Grafting of Styrene and Acrylonitrile onto Ethylene Polymers

20 Grafting of Styrene and Acrylonitrile onto

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Ethylene Polymers HEINRICH ALBERTS, HERBERT BARTL, and RAINER KUHN Zentrale Forschung, Wissenschaftliches Hauptlaboratorium der Bayer AG, Leverkusen, West Germany

Styrene was grafted onto low-density polyethylene under swelling conditions. The homogeneity of the graft copolymers depended not only on the temperature of the styrene diffusion, but also on the comonomer content and the crystallinity of the grafting backbone. The grafting efficiencies were affected primarily by the activity of the initiator radical and, to a smaller extent, by the mutual ratio of monomer to initiator to substrate. The supramolecular structure of the graft products could be elucidated by differential thermal analysis and by electron microscopy. The presumed molecular structure of the graft copolymer could be deduced from light scattering measurements.

I

n

1 9 6 8 w e r e p o r t e d o n t h e effect of t h e m o n o m e r r a d i c a l activities o n the g r a f t i n g of various v i n y l m o n o m e r s onto e t h y l e n e - v i n y l acetate c o p o l y m e r s ( E V A ) ( i ) . W e f o u n d that the g r a f t i n g activity of v i n y l m o n o m e r s corresponds to the m o n o m e r radical activity p u b l i s h e d b y M a y o a n d W a l l i n g (2) a n d o t h e r s (3, 4). T h i s f e a t u r e o f v i n y l m o n o m e r s is d e p i c t e d i n F i g u r e 1 . V i n y l c h l o r i d e is o n e of t h e most active m o n o m e r s i n g r a f t i n g reactions. W h e n E V A w a s u s e d as g r a f t i n g s u b s t r a t e , t h e r e s u l t a n t g r a f t c o p o l y m e r h a d i m p r o v e d c o m p a t i b i l i t y w i t h p o l y ( v i n y l c h l o r i d e ) ( P V C ) . B l e n d i n g of these grafts w i t h P V C p r o d u c e d h i g h - i m p a c t P V C . W h e n styrene w a s grafted onto E V A , the graft p r o d u c t h a d a very l o w graft copolymer content. T h e grafting of a c r y l o n i t r i l e o n t o E V A w a s also s t u d i e d b y B a r t l a n d H a r d t ( I ) . U n l i k e the v i n y l c h l o r i d e graft p r o d u c t s , the c o r r e s p o n d i n g a c r y l o n i t r i l e grafts w e r e i n c o m p a t i b l e despite c o m p a r a b l e g r a f t i n g efficiencies. W h e n s t y r e n e - a c r y l o nitrile combinations were grafted onto E V A , the graft products h a d a n average graft c o p o l y m e r content of about 5 0 w t % ; however, the reaction products w e r e i n c o m p a t i b l e i n a l l cases. T h e objective of these studies w a s to o b t a i n m o r e i n f o r m a t i o n about the factors that affect the g r a f t i n g efficiencies a n d the c o m p a t i b i l i t y of graft p o l y mers. Since i n c o m p a t i b l e grafts often h a v e v e r y p o o r m e c h a n i c a l properties, it w a s of interest to explore grafts of i m p r o v e d c o m p a t i b i l i t y w h i c h s h o u l d h a v e i m p r o v e d m e c h a n i c a l properties. O n e a p p r o a c h to this p r o b l e m w a s to obtain i n f o r m a t i o n as e x a c t as p o s s i b l e a b o u t t h e m o l e c u l a r s t r u c t u r e o f t h e g r a f t 214

Platzer; Copolymers, Polyblends, and Composites Advances in Chemistry; American Chemical Society: Washington, DC, 1975.


20.

ALBERTS

E T

Grafting

AL.

styrene

Figure 1.

onto Ethylene

methyl methacrylate

ethyl acrylate

215

Polymers

vinyl acetate

vinyl chloride

Grafting of various monomers onto Levapren 450, Bayer AG

a product of

p o l y m e r s a n d t h e n u m b e r of t h e g r a f t c h a i n s p e r m o l e c u l e as w e l l as t h e i r m o l e c u l a r w e i g h t , 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 , a n d c o m p a t i b i l i t y w i t h the g r a f t i n g substrate. F o r this s t u d y , the g r a f t i n g of styrene onto l o w - d e n s i t y p o l y e t h y l e n e ( L D P E ) w a s c h o s e n as t h e m o d e l r e a c t i o n . T h e t h e o r e t i c a l a s s u m p t i o n s u n d e r l y i n g t h e e v a l u a t i o n o f t h e m e a s u r e m e n t s of l i g h t s c a t t e r i n g have been discussed elsewhere (5). Materials

and

Experimental

Techniques

Materials. L D P E o f g r a d e A or Β w a s u s e d as t h e g r a f t i n g s u b s t r a t e ; it w a s i n t h e f o r m of p e l l e t s . G r a d e A is a h o m o p o l y e t h y l e n e w i t h a w e i g h t average molecular weight M = 5.7 Χ 10 , a m o l e c u l a r n o n u n i f o r m i t y U = (M /M — 1) ~ 18, a m o l e c u l a r w e i g h t of t h e u n b r a n c h e d c h a i n s e c t i o n s ( l o n g - c h a i n b r a n c h i n g ) of M = 3 . 5 Χ 1 0 , a n d a d e n s i t y p = 0 . 9 1 6 g / c m . G r a d e Β is a c h e m i c a l l y u n i f o r m e t h y l e n e - v i n y l a c e t a t e r a n d o m c o p o l y m e r ( E V A ) w i t h h i g h l y b r a n c h e d s h o r t a n d l o n g c h a i n s , v i n y l a c e t a t e c o n t e n t = 8.5 w t % , M = 5 . 2 Χ 1 0 , C7 = 14, M = 5 Χ 1 0 , a n d = 0.925 g / c m . T h e styrene a n d acrylonitrile m o n o m e r s used w e r e freshly distilled. A z o d i (isobutyronitrile) (Porofor Ν ) ( P N ) , benzoyl peroxide ( B P O ) , and tert-butyl p e r o c t o a t e ( f - B O ) w i t h a p u r i t y h i g h e r t h a n 9 9 % w e r e u s e d as i n i t i a t o r s . G r a f t i n g under Swelling Conditions. G r a f t i n g u n d e r s w e l l i n g c o n d i t i o n s was c a r r i e d out i n aqueous dispersion. T h i s g r a f t i n g m e t h o d increased the p r o b l e m of d i f f u s i o n a l c o n t r o l d u r i n g t h e r e a c t i o n . O n t h e o t h e r h a n d , t h i s m e t h o d a v o i d e d t r a n s f e r r e a c t i o n s b y s o l v e n t s a n d p e r m i t t e d t h e use of l o w e r temperatures than d i d b u l k grafting i n the melt. A t constant temperature, the i n d i v i d u a l components diffused into the pellets. S u b s e q u e n t l y , the p o l y m e r i z a t i o n process was started b y increasing 5

w

w

n

3

r

w

5

r

3

3

P


3

216

COPOLYMERS,

T a b l e I.


