Polymer Characterization - American Chemical Society

5 The PL-Dynamic Mechanical Thermal Analyzer and Its Application to the Study of Polymer Transitions

Downloaded by CORNELL UNIV on May 15, 2017 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch005

R. E. W E T T O N Loughborough University, Department of Chemistry, Loughborough, United Kingdom

T . G . C R O U C H E R and J. W. M . F U R S D O N Polymer Laboratories, Ltd., Church Stretton, Salop, United Kingdom

A new instrument measurements

for small strain dynamic

of polymers

ature can be programmed Special spaced

Head

is reported. from

arrangement).

drive frequencies

selected on the front

-150

to 300

Eight

from

panel.

0.33

Applications

to 90

Clamping

using

thermal scans and computer quencies polymers, and

copolymers,

and

both

°C

Hz

can

be

geometrics

for

deformation

are

single-frequency

multiplexing

are given. Examples

temper

°C (500

logarithmically

bending beam, shear, and elongational described.

mechanical

Sample

of several fre

of application composites

are

to

homo-

presented

discussed.

^MEASUREMENTS

of

polymeric

materials, w h i c h were previously laborious, can n o w be

OF DYNAMIC

MECHANICAL

PROPERTIES

performed

automatically w i t h microprocessor-controlled

instrumentation. T h e

P L - d y n a m i c m e c h a n i c a l t h e r m a l analyzer ( P L - D M T A ) uses the w e l l established phase a n d amplitude technique w i t h digital measurement o f p h a s e to g i v e t a n δ r e s o l u t i o n s o f 0 . 0 0 0 1 . M e a s u r e m e n t s c a n

be

p e r f o r m e d at f i x e d f r e q u e n c y o v e r a t h r e e a n d o n e - h a l f d e c a d e r a n g e . A d y n a m i c stiffness range of o v e r four d e c a d e s a l l o w s m e a s u r e m e n t o f all transitions w i t h one sample. T h i s , together w i t h precise tempera ture p r o g r a m m i n g , i n the range f r o m - 1 5 0 to 300 °C, o p e n s u p n e w p o s s i b i l i t i e s for r e s e a r c h , a n a l y t i c a l , a n d q u a l i t y c o n t r o l 0065-2393/83/0203-0095$06.00/0 © 1983 A m e r i c a n C h e m i c a l Society

Craver; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

measure-


96

POLYMER CHARACTERIZATION

Figure 1. The PL-dynamic mechanical thermal analyzer. Key: the mechanical measuring head, left; the dynamic analyzer unit, top right; and the temperature programmer, bottom right. m e r i t s . T h e i n s t r u m e n t , as w e l l as m e a s u r e m e n t s o n a r a n g e o f p o l y m e r systems, is d e s c r i b e d . Instrumentation T h e P L - D M T A c o m p r i s e s t h r e e b e n c h t o p u n i t s as s h o w n i n F i g u r e 1. T h e m e c h a n i c a l h e a d c o n t a i n s t h e s a m p l e i n a t e m p e r a t u r e e n c l o s u r e . T h e m a i n d e f o r m a t i o n m o d e s are b e n d i n g b e a m (either d o u b l e or single cantilever) a n d shear. T h e s a m p l e c l a m p i n g details i n these t w o a r r a n g e m e n t s are s h o w n i n F i g u r e s 2 a a n d b . T h e s a m p l e d i s k s for s h e a r s a n d w i c h o p e r a t i o n are m o u n t e d n o r m a l l y o n t h e f r a m e e x t e r n a l to t h e i n s t r u m e n t , a n d t h e c o m p l e t e a s s e m b l y is t h e n l o c a t e d o n the i n s t r u m e n t s . T h i s m e t h o d o f m o u n t i n g is p a r t i c u l a r l y u s e f u l w h e n m e a s u r i n g d i f f i c u l t s a m p l e s , s u c h as a d h e s i v e s . R e s u l t s q u o t e d i n this chapter were taken i n the b e n d i n g b e a m m o d e (typical sample s i z e 10 x 5 x 2 m m ) u n l e s s o t h e r w i s e s t a t e d . A n a l t e r n a t i v e h e a d a s s e m b l y for t h e t e n s i l e d e f o r m a t i o n o f f i l m s a n d f i b e r s at c o n s t a n t t e n s i o n is c u r r e n t l y b e i n g m a d e a v a i l a b l e . T h e o s c i l l a t i n g stress e x p e r i e n c e d b y t h e s p e c i m e n i s p r o p o r t i o n a l to t h e d r i v e c u r r e n t f r o m t h e o s c i l l a t o r . T h e s t r a i n p r o d u c e d i s c o n v e r t e d to a p r o p o r t i o n a l v o l t a g e b y a n o n l o a d i n g t r a n s d u c e r . T h e d y n a m i c a n a l y z e r u n i t c o m p a r e s t h e p h a s e a n d a m p l i t u d e o f stress a n d s t r a i n a n d c o m p u t e s a b s o l u t e v a l u e s o f l o g m o d u l u s a n d t a n δ. T h e i n s t r u m e n t is e a s i l y c a l i b r a t e d b y an a d d e d mass t e c h n i q u e that t h e n m a k e s the i n s t r u m e n t absolute w h e n the g e o m e t r y constant for the s a m p l e u n d e r m e a s u r e m e n t is d i a l e d i n o n the front p a n e l . C a l i b r a t i o n needs checking only periodically.


