Physical Techniques for Profiling Heterogeneous Polymer Degradation

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Physical Techniques for Profiling Heterogeneous Polymer Degradation R. L . C L O U G H and K. T. G I L L E N Sandia National Laboratories, Albuquerque, N M 87185

Three general techniques for studying heterogeneous degradation in polymers are described, and an example of heterogeneous effects on degradation behavior is illustrated. Heterogeneous degradation frequently occurs in materials subjected to high dose rate irradiation in air and results in oxidation only near surfaces. Optical examination of cross-sectioned,metallographically-polished samples provides qualitative information on oxidation depth. Quantitative profiles of heterogeneous material property changes are provided by relative hardness measurements across cross-sectioned surfaces using e i ther of two experimental apparatuses. Quantitative profiles of oxidation are obtained using density gradient columns. Viton is found to become embrittled when irradiated at high dose rate but becomes soft when irradiated at low dose rate; this i s shown to result from differences in oxidative penetration depth at different dose rates. Degradation in polymeric materials frequently takes place in a heterogeneous manner, such that the nature of the degradation in the i n terior of a sample may be very different from that near the edges . Some examples where degradation can be strongly heterogeneous are: 1) photochemical aging, where light is absorbed near the surface, 2) thermal aging, where plasticizer might be volatilized from near the surface or 3) chemical aging, where some corrosive gas or liquid is diffusing into a material. However, for polymers aged in air, probably the most common cause of heterogeneous effects is oxygen diffusion-limited degradation. This mechanism is relevant in many different environments where oxidation may be the predominant degradation mechanism. Examples include high-energy radiation, UV light, elevated temperature and mechanical stress. A long-standing goal in polymer science has been the development of accelerated aging tests for predicting polymer degradation rates in long-term applications. Design of meaningful accelerated aging tests must begin with replication of degradation mechanisms and modes which occur in the environment to be simulated (5-8). Hetero0097-6156/85/0280-0411$06.00/0 © 1985 American Chemical Society Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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geneous oxidative degradation i s frequently an impediment to this goal, due to the fact that at the high stress levels c h a r a c t e r i s t i c of accelerated t e s t s , the oxidation rate may be s u f f i c i e n t l y high that oxygen diffusion becomes a dominant rate-determining step. Thus in studying polymer degradation and p a r t i c u l a r l y i n attempting a c c e l erated aging simulations, the a b i l i t y to identify and characterize heterogeneous effects i s of fundamental importance. In this report we describe three techniques which we have developed and used for investigating heterogeneous oxidative degradation i n i r r a d i a t e d polymers. We believe these techniques should be widely applicable to heterogeneous degradation studies involving a variety of environmental c o n d i t i o n s . We also present an i l l u s t r a t i o n of the dramatic effects that d i f f e r i n g oxidation depths can exert over mechanical property changes. Oxygen Diffusion Effects If an air-saturated polymer sample i s placed i n a radiation environment, homogeneous oxidation w i l l take place i n i t i a l l y . At s u f f i c i e n t l y high oxidation rates, the i n i t i a l l y dissolved oxygen may be used up faster than i t i s replenished from the atmosphere. This gives r i s e to heterogeneous degradation with the oxidation rate i n i n t e r i o r regions of the material decreasing to zero (or to some fixed rate lower than that near the surfaces). For materials exposed to h i g h energy radiation conditions which result i n heterogeneous degradation, i t i s possible to estimate an absorbed dose by which strongly heterogeneous degradation i s already taking place. This can be accomplished by calculating the amount of oxygen which w i l l be dissolved i n a p o l y mer at equilibrium with the surrounding atmosphere, and then dividing by the amount of oxygen consumed (by chemical reaction) per rad of radiation absorbed by the polymer. We have obtained the following general expression for the equivalent dose, R, i n rads, required to use up a l l the oxygen i n i t i a l l y dissolved in a given polymer: R -

S • P —G(-0 )

• A • (1.6

x 10-12)

(1)

