Journal of Colloid and Interface Science 251, 78–84 (2002)doi:10.1006/jcis.2002.8383

Emulsion Polymerization Synthesis of Cationic Polymer Latexin an Ultrasonic Field

Melanie Bradley and Franz Grieser1

Particulate Fluids Processing Center, School of Chemistry, The University of Melbourne, Victoria, 3010, Australia

Received November 11, 2001; accepted March 23, 2002

Poly(methyl methacrylate) and poly(butyl acrylate) lattices havebeen synthesized under ultrasonic irradiation in the presence ofa cationic surfactant, dodecyltrimethylammonium chloride. Thepolymerization of oil-in-water emulsions of monomeric species wascarried out at 30◦C (±5◦C) in the absence of a chemical initiator.The lattices were formed as stable dispersions with particle diame-ters spanning the range of 40–150 nm and with polymer molecularweights greater than 106 g mol−1. The results obtained strongly sup-port a polymerization process involving a miniemulsion system, inwhich continuous nucleation of particles takes place throughout themonomer to polymer conversion reaction. C© 2002 Elsevier Science (USA)

INTRODUCTION

Synthetic latex is conventionally prepared by an emulsionpolymerization process (1). The main ingredients for conduct-ing these polymerizations are a water-insoluble monomer, anaqueous-phase-containing surfactant, and, commonly, a water-soluble radical initiator. In a batch emulsion polymerization, allingredients are added at the beginning of the polymerizationand an oil-in-water emulsion is formed. Polymerization is usu-ally initiated by heating the mixture, as this causes the thermaldegradation of the chemical initiator to produce free radicals thatreact with monomer. Chain polymerization then leads to the for-mation and growth of latex particles. These polymer particlesform a colloid stabilized against coalescence by surfactant thatis present on the surface of the particles.

There are some disadvantages in the use of a chemical initiatorin an emulsion polymerization in as much as it can affect the la-tex particles formed by altering molar mass, polydispersity, andthe colloid stability of the latex. In addition, some of the prod-ucts that are formed by aqueous phase termination of the initiatormay undergo subsequent reactions, resulting in discoloration ofthe final latex or any residual initiator present after polymer-ization may act as an undesirable contaminant. These problemscan be avoided by employing techniques that do not require achemical initiator. Thermal polymerization is one such processin which monomer is converted to polymer by thermal energy

1To whom correspondence should be addressed.

780021-9797/02 $35.00C© 2002 Elsevier Science (USA)All rights reserved.

alone. This type of polymerization generally requires substantialthermal energy and the radical production is restricted to specificmonomer structural functionalities. Another method of radicalinitiation is by the use of ionizing radiation. Gamma radiationor X-rays generate hydrogen and hydroxide radicals as the ini-tiating species via the radiolysis of water. The disadvantage ofthis method is largely associated with the substantial cost in theestablishment and maintenance of the radiation source.

Relatively recently, a new method for initiation of emulsionpolymerization was discovered using ultrasound (2). Insonationis widely employed in standard emulsion polymerization prepa-ration as a means of homogenization since sonication is an ef-ficient method of mixing and dispersing the monomer phasepreceding the reaction being initiated by an added chemical ini-tiator. Ostroski and Stambaugh (3) were the first to find thatsonication significantly accelerated the rate of a conventionalemulsion polymerization. It was postulated that the accelera-tion was caused by the enhanced decomposition of the chemicalinitiator in aqueous solution, as a result of faster and better emul-sification stemming from ultrasonic induced agitation.

Insonation actually plays a duel role in liquid systems. Thefirst is to cause large shear gradients in the interfacial regionof the cavitation bubbles that result as a consequence of thetransmission of ultrasound through a liquid (4). The second isthe production of radicals. When the ultrasonically generatedmicrobubbles rapidly collapse the gas and vapor within theircore is trapped and this leads to high local temperatures of theorder of 4000 to 5000 K (5, 6) within the bubble and at least1250 K (7) in the liquid immediately surrounding the interfacialregion. In an aqueous medium such high temperatures lead tothe homolysis of water, creating hydroxyl (OH) and hydrogen(H) radicals and the potential for radical chemistry (8).