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F r e q u e n c y a n d strain a m p l i t u d e are s e l e c t e d b y p u s h b u t t o n w i t h e i g h t l o g a r i t h m i c a l l y s p a c e d - f r e q u e n c i e s f r o m 0 . 0 3 to 9 0 H z a n d t h r e e c o n t r o l l e d strain l e v e l s c o v e r i n g a X 1 6 range. T e m p e r a t u r e is s e n s e d b y a p l a t i n u m resistance thermometer l y i n g i m m e d i a t e l y b e h i n d the s a m p l e . A t e m p e r a t u r e p r o g r a m m e r c o n t r o l s t h e b a l a n c e d h e a t o v e n to p r o d u c e u n i f o r m h e a t i n g rates f r o m 1 7 1 0 m i n (or less) to 2 0 7 m i n ( n o m i n a l ) . T h e t e m p e r a t u r e m a y b e c o n t r o l l e d i s o t h e r m a l l y at a n y point, or series of points, w i t h external c o m p u t e r control. Accuracy depends on the optimization of a n u m b e r of parameters. T e m p e r a t u r e u n i f o r m i t y i n t h i c k s p e c i m e n s c a n o n l y b e a c h i e v e d at a h e a t i n g rate o f a f e w d e g r e e s p e r m i n u t e . I n t h i n s p e c i m e n s t e m p e r a ture d e f i n i t i o n is easier, b u t errors from d i m e n s i o n a l m e a s u r e m e n t s m a y b e c o m e significant i n m o d u l u s d e t e r m i n a t i o n s . T a n δ is r e s o l v e d t o 0 . 0 0 0 1 for v a l u e s l e s s t h a n 1.0 a n d , b e c a u s e i t i s d e t e r m i n e d as kE"/kE', e r r o r s i n t h e g e o m e t r y t e r m (k) d o n o t p r o d u c e e r r o r s i n t h i s term. T h e h i g h resolution n o w achieved produces sufficiently fine steps i n o u t p u t vs. t e m p e r a t u r e s u c h that e s s e n t i a l l y s m o o t h c u r v e s are p r o d u c e d f r o m t r a c e s o n a n o r m a l t w o - p e n r e c o r d e r . E x t e r n a l c o m p u t e r control of the m a i n functions / , T, strain, a n d h e a t i n g rate i s a v a i l a b l e t h r o u g h t h e I E E E ( I n s t i t u t e o f E l e c t r i c a l E n g i n e e r s S t a n d a r d 488) i n t e r f a c e , a n d i n t h i s m o d e , r e s u l t s are d i s p l a y e d b o t h i n r e a l t i m e a n d s t o r e d for future m a n i p u l a t i o n . A n e x a m p l e o f t h e p o w e r o f e x t e r n a l c o m p u t e r c o n t r o l is s h o w n i n F i g u r e 3 i n w h i c h t h r e e f r e q u e n c i e s a r e m u l t i p l e x e d d u r i n g a s i n g l e t h e r m a l s c a n at 2 ° C / m i n . T h e s a m p l e is a n e x t r e m e l y h i g h m o d u l u s c a r b o n - f i l l e d e p o x y . b

Figure 2. (a) Sample clamping arrangement with a dual cantilever. Drive clamp vibrates perpendicular to sample, which can be mounted on one side only to accommodate expansion or contraction in length. Clamp frames are interchangeable to give variable length; and (b) shear sandwich geometry for rubbers and gels. The black discs repre sent the two samples, and the central plate is oscillated by the drive clamp.



