Block Size and Glass-Transition Temperatures of ... - ACS Publications

10 Block Size and Glass-Transition Temperatures of the Polystyrene Phase in Different Block Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2013 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0176.ch010

Copolymers Containing Styrene Blocks as the Hard Phase S. K R A U S E and M . ISKANDAR Department of Chemistry, Rensselaer Polytechnic Institute, Troy NY 12181

Glass-transition temperatures of the styrene microphases in diblock copolymers of styrene and dimethyl siloxane have been obtained as a function of the molecular weights of the styrene blocks using DSC and DTA techniques.

The mag-

nitudes of the glass-transition temperatures of the styrene microphases in these systems were, within

experimental

error, exactly the same as the glass-transition

temperatures

taken from the literature of the styrene microphases in styrene-isoprene

diblock and triblock copolymers and in

styrene-ethylene

oxide diblock and triblock copolymers in

which the styrene blocks had comparable molecular weights. It appears, therefore, that the chemical nature of the other blocks which are attached to the styrene blocks has no effect on the glass-transition temperature

of the styrene micro-

phase; some implications of this observation are discussed.

|^|ften, w h e n T

g

d a t a h a v e b e e n o b t a i n e d o n b l o c k c o p o l y m e r samples

c o n t a i n i n g s m a l l , l o w - m o l e c u l a r - w e i g h t b l o c k s , a systematic

varia-

t i o n of T w i t h b l o c k l e n g t h has b e e n o b s e r v e d . I f w e c o n s i d e r o n l y b l o c k g

copolymers i n w h i c h microphase separation has been demonstrated b y the o b s e r v a t i o n o f t w o different glass-transition t e m p e r a t u r e s , w e

find

t h a t t h e h i g h e r T , t o b e r e f e r r e d t o h e n c e f o r t h as t h e g l a s s - t r a n s i t i o n g

0-8412-0457-8/79/33-176-205$05.00/0 © 1979 American Chemical Society

In Multiphase Polymers; Cooper, S., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

206

MULTIPHASE POLYMERS

t e m p e r a t u r e of the glassy phase, varies q u i t e d i f f e r e n t l y w i t h c h a n g e s i n m o l e c u l a r w e i g h t of the r e l e v a n t b l o c k s t h a n does the l o w e r T , t o b e g

r e f e r r e d to h e n c e f o r t h as the g l a s s - t r a n s i t i o n t e m p e r a t u r e of t h e r u b b e r y phase. I n this c h a p t e r , w e s h a l l d e a l m a i n l y w i t h the T

of the glassy phase,

g

w h i c h , i n a l m o s t a l l cases t h a t h a v e b e e n i n v e s t i g a t e d , has b e e n p o l y s t y rene.

I n one of the earliest i n v e s t i g a t i o n s of this t y p e , K r a u s , C h i l d e r s ,

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2013 | http://pubs.acs.org Publication Date: June 1, 1979 | doi: 10.1021/ba-1979-0176.ch010

a n d G r u v e r ( I ) n o t i c e d that the t e m p e r a t u r e of the loss p e a k a t t r i b u t e d t o the styrene b l o c k s i n d y n a m i c m e c h a n i c a l m e a s u r e m e n t s of s t y r e n e butadiene block copolymers decreased

f r o m 104°

t o 60 ° C w h e n

the

styrene b l o c k l e n g t h d e c r e a s e d b y a f a c t o r of a b o u t f o u r ( exact m o l e c u l a r w e i g h t s of the samples w e r e n o t k n o w n ) . I n a m o r e r e c e n t w o r k , Kraus and R o l l m a n n (2)

s t u d i e d t r i b l o c k c o p o l y m e r s of styrene

isoprene over a copolymer-molecular-weight range f r o m 2 χ

with

10 to about 4

10 , a g a i n u s i n g d y n a m i c m e c h a n i c a l t e c h n i q u e s . T h e t e m p e r a t u r e of t h e 5

upper (polystyrene)

loss m a x i m u m v a r i e d f r o m 1 2 0 °

to 2 0 ° C

m o l e c u l a r w e i g h t of the styrene b l o c k s d e c r e a s e d f r o m 2 Χ 5000.

Differential scanning calorimetry ( D S C )

as

the

10 to a b o u t 4

was used b y Robinson

a n d W h i t e (3) to study glass-transition temperatures i n triblock copoly mers of styrene a n d i s o p r e n e a n d b y O ' M a l l e y et a l . (4)

to s t u d y t h e

m e l t i n g t r a n s i t i o n of the e t h y l e n e o x i d e a n d the glass t r a n s i t i o n of the styrene p h a s e i n d i b l o c k a n d t r i b l o c k c o p o l y m e r s of styrene a n d e t h y l e n e oxide.

