Polymer Bulletin 1, 743-748 (1979) Polymer Bulletin �9 by Springer-Verlag 1979

Effect of Cooling Rate on the Viscoelastic Properties in the Plastic Zone of Solid Polymers

Giovanni Rizzo and Giuseppe Titomanlio

Istituto di Ingegneria Chimica, Universit~ di Palermo, Viale delle Scienze, Palermo, Italy

ABSTRACT

PMMA and PC samples w e r e sub jec ted to s e v e r a l cool ing r a t e s f rom the g la s s t r a n s i t i o n t e mp e ra tu r e .

S t r e s s - r e l a x a t i o n data , obtained with these samples under s t r a i n s l a r g e r than the y ie ld s t r a i n , were not inf luenced by the cool ing r a t e p r e v i o u s l y expe r i enc ed by the mate r ia l . This shows that a f t e r y ie ld ing the ef fec t of p rev ious aging is quickly e r a s e d ; s imi la r ind ica t ions , d rawn from c a l o r i m e t r i c measuremen t s , w e r e a l r e a d y ava i lab le in ( 11-12 ) .

INTRODUCTION

The aging a n d / o r anneal ing effect on many p r o p e r t i e s of sol id po lymers , e s p e c i a l l y at small s t r a i n s , have been ex t ens i - ve ly s tudied by many au tho r s ; in p a r t i c u l a r S t ru ik ( 1 ) r e p o r t s a r e m a r k a b l e amount of r e s u l t s and rev iews most of the ma te r i a l ava i lab le in the l i t e r a t u r e .

Aging and anneal ing o r more in gene ra l the thermal h i s t o r y undergone by the ma te r i a l c e r t a i n l y ( lb ) have a s igni f icant e f fec t , a l though smal le r than at small s t r a i n s , a l so on the p r o - pe r t i e s at l a r g e s t r a i n s . F o r ins tance changes of y ie ld s t r e s s ( 1 b , 2, 3 ) and embri t t lement in t ens i l e and impact t e s t s have been o b s e r v e d ( 4 - 9 ) �9 F u r t h e r m o r e mater ia l dens i f i ca t ion ( 1, 39 13, 14 ) and en tha lpyre laxa t ion ( which pa ra l l e l s the en tha lpyabsorbed at the g l a s s t r a n s i t i o n t empe ra tu r e ) on aging or anneal ing have been m e a s u r e d ( 3, 8 -12 ) ~

On the o the r hand t h e r e a re indica t ions that the enthalpy r e l axed may be r e g a i n e d by the ma te r i a l by means of t ens i l e d rawing ( 1 1 - 1 2 ) ~

This work is o r i en ted towards a s c e r t a i n i n g the r o l e of aging in the behav io r a f t e r ma te r i a l y ie ld ing ~ The r e s u l t s

0170-0839/79/0001/0743/$01.20

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o b t a i n e d b y m e a n s of some c o m p r e s s i o n t e s t s up to v e r y l a r g e d e f o r m a t i o n s , p e r f o r m e d on b o t h p o l y c a r b o n a t e and p o l y m e t h y l - m e t a c r y l a t e s a m p l e s , w h i c h h a d b e e n s u b j e c t e d to d i f f e r e n t t h e r m a l h i s t o r i e s , a r e h e r e p r e s e n t e d o

E X P E R I M E N T AL

T h e m a t e r i a l s s t u d i e d w e r e b i s p h e n o l A p o l y c a r b o n a t e ( L e x a n d ) a n d p o l y m e t h y l m e t a c r y l a t e ( P l e x i g l a s a ) , w h i c h w e r e s u p p l i e d in t h e f o r m of r o d s o C y l i n d r i c a l s a m p l e s w i t h L = D = 1 c m f o r P M M A a n d L = D = 0 . 9 c m f o r p o l y c a r b o n a t e w e r e p r e p a r e d b y m a c h i n i n g .

A l l t h e s a m p l e s of e a c h m a t e r i a l w e r e a n n e a l e d t o g e t h e r f o r 0 . 5 h r a t a b o u t 10~ a b o v e t h e i r 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 a n d w e r e t h e n q u e n c h e d in i c e - w a t e r o They were h e a t e d

s e p a r a t e l y up to about t h e i r 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

10 E E

01

- 8 <)

6

4

2

mmm

mo ~ o

� 9

mmmmmm m

% rm

0

O

<>

<> O

0 <>

cooling rate

1 ~

10 ~

air quench

= 3 hr I

8 O ~ , e

F i g . 1 Compress ion s t r e s s - s t r a i n b e h a v i o r of P M M A samples subjected to d i f f e r e n t coo l ing ra tes . ~ is i n i t i a l de fo rma t ion ra te .

745

( i . e . t h e P C s a m p l e s to 150~ and the P M M A o n e s to IO0~ ) and w e r e s u b j e c t e d s e p a r a t e l y to d i f f e r e n t c o o l i n g r a t e s , t h a t i s a i r q u e n c h i n g , 6 0 ~ l o C / h r f o r t he P C s a m p l e s a n d a i r q u e n c h i n g , l O ~ a n d l ~ f o r t h e P M M A o n e s .

S t r e s s r e l a x a t i o n m e a s u r e m e n t s a f t e r c o n s t a n t v e l o c i t y c o m p r e s s i o n r a m p s w e r e p e r f o r m e d a t r o o m t e m p e r a t u r e b y m e a n s of a n I n s t r o n t e s t i n g m a c h i n e M o d . 1115 .

