Phase transitions and structural parameters of HIQ-40 liquid crystalline co-polyester

We describe the impact of thermal treatment on the structure and phase transitions of the liquid crystalline aromatic co-polyester, HIQ-40, comprising 40 mol% p-hydroxybenzoic acid (H), 30 mol% isophthalic acid (I), and 30 mol% p-hydroquinone (Q). Simultaneous, real-time wide and small angle X-ray scattering (WAXS, SAXS), differential scanning calorimetry, and optical ellipsometry were used to study initially isotropic, amorphous films of HIQ-40. Films were annealed above the glass transition temperature, Tg, at temperatures, Ta, from 130 to 290 °C. Depending upon Ta, thermal treatment results in formation of regions of nematic order and/or crystalline order in a disordered matrix. As Ta increases, molecular mobility in the amorphous phase increases resulting in a reduction in Tg. Two or three endothermic events are seen in all samples by thermal analysis. The lowest temperature endotherm is associated with melting of crystals formed either at Ta or during the thermal scan. The two higher temperature endotherm features result from transformation of crystal melt-to-nematic, and formation of more mobile nematic domains from constrained liquid, respectively, and are relatively insensitive to Ta. ng Bragg scattering peak is seen for Ta<290 °C corresponding to formation of two-phase structure comprising crystals and disordered phase. At higher temperatures, very strong scattered intensity in the SAXS pattern re-emerges, even after all WAXS crystal reflections have disappeared. Results suggest that a two-phase structure, of ordered nematic domains co-existing with less ordered regions, may be forming continuously above the crystal melting point.