Analytical and morphological studies of polymer-stabilised liquid crystals

This thesis examines a class of materials described as polymer-stabilised liquid crystals (PSLCs), in which a small amount of a monomer is cross-linked within a LC host to produce a network. The generation of such a network involves a variety of physical processes, leading to a composite material in which there is a high degree of interaction between network and host. PSLCs were prepared by photopolymerising 1 wt% of a diacrylate monomer within a nematic host. The progress of a polymerisation was monitored by separating the resultant network from the host, which contained unreacted monomer dissolved within it. The amount of unreacted monomer present for a particular set of reaction conditions was quantified by chemical analysis using high performance liquid chromatography, allowing the amount of monomer used to form a network to be deduced. The extent of reaction was found to be crucially dependent on the reaction conditions, with complex chemical kinetics involved. The effect of separating the network and host through dissolution of the host was investigated. The use of a non-polar solvent (heptane) produced relatively little disruption to the network, which was generally fibrillar in appearance. However, more polar solvents caused varying degrees of disruption, with increasing solvent polarity resulting in an increased tendency for aggregation of fibrils into lacy, strand-like structures. Network morphology was investigated using electron microscopy. Surface studies indicated that the fibrillar structures observed were made up of collections of phase-separated anisotropic droplets. For longer polymerisation times, these droplets became thinner and longer, suggesting that gradual expulsion of host from within the droplets takes place over the course of a reaction. Novel studies of the internal network structure via transmission electron microscopy suggested an open, porous arrangement. The results were used to produce a model of PSLCs, with particular reference to network growth and structure. (author)