Photosensitized Crosslinking of Polymers - ACS Publications

5 Photosensitized Crosslinking of Polymers ZDZISLAW

HIPPE

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 30, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch005

I. Lukasiewicz Technical University, 39-959 Rzeszów, Poland

Synopsis.Some r e s u l t s of broad research project on the photosensitized c r o s s l i n k i n g of p o l y ( v i n y l buty ral) and unsaturated polyester r e s i n , are presented. Various aspects of data obtained are b r i e f l y discussed. Introduction. I t is the purpose of t h i s a r t i c l e to report the present status of a research program which has as its goal photoinduced c r o s s l i n k i n g of va rious i n d u s t r i a l polymers. The r e s u l t s received f o r p o l y ( v i n y l butyral) and f o r unsaturated polyester r e s i n i n mentioned project, are dealt with. Photosensitized c r o s s l i n k i n g of polymers has been the subject of numerous publications [1 - 30] , concer ned mainly with poly(ethylene) , p o l y ( v i n y l a l c o h o l ) , various v i n y l copolymers, copolymers of maleic anhy dride and/or p h t a l i c anhydride with styrene and some polymers derivated from cinnamic a c i d . The following compounds were used as s e n s i t i z e r s : benzophenone, 4chloro- and 4,4-dimethylbenzophenone [1, 3-6, 8, 9] , -and β - d e r i v a t i v e s of anthraquinone [3, 23] aceto phenone, hydroquinone, triphenylmethane and p y r i d i n e [4] , chlorobenzene and no l e s s than t r i c h l o r i n a t e d n - p a r a f f i n s [6] , a complex of zink c h l o r i d e with o-dia n i z i d i n e [11], potassium bichromate [12] , anthracene [13, 14] 2,5-methoxy-4-amino-trans-stilbene [15], ben zylideneacetophenone [16-18] ,ω -thiophenylacetophenone, ω -thiophenyl-ω -phenylacetophenone, thiophenyl carbo nate and p,p-disubstituted dibenzoyldisulphide [19], / Note. Some parts of presented i n v e s t i g a t i o n s were c a r r i e d out at the Paint Research I n s t i t u t e , 44-100 Gliwice, Poland.

52 Labana; Ultraviolet Light Induced Reactions in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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b e n z i l m o n o a c e t a l 12JL^_^2^ ] h y d a n t o i n e 124 ], rhodamine ?25Ί, t r i p h e n y l p h o s p h i n e [26] , t r i e t h y l a l u m i n i u m [27.J , ferrocene + carbon t e t r a c h l o r i d e 2£ , t i t a n o c e n e d i c h l o r i d e [29] and some d o n o r - a c c e p t o r complexes [ 3 0 ] ; of t h e s e s u b s t a n c e s , benzophenone, a n t r a q u i n o n e and t h e i r d e r i v a t i v e s have a t t r a c t e d most a t t e n t i o n . r