The T

g

of the p o l y s t y r e n e m i c r o p h a s e s i n b o t h D S C studies d e

c r e a s e d w i t h d e c r e a s i n g p o l y s t y r e n e b l o c k lengths as u s u a l . Additional T

g

diblock

data on very l o w molecular weight

a n d triblock copolymers were

R o o v e r s ( 5 ) , also u s i n g D S C t e c h n i q u e s .

obtained

by

O n l y the T

e

m i c r o p h a s e s w a s g i v e n i n those cases i n w h i c h the T

g

styrene-isoprene Toporowski and

of the p o l y s t y r e n e of b o t h phases w a s

o b t a i n e d , i.e., w h e r e m i c r o p h a s e s e p a r a t i o n d e f i n i t e l y o c c u r r e d , a n d t h i s T

g

also d e c r e a s e d w i t h d e c r e a s i n g size of the styrene b l o c k s ; h o w e v e r , i t

i n c r e a s e d s l i g h t l y as the size of the i s o p r e n e b l o c k s w a s i n c r e a s e d w h i l e h o l d i n g constant the sizes of the styrene b l o c k s . T h i s is i n s h a r p contrast t o t h e d a t a of K r a u s a n d R o l l m a n n ( 2 ) , w h i c h i n d i c a t e d t h a t the T

e

of

the p o l y s t y r e n e m i c r o p h a s e d e p e n d e d o n l y o n the m o l e c u l a r w e i g h t of the styrene b l o c k s , w h e t h e r the other p h a s e w a s p o l y b u t a d i e n e or p o l y i s o p r e n e . It is of interest to n o t e that the d a t a of K r a u s a n d R o l l m a n n ( 2 ) w e r e o b t a i n e d o n b l o c k c o p o l y m e r s of m u c h h i g h e r m o l e c u l a r w e i g h t t h a n w e r e those of T o p o r o w s k i a n d R o o v e r s ( 5 ) ;

the latter a u t h o r s , as

a m a t t e r of fact, s t u d i e d b l o c k c o p o l y m e r s of m o l e c u l a r w e i g h t o n l y s l i g h t l y greater t h a n t h a t occurs.

at w h i c h m i c r o p h a s e

T h e y s t u d i e d , i n a d d i t i o n to t h e

copolymer

samples,

many

separation no

microphase-separated

lower-molecular-weight samples

in

microphase separation d i d not occur.


longer block which

10.

207

Styrene Blocks

KRAUSE AND i S K A N D A R

A n u m b e r o f e x p l a n a t i o n s exist f o r t h e decrease i n g l a s s - t r a n s i t i o n t e m p e r a t u r e of t h e glassy m i c r o p h a s e w i t h d e c r e a s i n g b l o c k l e n g t h o f t h e glassy b l o c k s . C h i l d e r s a n d K r a u s (6) s p e c u l a t e d t h a t this T

lowering

g

m i g h t p o s s i b l y b e a t t r i b u t e d t o some m i x i n g of t h e b l o c k segments i n styrene-butadiene

block copolymers, that is, they postulated

c o m p a t i b i l i t y of the b l o c k s as t h e b l o c k lengths d e c r e a s e d .

increased

M a n y investi-

gators h a v e p o s t u l a t e d that m i x i n g o f t h e c h e m i c a l l y d i f f e r e n t b l o c k seg-


ments occurs i n a n i n t e r l a y e r w i t h finite v o l u m e b e t w e e n

microphases;

t h e v o l u m e o f this m i x e d i n t e r l a y e r w o u l d p r e s u m a b l y d e p e n d o n t h e m o l e c u l a r w e i g h t s of the b l o c k s a n d o n t h e i r m u t u a l c o m p a t i b i l i t y . K r a u s a n d R o l l m a n n (2) a s s u m e d a n i n t e r l a y e r i n t h e i r s t y r e n e - b u t a d i e n e styrene-isoprene styrene,

and

block copolymers that w a s disproportionately r i c h i n

a n d they presented

calculations w h i c h

showed that

such a n

i n t e r p r e t a t i o n w a s consistent w i t h t h e i r d a t a w h i l e a n i n t e r p r e t a t i o n t h a t r e l i e d s o l e l y o n t h e free v o l u m e p r o v i d e d b y p o l y m e r c h a i n ends, w h o s e c o n c e n t r a t i o n increases w i t h d e c r e a s i n g m o l e c u l a r w e i g h t , p r e d i c t e d a v a r i a t i o n of T w i t h m o l e c u l a r w e i g h t that w a s m u c h s m a l l e r t h a n t h a t g

w h i c h was observed. T h e presence of a more u n i f o r m l y m i x e d interlayer h a d been postulated earlier b y F e s k o a n d Tschoegl ( 7 ) to account for the d y n a m i c m e c h a n i c a l p r o p e r t i e s o f some s t y r e n e - b u t a d i e n e - s t y r e n e

tri-

block copolymers. T h e o b s e r v e d T s of t h e p o l y s t y r e n e m i c r o p h a s e s i n a l l o f t h e w o r k g

quoted above

(1,2,3,4,5)

w e r e l o w e r t h a n those e x p e c t e d

for poly-

styrene h o m o p o l y m e r s h a v i n g t h e same m o l e c u l a r w e i g h t s as t h e b l o c k s in the block copolymers.

