Phase transitions in liquid crystalline poly(octamethylene p, p′-dibenzoate)

Abstract

A detailed study of the phase transitions in poly(octamethylene p, p′-dibenzoate) (P8MB) was carried out using differential scanning calorimetry. The thermodynamic parameters of the mesophase of P8MB have been determined by maintaining this phase at high temperatures to prevent its transformation into the crystal. If the crystal is formed, however, it exhibits a monotropic transition to the isotropic melt, and consequently the mesophase is not obtained on heating. This monotropic character is reflected in the free energy diagram of P8MB, as the crystal melting point is higher than the isotropization temperature ( T cr→i > T lc→i). From the free energy diagram it is also deduced that the crystal phase is thermodynamically the most stable one at any temperature below its melting point, although at low undercoolings the mesophase is always formed first for kinetic reasons. Moreover, the liquid crystalline (LC) glass is obtained by fast quenching from the isotropic melt, as revealed by X-ray analysis. This LC glass is stable at room temperature owing to a glass transition at 42°C, although it undergoes a relatively rapid ageing process at that temperature. Finally, the viscoelastic relaxations have been analysed for two samples of P8MB with different thermal histories.