effect of polyoxyethylene emulsifier composition on the course of emulsion polymerization of vinyl...

Polymer International 44 (1997) 413È420

Effect of Polyoxyethylene EmulsifierComposition on the Course of Emulsion

Polymerization of Vinyl Acetate

Magdy M. H. Ayoub,* H. E. Nasr & N. N. Rozik

Department of Polymers and Pigments, National Research Center, Dokki, Cairo, Egypt

(Received 14 November 1996 ; revised version received 12 March 1997 ; accepted 28 March 1997)

Abstract : Three di†erent emulsiÐer types from vinyl acetate monomer andmethoxypolyoxyethylene (35 : 65, 27 : 73 and 19 : 81 wt : wt%) were prepared inthe presence of benzoyl peroxide using a macroradical initiator technique.Fourier transform infrared spectroscopy and 1H nuclear magnetic resonancewere carried out to conÐrm the structure of the copolymers obtained(emulsiÐers). The emulsion polymerization of vinyl acetate initiated by sodiumpersulphate as an initiator in the presence of non-ionic (polyoxyethylene (POE)type) emulsiÐer has been kinetically investigated. The rate of polymerization wasfound to be proportional to the 0É33, 0É40 and 0É44 power of the emulsiÐer con-centration and to the 0É71, 0É79 and 0É87 power of the initiator concentration.The apparent activation energy was found to be 135, 56É5 and 38 kJmol~1 for65 wt%, 73 wt% and 81 wt% POE, respectively. The particle size was observed toincrease with increasing initiator concentration and to decrease with increasingemulsiÐer concentration. The reaction order with respect to the emulsiÐer con-centration (number of particles versus emulsiÐer concentration) was found to be0É10, 2É05 and 0É89 for 65 wt%, 73 wt% and 81 wt% POE, respectively.

Polym. Int. 44, 413È420 (1997)No. of Figures : 8 No. of Tables : 2 No. of References : 23

Key words : emulsiÐer, emulsion, polymerization, vinylacetate

INTRODUCTION

Surfactant plays several important roles in emulsionpolymerization.1,2 It regulates the particle size, thenumber of particles, the particle size distribution, thestability of particles and the rate of polymerization. Apolymerizable surfactant is considered to be one of themost efficient micelle generators.3 Polymerizable sur-factants may also be vinyl monomers, which have anamphiphilic structure composed of a hydrophobic tailand a hydrophilic head group.4h6 They have a charac-teristic property of forming molecular aggregates suchas micelles. Polymerizable surfactants were preparedand applied in di†erent systems of emulsion poly-merization by many authors.7h10

* To whom all correspondence should be addressed.

Urquiola et al.11 used sodium dodecyl allyl sul-phosuccinate as a polymerizable surfactant in emulsionpolymerization of vinyl acetate. They found that therate of polymerization increased with increasing initi-ator concentration and the reaction order with the initi-ator concentration (the rate of polymerization versusthe initiator concentration) was in the range 0É8È1É0. Areaction order close to 1É0 was also observed in the dis-persion copolymerization of polyethylene oxide (PEO)macromonomers.12 This was attributed to the forma-tion of stable or occluded growing radicals. These cangrow but bimolecular termination is restricted. It wasconcluded that the reaction order with respect to emul-siÐer concentration (the rate of polymerization versusthe emulsiÐer concentration) was small and thenumber of particles depended on surfactant concentra-tion to the power 0É75.11 A large reaction order on the

4131997 SCI. Polymer International 0959-8103/97/$17.50 Printed in Great Britain(

414 M. M. H. Ayoub, H. E. Nasr, N. N. Rozik

emulsiÐer (PEO type) concentration was observed inthe emulsion copolymerization of butyl acrylate.13 Thiswas attributed to a certain solubility of emulsiÐer in themonomer phase.

The purpose of the present study was to evaluate thee†ectiveness of polyvinyl acetate-block-methoxy-polyoxyethylene (PVAc-b-MPOE) copolymer as a non-ionic emulsiÐer in emulsion polymerization of vinylacetate. In addition, the e†ect of the emulsiÐer type, itsconcentration and the initiator concentration on thekinetic and colloidal parameters were investigated.

