Novel poly (N-substituted urethane):: 1.: Synthesis and characterization of liquid crystalline polyurethanes pendant with biphenyl mesogenic unit

Synthesis and characterization of new liquid crystalline poly(N-substituted urethane)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 dehydrogenated by sodium hydride, and then the prepared urethane polyanion was treated with C6-C4Br. The chemical structures were characterized by FTIR, H-1 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 degrees C to 370 degrees C because of different thermal-degradation mechanisms of parent and N-substituted polyurethanes. The glass-transition temperature (T.) 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-substituted polyurethanes. DSC Experiments were also found in accordance with mesophase formation. The liquid crystalline properties were strongly dependent on the N-substitution degree with increasing mesogen content from 25 to 100%, the mesophase-transition and isotropization temperature increased. The minimum 25% amount of mesogen is necessary to exhibit a liquid crystal property in the polymer.