W e present

a d e t a i l e d c o m p a r i s o n of s u c h

data below. E v e n i f t h e r e is n o m i x e d i n t e r l a y e r w i t h

finite

volume

between

m i c r o p h a s e s , i t is o b v i o u s t h a t contacts b e t w e e n t h e c h e m i c a l l y d i f f e r e n t b l o c k s o c c u r a t t h e surfaces b e t w e e n m i c r o p h a s e s a n d t h a t t h e n u m b e r of these contacts m u s t increase as t h e size o f t h e m i c r o p h a s e s decreases, t h a t i s , as t h e s u r f a c e a r e a b e t w e e n m i c r o p h a s e s Pegoraro

( 8 ) analyzed the observed

ethylene-propylene

increases.

glass-transition

Bares a n d

temperatures i n

c o p o l y m e r s w i t h v i n y l - c h l o r i d e grafts i n terms o f

s u c h surface m i x i n g . T h i s t y p e o f m i x i n g p r o v i d e s a n o t i c e a b l e decrease i n T o n l y w h e n the m i c r o p h a s e s are e x t r e m e l y s m a l l ; B a r e s a n d P e g o r a r o g

( 8 ) c a l c u l a t e d t h a t effects w o u l d o n l y b e o b s e r v e d w h e n s p h e r i c a l m i c r o phases w e r e less t h a n 2 5 m o n o m e r u n i t s i n d i a m e t e r o r w h e n c y l i n d r i c a l m i c r o p h a s e s w e r e less t h a n 10 m o n o m e r u n i t s i n d i a m e t e r .

I n a later

p u b l i c a t i o n , B a r e s ( 9 ) d e v i s e d a less r e s t r i c t i v e a r g u m e n t f o r c a l c u l a t i n g the T

g

o f t h e glassy m i c r o p h a s e s as t h e m o l e c u l a r w e i g h t of t h e b l o c k s

c o m p r i s i n g these- m i c r o p h a s e s

decreased

a n d as t h e s u r f a c e - t o - v o l u m e

r a t i o o f these m i c r o p h a s e s s i m u l t a n e o u s l y i n c r e a s e d . t h e F o x - F l o r y (10)

equation f o r T

g

H e essentially u s e d

l o w e r i n g caused b y l o w molecular


208

MULTIPHASE POLYMERS

w e i g h t i n homopolymers a n d i n r a n d o m copolymers a n d a d d e d another t e r m w h i c h p r o v i d e d f o r a n a d d i t i o n a l l o w e r i n g of T

g

u m e r a t i o of a p a r t i c u l a r m i c r o p h a s e i n c r e a s e d .

as the s u r f a c e : v o l -

Since the surface:volume

r a t i o of m i c r o p h a s e s w i t h d i f f e r e n t m o r p h o l o g y is n o t the same, the l o w e r i n g w o u l d be expected to vary w i t h morphology. Bares (9)

T

g

found

q u a l i t a t i v e a g r e e m e n t b e t w e e n his p r e d i c t i o n s a n d some of the d a t a of Robinson and White

(3).


C o u c h m a n a n d K a r a s z (11 ) r e c e n t l y h a v e m a d e some i n d i c a t i n g that

spherical microphases

transition temperatures

calculations

should exhibit increased

b e c a u s e of a n i n c r e a s e d

pressure

glass-

inside such

m i c r o p h a s e s a t t r i b u t e d to t h e surface t e n s i o n b e t w e e n m i c r o p h a s e s .

Since

t h e r e is some d o u b t a b o u t the existence of a surface of t e n s i o n i n t h e G i b b s ' sense

between

(12)

chemically l i n k e d microphases,

we

shall

s i m p l y n o t e t h a t these c a l c u l a t i o n s are the o n l y ones i n existence t h a t i n d i c a t e a p o s s i b l e reason f o r a n increase i n the T

g

of a glassy m i c r o p h a s e

a n d , i n a d d i t i o n , t h a t these c a l c u l a t i o n s also p o s t u l a t e differences i n w i t h differences

in morphology.

F o r e x a m p l e , this

p e n d e n t effect w o u l d n o t b e e x p e c t e d

T

g

surface-tension-de-

i n samples w i t h l a m e l l a r m o r -

p h o l o g y , n o m a t t e r h o w s m a l l the w i d t h of e a c h l a m e l l a . T o s u m m a r i z e the observations m a d e i n t h e p r e c e d i n g p a r a g r a p h s , w e c a n say that there is n o d o u b t a b o u t the f a c t t h a t t h e T

g

microphases

i n phase-separated

of the glassy

b l o c k c o p o l y m e r s decreases w h e n

the

lengths of the glassy b l o c k s decrease, a n d f u r t h e r m o r e , t h a t the T s g

of

s u c h m i c r o p h a s e s are g e n e r a l l y l o w e r t h a n are those of the c o r r e s p o n d i n g homopolymers

of c o m p a r a b l e

molecular

weight.

We

have

discussed

m o s t l y b l o c k c o p o l y m e r s i n w h i c h styrene c o m p r i s e s the glassy b l o c k s ; however, similar data have been obtained for the glass-transition temperatures of the p o l y c a r b o n a t e b l o c k s i n a l t e r n a t i n g m u l t i b l o c k c o p o l y mers of d i m e t h y l s i l o x a n e a n d b i s p h e n o l - A c a r b o n a t e b y K a m b o u r that

include:

a n e x p l a n a t i o n s i m i l a r to t h e m o l e c u l a r w e i g h t effect i n

( 1)

homopolymers, (2)

have

been

a d v a n c e d f o r this

general

(13).

