Electrical Conduction near Percolation Threshold in Polymer Composite Containing Conducting Polypyrrole-Coated Insulating Polymer Fiber

New types of conductor-insulator composites are prepared, in which conducting polymer-coated insulating polymers (polyethylene in spheres, nylon66 in fibers), whose size and shape are well controlled, are used as the conductor, and noncoated polyethylene spheres are used as the insulator. The dependence of electrical conductivity on the concentration of coated insulating polymers near the percolation threshold is studied in detail. The percolation threshold decreased with increasing length of coated nylon66 fibers. In the composite made of polypyrrole-coated polyethylene spheres and noncoated polyethylene spheres, the critical exponent t is evaluated to be 2.0±0.2. In the composite made of polypyrrole-coated nylon fibers (400 µ m, 800 µ m) and noncoated polyethylene spheres, a larger value t (3.0±0.4) is obtained. The dependence of t on applied voltage is also studied. In the composites made of polypyrrole-coated nylon66 fibers, t increases with increasing applied voltage. This can be explained by taking interparticle tunneling conduction into consideration in the percolation model.