High-performance polymeric materials through liquid crystallinity: In situ formed polymer composites and novel liquid crystalline epoxy thermosets.

Abstract

High performance polymeric materials offer great potential as structural materials in automobile, aircraft, and space industries where weight saving is critical. The aim of my dissertation research is to develop high performance polymeric materials (both thermoplastics and thermosets) by orienting polymer chains via liquid crystalline states. In the thermoplastic system, in-situ composite fibers were prepared by extrusion and subsequent drawing of blends of a liquid crystalline copoly(ester amide) (Vectra B950) and polycarbonate, where the liquid crystalline polymer (LCP) serves as the reinforcing phase. It was found that the liquid crystalline polymer and polycarbonate are partially miscible due to the exchange reactions between the two polymers. It was also observed that both the longitudinal elastic modulus and tensile strength of the in-situ composites increase with LCP concentration and the extent of drawing. The reinforcement effect originates from the stretching of the dispersed LCP phase and the increased molecular orientation of the LCP chains during drawing. Based on this understanding, a model is proposed to correlate the elastic moduli of in-situ composites with LCP concentration and the draw ratio. Equations for the longitudinal and the transverse elastic moduli of in-situ composites are then derived based on the Halpin-Tsai equation, and a composite model or Northolt's aggregate model of the reinforcing LCP phase. Theoretical predictions of longitudinal and transverse elastic moduli are found to be in good agreement with experimental results for Vectra/polycarbonate composite fibers. In the thermoset system, the effects of curing agent, cure time, cure temperature and shearing on the phase transformations, solid state structure, and molecular orientation of a monotropic nematic liquid crystalline epoxy, diglycidyl ether of 4,4$\sp\prime$-dihydroxy-$\alpha$-methylstilbene (DGEDHMS) have been investigated. It was found that isothermal curing of the DGEDHMS in its isotropic phase with di- and tetra-functional amines leads to the development of liquid crystallinity. The resultant cured networks show a layered structure with the mesogenic units aligned perpendicular to the layer surface. Shearing a partially cured liquid crystalline resin orients the mesogenic core perpendicular to the shear direction.