (27)

Substituting this equation into Equation 25

AG = AH - T AH /Tv v v' m

= AH (1 - T/T )v m J

= AHvAT/Tm (28)

32

where AT (* Tm - T) is the supercooling.

Combining Equation 28 with Equations 13 and 17, one obtains

the rate of homogeneous nucleation (folded chain nuclei):

I = Iqexp (-AG

t/kT) exp

2 9-32 o o T

e mT (AT)

2k (AH )

2 (29)

Experimental studies of homogeneous nucleation in the crystalli-

zation of polyethylene 85' 86

and polypropylene87

have been performed, using

the "droplet" technique to annul the effect of foreign heterogeneities. Before

the crystallization, the polymer samples were divided into sufficiently large

number of parts so that the heterogeneities, of finite number, resided in only

a small fraction of the parts. Supercoolings of about 60°C and 100°C were

necessary for polyethylene and polypropylene, respectively, for homogeneous

nucleation to occur.

Heterogeneous Nucleation in Polymers . Referring to the model

of Figure 6, let one AC face of the crystal be in contact with a foreign surface.

Also, let a be the substrate-crystal interfacial energy and a be themsubstrate-melt interfacial energy . The free energy of formation of the

crystal will be given by

AG - - abc AG + 2 ab o + 2 be a + ac AaO V c (30)

33

where

Aa = a + a - ac m (31)

es

The term oerepresents the interfacial energy of the fold surfaces (AB)

,

while o represents the interfacial energy of all the other fac

The situation just described is referred to as "non-coherent"

nucleation, which is characterized by the condition Aa £0. The expressions

for the size and free energy of formation of the critical nucleus will be

derived now.

From Equation 30,

-^-= - be AG +2 bo +c Ao

3AGW = " ac AGy

+ 2 a ae+ 2 c a

9AG-j^r = - ab AG

v+ 2 bo + a Aa

Equating the partial derivatives to zero, and solving, one obtains the size

of the critical nucleus:

34

a = 4o/AGv (32)

*

b s 2Ao/AGv (33)

c = 4o /AGe v (34)

Substitution of Equations 32 through 34 in Equation 30 gives the free energy of

formation of the critical nucleus:

AGg

16 aoeAo/ (AG

y ) (35)

Using Equation 28, and substituting Equation 35 into Equation 24, one obtains

the rate of heterogeneous nucleation per unit area of substrate (folded chain

nuclei)

:

Ig= K

seXp (-AG /kT) exp

-16 oa Ao Te m

T (AT) k(AHv )

2 (36)

or

35

16 aa Aa Tlog I = log K - AG./ (2.3 kT) e "

"mb St „ 9 r (37)

2.3 T(ATr k (AH )

2

This equation describes the dependence of the rate of heterogeneous nuclea-

tion on supercooling.

When the substrate is absent, am= 0 and o = a

c. Under this con-

dition.Equation 32 reduces to Aa = 2a, and Equation 36 reduces to Equation 29,

the case for homogeneous nucleation.

Comparison of Equations 29 and 36 shows that if Aa < 2a at a given

temperature, Ig

will exceed I; that is, heterogeneous nucleation will be

favored. According to Equation 31, Ao < 2a when (a - a ) < a. It should

be noted that this condition is identical to that due to Turnbull for the

spherical cap model of heterogeneous nucleation, namely W < a (sees ap

Equation 18) . The quantity aR is the same as a , a is the same as a ,P° c as mand o^p is the same as a

.

Heterogeneous nucleation processes are always accompanied by

a transient in the number of nuclei. However, the interaction between the

substrate and the polymer crystal may be strong or weak.

When the substrate-polymer crystal interaction is strong, the

wetting of the substrate by the crystal is good, and a has a low value.c

The rate of heterogeneous nucleation will be high in such cases, at tempera-

tures well above the range of homogeneous nucleation. If there exists a

finite, but very large, number of heterogeneities, on cooling the melt all

the crystals will appear close to zero time and will seem to develop from a

«xed number „ nuclei . The transiem in the number of ^ thjs ^of very short duration and large magnitude.

When the substrate-polyrner crystal interaction is weak, o is

large and Ao may approach 2o. In such a case, the substrate affects the

nucleation rate only slightly and crystals may appear sporadically i„ ,imeand space. This heterogeneous nucleation process mimics the homogeneousprocess and has been called "pseudohomogeneous nucleate 18 ' 88

. How-ever, the number of heterogeneities being finite, if the crystals do no, growso rapidly as to fill the vo.ume ,00 soon, eventually the number of growingcenters will become fixed (this cannot happen in homogeneous nucleation) .

In this case the transient in the number of nuclei can become quite long in

duration.

Figure 6a schematically illustrates the nucleation rate curves for

homogeneous, heterogeneous and pseudohomogeneous nucleation, the third

being a special case of the second.

Nucleation Control of Spherulite Growth

The very large temperature dependence of spherulite growth rate

at relatively small supercoolings led to the idea that the growth rate of

spherulites is nucleation-controlled16 '

17. The idea is based on an earlier

concept due to Volmer 89that the addition of a new layer of crystalline

material to an already completed crystal face is determined by the rate of

appearance upon that face of small crystal patches, which are extensions of

the underlying parent crystal and have been called "coherent growth nuclei".

The problem of crystal growth then reduces to one of describing the forma-

tion of coherent growth nuclei9,15

.

37

Figure 7 is a schematic representation of a coherent growth

nucleus of dimensions a,b

, and 1 on the tip of a crystal at the spherulite

boundary. The free energy of formation of such a nucleus from the melt is

given by

AG = - abl AG + 2 blv a + 2 ab

e (38)

Here AGy

is the bulk free energy of fusion per unit volume and o and o aree

'

respectively, the interfacial energies of the bl and ab faces. No term involving

the al faces appears because building the nucleus destroys as much such a

face as it produces. Setting dAG/da = 0 and 3AG/91 - 0, and solving, one

obtains for the critical nucleus:

a* = 2o/AGv (39)

1 =2a/AG (40)

Substituting Equations 39 and 40 into 38,

AG = 4b 00 /AGe v

38

(41)

Using Equation 28, this yields

* 4b oo TAG = e m

AHy

(AT) (42)

The growth rate (G) of the polymer spherulite is proportional to I,

the rate of formation of coherent growth nuclei on the tips of the crystals at

the spherulite boundary. Using the Turnbull-Fisher equation, one can

write

G = zl = zIq

eXp (-AGt/kT) exp (-AG /kT) (43)

where z is the proportionality constant. If z!Q

= GQ , combination of Equation

42 and 43 yields

G = Gqexp(-AG

t/kT) exp

- 4b oo Te m

T ( AT) k (AH )v(44)

The term Gq

is a constant depending on molecular parameters, but

essentially independent of temperature. Equation 44 describes the

temperature dependence of the growth rate of polymer spherulites.

Taking logarithms

,

i n ,4b oa T

log G = log G - AG./ (2.3 kT) - —.

e m0 1 2.3 k (AH ) T (AT) (45 ^

According to this equation, it is possible to calculate ooefrom

the temperature coefficient of spherulitic growth rates. Also, one can

obtain the quantity aoeAo from the temperature coefficient of heterogeneous

nucleation rates according to Equation 37. By dividing oa Aobyaae J

e'one gets Ao (= a + o

q- aj . The quantity o is independent of the sub-

strate; (om - CM,the difference between the. substrate-melt and

substrate-crystal interfacial energies, reflects the nucleating ability of

substrates. A higher value of (om" aj indicates a better heterogeneous

nucleating agent since, relative to the melt, less energy is expended in

creating a unit area of substrate-crystal interface. This approach has

been used in the present study to characterize the nucleating efficiency

of substrates

.

CHAPTER V

EXPERIMENTAL

5 . 1 Materials

Table I describes the materials used in the present investiga-

tion.

Melting points of the samples of iPP, poly (ethylene oxide),(PEO)

,

poly(butene-l) (PB1) and polycaprolactone (PCL) were determined by

heating the sample in the hot stage of a polarizing microscope at 0.2°C/min.

and observing the spherulites under crossed polars. The melting point

was taken as the temperature at which the birefringence of the sample dis-

appeared .

"Ash",Ti and Al contents were analyzed at the Microanalysis

Laboratory at the Department of Chemistry, University of Massachusetts.

For the determination of "ash" content, the sample was burned in oxygen

at 1000°C; the residue consists usually of metallic oxides. Mass of this

residue, as a percentage of the initial mass of the sample, is reported as

"ash content"

.

Ti and Al contents of isotactic polystyrene samples were deter-

mined by atomic absorption spectroscopy.

Molecular weights of PEO were obtained by Gel Permeation

Chromatography, through the courtesy of Dr. Robert Ulrich90

.

The intrinsic viscosities of two fractions of isotactic polystyrene

(entry 5d in Table I) were determined following the conventional proce-

91dure . Viscosity average molecular weights were calculated from the

intrinsic viscosities, using the Mark-Houwink equation. Details are

included in this chapter.

The remainder of the characterization data shown in Table I

was supplied by the vendors.

5.2 Equipment

Figure 8 gives an overall view of the equipment constructed forthe present study. The essential parts of this photomicrographs equip-ment, which is supported on a drill press stand (a,

, are the following:

(i) A Zeiss Polarizing Microscope (b) , equipped with a vertical

eyepiece (inside c) and a binocular eyepiece (d) .

(ii) A Mettler FP2 Hot Stage (e), placed on the microscope

stage. Temperature programming is accomplished using the control unit

(f).

(iii) A Prontor-Press shutter (g) (Geiss America, Inc.),

fitted on top of the vertical eyepiece. The shutter is provided with a cable

release and can give exposure times down to 1/125 second.

(iv) A light-tight black wooden box (h) tapering downwards,

at the bottom of which the shutter is held in place with screws. This box

is also fitted on the outside with a viewing telescope (i) facing the observer

and a solenoid (j) on the observer's right.

(v) A photographing device at the top. In Figure 8, a custom

70 mm. camera (k) is seen. In its place a Polaroid Film Holder or a motion

picture camera can also be used.

At the bottom of the wooden box is a 45° mirror which , when

vertical, allows light to reach the photographic film just above the top of

the wooden box. When the mirror is at 45°, the light coming through the

vertical eyepiece of the microscope, after passing through the shutter, is

reflected by the mirror and reaches the observer's eye through the tele-

scope (i).

This arrangement allows one to focus the image at the film plane

by focusing it on the ground glass in the telescope for magnifications of 80,

160, 200 and 400 X. The telescope in its present state suffers from the

following two disadvantages: (i) focusing at the ground glass of the tele-

scope does not insure focusing at the film plane for all magnifications, and

(ii) the telescope allows one to see only a small part of the field of view of

the microscope (as seen through the eyepiece) .

Focusing can alternatively be effected by placing an acrylic

sheet (size 6" x 4.75" x 0.25") with one face ground at the film plane

(that is, on top of the wooden box) and viewing through a comparator.

The custom 70 mm. camera (k) seen at the top is linked with

an Intervalometer (1) with which the exposure time and the time between

exposures can be controlled. For the present study, a Polaroid Film Holder

was used for photographing the crystallization process on a microscope

slide inside the hot stage. Motion pictures of transcrystalline nucleation

and growth processes were also taken using a Bolex 16 mm movie camera.

/ At the bottom right corner in Figure 8 can be seen an arrange-

ment (m) consisting of solid state rectifiers for converting the 110 volt

alternating current (A.C.) from the main to direct current (D.C.) which

drives the solenoid activating the mirror. The D.C. source is schematically

represented in Figure 9. The light dimmer in Figure 9 provides a contin-

uously variable wattage (zero to 600 watts) . A side view of each silicon

rectifier (6 ampere, 200 PIV , stud mount) , represented by a triangle in

Figure 9, is shown in Figure 10.

