Emulsion copolymerization of hydrophobic with hydrophilic monomers leading to products with anomalous chemical composition distributions

Emulsion copolymerization of hydrophobic with hydrophilic

monomers leading to products with anomalous chemical

composition distributions

Citation for published version (APA):

van Doremaele, G. H. J., Herk, van, A. M., Ammerdorffer, J. L., & German, A. L. (1988). Emulsion

copolymerization of hydrophobic with hydrophilic monomers leading to products with anomalous chemical composition distributions. Polymer Communications, 29(10), 299-301.

Document status and date: Published: 01/01/1988

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Local segmental dynamics in solution: D. A. Waldowet al. times to be reliably interpreted in terms of molecular

processes.

Experiments are currently being conducted with the labeUed polyisoprene chains in order to further our understanding of the influence of solvent power on local segmental dynamics. Preliminary results for a good solvent indicate that there is essentially no molecular weight dependence after overall molecular rotation is taken into account. These results will be reported in the near future19

•

Aeknowledgements

This research was supported by Johnson Wax and the National Science Foundation (Grant DMR-8513271). DAW thanks Engelhard Corporation for fellowship support. For their help in the preparation and characterization of the polymers, we thank Mr H. Tomonaga and Mr N. Ota at the Toyohashi University of Technology.

Referenees

1 Hyde,P. D., Waldow,D. A., Ediger, M. D.,Kitano, T. and Ito, K. Macromolecules 1986,19,2533

2 Waldow, D. A., Hyde, P. D., Ediger, M. D., Kitano, T. and Ito, K. in 'Photophysics of Polymers (ACS Symposium Series 358)' (Eds. C. E. Hoyle and J. M. Torkeison) American Chemical Society, Washington, 1987, p. 68

3 Viovy, J. L., Monnerie, L. and Brochon, J. C. Macromolecules 1983, 16, 1845

4 Viovy, J. L., Frank, C. W. and Monnerie, L. Macromolecules

1985, 18, 2606; Viovy, J. L., Monnerie, L. and Merola, F. Macromolecules1985,18, 1130

5 Viovy, J. L. and Monnerie, L. Polymer 1986,27, 181

6 Ricka, J., AmIser, K. and Binkert, Th. Biopolymers 1983, 22, 1301

7 Phillips, D. 'Polymer Photophysics: Luminescence, Energy Migration and Molecular Motion in Synthetic Po!ymers', Chapman and Hall, London, 1985

8 Saski, T., Yamamoto, M. and Nishijima, Y. Makromol. Chem. Rapid Commun.1986,7,345

9 Wagner, H. L. and Flory, P. J.J.Chem. Phys. 1952,74, 195; Poddubnyi,1. Va. and Ehrenburg, E. G.J. Polym. Sci. 1962,57, 545; Hadjichristidis, N. and Roovers,J.E.L.J. Polym. Sci. 1974, 12,2521

10 Heatley, F. 'Progress in NMR Spectroscopy', Pergamon, London, 1979, Vol. 13, p. 47

11 Hall, C. K. and He1fand, E. J. Chem. Phys. 1982,77,3275 12 Kubo, R., Toda, M. and Hashitsume, H. 'Statistical Physics II:

Nonequilibrium Statistica! Mechanics', Springer-Verlag, Berlin, 1985,p.42

13 Kramers, H. A. Physica 1940,22,613

14 Brandrup, J. and Immergut, E. H. 'Polymer Handbook', 2nd Edn., Wiley, New Vork, 1975; Bauer, D. R., Brauman, J. 1. and Pecora, R. Macromolecules 1975,8,443

15 Riseman, J. and Kirkwood, J. G.J. Chem. Phys. 1949,17,442; Zimm, B. H.J.Chem. Phys.1956,24,269; Isihara, A.J. Chem. Phys.1967,47,3821

16 Flory, P ..J. 'Statistical Mechanics of Chain Molecules', Wiley Interscienee, New Vork, 1969, p. 52

17 Valeur, B., Kasparyan, N. and Monnerie, L. 26th International Symp. Macromol.Mainz, 1979,2,989

18 Bullock, A. T., Cameron, G. G. and Smith, P. M.J. Chem. Soc., Faraday Trans. 2 1974,70, 1202

19 Waldow, D. A., JoOOson, B. S., Hyde, P. D., Ediger, M. D., Kitano, T. and Ito, K. Macromolecules submitted

Emulsion copolymerization of hydrophobic with hydrophilic monomers leading to products with anomalous chemical composition

distributions

G. H. J. van Doremaele, A. M. van Herk, J. L. Ammerdorffer and A. L. German*

La,boratory of Polymer Chemistry, Eindhoven University of Technology, PO Box 513,5600 MB Eindhoven The Netherlands

(Received6 May 1988)

The particle nucleation mechanism in styrene (Sty}-methyl acrylate (MA) emulsion copolymerization has been studied by means ofthe experimental determination ofthe chemical composition distribution (CCD) of the copolymers formed. The various possible loci of particle nucleation (polymer growth) may differ in their monomer concentration regimes leading to different copolymer compositions. Strong indications have been found that the occurrence of a homogeneous particle nucleation mechanism is reflected in the CCD of the Sty-MA emulsion copolymers prepared under conditions of high conversion rates.