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Figure 3. Control by an external computer allows collection of data at a number of frequencies during a single thermal scan. Data collected for a carbon filled epoxy through the main T region.

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-1

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Tan S


5.

WETTON ET A L .

PL-Dynamic

Mechanical

Thermal

Analyzer

99

O n e advantage of t h i s m e t h o d is that a n y t e m p e r a t u r e errors i n the s a m p l e are i d e n t i c a l for e a c h f r e q u e n c y .


Results

and

Discussion

D y n a m i c m e c h a n i c a l t h e r m a l analysis ( D M T A ) senses any change i n m o l e c u l a r m o b i l i t y i n t h e s a m p l e as t h e t e m p e r a t u r e is r a i s e d o r l o w e r e d . T h e t i m e s c a l e r e q u i r e d for t h e m o l e c u l a r m o t i o n to m a n i f e s t i t s e l f is d e t e r m i n e d b y t h e f r e q u e n c y , / , o f t h e i m p r e s s e d s i n u s o i d a l stress. A p r o g r e s s i v e c h a n g e i n s t o r a g e m o d u l u s a n d p e a k i n g i n tan δ (energy lost/energy stored per cycle) occurs w h e n the average m o l e c u l a r r e l a x a t i o n t i m e , r , i s V2 π/. D M T A i s n o t d e p e n d e n t o n t e m p e r a t u r e s c a n n i n g rate i n t h e s a m e w a y t h a t D S C - t y p e m e a s u r e m e n t s a r e . T h i s c a n b e d e m o n s t r a t e d m o r e a c u t e l y i n t h e a b i l i t y to o b t a i n i s o t h e r m a l data b y s c a n n i n g t h r o u g h the m o t i o n a l t i m e scale b y c h a n g i n g the frequency. T h i s information yields t h e n the relaxation t i m e s p e c t r u m (I, 2). M o r e than one transition w i l l n o r m a l l y be encountered i n the temperature plane. Figure 4 shows an example using a prequenched p o l y ( e t h y l e n e t e r e p h t h a l a t e ) . T h e m a i n t r a n s i t i o n (a ) at 8 5 ° C , 1 H z i s d u e to t h e o n s e t o f m i c r o - B r o w n i a n m o t i o n o f t h e m a i n c h a i n s a b o u t T . a

g

-100

- 50

0

50

100

150

200

250

Temperature (°C) Figure 4. PL-DMTA (1 Hz) scan for a quenched poly(ethylene tere phthalate) sample heated 5 °C/min. The scan shows a and β relaxation, crystallization, and melting.