The synthesis of anionic and cationic lattices by the conven-tional emulsion polymerization process using a chemical initia-tor has been widely reported (9–11). The use of ultrasound toinitiate a free radical emulsion polymerization in the absence ofa chemical initiator is one area of research that has positiveimplications for latex technology. The advantages include theability to conduct polymerizations at room temperature and theelimination of problems usually associated with a chemical ini-tiator. Research to date in the field of sonochemically inducedemulsion polymerization has resulted in the preparation of many

EMULSION POLYMERIZATION SYNTH

types of anionic latex (2, 12–14). Small particle size and highmolecular weight are common features of these particles but lit-tle is known about the mode of polymerization. In the presentstudy we report on the preparation and physical characteriza-tion of poly(methyl methacrylate) and poly(butyl acrylate) latexparticles by the ultrasonic initiation of monomer emulsions sta-bilized by a cationic surfactant.

EXPERIMENTAL SECTION

Reagents

Methyl methacrylate (MMA) and butyl acrylate (BA) (sup-plied by Dulux Australia) were filtered through basic aluminiumoxide twice to remove the inhibitor. The sealed purified samplewas stored at 4◦C until required. Dodecyltrimethylammoniumchloride (DTAC; TCI Chemicals) was used as supplied. Four-stage Milli Q-filtered water was used to prepare all aqueous so-lutions. High-purity argon (BOC Gases) was used without anyfurther purification.

Polymerization

All the ultrasonically initiated polymerization reactions re-ported here were performed using a conventional 19-mm-diameter 20-kHz horn sonifier (Branson 450); the horn wasinserted into a custom-built reaction vessel. The experimentalapparatus is shown elsewhere (2). The power generated at thetip of the horn and adsorbed by the solution was measured bycalorimetry to be between 7 and 9 W/cm2. For all the poly-merizations reported here the concentration of monomer was10% (w/w) in water. The effects of different concentrationsof surfactant (DTAC) on the resultant lattices produced wereinvestigated.

The following description for a polymerization at the abovemonomer concentration and with a DTAC concentration of 0.1 Mis typical of all the polymerizations described in this study. Anemulsion consisting of 7.5 g monomer, 2.0 g DTAC, and 67.5 gwater was thoroughly deaerated by gently purging with high-purity argon for 45 min in the reaction vessel at room temper-ature. Thereafter the argon gas stream was removed from theemulsion and was allowed to pass over the surface of the liq-uid mixture. The polymerization reaction was then initiated bysubjecting this mixture to continuous cycles of 10 min of soni-cation with 5-min pauses between sonication periods. This pe-riodic sonication cycle and the immersion of the reaction vesselin an ice/water bath was sufficient to maintain the temperature at30 ± 5◦C. Small samples of the reaction mixture were removedwith a syringe, via the free inlet, throughout the reaction. Thesamples were left to air dry overnight and then oven dried ata moderate temperature (<80◦C) for 3 h. The error associatedwith this technique is approximately 2%. The dried sampleswere used to monitor the progress of the reaction by gravimet-ric analysis. The final lattices (10 mL) were dialyzed against

water (200 mL) for at least 7 days with regular replacement ofwater.

ESIS OF CATIONIC POLYMER LATEX 79

Polymer Conversion

The percentage of monomer converted to polymer was de-termined by weighing the polymer produced as a function ofsonication time. During sonication small aliquots of the emul-sion solution were removed. These samples were dried leavingonly polymer and surfactant. Based on the assumption that thealiquots were homogeneous, correction was made for the sur-factant in the sample thereby giving the mass of polymer. The“uncorrected” percentage conversion does not take into con-sideration any coagulum formed (small amounts, <1%, of ag-gregated latex formed around the horn) because it was onlypossible to account for this at the end of the polymerizationprocess. With coagulum taken into account the overall conver-sion of monomer at the end of sonication was of the order of85–90%.

