AbstractThe structural and thermal behavior of liquid‐crystalline polyesters at hydrostatic pressure was investigated by using a high‐pressure DTA 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 Å at the crystal (K)‐smectic H(SH) 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‐SH transition changes slightly from ca. 0,29 Å at 0,1 MPa to 0,22 Å at 300 MPa. It is concluded that the transition mechanisms of the K‐SH transition of PB‐8 and the K‐N transition of PB‐7 are maintained at pressures up to 300 MPa.