- Email: [email protected]
An emulsion pathway to electrically conductive polyaniline-polystyrene composites
Synthetic Metals, 53 (1993) 283-292
283
An emulsion pathway to electrically conductive polyaniline-polystyrene composites Eli R u c k e n s t e i n * a n d S h i y o n g Yang Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, N Y 14260 (USA)
(Received June 25, 1992; accepted August 31, 1992)
Abstract A method for the preparation of electrically conductive polyaniline-polystyrene composites is described. The composite is obtained starting from an emulsion in which a solution of sodium dodecylsulfate in water constitutes the continuous phase and a solution of aniline and polystyrene in benzene the dispersed phase. Polymerization of aniline takes place by introducing an oxidant dissolved in an aqueous solution of HC1 under vigorous stirring in the emulsion. The composite either precipitates or is precipitated by using a suitable agent. The conductivity can reach values as high as 3-5 S/cm. In contrast to polyaniline, the composite obtained is processable due to the insulating polymer.
Introduction Polyaniline is r e c o g n i z e d as an air-stable, b u t u n p r o c e s s a b l e , electrically c o n d u c t i v e p o l y m e r [ 1 ]. N u m e r o u s studies h a v e b e e n c o n d u c t e d a n d m e t h o d s d e v e l o p e d for t h e i m p r o v e m e n t of its p r o c e s s a b i l i t y . T h e s e include the addition o f side g r o u p s to the m a i n c h a i n [2], g r a f t i n g the c o n d u c t i n g p o l y m e r chain to a n o n - c o n d u c t i v e p o l y m e r [3], i m b i b i n g a p o r o u s m e d i u m with aniline a n d p o l y m e r i z i n g the aniline inside [ 4 ], p r e p a r a t i o n o f c o m p o s i t e s o f c o n d u c t i v e a n d n o n - c o n d u c t i v e ( b u t easily p r o c e s s a b l e ) p o l y m e r s [5] a n d e l e c t r o c h e m i c a l p o l y m e r i z a t i o n o f aniline in a p o l y m e r m a t r i x [ 6 - 8 ] . U n f o r t u n a t e l y , t h e s e t e c h n i q u e s often yield m a t e r i a l s with m u c h l o w e r c o n d u c t i v i t y t h a n the p u r e parent polymer. Recently, A n d r e a t t a et al. [9] r e p o r t e d a m e t h o d f o r t h e p r e p a r a t i o n of a b l e n d fiber o f polyaniline a n d p o l y ( p - p h e n y l e n e t e r e p h t h a l a m i d e ) f r o m their solution in c o n c e n t r a t e d sulfuric acid b y p r e c i p i t a t i n g with a n a q u e o u s solution o f H2SO4. B e a d l e a n d A r m e s [ 10 ] p r e p a r e d a series of p o l y a n i l i n e - c o p o l y m e r l a t e x c o m p o s i t e s b y oxidative p o l y m e r i z a t i o n o f aniline in t h e p r e s e n c e of a film-forming c h l o r i n a t e d c o p o l y m e r latex. Our g r o u p h a s r e p o r t e d a r o u t e to p r e p a r e a p o l y p y r r o l e - p o l y u r e t h a n e c o m p o s i t e b y c o p r e c i p i t a t i o n f r o m a concentrated emulsion containing polyurethane and an aqueous suspension o f p o l y p y r r o l e p o w d e r [ 11 ]. *Author to whom correspondence should be addressed.
0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
284 In this paper, a method is pr e s ent ed for the preparation of polyaniline-polystyrene composites with good processability. Two steps are involved in this method. First, an emulsion with the appearance of a gel was p r e p a r e d starting from a solution of polystyrene and aniline in benzene as the dispersed phase and a solution of sodium dodecylsulfate (SDS) in water as the continuous phase. Secondly, an oxidant dissolved in an aqueous solution of HC1 was added dropwise to the emulsion with vigorous mechanical stirring in order to polymerize the aniline and to dope the polyaniline formed. The polymerization was allowed to p r o c e e d for 3 h. Depending upon the polyaniline content in the composites, a green-to-dark-blue solid material was obtained. The composites obtained were characterized by scaIming electron m i c r o s c o p y (SEM) and Fourier transform infrared s pect roscopy (FF-IR).
Experimental Chemicals Aniline (Aldrich, 99.5%) was purified by distillation in vacuum before use. Polystyrene (PS, Aldrich, ~;/w: 2 8 0 0 0 0 ) , ammonium persulfate ((NH4)2S2Os, Polysciences), sodium dodecylsulfate (SDS, Polysciences), benzene (Aldrich, 99%) and methanol (Aldrich, 99.9%) were used as received. Water was de-ionized and distilled.
Preparation o f the electrically conductive composite In a typical experiment, 0.2 g of SDS and 2 ml of water were placed in a 100 ml flask. To this, 5 ml of a solution containing 0.4 g polystyrene, 0.2 g aniline and the balance benzene was added dropwise, with stirring. An emulsion with the appearance of a gel, in which the water solution constitutes the continuous phase and the benzene-based solution the dispersed phase, was thus obtained. Then, 10 ml of 1 N HC1 aqueous solution containing 0.5 g of ammonium persulfate were introduced dropwise, with stirring, to polymerize the aniline and dope the polyaniline formed. The polymerization reaction was allowed to p r o c e e d for 3 h, with stirring. The benzene of the polymer solution was separated after the composite was formed. The solid material thus obtained was washed with 20 ml of methanol, filtered, washed again with 1 N HCI solution and finally dried at 40 °C in vacuum for 24 h. An amount of 0.52 g of polyaniline-polystyrene composite was thus obtained. The yield of the composite defined as the ratio of grams of composite to grams of polystyrene plus grams of aniline was 87%. When the amount of aniline was smaller than about 0.1 g and the other quantities remained the same, a precipitating agent had to be used to precipitate the composite. Details are given later in the paper.
