PHASE-TRANSITION OF LIQUID-CRYSTALLINE POLYESTERS WITH BIPHENYL MESOGEN UNDER HIGH-PRESSURES

The structural and thermal behavior of liquid-crystalline polyesters at hydrostatic pressure was investigated by using a high-pressure DT-A and a wide-angle X-ray scattering apparatus equipped with a high-pressure sample vessel. The thermotropic polyesters (two homopolymers and the corresponding copolymer) are based upon 4,4'-biphenyldiol as the mesogen and aliphatic dibasic acids containing 7 and 8 methylene groups as flexible spacer, which are labeled as PB-7, PB-8, and PB-7,8 (30/70 mol-%), respectively. The phase diagrams of the PB-7, PB-8 and PB-7,8 samples were constructed for pressures up to 300 MPa. At room temperature both the PB-7 and the PB-8 crystals are compressed hydrostatically in the molecular chain and lateral packing directions. The d spacing of the low-angle reflection of the PB-8 crystal, corresponding to the layer spacing, decreases by about 2,6 +/- 0,2 angstrom at the crystal (K)-smectic H(S(H)) transition under all pressures, while the crystal reflections of PB-7 disappear at the K-nematic (N) transition. The d spacings of the wide-angle reflections of both crystals, corresponding to the lateral packing in the crystals, expand gradually with increasing temperature up to the K-mesophase transition. The increase in lateral spacing of the PB-8 crystal at the K-S(H) transition changes slightly from ca. 0,29 angstrom at 0,1 MPa to 0,22 angstrom at 300 MPa. It is concluded that the transition mechanisms of the K-S(H) transition of PB-8 and the K-N transition of PB-7 are maintained at pressures up to 300 MPa.