Multicomponent Polymer Systems - American Chemical Society


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10 M u l t i c o m p o n e n t P o l y m e r Systems NORBERT PLATZER

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Springborn Laboratories, Inc., Enfield, CT 06082

Copolymers Random Copolymers Alternating Copolymers Graft Polymers Block Copolymers Network Copolymers and Interpenetrating Polymer Networks Polyblends Compatible Polyblends Incompatible Polyblends Composites Reinforced Thermoplastics Fillers in Rubber Fillers in Thermosets Reinforced Thermosets

Multicomponent polymer systems, frequently defined as polymer hybrids or polymer alloys, represent physical or chemical polymeric blends. Chemical polymeric blends are linked together by covalent bonds formed of random, alternating, graft, block, or network copolymers. These blends may also be linked topologically with no covalent bonds as interpenetrating polymer networks, or they may not be linked at all except physically through polyblending. In addition, the copolymers may be thermoplastics or thermosets, and the polyblends may be compatible or incompatible. Furthermore, distinction is made between single-phase homogeneous systems and multiphase heterogeneous systems. The term multicomponent polymer system is frequently extended to composites comprised of a polymeric matrix in which a filler or reinforcing material is dispersed. The large-volume commodity plastics known for decades are generally homopolymers. In the process of modifying the properties of these homopolymers, a large number of specialty and value-added plastics have been introduced. 0097 6156/85/0285-0219S06.00/0 © 1985 American Chemical Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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These polymers are not new, primarily, but are multicomponent systems of w e l l - e s t a b l i s h e d polymeric materials.

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Copolymers Random Copolymers. O r i g i n a l l y , only random copolymers (see Figure 1) could be made by charging two monomers simultaneously and using f r e e - r a d i c a l i n i t i a t o r s . T y p i c a l examples from the 1930s are the f i r s t synthetic rubber m a t e r i a l s , which were the random copolymers of butadiene w i t h e i t h e r s t y r e n e or a c r y l o n i t r i l e , and the f i r s t f u l l y s y n t h e t i c f i b e r , a random copolymer of v i n y l c h l o r i d e w i t h v i n y l acetate. A random copolymer of s t y r e n e and a c r y l o n i t r i l e e x h i b i t e d a 20 °C h i g h e r use temperature than t h a t of p o l y s t y r e n e ; a random copolymer of styrene with b u t y l a c r y l a t e was more f l e x i b l e than the styrene homopolymer. S c i e n t i s t s soon r e a l i z e d that the composition of these copolymers prepared from unsaturated monomers i s governed by d i f f e r e n t r u l e s than those c o n t r o l l i n g condensation r e a c t i o n s . The f i r s t attempt to t r e a t c o p o l y m e r i z a t i o n i n a s y s t e m a t i c way was made by F. T. W a l l (3.) i n 1941. He concluded t h a t the r e l a t i v e r a t e s of monomer addition to growing r a d i c a l s were dependent on the nature of the monomers and on t h e i r r e l a t i v e proportion, as expressed i n the top equation of Figure 2. There, M j / M i s the r a t i o of monomers i n the feed, m^/n^ i s the r a t i o of added monomers i n the r e s u l t i n g copolymer, and *\/*2 * l i rate of the two monomers. Soon afterwards s c i e n t i s t s observed that W a l l ' s equation did not h o l d f o r numerous random c o p o l y m e r s y s t e m s s u c h as s t y r e n e / a c r y l o n i t r i l e (SAN) and styrene/methyl methacrylate (SMMA). In such cases the copolymer d e p l e t e s the monomer w i t h r e s p e c t to e i t h e r r e a c t a n t and thus causes a d r i f t i n copolymer c o m p o s i t i o n with conversion, as i l l u s t r a t e d i n the graph of Figure 2. The point at which the curve crosses the straight l i n e was designated as the "azeotropic" composition. The o r i g i n a l equation had to be modified, as shown at the bottom of Figure 2. In order to produce a copolymer of a c o m p o s i t i o n t h a t d i f f e r e d from t h a t of the a z e o t r o p i c c o m p o s i t i o n , feeding of the f a s t e r r e a c t i n g monomer i n p o r t i o n s d u r i n g the c o p o l y m e r i z a t i o n r e a c t i o n was necessary (J5). By t h i s c o n t r o l l e d f e e d i n g of the monomers, r e g u l a t i o n of the sequence d i s t r i b u t i o n i n random copolymers and p r o d u c t i o n of SAN of h i g h a c r y l o n i t r i l e content were possible (6). Taking i n t o account p o l a r i t y , s t e r i c f a c t o r s , and resonance s t a b i l i z a t i o n , T. Alfrey and C. C. P r i c e (7) developed a Q-e scheme and predicted monomer r e a c t i v i t y . The effect of p o l a r i t y on v i n y l monomer copolymerization was recognized by F. R. Mayo and coworkers (8}, who d i s t i n g u i s h e d between monomers of average a c t i v i t y and those a c t i n g as e l e c t r o n donors or a c c e p t o r s . By combining these t h e o r i e s w i t h e x p e r i m e n t a l data, c a l c u l a t i o n of product p r o b a b i l i t i e s of various monomer combinations and determination of monomer r e a c t i v i t y parameters were p o s s i b l e . Random c o p o l y m e r s h a v e grown t o be the most v e r s a t i l e , e c o n o m i c a l , and e a s i l y s y n t h e s i z e d types of copolymers. A wide v a r i e t y of f r e e - r a d i c a l , i o n i c - a d d i t i o n , and r i n g - o p e n i n g p o l y merization techniques, as w e l l as many step-growth reactions, are employed. 2