Instruments The powder was shaped into a disc (2.5 cm d i a m e t e r × 0 . 1 cm) at an applied pressure of 120 MPa and r o o m temperature. The standard four-point
285
method was employed to measure the conductivity of the disc. The morphology of the composite was investigated by scanning electron microscopy (SEM, Hitachi S-800). Fourier transform infrared spectroscopy (F]?-IR) experiments were performed on a Mattson Alpha Centauri instrument. Results and discussion
The chemical polymerization of aniline in a solution containing polystyrene produces a polyaniline-polystyrene composite. It was found that by keeping the ratio of aniline to oxidant constant and varying the aniline/polystyrene ratio, composites with variable nitrogen content can be obtained (Table 1). Since polystyrene does not contain nitrogen, the polyaniline content of the composite (PANI wt.%) can be estimated from the elemental analysis of nitrogen by comparing with the nitrogen content of the bulk polyaniline (11.53 wt.% N). Elemental analyses for C, H and N were carried out by Quantitative Technologies Inc. (Bound Brook, NJ). From Table 1 one can see that the polyaniline content increases with an increase in the aniline/ polystyrene ratio. Thus, the present approach enables us to prepare polyaniline-polystyrene composites which contain variable amounts of conductive component and therefore possess conductivities in a wide range. For electrically conductive polymer composites, there exists a sharp change in conductivity when the volume fraction of the conductive component reaches a percolation threshold. This critical volume percent depends on the shape and distribution of the conductive particles. Figure 1 shows the TABLE 1 Polyaniline content in the composites prepared" starting from different initial ratios of aniline to polystyrene (PS) No.
Initial ratio of aniline/PS (wt./wt.)
C (wt.9/0)
H (wt.%)
N (wt.%)
Calculated polyaniline content (wt. 9/0) b
1 2 3 4 5 6 7
1:16 1:8 1:2 1:1 2:1 3:1 aniline
90.41 85.66 80.94 69.19 70.32 41.84 49.73
7.63 7.65 7.35 7.05 7.32 5.27 4.35
0.36 1.11 2.581 5.25 5.11 10.30 11.53
3.1 9.6 22.4 45.5 44.4 89.3 100
aThe composites were prepared under the following conditions: concentration of H C l = 0 . 5 8 tool/1 of the system; [(NH4)2S2Os]/[aniline] = 1:1 mole ratio; volume fraction of the dispersed phase in the emulsion prepared in the first step ((b)= 0.56; concentration of the surfactant (SDS) in the continuous phase = 0.1 g/ml; concentration of polystyrene = 0.08 g/ml of dispersed phase. bCalculated from the nitrogen elemental analysis, without considering the doping of the polyaniline.
286 0.4
1 0.3
o o~
~'-1
.4 ..>
"d
0.2
c o
o
0.1 o ,-I
-4
0
20
40
60
80
100
120
0.0
0.2
0.4
0.6
0.8
1.0
1.2
PANI content (vol.%)
Concentration of HCI (Mole/1 of the system) Fig. 1. Dependence of the conductivity of the composite on the polyaniline content (vol.%). The composites were prepared under the following conditions: concentration of the HC1= 0.58 mol/l of the system; [CNH4)2S2Os]/[aniline]= 1:1 mole ratio; volume fraction of the dispersed phase (¢)=0.56; concentration of the surfactant in the continuous phase=0.1 g/ml; concentration of polystyrene=0.08 g/ml of the dispersed phase. Fig. 2. Dependence of the conductivity of the composite on the concentration of HC1 (mol/l of the system). The composites were prepared under the following conditions: initial ratio of aniline/PS (wt./wt.)= 1:2; [(NH4)2SzOs]/[aniline]=1:1 mole ratio; volume fraction of the dispersed phase (¢)= 0.56; concentration of the surfactant in the continuous phase = 0.1 g/ml; concentration of polystyrene = 0.08 g/ml of the dispersed phase.
relationship b e t w e e n the polyaniline c o n t e n t and the c o n d u c t i v i t y o f the c o m p o s i t e . Conductivities as high as 3 - 5 S/cm have b e e n obtained. A typical p e r c o l a t i o n t h r e s h o l d b e h a v i o r is indeed observed. The critical v o l u m e conc e n t r a t i o n is in the r a n g e o f 2 - 1 0 vol.%, w h i c h is m u c h lower t h a n the 2 0 - 3 0 vol.% o b s e r v e d f o r p o l y p y r r o l e - - p o l y m e r c o m p o s i t e s [12]. Similar results have b e e n r e p o r t e d for c o m p o s i t e films p r e p a r e d f r o m sterically stabilized polyaniline m i x e d with sterically stabilized p o l y ( m e t h y l m e t h a c r y l a t e c o b u t y l a c r y l a t e ) latex particles [12 ]. The lower critical v o l u m e indicates t h a t the polyaniline c o m p o n e n t is well d i s p e r s e d within the c o m p o s i t e . The d e p e n d e n c e o f c o n d u c t i v i t y of the c o m p o s i t e o n the c o n c e n t r a t i o n of HCI u s e d in the p o l y m e r i z a t i o n of aniline is p r e s e n t e d in Fig. 2. The c o n d u c t i v i t y increases with increasing c o n c e n t r a t i o n o f HC1 and b e c o m e s c o n s t a n t for a c o n c e n t r a t i o n a b o v e 0.6 M. Of course, the h i g h e r d o p i n g o f polyaniline is r e s p o n s i b l e for the h i g h e r conductivity. The effect o f the mole ratio o f o x i d a n t ( a m m o n i u m persulfate) to aniline on the c o n d u c t i v i t y and yield o f the c o m p o s i t e is p r e s e n t e d in Fig. 3. An o p t i m u m m o l a r fraction o f oxidant/(aniline + oxidant) (0.5) is observed. The c o n d u c t i v i t y of the c o m p o s i t e increases rapidly b e l o w this o p t i m u m a n d d e c r e a s e s with a further increase in oxidant c o n c e n t r a t i o n . The yield o f c o m p o s i t e i n c r e a s e s with increasing o x i d a n t c o n c e n t r a t i o n . The d e c r e a s e in c o n d u c t i v i t y of the c o m p o s i t e c a n be p r o b a b l y attributed to the lower a v e r a g e
287 0.4
- 90
- 80
E
0.3 - 70
r.~
.>
0.2
- 60
5O
o
0.1 4O
0.0
u.0
•
i
0.2
.
i
0.4
,
J
0.6
.
