Heat-Resistant Polyarylsulfone Exhibiting Improved Flow during

9

Heat-Resistant Polyarylsulfone Exhibiting Improved Flow during Processing

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R O B E R T J. C O R N E L L Uniroyal Chemical, Division of Uniroyal, Inc., Naugatuck, Conn. 06770

The

reaction of bis(4-chlorophenyl)sulfone with the anhy

drous dialkali salt of α,α'-bis(4-hydroxyphenyl)-p-diisopropyl benzene, which is dissolved in an aprotic solvent, leads to a new heat-resistant polyarylsulfone. monomer

and

polymer

physical and mechanical The

polyarylsulfone

A general discussion of

preparation

is given as well as

properties of this polyarylsulfone.

thermoplastic exhibits essentially the

same heat resistance of the polyarylsulfone based on bis (4-chlorophenyl)sulfone

and

2,2-bis(4-hydroxyphenyl)pro-

pane, along with the added plus of a lower melt viscosity at equivalent processing temperatures.

The flow improve

ment is demonstrated by comparison with Brabender data, injection-molding conditions, and melt-viscosity data.

J n

recent years, there has b e e n a c o n s i d e r a b l e a m o u n t of interest i n high-molecular-weight polyarylsulfone polymers

(1-3).

These poly

m e r i c materials possess heat-deflection temperatures of 165°-260°C p l u s excellent t h e r m a l s t a b i l i t y at the h i g h p r o c e s s i n g temperatures r e q u i r e d . T h i s p a p e r deals w i t h the synthesis a n d the p h y s i c a l a n d m e c h a n i c a l properties of a n e w heat-resistant p o l y a r y l s u l f o n e .

This new polyaryl

sulfone t h e r m o p l a s t i c exhibits a significant r e d u c t i o n i n a p p a r e n t

(melt)

viscosity w i t h o n l y a s l i g h t r e d u c t i o n i n heat deflection t e m p e r a t u r e w h e n c o m p a r e d w i t h the p o l y a r y l s u l f o n e b a s e d o n bis ( 4 - c h l o r o p h e n y l )sulfone a n d 2,2-bis ( 4 - h y d r o x y p h e n y l ) p r o p a n e .

Experimental R e a g e n t . T e t r a h y d r o t h i o p h e n e 1,1-dioxide ( c o m m o n l y c a l l e d s u l f o l a n e ) was o b t a i n e d f r o m S h e l l C h e m i c a l c o n t a i n i n g 3% w a t e r . A n h y drous sulfolane was o b t a i n b y d i s t i l l a t i o n u n d e r r e d u c e d pressure. Re131 Platzer; Polymerization Reactions and New Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

132

POLYMERIZATION

REACTIONS AND N E W

POLYMERS


agent g r a d e p o t a s s i u m h y d r o x i d e , c h l o r o f o r m , a n d b e n z e n e w e r e u s e d without further purification. M o n o m e r s . B I S ( 4 - C H L O R O P H E N Y L ) S U L F O N E . W e p r e p a r e d bis ( 4 c h l o r o p h e n y l ) s u l f o n e ( I i n R e a c t i o n 1) b y d r o p w i s e a d d i t i o n of c h l o r o s u l f o n i c a c i d (0.4 m o l e ) to c h l o r o b e n z e n e (0.2 m o l e ) at 0 ° C . One h o u r after the c h l o r o s u l f o n i c a c i d a d d i t i o n was c o m p l e t e d , a n a d d i t i o n a l q u a n t i t y of c h l o r o b e n z e n e (0.16 m o l e ) was a d d e d to the c o l d s o l u t i o n .

