Photodegradation of Poly(n-butyl acrylate) - American

18 Photodegradation of Poly(n-butyl acrylate) Polymers in Solar Energy Utilization Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 04/24/17. For personal use only.

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H. R. DICKINSON, C. E. ROGERS, and R. SIMHA Case Western Reserve University, Department of Macromolecular Science, Cleveland,OH44106

The photodegradation of poly(n-butyl acrylate) results in the loss of the n-butyl ester side group to yield crosslinks, carboxylic acid groups, keto and aldo groups, and volatile products including butanol and butene. The degradation follows an apparent first order reaction with an initial rate (up to 2000 hours exposure in a QUV apparatus) that is faster than the subsequent rate. An assessment of the kinetic data in terms of proposed reaction mechanisms and concurrent changes in properties such as dynamic mechanical behavior can serve as a preliminary basis for evaluation of the material's ability to retain useful properties for time periods consistent with certain design requirements for solar energy system applications. The chemical and mechanical stability of poly(n-butyl acrylate)(PnBA) to weathering, especially to solar radiation, is of interest for possible use of this material as an encapsulant/ pottant for silicon cell solar energy arrays. This application requires that the material retain an acceptable level of its desirable properties, such as transparency, elastic modulus, etc., over several years of exposure to intermittent moisture, temperatures ranging from -10 to 50°C, solar radiation, and other norms and extremes of exposure conditions. Knowledge of the dependence of changes in properties and composition of the material on exposure conditions is a requisite for establishing reasonable estimates of its prospective performance lifetime characteristics. Degradation of poly(alkyl acrylates) and poly(alkyl methacrylates) has been the subject of several studies (e.g., 1-5).

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Current address: Ayerst Laboratories, Inc., Pharmaceutical Research and Development, Rouses Point, NY 12979. 0097-6156/83/0220-0275$06.00/0 © 1983 American Chemical Society

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Polymers in Solar Energy Utilization Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 04/24/17. For personal use only.

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Morimoto and Suzuki ( 1 ) , i n t h e i r i n v e s t i g a t i o n of the k i n e t i c s of chain s c i s s i o n and c r o s s l i n k i n g during p h o t o l y s i s of PnBA, observed only an i n c r e a s e i n molecular weight ; the presence of chain s c i s s i o n was not apparent. M c G i l l and Ackerman ( 2 ) , u s i n g a high pressure mercury lamp, found that approximately 80% g e l f r a c t i o n was obtained a f t e r twenty minutes of exposure. An i n crease i n the weight to number average molecular weight r a t i o was observed during the e a r l y stages of degradation. In a study of the e f f e c t s of γ-radiation on a s e r i e s of p o l y ( a l k y l a c r y l a t e s ) , B u r l a n t , e t . a l . (3) found that the extent of degradation was independent of polymer s t r u c t u r e and r e a c t i o n temperature. The concurrent temperature dependence of the c r o s s l i n k i n g r e a c t i o n , below and above the polymer s o f t e n i n g p o i n t , i n d i c a t e s that the segmental m o b i l i t y of the r a d i c a l s i s an important f a c t o r i n the c r o s s l i n k i n g step. A comparable c o n s i d e r a t i o n should be a p p l i c a b l e t o photodegradation processes. More r e c e n t l y , Gupta and coworkers (4,5), u s i n g the same PnBA samples as were used i n the present study, i n v e s t i g a t e d photodegradation at 293°K and 77°K w i t h r a d i a t i o n of 253.7 nm and 310 nm. They i d e n t i f i e d c e r t a i n p r i n c i p a l photoproducts and measured t h e i r quantum y i e l d s of formation. The data obtained from a number of d i f f e r e n t experimental methods were used t o formulate a mechanism of p h o t o l y s i s of PnBA which has a c l o s e r e l a t i o n s h i p to the r e s u l t s of t h i s present study. EXPERIMENTAL Sample P r e p a r a t i o n : The PnBA used was provided by the J e t Pro p u l s i o n Laboratory. The m a t e r i a l was prepared by thermal p o l y m e r i z a t i o n of nBA by r e f l u x i n g the monomer i n cyclohexane under h i g h p u r i t y n i t r o g e n f o r periods up t o 48 hours ( 5 ) . This me thod was chosen to avoid contamination of the polymer by t r a c e amounts of i n i t i a t o r which might a f f e c t the photooxidation k i n e t i c s . Molecular weight, as measured^ by HPLC, was about_ 8 χ 10 . Other measurements gave a ^ of 940,000 and a ^ of 620,000. The polymer was c l e a r , t r a n s p a r e n t , and very tacky. The g l a s s t r a n s i t i o n temperature i n the l i t e r a t u r e i s 219°K. Samples of PnBA were d i s s o l v e d i n dichloromethane, f i l t e r e d , and cast i n t o t h i n f i l m s on v a r i o u s s u b s t r a t e s . Quartz s u b s t r a t e s were used f o r U V - v i s i b l e s p e c t r o s c o p i c analy s i s , as w e l l as f o r g e l content and weight l o s s measurements. S a l t (NaCl) f l a t s were used as s u b s t r a t e s f o r FTIR a n a l y s i s . The f i l m s were d r i e d slowly to remove solvent f o l l o w e d by vacuum d r y i n g f o r at l e a s t 24 hours. 5

