This dissertation explores the potential of improving the performance of poly(ethylene terephthalate) fibers by incorporating novel thermotropic liquid crystalline polymers. To determine if a system exhibited desirable characteristics, a screening procedure was developed to assess the various blends. Evaluations focused on blend compositions ranging from 2 to 20 wt.% LCP. Fibers were obtained by melt extrusion and the effect of processing conditions, i.e. spinning temperature, stretch ratio, and post treatment evaluated. The fibers were tested for mechanical performance, dimensional instability (shrinkage), and the development of shrinkage stresses. Test results were used to determine the critical parameters necessary for in-situ reinforcement and to develop strategies for improving LCP architecture and processing techniques. The novel TLCP's incorporated into the PET were mesogenic copolymers containing either alternating or random flexible groups within the polymer backbone. The flexible moieties were used to promote compatibility between the PET matrix phase and the TLCPs. Two systems were found to significantly improve fiber stiffness compared to neat PET fibers. A Random Copolymer based on the reaction of oxyethylene substituted hydroquinone, ethylene glycol, and terephthaloyl chloride was found to effectively enhance the performance of PET fibers. Fibers containing only 5% TLCP exhibited a 50% increase in modulus, while maintaining an ultimate strength equivalent to the PET control. The thermal behavior of the 5% blend, as determined by free shrinkage and force-temperature experiments, was similar to the PET control. A segmented block copolymer consisting of rigid-rod, diad, and flexible coil segments was also found to improve the performance of PET fibers. At a concentration of 20 wt. percent, the alternating block copolymer, Triad2 (2:6:7), increased the tensile modulus of the fibers 40% and decreased free shrinkage by 20% compared to neat PET. The mechanism of reinforcement for these systems is unclear, but morphological, thermal and mechanical evidence suggest that the TLCPs are modifying the PET matrix and not providing true mechanical reinforcement.

Disclinations and inversion walls in the director field of nematic thermotropic liquid crystal polymers (TLCP's) are imaged at high resolution using the lamellar decoration technique of Wood and Thomas. The utility of the lamellar decoration technique has also been greatly extended by using an etch and replication technique to image the director in sections of a bulk sample. The interaction of disclinations in the presence of an applied magnetic or extensional flow field which tends to align the director is studied. At intermediate field strengths, where the disclination separation is comparable to the characteristic length for director field distortions, pairwise interaction no longer dominates, but clusters of disclinations are observed to form. The applied field suppresses fluctuations of the disclination positions which increase the topological dipole moment of a cluster while enhancing those that preserve zero dipole. The applied field tends to minimize the long range distortional energy which is proportional to the square of the dipole moment. As a result, quadrupolar (Lehmann) clusters of disclinations are commonly observed in either magnetically or extensionally flow aligned samples. Many-bodied interaction of disclinations in the absence of an applied field is studied via computer simulation, and the results agree favorably with recent experimental work. The structure of the cores of disclinations has been observed. The core size is on the order of a few molecular lengths, and its structure depends upon polymer architecture. A rigid polymer splays more within the core of a $\pi$ wedge disclination, and an excess of chain ends aggregates. A semi-flexible polymer, in contrast, bends more within the core. Hairpins are not thought to be abundant in the semi-flexible TLCP. The morphology of the TLCP semi-crystalline state has also been examined. It is observed to depend upon the relative primary crystal nucleation and crystal growth rates. The lamellar decoration morphology, where the crystalline lamellae are everywhere perpendicular to the director of the precursor nematic, is favored by rapid nucleation and slow growth. This morphology is unique to TLCP's, but other morphologies are possible. Spherulites have also been grown from the nematic melt. Their growth is favored by slow nucleation and rapid growth.