The D.C. source is used in conjuncUon with the Intervalometerto operate the solenoid. For taking Polaroid pictures, neither the Inter-

valometer nor the D.C. source is needed. In this case, the 45° mirror is

kept vertical (no obstruction to light passage), the exposure time being

controlled by the cable-release shutter (g) .

5.3 Procedure

The polymers were crystallized in contact with different sub-strates inside the hot stage mentioned in the preceding section. Both the

substrate and the crystallizing polymer were used in the form of films of

thickness 2 to 6 mils, prepared as described below.

5.3.1 Sample Preparation

5.3.1.1 Substrates. Films of polymers to be used subsequently as sub-

strates were prepared by pressing the polymer between two aluminum foils

in a Carver press at 15°C above the melting temperature and 2500-5000 psi

pressure, followed by quenching with cold water (15°C) . The films were

stored in a desiccator.*

The polymer substrates in many cases were initially quenched

to below their glass transition temperatures, while being made into films.

Crystallinity developed in the crystallizable polymer substrates (e.g.

,

isotactic polystyrene, nylons 6, 66, and 610) while the polymer whose

crystal nucleation was being studied was being melted and crystallized

in contact with the substrate.

The nylon 6, 66, and 610 substrates were heated in an oven

(105°C) overnight before use, in order to remove any residual moisture.

Since the glass transition temperatures of these nylons lie in the range

44

50 - 75 C.annealing at !05°C is expected to increase the crystallinity of

the nylons. The degrees of crystallinity of the substrates were „o, measuredAluminum and gold substrates were used as received, in the form of foils.^^W^Sl^. Films of the polymer ,o be crystalli2ed

prepared following the same method as used for the substrates. and stored

in a desiccator

.

5.3.2 Crystallization

Freshly-cut surfaces of the substrate films were used for

studying their nucleating ability. The substrate films were cut

perpendicular to the film plane with a clean pair of scissors. The sample

mounting is shown in Figure 11. Two films of the polymer to be crystallized

and one cut substrate film were assembled on a microscope slide and covered

with a glass cover slip. The microscope slide was then mounted inside the

Mettler FP2 Hot Stage. The crystallizing polymer was in contact with the

substrate at the AB and CD interfaces (Figure 11) , while in the regions

AEFB and CGHD, bulk crystallization of the polymer took place.

For crystallization,the temperature of the hot stage was first

raised to a point (T^ above the melting temperature (TJ of the polymer

to be crystallized (but below the melting temperature of the substrate),

held at Tj for a few minutes, and then the temperature dropped rapidly to

the crystallization temperature. The exact temperatures and times are shown

in Tables II through Vin. The morphology of the crystallizing polymer

induced by the substrate was then observed directly under the microscope

and photographed.

Calibration, using pure compounds of known melting points,

namely para-nitro toluene, biphenyl, naphthalene, and benzoic acid,

showed that the indicated temperature was accurate within ±0.5°C.

5.3.3 Nucleation Densities

The number of spherulites nucleated on the substrate surface

and in bulk were counted directly under the microscope, during isothermal

crystallization. Nucleation densities were calculated from these numbers bydetermining the area of the substrate and the volume of the bulk from the

measured sample thickness and the diameter of the field of view of the microscope.

Figure 12 shows the experimental arrangement for counting the

number of nuclei. The circle ACBD represents the field of view of the

microscope. The substrate was placed so that its cut surface (AB) was

along a diameter of the field of view. Figure 12 shows five hemispherical

spherulites nucleated on the substrate and four spherulites nucleated in

the bulk. As the polymer sample is sandwiched between two glass sur-

faces (Figure 11) ,the number of nuclei counted in the bulk includes those

originating from the glass surfaces.

The diameter of the field of view of the microscope was measured

with the help of a stage micrometer.

The total thickness of the assembly (glass slide plus polymer

sample plus cover slip) in Figure 11 was measured after the crystallization

with a micrometer. Subtraction of the thickness of the glass slide plus the

cover slip from the total thickness gave the thickness of the polymer sample.

5.3.4 Nucleation Rates

Rates of nucleation on the substrate and in bulk were measured

by counting the number of nuclei as a function of time , as crystallization

progressed isothermally, and determining the slopes of nucleation density

vs. time plots.

The temperature dependence of the nucleation rates was studied

by performing crystallization experiments at different temperatures, keeping

46

to the same portion of the sample (substrate and bulk, under observabonwithout disturbance between experiments. This technique ehminatcd thevariation of the nucleation arising from one area eiement of the substrate to

another at the same temperature. Variations in the volume elements in thebulk were also thereby minimized.

5.3.5 Growth Rates

Linear growth rates of the transcrystalline region and the radial

growth rates of the spherulites in the bulk were measured at the same crys-tallization temperature by direct observation of the growth process underthe microscope, using a micrometer eyepiece. The scale in the micrometer

eyepiece was calibrated using a stage micrometer. Growth rates were

obtained from the slopes of linear distance-time plots.

5.3.6 Ti and Al Imp uritie s in Isotactic Polystyrene Samgles

In the present investigation, it was observed that samples of

isotactic polystyrene (iPS) are able to induce transcrystallinity in isotactic

polypropylene (iPP) as well as in polycaprolactone. A series of experiments

was performed to ascertain whether the high nucleating ability of the isotactic

polystyrene substrate was due to the polymer itself or to the catalyst resi-

dues (Ti and Al compounds) in the sample of iPS, which was synthesized

by Ziegler-Natta polymerization93

. The iPS sample was subjected to (i)

centrifugation,

(ii) treatment with a chelating agent, and (iii) fractionation,

as described below.

5.3.6.1 Centrifugation. In anticipation of the lower solubility and higher

density of the metal-containing impurities, compared to the polymer, the

following experiment was performed. A sample of iPS (3 g .) was taken in

toluene (125 ml.) and stirred until partial solution of the polymer resulted.

47

The supernatant was centrifuged at 12000 rpm for 6 . 5 hours, usjng ,^

centrifuge. From the centrifugate IPS was recovered oy precipitaUon in

methanol; iPS was then filtered, washed with methanol and dried undervacuum

This technique did not reduce the ash content of iPS (initial

2.07%, after centrifugation 2.17%)

5-3.6.2 Use of a Chelat^A^^Both Ti and Al are

known to form organo-soluble chelates with acetylacetone (AA)94

' 95and it

was possible to remove these metals, though not completely, from the iPS

sample by this method.

Acetylacetone (7 ml.) was suspended in distilled water (40 ml.)

and ammonium hydroxide added dropwise with stirring until a clear solution

resulted. Then 0.5 g of the iPS sample was added and the suspension was

stirred at 29°C for 5 hours with a magnetic stirrer. The solid polymer was

then filtered and washed successively with benzene, carbon tetrachloride,

methanol and acetone, to dissolve and remove Ti- and Al-acetylacetonates.

Finally, acetone was removed from the sample by drying under vacuum.

Ash contents as well as Ti and Al contents of the initial iPS and

AA-treated iPS samples were determined. The difference in the nucleating

abilities of these two iPS samples in the crystallization of iPP was then

investigated by crystallizing iPP in contact with the iPS substrates and

observing the polypropylene morphologies induced by the iPS substrates.

The results are included in the following chapter.

5.3.6.3 Fractionation of Isotactic Polystyrene . The sample of iPS

(2g.) was taken in 200 ml. toluene and dissolved by heating at 100°C and

48

stirring „ith a magnetic^ A^ ^^^ ^theBM

.

four fractions of iPS were iso.ated, usln s the foUowing tech.nique.

Methanol was added dropwise with stirring to the solution ofiPS. When turbidity developed, the nature was warmed gently unti. theturbidity disappeared and then allowed to cool slowly to room temperatureThe precipitated first fraction was separated by decanting the supernatantFrom the supernatant three more fractions were collected using the sameprocedure. The precipitated fractions were washed with methanol andthen dried under vacuum to remove methanol.

The first fraction was found to have ash content twenty times

higher than that of the fourth fraction. The nucleating abilities of these

two fractions of iPS in the crystallization of iPP were compared by observingthe polypropylene morphologies induced by the iPS substrates.

5.3.7 Determination of Intrinsic Viscosities

Intrinsic viscosities of the two fractions of iPS in toluene at 30°C

were determined following the conventional technique2

. An Ubbelohde-type

capillary viscometer was used for flow time measurements, so that successive

dilution of the polymer solution within the viscometer could be made.

Intrinsic viscosity is defined as

49

where c is the concentration of the polymer solution; nsp

is the specific

viscosity defined as

and r, and r,

oare the viscosities of the polymer solution and the solvent, res-

pectively

.

For negligible kinetic energy contributions,

n/ri0 B t/t

o

where t and tQare the viscometric flow times for the polymer solution and

the solvent, respectively. Plots of both n /c vs. c and (In r, )/c vs. csp

were extrapolated to zero concentration to obtain intrinsic viscosities.

Experimentally-determined values of the intrinsic viscosities

were converted to viscosity average molecular weights (M ) using the

, * 96,97equation '

[nj = 9.3 x 10~ 5 M °' 72

50^^^^^^^^The term "surface morphology „m be used t0 refer tQ ^ mor _

phology of the crystallizing polymer in contact with the substrate. Twotypes of surface morphologies should be distinguished: "spherulitic"

(SPH) and "transcrystalline" (TC) . It should be noted that although a

transcrystalline layer is also spherulitic in nature, being composed of

very closely-spaced spherulites. the term "spherulitic surface morphology"

will be used to refer to a situation where the spherulites nucleated on the

substrate are isolated from one another, as shown in Figure 12. With con-

tinued growth, the spherulites shown in Figure 12 will eventually meet

one another.

"Transcrystalline surface morphology", on the other hand, is

characterized by intense lateral crowding of the spherulites nucleated by

the substrate and an easily recognizable columnar layer growing toward

the melt. It is clear that a substrate giving rise to a transcrystalline sur-

face morphology is of higher nucleating ability than a substrate giving

rise to spherulitic surface morphology.

In this study the substrates have been classified into the

following three groups, in order of decreasing nucleating ability.

Type I. Very Active Substrates. The very large number of

nuclei generated on such a substrate gives rise to a transcrystalline

surface morphology. Nucleation is overwhelmingly favored at the sub-

strate, compared to the bulk.

Type II. Moderately Active Substrates. This group is charac-

terized by surface morphologies of the following four types, in order of

51

decreasing nucleating ability of the substrate: (i, TC (mostly, + SPH,(ii) TC + SPH, (iii) SPH (mostly) TC, (iv) SPH.

Subdivisions (i, through (iv, represent cases of mixed surfacemorphology; examples can be seen in Chapter VI. Case (i) represents a

situation where more than half of the substrate gives rise to transcrystal-

lini.y and the rest gives rise to spherulitic surface morphology. Case (ii)

represents a substrate, one half of which gives rise to transcrystallinity andthe other half to spherulitic surface morphology. When the dominant surface

morphology is spherulitic, case (iii) results. Finally, case (iv) refers to

a completely spherulitic surface morphology

.

Type III. Inactive Substrates. These substrates generate very

few or no spherulites at all. In this case spherulites are nucleated prefer-

entially in the bulk and those close enough to the substrate grow and meet

the substrate. At the end of crystallization, the substrate is in contact with

bulk-nucleated spherulites.

52

CHAPTER VI

RESULTS

The Spectrum of Nucleating Ability of Substrates

The morphologies characterizing the three types of substrates

referred to at the end of the preceding chapter will be illustrated. Figure 13

shows an example of a Type I substrate, namely iPS which has induced trans-

crystallinity in iPP. Nucleation is strongly favored at the substrate. Figure 14

shows a Type II substrate,namely Kel-F or poly (chlorotrifluoroethylene)

,

which is only moderately active in the crystallization of iPP. Nucleation

in this case is seen to have occurred at the substrate surface as well as

in the bulk, without any preference for either. An example of a Type III

substrate is shown in Figure 15. The nylon 610 substrate is inactive as

a nucleating agent in the crystallization of PEO . Spherulites are nucleated

preferentially in the bulk of PEO.