Explanations

T

g

lowering

m i x i n g of segments f r o m c h e m i c a l l y d i f f e r e n t b l o c k s

b y surface c o n t a c t b e t w e e n m i c r o p h a s e s , a n d ( 3 )

m i x i n g of segments

f r o m c h e m i c a l l y d i f f e r e n t b l o c k s i n i n t e r l a y e r s of finite v o l u m e . T h e decrease i n h o m o p o l y m e r T

w i t h decreasing molecular weight

g

has g e n e r a l l y b e e n

a t t r i b u t e d to a n increase

of free v o l u m e i n l o w -

m o l e c u l a r - w e i g h t b u l k p o l y m e r s c a u s e d b y the i n c r e a s e d of c h a i n ends.

concentration

H o w e v e r , end blocks i n block copolymer molecules have

only a single-chain e n d w h i l e center blocks have no free-chain O n e w o u l d t h e r e f o r e expect t h a t the T

g

ends.

of a m i c r o p h a s e c o m p r i s e d of e n d

b l o c k s w o u l d b e l o w e r t h a n t h a t of a m i c r o p h a s e c o m p r i s e d of c e n t e r b l o c k s of c o m p a r a b l e m o l e c u l a r w e i g h t .


10.

KRAUSE AND isKANDAR The

209

Styrene Blocks

v o l u m e of a m i x e d i n t e r l a y e r b e t w e e n

microphases

should

d e p e n d o n c o m p a t i b i l i t y b e t w e e n t h e b l o c k s as m e a s u r e d b y t h e d i f f e r ence between their solubility parameters or b y their interaction parameter. S u r f a c e contacts b e t w e e n m i c r o p h a s e s , o n t h e o t h e r h a n d , s h o u l d b e affected o n l y b y m i c r o p h a s e size a n d m o r p h o l o g y . T o d i s e n t a n g l e t h e v a r i o u s p o s s i b l e effects o n t h e g l a s s - t r a n s i t i o n t e m p e r a t u r e s of glassy m i c r o p h a s e s , w e h a v e b e g u n studies o n a b l o c k


copolymer system i n w h i c h the blocks are expected to b e

extremely

i n c o m p a t i b l e so t h a t t h e f o r m a t i o n of a m i x e d i n t e r l a y e r w i t h finite v o l u m e is m u c h less p r o b a b l e t h a n i t is i n a s y s t e m l i k e

styrene-isoprene.

T h e s l i g h t b u t s i g n i f i c a n t c o m p a t i b i l i t y of styrene b l o c k s w i t h i s o p r e n e b l o c k s is s h o w n d i r e c t l y i n t h e d a t a of T o p o r o w s k i a n d R o o v e r s ( 5 ) , w h o w e r e a b l e t o observe c o m p l e t e m i x i n g b e t w e e n

styrene a n d i s o p r e n e

blocks i n their block copolymers w h e n molecular weights were very l o w . T h i s c o m p a t i b i l i t y m a k e s t h e p r e s e n c e of a m i x e d i n t e r l a y e r m i c r o p h a s e s reasonable.

between

S o m e e x p e r i m e n t a l studies of p o s s i b l e d i m e n -

sions of this i n t e r l a y e r u s i n g s m a l l a n g l e x-ray s c a t t e r i n g b e g i n n i n g t o a p p e a r i n t h e l i t e r a t u r e (14),

( S A X S ) are

a l t h o u g h o l d e r studies u s i n g

S A X S i n d i c a t e d a sharp b o u n d a r y between microsphases

(15,16).

T h e s y s t e m w i t h w h i c h w e h a v e b e g u n o u r i n v e s t i g a t i o n s is t h e s t y r e n e - d i m e t h y l s i l o x a n e system. T h e d i m e t h y l s i l o x a n e b l o c k s s h o u l d b e c o n s i d e r a b l y less c o m p a t i b l e w i t h p o l y s t y r e n e b l o c k s t h a n either p o l y b u t a d i e n e o r p o l y i s o p r e n e since t h e s o l u b i l i t y p a r a m e t e r

of d i m e t h y l -

siloxane is m u c h f a r t h e r f r o m t h a t of p o l y s t y r e n e t h a n are t h e s o l u b i l i t y parameters of p o l y b u t a d i e n e s o r of p o l y i s o p r e n e s ( 1 7 ) , n o m a t t e r w h a t t h e i r m i c r o s t r u c t u r e . F u r t h e r m o r e , e v e n hexamers of p o l y s t y r e n e a n d of polydimethylsiloxane are i m m i s c i b l e at r o o m temperature a n d have a n upper critical-solution temperature above 3 5 ° C

(18).

I n addition, the

m i c r o p h a s e s i n this s y s t e m c a n b e o b s e r v e d w i t h o u t s t a i n i n g a n d w i t h n o a m b i g u i t y a b o u t t h e i d e n t i t y of t h e phases i n t h e t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e ( T E M ) ; s i l i c o n has a m u c h h i g h e r a t o m i c n u m b e r t h a n carb o n or oxygen, m a k i n g the polydimethylsiloxane microphases the dark phases i n T E M (19,

20).

