Phase Behavior of Crystal Polymorphs of Thermotropic Poly(hexamethylene 4,4‘-biphenyldicarboxylate) under Hydrostatic Pressure

Abstract

Structural phase behavior of a thermotropic polyester, poly(hexamethylene 4,4‘-biphenyldicarboxylate), abbreviated as BB-6, was studied under hydrostatic pressures up to 200 MPa by wide-angle X-ray diffraction apparatus equipped with a high-pressure vessel. The Cα form of BB-6, cooled slowly from the melt under atmospheric pressure, is the most stable in the low-pressure region below 70 MPa, and this sample exhibits the reversible phase transition of Cα ↔ SA ↔ I. An application of high pressure above about 100 MPa induces another crystal polymorph (Cδ) at high temperature: the Cα → Cδ → SA → I transition process is observed on heating under high pressure. Since the Cδ ↔ SA ↔ I transition occurs reversibly at high pressure, the Cδ form can be regarded as the stable form in the high-pressure region. But this form becomes metastable in the low-pressure region because the Cδ → Cα → SA → I transition is observed at pressures below 50 MPa. The shrinkage of the fiber repeat of the Cδ form by about 2 Å would suggest the conformational change from all-trans to an introduction of gauche (g+ or g−) conformation in the aliphatic sequence. Since the X-ray fiber photograph of the Cδ form shows intense fiber reflections along the direction tilted by 25° from the equatorial line, the molecular chains in the unit cell would be arranged in a herringbone pattern. The phase stability between the crystal modifications of BB-6 was analyzed as s function of temperature and pressure.