II T h e r e a c t i o n m i x t u r e was a l l o w e d to w a r m g r a d u a l l y to 5 0 ° C . The r e a c t i o n was q u e n c h e d b y p o u r i n g into ice w a t e r . T h e aqueous sus p e n s i o n was h e a t e d to h y d r o l y z e p - c h l o r o b e n z e n e s u l f o n y l c h l o r i d e ( I I ) to the w a t e r - s o l u b l e s u l f o n i c a c i d . T h e d e s i r e d p r o d u c t was filtered a n d w a s h e d w i t h w a t e r u n t i l essentially n e u t r a l . B i s ( 4 - c h l o r o p h e n y l )sulfone was p u r i f i e d b y r e c r y s t a l l i z a t i o n f r o m b e n z e n e — m . p . 145°-147°C ( 4 ) . E x t r e m e l y l o w y i e l d s w e r e o b t a i n e d b y this process. B y contrast, y i e l d s i n excess of 90% have b e e n r e p o r t e d i n processes t h a t i n v o l v e the r e a c t i o n of c h l o r o b e n z e n e w i t h s u l f u r t r i o x i d e a n d d i m e t h y l or d i e t h y l sulfate (5, 6). a,a -Bls(4-HYDROXYPHENYL)-p-DIISOPROPYLBENZENE. «,a:'-Bis(4-hydrOXyphenyl)-p-diisopropylbenzene ( I I I i n R e a c t i o n 2 ) was p r e p a r e d b y /

III

Platzer; Polymerization Reactions and New Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1973.


9.

CORNELL

Heat-Resist ant

133

Polyarylsulfone

d r o p w i s e a d d i t i o n of p - d i i s o p r o p e n y l b e n z e n e i n toluene to a 3 M excess of p h e n o l s a t u r a t e d w i t h gaseous h y d r o g e n c h l o r i d e . A f t e r a f e w h o u r s , the b u l k of the a,a'-bis ( 4 - h y d r o x y p h e n y l ) - p - d i i s o p r o p y l b e n z e n e f o r m e d c r y s t a l l i z e d out. T h e excess p h e n o l c a n b e r e m o v e d b y steam s t r i p p i n g . T h e c o m p o u n d was p u r i f i e d b y r e c r y s t a l l i z a t i o n f r o m a c e t o n e — m . p . 191°-192°C (7). T h e b i s p h e n o l c a n also b e p r e p a r e d f r o m a , a ' - d i h y droxy-p-diisopropylbenzene and phenol (8,9). P o l y m e r i z a t i o n . T y p i c a l l y , the d i h y d r i c p h e n o l ( 1 m o l e ) a n d aqueous a l k a l i m e t a l h y d r o x i d e (2 m o l e s ) are m i x e d u n d e r a n i n e r t at m o s p h e r e i n sulfolane a n d benzene. T h e w a t e r f r o m the aqueous s o l u t i o n p l u s m e t a l p h e n o x i d e f o r m a t i o n is r e m o v e d b y d i s t i l l a t i o n of a b e n z e n e - w a t e r azeotrope b e t w e e n 110° a n d 140°C. A f t e r w a t e r r e m o v a l has b e e n c o m p l e t e d , the excess b e n z e n e is d i s t i l l e d off, t h e a n h y d r o u s salt i n sulfolane c o o l e d to 7 0 ° - 8 0 ° C , a n d bis ( 4 - c h l o r o p h e n y l ) sulfone ( I ) a d d e d . T h e t e m p e r a t u r e is i n c r e a s e d g r a d u a l l y to 2 0 0 ° C a n d h e l d for f o u r t o five hours. M e t h y l c h l o r i d e is b u b b l e d i n at the e n d of the p o l y m e r i z a t i o n to convert a n y t e r m i n a l p h e n o x i d e groups to m e t h y l ethers (10). CH

3

CH

3

IV T h e r e s i n ( I V ) c a n b e i s o l a t e d f r o m the mass b y filtration of t h e a l k a l i m e t a l c h l o r i d e a n d s t r i p p i n g of t h e solvent, or b y p r e c i p i t a t i o n i n a nonsolvent. A s e c o n d m e t h o d involves d i s p e r s i o n of t h e p o l y m e r mass i n w a t e r to r e m o v e the a l k a l i m e t a l h a l i d e , f o r m e d d u r i n g the p o l y m e r i z a t i o n , a n d the sulfolane. D r y i n g m a y b e a c c o m p l i s h e d b y heat i n g the p o l y a r y l s u l f o n e to 100°-120°C in vacuo for 8 to 12 h o u r s . T h e d r i e d p o l y a r y l s u l f o n e ( I V ) is s o l u b l e i n h a l o g e n a t e d h y d r o c a r bons s u c h as c h l o r o f o r m , c h l o r o b e n z e n e , a n d m e t h y l e n e c h l o r i d e . M o l d i n g . T e s t specimens w e r e either c o m p r e s s i o n m o l d e d i n a h y d r a u l i c press at 2 4 5 ° C or i n j e c t i o n m o l d e d at 2 7 5 ° C u s i n g a 2-ounce A n k e r w e r k , M o d e l 75. P h y s i c a l P r o p e r t y M e a s u r e m e n t s . F l e x u r a l m o d u l u s a n d strength w e r e m e a s u r e d a c c o r d i n g to A S T M M e t h o d D - 7 9 0 . I m p a c t resistance was m e a s u r e d b y the I z o d n o t c h A S T M D - 2 5 6 , a n d R o c k w e l l H a r d n e s s



