Semiflexible liquid crystalline polyesters based on twin di(p-oxybenzoyl) units: synthesis and characterization

A new series of thermotropic liquid-crystalline polyesters C₅C_n has been prepared, comprising two di(p-oxybenzoyl) units connected by a pentamethylene segment and a linear alkylene spacer of variable length. All the polymers present a disordered smectic mesophase while a nematic mesophase is also shown by polymer samples with relatively short alkylene segments (n=5-8, 10). The nematic to isotropic transition temperatures fall with increasing n with a typical odd-even alternation. In complete contrast, a rising trend with a reversed odd-even alternation is observed for the smectic to nematic or isotropic transition temperatures. The polymer mesophases, once formed in the melt, can be frozen by cooling them to room temperature in a glassy state.