RESULTS AND DISCUSSION

T h e s t r e s s G o b t a i n e d d u r i n g the l o a d i n g r a m p s p r e v i o u s to r e l a x a t i o n t e s t s i s r e p o r t e d in f i g s 1 and 2 v e r s u s t he s t r a i n e m /11/1 o , w h e r e lo i s t he i n i t i a l s a m p l e h e i g h t and /11 the s a m p l e h e i g h t d e c r e a s e d u r i n g c o m p r e s s i o n . B o t h t h e m a t e r i a l s e x a m i n e d s h o w e d the s a m e f e a t u r e s . In p a r t i c u l a r , a n a l o g o u s l y

10

8

6

4

Fig. 2

0

E

mr

O" d. = 3 3 h V 1 v v

v ~ �9 �9 O v �9 �9 �9

t

4

�9 vV v d= 3"3 hr-~

O Ov V �9 u ~ uv ~ ' o

f u

v cooling rate ~p �9 v l~

�9 o 50~ V o �9 u air quench

I I I

0 G1 0.2 0.3

w e

10

E 8

m ~

~ 1 7 6 O - e

4

2

e t ~ 0

0.4 0.5

Compression stress-strain behavior of PC samples subjected to different cooling rates . (% is initial deformation rate .

746

to what was observed in tensile tests ( ib, 2, 3 ) samples sub- jected to smaller cooling rates showed a larger yield stress . Furthermore both in figs 1 and 2, where two values of the strain rate (Z are considered, the curves approach each other after yielding .

Let us briefly discuss the meaning of these observations . Certainly the material relaxation time ( or the spectrum of relaxation times ) decreases as the stress increases and only when it is sufficiently sma11, may yielding take place . The changes induced in the material by large strains may thus ( 15 ) be considered the inverse of aging and in fact a larger yield stress is observed with samples more efficiently aged by means of a smaller cooling rate . Also, the fact that, after yielding, the stress-strain curves of figs 1 and 2 approach each other suggests that the differences induced in the samples by previous ( aging ) treatments are quickly erased .

Similar indications were in fact drawn in ( 11-12 ) from the observation that tensile drawing of ABS resins produced an enthalpy change matching the entalpy relaxation due to the thermal history undergone by the sample .

More detailed indications may be obtained from the stress- relaxation data reported in figs 3 and /+ as (I/ (I (where (/

O O

Fig . 3

.8

.6

. 4

0 / 0 o <>~ OL-- 3 h~' �9 r = .38

~ ~ 8

cooling rate

�9 1 ~ Qm. o 10 ~ - ' 8 ~ : ,

q W 0 air quench

t , sec | I I

1 10 102 103 1~

Compression stress-relaxation behavior of PMMA samples subjected to different cooling rates . er is relaxation strain .

747

i s the s t r e s s a t the end of the l o a d i n g r a m p ) v e r s u s t ime t . F o r e a c h m a t e r i a l a l l the t e s t s w e r e p e r f o r m e d a t the s ame s t r a i n e r ( l a r g e r t h a n the y i e l d s t r a i n ) on the s a m p l e s w h o s e l o a d i n g was a l r e a d y c o n s i d e r e d in f igs I and 2 .

The da t a of f igs 3 and s show tha t the c oo l i ng r a t e does no t a f f ec t the r e l a x a t i o n b e h a v i o r a f t e r y i e l d i n g . In o t h e r w o r d s , in th i s z o n e , the r e l a x a t i o n t ime does no t s e e m to d e p e n d u p o n the r e l a x a t i o n t ime of the m a t e r i a l p r i o r to the t e s t , w h i c h c o n - v e r s e l y is c e r t a i n l y i n f l u e n c e d b y the p r e v i o u s t h e r m a l h i s t o r y a n d / o r a g i n g : the i n c r e a s e of sma l l s t r a i n r e l a x a t i o n t ime is c o m p l e t e l y r e v e r s e d b y the y i e l d i n g p r o c e s s .

On the o t h e r h a n d the da t a of f i g . 4 show t h a t , as a l r e a d y o b s e r v e d in o t h e r c a s e s ( 16 ) , the m a t e r i a l v i s c o e l a s t i c b e h a v i o r in the p l a s t i c zone ( a f t e r y i e l d i n g ) d e p e n d s on the s t r a i n r a t e a d o p t e d in the l o a d i n g r a m p .

Bo th the e f f ec t s m e n t i o n e d a b o v e may be s u m m a r i z e d a s fo l lows . W h a t e v e r i s the ag ing h i s t o r y e x p e r i e n c e d p r e v i o u s l y

t

.9

.8

.7

.6

.5

" 4 - ~ ~ ~ ~=3.3hf '

~- ~ ~ cooli.ng rate # %

�9 v l~ �9 O 50~ o B �9 0 air quench ~ c ~

U ve~r n tm v

d = 3 3 hr-1

e , : O . 5

|

ONly o - z

" ; , ; ,

i i

10 -1 1 10

1

9

8

1 a l V l l a �9

�9 *m'Ov e

5

I I t , $ e c

10 2 10 ~

F i g . s C o m p r e s s i o n s t r e s s - r e l a x a t i o n b e h a v i o r of PC s a m p l e s s u b j e c t e d to d i f f e r e n t c o o l i n g r a t e s . e r i s r e l a x a t i o n s t r a i n .

748

by the material, in order to have yielding, the molecular mobili- ty has to increase to a level which depends only on the strain rate (~ ; during this process the effect of previous aging is completely erased .

REFERENCES

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Ii. MATSUOKA, S.,BAIR, H.E. and ALOISIO, C.]., J. Polym. Sci., f+6 , 115 (1971+)

12. MATSUOKA, S., ALOISIO, C.]. and BAIR, H.E., ]. Appl. Phys., ___~ , I+265 ( 1973 )

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15. MATSUOKA, S. BAIR, H.E., BEARDER, S.S., KERN, H.E. and RYAN, J.T. ,Polym. Eng. Sci., 18, 1073 ( 1978 )

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Received July 17, 1979