Copolymers, Polyblends, and Composites

23 Creep as Related to the Structure of

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Block Polymers LAWRENCE E. NIELSEN Monsanto Co., St. Louis, Mo. 63166

The creep of five commercial block copolymers, both amorphous and crystalline, was measured at 23° and 105°C with several loads. For a given load, the creep of block polymers was greater than that of the homopolymer making up the hard block. Specimens quench-cooled from the melt had more creep and a faster creep rate than slow-cooled specimens. The effect of molding conditions was most noticeable when creep was tested at 23°C, but annealing at 105°C for many hours did not eliminate the differences caused by initial molding conditions. Thermoplastic rubbers (block polymers with a high percent of soft block) had more creep than conventional crosslinked rubbers. For crystallizable block polymers, increasing the amount of crystalline phase reduced the creep.

A

l t h o u g h the d y n a m i c m e c h a n i c a l properties a n d the stress-strain behavior k> o f b l o c k c o p o l y m e r s h a v e b e e n s t u d i e d e x t e n s i v e l y , v e r y l i t t l e c r e e p d a t a are a v a i l a b l e o n t h e s e m a t e r i a l s ( 1 - 1 7 ) . A n u m b e r o f b l o c k c o p o l y m e r s a r e n o w c o m m e r c i a l l y a v a i l a b l e as t h e r m o p l a s t i c e l a s t o m e r s t o r e p l a c e c r o s s l i n k e d r u b b e r f o r m u l a t i o n s a n d o t h e r p l a s t i c s (16). F o r applications i n w h i c h the finished o b j e c t m u s t b e a r l o a d s f o r e x t e n d e d p e r i o d s o f t i m e , i t is i m p o r t a n t t o k n o w h o w these n e w m a t e r i a l s c o m p a r e w i t h c o n v e n t i o n a l c r o s s l i n k e d r u b b e r s a n d more r i g i d plastics i n d i m e n s i o n a l stability or creep behavior. T h e creep of five c o m m e r c i a l b l o c k p o l y m e r s w a s m e a s u r e d as a f u n c t i o n o f t e m p e r a t u r e a n d m o l d i n g conditions. F o u r of the polymers h a d crystalline h a r d blocks, a n d o n e h a d a g l a s s y p o l y s t y r e n e h a r d b l o c k . T h e soft b l o c k s w e r e v a r i o u s k i n d s of e l a s t o m e r i c m a t e r i a l s . T h e c r e e p o f t h e b l o c k p o l y m e r s w a s also c o m p a r e d w i t h that of a n o r m a l , crosslinked natural rubber a n d crystalline p o l y ( t e t r a methylene terephthalate ) ( P T M T ). Materials

and

Techniques

T h r e e of the b l o c k p o l y m e r s were D u P o n t H y t r e l s 4 0 5 5 , 5 5 5 5 , a n d 6 3 5 5 (17, 18, 19). These materials have a crystallizable P T M T h a r d block a n d a p o l y ( t e t r a m e t h y l e n e g l y c o l ) ( P T M G ) soft b l o c k w i t h a m o l e c u l a r w e i g h t o f about 1000. Hytrels 4055, 5555, a n d 6355 contained about 42.6, 35, a n d 2 7 % P T M G , respectively. T h e fourth p o l y m e r was U n i r o y a l T P R - 1 9 w h i c h has a 257

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


258

COPOLYMERS,

o

POLYBLENDS,

Boo

oo

A N D COMPOSITES

IF

TIME (min) Figure 1. Creep of Kraton 1101 and a vulcanized natural rubber at 22°C (load in lbs/in. is noted on curves) 2

c r y s t a l l i n e p o l y p r o p y l e n e h a r d b l o c k a n d a soft b l o c k o f e t h y l e n e - p r o p y l e n e elastomer w i t h r o u g h l y 3 0 % total ethylene. T h e fifth p o l y m e r w a s Shell K r a t o n 1101 triblock p o l y m e r w i t h polystyrene ends a n d a polybutadiene cent r a l b l o c k ; t h i s m a t e r i a l is a b o u t 2 4 . 5 % s t y r e n e . E a c h p o l y s t y r e n e b l o c k h a s a molecular weight of about 12,500 w h i l e the polybutadiene block has a molecular w e i g h t of 75,000.

(00 TIME (MINUTES) Figure 2.

Creep of TPR-19 with crystalline polypropylene hard blocks at 23°C


23.

NIELSEN

Creep

v s . Structure

259

SLOW-COOLED QUENCHED RUBBER

20psi


. 4 0 0 PSI

„

—

-

^ - 2 0 0 200

•100

1000

10,000

TIME (MIN) Figure 3.