R a d i a t i o n Procedures: The samples were i r r a d i a t e d at 40°C w i t h e i t h e r a QUV apparatus (Q-Panel Co., C l e v e l a n d , Ohio) which uses mercury f l u o r e s c e n t sunlamps (Westinghouse FS-40) w i t h an emis s i o n maximum at 313 nm, or a more intense (by a f a c t o r of about 40) medium pressure mercury lamp (Hanovia) w i t h an emission maximum at 366 nm. A l l i r r a d i a t i o n s were pyrex g l a s s f i l t e r e d .


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For the t o r s i o n a l b r a i d analyses, a f i b e r g l a s s b r a i d was coated w i t h polymer, d r i e d , then exposed to the medium pressure mercury lamp f o r 0, 42 and 160 hours. Since the f i b e r g l a s s b r a i d i s not transparent, i t was r o t a t e d at one r e v o l u t i o n per minute about 1 i n c h from the water j a c k e t e d lamp. R a d i a t i o n f l u x at the sample plane was measured u s i n g onitrobenzaldehyde (oNBA) as an actinometer. The f i l m s were h e l d p a r a l l e l to the plane of the QUV lamps at 7 cm from the outer edge of the nearest lamp. The f i l m s were contained i n a l i g h t proof c o l l i m a t o r box equipped w i t h a photographic s h u t t e r . The c o n c e n t r a t i o n of p h o t o s e n s i t i v e o-nitrobenzaldehyde i n the a c t i nometer f i l m s i s c a l c u l a t e d from the o p t i c a l d e n s i t y at 320 nm and the molar e x t i n c t i o n c o e f f i c i e n t found by P i t t s et a l ( 6 ) . The t o t a l i n t e n s i t y of l i g h t emitted from the FS-40 lamps was estimated to be 2.1 χ 10"^ moles of photons/cm sec. The i n t e n s i t y at 5 cm, measured by e l e c t r o n i c spectroradiometer at the Q-Panel Company, i s 18 uwatts corresponding to 4.7 χ 10"H moles of photons/cm sec. 2

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Sample C h a r a c t e r i z a t i o n : The procedures f o r the v a r i o u s analy t i c a l methods used f o r sample c h a r a c t e r i z a t i o n i n the study are described below. The procedures g e n e r a l l y f o l l o w e d e s t a b l i s h e d p r a c t i c e w i t h m o d i f i c a t i o n s to account f o r l o s s of m a t e r i a l and s p e c t r a l band broadening w i t h p r o g r e s s i v e degradation. F o u r i e r transform i n f r a r e d (FTIR) s p e c t r a were measured using a D i g i l a b FTS-14 spectrometer w i t h the Real Time Disk Operating System (RDOS). The samples were scanned 256 times at a r e s o l u t i o n of one p o i n t every four wavenumbers w i t h double precision. The changes i n c o n c e n t r a t i o n of PnBA groups and r e a c t i o n products were determined from the changes i n area of s e l e c t e d assigned bands c h a r a c t e r i s t i c of the groups. The group band assignments were made on the b a s i s of l i t e r a t u r e data and by our FTIR s t u d i e s of model compounds r e p r e s e n t a t i v e of expected groups. The e x t i n c t i o n c o e f f i c i e n t s of the model compound bands were estimated from knowledge of the d e n s i t i e s and the f i l m t h i c k n e s s estimated from i n t e r f e r e n c e f r i n g e s i n the i n f r a r e d . Dye b i n d i n g s t u d i e s were used t o o b t a i n estimates of c a r boxy l i e a c i d , aldehydes, and ketones i n the weathered f i l m s . For the a c i d determinations, the samples were exposed to a s a t u r a t e d s o l u t i o n of p r o f l a v i n e (3,6-diaminoacridine) hemisulfate for 45 to 50 minutes. The samples were e x t r a c t e d s e v e r a l times w i t h e t h a n o l . The amount of p r o f l a v i n bound i n the f i l m was determined from the o p t i c a l d e n s i t y at 450 nm f o r an e x t i n c t i o n c o e f f i c i e n t of 3.89 χ 10*. There are s e v e r a l l i t e r a t u r e values f o r the b i n d i n g r a t i o between p o l y ( a c r y l i c a c i d ) and v a r i o u s a c r i d i n e d e r i v a t i v e s . Tan and Schneider (7) s t a t e that 4 to 8 a c i d groups i n a 3 to 1 copolymer of e t h y l a c r y l a t e - a c r y l i c a c i d w i l l bind one molecule