Experimental P o l y m e r s . T h e p o l y s t y r e n e s t a n d a r d s u s e d i n this w o r k w e r e A r r o L a b o r a t o r i e s * " M o n o d i s p e r s e . " O n e g r o u p of s t y r e n e - d i m e t h y l s i l o x a n e d i b l o c k c o p o l y m e r s w e r e those p r e p a r e d a n d c h a r a c t e r i z e d b y Z i l l i o x , R o o v e r s , a n d B y w a t e r (21), k i n d l y sent t o us b y J . E . L . R o o v e r s . T h e s e polymers have been given the designation R i n Table II below; molecul a r w e i g h t s of t h e b l o c k s w e r e c a l c u l a t e d f r o m t h e e x p e r i m e n t a l l y determ i n e d w e i g h t - a v e r a g e m o l e c u l a r w e i g h t s a n d c o m p o s i t i o n s of t h e c o p o l y mers (21 ) e x c e p t f o r s a m p l e R 1 3 , f o r w h i c h o n l y t h e n u m b e r - a v e r a g e


MULTIPHASE POLYMERS

210

molecular weight was available. G e l permeation chromatograms were o b t a i n e d i n t o l u e n e s o l u t i o n o n a W a t e r s Associate's A n a p r e p G P C r u n i n t h e a n a l y t i c a l m o d e u s i n g five c o l u m n s r a n g i n g f r o m 2 Χ 10 to 7 Χ 1 0 A i n exclusion limit and were compared w i t h chromatograms supplied t o us b y R o o v e r s . T h e c h r o m a t o g r a m s w e r e v i r t u a l l y i d e n t i c a l . A s e c o n d g r o u p of d i b l o c k c o p o l y m e r s w a s k i n d l y s u p p l i e d t o us b y J . W . D e a n of the S i l i c o n e P r o d u c t s D e p a r t m e n t of the G e n e r a l E l e c t r i c C o . T h e s e samples are d e s i g n a t e d D i n T a b l e I I ; m o l e c u l a r w e i g h t s of the b l o c k s w e r e c a l c u l a t e d f r o m w e i g h t - a v e r a g e m o l e c u l a r w e i g h t s o b t a i n e d u s i n g G P C i n o u r l a b o r a t o r y a n d c o m p o s i t i o n s of the c o p o l y m e r s as d e t e r m i n e d b y D e a n ( 2 2 ) . Weight-average molecular w e i g h t s o b t a i n e d f r o m G P C i n o u r l a b o r a t o r y w e r e c a l c u l a t e d as f o l l o w s : the set of five c o l u m n s d e s c r i b e d a b o v e w e r e c a l i b r a t e d u s i n g A r r o Laboratories' " M o n o d i s p e r s e " polystyrenes, a n d then w e assumed that the b l o c k c o p o l y m e r s f o l l o w e d the same c a l i b r a t i o n c u r v e as p o l y s t y r e n e . T h e v a l i d i t y of this p r o c e d u r e is p r e s e n t l y u n d e r e x a m i n a t i o n i n o u r laboratory. Thermal A n a l y s i s . D i f f e r e n t i a l t h e r m a l analysis ( D T A ) d a t a w e r e o b t a i n e d u s i n g a F i s h e r T h e r m a l y z e r M o d e l 300 Q D T A w i t h a h e a t i n g r a t e of 2 0 ° C / m i n a n d static a i r i n the s a m p l e h o l d e r , w h i l e d i f f e r e n t i a l scanning calorimetry ( D S C ) data were obtained using a P e r k i n - E l m e r D S C - 1 B w i t h a h e a t i n g rate of 1 0 ° C / m i n a n d a steady flow of n i t r o g e n . T h e c o o l i n g rate w a s also c o n t r o l l e d at 1 0 ° C / m i n b e t w e e n runs o n the same s a m p l e i n the D S C e x p e r i m e n t s ; c o o l i n g rates v a r i e d i n t h e D T A r u n s . T h e t e m p e r a t u r e scales o n b o t h i n s t r u m e n t s w e r e c a l i b r a t e d u s i n g the m e l t i n g transitions of p - n i t r o t o l u e n e a n d n a p h t h a l e n e b e l o w 1 0 0 ° C a n d those of l e a d , t i n , i n d i u m , a n d b e n z o i c a c i d a b o v e 1 0 0 ° C . O n b o t h i n s t r u m e n t s , the average e r r o r of the t e m p e r a t u r e scale f o r a l l these t r a n sitions w a s ± 1 . 0 % ( ° C ) . A l l g l a s s - t r a n s i t i o n t e m p e r a t u r e s r e c o r d e d i n T a b l e s I a n d I I are i n the m i d d l e of the t r a n s i t i o n r a n g e of the heat c a p a c i t i e s . W e are a w a r e t h a t some w o r k e r s (23) f e e l that the t e m p e r a t u r e at w h i c h t h e h e a t c a p a c i t y just b e g i n s to c h a n g e d u r i n g h e a t i n g is s o m e w h a t m o r e r e p r o d u c i b l e , t h o u g h p r o b a b l y less m e a n i n g f u l t h a n the m i d d l e of t h e t r a n s i t i o n range. R e c o r d e d t e m p e r a t u r e s are t h e averages of m a n y r u n s f o r e a c h s a m p l e ; i f the T w a s n o t c o m p l e t e l y r e p r o d u c i b l e , the r a n g e of v a l u e s o b t a i n e d is s h o w n i n the tables. T h e r e w a s one s a m p l e , R 1 4 , f o r w h i c h t w o Tg-like t r a n s i t i o n s sometimes a p p e a r e d o n first h e a t i n g t h e


3

K

Table I.

Glass-Transition Temperatures of Standard Polystyrene Samples Obtained in This Laboratory

Mol Wt (X 10~4) 0.20 LOO 2.00 5.00 9.72 180

T

g

DSC

DTA 63 95 ± 100.5 ± 101.5 ± 101 ± 104 ±

(°C)

55

1 0.5 0.5 1 2

95 db 2 99 ± 2 104 106 105


5

10.