134

POLYMERIZATION

REACTIONS AND N E W POLYMERS

a c c o r d i n g to A S T M D - 7 8 5 . H e a t - d i s t o r t i o n temperatures w e r e deter m i n e d b y A S T M M e t h o d D - 6 4 8 at 264 p s i stress l o a d i n g . M i x i n g or t o r q u e generation d a t a w e r e o b t a i n e d u s i n g a B r a b e n d e r P l a s t i C o r d e r . T h e B r a b e n d e r P l a s t i C o r d e r has a s m a l l m i x i n g c a v i t y c o n t a i n i n g t w o r o t a t i n g m i x i n g b l a d e s ; the s p e e d of t h e m i x i n g blades m a y b e v a r i e d . T h e m i x i n g c a v i t y has a jacket t h r o u g h w h i c h h e a t e d o i l m a y b e c i r c u l a t e d . T h e p o l y m e r to b e e x a m i n e d is a d d e d to the c a v i t y . T h e r o t a t i n g blades exert a t o r q u e t h a t c a n b e m e a s u r e d a n d d e p e n d s o n the viscosity of the p o l y m e r . A p p a r e n t v i s c o s i t y d a t a w e r e o b t a i n e d o n the I n s t r o n C a p i l l a r y R h e o m e t e r , M o d e l T T . F o r this p u r p o s e , rods 5 inches b y % i n c h w e r e p r e p a r e d b y c o m p r e s s i o n m o l d i n g f r o m m a t e r i a l to b e tested. E a c h r o d was h e a t e d to 260° or 2 7 5 ° C i n the b a r r e l of the rheometer for five m i n u t e s . A p i s t o n p l u n g e r is t h e n pressed d o w n o n t o p of the h e a t e d r o d , f o r c i n g the r o d to flow t h r o u g h a 0.060-inch d i a m e t e r c a p i l l a r y , h a v i n g a l e n g t h - t o - d i a m e t e r r a t i o of 33. T h e p i s t o n p l u n g e r descends at a constant s p e e d of f r o m 0.005 to 5 inches p e r m i n u t e . T h e force r e q u i r e d to e x t r u d e the r o d t h r o u g h the c a p i l l a r y is m e a s u r e d . 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 of resins I V a n d V (see R e a c t i o n 4 ) were determined on an A n a p r e p gel permeation chromatograph. F o u r c o l u m n s w e r e u s e d : 1 0 A , 1 0 A , 1 0 A , a n d 1 0 A . T w o - m i l l i l i t e r samples of a 0.25% s o l u t i o n i n o - d i c h l o r o b e n z e n e at 1 0 0 ° C w e r e i n j e c t e d . The s o l u t i o n was c o l l e c t e d at a rate of 2 m l p e r m i n u t e . 6

Results

and

5

4

3

Discussion

Polymer Evaluation.

P h y s i c a l properties, m e l t - v i s c o s i t y d a t a , a n d

g e l p e r m e a t i o n c h r o m a t o g r a p h y d a t a of our p o l y a r y l s u l f o n e I V a n d the p o l y a r y l s u l f o n e V b a s e d o n bis ( 4 - c h l o r o p h e n y l )sulfone h y d r o x y p h e n y l ) p r o p a n e (11)

a n d 2,2-bis(4-

are l i s t e d i j i T a b l e s I t h r o u g h I V a n d F i g

ures 1 t h r o u g h 3.

(4)

V R e s i n I V has a l o w e r specific g r a v i t y t h a n does r e s i n V ( T a b l e I ) , p r o b a b l y e x p l a i n e d b y the presence of t w o i s o p r o p y l i d e n e groups p e r mer unit.

