Modification of Polystyrene with Polybutadiene and the Method of

25 Modification of Polystyrene w i t h Polybutadiene and the M e t h o d of Studying Morphology of the Obtained Multiphase H i g h Impact Systems V. D. YENALYEV, N. A. NOSKOVA, and Β. V. KRAVCHENKO

Downloaded by UNIV LAVAL on December 15, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch025

Donetsk State University, Donetsk, 340055, U.S.S.R.

High Impact Polystyrene (HPS) obtained by means of copolymerization of styrene with rubber represents heterogeneous system consisting of polystyrene matrix and the particles of rubber phase dispersed in i t ; the particles in their turn keep the graft copolymer and a great number of occluded polystyrene (I). Phy sical and mechanical properties of HIPS are defined by its morphology (2). Detailed investigation of mechanism of formation of HIPS microstructure and its properties is done in the works of many researchers: the influence of the agitation speed (3,4) graft copolymer (1,5), the degree of grafting (6), thickness of the intermediate layer (7). It was shown that mechanical properties of HIPS depend in the size of rubber particles and the increase of rubber phase volume raises rubber effici ency (2,8). In the work (9) the attempt was made to find the connection between dynamic-mechanical pro perties, rubber concentration and impact strength of different commercial samples of HIPS. However size of low-temperature damping peak in general does not ap pear to be related to the actual degree of toughness, except within a fairly narrow family of similarly pre pared polymers. In literature the following methods of prepara tion of polymer material specimen and studying their morphology are described: methods of ultrathin sec tion and films with contrasting of osmium tetroxide (10*11). method of replication of the brittle fractured surface (11.12) oxygen and chemical etch of the polished surface or the fractured surface with the following replication for electron microscopy (11. MiThe most spread methods for studying HIPS are the one of ultrathin section and films as well as the 379

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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r e p l i c a technique (without e t c h ) . These methods sup plement one another t o a c e r t a i n degree, but give l i m i t e d i n f o r m a t i o n r e g a r d i n g HIPS morphology. At the s e c t i o n when c o n t r a s t i n g the s t r u c t u r e of rubber phaa& i s seen w e l l , but the i n f o r m a t i o n of p o l y s t y r e n e mat r i x s t r u c t u r e i s i n e v i t a b l y l o s t . When r e p l i e of f i l m s and s e c t i o n i s used the s t r u c t u r e d i s t o r t i o n i s out, n e v e r t h e l e s s i d e n t i f i c a t i o n of rubber and p o l y styrene phases of HIPS becomes more d i f f i c u l t because of s t r o n g l y developed p o l y s t y r e n e m a t r i x r e l i e f . Be s i d e s more o f t e n than not changing of rubber p a r t i c l es forms takes place ( t h e i r "smoothing") w h i l e being prepared. The present paper d e a l s w i t h working out e l e c t ron microscope method of HIPS s t u d y i n g , l e s s d i f f i c u l t than above mentioned but l e t t i n g get f u l l e r i n f o r m a t i o n about m a t e r i a l morphology; i n choosing the way of f o r m a l i z e d q u a n t i t a t i v e i n t e r p r e t a t i o n of the obtained s t r u c t u r e ; i n e s t a b l i s h i n g dependence bet ween morphology of HIPS and t h e i r p h y s i c a l and mecha n i c a l p r o p e r t i e s as w e l l as i n making exact the mecha nism of m i c r o s t r u c t u r e f o r m a t i o n . I n v e s t i g a t i o n of HIPS morphology. For working out a new e l e c t r o n microscope method of s t u d y i n g HIPS m i c r o s t r u c t u r e s e l e c t i v e s w e l l i n g of d i f f e r e n t p a r t s of HIPS being i n f l u e n c e d by the va pour of the s o l v e n t s was used, s e l e c t i v e l y i n f l u e n c ed e i t h e r the rubber or p o l y s t y r e n e phases. Compres sion-molded specimens of HIPS obtained by block copo l y m e r i z a t i o n of styrene w i t h polybutadiene s y n t h e t i c rubber i n the form of bar i n the s i z e of 100x6x4 mm, were cooled i n l i q u i d n i t r o g e n f o r 30 minutes, then w i t h the help of a k n i f e made a f r a c t u r e d surface and a f t e r h e a t i n g specimen t o the room temperature they were placed above the solvent (at 2-3 cm from the surface d i s t a n c e ) f o r 0.5· 1.2, 3.5, 10 and 15 minutes. From the worked i n t h i s way f r a c t u r e d s u r f a ce carbon-palladium r e p l i e was made i n a u s u a l man ner (12) and was s t u d i e d w i t h e l e c t r o n microscope UEMV - 100. For s o l v e n t s e t h y l - a c e t a t e , d i m e t h y l f o r mamide, doixan, benzene were used. The vapour a c t i o n of s o l v e n t s was i n r e l i e f of p o l y s t y r e n e m a t r i x and p o l y s t y r e n e o c c l u s i o n s becoming smoother ( f o r because of s e l e c t i v e s w e l l i n g ) without changing rubber phase s t r u c t u r e and as a r e s u l t of i t b e t t e r r e v e a l i n g rub ber p a r t i c l e s on the surface of the HIPS f r a c t u r e . Best r e s u l t s were achieved when the s u r f a c e was worked out w i t h e t h y l a c e t a t e f o r 1 minute, d i m e t h y l -