Poly(vinyl Butyral) E x p e r i m e n t a l . T h i n f i l m s o f i n v e s t i g a t e d polymers, c o n t a i n e d about 125 χ 1 0 ~ mole o f s e n s i t i z e r p e r 100g of pure polymer, were spread o u t on g l a s s s u b s t r a t e . For c r o s s l i n k i n g , l i g h t was used w i t h t h e r a d i a t i o n below 3l5m e l i m i n a t e d by g l a s s f i l t e r . The s o u r c e was 120W h i g h - p r e s s u r e Q-400 mercury lamp w i t h t o t a l effi c i e n c y o f 30 lumens/W. The spectrum o f i n c i d e n t beam c o n t a i n e d 254 (2.09), 302 (l.74) and 313m/i (3.48W) l i n e s . The i n c i d e n t l i g h t i n t e n s i t y measured by means o f Hatchard and P a r k e r t e c h n i q u e f 3 l ] was 18 χ 1 0 qu a n t a , c m . " s e c . " l ; t h e doses a p p l i e d d i f f e r e d a c c o r d i n g to aim o f t h e t e s t performed. I n c o n s t a n t temperature c r o s s l i n k i n g e x p e r i m e n t s , t h e temperatures o f t h e s p e c i mens d u r i n g exposure d i d n o t exceed 23°C - 2°C. The s e n s i t i z e r e f f i c i e n c y was f o l l o w e d by e s t i m a t i o n o f sol-gel r e l a t i o n (extraction with ethyl alcohol f o r 1 hour i n a S o x h l e t t a p p a r a t u s ) . IR - s p e c t r a o f t h e c r o s s l i n k e d polymer ( n o t c i t e d h e r e ) were r u n on UR-10 (c. Z e i s s , J e n a ) double-beam s p e c t r o m e t e r ; t h e r m a l p r o p e r t i e s o f t h e polymer were e s t i m a t e d w i t h t h e u s e H i t a c h i - P e r k i n - E l m e r c a l o r i m e t e r model DSC-1B; elastic modulus d e t e r m i n a t i o n was done on u l t r a s o n i c e l a s t o meter t h e t y p e 501, ( U n i p a n , P o l a n d ) ; s t u d i e s o f s u r f a c e topography were performed on e l e c t r o n m i c r o s c o p e t y p e UM-100A (u.S.S.R.j u s i n g samples p r e p a r e d by means of a two-stage r e p l i c a system [ p o l y v i n y l a l c o h o l chromium preshadowed c a r b o n r e p l i c a ] . A l l computations were done on t h e ODRA-1013 e l e c t r o n i c d i g i t a l computer E l w r o , Poland and, i n some c a s e s , on H e w l e t t - P a c k a r d model 9100A d e s k - t o p e l e c t r o n i c c a l c u l a t o r . 3

1 5

2

R e s u l t s and d i s c u s s i o n . I n t h e c o u r s e o f our s t u d i e s on l i g h t i n d u c e d c r o s s l i n k i n g o f p o l y ( v i n y l b u t y r a l ) , new c l a s s o f e f f e c t i v e p h o t o s e n s i t i z e r s , namely o r g a n i c s u l p h o x i d e s , was d i s c o v e r e d . f 4 l ] I t s h o u l d be noted, t h a t f o u r y e a r s a f t e r o u r i n v e n t i o n , t h e u s e diphenyl sulphoxide simultaneously with o-ethylbenzoine and H2O2 a s e f f e c t i v e s e n s i t i z e r f o r c r o s s l i n k i n g of polymers, was r e p o r t e d [23] . In t h e a l i p h a t i c s e r i e s o f s a t u r a t e d , s y m m e t r i c a l

Labana; Ultraviolet Light Induced Reactions in Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