(Keywords: t.i.c.; emulsion copolymerization; oligomers; microstructure; particIe nucleation)

*To whom correspondence should be addressed

POLYMER COMMUNICATIONS, 1988, Vol 29, October 299

l~

i~

I

l

Introduetion

Emulsion polymerization is a very complex process due to the heterogeneity of the system. In 1948 Smith and Ewart1

proposed a mechanism for emulsion polymeri-zation of hydrophobic monomers, such as styrene (Sty). Subsequently, several more refined mechanisms have been proposed, for instance those taking into account the different water solubilities of the monomers or the initiators used. The water solubilities of monomers and initiator are important parameters that determine the locus of particle nucleation. The possible loci are: (1)

monomer swollen miceUes, (2) adsorbed emulsifier layer, (3) aqueous phase and (4) monomer droplets. Strong indications were found2_{that emulsion copolymerization}

involving two monomers ofvery different water solubility (e.g. styrene-methyl acrylate (Sty-MA») will exhibit

0263-6476/88/100299-03$03.00

©1988 Butterworth& Co. (Publishers) Ltd.

different particle nucleation mechanisms simultaneously, namely the mechanism of micellar entry as weU as the homogeneous nucleation mechanism. Guillot3 _showed that monomer partitioning between aqueous phase, latex particles and monomer droplets is the main cause of the large discrepancy between theapparentr-values found in emulsion copolymerizations (based onoverallmonomer feed ratio data in combination with copolymer compositional data) and the 'real' r-values determined in solution or bulk experiments. The monomer composition drift during emulsion copolymerization could be successfully described using these 'rea/' r-values in combination with theloealmonomer ratio inside the latex particles. Thus, the various possible loci of particle nucleation and growth mayalso be subject to different monomer concentration regimes. Therefore, in the present Sty-MA system, it may be exp~cted that copolymers of different chemical composition wiU be

Emu/sion copo/ymerization: G. H. J. van Doremae/eet al. 1 5 , - - - ,

a

b

J/'t

d

Figure I Observed CCDs of emulsion copolymers of styrene and methyl acrylate. Relative weight(Rw)versus the fraction of styrene in the copolymer (Fs1y).The peak area is proportional to conversion. (a) Exp. 1,21 % conversion (... ) and 77% conversion ( - - ) ; (b) expo 2, 94 % conversion; (c) expo

3,20% conversion (...) and 99% conversion ( - - ) ; (d) expo 4, 90% conversion

formed simultaneously. These are sufficient reasons to investigate the Chemical Composition Distribution (CCD) ofthe emulsion copolymers formed. These CCDs may provide important insight into emulsion polymerization nucleation mechanisms.

Experimental

The recipe of theab initio copolymerizations and the reaction rates in stageI!(RpI!)are shown in Table1. The copolymer latices were prepared in a Ilitre glass vessel. Before use, the monomers (Merck) were distilled at reduced pressure under nitrogen. Monomers and n-dodecyl mercaptan (Fluka) were added dropwise to a sodium dodecylsulphate (Merck p.a.) solution, thermostated at reaction temperature. Subsequently, a potassium persulphate (Merck p.a.) solution (in 25 mI water) was added to the reaction mixture. The total weight conversion was determined by solid content analyses. The feed ratio was continually monitored by means of gas chromatography. Mter reaction, the copolymer was isolated and purified from emulsifier, unreacted monomers and initiator by careful coagulation with an aluminium nitrate (Merck) solution (0.001 mol dm-3_{), subsequent decantation and removal}

of water and coagulant by filtration. The final products were thoroughly washed 10 times with hot water (SO°C) and dried at 10 -5Torr for at least Sh. The experimental

procedures, including the CCD determination by means of thin layer chromatographyjflame ionization detection (t.l.c.jf.i.d.), have been described in detail by Tacx4

.

Results

Since the water solubility of ethyl methacrylate is far less than that of MA, for the sake of comparison a series of Sty-ethyl methacrylate copolymers was prepared over a wide range of reaction conditions (e.g. temperatures 50-SO°C). A typical experiment with a high rate is given as an example in Table1 (exp. 5). We found that all these Sty-ethyl methacrylate copolymers exhibit monomodal (i.e. 'single-peaked') CCDs. The overall copolymer com-positions can be rather well predicted by the well-known Alfrey-Mayo model, using solution r-values.

Also the copolymers of styrene (Sty) and methyl acrylate (MA) prepared at moderate reaction rates and starting from a Sty-rich monomer feed composition, showed monomodal CCDs (Figure la).

However, athigh reactionrates and the same initial feed composition, produets with two-peaked CCDs (Figures

1b, c and d) are obtained, indicating anomalous behaviour in Sty-MA copolymerization. Furthermore, as reaction rate increases (by using higher temperatu re andjor higher initiator concentrations), the distance

Emu/sion copo/ymerization: G. H. J. van Doremaele et al.