Initiator Expertment

ΡΕ Grade

M R ZB ZA U A Ν Ε L ZG S G OA 0 OC OE Q

A

a b c

Β

1.0 0.70

t-EO

0.784 0.784

BPO BPO *-BO

0.091 0.70 0.23 0.72 0.784

t-BO t-BO

A N DC O M P O S I T E S

G r a f t i n g Efficiencies" Swelling

Cone, Type mole % T, ° c

PN BPO

POLYBLENDS,

65 50 60 70 70/50 85/50 85/50 50 50 70 70/50 85/50 50 85/50 75 80/50 85/50

Time, hrs

5 24 3 3 3/3 3/1 3/5 3 24 3 3/5 3/1 5 3/5 3 2/3 3/5

merization

T, °C

Time, hrs

80

8

85 85 90 90 85

5 8

„ Grafting PrepStyEffi- arahon rene, ciency , Prowt % % cedure h

38.6 35.9 44.3 41.5 43.9 43.7 42.7 42.4 36.5 44.0 42.9 43.7 20.2 43.2 20.4 18.5 44.3

6.2 12.9 10.9 9.1 10.0 11.0 14.9 56.4 59.7 51.8 52.2 51.6 17.0 23.2 76.0 69.0 68.8

e

I I I I II II II I I I II II III II III III II

Referred to the styrene polymerized in the pellets. See text. Chain regulator, isobutene.

the temperature, a n d it w a s c o m p l e t e d at constant temperature. I n several cases, t h e i n i t i a t o r w a s n o t a d d e d t o t h e s y s t e m u n t i l a f t e r t h e p e l l e t s h a d b e e n swollen w i t h styrene i n order to allow i t to diffuse into t h e pellets at a l o w e r temperature thereby preventing premature, u n w a n t e d decomposition of the p e r o x i d e . T h e s w e l l i n g p e r i o d w a s g e n e r a l l y 3 - 5 h r s , b u t i t w a s also e x t e n d e d u p to 2 4 hrs. Preparation Procedures. P R E P A R A T I O N P R O C E D U R E I. I n a 6-1 s t i r r i n g vessel, 2 0 0 0 g d e i o n i z e d water, 6 0 0 g L D P E , 8 0 m l 1 0 % dispersant solution ( 1 : 1 c o p o l y m e r of m e t h a c r y l i c a c i d a n d m e t h y l methacrylate ) , 4 9 0 g styrene, a n d 8 g r a d i c a l f o r m e r w e r e s t i r r e d u n d e r n i t r o g e n w i t h c o n d i t i o n s as i n d i c a t e d i n T a b l e I. S u b s e q u e n t l y t h e p e l l e t s w e r e c o l l e c t e d o n a s u c t i o n filter, w a s h e d w i t h p l e n t y o f w a t e r , a n d d r i e d f o r 4 8 - 7 2 h r s in vacuo a t 5 0 ° C . T h e s t y r e n e content of the pellets w a s d e t e r m i n e d f r o m the w e i g h t increase of the pellets:

/ χ vield - 600 styrene (wt %) = * ^ j - j PREPARATION PROCEDURE II. I n a 6-1 s t i r r i n g v e s s e l , 2 0 0 0 g d e i o n i z e d water, 6 0 0 g L D P E , 4 0 0 g styrene, a n d 8 0 m l 1 0 % dispersant solution were s t i r r e d u n d e r n i t r o g e n w i t h c o n d i t i o n s as i n d i c a t e d i n T a b l e I. T h e m i x t u r e was then cooled to 5 0 ° C , a n d a solution of 8 g r a d i c a l f o r m e r i n 9 2 g styrene was a d d e d . T h e b a t c h was stirred at 5 0 ° C f o r the time i n d i c a t e d . P o l y m e r i z a tion w a s started b y increasing t h e temperature. A t the e n d of the reaction, t h e p e l l e t s w e r e s u c t i o n e d off, w a s h e d , a n d t h e n d r i e d f o r 4 8 - 7 2 h r s in vacuo at 5 0 ° C . T h e s t y r e n e c o n t e n t o f t h e b e a d s w a s c a l c u l a t e d f r o m t h e w e i g h t i n c r e a s e as i n P r o c e d u r e I. P R E P A R A T I O N P R O C E D U R E III. T h e p r o c e d u r e w a s t h e s a m e as i n I a n d II except that 2 5 0 g styrene a n d 1 0 0 0 g L D P E were used. T h e aliphatic m o n o o l e f i n s u s e d as c h a i n r e g u l a t o r s w e r e p a s s e d t h r o u g h t h e b a t c h via a n a s c e n d i n g pipe. Analysis of the Polymer. G r a f t i n g e f f i c i e n c y a n d d e g r e e o f g r a f t i n g w e r e determined b y fractional precipitation. A f t e r the graft p r o d u c t w a s dissolved


20.

ALBERTS

E T

Grafting

AL.

onto Ethylene

217

Polymers

i n t o l u e n e or t o l u e n e - d i m e t h y l f o r m a m i d e m i x t u r e s at 1 0 0 ° C , u n g r a f t e d p o l y styrene ( P S ) or p o l y s t y r e n e - a c r y l o n i t r i l e r e m a i n e d i n solution after c o o l i n g a n d was separated. A f t e r t h e p r e c i p i t a t e d p r o d u c t w a s i s o l a t e d a n d d r i e d , its styrene content was d e t e r m i n e d , a n d f r o m this the g r a f t i n g efficiency w a s calculated:


grafting efficiency (%)