Particle Sizing

Particle size distributions and averages were determined pri-marily using a Malvern dynamic light scattering instrument.While capillary hydrodynamic fractionation (CHDF)–1100 par-ticle size analysis was used for comparison.

Molecular Weight Determination

Polymer samples dissolved in HPLC grade tetrahydrofuran(THF) were analyzed for molecular weight using gel permeationchromatography (GPC) using a Waters HMW7 Styragel columnattached to a Waters 410 differential refractometer, and the datawas collected and processed on a Waters Millenium 32 workstation calibrated against polystyrene standards.

Latex Surface Potential

The surface potentials on the latex particles were derivedfrom microelectrophoresis measurements of the particles in so-lution using a Coulter DELSA (Doppler electrophoretic light-scattering analyzer) 440. The measured mobilities were con-verted to zeta potential using the Helmholtz–Smoluchowskiequation (15).

Interfacial Tension

Interfacial tension (monomer/aqueous solution) measure-ments were performed on a FTA 200 interfacial tension deviceusing a pendant drop of oil (monomer). The measurements werecarried out at 20◦C and the instrument was calibrated againsthigh-purity octane that has an oil/water interfacial tension of50.8 mNm−1 at 20◦C (15).

RESULTS

The experimental conversion–time curves for the ultrasoundinitiated emulsion polymerization of MMA and BA as a func-tion of surfactant concentration are shown in Figs. 1 and 2, re-

spectively. All emulsion polymerizations were conducted abovethe critical micelle concentration of DTAC (20 mM) (16). In

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0

20

40

60

80

100

0 10 20 30 40 50 60 70 80

Un

corr

ecte

d %

Co

nv

ersi

on

Sonication Time / minutes

FIG. 1. Ten weight percent MMA monomer conversion as a function ofsonication time in the presence of different concentrations of DTAC: 0.025 M(�), 0.050 M (�), 0.075 M (�), and 0.100 M (�).

Fig. 1 it is evident that the rate of polymerization of MMA isapproximately the same over the concentration range of DTACexamined. However, the conversion–time profiles for BA (Fig. 2)indicate a decreased rate of BA conversion at 0.025 M DTAC.This latter observation coincides with the observed poor emul-sification of the BA system, particularly at this surfactant con-centration, in comparison with the MMA counterpart.

The molecular weight data for the number- and weight-average molecular weights, Mn and Mw, respectively, as wellas the molecular weight distribution (Mw/Mn) for each polymersample examined are given in Table 1. The molecular weightvalues for the latex obtained by ultrasonically initiated emul-sion polymerization are large in magnitude compared to typ-ical latex prepared conventionally. However, it is possible inconventional emulsion polymerizations to achieve similar mag-nitudes in molecular weight if low initiator concentrations are

0

20

40

60

80

100

0 10 20 30 40 50 60 70 80

Un

corr

ecte

d %

Co

nv

ersi

on

Sonication Time / minutes

FIG. 2. Ten weight percent BA monomer conversion as a function of soni-

cation time in the presence of different concentrations of DTAC: 0.025 M (�),0.050 M (�), 0.075 M (�), and 0.100 M (�).

D GRIESER

TABLE 1Polymer Molecular Weight Data as a Function of DTAC

Concentration for Poly(MMA) and Poly(BA) Latex

Molecular weight (g mol−1)

MMA BA[DTAC](mM) Mw(×106) Mn(×106) Mw/Mn Mw(×106) Mn(×106) Mw/Mn

25 2.5 0.8 3.0 1.5 0.4 3.450 2.8 0.9 3.1 2.3 0.7 3.375 2.8 0.9 3.1 2.8 0.7 3.9

100 3.2 1.1 2.9 2.7 0.8 3.6

used (17). The molecular weight results obtained using GPCshould only be taken as a lower limit because the technique haslimited sensitivity at the molecular weight magnitudes obtained.Other analytical techniques are currently being investigatedto better characterize the molecular weight of the polymersproduced.

