Structure-dependent conductivity and microhardness of metal-filled PVC composites

Metal-filed composites of a commercial PVC (polyvinyl chloride) powder (mea n particle size d(p) approximate to 100 microns) and a metal powder (mean p article size d(f) about 100 microns for copper, Cu, and about 10 microns fo r nickel, Ni) prepared by mechanical mixing in a ball mill, subsequent hot- pressing at 443 K and rapid cooling to 300 K, were characterized by the roo m-temperature measurements of electrical conductivity sigma density rho and microhardness H. The sudden jumps of about 17 orders of magnitude followed by a much slower growth up to the limiting filler fraction phi* on the log sigma vs. phi plo ts are the evidence for the onset of percolation transitions. at filler vol ume contents phi (cl) = 0.05 and 0.04 for PVC/Cu and PVC/Ni, respectively F or both systems, the values of H exhibited an initial steep increase up to phi (c2) = 0.07, followed by an apparent plateau extending up to phi = 0.18 . However, drastic differences in the patterns of composition dependence of H were observed at higher metal loadings, i.e., a continuous increase of H up to the leveling-off at phi for PVC/Cu, in contrast to a sudden drop of H at phi = 0.20 and subsequent slow increase for PVC/Ni. For both composite s the apparent density rho ' of a polymer matrix remained the same as that of the neat PVC in the composition interval phi < 0.20, white at phi < 0.20 a precipitous drop of rho (1) was observed due to the formation of polymer -free voids between filler particles (crowding effect) as phi approaches ph i*. The observed effects were analyzed in terms of a tentative model envisaging cross-overs from "dilute suspension regime" to "semi-dilute suspension reg ime" in the concentration range to phi (c1), to phi (c2), and from "semi-di lute suspension regime" to "concentrated suspension regime" above phi = 0.2 0. Different behavior in this latter regime was explained by intrinsic diff erences in the structure of conductive infinite clusters between mixtures o f particles of about the same size (PVC/Cu) and of widely different sizes ( PVC/Ni).