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I n d . Eng. Chem. Res. 1987,26, 678-680
Conductive Poly(acetylene) Blends Karl F. Schoch, Jr.,* and Rajender K. Sadhir Dielectrics and Insulation Department, Westinghouse R&D Center, Pittsburgh, Pennsylvania 15235
Blends of poly(acety1ene) and various fabrics (Kevlar, glass) were prepared and characterized. The conductivities of the films were followed for up to 6 weeks in air. All were more stable to ambient conditions than conventional poly(acety1ene) films, suggesting that poly(acety1ene) may have been grafted to the fibers. Low-magnification SEM photographs showed only a coating of poly(acety1ene) on the fabric materials. A conductive polymer offers several technologically attractive characteristics in addition to high conductivity, including low cost, simple preparation and processing, and properties variable over a wide range. Considerable effort has been expended in the synthesis and characterization of these materials, as described by Greene and Street (1984). Unfortunately, poly(acety1ene) (PA), one of the most conductive and easiest polymers to prepare, is also not stable to ambient oxygen. Pochan et al. (1981) report that the polymer is first doped and then decomposed by oxygen, with the decomposition being evident after 30 min in air. Another shortcoming of this material is that it is not readily processed since it is insoluble and infusible. Many other conductive polymers have similar limitations. One strategy for overcoming the poor stability of P A is to incorporate it into a polymer blend or copolymer with another polymer of greater stability and processability. Several PA copolymers have been prepared with such polymers as poly(ethy1ene) by Galvin and Wnek (1982), EPDM elastomer by Lee and Jopson (1983), poly(butadiene) by Rubner et al. (1983), and poly(pheny1ene vinylene) by Wnek et al. (1979), among others. There are several methods for preparation of graft copolymers using chemical initiators and radiation techniques. Chemical initiations include chain-transfer reactions, radical attack on unsaturated polymers, hydroperoxidation, diazotization, and redox systems. Preparation of polymer blends or copolymers incorporating PA is generally accomplished by a chemical initiation process which involves dissolving or swelling the copolymer of interest in the solvent used in the catalyst solution (usually toluene). The catalyst solution is then removed and acetylene gas admitted. As with PA homopolymer synthesis, the reaction can be carried out at low temperature to produce the predominant cis isomer, as described by Ito et al. (1974). Doping in this case usually requires milder oxidizing agents, like the halogens, because the strong oxidizing agents, like AsF,, will decompose the other constituent of the blend or copolymer. In the work described in this paper, Kevlar and glass fabrics were used as the matrices for the polymerization of acetylene. The polymer blends were then doped with iodine to render them conductive. The conductivity of the doped blends was monitored over a period of time exposed to ambient conditions. In addition, SEM photographs were taken to elucidate the microstructure of the blends.
Experimental Section Preparation of PA Blends. Polymer blends of poly(acetylene) and various fabrics (Kevlar or CS-181 glass) were prepared in the following way. A piece of the fabric was soaked in 10 mL of A1(C2H5)3in toluene (25 wt %, Aldrich) in a glovebox for 1-3 days. Then 1.7 mL of Ti(OC4H9I4(Aldrich, distilled) was added and the catalyst 0888-5885/87/2626-0678$01.50/0
allowed to age for 30 min at room temperature. The glass reactor was then removed from the glovebox, attached to the vacuum line, and cooled to -78 "C. With the impregnated fabric suspended above the catalyst solution, the vessel was evacuated and C2H2admitted. The acetylene was first passed through a -78 OC trap. Approximately 600 torr of CzHzwas added and the reaction allowed to run for 10-15 min. After the reaction was over, the acetylene was pumped out and the reactor warmed to room temperature. The catalyst solution was removed by syringe and the film rinsed with freshly distilled toluene until the rinses were clear. On the basis of weight changes, the PA/glass blend was estimated to contain 20% PA by weight. The resulting composite was doped with iodine by loading the sample into a 100-mL three-neck flask and adding iodine crystals. Doping was allowed to proceed over 24 h at room temperature. Afterward the iodine crystals were removed, and the flask was evacuated for 1-2 h. The weight change of the PA/glass blend after doping suggested a doping level of 0.015 mol % iodine. Doping was carried out in the absence of air. Electrical Measurements. The resistance of the samples was measured by using a Keithley 610A electrometer. The resistance was measured through the thickness of the sample as well as along one side of the sample. No attempt was made to calculate a resistivity because it was not possible to estimate an accurate value for the thickness of the conducting part of t_he sample. Instead, the resistance was followed over a period up to a month in air a t room temperature. The resistance of a conventional PA film doped with iodine was monitored simultaneously for comparison purposes. SEM Photographs. Samples of undoped blends of glass and Kevlar fabrics as well as glass and Kevlar fibers were examined by scanning electron microscopy. The fabric samples were coated with evaporated gold before examination. The fibers were embedded in a resin and microtomed in order to examine them in cross section.