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of a c r i d i n e orange. Schwarz and Klose (8) r e p o r t 2 to 3 groups p o l y ( a c r y l i c acid) per p r o f l a v i n e molecule. N i s h i d a and Watanabe (9) found a r a t i o of 20 sodium a c r y l i c a c i d monomer u n i t s per p r o f l a v i n e h e m i s u l f a t e molecule. This range i n b i n d i n g r a t i o gives a f a i r l y wide range i n the estimate of a c i d groups. Aldehyde and ketone concentrations were estimated by the extent of formation of hydrazones (10). The sample f i l m s were exposed to s a t u r a t e d e t h a n o l i c s o l u t i o n s of 2 , 4 - d i n i t r o p h e n y l hydrazine f o r ten minutes. The samples were r i n s e d e x t e n s i v e l y w i t h e t h a n o l . The c o n c e n t r a t i o n of bound dye was determined from the absorbance between 450 and 220 nm u s i n g l i t e r a t u r e values f o r the e x t i n c t i o n c o e f f i c i e n t s . In both dye b i n d i n g s t u d i e s , experiments were c a r r i e d out w i t h undegraded f i l m s to confirm the absence of concurrent phys i c a l a d s o r p t i o n of the dyes i n the absence of the s p e c i f i c b i n d i n g f u n c t i o n a l groups. Studies were a l s o c a r r i e d out to confirm the attainment of apparent e q u i l i b r i u m i n terms of dye exposure time and r i n s i n g time to e l i m i n a t e any d i f f u s i o n (sorpt i o n time) c o n t r o l of dye b i n d i n g . Nevertheless, there i s s t i l l some p o s s i b i l i t y that very slow s o r p t i o n r a t e s would give a q u a s i e q u i l i b r i u m c o n d i t i o n so that the estimated values of the f u n c t i o n a l groups may be too low. D i l u t e s o l u t i o n v i s c o s i t i e s of the untreated m a t e r i a l and the two i r r a d i a t e d f i l m s were measured i n a Cannon 50 viscometer i n toluene at 25°C. The c r o s s l i n k d e n s i t i e s were estimated from the s w e l l r a t i o s and the s o l / g e l r a t i o s . The t r e a t e d f i l m s were scraped o f f the g l a s s s l i d e s , weighed, then swollen i n toluene. The s o l v a t e d g e l was separated from solvent by c e n t r i f u g i n g at low speed i n a graduated c e n t r i f u g e tube. A f t e r the volume of the g e l was recorded, the mass was determined by pressure f i l t e r i n g the g e l through a preweighed t e f l o n f i l t e r , then d r y i n g the m a t e r i a l and weighing i t . Mechanical r e l a x a t i o n behavior of p o l y ( n - b u t y l a c r y l a t e ) was s t u d i e d u s i n g a f r e e l y o s c i l l a t i n g t o r s i o n a l pendulum at frequencies of about 1 c y c l e / s e c i n the temperature range of 100 to 300°K. Two damping measurements were made every 5°. RESULTS AND DISCUSSION Sol-Gel and Weight Change S t u d i e s : Studies of i n t r i n s i c v i s c o s i t y and per cent s o l of f i l m s i r r a d i a t e d f o r up to 21 hours under the medium pressure mercury lamp showed evidence of extens i v e network formation, see Figure 1. These r e s u l t s i n d i c a t e that s i g n i f i c a n t c r o s s l i n k formation began i n the time between two and seven hours of exposure. By f i f t e e n hours the m a t e r i a l i n s o l u t i o n was of lower molecular weight than at seven hours. At twenty-one hours, g e l formation i s e s s e n t i a l l y complete. This time p e r i o d corresponds to about 840 hours exposure i n the QUV apparatus. Samples exposed i n the QUV showed an i n c r e a s e i n g e l content from 81% to 91% f o r periods ranging from 300 to 2500 hours.