KRAUSE

AND

211

Styrene Blocks

ISKANDAR

Table II. Glass-Transition Temperatures of Styrene-Dimethyl siloxane Diblock Copolymers Obtained in This Laboratory

Sample

Wtfo Styrene

R13 R14

9,100 19,300

53.5 56.8

1.06 1.10

D2 Dl

20,300

89.7 91.9 47.1 59.6 73.5 66.5 58.3

1.26 1.30 1.14 1.16 1.19 1.15 1.13

23,900 47,100 121,000 136,000 327,000 362,000

R15

R8 R5 R25 R26

T,

n

w


MJM (Whole Sample)

M ( Styrene Block)

(°C) DSC

DTA

77

75±2 98 ± 3 e

— —

103 ± 101.5 ± 102 rb 98.5 103 ±

ft

3 0.5 1 2

79 db 72 ± 102 ± 96 95 ± 104 ± 108 ± 107 ± : 104 109

4 V 2

b

1 2 1 1

° The lower T appeared on the first heating of the sample only, sometimes i n conjunction with the higher T . The higher Ί\ sometimes appeared in conjunction with the lower T on the first heating of the sample ; when a transition appeared on subsequent heating of the sample, it was always the higher T . g

g

b

K

g

s a m p l e ; b o t h of these are s h o w n e v e n t h o u g h the l o w e r t r a n s i t i o n d i s a p p e a r e d o n subsequent h e a t i n g . W h e n a large e n d o t h e r m or, i n one case, a n e x o t h e r m , a p p e a r e d i n the glass-transition r a n g e of a s a m p l e o n first h e a t i n g , n o a t t e m p t w a s m a d e to d e d u c e a T f r o m it. Transmission Electron Microscopy. F i l m s of a l l samples d e s i g n a t e d R w e r e o b t a i n e d b y e v a p o r a t i o n of t o l u e n e f r o m solutions of the b l o c k copolymers and were observed without staining using a H i t a c h i Hu-125 o r a J E O L J E M 100 S e l e c t r o n m i c r o s c o p e . M e t h o d s of p r e p a r i n g t h e films h a v e b e e n d e s c r i b e d p r e v i o u s l y (24). So far, w e h a v e o b t a i n e d e v i d e n c e f o r m i c r o p h a s e s e p a r a t i o n i n o n l y the f o u r h i g h e s t m o l e c u l a r w e i g h t samples b y T E M . W e h a v e not o b t a i n e d c o n t i n u o u s films of t h e l o w e r m o l e c u l a r - w e i g h t s a m p l e s ; w e p l a n to e x a m i n e sections of these samples later. B e c a u s e of the v e r y s m a l l c o m p a t i b i l i t y of styrene a n d p o l y d i m e t h y l s i l o x a n e , h o w e v e r , w e expect phase s e p a r a t i o n i n a l l of these samples. g

Results and

Discussion

T a b l e I shows the T

g

values o b t a i n e d i n this w o r k o n s t a n d a r d p o l y

styrene samples; i n most cases, the values o b t a i n e d f r o m D T A d a t a are s l i g h t l y h i g h e r t h a n those o b t a i n e d b y D S C . Since these d a t a w e r e to b e u s e d f o r d e t a i l e d c o m p a r i s o n s w i t h d a t a o b t a i n e d b y others as w e l l as b y ourselves o n styrene b l o c k c o p o l y m e r s , w e first c o m p a r e d o u r d a t a o n a n i o n i c a l l y p o l y m e r i z e d s t a n d a r d p o l y s t y r e n e s w i t h those o b t a i n e d o n other a n i o n i c a l l y p o l y m e r i z e d p o l y s t y r e n e s b y other w o r k e r s also u s i n g DTA

a n d D S C techniques.

T h i s c o m p a r i s o n is s h o w n g r a p h i c a l l y i n

F i g u r e 1. I n F i g u r e 1, the s o l i d c u r v e is d r a w n t h r o u g h a c o m b i n a t i o n of o u r d a t a w i t h that o b t a i n e d b y W a l l et a l . (25)

u s i n g a D u p o n t 900



Figure 1. Glass-transition temperatures of anionically polymerized polystyrenes as determined by DTA or DSC. The solid curve is drawn through a combination of our data and that of Wall et al. (25); the dashed line is drawn through the data of Toporowski and Roovers (5).

r4

MOLECULAR WEIGHT x 10"

THIS WORK DSC THIS WORK DTA WALL (25) DTA TOPOROWSKI {5}DCS RIETSCH (26) DSC COWIE (27) DSC


10.

KRAUSE AND ISKANDAR

213

Styrene Blocks

D i f f e r e n t i a l T h e r m a l A n a l y z e r at a h e a t i n g rate of 2 0 ° C / m i n .

This curve

w i l l be considered our standard curve for polystyrene h o m o p o l y m e r a n d is r e d r a w n o n F i g u r e 2. O n F i g u r e 1, the d a s h e d c u r v e is d r a w n t h r o u g h the d a t a of T o p o r o w s k i a n d R o o v e r s ( 5 ) ;

only their lowest molecular-

w e i g h t sample falls on our standard curve.

T h e discrepancy

between

the t w o curves m a y p o s s i b l y b e c a u s e d b y s o m e p o l y d i s p e r s i t y i n T o p o r o w s k i a n d Roovers* s a m p l e s ; t h e y r e p o r t e d o n l y M „ v a l u e s f o r t h e i r


homopolymers.