EXPERIMENTAL

Materials

Vinyl acetate monomer was redistilled and stored at[20¡C before use. Sodium persulphate andNa2S2O8benzoyl peroxide were provided by Merck-Schuchardt(Germany). Sodium bicarbonate, to adjust the pH of themedium, and hydroquinone, to stop the polymerizationwere obtained from ADWIC (Egypt). Methoxy-polyoxyethylene (MPOE), molecular weight (MW)2000, was supplied by Union Carbide Company (USA).Hexane and toluene were supplied by El-Naser Phar-maceutical Chemical Company (Egypt). Ammoniummolybdate for particle staining was supplied by Merck-Schuchardt (Germany).

Preparation of PVAc /POE emulsifier (PVAc-b-MPOEblock copolymer)14

Methoxypolyoxyethylene (MW 2000) 65%, 73% and81%wt%, benzoyl peroxide 0É5 g and distilled toluenewere heated under reÑux at 115È120¡C for 2 h. Aftercooling at room temperature, vinyl acetate monomer(required amount) was added drop by drop over 30 min.The reaction was then allowed to proceed for another4 h at 65¡C with stirring. The resultant solution wasconcentrated using a rotary evaporator, and then pre-cipitated into distilled hexane. The crude products weredried in a vacuum oven at 40¡C overnight.

Characterization of POE emulsifier

Copolymers of (PVAc-b-MPOE) were characterized byFourier transform infrared (FTIR) spectroscopy(JASCO type 300E, Japan), and by 1H nuclear magneticresonance (NMR) spectroscopy (JEOL type EX 270MH, Japan).

Polymerization procedure

Batch emulsion polymerization of vinyl acetate wascarried out at 60È70¡C. In all cases the recipes con-

tained 51É75 g water, 11É02 g vinyl acetate, 0É125 gsodium bicarbonate, 0É026È0É750 g initiator, and thePVAc-b-MPOE emulsiÐer in di†erent compositionsvarying from 0É125 to 0É5 g (35 : 65 PVAc : POE, wt%/wt%), from 0É25 to 0É75 g (27 : 73) and from 0É05 to0É5 g (19 : 81). All experiments were run with mechanicalstirring at 500 rpm. This speed is in the range where theagitation has no noticeable e†ect on the rate of poly-merization.15 Samples of the reaction mixture weretaken at various time intervals. These samples were verysmall so that the overall composition in the reactionvessel was not seriously a†ected. Once a sample wasremoved and put on a watch glass, the reaction wasstopped with 1 ppm hydroquinone solution and thesample was evaporated at room temperature, then driedin an electric oven to constant weight. Conversion ofmonomer was determined gravimetrically.

Colloidal parameters of polymer latices

Ammonium molybdate was used for staining the par-ticles of PVAc emulsion latices using a transmissionelectron microscope (TEM) (JEOL type JEM 100 S,Japan). Conditions, 60È100 kV, magniÐcation 1000È140 000 and resolution The particle size, the4 Ó.number of particles and the polydispersity(Nt), (Dw/Dn)of PVAc emulsion latices were determined.

RESULTS AND DISCUSSION

Characterization of POE emulsifier

FTIR spectra of MPOE and PVAc-b-MPOE copoly-mers with three di†erent compositions (65%, 73% and81% POE) are shown in Fig. 1. The repeating unit ofthe copolymer is with a strong etherwCH2CH2Ow,vibration at 1110 cm~1 and OH stretching at 3200È3500 cm~1. The three ratios of PVAc-b-MPOE havecarbonyl groups at 1700 cm~1 and CwO stretching at1250 and 1170 cm~1. All samples have groupsCH2wstretching at 713 cm~1.

The 1H NMR spectra give further support for thepreparation of a polymerizable surfactant. The chemicalshifts appear at ranges d 3É2 to 3É8 ppm for the protonsin MPOE and at d 2É5 ppm for the(wOCH2wCH2w)protons in PVAc. A typical 1H NMR spectrum forPVAc-b-MPOE block copolymer is shown in Fig. 2.

The polymerizable surfactant is expected to have thefollowing structure :

POLYMER INTERNATIONAL VOL. 44, NO. 4, 1997

E†ect of PEO emulsiÐer composition 415

Fig. 1. FTIR spectra of MPOE and di†erent ratios ofPVAc-b-MPOE; (a) MPOE; (b) 35 : 65 wt%; (c) 27 : 73 wt%;

(d) 19 : 81 wt% VAc : POE.

