Photophysical Studies of Spin-Cast Polymer Films - American

Chapter 7

Photophysical Studies of Spin-Cast Polymer Films Downloaded by NORTH CAROLINA STATE UNIV on December 13, 2012 | http://pubs.acs.org Publication Date: December 13, 1989 | doi: 10.1021/bk-1989-0381.ch007

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2,4

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L. L. Kosbar , S. W. J. Kuan , C. W. Frank , and R. F. W. Pease 1

Department of Chemistry, Stanford University, Stanford, CA 94305 Department of Chemical Engineering, Stanford University, Stanford, CA 94305 Department of Electrical Engineering, Stanford University, Stanford, CA 94305 Spin casting is a commonly used technique for producing thin, uniform polymer films, especially for semi-conductor fabrication. There has been little attention given to the response of the polymers at the molecular level, however. We have reviewed previous literature on the spin casting process, as well as pertinent aspects of solvent cast and oriented polymer films. Our experimental work has focused on the use of excimer fluorescence to elucidate polymer chain conformation and fluorescent probe environment. The effects of spin speed and polymer molecular weight on spin cast polystyrene films and subsequent annealing behavior have been studied. In addition, the aggregation of a small molecule dye (pyrene) in novolac films was investigated. 2

3

The U.S. - Australia Symposium on Radiation Effects on Polymeric Materials contained research presentations on fundamental radiation chemistry and physics as well as on technological applications of polymer irradiation. This paper represents a hybrid contribution of these two areas, examining a field of extensive technological importance. Spin casting of radiation sensitive polymer resists for microelectronic fabrication was studied using photophysical techniques that are sensitive to the fundamental radiation response in the ultraviolet range. Thin, uniform polymer films can be formed by casting a polymer solution onto a rotating disc. Investigations of spin casting have, to date, focused on the process at a macroscopic level. Of primary concern has been the ability to predict the final film thickness and any radial dependence of the thickness induced by spinning. Various models have been derived to relate the spinning and solution characteristics to film thickness and uniformity. These parameters are of very practical importance in the present applications of spin casting, especially for resists. Address correspondence to this author.

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0097-6156/89/0381-O095$06.00/0 1989 American Chemical Society

In The Effects of Radiation on High-Technology Polymers; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

Downloaded by NORTH CAROLINA STATE UNIV on December 13, 2012 | http://pubs.acs.org Publication Date: December 13, 1989 | doi: 10.1021/bk-1989-0381.ch007