2

where S i s oxygen s o l u b i l i t y i n the polymer i n mol/g'bar, P i s oxygen pressure ( i n bars) i n the atmosphere surrounding the polymer sample, G(-02) i s the oxygen consumption y i e l d ( i n molecules per 100 eV absorbed energy), and A i s Avogadro's number. The numerical constant serves to convert eV to rads, and has the units of rad»g/100 eV. For the majority of common polymeric materials, we have calculated that the t r a n s i t i o n from homogeneous to strongly heterogeneous degradation w i l l occur at quite low doses—generally less than a few tenths of a megarad. For v i r t u a l l y a l l polymers, measurable degradation occurs only after substantially higher doses. Thus, where oxidation inhomogeneities occur i n high-energy radiation environments, the degradation can t y p i c a l l y be treated as coming e n t i r e l y from a heterogeneous mechanism. If the oxygen permeation constant and the rate of oxygen consumption for the material remain r e l a t i v e l y constant as a function of t o t a l absorbed dose, a steady state in heterogeneous oxidation w i l l be approached. If the rate of oxygen consumption by reaction with free radicals generated by the radiation exceeds the rate of supply of oxygen from the edges of the polymer sample, d i s t i n c t

Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Techniques for Profiling Polymer Degradation

r e g i o n s of o x i d i z e d and n o n o x i d i z e d polymer can r e s u l t . In comparing polymer samples i r r a d i a t e d a t d i f f e r e n t dose r a t e s , those exposed a t the highest dose rate may degrade heterogeneously, becoming o x i d i z e d o n l y near the s u r f a c e s . At s u c c e s s i v e l y lower dose r a t e s , o x i d a t i o n depth w i l l become p r o g r e s s i v e l y larger. Eventually, at s u f f i c i e n t l y low dose r a t e s , the oxygen can f u l l y penetrate the sample, g i v i n g r i s e t o o x i d a t i o n which i s homogeneous throughout the material. R a p i d I d e n t i f i c a t i o n of Heterogeneous D e g r a d a t i o n : Metallographic Polishing The f i r s t t e c h n i q u e i s a q u a l i t a t i v e or semiquantitative method for rapid identification of oxidative inhomogeneities. By this method, samples a r e mounted i n epoxy and a c r o s s - s e c t i o n a l s u r f a c e p o l i s h e d using standard metallographic techniques (10). I n the degraded m a t e r i a l , the p h y s i c a l and m e c h a n i c a l p r o p e r t i e s have undergone changes due to c h a i n s c i s s i o n , c r o s s l i n k i n g , p l a s t i cizer loss, etc. Material regions having different degradation have d i f f e r e n t p h y s i c a l p r o p e r t i e s , and so take on d i f f e r e n t l u s t e r s upon p o l i s h i n g . Oxidized and nonoxidized regions have d i f f e r e n t r e f l e c t i v i t i e s , r e s u l t i n g i n v i s u a l bands when examined under an o p t i c a l microscope. We r e f e r to the w i d t h s of such bands as the depth o f oxidation, although strictly speaking, a step function change from o x i d i z e d to n o n o x i d i z e d r e g i o n s r a r e l y o c c u r s , as w i l l be c l e a r from some of the examples g i v e n below. The photographs shown i n F i g . 