Rate of polymerization

Figure 3 shows the conversionÈtime data for the radicalemulsion polymerization of vinyl acetate initiated by

in the presence of VAc/POE as an emulsiÐer.Na2S2O8These conversion curves show the e†ect of emulsiÐercomposition (PVAc/POE) on the polymerizationprocess. The conversion curves show a sigmoidal shapetypical of a conventional emulsion polymerization. It isobserved that with increasing emulsiÐer concentrationthe Ðnal (limiting) conversion increases. These resultsare in agreement with those obtained by Piirma & Len-zotti.10

The rates of polymerization calculated from the inter-val 2 (from 20 to 60% conversion) are summarized inTable 1. These results show that the rate of poly-merization increases with increasing emulsiÐer concen-tration. The dependence of the rate of polymerizationon the emulsiÐer composition (PVAc : POE) isdescribed by a curve with a maximum at the composi-tion 27 : 73, wt%/wt%. Thus the rate of polymerizationincreases with increasing POE fraction in the emulsiÐermolecules up to c. 73 wt%. Above this concentration therate of polymerization is independent of emulsiÐer com-position. Table 1 shows that the rate of polymerizationparallels the number of particles. Thus, the numberincreases with increasing POE fraction in emulsiÐer upto 73 wt% and then decreases.

According to the micellar model, the relation betweenthe rate of polymerization, and the emulsiÐer con-Rp ,centration, [E], can be expressed as follows :16

RpP [E]0Õ6

The experimental results listed in Table 1 and Figs 3and 4, however, obey the following relationships :

Rp P [E]0Õ33 (65%), Rp P [E]0Õ44 (73%)

and RpP [E]0Õ44 (81%)

TABLE 1. Effect of varying surfactant concentration on emulsion polymerization

of vinyl acetate using different compositions of POE

POE (wt%) ÍEË (g) RpÃ10É4 mol lÉ sÉ1 D

n(nm) D

w(nm) D

w/D

nN

tÃ10É11

65 0·125 4·4 98·5 98·5 1·00 3·45

0·250 5·4 95·0 96·6 1·01 3·50

0·500 6·0 95·9 97·1 1·01 3·70

73 0·250 9·4 86·8 90·9 1·05 4·50

0·500 12·0 70·3 71·9 1·02 9·30

0·750 15·0 33·7 43·4 1·10 51·20

81 0·050 6·0 96·6 92·6 0·96 3·50

0·250 11·7 85·6 85·3 0·99 5·30

0·500 15·0 70·3 71·9 1·02 13·60

diameter of particles ; diameter of particles ;Dn¼number-average D

w¼weight-average

of particles per ml aq. phase.Dw/D

n¼polydispersity ; N

t¼number



Fig. 2. 1H NMR of PVAc-b-MPOE.

Similar results were observed in the emulsion copoly-merization of alkyl acrylate in the presence of PEO typeof emulsiÐer.17 This deviation from the micellar modelmay be ascribed to the type of interaction between theemulsiÐer and the polymer particle surface. Withincreasing hydrophobicity of polymer particle surfaceboth the emulsiÐer/polymer interaction and the expo-nent of the emulsiÐer concentration increase.

Figure 5 and Table 2 show the e†ect of the initiatorconcentration on the kinetic and colloidal parameters of

the emulsion polymerization of vinyl acetate. Thesedata show that the rate of polymerization and the limit-ing conversion increase with increasing initiator concen-tration. The reaction order, x (from versus initiatorRpconcentration, [I], Fig. 6), increases slightly withincreasing POE fraction in the emulsiÐer molecule asfollows :

RpP [I]x

where x \ 0É71 (65%), 0É79 (73%) and 0É87 (81%).

TABLE 2. Effect of initiator concentration on emulsion polymerization of vinyl

acetate

POE (wt%) ÍIË (g) RpÃ10É4 (mol lÉ1 sÉ1) D

n(nm) D

w(nm) D

w/D

nN

tÃ10É11

65 0·125 2·1 72·6 79·7 1·09 6·50

0·250 3·4 87·2 88·7 1·02 4·65

0·500 6·0 95·9 97·1 1·01 3·70

73 0·125 4·2 52·4 52·6 1·00 19·6

0·250 7·1 52·4 60·9 1·16 17·1

0·500 12·0 70·3 71·9 1·02 9·30

81 0·125 4·6 32·3 35·4 1·09 68·3

0·250 7·3 67·5 67·4 0·99 9·20

0·350 10·0 81·2 112·0 1·40 3·80

Nomenclature as in Table 1.



