Synthesis and properties of new liquid crystalline polyurethanes containing mesogenic side chain

Abstract

The segmented and unsegmented polyurethanes (PUs) containing hexamethylene diisocyanate (HDI), poly (tetramethylene oxide) (PTMO), and 2,2-bis (hydroxymethyl)propionic acid (DMPA) as chain extender at three molar ratios were synthesized through conventional technique. Furthermore, three, new side chain polyurethanes (PUM6s) containing 4-ω-hydroxyhexyloxycarbonyl-4-hexyloxyphenylbenzoate (M6) mesogenic pendant group on the DMPA chain extender were synthesized through esterification reaction of PUs. The M6 mesogen was successfully synthesized via multi step routes from 4-hexyloxybenzoic acid and 6-hydroxyhexyl-4-hydroxybenzoate. Characterization of all synthesized compounds was carried out by FT-IR and 1H NMR. The content of carboxylic groups of polyurethanes and the degree of M6 attachment on the chain extender were determined by titration and 1H NMR methods. The properties of polyurethane were also examined by solubility tests and inherent viscosity measurements using Ubbelhode viscosimeter. Thermal behavior of the M6 and PUM6s was investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The POM observation of M6 showed a focal conic texture formation of the SA phase on heating and cooling processes. The effect of the soft segment and M6 content on the liquid crystallinity of polymers was studied. The mesomorphic properties of PUM6 polyurethanes made up of both nonmesogenic pendant carboxyl group in the chain extender and mesogenic side chain were dependant polymer composition. Optical microscopy showed textures characteristic of the smectic A phase for PUM6-0.5 and PUM6-1 polyurethanes. The rigidity of the backbone along with the presence of the mesogenic side chains results in high Tg value for PUM6-1. Thermogravimetric analysis (TGA) displayed better thermal stability for the unsegmented polyurethane with high content of mesogenic side chain.