=

PS grafted X PS grafted + homo-PS

100

T h e l i g h t s c a t t e r i n g of t h i s m a t e r i a l c o u l d b e m e a s u r e d e v e n t h o u g h it s t i l l c o n t a i n e d u n g r a f t e d p o l y e t h y l e n e . I n t h e n e w m e t h o d of l i g h t s c a t t e r i n g , u n g r a f t e d p o r t i o n s of p o l y e t h y l e n e d o n o t i n t e r f e r e w i t h t h e d e t e r m i n a t i o n of the m o l e c u l a r w e i g h t of the graft chains, nor, after r e f r a c t i o n a t i o n a c c o r d i n g to t h e m o l e c u l a r w e i g h t o f t h e g r a f t c h a i n s , w i t h t h e d e t e r m i n a t i o n of t h e 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 of t h e g r a f t c h a i n s ( 5 ) . I n o r d e r to d e t e r m i n e t h e d e g r e e of g r a f t i n g of t h e s u b s t r a t e , h o w e v e r , i t w a s n e c e s s a r y to d e t e r m i n e t h e a m o u n t of u n g r a f t e d p o l y e t h y l e n e . B y c a r e f u l p r e c i p i t a t i o n of t h e g r a f t p r o d u c t a f t e r s e p a r a t i o n of u n g r a f t e d p o l y s t y r e n e f r o m a 1 : 1 t o l u e n e - n - h e p t a n e s o l u t i o n at 9 4 ° C w i t h 1 - b u t a n o l , i t w a s p o s s i b l e to s e p a r a t e t h e f r e e p o l y e t h y l e n e a l m o s t q u a n t i t a t i v e l y since it r e m a i n e d i n solution u n d e r these conditions. T h e p o l y s t y r e n e content of the i n d i v i d u a l fractions w a s d e t e r m i n e d b y Ν M R s t u d i e s i n t e t r a c h l o r o e t h y l e n e at 8 0 ° C f r o m t h e r a t i o of t h e p e a k areas of t h e a r o m a t i c a n d a l i p h a t i c p r o t o n s . W h e n s t y r e n e c o n t e n t w a s l o w , d e t e r m i n a t i o n w a s m a d e b y I R s p e c t r o s c o p y o f films, u s i n g a c a l i b r a t i o n curve. A c r y l o n i t r i l e was d e t e r m i n e d b y nitrogen analysis. L i g h t scattering w a s m e a s u r e d i n t o l u e n e at 9 0 ° C at t h e w a v e l e n g t h λ = 5 4 6 1 A . U n d e r t h e s e c o n d i t i o n s , s o l v e n t s a n d g r a f t i n g bases w e r e a l m o s t i s o r e f r a c t i v e , i.e. o n l y t h e graft chains w e r e visible. T h e solutions used i n a l l measurements w e r e p u r i f i e d b y c e n t r i f u g a t i o n at 9 0 ° C a n d a p p r o x i m a t e l y 1 0 , 0 0 0 g.

Temperature of the crystallisation start [°C]

100

90

80J

701

60H ?

50 λ

cooling rate 0,8

°C/min

AOH

30 0

10

20

30

40

50

60

Concentration of the polyethylene solution [wt%

Figure 2.

70

80

90

100

polyethylene]

Temperature of the start of crystallization as a function of polyethylene concentration in xylene solution • , LDPE

A; and

·,

LDPE

Β


218

COPOLYMERS,

POLYBLENDS,

A N D

COMPOSITES

Results Styrene G r a f t i n g .

I n a l l phases of grafting u n d e r s w e l l i n g conditions, the

spherical polyethylene pellets retained their shape; the only changes were a n i n c r e a s e i n d i a m e t e r as a f u n c t i o n o f t h e s t y r e n e c o n t e n t ,

a n d a n increase i n

t r a n s p a r e n c y as a f u n c t i o n o f t h e d e g r e e o f g r a f t i n g a n d t h e s t y r e n e F r o m the increase clude

that

there

i n the transparency w a s a n increase

content.

of the grafted pellets, one c o u l d con

i n compatibility with

increasing

grafting

efficiency. DIFFUSION

PROCESSES.

T h e rate of diffusion of styrene into L D P E w a s a


complex function of the temperature LDPE

(8).

(6, 7) a n d also of t h e c r y s t a l l i n i t y of t h e

A s t h e temperature a n d content of solvents like xylene a n d styrene

increased, the degree of crystallinity of polyethylene decreased In other words, w h e n a homogeneous at a c o n s t a n t

rate,

(Figure 2 ) .

polyethylene solution was cooled

the p o i n t of b e g i n n i n g crystallization w a s reached.

p o i n t w a s i n d i c a t e d b y t h e first v i s u a l t u r b i d i t y . A t a l l c o n c e n t r a t i o n s ,

This homo-

p o l y m e r A h a d a p o i n t o f b e g i n n i n g c r y s t a l l i z a t i o n a b o u t 10 ° C h i g h e r t h a n t h a t of c o p o l y m e r B . T h i s

finding

seemed reasonable

since the crystallization tem

perature depends o n the comonomer content a n d o n the degree of short-chain b r a n c h i n g of the polyethylene.

F r o m the curve w e c o n c l u d e d that t h e tem

perature at w h i c h styrene diffused into L D P E

A was about

1 0 ° C above

that

f o r d i f f u s i o n i n t o L D P E Β at a c o m p a r a b l e r a t e o f d i f f u s i o n . T h e homogeneity of styrene distribution i n t h e swollen pellets after i n c i p i ent styrene p o l y m e r i z a t i o n w a s another determinant of p r o p e r process

condi

tions ( F i g u r e 3 ) . W i t h h o m o p o l y e t h y l e n e A , f o r example, 24 hrs of s w e l l i n g

Density of pellets 0,945-

50°C

0,955-J

*x 50°C 24

h

• 65°C x

5

H

•χ.— 85°C x

0,9751

î

2

-i

3

1

1

4

5

3

Γ"

6

~7

8

9

TO"

Number of pellets Figure 3.

Effect of swelling conditions on the styrene content of the pellets


H

20.

ALBERTS

10090-

E T

Grafting

A L .

onto Ethylene

Polymers

219

A polyethylene A Β EVA copolymer Β

Grafting efficiency 10

80 70Β


6050 40 30 20 10 0 α-a'azodiisobutyronitrile Figure 4.

