Layer structures .3. Poly(p-phenylene-terephthalate)s with one, two, or four alkyl substituents: Thermotropic and isotropic rigid rods

Nine polyesters were synthesized by polycondensation of silylated hydroquinone with substituted terephthaloyl chlorides. Three mono(alkylthio), three bis(alkylthio), and three tetra(alkylthio)terephthaloyl chlorides were used as reaction partners. The length of the alkyl side chains was varied among 8, 12, and 16 carbons. WAXS powder patterns indicate that these polyesters form three different types of sanidic layer structures in the solid state depending on the number of side chains per repeating unit. All three layer structures have an interdigitating of the alkyl side chains in common. As evidenced by C-13 NMR CP/MAS spectroscopy and DSC measurements, the longer side chains form crystalline paraffin domains between the stacks of the main chains. The monosubstituted polyesters form two different LC phases, a biaxial nematic phase with layer structure and at higher temperatures a normal nematic phase. The disubstituted polyesters exclusively form a biaxial nematic melt. Increasing length and number of the side chains reduce the isotropization temperature of all polyesters, and the tetrasubstituted polyesters form an isotropic melt directly upon melting of the crystallized side chains. This result demonstrates that rigid rod polymers are not necessarily mesogenic. The failure to form a LC phase and the good solubility in many common solvents is attributed to poor attractive electronic interactions between the main chains. A more detailed interpretation was achieved by computer modeling.