Creep of Hijtrel 4055 and a vulcanized natural rubber at 23°C

Specimens of each material were prepared under t w o very different m o l d ing conditions: slow-cooled a n d quench-cooled. Hytrels 6355 a n d 5555 were c o m p r e s s i o n - m o l d e d at 2 7 0 ° C ; q u e n c h - c o o l e d specimens w e r e t h e n p l u n g e d into ice water whereas slow-cooled specimens were cooled to 2 0 0 ° C over 10 m i n a n d then cooled f r o m 2 0 0 ° C to room temperature i n about another 10 m i n .

' 8 0 0 PSI

SLOW-COOLED QUENCHED .600

-400

100

1000

10,000


Creep of Hytrel 5555 at 23°C


260

COPOLYMERS,

POLYBLENDS, A N D COMPOSITES

SLOW-COOLED QUENCHED

* 800 PSI


«X. Ο ,-400 —

800

•400

100

1000

10,000


Creep of Hytrel 6355 at 23°C

H y t r e l 4 0 5 5 w a s m o l d e d u n d e r the same conditions except that the m o l d i n g temperature was 2 6 0 ° C , a n d the slow-cooled specimens were cooled to 1 8 0 ° C i n 10 m i n . T h e initial m o l d i n g temperature for P T M T w a s 2 5 5 ° C . T P R - 1 9 was m o l d e d similarly except that the initial m o l d i n g temperature w a s 2 0 0 ° C . K r a t o n 1 1 0 1 w a s m o l d e d at 1 3 5 ° C . M o s t o f t h e s p e c i m e n s m e a s u r e d 4 X 3 / 8 X 0.030 i n . except for the crosslinked natural rubber specimens w h i c h were cut f r o m large rubber bands. T h e creep of the lowest m o d u l u s materials w a s t h e change i n s p a c i n g of t w o i n k d o t s o n t h e s p e c i m e n s as m e a s u r e d w i t h a 3 0 - p o w e r t r a v e l i n g m i c r o -

SL0W-COOLED QUENCHED 22*C •5|

Ε

1000 psi

g.4|

ι— ζ

ο

500 pti

-*

1000

TIME (min) Figure 6.

Creep of poly(tetramethylene terephthalate) at 22°C



23.

261

v s . Structure

Creep

NIELSEN

Ol I

I

I

I

10

100

DOO

Figure 7.

L K)

TIME (min) Creep of Kraton 1101 at 60°C

s c o p e . T h e c r e e p o f t h e m o r e r i g i d m a t e r i a l s , s u c h as H y t r e l 6 3 5 5 , w a s m e a s u r e d b y a r e c o r d i n g L V D T e x t e n s o m e t e r (20). T e s t s w e r e c o n d u c t e d at 2 3 ° ± 1 ° C at a r e l a t i v e h u m i d i t y o f a b o u t 5 0 % . C r e e p tests w e r e also m a d e at 1 0 5 ° C o n t h e H y t r e l s a n d T P R - 1 9 , a n d at 6 0 ° C o n K r a t o n . Results and

Discussion

T y p i c a l creep curves are presented i n F i g u r e s 1 - 1 2 . T h e elastic m o d u l i of t h e b l o c k c o p o l y m e r s w e r e b e t w e e n those o f c o n v e n t i o n a l crosslinked rubbers a n d t h o s e o f r i g i d c r y s t a l l i n e p o l y m e r s s u c h as P T M T a n d p o l y e t h y l e n e . T h u s ,

τδ

Rfej

lute

rctar


Creep of TPR-19 propylene block polymer at 105°C



262

COPOLYMERS,

Ol I

ι K)

ι KX)

POLYBLENDS,

ι 1000

A N D COMPOSITES

ι K)j000


Creep of Hytrel 4055 at

105°C

i t is n o t s u r p r i s i n g t h a t t h e i n i t i a l c r e e p d e f o r m a t i o n s o f b l o c k p o l y m e r s at a g i v e n l o a d w e r e g r e a t e r t h a n t h o s e o f t h e r i g i d c r y s t a l l i n e p o l y m e r s b u t less t h a n that of a crosslinked rubber. H o w e v e r , t h e creep rate, o r t h e slope of the c r e e p c u r v e , w a s g e n e r a l l y g r e a t e r f o r b l o c k p o l y m e r s t h a n f o r e i t h e r crosslinked rubbers or conventional rigid polymers. C r e e p o f b l o c k p o l y m e r s is d e t e r m i n e d p r i m a r i l y b y t h e d i m e n s i o n a l sta b i l i t y of t h e h a r d blocks w h i c h c a n b e either glassy o r crystalline. T h e i m p l i c a t i o n o f t h i s w o r k is t h a t t h e s m a l l size o f t h e h a r d b l o c k s l e a d s t o m o r p h o logical structures w i t h poorer d i m e n s i o n a l stability t h a n c o n v e n t i o n a l p o l y m e r s . T h e h a r d blocks must either break u p or d e f o r m relatively easily w h e n a l o a d is a p p l i e d t o t h e m . W h e n t h e h a r d b l o c k is a c r y s t a l l i n e m a t e r i a l , as i n t h e Hytrels a n d T P R - 1 9 , the creep depends o n the degree of crystallinity of the

Τ SLOW-COOLED

1

ώ

ate

içtar

TIME (MIN.) Figure 10.


