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Properties of composite membranes from ion exchange membranes and conducting polymers—I. Conductivity of composite membrane from cation exchange membranes and polypyrrole
Ekcrrochimica Acta, Vol. 39, No. 1, pp 131-134 Printed in Great Britain.
1994
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0013-4686/?34 s6.00 + 0.00 1993. Pcrgamon Prem Ltd
PROPERTIES OF COMPOSITE MEMBRANES FROM ION EXCHANGE MEMBRANES AND CONDUCTING POLYMERS-I. CONDUCTIVITY OF COMPOSITE MEMBRANE FROM CATION EXCHANGE MEMBRANES AND POLYPYRROLE T~~HIKATSUSATA Division of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai 2557, Ube City, Yamaguchi Prefecture, 755, Japan (Received 30 November 1992; in revisedform 12 July 1993) Abstract-In order to prepare a free-standing polypyrrole hhn with high mechanical strength, Fe(II1) form or Cu(I1) form cation exchange membranes were immersed in an aqueous pyrrole solution. Polymerization speed of pyrrole in the Fe(II1) form membrane was faster than that in the Cu(I1) form membrane. A linear relationship was observed between conductivity of the composite membrane and immersion period in the pyrrole solution. Conductivity of the composite membrane from the Fe(II1) form membrane reached about 10e3 Scm-‘, which was the ultimate vahte of this method. Properties of formed polypyrrole in the membrane matrix was discussed in relation to weight increase of the membrane and species of oxidative cation. Burst strength of the composite membrane increased with increasing polymerization period of pyrrole. Key words: cation exchange membrane, polypyrrole, chemical polymerization, composite membrane, conducting film.
INTRODUCITON Polypyrrole, polyaniline, other conducting polymers, and their analogues have been widely studied in the
field of pure and applied chemistry[l-51. Examples of their possible applications include batteries[6-121, electrolytic display[13-151, electrochromic capacitors[16], etc. One of requirements of these applications is to prepare a strong free.-standing lilm of the conducting polymer. Early works on this point were to polymerize electrochemically pyrrole or aniline within a predeposited, solvent-swollen thermoplastic polymer matrix such as poly(viny1 chloride)[ 173, poly(viny1 alcohol)[ 181, poly(methy1 methacrylate)[19], polystyrene[19], etc. Composite films of poly(3-methylthiophene) with poly(methy1 methacrylate) and poly(viny1 chloride) were reported to have been prepared by one-step procedure involving electrochemical polymerization on metal electrode from a solution containing 3_methylthiophene, supporting electrolyte, and the dissolved insulating polymer[20-211. Thereafter, a number of anionic polyelectrolytes such as poly(vinyl sulfate)[22-241, poly(viny1 sulfonate)[25], poly(styrene sulfonate)[22281, and other anionic copolymers have been used to give the film mechanical strength and as polymeric dopants. Chemically oxidative polymerization of pyrrole and aniline provides intractable, bulky powders with low processibility. Methods to obtain a polypyrrole film by chemical polymerization are to polymerize pyrrole in conventional films after pyrrole was adsorbed in the film. When a cation exchange membrane was used as the tilm, pyrrole was easily adsorbed in the membrane by ion exchange. reaction. The weight increase of the membrane was more than
50% when the membrane had been immersed in an aqueous pyrrole solution and then in an aqueous iron(II1) chloride solution[29]. However, the obtained film (composite membrane) was brittle and mechanically weak. In order to obtain the mechanically strong composite membrane, a Fe(II1) form cation exchange membrane was immersed in the aqueous pyrrole solution. In the previous paper, an anisotropically structured composite membrane was obtained by contacting one surface of the Fe(II1) form cation exchange membrane with an aqueous pyrrole solution[31]. Though thickness of a polypyrrole layer increased with increasing polymerization period, polypyrrole was distributed heterogeneously throughout the cross-section of the membrane even in 50 h polymerization[32]. This was because the polypyrrole layer on the membrane surface was so tight that permeation of pyrrole molecules was retarded. If the Fe(II1) form cation exchange membrane is immersed in the aqueous pyrrole solution, polypyrrole layers grow into the membrane from both surfaces and finally could be connected. It might be possible to obtain the composite membrane in which polypyrrole exists homogeneously. In this work, the Fe(II1) form and Cu(I1) form cation exchange membranes were immersed in the pyrrole solution for different period and the change in the conductivity of the composite membrane was observed. EXPERIMENTAL Materials Cation exchange membranes. Commercial cation exchange membrane, NEOSEPTA CH-45T, which is made by Tokuyama Soda Co., Ltd. and Nafion 117 131
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made by E. I. du Pont de Nemours Inc. were used. Table 1 shows characteristics of NEOSEPTA CH-45T and Nafion 117. Before use, NEOSEPTA CH-45T was alternately equilibrated with 0.5M sodium chloride solution and l.OM hydrochloric acid solution, and then equilibrated with an aqueous 0.173 M iron(II1) chloride solution or 0.2OOM copper(H) sulfate solution. Nafion was boiled in pure water for at least 60min after immersion in 60% nitric acid for 7 days at room temperature, and then equilibrated in the 0.173M iron (III) chloride solution. Before the composite membrane was prepared, the starting membrane was washed with pure water to remove excess salt. Gemicals. Pyrrole, FeCl, .6 H,O and CuSO, .5 H,O were obtained from Wako Pure Chemicals Ind. and used without further purification. Deionized water was used in all experiments.
