A Calorimetric Study of Liquid Crystal/Polymer Mixtures: Cure Temperature Effects and Unusual Phase Behavior

Abstract

Cure kinetics and energetics and phase behavior of a liquid crystal (LC)/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 microvolumes). The effects of cure temperature. T _cure, on the cure kinetics and energetics were similar to those found for previously studied UV-cured systems. Plots of the heats of cure for the pure matrix and LC/matrix mixtures go through maxima at temperatures where the extent of matrix cure is greatest. The fact that the nematic-isotropic (NI) transition enthalpy is maximum in the same temperature range confirms previous conclusions that phase separation is greatest when the matrix is most fully cured, The time constants for the cure process exhibit minima at temperatures slightly above ambient (as observed previously for other systems). Phase behavior of mixtures was studied as a function of degree of cure, D _cure. Differential scanning calorimeter scans of two uncured or partially cured samples exhibited an abrupt decrease near 390 K. This transition may be a decrease in specific heat due 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-existed with a NI transition for several values of D _cure, the possibility cannot be ruled out that the transition is in fact due to a NI phase change (perhaps in LC which is confined to micro-volumes other than droplets).

Key Words:

• phase separation
• phase behavior
• mixing
• binary systems
• liquid crystals
• polymers
• polymer-dispersed liquid crystals