A CALORIMETRIC STUDY OF LIQUID-CRYSTAL POLYMER MIXTURES - CURE TEMPERATURE EFFECTS AND UNUSUAL PHASE-BEHAVIOR

Cure kinetics and energetics and phase behavior of a liquid crystal (L C)/matrix mixture have been investigated. This research complements a previous study of phase separation in mixtures of low molecular weight liquid crystals with an organic matrix. In the present case we used a LC with a molecular weight about twice that of the previous ones. For some samples we found evidence for phase separation and an anomalous phase transition (possibly a smectic induced by confinement of the LC in micro-volumes). The effects of cure temperature, T-cure, on the cur e kinetics and energetics were similar to those found for previously s tudied UV-cured systems. Plots of the heats of cure for the pure matri x and LC/matrix mixtures go through maxima at temperatures where the e xtent of matrix cure is greatest. The fact that the nematic-isotropic (NI) transition enthalpy is maximum in the same temperature range conf irms previous conclusions that phase separation is greatest when the m atrix is most fully cured. The time constants for the cure process exh ibit minima at temperatures slightly above ambient (as observed previo usly for other systems). Phase behavior of mixtures was studied as a f unction of degree of cure, D-cure. Differential scanning calorimeter s cans of two uncured or partially cured samples exhibited an abrupt dec rease near 390 K. This transition may be a decrease in specific heat d ue to mixing. However, the step was an order of magnitude smaller than that for mixtures based on lighter LCs. In addition, its temperature remained fixed, rather than increasing with degree of cure, as was the case for previous systems. Although in one system this step co-existe d with a NI transition for several values of D-cure, the possibility c annot be ruled out that the transition is in fact due to a NI phase ch ange (perhaps in LC which is confined to micro-volumes other than drop lets).