Preparation of composite membrane Fe(II1) form cation exchange membrane (50 x 50mm) or Cu(I1) form cation exchange membrane (50 x 50mm) was immersed in an aqueous 0.745 M pyrrole solution under stirring for a given period at room temperature. After immersion, the membrane was taken out from the solution, washed with pure water, dried with nitrogen gas and stored under nitrogen atmosphere. The composite membrane in which polypyrrole existed anisotropically was also prepared by contacting one surface of the membrane with the pyrrole solution according to the method described in the previous paper[29]. Measurement ofconductivity composite membrane A strip of the (5.0 x 25.0 x 0.16mm) was used to measure conductivity parallel to the plane of the membrane. Strips were cut out from several places of the composite membrane (50 x 50mm). Silver conductive paste (Electroconductive DOTITE D-550 made by Fujikura Kasei Co., Ltd.) was coated on both surfaces from both ends of the strip (1Omm length each) and 5.0 x 5.0mm of the center of the strip was not coated. When conductivity was measured, both ends
of the strip, which were coated with silver paste, were
!hTA
tightly clamped between two platinum plates, respectively. Voltage was applied to both ends of the platinum plates by direct current supply (KIKUSUI Electronics Corp., Regulated DC Power Supply, Model PAB 350-0.1 A) and current which passed through the strip was measured by a Zero Shunt Ammeter (Hokuto Denko Ltd., HM-103). The voltage was increased from 0.1 V to 5.OV (measured by an electrometer: Hokuto Denko Ltd., HE-104) and current was recorded. Conductivity was calculated from a slope of voltagecurrent relation. Measurement of the voltage-current relation was carried out in desiccator (relative humidity: below 20%) at 30 f 1°C. Measurement of burst strength of the membrane To measure burst strength of the composite membrane, Bursting Tester Motordrive Mtillen type, Toyoseiki Seisakusho Ltd. No. 167, was used. Measurement was carried out under relative humidity of 5060% at room temperature. Analysis of iron(I1) and iron(III) in the composite membrane Composite membranes stored in nitrogen atmosphere were immersed in a 0.5 M sodium chloride solution under stirring several times to elute ion ions from the membranes. Collected solutions were divided into two parts. An aqueous solution of l,lOphenanthroline (0.5% solution adjusted to pH 2) was added to one of the solution. Ascorbic acid was added to the other part to reduce iron(II1) to iron( then l,lO-phenanthroline was added to that solution too. The concentration of iron(II)-l,lO-phenanthroline complex (maximum absorption peak was 5lOnm)[30] was determined by a U-3400 Hitachi spectrophotometer. The concentration of iron(II1) ions was calculated from the difference between total iron ions and iron(I1) ions.
RESULTS
AND DISCUSSION
Figure 1 shows examples of voltage-current relation of the composite membranes which were pre-
Table 1. Characteristics of cation exchange membranes used NEOSEPTA CH-45T* Backing Thickness (mm) Electric resistance (ncm’)t Transport number$ Ion exchange capacity5 Water content 11 Burst strength (kg cm - ‘H
Polyvinylcbloride 0.16 2.3 > 0.98 211 0.38 4.0
Na8on 117 0.175 3.5 >0.95 0.92 0.22 -
* Sulfonated styrenedivinylbenxene copolymer type. t Equilibrated with O.MON N&l solution at 25.O”C.measured with 1000 cycle ac. $ Measured by electrodialysis of 0.5N NaCl solution, current density: 20mA; 250°C. 0 Meq g-i Na+ form dry membrane. 11Equilibrated with OSN NaCl solution (gH,Og-’ Na+ form dry membrane). 7 Mtillen bursting strength.
Properties of composite membrane-I.
133
toward inside of the cation exchange membrane with increasing polymerization period. In this case, two conductive layers exist between direct current supply in parallel and resistance is shown as follows, +_$++ 1
2
where R is resistance of the composite membrane, R, and R, are resistance of the polypyrrole layer on each membrane surface. R, and R, are expressed as follows,
Fig. 1. Voltage and current relation of composite membrane with different polymerization period. FejIII) form cation exchange membranes were immersed in an aqueous pyrrole solution for 1,4 and 48 h.
pared by immersing the Fe(II1) form membrane in the pyrrole solution for 1, 4, and 48 h. Linear relations were observed in all composite membranes until voltage was S.OV. However, when higher voltage was applied to the strip, the strip generated heat. Though no current passed through the cation exchange membrane, the composite membrane of which the polymerization period was only 15 min showed conductivity. According to results of the previous work (XMA analysis of cross-section of the composite membrane)[32], very thin polypyrrole layers were formed on the surfaces of the cation exchange membrane and thickness of the layers increased with increasing polymerization period by immersing the membrane in the pyrrole solution. Figure 2 shows the relationship between conductivity of the composite membrane and polymerization period when the Fe(III) form membrane was used. Conductivity increased linearly until about 240min and increased gradually. Thereafter, it is expected that polypyrrole layers grew linearly
ltd10
1
1
13
10’
1 lo’
Fig. 2. Change in conductivity of composite membranes prepared from FejIII) form cation exchange membranes and polypyrrole with immersion period in an aqueous pyrrole solution. Dotted line shows the composite membrane with a single polypyrrole layer.