100



T h i s t r a n s i t i o n occurs i n the a m o r p h o u s phase, a n d has a large m a g nitude because of the h i g h v o l u m e fraction of amorphous phase pres e n t i n t h e q u e n c h e d s a m p l e . T h e t a n δ m a g n i t u d e , 0.6, a n d h a l f - w i d t h ( i g n o r i n g r e c r y s t a l l i z a t i o n o n s e t ) at 13 ° C , c o m p a r e to q u e n c h e d p o l y s t y r e n e v a l u e s o f 1.32 a n d 13 ° C at t h e s a m e s c a n n i n g r a t e . T h e r e d u c t i o n i n loss peak m a g n i t u d e w i t h o u t peak b r o a d e n i n g shows some crystallinity i n the q u e n c h e d sample. Quantitative information o f t h i s t y p e h a s n o t b e e n a v a i l a b l e p r e v i o u s l y as a l l e x p e r i m e n t a l parameters must be controlled. A t l o w t e m p e r a t u r e s t h e b r o a d β r e l a x a t i o n p r o c e s s i s o b s e r v e d at —60 ° C / H z , a n d i s d u e t o l o c a l i z e d m o l e c u l a r m o t i o n ( l o c a l m o d e ) o f t h e m e t h y l e n e a n d c a r b o x y l g r o u p s . I t c o m p r i s e s at l e a s t t w o o v e r l a p p i n g r e l a x a t i o n s (3) a n d is r e l a t i v e l y i n s e n s i t i v e to c r y s t a l l i n i t y . W h e n m a i n c h a i n relaxations are i n v o l v e d i n the β process, g o o d i m p a c t strength normally results. A s the temperature is r a i s e d above T , the m o l e c u l a r freedom a c q u i r e d a l l o w s r a p i d c r y s t a l l i z a t i o n at t h i s h i g h d e g r e e o f s u p e r c o o l i n g . T h e m o d u l u s t h u s r i s e s as c r y s t a l l i z a t i o n p r o c e e d s i n t h e r a n g e 1 1 0 — 1 5 0 ° C . F i n a l l y , at h i g h t e m p e r a t u r e s c a t a s t r o p h i c m e l t i n g o c c u r s at 2 0 0 ° C , a n d c l a m p i n g i s l o s t . I n p r i n c i p l e , t a n δ s h o u l d go t h r o u g h a s t e p at t h e m e l t i n g p o i n t a n d i n c r e a s e w i t h o u t l i m i t as t h e t e m p e r a t u r e is r a i s e d f u r t h e r . g

T h e r m a l s c a n n i n g at r e a s o n a b l y h i g h rates (5 ° G / m i n ) y i e l d s i n formation that w o u l d not be o b t a i n e d i n very s l o w s c a n n i n g or isothermal experiments. In isothermal experiments annealing pro c e s s e s m a y o c c u r a n d t h e s a m p l e w o u l d c r y s t a l l i z e at a s i g n i f i c a n t l y lower temperature. D a t a t h r o u g h the m e l t i n g r e g i o n are d i f f i c u l t to o b t a i n w i t h r e l a t i v e l y l o w m o l e c u l a r w e i g h t p o l y m e r s that have h i g h m e l t i n g points, s u c h as p o l y e s t e r s a n d n y l o n s . W h e n s u c h m a t e r i a l s m e l t t h e p r o c e s s is c a t a s t r o p h i c b e c a u s e t h e e q u i l i b r i u m l i q u i d h a s a l o w v i s c o s i t y . T h u s , c l a m p i n g i n t e g r i t y i s l o s t as s o o n as t h e c r y s t a l l i n e i n t e r a c t i o n s a r e l o s t . I t i s , h o w e v e r , r e l a t i v e l y e a s y to o b t a i n d a t a t h r o u g h t h e m e l t i n g range of a h i g h m o l e c u l a r w e i g h t , l o w m e l t i n g p o l y m e r , such as p o l y e t h y l e n e o x i d e . D a t a i n F i g u r e s 5 a n d 6 a r e f o r p o l y e t h y l e n e oxide of a m o l e c u l a r w e i g h t of approximately one m i l l i o n (Polyox grade). A r e l a t i v e l y short, t h i c k s a m p l e w i t h aspect ratio (length/ t h i c k n e s s ) o f t h r e e w a s u s e d to o b t a i n r e a s o n a b l e s a m p l e s t i f f n e s s o n the h i g h temperature side of the m e l t i n g region. Some sacrifice of m o d u l u s a c c u r a c y d u e to u n k n o w n e n d c o r r e c t i o n s is m a d e , p a r t i c u l a r l y w h e r e m o d u l u s l e v e l s are h i g h . F i g u r e 5 s h o w s results for a s a m p l e c o o l e d q u i c k l y f r o m t h e m e l t to r o o m t e m p e r a t u r e , w h e r e it w a s h e l d for 2 h b e f o r e s c a n n i n g at 4 ° C / m i n a n d 1 H z . T w o p e a k s are e x h i b i t e d i n tan δ i n sympathy w i t h the decreases i n m o d u l u s . T h e