The final particle size of the polymer latex was examined as afunction of surfactant concentration and the results are listed inTable 2. The particle size of poly(MMA) decreases as the surfac-tant concentration increases, whereas that of poly(BA) remainsthe same, within experimental error. The latex particle sizesobtained sonochemically are smaller than those obtained by con-ventional emulsion polymerization at similar surfactant concen-trations (18). The particle size of the latex was also measured asa function of sonication time at 0.1 M DTAC. For poly(MMA)the particle size is constant throughout the reaction time (Fig. 3).However, poly(BA) synthesis proceeds with a constant particlesize up to 50 min. of sonication after which a dramatic increasein size is observed (Fig. 4).

Latex surface potentials were measured for each surfactantconcentration investigated. The samples were then dialyzed andthe zeta potentials calculated again. The results are presentedin Table 3. The results show that following dialysis the elec-trostatic potential on the latex surface is greatly reduced. Thisis indicative that DTAC is adsorbed at the latex interface ratherthan chemically bound to the polymer and is in equilibrium with

TABLE 2Number Average (dn) and Volume Average (dv) Particle Diam-

eters for Poly(MMA) and Poly(BA) Latex as a Function of DTACConcentration

Particle diameter (nm) (±15 nm)

MMA BA[DTAC](mM) dn dv dn dv

25 63 74 107 11650 56 56 132 15075 37 42 123 138

100 37 41 119 136

E

EMULSION POLYMERIZATION SYNTH

10

20

30

40

50

60

70

20 25 30 35 40 45 50 55 60

dv /

nm

Sonication Time / minutes

FIG. 3. Volume average particle diameter (dv) as a function of sonicationtime for poly(MMA) latex at 0.10 M DTAC. (|) Indicates the error range thatapplies to each data point.

that present in solution. Also, there was a marked difference inthe physical state of the dialyzed MMA and BA samples. Dia-lyzed poly(BA) latex coagulated out of solution forming a poly-mer mass, whereas the dialyzed poly(MMA) latex dispersionformed a gel.

In addition, the interfacial tension of a monomer droplet inmonomer-saturated surfactant solution was measured to gaugethe ease with which formation of the emulsion occurs priorto polymerization. The interfacial tension for pure MMA was17 mN m−1 and that of BA was 24 mN m−1. On addition ofsurfactant, in the range 0.025 to 0.100 M, the interfacial tensiondecreased to 4 ± 1 mN m−1 for both monomer systems. Thesedata suggest that the ease of emulsification of monomer shouldnot be dependent on interfacial tension over the surfactant rangeused in the polymerization experiments.

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80

dv /

nm

Sonication Time / minutes

FIG. 4. Volume average particle diameter (dv) as a function of sonicationtime for poly(BA) latex at three different DTAC concentrations: 0.05 M (�),

0.10 M (�), and 0.20 M (O). (|) Indicates the error range that applies to eachdata point.

SIS OF CATIONIC POLYMER LATEX 81

TABLE 3Zeta Potentials of as Prepared and Dialyzed Poly(MMA) and

Poly(BA) Latex Particles as a Function of DTAC Concentration

Zeta potential (mV) (±10 mV)[DTAC](mM) MMA Dialyzed MMA BA Dialyzed BA

25 +36 +7 +25 +350 +29 +7 +43 +1375 +43 +7 +37 +13

100 +52 +18 +50 +17

Note. All solutions had an ionic strength of (1.3 ± 0.3) × 10−3 mol kg−1.