where o is specific resistance of the area in which polypyrrole was impregnated. L and A are the length of the composite membrane and area of the crosssection of the membrane in which polypyrrole exists. If the thickness of the polypyrrole layers increases linearly with increasing immersion period of the membrane and the specific resistance of the area in which polypyrrole exists is constant, there should be a linear relationship between the logarithm of resistance and the logarithm of immmersion period, logarithm of conductivity and logarithum of immersion period. Until about 24Omin, there is a linear relationship between log(conductivity) and log(immersion period). Figure 2 proves that polypyrrole grew from both surfaces in proportion to the immersion period and the specific resistance of the area in which polypyrrole existed was constant until 240min. According to a previous paper wherein NEOSEPTA CM-l (ion exchange capacity is almost the same as NEOSEPTA CH-45T) was used, the polypyrrole layer had progressed in the middle of the cross-section of the membrane at 240min when polymerization was carried out from one surface of the membrane (analysed by an X-ray microanalysis)[32]. After 240min the rate of conductivity increase was much slower, as shown by the change of slope in Fig. 2. It is thought that pyrrole molecules diffused into the membrane matrix and polymerized further. The highest conductivity obtained by this method was about lo-’ Scm- ’ when NEOSEPTA CH-45T was used. Figure 3 shows a microscopic photograph of the cross-section of the composite membrane when one surface of the Fe(II1) form membrane contacted the pyrrole solution for 2 h. The black part of the membrane is the area where polypyrrole exists. When pyrrole was polymerized from one surface of the membrane, the area of polypyrrole impregnation can be seen clearly. NEOSEPTA CH-45T is composed of copolymer of styrene-divinylbenzene, finely powdered poly(viny1 chloride) and poly(viny1 chloride) fabric. When base membranes for the ion exchange membrane were immersed in the aqueous pyrrole solution, weight gain of the membrane decreased with increasing cross-linkage and increased with increasing a ratio of resin part to poly(viny1 chloride).[33] It is apparent that pyrrole molecules were selectively adsorbed in the copolymer part of the membrane. Microscopic heterogeneity has been observed in morphological studies of the base membrane[34]. Fine particles of
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Fig. 3. Microscopic photograph of a cross-section of a composite membrane (NEOSEP’I’A CH-45T and polypyrrole). An aqueous pyrrole solution contacted one surface of the Fe(U) forru NEOSEPTA CH-45T for 2 h. the copolymer are distributed in the PVC matrix, ie added PVC and PVC fabric, because of polymer incompatibility. Since the weight ratio of the PVC part to the copolymer is about 1:l in NEOSEPTA CH-45T, it is thought that the increase in conductivity of the composite membrane was restricted by the PVC and such micro-structure of the cation exchange membrane. Cu(I1) form cation exchange membrane was also used to prepare the composite membrane with the same method. Figure 4 shows the change in conductivity of the composite membrane with polymerization period. Though the similar relationship was observed, conductivity of the membrane was relatively low compared with that prepared from Fe(II1) form membrane. Standard redox potential of Cu2+ + Cu+ + eis 0.153V and that of
1r9
’ 10
/
, 102
Polymerization
I
I
ld
10'
periodlmin
Fig. 4. Change in conductivity of composite membranes prepared from Cu(I1) form cation exchange membranes and polypyrrole with immersion period in an aqueous pyrrole solution. Dotted line shows the composite membrane with a single polypyrrole layer.
Fe’+ + Fe’+ + e- is 0.771 V.[35] Accordingly, polymerization speed of pyrrole in the Cu(I1) form membrane was slower than that of Fe(II1) form membrane. Slope can be obtained from Figs 2 and 4. The slope of Fig. 2 was 0.746 and that of Fig. 4 was 1.76. This is not consistent with results of conductivity and polymerization behavior of pyrrole. Namely, 10e3 S cm-’ of conductivity was attained at about 240min in the Fe(II1) form membrane and that was at 2800min in the Cu(I1) form. It is thought that there are three steps to form the composite membrane : (1) pyrrole molecules are adsorbed on membrane surfaces; (2) adsorbed pyrrole molecules polymerize to be polypyrrole; and (3) further pyrrole molecules diffuse through layers composed of the cation exchange membrane and polypyrrole, and polymerize in the inner part of the membrane. It is thought that adsorption of pyrrole on the membrane surface is not significantly different between the Fe(II1) form membrane and the Cu(I1) form because this is one kind of ion-exchange reaction at high concentration of pyrrole. Polymerization of pyrrole in the Fe(II1) form membrane was faster than that in the Cu(I1) form because of difference of standard redox potential[35]. In general, it is thought that polypyrrole layers are so tight and rigid that permeation of pyrrole molecules are restricted[32]. In fact, when polypyrrole layers were formed on an anion exchange membrane, the diffusion coefftcient of hydrochloric acid through the membrane decreased by about l/(1000-10000) of that of the conventional anion exchange membrane without remarkable increase in the electric resistance of the membrane (acids are remarkably easy to permeate through the membrane by concentration gradient)[36]. Although it is not clear which is tighterpolypyrrole formed in the Fe(III) form membrane and that formed in the Cu(I1) form-a conductivity of 10e4 S cm- ’ was observed in the composite membrane from Fe(II1) form at only 15min polymerization [Cu(II) form membrane required about 50h to attain the same conductivity]. It is thought that this was because of faster polymerization by Fe(II1). Weight increase of the membrane after polymerization was measured to compare polymerization speed in both membranes. The weight increase in the Fe(II1) form membrane was 5.4% after 60min polymerization and that of the Cu(I1) form was 3.9%. After 360min polymerization of the Fe(II1) form membrane and the Cu(I1) form, those were 6.1% and 5.1%, respectively. A rate of the weight increase of the Fe(II1) form membrane decreased with increasing polymerization period. Comparing conductivity of the Fe(II1) form membrane with that of the Cu(I1) form at the same weight increase (according to Figs 2 and 4), conductivity of the Cu(I1) form membrane at 360min was 9 x 10-6Scn-’ and that of the Fe(II1) form at 60min was about 3.4 x 10-4Scm-‘. This suggests that properties of polypyrrole in the Fe(II1) form membrane are different from that in the Cu(I1) form.