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h i g h e r p e a k (a ) c o i n c i d e s w i t h t h e o b s e r v e d m e l t i n g p o i n t , b u t t h e l o w e r t e m p e r a t u r e p e a k (a ') d o e s n o t a g r e e w i t h a n y m a j o r t h e r m a l event seen by D S C - t y p e techniques. Different thermal treatment a l t e r s t h e p o s i t i o n a n d m a g n i t u d e o f t h e a ' i n F i g u r e 6. T h e s a m p l e w a s c r y s t a l l i z e d at 6 0 ° C f o r 5 h t o a l l o w c o m p l e t e p r i m a r y c r y s t a l l i z a t i o n , a n d t h e a ' p r o c e s s o c c u r s at a h i g h e r t e m p e r a t u r e . T h e a process a p p a r e n t l y d e p e n d s o n the p e r f e c t i o n , or size of l a m e l l a e , a n d there is a strong parallel w i t h similar relaxations i n different types of polyethy l e n e (4). c

c

c

c

r

c

A n inherent consequence of the d y n a m i c m e c h a n i c a l method's r e s p o n s e to m o t i o n a l t i m e s c a l e i s t h a t m e a s u r e m e n t at h i g h e r f r e q u e n c i e s w i l l c a u s e t h e l o s s p e a k to b e o b s e r v e d at h i g h e r t e m p e r a tures. I n a m o r p h o u s h o m o p o l y m e r s the loss peak shifts w i t h m i n o r shape changes i n the t e m p e r a t u r e p l a n e . T h e shift factor is g i v e n b y t h e s e m i e m p i r i c a l W i l l i a m s , L a n d e l , a n d F e r r y ( W L F ) e q u a t i o n (5), w h i c h a p p r o x i m a t e s t o a s i m p l e A r r h e n i u s r e l a t i o n at t e m p e r a t u r e s far



102


a b o v e T . H o w e v e r , m a n y c o m m e r c i a l p o l y m e r s are p a r t i a l l y c r y s t a l l i n e , a n d c o p o l y m e r s are i n c r e a s i n g l y c o m m o n . I n s u c h cases the loss c u r v e s w i l l u s u a l l y change shape r a d i c a l l y w i t h change of m e a s u r e m e n t freq u e n c y . F i g u r e 7 s h o w s t h i s for a n e t h y l e n e - v i n y l acetate c o p o l y m e r ( 2 4 % v i n y l acetate) w h e r e t h e s h a r p e n i n g o f t h e l o s s c u r v e w i t h i n c r e a s i n g t e m p e r a t u r e results f r o m t w o effects. F i r s t t h e r e is a s h a r p e n i n g of the r e l a x a t i o n t i m e s p e c t r u m that is i n h e r e n t l y b r o a d for r a n d o m c o p o l y m e r s , a n d s e c o n d t h e r e is a n i m p r o v e m e n t i n s e g m e n t c o m p a t i b i l i t y as t h e t e m p e r a t u r e i s r a i s e d , l e a d i n g t o a m o r e h o m o g e n e o u s m a t e r i a l o n t h e m o l e c u l a r s c a l e . T h e l o s s p e a k h a l f - w i d t h at 3 0 H z i s s t i l l 18 ° C , a n d i s m u c h b r o a d e r t h a n o b s e r v e d f o r h o m o p o l y m e r s i n the same t e m p e r a t u r e range. g

T h e P L - D M T A c a n b e u s e d as a n a n a l y t i c a l i n s t r u m e n t i n t h e s a m e s e n s e as I R i s u s e d to d e t e r m i n e u n k n o w n s . T h e t h e o r y i s l e s s far d e v e l o p e d , h o w e v e r , a n d t h e t e c h n i q u e i s i n s o m e c a s e s e m p i r i c a l .


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WETTON ET AL.

