Characteristic smectic structures of main-chain liquid-crystalline polyimides driven by a microphase separation between aromatic imide mesogen and a siloxane spacer

Abstract

We investigated the phase transition behavior and liquid-crystalline structure for main-chain liquid-crystalline polyimides with aromatic imide cores and alkyl and dimethyl siloxane spacers, BPDA/3SiO4-3F. They formed crystal, smectic C (S_C), smectic A (S_A), and isotropic phases in order of increasing temperature. The characteristic structures of the S_A and S_C phases were extracted with wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy measurements. In the S_C phase, the mesogenic groups were tilted by approximately 40° to the smectic layer and the unusual quasi two-dimensional structural order exists irrespective of the liquid-like packing of aromatic cores within a layer. On heating, the S_C phase transformed into the S_A phase with a nature of first-order transition. The resulting S_A structure was also unusual. The orientational order parameter was 0.43, which was lower than 0.71 in the S_C phase. This indicates that a “de Vries”-type of smectic A is formed in which the mesogenic groups are tilted away from the optic axis, but in azimuthally random directions. The average tilt angle was estimated to be 42°. These characteristics were produced because of the strong segregation effect of the dimethyl siloxane spacer groups with a relatively larger molecular area compared to alkyl chain and aromatic imide cores.