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Progress Toward Processable, Environmentally Stable Conducting Polymers Gregory L. Baker Bell Communications Research, 331 Newman Springs Road, R e d Bank, NJ 07701-7020
The progress toward the development of tractable, environmentally stable conducting polymers is reviewed. Many of the difficulties in developing such materials are a direct result of the high reactivity of the electronic states responsible for high conductivity. Although conducting polymers based on polyenes are generally unstable, environmentally stable substituted polythiophenes, polypyrroles, and poly(arylene vinylene)s have been prepared. Efforts to synthesize tractable conducting polymers have yielded a rich variety of blends, random copolymers, and graft and block copolymers with enhanced processability. The use of soluble precursor polymers has been an especially effective strategy for the preparation of dense anisotropic conducting polymer films.
THE IDEA THAT ORGANIC POLYMERS
c o u l d b e m a d e electrically c o n d u c t i v e has s t i m u l a t e d research for m o r e than 20 years (1). I n d e e d , the first m e a surements of the c o n d u c t i v i t y of poly(acetylene) a p p e a r e d i n 1961 (2), o n l y 3 years after N a t t a s report of the synthesis of polyacetylene (3). A l t h o u g h extensive conjugation was generally c o n s i d e r e d to b e r e q u i r e d i n an effective organic c o n d u c t o r (or superconductor) (4), the n e e d for the oxidation or r e d u c t i o n of the conjugated system was not f u l l y appreciated. E a r l y w o r k (1) was h a m p e r e d b y the lack o f a general t h e o r y , or a successful organic c o n d u c t o r to use as a m o d e l for the p r e p a r a t i o n of n e w materials. E q u a l l y i m p o r t a n t , the intractability o f the existing materials, often r e f e r r e d to as " b r i c k d u s t " , discouraged characterization efforts.
0065-2393/88/0218-0271$07.50/0 © 1988 American Chemical Society
In Electronic and Photonic Applications of Polymers; Bowden, Murrae J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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A materials b r e a k t h r o u g h l e d to the c u r r e n t h i g h interest i n c o n d u c t i n g p o l y m e r s . P o l y a c e t y l e n e , p r e v i o u s l y k n o w n o n l y as an intractable p o w d e r , was p r e p a r e d as s h i n y free-standing films (5). T h e later observation that the c o n t r o l l e d oxidation or r e d u c t i o n of polyacetylene films l e d to h i g h l y c o n d u c t i n g materials (6) s p a r k e d t r e m e n d o u s interest i n c o n d u c t i n g p o l y m e r s a n d t h e i r applications ( F i g u r e 5.1). T h i s interest was i n s p i r e d p a r t l y b y the b e l i e f that a p o l y m e r w i t h the c o n d u c t i v i t y of a m e t a l a n d the p h y s i c a l characteristics of c o n v e n t i o n a l thermoplastics (the " i d e a l " c o n d u c t i n g p o l y mer) w o u l d soon be available. P r o p o s e d uses for these materials i n c l u d e d l i g h t w e i g h t conductors, battery electrodes, solar cells, semiconductors, a n d the basic m a t e r i a l for m o l e c u l a r - s i z e d e l e c t r o n i c devices. T h e i m p l e m e n t a t i o n of c o n d u c t i n g p o l y m e r s i n these applications has lagged b e h i n d the early predictions o f w i d e s p r e a d n e a r - t e r m use. T h i s delay can b e traced to the difficult p r o b l e m o f p r e p a r i n g stable, tractable c o n d u c t i n g p o l y m e r s for g e n eral use. A l t h o u g h the i d e a l c o n d u c t i n g p o l y m e r has yet to b e s y n t h e s i z e d , spectacular i m p r o v e m e n t s i n the c o n d u c t i v i t y a n d tractability of c o n d u c t i n g o r ganic p o l y m e r s have b e e n r e p o r t e d . T w o p r i n c i p a l goals have g u i d e d synthetic chemists: 1. the p r e p a r a t i o n o f heat- or solution-processable
conducting
polymers, and
1
filled p o l y m e r s
|
poiyphenylene sulfide
I
"1 |
poiyphenylene vinylene
I
1
poiyphenylene I
1
polyacetylene
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CdS
graphite
I
I
I
I
I
I
I
-10
-8
-6
-4
-2
log conductivity
copper I
I
I
I
I
0
2
4
6
(S/cm)
Figure 5.1. Polymers with very different chemical structures can he converted from insulators to conducting materials by controlled oxidation or reduction. The conductivity of the conducting polymer may be as much as 10 times as large as the pristine polymer. 12
In Electronic and Photonic Applications of Polymers; Bowden, Murrae J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
5.
