Orientational Behavior of Liquid-Crystalline Polymers with Amide Groups

Abstract

Liquid-crystalline polyacrylamides with mesogenic side-chains were synthesized by the radical polymerization of a mesogenic acrylamide derivative. Their thermal properties and orientational behavior were examined by polarizing microscopy, differential scanning calorimetry, temperature-variable IR, and X-ray diffraction measurements. The liquid-crystalline polyacrylamides containing secondary amide groups formed smectic A and smectic B phases during heating and cooling processes. The glass-smectic B, smectic B-A, and smectic A-isotropic phase transition temperatures increased with increasing molecular weight. The liquid-crystalline polyacrylamide showed higher phase transition temperatures than a liquid-crystalline polyacrylate, in which the secondary amide group was replaced with an ester group. The X-ray diffraction pattern of a smectic B-oriented sample of the liquid-crystalline polyacrylamide consisted of sharp inner and very sharp outer reflections. The very sharp reflection in the wide-angle region of the X-ray diffraction pattern indicated the formation of hexatic packing within the layer. The relationship between the layer distance and the extended mesogenic side-chain length suggested that the liquid-crystalline polyacrylamide formed an interdigitated bilayer structure. The IR spectra of the liquid-crystalline polyacrylamide exhibited that the number of hydrogen bonds between the secondary amide groups decreased with increasing temperature. In the liquid-crystalline polyacrylamide, smectic A and smectic B phases, with enhanced thermal stability, were formed through the formation of hydrogen bonds between the secondary amide groups.