Effect of flexible spacer length on the phase transitions and mesophase structures of main-chain thermotropic liquid-crystalline polyesters having bulky pendent side groups

The effect of flexible spacer length on the phase transitions and mesophase structures of main-chain thermotropic liquid-crystalline polyesters having bulky pendent side groups was investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarizing optical microscopy (POM). For the investigation, a homologous series of poly[(phenylsulfonyl)-p-phenylene alkylenebis(4-oxybenzoate)]s (PSHQn) having varying lengths (n) of methylene groups of flexible spacers (n = 3-12) were synthesized in our laboratory. Using POM we observed that all PSHQn synthesized, with the exception of PSHQ3, have only a schlieren-textured nematic mesophase at temperatures between a glass transition temperature (or melting point) and the isotropization temperature; the nematic-isotropic transition temperature exhibits odd-even fluctuation with the number of methylene groups of the flexible spacer. We found that the isotropization enthalpy and isotropization entropy for PSHQn do not follow a linear relationship with the number of methylene groups when the flexible spacer length becomes shorter than 7, which deviates from the findings for linear main-chain TLCPs reported in the literature. We ascribe this difference to the presence of bulky pendent arylsulfonyl side groups in the main-chain of PSHQn. We found from both DSC and WAXD that (i) PSHQn having odd numbers (5-11) of methylene groups exhibit a glassy nematic mesophase, while PSHQ3, having three methylene groups, has no liquid crystallinity, becoming an amorphous polymer with a glass transition temperature of 129 degrees C, and (ii) among the PSHQn having even numbers (4-12) of methylene groups, PSHQ4 showed the highest degree of ordering in the crystalline structure. WAXD of PSHQn melt-drawn fibers showed an odd-even fluctuation of chemical repeat unit d-spacing, very similar to that reported in the literature for linear main-chain TLCPs.