Emulsion Polymers and Emulsion Polymerization

10 Stabilization in Nonaqueous Radical Dispersion Polymerization with AB Block Copolymers of Polystyrene and Poly(dimethyl siloxane)

Downloaded by PURDUE UNIVERSITY on June 13, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch010

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J. V. DAWKINS, G. TAYLOR , S. P. BAKER, and R. W. R. COLLETT Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LE11 3TU, England J. S. HIGGINS Department of Chemical Engineering and Chemical Technology, Imperial College, London SW7 2BY, England

Nonaqueous polymer dispersions are prevented from flocc ulation by surrounding each particle by a surface layer of adsorbed polymeric stabilizer. Such dispersions are conveniently prepared by polymerizing in the presence of a preformed block or graft copolymer a monomer dissolved in a diluent which is a precipitant for the polymer. The stabilizing copolymer contains A blocks which are insoluble in the dispersion medium and act as anchors for the Β blocks which are swollen by the diluent and which extend away from the particle surface. Most of the work on the preparation of nonaqueous dispersions has been concerned with the radical initiation of acrylic monomers in the presence of co polymer stabilizers having the A block the same as the acrylic polymer in the particles (1). Our work has involved the prepar ation of nonaqueous polystyrene dispersions in aliphatic hydro carbons stabilized with well-defined AB block copolymers of poly styrene (PS) and poly (dimethyl siloxane) (PDMS) (2, 3), giving particles stabilized by surface layers of silicone. The prepara tion of dispersions of other polymers in the presence of the same PS-PDMS block copolymers in aliphatic hydrocarbons is of interest because effective anchoring of the copolymer may be influenced by the degree of compatibility between the A block and the polymer molecules in the particles. The present paper describes the prop erties of poly(methyl methacrylate) (PMMA) particles in aliphatic hydrocarbons, together with initial results on the radical dispersion polymerization of vinyl acetate (VA). Experimental Dispersion Polymerization. AB block copolymer stabilizers were prepared using anionic polymerization techniques by the addition of hexamethylcyclotrisiloxane to "living" polystyryl1

Current address: Unilever Research, Port Sunlight, Wirral, Merseyside, U.K. Current address: Berger, Newcastle upon Tyne, U.K. 0097-6156/81/0165-0189$05.00/0 © 1981 American Chemical Society 2

In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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lithium. P o l y m e r i z a t i o n s were p e r f o r m e d u n d e r c o n d i t i o n s o f r i g o r o u s p u r i t y u s i n g an i n e r t g a s b l a n k e t t e c h n i q u e o r a h i g h vacuum p r o c e d u r e a s d e s c r i b e d e l s e w h e r e (£, 5 ) . B l o c k c o p o l y m e r s p r e p a r e d o v e r a r a n g e o f m o l e c u l a r w e i g h t s a n d c o m p o s i t i o n s were c h a r a c t e r i z e d b y g e l p e r m e a t i o n c h r o m a t o g r a p h y (GPC), osmometry and s i l i c o n a n a l y s i s . The s a m p l e s h a d a n a r r o w m o l e c u l a r w e i g h t d i s t r i b u t i o n (M /M t y p i c a l l y 30000; (O)PS par ticles (6) n

n

n

J. C. S. Faraday I

20

10

Figure 2. Dependence of surface layer thickness on M (PDMS): (O) PMMA particles; A) particles (6)

30

M (PDMS)X 1 0 '

40

50

3

n

n

J. C. S. Faraday I


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e t h a n o l ( 5 1 / 4 9 , v / v l . The f l o c c u l a t i o n t e m p e r a t u r e was s t u d i e d a s a f u n c t i o n o f t h e m o l e c u l a r w e i g h t a n d c o m p o s i t i o n o f PS-PDMS, t h e p a r t i c l e s i z e o f t h e d i s p e r s i o n and t h e surface coverage o f t h e p a r t i c l e s (9) . V a l u e s o f t h e f l o c c u l a t i o n t e m p e r a t u r e i n T a b l e I a r e i n t h e r a n g e 338.2-340.5 Κ w h i c h may b e c o m p a r e d w i t h v a l u e s o f t h e t h e t a t e m p e r a t u r e o f 339 ± 1 Κ a n d 341.2 ± 2.0 Κ d e t e r m i n e d b y two s e p a r a t e p r o c e d u r e s f o r PDMS homopolymer d i s s o l v e d i n t h e same b i n a r y m i x t u r e . We may c o n c l u d e t h a t t h e mechanism o f s t e r i c s t a b i l i z a t i o n (10) o p e r a t e s f o r t h e PMMA d i s p e r s i o n s .