In Tables II through VIII are listed the surface morphologies

observed and surface types (I, II or III) in the crystallization of iPP, PB1,

PEO and PCL in contact with the indicated substrates at different tempera-

tures. Four different samples of iPP were used, as indicated in Tables

II through V. For iPS substrate , data are available for different samples,

different fractions from the same sample as well as for an acetylacetone-

treated sample (of reduced Ti and Al content) . Each of these iPS sub-

strates was able to induce transcrystalline nucleation of iPP, as can be seen

from Tables II through V

.

Effect of Crystallization Temperature on Morphology Induced by Substrates

The surface morphology induced in a crystallizing polymer

53

by a substrate was observed to depend on the crystallization temperature.

Examination of a particular substrate-crystallizing polymer pair in TablesII through V and VEI shows invariably that as the crystallization temperature

is raised, the surface morphology changes from transcrystalline to spheru-

litic. At intermediate temperatures, mixed surface morphologies (trans-

crystalline plus spherulitic) are observed.

Figure 16 illustrates the reduced nucleating ability of iPS sub-

strate with increasing temperature in the crystallization of iPP. The surface

morphology is transcrystalline at 125°C (Figure 16 i) and at 130°C (Figure

16 ii);

it is half transcrystalline and half spherulitic at 133°C (Figure

16 iii),while at 135°C (Figure 16 iv) , the surface morphology is completely

spherulitic.

Surface Morphology

Examination of Tables II through V shows that iPP (monoclinic

unit cell) can transcrystallize in contact with iPS (rhombohedral), nylon 6

(monoclinic),nylons 66 and 610 (both triclinic)

, Penton (orthorhombic)

and PET (triclinic) but Kel-F (hexagonal), PPO or poly (2 ,6-dimethyl

phenylene oxide), atactic polystyrene and Al substrates have lower

nucleating ability towards iPP .

Figures 17 through 20 show the transcrystalline surface mor-

phology of iPP crystallizing at 125°C in contact with nylons 6, 66 and 610,

and Penton, respectively. Spherulitic surface morphology of iPP crystal-

lizing at the same temperature in contact with PPO is shown by Figure 21.

Table VIII shows that PCL (orthorhombic) can transcrystallize

at 50°C in contact with high density polyethylene (orthorhombic), nylons

6, 66, and 610, iPS and polyoxymethylene (hexagonal) . However, iPP,

54

Penton, Kel-F, low density polyethylene, atactic PS, PET, PB1 (rhombo-

hedral),gold and aluminum substrates could not induce transcrystallinity

at 50°C. At 45°C, iPP, Penton and Kel-F were able to induce transcrystal-

linity, but atactic PS failed to do so.

Figures 22 through 24 are photomicrographs showing trans-

crystallization of PCL at 50°C in contact with nylons 6 and 66, and poly-

oxymethylene,respectively. Spherulitic surface morphology generated in

contact with gold at 50°C is shown in Figure 25.

Figures 26 through 28 show the moderate activity of three crys-

talline substrates, namely PET, PB1 and Kel-F, respectively, in the nuclea

tion of PCL crystals at 50°C . In each case, nucleation is favored neither

on the substrate nor in bulk, and the substrate should be classified as

Type II.

None of the substrates used in the crystallization of PEO and

PB1 was found to induce transcrystallinity in these polymers, as can be

seen from Tables VI and VII. For PEO, Figure 29 illustrates a moderately

active (Type II) substrate (iPP), while Figure 30 illustrates an inactive

(Type III) substrate (nylon 6) . Nylon 66 (Figure 31) behaves similar to

nylon 6 ,but since occasionally one or two PEO spherulites were found to

be nucleated on nylon 66 substrate , the latter has been classified as

Type II substrate in Table VII.

It is also worth noting that in the crystallization of iPP (Tables

II through V) , the substrate activity was either of Type I or Type II, no

example of Type in being found. The same is true for the crystallization

of PCL (Table VHI) . In the case of PB1 (Table VI) , all the substrates used

55

were found to be moderately active (Tvoe III in *y U ype II). in the crystallization of PEO

the substrates used were either of Type II or TvnP tttiype 11 or Type III, no example of trans-crystallinity being found.

In the crystallization of iPP in contact with ips

(Table II) at 125°C, it can be seen that increasing the holding time of the

transcrystallinity was generated in each case

.

Crystallinity of the Substrate

Figure 32 shows the higher nucleating ability of iPS, compared

to atactic PS.

in the crystallization of iPP . Isotactic polystyrene was foundto induce transcrystallinity within 2 minutes at 125°C (Figure 32 i) . Atthe same temperature, atactic PS showed only moderate activity at the most(Figure 32 ii)

. Transcrystallinity was found not to develop from the atactic

PS substrate even on reducing the crystallization temperature to 115°c

(Table II).Below this temperature, the bulk nucleation densities in com-

mercial isotactic polypropylenes were found to be high enough to interfere

with the study of morphological feature on substrates.

The higher nucleating ability of iPS compared to atactic PS wasalso found to be true in the crystallization of PCL. Thus at 50°C, while

iPS could induce transcrystallinity (Figure 33 i), atactic PS could not

(Figure 33 ii)

.

Thirdly, at 50°C PCL was found to transcrystallize in contact

with high density polyethylene (Figure 34 i) , but low density polyethylene

gave rise to spherulitic surface morphology (Figure 34 ii) . These results

indicate higher nucleating ability of substrates with higher crystallinity.

56

Inspection of Tables II through VIII reveals that all the substrateswhich are able to induce transcrystallinity are crystalline. On the other

hand, there are many crystalline substrates which are either moderately

active or inactive nucleants. This suggests that crystallinUy is a necessarybut not sufficient condition for a substrate to be a strong nucleating agent.

The author speculates that the requirement of crystallinity in

a substrate for strong nucleating action is probably tied with the fact that

the newly emerging phase is crystalline. In other words, crystallinity of

the substrate may not be a necessary condition for strong nucleaUon action

in the case of heterogeneous nucleaUon of vapors from a liquid or vice

versa.

Figure 32 i shows a feature of the development of crystallinity

in iPS, previously quenched from the melt to about 15°C. Birefringent

striations (e.g. ab) on the iPS film at about 45° to the iPS-iPP interface

were observed to develop while the iPS was being annealed. The annealing

of iPS took place unavoidably while iPP was being melted and crystallized.

The striations can also be seen in Figures 13 and 16 (ii - iv) . Examination

of Figures 16 (iii) and 32 (i) reveals that the striations are not seen at the

lower part of the photomicrographs, that is, away from the iPS-iPP inter-

face. This suggests that the striations may result from the shearing action

due to the cutting of the iPS film with a pair of scissors.

Ti and Al Impurities in Isotactic Polystyrene

Table IX shows the reduction of Ti and Al contents of

iPS sample by treatment with acetylacetone (AA) . Ti content was

reduced to less than half of its initial value , while Al content went

down to one-seventh of the initial value. Comparison

of the nucleating ability of untreated and AA-treated iPS samples in the

crystallization of iPP is shown in Table X. Experiments were performed

with three samples of iPP, at indicated temperatures. It can be seen that

the AA-treated sample did not lose its original nucleating ability. For all

the three samples of iPP, identical surface morphologies resulted in con-

tact with iPS substrates of markedly different Ti and Al content.

It should also be noted in Table X that at 135°C, for the same

sample of substrate, the surface morphology varies, depending on the

sample of the crystallizing polymer (iPP); this is true for both untreated

and AA-treated iPS substrates. The crystallization temperature (135°C)

is within the range where the transition from transcrystalline to spherulitic

surface morphology takes place.

Table XI shows the ash contents of the fractions of iPS . As suc-

cessive fractions were precipitated, their ash contents decreased pro-

gressively. Measurement of the intrinsic viscosities of the iPS fractions

in toluene at 30°C yielded the following values: 3.55 dl/g. for Fraction I

and 1.75 dl/g. for Fraction IV. The calculated viscosity average molecular

weights corresponding to these intrinsic viscosities are 2.24 x 106and

g0.85 x 10 , respectively.

Table XII shows the nucleating ability of the first and fourth

fractions of iPS in the crystallization of iPP (two samples) . The two frac-

tions of iPS differ twenty-fold in ash content, as can be seen from Table XI.

At 130°C, the two substrates had similar nucleating ability. However, at

135°C, Fraction IV had a lower nucleating ability than Fraction I, as indica-

ted by the surface morphologies. This difference might be due to a lower

58

degree *cry™, of Fraction w . Cl„nity^ ^ ^*S substrate while iPP „ being nielted and ^^Unity could result from the lower molecular weight of Fraction IV (highernumber of chain ends) .

Finally, iPP was crystallized in contact with a sample of iPS(Dow EP 1340-128) which by atomic absorption spectroscopy showed < 1

Ti and < 1 ppm Al and which showed ash content of 0.025%. Even at

were

ppm

thesenegligible metal contents, transcrystailinity was generated, as shown in

Table II, Table III and Figure 13.

The results mentioned in this section lead to the conclusion that

the high nucleating ability of iPS substrate in the crystallization of iPP is

due to the polymer (iPS) itself.

Nucleation Densities

Quantitative measures of the nucleating ability of substrates

obtained by counting the number of spherulites nucleated on the substrate

and in bulk and calculating the nucleation densities, namely ng(number

of nuclei/cm 2) for substrate and n

fe(number of nuclei/cm 3

) for bulk.

The experimentally-determined nucleation densities are shown in

Tables XIII through XV, for the crystallization of iPP, PB1 and PEO,

respectively. The substrates have been listed in order of increasing

ns/n

brati0

'The temperatures of crystallization and the times of

measurement have been indicated in the tables . Zero time was taken as

the time when the hot stage reached the crystallization temperature. After

cooling from the melting temperature, the crystallization temperature was

reached in two to four minutes . The times of observation in Tables Xin

59

active at the particular temperature.

The ns/n

bratios listed in Tables XIII through XV are repre-

sented as heights of the bars in Figures 35 through 37. Referring to theIPS and nylon 610 substrates in Table XIII, ,he exact number of nucleicould not be counted by visual observation due to the overcrowding of

spherulites in the transcrystalline region. The minimum number of

nuclei that could be counted is shown. I, follows that the real heights

corresponding to the last two substrates in Figure 35 are larger than the

indicated heights.

The selective nature of heterogeneous nucleahon processes is

evident from the results of Tables II through VIII and XIII through XV . Forexample, Kel-F and iPS have comparable nucleating ability in the crystal-

lization of FBI,but when IFF is the crystallizing polymer, iPS is a much

better nucleant than Kel-F.

Examination of the nucleation density data in Tables XIII through

XV indicate that the nucleation density in bulk (nb

) is of the order of

1010/cm

3.

For PEG at 53°C the order is 109

. For spherulitic surface

morphology, the nucleation density on the substrate (n ), is of the order

5 2s

of 10 /cm and the n^ ratio is of the order of 1QS^ fo^

transcrystallinity is generated, ng

is in excess of 106/cm

2(probably of

the order of 106

- 107/cm

2) .

Figure 38 shows the evolution of nuclei in the early stage of

the development of transcrystallinity in PCL, the substrate being high

density polyethylene. The photographs were printed from motion picture

60

negatives filmed at 2 frames/ sec and 1 frame/ 6 sec. Only four or five

spherulites can be seen to be nucleated at 9 minutes (Figure 38 i) at ADinterface. With the passage of time more spherulites are nucleated on the

substrate (Figures 38 ii to iv) . These photographs show that in the early

stage of the formation of a transcrystalline layer, nuclei appear sporadi-

cally over the substrate. However

, heterogeneous, and not homogeneous

,

nucleation process is involved since Figure 38 shows the virtual absence

of nucleation in the bulk PCL (region AED) .

