Fabrication and Characterization of a Carbon Nanofiber Reinforced Liquid Crystalline Polymer

Our group at the Naval Research Laboratory is studying hierarchical arrangements of materials at multiple length scales and how such arrangements can be used to yield novel properties. We are investigating nanocomposites comprising a thermotropic liquid crystalline polymer (LCP) matrix reinforced with carbon nanofibers for potential structure + conduction multifunctional applications. The LCP matrix is known for its inherent hierarchical microstructure, and its fracture surface is typically characterized by fibrils ranging in size from nanometer to micrometer. The carbon nanofibers being compounded with the LCP matrix are vapor-grown carbon nanofibers (VGCF) and pre-processing techniques are being developed to eventually replace VGCF with single-wall carbon nanotubes (SWNT). Composites with VGCF content of 0, 1, 2, 5 and 10 wt.% were extruded using a twin-screw extruder to yield monofilaments in the range of 0.5 to 2 mm in diameter. The mechanical properties of extruded filaments were determined via quasi-static tensile tests and fracture surfaces examined under a scanning electron microscope. Porosity and hierarchical fibrillar structures were commonly observed in the fracture surfaces of tensile tested LCP and LCP-VGCF filaments. The LCP-VGCF filaments showed a maximum increase in strength and modulus of 20% and 35%, respectively, at 1-2 wt.% VGCF content. The dependence of mechanical properties on VGCF content was attributed to the interplay between the extrusion process parameters, VGCF dispersion and molecular alignment of LCP. In another set of experiments, LCP was thermo-mechanically pre-processed using a laboratory scale double-roll mixer and extruded using a Maxwell mixing extruder to yield monofilaments in the range of 0.2 to 0.7 mm. At 0.2 mm diameter, filaments of un-pre-processed and pre-processed neat LCP showed almost identical mechanical properties. At 0.7 mm diameter, however, pre-processed LCP filaments showed 10% and 30% degradation in strength and modulus, respectively, relative to un-pre-processed LCP. The lowered mechanical properties of pre-processed LCP were attributed to its chemical degradation during thermo-mechanical processing. Over the diameter range from 0.2 to 2 mm and irrespective of prior processing or extrusion method, the modulus and strength of neat LCP filaments increased with decreasing diameter. The strength and modulus dependence on filament diameter could be explained by the "skin-core" effect typically seen in liquid crystalline polymers. Future work will involve optimizing processing parameters for simultaneous enhancements in mechanical properties and electrical/thermal conductivity in LCP-VGCF/LCP-SWNT filaments.