i
0.8
,
3O
l.O
Oxidant/(Oxidant + Aniline), (mole/mole) Fig. 3. Effect of oxidant/(aniline+ oxidant) mole ratio on the conductivity and yield of the composite. The composites were prepared under the following conditions: initial ratio of aniline/ PS (wt./wt.)=l:2; concentration of HCl=0.58 mol/1 of the system; volume fraction of the dispersed phase (~b)=0.56; concentration of the surfactant in the continuous phase=0.1 g/ ml; concentration of polystyrene = 0.08 g/ml of the dispersed phase. TABLE 2 Conductivity of the composite precipitateda with various solvents Solvent
Yield (%)
Conductivity (S/cm)
Methanol Mixture of 1 N HC1 and methanol (1:1 volJvol.) Ethanol
100 92 85
1.2 1.0 0.15
aThe composites were prepared under the following conditions: initial ratio of aniline to PS (wt./wt.) = 1: 1; [(NH4)2S2Os]/[aniline] = 1: 1 mole ratio; concentration of HC1 = 0.58 mol/1 of the system; volume fraction of the dispersed phase (¢) = 0.56; concentration of the surfactant in the continuous phase = 0.1 g/ml; concentration of polystyrene = 0.08 g/ml of dispersed phase; 20 ml of precipitating agent were used for 25 ml of the system containing the composite.
d e g r e e of p o l y m e r i z a t i o n of aniline and, hence, to the s h o r t e r c o n j u g a t i o n l e n g t h at higher o x i d a n t c o n c e n t r a t i o n s . At high c o n c e n t r a t i o n s of oxidant, t h e c o m p o s i t e o b t a i n e d is a b l a c k p o w d e r , w h i l e a t l o w e r o x i d a n t c o n c e n t r a t i o n s it is d a r k b l u e . T h e c o n d u c t i v i t y o f t h e c o m p o s i t e is a f f e c t e d b y t h e n a t u r e o f t h e a g e n t u s e d f o r t h e p r e c i p i t a t i o n o f t h e c o m p o s i t e ( T a b l e 2). T h e c o m p o s i t e m u s t be precipitated from the emulsion using a suitable precipitating agent when t h e a n i l i n e / p o l y s t y r e n e w e i g h t r a t i o is s m a l l e r t h a n a b o u t 1/2. M e t h a n o l , b e i n g the b e s t a g e n t for the p r e c i p i t a t i o n of polystyrene, was selected to precipitate the polyaniline-polystyrene composite. Compared to methanol, e t h a n o l is n o t a s g o o d a p r e c i p i t a t i o n a g e n t . T h e s m a l l e r c o n d u c t i v i t y o f t h e
288
composite treated with ethanol may be attributed to the dedoping of the polyaniline. The effect of the solvent employed for the polystyrene solution on the conductivity of the composites is presented in Table 3. One can see that the nature of the solvent employed in the dispersed phase affects the conductivity of the composite. For instance, the conductivity of the composites prepared with chloroform and toluene as solvents for polystyrene are 0.55 and 0.07 S/cm, respectively. Table 4 lists the conductivities of the composites prepared by changing the volume fraction (~b) of the dispersed phase and the amount of surfactant used. These experimental results indicate that the composite can be prepared both starting from common emulsions (~b<0.74) and from concentrated emulsions (~b > 0.74). In a concentrated emulsion, the volume fraction of the dispersed phase is larger than 0.74, which represents the most compact arrangement of spheres of equal radius. A concentrated emulsion has the TABLE 3 Effect o f t h e s o l v e n t u s e d f o r t h e p o l y s t y r e n e s o l u t i o n o n t h e c o n d u c t i v i t y o f t h e c o m p o s i t e a Solvent
Yield (%)
Conductivity (S/cm)
Benzene Chloroform DMF Toluene
83 100 69 47
0.34 0.55 0.40 0.07
a T h e c o m p o s i t e s w e r e p r e p a r e d u n d e r t h e following c o n d i t i o n s : initial ratio (wt.Avt.) = 1:2; [(NH4)2S2Os]/[aniline ] = 1:1 m o l e ratio; c o n c e n t r a t i o n o f HC1 = s y s t e m ; v o l u m e f r a c t i o n o f t h e d i s p e r s e d p h a s e (4)) = 0 . 5 6 ; c o n c e n t r a t i o n o f c o n t i n u o u s p h a s e = 0.1 g/rift; c o n c e n t r a t i o n o f p o l y s t y r e n e = 0 . 0 8 g / m l of t h e
o f aniline to PS 0 . 5 8 mol/1 o f t h e s u r f a c t a n t in t h e dispersed phase.
TABLE 4 Effect o f t h e v o l u m e f r a c t i o n (~b) o f t h e d i s p e r s e d p h a s e a n d t h e a m o u n t o f s u r f a c t a n t u s e d o n t h e c o n d u c t i v i t y of t h e c o m p o s i t e a 4)
Amount of suffactant
Conductivity (S/cm)
(g) 0.56 0.71 0.91 0.71 0.71
0.20 0.20 0.20 0.I0 0.05
0.34 0.35 0.25 0.23 0.14
a T h e c o m p o s i t e s w e r e p r e p a r e d u n d e r t h e following c o n d i t i o n s : initial ratio of aniline to PS (wt.Avt.) = 1:2; [(NH4)zSzOs]/[aniline] = 1:1 m o l e ratio; c o n c e n t r a t i o n o f HC1 = 0 . 5 8 mol/1 o f t h e s y s t e m ; c o n c e n t r a t i o n o f p o l y s t y r e n e = 0 . 0 8 g / m l o f t h e d i s p e r s e d p h a s e ; total v o l u m e o f t h e s y s t e m = 2 5 ml.
289
appearance of a gel and consists, when ~b is sufficiently large, of polyhedral droplets separated by thin layers of continuous phase. If the amount of surfactant used is below 0.05 g and the other conditions are as in Table 4, the emulsion breaks at the beginning of the polymerization process. The materials thus prepared are not brittle and are processable by hot pressing. Fr-IR spectra of the composite and of its polymer components are presented in Fig. 4. The spectrum of the polyaniline-polystyrene composite (22 wt.% PANI, Fig. 4(c)) exhibits strong absorption in the range 4 0 0 0 - 5 0 0
o)
=o o~
t--
i
4000
36oo
26oo
'V
I
Idoo soo
Wavenumbers, (cm-1) Fig. d. FT-IR spectra of (a) polystyrene, Co) polyaniline and (c) polyaniline-polystyrene composite (22 wt.% PANI, specimen 3 of Table 1).
