Phase Transitions and the Effect of External Fields in Thermotropic Main Chain Liquid Crystal Systems
Martin, Philip Gene
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https://hdl.handle.net/2142/71713
Description
Title
Phase Transitions and the Effect of External Fields in Thermotropic Main Chain Liquid Crystal Systems
Author(s)
Martin, Philip Gene
Issue Date
1988
Doctoral Committee Chair(s)
Stupp, Samuel I.
Department of Study
Ceramics Engineering
Discipline
Ceramics Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Abstract
The aim of this research was to investigate the unique behavior of thermotropic main chain liquid crystal polymers with regard to phase transitions and their response to surfaces and external fields. Three different aromatic polymers were utilized in this study, including two constitutional isomers with periodic and aperiodic chemical sequences. The experimental techniques employed were x-ray diffraction, optical microscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermally stimulated discharge (TSD). It was discovered that grooved surfaces can strongly influence the molecular orientation of macromolecular liquid crystals especially when coupled with a magnetic field. Specifically, grooves parallel to a magnetic field were found to increase both the rate and ultimate degree of molecular orientation relative to samples exposed to magnetic fields in contact with featureless surfaces. TSD spectra of one of the polymers previously crystallized from the liquid crystalline melt under the influence of an electric field revealed spontaneous repoling upon cooling which is atypical of ordinary polymers. This phenomenon was attributed to unmelted poled crystals that repole the material at lower temperatures. Optical microscopy experiments on the constitutional isomers revealed that birefringent and isotropic phases coexist over a range of 120$\sp\circ$C in the aperiodic isomer but only over a range of 5$\sp\circ$C in the periodic isomer. A computer simulation suggested that the distribution in chain flexibility in the aperiodic isomer was the source of the broad biphasic range. Finally, measurements of the kinetic rate constant for crystallization from DSC scans revealed that a medium molecular weight random sequence polymer crystallized more rapidly than the ordered sequence polymer and random sequence polymers of high and low molecular weight. This phenomenon has been attributed to an optimum balance between the molecular diffusion rate and the disruption caused by thermal agitation. Values of the relative percent crystallinity were found to monotonically increase with increasing isothermal aging time in the periodic isomer, which may be a reflection of the more facile uniaxial orientational ordering present in this system.
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