40 Conductive Hybrids Based on Polyacetylene: Copolymers and Blends
Polymers in Electronics Downloaded from pubs.acs.org by YORK UNIV on 12/18/18. For personal use only.
M. E. GALVIN, G. F. DANDREAUX, and G. E. WNEK Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
Several approaches used to prepare hybrid polymers i n which polyacetylene i s an electroactive component are presented. S p e c i f i c a l l y , these involve the prepara tion of (1) composites by in-situ polymerization, (2) graft copolymers utilizing carbanions i n n-type (CH) as polymerization initiators, and (3) A-B diblock co polymers exploiting anionic-to-Ziegler-Natta trans formation reactions. X
The h i g h l e v e l o f i n t e r e s t i n p o t e n t i a l a p p l i c a t i o n s o f p o l y a c e t y l e n e ( 1 ) , ( C H ) , i s t e m p e r e d i n many i n s t a n c e s b y t h e p r o s p e c t s o f i n t r a c t a b i l i t y and p o o r p h y s i c a l and m e c h a n i c a l p r o p e r t i e s . I n an a t t e m p t t o m i t i g a t e s u c h u n d e s i r a b l e c h a r a c t e r i s t i c s , we h a v e a t t e m p t e d t o p r e p a r e c o p o l y m e r s and b l e n d s ( o r c o m p o s i t e s ) i n w h i c h t h e e l e c t r o a c t i v e component i s ( C H ) . C o p o l y m e r i z a t i o n has t r a d i t i o n a l l y been a f r u i t f u l approach for the c o n s t r u c t i o n o f o r g a n i c m a t e r i a l s p o s s e s s i n g s p e c i f i c c h e m i c a l , p h y s i c a l and m e c h a n i c a l p r o p e r t i e s . Copolymers are c l a s s i f i e d a s random, a l t e r n a t i n g , g r a f t o r b l o c k d e p e n d i n g upon t h e s t r u c t u r a l a r r a n g e m e n t o f t h e comonomer u n i t s . Random ( o r n e a r l y so) c o p o l y m e r s o f a c e t y l e n e and m e t h y l a c e t y l e n e h a v e b e e n p r e p a r e d (2) w h i c h e x h i b i t , a s e x p e c t e d , e l e c t r i c a l c o n d u c t i v i t i e s upon d o p i n g w h i c h a r e h i g h l y d e p e n d e n t upon comonomer c o n c e n tration. The d e c r e a s e i n c o n d u c t i v i t y w i t h i n c r e a s i n g m e t h y l a c e t y l e n e c o n c e n t r a t i o n h a s b e e n r a t i o n a l i z e d ( 2 , 3 ) as b e i n g due t o d i s r u p t i o n o f p l a n a r i t y of the backbone. A s i m i l a r argument a p p a r e n t l y a p p l i e s to the a c e t y l e n e / p h e n y l a c e t y l e n e system(4). Inte r e s t has d e v e l o p e d i n t h i s l a b o r a t o r y c o n c e r n i n g the p o s s i b i l i t y of a t t a c h i n g a wide v a r i e t y o f polymer chains to the ( C H ) back bone ( i . e . , grafting). A m o t i v a t i o n f o r such work i s the p o s s i b i l i t y of t a i l o r i n g the surface p r o p e r t i e s of ( C H ) f o r e l e c t r o d e applications. The a b i l i t y o f a l k a l i m e t a l g r a p h i t i d e s (5) t o i n i t i a t e p o l y m e r i z a t i o n o f s e v e r a l monomers s u g g e s t e d t h e u s e o f doped ( C H ) i n t h i s c a p a c i t y w i t h t h e p r o s p e c t o f c o v a l e n t l y X
X
X
X
X
0097-6156/ 84/0242-0507506.00/0 © 1984 American Chemical Society
508
POLYMERS IN ELECTRONICS