Fig. 3. Rate dependence on emulsiÐer concentration in di†erent compositions : (a) 65 wt% POE; (b) 73 wt% POE; (c) 81 wt% POE.Emulsion polymerization of vinyl acetate. Initiator\ 0É5 g, monomer\ 11É02 g and temperature\ 60¡C.

A reaction order of 0É5 supports bimolecular termina-tion of growing radicals,18 or instantaneous terminationcaused by entered radicals (complete re-entry19). Thisapproach was discussed and applied in the emulsionpolymerization of acrylates in the presence of PEO typeemulsiÐer.17 However, a value of x greater than 0É5results from a contribution of the Ðrst radical lossprocess. The present results indicate that monomolecu-lar termination contributes to the deactivation ofgrowing radicals. Thus, the entanglement of growingradicals, the formation of stable radicals by chain trans-fer to emulsiÐer (PEO segments), and/or the delayed re-

entry, or complete re-escape, of desorbed radicalsfavour a Ðrst-order radical loss process.19 Similar valuesof reaction order x were reported by Urquiola et al.11and Capek et al.12

Figure 7 shows the conversionÈtime data for theemulsion polymerization of VAc at di†erent tem-peratures (60È70¡C). These curves have a sigmoidalshape and the limiting conversion strongly decreaseswith increasing reaction temperature. The lowest limit-ing conversions are observed in the emulsiÐer systemwith the largest POE fraction. The gradual decrease inthe limiting conversion with temperature may be attrib-



Fig. 4. Double logarithmic plot of the initial rate of poly-merization versus emulsiÐer concentration.

uted to increased solubility of emulsiÐer, increased poly-merization in water, thermal disturbance of hydrogenbonds, or dehydration of surface hydrophilic OE groupsin the temperature range close to the cloud point forPOE emulsiÐer.

The apparent activation energy was estimated(Ea)from the dependence of the initial rate of poly-merization on 1/T (an Arrhenius plot, Fig. 8). The ofEathe polymerization process of vinyl acetate was calcu-lated according to the Arrhenius equation to be equalto 135, 56É5 and 38 kJmol~1 with the 65%, 73% and81% POE emulsiÐers, respectively.

In the case of bulk polymerization the overall activa-tion is expressed as follows :(Ea)

Ea\ Ep [ Et/2 ] Ed/2where is the activation energy for propagation, isEp Etthe activation energy for termination, and is the acti-Edvation energy for decomposition of initiator. It is seen

Fig. 5. Rate dependence on initiator concentration using PVAc-b-MPOE emulsiÐer of di†erent compositions : (a) 65 wt% POE; (b)73 wt% POE; (c) 81 wt% POE. EmulsiÐer\ 0É5 g, monomer\ 11É02 g and temperature\ 60¡C.



Fig. 6. Double logarithmic plot of the initial rate of poly-merization versus initiator concentration.

that the activation energy strongly decreases withincreasing POE fraction in the emulsiÐer molecule.

for homogeneous polymerization is c. 90ÈEa100 kJmol~1.20 However, in the emulsion poly-merization of classic monomers, was observed to beEamuch smaller. For example, in the emulsion poly-merization of acrylates the was observed to be in theEarange 30È50 kJmol~1.21 Similar values were observedin the dispersion copolymerization of PEO macro-monomers.22 These results were discussed in terms of alarger activation energy for termination due to theimmobilization of growing radicals in the polymer par-ticles. Thus, with increasing POE fraction the activationenergy for termination increases. In the 65 wt% POEthe polymerization seems to be regulated by homo-geneous kinetics. For the other two ratios, the kineticsare governed by the dispersion process.

Fig. 7. Rate dependence on temperature using PVAc-b-MPOE emulsiÐer of di†erent compositions : (a) 65 wt% POE; (b) 73 wt%POE, (c) 81 wt% POE. Temperature : 60¡C, (] ) 65¡C, 70¡C.(…) (L)



Fig. 8. Arrhenius plot of the initial rate of polymerizationversus 1/T .