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Analyzer

103

T h e loss p e a k l o c a t i o n for a r a n d o m c o p o l y m e r m o v e s n o r m a l l y b e t w e e n t h e l o c a t i o n for the p a r e n t h o m o p o l y m e r s i n a s i m i l a r w a y to T itself. H o w e v e r , i n e t h y l e n e - v i n y l acetate c o p o l y m e r s c r y s t a l l i n i t y l e v e l changes of the p o l y e t h y l e n e sequences disturb the average a m o r p h o u s phase c o m p o s i t i o n , a n d , c o n s e q u e n t l y , the a loss p e a k p o s i t i o n c h a n g e s o n l y s l i g h t l y (see F i g u r e 8), b u t i t s m a g n i t u d e d e creases i n s y m p a t h y w i t h the a m o r p h o u s phase content. T h e peak h a l f - w i d t h i n t h e t e m p e r a t u r e p l a n e (30 H z ) i n c r e a s e s w i t h e t h y l e n e c o n t e n t u n t i l i t is t o o b r o a d to m e a s u r e s e n s i b l y at 7 . 5 % v i n y l a c e t a t e composition. As more and more ethylene sequences crystallize, w i t h increasing ethylene content, the amorphous phase becomes relatively r i c h e r i n v i n y l a c e t a t e c o m p o s i t i o n , a n d t h i s fact c a u s e s t h e l a c k o f shift i n p e a k p o s i t i o n . C l e a r l y , after p r o p e r t h e r m a l h i s t o r y c o n t r o l , a D M T A scan c a n p r o v i d e a g o o d i n d i c a t i o n of the c o m p o s i t i o n , or a l t e r n a t i v e l y , i f c o m p o s i t i o n is k n o w n , i n f o r m a t i o n is o b t a i n e d o n the nature of the amorphous phase a n d crystallite interaction/crosslinking. g


a

T h e e x t r e m e cases of h e t e r o g e n e i t y i n c o p o l y m e r s is p r o v i d e d b y b l o c k a n d graft c o p o l y m e r s p a r t i c u l a r l y as e x e m p l i f i e d b y t h e r m o plastic elastomers. I n these materials p h y s i c a l phase separation of g l a s s y o r c r y s t a l l i n e d o m a i n s i s n e c e s s a r y to p r o v i d e c r o s s - l i n k i n g . G o o d clear cut phase separation, w i t h e a c h phase e x h i b i t i n g a loss

9.0 h

οι ε

-60

-40

-20

0

20

40

Temperature (° C) Figure 7. Effect of frequency of measurement on loss peak position and shape for ethylene—vinyl acetate copolymer. Heating rate was 5 °C/min.


104



c

Temperature °C Figure 8. PL-DMTA scan (30 Hz) of ethylene-vinyl acetate copolymers with varying acetate contents. Crystallinity perturbs the amorphous phase composition. Key: —, 25% vinyl acetate; ,15% vinyl acetate; and —, 7.5% vinyl acetate.

p e a k c l o s e to its h o m o p o l y m e r p o s i t i o n a n d n o t s i g n i f i c a n t l y b r o a d e n e d , is the r e q u i r e m e n t . D M T A characterizes f u r t h e r the r u b b e r a n d g l a s s y state m o d u l u s l e v e l s . T h e s e f e a t u r e s a r e n o t a t t a i n a b l e b y any other single t e c h n i q u e . Phase studies of this type can be ex t e n d e d to p o l y u r e t h a n e s y s t e m s , w h i c h a r e e s s e n t i a l l y m u l t i - b l o c k copolymers with poor chemical and physical definition. Figure 9 s h o w s data for t w o t y p e s o f p o l y u r e t h a n e . S a m p l e A is a n o r m a l for m u l a t i o n b a s e d o n polyester, s o m e w h a t greater t h a n b i f u n c t i o n a l i n — O H g r o u p s w i t h b u t a n e d i o l / m e t h y l e n e d i p h e n y l i s o c y a n a t e as c h a i n extenders. P h a s e s e p a r a t i o n is c l e a r cut w i t h the p o l y e s t e r a l o s s p e a k at - 8 ° C a n d 1 H z , a n d s o m e s t r u c t u r a l d i s o r d e r i n g at 7 0 ° C . S t r u c t u r a l i n t e g r i t y o f t h e n e t w o r k p e r s i s t s u p to 1 8 0 ° C . I n c o n t r a s t , m i c r o p h a s e s e p a r a t i o n w i t h s o m e p h a s e m i x i n g is a c h i e v e d i n t e n t i o n a l l y i n S a m p l e Β to p r o d u c e a n o p t i c a l l y c l e a r p r o d u c t . T h e e x t e n t o f a


WETTON ET AL.

PL-Dynamic

Mechanical

Thermal Analyzer


5.