DISCUSSION

From the conversion traces shown in Figs. 1 and 2 it is evi-dent that the maximum conversion reached is consistently lessthan 100% conversion. Free monomer analysis on the final latexsamples provided proof that there was no monomer left at thecompletion of the polymerizations. The possibility of monomerevaporation during the course of the reaction was also investi-gated but this proved that a negligible amount was lost by thisroute. Therefore, it is assumed that the missing monomer hasbeen consumed in side reactions upon sonication to producea water-soluble product. Apart from the missing monomer theconversion traces show that in general the polymerization mech-anism under ultrasonic initiation is independent of surfactantconcentration. The remainder of this discussion centers on theelucidation of the ultrasonically initiated reaction mechanism.

There are three conventional heterogeneous polymerizationsystems all of which incorporate a water-soluble initiator andare classified according to the characteristics of the oil-in-wateremulsion they form: macroemulsions, miniemulsions, and mi-croemulsions.

A macroemulsion is composed of monomer droplets in the 1-to 100-µm size range (19). Most conventional emulsion poly-merizations where the surfactant concentration is at or above itscritical micelle concentration are conducted in macroemulsions.Particle nucleation in macroemulsions takes place by either mi-cellar or homogenous nucleation (20). The process of nucleationresults in the creation of polymer particles that take up and swellwith monomer. This monomer swollen particle phase acts as thelocus of further polymerization. Therefore, the particle diameterincreases during the polymerization process as a result of equi-librium swelling to yield particles with final diameters between100 and 600 nm (19).

Miniemulsions lie in between macro- and microemulsionsin terms of droplet size and emulsion stability. Miniemulsiondroplet sizes range from 50 to 500 nm and are often generatedusing a high-shear device, such as a sonicator. The emulsionsare thermodynamically unstable and are often prepared with theintent of eliminating nucleation via micellar and aqueous phase

routes (homogeneous nucleation), thereby restricting the locusof polymerization to the miniemulsion droplets. In general, final

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latex particle diameters range from 60 to 200 nm in a miniemul-sion polymerization (19).

Microemulsions are transparent solutions that are thermo-dynamically stable, form spontaneously on mixing, and con-tain droplet sizes varying from 10 to 100 nm. Akin to theminiemulsion system the polymerization locus is in the preexist-ing droplets but with the difference being that the latex formedhas a particle diameter between 20 and 60 nm (19).

Based on these conventional classifications that are commonlyused to describe an emulsion polymerization process, the onethat best fits the experimental data for latex produced throughultrasonic initiation is the miniemulsion polymerization model.There are several qualitative comparisons that can be made thatsupport this postulate. First, the emulsions produced by sonica-tion were thermodynamically unstable—phase separation wasobserved to occur when the sonicator was switched off. Second,Sakai et al. (21) recently reported that the ultrasonic (40 kHz)dispersion of benzene and other hydrocarbons in water produceddrops of <100 nm in diameter as measured by dynamic lightscattering and freeze-fracture electron microscopy. Finally, thedata presented in Table 2 show that the fully converted sono-chemically produced latex particle diameters span from 40 to150 nm.

The ultrasonic initiation of the polymerization process thatforms latex particles is thought to occur through the generationof monomeric radical species in an emulsion. The individualsteps that take place are summarized in Scheme I and showndiagrammatically in Fig. 5. The cavitation event that occurs asultrasound travels through a liquid medium produces microbub-bles in solution. On collapse of the microbubble in an aqueoussolution the foremost primary radical species present are mostlikely ·OH and ·H radicals. Although MMA and BA are volatilemonomers and could enter a cavitation bubble to be thermallydecomposed to produce a variety of primary organic radicals,we do not believe this occurs to any significant extent at 20 kHz.Recent work in this laboratory has shown that little pyrolysis ofmethacrylic acid or MMA occurs at an acoustic frequency of

(a) InitiationH2O ))) ·H + ·OH (1)·OH(·H) + Ms → HOM·

s(HM·S) ≡ (M·

s) (2)

(b) PropagationM·

s → M·b (3)

M·b + Mb → M·

ib (4)M·

b (M·ib) + D → D· (5)

M·b + micelle → M·

micb (6)(a)M·

ib + micelle → M·micib (6)(b)

D· → D·p (7)

(c) TerminationD·

p + M·b(M·

ib) → L (8)M·

micib + M·b(M·

ib) → L (9)

SCHEME I. Mechanism for ultrasound initiated polymerization. ))) repre-sents the applied ultrasonic field. M stands for the monomer, s for the surface

of the cavitation bubble, b for species in bulk solution, D for monomer droplet,and L for a latex particle.