96

EFFECTS OF RADIATION ON HIGH-TECHNOLOGY POLYMERS

The i n t e n t o f our i n v e s t i g a t i o n s i s t o s t u d y the e f f e c t s o f s p i n c a s t i n g on polymers a t a m o l e c u l a r l e v e l . The polymers may be r a d i a t i o n s e n s i t i v e m a t e r i a l s such as p o l y ( m e t h y l methacrylate) (PMMA), p o l y s u l f o n e s , o r p o l y s t y r e n e d e r i v a t i v e s , o r r a d i a t i o n i n s e n s i t i v e m a t e r i a l s such as n o v o l a c o r pure p o l y s t y r e n e . In a l l c a s e s , p h o t o p h y s i c a l t e c h n i q u e s w i l l be u s e d t o i n t e r r o g a t e the polymer s t r u c t u r e and f i l m homogeneity ( f o r the f i l m s w h i c h a r e m i x t u r e s o f a r a d i a t i o n i n s e n s i t i v e polymer and a r a d i a t i o n s e n s i t i v e m a t e r i a l ) . There has been l i t t l e e f f o r t g i v e n t o u n d e r s t a n d i n g the e f f e c t s o f s p i n c a s t i n g on c o n f i g u r a t i o n o f polymer c h a i n s up u n t i l now. Indeed, f o r c i r c u i t g e o m e t r i e s above the sub-micron regime, i t i s l e s s i m p o r t a n t because any s m a l l i n c o n s i s t e n c i e s o r i n h o m o g e n e i t i e s i n the r e s i s t f i l m are w i t h i n the a l l o w a b l e t o l e r ances. But as image dimensions creep down i n t o the 0.5 range, and t o l e r a n c e s a r e reduced i n t o the r e a l m o f polymer d i m e n s i o n s , the b e h a v i o r o f the f i l m on the m o l e c u l a r l e v e l may p l a y an i m p o r t a n t r o l e i n e n h a n c i n g r e s i s t c o n t r o l and r e p r o d u c i b i l i t y . S p i n c a s t i n g a p p l i e s r a d i a l s t r e s s t o the polymer f i l m as the s o l v e n t evaporates. As a r e s u l t , i t i s l i k e l y t h a t the polymer c h a i n s w i l l be f r o z e n i n t o s t r e t c h e d , n o n - e q u i l i b r i u m c o n f i g u r a t i o n s . R a d i a l o r i e n t a t i o n and s t r e s s i n the polymer backbone c o u l d s l i g h t l y change the response o f the polymer t o r a d i a t i o n o r s o l v e n t developers. R e l e a s e o f the s t r e s s may appear i n the form o f m i c r o c r a c k i n g (1) o f the f i l m , o r l o s s o f a d h e s i o n w i t h the s u b s t r a t e ( 2 ) . R e s i s t f i l m s g e n e r a l l y r e c e i v e a p o s t s p i n n i n g bake a t a t e m p e r a t u r e w h i c h may be above o r below the g l a s s t r a n s i t i o n tempera t u r e (Tg) o f the polymer, depending on the r e s i s t . Many r e s i s t s , such as most p o s i t i v e d i a z o q u i n o n e r e s i s t s are too t h e r m a l l y s e n s i t i v e t o be baked above the Tg o f the polymer. The bake i s i n t e n d e d t o remove any r e m a i n i n g t r a p p e d s o l v e n t and t o r e l a x s t r e s s e s i n d u c e d by s p i n n i n g , b u t t h e r e has been no thorough s t u d y o f the e f f e c t i v e n e s s o f commonly u s e d b a k i n g c o n d i t i o n s . A g g r e g a t i o n o f i n d i v i d u a l components o f the r e s i s t d u r i n g s p i n n i n g c o u l d change the development c h a r a c t e r i s t i c s o f the r e s i s t , the e f f e c t o f w h i c h would be dependent on the s i z e o f the a g g r e g a t e d domains. I n p a r t i c u l a r , some r o u g h e n i n g o f the image s i d e w a l l s o r o v e r a l l d i m e n s i o n a l v a r i a t i o n w o u l d be e x p e c t e d . I n t r i n s i c and e x t r i n s i c f l u o r e s c e n c e can be u s e d as v e r y s e n s i t i v e probes o f the polymer environment and c o n f i g u r a t i o n . A p a r t i c u l a r example i s excimer f l u o r e s c e n c e , w h i c h can o c c u r when two a r o m a t i c groups i n t e r a c t i n a c o p l a n a r s t r u c t u r e . The r i n g s must be w i t h i n t h r e e t o f o u r angstroms t o produce a s u i t a b l e excimer f o r m i n g s i t e , so the e x c i m e r f l u o r e s c e n c e y i e l d s i n f o r m a t i o n on the l o c a l c o n c e n t r a t i o n o f t h e s e p r o p e r l y o r i e n t e d a r o m a t i c chromophores. Exc i m e r f o r m a t i o n may be i n t e r m o l e c u l a r i f the two a r o m a t i c r i n g s a r e on d i f f e r e n t polymer c h a i n s , o r i n t r a m o l e c u l a r i f t h e y are on r e p e a t u n i t s o f the same polymer c h a i n . Changes i n the r a t i o o f e x c i m e r t o monomer f l u o r e s c e n c e i n t e n s i t y (Ie/Im) o f a r o m a t i c polymers such as p o l y s t y r e n e have been used t o measure the l e v e l o f o r i e n t a t i o n i n a u n i a x i a l l y s t r e t c h e d f i l m ( 3 ) . T h i s t e c h n i q u e can be a p p l i e d t o the r a d i a l o r i e n t a t i o n caused by s p i n c a s t i n g . The f l u o r e s c e n c e o f probes such as pyrene can a l s o y i e l d a v a r i e t y o f i n s i g h t s i n t o the e f f e c t s o f s p i n c a s t i n g . Probes t h a t a r e c h e m i c a l l y a t t a c h e d t o the polymer c h a i n can y i e l d c o n f i g u r a t i o n a l i n f o r m a t i o n on o t h e r w i s e n o n - f l u o r e s c e n t polymers. A g g r e g a t i o n o f f r e e f l u o r e s c e n t probes