benzoyl peroxide

t-butyl peroctoate

Initiator and substrate activity in grafting styrene

at 5 0 ° C r e s u l t e d i n a m u c h less u n i f o r m s t y r e n e d i s t r i b u t i o n i n t h e p e l l e t s t h a n s w e l l i n g at 8 5 ° C f o r 3 h r s . W h e n s w e l l i n g w a s a c c o m p l i s h e d at a m b i e n t t e m p e r a t u r e w i t h s u b s e q u e n t p o l y m e r i z a t i o n at 6 0 ° - 9 0 ° C , h o m o g e n e o u s s t y r e n e distribution i n the pellets c o u l d not be obtained. T h e polystyrene w a s con centrated at the surface of the pellets f o r m i n g a n outer s k i n ; t h e g r a f t i n g efficiency w a s very poor. H o w e v e r , a homogeneous styrene distribution c o u l d b e o b t a i n e d b y s w e l l i n g at e l e v a t e d t e m p e r a t u r e ( 6 5 ° - 8 5 ° C , see F i g u r e 3 ) . T h i s c o n d i t i o n w a s essential f o r o b t a i n i n g h i g h g r a f t i n g efficiencies. I n 1 9 5 9 , H o f f m a n et al. ( 9 ) r e p o r t e d t h e i m p o r t a n c e o f t h e d i f f u s i o n t e m perature i n r a d i a t i o n - i n d u c e d grafting of styrene under s w e l l i n g conditions. T h e y used the term "diffusion-controlled grafting," a n d they noted that w h e n s t y r e n e w a s g r a f t e d s t e p w i s e (first s w e l l i n g , t h e n g r a f t i n g ) , o n l y t h e first step w a s d i f f u s i o n - c o n t r o l l e d . O f c o u r s e , s i n c e t h e m e d i u m i n s i d e t h e p e l l e t s is highly viscous, the termination reaction should be diffusion-controlled. T h i s is d i s c u s s e d b e l o w . GRAFTING EFFICIENCY. I n a l l the batches mentioned above, polymeriza tion yields exceeded 9 0 % . T h e graft products contained 1 0 - 4 5 w t % poly styrene. W h e n the g r a f t i n g efficiencies attained w i t h t h e various peroxides under otherwise identical conditions were compared, the following activity scale w a s established: a z o d i ( i s o b u t y r o n i t r i l e ) ( I ) < b e n z o y l peroxide (10, 11, 12) < tert-buty\ p e r o c t o a t e ( F i g u r e 4 ) . U n d e r i d e n t i c a l reaction conditions, the v i n y l acetate content of t h e back b o n e p o l y m e r h a d a s l i g h t effect o n t h e g r a f t i n g e f f i c i e n c y . T h e g r a f t i n g effi ciency w i t h E V A w a s about 3 0 w t % greater t h a n that w i t h h o m o p o l y e t h y l e n e A (Figure 4 ) . Saponification of the E V A backbone d i d not change the poly-


220

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

styrene content of the graft c o p o l y m e r ; this meant that styrene w a s grafted m a i n l y i n the C - c h a i n of t h e substrate m o l e c u l e .


T h e f a c t o r s t h a t d e c i s i v e l y a f f e c t e d t h e g r a f t i n g e f f i c i e n c y i n g r a f t i n g styr e n e w e r e : (a) t h e a c t i v i t y o f t h e i n i t i a t o r r a d i c a l ( t h e p r e d o m i n a n t f a c t o r ) , a n d (b) t h e a c t i v i t y o f t h e g r a f t i n g s u b s t r a t e , i.e. t h e c h e m i c a l c o m p o s i t i o n , structure, a n d crystallinity of t h e substrate. I n o r d e r t o i n v e s t i g a t e t h e m u t u a l effects o f t h e p o l y e t h y l e n e , s t y r e n e , a n d initiator ratios o n the g r a f t i n g efficiency, the f o l l o w i n g experiments w e r e performed.

40302010-1 0-1

([J]/fMl)x10 1

1

1 2 Figure 5.

1

1

1

3 4 5

1

10

2

r

15

20

Grafting efficiency as a function of monomer concentration [LDPE]

and [I], constant; and [M], decreasing

(a) T h e s t y r e n e c o n c e n t r a t i o n w a s v a r i e d w h i l e t h e q u a n t i t i e s o f i n i t i a t o r a n d polyethylene A w e r e kept constant. T h e grafting efficiency increased l i n e a r l y as s t y r e n e c o n c e n t r a t i o n d e c r e a s e d (see F i g u r e 5 ) . T h i s f i n d i n g c o u l d b e i n t e r p r e t e d as f o l l o w s : w i t h d e c r e a s i n g s t y r e n e c o n c e n t r a t i o n , t h e p r o b a b i l i t y of a transfer reaction b e t w e e n initiator r a d i c a l a n d p o l y m e r c h a i n i n creased, a n d hence the possibility of c h a i n - g r o w i n g started b y substrate radicals also increased. (h) T h e q u a n t i t y o f p o l y e t h y l e n e w a s k e p t c o n s t a n t w h i l e t h e s t y r e n e a n d initiator concentrations were decreased i n such a w a y that the ratio of their concentrations w a s kept constant. T h e grafting efficiency increased u p to a p r a c t i c a l l y c o n s t a n t v a l u e ( F i g u r e 6 ) . T h i s series o f e x p e r i m e n t s s u g g e s t e d t h a t , a l t h o u g h t h e t r a n s f e r r e a c t i o n t o w a r d s t h e s u b s t r a t e w a s p r o m o t e d a t first,


20.

ALBERTS

E T

Grafting

A L .

onto Ethylene

221

Polymers


grafting efficiency

-ι

1

1

2

Figure 6.

1

1

1

1

1

r

3

4

5

6

7

8

Grafting efficiency as a function of initiator concentration

[LDPE],

constant; [M] and [I], decreasing; and [M]/[I], constant

Grafting efficiency %

704

I

1

0,5 Figure 7.

1

1

1

1

1

1,0

1,5

2,0

2,5

3,0

Grafting efficiency as a function of initiator concentration [LDPE]

and [M], constant; and [I], increasing


222

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

Grafting efficiency % 10CM

90 H


80

70-

60-

50

2

3

5

Figure 8.

Τ -

6

8

Grafting styrene onto EVA Β

Chain transfer agent, isobutene; [LDPE]

Enthalpies

—Γ"

Initiator wt % T ι— 9 10

and [M], constant; and [I], increasing

[joule/g] 100%

1007.

-ΔΗ,

100T-. 90-I ο

a.

80· 70

βοΗ à'

50· Α0· 3020Η 10

_^

Styrene content

[wt %]

0 20

Figure 9.

40

30

50

Heat of melting ( A H ) and heat of crystallization ( A H ) vs. styrene content of LDPE A-g-styrene m

C


20.

ALBERTS

E T A L .

its s t a t i s t i c a l f r e q u e n c y

Grafting decreased

onto Ethylene

223

Polymers

as t h e r e s u l t o f t h e c o n s t a n t

reduction i n

a m o u n t of initiator.


