EFFECT OF THERMAL HISTORY ON β RELAXATIONS AND COMPRESSION YIELD BEHAVIOR OF POLYCARBONATE

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EFFECT OF THERMAL HISTORY ON β RELAXATIONS AND COMPRESSION YIELD

BEHAVIOR OF POLYCARBONATE

C. Bauwens-Crowet, J.-C. Bauwens

To cite this version:

C. Bauwens-Crowet, J.-C. Bauwens. EFFECT OF THERMAL HISTORY ON β RELAXATIONS

AND COMPRESSION YIELD BEHAVIOR OF POLYCARBONATE. Journal de Physique Colloques,

1983, 44 (C9), pp.C9-185-C9-190. �10.1051/jphyscol:1983923�. �jpa-00223369�

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E F F E C T OF THERMAL H I S T O R Y ON

0

R E L A X A T I O N S AND COMPRESSION Y I E L D B E H A V I O R O F POLYCARBONATE

C . Bauwens-Crowet and J . - C . Bauwens

Universitg Libre de BruxeZZes, Physique des Mat6riaz.m de Synthe'se, 67 avenue AdoZphe BuyZ, 194/8, 1050, BmxeZles, Belgium

Rksumk

-

Les pics de p e r t e s mkcaniques m e s u r k s

B

l ' a i d e dlune pendule de flexion, dans l e domaine de l a t r a n s i t i o n

13

du Polycarbonate, sont r e l e - vks pour des kchantillons ayant un passk thermique diffkrent. Aucune influence de c e p a s s 6 thermique n ' a pu d t r e m i s e e n kvidence, comme c'ktait d ' a i l l e u r s l e c a s pour l e s p i c s m e s u r k s e n t o r s i o n . L ' e f f e t du passk t h e r m i - que s u r l a limite d'kcoulement en c o m p r e s s i o n e s t ktudik dans un grand do- m a i n e de t e m p k r a t u r e s

B

v i t e s s e constante. L e s r k s u l t a t s sont a n a l y s e s en f a i s a n t l'hypoth'ese que deux p r o c e s s u s activks d e t (3 nkcessitant d e s con- t r a i n t e s additives TCd et

rCp

sont impliquks dans l a dkformation a u

s e u i l d' kcoulement. Suivant un t r a i t e m e n t pmposk a n t k r i e u r e m e n t , une c o r r k l a t i o n e s t Btablie e n t r e l e pic

IJ

e t l a contrainte 0- pour des itchan-

c ^Y,

tillons vieillis; e l l e e s t en bon a c c o r d a v e c l e s r k s u l t a t s expkrimentaux. C e s r k s u l t a t s confirment que s e u l e l'entropie configurationnelle du p r o c e s s u s d e s t affect6e p a r l e v i e i l i s s e m e n t tandis que l a p r e s s i o n hydrostatique influence l'enthalpie d'activation d e s deux p r o c e s s u s d et (3. 11s p e r m e t t e n t d e c e r n e r l a n a t u r e du volume l i b r e impliquk dans l a deformation a u s e u i l d'kcoulement.

A b s t r a c t

-

The l o s s c u r v e r e l a t e d t o Young's modulus of s a m p l e s of P o l y - carbonate c h a r a c t e r i z e d by different t h e r m a l h i s t o r i e s i s m e a s u r e d in the (3 t r a n s i t i o n r a n g e . P e a k s a r e found t o be insensitive t o t h e r m a l h i s t o r i e s a s was t h e c a s e f o r the s h e a r l o s s c u r v e . On s i m i l a r s a m p l e s , the c o m p r e s - sion yield behavior i s investigated a t constant s t r a i n r a t e over a wide range of t e m p e r a t u r e s . Assuming that two activated p r o c e s s e s d and (3 r e q u i r i n g additive s t r e s s e s 6 and 0- a r e involved a t yield, and using a t r e a t m e n t

C d C

CJ

previously proposed, a good c o r r e l a t i o n i s found between the (3 peak and r e l a t e d t o aged s a m p l e s . T h e s e r e s u l t s confirm that only the configura- .c @

tional entropy of the A p r o c e s s i s affected by aging while the hydrostatic p r e s s u r e influences the activation enthalpies of both d and

IJ

p r o c e s s e s . They allow to d i s c e r n the n a t u r e of the f r e e volume involved in yield d e f o r - mation.

