Electronic Structure of Polymers - American Chemical Society

13 Electronic Structure of Polymers

Downloaded by UNIV OF MICHIGAN ANN ARBOR on March 27, 2017 | http://pubs.acs.org Publication Date: August 13, 1981 | doi: 10.1021/bk-1981-0162.ch013

X-Ray Photoelectron Valence Band Spectra J. J .

1

PIREAUX ,

J. R I G A , R . C A U D A N O , and J. V E R B I S T

Facultés Universitaires Notre-Dame de la Paix, Laboratoire de Spectroscopie Electronique, 61 rue de Bruxelles, 5-5000 Namur, Belgium

The first X-ray photoelectron spectra of polymers valence bands and core levels were recorded almost ten years ago (exactly in 1971 and 1972) for polytetrafluoroethylene (1), polyethylene (2), and nitroso rubbers (3). That same period was the beginning of an exploding interest of the spectroscopists for the technique better known under the acronyms XPS (X-ray Photoelectron Spectroscopy) and ESCA (Electron Spectroscopy for Chemical Analysis). Indeed, since that time, the technique has grown into a valuable method to study the electronic structure of materials, particularly polymers. But, contrary to all the other fields of research (on metals, semiconductors, compounds,...), applications to the polymeric materials were, until recently, almost only concerned with core levels analysis; in numerous series of polymers, emphasis was put on the study of the gross and fine chemical composition, the chemical bonds, the electronic charge distributions, of (copolymers as clean and pure materials, or after some reaction (aging, casing, adhesion, oxidation, sputtering, electrical or plasma discharge...). Until recently, the valence bands of polymers have received the attention of very few scientists: XPS studies of valence band spectra are still scarce; although they are measuring the less bound electrons of the materials, those that are directly i n volved in the bonds between the atoms of the molecules, and consequently those that are containing the best potential inform1

Chercheur Qualifie of the Fonds National de la Recherche Scientifique, Belgium. **Work performed under the auspices of the IRIS program-Institute of Research in Interface Science-supported by the Belgian Ministry for Science Policy. 0097-6156/81/0162-0169$08.25/0 © 1981 American Chemical Society Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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PHOTON, ELECTRON, AND ION PROBES

a t i o n t o u n d e r s t a n d the m o l e c u l a r s t r u c t u r e o f t h e p o l y m e r s . In f a c t , t h e r e c o r d i n g o f p o l y m e r v a l e n c e band s p e c t r a and the i n t e r p r e t a t i o n o f t h e s e d a t a r e q u i r e on one hand v e r y c a r e f u l and patient experimentation, on t h e o t h e r hand, the support of e l a b o r a t e and a c c u r a t e quantum m e c h a n i c a l c a l c u l a t i o n s a n d / o r t h e comparison w i t h other types o f e x p e r i m e n t a l d a t a , . . . i n order to e x t r a c t a l l the i n f o r m a t i o n h i d d e n b e h i n d the e x p e r i m e n t a l d a t a . I t a p p e a r e d r e c e n t l y t h a t XPS v a l e n c e band d a t a o f p o l y m e r s , t h a t were known t o c o n t a i n v a l u a b l e i n f o r m a t i o n r e l a t e d t o t h e s u b s t i t u e n t s e f f e c t s ( p r i m a r y s t r u c t u r e s ) , c o u l d a l s o be u s e d as p o w e r f u l - a n d sometimes u n i q u e , compared t o a c o r e l e v e l a n a l y sisf i n g e r p r i n t s t o c h a r a c t e r i z e as s u b t l e s y s t e m s as those c o n c e r n e d w i t h p u r e l y c a r b o n c o n t a i n i n g c h a i n s , i s o m e r i s m , and c r y s t a l l i n e s t r u c t u r e . . . what w i l l be c a l l e d the s e c o n d a r y s t r u c t u r e o f the p o l y m e r s . The p u r p o s e o f t h e p r e s e n t p a p e r i s t o r e v i e w t h i s f i e l d o f r e s e a r c h - a summary o f t h i s r e v i e w a p p e a r e d i n ( 4 ) . After a b r i e f i n t r o d u c t i o n ( p a r t 2) t o t h e XPS t e c h n i q u e i t s e l f , s p e c i a l l y o r i e n t e d to the study o f the polymer v a l e n c e bands, g i v e n i n o r d e r t o s t r e s s upon t h e i n h e r e n t d i f f i c u l t i e s ( a n d l i m i t a t i o n s ) o f t h e e x p e r i m e n t , we s h a l l p r e s e n t s p e c i f i c examples o f the v a r i o u s t y p e s o f i n f o r m a t i o n s t h a t c a n be a c q u i r e d on t h e p o l y m e r m o l e c u l a r and e l e c t r o n i c s t r u c t u r e ; ( p a r t 3) s u b s t i t u t i o n e f f e c t s i n the v a l e n c e bands; ( p a r t 4) s t r u c t u r a l i s o m e r i s m , s t e r e o i s o m e r i s m , and g e o m e t r i c a l c r y s t a l l i n e s t r u c t u r e ; ( p a r t 5) other types of i n f o r m a t i o n . Experiment I n v i e w o f t h e number o f m o n o g r a p h i e s , r e v i e w a r t i c l e s , . . . p u b l i s h e d on t h e XPS t e c h n i q u e , i t i s u s e l e s s t o d e s c r i b e h e r e i n l e n g t h a l l the d e t a i l s o f the e x p e r i m e n t . H e r e , we w i l l o n l y r e c a l l the e l e m e n t a r y concepts f o r the n o n - e x p e r t s i n the f i e l d and d i r e c t the r e a d e r s t o t h e l i t e r a t u r e f o r a c o m p l e m e n t a r y presentation i f necessary (.5). P a r t i c u l a r topics, that are p r o p e r t o the s t u d y o f p o l y m e r s w i l l be d e s c r i b e d i n g r e a t e r length. The XPS M e a s u r e m e n t . I n an XPS s p e c t r o m e t e r , t h e s t u d i e d m a t e r i a l i s e x p o s e d i n s i d e a vacuum chamber t o a f l u x o f X - r a y s ( e n e r g y 1 k e V ) . The k i n e t i c e n e r g y o f t h e p h o t o e l e c t r o n s e j e c t e d from t h e sample i s m e a s u r e d by an a p p r o p r i a t e a n a l y z e r . This energy i s d i r e c t l y r e l a t e d to the b i n d i n g energy o f the e l e c t r o n s i n s i d e t h e s a m p l e ; on a w i d e s c a n XPS s p e c t r u m , t h e u n s e a t t e r e d electrons r e s u l t i n c h a r a c t e r i s t i c peaks: t h e i r energies serve to i d e n t i f y the elements i n the m a t e r i a l ( a t o m i c c o m p o s i t i o n ) , to c h a r a c t e r i z e t h e m o l e c u l a r e n v i r o n m e n t o f t h e s e atoms ( c h e m i c a l a n a l y s i s , see i n s e t A o f F i g u r e 1 ) , a n d , by t h e measurement o f t h e p h o t o e l e c t r i c l i n e s r a t i o s , t o r e a c h some q u a n t i t a t i v e r e s u l t s . Such t y p e o f measurement from t h e c o r e l e v e l p e a k s c a n u s u a l l y be

Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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X-Ray Photoelectron Valence Band Spectra

Figure 1. Wide survey scan of an ethylene tetrafluoroethylene copolymer. Inset A represents the G-ls core level peaks; inset B pictures the copolymer valence band.



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a c h i e v e d r a t h e r a c c u r a t e l y i n a few t e n s o f m i n u t e s . make a few r e m a r k s o f i m p o r t a n c e : 1.


2.

3.

L e t us h e r e

ThegSample b e i n g s t u d i e d i n vacuum ( u s u a l l y b e t t e r t h a n 10 T o r r ) , i t must be a s o l i d , a n d , i f l i q u i d a t room temperature ( l i k e f o r pre-polymers w i t h low m o l e c u l a r w e i g h t ) , i t must be c o o l e d by some r e f r i g e r a n t c i r c u l a tion. T h i s l i m i t s a l s o the p o s s i b i l i t y to study h i g h t e m p e r a t u r e e f f e c t s on s u c h m a t e r i a l s . As t h e p h o t o e l e c t r o n s r e a c h i n g t h e a n a l y z e r e s c a p e from a 5 0 - 1 0 0 X l a y e r a t the t o p o f t h e s t u d i e d s o l i d , t h e technique is extremely surface sensitive, i.e. it c o u l d d e t e c t i m p u r i t i e s ( a c o m p o s i t i o n d i f f e r e n t from t h e b u l k ) at t h e e x t r e m e s u r f a c e o f the sample (6). G r e a t c a r e must t h e n be t a k e n i n t h e p r e p a r a t i o n and h a n d l i n g o f the samples to a v o i d s p u r i o u s c o n t r i b u t i o n s from c o n t a m i n a t i o n . T h i s drawback o f t h e t e c h n i q u e c a n be advantageously t u r n e d o u t to an a d v a n t a g e when s t u d y i n g ( i n an a n g l e r e s o l v e d e x p e r i m e n t ) r e a c t i o n s a t the s u r f a c e s , e . g . note t h a t the s u b s t a n t i a l d i f f e r e n c e o f escape depth for e l e c t r o n s o r i g i n a t i n g from the v a l e n c e l e v e l s compared t o c o r e l e v e l s may a l s o be u s e d t o o b t a i n i n f o r m a t i o n on a d e p t h p r o f i l e (7). I t h e l p s to the p r e c i s i o n o f the i n t e r p r e t a t i o n o f the s p e c t r a , t o use a m o n o c h r o m a t i z e d X - r a y s o u r c e t o i r r a d i a t e the sample. T h i s c a n be c o n v e n i e n t l y done f o r t h e A l K l i n e ( e n e r g y 1 4 8 6 . 6 e V ) , the a p p a r e n t w i d t h o f t h e p h o t o e l e c t r o n p e a k s b e i n g r e d u c e d by t h e use o f a monochromator from 0 . 9 to 0 . 6 e V ( i n t h e o r y ) . This h e l p s a l s o t o remove s p u r i o u s e x c i t a t i o n l i n e s and r e d u c e a p p r e c i a b l y the b a c k g r o u n d o f t h e r e c o r d e d s p e c tra. F o r a v a l e n c e band a n a l y s i s , t h e need t o use a monoc h r o m a t i z e d X - r a y source i s almost e v i d e n t ; the e x c i t a t i o n l i n e h a v i n g s a t e l l i t e s a t 10-12eV ( f o r A l K , w i t h a r e l a t i v e i n t e n s i t y o f 10%), c a n n o t p r o b e w i t h o u t d i s t u r b i n g the s p e c t r u m a v a l e n c e e l e c t r o n r e g i o n t h e w i d t h o f w h i c h r a n g e s from 20 t o 35eV ( f o r c a r b o n polymers, to s u b s t i t u t e d polymers) (11). It is also of p r i m e i m p o r t a n c e t o remove t h e B r e m s s t r a h l u n g c o n t i n u u m o f the r a d i a t i o n s o u r c e , t o i m p r o v e c o n s i d e r a b l y t h e s e n s i t i v i t y and r e l i a b i l i t y o f the f i n e d e t a i l s c o n t a i n e d i n the v a l e n c e bands. Being i n s u l a t o r s , the polymers present a p a r t i c u l a r p r o b l e m d u r i n g the XPS a n a l y s i s due t o t h e e j e c t i o n o f t h e p h o t o e l e c t r o n t h e i r s u r f a c e becomes ( s o m e t i m e s i n homogeneous l y ) c h a r g e d , what p e r t u r b s the measurements by d i s p l a c i n g t h e p o s i t i o n , and d i s t o r t i n g t h e shape o f the p h o t o e l e c t r o n peaks, and p r e v e n t i n g any d i r e c t c a l i b r a t i o n o f the e l e c t r o n b i n d i n g energy s c a l e . a

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R a d i a t i o n damage m i g h t o c c u r d u r i n g the s t u d y o f p o l y mers. I n our i n v e s t i g a t i o n s , o n l y the c h l o r i n e - c o n t a i n i n g compounds were found t o p r e s e n t a c o l o r c h a n g e , b u t no l o s s o f c h l o r i n e c o u l d be d e t e c t e d on t h e XPS core l e v e l spectra. The a d v a n t a g e s o f u s i n g a X - r a y s o u r c e i n s t e a d o f a UV lamp s t u d y the p o l y m e r s a r e many f o l d :


1.