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Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Multicomponent Polymer Systems

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10. PLATZER

Figure 2.

Copolymerization of SAN and SMMA.

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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During the past four y e a r s , l i n e a r l o w - d e n s i t y p o l y e t h y l e n e (LLDPE) has probably become the most important of the thermoplastic copolymers. In c o n t r a s t to the customary p r a c t i c e of producing branched ethylene homopolymer i n a high-pressure reaction, a system of copolymerizing ethylene with a-C^_g o l e f i n s at low pressure i s used to make LLPDE copolymer. T h i s random c o p o l y m e r i z a t i o n i s c o m m e r c i a l l y c a r r i e d out i n gas-phase, s l u r r y , and s o l u t i o n processes i n the presence of a t r a n s i t i o n metal c a t a l y s t ; 1-butene, 1-hexene, 4-methy1-1-pentene, or 1-octene are choices of comonomer. In the face of p l a n t o v e r c a p a c i t y and i d l e equipment e x i s t i n g at t h i s time, LLDPE can a l s o be made i n h i g h - p r e s s u r e a u t o c l a v e s and tubular reactors. Other large-volume random copolymers are e t h y l e n e / v i n y l acetate ( E V A ) , e t h y l e n e / p r o p y l e n e / d i e n e monomer (EPDM), and v i n y l c h l o r i d e / a c r y l o n i t r i l e copolymer. Uniform random copolymers g e n e r a l l y d i s p l a y a s i n g l e - p h a s e morphology. The ordered sequence d i s t r i b u t i o n (see Figure 1) i s too s h o r t to induce microphase s e p a r a t i o n i n amorphous systems. For example, a transparent impact polystyrene i s obtainable by the order sequence d i s t r i b u t i o n of butadiene between styrene segments (9). Furthermore, monomers from which c r y s t a l l i n e homopolymer can be produced, such as high-density polyethylene and polypropylene, can be copolyraerized to produce r e s i n s w i t h c o n t r o l l a b l y reduced c r y s t a l l i n i t y and thus greater transparency. The ethylene/propylene copolymers may range from p a r t i a l l y c r y s t a l l i n e p l a s t i c s to amorphous elastomers. A l t e r n a t i n g Copolymers. A l t e r n a t i n g copolymers, comprised of monomers i n uniform a l t e r n a t i n g s u c c e s s i o n a l o n g the c h a i n (see F i g u r e 1), are r a t h e r r a r e because they r e q u i r e h i g h l y s p e c i f i c copolymerization r e a c t i v i t y r a t i o s . Styrene/maleic anhydride i s a copolymer that i s h i g h l y a l t e r n a t i n g i n nature. The r a t e of p o l y m e r i z a t i o n of p o l a r monomers, f o r example, m a l e i c a n h y d r i d e , a c r y l o n i t r i l e , or methyl m e t h a c r y l a t e , can be enhanced by complexing them w i t h a metal h a l i d e ( z i n c or vanadium c h l o r i d e ) or an organoaluminum h a l i d e ( e t h y l aluminum s e s q u i c h l o r i d e ) . These complexed monomers p a r t i c i p a t e i n a one-electron t r a n s f e r r e a c t i o n w i t h e i t h e r an uncomplexed monomer or another electron-donor monomer, for example, o l e f i n , diene, or styrene, and thus form a l t e r n a t i n g copolymers (11) with f r e e - r a d i c a l i n i t i a t o r s . An a l t e r n a t i n g s t y r e n e / a c r y l o n i t r i l e copolymer (12) has been prepared by f r e e - r a d i c a l i n i t i a t i o n of equimolar m i x t u r e s of the monomers i n the presence of n i t r i l e - c o m p l e x i n g agents such as aluminum a l k y l s . Graft Polymers. At the F a l l 1950 American Chemical Society meeting i n Chicago, the term " g r a f t i n g " was i n t r o d u c e d by T. A l f r e y , and s h o r t l y t h e r e a f t e r i t was r e p o r t e d and adopted by H. F. Mark (13). In g r a f t p o l y m e r i z a t i o n (see F i g u r e 1) a preformed polymer h a v i n g r e s i d u a l double or polar groups i s either dispersed or d i s s o l v e d i n the monomer i n the presence or absence of a s o l v e n t . Onto t h i s backbone sequences of the monomer are grafted, g e n e r a l l y i n a freer a d i c a l reaction. G r a f t i n g has become the most w i d e l y used t e c h n i q u e f o r toughening b r i t t l e polymers w i t h an e l a s t o m e r . G r a f t polymers

