Novel Poly (N-substituted urethane): 1. Synthesis and Characterization of Liquid Crystalline Polyurethanes Pendant with Biphenyl Mesogenic Unit

摘要

Synthesis and characterization of new liquid crystalline poly(N-substituted ure-thane)s using 4-bromo-(4-hexyloxy-biphenyl-4′-oxy)butane (C6-C4Br) as mesogenic pendant group is described. The liquid crystalline polyurethanes were synthesized through N-substitution by a two-step process. First the polymer was dehy-drogenated by sodium hydride, and then the prepared urethane polyanion was treated with C6-C4Br. The chemical structures were characterized by FTIR, ~1H NMR, and elemental analysis. The solubility and thermal properties of N-substituted polyurethanes were investigated in view of the N-substitution degree of mesogenic unit. Thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) in combination with polarizing optical microscopy (POM) were used to investigate the thermal properties of N-substituted polyurethane. As the degree of N-substitution increased, the solubility of polyurethane decreased in common organic solvents, and thermal stability increased from 285℃ to 370℃ because of different thermal-degradation mechanisms of parent and N-substituted polyurethanes. The glass-transition temperature (T_g) was shifted to higher temperature with an increasing degree of substitution due to the increasing rigidity of polymer chains. Optical microscopy showed focal conic texture characteristic of the smectic A phase for both C6-C4Br and N-substitut-ed polyurethanes. DSC Experiments were also found in accordance with mesophase formation. The liquid crystalline properties were strongly dependent on the N-substitu-tion degree with increasing mesogen content from 25 to 100%, the mesophase-tran-sition and isotropization temperature increased. The minimum 25% amount of mesogen is necessary to exhibit a liquid crystal property in the polymer.