1 were o b t a i n e d on a s e r i e s of c l a y - f i l l e d e t h y l e n e - p r o p y l e n e rubber (EPR) m a t e r i a l s which were i r r a d i a t e d and then s u b j e c t e d to c r o s s - s e c t i o n a l p o l i s h i n g . The occ u r r e n c e of h e t e r o g e n e o u s o x i d a t i o n i s c l e a r l y i l l u s t r a t e d i n photo B f o r a sample i r r a d i a t e d at h i g h dose r a t e (6.7 x 10 rad/h). Photo C r e p r e s e n t s a sample i r r a d i a t e d to s i m i l a r t o t a l dose, but a t lower dose r a t e (1.1 x 10~* r a d / h ) ; h e r e , oxygen permeates t h r o u g h out the sample g i v i n g r i s e to e s s e n t i a l l y homogeneous oxidation. Photo A r e p r e s e n t s an u n i r r a d i a t e d sample. Photo D r e p r e s e n t s a sample i r r a d i a t e d a t h i g h dose r a t e (1.1 x IQr r a d / h ) but i n the absence of oxygen. The two samples (B and C) which were i r r a d i a t e d at d i f f e r e n t dose r a t e s but t o s i m i l a r t o t a l doses showed s i g n i f i c a n t d i f f e r e n c e s i n u l t i m a t e t e n s i l e s t r e n g t h ; t h i s stems from d i f f e r e n c e s i n the ext e n t of o x i d a t i o n a t the d i f f e r e n t dose r a t e s . Sample B had a t e n s i l e s t r e n g t h t h a t was 85% (+7%) o f the t e n s i l e s t r e n g t h o f unaged m a t e r i a l , w h i l e C e x h i b i t e d a t e n s i l e s t r e n g t h o f 53% (+7%) compared w i t h unaged. As might be e x p e c t e d , the o p t i c a l r i n g s became p r o g r e s s i v e l y f a i n t e r on g o i n g t o s u c c e s s i v e l y lower d o s e s ; r i n g s were not v i s i b l e a t doses below about 50 Mrad. However, the s i z e of the r i n g s i n t h i s m a t e r i a l d i d not change s i g n i f i c a n t l y as a f u n c t i o n of dose a t c o n s t a n t dose r a t e . T h i s o b s e r v a t i o n i n d i c a t e s t h a t oxygen p e r m e a t i o n and consumption r a t e s i n t h i s m a t e r i a l a r e not s i g n i f i c a n t l y dependent on dose. The v i s u a l r i n g s seen i n cross-sectioned, polished samples c o r r e s p o n d t o a r e a s h a v i n g l a r g e d i f f e r e n c e s i n the e x t e n t of o x i d a t i o n , and a r e u s e f u l f o r i d e n t i f i c a t i o n of h e t e r o g e n e o u s d e g r a d a t i o n and f o r a q u a l i t a t i v e or s e m i q u a n t i t a t i v e d e t e r m i n a t i o n of the d e p t h of o x i d a t i o n . However, the exact shape of the d e g r a d a t i o n p r o f i l e 5