(c) T h e quantities of p o l y e t h y l e n e a n d styrene w e r e k e p t constant w h i l e the initiator concentration was increased. F i r s t a n increase i n g r a f t i n g efficiency was observed a n d then a decrease ( F i g u r e 7 ) . T h e decrease i n grafting efficiency at v e r y h i g h initiator concentrations suggested a n increase i n the n u m b e r of c h a i n t e r m i n a t i o n reactions b e t w e e n E V A p o l y m e r radicals a n d p r i m a r y r a d i c a l s . O n t h e o t h e r h a n d , w i t h i n c r e a s i n g q u a n t i t y of p r i m a r y radicals, the starting reaction of styrene p o l y m e r i z a t i o n w i l l b e p r o m o t e d . (d) T h e g r a f t i n g r e a c t i o n w a s a c c o m p l i s h e d as i n c b u t i n t h e p r e s e n c e of a c h a i n t r a n s f e r a g e n t s u c h as i s o b u t e n e . G r a f t i n g efficiency r e m a i n e d constant w h e n initiator concentration w a s increased ( F i g u r e 8 ) . T h i s striking effect c o u l d b e i n t e r p r e t e d e a s i l y . B e c a u s e of t h e m e c h a n i s m o f r a d i c a l p o l y m e r i z a t i o n i n t h e p r e s e n c e o f c h a i n t r a n s f e r a g e n t s , t h e i n i t i a t o r first e f f e c t e d t h e f o r m a t i o n of a r e g u l a t o r r a d i c a l . W i t h m o n o o l e f i n s as c h a i n t r a n s f e r a g e n t s , the regulator r a d i c a l d i d n o t initiate c h a i n - g r o w i n g , b u t o n l y t h e transfer reac t i o n to t h e E V A s u b s t r a t e . W h e n i n i t i a t o r c o n c e n t r a t i o n i n c r e a s e d , t h e c o n t e n t of u n g r a f t e d s u b s t r a t e m o l e c u l e s d e c r e a s e d i n a c c o r d a n c e w i t h t h e p o s t u l a t e d transfer m e c h a n i s m . STRUCTURAL INVESTIGATIONS. T h e m e l t i n g a n d crystallization points a n d the m e l t i n g a n d crystallization enthalpies of styrene graft p o l y m e r s w i t h dif ferent styrene contents a n d different g r a f t i n g efficiencies w e r e d e t e r m i n e d b y differential scanning calorimetry. T h e m e l t i n g a n d crystallization points were

% Elongation 700 *

600

500J

400H

300H

200 H

100 wt % Polystyrene 10 Figure 10.

-τ—

20

~30~

Grafting styrene onto EVA Β—elongation styrene content

40

50

at break vs. poly


224

COPOLYMERS,

dyn /cm

POLYBLENDS,

A N D COMPOSITES

2

LDPE - Β : a tan$ · G " B-g-styrene: Δ tan $ ο G"( product OC ]


10*

temperature °C

ν ° -160

Figure

- K O

11.

-120

-100

Dynamic

- 80

- 60

--.0

- 20

0

+20

+40

+ 100

+ 120

+ KQ

mechanical behavior of LDPE Β and LDPE (Experiment OC in Table I)

+160

+180

B-g-styrene

The transitions were obtained from the maximum in G " at about —25°C. The simultaneous transi tion at about —140°C is the y-transition of polyethylene.

not affected b y t h e g r a f t i n g process ( F i g u r e 9 ) . T h e levels of the c r y s t a l l i z a tion a n d m e l t i n g enthalpies of the graft polymers decreased w i t h increasing polystyrene content. H o w e v e r , w h e n the measured data were standardized to 1 0 0 % p o l y e t h y l e n e , n o d e v i a t i o n f r o m t h e m e l t i n g e n t h a l p y of the o r i g i n a l p o l y e t h y l e n e w a s detectable w i t h i n the l i m i t of error of this m e t h o d . T h e g r a f t i n g process h a d n o a p p r e c i a b l e effect o n t h e c r y s t a l l i n i t y of t h e g r a f t e d s u b s t r a t e , a n d o n l y a s l i g h t effect o n p o s t - c r y s t a l l i z a t i o n . F r o m t h e f a c t t h a t t h e c r y s t a l l i z a t i o n a n d m e l t i n g e n t h a l p i e s d e c r e a s e d l i n e a r l y as s t y r e n e c o n t e n t i n c r e a s e d , i t w a s c o n c l u d e d t h a t p o l y s t y r e n e w a s p r e s e n t as a v i r t u a l l y i n e r t filler i n a c o n t i n u o u s p o l y e t h y l e n e p h a s e .

Figure 12. Electron photomicrograph of styrene grafted onto LDPE Β (Ex periment OC in Table I)

Figure 13. Electron photomicrograph of styrene grafted onto LDPE Β (Ex periment OA in Table I)

Styrene content, 20.4 wt %; and grafting efficieny, 76%

Styrene content, 20.2 wt %; grafting effi ciency, 17%; and ungrafted polystyrene re moved with THF



20.

ALBERTS

Grafting

ET AL.

Figure 14. Electron photomicrograph of styrene grafted onto LDPE A (Expériment A in Table I) Styrene content, 43.7 wt %; and grafting efficiency, 11.Οψο

onto Ethylene

225

Polymers

Figure 15. Electron photomicrograph of graft product A at higher magnification after removal of ungrafted polystyrene with THF (cf. Figure 14)

T h i s c o n c e p t i o n w a s s u p p o r t e d b y the elongation measurements of t h e graft p o l y m e r s ( F i g u r e 1 0 ) . A s styrene content increased, the m a x i m u m elongation decreased linearly, regardless of the grafting efficiency. T h i s be h a v i o r is w e l l k n o w n f o r e t h y l e n e p o l y m e r s l o a d e d w i t h f i l l e r s ( 1 3 ) . T h e d a t a suggested a decrease i n the entanglement between the polyethylene crystallites as a r e s u l t o f t h e s u r r o u n d i n g , i n c o m p a t i b l e p o l y s t y r e n e m o l e c u l e s . I n F i g u r e 11, the d y n a m i c m e c h a n i c a l behaviors of E V A Β a n d of the g r a f t p r o d u c t f r o m E x p e r i m e n t O C (see T a b l e I ) , w h i c h h a d a s t y r e n e c o n t e n t of 2 0 . 4 w t % a n d a g r a f t i n g e f f i c i e n c y o f 7 6 . 0 % , a r e c o m p a r e d . T h e glass transition temperature T of t h e graft p o l y m e r w a s — 2 6 ° C whereas the T of E V A Β w a s — 2 7 ° C ; therefore, the difference w a s not significant. T h e m o d u l u s of t h e g r a f t w a s s l i g h t l y c h a n g e d f r o m t h a t o f L D P E B , so a s l i g h t m u t u a l effect of t h e p o l y s t y r e n e a n d p o l y e t h y l e n e regions w a s reasonable. g

