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Title: Liquid crystal-polymer composites and the stabilisation of defect phases
Author: Kasch, Nicholas
ISNI:       0000 0004 5356 3907
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2015
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A simple method for increasing the stable temperature range of the liquid crystalline blue phase is demonstrated, by mixing a non-mesogenic polymer of low molecular weight into the blue phase material. In a mixture of cholesteryl benzoate and cholesteryl nonanoate the addition of polystyrene increased the stable blue phase range from 0.5K to 12K. This was measured strictly on heating from the chiral nematic phase through the blue phase in order to minimise non-equilibrium effects, and is one of the largest ranges so measured. The stability range can be closely tuned by changing the polymer concentration and molecular weight. The maximum range found by adding a particular compound seems only to depend on its saturation point in the liquid crystal, and the dependence of the range on concentration is non-linear. These features were explained by a numerical model of a blue phase unit cell incorporating the mean field Flory-Huggins and Maier-Saupe theories where the polymer could fill the high energy defect regions. Two of the oligomers which are shown to stabilise the blue phase are fluorescent, at 450nm and 500nm respectively, and it is proposed that tests on these mixtures could reveal photonic effects caused by the concentration of the fluorophores in the blue phase defect regions. The twist-grain boundary (TGB) phase is present in mixtures of cholesteryl oleyl carbonate and cholesteryl nonanoate over a range of up to 0.3K. The addition of polystyrene has no effect on the stability of the TGB phase. Conventional, in situ UV-initiated polymer stabilisation does not appear to stabilise the TGB phase, but is capable of stabilising over at least 30K the micron-size filaments which appear in the TGB phase when it is heated from the smectic phase in a cell with homeotropic alignment. Some notes are made on the causes and structure of this filament texture, and it is observed that the filaments tend to grow with a characteristic curvature. It is shown theoretically that the correct material could stabilise the TGB phase similarly to the polymers in the blue phase, by extending the previous model to include the Kobayashi-McMillan theory of smectic ordering. A second theoretical model of chirality around the transition to the smectic phase is then presented which takes account of fluctuations, based on an analogy with the state of a smectic-forming material infiltrated into an aerogel. A phase resembling the TGB phase emerges from this model. The model gives two first order transitions in accordance with experiments on the TGB phase, and reflects other experimental pitch and calorimetry measurements too. The electrochemical polymerisation of an acrylate monomer in the nematic and smectic-C* phases is investigated. 30-100V is applied across a cell containing the liquid crystal-monomer mixture, with no additional initiating compound. In both phases, the texture during polymerisation is frozen in by the polymer formed. In a nematic phase in a cell with initially planar alignment, the director in the field off state can be observed to tilt toward the homeotropic over a number of hours. In the ferroelectric case, as well as the textural freezing there is a somewhat reversible agglomeration of polymer strands into micron-scale structures. Scanning electron microscopy reveals a range of structures on both electrode surfaces, including in the nematic case corrugations with a periodicity of 500-750nm. There is no evidence of a polymer network spanning the thickness of the cell - rather the liquid crystal seems to be realigned by a polymer film at the electrode surfaces.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: liquid crystal ; blue phase ; twist grain boundary phase ; dislocation ; disclination ; polarising optical microscopy ; polymer ; oligomer ; electrochemical