290
Fig. 5. Scanning electron micrographs of the polyaniline-polystyrenecomposites: (a) polystyrene sample prepared as the composite but without aniline; and the composites with (b) 10 wt.%, (c) 22 wt.% and (d) 45 wt.% PANI; specimens 2, 3 and 4, respectively, of Table 1. c m - i a n d is s i m i l a r t o t h a t o f p u r e p o l y a n i l i n e (Fig. 4 ( b ) ) . A l m o s t all t h e main vibrational bands of polystyrene are absent. The vibrational bands o b s e r v e d for the c o m p o s i t e can be reasonably assigned to the normal m o d e s of p o l y a n i l i n e : t h e b a n d at 3 2 0 0 - 3 6 0 0 c m - 1 is a s s i g n e d to t h e N - H s t r e t c h i n g of an a r o m a t i c a m i n e a s well a s of its s a l t ( t h e d o p e d p o l y a n i l i n e ) . T h e 1 5 5 6
291 a n d 1 4 7 5 c m -1 b a n d s are a s s i g n e d to a b e n z e n e ring a n d / o r to a q u i n o n e ring d e f o r m a t i o n [ 13 ] a n d t h a t at 1302 c m - 1 to C - N s t r e t c h i n g in a s e c o n d a r y a r o m a t i c a m i n e . In t h e r a n g e 1 0 1 0 - 1 1 7 0 c m -1, a s t r o n g a r o m a t i c C - H inp l a n e - b e n d i n g m o d e is o b s e r v e d . A s t r o n g c h a r a c t e r i s t i c b a n d a p p e a r s at a b o u t 1 1 6 0 c m -1, w h i c h h a s b e e n c o n s i d e r e d an e l e c t r o n i c b a n d [14] or a v i b r a t i o n a l b a n d of C - N in a q u i n o n e ring [13] c a u s e d b y doping. S o m e v i b r a t i o n s b e t w e e n 1 5 0 0 - 5 0 0 c m -1 are similar to t h o s e o f polyaniline a n d a f e w o f t h e m to t h o s e o f p o l y s t y r e n e . One c a n p e r h a p s c o n c l u d e t h a t the c o m p o s i t e is n o t a s i m p l e c o m b i n a t i o n of its two c o n s t i t u e n t s . Figure 5 presents scanning electron micrographs of polyanilinep o l y s t y r e n e c o m p o s i t e s c o n t a i n i n g 0, 10, 2 2, 4 5 wt.% polyaniline, respectively. In t h e p o l y s t y r e n e s a m p l e p r e p a r e d as the c o m p o s i t e a n d p r e c i p i t a t e d in m e t h a n o l , o n e c a n identify s p h e r i c a l p a r t i c l e s (Fig. 5(a)). This s t r u c t u r e c a n also b e o b s e r v e d in the c o m p o s i t e c o n t a i n i n g a small a m o u n t o f polyaniline (Fig. 5(b), 10 wt.% PANI), b u t d i s a p p e a r s as the PANI c o n t e n t i n c r e a s e s (Figs. 5 ( c ) a n d 5(d)). In t h e latter m i c r o g r a p h s the s t r u c t u r e is u n i f o r m and nonparticulate. A o n e - s t e p m e t h o d w a s also e m p l o y e d , b y g e n e r a t i n g an e m u l s i o n b e t w e e n b e n z e n e c o n t a i n i n g aniline a n d p o l y s t y r e n e , a n d a n a q u e o u s solution o f HC1 c o n t a i n i n g t h e s u r f a c t a n t a n d the oxidant. T h e t w o - s t e p p r o c e d u r e h a s the a d v a n t a g e of a b e t t e r distribution o f the o x i d a n t a m o n g the m i c r o m e t e r sized cells of the d i s p e r s e d p h a s e o f the e m u l s i o n o b t a i n e d in t h e first step.
Conclusions It is s h o w n t h a t p o l y a n i l i n e - p o l y s t y r e n e c o m p o s i t e s c a n b e p r e p a r e d b y t h e o x i d a t i v e p o l y m e r i z a t i o n of aniline in a n e m u l s i o n t h a t c o n t a i n s a solution o f aniline a n d p o l y s t y r e n e as the d i s p e r s e d p h a s e a n d an a q u e o u s solution o f s o d i u m d o d e c y l s u l f a t e as the c o n t i n u o u s p h a s e . T h e c o n t e n t o f polyaniline c o m p o n e n t in s u c h c o m p o s i t e s c a n b e c o n t r o l l e d o v e r a w i d e r a n g e b y v a r y i n g t h e initial r a t i o o f p o l y s t y r e n e to aniline. T h e c o n d u c t i v i t y o f t h e s a m p l e s i n d i c a t e s a p e r c o l a t i o n t h r e s h o l d for a low polyaniline v o l u m e f r a c t i o n o f a p p r o x i m a t e l y 2 - 1 0 vol.%. T h e p r e p a r e d c o m p o s i t e s a r e n o t brittle a n d h a v e e l e c t r i c a l c o n d u c t i v i t i e s as high as 3 - 5 S/cm.
References 1 2 3 4
A. G. MacDiarmid and A. J. Epstein, Faraday Discuss. Chem. Soc., 88 (1989) 317. L. H. Dao, M. Leclerc, J. Guay and J. W. Chevalier, Synth. Met., 29 (1989) E377. S. Li, Y. Cao and Z. Xue, Synth. Met., 20 (1987) 141. (a) E. Ruckenstein and J. S. Park, J. Appl. Polym. Sci., 42 (1991) 925; (b) E. Ruckenstein and J. S. Park, Synth. Met., 44 (1991) 293. 5 Y. Murakoshi, T. Ikezaki and K. Naito, Jpn. Patent Tokkyo Koho JP 62/64 862 (1987). 6 S. A. Chen and W. G. Fang, Macromolecules, 24 (1991) 1242. 7 G. Bidan and B. Ehui, J. Chem. Soc., Chem. Commun., (1989) 1568.
292 E. L. Tassi and M. A. DePaoli, J. Chem. Soc., Chem. Commun., (1990) 155. A. Andreatta, A. J. Heeger and P. Smith, Polym. Commun., 31 (1990) 275. P. Beadle and S. P. Armes, Macromolecules, 25 (1992) 2526. E. Ruckenstein and J. H. Chen, Polymer, 32 (1991) 1230. E. C. Cooper and B. Vincent, J. Phys. D: Appl. Phys., 22 (1989) 1580. W. R. Salaneck, B. Liedberg, O. Ingan/4s, R. Erlandsson, I. Lundstr6m, A. G. MacDiarmid, M. Halpern and N. L. D. Somasiri, Mol. Cryst. Liq. Cryst., 121 (1985) 191. 14 T. Jiang, X. Jing, B. W a n g and F. Wang, Synth. Met., 24 (1988) 231.