2.

3.

4.

5.

W i t h X - r a y e x c i t a t i o n , i t i s p o s s i b l e to probe deeper i n b i n d i n g energy i n the m o l e c u l a r o r b i t a l s ( M . O . ) o f t h e v a l e n c e bands and a c c e s s o r i l y , t o l e t the user b e n e f i t o f the p o s s i b i l i t y o f a core l e v e l a n a l y s i s . I n t r i n s i c a l l y , a UV lamp has a much b e t t e r r e s o l u t i o n ( a few m e V ) , t h a t i n f a c t t u r n s o u t t o be v e r y u s e f u l t o study g a s e s . But i n a s o l i d , the m o l e c u l a r o r b i t a l s are g r o u p e d i n t o b a n d s , b r o a d e n e d by v i b r a t i o n a l e x c i t a t i o n s , and become u n r e s o l v e d so t h a t i t h a s b e e n p r o v e n t h a t a p h o t o e l e c t r o n v a l e n c e band s p e c t r u m r e c o r d e d w i t h a UV lamp do n o t o f f e r s u b s t a n t i a l advantages compared t o a XPS e x p e r i m e n t . The p h o t o e l e c t r i c c r o s s s e c t i o n f o r a g i v e n e l e c t r o n i c orbital varies with the energy o f the excitation source: on a UV v a l e n c e band s p e c t r u m , t h e c a r b o n 2p o r b i t a l s a p p e a r w i t h l a r g e r i n t e n s i t y t h a n t h e C2s ones. The r e v e r s e i s t r u e when a X - r a y e x c i t a t i o n source i s used: the c r o s s s e c t i o n i s g r e a t e r for M . O . w i t h major 2s c h a r a c t e r . T h i s enhances the i n t e n s i t y o f t h e 2s b a n d s t h a t a r e g e n e r a l l y s i m p l e r t o i n t e r p r e t t h a n t h e 2p o n e s . I t must be s a i d t h a t t h e c o m p a r i s o n b e t w e e n t h e two ( X - r a y and UV) e x c i t e d s p e c t r a c a n be o f g r e a t h e l p t o a s s i g n t h e s y m m e t r i e s o f the p r o b e d M . O . (compare e . g . p o l y s t y r e n e ( X - r a y ) o f ( 1 0 ) , and (UV) o f (9). When e x c i t e d w i t h a UV l a m p , the p h o t o e l e c t r o n s have a k i n e t i c e n e r g y r a n g e ( 0 - 4 0 e V ) where t h e e s c a p e d e p t h i s s m a l l ( ^ 5&) : t h e c o n t a m i n a t i o n p r o b l e m i s t h e n c r u c i a l i n t h i s case. On t h e c o n t r a r y , when e x c i t e d w i t h a X ray source, the e l e c t r o n s e m i t t e d from t h e v a l e n c e l e v e l s have a l a r g e r e s c a p e d e p t h , so t h a t t h e s u r f a c e s f e a t u r e s do n o t c o n t r i b u t e as c r u c i a l l y t o t h e v a l e n c e band s p e c t r u m t h a n t o the c o r e l e v e l p e a k s . F o r a v a l e n c e band a n a l y s i s , t h e need t o use a monoc h r o m a t i z e d X - r a y source i s almost e v i d e n t ; the e x c i t a t i o n l i n e h a v i n g s a t e l l i t e s a t 10-12eV ( f o r A l K , w i t h a r e l a t i v e i n t e n s i t y o f 10%), c a n n o t p r o b e w i t h o u t d i s t u r b i n g t h e s p e c t r u m a v a l e n c e e l e c t r o n r e g i o n the w i d t h o f w h i c h r a n g e s from 20 t o 35eV ( f o r c a r b o n p o l y m e r s , t o s u b s t i t u t e d p o l y m e r s ) (11). It is also of p r i m e i m p o r t a n c e t o remove t h e B r e m s s t r a h l u n g c o n t i n u u m o f the r a d i a t i o n s o u r c e , t o i m p r o v e c o n s i d e r a b l y t h e a



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s e n s i t i v i t y and r e l i a b i l i t y o f t a i n e d i n the v a l e n c e bands.

the

fine

details

con-

The R o l e o f t h e V a l e n c e B a n d . The v a l e n c e band r e p r e s e n t s only a v e r y s m a l l p a r t o f the p h o t o e l e c t r o n spectrum ( i n s e t B o f Figure 1): due t o t h e v e r y l o w p h o t o e l e c t r i c c r o s s - s e c t i o n s f o r e l e c t r o n s h a v i n g low b i n d i n g e n e r g i e s , t h e r e c o r d i n g o f one v a l e n c e band s p e c t r u m r e q u i r e s many h o u r s . I n s p i t e o f t h i s f a c t , i t i s i n p r i n c i p l e p o s s i b l e t o p e r f o r m on v a l e n c e band d a t a a similar ( e l e m e n t a l and c h e m i c a l ) a n a l y s i s t h a n on t h e core levels. B u t , i n a v a l e n c e b a n d , and f o r t h e same e n e r g y w i n d o w , the photoelectron lines are more numerous and generally a s s o c i a t e d w i t h i n bands. Why spend more t i m e f o r a v a l e n c e b a n d a n a l y s i s t o o b t a i n s i m i l a r r e s u l t s t h a n w i t h a XPS a n a l y s i s o f t h e c o r e l e v e l s ? T h i s e x p e r i e n c e i s n o t i n f a c t u n r e a l i s t i c , as t h e m o l e c u l a r o r b i t a l s a n a l y z e d i n a v a l e n c e band s p e c t r u m i n v o l v e those e l e c t r o n s t h a t are d i r e c t l y p a r t i c i p a t i n g i n the m o l e c u l a r b o n d s : i t w i l l be shown i n d e e d t h a t s u c h e x p e r i m e n t s c a n f u r n i s h o t h e r t y p e s o f i n f o r m a t i o n on t h e p o l y m e r m o l e c u l a r s t r u c t u r e , t h a t a r e n o t a v a i l a b l e from t h e c o r e l e v e l p h o t o e l e c t r o n s . Practical Details. To f a c e t h e p r o b l e m s and r e q u i r e m e n t s announced a b o v e , t h e s p e c t r a p r e s e n t e d h e r e and c h o s e n among t h e most r e c e n t c o n t r i b u t i o n s from the a u t h o r s l a b o r a t o r y , were o b t a i n e d under the f o l l o w i n g c o n d i t i o n s : 1

1.

The p h o t o e l e c t r o n s p e c t r a have been r e c o r d e d w i t h a H e w l e t t - P a c k a r d s p e c t r o m e t e r (HP5950A) u s i n g a monochromatized A l K r a d i a t i o n . The c h a r g i n g e f f e c t on the p o l y m e r s i s c o n t r o l l e d by t h e use o f an e l e c t r o n f l o o d - g u n whose c u r r e n t and e n e r g y a r e a d j u s t e d t o n e u t r a l i z e the s u r f a c e e l e c t r o s t a t i c charges: t h e c h a r g i n g e f f e c t become s t a b i l i z e d , but not a b s o l u t e l y compensated. This external e l e c t r o n s o u r c e does n o t a l l o w t h e r e f o r e t o m e a s u r e a b s o l u t e e n e r g i e s a u t o m a t i c a l l y , but h e l p s to p a r t i a l l y c a n c e l , o r h o m o g e n i z e t h e c h a r g i n g e f f e c t on the w h o l e i n s u l a t o r s u r f a c e , i n s u c h a way t h a t t h e r e c o r d e d s p e c t r a a r e much b e t t e r r e s o l v e d and v e r y s t a b l e on t h e e n e r g y s c a l e d u r i n g the l o n g a c c u m u l a t i o n runs o f the v a l e n c e band s p e c t r a . The p r e p a r a t i o n o f t h e p o l y m e r m a t e r i a l as a v e r y t h i n f i l m ( d e p o s i t e d from a s o l u t i o n o n t o a m e t a l l i c s u b s t r a t e ) was f o u n d t o be t h e most s a t i s f a c t o r y to r e d u c e the charging e f f e c t . A r e p r o d u c i b l e c a l i b r a t i o n o f t h e r e c o r d e d s p e c t r a was o b t a i n e d by m i x i n g t h e p o l y m e r w i t h a r e f e r e n c e compound h a v i n g s i m i l a r e l e c t r i c a l p r o p e r t i e s ( e . g . polye t h y l e n e , o r p o l y t r i f l u o r o e t h y l e n e ) , and s t u d y i n g t h e m i x t u r e u n d e r d i f f e r e n t s e t t i n g s o f t h e e l e c t r o n boma

