Molecular Mechanistic Analysis of Liquid-Crystalline Polymers Composed of <i>p</i>-Hydroxybenzoic Acid I: Thermal Properties

All-atom molecular dynamics (MD) calculations of the crystalline polymeric <i>p</i>-hydroxybenzoic acid (<i>p</i>HBA) were conducted at various temperatures to investigate its thermal response. The calculated structure factor equivalent to the X-ray diffraction pattern of <i>p</i>HBA clearly showed two phase transitions occurring at 600 and 650 K. The first transition at 600 K occurred from the orthorhombic phase to the pseudohexagonal phase, identified by the presence of the 211-peak. This peak disappeared during the second transition at 650 K, indicating that the phase at 650 K was hexagonal. The structure of the pseudohexagonal phase was anisotropic with respect to the <i>ab</i> plane but isotropic in the hexagonal phase. Discontinuous changes in the calculated unit cell volume and unit cell length were observed at 600 K, associated with the first phase transition. We also calculated the linear expansion coefficients in three directions. An anisotropic expansion was observed in three directions for the orthorhombic crystal. In particular, the linear expansion coefficient in the <i>c</i>-direction was negative. In contrast to this, an isotropic expansion was found in the <i>a</i>- and <i>b</i>-directions for the hexagonal crystal, while the expansion in the <i>c</i>-direction is still negative. This study provides valuable insights into the thermal behavior of polymeric <i>p</i>HBA, which is essential for understanding its structural transformations and designing crystalline polymers with tailored thermal properties.