Effect of the nature of the liquid crystal on the phase separation kinetic behaviour in polymer/liquid crystal mixtures

 Polymer Dispersed Liquid Crystals (PDLC) materials are generally prepared by a phase separation process induced by ultraviolet (UV) polymerization from a homogeneous mixture of pre-polymer and liquid crystal (LC). Phase separation kinetic was investigated by studying the evolution of the transmission over time of a film composed of a polymer/liquid crystal mixture. Two systems were considered: polyacrylate/E7 and polyacrylate/5CB. These mixtures consist of 30% by weight of Tripropylene glycoldiacrylate (TPGDA) and 70% by weight of LC, in the presence of 1% photoinitiator. Kinetics was studied by measuring, in the absence of external fields, the intensity of the light transmitted by the film before, during, and after irradiation of the films by UV light for different doses. Under UV light, the initial mixture (monomers/LC) will undergo polymerization-induced phase separation. One observes a rapid relaxation of the transmission at the beginning of the irradiation followed by an increase in the transmission for longer irradiation times. This surprising effect depends on the nature of LC as well as the distance between UV source and the sample.