105

106


p h a s e m i x i n g is a s s e s s e d b y t h e D M T A t r a c e . A g a i n , c r o s s - l i n k i n g i n t e g r i t y is p r e s e r v e d u p to 1 8 0 ° C . A l t e r n a t i v e c l a m p i n g g e o m e t r y a l l o w s m e a s u r e m e n t o f soft s a m p l e s ( < 1 0 N / m ) i n s h e a r , b u t t h e m a i n i n t e r e s t i n t h e s e c a s e s i s for e n g i n e e r i n g d a t a o n r u b b e r s t h a t c a n b e g e n e r a t e d u p to a 1 0 % s t r a i n l e v e l . M e a s u r e m e n t s o f this t y p e are not r e l e v a n t to t h e m a i n t h e m e o f t r a n s i t i o n m e a s u r e m e n t s , b u t this m o d e o f d e f o r m a t i o n is u s e f u l for s t u d y i n g t r a n s i t i o n p h e n o m e n a i n a d h e s i v e s a n d for s t u d y i n g the e a r l y part of cure i n c r o s s - l i n k i n g resins. F i g u r e 10 s h o w s a n a l t e r n a t i v e g e o m e t r y t h a t a l l o w s t e n s i l e m o d u l u s m e a s u r e m e n t o f t h i n p o l y m e r f i l m s , t y p i c a l l y b e l o w 0.05 m m i n thickness. T h e geometry constant i n this m o d e is s h o w n i n the figure a n d i s m e r e l y t h e n o r m a l t e n s i l e v a l u e m o d i f i e d b y c o s 0 (the a n g l e b e t w e e n d r i v e d i r e c t i o n a n d film). T h e t e n s i o n is m o n i t o r e d b y the d i s p l a c e m e n t p r o d u c e d i n t h e d r i v e s p r i n g a n d i s s e e n d i r e c t l y as a p r o p o r t i o n a l c h a n g e i n t h e t r a n s d u c e r off-set v o l t a g e . A f t e r t e n s i o n a d j u s t m e n t s h a v e b e e n m a d e , t h e s a m p l e is c l a m p e d b y t h e c e n t r a l c l a m p w i t h n a r r o w c l a m p e d g e s a n d the t r a n s d u c e r is a d j u s t e d m a n u a l l y to its n o r m a l w o r k i n g r a n g e . G o o d d a t a c a n b e g e n e r a t e d o n f i l m s i n regions o f m i n o r changes i n t e n s i o n t h r o u g h r e l a x a t i o n effects, b u t i t is n o t p r a c t i c a l i f stress r e l a x a t i o n o c c u r s s t r o n g l y . T h e n e w t e n s i o n h e a d i s d e s i g n e d t o r e m e d y t h i s p r o b l e m . F i g u r e 11 s h o w s d a t a o n r u b b e r t o u g h e n e d p o l y p r o p y l e n e f i l m s m o u n t e d as s h o w n i n F i g u r e 10. T h e r e g i o n o f e t h y l e n e - p r o p y l e n e r u b b e r r e l a x a t i o n c a n b e s e e n clearly i n Samples A a n d B . T h e rubber segments have b e e n incorpo rated d i f f e r e n t l y , a n d this is r e f l e c t e d i n the shape of the loss process. T h e D M T A t e c h n i q u e is o n e o f t h e f e w w a y s o f s t u d y i n g f i n e d i f f e r 8

2


2

Drive

Drive spring

Film. sample

2bt (cos 0) 2

K - - J -

Figure 10. Arrangement for tensile dynamic modulus measurements in thin films. The angle θ is between film and drive directions, and b, t, and I are width, thickness, and length, respectively.


5.

WETTON ET A L .

