Synthesis and Characterization of a Combined Main-Chain/Side-Chain Liquid-Crystalline Polymer Exhibiting Both Thermotropic and Lyotropic Characteristics and Its Lyotropic Phase Behavior

A combined main-chain/side-chain liquid-crystalline polymer (PSHQ4−7CNCOOH), which is composed of poly[(phenylsulfonyl-p-phenylenene 1,4-tetramethylene bis(oxybenzoate)] (PSHQ4) as the main-chain backbone and 8-[(4-cyano-4‘-phenyl)oxy]octanoic acid (7CNCOOH) as the side-chain mesogenic groups, has been synthesized. It has been found that 7CNCOOH has a clearing temperature of 150 °C and exhibits nematic mesophase, and PSHQ4−7CNCOOH exhibits both thermotropic and lyotropic characteristics. The thermotropic characteristics of PSHQ4−7CNCOOH have been investigated using differential scanning calorimetry (DSC), fiber and powder patterns of wide-angle X-ray diffraction (WAXD), and polarized optical microscopy (POM). It has been found that in the thermotropic state PSHQ4−7CNCOOH has a glass transition temperature (T_g) of 85 °C and a nematic-to-isotropic (N−I) transition temperature (T_NI) of 172 °C, and it is a glassy polymer. Interestingly, PSHQ4−7CNCOOH is found to exhibit lyotropic characteristics, undergoing isotropic-to-nematic biphasic separation over a wide range of concentrations in m-dibromobenzene, o-dichlorobenzene, and tetrachloroethane, enabling us to construct a phase diagram. The thermal transitions and phase structures of the main-chain backbone of PSHQ4−7CNCOOH, PSHQ4, were compared with those of PSHQ4−7CNCOOH. It has been found that PSHQ4 has a T_g of ca. 130 °C and a T_NI of ca. 260 °C, and it undergoes, during isothermal annealing, crystallization at about 240 °C as determined from DSC. Both the fiber and powder patterns of WAXD indicate that PSHQ4 is a semicrystalline polymer, which is consistent with the findings from DSC. It has been found that, in contrast to PSHQ4−7CNCOOH, PSHQ4 does not exhibit lyotropic characteristics. Both the very large decrease (about 90 °C) in T_NI after the attachment of side-chain mesogenic groups (7CNCOOH) onto the main-chain backbone (PSHQ4) and the lyotropic characteristics of PSHQ4−7CNCOOH are attributable to the significant increase in the bulkiness of PSHQ4−7CNCOOH, which apparently cannot pack well, giving rise to a glassy state.