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formamide f o r 2-3 m i n . , benzene and d i o x a n f o r 0.5 m i n . When t i m e i s b e i n g p r o l o n g e d f o r v a p o u r i n f l u ence of s o l v e n t s i t g i v e s the c o n t r a s t d i m i n i s h i n g and w a s h i n g out of r u b b e r p a r t i c l e s c o n t o u r s . Not suf f i c i e n t time of working out w i t h vapour of solvents d o e s n t smooth out the p o l y s t y r e n e m a t r i x r e l i e f and makes d i f f i c u l t microgramms i n t e r p r e t i n g · Rate of polymer s w e l l i n g i s known t o depend on i t s m o l e c u l a r weight and composition and m o l e c u l a r s t r u c t u r e . T h a t ' s why f o r a c h i e v i n g b e s t c o n t r a s t ( r e l i e f ) when working with different copolymers the optimal time o f w o r k i n g out w i t h t h e s o l v e n t v a p o u r i s t o be c h o s en e x p e r i m e n t a l l y f o r each type of materials. Microgramms of HIPS morphology r e v e a l e d without w o r k i n g out w i t h t h e s o l v e n t v a p o u r and by means of above d e s c r i b e d method are g i v e n i n f i g u r e 1,a,b. When c o m p a r i n g t h e m w i t h t h e p h o t o s o b t a i n e d w i t h the h e l p of g e n e r a l l y known Kato method (10). one c a n see b e s i d e s i m i l a r i t y of g e n e r a l p i c t u r e of rubber distribution also difference: membranes between o c c l u d e d p o l y s t y r e n e and r u b b e r p a r t i c l e s i n our p i c t u r e a r e m u c h t h i c k e r . I n o u r o p i n i o n i t c a n be ex p l a i n e d by t h e f a c t t h a t when b e i n g w o r k e d out one can see only c o n t r a c t e d w i t h osmium-tetroxide rub ber and i n f i g . 1 b membranes between o c c l u s i o n s represent rubber together w i t h the intermediate lay er, c o n s i s t i n g of g r a f t polymer of poly(styrene-gr-bu tadiene). S i d e by s i d e w i t h good v i s i a l q u a l i t a t i v e obser v a t i o n of HIPS morphology o b t a i n e d by means o f above d e s c r i b e d m e t h o d we a p p l i e d t h e m e t h o d u s e d i n m e t a l l o g r a p h y (14) and adapted f o r polymer m a t e r i a l for o p t i c a l m i c r o s c o p y (15) for getting distribution c h a r a c t e r i s t i c s of r u b b e r phase i n the m a t e r i a l and q u a n t i t a t i v e value of m i c r o s t r u c t u r e elements i n electron microscope pictures. When a p p l y i n g t h i s m e t h o d a c c o r d i n g t o m i c r o grams one c a n c a l c u l a t e : volume f r a c t i o n o f HIPS r u b ber phase Vf i n per centage; Intermediate surface of rubber and polystyrene phases S i n mm /mm3 ; M e a n cord spheres S which i s proportioned to diameter of rubber particles S i n ; Mean f r e e d i s t a n c e among t h e p a r t i c l e s M F D i n Ju. . The m e n t i o n e d s t r u c t u r a l c h a r a c t e r i s t i c s o f HIPS at electron-microscope micrograms are defined with greater exactness than at o p t i c a l because only r u b b e r p a r t i c l e s l o c a t e d i n one s u r f a c e c a n be calcula ted w h i l e at phase c o n t r a s t o p t i c a l microscope par t i c l e s l o c a t e d i n a l l volume of the observed f i l m are taken i n t o account.


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I t was found that f o r q u a n t i t a t i v e characte r i s t i c s of HIPS morphology i t i s s u f f i c i e n t t o make c a l c u l a t i o n according t o 4 micrographs from the square of the surface 20x30 , obtained from d i f f e rent samples of the material.under general increase 5000x. While doing i t the r e l a t i v e mistake of d e f i n i n g f o r a l l s t r u c t u r a l c h a r a c t e r i s t i c s d o e s n t ex ceed 9 %. The above described method was a p p l i e d f o r cha r a c t e r i s t i c s of HIPS both commercial and obtained i n l a b o r a t o r i e s by means of d i f f e r e n t methods u s i n g d i f f e r e n t types of polybutadiene rubbers. M i c r o s t r u c t u r e elements of t h i s m a t e r i a l are compared t o t h e i r p h y s i c a l and mechanical p r o p e r t i e s i n t a b l e I . Table I . P h y s i c a l and mechanical p r o p e r t i e s and s t r u c t u r a l paramètres of HIPS being s y n t h e s i z e d under d i f f e r e n t c o n d i t i o n s . Rubber contents i n a l l specimens i s 5 %·

J Polymers


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A Β C D Ρ

Mechanical p r o p e r t i e s Jotched Tensible Elonga Izod strengtt t i o n at break kg kg/cm % cm/cm 2

2

12.0

11.0 8.8 5.9 5.6 4.5

290 310 300 314 265 450

32 30 28 16 22.8 20.0

Structure properties Disper volume Mean Mean s i o n of f r a c cord f r e e mean t i o n cf dis rubber tance cord phase between s2 part Vf t % icles 21.1 18.4 13.1 10.2 8.7 4.8