D GRIESER

FIG. 5. Schematic diagram of proposed miniemulsion polymerization path-way induced by ultrasound. The ultrasound coming from the horn tip producescavitation bubbles that on collapse (implosion) generate the conditions that leadto primary radical formation (·H, ·OH) and emulsification of the monomer.Secondary radicals M· are formed at the surface of the collapsed bubble andsubsequently react with monomer droplets in bulk solution producing latexparticles.

20 kHz. This suggests that primary organic radicals are unlikelyto play a major role in these cases in the ultrasonically initiatedemulsion polymerization (22).

The H/OH radicals generated by the homolysis of water areintercepted before they reach the aqueous phase by the solutesin the boundary layer of the bubble–solution interface. In anemulsion these solutes are monomer and surfactant (8). The re-sulting H/OH radicals can undergo addition to the monomeradsorbed at the bubble solvent interface (23) (reaction (2); forsimplicity this is generically denoted by M·

s ). Thereafter, theoutcome is the formation of monomeric radicals (which are thesecondary radicals referred to earlier) in bulk solution (step (3))that have three alternative pathways to initiate polymerization.They can add to a monomer molecule and undergo polymer-ization in the bulk phase (reaction (4); the “i” refers to two ormore monomer molecules per radical), enter a droplet (reac-tion (5)) or a micelle (reaction(6)). Reaction (7) represents amonomer droplet undergoing polymerization. Termination willoccur by reaction with another growing radical by recombina-tion (reactions (8) and (9)). A number of other steps could alsobe considered; however, Scheme I we suggest represents themain reactions likely to be occurring in the formation of latexparticles.

If butyl acrylate is taken as an example, it is reasonable toconclude from the kinetic data presented in Table 4, which shows

H

EMULSION POLYMERIZATION SYNT

TABLE 4Kinetic Data for the Possible Pathways of Polymer Initiation

Rate RelativeConcentration constant initial

No. Reaction (mol L−1) (L mol−1s−1) rate

1 M·b + Mb . . . → M·

iba10 × 10−3 d 5 × 102 1

2 M·b (M·

ib) + D → D· b5.6 × 10−6 e2 × 1011 2 × 105

M·b + micelle → M·

micb3 c0 f 7 × 109 0M·

ib + micelle → M·micib

Note. Where the monomer (M) is BA, D is a monomer droplet and Mmic isthe BA swollen micelle. The superscript ( · ) represents a radical.

a The water solubility of BA is 10 mmol L−1 (24).b Assuming that 10 wt% monomer is dispersed as emulsion droplets 40 nm

in diameter.c If 10 wt% BA and 0.05 M DTAC produces droplets 40 nm in diameter and

DTAC adsorbs to the surface of the droplets (60 A2/molecule (25)), the amountof free DTAC will be less than the cmc (20 mM) and therefore no micelles willbe present in the emulsion.

d The propagation rate coefficient for BA (24).e Calculated (26, 27) using k = 4πDRN with the monomer diffusion coeffi-

cient D of 1 × 10−5 cm2 s−1 (28) and assuming a droplet radius (R) of 20 nm;N is Avogadro’s constant.

f Entry rate constant of low-molecular-weight hydrocarbon molecules into amicelle (26, 27).

the three main steps in monomer polymerization, that dropletnucleation is the dominant pathway for latex formation.