413

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w i l l depend on t h e u n d e r l y i n g oxidation kinetics. Equations f o r c a l c u l a t i n g g r a d i e n t s i n systems h a v i n g s i m u l t a n e o u s d i f f u s i o n and c h e m i c a l r e a c t i o n have been d e s c r i b e d (11-14). Using f r e e r a d i c a l r e a c t i o n k i n e t i c s , o x i d a t i o n p r o f i l e s r a n g i n g from g e n t l e " p a r a b o l i c " shapes t o s t e p - f u n c t i o n t r a n s i t i o n s c a n be o b t a i n e d ( 1 4 ) . The shapes depend on t h e r e l a t i v e r a t e s o f t e r m i n a t i o n and p r o p a g a t i o n steps. Thus o x i d a t i v e rings visible from c r o s s - s e c t i o n a l p o l i s h i n g may r e p r e s e n t s i t u a t i o n s r a n g i n g from sharp t r a n s i t i o n s between o x i d i z e d and n o n o x i d i z e d r e g i o n s , t o more g r a d u a l t r a n s i t i o n s between h e a v i l y o x i d i z e d r e g i o n s and r e g i o n s h a v i n g e i t h e r no o x i d a t i o n o r l i g h t oxidation. The term o x i d a t i o n depth has q u a n t i t a t i v e meaning i n t h e former l i m i t , but has a somewhat more q u a l i t a t i v e meaning i n t h e latter. D e g r a d a t i o n P r o f i l i n g i n Terms o f R e l a t i v e Hardness More q u a n t i t a t i v e p r o f i l e s o f heterogeneous d e g r a d a t i o n c a n be o b t a i n e d by measuring changes i n r e l a t i v e hardness a c r o s s polished cross-sectional surfaces o f degraded samples. Relative hardness p r o f i l e s a r e o b t a i n e d u s i n g one of two types o f i n s t r u m e n t a t i o n . For r e l a t i v e l y hard p l a s t i c s , a commercial Knoop Hardness T e s t e r g i v e s good r e s u l t s . T h i s a p p a r a t u s employs a t h i n , convex diamond b l a d e which i s p r e s s e d i n t o t h e sample under c o n s t a n t weight f o r a s e t p e r i o d of time. The s o f t e r the m a t e r i a l , t h e deeper t h e b l a d e p e n e t r a t e s i n t o t h e sample. Data i s o b t a i n e d by m e a s u r i n g t h e l e n g t h o f t h e i m p r e s s i o n l e f t by t h e b l a d e . F o r t h i s e x p e r i m e n t , t h e sample i s p l a c e d on a c a l i b r a t e d t r a n s l a t i o n a l m i c r o s c o p e s t a g e , and measurements made a t r e g u l a r i n t e r v a l s . We can r e a d i l y o b t a i n 20 t o 30 d a t a p o i n t s o v e r a d i s t a n c e o f 1 mm. By o b t a i n i n g measurements o v e r a c r o s s - s e c t i o n o f a h e t e r o g e n e o u s l y degraded m a t e r i a l , a u s e f u l p r o f i l e o f t h e h e t e r o g e n e i t y i n m a t e r i a l p r o p e r t y changes i s o b t a i n e d (10). M a t e r i a l h a r d n e s s i s r e l a t e d t o modulus: i n c r e a s e d p e n e t r a t i o n c o r r e s p o n d s t o d e c r e a s e d modulus. ( I n f a c t , modulus c a n be c a l c u l a t e d from such p e n e t r a t i o n e x p e r i m e n t s , dependent upon t i p geometry and experimental procedure.) ( 1 5 ) . F i g u r e 2 g i v e s an example of a p r o f i l e of changes i n r e l a t i v e h a r d n e s s f o r a c l e a r p o l y p r o p y l e n e m a t e r i a l exposed t o UV l i g h t i n a Rayonet chamber. The data i n d i c a t e t h a t t h e p o l y p r o p y l e n e becomes p r o g r e s s i v e l y h a r d e r near t h e edges w i t h UV exposure, y i e l d i n g a b r o a d profile. The m a t e r i a l i n t h e i n t e r i o r o f t h e sample has undergone e s s e n t i a l l y no change i n r e l a t i v e h a r d n e s s . F o r s o f t e r , r u b b e r y polymers, a d i f f e r e n t e x p e r i m e n t a l p r o c e d u r e g i v e s b e t t e r r e s u l t s . F o r t h e s e e x p e r i m e n t s we have measured d i r e c t l y the p e n e t r a t i o n d i s t a n c e of a t i n y weighted probe i n t o t h e c r o s s s e c t i o n e d s u r f a c e o f polymer samples. F o r t h i s purpose we have made use o f a P e r k i n - E l m e r Thermomechanical A n a l y z e r equipped w i t h a t i p m o d i f i e d t o be s m a l l enough t o p r o v i d e measurements o f t h e d e s i r e d resolution. (We a r e c u r r e n t l y u s i n g a c o n i c a l diamond phonograph needle having a t i p angle of 60°.) F i g u r e 3 shows p r o f i l e s i n d i c a t i n g changes i n r e l a t i v e h a r d n e s s on c r o s s - s e c t i o n e d samples o f t h e EPR m a t e r i a l o f F i g . 1. F o r u n i r r a diated material, the p r o f i l e i s e s s e n t i a l l y flat (x's). F o r the samples i r r a d i a t e d a t 6.7 x 10^ r a d / h , a d i s t i n c t flat-bottomed, U-shaped p r o f i l e i s seen ( c i r c l e s ) . The boundary p o s i t i o n between o p t i c a l bands (photo B, F i g u r e 1) c o r r e s p o n d s t o t h e steep p a r t of