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bardment s o u r c e . Absolute b i n d i n g energies, expressed r e l a t i v e l y to the Fermi l e v e l o f the s p e c t r o m e t e r are o b t a i n e d w i t h a p r e c i s i o n u s u a l l y o f the order o f 0 . 1 0 . 2 e V ( t h a t means t h a t s e v e r a l i n d e p e n d e n t measurements were c o n s i s t e n t w i t h each o t h e r to t h i s A E ) . 5. The r e c o r d i n g o f one v a l e n c e band s p e c t r u m r e q u i r e s t y p i c a l l y one o v e r n i g h t a c c u m u l a t i o n . B u t one v a l e n c e band s p e c t r u m i s r e c o g n i z e d as s a t i s f a c t o r y o n l y when a few s i m i l a r s p e c t r a have b e e n r e c o r d e d s e p a r a t e l y on d i f f e r e n t samples. P a r t o f the s p e c t r a p r e s e n t e d here ( F i g u r e s 7 , 8 , 9 , 1 0 , 12) were d i g i t a l l y s m o o t h e d ; t h e o t h e r s a r e shown as r e c o r d e d . 6. The p o l y m e r s were a l l s t u d i e d , w i t h o u t f u r t h e r c h e m i c a l t r e a t m e n t , as r e c e i v e d from t h e i r s o u r c e s , most g e n e r a l l y u n i v e r s i t i e s or i n d u s t r i a l r e s e a r c h l a b o r a t o r i e s , where i n t h e most c a s e s t h e y were c h a r a c t e r i z e d by t h e conventional techniques. L i k e f o r the c o r e l e v e l s , a v a l e n c e band p h o t o e l e c t r o n s p e c t r u m i s composed o f p e a k s or bands h a v i n g t h e following characteristics: the b i n d i n g energy (expressed r e l a t i v e l y to a r e f e r e n c e value); the i n t e n s i t y ; t h e p e a k / b a n d w i d t h and s h a p e . E a c h o f t h e s e c h a r a c t e r i s t i c s c a n be i n t e r p r e t e d t o i n f e r t h e m o l e c u l a r and e l e c t r o n i c s t r u c t u r e o f t h e p o l y m e r s . Comparison w i t h T h e o r e t i c a l C a l c u l a t i o n s . I t appears that the p o l y m e r v a l e n c e bands a r e ( v e r y ) d i f f i c u l t to interpret w i t h o u t the a i d o f a t h e o r e t i c a l b a s i s , o r a m o d e l , o r o f t h e use o f a r e f e r e n c e s p e c t r u m o b t a i n e d from a m o d e l compound. Indeed, Quantum C h e m i c a l t h e o r y i s nowadays a b l e t o c a l c u l a t e band s t r u c t u r e and d e n s i t y o f s t a t e s f o r p o l y m e r s , t o s i m u l a t e t h e l i m i t e d r e s o l u t i o n o f t h e s p e c t r o m e t e r , and t o m o d u l a t e t h e s e t h e o r e t i c a l d e n s i t y o f s t a t e s to account f o r the p h o t o i o n i z a t i o n c r o s s s e c t i o n s t h a t v a r y c o n s i d e r a b l y f o r v a l e n c e bands o f p o l y m e r s c o n t a i n i n g d i f f e r e n t t y p e s o f a t o m s , and e l e c t r o n s w i t h v a r i o u s symmetries. C o n s e q u e n t l y , one i s a b l e now t o p r e d i c t t h e o r e t i c a l l y the e n e r g i e s o f the v a r i o u s m o l e c u l a r o r b i t a l s , but a l s o t h e i r r e l a t i v e i n t e n s i t i e s . A s i t i s p r o v e n e l s e w h e r e (^12) and i n a n o t h e r r e v i e w at t h i s c o n f e r e n c e ( 1 3 ) , t h e s u p p o r t o f s u c h t h e o r e t i c a l r e s u l t s o f an a p p r e c i a b l e and a p p r e c i a t e d v a l u e i s t o e x t r a c t a l l the i n f o r m a t i o n t h a t a r e h i d d e n b e h i n d t h e fine d e t a i l s o f t h e v a l e n c e b a n d s o f p o l y m e r s . B u t we w i l l n o t d i s c u s s f u r t h e r t h i s t h e o r e t i c a l aspect o f the study o f the e l e c t r o n i c s t r u c t u r e o f t h e p o l y m e r s . The p u r p o s e o f t h e n e x t s e c t i o n s w i l l be t o show t h a t , i n s p i t e o f t h e t i m e and s p e c i a l c a r e r e q u i r e d t o a n a l y z e v a l e n c e band s p e c t r a , t h e p a r t i c u l a r and u n i q u e r o l e p l a y e d by t h e l o w e n e r g y o r b i t a l s t o d e s c r i b e t h e b o n d i n g o f t h e atoms w i t h i n t h e m o l e c u l e , makes i t w o r t h w h i l e t o u n d e r t a k e t h e i r s tudy.


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PHOTON, ELECTRON, AND ION PROBES Substitution Effects

i n t h e V a l e n c e Bands


To d i s c u s s t h e i n f o r m a t i o n a v a i l a b l e from XPS s p e c t r a o f p o l y m e r s , i t i s n e c e s s a r y f i r s t to u n d e r s t a n d t h e e l e c t r o n i c s t r u c t u r e o f t h e s i m p l e s t o f t h e s e compounds, p o l y e t h y l e n e ( P E ) , the spectrum o f which w i l l c o n s t i t u t e the b a s i s f o r the i n t e r p r e t a t i o n o f a l l t h e o t h e r s t u d i e d compounds. P o l y e t h y l e n e and O t h e r M o d e l Compounds. The p o l y e t h y l e n e XPS v a l e n c e band s p e c t r u m i s r e p o r t e d on F i g u r e 2 . Superimposed on a b a c k g r o u n d g e n e r a t e d by t h e s c a t t e r i n g o f t h e p h o t o e l e c t r o n s on t h e i r way o u t s i d e t h e s o l i d , two s t r u c t u r e s a r e r e s o l v e d on t h i s spectrum. The f i r s t one a t h i g h b i n d i n g e n e r g y has two p e a k s w i t h d i f f e r e n t m a g n i t u d e s a t 1 8 . 8 and 1 3 . 2 e V ; t h e s e c o n d s t r u c t u r e c o n t a i n s s e v e r a l f i n e components ( t h e y a r e j u s t e m e r g i n g from t h e s t a t i s t i c a l b a c k g r o u n d on a b e t t e r s p e c t r u m (JL5); t h e y a r e n o t e a s i l y d i s c e r n e d due t o t h e low i n t e n s i t y o f t h i s b a n d . A s a t i s f a c t o r y i n t e r p r e t a t i o n - i n agreement w i t h t h e t h e o r e t i c a l c a l c u l a t i o n s - o f t h i s v a l e n c e band i s t o a s s i g n t h e l o w e n e r g y s t r u c t u r e t o a C - H band ( w i t h C2p and H i s p o l y m e r o r b i t als), and the h i g h e n e r g y s t r u c t u r e s to t h e C - C b a n d ( w i t h a d o m i n a n t C2s c h a r a c t e r ) i n t h e p o l y m e r : t h e l e f t m o s t peak i s t h e bonding c o m b i n a t i o n o f the C2s-C2s o r b i t a l s , the r i g h t m o s t i s the a n t i b o n d i n g a n a l o g u e (14-, 15_, 1 6 ) . I n o r d e r t o b e t t e r u n d e r s t a n d t h i s v a l e n c e band s p e c t r u m , we found i t i n t e r e s t i n g t o i n v e s t i g a t e t h e e f f e c t o f o n e - d i m e n s i o n a l p e r i o d i c i t y on t h e b r o a d e n i n g o f e n e r g y l e v e l s ( i n an atom o r i n a m o l e c u l e ) , which g i v e s r i s e to the energy bands ( i n a s o l i d ) . Indeed, polymers e x i s t w i t h a wide p o s s i b l e range o f m o l e c u l a r w e i g h t ( M . W . ) - o r c h a i n l e n g t h s - ; c o u l d we t h i n k o f d i s t i n g u i s h ing polymers w i t h d i f f e r e n t M.W.'s? The a l k a n e s s e r i e s i s s u i t a b l e t o s t u d y s u c h an e f f e c t ; t h e s u c c e s s i v e m o l e c u l e s w i t h an i n c r e a s i n g number o f c a r b o n atoms i n t h e a l m o s t l i n e a r c h a i n a r e c o n s i d e r e d as p r o g r e s s i v e s t e p s i n t h e f o r m a t i o n o f an i d e a l one d i m e n s i o n a l s o l i d , p o l y e t h y l e n e ( 1 7 , 18). On the M o l e c u l a r O r b i t a l p o i n t o f v i e w , t h e e l e c t r o n i c l e v e l s o f c a r b o n i n c r e a s e i n d e n s i t y , t o f i n a l l y form a b a n d structure. S e l e c t e d s p e c t r a r e c o r d e d f o r the l i g h t e s t a l k a n e s a r e r e p o r t e d on F i g u r e 3 . A l l show d i s t i n c t C2s and C2p r e g i o n s s e p a r a t e d by an e n e r g y g a p ; t h e number o f l e v e l s i n t h e C - C ( C 2 s ) r e g i o n i s e q u a l t o t h e number o f c a r b o n atoms i n t h e a l k a n e c h a i n . As t h e s e l e v e l s a r e c o n f i n e d i n a l i m i t e d e n e r g y r a n g e , the s p a c i n g b e t w e e n e a c h o f them d e c r e a s e s and s o o n , a band w i l l b e formed. From w h i c h m i n i m a l c h a i n l e n g t h , w i l l t h e v a l e n c e band s p e c t r u m a p p r o x i m a t e t h e band s t r u c t u r e o f t h e i n f i n i t e s o l i d polyethylene? F i g u r e 4 shows t h a t t h e e s s e n t i a l f e a t u r e s o f t h e p o l y e t h y l e n e v a l e n c e band a r e a l r e a d y p r e s e n t i n t h e n - t r i d e c a n e s o l i d : t h e e x p e r i m e n t a l r e s o l u t i o n does n o t a l l o w t o r e s o l v e a l l



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Figure 3. XPS valence band spectra of the lightest alkanes: (from top to bottom and left to right) methane, ethane, propane, n-butane, n-pentane, n-nonane, successively. The measurements were performed in the gas phase.


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Figure 5.

XPS valence band spectra of the acenes (from top to bottom,): benzene, naphthalene, anthracene, and tetracene (20)