The results on particle diameters as a function of monomerconversion provide additional insight into the polymeriza-tion process. The particle diameter during monomer conver-sion/sonication was observed to remain approximately constantduring the synthesis of poly(MMA) latex and is consistent witha miniemulsion polymerization process. A knowledge of theamount of monomer reacted and the particle diameter makes itpossible to calculate the number of particles produced through-out the reaction. The results presented in Table 5 show that

TABLE 5Particle Number (Nc) per Milliliter as a Function

of Monomer Conversion

MMA BA

Percentage Percentageconversion of conversion of

monomer dn (nm) Nc (×1015) monomer dn (nm) Nc (×1015)

25 35 0.9 18 28 1.530 35 1.1 26 28 2.134 35 1.3 29 28 2.436 35 1.3 34 28 2.840 35 1.5 36 28 3.041 35 1.6 45 28 3.744 35 1.6 53 28 4.349 35 1.9 60 28 4.9

60 35 2.3 70 46 1.167 35 2.5 74 130 0.05

ESIS OF CATIONIC POLYMER LATEX 83

particle number per milliliter of solution increases throughoutthe conversion. This is interpreted as continuous nucleation ofmonomer droplets occurring to form latex particles. Indeed, thisis what would be expected in a miniemulsion process if the num-ber of latex particles increase with a decrease in the number ofdroplets.

For the BA system not all the experimental observations, atfirst glance, appear to be consistent with a miniemulsion poly-merization process. For example, the 0.1 M DTAC data in Fig. 4indicate that with longer sonication times a rapid particle growthoccurs after about 50 min of sonication (60% conversion topolymer). This observation is inconsistent with the argumentsthat support a polymerization process involving a miniemulsion.However, on a closer inspection of this figure it can be noticedthat the particle size of poly(BA) latex prior to 50 min of son-ication is constant with an average particle diameter of about30 nm ± 15 nm. This is comparable, within experimental er-ror, to the average particle diameter determined for poly(MMA)latex (30 ± 15 nm; shown in Fig. 3). This particle growth, whichresults from prolonged sonication time, need not be due to achange in the mechanism of polymerization.

One important difference between poly(BA) and poly(MMA)is the glass transition of the polymer, (−)55◦C and (+)105◦C,respectively. Under the given reaction conditions poly(MMA)latex particles would be glassy, whereas poly(BA) latex particleswould be soft and deformable. Hence, at longer sonication timeswhere latex particle numbers are high and the rate of particle–particle collisions increases, it is then probable that for a softpolymer latex particle coalescence/fusion occurs. Further evi-dence that particle coalescence is occurring in the BA systemcomes from the effects on particle size associated with chang-ing the surfactant concentration. It can be seen in Fig. 4 that onreducing the DTAC concentration, and consequently reducingparticle stability, the particle size increases at shorter sonicationtimes. In essence, it is reasonable to conclude that BA latex par-ticles are formed in the same way as MMA particles with anadded reaction step involving particle–particle fusion at longersonication times.

CONCLUSIONS

The ultrasonically initiated emulsion polymerization of MMAand BA in the presence of DTAC produces cationic lattices in thesize range of 30–100 nm. Collectively, the results obtained areconsistent with a miniemulsion polymerization process that in-volves the direct polymerization of the small monomer dropletsthat are formed by the effects of ultrasound in mixing monomerand water to produce an emulsion. For the case of BA latex,there is some evidence that suggests that prolonged sonicationleads to particle–particle fusion resulting in the growth of largerdiameter latex. Overall it has been found that emulsion polymer-ization using ultrasound as the initiator is readily achieved and

is a convenient method for producing nanosized cationic latexparticles.

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ACKNOWLEDGMENTS

M.B. acknowledges the receipt of an Australian Postgraduate Award. Theproject funding is in part supported by Dulux Australia Pty. Ltd. and the ARCParticulate Fluids Processing Centre. We are also grateful for the contributionsto the program made by Michelle Carey and Chris Such of Dulux, Australia.