T a b l e I . S t a b i l i z i n g Copolymers, P a r t i c l e S i z e s and S u r f a c e C o v e r a g e o f PMMA D i s p e r s i o n s i n F l o c c u l a t i o n E x p e r i m e n t s (9) Block Copolymer M (PS) η

M (PDMS) η

12700 8800 44000 16400 12700 44000 33400

3200 11200 13700 16100 23800 29800 48000

D (nm)

A (nm )

110 250 670 330 74 69 95

6.4 12.6 19.4 24.6 35.4 44.5 51.3

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Flocculation Temperature (K) 339.6 340.1 339.0 340.4 340.5 340.4 338.2 Colloid and Polymer Science

s t a b i l i t y o f PMMA d i s p e r s i o n s was e x a m i n e d f u r t h e r b y r e d i s p e r s i o n o f t h e p a r t i c l e s i n c y c l o h e x a n e a t 333 K. Above 307 Κ c y c l o h e x a n e i s a good s o l v e n t f o r PS a n d PDMS, a n d i f t h e PS-PDMS b l o c k c o p o l y m e r was n o t f i r m l y a n c h o r e d , d e s o r p t i o n o f s t a b i l i z e r by d i s s o l u t i o n s h o u l d o c c u r a t 333 K, f o l l o w e d b y f l o c c u l a t i o n o f t h e PMMA d i s p e r s i o n . However, l i t t l e change i n d i s p e r s i o n s t a b i l i t y was o b s e r v e d o v e r a p e r i o d o f 6 0 h . C o n s e q u e n t l y , we may c o n c l u d e t h a t t h e PS b l o c k s a r e f i r m l y a n c h o r e d w i t h i n t h e h a r d PMMA m a t r i x . E x p e r i m e n t s s u g g e s t i n g t h a t g r a f t i n g o f PS-PDMS t o PMMA i s u n l i k e l y , have b e e n r e p o r t e d p r e v i o u s l y ( 4 ) . The s t a b i l i t y o f a_PMMA d i s p e r s i o n , w i t h a b l o c k c o p o l y m e r h a v i n g M (PS) = 12700 a n d M (PDMS) = 23800, was a l s o e x a m i n e d a f t e r r e d i s p e r s i o n i n n-dodecône b y h e a t i n g above t h e g l a s s t r a n s i t i o n t e m p e r a t u r e o f PMMA. A t e n d e n c y t o w a r d s f l o c c u l a t i o n o c c u r r e d above 393 K_. S o l u b i l i t y s t u d i e s (3) o f PS i n a l k a n e s s u g g e s t t h a t PS w i t h M = 12700 w i l l d i s s o l v e i n n-dodecane a t 393 K. T h e r e f o r e , i t i s p r o p o s e d t h a t s i n c e t h e r e s u l t s i n F i g u r e 3 i n d i c a t e t h a t t h e PS b l o c k s a r e n o t c o m p a t i b l e w i t h PMMA, t h e PS b l o c k s above 393 Κ d i f f u s e t h r o u g h t h e s o f t PMMA m a t r i x t o t h e p a r t i c l e s u r f a c e where t h e b l o c k s a r e d i s s o l v e d b y n-dodecane s o t h a t r a p i d f l o c c u l a t i o n occurs. This suggests t h a t f o r polymer p a r t i c l e s having a g l a s s t r a n s i t i o n t e m p e r a t u r e above t h e d i s p e r s i o n p o l y m e r i z a t i o n


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3000


?000 1000

Polymer

Figure 3. Scattering intensity from PS(D) blocks in PS particles (a) and PMMA particles (b): (+, X , A , O) different PS(D)/PS(H) ratios between 1/2 and 1/12 in the particles; (Φ) undeuterated particles ($)

Figure 4.