g

T h e m o r p h o l o g y of the graft products w a s elucidated f r o m electron p h o t o m i c r o g r a p h s of m i c r o t o m e d , t h i n sections of t h e grafts. I n F i g u r e s 12 a n d 13, the g r a f t i n g p r o d u c t s h a d a styrene content of about 2 0 w t % , a n d E V A Β w a s the substrate i n the g r a f t i n g reaction. D e s p i t e t h e w i d e l y different g r a f t i n g efficiencies, o n l y slight structural differences w e r e apparent. When L D P E A w a s the substrate, it w a s reasonable to assume styrene occlusions i n a polyethylene matrix ( F i g u r e 1 4 ) . T h e styrene content of this graft p o l y m e r was about 4 0 w t % . A f t e r the ungrafted polystyrene units were r e m o v e d w i t h tetrahydrofuran ( T H F ) , the styrene occlusions were clearly visible ( F i g u r e 1 5 ) . S i n c e i n t h i s case t h e s u b s t r a t e w a s L D P E A w h i c h h a s a c o n s i d e r a b l y greater tendency t o w a r d crystallization ( F i g u r e 2) a n d a crystallization t e m perature about 1 0 ° C h i g h e r t h a n that of E V A B , the f o l l o w i n g c o u l d b e assumed. C o o l i n g to 5 0 ° C d u r i n g the peroxide diffusion p e r i o d resulted i n the f o r m a t i o n of crystalline regions w h i c h caused the styrene to a c c u m u l a t e i n occlusions. W h e n p o l y m e r i z a t i o n w a s started b y a n increase i n temperature, the rate of styrene p o l y m e r i z a t i o n w a s faster t h a n that of styrene d i f f u s i o n t h r o u g h the s l o w l y d i s a p p e a r i n g crystalline regions, a n d , hence, the styrene occlusions w e r e retained. W h e n E V A w a s the g r a f t i n g substrate, the styrene o c c l u s i o n s d i d n o t f o r m t o t h e s a m e e x t e n t as i n p r o d u c t A b e c a u s e o f t h e clearly l o w e r crystallization degree, tendency, a n d temperature of E V A . Compatibility of Graft Copolymers. F r o m t h e i n c r e a s e i n t h e t r a n s p a r e n c y of t h e g r a f t e d pellets w i t h i n c r e a s i n g degree of g r a f t i n g , a n increase i n c o m -


226

COPOLYMERS,

\


2

ο without propylene

3

4

5

6

Fraction no. Figure 16.

A N D COMPOSITES

χ propylene a s chain transfer agent \

1

POLYBLENDS,

7 •

Intrinsic viscosities of different fractions of EVA-g-styreneco-acrylonitrile

p a t i b i l i t y w i t h i n c r e a s i n g g r a f t i n g e f f i c i e n c y c o u l d also b e c o n c l u d e d . A q u a n t i tative measure of p o l y m e r c o m p a t i b i l i t y c o u l d be d e r i v e d f r o m measurements of l i g h t s c a t t e r i n g f r o m t h e d e t e r m i n a t i o n o f t h e s e c o n d v i r i a l c o e f f i c i e n t a n d t h e r a d i i o f g y r a t i o n (5, 14, 15) as w e l l as f r o m t h e d e t e r m i n a t i o n o f t h e first v i s u a l t u r b i d i t y p o i n t ( 5 ) . H e n c e , the shorter the graft chains, the m o r e c o m p a t i b l e they w e r e w i t h the g r a f t i n g base. W h e n t h e ratio of m o n o m e r - t o g r a f t i n g substrate a n d the g r a f t i n g efficiency w e r e k e p t constant, a decrease i n t h e m o l e c u l a r w e i g h t o f t h e g r a f t c h a i n i n c r e a s e d t h e n u m b e r o f g r a f t i n g sites. T h e resultant consequences f o r the p r o p e r t y pattern of graft polymers c o u l d be e x e m p l i f i e d b y t h e g r a f t i n g of s t y r e n e - a c r y l o n i t r i l e c o m b i n a t i o n s onto a n e t h y l e n e - v i n y l acetate c o p o l y m e r . Styrene-Acrylonitrile G r a f t i n g . T h u s , t h e k e y t o i m p r o v i n g t h e c o m p a t i b i l i t y of graft copolymers w a s n o t o n l y a n i m p r o v e m e n t i n the grafting e f f i c i e n c y , b u t i t w a s also c o n t r o l o f t h e n u m b e r a n d m o l e c u l a r w e i g h t o f t h e graft chains. A l t h o u g h the molecular w e i g h t of the graft chains c o u l d be r e d u c e d b y r e g u l a t o r s s u c h as m e r c a p t a n s , a h i g h e r m e r c a p t a n c o n c e n t r a t i o n also r e s u l t e d i n a d e c r e a s e d g r a f t i n g e f f i c i e n c y (16). A n o p t i m u m system of regulators w a s f o u n d ; i t n o t o n l y r e d u c e d the m o l e c u l a r w e i g h t s of the graft c h a i n s b u t also i n c r e a s e d t h e i r n u m b e r t h r o u g h c h a i n t r a n s f e r t o t h e p o l y m e r i c s u b s t r a t e . T h e s e r e g u l a t o r s w e r e m o n o o l e f i n s s u c h as e t h y l e n e , p r o p y l e n e , a n d isobutene. W h e n a c o m b i n a t i o n of styrene a n d acrylonitrile w a s grafted onto ethylene p o l y m e r s i n the presence of these monoolefins, t h e graft p r o d u c t s w e r e e x t r e m e l y h o m o g e n e o u s , as w a s p r o v e d b y a n a l y s i s o f t h e g r a f t p r o d u c t s a f t e r p r e p a r a t i v e p r e c i p i t a t i o n f r a c t i o n a t i o n ( F i g u r e 1 6 ) . B e c a u s e these graft p r o d -


20.

ALBERTS

E T

Grafting

A L .

