A CALORIMETRIC STUDY OF POLYMER-DISPERSED LIQUID-CRYSTALS - CURE ENERGETICS AND KINETICS

Polymer-dispersed liquid crystal (PDLC) films are formed by the phase separation of liquid crystal (LC) microdroplets from a polymer matrix during cross-linking (cure) of the matrix. Thus, studies of the cure e nergetics and kinetics of PDLCs provide insights into the mechanisms o f their formation. We have carried out such studies for PDLCs cured in two different ways. We have measured the heat released during cure (D elta Q(cure)) and the time constant for the cure process (tau(cure)) a s a function of cure temperature (T-cure) for two thermally-cured and four UV-cured systems. The heat of cure for both types of systems exhi bits similar behavior: Delta Q(cure) goes through a broad maximum at a value of T-cure where the degree of matrix cure is greatest. On the o ther hand, the behavior of the cure time constant differs for the two systems: tau(cure) for thermally-cured PDLCs decreases monotonically w ith temperature, whereas that for UV-cured PDLCs exhibits a minimum. B oth quantities are of importance for optimization of PDLC electro-opti c properties. Delta Q(cure) is proportional to the degree of matrix cu re and is therefore a measure of the extent of phase separation of LC from the matrix; tau(cure) plays a role in determining liquid crystal microdroplet size.