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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10. PLATZER

Multicomponent Polymer Systems

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i n c l u d e impact p o l y s t y r e n e , a c r y l o n i t r i l e / b u t a d i e n e / s t y r e n e (ABS), and methacrylate/butadiene/styrene (MBS) and have been treated i n a number of reviews and books (14). Impact p o l y s t y r e n e i s produced c o m m e r c i a l l y i n t h r e e s t e p s : s o l i d polybutadiene rubber i s cut up and dispersed as s m a l l p a r t i c l e s i n styrene monomer; mass prepolymerization; and completion of the polymerization either i n mass or aqueous suspension. During the prepolymerization step, styrene s t a r t s to polymerize by i t s e l f by forming droplets of polystyrene upon phase separation. When equal phase volumes are a t t a i n e d , phase i n v e r s i o n occurs (15). The d r o p l e t s of p o l y s t y r e n e become the continuous phase i n which the rubber p a r t i c l e s are d i s p e r s e d . Impact s t r e n g t h i n c r e a s e s w i t h rubber p a r t i c l e s i z e and concentration, whereas g l o s s and r i g i d i t y decrease. The type and configuration of the dispersed rubber a l s o have a s i g n i f i c a n t influence on properties. For example, four configurat i o n s of p o l y b u t a d i e n e e x i s t , as shown i n F i g u r e 3. C u r r e n t l y , polybutadiene having a medium c i s - 1 , 4 configuration of about 36% and made by using a b u t y l l i t h i u m c a t a l y s t i s the rubber most widely used i n impact polystyrene. This c i s - 1 , 4 configuration i s characterized by a lower g l a s s t r a n s i t i o n temperature (-108 °C) than that of the t r a n s - 1 , 4 c o n f i g u r a t i o n (-14 °C) and thus r e s u l t s i n s a t i s f a c t o r y impact s t r e n g t h at low temperatures. T h i s rubber a l s o c o n t a i n s about 12% l , 2 - ( v i n y l ) configuration which i s more subject to crossl i n k i n g and o x i d a t i v e degradation. I t has a l s o been shown (16) t h a t g r a f t i n g f i r s t takes p l a c e on the v i n y l s i t e s and l a t e r on the l e s s exposed 1,4 configurations. A polybutadiene of high v i n y l configuration (70%) has been evaluated i n Japan (17) f o r making photodegradable impact p o l y s t y r e n e f o r disposable containers. T h i s p o l y b u t a d i e n e was made by a n i o n i c polymerization with a s p e c i a l aluminum/chromium c a t a l y s t . A polybutadiene with 60% trans-1,4, 20% c i s - 1 , 4 , and 20% v i n y l configuration i s made i n an emulsion polymerization system for use as a substrate i n the manufacture of ABS. The rubber p a r t i c l e s are l e s s than one-tenth the s i z e of the rubber p a r t i c l e s used i n impact polystyrene, as shown i n Figure 4. With the large rubber p a r t i c l e s , g r a f t i n g takes p l a c e f i r s t i n s i d e the v o i d s and l a t e r as warts on the outside (18). With the use of s m a l l s o l i d p a r t i c l e s , the graft polymer forms a s h e l l around the rubbery c o r e . T h i s s h e l l grows with increasing grafting l e v e l and provides a better separation of the p a r t i c l e s and more uniform d i s t r i b u t i o n i n the SAN matrix. A higher c r o s s - l i n k i n g density of the larger p a r t i c l e s leads to an i r r e g u l a r l y shaped graft s h e l l and to a f i n e r d i s t r i b u t i o n of the g r a f t e d p o l y s t y r e n e i n s i d e the rubber p a r t i c l e s . Impact strength p l o t t e d versus grafting l e v e l goes through a maximum, the p o s i t i o n of which depends on the s i z e of the rubber p a r t i c l e s . A steady r i s e i n melt v i s c o s i t y i s observed with increasing grafting. Differences i n y i e l d strength and elongation are the r e s u l t of the reduction of the shrinkage s t r e s s i n s i d e the rubber p a r t i c l e s a r i s i n g from the incorporation of r i g i d graft polymer. Constant stresses at f a i l u r e are the r e s u l t of the v i s c o e l a s t i c behavior of the crazes inside the matrix, a behavior pattern that i s unaffected by p a r t i c l e s i z e . To obtain better outdoor w e a t h e r a b i l i t y , polybutadiene rubber i s frequently replaced as the substrate i n impact polystyrene and ABS by EPDM, EVA (19), or p o l y b u t y l a c r y l a t e . G r a f t i n g of s t y r e n e ,

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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