Polyimides Containing Benzoxazole Units and Their Liquid-Crystalline Behavior

Herein, we report the liquid-crystalline behavior of polyimides (PIs) containing benzoxazole (BO) units and their model compounds, and provide fundamental information in relation to their potential as heat-releasing materials. A series of BO-containing tetracarboxylic dianhydrides were synthesized using two bis(o-aminophenol)s as starting materials, namely, 3,3-dihydroxy-4,4-diaminobiphenyl (p-HAB) and 4,6-diaminoresorcinol (DAR). When a p-HAB-type BO-containing tetracarboxylic dianhydride with ester linkages was combined with a siloxanediamine (amine equivalent: 430 g/mol), the resulting PI barely exhibited optical anisotropy with an unclear and undeveloped texture with the assistance of continuous shear stimulus. On the other hand, the combination of DAR-type BO-containing tetracarboxylic dianhydride with ester linkages and the siloxanediamine led to the formation of an optically anisotropic texture with particle-like domains in the heating process over a wide temperature range, although continuous shear stimulus in the molten state was still required for this system. The thermal diffusivity of BO-containing PI films quenched from their liquid-crystalline states is also reported.