onto Ethylene

227

Polymers


ucts have drastically r e d u c e d proportions of free styrene-acrylonitrile c o p o l y m e r a n d because they have more graft chains w i t h r e d u c e d molecular w e i g h t t h a n do t h e graft products o b t a i n e d w i t h o u t regulators, their c o m p a t i b i l i t y w a s c o n s i d e r a b l y i m p r o v e d . A s a result of their h o m o g e n e i t y , their processing has been improved. I n order to obtain a better u n d e r s t a n d i n g of the relations b e t w e e n m o l e c u lar w e i g h t a n d t h e c o m p a t i b i l i t y of t h e g r a f t i n g substrate, it w a s necessary to d e t e r m i n e the m o l e c u l a r w e i g h t of the grafted chains of the graft c o p o l y m e r . S u c h investigations c o u l d be facilitated b y oxidative degradation of the backbone p o l y m e r a n d determination of the molecular weights a n d molecular w e i g h t d i s t r i b u t i o n s o f t h e i s o l a t e d g r a f t c h a i n s w h e n p o l y b u t a d i e n e w a s u s e d as t h e g r a f t i n g s u b s t r a t e ( 1 7 , 18, 19). Ethylene polymers, however, could not be subjected to s u c h d e g r a d a t i o n reactions since this w o u l d l e a d u n a v o i d a b l y to degradation of the grafted chains too. M o l e c u l a r Weights of the Graft Chains. T h e m o l e c u l a r w e i g h t s o f t h e graft chains were d e t e r m i n e d b y m e a s u r i n g the light scattering u n d e r those conditions w h e n the grafting substrate w a s isorefractive w i t h the solvent ( 5 ) . T h i s n e w m e t h o d of determination w a s tried o n E V A - p o l y s t y r e n e graft c o p o l y m e r s w h i c h were o b t a i n e d b y grafting a p p r o x i m a t e l y 2 0 w t % styrene onto L D P E B . W h e n the polyethylene/styrene ratio w a s kept constant, the initiator concentration c o u l d b e v a r i e d to obtain graft chains differing i n length (Figure 17). (5)

E v a l u a t i o n of the anomalous Z i m m diagrams f o r graft p o l y m e r solutions r e v e a l e d o n l y a n e g l i g i b l e d e v i a t i o n of t h e m o l e c u l a r weights of the graft

1 Figure 17.

2

3

4

5

6

7

8

9

10

Molecular weight of grafted and ungrafted chains as a function of initiator concentration (wt % )



228

COPOLYMERS,

POLYBLENDS,

A N D COMPOSITES

chains f r o m those of the u n g r a f t e d p o l y s t y r e n e . I n a d d i t i o n to t h e e x p e c t e d d e c r e a s e i n m o l e c u l a r w e i g h t s as i n i t i a t o r c o n c e n t r a t i o n i n c r e a s e d , t h e r e w a s a decrease i n the a m o u n t of u n g r a f t e d polyethylene a n d a n increase i n the c o m p a t i b i l i t y of the graft chains w i t h the g r a f t i n g substrate ( 5 ) . F i g u r e 18 repre sents t h e i n t e g r a l 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 s o f t h e g r a f t c h a i n s as d e t e r m i n e d b y m e a s u r i n g the light scattering after preparative fractionation. T h e w e i g h t averages of the m o l e c u l a r w e i g h t s c a l c u l a t e d f r o m these d i s t r i b u t i o n s a g r e e d w e l l w i t h t h e v a l u e s o b t a i n e d f o r t h e u n g r a f t e d p o l y s t y r e n e as w e l l as f o r t h a t f r a c t i o n f r o m w h i c h o n l y t h e u n g r a f t e d p o l y s t y r e n e w a s r e m o v e d by fractionation. I n a d d i t i o n , the m o l e c u l a r non-uniformities of the graft c h a i n s c a l c u l a t e d f r o m F i g u r e 18 w e r e t h e s a m e as t h o s e d e t e r m i n e d f o r t h e u n g r a f t e d polystyrene b y g e l permeation c h r o m a t o g r a p h y ( 5 ) . F r o m the corre s p o n d e n c e of the w e i g h t averages of the m o l e c u l a r w e i g h t s a n d of the n o n uniformities of the grafted a n d ungrafted polystyrenes, it w a s c o n c l u d e d that, at b e s t , o n e g r a f t c h a i n w a s l i n k e d t o o n e s u b s t r a t e m o l e c u l e . Conformation of the Graft Chains in Solution. T h e f i n d i n g s f r o m d i f f e r e n t i a l t h e r m a l a n a l y s i s ( D T A ) c o u l d b e i n t e r p r e t e d to m e a n t h a t g r a f t c h a i n s were practically not incorporated into the polyethylene crystalline regions. I n f o r m a t i o n a b o u t t h e g r a f t i n g site, i.e., w h e t h e r t h e g r a f t i n g p r o c e s s o c c u r r e d in fact w i t h i n the polyethylene molecule, c o u l d be derived f r o m the measured r a d i i of g y r a t i o n of the graft chains. A s the m o l e c u l a r w e i g h t of the graft chains decreased, the values for the r a d i i of gyration of the grafted polystyrene increased relative to those f o r the h o m o p o l y s t y r e n e ( F i g u r e 1 9 ) . T h i s increase c o u l d b e i n t e r p r e t e d as f o l l o w s . T h e L D P E Β u s e d as t h e g r a f t i n g s u b s t r a t e was h i g h l y l o n g - c h a i n - b r a n c h e d . E v e n i n solution, this p o l y e t h y l e n e m o l e c u l e was rather compact. T h e graft c o p o l y m e r ( p r o d u c t O C i n T a b l e I) contained 2 0 w t % styrene. A f t e r r e m o v a l of the u n g r a f t e d polystyrene a n d fractional p r e c i p i t a t i o n of the graft c o p o l y m e r , the r a d i i of g y r a t i o n of the fractions were measured. B y the n e w m e t h o d of l i g h t scattering measurements, o n l y the polystyrene molecules were visible. T h e radii of gyration of the polystyrene molecules d e p e n d o n their conformation i n solution. Molecules w i t h coiled c o n f o r m a t i o n i n solution have smaller r a d i i of gyration than d o stretched molecules w i t h the same molecular weight.

Figure 18.

Molecular weight distributions of grafted chains

Initiator concentration: X, 0.48 wt % and ·, 5.0 wt %


20.

A L B E R T S

E T

Grafting

A L .

onto Ethylene

229

Polymers

100 τ


10 -=- * · 10"

[

1

/ / /

0,1

:

^ M ^ w 1—ι

10* Figure 19. ·,

1—ι—I

M i l l

10

1—ι 5

1—I

ι

ι ι ι 11

10

1—ι

1—ι—I

6

I I I 11

10

7

Radii of gyration of polystyrene grafted onto different substrates

LDPE B; A, EVA (vinyl acetate content, 45 wt %); and polystyrene with a molecular non-uniformity of zero