REFERENCES

1. Bovey, F. A., Kolthoff, I. M., Medalia, A. I., and Meehan, E. J., “EmulsionPolymerization,” Interscience, New York, 1955.

2. Biggs, S., and Grieser, F., Macromolecules 28, 4877 (1995).3. Ostroski, A. S., and Strambaugh, R. B., J. Appl. Phys. 21, 478 (1950).4. Price, G. J., “New Methods of Polymer Synthesis,” Blackie, Glasgow, UK,

1995.5. Didenko, Y. T., McNamara, III, W. B., and Suslick, K. S., J. Am. Chem.

Soc. 121, 5817 (1999).6. Misik, V., Miyoshi, N., and Riesz, P., J. Phys. Chem. 99, 3605 (1995).7. Suslick, K. S., “Ultrasound: Its Chemical, Physical and Biological Effects,”

VCH Publishers, New York, 1988.8. Tauber, A., Mark, G., Schuchman, H. P., and Sonntag, C. V., J. Chem. Soc.,

Perkin Trans. 2, 1129 (1999).9. Hammond, A., Budd, P. M., and Price, C., Prog. Colloid Polym. Sci. 113,

142 (1999).10. Landfester, K., Bechthold, N., Tiorks, F., and Antonietti, M., Macro-

molecules 32, 679 (1999).11. Cooper, G., Grieser, F., and Biggs, S., J. Colloid Interface Sci. 184, 52

(1996).

12. Kim, H. J., Kim, W. I., Lee, S. B., and Hong, I. K., J. Korean Ind. Eng.

Chem. 8 (1997).

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13. Chou, H. C. J., and Stoffer, J. O., J. Appl. Polym. Sci. 72, 797 (1999).14. Ooi, S. K., and Biggs, S., Ultrason. Sonochem. 7, 125 (2000).15. Shaw, D. J., “Introduction to Colloid and Surface Chemistry,” Butterworths,

London, 1980.16. Mukerjee, P., and Mysels, K. J., “Critical Micelle Concentrations of Aque-

ous Surfactant Systems,” U.S. Govt. Printing Office, Washington DC,1970.

17. Ferrick, M. R., Murtagh, J., and Thomas, J. K., Macromolecules 22, 1515(1989).

18. Sajjadi, S., and Brooks, B. W., J. Polym. Sci. Part A: Polym. Chem. 37, 3957(1999).

19. Sudol, E. D., and El-Aasser, M. S., in “Emulsion Polymerisation and Emul-sion Polymers” (P. A. Lovell and M. S. El-Aasser, Eds.), Wiley and Sons,London, 1997.

20. El-Aasser, M. S., in “An Introduction to Polymer Colloids” (F. Candau andR. H. Ottewill, Eds.), Kluwer Academic Press, Dordrecht, 1990.

21. Sakai, T., Kamogawa, K., Harusawa, F., Momozawa, N., Sakai, H., andAbe, M., Langmuir 17, 255 (2001).

22. Price, G. J., Ashokkumar, M., Cowan, T. D., and Grieser, F., J. Chem. Soc.,in press.

23. O’Donnell, J. H., and Sangster, D. F., “Principles of Radiation Chemistry,”Edward Arnold, London, 1970.

24. Napper, D. H., and Gilbert, R. G., “Comprehensive Polymer Science,”Vol. 4, Pergamon, Oxford, 1989.

25. Kong, X. Z., Pichot, C., and Guillot, J., Colloid Polym. Sci. 265, 791 (1987).26. Almgren, M., Grieser, F., and Thomas, J. K., J. Am. Chem. Soc. 101, 279

(1979).27. Almgren, M., Grieser, F., and Thomas, J. K., J. Am. Chem. Soc. 102, 3188

(1980).

28. Herrera-Ordonez, J., and Olayo, R., J. Polym. Sci. Part A: Polym. Chem.

38, 2201 (2000).