Electron micrograph of poly vinyl acetate) particles


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t e m p e r a t u r e t h e ΡS-PDMS b l o c k c o p o l y m e r becomes e n t r a p p e d w i t h i n a hard polymer m a t r i x d e s p i t e t h e polymer i n c o m p a t i b i l i t y e f f e c t and s t a b i l i z e s p o l y m e r p a r t i c l e s a f t e r r e m o v a l o f e x c e s s b l o c k c o p o l y m e r b y r e d i s p e r s i o n . The p r e p a r a t i o n o f s t a b l e p o l y a c r y l o n i t r i l e d i s p e r s i o n s confirms t h i s view. Nonaqueous d i s p e r s i o n s o f PMMA a n d p o l y a c r y l o n i t r i l e s t a b i l i z e d b y BAB b l o c k c o p o l y m e r s o f PS a n d PDMS h a v e a l s o b e e n r e p o r t e d b y E v e r e t t a n d Stageman (11, 1 2 ) . PVA P a r t i c l e s . I n i t i a l s t u d i e s (13) o n p o l y m e r s w i t h g l a s s t r a n s i t i o n temperatures below t h e d i s p e r s i o n p o l y m e r i z a t i o n temperature suggest t h a t s t a b l e d i s p e r s i o n s o f p o l y ( v i n y l acetate) ( P V A ) , a n d a l s o o f p o l y ( e t h y l a c r y l a t e ) , may b e p r e p a r e d i n t h e p r e s e n c e o f e x c e s s PS-PDMS. A n e x a m p l e i s shown i n F i g u r e 4 i n w h i c h PVA p a r t i c l e s a r e s u p e r i m p o s e d o n a b a c k g r o u n d c o n s i s t i n g o f m i c e l l e s o f PS-PDMS. PVA p a r t i c l e s p r e p a r e d w i t h PS-PDMS h a v i n g M (PS) ^ lOOOO showed a t e n d e n c y t o f l o c c u l a t e a t a m b i e n t t e m p e r a t u r e d u r i n g r e d i s p e r s i o n c y c l e s t o remove e x c e s s b l o c k copolymer, p a r t i c u l a r l y i f t h e d i s p e r s i o n p o l y m e r i z a t i o n had n o t p r o c e e d e d t o 1 0 0 % c o n v e r s i o n o f monomer. I t i s w e l l documented t h a t on m i x i n g s o l u t i o n s o f p o l y s t y r e n e and p o l y ( v i n y l acetate) homopolymers p h a s e s e p a r a t i o n t e n d s t o o c c u r (14, 15) , a n d s o l u b i l i t y s t u d i e s (3_) o f PS i n n - h e p t a n e s u g g e s t t h a t PS b l o c k s w i t h M (PS) ^ 10000 w i l l be c l o s e t o d i s s o l u t i o n when d i s p e r s i o n p o l y m e r i z a t i o n s a r e p e r f o r m e d a t 34 3 K. C o n s e q u e n t l y , we may p o s t u l a t e t h a t f o r s o f t polymer p a r t i c l e s t h e b l o c k copolymer i s r e j e c t e d from t h e p a r t i c l e because o f an i n c o m p a t i b i l i t y e f f e c t and i s a d s o r b e d a t t h e p a r t i c l e s u r f a c e . I f t h e block copolymer desorbs from the p a r t i c l e s u r f a c e , then p a r t i c l e agglomeration w i l l occur unless r a p i d adsorption o f other copolymer molecules occurs from a r e s e r v o i r o f excess b l o c k copolymer. With a c a r e f u l r e d i s p e r s i o n technique s t a b l e d i s p e r s i o n s f r e e o f e x c e s s b l o c k c o p o l y m e r _ a r e p r o d u c e d f o r PVA p a r t i c l e s w i t h t h e a n c h o r i n g PS b l o c k h a v i n g M (PS) = 33400. T h i s s u g g e s t s t h a t more e f f e c t i v e a n c h o r i n g o c c u r s w h e n t h e s o l u b i l i t y o f t h e b l o c k c o p o l y m e r i n t h e d i s p e r s i o n medium i s r e d u c e d . PVA d i s p e r s i o n s s t a b i l i z e d w i t h b l o c k c o p o l y m e r s h a v i n g M (PS) = 33400 were r e d i s p e r s e d i n n-dodecane a n d r e t a i n e d t h e i r s t a b i l i t y o n h e a t i n g t o 393 K. I n v i e w o f t h e i n c o m p a t i b i l i t y e f f e c t o f PS i n t h e s o f t PVA m a t r i x a n d t h e t e n d e n c y o f t h e PS b l o c k s t o w a r d s d i s s o l u t i o n a t elevated temperatures, e f f e c t i v e anchoring o f the PDMS c h a i n s may r e q u i r e c o v a l e n t g r a f t i n g o f t h e c o p o l y m e r t o t h e p a r t i c l e s b y r e a c t i o n b e t w e e n PVA r a d i c a l s a n d PS-PDMS d u r i n g dispersion polymerization. L i t e r a t u r e values f o r chain t r a n s f e r c o n s t a n t s f o r t h e r e a c t i o n b e t w e e n PVA r a d i c a l s a n d PS s u g g e s t t h a t t h e g r a f t i n g r e a c t i o n i s more l i k e l y t h a n t h e c h a i n t r a n s f e r r e a c t i o n b e t w e e n PVA r a d i c a l s a n d PVA homopolymer w h i c h p r o d u c e s b r a n c h e d PVA a t h i g h c o n v e r s i o n s o f monomer ( 1 6 ) . I n order t o c o n f i r m t h e e x i s t e n c e o f c o v a l e n t g r a f t i n g o f PS-PDMS t o t h e p a r t i c l e s , we h a v e i s o l a t e d b l o c k c o p o l y m e r f r o m a washed a n d n


