A conductive composite film by permeation method
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ABSTRACT A new method of making conductive composite films by permeation of the conducting guest species into the host is reported. A layer of poly(3-n-dodecyl thiophene) (P3ddt) is embedded at the surface of polyimide by permeation of the monomer or polymer (in solution in tetrahydrofuran or chloroform) into a solution of polyamic acid in n-methyl pyrrolidinone or dimethyl acetamide. The resulting composites were imidised and polymerized (if necessary). Chemical imidisation yielded composite -films that retained the conducting polymer even when the composite was subjected to solvent extraction. The films were conductive upon doping with iodine and recovered conductivity when they were exposed to iodine vapor subsequent to thermal de-doping. Thermogravimetry showed that the amount of thiophene incorporated into the polyimide was higher for permeation of the polymer than that of the monomer; however, the amount of p3ddt incorporated by the latter method was still higher than the amount that could be incorporated by blending polyamic acid with p3ddt. The levels of conductivity and speed of recovery for doped films were also higher for the permeated films. Results of scanning electron microscopy suggested that the higher mobility afforded by contact in the liquid state have contributed greater entanglement between the constituents leading to higher thermal and solvent resistance of the conducting constituent. The permeation method could be adopted to form composite films in solvent systems that are not completely miscible.
INTRODUCTION Conducting polymers have been intensively studied in the past two decades. New conductive polymers such as polythiophene, polypyrrole, poly(p-phenylene), poly(p-phenylene sulphide), polyaniline and their derivatives have been developed. Among these polymers, the pure, unsubstituted polythiophene is an intractable material. By introducing a flexible side chain at the 3-position of the thiophene ring, the polymer is made fusible, melt processable, and soluble in common organic solvents [1]. The processability of the PATs implies that it is possible to obtain a wide variety of polymer blends or composites with thermosetting or thermoplastic polymers. In addition to providing mechanical and thermal reinforcement, these new polymeric materials may yield further interesting properties. Conductive polymer composites which combine the electrical conductivity of polythiophene, polypyrrole or its derivatives with good mechanical properties of insulating polymers such as polystyrene [2,3], ethylene vinyl acetate copolymer [4,5], polyimide [6,7,8], and rubber [9] have been reported. Among insulating matrix polymers, polyimide has been popular because of its wellknown thermal and mechanical stability. Polypyrrole-polyimide composite films were prepared
209 Mat. Res. Soc. Symp. Proc. Vol. 600 © 2000 Materials Research Society
by electrochemical and chemical oxidation of pyrrole on polyimide substrates [7]. A surface absorption method was also reported [8]. These methods use the imidised polymer a
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