T h e s e r e d u c e the a b i l i t y of the p o l y m e r i c chains to b e as

t i g h t l y p a c k e d as i n r e s i n V .

T h e l o w e r specific g r a v i t y of I V c a n n o t b e

e x p l a i n e d b y the p e r cent c r y s t a l l i n i t y present i n b o t h p o l y m e r s I).

(Table

If c r y s t a l l i n i t y w e r e the major force, the specific g r a v i t y of r e s i n

I V w o u l d be higher.

T h e r e m a i n i n g properties of resins I V a n d V l i s t e d

i n T a b l e I e x h i b i t little v a r i a t i o n .


9.

Heat-Resistant

CORNELL

Table I.

Physical Properties of Polyarylsulfone Resins I V and V Property

Rockwell " R " Specific G r a v i t y H D T 264 p s i (°C) I z o d M" ( R T ) (ft. l b s . / i n . notch) I z o d \i" ( - 2 9 ° C ) Izod % " ( R T ) Izod V" ( - 2 9 ° C ) T e n s i l e S t r e n g t h (psi) T e n s i l e M o d u l u s (psi) X 1 0 T e n s i l e E l o n g a t i o n (%) a t break ° F l e x u r a l S t r e n g t h (psi) X 1 0 F l e x u r a l M o d u l u s (psi) P e r cent C r y s t a l l i n i t y s

6


135

Polyarylsulfone

5

Resin IV

Resin V

127 1.19 158 0.8 0.8 0.8 0.8 11,350 3.7 10 16,730 4.0 4.1

128 1.24 166 1.1 1.3 1.3 1.3 10,370 3.5 6.7 14,370 3.4 ~0.0

° Samples annealed 120°C for 100 hours.

Table II.

Torque-Generation D a t a Obtained During Brabender Mixing of Resins I V and V

Material

RPM

Melt Temp. (°C)

Torque (gram-meters)

Mixing time (minutes)

Resin I V

50 50 60

227 235 235

2,300 1,500 1,800

4 10 8

Resin V

50 60

235 250

3,800 2,600

12 15

Table III.

Injection-Molding Conditions for Polyarylsulfone Resins I V and V Resin IV

Resin V

M e l t Temperature (°C):

290

290

Injection Cylinder Temp. Nozzle (°C): Zones 1 a n d 2 ( ° C ) :

275 275,

I n j e c t i o n Pressure T - b a r (psi) Shots (psi) M o l d Temperature (°C): B a c k Pressure (psi)

275

275 275,

1300 550

1600 800

85

85

200

200


275

136

POLYMERIZATION

REACTIONS AND N E W

POLYMERS

T h e significant i m p r o v e m e n t i n flow properties of r e s i n I V vs. V is e v i d e n t f r o m the d a t a .

F i r s t , the t o r q u e f r o m B r a b e n d e r m i x i n g i n d i

cates that r e s i n I V is a n easier-flowing m a t e r i a l ( T a b l e I I ) .

T h e lower

t o r q u e values for I V i n d i c a t e the necessity of a l o w e r - e n e r g y i n p u t to m i x the p o l y m e r m e l t .

T h i s l o w e r r o t a t i o n a l force therefore

the p o l y m e r m e l t has a l o w e r m e l t viscosity. ing

conditions

demonstrate

the i m p r o v e d

( T a b l e III) i n comparison w i t h resin V .

indicates

Secondly, injection-mold processability

of

resin I V

A t the same i n j e c t i o n c y l i n d e r

temperatures, the injection pressure for the tensile b a r a n d

Izod/heat


d i s t o r t i o n b a r m o l d s is l o w e r e d b y 300 p s i a n d 250 p s i , respectively. F i n a l l y , the a p p a r e n t viscosity, at v a r i o u s shear rates, of r e s i n I V at 500° a n d 5 2 5 ° F is l o w e r e d b y a factor of 3 to 4 ( T a b l e I V a n d Figures 1 and 2).

T h e a p p a r e n t viscosity of r e s i n I V at 5 0 0 ° F s t i l l is

l o w e r t h a n r e s i n V at 5 2 5 ° F . Table IV.