b i n d i n g polymer chains t o (CH) backbones. Block copolymers are a l s o o f i n t e r e s t because the p l a n a r i t y of the ( C H ) b l o c k i s expected to remain e s s e n t i a l l y i n t a c t , a l l o w i n g the e l e c t r i c a l prop e r t i e s t o be p r i m a r i l y d e t e r m i n e d by p e r c o l a t i o n o f ( C H ) d o m a i n s . F u r t h e r m o r e , s o l u b l e and t r a c t a b l e d e r i v a t i v e s c o u l d be e n v i s i o n e d by j u d i c i o u s c h o i c e o f t h e r e m a i n i n g b l o c k ( s ) . S i n c e a c e t y l e n e p o l y m e r i z a t i o n w i l l n e c e s s a r i l y be i n i t i a t e d by c o o r d i n a t i o n c a t a l y s t s , we have c o n s i d e r e d t h e p o s s i b i l i t y o f u s i n g t h e " a l t e r n a t e f e e d " method t o p r o d u c e , f o r e x a m p l e , e t h y l e n e - a c e t y l e n e b l o c k copolymers from a p p r o p r i a t e t r a n s i t i o n metal systems. However, such c a t a l y t i c systems are t y p i c a l l y not s u i t a b l e f o r the synthes i s o f w e l l - d e f i n e d b l o c k s (6) due t o v a r i a b l e i n i t i a t i o n r a t e s o f t h e c a t a l y t i c c e n t e r s and u n d e s i r a b l e t e r m i n a t i o n r e a c t i o n s . The e x p l o i t a t i o n o f a c t i v e p o l y m e r - t r a n s i t i o n m e t a l bonds f o r t h e s y n t h e s i s o f b l o c k c o p o l y m e r s (7,8) suggested a p o t e n t i a l l y v i a b l e approach. S p e c i f i c a l l y , we h a v e a t t e m p t e d t o use " l i v i n g " p o l y s t y r e n e t o a l k y l a t e Ti(0Bu)4 f o l l o w e d by a c e t y l e n e p o l y m e r i z a t i o n , viz X
X
X
A p o t e n t i a l a d v a n t a g e o f s u c h a t r a n s f o r m a t i o n r e a c t i o n (8) i s t h a t a t l e a s t t h e f i r s t b l o c k ( p o l y s t y r e n e ) c a n be s y n t h e s i z e d w i t h a w e l l - d e f i n e d c h a i n l e n g t h . D i f f e r e n t a p p r o a c h e s t o g r a f t s and b l o c k s b a s e d on t e r m i n a t i o n o f a c e t y l e n e p o l y m e r i z a t i o n by a p p r o p r i a t e c a r r i e r polymers hava r e c e n t l y been r e p o r t e d (9)· C o m p o s i t e s h a v e b e e n p r e p a r e d (10) t h r o u g h t h e p o l y m e r i z a t i o n o f a c e t y l e n e i n l o w d e n s i t y p o l y e t h y l e n e (LDPE) i m p r e g n a t e d w i t h t h e S h i r a k a w a (11) c a t a l y s t . T h i s a p p r o a c h may be p o t e n t i a l l y
40.
GALVIN ET AL.
509
Conductive Hybrids Based on PA
u s e f u l i n t h a t t h e ( 1 ) i n - s i t u p o l y m e r i z a t i o n c o u l d p r o v i d e a more i n t i m a t e m o l e c u l a r m i x i n g o f t h e components a s compared w i t h mecha n i c a l l y p r e p a r e d d i s p e r s i o n , (2) a v a r i e t y o f m a t r i c e s w i t h d e s i r a b l e p h y s i c a l p r o p e r t i e s may be e m p l o y e d , (3) a r t i c l e s o f t h e s e l e c t e d m a t r i x m a t e r i a l s may be p r e - f a b r i c a t e d t o a d e s i r e d s t r u c t u r e f o l l o w e d b y c a t a l y s t i m p r e g n a t i o n , p o l y m e r i z a t i o n and d o p i n g , and (4) ( C H ) c h a i n s may be i s o l a t e d i n a m a t r i x i n o r d e r t o s t u d y t h e i n f l u e n c e o f t h e l o c a l m o l e c u l a r e n v i r o n m e n t ( c h e m i c a l compo s i t i o n , morphology, e t c . ) on, f o r example, s o l i t o n m o b i l i t y . X
Experimental Composites. The ( C H ) / L D P E c o m p o s i t e s w e r e p r e p a r e d u s i n g t h e T i ( 0 B u ) 4 / E t 3 A l Z i e g l e r - N a t t a c a t a l y s t system as p r e v i o u s l y des c r i b e d ( 1 0 ) . The amount o f ( C H ) i n c o r p o r a t e d was d e t e r m i n e d b y monitoring the acetylene uptake during the p o l y m e r i z a t i o n . E l e c t r i c a l l y c o n d u c t i v e d e r i v a t i v e s were p r e p a r e d b y i m m e r s i o n o f t h e c o m p o s i t e s i n a s a t u r a t e d 1 2 / p e n t a n e s o l u t i o n f o r 24-48 h o u r s . E l e c t r i c a l c o n d u c t i v i t i e s were measured by s t a n d a r d f o u r - p r o b e techniques. X