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t h e components o f t h e C2s b a n d , t h e w i d t h o f w h i c h ( 7 . 6 e V ) approaches the v a l u e measured f o r p o l y e t h y l e n e ( 7 . 7 e V ) . As we o b s e r v e d t h a t a dozen o f c a r b o n atoms i n t h e a l k a n e c h a i n i s s u f f i c i e n t to s i m u l a t e t h e e l e c t r o n i c p r o p e r t i e s o f p o l y e t h y l e n e , i t becomes e v i d e n t t h a t t h e XPS t e c h n i q u e , by t h e a n a l y s i s o f the v a l e n c e b a n d s , w i l l n o t be a b l e t o d i s t i n g u i s h between polymers w i t h d i f f e r e n t h i g h m o l e c u l a r w e i g h t s . B u t , t h e same e x p e r i m e n t p r o v e s on a n o t h e r hand t h a t t h e XPS technique i s not too short range s e n s i t i v e : i t i s shown t h a t t h e v a l e n c e band p h o t o e l e c t r o n s l e a v i n g t h e p o l y m e r sample b r i n g i n f o r m a t i o n on t h e e l e c t r o n i c s t r u c t u r e o f one " s l i c e " o f the p o l y m e r i c s k e l e t o n c o n t a i n i n g a t l e a s t a dozen c a r b o n a t o m s . Another experimental evidence for t h i s type o f l i m i t a t i o n i s p r o v i d e d by t h e XPS v a l e n c e band measurements o f t h e p o l y a c e n e s (benzene, naphthalene, anthracene, and t e t r a c e n e ) (19, 20). P a s s i n g f r o m b e n z e n e t o t e t r a c e n e ( s e e F i g u r e 5 ) , t h e number o f m o l e c u l a r o r i b i t a l s i n c r e a s e s and v e r y soon t h e C2s b a n d s ( p e a k s A , B , C) become l e s s r e s o l v e d . P a r t i c u l a r i n t h i s case i s the e v o l u t i o n o b s e r v e d f o r the TT e l e c t r o n s ( p e a k E ) t h a t p r o g r e s s i v e l y become more d e l o c a l i z e d ; t h i s e f f e c t is important t o be m e a s u r e d as i t i s d i r e c t l y r e l a t e d t o t h e e l e c t r i c a l p r o p e r t i e s o f the polyacenes. A t h i r d e x a m p l e o f t h i s e f f e c t i s e v i d e n c e d by a s t u d y o f t h e v a l e n c e band s t r u c t u r e s o f the s e r i e s o f n - p h e n y l m o l e c u l e s c o n v e r g i n g t o t h e p o l y m e r p - p o l y p h e n y l whose v a l e n c e band s p e c t r u m c a n be seen i n F i g u r e 6 ( 2 1 ) . T h i s experience confirms the r e s u l t s o b t a i n e d f o r t h e a l k a n e s and t h e p o l y a c e n e s . When one ( o r more) h y d r o g e n a t o m ( s ) o f t h e p o l y e t h y l e n e monomeric u n i t i s ( a r e ) r e p l a c e d by a h e t e r o a t o m , an a l i p h a t i c o r an a r o m a t i c g r o u p , m o d i f i c a t i o n s a r e i n d u c e d i n t h e v a l e n c e band s p e c t r u m ; new p e a k s , band s h i f t a n d / o r s p l i t t i n g , r e d i s t r i b u t i o n o f the e l e c t r o n i c p o p u l a t i o n among t h e m o l e c u l a r o r b i t a l s w i l l d e n o t e the new bonds c r e a t e d i n t h e m o l e c u l e . Similar effects w i l l be o b s e r v e d f o r t h e i n s e r t i o n o f h e t e r o a t o m ( s ) b e t w e e n two c a r b o n elements o f the polymer s k e l e t o n . H y d r o c a r b o n B a s e d P o l y m e r s . The s u b s t i t u t i o n o f one h y d r o g e n atom i n t h e - C ^ - C ^ - u n i t by some s h o r t c a r b o n c h a i n s i n d u c e s s u b t l e m o d i f i c a t i o n s i n the e l e c t r o n i c structure (molecular o r b i t a l s ) o f the p o l y m e r s . Though t h e s e m o d i f i c a t i o n s c a n n o t be e a s i l y e v i d e n c e d on t h e XPS c a r b o n I s c o r e l e v e l s p e c t r a , it a p p e a r s t h a t the XPS v a l e n c e band s t r u c t u r e s a r e much more s e n s i t i v e t o t h e s e s u b s t i t u t i o n s and t h a t t h e y become u n i q u e and r e a d a b l e f i n g e r p r i n t s o f t h e p o l y m e r s ( l f J , 2 2 ) . We w i l l n o t speak h e r e o f t h e C l s s h a k e - u p d a t a t h a t were r e v e a l e d u s e f u l to d i s t i n g u i s h b e t w e e n s a t u r a t e d and u n s a t u r a t e d bonds ( t h i s f i e l d w i t h v a r i o u s a p p l i c a t i o n s was r e c e n t l y r e v i e w e d ( 2 3 ) ) . Up to now, o n l y t h e v a l e n c e b a n d s o f t h e s i m p l e s t and c o m m e r c i a l l y most u s e d p o l y m e r s have b e e n r e c o r d e d , and t h e i r m o l e c u l a r o r b i t a l s assigned ( 1 0 ) ; these are p o l y p r o p y l e n e ( P P ) ,




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Figune 6.

and XPS valence band spectra of benzene (solid phase), polystyrene, p-poly phenyl (4)



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poly(ljbutene) (PB), poly(vinyl cyclohexane) (PVCH) f o r the s a t u r a t e d compounds, and p o l y ( s t y r e n e ) ( P S ) , and p o l y p h e n y l (PPH) for the compounds c o n t a i n i n g u n s a t u r a t e d bonds. Some o t h e r s e r i e s o f p o l y m e r s were a l s o s t u d i e d , b u t t h e s t r e s s was p u t on a n o t h e r s u b j e c t t h a n t h e c o m p l e t e u n d e r s t a n d i n g o f the v a l e n c e m o l e c u l a r o r b i t a l s ; t h e y w i l l be d i s c u s s e d i n a n o t h e r s e c t i o n o f t h i s paper. I n p o l y p r o p y l e n e , one m e t h y l g r o u p r e p l a c e s an h y d r o g e n atom i n the p o l y e t h y l e n e r e p e t i t i v e u n i t ; t h i s r e s u l t s i n an i n t e n s e structure l o c a t e d i n the m i d d l e o f the C - C ( C 2 s ) b a n d . In i s o t a c t i c p o l y p r o p y l e n e m o r e o v e r , t h e b o n d i n g and a n t i b o n d i n g s t r u c t u r e s o f t h i s band a r e s p l i t i n a c c o r d a n c e w i t h t h e o r e t i c a l c a l c u l a t i o n s p e r f o r m e d on i s o t a c t i c p o l y p r o p y l e n e i n an h e l i c a l conformation (lfj, 44). S u b s t i t u t i n g a h y d r o g e n i n PE by a - C H ^ - C H ^ r a d i c a l m o d i f i e s more d r a s t i c a l l y t h e v a l e n c e band s p e c t r u m o f p o l y ( l - b u t e n e ) . An i n t e r p r e t a t i o n o f t h i s s p e c t r u m c a n be e x t r a p o l a t e d from t h e d a t a r e c o r d e d f o r t h e s a t u r a t e d h y d r o c a r b o n i d e n t i f i e d w i t h t h e monomeric u n i t , n-butane. The two s p e c t r a d i f f e r however i n some d e t a i l s ( r e l a t i v e i n t e n s i t i e s o f the bands, or s p l i t t i n g s o f the p e a k s ) due t o t h e new bonds c r e a t e d i n t h e p o l y m e r by t h e b o n d i n g o f the - C H - C H group to the p o l y e t h y l e n e s k e l e t o n ( 1 0 ) . The e l e c t r o n i c s t r u c t u r e o f p o l y ( v i n y l c y c l o h e x a n e ) i s i n a s i m i l a r way e x p l a i n e d w i t h t h e XPS s p e c t r u m o f s o l i d c y c l o h e x a n e (1TJ); b u t i n t h i s c a s e , t h e s i m i l a r i t y w i t h t h e PE s p e c t r u m i s v e r y l o w . Coming from P E , t o P P , PB and PVCH, t h e l e n g t h o f t h e monomeric u n i t becomes l o n g e r t h r o u g h the s i d e group t h a t i s i m p o s i n g i t s shape t o t h e CC b a n d . T h i s C2s s t r u c t u r e constitutes a r e a l f i n g e r p r i n t o f t h e e l e c t r o n i c s t r u c t u r e o f the p o l y m e r ; a l m o s t i m m e d i a t e l y , t h e p o l y m e r c a n be i d e n t i f i e d t h r o u g h i t s C - C band shape ( 2 2 ) . P o l y s t y r e n e ( P S ) and p - p o l y p h e n y l (PPH) a r e two TT e l e c t r o n i c s y s t e m s whose p h o t o e l e c t r o n v a l e n c e band s p e c t r a c a n be compared t o t h e d a t a r e c o r d e d f o r s o l i d b e n z e n e ( F i g u r e 6 ) . The a s s i g n m e n t o f t h e p e a k s i s s t r a i g h t f o r w a r d ; t h e bands a r e s i m i l a r i n s h a p e , t h e p h o t o e l e c t r o n p e a k s a r e l o c a t e d a t a b o u t t h e same b i n d i n g energies. L o o k i n g more c a r e f u l l y t o t h e s p e c t r a , we n o t e s l i g h t d i f f e r e n c e s ; i n P P H , b a n d D i s b r o a d e n e d and s p l i t due t o an i n t e r a c t i o n o f the l o w e s t l y i n g c a r b o n 2s a and t h e h i g h e s t C2 p o M . 0 . s from v a r i o u s u n i t s i n t o t h e p o l y m e r c h a i n ; t h e PPH TT b a n d s have a d i f f e r e n t shape, d i s c l o s i n g an i n t e r a c t i o n o f t h e TT e l e c t r o n s between d i f f e r e n t p h e n y l groups w i t h i n d i f f e r e n t p o l y mer m o l e c u l e s ; t h e b i n d i n g e n e r g y o f the l o w e s t o c c u p i e d n m o l e c u l a r o r b i t a l i s m e a s u r e d a t 1 . 8 e V , compared t o 2 . 9 e V i n polystyrene. T h i s s o l e measurement e x p l a i n s t h e d i f f e r e n t e l e c t r i c a l p r o p e r t i e s o f t h e two p o l y m e r s ; t h e low b i n d i n g e n e r g y o f t h e s e TT e l e c t r o n s f o r PS (compared t o t h e C2p band o f t h e p u r e l y s a t u r a t e d h y d r o c a r b o n p o l y m e r s ) shows t h a t t h e s e e l e c t r o n s r e m a i n l o c a l i z e d i n the p e n d a n t benzene r i n g s , and t h a t t h e y do n o t p a r t i c i p a t e i n the b o n d i n g o f the carbon s k e l e t o n o f the polymer 2

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182


( 1 0 ) . A much l o w e r e n e r g y f o r t h e s e T e l e c t r o n s i n PPH r e v e a l s an i n t e r a c t i o n b e t w e e n a l l t h e ^ e l e c t r o n s , t h a t become more d e l o c a l i z e d i n t o the p o l y m e r m a t e r i a l . I n o t h e r w o r d s , t h e TT e l e c t r o n s o f PS a r e shown t o r e m a i n i n t h e p e n d a n t group on t h e p o l y m e r c h a i n , whereas t h o s e o f PPH a r e s u b j e c t t o i n t e r m o l e c u l a r i n t e r a c t i o n s , w h i c h e x p l a i n s a much more p r o n o u n c e d c o n d u c t o r c h a r a c t e r f o r PPH ( 2 1 ) . When t h e p o l y m e r b a c k b o n e c o n t a i n s a l a r g e number o f s u b s t i t u e n t s , or l o n g s i d e c h a i n s , i t i s c l e a r t h a t i t s v a l e n c e b a n d p h o t o e l e c t r o n spectrum w i l l c o n t a i n a l o t o f peaks; each s i d e g r o u p w i l l b r i n g i t s own f i n g e r p r i n t i n t h e XPS s p e c t r u m , and t h e i n t e r p r e t a t i o n o f t h e d a t a w i l l be v e r y d i f f i c u l t . An example o f t h i s l i m i t a t i o n has been found i n the study o f p o l y d i a c e t y l e n e s , w h i c h u n t i l now were n o t s y n t h e s i z e d w i t h s u f f i c i e n t l y s h o r t s i d e c h a i n s to r e d u c e t h e number o f b a n d s ( m o l e c u l a r o r b i t a l s ) a p p e a r ing i n the v a l e n c e spectrum ( 2 4 , 2 5 ) . I t i s t h e r e a s o n why i t was a l s o i m p o r t a n t t o s t u d y s y s t e m a t i c a l l y t h e XPS v a l e n c e band s p e c t r a o f s i m p l e m o d e l p o l y m e r s s u b s t i t u t e d by an h e t e r o a t o m ; i t i s n e c e s s a r y to d i s c o v e r a l s o t h e f i n g e r p r i n t ( s ) o f these s u b s t i t u e n t s i n order to understand i n t h e f u t u r e more c o m p l i c a t e d s y s t e m s . Fluoro-substituted Polymers. The f l u o r o p o l y m e r s were b e tween t h e f i r s t t o be s t u d i e d by the XPS t e c h n i q u e b e c a u s e t h e s u b s t i t u t i o n o f F atom(s) i n the - C H - C ^ - u n i t induced v e r y l a r g e m o d i f i c a t i o n s i n t h e XPS c o r e l e v e l s p e c t r a ( s h i f t s up t o 8eV) t h a t were e a s y to d e t e c t and i n t e r p r e t . The XPS v a l e n c e band s p e c t r a o f s i m i l a r compounds, namely p o l y ( v i n y l f l u o r i d e ) ( P V F ) , poly(vinylidene fluoride) (PVF2), poly(trifluoroethylene) ( P V F 3 ) , and p o l y ( t e t r a f l u o r o e t h y l e n e ) (PTFE) ( 2 6 , 2 7 , 28) a r e a l s o e x p e c t e d to r e f l e c t the i n d u c t i o n o f such s t r o n g e l e c t r o n i c e f f e c t s at the v a l e n c e m o l e c u l a r l e v e l . The f i r s t , and a l m o s t e v i d e n t f i n g e r p r i n t o f t h e s u b s t i t u t i o n o f h y d r o g e n a t o m ( s ) by f l u o r i n e i s t h e a p p e a r a n c e i n t h e v a l e n c e band s p e c t r u m o f two new s t r u c t u r e s r e l a t e d to the e l e c t r o n i c p r o p e r t i e s o f f l u o r i n e , namely i t s 2s and 2p o r b i t a l s whose b a n d s a r e e x p e c t e d t o a p p e a r i n t h e b i n d i n g e n e r g y r e g i o n b e t w e e n 0 and 4 0 e V . The s p e c t r a p r e s e n t e d a t F i g u r e 7 show c l e a r l y the F 2 s peak a p p e a r i n g w i t h a v e r y h i g h c r o s s s e c t i o n b e t w e e n 30 and 40eV ( t h e t h e o r e t i c a l r e l a t i v e p h o t o i o n i z a t i o n c r o s s s e c t i o n s f o r the F 2 s , C 2 s , F2p and C2p o r b i t a l s a r e r e s p e c t i v e l y 4 . 2 , 1 . 0 , 0 . 3 5 , and 0 . 0 3 2 ( 2 9 ) ) . The F2p peak i s l e s s s h a r p , b u t g a i n s i n r e l a t i v e i n t e n s i t y when t h e number o f f l u o r i n e atoms i n t h e monomeric u n i t i n c r e a s e s ; i t i s l o c a t e d a t a b o u t 1 0 12eV. The C - C band o f the f l u o r o p o l y m e r s e r i e s s t i l l a p p e a r s w i t h the same shape t h a n i n P E , i t s low e n e r g y l i m i t s h i f t i n g to h i g h e r b i n d i n g e n e r g y , w h i l e i t s h i g h e n e r g y l i m i t moves s l i g h t l y l e s s ; t h i s r e s u l t s i n a n a r r o w i n g o f t h e band i n t h e s e r i e s . It is also n o t e w o r t h y to r e m a r k t h a t t h e e n e r g y s e p a r a t i o n b e t w e e n t h e F 2 s