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dried sample of a PVA dispersed phase. Acetonitrile was used as solvent in a Soxhlet extraction for 60 h in order to remove PVA homopolymer. The residue was washed, dried and characterized by IR spectroscopy and GPC. The molecular weight distribution of the residue was broader than that of the original block copolymer, and the residue displayed an intense IR absorption at about 1730 cm" 1


Conclusions Our results demonstrate that PS-PDMS block copolymers stabilize both hard and soft polymer particles during nonaqueous dispersion polymerization. The dispersions have been characterized by electron microscopy, silicon analysis and viscometry. It was observed that the surface coverage and surface layer thickness of the stabilizing PDMS blocks were similar for PS and PMMA particles and that the PDMS chains are in a somewhat extended chain conformation. The anchoring of the PS blocks depends on the degree of compatibility between PS and the core polymer and on the incidence of radical grafting reactions during dispersion polymerization. From the results of the flocculation experiments, it is suggested that hard particles are stabilized because the PS blocks although incompatible with the PMMA chains are trapped within the PMMA matrix. The phase separation of the PS blocks into domains in the particles may be identified by small angle neutron scattering. Acknowledgements This work was supported by the Science Research Council and by an S.R.C. CASE award in collaboration with Dow Corning. Literature Cited 1.

Barrett, K.E.J., Ed. "Dispersion Polymerization in Organic Media"; Wiley: New York, 1975. 2. Dawkins, J.V.; Taylor, G. In "Polymer Colloids II"; Fitch, R.M., Ed.; Plenum: New York, 1980; p. 447. 3. Dawkins, J.V.; Taylor, G. Eur.Polym.J. 1979, 15, 453. 4. Dawkins, J.V.; Taylor, G. Polymer 1979, 20, 599. 5. Dawkins, J.V.; Taylor, G. Makromol. Chem. 1979, 180, 1737. 6. Dawkins, J.V.; Taylor, G. J.C.S. Faraday I 1980, 76, 1263. 7. Goodwin, J.W. In "Colloid Science"; Everett, D.H., Ed.; Specialist Periodical Report, The Chemical Society: London, 1975; Vol. 1, Chapter 7. 8. Higgins, J.S.; Dawkins, J.V.; Taylor, G. Polymer 1980, 21, 627. 9. Dawkins, J.V.; Taylor, G. Colloid Polym.Sci. 1980, 258, 79. 10. Napper, D.H. J.Colloid Interface Sci. 1977, 58, 390. 11. Everett, D.H.; Stageman, J.F. Colloid Polym.Sci. 1977, 255, 293.


10. DAWKINS ET AL. 12. 13. 14. 15. 16. 17.

Nonaqueous Radical Dispersion Polymerization

Everett, D.H.; Stageman, J.F. Faraday Disc. Chem. Soc. 1978, 65, 230. Dawkins, J.V.; Collett, R.W.R. unpublished work, 1979. Dobry, A.; Boyer-Kawenoki, F. J.Polym.Sci. 1947, 2, 90. Kern, R.J.; Slocombe, R.J. J.Polym.Sci. 1955, 15, 183. Brandrup, J.; Immergut, E.H. Eds. "Polymer Handbook"; Wiley-Interscience: New York, 1976. Dawkins, J.V.; Gatehouse, P. unpublished work, 1980. April 6, 1981.


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Emulsion Polymers and Emulsion Polymerization

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