7.

KOSBAR ETAL.

Photophysical Studies ofSpin-Cast Polymer Films


can be m o n i t o r e d and used as a model system f o r polymer/dye type p o s i t i v e r e s i s t systems. We w i l l p r e s e n t the i n i t i a l r e s u l t s obt a i n e d u s i n g these f l u o r e s c e n t t e c h n i q u e s t o e v a l u a t e the e x t e n t o f o r i e n t a t i o n and dye a g g r e g a t i o n i n d u c e d by s p i n c a s t i n g and the e f f e c t i v e n e s s o f a n n e a l i n g t o remove o r i e n t a t i o n and s t r e s s i n polymer systems. Background on S p i n C a s t i n g . As e a r l y as 1958, E m s l i e , e t a l . (4) proposed a t h e o r e t i c a l treatment of s p i n c a s t i n g f o r n o n v o l a t i l e Newtonian f l u i d s . T h i s t h e o r y p r e d i c t e d t h a t f i l m s formed on a f l a t r o t a t i n g d i s c would have r a d i a l t h i c k n e s s u n i f o r m i t y . They p r e d i c t e d t h a t the f i n a l f i l m t h i c k n e s s w o u l d depend on s p i n speed (o>) and v i s c o s i t y (rj) as w e l l as o t h e r v a r i a b l e s such as l i q u i d d e n s i t y and i n i t i a l f i l m t h i c k n e s s . The dependence o f t h i c k n e s s on w and rj was a l s o r e c o g n i z e d by many o f the o t h e r a u t h o r s r e v i e w e d i n t h i s p a p e r , and t h e i r p r o p o s e d r e l a t i o n s h i p s a r e compared i n T a b l e I . A c r i v o s , e t a l . (5) extended the E m s l i e t r e a t m e n t t o the g e n e r a l case o f non-Newtonian f l u i d s , a c a t e g o r y i n t o w h i c h most polymers fall. A c r i v o s p r e d i c t e d t h a t non-Newtonian f l u i d s w o u l d y i e l d f i l m s w i t h non-uniform r a d i a l t h i c k n e s s . M e y e r h o f e r (6) i n c l u d e d the e f f e c t s o f s o l v e n t e v a p o r a t i o n d u r i n g s p i n n i n g i n h i s model, w h i c h he f o u n d t o be i n good agreement with experimental r e s u l t s f o r p h o t o r e s i s t f i l m s . Chen (7) a l s o r e c o g n i z e d the i m p o r t a n c e o f s o l v e n t e v a p o r a t i o n on f i n a l f i l m t h i c k n e s s . Chen p o i n t e d out t h a t as the s o l v e n t e v a p o r a t e s , i t w i l l c o o l the s o l u t i o n , c a u s i n g an i n c r e a s e i n v i s c o s i t y . He c l a i m e d t h a t the f i l m t h i c k n e s s s h o u l d be r e l a t e d t o the p h y s i c a l p a r a m e t e r s o f the s o l v e n t such t h a t the same t h i c k n e s s f i l m c o u l d be a t t a i n e d w i t h a low v i s c o s i t y s o l u t i o