Table 1 Copolymerization recipe in parts by weight and reaction rates observed during stageIJ (RpIJ)

Exp.l Exp.2 Exp. 3 Exp. 4 Exp. 5

Styrene 79.1 42.0 78.6 72.7 141.2

Methyl acrylate 31.9 17.4 29.6 27.3

Ethyl methacrylate 154.9

Water 750 600 750 750 700

Sodium dodecyl sulphate 4.0 2.4 4.0 4.0 3.4

Potassium persulphate 0.126 3.28 3.50 3.50 1.00

n-dodecyl mercaptan 0.90 0.49 0.90 0.80 0.75

Temperature

eq

RpiI(wt% min-I) 0.4 2.9 6.6 8.0 12

"Temperature rise during reaction due to high polymerization rate

between the two peaks becomes larger. At the same time the fraction ofthe relatively MA-rich copolymer increases

(Figures Ib,c and d). Apparently, the latter copolymer fraction is mainly formed at low conversion (Figure Ic,

broken line), whereas higher conversion favours the formation of the relatively Sty-rich product (Figure Ic,

solid line). Replacement of the water-soluble potassium persulphate initiator by the oil soluble AIBN (2,2'-azo-bis-isobutyronitrile) or the amphiphilic ACPA (4,4'-azo-bis-4-cyanopentanoic acid) did not significantly affect the observed CCOs.

Discussion and Conclusions

In the case of emulsion homopolymerization of hydrophobic monomers, such as Sty, oligomers with only a few monomeric unIts will already tend to become insoluble in the aqueous phase. These short oligomers will either coprecipitate to form new particles orbecome incorporated in existing particles, depending upon the em\llsifier concentration in the aqueous phase5

•

Therefore, in that case it is very difficult to discriminate between possibly occurring homogeneous and micellar nucleation mechanisms. In the case of hydrophilic monomers the oligomeric length at which precipitation occurs, is considerably larger. For example, according to literature data5, oligomers containing up to 65 units of

methyl methacrylate are still soluble in the aqueous phase, and this number will probably even be larger for the more hydrophilic monomer MA. These large oligomeric radicals will be stabilized by adsorbing soap molecules resulting in a polyelectrolyte-type complex6

•

On the grounds of the above-mentioned phenomena the following mechanism is proposed to explain the present anomalous behaviour. In the first instance homogeneous nucleation will occur during stage 1 of the Sty-MA emulsion copolymerization. The formed oligomeric radicals mainly contain MA due to the high water solubility of MA relative to Sty. Assuming solution kinetics and deriving relevant rate constants from literature data7,8,the maximum average chain length that

kinetically can be obtained in the water phase is estimated to lie in between 100 MA and 200 MA units (60°C). However, before reaching this kinetically estimated length, the growing oligomeric radicals will (co)precipitate, and then continue to grow in the primary particles under different conditions. The monomer content of these growing primary particles wilI.initially be relatively MA rich. Gradually, a separate phase begins to develop with an increasingly less hydrophilic interior,

stabilized by adsorbed surfactant. Coagulation of the soap stabilized primary particles is retarded by electrostatic repulsion. The copolymer formed up to this moment will be relatively rich in MA.

Eventually, latex particles are formed either by coagulation, as the soap concentration decreases, or by growth ofthe primary particles. The Sty/MA ratio will be higher in the latex particles than in the primary particles. Thus, the fraction of relatively MA-rich copolymer is formed during homogeneous nucleation and primary particle formation, whereas the relatively Sty-rich copolymer is formed during the (further) growth of the latex particles. The discussed mechanism involves the formation of MA-rich copolymer in Smith-Ewart stage1. The present concept explains that at higher reaction rates, when more primary particles are being formed, more MA-rich material is obtained. Also, at higher temperatures the critical oligomeric length will increase and therefore the difference in chemical composition between the two fractions of copolymer molecules will increase. Apparently, the amount of MA-rich copolymer formed at the very beginning of the low rate reactions (i.e. far less primary particles) is too small to be detected in the product at moderate conversion.

Present investigations are aimed at testing this concept by studying different reaction conditions, such as varying monomer ratios, potassium persulphate and sodium dodecyl sulphate concentrations, and temperature. Furthermore, emulsion copolymerizations will be carried out with other monomer pairs differing in water solubility.

Acknowledgement

This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for Scientific Research (NWO).

References

1 Smith, M. V. and Ewart, R. H.J.Chem. Phys. 1948,16,592 2 Dunn, A. S. Makromol. Chem. 1985, Suppl. 10/11, 1 3 Guillot, J. Makromol. Chem. 1985, Suppl. 10/11, 165

4 Tacx, J. C. J. F. PhD Thesis, Eindhoven University of Technology, 1986

5 Piinna,I.'Emulsion Polymerization', Academie Press, New York, 1982

6 Litt, M., Patsiga, R. and Stannet, V.J.Polym. Sci. Al 1970,8,3607 7 Brandrup, J. 'Polymer Handbook', 2nd Edn., Joho Wiley and Sons,

New York, 1975

8 Blackley, D. C. 'Emulsion Polymerisation', Applied Science Publishers Ltd, London, 1975