| Abstract: | Thermal and electron diffraction studies of single crystals derived from the tetramer of p-acetoxybenzoic acid have revealed the presence of two consecutive crystallographic transformations, i.e., topotactic transitions. Thus, differential scanning calorimetry of the oligomer indicated two irreversible phase transitions, an endotherm followed by an exotherm, between 150 and 180°C, in addition to the crystal-to-nematic transition at 260°C. Electron diffraction analysis further elucidated the transformation occurring during thermal treatment of the original oligomer. Hence it could be shown that the solution-crystallized material (α form) underwent a solid–solid phase transition to the β modification at a temperature corresponding to the observed endotherm. This β form then converted to a more stable modification (β′ form) which represented a superlattice structure derived from the β modification. |