n o f a h i g h l y v o l a t i l e s o l v e n t o r w i t h a h i g h v i s c o s i t y s o l u t i o n o f a l o w - v o l a t i l i t y s o l v e n t . W e i l l (8) n o t e d t h a t the v i s c o s i t y o f a polymer s o l u t i o n i s dependent on b o t h the polymer c o n c e n t r a t i o n and m o l e c u l a r w e i g h t (MW). W e i l l found t h a t the t h i c k n e s s o f f i l m s c a s t from s e v e r a l s o l u t i o n s w i t h the same v i s c o s i t y b u t v a r y i n g MW i n c r e a s e d w i t h MW, a l t h o u g h no e x a c t r e l a t i o n between MW and f i n a l t h i c k n e s s was d e r i v e d . Jenekhe (9.10) a l s o i n c o r p o r a t e d the c h a n g i n g r h e o l o g i c a l prope r t i e s o f the d r y i n g f i l m i n t o h i s t h i c k n e s s model. He d e f i n e d an e x p e r i m e n t a l l y d e t e r m i n e d parameter (a) t h a t measures the timedependent change i n v i s c o s i t y due t o s o l v e n t e v a p o r a t i o n d u r i n g c a s t i n g , w i t h a-0 f o r n o n v o l a t i l e f l u i d s . A range o f a-0.44-3.00 was f o u n d t o a c c o u n t f o r p r e v i o u s e m p i r i c a l r e s u l t s f o r s p i n c a s t f i l m s . Jenekhe f o u n d t h a t many polymer s o l u t i o n s , i n c l u d i n g those o f i n t e r e s t t o the m i c r o e l e c t r o n i c s i n d u s t r y , can be d e s c r i b e d by the C a r r e a u non-Newtonian v i s c o s i t y e q u a t i o n , w h i c h was shown t o p r e d i c t r a d i a l l y uniform f i l m s a t s u f f i c i e n t l y long s p i n n i n g times. F l a c k , e t a l . (11) took the c h a n g i n g r h e o l o g i c a l p r o p e r t i e s i n t o a c c o u n t by a l l o w i n g v i s c o s i t y t o v a r y w i t h c o n c e n t r a t i o n and s h e a r r a t e d u r i n g s p i n n i n g . They d e t e r m i n e d t h a t c o n v e c t i v e r a d i a l f l o w dominated the c a l c u l a t e d t h i c k n e s s d u r i n g the e a r l y s t a g e s , and t h a t s o l v e n t e v a p o r a t i o n became more i m p o r t a n t i n the l a t e r s t a g e s o f the spinning process. They a l s o i n d i c a t e d t h a t a r a d i a l t h i c k n e s s dependence s h o u l d be e x p e c t e d i f a ramp o f the s p i n speed were used. B o r n s i d e , e t a l . (12) a r e a l s o a t t e m p t i n g t o s e p a r a t e the s p i n n i n g p r o c e s s i n t o d e p o s i t i o n , s p i n up, s p i n o f f , and e v a p o r a t i o n s t a g e s , and t o model these s t a g e s u s i n g fewer o f the s i m p l i f y i n g assumptions