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Figure 7. XPS valence band spectra of the fluoropolymers: ΡΕ, PVF, PVF2, PVF3, and PTFE (27)



184


and t h e a n t i b e n d i n g p a r t ( s t r u c t u r e a t l o w e s t e n e r g y ) i n t h e C - C band i s c o n s t a n t t o w i t h i n 0 . 3 e V . These peaks may t h e n s e r v e as m a r k e r s i n e a c h s p e c t r u m , e s p e c i a l l y as t h e F 2 s peak c a n a l w a y s b e unambiguously assigned. The c o m p i l a t i o n o f s u c h d a t a c o n s t i t u t e d a f i r m b a s i s t h a t was u s e d t o s t u d y a s p e c i f i c and more c o m p l i c a t e d s y s t e m ; t h e e l u c i d a t i o n o f the e l e c t r o n i c s t r u c t u r e o f a copolymer o f e t h y l ene (48%) and t e t r a f l u o r o e t h y l e n e (52%) whose s y n t h e s i s was c o n d u c t e d i n o r d e r t o m a x i m i z e the a l t e r n a t i n g s e q u e n c e s . The v a l e n c e band s p e c t r u m o f s u c h a compound ( F i g u r e 8) was f o u n d v e r y s i m i l a r t o t h e one measured e . g . f o r p o l y ( v i n y l i d e n e f l u o r i d e ) . B u t , by l o o k i n g t o t h e f i n e d e t a i l s o f t h e s p e c t r u m , by s i m u l a t i n g t h e v a l e n c e band o f a b l o c k c o p o l y m e r (by a d d i t i o n o f PE and PTFE spectra), and by c o m p a r i s o n w i t h model c a l c u l a t i o n s , i t was p o s s i b l e t o show t h a t t h e C - C band w i d t h and t h e d i s t a n c e F 2 s - t o p o f the C - C band were c h a r a c t e r i s t i c o f an e t h y l e n e - t e t r a f l u o r o e t h y l e n e copolymer w i t h dominant a l t e r n a n t s t r u c t u r e ( 2 8 ) . Chlorine-containing Polymers. Polymers containing one c h l o r i n e atom i n v a r i o u s e n v i r o n m e n t s ( o t h e r s u s t i t u e n t s ) were s t u d i e d by X P S ; p o l y ( v i n y l c h l o r i d e ) P V C , p o l y ( c h l o r o t r i f l u o r o e t h y l e n e ) P C T F E , an ( e t h y l e n e - c h l o r o t r i f l u o r o e t h y l e n e ) copoly^ m e r , and p o l y ( e p i c h l o r o h y d r i n e ) P E P I , were c h o s e n b e c a u s e b e s i d e s c a r b o n atoms t h e y c o n t a i n c h l o r i n e i n p r e s e n c e of hydrogen, f l u o r i n e , and o x y g e n a t o m s . The v a l e n c e band s p e c t r a o f t h e s e compounds ( s e e F i g u r e 9 ) show t h a t f e a t u r e s c a n be e a s i l y and u n a m b i g u o u s l y a s s i g n e d t o a c o n t r i b u t i o n from t h e c h l o r i n e m o l e c ular orbitals. From t h e C13s and C13p l e v e l s t h a t a r e e x p e c t e d t o a p p e a r i n the s t u d i e d e n e r g y r a n g e , t h e C13p band i s t h e d o m i n a n t s t r u c t u r e o f t h e s p e c t r u m , as i t i s d e t e c t e d as t h e f i r s t ( c l o s e s t t o t h e F e r m i l e v e l ) and most i n t e n s e p e a k . I t s energy i s not c o n s t a n t ; i t v a r i e s i n t h e r a n g e o f 5 ( f o r PVC and P E P I ) t o 8eV ( f o r PCTFE and t h e c o p o l y m e r ) . The p r e s e n c e o f s u c h an i n t e n s e structure a f f e c t s t h e r e s t o f t h e s p e c t r u m as i t c a u s e s a l i f t i n g o f a l l t h e o t h e r s t r u c t u r e s o v e r an i n t e n s e b a c k g r o u n d , r e s u l t i n g from t h e i n e l a s t i c a l l y s c a t t e r e d p h o t o e l e c t r o n s t h r o u g h t h e m a t e r i a l . The C13s l e v e l a p p e a r s a t a v a r y i n g p o s i t i o n i n t h e m i d d l e o f t h e C - C ( C 2 s ) b a n d ; i t i s much l e s s r e s o l v e d , b u t a t an a l m o s t c o n s t a n t e n e r g y o f 12eV f r o m t h e C l 3 p p e a k . We n o t e t h a t t h e C13p band i s more i n t e n s e t h a n t h e C I 3 s o n e , w h i l e the r e v e r s e was f o u n d f o r t h e c o r r e s p o n d i n g s y m m e t r i e s o f the o u t e r m o s t F o r b i t a l s i n t h e f l u o r o p o l y m e r s (compare F i g u r e s 7 and 9 ) . A more c o m p l e t e d e s c r i p t i o n o f t h e f e a t u r e s a p p e a r i n g i n t h e v a l e n c e band s p e c t r a o f t h e s e compounds w i l l be p r e s e n t e d elsewhere ( 3 0 ) . We n o t e h e r e t h a t , when t h e p o l y m e r c o n t a i n s c h l o r i n e and f l u o r i n e a t o m s , i t s v a l e n c e band s p e c t r u m becomes (very) c o m p l i c a t e d , c o n s i s t i n g o f an a l m o s t c o n t i n u o u s s u c c e s s i o n o f molecular orbitals with similar i n t e n s i t i e s . Their interpretat i o n c o u l d not have been g i v e n w i t h o u t the a i d o f a r e l a t i v e l y



185


PIREAUX ET AL.

Figure 9. XPS valence band spectra of (A) polyvinyl chloride, (B) polyfepi chlorohydrine), (C) poly(chlorotrifluoroethylene), (D) cofethylene-tetrafluoroethylene


186



complete set (10, 30).

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Oxygen C o n t a i n i n g P o l y m e r s . Oxygen c a n be i n c o r p o r a t e d i n t o a p o l y m e r c h a i n as p a r t o f d i f f e r e n t f u n c t i o n a l g r o u p s : a l c o h o l , e t h e r ( l i n e a r and b r a n c h e d ) , e s t e r , c a r b o n a t e , . . . V a l e n c e band d a t a on p o l y a c r y l a t e s (7) and p o l y m e t h y l a c r y l a t e s (34) have a l r e a d y been p u b l i s h e d , b u t w i t h o u t a c o m p l e t e a s s i g n m e n t o f t h e molecular o r b i t a l s . Compounds b e l o n g i n g to o t h e r s e r i e s were s t u d i e d i n o u r l a b o r a t o r y 0 3 1 , 32) w i t h the a i d o f t h e o r e t i c a l calculations (34). B u t we c h o s e t o p r e s e n t h e r e s p e c i a l t o p i c s t h a t w i l l shed a l i t t l e more l i g h t on o u r u n d e r s t a n d i n g o f t h e v a l e n c e band s p e c t r a o f p o l y m e r s . The l i n e a r p o l y e t h e r s a r e model compounds t o show t h e e f f e c t o f t h e i n s e r t i o n o f one o x y g e n atom a t d i f f e r e n t p o s i t i o n s i n t o the p o l y e t h y l e n e - C H ^ - C H - r e p e t i t i v e u n i t . What w i l l be t h e i n f l u e n c e on the p o l y m e r XPS v a l e n c e band s p e c t r u m , compared t o the one o f p o l y e t h y l e n e ( F i g u r e 2 ) ? Our s t u d y was c o n d u c t e d on t h r e e m a t e r i a l s ; p o l y ( m e t h y l e n e o x i d e ) o r PMO, p o l y ( e t h y l e n e o x i d e ) o r PEO, and p o l y ( t e t r a m e t h y l e n e o x i d e ) o r PTMO, t h a t a r e c o n v e n i e n t l y p i c t u r e d by the f o r m u l a —P 2^ > with x=l, 2 , and 4 r e s p e c t i v e l y . F i g u r e 10 r e p o r t s t h e v a l e n c e band s p e c t r a o f t h e s e p o l y m e r s , where from two t y p e s o f i n f o r m a t i o n s c a n be deduced: CH

1.