1.4 1.1 0.7 0.6 1.6 1.5

5.7 4.6 7.0 5.3 19.0 32.0

2.91 3.14 1.14 1.04 7.2 6.15

The d a t a of t a b l e I . witness the q u a l i t a t i v e corr e l a t i o n between morphology and p r o p e r t i e s of HIPS. E.g. r e g u l a r l y toughness i n c r e a s e s w i t h the increase of volume f r a c t i o n of rubber phase. The c o n c l u s i o n made i n l i t e r a t u r e about the dependence of p h y s i c a l and mechanical p r o p e r t i e s of HIPS on the s i z e of rub ber p a r t i c l e s (8.15.17) i s confirmed. However the s t a t e d optimal s i z e of the p a r t i c l e s (1-10>t) i s a necessary but not s u f f i c i e n t c o n d i t i o n f o r p r o v i d i n g h i g h p r o p e r t i e s of the m a t e r i a l ( p o l y mers E.P, t a b l e I ). Due t o a n a l y s i s of many data


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a n a l o g o u s t o g i v e n i n t a b l e I. one c a n come t o c o n c l u s i o n t h a t f o r a c h i e v i n g maximal f i r m n e s s of HIPS under the minimal r u b b e r contents, i.e. at the least e x p e n d i t u r e s i t i s n e c e s s a r y by means of different technological procedures to obtain the m a t e r i a l to satisfv the following requirements: a ) o p t i m a l s i z e o f r u b b e r p h a s e p a r t i c l e s (1-3JuC% b) not a wide d i s t r i b u t i o n of t h e r u b b e r phase p a r t i c l e s on the s i z e - q u a d r a t i c d i s p e r s i o n of p a r t i c l e d i a m e t e r m u s t n o t e x c e e d 4; c) volume f r a c t i o n of r u b b e r phase must not be l e s s t h a n 13-15%. Definition

of

the

residual

internal microtension

i n

Hifts, B e s i d e s the known f r o m l i t e r a t u r e on HIPS morpho logy elements of m i c r o s t r u c t u r e i n e l e c t r o n microscope p h o t o s o b t a i n e d by means o f t h e d e s c r i b e d above me t h o d a new phenomenon i s o b t a i n e d w h i c h i s n o t s e e n i n t h e p i c t u r e s m a d e b y K a t o m e t h o d (10) and i n the p i c t u r e s o f t h e f r a c t u r e s w h i c h were not worked out by the s o l v e n t v a p o u r s - on the smoothed s u r f a c e of po l y s t y r e n e m a t r i x a net of crazes i s c l e a r l y seen. These crazes have i n t e r n a l s t r i p e d s t r u c t u r e analogouja, t o m l c r o c r a c k s t r u c t u r e w h i c h a p p e a r i n H I P S . When t n e e x t e r n a l t e n s i o n i s a p p l i e d . The appearance of these c r a z e s on the polymer m a t e r i a l s u r f a c e u n der the i n f l u e n c e of solvents and t h e i r vapours is due t o e x i s t e n c e of r e s i d u a l t e n s i o n i n them (19« 20) which appear during forming specimens or t e n s i o n s c a u s e d by s t r e s s . In our method a p p l y i n g the e l e c t r o n microscope there arose the p o s s i b i l i t y to observe crazes which appeared u n d e r the i n f l u e n c e of s o l v e n t v a p o u r on the s u r f a c e of the HIPS f r a c t u r e that w i t n e s s the e x i s t e n c e and d i s t r i b u t i o n of the r e s i d u a l i n t e r n a l microtension i n the m a t e r i a l i t s e l f , these crazes p l a y i n g i n our mind a great importance i n toughning p o l y s t y r e n e by means o f r u b b e r and t h e y a r e n o t d i s c o v e r e d up t o the p r e s e n t time. F o r c l e a r i n g out the nature of these m i c r o t e n s i o n s we m i c r o s c o p e d H I P S e x a m p l e s a c c o r d i n g t o the above d e s c r i b e d m e t h o d : a) b e f o r e and a f t e r extru s i o n and m o l d i n g u n d e r p r e s s u r e ; b) a f t e r annealing f o r 24 h o u r s a t 6o°-80° C b e f o r e a n d a f t e r obtain i n g f r a c t u r e : c) a f t e r h e a t i n g w i t h the f i e l d of h i g h f r e q u e n c y 10 H z f o r 15 m i n u t e s . I n a l l t h e described c a s e s t h e c h a r a c t e r o f d i s t r i b u t i o n o f c r a z e s was not changed. I t p r o v e s t h a t r e v e a l e d c r a z e s have no