INTRODUCTION.

T h e c o m p r e s s i o n yield behavior of Polycarbonate ( P C ) , a t constant s t r a i n r a t e , o v e r a wide range of t e m p e r a t u r e s , h a s been previously c o r r e l a t e d /1/ with the s h e a r l o s s peak exhibited by this g l a s s y polymer a t about -100°C and 1 Hz.

The proposed t r e a t m e n t r e l i e s on a p r e s s u r e dependant yield c r i t e r i o n and the a s - sumption that two additive a c t i v a t e d yield p r o c e s s e s o( and (3 a r e involved in yield deformation, t h e l a t t e r one being r e l a t e d t o t h e

f3

relaxation. This relaxation h a s been revealed by the s h e a r l o s s c u r v e (i. e. t a n d G v e r s u s t e m p e r a t u r e ) simply

because such m e a s u r e m e n t s a r e a c c u r a t e and e a s y t o p e r f o r m , admitti.ng i m p l i c i t - ly that Young's modulus.E, f o r example, will give t h e s a m e dissipation p r o c e s s r e l a t e d t o the s a m e s p e c i f i c m o l e c u l a r motion.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983923

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C9-186 JOURNAL DE PHYSIQUE

We intend h e r e t o check e x p e r i m e n t a l l y t h i s a s s u m p t i o n on PC s a m p l e s differing by t h e i r t h e r m a l h i s t o r y . The f? s h e a r peak of such s a m p l e s h a s b e e n found i n s e n - s i t i v e t o t h e r m a l t r e a t m e n t s / 2 - 5 / ; s i m i l a r d a t a r e l a t e d t o Young's modulus a r e lacking i n the l i t t e r a t u r e .

Knowledge of both t y p e s of l o s s peaks allows a complete d e s c r i p t i o n of t h e e l a s t i c p r o p e r t i e s of t h e m a t e r i a l .

In p a r a l l e l , t h e c o m p r e s s i o n yield behavior of annealed (below T ) a n d " a s r e c e i - ved" s a m p l e s of P C , i s investigated a t constant s t r a i n r a t e fro$ -100°C t o 8 0 ° C . T h e d a t a a r e c o m p a r e d with t e n s i l e yield s t r e s s e s conducted at t h e s a m e s t r a i n r a t e and t e m p e r a t u r e conditions, r e p o r t e d previously /5/.

The p r e s e n t r e s u l t s a r e in a g r e e m e n t with o u r p r e v i o u s s t a t e m e n t s :

- aging only influences the configurational entropy a s s o c i a t e d with m o l e c u l a r m o - tion a t yield; t h r o u g h t h i s effect, the 6 contribution t o t h e yield s t r e s s i s r a i s e d a l l o v e r the r a n g e e x p l o r e d , i n c o m p r e s s i o n a s well a s i n t e n s i l e t e s t s ;

-

on t h e c o n t r a r y , i n both t e s t s , t h e (3 contribution t o t h e yield s t r e s s i s unaffec- t e d by t h e r m a l t r e a t m e n t s ; t h e s a m e holds f o r the l o s s peaks r e l a t e d t o G o r E.

EXPERIMENTA L.

M a t e r i a l .

- - - - -

Makrolon ( B a y e r ) i s u s e d throughout. It h a s a v i s c o s i t y a v e r a g e m o l e c u l a r weight of 26 000 which i s not affected by annealing t r e a t m e n t s . T h e i n i t i a l u n t r e a t e d s t a t e i s t h e " a s r e c e i v e d " one, i t i s called : o r i g i n a l .