T h e d a t a of R i e t s c h et a l . (26),

w h i c h were

obtained

u s i n g a P e r k i n - E l m e r D S C - 2 at v a r i o u s h e a t i n g rates a n d e x t r a p o l a t i n g t h e c a l c u l a t e d T s to v e r y l o w h e a t i n g rate, f a l l w e l l b e l o w o u r s t a n d a r d g

c u r v e ; this is to b e e x p e c t e d b e c a u s e of the e x t r a p o l a t i o n to l o w h e a t i n g rate. T h e d a t a of C o w i e (27),

o b t a i n e d u s i n g a D u P o n t 900 D S C m o d u l e

at a h e a t i n g rate of 2 0 ° C / m i n ,

cannot be explained simply b y noting

t h a t C o w i e r e p o r t e d the i n i t i a l c h a n g e of h e a t c a p a c i t y o n h e a t i n g as h i s glass-transition temperature.

, V Δ Ο • Ο χ -f

,

,—_

,

,

THIS WORK D T A , S - D M S , T O N E O F T W O T g ' s O B S E R V E D THIS WORK D S C , S - D M S , • ONE O F T W O Tg's O B S E R V E D TOPOROWSKI (5)S- I , PHASE SEPARATED TOPOROWSKI ( 5 ) S - I - S , PHASE SEPARATED ROBINSON (3) S - l - S , φ I - S - I O'MALLEY O'MALLEY

ON SAME SAMPLE ON SAME S A M P L E

P O L Y S T Y R E N E HOMOPOLYMER STANDARD C U R V E (FROM FIGURE I)

50-

B

4oLMOL. WT. EACH STYRENE BLOCK x 10"

Figure 2. Class-transition temperatures of the glassy microphases of various block copolymers containing styrene blocks vs. the molecular weights of the styrene blocks as determined by DTA or DSC.

S-DMS refers to styrene-dimethylsiloxane diblock copolymers; S—I refers to styrene-isoprene diblock copolymers; S-I-S and I-S-I refer to styrene-isoprenestyrene and to isoprene-styrene-isoprene triblock copolymers, respectively; an S-EO and EO-S-EO refer to styrene-ethylene oxide diblock copolymers and to ethylene oxide—styrene-ethylene oxide triblock copolymers, respectively.


214

MULTIPHASE POLYMERS

T a b l e I I shows the T

g

values o b t a i n e d f o r the glassy p h a s e of the

s t y r e n e - d i m e t h y l s i l o x a n e b l o c k c o p o l y m e r s u s e d i n this w o r k ; i n the case of the b l o c k c o p o l y m e r s , n e i t h e r D S C n o r D T A gave a consistently h i g h e r v a l u e of T .

T h e o n l y p e c u l i a r s a m p l e i n the t a b l e is s a m p l e R 1 4 , w h i c h

g

e x h i b i t e d t w o T s o n b o t h i n s t r u m e n t s at least o c c a s i o n a l l y d u r i n g the g

first h e a t i n g c y c l e .

A l t h o u g h the p r e s e n c e of t w o T s i m p l i e s some k i n d g

of p h a s e s e p a r a t i o n , p o s s i b l y i n a d d i t i o n to m i c r o p h a s e s e p a r a t i o n ,

the


G P C of this s a m p l e shows n o d o u b l e peak, s h o u l d e r , or other p e c u l i a r i t y w h i c h m i g h t e x p l a i n its p e c u l i a r p h a s e

behavior.

This sample

is a n

a n o m a l y unless one w i s h e s to d i smi ss a l l first h e a t i n g d a t a . F i g u r e 2 shows o u r p o l y s t y r e n e s t a n d a r d c u r v e , the d a t a f r o m T a b l e I I , a n d d a t a o b t a i n e d f o r the T

g

of the glassy m i c r o p h a s e i n v a r i o u s

s t y r e n e - r u b b e r b l o c k c o p o l y m e r s b y other investigators u s i n g D T A a n d D S C techniques.

It is r e m a r k a b l e that the d a t a f o r a l l b u t a f e w samples

f a l l i n a n a r r o w b a n d of the o r d e r of 10 ° C b e l o w the s t a n d a r d p o l y s t y r e n e homopolymer curve.

T h e l o w e s t m o l e c u l a r - w e i g h t styrene b l o c k s

have

T s that are s o m e w h a t f a r t h e r t h a n 10 ° C b e l o w the s t a n d a r d c u r v e . g

It

s h o u l d be n o t e d that some of the b l o c k c o p o l y m e r s f o r w h i c h d a t a a p p e a r o n F i g u r e 2 h a v e styrene as one of the b l o c k s of a d i b l o c k c o p o l y m e r , some h a v e styrene as the c e n t e r b l o c k i n a t r i b l o c k c o p o l y m e r , a n d h a v e styrene as the t w o e n d b l o c k s i n a t r i b l o c k c o p o l y m e r .

others

F o r those

samples i n w h i c h styrene appears as the t w o e n d b l o c k s i n a t r i b l o c k c o p o l y m e r , the m o l e c u l a r w e i g h t of o n l y one of the styrene b l o c k s has b e e n u s e d i n the

figure.