A p p a r e n t V i s c o s i t y D a t a of P o l y a r y l s u l f o n e R e s i n s I V a n d V Apparent Viscosity (Dynes sec/cm ) 2

Resin IV ir rate (sec ) -1

Resin V

—

0.173 0.433 0.865 1.73 4.33 8.65 17.3 43.3 86.5 173 433 865 1730

8.70 X 9.81 9.25 8.08 7.78 7.10 6.50 5.67 5.06

— — —

10

— —

4

2.71 X 3.54 3.90 3.81 3.67 3.25 3.17 2.40 1.44 0.96 0.70

275°C

260°C

175° C

260°C

10

4

35.3 X 34.7 31.5 30.0 28.2 27.8 28.3 48.8

— — — — —

10

—

4

16.8 X 10 15.2 13.3 11.8 10.8 10.1 8.90 X 10 7.39 5.71 4

4

— — —

F i g u r e 1, shows a r a p i d increase i n a p p a r e n t viscosity for r e s i n V as the shear rate reaches

17.3 s e c . -1

T h e s h a r p increase i n a p p a r e n t

viscosity is not any c h e m i c a l change, s u c h as c r o s s l i n k i n g of p o l y m e r i c chains.

T h e a p p a r e n t viscosity c u r v e c a n be r e t r a c e d b y l o w e r i n g the

shear rate.

T h i s increase i n a p p a r e n t viscosity c a n b e e l i m i n a t e d b y

i n c r e a s i n g t h e m e a s u r e m e n t t e m p e r a t u r e , as s h o w n i n F i g u r e 2. same p h e n o m e n o n Porter

(12).

The

This

has b e e n r e p o r t e d for p o l y s t y r e n e b y P e n w e l l a n d e x p l a n a t i o n of

t h e a p p a r e n t v i s c o s i t y increase i n

c a p i l l a r y flow of p o l y s t y r e n e was q u a n t i t a t i v e l y e x p l a i n e d t h r o u g h the


9.

Heat-Resistant

CORNELL

137

Polyarylsulfone

pressure d e p e n d e n c e of glass t r a n s i t i o n , T .

T h e same e x p l a n a t i o n c o u l d

g

a c c o u n t f o r t h e increase i n a p p a r e n t v i s c o s i t y o f r e s i n V . l a r y pressure increases w i t h i n c r e a s i n g shear rate, T

A s the c a p i l

approaches t h e

g

m e a s u r e m e n t t e m p e r a t u r e , c a u s i n g a r a p i d rise i n viscosity. T h e a b o v e d a t a substantiate this e x p l a i n its o c c u r r e n c e . I0 P

I

6

flow

I I I I Mil

I

I

I

I I 11IIII

I III!


J

•o

|0

improvement b u t do not

A possible reason f o r the i m p r o v e m e n t i n flow I I I 11111

I

I I

Resin 3 £

S

Resin TSL

5 I0

4

|Q3

•

•

I

I I I M i l

10"'

I

10°

'

|

* * M i l l

|

| | | Mil

10'

|

I I lllll

|

10*

I

I

10*

Shear Rote ( sec." ) 1

Figure

1.

10*

s

, 0

viscosity vs. shear rate of resins IV and V at 260° C

Apparent

i

Ml

i i

I

1 I I I Mil

'e

I—I

I I I III

Resin 3 £

Resin HZ. > 10" c a> QL

1—i—i—i—r-

15

10

3. Molecular-weight distribution of resins IV and V determined by gel permeation chromatography

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

I t is

v e r y difficult to b e l i e v e that this is t h e major c o n t r i b u t o r t o t h e flow improvement.

A n o t h e r p o s s i b i l i t y c o u l d b e a d r a s t i c difference i n t h e

molecular weight distribution or weight-average ( M ) molecular weight w

( o r b o t h ) o f the t w o resins.

G e l permeation chromatography data indi

cate that this is n o t t h e case.

A s s h o w n i n F i g u r e 3, t h e m e l e e u l a r -

w e i g h t d i s t r i b u t i o n s are q u i t e s i m i l a r . W o r k i n g o n t h e a s s u m p t i o n that p o l y s t y r e n e standards a r e v a l i d c a l i b r a t i o n standards f o r p o l y a r y l s u l f o n e s , t h e m o l e c u l a r w e i g h t s

(M ) w

for resins I V a n d V w e r e c a l c u l a t e d f r o m the 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 curves b y the f o l l o w i n g e q u a t i o n . M

w

= XWiMi] Wi =

Hi/ZHi

Hi = height of c u r v e a t v a r i o u s counts 2Hi = t o t a l of a l l heights measured Mi = m o l e c u l a r weight of p o l y s t y r e n e a t v a r i o u s counts.