After counting the number of nuclei by analyzing the film in a

viewer, the nucleation density corresponding to Figure 38 (iv) was

determined to be 6.7 x 105/cm

2. This may be taken to be a stage where

approximately half of the substrate is occupied by spherulites. Considering

the fact that these spherulites were growing, it was assumed that at complete

coverage of the substrate by spherulites, half as many more nuclei will

appear on the substrate. This assumption leads to a nucleation density

(for transcrystalline formation) of 1.5 x 6.7 x 105/cm

2or 10^/cm

2, at the

point of total occupancy of the substrate by spherulites.

To check the reproducibility of the numbers of nuclei, the

experiment of crystallization and counting the number of nuclei was

repeated four times with attention to the same portion of the sample , with-

out any disturbance between crystallizations. The result is shown in

Table XVI. The data indicate that the numbers counted in bulk vary on

repeating the crystallization. The coefficient of variation, defined as (s/m) X

100, was 21% in this case. Here s is the standard deviation and m is the mean.

The number of nuclei on the substrate is too small to derive any useful

61

conclusion. However,the inactive nature of nylon 66 as a nucleant for PEG

crystals is evident.

The bulk nucleation densities in each of Tables XIII through XV at

a particular temperature and time represent nucleation densities in different

volume elements of the same sample of the crystallizing polymer. The mean,

standard deviation and the coefficient of variation for each case are shown

in Table XVII. A comparison of the sixth and last lines (PEO at 51°C) of

the table shows that the coefficient of variation for different volume elements

is greater than that calculated after repeated crystallizations within the

same volume element. It should also be noted that the coefficient of varia-

tion is higher at higher crystallization temperatures; this probably is due

to the smaller number of nuclei counted at higher crystallization temperatures

The variation of nucleation density on substrate (given sample)

from one sample of a crystallizing polymer to another is shown by Tables

XIII (part i) and XVIII. For these two tables, the same substrate sample

was used, but the iPP samples were different, namely Pro-fax X 12886-38

(Hercules, Inc.) for Table XIII and Shell 5520 for Table XVIII. A comparison

of the two tables shows that for each of the three substrates, namely Al

,

atactic PS and Kel-F , the ngand n

g/n

bvalues obtained with the Pro-fax

iPP sample are different from the values for the Shell iPP sample. The

ratios of ng/n

bvalues (Pro-fax to Shell) for the three substrates are shown

in Table XIX. Their pronounced deviation from unity in both directions

shows that ng/n

bratios for a given substrate sample vary from one sample

of a crystallizing polymer to another. It is evident that the ranking of sub-

strates based on ng/n

bratios depends on the particular sample of the crystal-

lizing polymer used.

62

Nucleation Rates

Figure 39 shows typical curves of nucleation density on sub-

strate vs. time of crystallization at three crystallization temperatures. Theinhibition period increases as the supercooling decreases. The surface

morphology at 126°C was transcrystalline and after 3 minutes the number of

nuclei were too many to count accurately by direct observation. At the other

two temperatures, the number of nuclei could be followed until the substrate

area was completely occupied.

It can be seen in Figure 39 that the number of nuclei on the sub-

strate eventually becomes fixed and the growth of these fixed numbers of

spherulites fills the total available substrate area. The number of growing

centers was observed to become fixed before the available area on the sub-

strate was totally occupied. In other words , the horizontal part of the

curve is true up to the complete coverage of the substrate area. A fixed

number of active nucleating sites on substrate surfaces is thus indicated.

The shape of the curve is general; iPP , PB1 and PEO crystallizing in con-

tact with various substrates showed this behavior.

At higher crystallization temperatures the nucleation density

decreases. The height of the plateau in Figure 39 gives the total number

of nuclei active at a particular temperature. The difference between the

plateau heights gives the additional number of nuclei that become active

for a given drop in the crystallization temperature. Thus, according to

Figure 39, 30% more nuclei became active on the substrate when the crys-

tallization temperature was dropped from 128° to 127°C.

Curves of similar shape were obtained when the nucleation den-

sity in bulk was plotted against time of crystallization, as shown by

63

Figure 40. Here too, the number of growing centers, after initial risebeco.es constant before the total volume is filled with spherulites. Thus» bulk also there exists a fixed number of active heterogeneities which areresponsible for nucleation at a given temperature.

For both substrate and bulk, two cases have been observed in

connection with the nucleation density-time curves, namely (i, the initial

part of the curve is linear (e.g. Figure 40, , and (ii, the curve is S-shaped(e.g. Figure 41) . Nucleation rates have been obtained for the first casefrom the slope of the initial linear part, and for the second case from the

maximum slope of the curve.

Experimentally determined nucleation rates on substrate (I )

ssand in bulk (I

sb ) as well as1^ ratios are listed in Tables XX and XXI,with PB! and PEO, respectively, as the crystallizing polymer. The I /I

ratios for different substrates have been represented as the heights of the

bars in Figures 42 and 43.

To study the reproducibility of the bulk nucleation rates, the

crystallization of PEG was performed four times, with the same portion of

the sample under observation. The plots of nucleation density vs. time for

the four runs are shown in Figure 41 . It can be seen that the curves are

not exactly reproduced on repeating the experiment, though the same shape

is obtained each time. However, no definite trend is noticeable in these

curves. The nucleation rates have been calculated from the maximum slopes

of the S-shaped curves in Figure 41 and shown in Table XXII. Their mean,

standard deviation and coefficient of variation have also been included.

64

The different bulk nuclea(ion raies jn TaMes ^sent variation from one volume ,tament t0 another h ^ bu[k Qf^^P-^r sample. The values of the mfian

, standard deviatjon coefflcjen[of variation are entered in Tabie XX,„ . The large coeffjclent Qf

for PEO is probably due to the smaller number of spherulites counted, thenature of heterogeneities in the PEO samole and th» o „fcsampie, and the S- shaped nucleationrate curves which introduce arrnr i«ntrociuce error m the measurement of the slope of thenucleation density vs. time curves.

In one instance, the reproducibility of the nucleation rate onsubstrate was checked. The determinabon of nucleation rate was repeatedwith the same pair of samples but different portions in contact. The agree-ment, as shown in Table XXfV, is good. However, whether this goodagreement will be observed consistently can only be ascertained by fur-ther repetition of the experiment.

Finally, it should be noted that the nucleation rates on substrates

studied are of the order of 104

- 105/min/cm 2

and the bulk nucleation rates

are of the order of 109

- 1010/min/cm 3

.

Temperature Coefficient of Spherulitic Growth Rates

Spherulitic growth rate (G) data for poly (butene-1) at different

temperatures of crystallization are shown in Table XXV. The plot of log Gvs. 1/CTAT) is shown in Figure 44. The calculated least square slope of

the straight line is - 0.622 X 105 V. According to Equation 45 (Chapter IV)

for spherulitic growth rate, the slope is equal to - 4b oa T /(2.3k AH ) .e m v

The melting temperature (TJ of PB1 was determined to be

134. 3°C, the method having been described in Chapter 5. From the litera-98

ture,the heat of fusion of poly (butene-1) was taken as AH =

v

65

35.7 cal/g. . 1.42 X 10yerg/cm 3

. Using these data and assuming b = 5A,

the interfacial energy product ooqwas calculated to be 243 erg

2/cm

4.

Temperature Coefficients of Heterogeneous Nucleation Rates

Figure 45 shows the number of nuclei counted on iPS substrate as

a function of time in the crystallization of PB1 at different temperatures. In

each case PB1 was premelted at 140°C for 4 minutes. The decreasing inhi-

bition period and increasing nucleation density with increasing supercooling,

as well as the fixed number of active nucleating sites on the substrate, as

indicated by the levelling off of the curves, have already been pointed out.

The number of nuclei on the substrate became constant before the available

area on the substrate was occupied totally by spherulites.

The nucleation rates were calculated from the slopes (N ) of the» s

plots in Figure 45. The data are shown in Table XXVI. The quantity log

Ngwas plotted against 1/[T(AT)

2] (Figure 46) and the least square

straight line was drawn through the points. According to Equation 37 for

heterogeneous nucleation (Chapter IV) , the slope of the straight line is

equalto- 16 aaeAo Tm

2/ [2. 3 k (AH

y )

2]. To make this clear , it should

be noted that Nghas the unit # nuclei/min, while I has the unit #

s2

nuclei/min/cm and if a = area of the substrate,

I = N /as s

66

Differentiating,

d log I d log N1

s

r1

J (AT)2 d

T (AT)2

since a = constant, by experimental setup.

Using the previously-mentioned values of Tm and AHy

for poly-

(butene-1),the triple product oo

qAc for iPS substrate was calculated to be

531 erg3/cm 6

.

Similar experiments, namely measurement of nucleation rates

of PB1 at different crystallization temperatures, were performed using iPP as

the substrate. The data are shown in Table XXVII and Figures 47 and 48.

Plots of nucleation density vs. time for the bulk crystallization

of PB1 at different temperatures are shown in Figure 49. The existence of

a fixed number (decreasing with increasing crystallization temperature) of

active heterogeneities in the bulk polymer, and the increasing inhibition

period and decreasing nucleation rate with increasing crystallization tem-

perature are in agreement with the results of Sharpies" for the crystalli-

zation of poly (decamethylene terephthalate) . The temperature dependence

of the nucleation rates of PB1 is shown in Table XXVHI and Figure 50. In

this case the value of oa Aa was calculated to be 283 erg3/cm

6.

67

The calculated values of the interfacial energy parameters are

summarized in Table XXIX. The value of Ac was obtained by dividing 00*0by aa .J e

The quantity (om - 0(J in Table XXIX represents the difference

between the substrate-melt and substrate-crystal interfacial energies and

hence is a measure of the nucleating ability of substrates; larger values of

(om - oj indicate better nucleating ability. Table XXIX shows that iPS and

iPP substrates have practically the same nucleating ability toward PB1. This

is in agreement with the surface morphology studies (Table VI) . Both sub-

strates showed moderate activity (Type II) in the nucleation PB1 crystals.

Table XXIX also shows that the nucleating ability of the surfaces

of the foreign particles present in the PB1 sample is comparable to those of

iPS and iPP substrates.

Transcrystalline Growth Rates

Transcrystalline layers of iPP and PCL were found to grow with

a constant linear velocity. Comparison of the linear growth rate of the trans-

crystalline region with the radial growth rate of bulk-nucleated spherulites

is shown in Table XXX. For iPP, the values of the radial growth rates of

spherulites are in good agreement with those reported in the literature100

.

For PCL, no growth rate data at 50°C was available for comparison at this

writing

.

Table XXX indicates that the linear growth rate of the transcrys-

talline region is the same as the radial growth rate of the spherulites. Exam-

ination of the last two lines of this table also shows that the transcrystal-

line growth rate is independent of the substrate responsible for the trans-

crystalline nucleation.

68

Melting Temperature

The meltag temperatures of the transcrystalline polymer-d that of the bulh-nucleated spherulites were determined by directobservation of the disappearanoe of the birefringence of these tworegions, under the same field of view of a polarizing microscope, a,

a beating rate of 0.2°C/min. The two melting temperatures were foundto be identical, namely 171. 8°C for iPP and 64.2°C for PCL.

*

69

CHAPTER VII

DISCUSSION

Examination of the forty three substrate-crystallizing polymer

pairs during the present study has confirmed the selectivity of heterogeneous

nucleation processes. However

, the necessary and sufficient conditions for

effective heterogeneous nucleation are not yet definitely established. The

present study represents progress towards this goal.

It has been noted during the present investigation, particularly

in the crystallization of PEO and PB1, that a number of particles in the melt

visible under the microscope gave rise to no spherulites at all. This indi-

cates that the mere presence of a foreign surface in a supercooled melt is not

enough for nucleation to occur.

No definite correlation has been found between nucleating ability

and similarity of chemical structure of the substrate and the crystaUizing

polymer. In fact, examples have been found for all the four classes, shown

in Table XXXI. The conclusion must be drawn that similarity in chemical

structure is no certain criterion for strong nucleating ability.