105°C


23.

Creep

NIELSEN


5-

263

v s . Structure

SLOW-COOLED QUENCHED I05*C

01

ι K)

I

I 100

I 1000

I IOJ0OO



105°C

polymer. T h e higher the total crystallinity, the smaller the creep a n d the creep rate. T h e m o r p h o l o g y o f t h e m a t e r i a l s m u s t also b e i m p o r t a n t . L a r g e c h a n g e s i n c r e e p w e r e f o u n d f o r v e r y s m a l l c h a n g e s i n t h e d e g r e e o f c r y s t a l l i n i t y as measured b y x-ray techniques or b y differential scanning calorimetry ( D S C ) . T h i s is a p p a r e n t f r o m a c o m p a r i s o n o f t h e c r e e p d a t a w i t h d a t a o n c r y s t a l l i n i t y i n d e x a n d a p p a r e n t h e a t o f f u s i o n ( T a b l e I ) . T h e c r y s t a l l i n i t y i n d e x is t h e area above the amorphous curve d i v i d e d b y the total area o b t a i n e d f r o m a n

• Slow-Cooled

/

500 PSI

Ol I

ι 10

ι 100

ι 1000

ι IOJ0OO


Creep of poly(tetramethylene terephthalate) at

105°C


264

COPOLYMERS,

Table I.

A N D COMPOSITES

X-ray CrystallinityIndex a n d D S C Apparent Heats of F u s i o n Apparent of Fusion,

Heat Treatment

Crystallinity Index

TPR-19

slow-cooled quenched

.24 .21

9.8 8.0

Hytrel 4055


0.9 .07

4.8 4.5

Hytrel 5555


.34 .19

7.6 7.5

Hytrel 6355


.24 .20

7.6 8.6

Polymer


POLYBLENDS,

x-ray scan. B o t h t h e crystallinity index a n d t h e apparent heat of fusion are relative indicators of the degree of crystallinity. B o t h t h e x-ray a n d t h e D S C c u r v e s s h o w e d s o m e u n e x p e c t e d c h a n g e s i n s h a p e i n t h e H y t r e l series w h i c h sometimes reversed the expected trends i n crystallinity. D S C curves sometimes indicated higher crystallinities f o r the quench-cooled than for t h e slow-cooled H y t r e l s as m e a s u r e d b y t h e a r e a u n d e r t h e D S C m e l t i n g p e a k . D i f f e r e n c e s i n m o r p h o l o g y are i m p l i e d b y the b r o a d w i d t h of the m e l t i n g peak of the q u e n c h e d p o l y m e r c o m p a r e d w i t h t h e peak f o r t h e s l o w - c o o l e d p o l y m e r . D e t a i l e d studies of t h e m o r p h o l o g y h a v e n o t b e e n m a d e o n these materials, b u t C e l l a ( 1 7 ) f o u n d a lamellar structure of the crystalline phase i n similar H y t r e l s . E x a m p l e s

TEMPERATURE Figure 13. DCS curves for A, Hytrel 6355 and B, Hytrel 4055 (polymers were annealed for 8 days at 105°C after molding)


23.