S p e c i m e n s of o r i g i n a l P C a r e c o n v e r t e d into t h e annealed f o r m s by heating a t 13OoC, 2 h o u r s a n d 400 h o u r s r e s p e c t i v e l y and a i r cooling. The annealing conditions below the g l a s s t r a n s i t i o n (Tg" 1 4 5 ° C ) a r e chosen t o give a n a p p r e c i a b l e i n - c r e a s e of t h e c o m p r e s s i o n yield s t r e s s . The t r e a t m e n t w a s p e r f o r m e d inside a d r y oven.

M e a s u r e m e n t s .

The damping t e s t s w e r e c a r r i e d out using a f r e e o s c i l l a t i o n f l e x u r a l pendulum a t f r e q u e n c i e s of about 1 Hz f r o m -145 t o 22°C. M e a s u r e m e n t s w e r e m a d e a t nomi- nally 5 ° C i n t e r v a l s a n d 10 m i n u t e s w e r e allowed a t e a c h t e m p e r a t u r e b e f o r e r e c o r - ding. The f l e x u r a l t e s t - p i e c e 8 have the following d i m e n s i o n s : 8.2 c m x 0.202 c m x 0 . 8 9 3 c m ,

The equipment u s e d in c o m p r e s s i o n t e s t s i n o r d e r t o d e t e r m i n e t h e yield s t r e s s i s d e s c r i b e d e l s e w h e r e /6/ a s well a s the d i m e n s i o n s of the s p e c i m e n s used. T e s t s

-4 s - l

w e r e p e r f o r m e d a t a s t r a i n r a t e e q u a l t o 4. 16 10 f r o m -100 t o +80°C.

E n g i n e e r i n g s t r e s s e s a r e c o n s i d e r e d throughout.

We avoided t h e r a n g e of t e m p e r a t u r e s w h e r e a n involontary t h e r m a l t r e a t m e n t t a - k e s p l a c e during t h e t e s t ; t h i s i s t h e r e a s o n why 80°C i s t h e highest t e m p e r a t u r e e x p l o r e d /2/.

RESULTS

.

Damping t e s t s . - - -

In f i g u r e 1, the t a n

6

l o s s c u r v e i s given f o r two P C s a m p l e s : o r i g i n a l and a n - nealed 400 h a t 130

"?

p r i o r t e s t i n g . No a p p r e c i a b l e d i f f e r e n c e c a n be d i s c e r n e d i n the position, the s h a p e and t h e m a x i m u m value of the peak, s o that we c o n s i d e r t h a t i t is not affected by t h e r m a l h i s t o r y .

The t a n A G c u r v e r e l a t e d t o a n o r i g i n a l s a m p l e /5/ i s given on the s a m e f i g u r e , c o m p a r e d t o t a n

S E

t h e m a x i m u m value i s slightly h i g h e r (by about l o % ) , but i s located (within e x p e r i m e n t a l e r r o r s ) a t t h e s a m e t e m p e r a t u r e . A l l t h e peaks have

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-

t - -

a5

- -

F i g . 1 - The Young's modulus l o s s c u r v e a t about 1 Hz for two s a m p l e s of P C : o r i g i n a l and annealed. The s h e a r l o s s c u r v e of the o r i g i n a l f o r m i s given f o r c o m p a r i s o n (dashed l i n e ) .

v e r y s i m i l a r s h a p e s .

Yee and T a k e m o r i /7/ p r o p o s e d t o d e s c r i b e t h e d e f o r m a t i o n of p o l y m e r s in t e r m s of complex P o i s s o n ' s r a t i o 3 , a s well a s complex moduli. It follows t h a t :

- J * =

v 1

^{- i ; I}

( 1

^{I t}

The a p p r o x i m a t i o n t h a t t a n bE 5 t a n

6

l e a d s t o a d m i t t h a t IIequals z e r o . But l e t u s c o n s i d e r t h a t , although s m a l l , t h e difference i n the m a x i m u m v a l u e s of t h e -

-

s e l o s s t a n g e n t s m u s t be t a k e n i n t o account. It m a y be e a s i l y e s t a b l i s h e d t h a t if t a n d G

>

^{t a n}

⁵

^{E ,} a s i n our m e a s u r e m e n t s , i t follows that t a n 6,,

>

0. T h i s i s not i n a g r e e m e n t with Y e e l s investigations on P C /4/, w h e r e t a n

6 . , ,

i s found t o be nagative in the (3 t r a n s i t i o n region. T h e s e r e s u l t s w e r e obtained f r o m d i r e c t m e a -

s u r e m e n t i n o s c i l l a t i n g t e n s i o n on a t e n s i l e m a c h i n e ; s m a l l v a r i a t i o n s a s a function of t h e r m a l h i s t o r y m a y be d i s c e r n e d on the l o s s c u r v e

.