Properties of composite membranes--I.
135
In fact, although the color of the membrane changed from pale yellow to black within a minute by immersing the Fe(III) form membrane in the pyrrole solution, the Cu(I1) form membrane became black after 30min in the same condition. The weight increase was also examined in Nafion 117 because the weight increase was related to conductivity when the oxidative cation was the same. The weight increase of Nafion membrane was relatively low compared with NEOSEPTA membrane. Though the weight increase of NEOSEPTA CH45T was 12.6% by immersing the Fe(II1) form membrane in the pyrrole solution for 35 h, that of Nafion 117 was only 3.3% in the same condition. Moreover, when a lithium ion form NEOSEPTA CH-45T was equilibrated in the pyrrole solution and then immersed in the aqueous 0.175M iron(II1) chloride solution for 48 h, the weight increase was 33.0%. The weight increase of Nafion 117 was 20.9% in the same condition. These results suggest that much polypyrrole could not be formed in Nafion membrane. It is not clear whether this is based on a difference in ion exchange capacity (the ion exchange capacity of NEOSEPTA CH-45T was 2.11 meq g- ’ of dry membrane and that of Nafion 117 was 0.92meqg- ’ of dry membrane) or of membrane matrix. Dotted lines in Figs 2 and 4 show conductivity of the composite membrane of which the pyrrole solution contacted one surface of the membrane. Since the polypyrrole layer exists in only one surface of the membrane. Conductivity should be a half of the composite membrane with two polypyrrole layers. However, conductivity of the composite membrane with a single polypyrrole layer is only one-tenth of the value for the composite membrane with two layers in the case of the Fe(II1) form membrane and about one-hundredth in the case of the Cu(I1) form membrane. Though the increase in conductivity with immersion period was also linear, this phenomenon cannot be explained clearly. When NEOSEPTA CH-45T was immersed in the aqueous pyrrole solution, the membrane swelled with pyrrole. Since one surface of the cation exchange membrane swelled with the pyrrole solution and the other side did not swell, pyrrole molecules might penetrate into the inner part of the membrane with difficulty when the composite membrane with a single polypyrrole layer was prepared. Figure 5 shows a ratio of Fe(I1) to total iron ions in the composite membrane and an equivalent ratio of iron ions to ion exchange capacity of the membrane related to polymerization period of pyrrole. Since pyrrole molecules are oxidized to be polypyrrole, Fe(III) ions in the membrane are reduced to Fe(I1). It is well known that the anionic polyelectrolyte such as sulfonated polystryrene becomes a dopant of polypyrrole when pyrrole is electrochemically polymerized in the presence of the anionic polyelectrolyte on an anode, and the obtained polypyrrole becomes n-type conducting polymer[37]. The equivalent ratio of total iron ions to the ion exchange capacity of the cation exchange membrane was more than unity as shown in Fig. 5. Since Fe(II1) ion-exchanges with the membrane as FeCI:, FeCl’+ as well as Fe ‘+, the ratio should not be unity. Though the fact that the ratio approached
ioId Polymc&ation puled/min Fig. 5. Relationship between Fe’+/(Fe*+ + Fe3+) and (Fez+ + Fe3+)/(ion exchange capacity), and polymerization period. Solid line shows an equivalent ratio of Fe(U) to total iron ions and dotted line shows an equivalent ratio of total iron ions to ion exchange capacity.
to unity means that a part of sulfonic acid groups of the membrane became a dopant of polypyrrole in longer polymerization period, and most of the dopant of polypyrrole were not sulfonic acid groups of the membrane, but Cl-, FeC12- and FeCl;. Figure 6 shows the change in burst strength of the composite membrane with polymerization period of pyrrole. NEOSEPTA membrane is composed of sulfonated styrene-divinylbenzene copolymer and backing fabric (PVC). In general, as the copolymer of styrene-divinylbenzene is brittle and introducing ion exchange groups makes the copolymer film weaken. At the same time, polypyrrole is also brittle. However, the burst strength of the composite membrane increased unexpectedly with increasing polymerization period. The same phenomena were also observed in the composite membrane from NEOSEPTA CM-1 and polypyrrole. The burst strength of sodium ion form NEOSEPTA CH-45T was 4.5 kgcmm2 and that of ferric ion form membrane was 2.3 kgcme2. The burst strength of the composite membrane approached that of sodium ion form membrane. It is thought that polypyrrole in the membrane strengthened the mechanically weak sulfonated styrene-divinylbenzene copolymer membrane. Appearance of the composite membrane from the Fe(II1) form was the same as that from the Cu(I1)
& 10 ’
1
102
I
103
Polymeriaation periodftnin Fig. 6. Change in burst strength of composite membranes with polymerization period.
Tostit~~xx~ SATA
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form such as color (black), roughness of the membrane surfaces, thickness of the composite membrane, etc. Ac&nowle&ement-Many suggestions made by Dr Thomas A. Davis are gratefully acknowledged.
