Effects of Ozone Exposure on the Physical Properties of Butadiene

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Effects of Ozone Exposure on the Physical Properties of Butadiene and Styrene/Butadiene Copolymers Κ. E. STEPHENS and C. L. BEATTY University of Florida, Department of Materials Science and Engineering, Gainesville, FL 32611

Polystyrene and polybutadiene homopolymers as well as random and block copolymers of these mers have been studied via dielectric relaxation spectroscopy and ten sile stress-strain measurements. The results suggest that some block copolymer systems studied have styrene rich surfaces which appear to partially crosslink upon initial exposure to ozone even though surface oxygen concentrations are not significantly affected. After continued exposure these samples appear to then under go chain scission. Complex plane analysis implies that after degradation has occured, the Tg process for PS is reduced in temperature and occurs by the same mechanism as in non degraded PS. The butadiene rich materials appear to undergo predominantly chain scission with increases in oxygen concentrations at the surface of the sample. Photo-oxidation o f p o l y s t y r e n e , PS, (1-5) and high impact p o l y s t y r e n e , HIPS, (6) have been w e l l s t u d i e d by n o t i n g changes i n the i n f r a r e d s p e c t r a (1,2,3,6) contact angle (_1) u l t r a v i o l e t s p e c t r a (2), dynamic mechanical (6) and d i e l e c t r i c r e l a x a t i o n (4,5) p r o p e r t i e s . The e f f e c t o f ozone exposure on the p r o p e r t i e s of elastomer i n c l u d i n g styrenebutadiene random (SBR) copolymer has been studied v i a i n f r a r e d spectroscopy (7). In a d d i t i o n , the XPS s p e c t r a of ozone t r e a t e d p o l y s t y r e n e , polybutadiene (PB) and block and random copolymers has been reported (8). Ozone e f f e c t s on r e l a t i v e s u r f a c e atomic concentrations and s u r f a c e energy has a l s o been examined and reported f o r PS, PB and both b l o c k and random styrene copolymers (20). In t h i s study we wish to s y s t e m a t i c a l l y i n v e s t i g a t e the e f f e c t s o f chemical composition, chain micros t r u c t u r e and morphology with regard to the e f f e c t o f treatment as manifested by changes i n a v a r i e t y o f p h y s i c a l p r o p e r t i e s . Experimental Chloroform

