Liquid-Crystalline Complexes of Polyurethane Containing an Isonicotinamide Moiety with 4-Dodecyloxybenzoic Acid

Abstract

A series of supramolecular side-chain liquid crystalline polyurethanes (SCLCPUs) was prepared by utilizing the selective hydrogen bonding interaction between polyurethane containing pyridine moieties (1) and 4-dodecyloxybenzoic acid (2). Hydrogen bonding was confirmed by FTIR spectroscopy and supported by medium high level ab-initio MO calculations. The components are miscible up to 0.7 mole of 2 versus the polyurethane repeat unit and the resulting complexes behave as uniform liquid-crystalline polymers, which exhibit stable, reversible mesophases. The liquid crystalline behavior of these supramolecular polymeric complexes was established by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffractometry (XRD). The complexes show a highly ordered smectic phase, which does not appear for either of components 1 and 2 separately, and a nematic phase. Above the concentration limit of 0.7 mole of the acid 2, the excess low molar mass molecules emerge to form a two phase system consisting of a polyurethane supramolecular complex and 4-dodecyloxybenzoic acid (2). For complexes derived from polyurethane 1 and 4-octyloxybenzoic acid (3) or 4-butyloxybenzoic acid (4) with alkyl tails shorter than 2, the phase separation occurs at lower molar ratios of 3 and 4 versus 1. This means that the length of the alkyl tail in 4-(alkoxy)benzoic acids plays an important role in the stabilization of supramolecular polyurethane complexes.