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ELECTRICALLY CONDUCTIVE POLYMER-POLYMER COMPOSITES
M. Galvin, G. Wnek
To cite this version:
M. Galvin, G. Wnek. ELECTRICALLY CONDUCTIVE POLYMER-POLYMER COMPOSITES.
Journal de Physique Colloques, 1983, 44 (C3), pp.C3-151-C3-153. �10.1051/jphyscol:1983329�. �jpa-
00222680�
JOURNAL DE PHYSIQUE
Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-I51
ELECTRICALLY CONDUCTIVE POLYMER-POLYMER COMPOSITES
M.E. G a l v i n and G.E. Wnek
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.
Résumé - Des composites conducteurs ont été préparés par polymérisation de l'acétylène dans des films de polyéthylëne suivie par un dopage à l ' i o d e . L'obtention de conductivités élevées (5-10 S/cm) à faible concentration en (CH) (-3% poids) est expliquée à l'aide d'un modèle simple de percolation.
A b s t r a c t - Conductive composites have been prepared by p o l y m e r i z a t i o n o f ace- lene i n polyethylene f i l m s f o l l o w e d by i o d i n e doping. The attainment of high c o n d u c t i v i t y (5-10 S/cm) a t low (CH)X concentrations (~3 wt%) i s explained w i t h a simple p e r c o l a t i o n model.
I - INTRODUCTION
The high 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 , (CH)X, is tempered i n many instances by the prospects o f i n t r a c t a b i l i t y and poor environmental and physical p r o p e r t i e s . In an attempt t o m i t i g a t e such undesirable c h a r a c t e r i s t i c s , we have prepared composites / l / through the p o l y m e r i z a t i o n o f acetylene i n low den- s i t y polyethylene (LDPE) impregnated w i t h the Shirakawa / 2 / c a t a l y s t . This approach may be p o t e n t i a l l y useful i n t h a t (1) i n - s i t u p o l y m e r i z a t i o n could provide a more i n t i m a t e molecular mixing o f the components as compared w i t h mechanically prepared d i s p e r s i o n s , ( 2 ) a v a r i e t y o f matrices w i t h d e s i r a b l e physical p r o p e r t i e s may be employed, (3) a r t i c l e s o f the selected m a t r i x m a t e r i a l may be p r e - f a b r i c a t e d t o a desired s t r u c t u r e f o l l o w e d by 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 doping, and (4) (CH)X chains may be i s o l a t e d i n a m a t r i x i n order t o study the i n f l u e n c e o f the l o c a l molecular environment (chemical c o m p o s i t i o n , morphology, e t c . ) o n , f o r example, s o l i t o n m o b i l i t y . Recent studies concerning the (CH)X/LDPE system and p r e l i m i n a r y r e s u l t s from syntheses employing e t h y l e n e / v i n y l a c e t a t e copolymers as matrices are presented i n t h i s r e p o r t .
I I - EXPERIMENTAL
The (CH)X/LDPE composites were prepared using the Ti(0Bu)4/Et3Al 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 described / ! / . The h i g h l y amorphous EVA f i l m s (ob- t a i n e d from E n s i g n - B i c k f o r d Co.) allowed f a c i l e impregnation and acetylene polymeri- z a t i o n a t 25CC. The amount of (CH)X incorporated was determined by m o n i t o r i n g 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 conductive d e r i v a t i v e s were prepared by immersion o f the composites i n a s a t u r a t e d 12/pentane s o l u t i o n f o r 24-48 hours. 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 standard four-probe t e c h - niques.
I l l - RESULTS AND DISCUSSION
The r e l a t i o n s h i p between the four-probe e l e c t r i c a l c o n d u c t i v i t y , a, and the wt.%
(CH)x i n iodine-doped (CH)X/LDPE composites i s shown i n Figure 1 . An apparent per- c o l a t i o n t h r e s h o l d e x i s t s between 2-4 wt.% (CH)X. Many a d d i t i o n a l data p o i n t s have been c o l l e c t e d since our previous communication / I / which show (as expected) a con-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983329
JOURNAL DE PHYSIQUE
WT. 010 /oCH)x
FIGURE 1 : CC1lDUCTIVITY OF r2-DOPEO COMPDSITES VERSUS UT.% (CH),.
