Studies on Thermotropic Main Chain Liquid Crystalline Segmented Polyurethanes. II. Synthesis and Properties of H 12 MDI- and MDI-Based Polyurethanes from High Aspect Ratio Mesogenic Diol as Chain Extender

Abstract

Previous studies on thermotropic main chain liquid crystalline polyurethanes (TLCPUs) prepared from 4-{[4-(6-hydroxyhexyloxy)-phenylimino]-methyl}-benzoic acid 4-{[4-(6-hydroxyhexyloxy)-phenylimino-methyl}-phenyl ester (mesogenic diol) as chain extender were continued. In this article, the main chain liquid crystalline polyurethanes are based on the mesogenic diol as chain extender, poly(tetramethylene oxide)glycol (PTMG) soft segment of different molecular weight PTMGs (M n =650, 1000, 2000), different diisocyanates including 4,4′-methylene bis(cyclohexyl isocyanate) (H 12 MDI) and 4,4′-methylene bis(phenyl isocyanate) (MDI). The polyurethanes were investigated by FTIR spectroscopy for structural confirmation and DSC and hot stage optical polarizing microscope to study the mesomorphic properties. The molecular weight distribution was determined by GPC. The effect of partial replacement of the mesogenic diol by 20-50 mol% PTMG and the effect of the diisocyanates on the liquid crystalline properties were studied. It was observed that the partial replacement of the mesogenic diol by PTMG of varying molecular weights and also the diisocyanates influenced the phase transitions and the occurrence of mesophase textures. Characteristic liquid-crystalline textures were observed when sufficient content of the mesogenic diol was present. If the flexible spacer was longer, a higher content of mesogenic agent was needed to obtain LC properties. Depending on the flexible spacer length and the mesogenic content, grained and threadlike textures were obtained for the H 12 MDI series polymers while the PUs prepared from MDI showed only grained texture for all the compositions. Elastomeric behavior was observed in the polymers from PTMG 2000 and H 12 MDI with mesogen content 20-49 mol%.