PL-Dynamic

Mechanical

Thermal

Analyzer

107

.08

.01


-100

Quenched P.P. film -50 0 + 40 Temperature (°C)

Figure 11. Rubber-modified polypropylene films measured through the ethylene—propylene rubber loss region. Geometry used is shown in Figure 10.

e n c e s i n f i l m structure of t h i s t y p e . S a m p l e A has a w i d e r range of m o l e c u l a r heterogeneity associated w i t h the r u b b e r segments than Sample B. C o m p a r i s o n of m o d u l i o b t a i n e d from b e n d i n g of short f i l m sam ples w i t h those o b t a i n e d from tensile measurements provides a m e t h o d f o r l o o k i n g at s k i n e f f e c t s i n f i l m s . I n n e r a n d o u t e r l a y e r s are w e i g h t e d e q u a l l y i n the tensile case, b u t the outer layers contribute m o r e to t h e b e n d i n g m o d u l u s . H i g h accuracy m o d u l u s a n d d a m p i n g characteristics m e a s u r e d on the P L - D M T A have p r o v e d u s e f u l i n s t u d y i n g the a g i n g characteris t i c s o f p o l y m e r g l a s s e s . I n b o t h , c r o s s - l i n k e d e p o x y s y s t e m s (6) a n d u n c r o s s - l i n k e d p o l y s t y r e n e (7, 8) t h e b a s i c e f f e c t s s e e m t h e s a m e . C o m p a r e d to t h e c o r r e s p o n d i n g a n n e a l e d (aged) g l a s s a q u e n c h e d s a m p l e h a s a l o w e r g l a s s y state m o d u l u s , b u t s u r p r i s i n g l y a h i g h e r d a m p i n g peak, w h e n scanned i n the temperature plane. T h e m o d u l u s c h a n g e o v e r a p e r i o d o f 1 w e e k , a n n e a l e d at 2 0 ° C b e l o w T , i s i n e x c e s s o f 1 0 0 % a n d t h e d a m p i n g l e v e l i n t h e g l a s s , at 3 0 ° C b e l o w T , decreases b y a factor of five i n the case of a n n e a l e d p o l y s t y r e n e . T h e l o w t e m p e r a t u r e (β) l o s s p r o c e s s i n t h e e p o x y s y s t e m s s h o w s a s h a r p e n i n g w i t h a n n e a l i n g , b u t no significant shift i n c e n t r a l p o s i t i o n . g

g

Conclusions T h e P L - D M T A is a b l e to g i v e accurate data o n the w i d e s t p o s s i b l e r a n g e o f p o l y m e r i c m a t e r i a l s . T h e r m a l s c a n n i n g at c o n s t a n t f r e q u e n c y t y p i c a l l y c a n b e a c h i e v e d i n 1 h , c o m p a r a b l e to D S C t e c h n i q u e s . T h e t e c h n i q u e is c o m p l e m e n t a r y to D S C i n t h a t m o t i o n a l t r a n s i t i o n s are o b s e r v e d , a l t h o u g h t h e r m o d y n a m i c t r a n s i t i o n s a r e a l -


108


ways detected b y the structural changes they produce.

Secondary

transitions are not o b s e r v a b l e b y t h e D S C m e t h o d n o r c a n s m a l l r e i n forcing phases be studied. T h e P L - D M T A m e t h o d detects a l l these transitions a n d a l l o w s f o r m u l a t i o n o f définitive c o n c l u s i o n s sample morphology

a n d phase

concerning

composition.

External computer control allows the full capabilities of thei n s t r u m e n t to b e e x p l o i t e d a n d i t is s h o w n that several different

mea-

surement frequencies can be multiplexed during a single temperature scan. A s data are stored i n the c o m p u t e r a n y r e q u i r e d data m a n i p u l a -


tion can be achieved

subsequently.

Literature Cited 1. Ferry, J. D. In "Viscoelastic Properties of Polymers"; Wiley: New York, 1961; p. 63. 2. Schwarzl, F.; Stavermann, A. J. Appl. Sci. Res. 1953, A4, 127. 3. Illers, K. H.; Breuer, H. J. Colloid Sci. 1963, 18, 1. 4. Mills, P. J.; Hay, J. N.; Cox, C., First PL-DMTA Users Meeting, Univ. of Warwick, 1981. 5. Ferry, J. D. In "Viscoelastic Properties of Polymers"; Wiley: New York, 1961; p. 203. 6. Richardson, M. J., First PL-DMTA Users Meeting, Univ. of Warwick, 1981; p. 9. 7. Wetton, R. E.; Woo, Y. H. Polymer, in press. 8. Wetton, R. E . Chemical Soc., Analytical Proc. Sept/Oct 1981. RECEIVED for review October 30, 1981. ACCEPTED October 11, 1982.


Polymer Characterization - American Chemical Society

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