Crystal structure and transitions in nascent, lamellar polyoxybenzoate single crystals

The crystal-liquid crystal and liquid crystal-liquid crystal transitions (T c-lc and T c-lc) of poly(p-oxybenzoate), nominally at 350° and 430°C, have been followed by electron diffraction (ED) using thin film polymerized single crystals. The ED patterns indicate the transition occurs in both phase I and II individual crystals over a broad temperature range, with high-order hk0 reflections being observable in the ED patterns of some samples. The molecular packing for phase II at room temperature, phase I and II high-temperature forms, and the relationship between the high- and low-temperature forms are proposed. Cooling from above the T c-lc transition temperature results in an apparent memory of the original structure. It is proposed that the high-temperature lattices, between 350° and 430°, should be described as molecular columnar or condis crystals. Above 430°C the hk0 ED patterns are consistent with a nematic-type packing of the molecules.