A Calorimetric Study of Phase Separation in Liquid Crystal/Matrix Systems: Determination of the Excess Specific Heat of Mixing

Abstract

The effects of concentration and state of cure on mixing and phase separation of binary systems of a low molecular weight liquid crystal (LC) and an organic matrix have been determined for the first time. We found that, as temperature is increased, mixing in uncured LC/matrix systems is accompanied by a step-like decrease in specific heat at T _mix, the mixing temperature. This decrease is due, for the most part, to the (negative) excess specific heat of mixing. ΔC _mix. The excess enthalpy of mixing. ΔH _mix, may also contribute to the thermal effect. A plot of ΔC _mix versus LC concentration exhibits a minimum, as expected from theory. Such a plot for a mixture of a specific LC (E7) with uncured matrix (NOA65) allows us to estimate the solubility limit of E7 in the matrix-rich phase (∼15%) and of NOA65 in the LC-rich phase (∼5%).

Uncured E7/NOA65 mixtures exhibit phase diagrams with an upper critical solution temperature (UCST) slightly above 300 K. To avoid undesired phase separation prior to final cure, the formation temperature for an E7/NOA65 polymer-dispersed liquid crystal (PDLC) should thus be suitably higher than 300 K. The phase diagram of uncured LC/matrix mixtures can be computed using Flory-Huggins theory. The F-H model also predicts the correct sign for ΔC _mix, but is less successful in calculating its magnitude.

The extent (or degree) of matrix cure plays the major role in phase separation of LC/matrix mixtures to form a PDLC. Partially-cured samples exhibit mixing transitions which do not disappear until the final stages of the cure process. Only during the last 10%, or so, of cure does phase separation of the LC from the matrix become fixed. Once this has occurred, a fraction of the LC is permanently phase-separated, with the rest dissolved in the matrix. The appearance of a nematic-isotropic (NI) transition during the final cure stages confirms that phase separation has become more or less “frozen in”; the magnitude of the NI transition enthalpy makes it possible to estimate the amount of LC dissolved in the matrix. The temperature of the NI transition for incomplete cure is nearly coincident with T _mix, perhaps due to greater solubility of the polymer in the isotropic phase than in the nematic.

Key Words:

• phase separation
• mixing
• binary systems
• liquid crystals
• polymers
• solubility