The complex Young's m o - dulus E* w a s a l s o m e a s u r e d but not r e p o r t e d . F r o m t h e d a t a r e l a t e d t o E and

.I* ,

t h e l o s s tangent of t h e bulk modulus, t a n

6

and t a n

6 G

w e r e c a l c u l a t e d K '

a n d plotted a s a function of t e m p e r a t u r e . The s h e a r modulus i s found t o be i n s e n - s i t i v e t o t h e r m a l h i s t o r y , while t h e bulk modulus i s s t r o n g l y affected. The m a x i - m u m value of t h e t a n

d

of quenched P C i s shifted f r o m - 100°C t o - 5 0 ° C when a slow cooled s p e c i m e n i s c o n s i d e r e d (slow cooling p r o d u c e s s i m i l a r e f f e c t s t o a n - K nealing below T ). L e t u s point out t h a t i n the p r o c e d u r e followed t o obtain t a n 6K,

g

s m a l l d i f f e r e n c e s i n t a n 6

,,

(which l i e s r a t h e r n e a r z e r o a l l o v e r the r a n g e explo- r e d ) r e f l e c t v e r y high v a r i a t i o n s i n t a n 6 K (about 7 t i m e s ) . D i r e c t m e a s u r e m e n t s of t a n S K would c l e a r t h i s p r o b l e m but they a r e v e r y difficult t o p e r f o r m .

Uniaxial c o m p r e s s i o n t e s t s .

. . . -

The a range.

In f i g u r e 2, we have plotted the c o m p r e s s i o n yield s t r e s s

rc

v e r s u s t e m p e r a t u r e -

a t constant s t r a i n r a t e , f o r two P C : o r i g i n a l and annealed 400 h a t 130°C. A s i m i - l a r plot r e l a t e d t o s a m p l e s annealed 2 h a t 130°C i s given i n f i g u r e 3. T e n s i l e yield s t r e s s c u r v e s ( B ) c o r r e s p o n d i n g t o the o r i g i n a l P C and t o the one annealed

t

2 h a t 1 3 0 ° C /5/ a r e r e c a l l e d on f i g u r e s 2 and 3 r e s p e c t i v e l y . E a c h of t h e s e c u r - v e s a d m i t s two a s y m p t o t i c d i r e c t i o n s i n t e r s e c t i n g a t the s a m e t e m p e r a t u r e

To = -70°C. T h i s t e m p e r a t u r e i s chosen t o c h a r a c t e r i z e t h e obvious b r e a k on t h e c u r v e s r e f l e c t i n g , i n our opinion, t h e (3 d i s p e r s i o n .

Throughout t h e c o m p r e s s i o n data i t i s a l s o p o s s i b l e t o d r a w f o r e a c h of t h e t h r e e c o n s i d e r e d P C , two a s y m p t o t i c d i r e c t i o n s i n t e r s e c t i n g a t Tp ( f i g u r e s 2 a n d 3).

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PHYSIQUE

F i g u r e 2.

-

P l o t of the c o m p r e s s i o n yield s t r e s s a t constant s t r a i n r a t e v e r s u s tem- p e r a t u r e , f o r o r i g i n a l and annealed PC. The t e n s i l e yield s t r e s s c u r v e /5/ of the o r i g i n a l P C a t t h e s a m e s t r a i n r a t e i s given f o r c o m p a r i s o n .

F i g u r e 3.