8 9 10 11 12 13
283
An emulsion pathway to electrically conductive polyaniline-polystyrene composites Eli R u c k e n s t e i n * a n d S h i y o n g Yang Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, N Y 14260 (USA)
(Received June 25, 1992; accepted August 31, 1992)
Abstract A method for the preparation of electrically conductive polyaniline-polystyrene composites is described. The composite is obtained starting from an emulsion in which a solution of sodium dodecylsulfate in water constitutes the continuous phase and a solution of aniline and polystyrene in benzene the dispersed phase. Polymerization of aniline takes place by introducing an oxidant dissolved in an aqueous solution of HC1 under vigorous stirring in the emulsion. The composite either precipitates or is precipitated by using a suitable agent. The conductivity can reach values as high as 3-5 S/cm. In contrast to polyaniline, the composite obtained is processable due to the insulating polymer.
Introduction Polyaniline is r e c o g n i z e d as an air-stable, b u t u n p r o c e s s a b l e , electrically c o n d u c t i v e p o l y m e r [ 1 ]. N u m e r o u s studies h a v e b e e n c o n d u c t e d a n d m e t h o d s d e v e l o p e d for t h e i m p r o v e m e n t of its p r o c e s s a b i l i t y . T h e s e include the addition o f side g r o u p s to the m a i n c h a i n [2], g r a f t i n g the c o n d u c t i n g p o l y m e r chain to a n o n - c o n d u c t i v e p o l y m e r [3], i m b i b i n g a p o r o u s m e d i u m with aniline a n d p o l y m e r i z i n g the aniline inside [ 4 ], p r e p a r a t i o n o f c o m p o s i t e s o f c o n d u c t i v e a n d n o n - c o n d u c t i v e ( b u t easily p r o c e s s a b l e ) p o l y m e r s [5] a n d e l e c t r o c h e m i c a l p o l y m e r i z a t i o n o f aniline in a p o l y m e r m a t r i x [ 6 - 8 ] . U n f o r t u n a t e l y , t h e s e t e c h n i q u e s often yield m a t e r i a l s with m u c h l o w e r c o n d u c t i v i t y t h a n the p u r e parent polymer. Recently, A n d r e a t t a et al. [9] r e p o r t e d a m e t h o d f o r t h e p r e p a r a t i o n of a b l e n d fiber o f polyaniline a n d p o l y ( p - p h e n y l e n e t e r e p h t h a l a m i d e ) f r o m their solution in c o n c e n t r a t e d sulfuric acid b y p r e c i p i t a t i n g with a n a q u e o u s solution o f H2SO4. B e a d l e a n d A r m e s [ 10 ] p r e p a r e d a series of p o l y a n i l i n e - c o p o l y m e r l a t e x c o m p o s i t e s b y oxidative p o l y m e r i z a t i o n o f aniline in t h e p r e s e n c e of a film-forming c h l o r i n a t e d c o p o l y m e r latex. Our g r o u p h a s r e p o r t e d a r o u t e to p r e p a r e a p o l y p y r r o l e - p o l y u r e t h a n e c o m p o s i t e b y c o p r e c i p i t a t i o n f r o m a concentrated emulsion containing polyurethane and an aqueous suspension o f p o l y p y r r o l e p o w d e r [ 11 ]. *Author to whom correspondence should be addressed.
0379-6779/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
284 In this paper, a method is pr e s ent ed for the preparation of polyaniline-polystyrene composites with good processability. Two steps are involved in this method. First, an emulsion with the appearance of a gel was p r e p a r e d starting from a solution of polystyrene and aniline in benzene as the dispersed phase and a solution of sodium dodecylsulfate (SDS) in water as the continuous phase. Secondly, an oxidant dissolved in an aqueous solution of HC1 was added dropwise to the emulsion with vigorous mechanical stirring in order to polymerize the aniline and to dope the polyaniline formed. The polymerization was allowed to p r o c e e d for 3 h. Depending upon the polyaniline content in the composites, a green-to-dark-blue solid material was obtained. The composites obtained were characterized by scaIming electron m i c r o s c o p y (SEM) and Fourier transform infrared s pect roscopy (FF-IR).
Experimental Chemicals Aniline (Aldrich, 99.5%) was purified by distillation in vacuum before use. Polystyrene (PS, Aldrich, ~;/w: 2 8 0 0 0 0 ) , ammonium persulfate ((NH4)2S2Os, Polysciences), sodium dodecylsulfate (SDS, Polysciences), benzene (Aldrich, 99%) and methanol (Aldrich, 99.9%) were used as received. Water was de-ionized and distilled.
Preparation o f the electrically conductive composite In a typical experiment, 0.2 g of SDS and 2 ml of water were placed in a 100 ml flask. To this, 5 ml of a solution containing 0.4 g polystyrene, 0.2 g aniline and the balance benzene was added dropwise, with stirring. An emulsion with the appearance of a gel, in which the water solution constitutes the continuous phase and the benzene-based solution the dispersed phase, was thus obtained. Then, 10 ml of 1 N HC1 aqueous solution containing 0.5 g of ammonium persulfate were introduced dropwise, with stirring, to polymerize the aniline and dope the polyaniline formed. The polymerization reaction was allowed to p r o c e e d for 3 h, with stirring. The benzene of the polymer solution was separated after the composite was formed. The solid material thus obtained was washed with 20 ml of methanol, filtered, washed again with 1 N HCI solution and finally dried at 40 °C in vacuum for 24 h. An amount of 0.52 g of polyaniline-polystyrene composite was thus obtained. The yield of the composite defined as the ratio of grams of composite to grams of polystyrene plus grams of aniline was 87%. When the amount of aniline was smaller than about 0.1 g and the other quantities remained the same, a precipitating agent had to be used to precipitate the composite. Details are given later in the paper.
