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Polymer dispersed liquid crystal (PDLC) films, consisting of micro-sized domains of a liquid crystal dispersing in a polymer matrix, serve as the basis of a variety of high-efficiency electro-optical effects. The thermo-optical properties of the PDLCs were investigated in this thesis. The thermal properties and the morphologies of four low molar mass mesogens were studied by DSC and polarized optical microscope (POM). There were significant super cooling/heating effects on the first order phase transitions but not on the mesophase transitions. The structural effects on the transition temperature were investigated. Between the two 4-alkoxybenzoic acids mesogens, the clearing temperature of 4-(octyloxy)benzoic acid was higher than 4-(decyloxy)benzoic acid because of the increasing chain length. Trans-4-methoxycinnamic acid had the highest melting temperature among the four mesogens despite the molar mass because the carboxylic acid termini of trans-4-methoxycinnamic acid gave rise to strong intermolecular attractions. The smectic phases of 4-(octyloxy)benzoic acid were classified as head-to-head bilayer orientational smectic structures, SmA2 and SmC2, respectively, by wide angle X-ray diffraction through measuring the d spacing of the liquid crystal. The total solubility parameter was used to evaluate matching a polymer-LC-solvent combination. PDLC films were prepared by the solvent induced phase separation method and suitable morphologies were achieved by thermal induced phase separation. The phase transition temperatures of PDLCs were shifted to a lower temperature due to the polymer dispersion effects. Different mesophases were observed in PDLC films when LC exhibited different mesophases. The LC fractions in the droplets were calculated from the nematic to isotropic enthalpies through the Smith equation. Two factors, thermal cooling rate and the LC concentration, which affect the size of the droplet dispersed in the polymer matrix, were investigated in the PVC dispersed 4,4'-azoxyanisole. The phase transitions of pHEMA dispersed 4-(octyloxy)benzoic acid and PVC dispersed 4,4'-azoxyanisole were investigated by TMDSC and quasi-isothermal TMDSC. The TMDSC results were analysed by the two approaches, reversing and non-reversing heat flow and complex heat capacity. The results of the phase transitions of the two PDLCs illustrated that in the PDLCs it involved both non-reversing, melting, and reversing, clearing and the transition between two mesophases. In the non-reversing transition, the transition temperature would be affected by super cooling/heating and the results obtained in the experiments were dependent on the experimental conditions, such as the heating or cooling rate, sample size and purge gas flow rate. However, in the reversing transition process, there were no super cooling/heating effects observed and it seemed that the experimental conditions were not so critical. Results could be monitored by Lissajous figures obtained from the quasi-isothermal TMDSC. The plots of modulated heat flow versus the derivative of modulated temperature can be used to alert to unfavorable experimental conditions where loss of system linearity could be seen.

Owing to their excellent infrared transmittance and good rheological properties, selenium based chalcogenide glasses have been materials of choice for a number of technological applications. However, chalcogenide glasses can undergo substantial structural relaxation even at room temperature due to their low glass transition temperatures. The origins of these dynamic processes and their correlation to the glass structure is therefore of fundamental and practical interest. In particular, a deep understanding of the dynamic response near the glass transition region could help elucidate the mechanism of these structural relaxation processes. The correlation between structure and dynamic properties of selenium based glass systems were therefore investigated. NMR and Raman spectroscopy measurements reveal that the structure of AsₓSe₁₋ₓ glass follow the chain crossing model in selenium-rich glasses but contain increasing amounts of cage molecules in arsenic-rich compositions. This structural pattern leads to systematic extrema in physical properties at the stoichiometric composition As₄₀Se₆₀.The dynamic response of AsₓSe₁₋ₓ glasses investigated by heat capacity spectroscopy shows two minima in melt fragility as a function of composition which correlate well with the dimensionality of the glassy network. The structure evolves from 2D to 3D during crosslinking of selenium chains by arsenic but reduces into a 2D layer-like structure at the stoichiometric composition. Upon precipitation of arsenic-rich cages the network first reverts back to 3D and eventually becomes a mix of 2D and 0D structural units. The presence of molecular clusters in the network is evidenced by a strong bimodal dynamic response at high arsenic contents. NMR and Raman spectroscopy measurements of GeₓSe₁₋ₓ glasses suggest a structure composed of aggregated tetrahedral units and long selenium chains with little or no connectivity. Distinct dynamic responses of these two separated structural motifs are revealed by heat capacity spectroscopy. A non-Gaussian distribution of the imaginary heat capacity peak provided further evidence for the structural heterogeneity. This behavior is consistent with high temperature NMR measurements which show that the dynamic response of floppy selenium chains is distinct from that of rigid tetrahedral units. Finally, heat capacity spectroscopy applied to pure selenium provides strong evidence for the microscopic origin of the non-exponential structural relaxation, a universal feature of fragile glasses. The evolution of the imaginary heat capacity peak shape during annealing shows a non-monotonic trend which remarkably matches model predictions based on the enthalpy landscape. These results indicate that the non-exponential character of the relaxation process is linked to density fluctuations in the glass.

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