-

S a m e plot a s i n f i g u r e 2 but r e l a t e d . t o P C s a m p l e s annealed in other conditions.

In the t e m p e r a t u r e r a n g e f r o m -70°C t o 8 0 ° C (i. e . t h e d range), it m a y be c o n s i - d e r e d that only t h e d p r o c e s s r e q u i r e s a s t r e s s contribution a t yield :

rd

which can be e x p r e s s e d by the exponential f o r m of the E y r i n g equation :

ad

= A 4 ( T l n 2 C d E + Q ~ / R ) ( 2 )

R denoting t h e u n i v e r s a l g a s constant, T the t e m p e r a t u r e and E t h e s t r a i n r a t e ; t h e p a r a m e t e r s Ao(, _{Cd and}Qd having constant values which have been developed by one of u s a s a function of the s e g m e n t a l motion leading t o yield deformation /8/.

It h a s been shown previously t h a t :

-

t h e r m a l h i s t o r y a f f e c t s only the p a r a m e t e r C A , i n t h e e x p r e s s i o n of the t e n s i l e yield s t r e s s 0- t d r e l a t e d to t h e d range /5/;

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e _ -.

It follows that t h e valueLgf

r6_

e x t r a p o l a t e d t o z e r o Kelvin does not depend on t h e r m a l h i s t o r y . A s i m p l e $&ce a t f i g u r e s 2 and 3 give a n e x p e r i m e n t a l proof:

the a s y m p t o t e of the c u r v e s r e l a t e d t o t h e a( r a n g e converge a t z e r o Kelvin w h e r e the e x t r a p o l a t e d value of 16

1

r e a c h e s Acd Qd /R= 19. 5 kg. m m - Z , independent

Cd of t h e r m a l h i s t o r y .

On t h e o t h e r hand, on t h e s a m e f i g u r e , i t can b e s e e n that, i n the & r a n g e , c o m - p r e s s i o n and t e n s i l e s t r e s s c u r v e s converge a t z e r o s t r e s s , provided they a r e r e l a t e d t o t h e s a m e P C , original o r aged samples.

Let u s point out t h a t , i n t h e d r a n g e , t h e s t r a i g h t line drawn on t h e g r a p h of f i - g u r e 3, giving

6c c4 v e r s u s T , i s t h e o r e t i c a l : the i n t e r c e p t a t z e r o s t r e s s i s g i - ven by the t e n s i l e yteld s t r e s s c u r v e ; the i n t e r c e p t a t z e r o Kelvin i s i m p o s e d by t h e c o m p r e s s i o n yield s t r e s s c u r v e s of both o t h e r kinds of P C . The excellent c o r - r e l a t i o n with the data c o n f i r m s the s t a t e m e n t s previously obtained. Let u s point out t h a t Cd i s r e l a t e d t o the configurational entropy of the yield p r o c e s s and Ad r e f l e c t s t h e activation enthalpy which h a s been found t o equal Q _d

-

R

5

/A

. -

T h e [3 range.

Below Tp

,

i t follows f r o m t h e proposed model that the m e a s u r e d yield s t r e s s in- c l u d e s two additive components :

6 = r

+

⁶

P

⁽³⁾

-

0 : original

0 : annealed 400h at 130°C-

A : annealed 2 h at 130°C

F i g u r e 4.

-

A plot of the (3 component of the c o m p r e s s i o n yield s t r e s s v e r s u s t e m - p e r a t u r e a t constant s t r a i n r a t e ; f o r the t h r e e c o n s i d e r e d f o r m s of PC. The d a s - hed line i s obtained by integrating the tan

b E

peak of f i g u r e 1 (original) using a p r e v i o u s t r e a t m e n t / I

/.

The a s y m p t o t i c value of is e x p r e s s e d i n t h e s a m e m a n n e r a s

rd

by :

6 6 = A (, ( T ~ ~ z c ~ L + Q ~ / R ) (4)

Only the value of Al, differs when c o m p r e s s i o n yield s t r e s s instead of tensile yield s t r e s s i s c o n s i d e r e d ( p e r f o r m e d a t the s a m e conditions of t e m p e r a t u r e and s t r a i n r a t e ) . The g r a p h of f i g . 4 gives

IF

J a s a function of t e m p e r a t u r e f o r the t h r e e PC. The data w e r e obtained by s u b t r a c t i n g f r o m the m e a s u r e d yield s t r e s s the e x - cc!

t r a p o l a t e d value of 10-

1.