Instruments The powder was shaped into a disc (2.5 cm d i a m e t e r × 0 . 1 cm) at an applied pressure of 120 MPa and r o o m temperature. The standard four-point
285
method was employed to measure the conductivity of the disc. The morphology of the composite was investigated by scanning electron microscopy (SEM, Hitachi S-800). Fourier transform infrared spectroscopy (F]?-IR) experiments were performed on a Mattson Alpha Centauri instrument. Results and discussion
The chemical polymerization of aniline in a solution containing polystyrene produces a polyaniline-polystyrene composite. It was found that by keeping the ratio of aniline to oxidant constant and varying the aniline/polystyrene ratio, composites with variable nitrogen content can be obtained (Table 1). Since polystyrene does not contain nitrogen, the polyaniline content of the composite (PANI wt.%) can be estimated from the elemental analysis of nitrogen by comparing with the nitrogen content of the bulk polyaniline (11.53 wt.% N). Elemental analyses for C, H and N were carried out by Quantitative Technologies Inc. (Bound Brook, NJ). From Table 1 one can see that the polyaniline content increases with an increase in the aniline/ polystyrene ratio. Thus, the present approach enables us to prepare polyaniline-polystyrene composites which contain variable amounts of conductive component and therefore possess conductivities in a wide range. For electrically conductive polymer composites, there exists a sharp change in conductivity when the volume fraction of the conductive component reaches a percolation threshold. This critical volume percent depends on the shape and distribution of the conductive particles. Figure 1 shows the TABLE 1 Polyaniline content in the composites prepared" starting from different initial ratios of aniline to polystyrene (PS) No.
Initial ratio of aniline/PS (wt./wt.)
C (wt.9/0)
H (wt.%)
N (wt.%)
Calculated polyaniline content (wt. 9/0) b
1 2 3 4 5 6 7
1:16 1:8 1:2 1:1 2:1 3:1 aniline
90.41 85.66 80.94 69.19 70.32 41.84 49.73
7.63 7.65 7.35 7.05 7.32 5.27 4.35
0.36 1.11 2.581 5.25 5.11 10.30 11.53
3.1 9.6 22.4 45.5 44.4 89.3 100
aThe composites were prepared under the following conditions: concentration of H C l = 0 . 5 8 tool/1 of the system; [(NH4)2S2Os]/[aniline] = 1:1 mole ratio; volume fraction of the dispersed phase in the emulsion prepared in the first step ((b)= 0.56; concentration of the surfactant (SDS) in the continuous phase = 0.1 g/ml; concentration of polystyrene = 0.08 g/ml of dispersed phase. bCalculated from the nitrogen elemental analysis, without considering the doping of the polyaniline.
286 0.4
1 0.3
o o~
~'-1
.4 ..>
"d
0.2
c o
o
0.1 o ,-I
-4
0
20
40
60
80
100
120
0.0
0.2
0.4
0.6
0.8
1.0
1.2
PANI content (vol.%)
Concentration of HCI (Mole/1 of the system) Fig. 1. Dependence of the conductivity of the composite on the polyaniline content (vol.%). The composites were prepared under the following conditions: concentration of the HC1= 0.58 mol/l of the system; [CNH4)2S2Os]/[aniline]= 1:1 mole ratio; volume fraction of the dispersed phase (¢)=0.56; concentration of the surfactant in the continuous phase=0.1 g/ml; concentration of polystyrene=0.08 g/ml of the dispersed phase. Fig. 2. Dependence of the conductivity of the composite on the concentration of HC1 (mol/l of the system). The composites were prepared under the following conditions: initial ratio of aniline/PS (wt./wt.)= 1:2; [(NH4)2SzOs]/[aniline]=1:1 mole ratio; volume fraction of the dispersed phase (¢)= 0.56; concentration of the surfactant in the continuous phase = 0.1 g/ml; concentration of polystyrene = 0.08 g/ml of the dispersed phase.
relationship b e t w e e n the polyaniline c o n t e n t and the c o n d u c t i v i t y o f the c o m p o s i t e . Conductivities as high as 3 - 5 S/cm have b e e n obtained. A typical p e r c o l a t i o n t h r e s h o l d b e h a v i o r is indeed observed. The critical v o l u m e conc e n t r a t i o n is in the r a n g e o f 2 - 1 0 vol.%, w h i c h is m u c h lower t h a n the 2 0 - 3 0 vol.% o b s e r v e d f o r p o l y p y r r o l e - - p o l y m e r c o m p o s i t e s [12]. Similar results have b e e n r e p o r t e d for c o m p o s i t e films p r e p a r e d f r o m sterically stabilized polyaniline m i x e d with sterically stabilized p o l y ( m e t h y l m e t h a c r y l a t e c o b u t y l a c r y l a t e ) latex particles [12 ]. The lower critical v o l u m e indicates t h a t the polyaniline c o m p o n e n t is well d i s p e r s e d within the c o m p o s i t e . The d e p e n d e n c e o f c o n d u c t i v i t y of the c o m p o s i t e o n the c o n c e n t r a t i o n of HCI u s e d in the p o l y m e r i z a t i o n of aniline is p r e s e n t e d in Fig. 2. The c o n d u c t i v i t y increases with increasing c o n c e n t r a t i o n o f HC1 and b e c o m e s c o n s t a n t for a c o n c e n t r a t i o n a b o v e 0.6 M. Of course, the h i g h e r d o p i n g o f polyaniline is r e s p o n s i b l e for the h i g h e r conductivity. The effect o f the mole ratio o f o x i d a n t ( a m m o n i u m persulfate) to aniline on the c o n d u c t i v i t y and yield o f the c o m p o s i t e is p r e s e n t e d in Fig. 3. An o p t i m u m m o l a r fraction o f oxidant/(aniline + oxidant) (0.5) is observed. The c o n d u c t i v i t y of the c o m p o s i t e increases rapidly b e l o w this o p t i m u m a n d d e c r e a s e s with a further increase in oxidant c o n c e n t r a t i o n . The yield o f c o m p o s i t e i n c r e a s e s with increasing o x i d a n t c o n c e n t r a t i o n . The d e c r e a s e in c o n d u c t i v i t y of the c o m p o s i t e c a n be p r o b a b l y attributed to the lower a v e r a g e
287 0.4
- 90
- 80
E
0.3 - 70
r.~
.>
0.2
- 60
5O
o
0.1 4O
0.0
u.0
•
i
0.2
.
i
0.4
,
J
0.6
.