X
Grafts. P o l y a c e t y l e n e f i l m s w e r e s y n t h e s i z e d a t -78°C u s i n g t e c h n i q u e s s i m i l a r t o t h o s e d e v e l o p e d b y S h i r a k a w a and c o w o r k e r s ( 1 1 ) . R e d u c t i v e d o p i n g was c a r r i e d o u t i n a d r y b o x b y i m m e r s i o n o f ( C H ) f i l m s i n 1 M sodium n a p h t h a l i d e / T H F s o l u t i o n s f o r 2 minutes. The f i l m s w e r e t h e n washed s e v e r a l t i m e s w i t h d r y , 0 2 - f r e e THF and a l l o w e d t o s t a n d i n f r e s h THF f o r a p p r o x i m a t e l y 1 h o u r . The c o n d u c t i v i t i e s and c o m p o s i t i o n s o f t h e f i l m s w e r e i n t h e r a n g e 5-50 S/cm and [CHNaQ 20-0.25lx» r e s p e c t i v e l y . E x p o s u r e o f t h e η-type f i l m s t o e i t h e r l i q u i d ( s t y r e n e , m e t h y l m e t h a c r y l a t e ) o r g a s e o u s ( e t h y l e n e o x i d e , i s o p r e n e ) mono mers r e s u l t e d i n p o l y m e r i z a t i o n . Much o f o u r i n i t i a l w o r k h a s f o c u s e d on g r a f t i n g o f p o l y ( e t h y l e n e o x i d e ) (PEO) t o ( C H ) i n an e f f o r t t o r e n d e r t h e ( C H ) s u r f a c e more h y d r o p h i l i c and t o p r o v i d e c o v a l e n t attachment o f a m a t e r i a l c a p a b l e o f f u n c t i o n i n g as a s o l i d e l e c t r o l y t e ( 1 2 ) . F i l m s o f η-type ( C H ) w e r e e x p o s e d t o d r y ( C a H 2 - t r e a t e d ) , g a s e o u s e t h y l e n e o x i d e i n t h e r a n g e 55-75°C w i t h i n i t i a l p r e s s u r e s b e i n g c a . 500 t o r r . R e a c t i o n times were t y p i c a l l y 5 hours. The f i l m s w e r e washed w i t h d r y , 0 2 - f r e e m e t h y l e n e c h l o r i d e t o remove n o n - c o v a l e n t l y bound PEO and t h e n w i t h d e a e r a t e d H2O t o p r o t o n a t e o x y a n i o n s and remove t h e NaOH b y p r o d u c t . The p r e s e n c e o f bound PEO a f t e r e x t r a c t i o n was c o n f i r m e d b y I R s p e c troscopy. X
e
X
X
X
Blocks. The s y n t h e s i s o f b l o c k c o p o l y m e r s w e r e a t t e m p t e d i n t h e f o l l o w i n g manner. " L i v i n g " p o l y s t y r e n e was f i r s t p r e p a r e d b y c o n v e n t i o n a l a n i o n i c t e c h n i q u e s u s i n g n - B u L i a s t h e i n i t i a t o r i n THF a t -78°C. I n i n i t i a l e x p e r i m e n t s , t h i s p o l y s t y r y l l i t h i u m p r o d u c t was t r e a t e d w i t h A I C I 3 t o a f f o r d a p o l y s t y r y l A l s p e c i e s w h i c h c o u l d be c a p a b l e o f a l k y l a t i n g T i ( 0 B u ) 4 . However, we f i n d i t t o
POLYMERS IN ELECTRONICS
510
be more u s e f u l t o d i r e c t l y a k l y l a t e T i ( 0 B u ) 4 w i t h t h e p o l y s t y r y l Li. The o r a n g e - r e d p o l y s t y r y l L i s o l u t i o n became deep r e d upon t r e a t m e n t w i t h T i ( 0 B u > 4 a t -78°C ( L i / T i = 2 : 1 ) . The s o l u t i o n was warmed t o room t e m p e r a t u r e and d r y , 0 2 - f r e e a c e t y l e n e ( i n i t i a l p r e s s u r e s c a . 700 t o r r ) was i n t r o d u c e d on a vacuum l i n e . The s o l u t i o n became deep b l u e i n c o l o r and i n some c a s e s , f i n e b l u e b l a c k p a r t i c u l a t e s w e r e o b s e r v e d . The p o l y m e r i z a t i o n s w e r e t e r m i n a t e d by a d d i t i o n o f MeOH. The p r e c i p i t a t e d p o l y m e r s w e r e e x t r a c t e d w i t h T H F / a c e t o n e w h i c h removed h o m o p o l y s t y r e n e . The r e s i dues w e r e b r o m i n a t e d i n THF and t h e n s u b j e c t e d t o GPC a n a l y s i s . R e s u l t s and