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n a t u r e of t e n s i o n s w h i c h appear i n HIPS w h i l e f o r m ing specimen disappearing during annealling material a s w e l l a s t h e y d o n ' t a p p e a r a t t h e moment o f des t r o y i n g specimen when the s u r f a c e i s b e i n g p r e p a r e d f o r the r e p l i c a t i o n * F o r c o m p a r i n g we m i c r o s c o p e d i n t h e described manner the specimen of homopolymer of s t y r e n e where t h e r e i s no r u b b e r . In the not annealled specimens of the m a t e r i a l the c r a z e s were formed w h i c h were t h e same a s t h o s e i n H I P S ( a c c o r d i n g t o their s i g h t ) . I n t h o s e a n n e a l l e d at 80° C f o r 24 h o u r s c r a z e s d o n ' t e x i s t . I t shows t h a t the cause for f o r m i n g r e s i d u a l m i c r o t e n s i o n i n HIPS i s presence of rubber p a r t i c l e s i n i t . If the surface of the f r a c t u r e i s worked w i t h t h e s o l v e n t v a p o u r s when a p p l y i n g the e x t e r n a l stress ( s t r a i n or compression) i n the microgramm a great number of w i d e r c r a z e s appear ( f i g . 2 ) . T h e external l o a d i n g was 30 % l e s s t h a n t h a t o f d e s t r o y i n g one, i.e. at these loadings without the solvent vapours influence don't appear e i t h e r "whitening" or " s i l v e r " . However i n the HIPS specimens the e f f e c t of o r i e n t a t i o n of crazes perpendicular to the d i r e c t i o n of t h e a p p l i e d t e n s i o n d o e s n ' t appear. I n t h e homopolystyrene specimen under the influence of solvent v a p o u r s a g r e a t many o f c r a z e s a p p e a r orientated p e r p e n d i c u l a r l y towards the d i r e c t i o n of the a p p l i e d tension (fig. 3). In the l a s t case the working out of specimen under s t r e s s w i t h the solvent vapour lasted for 1 minute (i.e. less than the minimal time) to define the d i r e c t i o n of the applied t e n s i o n ac cording to the d i r e c t i o n of o r i e n t a t i o n of the micros t r u c t u r e elements on the f r a c t u r e d s u r f a c e as i t is d i f f i c u l t t o do i n t h e o t h e r way b e c a u s e o f manystage process of making r e p l i e from the f r a c t u r e of p o l y s t y r e n e . A l l t h i s makes p o s s i b l e t o g i v e the statement on the n a t u r e of the r e v e a l e d i n t e r n a l m i c r o t e n s i o n s by means o f t h e above d e s c r i b e d m e t h o d . HIPS r e p r e s e n t s a heterogeneous system i n w h i c h r u b ber phase p a r t i c l e s and polystyrene m a t r i x are con n e c t e d t o g e t h e r due t o " s e i z u r e " of g r a f t e d chains of copolymer b o t h r u b b e r and homopolystyrene phases. When c o o l i n g H I P S d i f f e r e n t deformation of polysty rene and r u b b e r phases takes place and as a c o n s e q u e n c e m i c r o t e n s i o n s a p p e a r . The o b t a i n e d r e s u l t s g i v e p r o o f t o s u p p o s i t i o n s made b y S c h m i t t ( 2 1 ) and S t e r n s t e i n (22) s t a t e d on the b a s i s of i n d i r e c t d a t a . The s i z e s of t e n s i o n regions of polystyrene matrix stretch from the interface surface for several micr o n e s i n t o m a t r i x . The e x t e r n a l s t r e s s a p p l i e d t o



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Figure 1. Electron-micrographs of HIPS microstructure, a. (left) fracture without treatment by the vapors of solvent, increase of X12000. b. (right) fracture, treated by the vapors of ethyl acetate for 2 min, increase of X7000.

Figure 2.

Surface of the HIPS fracture treated by the solvent vapors while applying the external straining stress, increase X3500


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t h e s p e c i m e n i s summed u p w i t h i n t e r n a l m i c r o t e n s i o n s a c c o r d i n g t o t h e scheme of summing, described f o r m a t i o n of a l o t of by a u t h o r s crazes being developed i n d i r e c t i o n s which are not perpendicular to the d i r e c t i o n of the a p p l i e d stress. F r o m t h i s p o i n t o f v i e w we c o m e t o u n d e r s t a n d c r a c k i n g i n not o r i e n t e d w i t h respect to the s t r e s s ap p l i e d w h i c h was o b s e r v e d by S c h m i t t and K e s k k u l a by the o p t i c a l microscope (24/ and by t h e a u t h o r s (23) by t h e e l e c t r o n m i c r o s c o p e when t h e m a t e r i a l was d e s t r o y e d b e i n g e f f e c t e d by t h e c y c l e sign-variable s t r e s s . The i n t e r n a l m i c r o t e n s i o n s f o u n d by t h e above mentioned method i n HIPS f i n a l l y determine the d i s t r i b u t i o n of the d e s t r o y i n g s t r e s s applied to the specimen. Therefore the character of the d i s t r i b u t i o n of these m i c r o t e n s i o n s i n the material i s of importance i n t o u g h n i n g p o l y s t y r e n e by r u b b e r . We f i n d e x p e r i m e n t a l c o r r o b o r a t i o n o f t h e s a i d a b o v e i n q u a l i t a t i v e c o r r e l a t i o n of s i z e s of crazes on photographs and t h e i r d i s t r i b u t i o n s w i t h t o u g h n i n g c h a r a c t e r i s t i c s of HIPS - the m a t e r i a l possesses h i g h e r toughness i f i t has a s u f f i c i e n t l y great num ber of crazes connecting the p a r t i c l e s of rubber phase. Thus i t f o l l o w s t h a t to o b t a i n HIPS w i t h h i g h physico-mechanical properties independent of the manner of i t s o b t a i n i n g , i t i s necessary to create c o n d i t i o n s f o r forming d i s c r e t e p a r t i c l e s of rubber phase, w h i c h are at most f i l l e d w i t h p o l y s t y r e n e oc c l u s i o n s and w h i c h have an o p t i m a l e x t e r n a l diameter as w e l l as possessing s u f f i c i e n t connection with po l y s t y r e n e m a t r i x due t o the i n t e r m e d i a t e adhesion l a y e r . A s shown by t h e a u t h o r s (25) the thickness of t h i s l a y e r e s s e n t i a l l y depends on the number and s i z e of p o l y s t y r e n e occlusions inside the rubber particles. The i n v e s t i g a t i o n formation.