U V L I G H T INDUCED REACTIONS IN


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and u n s y m m e t r i c a l s u l p h o x i d e s , t h e s e n s i t i z i n g a c t i o n r i s e d l i n e a r l y w i t h t h e SO mole f r a c t i o n i n t h e s e n s i t i z e r m o l e c u l e . T h i s f i n d i n g may be u n d e r s t o o d i f we consider the S-*»0 bond c h a r a c t e r . E x a m i n a t i o n o f the molar r e f r a c t i o n o r o f t h e p a r a c h o r l e a d s t o t h e c o n c l u s i o n t h a t i n the a l i p h a t i c sulphoxides the S 0 bond i s p r e d o m i n a n t l y a s i n g l e c o o r d i n a t e c o v a l e n t bond 32, 33 . S a t u r a t e d a l i p h a t i c s u l p h o x i d e s d i s p l a y n o t w i t h s t a n d i n g t h e c h a i n l e n g t h - a chromophoric ab s o r p t i o n near 210iryi , which i n d i c a t e s t h a t t h e S-^0 bond i s r a t h e r unchangeable i n c h a r a c t e r . T h e r e f o r e , a d e f i n i t e and c o n s t a n t i n f l u e n c e o f t h e s u l p h u r - o x y g e n bond on t h e s e n s i t i z e r e f f i c i e n c y may be e x p e c t e d . T h i s i n f l u e n c e d i m i n i s h e d as t h e SO mole f r a c t i o n i n the s u l p h o x i d e m o l e c u l e i s decreased. As r e g a r d the aroma t i c s u l p h o x i d e s , even though t h e s u l p h o x i d e s a r e not p l a n a r (and'H-IT c o n j u g a t i o n a c t u a l l y does not e x i s t ) , p o s s i b l y t h e p-d o r b i t a l i n t e r a c t i o n s may be i n v o l v e d [34], which r e s u l t e d i n a more r i g i d S-0 bond. This phenomenon may a l s o be e x p l a i n e d on t h e b a s i s of the i n d u c t i v e e f f e c t of the e l e c t r o n - withdrawing aromatic r i n g which reduc^es^ t h e c o n t r i b u t i o n o f t h e r e s o n a n c e form >S=0 -*~^ÎS-0~ and i n c r e a s e s t h e f o r c e c o n s t a n t o f t h e bond. T h i s was r e v e a l e d by t h e s p e c t r a l s h i f t o f t h e SO band i n t h e i n f r a r e d , i n a c c o r d a n c e w i t h the e l e c t r o n - a t t r a c t i n g a b i l i t y of the S-attached s u b s t i t u e n t s ( i n t r a n s i t i o n from n - a l k y l - S O - n - a l k y l t o Ph-SOPh t h e s p e c t r a l s h i f t o f t h e band 1008.0cm"" up t o 1042.0cm"" , was d i s c o v e r e d ) . I t seems t h a t i n t h e c a s e o f a r o m a t i c o r mixed s u l p h o x i d e s , t h e S-0 bond c h a r a c t e r w i l l be t h e most i m p o r t a n t f a c t o r d e t e r m i n i n g t h e s e n s i t i z e r e f f e c t i v e n e s s ; t h e mole f r a c t i o n o f t h e SO grouping i s here of secondary importance. Among t h e i n v e s t i g a t e d compounds, d i p h e n y l s u l p h o x i d e , t h e s t r u c t u r a l analogue o f c l a s s i c a l s e n s i t i z e r - benzophenone - was f o u n d t o be t h e most e f f e c t i v e . According to s e v e r a l authors [ l ] , e f f e c t i v e sen s i t i z e r s seem t o be s u b s t a n c e s t h a t decompose p h o t o c h e m i c a l l y t o f r e e r a d i c a l s . I t has a l r e a d y been found [8] t h a t , d u r i n g t h e p h o t o l y s i s o f benzophenone, t h e r a d i cals C H and ÇgHsÔO a r e p r o d u c e d , t h e l a t t e r decom p o s i n g p a r t l y t o C H and CO. T h i s f i n d i n g means i n f a c t t h a t i n t h e i r r a d i a t e d system, t h e c o n c e n t r a t i o n o f t h e s e n s i t i z e r d e c r e a s e s owing t o i t s g r a d u a l decom p o s i t i o n . Speaking o f benzophenone [9, 35] t h e t r i p l e t s t a t e f o r m a t i o n i s p o s t u l a t e d i n which t h e hydrogen atom i s a b s t r a c t e d from t h e polymer m o l e c u l e g i v i n g a m a c r o r a d i c a l [36], whereas benzophenone i t s e l f gives 1