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26. R. E. Noftle and D. Pletcher, J. electrochem. Sot. 227, 229 (1987). 27. D. T. Glatzhofer, J. Ulanski and G. Wegner, Polymer 28,449 (1987). 28. C. K. Baker, Y. Qui and J. R. Reynolds, J. pkys. C/tern. %,4446 (1991). 29. T. Sata, Electrochim. Acta 37,555 (1992). 30. G. Muto in Hishoku-Bunseki-Ho (Calorimetric Analysis Method), Kyoritsu Shuppan, p. 161 Tokyo (1955). 31. T. Sata. J. membrane Sci. 66.289 (1992). 32. T. Sata; Chem. Mater. 3,838 (199i). 33. T. Sata, J. membrane Sci. 72,43 (1992). 34. K. Takata, K. Kusumoto, T. Sata and Y. Mizutani, J. macromol. Sci. Ckm. A24,645 (1987). 35. Kagaku Binran, Kiso-Hen II (Handbook of Chemistry) (Edited by Chemical Society of Japan), Vol. II, p. 473. 36. T. Sata, submitted to J. membrane Sci. 37. T. Hirai, S. Kuwabata and H. Yoneyama, J. electrochem. Sot. 135,1132 (1988).
1994
0
0013-4686/?34 s6.00 + 0.00 1993. Pcrgamon Prem Ltd
PROPERTIES OF COMPOSITE MEMBRANES FROM ION EXCHANGE MEMBRANES AND CONDUCTING POLYMERS-I. CONDUCTIVITY OF COMPOSITE MEMBRANE FROM CATION EXCHANGE MEMBRANES AND POLYPYRROLE T~~HIKATSUSATA Division of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai 2557, Ube City, Yamaguchi Prefecture, 755, Japan (Received 30 November 1992; in revisedform 12 July 1993) Abstract-In order to prepare a free-standing polypyrrole hhn with high mechanical strength, Fe(II1) form or Cu(I1) form cation exchange membranes were immersed in an aqueous pyrrole solution. Polymerization speed of pyrrole in the Fe(II1) form membrane was faster than that in the Cu(I1) form membrane. A linear relationship was observed between conductivity of the composite membrane and immersion period in the pyrrole solution. Conductivity of the composite membrane from the Fe(II1) form membrane reached about 10e3 Scm-‘, which was the ultimate vahte of this method. Properties of formed polypyrrole in the membrane matrix was discussed in relation to weight increase of the membrane and species of oxidative cation. Burst strength of the composite membrane increased with increasing polymerization period of pyrrole. Key words: cation exchange membrane, polypyrrole, chemical polymerization, composite membrane, conducting film.
INTRODUCITON Polypyrrole, polyaniline, other conducting polymers, and their analogues have been widely studied in the
field of pure and applied chemistry[l-51. Examples of their possible applications include batteries[6-121, electrolytic display[13-151, electrochromic capacitors[16], etc. One of requirements of these applications is to prepare a strong free.-standing lilm of the conducting polymer. Early works on this point were to polymerize electrochemically pyrrole or aniline within a predeposited, solvent-swollen thermoplastic polymer matrix such as poly(viny1 chloride)[ 173, poly(viny1 alcohol)[ 181, poly(methy1 methacrylate)[19], polystyrene[19], etc. Composite films of poly(3-methylthiophene) with poly(methy1 methacrylate) and poly(viny1 chloride) were reported to have been prepared by one-step procedure involving electrochemical polymerization on metal electrode from a solution containing 3_methylthiophene, supporting electrolyte, and the dissolved insulating polymer[20-211. Thereafter, a number of anionic polyelectrolytes such as poly(vinyl sulfate)[22-241, poly(viny1 sulfonate)[25], poly(styrene sulfonate)[22281, and other anionic copolymers have been used to give the film mechanical strength and as polymeric dopants. Chemically oxidative polymerization of pyrrole and aniline provides intractable, bulky powders with low processibility. Methods to obtain a polypyrrole film by chemical polymerization are to polymerize pyrrole in conventional films after pyrrole was adsorbed in the film. When a cation exchange membrane was used as the tilm, pyrrole was easily adsorbed in the membrane by ion exchange. reaction. The weight increase of the membrane was more than
50% when the membrane had been immersed in an aqueous pyrrole solution and then in an aqueous iron(II1) chloride solution[29]. However, the obtained film (composite membrane) was brittle and mechanically weak. In order to obtain the mechanically strong composite membrane, a Fe(II1) form cation exchange membrane was immersed in the aqueous pyrrole solution. In the previous paper, an anisotropically structured composite membrane was obtained by contacting one surface of the Fe(II1) form cation exchange membrane with an aqueous pyrrole solution[31]. Though thickness of a polypyrrole layer increased with increasing polymerization period, polypyrrole was distributed heterogeneously throughout the cross-section of the membrane even in 50 h polymerization[32]. This was because the polypyrrole layer on the membrane surface was so tight that permeation of pyrrole molecules was retarded. If the Fe(II1) form cation exchange membrane is immersed in the aqueous pyrrole solution, polypyrrole layers grow into the membrane from both surfaces and finally could be connected. It might be possible to obtain the composite membrane in which polypyrrole exists homogeneously. In this work, the Fe(II1) form and Cu(I1) form cation exchange membranes were immersed in the pyrrole solution for different period and the change in the conductivity of the composite membrane was observed. EXPERIMENTAL Materials Cation exchange membranes. Commercial cation exchange membrane, NEOSEPTA CH-45T, which is made by Tokuyama Soda Co., Ltd. and Nafion 117 131
Tosum~rsu
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made by E. I. du Pont de Nemours Inc. were used. Table 1 shows characteristics of NEOSEPTA CH-45T and Nafion 117. Before use, NEOSEPTA CH-45T was alternately equilibrated with 0.5M sodium chloride solution and l.OM hydrochloric acid solution, and then equilibrated with an aqueous 0.173 M iron(II1) chloride solution or 0.2OOM copper(H) sulfate solution. Nafion was boiled in pure water for at least 60min after immersion in 60% nitric acid for 7 days at room temperature, and then equilibrated in the 0.173M iron (III) chloride solution. Before the composite membrane was prepared, the starting membrane was washed with pure water to remove excess salt. Gemicals. Pyrrole, FeCl, .6 H,O and CuSO, .5 H,O were obtained from Wako Pure Chemicals Ind. and used without further purification. Deionized water was used in all experiments.