9.

Heat-Resist ant

CORNELL

139

Polyarylsulfone

E v e n i f the a s s u m p t i o n that p o l y s t y r e n e standards c a n b e u s e d is not c o m p l e t e l y v a l i d , the error i n the d e t e r m i n e d M

molecular weights

w

w o u l d b e a constant, a n d s h o u l d not alter the p e r c e n t difference i n m o l e c u l a r w e i g h t of the t w o p o l y a r y l s u l f o n e s .

The M

w

M

w

molecular weight

for resins I V a n d V are 46,100 a n d 52,300, r e s p e c t i v e l y .

T h i s 10% differ

ence c o u l d b e significant i f resins I V w e r e to l i e b e l o w the c r i t i c a l m o l e c u l a r w e i g h t (Mj))

for p o l y m e r c h a i n e n t a n g l e m e n t a n d r e s i n V l i e above.

M u c h e x p e r i m e n t a l d a t a are a v a i l a b l e for the v i s c o s i t y of p o l y m e r m e l t s . T h e s e d a t a , w i t h o u t exception, o b e y this e q u a t i o n for M greater t h a n


some c r i t i c a l m o l e c u l a r w e i g h t M&:

T h i s r e l a t i o n s h i p is s h o w n i n F i g u r e 4 f o r p o l y s t y r e n e ( 1 3 ) .

Figure 4. polystyrene

T h e l i n e for M greater t h a n M

h

Variation of melt viscosity of as a function of molecular weight at 217°C

has a slope of 3.5.

If resins I V a n d

V do, i n fact, l i e o n o p p o s i t e sides of the c r i t i c a l m o l e c u l a r w e i g h t ( M ) , 6

a 10% increase i n M

w

m o l e c u l a r w e i g h t c o u l d b e a c c o m p a n i e d b y a 40%

or h i g h e r increase i n m e l t viscosity.

T o substantiate this possible e x p l a

n a t i o n for the significant differences i n m e l t viscosity of resins I V a n d V , Mb f o r b o t h resins w o u l d h a v e to b e d e t e r m i n e d .

Acknowledgment T h e a u t h o r wishes to t h a n k the entire P o l y m e r P h y s i c s G r o u p f o r helpful

assistance

on

polymer

characterization, Peter

M . Byra

for

h a n d l i n g the i n j e c t i o n - m o l d i n g operations, a n d A n g e n e t t e G r a n t for h e r assistance i n the m o n o m e r a n d p o l y m e r syntheses.

Literature

Cited

1. Lee, H., Stoffy, D., Neville, K., "New Linear Polymers," McGraw-Hill, New York (1967). 2. Rose, J. B., Jennings, B. E., Jones, M. E. B., J. Polym. Sci., Part C (1967) 715. 3. Vogel, H. A., J. of Polym. Sci., Part A-1 (1970) 8, 2035.


140

4. 5. 6. 7. 8. 9. 10. 11.


12. 13.

POLYMERIZATION

REACTIONS A N D N E W POLYMERS

Erickson, A., U.S. Patent 2,860,168 (1958). Joyl, R., Bucourt, R., Fabignon, C., U.S. Patent 2,971,985 (1961). Keogh, M. J., Ingberman, A. K., U.S. Patent 3,415,887 (1968). Ruppert, H., Schnell, H., British Patent 935,061 (1963). Simmons, P., Ulyatt, J. M., British Patent 932,811 (1963). Broderick, G. F., Oxenrider, B. C., Vitrone, J., U. S. Patent 3,393,244 (1968). Cornell, R. J., U.S. Patent 3,554,972 (1971). Johnson, R. N., Farnham, A. G., Clendinning, R. A., Hale, W. F., Merrian, C. N.,J.Polym.Sci.,PartA-1,(1967) 5, 2375. Penwell, R. C., Porter, R. S., Appl. Polym.Sci.,(1969) 13, 2427. Bueche, F., "Physical Properties of Polymers," Interscience, New York (1962).

RECEIVED April 10, 1972.


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