, homo-

W h e n the polystyrene graft c h a i n w a s a n c h o r e d to the substrate w i t h i n the c o m p a c t p o l y e t h y l e n e m o l e c u l e , the graft c h a i n s h o u l d h a v e one section w i t h i n a n d one section outside the polyethylene molecule. T h e conformations of t h e s e s e c t i o n s w o u l d b e d i f f e r e n t . T h e i n n e r s e c t i o n w a s f o r c e d t o e n t e r i n t o as f e w i n t e r a c t i o n s w i t h t h e s u r r o u n d i n g p o l y e t h y l e n e m o l e c u l e as p o s s i b l e , w h i c h s h o u l d l e a d to stiffening of the i n n e r p o l y s t y r e n e c h a i n . T h e section of graft c h a i n outside the polyethylene molecule w o u l d be free a n d m o v a b l e , a n d it w o u l d have a n ordinary, c o i l e d conformation i n solution. T h e m e a s u r e d v a l u e of g y r a t i o n s h o u l d be c o n s i d e r e d the c o m b i n e d v a l u e f o r a n e x t e n d e d c h a i n component a n d a coiled c h a i n component. I n order to explain the increas i n g d e v i a t i o n of t h e r a d i i of g y r a t i o n of the graft chains f r o m those of h o m o p o l y s t y r e n e as t h e m o l e c u l a r w e i g h t d e c r e a s e d , w e h a v e t o a s s u m e t h a t t h e a v e r a g e l e n g t h o f t h e i n n e r g r a f t c h a i n w a s c o n s t a n t . T h e n , as t h e m o l e c u l a r w e i g h t of the graft c h a i n decreased, the c o i l e d c h a i n c o m p o n e n t of the radius of g y r a t i o n d e c r e a s e d , t o o , a n d t h e e x t e n d e d c h a i n c o m p o n e n t i n c r e a s e d .



230

COPOLYMERS,

POLYBLENDS,

AND

COMPOSITES

If t h i s i n t e r p r e t a t i o n i s c o r r e c t , t h e e f f e c t o f i n n e r g r a f t - c h a i n s t i f f e n i n g should be reduced w h e n the grafted polymer becomes more compatible w i t h t h e g r a f t e d s u b s t r a t e a n d / o r w h e n t h e s u b s t r a t e m o l e c u l e b e c o m e s less c o m pact. Therefore styrene was grafted onto a n e t h y l e n e - v i n y l acetate c o p o l y m e r ( v i n y l acetate content, 4 5 w t % ) w i t h a l o w e r l o n g - c h a i n b r a n c h i n g f r e q u e n c y (20) t h a n L D P E a n d i m p r o v e d c o m p a t i b i l i t y w i t h p o l y s t y r e n e ( 5 ) b y t h e s a m e p r e p a r a t i o n p r o c e d u r e (///) as f o r g r a f t i n g s t y r e n e o n t o L D P E B . A f t e r r e m o v a l of u n g r a f t e d p o l y s t y r e n e a n d f r a c t i o n a l p r e c i p i t a t i o n o f t h e E V A - g - s t y r e n e c o p o l y m e r , t h e r a d i i o f g y r a t i o n o f t h e f r a c t i o n s w e r e m e a s u r e d (see F i g u r e 1 9 ) . I n fact, the radii of gyration o f E V A - g - s t y r e n e , w h e n c o m p a r e d w i t h those of L D P E B-g-styrene, s h o w e d t h e expected smaller deviation f r o m t h e radii of gyration of u n g r a f t e d polystyrene. Perhaps, this r e m a i n i n g d e v i a t i o n c o u l d b e r e d u c e d if t h e r a d i i of gyration of t h e graft chains w o u l d b e calculated w i t h c o r r e c t i o n s f o r t h e s m a l l n o n - u n i f o r m i t y . F u r t h e r m o r e , t h e l i t t l e effect o f m o l e cules w i t h t w o or m o r e graft chains was neglected ( 5 ) i n t h e calculations. Conclusion T h e objective o f this s t u d y , n a m e l y i m p r o v e m e n t of t h e c o m p a t i b i l i t y of g r a f t p o l y m e r s , as m e n t i o n e d a t t h e b e g i n n i n g o f t h i s p a p e r , w a s r e a c h e d . C o n t r a r y t o t h e findings t h a t t h e g r a f t i n g o f s t y r e n e - a c r y l o n i t r i l e o n t o E V A c o p o l y m e r s r e s u l t s i n i n c o m p a t i b l e g r a f t p r o d u c t s — a s w a s r e p o r t e d i n 1 9 6 8 (1 ) — a n i m p r o v e d g r a f t i n g \' j h n i q u e n o w p e r m i t s t h e p r o d u c t i o n o f g r a f t t h e r m o plastics w i t h i m p r o v e d c o m p a t i b i l i t y . Acknowledgment T h e authors are grateful to M . H o f f m a n n f o r interesting discussions o n structural problems, to G . K a e m p f a n d L . M o r b i t z e r for investigations o n the m o r p h o l o g y of the graft copolymers, a n d to H . K r o e m e r for differential t h e r m a l analysis.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Bartl, H., Hardt, D., ADVAN. CHEM. SER. (1969) 91, 477. Mayo, F. R., Walling, C., Chem. Rev. (1950) 46, 191. Bamford, C. H., Jenkins, A. D., Johnston, R., Trans. Faraday Soc. (1959) 55, 41. Burnett, G. M., Wright, W. W., Proc. Roy. Soc. London (1954) A 221, 41. Kuhn, R., Alberts, H., Bartl, H., Makromol. Chem. (1974) 175, 1471. Bent, Η. Α., J. Polym.Sci.(1957) 24, 387. Pinsky, J., Mod. Plast. (1957) April, 145. Fuhrmann, J., Diremeyer, M., Rehage, G., Ber. Bunsenges. Phys. Chem. (1970) 74, 842. Hoffman, A. S., Gilliland, E. R., Merrill, E. W., Stockmayer, W. H.,J.Polym. Sci. (1959)34,461. Fischer, J. P., Angew.Makromol.Chem. (1973) 33, 35. Allen, P. W., Ayrey, G., Moore, C. G., J. Polym. Sci. (1959) 36, 55. Czvikovszky, T., Dobo, J., J. Polym. Sci. Part C (1967) 16, 2973. Miller, S. Α., "Ethylene and Its Industrial Derivatives," p. 473, Ernest Benn, London, 1969. Kuhn, R., Cantow, H.-J., Liang, S. B., Angew. Makromol. Chem. (1971) 18, 93. Kuhn, R., Bugdahl, V., Cantow, H.-J., Angew. Makromol. Chem. (1971) 18, 109. Hayes, R. Α.,J.Polym. Sci. (1953) 11, 531. Rieke, J. K., Hart, G. M., Saunders, F. L., J. Polym. Sci. Part C (1964) 4, 589. Locatelli, J. L., Riess, G., Angew. Makromol. Chem. (1973) 28, 161. Hoffmann, M., Pampus, G., Marwede, G., Kaut. Gummi Kunstst. (1969) 22, 691. Bartl, H., Kaut. Gummi Kunstst. (1972) 25, 452.

RECEIVED April 3, 1974.


Grafting of Styrene and Acrylonitrile onto Ethylene Polymers

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