Preparation of composite membrane Fe(II1) form cation exchange membrane (50 x 50mm) or Cu(I1) form cation exchange membrane (50 x 50mm) was immersed in an aqueous 0.745 M pyrrole solution under stirring for a given period at room temperature. After immersion, the membrane was taken out from the solution, washed with pure water, dried with nitrogen gas and stored under nitrogen atmosphere. The composite membrane in which polypyrrole existed anisotropically was also prepared by contacting one surface of the membrane with the pyrrole solution according to the method described in the previous paper[29]. Measurement ofconductivity composite membrane A strip of the (5.0 x 25.0 x 0.16mm) was used to measure conductivity parallel to the plane of the membrane. Strips were cut out from several places of the composite membrane (50 x 50mm). Silver conductive paste (Electroconductive DOTITE D-550 made by Fujikura Kasei Co., Ltd.) was coated on both surfaces from both ends of the strip (1Omm length each) and 5.0 x 5.0mm of the center of the strip was not coated. When conductivity was measured, both ends
of the strip, which were coated with silver paste, were
!hTA
tightly clamped between two platinum plates, respectively. Voltage was applied to both ends of the platinum plates by direct current supply (KIKUSUI Electronics Corp., Regulated DC Power Supply, Model PAB 350-0.1 A) and current which passed through the strip was measured by a Zero Shunt Ammeter (Hokuto Denko Ltd., HM-103). The voltage was increased from 0.1 V to 5.OV (measured by an electrometer: Hokuto Denko Ltd., HE-104) and current was recorded. Conductivity was calculated from a slope of voltagecurrent relation. Measurement of the voltage-current relation was carried out in desiccator (relative humidity: below 20%) at 30 f 1°C. Measurement of burst strength of the membrane To measure burst strength of the composite membrane, Bursting Tester Motordrive Mtillen type, Toyoseiki Seisakusho Ltd. No. 167, was used. Measurement was carried out under relative humidity of 5060% at room temperature. Analysis of iron(I1) and iron(III) in the composite membrane Composite membranes stored in nitrogen atmosphere were immersed in a 0.5 M sodium chloride solution under stirring several times to elute ion ions from the membranes. Collected solutions were divided into two parts. An aqueous solution of l,lOphenanthroline (0.5% solution adjusted to pH 2) was added to one of the solution. Ascorbic acid was added to the other part to reduce iron(II1) to iron( then l,lO-phenanthroline was added to that solution too. The concentration of iron(II)-l,lO-phenanthroline complex (maximum absorption peak was 5lOnm)[30] was determined by a U-3400 Hitachi spectrophotometer. The concentration of iron(II1) ions was calculated from the difference between total iron ions and iron(I1) ions.
RESULTS
AND DISCUSSION
Figure 1 shows examples of voltage-current relation of the composite membranes which were pre-
Table 1. Characteristics of cation exchange membranes used NEOSEPTA CH-45T* Backing Thickness (mm) Electric resistance (ncm’)t Transport number$ Ion exchange capacity5 Water content 11 Burst strength (kg cm - ‘H
Polyvinylcbloride 0.16 2.3 > 0.98 211 0.38 4.0
Na8on 117 0.175 3.5 >0.95 0.92 0.22 -
* Sulfonated styrenedivinylbenxene copolymer type. t Equilibrated with O.MON N&l solution at 25.O”C.measured with 1000 cycle ac. $ Measured by electrodialysis of 0.5N NaCl solution, current density: 20mA; 250°C. 0 Meq g-i Na+ form dry membrane. 11Equilibrated with OSN NaCl solution (gH,Og-’ Na+ form dry membrane). 7 Mtillen bursting strength.
Properties of composite membrane-I.
133
toward inside of the cation exchange membrane with increasing polymerization period. In this case, two conductive layers exist between direct current supply in parallel and resistance is shown as follows, +_$++ 1
2
where R is resistance of the composite membrane, R, and R, are resistance of the polypyrrole layer on each membrane surface. R, and R, are expressed as follows,
Fig. 1. Voltage and current relation of composite membrane with different polymerization period. FejIII) form cation exchange membranes were immersed in an aqueous pyrrole solution for 1,4 and 48 h.
pared by immersing the Fe(II1) form membrane in the pyrrole solution for 1, 4, and 48 h. Linear relations were observed in all composite membranes until voltage was S.OV. However, when higher voltage was applied to the strip, the strip generated heat. Though no current passed through the cation exchange membrane, the composite membrane of which the polymerization period was only 15 min showed conductivity. According to results of the previous work (XMA analysis of cross-section of the composite membrane)[32], very thin polypyrrole layers were formed on the surfaces of the cation exchange membrane and thickness of the layers increased with increasing polymerization period by immersing the membrane in the pyrrole solution. Figure 2 shows the relationship between conductivity of the composite membrane and polymerization period when the Fe(III) form membrane was used. Conductivity increased linearly until about 240min and increased gradually. Thereafter, it is expected that polypyrrole layers grew linearly
ltd10
1
1
13
10’
1 lo’
Fig. 2. Change in conductivity of composite membranes prepared from FejIII) form cation exchange membranes and polypyrrole with immersion period in an aqueous pyrrole solution. Dotted line shows the composite membrane with a single polypyrrole layer.