NIELSEN

Creep

v s . Structure

265


of D S C c u r v e s f o r t w o p o l y m e r s a n n e a l e d f o r 8 d a y s at 1 0 5 ° C a r e p r e s e n t e d in Figure 13. M o l d i n g conditions greatly affected the creep of block p o l y m e r s , especially c r y s t a l l i n e ones m e a s u r e d a t 2 3 ° C . B o t h i n i t i a l c r e e p d e f o r m a t i o n a n d c r e e p rate w e r e g e n e r a l l y h i g h e r f o r q u e n c h e d s p e c i m e n s t h a n f o r s l o w - c o o l e d o n e s . T h e s a m e effects, a l t h o u g h less d r a m a t i c , w e r e f o u n d at 1 0 5 ° C ( s p e c i m e n s w e r e h e l d at 1 0 5 ° C at least 1 5 h r s b e f o r e t h e c r e e p tests w e r e s t a r t e d ) . E v e n w i t h this l o n g a n n e a l i n g t i m e , t h e d i f f e r e n c e s p r o d u c e d b y t h e m o l d i n g c o n d i t i o n s were noteliminated, b u t only reduced i n magnitude. A n o t h e r e x a m p l e o f t h e l a r g e effect o f m o r p h o l o g y o n c r e e p b e h a v i o r m a y have been f o u n d w i t h t h e K r a t o n p o l y m e r . It was impossible to obtain g o o d r e p r o d u c i b l e creep values w i t h this p o l y m e r , a n d s u p p o s e d l y i d e n t i c a l speci m e n s o f t e n d i f f e r e d g r e a t l y i n c r e e p b e h a v i o r . T h e c a u s e o f this e r r a t i c b e havior w a s n o t d e t e r m i n e d , b u t it m i g h t b e related to cylinder-to-lamellae m o r p h o l o g i c a l c h a n g e s . T h e c o m p o s i t i o n o f t h e K r a t o n is a b o u t t h a t at w h i c h this transformation occurs. A s a n e x a m p l e o f this erratic b e h a v i o r , t h e creep o f t h e s l o w - c o o l e d p o l y m e r at b o t h 2 0 a n d 4 0 p s i f e l l b e t w e e n t h e c o r r e s p o n d i n g values of the quench-cooled material ( F i g u r e 1 ) . Summary B l o c k p o l y m e r s have a higher creep rate t h a n either crosslinked rubbers or crystalline h o m o p o l y m e r s . T h e h a r d blocks are a p p a r e n t l y n o t large e n o u g h to b e s t a b l e t o h i g h l o a d s , b u t t h e y a p p a r e n t l y b r e a k u p o r a l l o w c h a i n s l i p p a g e to o c c u r so t h a t v e r y h i g h c r e e p rates a r e f o u n d at h i g h l o a d s . T h e r e a r e g r e a t differences b e t w e e n s l o w - c o o l e d a n d q u e n c h e d specimens. Q u e n c h e d speci mens h a d m o r e creep a n d a higher creep rate t h a n s l o w - c o o l e d ones. H i g h temperature annealing below the melting point of the h a r d block d i d not eliminate t h e differences b e t w e e n q u e n c h e d a n d slow-cooled specimens. This i n d i c a t e s t h a t t h e m o r p h o l o g y is p e r m a n e n t l y l o c k e d - i n at t h e t i m e o f i n i t i a l molding. Acknowledgment M o s t of the data were obtained b y Jerry S u g a r m a n .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Baer, M., J. Polym. Sci. Part A (1964) 2, 417. Kenney, J. F., Polym. Eng. Sci. (1968) 8, 216. Zelinske, R., Childers, C. W., Rubber Chem. Technol. (1968) 41, 161. Beecher, J. F., Marker, L., Bradford, R. D., Aggarwal, S. L.,J.Polym. Sci. Part C (1969) 26, 117. Holden, G., Bishop, E. T., Legge, N. R., J. Polym. Sci. Part C (1969) 26, 37. Harrell, Jr., L. L., Macromolecules (1969) 2, 607. Morton, M., McGrath, J. E., Juliano, P.C.,J. Polym. Sci. Part C (1969) 26, 99. Estes, G. M., Cooper, S. L., Tobolsky, Α. V., J. Macromol. Sci. Rev. Macromol. Chem. (1970) C4, 313. Miyamoto, T., Kodama, K., Shibayama, K.,J.Polym. Sci. Part A-2 (1970) 8, 2095. Shen, M., Kaelble, D. H., J. Polym. Sci. Part B (1970) 8, 149. Charrier, J.-M., Ranchoux, R. J. P., Polym. Eng. Sci. (1971) 11, 318. Lim, C. K., Cohen, R. E., Tschoegl, N. W., ADVAN. CHEM. SER. (1971) 99, 397. Cohen, R. E., Tschoegl, N. W., Int. J. Polym. Mater. (1972) 2, 49. Fielding-Russell, G. S., Rubber Chem. Technol. (1972) 45, 252. Kraus, G., Rollmann, K. W., Gardner, J. O., J. Polym. Sci. Part A-2 (1972) 10, 2061.


266

COPOLYMERS,

POLYBLENDS,

AND

COMPOSITES

16. 17. 18. 19.

Scheiner, L. L., PlasticsTechnol.(1973) 19 (5), 36. Cella, R. J., J. Polym. Sci. Part C (1973) 42, 727. Brown, M., Witsiepe, W. K., Rubber Age (1972) 104 (3), 35. Hoeschele, G. K., Witsiepe, W. K., Angew. Makromol. Chem. (1973) 29/30, 267. 20. Nielsen, L. E., Trans. Soc. Rheol. (1969) 13, 141. 1974.


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