The p r e s e n c e o f o x y g e n i n the p o l y m e r i s e a s i l y deduced from the a p p e a r a n c e o f a new peak l o c a t e d a t a b o u t 27eV: t h e 02s m o l e c u l a r o r b i t a l , t h a t r e t a i n s p r o b a b l y most o f i t s a t o m i c l e v e l c h a r a c t e r ( i . e . does n o t f e e l t h e i n f l u e n c e o f the o t h e r m o l e c u l a r o r b i t a l s d e f i n i n g the r e a l v a l e n c e band a t l o w e r b i n d i n g e n e r g i e s ) . On t h e c o n t r a r y , t h e 02p l e v e l i s s t r o n g l y m i x e d w i t h the o t h e r ( c a r b o n and h y d r o g e n ) l e v e l s , as i t i s the medium t h a t bounds the o x y g e n t o t h e o t h e r a t o m s ; i t does n o t a p p e a r c l e a r l y on the XPS s p e c t r a . Noteworthy i s the s i g n a t u r e o f t h e o x y g e n l o n e p a i r s (02p TT non b o n d i n g molecular orbitals); t h e y a p p e a r as t h e rightmost s t r u c t u r e s , a t the l o w e s t b i n d i n g e n e r g y . They p l a y an i m p o r t a n t r o l e i n c h a r a c t e r i z i n g the p o l y m e r m o l e c u l a r s t r u c t u r e , as t h e y a r e i n t i m a t e l y r e l a t e d t o t h e p r o p e r t i e s o f s u r f a c e w e t t a b i l i t y o f s u c h t y p e s o f comp ound s.

2.

The i n t e r a c t i o n ( b o n d i n g ) o f o x y g e n w i t h t h e c a r b o n e o u s s k e l e t o n i s seen e . g . i n the C-C b o n d i n g band as new s t r u c t u r e s t h a t a r e r e l a t e d to the o v e r l a p between t h e C2s and 02p m o l e c u l a r o r b i t a l s t o form t h e new C - 0 bonds ( t h e y a r e b e s t seen i n the PTMO s p e c t r u m as t h e two new s t r u c t u r e s a p p e a r i n g i n the m i d d l e o f t h e C-C b a n d ) . The d i s c u s s i o n and a s s i g n m e n t o f the M . O . o f t h e p r e sented s p e c t r a are g i v e n elsewhere ( 3 1 , 3 2 ) . We want



187

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Figure 10. XPS valence band spectra of the linear polyethers: (1) poly(methylene oxide), (2) polyfethylene oxide), (4) poly(tetramethylene oxide) (4).




188

h e r e t o s t r e s s o n l y on t h e t r e n d s t h a t a r e c l e a r l y s e e n i n t h e b a n d w i d t h s and s h a p e s , as a c o n s e q u e n c e o f t h e i n t e r a c t i o n s b e t w e e n t h e c a r b o n and o x y g e n o r b i t a l s . F o r e x a m p l e , the 02s peak i s measured t o n a r r o w from 5 . 7 t o 3 . 8 and 3 . 5 e V f o r the p o l y m e r s PMO, PEO, and PTMO r e s p e c t i v e l y ; t h i s f i n g e r p r i n t s the p r o g r e s s i v e i s o l a t i o n o f t h e o x y g e n atom i n t h e c a r b o n c h a i n ( a s a reference, t h e 02s l i n e w i d t h o f p o l y ( v i n y l m e t h y l e t h e r ) PVME and p o l y ( v i n y l a l c o h o l ) PVOH a r e 3 . 7 and 3.8eV r e s p e c t i v e l y ) . C o r r e s p o n d l i n g l y , t h e C2s band b r o a d e n s o u t from 5.1 (PMO) t o 7 . 1 e V (PTMO), a p p r o a c h i n g t h e w i d t h measured f o r t h e i n f i n i t e - C H ^ - s e q u e n c e i n polyethylene; 7.7eV. I t i s noteworthy to p o i n t out h e r e t h a t t h e s e t r e n d s o b s e r v e d i n t h e v a l e n c e band s p e c t r a a r e c o r r e l a t e d by s e p a r a t e t h e o r e t i c a l c a l c u l a t i o n s at the Extended Huckel l e v e l ( 3 3 ) . Miscellaneous. To c o m p l e t e t h e l i b r a r y o f XPS p h o t o e l e c t r o n v a l e n c e band s p e c t r a i t w o u l d be u s e f u l t o add measurements on v a r i o u s n i t r o g e n , or s i l i c o n , or s u l f u r c o n t a i n i n g polymers. These d a t a a r e n o t y e t a v a i l a b l e n o w . . . t h e y w o u l d be o f i n t e r e s t f o r t h e XPS s t u d y o f p o l y u r e t h a n e s , p o l y s i l o x a n e s , r e s i n s , p o l y meric glues, p o l y s u l f o n e s . . . Isomerism Introduction. Substitution effects i n t h e v a l e n c e band s p e c t r a o f p o l y m e r s a r e t h e l a r g e s t e f f e c t s one c o u l d hope t o see w i t h t h e XPS t e c h n i q u e ; i n d e e d , l i k e f o r a c o r e l e v e l a n a l y s i s , i t i s when one h e t e r o a t o m i s i n c l u d e d i n or a s i d e t h e p o l y m e r s k e l e t o n t h a t one e x p e c t s t o see the most s u b s t a n t i a l m o d i f i c a t i o n s i n t h e s p e c t r a ; new p e a k s o r b a n d s , i n f l u e n c e ( s h i f t . . . ) on the e x i s t i n g s t r u c t u r a l c h a r a c t e r i s t i c s . B u t , a l l the polymers t h a t a r e p r o c e s s e d and s t u d i e d nowadays do n o t d i f f e r e x c l u s i v e l y by t h e i r c h e m i c a l c o m p o s i t i o n . W i t h i n one s e r i e s o r f a m i l y o f p o l y m e r s , t h e r e e x i s t s compounds w i t h v a r i o u s m o l e c u l a r w e i g h t (we d i d show p r e v i o u s l y t h a t t h e XPS t e c h n i q u e i s n o t a b l e t o d i s t i n g u i s h b e t w e e n two p o l y m e r s w i t h d i f f e r e n t h i g h m o l e c u l a r w e i g h t ) . , c r y s t a l l i n i t y , t a c t i c i t y , isomerism ( l i n e a r , branched, ramified s t r u c t u r e s ; head-to-head l i n k a g e s . . . ) . c h e m i c a l i n d u s t r i e s s y n t h e s i z e now more and more e l a b o r a t e d m a t e r i a l s , they e x p l o i t not o n l y a d d i t i v e s ( f o r the synthesis, t h e p r o c e s s i n g , t h e s t a b i l i t y . . . ) , b u t a l s o r e a c h now t h e v e r y p r o m i s i n g f i e l d o f t h e c o p o l y m e r s where a l l t h e p a r a m e t e r s c i t e d above c o u l d be p l a y e d w i t h f o r t w o , o r t h r e e . . . p o l y m e r s i n t h e same m a t e r i a l . These new p o l y m e r s a r e e x p e c t e d t o become v e r y s o p h i s t i c a t e d and d e d i c a t e d t o v e r y s p e c i f i c a p p l i c a t i o n s . . . . T h e r e f o r e , new t e c h n i q u e s a r e s c a n n e d now t o w i d e n t h e a v a i l a b l e a n a l y t i c a l t o o l s i n o r d e r t o h e l p the more c l a s s i c a l ones ( i n f r a red a b s o r p t i o n and n u c l e a r m a g n e t i c r e s o n a n c e ) .



13.

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X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y , through the a n a l y s i s o f the polymer core l e v e l s s p e c t r a d i d succeed i n g i v i n g p a r t i a l answer t o a few q u e s t i o n s o f t h e s e t y p e s . A l t h o u g h s t u d i e s o f t h e d e g r e e o f c r y s t a l l i n i t y were n e g a t i v e (35^ 36), some r e s u l t s were o b t a i n e d i n the study o f s t r u c t u r a l i s o m e r i s m , f o r n i t r o s o r u b b e r s (3), p o l y b u t y l a c r y l a t e s (_7) > and p a r a c y c l o p h a n e s (37). The l i m i t s o f t h e method a r e a l r e a d y n o t e d : t h e c o r e l e v e l s p e c t r a o f s t r u c t u r a l isomers are v i r t u a l l y i d e n t i c a l . I n some p a r t i c u l a r c a s e s , one m i g h t be a b l e t o i d e n t i f y i s o m e r s , u s i n g XPS w i t h m o n o c h r o m a t i z e d X - r a y s o u r c e , s t u d y i n g the s h a k e - u p s a t e l l i t e s i f a n y , and w i t h t h e h e l p o f a p p r o p r i a t e t h e o r e t i c a l c a l c u l a t i o n s . R e s u l t s o b t a i n e d on c o p o l y m e r s a r e more p r o m i s i n g t o s t u d y t h e s u r f a c e c o m p o s i t i o n , t o p o g r a p h y , m o r p h o l o g y , and t h e f i r s t o u t e r most l a y e r s ( e . g . c o ( p o l y d i m e t h y l s i l o x a n e and p o l y s t y r e n e ) , o r c o ( p o l y s t y r e n e and p o l y ( e t h y l e n e o x i d e ) ) (38, 39). The XPS v a l e n c e band s p e c t r a p i c t u r e d i r e c t l y t h e bonds b e t w e e n t h e atoms o f t h e m o l e c u l e , and a r e more c h a r a c t e r i s t i c o f t h e compounds (34) e s p e c i a l l y f o r p o l y m e r s c o n t a i n i n g o n l y c a r b o n a t o m s ; c o u l d t h e t e c h n i q u e ( w i t h t h e use o f c o m p l e m e n t a r y r e f e r ence s p e c t r a , a n d / o r t h e o r e t i c a l d e n s i t y o f s t a t e s c a l c u l a t i o n s ) b e s e n s i t i v e enough t o a l l o w an i d e n t i f i c a t i o n o f i s o m e r s ? I f i n t h e s t u d y o f t h e p o l y m e r s p r i m a r y s t r u c t u r e we d i d n o t e s p e c i a l l y take i n t o account p a r t i c u l a r c h a r a c t e r i s t i c parameters o f the compounds b e c a u s e " s u b s t i t u t i o n e f f e c t s " a r e so c l e a r l y marked, i t i s c l e a r t h a t i n s t u d y i n g the secondary s t r u c t u r e o f t h e p o l y m e r s , we must be v e r y c a r e f u l and c r i t i c a l , a b o u t t h e p r e p a r a t i o n and c h a r a c t e r i z a t i o n o f t h e compounds as f a r as t h e i s o m e r i c c o n f o r m a t i o n , t a c t i c i t y , and/or c r y s t a l l i n i t y . . . are concerned. I t i s i n d e e d e x p e c t e d t h a t t h e e f f e c t s t h a t must be e x p e r i m e n t a l l y e v i d e n c e d w i l l be ( v e r y ) f i n e , and s m a l l i n a m p l i tude. I t i s t h e r e a s o n why t h e compounds, whose XPS s p e c t r a a r e d i s c u s s e d b e l o w , were s y n t h e s i z e d and c h a r a c t e r i z e d by s p e c i a l i z e d l a b o r a t o r i e s i n t h e w o r l d ( p r a c t i c a l i n f o r m a t i o n s w i l l be g i v e n e l s e w h e r e (40, 41). Isomerism i s a term c o v e r i n g d i f f e r e n t m o l e c u l a r conformation effects. We w i l l n o t speak h e r e a g a i n on the t y p e o f i n f o r m a t i o n t h a t c a n be g a i n e d on the a l t e r n a t i n g o r b l o c k s t r u c t u r e o f c o p o l y m e r s , as was e x p l a i n e d above f o r an e t h y l e n e t e t r a f l u o r o e t h y l e n e c o p o l y m e r . F o r h o m o p o l y m e r s , we w i l l d i s t i n guish various categories: - constitution isomers, i.e. p o l y m e r s t h a t have the same c h e m i c a l f o r m u l a , b u t whose bonds a r e d i f f e r e n t l y o r g a n ized, l i k e i n l i n e a r or branched s t r u c t u r e s ; - head-to-head l i n k a g e s ; - s t e r e o i s o m e r i s m , i . e . p o l y m e r s whose f u n c t i o n a l g r o u p s a r e c i s - or t r a n s - , r e l a t i v e to a double bond; - o r i s o - and s y n d i o - p o l y m e r s , r e l a t i v e t o t h e m a i n s k e l e t o n (tacticity). S t r u c t u r a l Isomerism.