C d

Below Tp, a s i n g l e a s y m p t o t e m a y be drawn throughout the data r e l a t e d t o t h e t h r e e P C . In the vicinity of TR , t h e s e data deviate f r o m s t r a i g h t l i n e s and a r e found t o a g r e e with the c u r v e c a l c u l a t e d f r o m the Young's modulus l o s s peak of the o r i g i n a l s a m p l e , using the p r o c e d u r e d e s c r i b e d previously /1/. It c a n t h e r e f o r e be a d m i t t e d that aging h a s no influence on t h e following p a r a m e t e r s : Ace , A t p /5/, C and Q a s on the

P

p

l o s s peak.

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JOURNAL DE PHYSIQUE

CONCLUSIONS.

R e s u l t s obtained h e r e tend t o show that t h e r m a l h i s t o r y h a s no influence on t h e r e l a x a t i o n s of P C , n o r on t h e (3 contribution t o the yield s t r e s s . A s a consequen- c e t h e t e m p e r a t u r e T

P .

above which t h i s contribution m a y be n e g l e c t e d i s found t o r e m a i n t h e s a m e f o r o r i g i n a l o r aged s a m p l e s t e s t e d in t e n s i l e o r c o m p r e s s i o n e x p e r i m e n t s .

The c o m p r e s s i o n yield s t r e s s however, i s affected by t h e r m a l h i s t o r y , even below T g

,

through t h e o( contribution.

It is c o n f i r m e d t h a t only one p a r a m e t e r i m p l i e d i n our t r e a t m e n t r e f l e c t s a p p r e c i a - bly t h e effect of aging, namely Cd r e l a t e d t o t h e configurational entropy of t h e d p r o c e s s . The h y d r o s t a t i c component of the s t r e s s a f f e c t s two p a r a m e t e r s Ad a n d A r e s p e c t i v e l y r e l a t e d t o t h e a c t i v a t i o n e n t h a l p i e s of t h e c( and (3 p r o c e s s e s . A s t h e r m a l t r e a t m e n t s and h y d r o s t a t i c p r e s s u r e a r e known t o r e d u c e the s p e c i f i c volume of t h e p o l y m e r , t h e p r e s e n t r e s u l t s c o n f i r m t h a t two kinds of f r e e volume ought t o be distinguished: t h e f i r s t one depending on configurational entropy, t h e s e c o n d one linked t o c o m p r e s s i b i l i t y .

REFERENCES.

1. BAUWENS J - C . , J . M a t e r . S c i .

2

(1972) 577.

2. A L L E N G . , MORLEY D. C. W. a n d WILLIAMS T . , J . Mater. Sci.

8

(1973) 1449.

3. STRUIK L. C. E . , "Physical aging in amorphous p o l y m e r s and other m a t e r i a l s "

E l s e v i e r s , A m s t e r d a m , (1978) p. 22 and 23.

4. YEE A. F. , "Fifth i n t e r n a t i o n a l c o n f e r e n c e on d e f o r m a t i o n , yield and f r a c t u r e of p o l y m e r s " C a m b r i d g e (1982) p. 32-7, quoted with t h e p e r m i s s i o n of the a u - t h o r .

5. BAUWENS-CROWET C . a n d BAUWENS J - C . , P o l y m e r . (1983) in p r e s s . 6. BAUWENS-CROWET C . , BAUWENS J - C . a n d HOMES G. ; J . M a t e r . Sci.

1

(1972) 176.

7. YEE A . F . a n d TAKEMORI M. T . , J . P o l y m e r S c i . Pol. P h y s . Ed.

20

(1982) 205.

8 . BAUWENS J - C . , P o l y m e r

21

(1980) 699.