Date

2017

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Pearson, Raymond A.

Other advisers/committee members

Gilchrist, James; Coulter, John; Robeson, Lloyd

Abstract

Electrically conductive polymer composites consisting of conductive fillers dispersed in polymer systems continue to attract increasing research. Multiphase polymer blends provide unique morphologies to reduce the percolation concentration and increase conductivity of carbon-based polymer composites. The goal of this research is to further the current understanding of electrically conductive ternary polymer blends. The overall purpose is to leverage this work to design composite materials that achieve increased conductivity at reduced conductive filler loadings that can be extended to applications requiring conductivity and a balance of additional properties. The first part of this research investigated the kinetic and thermodynamic effects on a series of multiphase conductive polymer composites. The electrical conductivity and phase morphology of a carbon black (CB) filled polypropylene (PP)/poly(methyl methacrylate) (PMMA)/ethylene acrylic acid copolymer (EAA) ternary polymer blend was determined as a function of compounding sequence and annealing time. The phase morphology and conductivity at short annealing times were influenced by the compounding sequence; however, they were thermodynamically driven at longer annealing times. The resistivity was found to decrease by a statistically significant amount to similar levels for all of the composite systems with increasing annealing time. The increase in conductivity at longer annealing times was determined to be the result of changes in the phase morphology from sea-island, dispersed microstructure to a tri-continuous morphology. The second part of this research studied the influence of CB and multiwall carbon nanotube (CNT) conductive fillers with different colloidal properties on the phase morphology, electrical properties, and rheological behavior in the PP/PMMA/EAA ternary polymer blend. A PP/PMMA/(EAA-CNT) system was compared to two different PP/PMMA/(EAA-CB) systems. The critical electrical percolation threshold for the ternary conductive polymer composites was found to be more than 8 times lower than for the single phase systems. The rheological threshold coincided with the electrical resistivity percolation threshold inversion point. It was proposed that beyond a critical loading of conductive filler particles in the minor EAA phase, especially for fillers with effective aspect ratios that are high such as the CB2 and CNT, the kinetics of phase separation and resulting formation of a tri-continuous morphology are dictated by the viscosity of the minor phase relative to the two major phases. The last part of this research used the Cahn-Hilliard theory to model and predict the phase morphology and electrical conductivity as a function of the constituents' characteristics of the ternary system. A method for generating statistically representative microstructures of a co-continuous ternary polymer system and a numerical method for calculating the resultant electrical conductivity of these ternary polymer systems are presented. Excellent agreement between numerically calculated and experimentally measured results was observed. The combination of experimental and numerical results presented suggests the optimization of the conductive minor phase includes having a conductivity beyond the critical percolation threshold, is at least three orders of magnitude greater than either of the two non-conductive phases, and has a lower viscosity than the other two major phases in order to maximize the phase separation kinetics.

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