of

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HIPS

microstructure

The b a s e s o f HIPS m i c r o s t r u c t u r e , a s i t is known, are l a i d at the stage of prepolymerization, when p o l y m e r - p o l y m e r emulsion (POO-emulsion) is ob t a i n e d due t o i n c o m p a t i b i l i t y of homopolystyrene b e i n g o b t a i n e d and polybutadiene i n styrene solution. Due t o t h e f a c t t h a t t h e p a r t i c l e s o f t h e discrete phase of e m u l s i o n and l a y e r s of c o n t i n u o u s phase between them have s i z e s not more t h a n t e n s of , t h e p r o c e s s o f d e f u s i o n o f monomer o w i n g t o agitat i n g i s p r o c e e d i n g a t s u f f i c i e n t r a t e . T h a t i s why t h e



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composition of polymer emulsion phases approaches the equilibrium. With the purpose of finding the factors influencing the formation of this or that internal structure of rubber phase, the phase equi l i b r i a of multicomponent polymer systems of ρ ο l y b u tadiene-poly styrene-styrene and polybutadiene-polystyrene-graft eopolymer-styrene. The phase equilibria were studied on model emul sions with various ratios and concentrations of po lymers modelling different degrees of copolymeriza tion conversion. The emulsions have been prepared by mixing equal volumes of styrene solutions of ISR polybutadiene rubber " In te ne - 55 UFA, (PB) and nonfractionated polystyrene (PS), which has been ob tained by free radical polymerization in the presence of benzoyl peroxide(Bz Û2 ) and also by their simultaneous dissolving i n styrene. After centrifuging (separating factor G= 5000) polymers from each phase have been separated by precipitation with the help of methylalcohol. Molecular weight of homopolystyrene was determined by viscosity measurement. Studying the process of microstructure forming at prepolymerization stage was carried out with the help of phasecontrast microscopy and by separating polystyrene and by rubber phases by centrifuging POO obtained by copolimerization of styrene with polybutadiene. While studying polymer distribution between the emulsion phases i t was found that in the systems mentioned above obtained both by copolymerization of styrene with polybutadiene rubber and mixing styrene solutions of polymers when the composition i s far enough from the c r i t i c a l mixing point, thermodynamic equilibrium i s reached.At this thermodynamic equilibrium the ratio of polymer concentration (Cp) in rubber (index') as well as i n polystyrene (index ) phases i s practically constant (table II). The state of equilibrium in the investigated system i s proved to be true, thermodynamic by the fact that the phase distributing coefficient of polymers does not depend on the following: l.the way of preparing emulsion: styrene and rubber copolymerization (lines 1,2 of the table II) and the presence of graft copolymer i n the system as a result of i t ; simultaneous dissolving of polymers i n styrene (lines 12-14); mixing equal volumes of solutions of PS and PB (the concentration of i n i t i a l solutions varied widely); 2.Summed concentration of polymers in the emulsion (6 - 32%); 3.Molecular weight of homopolystyrene (lines ff

2

f t


388

EMULSION

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Table I I . E q u i l i b r i u m concentrations o f polymers i n POO-emulsions polybutadiene-polystyrene-styrene. Total ! concen tratiorl


N2

T°C

CPS in emul sions

o f polsl

mers i n emulj sion,% w/w

1. 16.0 2. 8 . 1

30 30

3. 4. 5. 6. 7.

30 30 30 30 30 30 30 30 30

8. 9.

10 11 12 13 14 15

16

19.2

12.0 32.0

8.0

12.0 6.0 8.0 8.1 8.0

6.0 6.0 6.0 6.0 7.0

CPB/

20 40

60

30 30

AO PS

Concentra tion of poly mers i n p h a s e s , % , w/w rubberj p o l y styre ne

fType of (emul sion*

Polymerization emulsions 0.4 1.3 PB/PS 13.0 tl 1.0 1.1 7.4 Model e m u l s i o n s PB/PS 15.0 0.1 3.0 tl 0.2 3.0 9.3 0.3 0.3 25.0 0.3 3.0 6.4 0.5 3.0 "Ti 10.3 0.5 3.0 4.7 0.6 3.0 6.5 0.6 6.6 2.3 0.6 0.9 6.7 1.0 2.9 5.0 1.0 2.9 5.0 1.0 2.9 5.0 PSTFB" 5.4 2.0 3.0 ft 2.5 3.0 6.4 F

—

F

F

L

— —

—

—

C

1

Ρ

17.6 10.2

0.74 0.72

21.4 13.3

0.70 0.70 0.72 0.72 0.72 0.71

34.8

8.9

14.1

6.6 9.5 9.5 9.2

0.68 0.69

8.0 9.2

0.67 0.70

7.3 7.3 7.3

χ PB/PS - preinversion emulsion M inversion emulsion PS/PB - post inversion emulsion


