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Polymer nematic liquid crystals: Disclination structure and interaction
Abstract
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.
Subject Area
Materials science|Polymer chemistry
Recommended Citation
Hudson, Steven David, "Polymer nematic liquid crystals: Disclination structure and interaction" (1990). Doctoral Dissertations Available from Proquest. AAI9110155.
https://scholarworks.umass.edu/dissertations/AAI9110155