s o l u t i o n s prepared u s i n g the "as r e c e i v e d " polymers

0097-6156/ 83/ 0229-0261 $06.00/ 0 © 1983 American Chemical Society

262

EFFECTS OF HOSTILE ENVIRONMENTS

(see Table I) were r e p r e c i p i t a t e d i n c o l d methanol and vacuum d r i e d overnight t o remove r e s i d u a l methanol. P o l y s t y r e n e , 310 and 308 r e p r e c i p i t a t e d polymers were r e d i s s o l v e d i n t o chloroform and c a s t u s i n g a micrometer a d j u s t a b l e f i l m c a s t e r onto g l a s s p l a t e s . The index cards had the center p o r t i o n removed so the index card " p i c t u r e frame" served as a support f o r the elastomer f i l m s and f a c i l i t a t e d h a n d l i n g o f the f i l m s . The Solprene 308 and 1205 f i l m s were removed from the g l a s s p l a t e s under water. The f i l m s were then placed i n a vacuum f o r a t l e a s t 3 days. To a c q u i r e uncrossl i n k e d polybutadiene and random copolymer f i l m s o f s u f f i c i e n t s t r e n g t h t o be handled and cut i n t o specimens, these s o l u t i o n s were poured i n t o a r e c t a n g u l a r form atop a g l a s s p l a t e and allowed to dry. Repeated c a s t i n g s were made u n t i l s u f f i c i e n t t h i c k n e s s had been b u i l t up f o r the f i l m t o r e t a i n i t s own shape upon r e moval from the g l a s s p l a t e . These f i l m s were d r i e d i n vacuum f o r one week. Polybutadiene c r o s s l i n k i n g was done by adding benzoyl peroxide e q u i v a l e n t to 1% o f polymer weight i n the r e p r e c i p i t a t e d s o l u t i o n . The mixture was then turned at approximately 30 rpm s f o r 1 hr. on r o l l e r t o f a c i l i t a t e mixing, p r i o r to pouring i n t o the form. Samples were placed i n t o a b e l l j a r having an a i r flow o f 1.5 l i t e r s / m i n . which contained 375 ppm+10% ozone. For the r e c i p r o c i t y study, ozone c o n c e n t r a t i o n was v a r i e d from 100 to 1000 ppm + 10%. Ozone generation was accomplished u s i n g a P.C.I. Model C2P-3C h i g h v o l t a g e ozone generator. Ozone c o n c e n t r a t i o n s were monitored by bubbling the gas flow through a b u f f e r e d (6.7 pH) potassium i o d i d e s a l t s o l u t i o n and t i t r a t i n g with sodium t h i o s u l f a t e (9). A l l f i l m s were kept i n vacuum f o r a t l e a s t one a d d i t i o n a l day. Upon opening the ozone chamber to introduce samples, a conc e n t r a t i o n gradient was produced. For t h i s reason the subsequent time i n v o l v e d f o r e q u i l i b r i u m t o be r e e s t a b l i s h e d , exposure time was not used as the only gauge o f exposure i n r e c i p r o c i t y measurements. Under these circumstances, the area under the concentrat i o n v s time curve appears to be a b e t t e r measure of exposure o f the sample t o ozone. T h i s area f a c t o r was kept constant over a range o f c o n c e n t r a t i o n c o n d i t i o n s . f

Tensile Stress-Strain T e n s i l e t e s t i n g was done u s i n g an I n s t r o n Model 1122 load frame and an I n s t r o n 1000 l b . maximum combination tension/comp r e s s i o n load c e l l . Samples were cut from f i l m s u s i n g ASTM 1822 Type D standard c u t t e r f o r t e n s i l e samples. Crosshead speed was 5 inches per minute f o r a l l samples. P r i o r to t e n s i l e t e s t i n g , p o l y s t y r e n e homopolymer t e n s i l e samples were annealled a t 85°C f o r 4 days i n order t o remove any a d d i t i o n a l i n t e r n a l s t r e s s e s i n t r o duced during the specimen c u t t i n g procedure.

* As r e c e i v e d ; before

FG-817-D9 Solprene 310 Solprene 308 Solprene 1205 Solprene 1206 Solprene 233

Polymer Code

reprecipitation.

0/100 52/48 70/30 75/25 75/25 100/0

Wt.*

100,000 85,000 160,000 83,000 212,000 300,000

Mol.

Homopolymer Block copolymer Block copolymer Block copolymer Random copolymer Homopolymer

Type

Polymers used i n the study of ozone degradation of bulk and s u r f a c e p r o p e r t i e s

Nominal Butadiene/styrene Content

Table I .

F o s t e r Grant Co. P h i l l i p s Pet. Co. P h i l l i p s Pet. Co. P h i l l i p s Pet. Co. P h i l l i p s Pet. Co. P h i l l i p s Pet. Co.