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Changes i n g e l content (•) and i n the i n t r i n s i c v i s c o s i t y of the s o l f r a c t i o n (o) w i t h i r r a d i a t i o n time.


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I t i s reasonable to assume that random s c i s s i o n , unzipping and c r o s s l i n k i n g may be o c c u r r i n g simultaneously. We a t t r i b u t e decreases i n the i n t r i n s i c v i s c o s i t y to a r i s e i n the p o p u l a t i o n of low molecular weight polymer due t o chain s c i s s i o n and an i n crease i n the i n t r i n s i c v i s c o s i t y to a r i s e i n the p o p u l a t i o n of h i g h molecular weight polymer due to c r o s s l i n k i n g as a precursor to g e l formation. An estimate of c r o s s l i n k d e n s i t y was made from s w e l l i n g r a t i o s using the Flory-Huggins-Rehner equation w i t h an assumed value of 0.3 f o r the polymer-solvent i n t e r a c t i o n parameter, χ. T h i s simple c a l c u l a t i o n ignores any changes i n polymer-solvent i n t e r a c t i o n s during p h o t o o x i d a t i o n due to conversion of the nb u t y l e s t e r s i d e group to c a r b o x y l i c a c i d , a l c o h o l and a l d e hydes. The l o g a r i t h m of the mole f r a c t i o n of c r o s s l i n k s i s a l i n e a r f u n c t i o n of i r r a d i a t i o n time as shown i n Figure 2. The FTIR spectrum of unexposed PnBA i s shown i n Figure 3 w i t h assignments of the predominant bands. The carbonyl band i d e n t i f i e d i s that of the carbonyl i n the e s t e r s i d e group of the chain repeat u n i t . The COC s t r e t c h band i s that of the e s t e r s i d e group to the b u t y l group. The r e s u l t s of exposure of PnBA i n the QUV apparatus were measured by FTIR d i f f e r e n c e s p e c t r a ; t y p i c a l s p e c t r a are shown i n Figure 4. These s p e c t r a were obtained by computer subtrac t i o n of the spectrum of unexposed PnBA from the spectrum of PnBA taken a f t e r exposure. Peaks below the b a s e l i n e i n d i c a t e a decrease i n those f u n c t i o n a l groups w h i l e peaks extending above the b a s e l i n e i n d i c a t e a gain i n those groups. A q u a l i t a t i v e i n t e r p r e t a t i o n of the s p e c t r a i n d i c a t e s a p r o g r e s s i v e l o s s of e s t e r carbonyl and COC groups w i t h an i n crease i n another type of c a r b o n y l , assumed to be k e t o n i c type. This i s c o n s i s t e n t w i t h a c r o s s l i n k i n g mechanism i n v o l v i n g the conversion of the e s t e r s i d e group i n t o a k e t o n i c carbonyl c r o s s l i n k group p l u s f r e e b u t a n o l . I n s p e c t i o n of the s p e c t r a does show evidence f o r h y d r o x y l formation i n the range of 3000 to 3600 wavenumbers. This broad band i s g e n e r a l l y not s u i t a b l e for precise quantitative analysis. C a l c u l a t i o n of the areas of these major bands before and a f t e r UV treatment i n d i c a t e s t h a t the CH (2900 cm-1), C=0 (1736 cm- ), CH (1450 cm" ), COC (1170 cm" ) and n - b u t y l (1064 cm' ) bands are decreasing; e.g., by an average of n e a r l y 40% f o r the sample exposed f o r 1885 hours. However, the magnitude of the area under the f i n g e r p r i n t r e g i o n has changed very l i t t l e . This suggests that under the c o n d i t i o n s of p h o t o o x i d a t i o n , the major r e a c t i o n i s cleavage of the e s t e r carbonyl group from the main c h a i n , w i t h l o s s of CO2 and butene by evaporation. S p e c t r a l changes i n the carbonyl r e g i o n i n d i c a t e conversion of the e s t e r carbonyl to two or more other types of carbonyl group. The shoulder at 1713 cnf~l which i s evident i n the d i f ference s p e c t r a suggests the presence of c a r b o x y l i c a c i d and/or 1