1

6

5

6

5



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b e n z o p i n a c o l [37, 38] o r b e n z o h y d r o l [ 1 ] . In c o n t r a s t , o t h e r workers e.g. Schenck [39] expected t h a t the s e n s i t i z i n g agent would t r a n s f e r t h e absorbed energy on t h e polymer m o l e c u l e s i n i t i a t i n g t h e r e a c t i o n o f c r o s s l i n k i n g , without e n t e r i n g i n t o the r e a c t i o n i t s e l f , i . e. i t s m o l e c u l e s would be r e g e n e r a t e d a f t e r t e r m i n a t i o n o f t h e k i n e t i c c h a i n . The i n v e s t i g a t i o n s performed p r o ved, t h a t i n c o n d i t i o n s u s e d , t h e s u l p h o x i d e s a c t as thrue s e n s i t i z e r s f o r c r o s s l i n k i n g : the e x c i t a t i o n energy was t r a n s f e r r e d t o polymer m o l e c u l e s but t h e s e n s i t i z e r s d i d not change p h o t o c h e m i c a l l y t h e m s e l v e s , i . e . t h e y r e g e n e r a t e t h e i r i n i t i a l m o l e c u l e s when t h e k i n e t i c c h a i n was completed. The above p o s t u l a t e d v i e w may be w e l l assumed i n t h e c a s e o f s e n s i t a t i o n o f s o l i d s t a t e r e a c t i o n s s i n c e the p o s s i b i l i t y of recombination o f t h e r a d i c a l s formed o f a s e n s i t i z e r m o l e c u l e has been advanced t o a c c o u n t f o r t h e c a g e - e f f e c t o f F r a n c k - R a b i n o v i t s c h [40] . The p r o p e r t i e s o f t r i d i m e n s i o n a l network formed i n t h e p o l y ( v i n y l b u t y r a l ) on i r r a d i a t i o n were e s t i m a t e d by means o f u l t r a s o n i c and t h e r m a l methods. I t was found, f o r i n s t a n c e , t h a t t h e e l a s t i c modulus changed o v e r from 3 . 3 3 x l 0 dynes cm." to 4.03xl0 dynes cm" f g e i c o n t e n t i n c r e a s e from 0.082 up t o 0.923. Simultaneously, the thermal s t a b i l i t y of the c r o s s l i n ked polymer r i s e d from 167°C up t o 417°C. [42] In c o m p l e t i o n o f above mentioned s t u d i e s , t h e dependence between i n c i d e n t l i g h t i n t e n s i t y , I , and t h e c r o s s l i n k i n g y i e l d , was e s t i m a t e d . I t has been found, t h a t the dependence e x p l o r e d may be r e p r e s e n t e d by t h e e q u a t i o n : 1 0

2

1 0

2

o

r

a

(l)

g e l content

(% by wt.)

=(l.5I

a

+ 45.l) +

I - ^

2

Assuming t h a t t h e y i e l d f o r g e l f r a c t i o n i s p r o p o r t i o n a l t o t h e r a t e o f c r o s s l i n k i n g , i t seemed t h a t t h e o b s e r v a t i o n on p o s s i b i l i t y o f r e s o l v i n g t h e e x p e r i m e n t a l r e s u l t s i n t o terms p r o p o r t i o n a l t o I and l a / , p o i n t s o u t t h e d u a l mechanism o f p h o t o i n d u c e d c r o s s l i n k i n g o f p o l y ( v i n y l b u t y r a l ) . The o v e r a l l c h a r a c t e r o f t h e e q u a t i o n ( l ) s u g g e s t s t h a t t h e main r o l e i n the d i s c u s s e d r e a c t i o n p l a y s the r e l a t i o n : g e l con t e n t R Q

L

59

case.

Energy ab sorption a t Gel content i n t e r the i n c i d e n t v a l surf. quanta 20 g 0.00032X>-0.2400

quanta/g, f i l m t h i c k n e s s

- i n the Table I I .

R ^ R* c a s e . L

Energy ab sorption a t Gel content i n t e r the incident v a l surface, R quanta 20 Q

x l Q

280.8 561.6 842.4 1123.2 2246.4 4492.8

0.00l20X>-0.2400 quanta/g, f i l m t h i c

The r e s u l t s p r e s e n t e d proved o u r assumption t h a t t h e use o f S h u l t z ' s e q u a t i o n s , s h o u l d be i n some c i r c u m s t a n c e s d i s s u a d e d . Even S h u l t z h i m s e l f suggested i n f47] t h a t i n some c i r c u m s t a n c e s ( a s i n t h e c r o s s l i n k i n g o f m e t h y l m e t h a c r y l a t e - e t h y l e n e d i m e t h a c r y l a t e system) n e g a t i v e , p h y s i c a l l y u n r e a l X v a l u e s were o b t a i n e d . I n t h i s c a s e , a s p e c i a l f a c t o r ( e . g . - l ) has t o be used



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f o r making a change o f t h e s i g n . The o r i g i n o f t h e f a c t o r i s not q u i t e c l e a r . Equations ( 4 ) , ( 5 ) and ( δ ) were d e r i v e d under the assumption t h a t t h e a b s o r p t i o n c o e f f i c i e n t k remains c o n s t a n t d u r i n g t h e e x p o s u r e . I t may t h u s be supposed t h a t the d e r i v a t i o n o f r e l a t i o n between X and g e l f r a c t i o n c o n t e n t o f t h e polymer, w i t h t h e assumption that t h e a b s o r p t i o n c o e f f i c i e n t changes d u r i n g i r r a d i a t i o n may y i e l d u s e f u l r e s u l t s . I f t h e l i g h t absorption by polymer f i l m i s p r o p e r l y measured, t h e f o l l o w i n g formu l a may be d e r i v e d : ^