S o l e l y hydrocarbon based polymers


(that



190

d i f f e r o n l y by s a t u r a t e d and u n s a t u r a t e d b o n d s ) c a n be c o n s i d e r e d i n a b r o a d e n e d s e n s e as s t r u c t u r a l i s o m e r s . I t has a l r e a d y b e e n shown t h a t XPS c o r e l e v e l s p e c t r a c a n e a s i l y d i s t i n g u i s h b e t w e e n t h e two t y p e s o f compounds as s h a k e - u p s a t e l l i t e s a r e r e l a t e d t o the e x i s t e n c e o f e l e c t r o n s i n t h e u n s a t u r a t e d p o l y m e r s (23^). We want t o m e n t i o n h e r e t h a t s i m i l a r r e s u l t s a r e a c h i e v e d by an o v e r l o o k t o the v a l e n c e band s p e c t r a o f t h e c o n c e r n e d compounds; t h i s was a l r e a d y shown f o r p o l y s t y r e n e and p o l y ( v i n y l cycloh e x a n e ) , see above and ( l f j ) . We p r e s e n t h e r e ( F i g u r e 11) a n o t h e r example o f t h i s p o t e n t i a l i t y o f the t e c h n i q u e ; it concerns p o l y ( l - b u t e n e ) and p o l y ( b u t a d i e n e l , 4 ) c i s . These v a l e n c e band s p e c t r a a r e so p r o f o u n d l y d i f f e r e n t t h a t t h e y u n d o u b t l y f i n g e r p r i n t t h e compounds. Examples o f r e a l i s o m e r i s m o f c o n s t i t u t i o n are v e r y numerous; we l i s t h e r e a few s e r i e s o f compounds w h i c h were more o r l e s s e a s i l y i d e n t i f i e d by t h e i r XPS v a l e n c e band s p e c t r a . Generally, they are r e l a t i v e l y s i m p l e polymers i n the sense t h a t their monomer i c u n i t c o n t a i n s few a t o m s ; f o r l a r g e r s y s t e m s , i t i s e x p e c t e d t h a t the d i s t i n c t i o n between two i s o m e r s w i l l be l e s s evident. We d i s t i n g u i s h h e r e t h e two f o l l o w i n g g r o u p s o f p o l y m e r s , and o n l y l i s t the compounds a l r e a d y s t u d i e d ( t h e list c o n t a i n s a l l t h e compounds a l r e a d y s t u d i e d i n t h e l a b o r a t o r y o r e l s e w h e r e ; i t i s n o t r e s t r i c t i v e : a l l t h e i s o m e r s s t u d i e d up t o now were found d i f f e r e n t by t h e i r v a l e n c e band s p e c t r a ) : 1. f o r p u r e l y h y d r o c a r b o n p o l y m e r s : - poly(3-methyl l-butene)and poly(l-pentene), with a C,.H monomeric u n i t . - p o l y C 4 - m e t h y l 1-pentene) and a copolymer(butene-2, e t h y l e n e ) w i t h a C ^ H ^ monomeric u n i t . l n

2.

f o r oxygen c o n t a i n i n g p o l y m e r s : - p o l y ( v i n y l a l c o h o l ) and p o l y ( e t h y l e n e o x i d e ) w i t h a C^H^O u n i t ; b u t i n t h i s c a s e t h e c a r b o n I s c o r e l e v e l s p e c t r a o f t h e two compounds a r e d i f f e r e n t ( 2 4 , 3 1 , 32); - p o l y ( p r o p y l e n e o x i d e ) and p o l y ( v i n y l m e t h y l e t h e r ) w i t h a C ^ O unit; - p o l y t t e t r a m e t h y l e n e o x i d e ) and p o l y ( v i n y l e t h y l e t h e r ) w i t h C^HgO u n i t ; - t h r e e i s o m e r i c p o l y ( b u t y l a c r y l a t e s ) ( 7 ) w i t h a CyH^^O^ unit; - four isomeric p o l y ( b u t y l methacrylates) (34) w i t h a C

H

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8 14°2 * The i n t e r p r e t a t i o n o f the v a l e n c e band s p e c t r a o f t h e s e p o l y m e r s c a n be found e l s e w h e r e ( 3 1 , 3 2 ) . F i g u r e 12 i s i n c l u d e d t o s u b s t a n t i a t e t h e a b i l i t y o f t h e XPS t e c h n i q u e t o d i s t i n g u i s h two c o n s t i t u t i o n i s o m e r s ; i t compares the c o r e and v a l e n c e s p e c t r a r e c o r d e d f o r PPO and PVME, and shows t h a t , w h e r e a s t h e C l s l e v e l s a p p e a r v e r y much a l i k e , t h e v a l e n c e b a n d s r e a d i l y d i s t i n g u i s h t h e two p o l y m e r s .



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Figure 12. XPS valence band and core level spectra of poly(propylene oxide) and poly(vinyl methyl ether) (4)



192


Head-to-Head and Head-to-Tail Linkages. Errors in the l i n k ages between successive monomeric units of the polymers are possible (and always s t a t i s t i c a l l y present). The effect of headto-head (HH) and head-to-tail (HT) bonds in the XPS core levels spectra of substituted polymers have been computed and found at the limit of the sensitivity of the technique (e.g. 42). The control of these linkages during the synthesis is d i f f i c u l t and the number of polymers that can be prepared with 100% of HH or HT linkages is small (43). Two compounds of this type were placed at our disposal; isotactic polypropylene and an alternating erythro-iso-copolymer of butene-2 and ethylene. Looking to the extended chemical formula of the latter (Figure 13), it is indeed immediately evident that this copolymer can be considered as an equivalent to a HH polypropylene. The XPS analysis of the valence band spectrum of this compound reveals that its electronic structure, reflected through the C-C (C2s) molecular orbitals is entirely different from that of polypropylene (Figure 14). In view of this favorable experience, i t is now highly desirable to test the potentiality of the technique on other series of HH and HT polymers. Stereoisomerism. The f i r s t t r i a l s to use the XPS valence band spectra to distinguish between two stereoisomers were unsuccessful. C i s - and transpoly(isoprene) - with short branched chain and small substitution effects-, as well as c i s - and transpoly (1,4dichloro-2,3epoxybutene) - with longer branched chain and more intense substituent effects- did not show in our f i r s t measurements significant differences in their valence band spectra that could be attributed to the searched effect. Before concluding in a limitation of the technique, we note: - that the poly(isoprene) samples were commercially available compounds, whose (chemical and stereoisomer ic) purity was not known, and that very l i k e l y they were highly cross linked as no care was taken to store adequately the polymers. - the poly(1,4 dichloro-2,3 epoxybutenes), two well characterized research products, are probably much too complicated systems to be distinguished by their XPS valence band spectra. Indeed, preliminary results just acquired for c i s - and trans- 1,4polybutadiene are more promising as substantial modifications appear in their valence band spectra (the Cis core level peaks are exactly comparable (41)). This indicates that more research in this field is worthwhile and w i l l be carried on in the near future. Tacticity. As the XPS core level investigation of the t a c t i c i t i e s in the polymers revealed i t s e l f a relatively i n sensitive technique (34), i t was worthwhile to test the poten-


PIREAUX ET AL.


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XPS valence band spectra of HH and HT polypropylenes



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t i a l i t y o f the XPS v a l e n c e band a n a l y s i s . S t u d i e s ( l f j , 44) o f p o l y p r o p y l e n e r e v e a l e d d i s c r e p a n c i e s i n the comparison o f e x p e r i m e n t a l and quantum c h e m i c a l c a l c u l a t i o n s on t h e v a l e n c e b a n d . The s i m p l i c i t y o f t h e monomer s t r u c t u r e and t h e a c t u a l c o m m e r c i a l interest o f the polymer render its s t u d y more attractive. S p e c i a l l y s y n t h e s i z e d and c h a r a c t e r i z e d i s o - (99%) and s y n d i o (80-95%) t a c t i c p o l y p r o p y l e n e were s t u d i e d , and t h e v a l e n c e band s p e c t r a o f t h e two s t e r e o i s o m e r s were f o u n d p r o f o u n d l y d i f f e r e n t by t h e i r C2s band shape ( F i g u r e 1 5 ) ; i t s w i d t h i s i d e n t i c a l i n t h e two compounds, b u t the b o n d i n g subband (at highest binding energy) i s m a r k e d l y r e d u c e d i n i n t e n s i t y and l o s s e s o f its substructures i n s y n d i o - t a c t i c PP. We n o t e i n a d d i t i o n t h a t no change i n t h e c a r b o n I s l e v e l c o u l d be d e t e c t e d . The d a t a a r e d i s c u s s e d i n l e n g t h w i t h t h e a i d o f new t h e o r e t i c a l c a l c u l a t i o n s elsewhere ( 1 ^ , 4 0 ) . P o l y m e r C o n f o r m a t i o n and C r y s t a l l i n i t y . Beyond t h e s t e r e o r e g u l a r i t y and t a c t i c i t y , t h e g e o m e t r i c a l c o n f o r m a t i o n o f the polymer c h a i n i n the s o l i d m a t e r i a l c o u l d i n f l u e n c e i t s e l e c t r o n i c s t r u c t u r e , t h r o u g h a m o d i f i c a t i o n o f i t s v a l e n c e band m o l e c ular orbitals. I n d e e d , a few y e a r s a g o , v e r y c h a r a c t e r i s t i c band s t r u c t u r e s were c a l c u l a t e d f o r T , G , T G , and TGTG p o l y e t h y l e n e s (45). More r e c e n t l y , E x t e n d e d H u c k e l c r y s t a l o r b i t a l c a l c u l a t i o n s showed t h a t f o r i s o t a c t i c p o l y p r o p y l e n e , a z i g - z a g p l a n a r or a h e l i c a l c o n f o r m a t i o n r e s u l t e d i n s i g n i f i c a n t changes i n t h e t h e o r e t i c a l v a l e n c e band s p e c t r a , s u p p o r t i n g t h e i d e a t h a t c o n formation effects c o u l d be d e t e c t e d e x p e r i m e n t a l l y by the XPS method ( 4 4 ) . I s o t a c t i c p o l y p r o p y l e n e does n o t e x i s t i n t h e two z i g - z a g and h e l i x c o n f o r m a t i o n s , b u t s y n d i o t a c t i c PP i s a good c a n d i d a t e for t h i s search. Indeed, a c c o r d i n g to the p r e p a r a t i o n procedure of the polymer ( 4 6 ) , i t can e x i s t i n a h e l i x or z i g - z a g p l a n a r conformation. S y n d i o - P P f i l m s were p r e p a r e d f o l l o w i n g b o t h w a y s , t h e i r c o n f o r m a t i o n c h e c k e d by I R , and s t u d i e d by X P S . Their v a l e n c e band s p e c t r a a g a i n show d i s t i n c t d i f f e r e n c e s i n t h e C2s band ( F i g u r e 1 6 ) ; f o r z i g - z a g PP t h a t i s p r o b a b l y h i g h l y amorp h o u s , t h e C-C band w i d t h i n c r e a s e d by a b o u t l e V , whereas the b o n d i n g subband i n c r e a s e d i n i n t e n s i t y and became more s t r u c t u r e d (40). That success l e d us t o t a c k l e a n o t h e r more c o m p l i c a t e d q u e s t i o n ; i s i t p o s s i b l e w i t h t h e XPS v a l e n c e band s p e c t r a t o d i s t i n g u i s h b e t w e e n c r y s t a l l i n e and amorphous p o l y m e r s ? I t has a l r e a d y b e e n shown t h a t c o r e l e v e l a n a l y s i s was n o t s u c c e s s f u l (36). From o t h e r m e a s u r e m e n t s , m a i n l y on s e m i c o n d u c t o r m a t e r i a l s , i t i s known t h a t , i f t h e c o r e l e v e l peaks o f t h e amorphous m a t e r i a l s were s l i g h t l y b r o a d e r , the m a i n m o d i f i c a t i o n s a p p e a r i n t h e v a l e n c e band where t h e f i n e d e t a i l s a r e smeared o u t compared to the c r y s t a l l i n e m a t e r i a l ( 4 7 ) . C r y s t a l l i n e and amorphous p o l y s t y r e n e were c h o s e n f o r t h e XPS s t u d y . B u t , t h e two r e c o r d e d v a l e n c e band s p e c t r a do n o t



