Microstructure-Property Relation in a Liquid Crystalline Polymer-Carbon Nanofiber Composite

Abstract

Microstructure-property relationship is being examined in a polymer matrix composite system consisting of vapor grown carbon nanofibers (VGCF) mixed in a thermotropic liquid crystalline polymer (LCP) matrix. These nanocomposites show an inherent hierarchical structuring, which we hope to utilize in the development of multifunctional structure-conduction composites with improved performance. Among unfilled polymers, extruded LCPs show relatively high strength and high stiffness that have been attributed in the literature to the preferential molecular alignment along the extrusion direction and the hierarchical nature of LCPs. Further, as is typical for polymers, LCPs have poor thermal and electrical conductivity relative to metals. By contrast, carbon nanofibers are known to possess high strength, high stiffness and high conductivity in the axial direction. It is expected that the combination of the extrusion process and the similarity of the length-scales of LCP fibrils and carbon nanofibers will lead to improved axial alignment of both phases within the nanocomposite filaments. This simultaneous alignment of the LCP matrix and that of the carbon nanofibers is expected to lead to interesting mechanical and conductive behavior in the nanocomposite filaments through hierarchical interactions at the nanometer to micrometer scale levels. Carbon nanofibers, 60-150 nm in diameter, were mixed with Vectra A950 LCP and the mixture was extruded as 0.5–2 mm diameter filaments. Nanocomposite filaments with 1%, 2%, 5% and 10 wt.% VGCF were characterized via tensile testing and fractography. The tensile modulus, failure strength and strain-to-failure were found to be sensitive to filament diameter, carbon nanofiber content and extrusion process. There was a noticeable increase in mechanical performance with decreasing filament diameter irrespective of carbon nanofiber content. Fracture surfaces showed hierarchical features from nanometer to micrometer scale and processing defects in the form of voids. The results of this research will be used to fabricate composite components that exploit structural hierarchy from nano-to macro-scale.