Discussion
C o m p o s i t e s . The r e l a t i o n s h i p b e t w e e n t h e f o u r - p r o b e e l e c t r i c a l c o n d u c t i v i t y , 0 , and t h e wt.% ( C H ) i n i o d i n e - d o p e d (CH) /LDPE c o m p o s i t e s i s shown i n F i g u r e 1. An a p p a r e n t p e r c o l a t i o n t h r e s h o l d e x i s t s b e t w e e n 2-4 wt.% ( C H ) . Many a d d i t i o n a l d a t a p o i n t s h a v e b e e n c o l l e c t e d s i n c e o u r p r e v i o u s c o m m u n i c a t i o n w h i c h show (as expected) a c o n t i n u o u s i n c r e a s e i n σ w i t h ( C H ) c o n t e n t beyond t h e " k n e e i n F i g u r e 1. I n F i g u r e 2 we r e p l o t t h e d a t a t o i n c l u d e a v a l u e (1) f o r Ο o f 12-doped, t r a n s ( C H ) ; e x t r a p o l a t i o n o f o u r data to t h i s value a f f o r d s a reasonable l i n e . The a p p a r e n t t h r e s h o l d a t s u c h a l o w l o a d i n g i s somewhat s u r p r i s i n g a l t h o u g h i t i s i n t e r e s t i n g t h a t t h r e s h o l d s o f c a . 4 v o l . % c a r b o n b l a c k i n LDPE h a v e b e e n o b s e r v e d ( 1 3 ) . T h i s has b e e n r a t i o n a l i z e d a s b e i n g due t o a c o m b i n a t i o n o f p a r t i c l e s i z e and w e t t a b i l i t y e f f e c t s . It s h o u l d be n o t e d t h a t t h e ( C H ) e n t i t i e s ( a s r e v e a l e d by t r a n s m i s s i o n e l e c t r o n microscopy of t h i n f i l m s ) i n the composites are i r r e g u l a r i n s h a p e w i t h s i z e s i n t h e r a n g e o f 600-2,000 Â. We f i n d t h a t ( C H ) "powder" (11) c o n s i s t s o f much l a r g e r e n t i t i e s ( h u n d r e d s o f m i c r o n s i n s i z e ) and t h a t s i m p l e d i s p e r s i o n s o f powd e r e d ( C H ) i n LDPE (made by c a s t i n g f i l m s f r o m h o t t o l u e n e i n a d r y box) o f much h i g h e r l o a d i n g l e v e l s (>40 wt.%) g e n e r a l l y f a i l t o y i e l d h i g h l y c o n d u c t i v e m a t e r i a l s upon I2 d o p i n g . T h u s , a v i r t u e o f t h e i n - s i t u p o l y m e r i z a t i o n a p p r o a c h a p p e a r s t o be t h e a b i l i t y t o f o r m much s m a l l e r ( C H ) d o m a i n s and i s p r e s u m a b l y r e s p o n s i b l e , at l e a s t i n p a r t , f o r the observed p e r c o l a t i o n t h r e s hold. I t i s p o s s i b l e that melt e x t r u s i o n of LDPE/(CH) p a r t i c u l a t e systems c o u l d a f f o r d m a t e r i a l s having lower p e r c o l a t i o n t h r e s h o l d s (compared w i t h s o l v e n t - c a s t s y s t e m s ) a l t h o u g h we h a v e n o t a t t e m p t e d t o p e r f o r m s u c h e x p e r i m e n t s . The i n t e r e s t i n g p r o p e r t i e s o f ( C H ) / e l a s t o m e r c o m p o s i t e s p r e p a r e d by t h e i n - s i t u approach have r e c e n t l y been d e s c r i b e d ( 1 4 ) . X
X
X
X
11
X
X
X
X
X
X
x
Grafts. The p r i m a r y r e a c t i o n o f η-type ( C H ) and e t h y l e n e o x i d e i s presumably a t t a c k of a carbanion of a methylene carbon r e s u l t i n g i n r i n g o p e n i n g and o x y a n i o n f o r m a t i o n , f o l l o w e d by s u c c e s s i v e monomer a d d i t i o n s : X
GALVIN ET A L .