where g i s g e l f r a c t i o n c o n t e n t o f t h e polymer a f t e r energy R^^ has been a b s o r b e d ; R.^ - i n t e g r a l energy a b s o r p t i o n by t h e f i l m , and R. - i n t e g r a l energy a b s o r p t i o n , a t g e l a t i o n p o i n t . R e l a t i o n (7) was t e s t e d f o r p o l y v i n y l butyra]) , and r e c e n t l y , f o r o t h e r p o l y m e r s . R

Polyester Glycol i n

of M a l e i c / P h t a l i c Anhydrides Styrène V

and

Ethylene

E x p e r i m e n t a l . The g e n e r a l c o n d i t i o n s o f experime n t s , and methods o f i n v e s t i g a t i o n s were s i m i l a r to those d e s c r i b e d previously.However, extremely p r e c i s e and r e p r o d u c i b l e r e s u l t s o f c r o s s l i n k i n g were a c h i e v e d owing o r i g i n a l l y d e v e l o p e d d e v i c e f o r exposure ( F i g l) i n which t h e specimens may be r o t a t e d w i t h chosen r a t e . A d d i t i o n a l l y , t h e s e l e c t i o n o f U V - s e n s i t i z e r s was ext e n t e d , and b e s i d e s d i p h e n y l s u l p h o x i d e , a l s o d i n a p h t y l sulphoxide, methyl - benzyl sulphoxide, b i s - p - c h l o r o phenyl sulphoxide, b i s - 2,4-dichlorophenyl sulpho x i d e , were u s e d . A l s o , some o t h e r compounds were e x a mined, namely: d i p h e n y l s u l p h i d e and i t s p - n i t r o - p amino- as w e l l ο,ο-dicarboxy-derivatives, methiomethyl e n e - d i e t h y l phosphate and u r a n y l a c e t a t e .

T h i s p a r t o f work was done w i t h c o l l a b o r a t i o n Mrs. B. Guzowska and Mr. J . L i p i n s k i


of


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Figure 1. Device for controlled irradiation of polymerfilms—rotatingplate with specimens (a), source (b), ventilator (c), and electronics for controlling of light intensity, specimens temperature, and rate of rotations, (d)

R e s u l t s and d i s c u s s i o n . The most e f f e c t i v e sub s t a n c e was found t o be (among t h e S-compounds) d i p h e n y l s u l p h i d e . On t h e o t h e r s i d e , p-nitro-p-amino-diphenyl s u l p h i d e .showed n e a r l y complete l a c k o f a c t i v i t y . I n the group o f s u b s t i t u t e d s u l p h o x i d e s and s u l p h i d e s , may be n o t i c e d t h e dependence o f s e n s i t i z i n g e f f i c i e n c y on the e l e c t r o m e r i c e f f e c t s i n the molecule. A t present s t a t u s o f t h e work i t i s i m p o s i b l e t o p r e d i c t whether t h i s r e s u l t has as i t s own b a s i s an I - o r M - e f f e c t . However, d e c r e a s e o f s e n s i t i z i n g a b i l i t y o f d i s c u s s e d compounds may a l s o be c o n n e c t e d w i t h d i s t u r b a n c e of coplanarity of photosensitizers structures. High a c t i v i t y o f u r a n y l a c e t a t e as c r o s s l i n k i n g s e n s i t i z e r s h o u l d be n o t e d . P o s s i b l y , t h i s may be ex p l a i n e d from d i f f e r e n t p o i n t s o f v i e w : u r a n y l a c e t a t e b e l o n g s t o t h e compounds t h a t y i e l d e a s i l y t o p h o t o c e m i c a l e x c i t a t i o n , and a l s o u r a n y l a c e t a t e as r a d i o a c t i v e compound may produce i t s e l f f r e e r a d i c a l s i n neighboring polyester molecules. The d e t a i l e d i n v e s t i g a t i o n s a l o n g t h e s e l i n e s a r e now i n t h e c o u r s e .


UV

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IN

POLYMERS


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