97

98



employed by p r e v i o u s a u t h o r s . The model p r o m i s e s t o be v e r y complex. S e v e r a l a u t h o r s have t r i e d t o determine e m p i r i c a l r e l a t i o n s t h a t w i l l a c c u r a t e l y p r e d i c t the dependence o f f i n a l f i l m t h i c k n e s s on w, 17, and o t h e r parameters. The r e l a t i o n s h i p s o b t a i n e d by Damon (13), Daughton and G i v e n s ( 1 4 ) , and Malangone and Needham (15) are i n c l u d e d i n T a b l e I f o r comparison w i t h the p r e d i c t i o n s o f the t h e o r e t i c a l models.

Table I .

The P r e d i c t e d Dependence o f F i l m T h i c k n e s s on u> and rj a c c o r d i n g t o h oc u> ri

Reference E m s l i e (4)

n -1

m 0. 5

Damon (13) M e y e r h o f e r (6)

-0. 5 -0. 67

0. 33

Daughton

-0.,5-0.8

0..29

-0.,5 -0.,5

0.,33 0. 36

-2/(2+a)

l/(2+a)

Malangone Chen (7)

(14) (15)

Jenekhe (9)

(h)

Other V a r i a b l e s d e n s i t y , s p i n n i n g time, i n i t i a l thickness i n i t i a l concentration evaporation rate, i n i t i a l concentration s p i n n i n g time, i n i t i a l concentration i n i t i a l concentration evaporation r a t e , heat capacity, l a t e n t heat of evaporation spinning time, i n i t i a l t h i c k n e s s , index of v i s c o s i t y change (a)

I t i s o b v i o u s from the p r e v i o u s d i s c u s s i o n and T a b l e I t h a t t h e r e has been no g e n e r a l agreement on the e f f e c t o f w and rj on t h i c k n e s s , and even l e s s agreement on what o t h e r v a r i a b l e s a r e s i g n i f i c a n t i n d e t e r m i n i n g f i n a l f i l m t h i c k n e s s . W h i l e the exponent f o r v i s c o s i t y tends t o be about 0.3 i n most o f the models, the emp i r i c a l exponent o f s p i n speed v a r i e s from -0.5 t o -0.8. The v a r i o u s models a l s o g i v e d i f f e r e n t r e l a t i v e w e i g h t i n g t o the two p a r a m e t e r s . The v a r i e t y o f o t h e r v a r i a b l e s t h a t were i n c l u d e d i n the models i n d i c a t e t h a t t h e r e i s s t i l l a g r e a t d e a l o f disagreement as t o what the c r i t i c a l parameters f o r s p i n c a s t i n g a r e . The macros c o p i c p r o p e r t i e s o f s p i n c a s t f i l m s a r e d i f f i c u l t t o p r e d i c t from f i r s t p r i n c i p l e s , and do n o t l e n d themselves t o e m p i r i c a l a n a l y s i s e i t h e r . Daughton and Givens (14) found d i f f e r e n t e m p i r i c a l r e l a t i o n s t o a p p l y f o r d i f f e r e n t polymer systems. These s i m p l e models a r e g i v i n g way t o much more c o m p l i c a t e d , m u l t i - s t e p models, b u t t h e r e i s o b v i o u s l y s t i l l much t h a t i s n o t u n d e r s t o o d about the "simple" macroscopic aspects of s p i n c a s t i n g . R e s i s t systems may be more c o m p l i c a t e d t h a n j u s t a s i n g l e polymer i n a s i n g l e s o l v e n t . They may be composed o f polymer, polymer/dye, o r polymer/polymer c o m b i n a t i o n s (where the s m a l l molec u l e dye o r a d d i t i o n a l polymer i n c r e a s e s the r a d i a t i o n s e n s i t i v i t y o f the r e s i s t f i l m ) w i t h one o r more s o l v e n t s . The more c o m p l i c a t e d polymer/dye o r polymer/polymer systems have the added p o s s i b i l i t i e s o f phase s e p a r a t i o n o r a g g r e g a t i o n d u r i n g the n o n - e q u i l i b r i u m c a s t ing process. Law (16) i n v e s t i g a t e d the e f f e c t s o f s p i n c a s t i n g on a