i
0.8
,
3O
l.O
Oxidant/(Oxidant + Aniline), (mole/mole) Fig. 3. Effect of oxidant/(aniline+ oxidant) mole ratio on the conductivity and yield of the composite. The composites were prepared under the following conditions: initial ratio of aniline/ PS (wt./wt.)=l:2; concentration of HCl=0.58 mol/1 of the system; volume fraction of the dispersed phase (~b)=0.56; concentration of the surfactant in the continuous phase=0.1 g/ ml; concentration of polystyrene = 0.08 g/ml of the dispersed phase. TABLE 2 Conductivity of the composite precipitateda with various solvents Solvent
Yield (%)
Conductivity (S/cm)
Methanol Mixture of 1 N HC1 and methanol (1:1 volJvol.) Ethanol
100 92 85
1.2 1.0 0.15
aThe composites were prepared under the following conditions: initial ratio of aniline to PS (wt./wt.) = 1: 1; [(NH4)2S2Os]/[aniline] = 1: 1 mole ratio; concentration of HC1 = 0.58 mol/1 of the system; volume fraction of the dispersed phase (¢) = 0.56; concentration of the surfactant in the continuous phase = 0.1 g/ml; concentration of polystyrene = 0.08 g/ml of dispersed phase; 20 ml of precipitating agent were used for 25 ml of the system containing the composite.
d e g r e e of p o l y m e r i z a t i o n of aniline and, hence, to the s h o r t e r c o n j u g a t i o n l e n g t h at higher o x i d a n t c o n c e n t r a t i o n s . At high c o n c e n t r a t i o n s of oxidant, t h e c o m p o s i t e o b t a i n e d is a b l a c k p o w d e r , w h i l e a t l o w e r o x i d a n t c o n c e n t r a t i o n s it is d a r k b l u e . T h e c o n d u c t i v i t y o f t h e c o m p o s i t e is a f f e c t e d b y t h e n a t u r e o f t h e a g e n t u s e d f o r t h e p r e c i p i t a t i o n o f t h e c o m p o s i t e ( T a b l e 2). T h e c o m p o s i t e m u s t be precipitated from the emulsion using a suitable precipitating agent when t h e a n i l i n e / p o l y s t y r e n e w e i g h t r a t i o is s m a l l e r t h a n a b o u t 1/2. M e t h a n o l , b e i n g the b e s t a g e n t for the p r e c i p i t a t i o n of polystyrene, was selected to precipitate the polyaniline-polystyrene composite. Compared to methanol, e t h a n o l is n o t a s g o o d a p r e c i p i t a t i o n a g e n t . T h e s m a l l e r c o n d u c t i v i t y o f t h e
288
composite treated with ethanol may be attributed to the dedoping of the polyaniline. The effect of the solvent employed for the polystyrene solution on the conductivity of the composites is presented in Table 3. One can see that the nature of the solvent employed in the dispersed phase affects the conductivity of the composite. For instance, the conductivity of the composites prepared with chloroform and toluene as solvents for polystyrene are 0.55 and 0.07 S/cm, respectively. Table 4 lists the conductivities of the composites prepared by changing the volume fraction (~b) of the dispersed phase and the amount of surfactant used. These experimental results indicate that the composite can be prepared both starting from common emulsions (~b<0.74) and from concentrated emulsions (~b > 0.74). In a concentrated emulsion, the volume fraction of the dispersed phase is larger than 0.74, which represents the most compact arrangement of spheres of equal radius. A concentrated emulsion has the TABLE 3 Effect o f t h e s o l v e n t u s e d f o r t h e p o l y s t y r e n e s o l u t i o n o n t h e c o n d u c t i v i t y o f t h e c o m p o s i t e a Solvent
Yield (%)
Conductivity (S/cm)
Benzene Chloroform DMF Toluene
83 100 69 47
0.34 0.55 0.40 0.07
a T h e c o m p o s i t e s w e r e p r e p a r e d u n d e r t h e following c o n d i t i o n s : initial ratio (wt.Avt.) = 1:2; [(NH4)2S2Os]/[aniline ] = 1:1 m o l e ratio; c o n c e n t r a t i o n o f HC1 = s y s t e m ; v o l u m e f r a c t i o n o f t h e d i s p e r s e d p h a s e (4)) = 0 . 5 6 ; c o n c e n t r a t i o n o f c o n t i n u o u s p h a s e = 0.1 g/rift; c o n c e n t r a t i o n o f p o l y s t y r e n e = 0 . 0 8 g / m l of t h e
o f aniline to PS 0 . 5 8 mol/1 o f t h e s u r f a c t a n t in t h e dispersed phase.
TABLE 4 Effect o f t h e v o l u m e f r a c t i o n (~b) o f t h e d i s p e r s e d p h a s e a n d t h e a m o u n t o f s u r f a c t a n t u s e d o n t h e c o n d u c t i v i t y of t h e c o m p o s i t e a 4)
Amount of suffactant
Conductivity (S/cm)
(g) 0.56 0.71 0.91 0.71 0.71
0.20 0.20 0.20 0.I0 0.05
0.34 0.35 0.25 0.23 0.14
a T h e c o m p o s i t e s w e r e p r e p a r e d u n d e r t h e following c o n d i t i o n s : initial ratio of aniline to PS (wt.Avt.) = 1:2; [(NH4)zSzOs]/[aniline] = 1:1 m o l e ratio; c o n c e n t r a t i o n o f HC1 = 0 . 5 8 mol/1 o f t h e s y s t e m ; c o n c e n t r a t i o n o f p o l y s t y r e n e = 0 . 0 8 g / m l o f t h e d i s p e r s e d p h a s e ; total v o l u m e o f t h e s y s t e m = 2 5 ml.
289
appearance of a gel and consists, when ~b is sufficiently large, of polyhedral droplets separated by thin layers of continuous phase. If the amount of surfactant used is below 0.05 g and the other conditions are as in Table 4, the emulsion breaks at the beginning of the polymerization process. The materials thus prepared are not brittle and are processable by hot pressing. Fr-IR spectra of the composite and of its polymer components are presented in Fig. 4. The spectrum of the polyaniline-polystyrene composite (22 wt.% PANI, Fig. 4(c)) exhibits strong absorption in the range 4 0 0 0 - 5 0 0
o)
=o o~
t--
i
4000
36oo
26oo
'V
I
Idoo soo
Wavenumbers, (cm-1) Fig. d. FT-IR spectra of (a) polystyrene, Co) polyaniline and (c) polyaniline-polystyrene composite (22 wt.% PANI, specimen 3 of Table 1).