Results and Discussion The poly(acety1ene) blends were characterized by scanning electron microscopy and electrical resistance measurements. We found that the resistance of the blends was substantially more stable to ambient conditions than conventional P A films. In order to investigate why that would be the case, SEM photographs were taken of the blends and of individual fibrils reacted with PA. These photographs suggested that the P A acted as a coating on the fabric and the fibrils. Because of the preliminary immersion of the fabric in the catalyst solution, it was plausible that there would be some grafting of the polymer to the fabric material. Certainly either Ti(OBu), or Al(C2H5)3from the catalyst solution could coordinate to 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 4,1987 679
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4
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! Figure 1. SEM photographs of puly(scetylene)/Kevlar blends.
Figure 2. SEhl phulcrgmphs i i,"l?(acei?.le~~ei/gli~*\ Mends.
either amine groups in the Kevlar or Si0 groups in the glass fabric. Because poly(acety1ene) homopolymer has no known solvent, however, it is impossible to determine by chemical means the extent of grafting in these blends. SEM Photographs. Scanning electron microscopy photographs of PA/Kevlar blend and PA/glass blend are shown in Figures 1 and 2, respectively. We wanted to establish whether the PA had actually impregnated the individual fibrils of the fabric or whether it was simply a coating over the fabric material. As can be seen in Figure 1,the poly(acety1ene) filmwas pulled away from the fibrils in some places, and there were areas where the fibrils went in and out of the coating. Therefore, the PA is apparently acting as a coating on the fibers. The poly(acety1ene) seemed to be particularly smooth in the PA/glass sample. That could be due to the more difficult cutting process required in handling the PA/Kevlar sample after it was prepared. In order to investigate further whether the polymer penetrated into the fibers themselves, we prepared a sample of blended fibers of each material by using the procedure described in the Experimental Section. These samples were then embedded in a resin which was cured a t room temperature. Thin slices of the fiber embedded
in cured resin were cut by a microtomy technique so as to expose the cross section of the fiber. The resulting SEM photographs are shown in Figure 3. These photographs also show the poly(acety1ene) coating on the fibers. Conclusive evidence of grafting of PA within the matrix of the fiherswas not observed from these low-magnificationSEM photographs. Electrical Properties. The changes of conductivity over time in air for each of the blends are given in Figures 4 and 5. There are several important observations to make about the behavior of the different materials in air. First, the conduction through the hulk of the samples is of the same order of magnitude as that along the surface. That was true for both the Kevlar- and the glass-reinforced blends. That observation suggested that there might be some grafting of the fibers themselves with poly(acety1ene). Second, in each case, the blend was more stable toward ambient conditions than a conventional PA film. Whereas the conventional films lost all their conductivity within 5 days, the blends were still a t least somewhat conductive even after 4-6 weeks. Third, the changes in resistance along the surface and through the bulk of the samples tracked each other consistently. If the enhanced stability of the blends was simply due to slowed diffusion of oxygen
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680 Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 L I
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Figure 5. Aging of poly(acetylene)/glass in air.
that the resistance for each of the blends was much more stable to ambient conditions than conventional poly(acetylene) films. SEM photographs showed that the poly(acety1ene) acted as a coating for the fabrics. The insolubility of the poly(acety1ene) homopolymer made it impossible to separate and identify any grafted material from the homopolymer. The electrical conductivity results suggest that there may have been grafting of PA onto the fibers, but we could not obtain any definitive evidence for that. Acknowledgment The experimental assistance of J. F. Chance and K. E. Pfeiffer is most appreciated. SEM photographs were taken by T. J. Mullen of the Materials Characterization Laboratory. Assistance of D. A. Smoody in the preparation of the manuscript is gratefully acknowledged. W s t r y No. Polyacetylene, 25067-58-7; iodine, 7553-56-2. Literature Cited Calvin, M.E.; Wnek. G. E. Poly. Commun. 1982,23,795. Greene. R. L.;Street, G. B. Science (Woshingfon,D.C.) 1984,226, 651.
Ito. T.;Shirskawa, H.;Ikeda, S.J. Polym. Sei., Polym. Chem. Ed. TlmelDayri
Figure 4. Aging of poly(aeetylene)/Kevlar in sir.
into the interior of the sample, the surface resistance should decay in a manner comparable to conventional PA films. Conclusions In this report, we have described the properties of blends of poly(acetylene) with Kevlar and glass fabrica. We found
1974, 12, 11.
Lee, K. I.; Jopson. H.Makromol. Chem. Rapid Commun. 1983.4, 375.
Pochan. J. M.;Pochan, D. F.; Rammelmann, H.;Gibson, H. W. Moeromoleeules 1981, 14, 110.
Rubner, M.F.; Tripathy. S. K.; Georger, J., Jr.; Cholewa, P. Maeromolecules 1983. 16, 870.
Wnek, G . E.; Chien. J. C. W.; Karasz, F. E.; Lillys, C. P. Pofym. Commun. 1979.20,1441. Receiued for reuiew May 23, 1986 Accepted November 24,1986