O n e c a n note that, i n the case of these samples,

t h e d a t a w o u l d n o l o n g e r be i n a b a n d w i t h most of the other d a t a if the c o m b i n e d m o l e c u l a r w e i g h t of the t w o styrene b l o c k s h a d b e e n u s e d i n the

figure.

between

O n e also s h o u l d note that there are n o systematic the

data

for

styrene-dimethylsiloxane

styrene-isoprene,

styrene-ethylene

block copolymers.

and

molecular weight

of

e a c h styrene b l o c k a p pe a rs to be the m a j o r v a r i a b l e that c o n t r o l s

its

glass-transition temperature,

The

differences oxide,

not its c o m p a t i b i l i t y w i t h its

a n d not its p o s i t i o n i n the b l o c k c o p o l y m e r m o l e c u l e .

comonomer

It is n o t o b v i o u s

f r o m F i g u r e 2, t h o u g h it m a y be d e d u c e d f r o m T a b l e II a n d f r o m a p e r u s a l of the p e r c e n t a g e c o m p o s i t i o n d a t a f o r a l l the b l o c k c o p o l y m e r s s h o w n i n the figure, t h a t the glass-transition t e m p e r a t u r e s of the

styrene

m i c r o p h a s e s d o not d e p e n d , i n a n y systematic w a y , o n the p e r c e n t a g e c o m p o s i t i o n s of the b l o c k c o p o l y m e r s w h i c h c o m p o s e t h e m ; this i m p l i e s t h a t the m o r p h o l o g y of the b l o c k c o p o l y m e r s , w h i c h at least at e q u i l i b r i u m is c o n t r o l l e d b y p e r c e n t a g e c o m p o s i t i o n , has n o major effect o n the g l a s s - t r a n s i t i o n t e m p e r a t u r e of the glassy m i c r o p h a s e . T h e " m a v e r i c k " p o i n t s o n F i g u r e 2 c a n b e r a t i o n a l i z e d . O n e set of t h e m belongs to our anomalous sample, R14, that exhibited t w o T s . g

If

o n e desires to di smi ss a l l " f i r s t - h e a t i n g " d a t a , the l o w e r t w o p o i n t s w i l l


10.

KRAUSE AND ISKANDAR

disappear.

215

Styrene Blocks

T h e other f o u r p o i n t s b e l o n g t o some o f T o p o r o w s k i a n d

Roovers' styrene-isoprene-styrene

block copolymers i n w h i c h the mo

l e c u l a r w e i g h t s of t h e i s o p r e n e b l o c k s w e r e n o t v e r y m u c h a b o v e t h e m o l e c u l a r w e i g h t a t w h i c h m i c r o p h a s e s e p a r a t i o n occurs i n b l o c k c o polymers w i t h styrene-block

m o l e c u l a r w e i g h t s e q u a l t o those i n t h e

b l o c k c o p o l y m e r s f o r w h i c h d a t a are s h o w n i n F i g u r e 2. T h i s i m p l i e s t h a t these p a r t i c u l a r b l o c k c o p o l y m e r s o f T o p o r o w s k i a n d R o o v e r s


ones i n w h i c h

segment m i x i n g o f t h e c h e m i c a l l y d i f f e r e n t

( 5 ) are b l o c k s is

e x t r e m e l y l i k e l y . W e w o u l d guess t h a t the l o w T s o f the styrene m i c r o g

phases i n these f o u r b l o c k - c o p o l y m e r samples a r e c o n n e c t e d compositions

o f these m i c r o p h a s e s ;

considerable

with the

i s o p r e n e is p r o b a b l y

a d m i x e d w i t h i n these m i c r o p h a s e s . W e p l a n t o c o n t i n u e studies o n t h e g l a s s - t r a n s i t i o n t e m p e r a t u r e s o f the glassy m i c r o p h a s e s i n b l o c k c o p o l y m e r s a n d , i n t h e f u t u r e , w e also shall

investigate

t h e glass-transition

temperatures

of the

rubbery

microphases. Conclusion

T h e g l a s s - t r a n s i t i o n t e m p e r a t u r e of the glassy m i c r o p h a s e i n a g l a s s rubber block copolymer depends almost entirely on the molecular weights of the glassy b l o c k s a n d n o t o n the c h e m i c a l n a t u r e o f the r u b b e r y b l o c k s , the p o s i t i o n o f t h e glassy b l o c k o r b l o c k s w i t h i n t h e b l o c k c o p o l y m e r molecule, o r the percentage composition of t h e block copolymer. T h e c h e m i c a l n a t u r e a n d / o r m o l e c u l a r w e i g h t of the r u b b e r y b l o c k influences the g l a s s - t r a n s i t i o n t e m p e r a t u r e of t h e glassy m i c r o p h a s e o n l y w h e n t h e m o l e c u l a r w e i g h t a n d c o m p o s i t i o n o f the b l o c k c o p o l y m e r is n e a r t h a t a t w h i c h m i c r o p h a s e s e p a r a t i o n n o l o n g e r takes p l a c e . Acknowledgment M a n y thanks to J . E . L . Roovers a n d to J . W . D e a n for p r o v i d i n g us with

the styrene-dimethylsiloxane

block

copolymers;

thanks

N a t i o n a l Science F o u n d a t i o n , Contract N o . D M R 7 6 - 1 9 4 8 8 , us w i t h this w o r k ; a n d t h a n k s t o t h e N a t i o n a l Institutes

to the

for helping

of H e a l t h f o r

p r o v i d i n g one of us ( S . K . ) w i t h a R e s e a r c h C a r e e r A w a r d .

Literature

Cited

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