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Title: Surface effects and director profiles in liquid crystal-polymer structures with spatially varying surface conditions
Author: Gilchrist, Graham
ISNI:       0000 0001 3498 7518
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2007
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This thesis presents a study of the role of surface effects in the reorientation mechanism in a photorefractive-like liquid crystal-polymer hybrid structure. The photorefractive properties of liquid crystals were first discovered in 1994. Liquid crystals are very attractive as photorefractive materials due to their high nonlinear optical response caused by collective molecular reorientation, which can be induced by light and applied electric field. They are also low cost compared with photorefractive crystals. Liquid crystals are also excellent adaptive materials, ideal for writing dynamic re-writable gratings. . Photorefractive effects have been observed in both pure liquid crystals and polymers such as PVK, but also in certain hybrid structures of both liquid crystal and polymer layers. Recent experiments have shown that these effects are only apparent for certain combinations of liquid crystals and polymer surface layer, suggesting the cause to be surface-mediated at the interface between the two, and not due to bulk effects in the liquid crystal or the polymer. In this thesis I present characterisation and analysis of such hybrid structures and discuss a hypothesis for the underlying mechanism, supported by experimental and theoretical investigation. When investigating surface effects, I also developed a system for characterisation of the molecular alignment in samples. This has been shown to be valid for molecular angles 0-16.30 and 48.8-900 from the surfaces. I also present an analysis of 'pretilt' measurement methods in which I have found the Crystal Rotation Method (CRM) to be most suited to our needs. I used this experimental method and the characterisation program to inspect cells that had been 'damaged' and exhibited long-term memory effects. I found that for some polymer-liquid crystal combinations, such as LC1294PVK, the surface 'damage' is due to applying electric field and light illumination. A mathematical model for liquid crystal reorientation under a spatially varying field is presented. This model is based on the Landau De-Gennes theory and is used to simulate photorefractive gratings in liquid crystal cells with varying thickness and grating size. It has been found that the grating spacing to cell thickness ratio governs the penetration of electric field into the liquid crystal bulk. These simulations are useful for predicting the conditions for highest diffraction efficiency. Finally, the birefringence measurements I carried out were used to validate the theoretical simulations and to obtain simulation parameters for the experimental cells investigated
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available