Manufacturer

264


D i e l e c t r i c R e l a x a t i o n Spectroscopy A l l polymer f i l m s were prepared and t r e a t e d w i t h ozone as s t a t e d above, w i t h the e x c e p t i o n o f the c r o s s l i n k e d butadiene and copolymer #1206 and #1205. For these three polymers, the s o l u t i o n was poured d i r e c t l y i n t o the d i e l e c t r i c c e l l and allowed t o d r y a t 1 atm pressure f o r 4 h r s . The d i e l e c t r i c c e l l was then p l a c e d i n a vacuum f o r a t l e a s t 4 h r s . t o remove r e s i d u a l chloroform. Thickness o f the c e l l and the c a s t f i l m were then measured and the d i e l e c t r i c s p e c t r a were attempted. The ozone t r e a t e d c r o s s l i n k e d butadiene, #1205 and #1206 specimens were c a s t as p r e v i o u s l y e x p l a i n e d f o r the t e n s i l e samples. Except f o r specimens that were cast i n t o the d i e l e c t r i c c e l l , a l l other samples were cut i n t o 1" c i r c u l a r specimens. The d i e l e c t r i c c e l l used was a Balsbaugh Model #MC100. The c e l l was connected t o a Hewlett Packard (H|) Model ^274A multifrequency d i g i t a l meter (frequency range 10 Hz - 10 H z ) . T h i s meter has automatic b r i d g e b a l a n c i n g c a p a b i l i t i e s and was i n t e r f a c e d w i t h an HP 85 desktop computor f o r data c o l l e c t i o n and p r o c e s s i n g . D i e l e c t r i c r e l a x a t i o n measurements were made by scanning the temperature a t a constant r a t e o f 0.35°C/min. i n a D e l t a Design tempera t u r e chamber (Model-No. 5100) as c o n t r o l l e d by a D e l t a Design r a t e programming accessory. The temperature of the sample was measured t o an accuracy o f + 0.1°C by a Fluke 2190A d i g i t a l thermometer. T h i s thermometer was coupled w i t h a Fluke 1120A IEEE-488 T r a n s l a t o r which was i n t e r f a c e d w i t h the HP 85 computer. The normal temperatures examined ranged from -150°C to +150°C however, some specimens were examined a t extended ranges (-180 to +200°C). R e s u l t s and D i s c u s s i o n In examining the e f f e c t s o f r e c i p r o c i t y , (concentrâtion-time of exposure r e l a t i o n s h i p s ) there does not appear to be an obvious r e l a t i o n s h i p form these s t u d i e s . In f a c t , n o n - r e c i p r o c i t y appears to be the case. I t can be seen i n F i g . 1 that exposure to ozone at h i g h c o n c e n t r a t i o n f o r short p e r i o d s o f time appear to have l i t t l e o r no e f f e c t on the butadiene g l a s s t r a n s i t i o n , Tg, process of the 70% butadiene 30% styrene b l o c k copolymer, (sample #308). One would p o s t u l a t e that the l e n g t h o f time needed f o r d i f f u s i o n i n t o the bulk and s i g n i f i c a n t degradation t o occur had not been reached such as to a l t e r t h i s Tg p r o c e s s . As the exposure time i s i n c r e a s e d to 75 min. and ozone c o n c e n t r a t i o n decreased, a marked i n c r e a s e i n r e a c t i o n has occured and the r e l a x a t i o n i s seen to occur a t higher temperatures (^4° Κ s h i f t ) . The minimum time f o r d e t e c t a b l e degradation has elapsed and the c o n c e n t r a t i o n of ozone i s s t i l l s i g n i f i c a n t l y h i g h to r e a c t a t a d e t e c t a b l e r a t e . As the exposure time i s f u r t h e r i n c r e a s e d and the ozone c o n c e n t r a t i o n decreased, the s h i f t o f the g l a s s t r a n s i t i o n process on the t r a n s i t i o n map appears l e s s s i g n i f i c a n t . T h i s i s assumed to be due to

Effects of Ozone Exposure

STEPHENS AND BEATTY

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1/T χ 1000 (K) Figure 1. Effects of ozone on butadiene/styrene block copolymer. Areas of ozone exposure curves (time in minutes vs. concentration ppm)for all specimens infigureare equivalent. Key to ozone treatment: *, untreated; Δ, 975ppmfor 33 min; B, 375ppmfor 75 min; and +, 110 ppm for 223 min.