No definite correlation was found between strong nucleating action

and similarity of crystallographic unit cell type of the substrate and the

crystallizing polymer. Again, examples found for each of the four classes

are shown in Table XXXII. Similarity of unit cell type evidently is not a

necessary condition for strong nucleating action.

The role of crystallographic unit cell parameters in heterogeneous

nucleation will now be discussed. In this study, strong nucleating action

has been observed in both cases, namely with and without similarity in

70

the unit cell parameters of the substrate and the crystallizing polymer. Someexamples will follow. The crystal structures of PCL and PE are very similar.

Both crystallize with orthorhombic unit cells, the dimensions (in % of which

have been reported to be as follows: 101

c

(fiber axis)

PE 7.400 4.930 2.534

PCL 7.496 4.974 17.279

During the present study, PCL has been found to transcrystallize

in contact with high density polyethylene (Figure 34 i); however, low den-

sity polyethylene showed a lower nucleating ability (Figure 34 ii) . Decreased

crystallite size or increased lattice distortion in low density polyethylene may

be responsible for this result.

The other substrates which have been observed to induce trans-

crystallinity in PCL are nylons 6, 66 and 610, isotacuc polystyrene and poly-

oxymethylene . However , comparison of the unit cell parameters of these

polymers102

with those of PCL 101shows no particular similarity.

In the case of iPP, transcrystallinity has been found to be induced

by iPS, nylons 6 , 66 and 610, PET and Penton substrates. The crystallo-

graphic b and c axes of the unit cells of iPP and iPS are very similar and

1 fi?both polymers form 3-fold helices in the crystalline state . However, no

significant similarity exists in the unit cell parameters of iPP and any of the

1 02other five polymer substrates mentioned above

71

The evidence of strong nucleating action without any close match of

lattice parameters is an exception to the theory of Turnbull and Vonnegut 103,

according to which nucleating efficiency should increase with increasing

closeness of match between the lattice parameters of the substrate and the

forming crystal. Exceptions to the Turnbull-Vonnegut theory have been

found earlier also, e.g. Beck35

noted that although sodium formate has a

better match of lattice parameters with PP than does lithium benzoate, the

latter is a much better nucleating agent. One must conclude that although

there are known examples where the Turnbull-Vonnegut theory is obeyed 103,

match of lattice parameters is not a necessary condition for strong nucleating

action.

The three nylons (6, 66 and 610) have been found in this study to

behave similarly in the nucleation of crystals of iPP. This is also true for the

crystallization of PCL and PEO . Thus , each of the three nylons can induce

transcrystallinity in iPP and PCL, but is inactive in the case of PEO. The

pronounced difference in the nucleating ability of a particular substrate,

from one crystallizing polymer to another, should be noted.

Transcrystalline nucleation and growth of polymers on polymer

substrates have been reported earlier, e.g. PE in contact with Mylar53

and iPP in contact with Mylar and TFE-Teflon54

. The present study has

brought to light further examples of this phenomenon, with iPP and PCL

crystallizing in contact with crystalline polymer substrates. To the author's

knowledge, no known example of transcrystallinity induced by an amor-

phous substrate exists.

72

It should be noted that well-developed transcrystalline morphologyis observed only in a certain range of crystallization temperature. At highertemperatures, transcrystallinity is not seen because of the reduced nuclea-te density on the substrate. On the other hand, at temperatures belowthe range, although a very high nucleation density initially exists on the

substrate, a large number of spherulites are also nucleated in the bulk of

the crystallizing polymer. These spherulites meet the transcrystalline

region very early in its development. Thus the volume is filled quickly anda well-developed transcrystalline morphology is not seen. The convenient

ranges of crystallization temperature for observing transcrystallinity have

been found to be: 120-130°C for iPP and 48-52°C for PCL

.

The results of the present investigation, as well as those of Fitchmun

and Newman 53' 54

,indicate that low energy surfaces of polymers are capable

of inducing transcrystallinity, contrary to claims in the literature that only

high energy surfaces can induce transcrystallinity52

' 58 ' 59. in fact, during

the present study, PCL has been found to transcrystallize in contact with a

number of low energy surfaces at 50°C (Table VIII), while gold and aluminum

(high energy surfaces) were unable to induce transcrystallinity (Figures 22

through 25, 33 i and 34 i) . Also , Table II reveals that Al could not induce

transcrystallinity in iPP at 125°C, while a number of crystalline polymers

were able to do so.

The results mentioned above lead one to believe that surface

energy of a substrate does not dictate its nucleating ability. This thesis

is supported by the results of a study by Koutsky, Walton and Baer on the

heterogeneous nucleation of PE from the melt on cleaved surfaces of alkali

73

halide singie Wtth»< Changing the surface energy ^ hajidesubstrates by a factor of more than four ^ ^ resuu ^^ signiacantchange in th. t, - ^ values for toe ^ ^ ^ ^^of 5 - 7 erg/cm . As indicated earlier

, a Mgher value of _ o^ indjcatesa better heterogeneous nucleating agent.

The melting temperature of the transcrystalline region has beenfound in this study to be identical with that of the bulk-nucleated spheruiitesfor both iPP and PCL as the orystailizing polymer. This finding is not in

agreement with that of Matsuoka Pt a i61 uiatsuoka

,et al

. ,who reported that transcrystalline

PE has a melting point 5°C lower than that of spherulitic bulk PE. It shouldbe noted that in the present study the melting process was directly observedunder the polarizing microscope at a heating rate of 0.2°C/min

( with the

transcrystalline region and the bulk-nucleated spheruiites occupying the

same field of view. The data of Matsuoka, et al., are based on differential

scanning calorimeter thermograms obtained at the heating rate of 10°C/min.

The change of the surface morphology from transcrystalline to

spherulitic on increasing the crystallization temperature (Figure 16) is due

to the reduced rate of nucleation at smaller supercooling, as predicted by

the theory of heterogeneous nucleation.

At intermediate temperatures (e.g. 133°C for Figure 16 iii),

mixed (transcrystalline plus spherulitic) surface morphologies have been

observed. Examples can be seen in Tables II through V and VIII. The sub-

strate-crystallizing polymer interface (ab) in Figure 16 iii is 2.015 mm long.

The left half of the interface shows spherulitic morphology while the right

half shows transcrystallinity. Evidently, the right half has a stronger

74

nucleaang effect. Any explanation for this observabon should take into

account the fact that mixed surface morphologies are observed only at the

intermediate temperatures of crystallizadon; above this range one observes

spherulitic surface morphology, while below this range, transcrystallinity

is seen.

One speculation will be made in order to explain the existence

of mixed surface morphologies, namely that the right half of the substrate

(Figure 16 iii) is more wettable by iPP crystals, compared to the left half,

thereby giving rise to a higher nucleation density in the right half. The

wettability differences do not result in a morphological difference at lower

temperatures (when transcrystallinity is generated) or at higher tempera-

ture (when spherulitic surface morphology results) . In other words,

probably there exists a difference in wettability between the right and

left sides of the substrate at all temperatures, but only at intermediate tem-

peratures this difference gives rise to different morphologies in different

regions of the substrate.

Apart from transcrystallinity, there exists a phenomenon involving

another type of oriented overgrowth on substrates, namely epitaxial crystal-

lization105 115

. However, it should be noted that while transcrystallinity

is defined by its characteristic morphology, for epitaxy a definite orienta-

tion relation exists between the crystalline substrate and the forming

crystal, namely that the planes and directions in the two crystals in which

the atomic arrangement is most similar are parallel103

. High polymers

(e.g. PE, polyamides, iPS, Penton) have been grown epitaxially from

75

solution onto selected faces of cleaved alkali halide single crystals107

' 109 '

110,113-115 _ .

.Oriented rod-like crystallites of the growing polymer on

the substrate are usually observed. The macromolecular chains lie

parallel to the substrate surface. Koutsky, et al.

104, observed that rod-

like PE crystallites form on the (001) face of alkali halide single crystals

when PE melt is nucleated on them .

Epitaxy of low molecular weight substances on oriented polymer

substrates was observed by Richards 116 and Willems108

. Richards found

that paraffin wax crystals grow epitaxially from solution onto cold-drawn

PE and polyethylene sebacate, but not onto cold-drawn PET, nylon 66,

nylon 610, natural rubber or gutta percha substrates. The chain molecules

in the substrates were oriented parallel to the direction of drawing. With

the PE substrate, the paraffin wax molecules were found to be oriented

parallel to the PE molecules.

108Willems observed the epitaxy of pentachlorophenol (PCP) and

hexaethylbenzene on the surface of a strip of cold-drawn PE and the epitaxy

of PCP, pentabromophenol, pentachloroaniline and anthraquinone on the

surface of cold-drawn strips of nylon 66.

117Takahashi, et al. , reported the epitaxial crystallization of

polymers from the melt on uniaxially drawn and annealed polymer substrates.

This study involved the crystallization of polymers from the melt on the

surface of other polymers and, therefore, among the studies mentioned,

bears the greatest resemblance to the present study. The results of

Takahashi, et al. , and those obtained during the present study are in

excellent agreement. Thus , Takahashi, et al. , reported that PCL grows

76

epitaxially on HOPE, nylon 6 and POM substrates, but not on LDPE or PP

According to the present study (Table VIII), PCL transcrystallizes at 50°C

in contact with HDPE,nylon 6 and POM, but LDPE induces a spherulitic

surface morphology and iPP induces a mixed surface morphology.

Based on the tendency of crystallographic planes of closest

atomic packing to appear on the surface (as a stable crystal habit),

Takahashi, et al., assumed that the (110) planes of PE are exposed

locally on the substrate and are responsible for the nucleation of PCL

crystals.

In the present study the polymer substrates used were cut

surfaces from pressed films (undrawn) . The orientation of the chain

molecules in the substrate was not determined. In the absence of the

necessary information, no particular crystal plane of the substrate can

be singled out to be responsible for generating transcrystallinity . It

certainly is desirable to clarify whether only particular crystal planes of

the substrates are responsible for transcrystalline nucleation or all the

crystallographic faces of the substrates are capable of inducing trans-

crystallinity .

The nucleation density vs. time curves for crystallization in

contact with a substrate (Figures 39, 45, 47) and for crystallization of

the bulk polymer (Figures 40, 49) exemplify pseudohomogeneous nucleation

in those cases where the transient in the number of nuclei is relatively long

in duration. The levelling off of the number of spherulites before the

available area (or volume) is filled indicates that homogeneous nucleation

is not involved. However, due to weak substrate-polymer crystal

77

interaction, spherulites are nucleated sporadically in hme on the substrate(and in bulk)

,thus minicking the process of homogeneous nucleation.

It has already been mentioned that the shape of the nucleation

density vs. time curve can have two variations, namely (i) with an initial

linear part (Figure 40) , and (ii) S-shaped curve (Figure 41) . When the

embryo size distribution adjusts itself to that corresponding to the crystal-

lization temperature at a rate greater than or equal to that at which the

crystallization temperature (T) is reached, curves of type (i) result. Onthe other hand, in cases where the hot stage has reached the crystalliza-

tion temperature, but the embryo size distribution has not yet adjusted

itself to the state corresponding to T, S-shaped curves result. This is

because even though the crystallization temperature is reached, the system

is still adjusting itself from a state of higher temperature to one of lower

temperature, and a lower nucleation rate is observed in the early stage,

giving rise to the foot of the S-shaped curve.

Tables XXVI through XXVIII reveal that for the crystallization

of PB1 in contact with substrates (iPS, iPP) and for the bulk crystallization

of this polymer,the heterogeneous nucleation rates increase by a factor

of approximately 5 for a 10°C increase in the supercooling. Such tempera-

ture coefficients are to be contrasted with the much stronger dependence

of homogeneous nucleation rate on supercooling. For PE, the homogeneous

nucleation rate changes by a factor of 10 for a degree change in super-

1

8

cooling.

