Network formation in mixtures of nematic liquid crystal and colloid particles

Cooling mixtures of thermotropic liquid crystal (5CB) and colloid (polymethylmethacrylate) particles through the isotropic-nematic phase transition forms an optically switchable material with an unusually high storage modulus. Previous work has shown that the particles form into an interconnected network. I used time-resolved laser scanning confocal microscopy (LSCM) to achieve near-single-particle resolution and observed the kinetics of the network formation. The particles were expelled by the growing droplets of nematic liquid crystal to form the walls of a three dimensional network. This process took on the order of 30 seconds (dependent upon cooling rate), but the I-N transition of the pure liquid crystal was much quicker. Could this be due to the presence of impurities adsorbed on the particles before they were dispersed in liquid crystal? These impurities open up a biphasic region in the phase diagram and slow down interface movement. Calorimetric data are consistent with this interpretation.