0.73

0.68 0.68 0.68


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9,10,11); 4. Temperature (lines 12-14); 5. Rubber and polystyrene concentration ratio · The l a t t e r i n f l u e n c e s o n l y t h e t y p e o f t h e e m u l s i o n , the presence of the g r a f t copolymer i n f l u e n c i n g only i t s s t a b i l i t y . While a n a l y s i n g the composition of phases of the f u l l s e p a r a t i o n o f t h e e m u l s i o n i t was f o u n d t h a t the experimentally found concentrations of polymers i n these solutions d i f f e r from those c a l c u l a t e d for t h e c a s e when e a c h p o l y m e r i s p r e s e n t i n one phase o n l y . I t c a n be s u p p o s e d t h a t due t o t h e partial c o m p a t i b i l i t y i n both emulsion phases there are both polymers present, but the " r u b b e r " phase i s a p o l y butadiene s o l u t i o n w i t h the admixture of small quan t i t y of PS, and the " p o l y s t y r e n e " phase r e p r e s e n t s a p o l y s t y r e n e s o l u t i o n w i t h t h e a d m i x t u r e o f P B . On t h e b a s i s t h a t i n model emulsions of equal compositions the volume of rubber phase i n c r e a s e s as the molecular weight of polystyrene decreases, a n d ffiy of homopolystyrene i n the polystyrene phase i n c r e a s e s ( t a b l e II]} we c a n d r a w a c o n c l u s i o n t h a t l o w - m o l e c u l a r frac t i o n s of polystyrene migrate i n t o the rubber phase. Table III. Dependence o f a volume f r a c t u r e (7) of the rubber phase of POO-emulsion on l y of polystyrene.Composi t i o n o f e m u l s i o n : P S - 5%; P B - 3%; My 10 o f PS u s e d the emulsion l y

1 0

rene V

of

5

of

PS

i n

to

the

prepare polysty

3.0

2.2

3.05

2 . 3 0 1.15

50

53

1.0

0.7 0.78

phase the

rubber

phase;

%

67

58

p a r t of i t defuses i n t o the p o l y s t y r e n e phase. Howev er, the rubber concentration i n the polystyrene phase i s n o t h i g h due t o n o n - e q u a l s o l v e n t d i s t r i b u t i o n between p h a s e s and use f o r i n v e s t i g a t i o n of polybu t a d i e n e o b t a i n e d by i o n i c p o l y m e r i z a t i o n w i t h n a r r o w MWD a n d h i g h m e d i u m v a l u e o f m o l e c u l a r w e i g h t , the c o n c e n t r a t i o n of rubber i n polystyrene phase is small.The selective d i s o l v e n t m e t h o d f o u n d i t t o be equal to 0.05 - 0.1%. The e q u i l i b r i u m c o m p o s i t i o n o f p h a s e s c a n be represented v i v i d l y w i t h the help of the diagram for t e r n a r y m i x t u r e . As seen i n f i g . 4 p o i n t s 1 , 2 , 3 of i n t e r s e c t i o n of c o n n e c t i n g l i n e w i t h b i n o d a l e s des c r i b i n g t h e c o m p o s i t i o n o f t h e r u b b e r phase o f POOf

f


f


390

EMULSION

POLYMERIZATION

Figure 3. Surface of the homopolystyrene fracture treated by the vapors of ethyl acetate for 1 min while applying the external straining stress. The arrows show the direction of the stress applied.

Figure 4. Diagram of ternary system of polybutadiene-polystyrene-styrene at t = 20°C for PS, the values of the molecular weight being: A—3.1 · JO ; Β—0.7 · ΙΟ ; C—0.3 · 10 5

5

s


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emulsion w i t h p o l y s t y r e n e of d i f f e r e n t molecular weight l i e near each other i n the f i e l d of low con centrations of rubber, while points 1 2 and 3" c h a r a c t e r i z i n g the composition of the polystyrene p h a s e move c o n s i d e r a b l y i n t o t h e t r i a n g l e a s t h e Mv PS decreases. The c o m p o s i t i o n o f e m u l s i o n p h a s e s c h a n g e s also when the t e m p e r a t u r e i n c r e a s e s . F o r example, concent r a t i o n of polystyrene i n the rubber phase increases. T h i s i s p r o v e d by i t s t u r b i d i t y when b e i n g c o o l e d i n case of s e p a r a t i o n of the emulsion at h i g h tempera tures. P r o c e e d i n g from s a i d above the c o e f f i c i e n t of polymer d i s t r i b u t i o n between the phases of P00e m u l s i o n c a n be w r i t t e n down i n t h e f o l l o w i n g way:


f ,

f

11

+ PS ^0.7 (1) C PS + C " P B w h e r e C p £ a n d C'ps - rubber and polystyrene concent r a t i o n s m the rubber phase; G"PB and C PS - r u b b e r and p o l y s t y r e n e concent r a t i o n s i n the polystyrene phase. A f t e r c o m p l e t e POO s e p a r a t i o n i n t h e centrifuge r u b b e r a n d p o l y s t y r e n e p h a s e v o l u m e s h a v e b e e n measuj*e d . The r e s u l t s of t h e s e measurements a r e represented i n f i g . 5 , i t b e i n g known t h a t the r e g i o n of existence of multiphase emulsions (corresponding to the phase i n v e r s i o n ) has been shaded. From the d a t a g i v e n i t is c l e a r t h a t the determining c o n d i t i o n s of phase inver s i o n of polymer emulsion i s n o t t h e PS c o n t e n t but the complex of f a c t o r s , p r o v i d i n g the d e f i n i t e volume r a t i o of d i s c r e t e and continuous phases - the con tent of b o t h polymers, t h e i r m o l e c u l a r weights and MWD. T h e e m u l s i o n m u l t i p h a s e c h a r a c t e r i s k e p t i n the wide i n t e r v a l r a t i o s of volumes, i . e . condition f o r t h e i n v e r s i o n o f P O O - e m u l s i o n p h a s e s may b e r e presented as follows: Κ =

g

t

p

B

C

>

f t

f

f ,

V/V

11

-

0.71.4

P r o c e e d i n g from the above m e n t i o n e d , e x a c t l y i t i s p o s s i b l e to define the mechanism of HIPS morpholo g y f o r m a t i o n . A c c o r d i n g t o M o l a u (1.5) the process of rubber p a r t i c l e s formation takes place d u r i n g and a f t e r the phase i n v e r s i o n s ( formations of polysty rene o c c l u s i o n s of I and I I - t y p e ) . In case of the formation at prepolymerization of low molecular homopolystyrene f r a c t i o n s , compa t i b l e w i t h the rubber i n the rubber phase, the poly mer c o n c e n t r a t i o n i n t h e s y s t e m i s r a t h e r h i g h by the end of i n v e r s i o n . D u r i n g the f o l l o w i n g polymeri-