where o is specific resistance of the area in which polypyrrole was impregnated. L and A are the length of the composite membrane and area of the crosssection of the membrane in which polypyrrole exists. If the thickness of the polypyrrole layers increases linearly with increasing immersion period of the membrane and the specific resistance of the area in which polypyrrole exists is constant, there should be a linear relationship between the logarithm of resistance and the logarithm of immmersion period, logarithm of conductivity and logarithum of immersion period. Until about 24Omin, there is a linear relationship between log(conductivity) and log(immersion period). Figure 2 proves that polypyrrole grew from both surfaces in proportion to the immersion period and the specific resistance of the area in which polypyrrole existed was constant until 240min. According to a previous paper wherein NEOSEPTA CM-l (ion exchange capacity is almost the same as NEOSEPTA CH-45T) was used, the polypyrrole layer had progressed in the middle of the cross-section of the membrane at 240min when polymerization was carried out from one surface of the membrane (analysed by an X-ray microanalysis)[32]. After 240min the rate of conductivity increase was much slower, as shown by the change of slope in Fig. 2. It is thought that pyrrole molecules diffused into the membrane matrix and polymerized further. The highest conductivity obtained by this method was about lo-’ Scm- ’ when NEOSEPTA CH-45T was used. Figure 3 shows a microscopic photograph of the cross-section of the composite membrane when one surface of the Fe(II1) form membrane contacted the pyrrole solution for 2 h. The black part of the membrane is the area where polypyrrole exists. When pyrrole was polymerized from one surface of the membrane, the area of polypyrrole impregnation can be seen clearly. NEOSEPTA CH-45T is composed of copolymer of styrene-divinylbenzene, finely powdered poly(viny1 chloride) and poly(viny1 chloride) fabric. When base membranes for the ion exchange membrane were immersed in the aqueous pyrrole solution, weight gain of the membrane decreased with increasing cross-linkage and increased with increasing a ratio of resin part to poly(viny1 chloride).[33] It is apparent that pyrrole molecules were selectively adsorbed in the copolymer part of the membrane. Microscopic heterogeneity has been observed in morphological studies of the base membrane[34]. Fine particles of
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134
Fig. 3. Microscopic photograph of a cross-section of a composite membrane (NEOSEP’I’A CH-45T and polypyrrole). An aqueous pyrrole solution contacted one surface of the Fe(U) forru NEOSEPTA CH-45T for 2 h. the copolymer are distributed in the PVC matrix, ie added PVC and PVC fabric, because of polymer incompatibility. Since the weight ratio of the PVC part to the copolymer is about 1:l in NEOSEPTA CH-45T, it is thought that the increase in conductivity of the composite membrane was restricted by the PVC and such micro-structure of the cation exchange membrane. Cu(I1) form cation exchange membrane was also used to prepare the composite membrane with the same method. Figure 4 shows the change in conductivity of the composite membrane with polymerization period. Though the similar relationship was observed, conductivity of the membrane was relatively low compared with that prepared from Fe(II1) form membrane. Standard redox potential of Cu2+ + Cu+ + eis 0.153V and that of
1r9
’ 10
/
, 102
Polymerization
I
I
ld
10'
periodlmin
Fig. 4. Change in conductivity of composite membranes prepared from Cu(I1) form cation exchange membranes and polypyrrole with immersion period in an aqueous pyrrole solution. Dotted line shows the composite membrane with a single polypyrrole layer.
Fe’+ + Fe’+ + e- is 0.771 V.[35] Accordingly, polymerization speed of pyrrole in the Cu(I1) form membrane was slower than that of Fe(II1) form membrane. Slope can be obtained from Figs 2 and 4. The slope of Fig. 2 was 0.746 and that of Fig. 4 was 1.76. This is not consistent with results of conductivity and polymerization behavior of pyrrole. Namely, 10e3 S cm-’ of conductivity was attained at about 240min in the Fe(II1) form membrane and that was at 2800min in the Cu(I1) form. It is thought that there are three steps to form the composite membrane : (1) pyrrole molecules are adsorbed on membrane surfaces; (2) adsorbed pyrrole molecules polymerize to be polypyrrole; and (3) further pyrrole molecules diffuse through layers composed of the cation exchange membrane and polypyrrole, and polymerize in the inner part of the membrane. It is thought that adsorption of pyrrole on the membrane surface is not significantly different between the Fe(II1) form membrane and the Cu(I1) form because this is one kind of ion-exchange reaction at high concentration of pyrrole. Polymerization of pyrrole in the Fe(II1) form membrane was faster than that in the Cu(I1) form because of difference of standard redox potential[35]. In general, it is thought that polypyrrole layers are so tight and rigid that permeation of pyrrole molecules are restricted[32]. In fact, when polypyrrole layers were formed on an anion exchange membrane, the diffusion coefftcient of hydrochloric acid through the membrane decreased by about l/(1000-10000) of that of the conventional anion exchange membrane without remarkable increase in the electric resistance of the membrane (acids are remarkably easy to permeate through the membrane by concentration gradient)[36]. Although it is not clear which is tighterpolypyrrole formed in the Fe(III) form membrane and that formed in the Cu(I1) form-a conductivity of 10e4 S cm- ’ was observed in the composite membrane from Fe(II1) form at only 15min polymerization [Cu(II) form membrane required about 50h to attain the same conductivity]. It is thought that this was because of faster polymerization by Fe(II1). Weight increase of the membrane after polymerization was measured to compare polymerization speed in both membranes. The weight increase in the Fe(II1) form membrane was 5.4% after 60min polymerization and that of the Cu(I1) form was 3.9%. After 360min polymerization of the Fe(II1) form membrane and the Cu(I1) form, those were 6.1% and 5.1%, respectively. A rate of the weight increase of the Fe(II1) form membrane decreased with increasing polymerization period. Comparing conductivity of the Fe(II1) form membrane with that of the Cu(I1) form at the same weight increase (according to Figs 2 and 4), conductivity of the Cu(I1) form membrane at 360min was 9 x 10-6Scn-’ and that of the Fe(II1) form at 60min was about 3.4 x 10-4Scm-‘. This suggests that properties of polypyrrole in the Fe(II1) form membrane are different from that in the Cu(I1) form.
