Thermotropic liquid-crystalline polymers having five-membered heterocyclesas mesogens V. Relationship between polymer structure and thermotropic liquid-crystallinity in semi-rigid polyesters composed of twin 2-phenyl-1,3,4-thiadiazoles having central aliphatic chains

Semi-rigid polyesters based on twin biphenyl analogues of 1,3,4-thiadiazole (2-phenyl-1,3,4-thiadiazoles) with hepta- and octamethylene chains in the central part were synthesized by melt polycondensation of bismethyl eater d erivatives of twin 2-phenyl-1,3,4-thiadiazole with alpha,omega -alkylenedio ls and dioxydialcohol derivatives of biphenyl. The relationship between pol ymer structure and thermotropic liquid-crystallinity (LC) was investigated by differential scanning calorimetry (DSC), polarizing microscope observati ons, miscibility tests and temperature-dependent X-ray analysis. All the po lymers derived from alpha,omega -alkylenediols appeared to form enantiotrop ic or monotropic nematic phase independent of the lengths of aliphatic chai ns. Among the polyesters containing the biphenyl unit, most polymers with o ctamethylene segment between the twin 2-phenyl-1,3,4-thiadiazoles showed th e nematic phase, but polymers composed of the twin biphenyl analogue having the central heptamethylene chain had no LC melts.