For normal alkane liquids, Turnbull and Cormia118

reported

increase of homogeneous nucleation rate by a factor of 5000 to 8000 per

degree decrease in temperature.

78

Examination of the temperature dependence of spherulitic growthrates of FBI (Table XXV) indicates a (5 to 10) fold increase in the growthrate for 10°C increase in the supercooling. This dependence is comparableto the temperature coefficient of the heterogeneous nucleation rates of FBIand is much weaker than the temperature dependence of homogeneous

nucleation rates. This result is not surprising since spherulitic growth is

believed to be controlled by coherent nucleation on already-formed crystal

faces, and not governed by homogeneous nucleation processes.

Measurements of the temperature coefficients of the heterogeneous

nucleation rates have permitted the calculation of Ao and (a - a ) values

for substrates in the crystallization of PB1 (Table XXIX) . The values of

(om - oc ) can be used to compare the nucleating ability of the indicated

substrates. Furthermore, the substrate-melt interfacial energy(0 ) can

be obtained from measurements of the contact angle of a drop of the melt on

the substrate. This will enable one to obtain absolute values of a , thec

substrate-crystal interfacial energy. To the author's knowledge, values

of ochave neither been directly measured nor indirectly calculated so far.

Glicksman and Childs119

also obtained values of (c - a ) fromm c

experimental studies of the heterogeneous nucleation of metallic tin from the

melt on inorganic substrates, namely metals, metallic oxides, carbides

and sulfides. They used the spherical cap model of the crystalline phase

nucleated on a plane substrate and experimentally measured the critical

supercooling necessary to induce a pre-estimated sensible nucleation rate.

Their calculated values of (a - a ) were in the range of 35-58 erg/cm2

.

The values indicated that in the crystallization of tin , metallic substrates

are better nucleants than metallic oxides, carbides or sulfides.

The nucleation density data (Tables yttt aUables XI" through XIX) are applicable for one temperature and one ^ ^

obtained from the temperatoe ^rates, the effects of both time and temperature are taken^^ ^

terizing the nucleating ability of =:iih«t,- a .„.my substrates compared to either nucleationdensities or nucleation rates by themselves.

in the present study the rates of heterogeneous nucleation havebeen measured by counting the number of spherulites every minute. Moreaccurate nucleation rates will be obtained by counting the number of

spheruHtes at smaller intervals of time. e.g. 10 Seconds. from motionPicture frames. This technique will enable one to investigate in detail

«he reproducibility of the nucleation density vs. time curves on repeatingthe crystallization experiment. Such a study by Abbs with the isotachc

polypropylene-isotactic polystyrene system is in progress now 120.

The present study has indicated the existence of a fixed numberof active nucleating sites on substrate surfaces. The work of Zettlemoyer

and his coworkers 121 indicated isolated nucleating sites on the surface of

silver iodide crystals which have been used as nucleating agents for ice

formation. They showed that silver iodide surface is largely hydrophobic,

but has hydrophihc sites on it. These hydrophilic sites are responsible

for the nucleation of ice crystals.

The existence of localized active nucleating sites on a substrate

thus appears to be a general phenomenon. The distinguishing feature of

80

these active sites is only a matter of speculation at this writing. It is also

unknown whether the nature of the active sites for polymer crystallization

is the same as or different from that of the active sites responsible for the

nucleation of low molecular weight substances. During the present study

direct light microscopic observations did not reveal any distinguishable

physical feature characterizing the active sites. Four speculations as to

the nature of the active nucleating sites will be offered now.

First, steps and dislocations on the substrate surface may be

responsible for nucleation. Secondly, the active sites may have cracks or

fissures inside which crystalline residues can survive at temperatures suf-

ficient to melt the polymer on the rest of the substrate, in accordance with a

theory due to Turnbull83

. On supercooling the melt, the crystalline resi-

dues act as seeds for the nucleation of spherulites. Thirdly, the active

sites may be envisioned to have more wettability (by the crystalline

embryo) compared to the inactive sites. In other words, (a - a ) valuesHi C

differ from one site on the substrate to another. Sites characterized by

(om "°c ) values lower than som e critical value are inactive. Finally, the

active sites on polymer substrates may have chain conformations which are

conducive to nucleation. Results of the present study do not favor any of

the four speculations.

This chapter will be concluded with some remarks on the poly-

morphism of poly (butene-1), 1,4-trans polybutadiene , and nylons 6, 66

and 610.

Three crystalline polymorphs of PB1 have been reported98

'122-127

,

namely forms I (rhombohedral) , II (tetragonal) , and III (orthorhombic),

81

with molting points of 135.5°, 124°, and 306. 5°C, respectively 127. On

crystallization from the melt, form II is formed first; it slowly transforms

into form I which is the more stable form124

' 126 - 127 - 128 o• torm I may be

taken to be responsible for the behavior of FBI substrate in the crystalli-

zation of PCL , shown in Table VIII

.

According to Natta and Corradini129

, a first order crystal-

crystal transition takes place in 1 , 4-trans polybutadiene (TPB) at 67°C.

The high temperature form is characterized by a packing mode in which

the macromolecules have increased freedom of rotation about the chain

axes, compared to the low temperature form. A density decrease of more

than 9% accompanies this phase change.

While studying the nucleating ability of TPB in the crystalliza-

tion of PEO (Table VII), the pair of polymers was first heated at 80°C for

5 minutes to melt the PEO, and then the temperature was dropped rapidly

to 51°C, in order to crystallize PEO. The low temperature form of TPB can,

therefore, be taken to be responsible for the behavior shown in Table VII.

According to Holmes, et al.130

, most samples of nylon G contain,

in addition to the normal crystalline form (a) , varying amount of another

crystal form (p) . These two forms differ in the side-by-side packing of

131hydrogen-bonded sheets . The p form is unstable and the transforma-

tion p -» a occurs almost completely by immersing nylon 6 in boiling water

1 30for 5 or 6 hours

. During the present study, the nylon 6 samples used

as substrates were heated in an oven at 105°C overnight to remove moisture

Based on these facts, the nylon 6 substrates used may be taken to contain

the a crystalline form almost completely.

82

According to Bonn and Garnor 132, fibers of nylon 66 also con-

tain two different crystalline forms: a and (3. These two forms involve

different packings of geometrically similar molecules 132•1 33

. Most

nylon 66 specimens contain predominantly the a form. These remarks

apply for nylon 610 also41,132

.

The above is a summary of information from the literature which

is relevant as to which crystalline polymorphs of the polymers used were

responsible for the results obtained in the present study. It should be

kept in mind that the substrates used were in the form of unoriented films

which were spherulitic for crystallizable polymers. Spherulitic specimens

are known to contain amorphous material also. The degrees of crystal-

linity of the substrates were not determined. The degree of crystallinity

of the substrate was increasing due to annealing, as the other polymer was

being melted and crystallized.

83

CHAPTER VIII

SUMMARY AND CONCLUSIONS

Summary

1.

Heterogeneous nucleation of isotactic polypropylene (iPP)

,

poly(butene-l) (FBI),poly (ethylene oxide) (PEO)

, and polycaprolaetone

(PCL) crystals have been studied using hot stage polarizing microscopy

coupled with still and motion photography. The polymers have been crys-

tallized from the melt in contact with various substrates (mostly polymeric) .

Attention has been paid to the morphologies of the growing polymer induced

by substrates. The morphologies have been used to characterize and com-

pare the nucleating ability of substrates qualitatively. The evolution of

spherulites on the substrate (as well as in the bulk polymer) has been

directly observed and studied in order to characterize the nucleating

ability of substrates quantitatively.

2. Heterogeneous nucleation in polymer crystallization is a

selective process. A given substrate has different nucleating abilities

for different crystallizing polymers. Also, different substrates have dif-

ferent nucleating abilities for the same crystallizing polymer.

3. The mere presence of a foreign surface in a supercooled

melt is not enough for nucleation to occur.

4. Similarity in chemical structure of the substrate and the

crystallizing polymer is no certain criterion for nucleating ability.

5. Similarity of crystallographic unit cell type of the substrate

and the crystallizing polymer cannot be correlated with nucleating ability.

Close match of the lattice parameters of the substrate and the crystallizing

polymer is not a necessary condition for strong nucleating action

.

6.

Crystallinity is a necessary but not sufficient condition for a

substrate to be a strong nucleating agent.

7. Surface energy of a substrate does not dictate its nucleating

ability. Low energy surfaces of polymers are capable of inducing trans-

crystalline nucleation of polymer crystals.

8. Substrates can be classified into the following three groups,

based on their nucleating ability: highly active (Type I), moderately

active (Type II),and inactive (Type III) . Substrates of Type I are able to

induce transcrystalhnity. In this case nucleation is favored on the sub-

strate. With a Type II substrate, nucleation occurs on the substrate as

well as in the bulk polymer, without any preference for either. With a

Type III substrate, nucleation is favored in the bulk polymer.

9. The morphology of a crystallizing polymer induced by a sub-

strate is temperature dependent. The morphology changes from transcrys-

talline to spherulitic on increasing the crystallization temperature. The

reduced nucleation density at the higher temperature is the result of lower

nucleation rate at smaller supercooling, in accordance with the theory of

heterogeneous nucleation.

10. At intermediate temperatures, that is in the temperature

range where the transition from transcrystalline to spherulitic surface

morphology takes place, one observes mixed surface morphology (trans-

crystalline plus spherulitic) . Three cases of the latter are observed,

namely (i) transcrystalhnity. predominating, (ii) transcrystalline and

85

spherulitic surface morphology in equal proportions, and m spherulWcsurface morphology predominating

.

11.

Well-developed transcrystalline morphology is ohserved onlyin a certain range of crystallization temperatures. The range for iPP is

120-130°C, and for PCL it is 48-52°C.

12. In the early stage of the development of transcrystallinity in

polycaprolactcne.sporadic nucleation on the substrate ,high density poly-

ethylene) has been observed.

13. The linear growth rates of the transcrystalline region in iPP

and PCL are equal to the radial growth rates of bulk-nucleated spherulites.

The transcrystalline growth rate is independent of the substrate which is

responsible for the transcrystalline nucleation.

14. The melting temperatures of transcrystalline iPP and PCL

are identical with those of the bulk-nucleated spherulites.

15. The high nucleating ability of isotactic polystyrene samples

in the crystallization of isotactic polypropylene, as evidenced by the genera-

tion of transcrystallinity, is due to the polymer (iPS) itself and not due to

the Ti and Al impurities in the sample of iPS . Ti and Al contents (residues

of Ziegler-Natta catalysts) of iPS samples can be reduced by two methods:

(a) by fractionating the polymer, and (b) by chelating Ti and Al with

acetylacetone

.

16. Nucleation densities on substrate and in bulk have been

determined by counting the number of spherulites nucleated under the

polarizing microscope. The nucleation density on the substrate (n ) is

5 2S

of the order of 10 /cm . In cases where transcrystallinity is generated,

86

n is of the order of 106

- i n7 /rm2 TK „ , .eroiAU 10/cm •

The nucleation density in the bulkpolymer (n, ) is of the order of m 10 /™ 3 *h )

me oraer ot 10 /cm, for commercial iPP, PB l and PEO

samples. These data were obtained for the crystallization of iPP (i 2 50c/

3 min, 129°C/14 min),PBl (85.5°C/9 min, 88°C/9 min) and PEO (50°C/

3 min, 51°C/5 min

, 53°C/34 min) .

17.

Bulk nucleation densities vary from one volume element of

the crystallizing polymer to another. The volume elements under observa-

tion were of the order of l(f9cm 3

. Coefficients of variation of 20-40% are

typical. The variation from one volume element to another is larger than

that within a given volume element with the crystallization experiment

being repeated.

18. Nucleating ability of substrates can be compared using the

ns/n

bratio as an index. However, for a given substrate, n /jl ratios

s bvary from one sample of a crystallizing polymer to another.