Conductive Hybrids Based on PA
10F
ο polymerization temperature
matrix
LDP Ε 90'c no'c
A • Ο
,0*-
e
polystyrene i05 c e
0 J
L
6
8
LDPE 60 c LDPE 120 *c HDPE n o t
J_
10 12 14 16 10'
WT. %(CH)
X
Figure 1. Conductivity/composition data for (CH) /polymer composites doped with I^.
512
POLYMERS IN ELECTRONICS
The fact that approximately half of the product can be removed by CH2CI2 extraction may suggest that electron transfer from a (CH) radical anion to ethylene oxide occurs leading to polymerization without attachment to the (CH) or that some attached PEO suffered mechanically induced scission. The o i l y character of the extrac ted PEO suggests a low molecular weight. We have no information regarding the molecular weight of the covalently bound PEO or the number of PEO branches. The (CH) /PEO materials can be doped with 12 to afford conductivities of approximately 1 Ω ™ " . Problems with e l e c t r i c a l contacts due to an insulating PEO layer may be a factor i n this lower conductivity compared with 12-doped (CH) but i t should be noted that the (CH) conjugation i s broken at the graft points and this is expected to reduce the intrachain hole mobility. At present, we do not believe that Na metal in the films i s responsible for i n i t i a t i o n since η-type (CH) films which con tained a 2:1 naphthalene/sodium ratio were s t i l l highly active as polymerization i n i t i a t o r s . A comparison of the number of (CH) carbanions which are i n i t i a t o r s with the number, i f any, of elec tron transfer sites may yield important information about the distribution of charge carrier reactivities in doped (CH) . X
X
x
- 1
1
X
X
X
X
X
Blocks. The IR spectra of films cast from the soluble, colored THF fraction are consistent with the presence of both polystyrene and (CH) segments, although do not prove the existence of a block copolymer. The required proof i s derived from GPC studies (Figure 3). A sample of homopolystyrene (taken from the " l i v i n g " polysty rene prior to reaction with Ti(0Bu)4> was found to have an M of 27,730. Also, a sample of homopolystyrene (from the THF/acetone extraction, treated with Br2 i n THF) yielded an of 27,470 i n d i cating that the extraction does not merely fractionate the polyX
n
40.
GALVIN ET AL.