266


the c o n c e n t r a t i o n of ozone now being below a p o i n t such that the s u r f a c e i s no longer s a t u r a t e d w i t h ozone and the maximum r a t e of d i f f u s i o n and degradation i s no longer o c c u r i n g f o r t h i s compos i t i o n sample a t room temperature. Therefore, even though the area f a c t o r s are e q u i v a l e n t , the e f f e c t s of the exposure are not, and appear to depend not only upon the c o n c e n t r a t i o n of ozone a v a i l a b l e i n the environment of the sample, but a l s o the l e n g t h of time a v a i l a b l e f o r d i f f u s i o n i n t o the sample. The g l a s s t r a n s i t i o n of butadiene appear to not be a f f e c t e d by the p a r t i a l c r o s s l i n k i n g done w i t h benzoyl peroxide. I t can be seen i n F i g u r e 2 curves (1,3) that the r e l a x a t i o n processes almost superimpose. Both curves appear to f o l l o w the expected W i l l i a m s , Landel, F e r r y (WLF) c u r v i l i n e a r behavior (17). On examining the t r a n s i t i o n map of butadiene c r o s s l i n k e d w i t h 1% benzoyl peroxide (sample XL) we can see (Figure 2) that the butadiene t r a n s i t i o n i s o c c u r i n g at approximately 9 degrees lower temperature than i n sample 308 (at 100 Hz) (Figure 1 ) . The lower Tg of the c r o s s l i n k e d polybutadiene compared to the 30 wt.% s t y rène b l o c k copolymer suggests that the styrene domains i n the b l o c k copolymer e f f e c t i v e l y provides more c o n s t r a i n t s to p o l y b u t adiene m o b i l i t y . A f t e r exposure to ozone f o r 150 mins., a t 375 ppm the tempe r a t u r e of the g l a s s t r a n s i t i o n f o r the p a r t i a l l y c r o s s l i n k e d sample i s not s i g n i f i c a n t l y a f f e c t e d a t lower f r e q u e n c i e s , however at higher frequencies (above 20 KHz) i t appears as though the Tg i s s h i f t e d to higher temperatures as compared to the samples unexposed to ozone. The exposed sample appears to f o l l o w an almost l i n e a r behavior r a t h e r than the expected WLF c u r v i l i n e a r behavior and would i n d i c a t e that f u r t h e r s t u d i e s would be warranted to examine higher frequencies (above 100 KHz) i n an e f f o r t to a s c e r t a i n when or i f c u r v i l i n e a r behavior would occur. Other s t u d i e s show the c o n c e n t r a t i o n of oxygen a t the s u r f a c e i n c r e a s i n g markedl y w i t h exposure to ozone (20). From the data a t hand, i t i s apparent t h a t ; the temperature and a c t i v a t i o n energies (19) (from t r a n s i t i o n map slopes) of g l a s s t r a n s i t i o n s f o r butadiene and p a r t i a l l y c r o s s l i n k e d butadiene are e q u i v a l e n t a t the frequency range examined; and f o r a sample exposed to ozone the temperatures and a c t i v a t i o n energies f o r t h i s process are not s i g n i f i c a n t l y a f f e c t e d a t lower f r e q u e n c i e s , but are a f f e c t e d a t h i g h e r f r e qunecies (above 20 KHz). At these h i g h e r f r e q u e n c i e s , these t r a n s i t i o n s appear to s h i f t to lower temperatures and higher a c t i v a t i o n energies vs the nonexposed samples. D i e l e c t r i c r e l a x a t i o n spectroscopy has been u t i l i z e d as a very s e n s i t i v e t o o l to d e t e c t photo-oxidation of PS (4,5) by the appearance of a r e l a x a t i o n process a t 230°K when measured at 24 KHz. Our measurements ( F i g u r e 3) i n i d i c a t e a s i m i l a r r e l a x a t i o n process i s o c c u r i n g at 270°K when measured a t 100 kHz. In a d d i t i o n a p o r t i o n of a l a r g e r e l a x a t i o n process that appears to be a t or below 115°K i s apparent i n the ozone t r e a t e d PS. The o r i g i n of t h i s r e l a x a t i o n process i s p r e s e n t l y u n c l e a r but may be due to a molecular charge t r a n s f e r complex s i m i l a r to that formed

STEPHENS AND BEATTY


1/T χ 1000 (K) Figure 2. Glass transition map. Key: +, butadiene: *, crosslinked butadiene; and o, crosslinked butadiene exposed to ozone at 375 ppm for 150 min.