19. Repetition of the crystallization of PEO with the same volume

element under observation does not produce identical nucleation density

(bulk) vs. time curves. However, no definite trend showing progressive

deactivation of nuclei was noticeable with this polymer, the number of

spherulites being counted every minute.

20. In the crystallization of polymers in contact with substrates,

there exists a fixed number of nucleating sites on the substrate. Similarly,

in the crystallization of the bulk polymer, a fixed number of heterogeneities

are responsible for nucleation at each temperature.

21. Pseudohomogeneous nucleation has been observed on sub-

strate and in bulk, in the crystallization of iPP, PBl and PEO.

87

22. The shape of the experimental nucleation density vs. time

curve has two variations, namely (i) curve with an initial linear part, and(ii) S-shaped curve.

23. Experimental heterogeneous nucleation rates of PB1 at

85-88°C are of the order of 105/min/cm 2

on substrates and 109-10 10/min/cm 3

in bulk. The coefficient of variation of bulk nucleation rates has been calcu-

lated to be 12-16%. For PEO crystallization at 51°C, measured heterogeneous

nucleation rates are of similar orders of magnitude, while the coefficient of'

variation of bulk nucleation rates has been calculated to be 56%.

24. The heterogeneous nucleation rates in the crystallization of

PB1 (on iPS and iPP substrates and in bulk) increase by a factor of about 5

for a 10°C increase in supercooling. The spherulitic growth rates of PB1

have comparable temperature coefficients. These temperature coefficients

are much lower than those of homogeneous nucleation rates.

25. Experimentally-measured temperature coefficients of hetero-

geneous nucleation rates of PB1 yielded values of the interfacial energy

parameter aaeAa. The temperature coefficients of spherulitic growth

rates yielded the value of ca • By division, values of Aa (= o + a - a )e cmwere obtained for different substrates. The values of Ao, in turn, yielded

values of (aQ

- om ) . The quantity aQ

is the substrate -crystal interfacial

energy, while om is the substrate-melt interfacial energy. Higher values

of (am - oQ

) indicate a better heterogeneous nucleating agent. Absolute

values of o may be obtainable by further measurements of o by contactu m J

angle techniques.

88

Conclusions

1.

Morphologies of crystallizing polymers induced by substratescan be used to characterize the nucleating ability of substrates qualitatively

The morphologies are temperature dependent.

2. Experimentally-determined nucleation densities, nucleation

rates and the interfacial energy parameters appearing in the theory of

heterogeneous nucleation can be used to characterize the nucleating ability

of substrates quantitatively. The third approach is preferred, since the

effect of both time and temperature are taken into account

.

3. Similarity in chemical structure and in crystallographic unit

cell type of the substrate and the crystallizing polymer are no certain cri-

teria for nucleating ability. Close match of the lattice parameters of the

substrate and the forming crystal is not a necessary condition for strong

nucleating action.

4. The surface energy of a substrate does not dictate its nuclea-

ting ability. Low energy surfaces of polymers are capable of inducing

transcrystalline nucleation of polymer crystals.

5. Crystallinity is a necessary but not sufficient condition for a

substrate to be a strong nucleating agent.

6. In the crystallization of polymers in contact with substrates,

there exists a fixed number of nucleating sites on the substrate. When

these nucleating centers become active successively during crystallization,

pseudohomogeneous nucleation on the substrate obtains.

7. The high nucleating ability of isotactic polystyrene samples

in the crystallization of isotactic polypropylene, as evidenced by the genera-

tion of transcrystallinity, is due to the polymer (iPS) itself. Titanium and

89

alundnum contents (residues of Zie gler-Natta catalysts) of ips ^be reduce, by two methods: W by fracUonatog fte^ ^ ^chelating Ti and Al with acetylacetone.

8. The growth rate and the melting temperature of transcrystal-hne polymer are identical with those of bulk-nucleated spherulites.

90

CHAPTER IX

SUGGESTIONS FOR FURTHER RESEARCH

In the present study the number of nuclei was counted by visual

observation of spherulites. When transcrystallinity is generated this tech-

nique is insufficient due to the crowding of spherulites on the substrate.

Motion pictures of the transcrystalline nucleation process will permit more

accurate determination of nuclcabon densities in the transcrystalline region

as well as their time dependence.

Molecular weight of the crystallizing polymer does not enter

directly as a parameter in the theories of heterogeneous nucleation of

polymer crystallization. Furthermore, the theories treat the substrate as

a plane surface and no distinction is made as to whether the substrate is

polymeric or not. It seems desirable to generate experimental data on the

effect, if any, of molecular weight of the crystallizing polymer and of the

substrate on the nucleating action.

One study of the effect of molecular weight of the crystallizing

polymer on the nucleating effect of a low molecular weight substance has

26been reported. A hundred-fold change in the molecular weight of

crystallizing iPP resulted in a negligibly small change in the nucleating

ability of basic aluminum dibenzoate . The effect of molecular weight of the

crystallizing polymer on the nucleating effect of a polymer substrate should

also be investigated.

91

The present study has indicated that probably the molecular weightof a polymer substrate does influence its nucleating ability. The molecular

weight of the substrate can affect its crystallization kinetics from the

quenched state and hence the degree of crystallinity of the substrate.

Further research in this direction is recommended.

The role of the adsorption of the chain molecules in the melt on

substrates in nucleation phenomena should be clarified. It is felt that for a

better understanding of the heterogeneous nucleation processes, it should

be inquired whether the nature of the adsorbed layer can be related to

the activity of the nucleating surface , and how far the conformation of

adsorbed molecules is conducive to nucleation.

From this study the crystallinity of the substrate has emerged as

an important factor in the heterogeneous nucleation of polymer crystals.

The dependence of the nucleating ability of polymer substrates on their

degree of crystallinity should be quantitatively studied. Also, the effect

of the nucleation density (number of spherulites per cm 2) of the substrate

and the orientation of the chain molecules on the substrate on the nucleatin

ability of the substrate should be investigated. The effect of orientation can

be studied by using stretched polymer films as substrates. The possibility

of epitaxy in such cases should also be investigated.

The bulk nucleation densities reported in the present study include

the contribution from nuclei generated by the glass surfaces. It is recommen-

ded that the effect of the glass surfaces be determined by performing crystal-

lization experiments at different thicknesses of the crystallizing polymer.

ao

92

During the course of repeated crystallization on a substrate in

this study, it seemed that spherulites reappeared at the same positions.

This point should be clarified.

On repeating the crystallization of PEO, identical nucleation

density (bulk) vs. time curves were not produced (Figure 41) . The first

crystallization produced the highest slope and the coefficient of variation

of the nucleation rates was 63% (Table XXII) . It is suggested that repeated

crystallization experiments be performed with other polymers in order to

clarify whether any definite trend exists or not. Nucleation both on sub-

strates and in bulk should be studied.

From measurements of temperature coefficients of heterogeneous

nucleation rates of PB1,values of (om

- oq

) have been obtained. It is recom-

mended that experiments on wettability of substrates by PB1 melts be per-

formed so that values of am can be calculated. In this way, absolute values

of acwill be obtained. Such data have not been reported so far, to the

author's knowledge.

The present study has shown the ability of polymer substrates

to induce transcrystallinity in other polymers. One can envision the

development of composite fibers using the technique of growing one polymer

onto another, e.g. iPP onto PET or nylons 6, 66, or 610. Such fibers will

have contributions from the properties of both polymers. The feasibility

of the process and the usefulness of the final materials should be investigated.

Heterogeneous nucleation by the dispersed phase of polyblends is

expected to influence their crystallization kinetics, morphology and physical

properties. Quantitative study of this area is recommended. Finally, a

93

systematic study of the role of contact surfaces in the manufacture of polymerfilms will help in controlling and improving film properties.

A comparative study of the nucleating ability of substrates with trans-

crystalline and spherulitic morphologies, respectively, will throw light on

the role of morphology of a substrate on its nucleating ability. Also, during

the present study, cut surfaces of polymer films and metals were used as

substrates. Fracture surfaces should also be studied as nucleating sub-

strates.

Corners and cavities in crystals are high energy sites and are con-

sidered to have significant nucleating potential134

. A study of the role of

crystal defects on the nucleating ability of substrates is recommended, with

the hope that a better understanding of the dependence of the nucleating

ability of substrates on their physical features will evolve.

94

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103

TABLE I

Material

MATERIALS USED

Source Characterization Data1. Isotactic —

Polypropylene(iPP)

(a) Shell 5520 Shell Chemical Isotactic content: -95%Company

M.Pt. : 170. 3°C

Ash Content : 1.6 5%

Molecular Weight:

Mn

M\

Mw

v

426,00

337,000

281,000

(b) Pro-fax 6323 HerculesInc.

Additives (stabilizers)::

(i) thioester : 0.15% (wt)(dimyristyl-thiodipropion-ate)

(ii) hindered phenol: 0.1% (wt)(3,5-di-t-buty1-4-methyl-phenol )

(iii) calcium stearate : 0 . 1%(wt)

Isotactic content: 96-97

M.Pt. : 171. 8°C

Ash content : 3.57%

Intrinsic viscosity in de-calin at 135°C:1.8 dl/g.

Mv 209,000

(Cont.

)

104

Material

(c) Profax 6509

(d) Pro-faxX 12886-38

(e) PR 84919

2. Poly (ethyleneoxide) (PEO)

Carbowax 20-M

3. Poly (butene-1

)

(PB1)

(a) Petrotex14988-1(for all theentries inTable VI ex-cept thelast two)

(b) Vestolen BT6000(for the lasttwo entriesin Table VI)

TABLE I

(cont.

)

Source Characterization Data

HerculesInc

.

HerculesInc

.

HerculesInc.

Union CarbideCorp.

Petrotex Co

Hiils Co.

Isotactic content: 96-97%

Isotactic content: 96%

Ash content: 0.76%

Mw

"7 X 10

Melt index: 0.35

M.Pt. 61.3°C

Molecular Weight

Mn 26 ,600

Mw : 395,000

Ash content: 0.90%

M.Pt. : 134. 3°C

(Cont.

)

105

Material

4. Polycaprolac-tone (PCL)

[<CH2

)5-C-0-]

n

TABLE I

(cont.

)

Source Characterization Data

PCL- 700

5. IsotacticPolystyrene

(iPS)

(a) iPS-MPL

(b) iPS-MPL-U-1

(c) iPS-MPL-AA-1

Union CarbideCorp

.

Monomer Poly-mer Labora-tories

, BordenChemical Co.

This sample wasrecovered fromthe centrifugatewhen a toluenesolution of iPS-MPL was centri-fuged (details insection 5.3.6.1of Ch. v)

This sample was ob-tained when iPS-MPLwas treated withacetylacetone toreduce Ti and Alcontent (details insection 5.3.6.2 ofCh. v)

M.Pt.

Mw

64.2°C

40,000

Chemical analysisTiAlAsh

Ash

0.60%0. 36%2. 07%

2.17%

Chemical AnalysisTiAlAsh

0.26%0.05%1.24%

(Cont.