392


zation the homopolystyrene concentration i n the rub ber drop w i l l exceed the c o m p a t i b i l i t y l i m i t , but the h i g h PB s o l u t i o n v i s c o s i t y makes t h e p o l y s t y r e n e mac romolecules diffusion more d i f f i c u l t f r o m t h e drop of rubber phase, homopolystyrene separates i n s i d e t h e r u b b e r phase a s d r o p s o f a new p o l y s t y r e n e phase. During t h e p o l y m e r i z a t i o n t h e volume o f these drops grows. The rubber s o l u t i o n volume n a t u r a l l y decreases w i t h o u t t h e change o f t h e t o t a l r u b b e r p h a s e v o l u m e . We s u g g e s t t o c a l l s u c h p o l y s t y r e n e p a r t i c l e s i n s i d e t h e r u b b e r ones formed a c c o r d i n g to such mechanism - t h e o c c l u s i a n s o f t h e I l l - t y p e . The I I - t y p e o c c l u s i o n s p a r t i a l l y d e g e n e r a t e i n s u c h a c a s e . T h e I l l - t y p e o c c l u s i o n s f o r m a t i o n may b e i l lustrated b y t h e f o l l o w i n g m o d e l e x p e r i m e n t . 5% h o mogeneous s t y r e n e s o l u t i o n o f p o l y b u t a d i e n e c o n t a i n i n g 1.55, of polystyrene with the molecular weight o f 4 * 1Cr i s a d d e d t o 50% p o l y s t y r e n e s o l u t i o n w i t h t h e m o l e c u l a r w e i g h t o f 9 · 10*" w i t h o u t b e i n g m i x e d . To r e a c h t h e e q u i l i b r i u m i n t h e s y s t e m t h e s t y r e n e would migrate into the s o l u t i o n of high-molecular p o l y s t y r e n e from t h e r u b b e r one , t h e l a t t e r would c o n c e n t r a t e . Due t o t h i s t h e l o w m o l e c u l a r polystyrene becomes i n c o m p a t i b l e w i t h t h e r u b b e r a n d t h e r u b b e r phase becomes t u r b i d ^ d r o p s o f a new p h a s e , represent ing the lowmolecular polystyrene solution i n styrene are formed i n s i d e i t . To o b s e r v e t h e f o r m a t i o n o f t h e I l l - t y p e occlu sions i s also possible at high stages of the thermal polymerization without the agitation of the rubber s o l u t i o n , w h i c h has been obtained by complete sepa r a t i o n i n the centrifuge of polymer w i t h lowmolecul ar homopolystyrene. Heterogeneity i n the rubber phase appeares immediately a f t e r t h e b e g i n n i n g o f p o l y m e r i z a t i o n . The momentary t u r b i d i t y o f t h e s o l u t i o n witnesses this. Small particles of the polystyrene phase a r e seen i n m i c r o s c o p e . The e l e c t r o n - m i c r o s cope photograph o f m i c r o s t r u c t u r e o f t h e f i n a l pro duct i s represented i n f i g . 6 . The i n f l u e n c e o f t h e m o l e c u l a r w e i g h t o f homo p o l y s t y r e n e POO o n t h e f o r m a t i o n o f p o l y s t y r e n e o c c l u s i o n s i s v i v i d l y seen i n f i g . 7 , where photographs of m i c r o s t r u c t u r e s o f HIPS specimens a r e r e p r e s e n t e d ; The s p e c i m e n s o f HIPS a r e o b t a i n e d b y t h e p o l y m e r i z a t i o n of model emulsions, prepared by m i x i n g s o l u t i o n s o f r u b b e r - 8% a n d p o l y s t y r e n e - 30% i n r a t i o 1:1. MW P S v a r i e d a t 0.7-r- 3 10^ . As seen i n fig.7 the rubber p a r t i c l e s , formed from multiphase model e m u l s i o n a s a r e s u l t o f r e d i s t r i b u t i o n o f monomer, d i f f e r g r e a t l y f r o m e a c h o t h e r . MW o f p o l y s t y r e n e 9



YENALYEV

ET AL.

Modification

5

of

Polystyrene

10

15

po(ystyrene in emuCsion % t

Figure 5. Dependence of the ratio of volumes of the rubber and polystyrene phases in polymer emulsion on PS concentration (M = 3 · 10 ); C being: 1—2%; 2—3%; 3—4%; 4—M PS being 0.5 · 10 its concentration—4% 5

v

v

PB

s

Figure 6. Microstructure of polymer, obtained by the polymerization of the rubber phase, separated from the prepolymer, increase X8000. Occlusions of the II-type are seen.


EMULSION

POLYMERIZATION


394

Figure 7. Structure of the rubber particles in HIPS, obtained by the polymeriza tion of model emulsions, MW of homopolystyrene in which being: a) 2.9 · JO X3500, b) 1.7 · 10 X2500, c) 1.17 · 10 χ5000, d) 7.0 · 10 X7000.

