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Title: Network formation in mixtures of nematic liquid crystal and colloids
Author: Cleaver, Julie
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2004
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Mixtures of thermotropic liquid crystal (5CB) and colloid (polymethylmethacrylate) particles have been studied. When these composites are cooled through the isotropic-nematic (IN) phase transition an optically switchable material is formed with an unusually high storage modulus. Previous studies have shown that the particles form into an interconnected network. In this thesis the mechanism of network formation, and the morphological and mechanical properties of the network are explored. Time-resolved laser scanning confocal microscopy (LSCM) is used to achieve near-single-particle resolution and observe the kinetics of the network formation upon cooling from the initial isotropic dispersion. As the mixture is cooled below the IN transition temperature (TIN), the particles are expelled by growing droplets of nematic liquid crystal to form the walls of a three dimensional network. This process takes the order of 30 seconds (dependent upon cooling rate), but the IN transition of the pure liquid crystal is much quicker. The presence of impurities adsorbed onto the particles before they are dispersed in liquid crystal could be responsible for this. These impurities open up a biphasic region in the phase diagram and slow down interface movement. Calorimetric data are consistent with this interpretation. Microscopy observations show that upon heating above TIN single particles become free and exhibit Brownian motion. As the sample is heated deep into the isotropic phase the network is broken up but clusters of particles remain. Sedimentation of these clusters causes a density gradient of particles to form across the sample with varying height and upon cooling a new network of particles or ‘clusters of particles’ is formed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available