Colloidal parameters

According to the micellar model the relation betweenthe number of particles and the emulsiÐer concentrationcan be expressed as follows :

NtP [E]0Õ6

The experimental results listed in Table 1 and Figs 3and 4, however, obey the following relationships :

Nt P [E]0Õ1 (65%), NtP [E]2Õ05 (73%)

and NtP [E]0Õ89 (81%)

Similar results were obtained in the emulsion copoly-merization of alkyl acrylates in the presence of non-ionic (PEO type) emulsiÐer,20 where the increase in thereaction order was attributed to the solubility of emulsi-Ðer. Indeed the increase in POE content in the emulsi-Ðer leads to the strong increase in the reaction order.This trend may also be a function of the di†erent colloi-dal stabilization of emulsiÐer.10,11,23 A similar devi-ation from the micellar model was also found inanother report.11

The data in Table 1 show that the particle size dis-tribution is very narrow. The polydispersity parameter

is very close to 1É0. It is also seen that the parti-(Dw/Dn)cle size distribution is independent of the emulsiÐer type(the POE fraction) and concentration and slightlydecreases with increasing initiator concentration. Thus,the decrease of the nucleation period favours the forma-tion of particles with higher monodispersity.

CONCLUSIONS

The rate of polymerization in the emulsion poly-merization of vinyl acetate in the presence of POE typeemulsiÐer increases with increasing initiator and emulsi-Ðer concentration. The data indicate that both Ðrst- andsecond-order radical loss processes take part in the ter-mination mechanism. The solubility of emulsiÐer in theaqueous medium is responsible for deviation of thepolymerization behaviour from the micellar model.

The apparent activation energy varies over a widerange showing the contribution of both homogeneousand heterogeneous polymerization kinetics. It is inter-esting to note that the present emulsiÐer favours thegeneration of monodisperse polymer particles.

REFERENCES

1 El-Aasser, M. S. in ScientiÐc Methods for the Study of PolymerColloids and their Applications, eds F. Candau & R. H. Ottewill.Kluwer Academic Publishers, Dordrecht, 1990.

2 Barton, J. & Capek, I., in Radical Polymerization in DisperseSystems, eds T. J. Kemp & J. P. Kenedy. Ellis Horwood, London,1994.

3 Napper, D. H., Polymeric Stabilization of Colloidal Dispersions.Academic Press, New York 1983.

4 Paleos, C. M. & Malliaris, A., J. Macromol. Sci. Rev. Macromol.Chem. Phys. C., 28 (1988) 403.

5 Paleos, C. M. in Polymerization in Organized Media, ed. C. M.Paleos. Gordon & Breach, Philadelphia, 1992, p. 183.

6 Nagai, K. Makromol. Chem. Macromol. Symp., 84 (1994) 29.7 Tauer, K., Goebel, K. H., Kosmella, S., Stahler, K. & Neelsen, J.,

Makromol. Chem. Macromol. Symp., 31 (1990) 107.8 Pichot, C. Makromol. Chem. Macromol. Symp., 35/36 (1990) 327.9 Jialanella, G. L., Firer, E. M. & Piirma, I. J. Polym. Sci., Part A:

Polym. Chem., 30 (1992) 1925.10 Piirma, I. & Lenzotti, J. R., Br. Polym. J., 21 (1989) 45.11 Urquiola, M. B., Dimonie, V. L., Sudol, E. D. & El-Aasser, M. S.,

J. Polym. Sci., Part A: Polym. Chem., 30 (1992) 2619.12 Capek, I., Riza, M. & Akashi, M., Eur. Polym. J., 31 (1995) 895.13 Capek, I., Mlynarova, M. & Barton, J., Makromol. Chem., 189

(1988) 207.14 Napper, D. H., J. Colloid Interface Sci., 32 (1970) 106.15 Dunn, A. S., & Taylor, P. A., Makromol. Chem., 83 (1952) 207.16 Smith, W. V. & Ewart, R. H., Chem. Phys., 16 (1948) 592.17 Capek, I., Barton, J., Tuan, L. Q., Svobodaand, V. & Novotny, V.,

Makromol. Chem., 188 (1987) 1733.18 Capek, I. & Tuan, L. Q., Makromol. Chem., 187 (1986) 2063.19 Gilbert, R. G. & Napper, D. H., JMS Macromol. Chem. Phys., C23

(1983) 127.20 Brandrup, J. & Immergut, E. H. (eds), Polymer Handbook, 3rd edn,

Wiley, New York, 1989.21 Capek, I. & Potisk, P., Eur. Polym. J., 31 (1995) 1269.22 Riza, M., Capek, I., Kishidaand, A. & Akashi, M., Angew. Makro-

mol. Chem., 60 (1993) 69.23 Feeney, P. J., Napper, D. H. & Gilbert, R. G., Macromolecules, 17,

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