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Figure 2) N a t u r a l l o g a r i t h m of the mole f r a c t i o n of c r o s s l i n k s (Ν ) as a f u n c t i o n of i r r a d i a t i o n time. χ


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ketone. The work of M c G i l l (2) i n d i c a t e s t h a t the c r o s s l i n k e r group i n i r r a d i a t e d PnBA i s a ketone. The shoulder at 1785 cm"! may be due t o low c o n c e n t r a t i o n s of a c i d anhydride, l a c t o n e , or to an i n t e r c h a i n e s t e r . I t i s i n t e r e s t i n g to note that the c a r bonyl s t r e t c h band f o r γ-butyrolactone i s l o c a t e d at 1773 cnr~l, f a i r l y c l o s e to the new carbonyl band at 1785 cm~l i n the weathered PnBA. The carbonyl bands f o r p r o p i o n i c anhydride at 1819 and 1752 cm"" are both too f a r from the band at 1785 cm i n the weathered PnBA to account f o r i t . The conversion of e s t e r groups to products f o l l o w s apparent f i r s t order k i n e t i c s . An i n s p e c t i o n of F i g u r e 5 shows t h a t the r a t e of conversion of e s t e r decreases by a f a c t o r of about two f o r exposure times g r e a t e r than 1800 hours i n the QUV. The c o n c e n t r a t i o n of h y d r o x y l groups formed was estimated by comparing the i n t e n s i t y of the 0-H s t r e t c h i n g bands centered at 3250 cm" to the i n t e n s i t y of the e s t e r carbonyl s t r e t c h band at 1735 cm" i n the untreated f i l m . The molar e x t i n c t i o n coef f i c i e n t s f o r the h y d r o x y l and carbonyl s t r e t c h bands were c a l c u l a t e d from the model s t u d i e s . This measure of the formation of h y d r o x y l groups, shown i n Figure 6, i n d i c a t e s an apparent l e v e l i n g o f f at times greater than about 2000 hours exposure. T h i s may be r e l a t e d to the change i n r a t e of e s t e r l o s s but the u n c e r t a i n t y of FTIR mea surement i n the r e g i o n of 3000 to 3600 cm does not permit a more r i g o r o u s i n t e r p r e t a t i o n of the behavior. The r e l a t i v e amount of c a r b o x y l i c a c i d i n each f i l m was es timated from the c o n c e n t r a t i o n of bound p r o f l a v i n e h e m i s u l f a t e . Our data i n d i c a t e that the r a t e of conversion of e s t e r to a c i d i s constant f o l l o w i n g a l a g time of the order of 200 hours, Figure 7. The data i n d i c a t e t h a t about 0.6% of the o r i g i n a l e s t e r groups i n the PnBA f i l m weathered f o r 2500 hours were able to bind one molecule of p r o f l a v i n e h e m i s u l f a t e . Since there are s e v e r a l l i t e r a t u r e values f o r the b i n d i n g r a t i o , t h i s estimate of 0.6% corresponds to a minimum of 1.8% and a maximum of 12% of c a r b o x y l i c a c i d groups i n the f i l m . Since the f i l m d i d not s w e l l i n the experiments and p r o f l a v i n e i s a bulky molecule, i t i s q u i t e p o s s i b l e t h a t the dye d i d not penetrate the f i l m com p l e t e l y . Therefore, the a c t u a l content of c a r b o x y l i c a c i d may be higher. The c o n c e n t r a t i o n of aldehydes and ketones i n the p h o t o o x i d i z e d f i l m s was estimated from the extent of formation of the y e l l o w 2,4 dinitrophenylhydrazones. The r e s u l t s are presented i n Table I along w i t h the r e s u l t s of other measurements. The r e s u l t s of measurements of dynamic mechanical behavior u s i n g t o r s i o n b r a i d a n a l y s i s on samples exposed to the medium pressure mercury lamp f o r 0 and 160 hours are shown i n Figures 8 and 9. In Figure 8, f o r 0 hours, the l o g decrement spectrum shows two t r a n s i t i o n peaks. The more i n t e n s e peak at higher temperature corresponds t o the g l a s s t r a n s i t i o n temperature. The lower temperature peak i s not now assigned, but may be r e l a t 1



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Photodegradation of Poly(n-butyl acrylate) - American

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