290
Fig. 5. Scanning electron micrographs of the polyaniline-polystyrenecomposites: (a) polystyrene sample prepared as the composite but without aniline; and the composites with (b) 10 wt.%, (c) 22 wt.% and (d) 45 wt.% PANI; specimens 2, 3 and 4, respectively, of Table 1. c m - i a n d is s i m i l a r t o t h a t o f p u r e p o l y a n i l i n e (Fig. 4 ( b ) ) . A l m o s t all t h e main vibrational bands of polystyrene are absent. The vibrational bands o b s e r v e d for the c o m p o s i t e can be reasonably assigned to the normal m o d e s of p o l y a n i l i n e : t h e b a n d at 3 2 0 0 - 3 6 0 0 c m - 1 is a s s i g n e d to t h e N - H s t r e t c h i n g of an a r o m a t i c a m i n e a s well a s of its s a l t ( t h e d o p e d p o l y a n i l i n e ) . T h e 1 5 5 6
291 a n d 1 4 7 5 c m -1 b a n d s are a s s i g n e d to a b e n z e n e ring a n d / o r to a q u i n o n e ring d e f o r m a t i o n [ 13 ] a n d t h a t at 1302 c m - 1 to C - N s t r e t c h i n g in a s e c o n d a r y a r o m a t i c a m i n e . In t h e r a n g e 1 0 1 0 - 1 1 7 0 c m -1, a s t r o n g a r o m a t i c C - H inp l a n e - b e n d i n g m o d e is o b s e r v e d . A s t r o n g c h a r a c t e r i s t i c b a n d a p p e a r s at a b o u t 1 1 6 0 c m -1, w h i c h h a s b e e n c o n s i d e r e d an e l e c t r o n i c b a n d [14] or a v i b r a t i o n a l b a n d of C - N in a q u i n o n e ring [13] c a u s e d b y doping. S o m e v i b r a t i o n s b e t w e e n 1 5 0 0 - 5 0 0 c m -1 are similar to t h o s e o f polyaniline a n d a f e w o f t h e m to t h o s e o f p o l y s t y r e n e . One c a n p e r h a p s c o n c l u d e t h a t the c o m p o s i t e is n o t a s i m p l e c o m b i n a t i o n of its two c o n s t i t u e n t s . Figure 5 presents scanning electron micrographs of polyanilinep o l y s t y r e n e c o m p o s i t e s c o n t a i n i n g 0, 10, 2 2, 4 5 wt.% polyaniline, respectively. In t h e p o l y s t y r e n e s a m p l e p r e p a r e d as the c o m p o s i t e a n d p r e c i p i t a t e d in m e t h a n o l , o n e c a n identify s p h e r i c a l p a r t i c l e s (Fig. 5(a)). This s t r u c t u r e c a n also b e o b s e r v e d in the c o m p o s i t e c o n t a i n i n g a small a m o u n t o f polyaniline (Fig. 5(b), 10 wt.% PANI), b u t d i s a p p e a r s as the PANI c o n t e n t i n c r e a s e s (Figs. 5 ( c ) a n d 5(d)). In t h e latter m i c r o g r a p h s the s t r u c t u r e is u n i f o r m and nonparticulate. A o n e - s t e p m e t h o d w a s also e m p l o y e d , b y g e n e r a t i n g an e m u l s i o n b e t w e e n b e n z e n e c o n t a i n i n g aniline a n d p o l y s t y r e n e , a n d a n a q u e o u s solution o f HC1 c o n t a i n i n g t h e s u r f a c t a n t a n d the oxidant. T h e t w o - s t e p p r o c e d u r e h a s the a d v a n t a g e of a b e t t e r distribution o f the o x i d a n t a m o n g the m i c r o m e t e r sized cells of the d i s p e r s e d p h a s e o f the e m u l s i o n o b t a i n e d in t h e first step.
Conclusions It is s h o w n t h a t p o l y a n i l i n e - p o l y s t y r e n e c o m p o s i t e s c a n b e p r e p a r e d b y t h e o x i d a t i v e p o l y m e r i z a t i o n of aniline in a n e m u l s i o n t h a t c o n t a i n s a solution o f aniline a n d p o l y s t y r e n e as the d i s p e r s e d p h a s e a n d an a q u e o u s solution o f s o d i u m d o d e c y l s u l f a t e as the c o n t i n u o u s p h a s e . T h e c o n t e n t o f polyaniline c o m p o n e n t in s u c h c o m p o s i t e s c a n b e c o n t r o l l e d o v e r a w i d e r a n g e b y v a r y i n g t h e initial r a t i o o f p o l y s t y r e n e to aniline. T h e c o n d u c t i v i t y o f t h e s a m p l e s i n d i c a t e s a p e r c o l a t i o n t h r e s h o l d for a low polyaniline v o l u m e f r a c t i o n o f a p p r o x i m a t e l y 2 - 1 0 vol.%. T h e p r e p a r e d c o m p o s i t e s a r e n o t brittle a n d h a v e e l e c t r i c a l c o n d u c t i v i t i e s as high as 3 - 5 S/cm.
References 1 2 3 4
A. G. MacDiarmid and A. J. Epstein, Faraday Discuss. Chem. Soc., 88 (1989) 317. L. H. Dao, M. Leclerc, J. Guay and J. W. Chevalier, Synth. Met., 29 (1989) E377. S. Li, Y. Cao and Z. Xue, Synth. Met., 20 (1987) 141. (a) E. Ruckenstein and J. S. Park, J. Appl. Polym. Sci., 42 (1991) 925; (b) E. Ruckenstein and J. S. Park, Synth. Met., 44 (1991) 293. 5 Y. Murakoshi, T. Ikezaki and K. Naito, Jpn. Patent Tokkyo Koho JP 62/64 862 (1987). 6 S. A. Chen and W. G. Fang, Macromolecules, 24 (1991) 1242. 7 G. Bidan and B. Ehui, J. Chem. Soc., Chem. Commun., (1989) 1568.
292 E. L. Tassi and M. A. DePaoli, J. Chem. Soc., Chem. Commun., (1990) 155. A. Andreatta, A. J. Heeger and P. Smith, Polym. Commun., 31 (1990) 275. P. Beadle and S. P. Armes, Macromolecules, 25 (1992) 2526. E. Ruckenstein and J. H. Chen, Polymer, 32 (1991) 1230. E. C. Cooper and B. Vincent, J. Phys. D: Appl. Phys., 22 (1989) 1580. W. R. Salaneck, B. Liedberg, O. Ingan/4s, R. Erlandsson, I. Lundstr6m, A. G. MacDiarmid, M. Halpern and N. L. D. Somasiri, Mol. Cryst. Liq. Cryst., 121 (1985) 191. 14 T. Jiang, X. Jing, B. W a n g and F. Wang, Synth. Met., 24 (1988) 231.
8 9 10 11 12 13