BAKER
Conducting
Polymers
273
2. the p r e p a r a t i o n o f materials w i t h m i n i m a l defects a n d a h i g h
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degree o f c h a i n a l i g n m e n t . Processability has obvious benefits, a n d samples w i t h fewer defects o r a l i g n e d chains can have significantly h i g h e r c o n d u c t i v i t i e s a n d stabilities relative to d i s o r d e r e d materials. T h e most d r a m a t i c example o f such benefits was the p r e p a r a t i o n of polyacetylene b y u s i n g a m o d i f i e d catalyst (7, 8). T h e r e s u l t i n g m a t e r i a l was i n m a n y ways spectroscopically i d e n t i c a l to that m a d e from standard preparations o f polyacetylene, except that it c o n t a i n e d few s p defects. T h e r e s u l t i n g conductivities for i o d i n e - d o p e d samples of s t r e t c h a l i g n e d polyacetylene w e r e 1.5 X 1 0 S / c m , a n d the e l e c t r i c a l anisotropics w e r e a r o u n d 1000. A c c o m p a n y i n g the increase i n c o n d u c t i v i t y was a n i n crease i n the e n v i r o n m e n t a l stability o f the d o p e d p o l y m e r . Significant p r o g ress has also b e e n r e p o r t e d i n m a k i n g tractable c o n d u c t i n g p o l y m e r s . Solutions o f c o n d u c t i n g p o l y m e r s i n b o t h t h e i r n e u t r a l a n d d o p e d states have b e e n p r e p a r e d , a n d e v e n w a t e r - s o l u b l e c o n d u c t i n g p o l y m e r s have b e e n s y n t h e s i z e d . Latexes are n o w available that d r y to y i e l d c o n d u c t i v e films. 3
5
I n this chapter, the progress t o w a r d m a k i n g c o n d u c t i n g organic m a t e rials tractable a n d stable e n o u g h for electronic applications is r e v i e w e d . Because the g e n e r a l field of c o n d u c t i n g p o l y m e r s has b e e n r e v i e w e d m a n y times (9, 10), this c h a p t e r w i l l focus almost e n t i r e l y o n the issues o f stability and tractability b y u s i n g the literature references available i n J u n e 1987. Because o f the b r e a d t h of the field, not a l l materials p o p u l a r l y d e s c r i b e d as c o n d u c t i n g p o l y m e r s are i n c l u d e d i n this r e v i e w . F o r e x a m p l e , the area o f c o n d u c t i n g composites is not c o m p r e h e n s i v e because a good r e v i e w o f c o m posites s h o u l d also i n c l u d e p o l y m e r s filled w i t h carbon black, m e t a l p a r t i c l e s , and inorganic fillers. Inorganic a n d organometallic conductors (e.g. the phthalocyanines) are b e y o n d the scope of this r e v i e w a n d w i l l not b e c o v e r e d . T h i s chapter w i l l first r e v i e w the electronic states responsible for h i g h c o n d u c t i v i t y i n organic p o l y m e r s (11) to p r o v i d e insight i n t o the d e s i g n o f stable organic conductors. N e x t , the efforts to i m p a r t e n v i r o n m e n t a l stability to the p o l y m e r s w i l l b e d e s c r i b e d . F i n a l l y , the p r i n c i p a l efforts t o w a r d the p r e p a r a t i o n o f tractable c o n d u c t i n g p o l y m e r s w i l l be discussed.