CLOUGH AND GILLEN


F i g u r e 1. C r o s s - s e c t i o n e d , p o l i s h e d samples o f gamma i r r a d i a t e d EPR. A: U n i r r a d i a t e d m a t e r i a l . B: 6.7 x l t V r a d / h ( i n a i r ) to 165 Mrad. C: 1.1 x 1 0 r a d / h ( i n a i r ) t o 175 Mrad. D: 1.1 x 1 0 r a d / h ( i n vacuum) t o 253 Mrad. A l l i r r a d i a t i o n s c a r r i e d o u t a t 70°C. Sample t h i c k n e s s - 3.15 mm. 5

6

F i g u r e 2. Hardness p r o f i l e f o r p o l y p r o p y l e n e exposed t o UV l i g h t i n a Rayonet. • - 6 day exposure A - 3 day exposure, X = unexposed m a t e r i a l . A Knoop Hardness T e s t e r was employed.


415


416

the p r o f i l e ( s l i g h t l y l e s s than 20 p e r c e n t of the way i n on both sides). The i r r a d i a t e d m a t e r i a l has become s i g n i f i c a n t l y harder throughout ( i . e . , i n c r e a s e d modulus) w i t h the l a r g e s t i n c r e a s e o c c u r r i n g a t the i n t e r i o r p o r t i o n where oxygen i s a b s e n t . F o r a sample i r r a d i a t e d a t a lower dose r a t e (1.1 x 10^ r a d / h ) the p r o f i l e approaches a homogeneous c o n d i t i o n , showing o n l y a s l i g h t , s h a l l o w curvature (squares). D e g r a d a t i o n P r o f i l i n g i n Terms of

Density

I n a n o t h e r t e c h n i q u e , the d e n s i t i e s of p i e c e s of degraded samples a r e p r o f i l e d u s i n g a s a l t g r a d i e n t ' column ( 7 ) . This technique provides a p r o f i l e on oxygen uptake i n the s a m p l e — i n f o r m a t i o n which i s comp l e m e n t a r y t o t h a t p r o v i d e d by the t e c h n i q u e s based on changes i n mechanical p r o p e r t i e s . O x i d a t i o n of samples n o r m a l l y l e a d s to i n c r e a s e s i n sample d e n s i t y . F i g u r e 4 shows d e n s i t y d a t a on two EPR samples (B and C of F i g . 1) t h a t were i r r a d i a t e d t o s i m i l a r dose, but a t d i f f e r e n t dose r a t e s . S t r o n g l y heterogeneous o x i d a t i o n i s a g a i n I n d i c a t e d f o r the h i g h - d o s e r a t e sample, i n c o n t r a s t to n e a r l y homogeneous o x i d a t i o n f o r the lowd o s e - r a t e sample. The r e s u l t s c o r r e l a t e w e l l w i t h r e s u l t s o b t a i n e d by o p t i c a l e x a m i n a t i o n of p o l i s h e d samples and by r e l a t i v e h a r d n e s s measurements. Figure 5 shows another example of density gradient results o b t a i n e d w i t h an EPR i n s u l a t i o n m a t e r i a l w h i c h had been e x t r u d e d o n t o a copper conductor. For the d e g r a d a t i o n s t u d i e s , the i n s u l a t i o n had been s t r i p p e d from the c o n d u c t o r and i r r a d i a t e d i n a i r as a h o l l o w t u b e . At h i g h dose r a t e s , a U-shaped g r a d i e n t c h a r a c t e r i s t i c o f oxygen d i f f u s i o n was o b t a i n e d ( d a t a not shown). However, a t low dose r a t e s , a p r o f i l e showing d r a m a t i c a l l y enhanced o x i d a t i o n o n l y near the i n t e r i o r surface ( i . e . , the s u r f a c e which had been a d j a c e n t t o the c o p p e r ) was f o u n d . The p r o f i l i n g r e s u l t s l e d t o the c o n c l u s i o n t h a t oxidation catalyzed by copper i o n s was a predominant d e g r a d a t i o n mechanism i n t h i s m a t e r i a l ( 7 ) . S u p p o r t i n g e v i d e n c e was o b t a i n e d by d e m o n s t r a t i n g a l a r g e copper c o n c e n t r a t i o n i n the r e g i o n near the i n t e r i o r s u r f a c e by means of e m i s s i o n s p e c t r o s c o p y . T h i s example i l l u s t r a t e s a c a s e of heterogeneous d e g r a d a t i o n w h i c h r e s u l t e d from a mechanism d i f f e r e n t from oxygen d i f f u s i o n . Effect Viton