7. KOSBARETAL.

Photophysical Studies ofSpin-Cast Polymer Films

s m a l l m o l e c u l e dye i n s o l u t i o n w i t h p o l y m e r s . He f o u n d a s l i g h t i n c r e a s e i n t h e a g g r e g a t i o n o f t h e dye a t h i g h dye c o n c e n t r a t i o n (>40%) and low s p i n speed. Law s u g g e s t e d t h a t s p i n n i n g c o n d i t i o n s c o u l d a f f e c t t h e k i n e t i c a l l y c o n t r o l l e d a g g r e g a t i o n o f polymer/dye systems w i t h poor c o m p a t i b i l i t y . Even a t t h e i r b e s t , t h e models a r e a b l e t o p r e d i c t o n l y macros c o p i c p r o p e r t i e s o f t h e f i l m s , y i e l d i n g no i n f o r m a t i o n on m i c r o s c o p i c parameters t h a t may a f f e c t r e s i s t performance. I t i s h i g h l y p r o b a b l e t h a t s p i n c a s t i n g i n d u c e s some s t r u c t u r e o r p r e f e r e n t i a l c h a i n o r i e n t a t i o n i n t o t h e f i l m s , o r causes s e c o n d a r y e f f e c t s such as t h e a g g r e g a t i o n o b s e r v e d by Law. These e f f e c t s a r e b a r e l y add r e s s e d i n t h e c u r r e n t l y a v a i l a b l e l i t e r a t u r e . However, some e a r l i e r works (3.17-19) on s o l v e n t ( s t a t i c ) c a s t f i l m s have i n v e s t i g a t e d t h e m o l e c u l a r o r i e n t a t i o n o f polymer c h a i n s as w e l l as c h a i n r e l a x a t i o n due t o t h e r m a l a n n e a l i n g . P r e s t and Luca (17) found t h a t t h e s o l v e n t - c a s t i n g p r o c e s s p r e f e r e n t i a l l y a l i g n e d p o l y m e r i c c h a i n s i n t h e p l a n e o f t h e f i l m , as measured b y t h e o p t i c a l a n i s o t r o p y o f t h e f i l m s . Cohen and R e i c h (18) r e p o r t e d t h a t f i l m s c a s t from monodisperse l o w m o l e c u l a r w e i g h t (< 30,000) p o l y s t y r e n e e x h i b i t h i g h o r d e r i n g c l o s e t o t h e s u b s t r a t e , b u t t h i s o r d e r i n g decays w i t h i n 1 pm from t h e s u b s t r a t e . F i l m s c a s t from monodisperse h i g h m o l e c u l a r w e i g h t p o l y s t y r e n e (> 30,000), on the o t h e r hand, e x h i b i t a much s m a l l e r , b u t v e r y l o n g - r a n g e (-10 /im) degree o f o r d e r . C r o l l (19) s t u d i e d t h e f o r m a t i o n o f s t r e s s e s i n s o l v e n t c a s t f i l m s as t h e s o l v e n t e v a p o r a t e s . The t h i c k n e s s o f t h e f i l m w i l l decrease, but the area i s constrained by adhesion t o the s u b s t r a t e . Because o f t h i s c o n s t r a i n t , i n t e r n a l s t r e s s a r i s e s i n the p l a n e o f t h e c o a t i n g . He f u r t h e r c o n c l u d e d t h a t t h e r e s i d u a l i n t e r n a l s t r e s s i s independent o f d r i e d c o a t i n g t h i c k n e s s and i n i t i a l s o l u t i o n concentration. Polymer o r i e n t a t i o n has a l s o been s t u d i e d w i t h s t r e t c h e d polymer f i l m s . J a s s e and K o e n i g (20) s t u d i e d u n i a x i a l l y oriented polystyrene f i l m s using Fourier transform i n f r a r e d s p e c t r o s c o p y and c o n c l u d e d t h a t t h e o r i e n t a t i o n p r o c e s s produces a l i g n m e n t o f t h e c h a i n s as w e l l as an i n c r e a s e i n t h e amount o f t r a n s c o n f o r m a t i o n a l segments. We b e l i e v e t h a t t h e p h o t o p h y s i c a l approach d e s c r i b e d i n t h i s paper w i l l a l l o w us t o i n v e s t i g a t e s i m i l a r microscopic properties of spin cast f i l m s . EXPERIMENTAL M a t e r i a l s . P o l y s t y r e n e samples o f m o l e c u l a r w e i g h t s 10,000, 50,000, 300,000 and 600,000 were o b t a i n e d from P o l y s c i e n c e s I n c . ; t h e y a l l have narrow m o l e c u l a r w e i g h t d i s t r i b u t i o n s , w i t h Mw/Mn < 1.1. They were p u r i f i e d by t h r e e p r e c i p i t a t i o n s from t e t r a h y d r o f u r a n (THF) i n t o methanol. The n o v o l a c sample was p r o v i d e d b y Kodak, and was s y n t h e s i z e d from pure m e t a - c r e s o l and formaldehyde. The m a t e r i a l was q u i t e p o l y d i s p e r s e w i t h Mw- 13,000 and Mw/Mn= 8.5, as measured by g e l p e r m e a t i o n chromatography (GPC) w i t h p o l y s t y r e n e s t a n d a r d s . The m a t e r i a l was p u r i f i e d by t h r e e p r e c i p i t a t i o n s from THF i n t o hexane. The pyrene and pyrene b u t y r i c a c i d (PBA) were r e c r y s t a l l i z e d from t o l u e n e p r i o r t o use. The THF was d r i e d b y r e f l u x i n g and d i s t i l l i n g o v e r sodium and benzophenone. S p e c t r a l grade d i o x a n e ( A l d r i c h ) , THF ( A l l t e c h ) , 2-methoxyethyl e t h e r ( A l d r i c h ) , and r e a g e n t grade methanol ( A l d r i c h ) were used.


