Analysis of hairy-rod polyimides for photoalignment

Droge, Stefan

Physical sciences
January 2008

Thesis or dissertation


Rights
© 2008 Stefan Droge. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Abstract

The evaluation of novel polyimides for nematic liquid crystal alignment by UV irradiation was the aim of this thesis. Homeotropic liquid crystal (LC) alignment was observed for the majority of the in-house synthesised hairy-rod polyimides (PIs). Oblique unpolarised UV irradiation of branched PIs results in pretilted LCs of 2 deg to 88 deg depending on chemical composition, processing history and fluence. The initial pretilted alignment collapsed to homogeneous alignment at higher fluences, which was dependent on the main chain, side chain combination. Minimal changes in the UV/VIS and FTIR absorption spectra were found for fluences at which pretilted LC alignment occurred. For higher fluences photodegradation could be observed in the FTIR spectra. AFM measurements revealed an increased surface roughness after UV irradiation, but no surface anisotropy could be observed. X-ray reflectivity measurements showed film shrinkage after in-situ UV irradiation with identical trends as was observed for the collapse of the LC pretilt. Modeling revealed an increased electron density at the surface and a less diffuse interface following UV exposure. Bragg features in the reflectivity curve suggested a periodical internal film morphology, which was confirmed by grazing incidence X-ray diffraction (GIXRD). Models of chain orientations are proposed for three PIs and the UV induced morphology change analysed. GIXRD probes the surface or bulk of the thin film and an anisotropic increase in order was found for the in-plane surface scan at low fluences, followed by a general order decrease for higher fluences. The LC tilts in the direction of UV irradiation and not toward higher order. An entropy-related photodegradation model is suggested whereby the LC tilts toward maximum surface disorder to minimise free energy. Polarised UV irradiation of main chain only PIs results in homogeneous LC alignment. This was used to photoalign a LC semiconductor material, resulting in a polarised UV absorption ratio of 11 to 1. 

Publisher
Department of Physics, The University of Hull
Supervisor
O'Neill, Mary
Sponsor (Organisation)
EPSRC; The University of Hull
Qualification level
Doctoral
Qualification name
PhD
Language
English
Identifier
hull:1604
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