)

106

fd

-Pcd

QcoH-Pfd

N•H

CD

-POfd

(d

x:u

IS

<D

o >. c•H p <DCO -H pC CO H•H 0 0M u 4J-P 03

C •H CH > •H

«—

<

r-—

«

co

<•

u

uooco

-P

OrHXCN

CM

lolo

00

CN

OHXLO00

i

o

LO

CO

LOCO

•PC0)

-p

coo

o•HPofd

-P

0CO

co

•Ho COo >1o rHrd

LO CiH fd

LOrHfd

«

•

0•HgCD

u

G LOfi Q.CN

HHOV V

"H rH CO

Eh r< <

-PC0u

HwrJ

<

-Pcou

<d

uu

oCO

4-1

o

u

o

-p

<D

cn

CD

4-)

0)

rH -P

13

fd o o0) CD

-p o.CO CO

u-H

H

O•H-PO I

fd co

r-3

03

co >

ftH

CO MHCD CO rH O

O fd roH -P •

-P CD vofd u ^ •

fd w roco UCD M-i > i LOH rHtt-p a) c2 co > ofd fd -h -hCO rH -p -P

>1cfd

e0u

fd

ah

u

0a

fd

ouo•HU-Pua)

w

a)

c<D

to

Hna)

rd«-<

H >

fa fal l

& &I I

CO COft ft•H -H

oQI

CO

ftH

00CNrH

I

oro

ftw

>1—x: <d

-p rdCD -H6 XH O*V

I 0)

^> C- CD

CN rH

rH CD

o &

L£>

ro

u o

7T1

u

uoft

ft

107

TABLE I

(Cont.

)

Material Source Characterization Data

Poly [3,3-bis(choromethyl)oxacyclobutane]

CH-C11

2

(0-CHo-C-CHo -)/-I 2 n

CH2C1

•Penton" HerculesInc

.

8. Polyoxymethy-lene (POM)

(CH9-0)

B 5849 DuPont Co.

9. Poly (chlorotri-f luoroethylene)

(CCIF-CFJ2 n

'Kel-F

'

KF-6060 3M Co. M . Pt . . 2 12°CDegree of Polymerization: 1600

10. Low density (Molecular weight: 1,85,600)Polyethy-lene (LDPE)

Monsanto 8011 Monsanto Density: 0.925 g./cm3

Co.

11. High densityPolyethylene

(HDPE)

Marlex TR-885 Phillips Density: 0.965 g./cm 3

Petro- m : <110,000leum Co. _w

M /M : <7w n

Ash: 0.085%(Cont.

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108

TABLE I

(cont.

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Material Source Characterization Data

13. Atacticpolystyrene

14 . Nylon 6

15. Nylon 66Zytel 101

16. Nylon 610Zytel 38

16. Aluminum

Monsanto Co.

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DuPont Co.

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TABLE IX

REDUCTION OF TITANIUM AND ALUMINUM CONTENT*? op tc™POLYSTYRENE SAMPLE BY TREATMENT S»^1S^tSS?^2) C

CodeAl °-

Untreated sample of iPS

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TABLE XI

FRACTIONATION OF ISOTACTIC POLYSTYRENE(1PS-MPL) OF ASH CONTENT 2?07%

Ash %

11.80

6. 92

1.52

0.51

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gSberment »yer of^

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151

CAPTIONS FOR FIGURES

Gibbs free energy (AG) as a function of the radius (r) of a spherical

nucleus.

A cylindrical nucleus of radius r and length 1 , with interfacial

energies o and a •

e s

Model for a small crystallite of low molecular weight substance.

Heterogeneous nucleation of a spherical cap of phase p from

phase a on a flat substrate s (0 is the contact angle) .

Retention of embryo in cylindrical cavity.

Model for the nucleus in polymer crystallization with regular folding

and adjacent re-entry

.

Nucleation rate curves (schematic) for homogeneous (A) , hetero-

geneous (B) , and pseudohomogeneous (C) nucleation.

Model of growth nucleus on spherulite boundary, illustrating

nucleation control of spherulite growth.

Overall view of photomicrographic equipment: (a) drill press stand,

(b) Zeiss polarizing microscope, (c) eyepiece to shutter connector,

(d) binocular eyepiece, (e) hot stage for sample mounting, (f)

temperature control unit for the hot stage, (g) shutter with cable

release, (h) wooden box,

(i) viewing telescope , (j) solenoid,

(k) camera, (1) intervalometer , and (m) A.C.-D.C. converter for

solenoid

.

Schematic diagram of the direct current source for the solenoid.

Side view of rectifier.

152

11. Arrangement for sample mounting before crystallization (top view) .

(ABDC: substrate; ABFE, CDHG: polymer to be melted and crystal-

lized)

12. Experimental arrangement (top view) for counting the number of

nuclei on the substrate (area AB) and in bulk (volume ABCA) .

13. Light micrograph showing transcrystalline surface morphology of

iPP crystallizing in contact with iPS (ab) (substrate type I) at 130°C

(time 15 min. ) .

14. Spherulitic surface morphology of iPP crystallizing in contact with

Kel-F or poly (chlorotrifluoroethylene) (substrate type II) at 125°C

(time 9 min. ) . (abed: Kel-F overlaid with iPP

.

)

15. Nylon 610 as an inactive substrate (type III) in the crystallization of

PEO at 50°C (time 2 min.)

16. Effect of crystallization temperature (T) on the surface morphology

of iPP crystallizing in contact with iPS: (i) T = 125°C (time 4 min.)

;

ab: iPS overlaid with iPP; ac: transcrystalline region; (ii) T =

130°C (time 15 min.) ; ab: iPS; ac: iPS overlaid with iPP; ad:

otranscrystalline region, (iii) T = 133 C (time 42 min .) ; abca: iPS;

abd: iPS overlaid with iPP . (iv) T = 135°C (time 84 min. ) .

17. Transcrystalline surface morphology of crystallizing iPP generated

by nylon 6 substrate at 125°C (time 4 min.) .

18. * Transcrystallinity of iPP crystallizing in contact with nylon 66 at

125°C (time 3 min.) . (abc: nylon 66)

19. Transcrystallinity of iPP crystallizing in contact with nylon 610 at

125°C (time 4 min.)

153

Transcrystallinity of iPP crystallizing in contact with Penton or

poly[3,3-bis(chloromethyl) oxacyclobutane] at 125°C (time 12 min.)

Spherulitic surface morphology of iPP generated in contact with

poly(2,6-dimethyl phenylene oxide) or PPO at 125°C (time 4 min.) .

(ac: PPO overlaid with iPP)

Transcrystallinity of PCL generated in contact with nylon 6 at 50°C

(time 2 hr. 57 min.) . (abc: nylon 6)

Transcrystalline morphology of PCL induced by nylon 66 substrate

at 50°C (time 3 hr. 22 min.) . (abed: nylon 66)

Transcrystalline morphology of PCL generated in contact with poly-

oxymethylene (POM) at 50°C (time 2 hr. 43 min.). (ab: POM-PCL

interface)

Spherulitic surface morphology of PCL crystallizing in contact with

gold (abed) at 50°C (time 2 hr. 42 min.) .


polyethylene terephthalate (PET) at 50°C (time 2 hr. 42 min.) .

(abc: PET overlaid with PCL)


poly(butene-l) (PB1) at 50°C (time 2 hr. 52 min.) . (abc: PB1

overlaid with PCL)


Kel-F at 50°C (time 2 hr. 35 min.) . (abc: Kel-F overlaid with PCL)

Isotactic polypropylene as a moderately active substrate (type II)

in the crystallization of PEO at 50°C (time 1 min. )

.

30. Nylon 6 as an inactive substrate (type III) in the crystallization of

PEO at 50°C (time 1 min.) .

31. Nylon 66 as an inactive substrate in the crystallization of PEO at

50°C (time 1 min. ) .

32. Crystallization of iPP at 125°C in contact with: (i) iPS (time 2 min.)

:

transcrystalline surface morphology, (ab: striation on iPS film)

(ii) atactic PS (time 5 min.) : spherulitic surface morphology,

(ab: atactic PS-iPP interface; abc: atactic PS overlaid with iPP)

33. Crystallization of PCL at 50°C in contact with: (i) iPS (time 1 hr.

53 min.): transcrystalline surface morphology, (ii) atactic PS

(timelhr. 15 min.): spherulitic surface morphology . (ab:

atactic PS overlaid with PCL; cad: atactic PS-PCL interface) .

34.. Crystallization of PCL at 50°C in contact with: (i) High density

polyethylene (HDPE) (time 2 hr. 50 min.): transcrystalline surface

morphology (abc: HDPE) . (ii) Low density polyethylene (LDPE)

(time 2 hr. 50 min.): spherulitic surface morphology, (abed:

LDPE)

.

35. Ratio of nucleation densities (substrate to bulk) in the crystallization

of iPP in contact with indicated substrates at 125°C (3 min.) and

129°C (14 min.) ; data of Table XIII. (Actual heights for iPS and

nylon 610 substrates are greater than the indicated heights.)


of PB1 in contact with indicated substrates at 85.5°C (9 min.) and

88°C (9 min.); data of Table XIV

.

155


of PEO in contact with indicated substrates at 50°C (3 min.), 51°C

(5 min. ) and 53°C (34 min

. ) ; data of Table XV .

38. Appearance of spherulites on substrate (high density polyethylene)

in the early stage of the formation of transcrystallinity in PCL at 50°C

at (i) 9 min., (ii) 14 min. , (iii) 19 min. , and (iv) 24 min. (ABCD:

HDPE; AED: supercooled PCL) .

39. Nucleation density on substrate as a function of time in the crystal-

lization of iPP in contact with nylon 66 at three crystallization tem-

peratures.

40. Nucleation density in bulk as a function of time in the crystallization

of PB1 at 85.5°C.

41. Bulk nucleation density as a function of time for repeated crystalliza-

tion of PEO at 51°C.

42. Ratio of nucleation rates (substrate to bulk) in the crystallization of

PB1 at 85.5°C and 88°C in contact with indicated substrates; data of

Table XX.

43. Ratio of nucleation rates (substrate to bulk) in the crystallization of

PEO at 51°C in contact with indicated substrates; data of Table XXI.

44. Dependence of the growth rate (G) of PB1 spherulites on supercooling

(AT) . (T = crystallization temperature; straight line drawn by

least square technique.)

45. Nucleation density on substrate as a function of time in the crystal-

lization of PB1 in contact with iPS , at indicated temperatures.

156

46. Heterogeneous nucleation rate (on iPS substrate) as a function of

supercooling in the crystallization of PB1;*N represents the slope

measured from Figure 45. (Straight line drawn by least square

technique.

)

47. Nucleation density on substrate (iPP) as a function of time in the

crystallization of PB1 at indicated temperatures.

48. Heterogeneous nucleation rate (on iPP substrate) as a function of

supercooling in the crystallization of PB1; N represents the slopes

measured from Figure 47. (Straight line drawn by least square

technique.

)

49. Bulk nucleation density as a function of time in the crystallization

of PB1 at indicated temperatures.

50. Heterogeneous nucleation rate (bulk) as a function of supercooling

in the crystallization of PB1; represents the slope measured from

Figure 49. (Straight line drawn by least square technique.)

AG

ZlG

t

157

Fig. 1. Gibbs free energy as afunction of the radiusof a spherical nucleus.

Fig. 2. A cylindricalnucleus.

Fig. 3. Model for a small crystallite of lowmolecular weight substance.

Fig. 4. Heterogeneous nucleation of a spherical cap

of phase from on a flat substrate s.

158

j<

fs

0

Fig. 5. Retention of embryo in cylindrical cavity

Pig. 6. Model for the nucleus in polymer crystallizationwith regular folding and adjacent re-entry.

Time

Pig. 6a. Nucleation rate curves (schematic) for homogeneous(A), heterogeneous (B) , and pseudohomogeneous (C

)

nucleation.

Pig. 7. Model of growth nucleus on spherulite boundary,illustrating nucleation control of spherulite

growth.

160

Figure 8

161

Light Dimmer

Maleplug

Rectifiers (to solenoid)

Figure 9

o

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Cathode

Figure 10

162

microscope slide

glass cover slip

polymer to be crystallized

substrate

Figure 11

Crystallizingpolymer

Substrate

Figure 12

163

Figure 13

Figure 14

164

Figure 15

166

Figure l6(Cont.)

Figure 17

Figure 18

Figure 20

170

Figure 23

Figure 26

173

Figure 29

Figure 30

(ii)Figure 32

176

Figure 33

Figure 34

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132

CRYSTALLIZATION TIKE, WIN.

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Heterogeneous nucleation in the crystallization of high ...

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