5

s

s

s


25.

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Modification

of Polystyrene

395

b e i n g 1 . 7 * l O ^ a n d 2 . 9 · 10 ( f i g . 7 , a , b ) when p o l y styrene i s p r a c t i c a l l y incompatible with polybutadiene i n s t y r e n e s o l u t i o n , t h e r u b b e r p a r t i c l e s w i t h high c o n c e n t r a t i o n o f PB and o c c l u s i o n s o f II-type only are formed. With t h e d e c r e a s e u p t o MW P S o f 1.17 · 1Cr t h e s i z e s o f t h e o c c l u d e d p a r t i c l e s incre ase u p t o 0 . 3 - 0 . 5 Λ ^ , ί-·β. t h e i n t e r n a l s t r u c t u r e of p a r t i c l e s i s analogous t o HIPS, obtained by copol y m e r i z a t i o n i n b l o c k . Sty o f p o l y s t y r e n e being 0.7 · 1Cr t h e contents of PB i n t h e rubber p a r t i c l e s i s minimal and the sizes of occlusions sometimes e x c e e d 2ju*, a n d t h i s c o n s i d e r a b l y i n c r e a s e s t h e t o t a l volume o f t h e rubber phase. Summing u p t h e above m e n t i o n e d , t h e mechanism of formation of the i n t e r n a l microstructure of the " r u b b e r " phase p a r t i c l e s a f t e r t h e i n v e r s i o n of p h a s e s c a n be r e p r e s e n t e d i n t h e f o l l o w i n g w a y : a) i n case of absence of lowmolecular weight fractures of homopolystyrene the process proceeds according to Molau (1)« i . e . with the formation of occlusions of I and Il-types; b) i n c a s e o f p r e s e n c e o f l o w m o l e c u l a r w e i g h t homopolystyrene after the i n v e r s i o n of phases a part of i t remains w i t h i n t h e " r u b b e r " phase, separating d u r i n g t h e f u r t h e r p o l y m e r i z a t i o n i n t o t h e new p o lystyrene phase, forming the occlusions of the I l l t y p e . M o l e c u l e s o f PS a p p e a r i n g d u r i n g t h e p r o c e s s of homopolymerization i n s i d e t h e rubber s o l u t i o n diffuse into the occluded drops of the polystyrene s o l u t i o n , i n c r e a s i n g i n such a manner t h e i r sizes. T h u s , t h e p o s s i b i l i t y o f f o r m i n g new o c c l u s i o n s o f the I I - t y p e d e c r e a s e s o r becomes i m p o s s i b l e .


s

LITERATURE CITED 1. Molau G . E . , Keskkula H.J. J. Polymer Sci.,(1966), A 1, 4, 1596. 2. Wagner E . R . , Robeson M.M., Ruber Chem. and Technolog, (1970), 43 , 1129. 3. Frugward G.W. Polymer, (1972), 13, 366. 4. Frugward G.W. Karmarkar M . , J. A p p l . Polymer Sci., (1972), 16, 69. 5. Molau I.E. Koll- Ζ u Ζ für Polymer, (1970), Bd.238, 493. 6. Bongardt J., Plaste und Kautschuk, (1973), 20 , 265. 7. Pohe G . , Plaste and Kauschuk, (1973), 23, 340. 8. Bender B.W., J . A p p l . Polym.Sci, (1965), 9, 2887. 9. Keskkula H., Turley S . G . , Boyer R.F., J . A p p l . Poly mer Sci., (1971), 15, 351. 10. Kato R . , Elektron Microscopy, (1965), 14 , 220.



396

11. Williams R.Y., Hudson R.W., Polymer, (1967), 8, 643. 12.

Πилянкевич A.H.Πрактика әлектрoннoй микрoскoпии, Машгиз,Мoсква,I96I.

13. Keskkula H.J. Tranlor P.A., J. Appl. Polym. Sci., (1967), 11 , 2361. 14. Underwood Ε., Amer. Soc.Metals. Eng. Quart., (1962), 11, 62. 15. Cigna G., J. Appl. Polym. Sci., (1970),14, 1781. 16. ΕналЪев

В.Д.,Зайцев

Ю.C.,Зайцева

В.В.

и

др.,


Πласт.массы, 17. Кулезнев системы, 1 8 . Kambour 237.

(I969),I0,II. В.Η. в кн.Мнoгoкoмпoнентные 27,Χимия,Мoсква,I974. R.P. and R u s e l R.R., P o l y m e r ,

пoлимерные (1971),

12,

19. Ziegler E.E., SPE - j o u r n . , (1965),10, 12. 20. G . A . Bernier, R . P . Kambour, Macromolecules, (1968) 1, 393. 21. J . A . Schmitt, J. Appl. Polym. Sci., (1968), 12, 533. 22. S. S t e r s t e i n , J . Paterno und Ongchink., Intern. Conf. on Weld, Deformation and Fracture of Poly mers, Cambridge, 31 March-3 A p r i l (1970). 23. J . A . Manson, R.W. Hertzberg, J.Polymer S c i . , (1966), A 1, 4, 1595. 24. J . A . Schmitt, H . J . Keskkula, J. Appl. Polym. Sci., (I960) 3, 132. 25. T.O. Graig, J Polym. Sci.,: Polym. Chem. E d . , (1974), 12, 2105.


Modification of Polystyrene with Polybutadiene and the Method of

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