99


100


Pvrene Tagging. The tagged n o v o l a c m a t e r i a l s were p r e p a r e d by the r e a c t i o n o f pyrene b u t y r i c a c i d c h l o r i d e (PBA-C1) w i t h the polymer. T h i s r e a c t i o n c o v a l e n t l y bonds the pyrene b u t y r i c a c i d t o the novol a c by f o r m i n g an e s t e r l i n k a g e t h r o u g h the p h e n o l i c group on t h e n o v o l a c . The PBA-C1 was s y n t h e s i z e d by r e a c t i n g pyrene b u t y r i c a c i d (3.42 g, 11.8 mmol) w i t h o x a l y l c h l o r i d e (17.5 m l , 20.0 mmol) i n d i c h l o r o m e t h a n e (75 ml) a t room t e m p e r a t u r e , and s t i r r e d o v e r n i g h t . The s o l v e n t and e x c e s s o x a l y l c h l o r i d e were p u l l e d o f f under vacuum. The c r u d e p r o d u c t was d r i e d under vacuum f o r two h o u r s . To o b t a i n t h e most h i g h l y tagged sample, the crude PBA-C1 (2.0 g, -6.5 mmol) was d i s s o l v e d i n d r y THF (15 ml) and added s l o w l y t o a s t i r r e d s o l u t i o n o f n o v o l a c (2.1 g, 17.7 mmol monomer) and p y r i d i n e (10 m l ) , as a b a s e , i n d r y THF (50 m l ) . The r e a c t i o n was p e r f o r m e d under n i t r o gen. The m i x t u r e was r e f l u x e d f o r t h r e e h o u r s , and t h e n s t i r r e d a t room t e m p e r a t u r e o v e r n i g h t . The s o l u t i o n was washed t w i c e w i t h 10% aqueous HC1 and t w i c e w i t h w a t e r . The o r g a n i c l a y e r was d r i e d o v e r MgSO^, t h e polymer was p r e c i p i t a t e d t h r e e t i m e s from THF/hexane, and the p r o d u c t d r i e d i n vacuum f o r 24 h o u r s . Four samples tagged w i t h l o w e r l e v e l s o f pyrene were p r e p a r e d i n a s i m i l a r manner u s i n g 1.2 g o f n o v o l a c w i t h 0.2, 0.12, 0.05, o r 0.02 g o f PBA-C1 and 0.5-1 ml o f N , N - d i i s o p r o p y l e t h y l a m i n e , w h i c h i s a b e t t e r base t h a n p y r i d i n e . About 64-75% o f the polymer was r e c o v e r e d a f t e r the r e a c t i o n s . To ensure t h a t the pyrene was a t t a c h e d t o the polymer c h a i n , a sample o f the p u r i f i e d polymer was a n a l y z e d by GPC w i t h tandem UV and r e f r a c t i v e i n d e x d e t e c t o r s . There was no d e t e c t a b l e (

Photophysical Studies of Spin-Cast Polymer Films - American

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