Conductive
Hybrids Based on PA
513
s t y r e n e i t s e l f and t h a t Br£ t r e a t m e n t h a s a n e g l i g i b l e e f f e c t on M. However, t h e GPC t r a c e o f t h e c o l o r e d r e s i d u e w h i c h h a d b e e n p r e v i o u s l y b r o m i n a t e d c l e a r l y shows a h i g h e r m o l e c u l a r w e i g h t . An a b s o l u t e M c a n n o t be d e t e r m i n e d s i n c e t h e h y d r o d y n a m i c v o l u m e o f p o l y ( l , 2 - d i b r o m o e t h y l e n e ) i s unknown a l t h o u g h t h e a p p a r e n t M i s 72,360. I t i s d i f f i c u l t t o r e c o n c i l e t h i s r e s u l t a s b e i n g due t o anything but the presence of a t r u e d i b l o c k copolymer. For the s a m p l e d i s c u s s e d a b o v e , t h e y i e l d o f b l o c k c o p o l y m e r b a s e d on t h e t o t a l w e i g h t o f p o l y s t y r y l L i u s e d i n i t i a l l y i s c a . 5.5%. The observation that the soluble diblocks slowly y i e l d particulates upon s t a n d i n g s u g g e s t s t h a t c r y s t a l l i z a t i o n o f ( C H ) d o m a i n s o c c u r s , a l t h o u g h a t a much s l o w e r r a t e compared w i t h p r e c i p i t a t i o n d u r i n g t h e p r e p a r a t i o n o f ( C H ) powders ( 1 1 ) . T h i s i s p r e s u m a b l y due t o s l o w e r c r y s t a l l i z a t i o n k i n e t i c s b y v i r t u e o f t h e p r e s e n c e of the s o l u b i l i z i n g p o l y s t y r e n e " t a i l " . Studies of the e f f i c i e n c y o f t h e s y n t h e s i s a n d t h e m o r p h o l o g y and e l e c t r i c a l p r o p e r t i e s o f the d i b l o c k copolymers a r e i n progress. n
n
n
X
X
Μ χ
10'
5
F i g u r e 3. GPC t r a c e s f o r a l i q u o t o f " l i v i n g " p o l y s t y r e n e ( s t a r t i n g P S ) , PS e x t r a c t e d from a c e t y l e n e / s t y r e n e b l o c k m a t e r i a l ( s o l u b l e i n 10% T H F / a c e t o n e ) , and b r o m i n a t e d acetylene/styrene b l o c k ( i n s o l u b l e i n 10% T H F / a c e t o n e ) .
514
P O L Y M E R S IN ELECTRONICS
A c k n o w l e dgment s We a r e g r a t e f u l f o r s u p p o r t f r o m t h e MIT C e n t e r f o r M a t e r i a l s S c i e n c e and E n g i n e e r i n g (NSF-MRL C o r e Fund DMR 7 8 - 2 4 1 8 5 ) , a DuPont Young F a c u l t y Award ( t o G . E . Wnek) and a f e l l o w s h i p f r o m P o l a r o i d Corporation (to M . E . G a l v i n ) .
Literature Cited 1. MacDiarmid, A.G.; Heeger, A.J. Synth. Met. 1979, 1, 101. 2. Chien, J.C.W.; Wnek, G.E.; Karasz, F.E.; Hirch, J.A. Macromolecules 1981, 14, 479. 3. Tripathy, S.K.; Kitchen, D.; Druy, M.A. ACS Polym. Preprints, 1982, 23, 109. 4. Deits, W.; Cubor, P.; Rubner, M.; Jopson, H. Synth. Met. 1982, 4, 199. 5. Gole, J.; Merle, G.; Pascault, J.P. Synth. Met. 1982, 4, 269. 6. Borr, J., Jr. in "Ziegler-Natta Catalysts and Polymerizations"; Academic Press: New York, 1979; chap. 21. 7. Sioue, Α.; Fontanille, M. Eur. Polym. J. 1981, 17, 1175. 8. Richards, D.H. Br. Polym. J. 1980, 12, 89. 9. Bates, F.S.; Baker, G.L. Macromolecules 1983, 16(4), 704. 10. Galvin, M.E.; Wnek G.E. Polym. Comm. 1982, 23, 795. 11. Shirakawa, H.; Ikeda,S. Synth. Met. 1980, 1, 172. 12. Armand, M.Β.; Chabagno, J.M.; Duclot, M.J. in "Fast Ion Trans port in Solids"; Vashista; Mundy; Shenoy, Eds.; Elsevier North Holland, 1979, p. 131. 13. Miyasaka, K.; Watanabe, K.; Jojima, E.; Aida, H.; Sumita, M.; Ishikawa, Κ. J. Mater. Sci. 1982, 17, 1610. 14. Rubner, M.F.; Tripathy, S.K.; Georger, J.,; Cholewa, P. Macromolecules 1983, 16(6), 870. RECEIVED December 19, 1983