PIREAUX ET AL.


BINDING

E N E R G Y .eV

I 40

Figure 15.

195

| 30

| 20

I IO

O

XPS valence band spectra of iso- and syndio-polypropylenes

1 HELIX

ZIG-ZAG

" "V* *

M

u BINDING .

Figure 16.

1

E N E R G Y .eV 1

1

1

XPS valence band spectra of syndio-polypropylene in the helical and zig-zag planar conformations



196


a p p e a r n o t a b l y d i f f e r e n t from e a c h o t h e r . We n o t i c e d o n l y a s l i g h t m o d i f i c a t i o n o f the w i d t h o f t h e a n t i b o n d i n g C2s b a n d , and a s m a l l ( 0 . 1 5 e V , at the i n s t r u m e n t a l d e t e c t i o n l i m i t ) s h i f t o f the w h o l e C - C band r e l a t i v e t o t h e C - H m o l e c u l a r o r b i t a l s . For c o m p l e t e n e s s , we add t h a t t h e two p o l y s t y r e n e s d i d n o t p r e s e n t d i f f e r e n t C i s c o r e p e a k s and C i s s h a k e - u p s a t e l l i t e s ( o n l y t h e C i s line itself i s s l i g h t l y broadened, from 1.1 t o 1 . 2 e V - i n t h e amorphous s t a t e ) . I s t h i s f a i l u r e due t o a damage o f t h e s a m p l e s by i r r a d i a t i o n ? ) ? I s t h i s e x p e r i m e n t a p r o o f o f one l i m i t a t i o n i n t h e p o t e n t i a l i t i e s o f t h e XPS t e c h n i q u e ? Further experiments are needed t o g i v e a d e f i n i t i v e a n s w e r , b u t i t i s a l r e a d y c l e a r a t t h e moment - t h r o u g h t h e r e s u l t s p r e s e n t e d i n t h i s r e v i e w - t h a t , l i k e f o r t h e s t u d i e s o f o t h e r t y p e s o f compounds, the d o m i n a n t s t r u c t u r e s a p p e a r i n g i n t h e v a l e n c e band s p e c t r a o f t h e p o l y m e r s a r e d i c t a t e d by the a t o m i c p r o p e r t i e s ( c h e m i c a l c o m p o s i t i o n d e f i n i n g the s u b s t i t u e n t e f f e c t ) , and by the s h o r t r a n g e o r d e r ( i n t h e l a r g e s t s e n s e , i s o m e r i s m and c o n f o r m a t i o n ) i n t h e m a t e r i a l s . As r e g a r d s the l o n g range o r d e r i n g o f the polymer c h a i n ( i . e . the p o l y m e r c r y s t a l l i n e form or the i n t e r c h a i n i n t e r a c t i o n s . . . ) , it p r o b a b l y i n f l u e n c e s the e l e c t r o n i c s t r u c t u r e o f the v a l e n c e bands through only (too) f i n e d e t a i l s . Other Informations

Available

from the V a l e n c e Band

Spectra

F o r s a k e o f c o m p l e t e n e s s and o n l y b r i e f l y ( a s o t h e r r e v i e w s a r e s p e c i a l l y d e d i c a t e d t o t h e s e s u b j e c t s ) , we w o u l d l i k e to m e n t i o n t h a t XPS v a l e n c e band s p e c t r a o f p o l y m e r s c a n a l s o be u s e d - g o i n g beyond a s o l e i n t e r p r e t a t i o n o f the m o l e c u l a r o r b i t a l s i n t h e v a l e n c e b a n d - t o p r o v i d e i n f o r m a t i o n s on some s o l i d s t a t e p a r a m e t e r s and on s u r f a c e s t r u c t u r e e f f e c t s o f t h e p o l y m e r s . The low b i n d i n g e n e r g y s t r u c t u r e s i n t h e v a l e n c e band o f t h e p o l y m e r s a r e b r o a d , l e s s s t r u c t u r e d and o f l o w i n t e n s i t y . Nevertheless, from t h e s e d a t a i t i s p o s s i b l e t o e x t r a c t valuable i n f o r m a t i o n s on t h e band gap ( b e t w e e n t h e v a l e n c e and c o n d u c t i o n b a n d s ) and the work f u n c t i o n o f the p o l y m e r s . These p r o p e r t i e s a r e r e l e v a n t t o u n d e r s t a n d e . g . t h e c o n d u c t i v i t y and a d h e s i o n o f commercial polymers. D a t a a r e a v a i l a b l e on m o d e l compounds, a l k a n e s (_18), b e n z e n e (_19) and o t h e r a c e n e s (2_0), p o l y p h e n y l ( 2 1 ) and o t h e r p o l y m e r s ( l f j , 4 8 ) . Being e s s e n t i a l l y a surface t e c h n i q u e , XPS v a l e n c e band s p e c t r a a l s o a l l o w s to m o n i t o r m o d i f i c a t i o n s o c c u r r i n g at the polymer surface d u r i n g a d s o r p t i o n , r e a c t i o n s , o f d e g r a d a t i o n . . . V e r y few c o n t r i b u t i o n s a r e , up t o now, d e a l i n g w i t h s u c h s t u d i e s (49). The most d i r e c t use o f the t e c h n i q u e i s a c t u a l l y a c o m p a r i s o n o f the c o r e and v a l e n c e p h o t o e l e c t r o n l i n e i n t e n s i t i e s t o deduce i n f o r m a t i o n s a b o u t t h e s u r f a c e and t h e i n - d e p t h comp o s i t i o n o f the p o l y m e r , as w e l l as a b o u t the o r i e n t a t i o n o f t h e m a c r o m o l e c u l a r c h a i n at the s u r f a c e boundary.


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Conclusions X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y u s e d t o s t u d y the e l e c tronic structure o f polymers through the analysis of their v a l e n c e band s p e c t r a was found t o p r e s e n t v e r y i n t e r e s t i n g p o t e n t i a l i t i e s , b u t a l s o some l i m i t s . If the t e c h n i q u e was n o t found s e n s i t i v e t o d i s t i n g u i s h p o l y m e r s w i t h d i f f e r e n t h i g h m o l e c u l a r w e i g h t , we p r o v e d t h a t i t g i v e s an i n s t a n t a n e o u s p i c t u r e o f t h e e l e c t r o n i c s t r u c t u r e o f a r a t h e r l a r g e (10 t o 20 c o n s e c u t i v e c a r b o n atoms) p o r t i o n o f t h e polymer s k e l e t o n . S u b s t i t u t i o n o f h y d r o g e n atom i n the C H ^ - C ^ - p o l y m e r skeleton results always i n s i g n i f i c a n t m o d i f i c a t i o n s i n the v a l e n c e bands; t h i s i s m o s t l y i n t e r e s t i n g f o r the s o l e l y h y d r o c a r b o n b a s e d p o l y m e r s , f o r w h i c h t h e v a l e n c e band d a t a r e p r e s e n t a u n i q u e s o u r c e o f i n f o r m a t i o n , as t h e y u s u a l l y f i n g e r p r i n t the p o l y m e r (when no s h a k e - u p s a t e l l i t e c a n r e v a l o r i z e t h e c o r e l e v e l data). G e n e r a l t r e n d s i n t h e v a l e n c e band s p e c t r a were a l s o p r e s e n t e d f o r p o l y m e r s c o n t a i n i n g f l u o r i n e , c h l o r i n e , and o x y g e n atom( s ) . Numerous examples o f c o n s t i t u t i o n i s o m e r i s m s t h a t c a n be s o l v e d w i t h t h e a i d o f t h e v a l e n c e band s p e c t r a were g i v e n . Also, for s p e c i a l l y s y n t h e s i z e d and c h a r a c t e r i z e d compounds, i t was p o s s i b l e t o show a p o t e n t i a l i t y o f t h e t e c h n i q u e t o e v i d e n c e h e a d to-head linkages, stereoisomers (?), t a c t i c i t i e s and (alternant/block) structure of copolymers. I f the i n f l u e n c e o f c o n f o r m a t i o n i n t h e v a l e n c e band c o u l d a l s o be e v i d e n c e d , no s u c c e s s was obtained for differentiating crystalline and amorphous polymers. A l m o s t t e n y e a r s a f t e r the r e c o r d i n g o f t h e f i r s t v a l e n c e band s p e c t r a o f p o l y m e r s , i t a p p e a r s t h a t t h e method has n o t y e t reached i t s m a t u r i t y . A f u n d a m e n t a l , c a u t i o u s , s y s t e m a t i c , and p r o g r e s s i v e i n v e s t i g a t i o n o f t h e s i m p l e s t p o l y m e r s i s needed t o d i s c o v e r a l l the i n f o r m a t i o n s t h a t c a n be e x t r a c t e d from t h e f i n e d e t a i l s c o n t a i n e d i n t h e p o l y m e r v a l e n c e band d a t a . This i s c e r t a i n l y one f i e l d o f r e s e a r c h t h a t w i l l p r o g r e s s i n t h e n e a r future. Ack nowledgements The authors w i s h to thank their coworkers from the L a b o r a t o i r e de S p e c t r o s c o p i e E l e c t r o n i q u e , and t h e L a b o r a t o i r e de Chimie Theorique Appliquee for t h e i r c o l l a b o r a t i o n . Gifts of samples and f r u i t f u l d i s c u s s i o n s a r e a k n o w l e d g e d from P r o f . Z a m b e l l i ( M i l a n o ) and D r . G o b i l l o n ( S o l v a y C o . ) . J . J . P i r e a u x i s i n d e b t e d to the Fonds N a t i o n a l de L a R e c h e r c h e S c i e n t i f i q u e f o r f i n a n c i a l support.


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Electronic Structure of Polymers - American Chemical Society

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