Electrically conducting polymer matrix composites: A morphological and structural perspective.

Abstract

Wide angle x-ray scattering (WAXS), conductivity, optical microscopy, stress relaxation, and differential scanning calorimetry (DSC) measurements, were used to show the effect of filler addition and uniaxial orientation on the morphology of the matrix and the physical properties of reticulate doped polymer composites. Reticulate doping consists of casting a composite film from a solution containing a polymer and a charge-transfer complex (CTC) and allowing the conductive, free radical salt to recrystallize as the solvent is removed from the polymer. In this study, the CTC, tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) was supported by PE. It has been shown that increasing TTF-TCNQ concentration shifts the preferential orientation of the crystalline phase of the PE from slightly perpendicular to slightly parallel to the casting surface.

Past research on conductive polymer-matrix composites has focused on the relationships between concentration, geometry, and dispersion of the discontinuous phase and the physical properties of the resulting material. However, the volume fraction - geometry approach does not take into account the influence of polymer morphology and polymer-filler interactions on the composite properties. The work presented in this dissertation addresses the changes in the polymer morphology and structure resulting from the addition of the conductive filler.

Isothermal crystallization experiments have been carried out for nickel - low-density polyethylene (LDPE) composites from 95° to 104° C. In this study, the effect of nickel in filled LDPE composites on the crystallization kinetics of the LDPE crystallites has been quantified and compared to the filler's effect on electrical and thermal properties. The crystallization kinetics were altered by the nickel addition in two ways; reducing the nucleation time for a given isothermal crystallization temperature and increasing the crystallization rate. The rates were compared by fitting the data to the Avrami equation. The Avrami exponent was not effected by the addition of nickel, indicating the change was due to increased crystal growth rate rather than a nucleation effect.

The electrical resistance of polymeric materials loaded with conductive fillers can be divided into three major categories: the intrinsic resistance of the filler and matrix, the particle-particle contact resistance, and the tunneling resistance. A method for decreasing both the particle-particle contact and tunneling resistance in particulate filled LDPE composites which involves coating the particles with polypyrrole (PPgamma) using admicellar polymerization has been developed. Nickel flake, alumina, and glass fibers were used as substrates for polymerization and represent conductive, resistive, and insulating materials, respectively. Addition of PPgamma to the conductive and resistive particulates lead to an increase of 2 to 4 orders of magnitude in composite conductivity at concentrations above the percolation threshold without significantly changing the thermal or mechanical properties of the composite. (Abstract shortened by UMI.)