10 L 1
.
0WT. % (CHI,
FlGURE 2 : CONDVCTIVITY OF 12-DOPED COMPOSITES VERSUS YT.",CH), EXTRRPOLATED TO THE CONDUCTlVlTY ?F 12-WPED.
LC" )% .
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*LOPE W k - W b Apdyttyrene 10s 'c OLDPE 00 -c OLDPE *no%
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1 I I I 1 I I I I
2 4 6 8 l0 12 14 16 18
tinuous increase i n a w i t h (CH)x c o n t e n t beyond t h e "knee" i n Figure 1.. I n F i g u r e 2 we r e p l o t t h e data t o i n c l u d e a value /3/ f o r a of 12-doped, t r a n s (CH),; e x t r a - p o l a t i o n o f o u r data t o t h i s value a f f o r d s a reasonable l i n e . The apparent t h r e s - h o l d a t such a low 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 thresholds o f
G.
4 vol.% carbon b l a c k i n LDPE have been observed /4/. T h i s has been r a t i o n a l i z e d as being due t o a combination 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 . I t should be noted t h a t t h e (CH)x e n t i t i e s (as revealed by transmission e l e c t r o n microscopy o f t h i n f i l s ) i n t h e composites a r e i r r e g u l a r i n shape w i t h s i z e s i n f h e range o f 600-2,000A.
We f i n d t h a t (CHIx "powder" /2/ c o o s i r t s o f much l a r g e r e n t i t i e s (hundreds o f microns i n s i z e ) and t h a t simple d i s p e r s i o n s o f pow- dered (CH)x i n LDPE (made by c a s t i n g f i l m s from h o t toluene 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 (240 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 conductive mate- r i a l s upon I 2 doping. Thus, 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 approach appears t o be the a b i l i t y t o form much s m a l l e r (CH)x domains and i s presumably responsible, a t l e a s t i n p a r t , for t h e observed p e r c o l a t i o n t h r e s h o l d . I t i s p o s s i b l e t h a t m e l t e x t r u s i o n o f 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 thresholds (compared w i t h s o l v e n t - c a s t systems) although we have n o t attempted t o perform such experiments.I n an attempt t o expand t h e scope o f t h e i n - s i t u method t o i n c l u d e p o l a r polymers, EVA m a t r i c e s were employed. Acetylene p o l y m e r i z a t i o n proceeded r a p i d l y a t room temperature. Composites c o n t a i n i n g as much as 20 wt .% (CH), were prepared b u t a f f o r d e d c o n d u c t i v i t i e s o f o n l y 0.5
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1.0 S/cm upon I 2 doping. The low c o n d u c t i - v i t y may be due t o m i c r o s t r u c t u r a l c o n s t r a i n t s (perhaps enhanced phase separation because o f t h e p o l a r i t y d i f f e r e n c e between the two polymers) o r t o i n t e r a c t i o n s be- tween t h e p o l a r v i n y l a c e t a t e groups and t h e doped (CH), o r 12.I V
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ACKNOWLEDGEMENTSWe a r e g r a t e f u l f o r support from the MIT Center f o r M a t e r i a l s Science and Engineering (NSF-MRL Core Fund DMR 78-24185), 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 from P o l a r o i d Corporation ( t o M.E. G a l v i n ) , and thank Dr. 6. S k u t n i k o f Ensign-Bickford Co. f o r t h e EVA samples.
V
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REFERENCES1. GALVIN M.E., WNEK G.E., Polym Comm.,
2
(1982) 795.2. SHIRAKAWA H., IKEDA S., Synth. Met., 1 (2) (1980) 175.
3. MACDIARMID A.G., HEEGER A.J., i b i d . ,
1
(1979) 101.4. MIYASAKA K., WATANABE K., JOJIMA E., AIDA H., SUMITA M., ISHIKAWA K., J. Mater.
Sci., (1982) 1610.