Properties of composite membranes--I.
135
In fact, although the color of the membrane changed from pale yellow to black within a minute by immersing the Fe(III) form membrane in the pyrrole solution, the Cu(I1) form membrane became black after 30min in the same condition. The weight increase was also examined in Nafion 117 because the weight increase was related to conductivity when the oxidative cation was the same. The weight increase of Nafion membrane was relatively low compared with NEOSEPTA membrane. Though the weight increase of NEOSEPTA CH45T was 12.6% by immersing the Fe(II1) form membrane in the pyrrole solution for 35 h, that of Nafion 117 was only 3.3% in the same condition. Moreover, when a lithium ion form NEOSEPTA CH-45T was equilibrated in the pyrrole solution and then immersed in the aqueous 0.175M iron(II1) chloride solution for 48 h, the weight increase was 33.0%. The weight increase of Nafion 117 was 20.9% in the same condition. These results suggest that much polypyrrole could not be formed in Nafion membrane. It is not clear whether this is based on a difference in ion exchange capacity (the ion exchange capacity of NEOSEPTA CH-45T was 2.11 meq g- ’ of dry membrane and that of Nafion 117 was 0.92meqg- ’ of dry membrane) or of membrane matrix. Dotted lines in Figs 2 and 4 show conductivity of the composite membrane of which the pyrrole solution contacted one surface of the membrane. Since the polypyrrole layer exists in only one surface of the membrane. Conductivity should be a half of the composite membrane with two polypyrrole layers. However, conductivity of the composite membrane with a single polypyrrole layer is only one-tenth of the value for the composite membrane with two layers in the case of the Fe(II1) form membrane and about one-hundredth in the case of the Cu(I1) form membrane. Though the increase in conductivity with immersion period was also linear, this phenomenon cannot be explained clearly. When NEOSEPTA CH-45T was immersed in the aqueous pyrrole solution, the membrane swelled with pyrrole. Since one surface of the cation exchange membrane swelled with the pyrrole solution and the other side did not swell, pyrrole molecules might penetrate into the inner part of the membrane with difficulty when the composite membrane with a single polypyrrole layer was prepared. Figure 5 shows a ratio of Fe(I1) to total iron ions in the composite membrane and an equivalent ratio of iron ions to ion exchange capacity of the membrane related to polymerization period of pyrrole. Since pyrrole molecules are oxidized to be polypyrrole, Fe(III) ions in the membrane are reduced to Fe(I1). It is well known that the anionic polyelectrolyte such as sulfonated polystryrene becomes a dopant of polypyrrole when pyrrole is electrochemically polymerized in the presence of the anionic polyelectrolyte on an anode, and the obtained polypyrrole becomes n-type conducting polymer[37]. The equivalent ratio of total iron ions to the ion exchange capacity of the cation exchange membrane was more than unity as shown in Fig. 5. Since Fe(II1) ion-exchanges with the membrane as FeCI:, FeCl’+ as well as Fe ‘+, the ratio should not be unity. Though the fact that the ratio approached
ioId Polymc&ation puled/min Fig. 5. Relationship between Fe’+/(Fe*+ + Fe3+) and (Fez+ + Fe3+)/(ion exchange capacity), and polymerization period. Solid line shows an equivalent ratio of Fe(U) to total iron ions and dotted line shows an equivalent ratio of total iron ions to ion exchange capacity.
to unity means that a part of sulfonic acid groups of the membrane became a dopant of polypyrrole in longer polymerization period, and most of the dopant of polypyrrole were not sulfonic acid groups of the membrane, but Cl-, FeC12- and FeCl;. Figure 6 shows the change in burst strength of the composite membrane with polymerization period of pyrrole. NEOSEPTA membrane is composed of sulfonated styrene-divinylbenzene copolymer and backing fabric (PVC). In general, as the copolymer of styrene-divinylbenzene is brittle and introducing ion exchange groups makes the copolymer film weaken. At the same time, polypyrrole is also brittle. However, the burst strength of the composite membrane increased unexpectedly with increasing polymerization period. The same phenomena were also observed in the composite membrane from NEOSEPTA CM-1 and polypyrrole. The burst strength of sodium ion form NEOSEPTA CH-45T was 4.5 kgcmm2 and that of ferric ion form membrane was 2.3 kgcme2. The burst strength of the composite membrane approached that of sodium ion form membrane. It is thought that polypyrrole in the membrane strengthened the mechanically weak sulfonated styrene-divinylbenzene copolymer membrane. Appearance of the composite membrane from the Fe(II1) form was the same as that from the Cu(I1)
& 10 ’
1
102
I
103
Polymeriaation periodftnin Fig. 6. Change in burst strength of composite membranes with polymerization period.
Tostit~~xx~ SATA
136
form such as color (black), roughness of the membrane surfaces, thickness of the composite membrane, etc. Ac&nowle&ement-Many suggestions made by Dr Thomas A. Davis are gratefully acknowledged.
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