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Temperature (K) Figure 3. Representative dielectric data for polystyrene taken at 100 kHz. Key: +, untreated: and ·, exposed to ozone at 375 ppm for 150 min.

15.

STEPHENS AND BEATTY


269

by the phenyl group i n PS with 0^ (11-13) or due to chemical r e a c t i o n of the styrene repeat u n i t analogous to a d d i t i o n of a p o l a r group to the phenyl s u b s t i t u e n t (14-16). Complex plane a n a l y s i s (18) of PS d i e l e c t r i c r e l a x a t i o n s p e c t r a , (Figure 4) shows that even though the g l a s s t r a n s i t i o n process has been s h i f t e d to a lower temperature, the mechanism i n v o l v e d i n the r e l a x a t i o n and the r e l a x a t i o n d i s t r i b u t i o n i s e s s e n t i a l l y unchanged. I f c r o s s l i n k i n g of the PS has occured i t does not appear to be very predominant as compared to c h a i n s c i s s i o n a f t e r 75 minutes of ozone exposure. F i g u r e 5 shows the e f f e c t s of ozone on PS. Degradation of the styrene main chain appears to be o c c u r r i n g as evidenced by the s h i f t i n the glass t r a n s i t i o n to lower temperatures. However, XPS s t u d i e s i n d i c a t e that oxygen c o n c e n t r a t i o n i s not s i g n i f i c a n t l y a f f e c t e d at the s u r f a c e of PS (20). Tensile E l o n g a t i o n to f a i l u r e measured i n t e n s i l e s t r e s s - s t r a i n measurements a t room temperature ( i . e . , approximately 25°C) vary s i g n i f i c a n t l y as the composition of the polymer i s v a r i e d from the g l a s s y PS to the elastomeric PB (Figure 6 ) . A l s o i t i s c l e a r that the t e n s i l e p r o p e r t i e s of the random copolymer i s s i g n i f i c a n t l y d i f f e r e n t from the same composition b l o c k copolymer. T h i s d i f ference i s due, i n p a r t , to the d i f f e r e n c e s i n molecular weight (Table 1). Upon examination of y i e l d s t r e s s vs % styrene, i t can be e a s i l y seen (Figure 7) that i n c r e a s i n g the % styrene content a l s o increases the t e n s i l e strength of the copolymer. Polystyrene appears to e x h i b i t an i n t e r e s t i n g phenomenon, when exposed to ozone f o r i n c r e a s i n g lengths of time, the y i e l d s t r e s s f i r s t i n creases then decreases w i t h i n c r e a s i n g exposure (Figure 8 ) . Sample 310 a l s o appears to e x h i b i t t h i s same phenomenon. Samples with lower % styrene content do not e x h i b i t t h i s a f f e c t . This a f f e c t may be due to an i n i t i a l PS c r o s s l i n k i n g (10) followed by chain degradation as ozone d i f f u s i o n continues. XPS s t u d i e s of these b l o c k systems (20) imply that there may be p r e f e r e n t i a l m i g r a t i o n of the PS blocks to the s u r f a c e under these specimen proparation conditions. Summary Polystyrene and polybutadiene homopolymers as w e l l as random and b l o c k copolymers of these mers have been s t u d i e d v i a d i e l e c t r i c r e l a x a t i o n spectroscopy and t e n s i l e s t r e s s - s t r a i n measurements. The r e s u l t s suggest that some b l o c k copolymer systems s t u d i e d have styrene r i c h surfaces which appear to p a r t i a l l y c r o s s l i n k upon i n i t i a l exposure to ozone even though s u r f a c e oxygen concentrations are not s i g n i f i c a n t l y a f f e c t e d . A f t e r continued exposure these samples appear to then undergo c h a i n s c i s s i o n . Complex plane a n a l y s i s i m p l i e s that a f t e r degradation


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Effects of Ozone Exposure on the Physical Properties of Butadiene

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