o f Heterogeneous D e g r a d a t i o n on M a c r o s c o p i c P r o p e r t y

Changes:

The d e g r a d a t i o n b e h a v i o r of a V i t o n m a t e r i a l p r o v i d e s a good example o f the e f f e c t s of heterogeneous o x i d a t i o n . Figure 6 provides a p l o t of m e c h a n i c a l p r o p e r t y changes i n V i t o n as a f u n c t i o n of absorbed dose I n a i r a t t h r e e d i f f e r e n t dose r a t e s . The d a t a show t h a t a t h i g h dose r a t e (5.5 x 10^ r a d / h , open s q u a r e s ) the e l o n g a t i o n drops markedl y w h i l e the t e n s i l e s t r e n g t h undergoes a more modest d e c r e a s e . At low dose r a t e (1.3 x 10^ r a d / h , c i r c l e s ) the e l o n g a t i o n changes v e r y l i t t l e , whereas the t e n s i l e s t r e n g t h d e c r e a s e s s h a r p l y . Data o b t a i n e d a t an i n t e r m e d i a t e dose r a t e (9.2 x 10^ r a d / h , diamonds) show i n t e r mediate b e h a v i o r . In terms of v i s u a l e x a m i n a t i o n , samples i r r a d i a t e d a t the h i g h dose r a t e a r e found to become p r o g r e s s i v e l y h a r d e r and e v e n t u a l l y so e m b r i t t l e d t h a t they break r e a d i l y when f l e x e d l i g h t l y by hand. Samples i r r a d i a t e d a t the low dose r a t e degrade i n j u s t the



28. CLOUGH AND GILLEN

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0.05

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Position On Sample F i g u r e 3. P r o f i l e s o f r e l a t i v e h a r d n e s s i n terms o f p e n e t r a t i o n d i s t a n c e o f a w e i g h t e d probe i n t o c r o s s - s e c t i o n e d samples o f i r r a d i a t e d EPR. X unirradiated material, O 6.7 x 1 0 r a d / h t o 165 Mrad. • - 1.1 x 1 0 r a d / h t o 175 Mrad. Probe l o a d was 5 g. s

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1.11 Edge

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Center Position On Sample

F i g u r e 4. D e n s i t y p r o f i l e s f o r i r r a d i a t e d EPR samples. Solid l i n e symbols a r e f o r 165 Mrad a t 6.7 x 1 0 rad/h. Dotted l i n e symbols a r e f o r 175 Mrad a t 1.1 x 1 0 r a d / h . The arrow i n d i c a t e s the d e n s i t y of u n i r r a d i a t e d m a t e r i a l , which g i v e s a f l a t profile. 5

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P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

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Center

Edge

Position On Sample F i g u r e 5. D e n s i t y p r o f i l e f o r an EPR c a b l e i n s u l a t i o n m a t e r i a l which had been s t r i p p e d from the copper c o n d u c t o r and i r r a d i a t e d as a h o l l o w tube. At low dose r a t e (1.6 x 10^ r a d / h ) t h e mater i a l shows enhanced o x i d a t i o n near the i n t e r i o r s u r f a c e ( s o l i d l i n e symbols). The d o t t e d l i n e symbols show t h e f l a t p r o f i l e of u n i r r a d i a t e d m a t e r i a l .

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Physical Techniques for Profiling Heterogeneous Polymer Degradation

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