5.1 Electronic States of Conducting Polymers A l t h o u g h the exact m e c h a n i s m s b y w h i c h charge is transported i n c o n d u c t i n g p o l y m e r s are s t i l l d i s p u t e d (12, 13), i t is still useful to examine the s i m p l e one-electron m o d e l for the e l e c t r o n i c states f o r m e d i n organic p o l y m e r s u p o n oxidation or r e d u c t i o n (doping). ( D e t a i l e d discussions of this m o d e l can b e f o u n d i n reference 9.) T h e s e states are not necessarily synonymous w i t h t h e i r b e i n g charge carriers, b u t they do represent the c h e m i s t r y of d o p i n g . T h e m o d e l also p r o v i d e s g u i d e l i n e s for the synthesis of n e w c o n d u c t i n g p o l y mers a n d illustrates w h y most c o n d u c t i n g p o l y m e r s have l i m i t e d stability.
In Electronic and Photonic Applications of Polymers; Bowden, Murrae J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.
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F o r this exercise, conjugated p o l y m e r s can be d i v i d e d into two groups: those w i t h degenerate g r o u n d states such as frans-polyacetylene a n d p o l y ( p h e n y l e n e m e t h i n e ) , a n d those w i t h o u t degenerate g r o u n d states, s u c h as n e a r l y a l l o t h e r c o n d u c t i n g p o l y m e r s ( F i g u r e 5.2). O n an infinite p o l y e n e c h a i n , the transposition o f single a n d d o u b l e bonds y i e l d s energetically e q u i v a l e n t structures. T h i s situation is to be contrasted w i t h p o l y ( p - p h e n ylene), w h e r e a s i m i l a r transposition leads to the formation o f a q u i n o i d structure of significantly h i g h e r energy. T h i s difference leads to q u a l i t a t i v e l y different e l e c t r o n i c structures for the two p o l y m e r s . T h e r e d u c t i o n (oxidation) process ( F i g u r e 5.3) consists of a d d i n g (removing) one e l e c t r o n to the p o l y m e r c h a i n , w h i c h causes the i n j e c t i o n of states from the top o f the valence b a n d a n d b o t t o m o f the c o n d u c t i o n b a n d i n t o the gap. T h i s radical a n i o n (cation), c o m m o n l y t e r m e d a polaron, carries b o t h s p i n a n d charge. A d d i t i o n (removal) o f a second e l e c t r o n forms a second p o l a r o n , w h i c h can l o w e r the total energy o f the system t h r o u g h d i m e r i z a t i o n a n d y i e l d a bipolaron. I n the special case o f p o l y m e r s w i t h degenerate g r o u n d states, the excitations can f u r t h e r l o w e r t h e i r energy b y separating to y i e l d two i n d e p e n d e n t states (solitons) at o n e - h a l f o f the gap energy. T h i s separation is possible because a p o l y e n e segment of e q u i v a l e n t energy is f o r m e d b e t w e e n the charges as t h e y separate. I n p o l y m e r s w i t h nondegenerate g r o u n d states, the c h e m i s t r y is s i m i l a r , b u t pairs of solitons cannot c o m p l e t e l y separate because a p o l y m e r segment o f h i g h e r e n e r g y w o u l d f o r m b e t w e e n the charges. T h u s , for p o l y ( p - p h e n ylene) (Scheme 5.1), the f o r m a t i o n of the less stable q u i n o i d structure acts as a restraint to the d e v e l o p m e n t o f i n d e p e n d e n t soliton pairs, a n d i n s t e a d , a b i p o l a r o n results. T h e signature o f the b i p o l a r o n is the presence o f t h r e e e l e c t r o n i c transitions w i t h energies less t h a n the gap energy, whereas the soliton exhibits o n l y one e l e c t r o n i c transition. E l e c t r o n i c structures s i m i l a r
A
aromatic
B
B
=
quinoid E A
E aromatic
