Synthesis of novel liquid crystalline polymers with cross-linked network structures

Abstract

Liquid crystalline (LC) polymers with cross-linked network structures containing azobenzene mesogens were synthesised from cardanol, the unsaturated C 15 hydrocarbon side chain of which is utilised for cross-linking reactions. The azobenzene group was introduced by the diazo coupling reaction between cardanol and 4-aminobenzoic acid. The resulting monomer, 4-[(4-cardanyl)azo]benzoic acid ( I) was polymerised by self-polycondensation using thionyl chloride and pyridine to get poly[4-[(4-cardanyl)azo]benzoic acid] ( III). ( I) was also converted to poly[4-[(4-acryloyloxycardanyl)azo]benzoic acid]( IV) through acryloylation of ( I) followed by free radical polymerisation. Cationic polymerisation of ( I) gave poly {4-[(4-cardanyl)azo]benzoic acid} ( V). The monomers and polymers were characterised using elemental analysis, IR, 1H and 13C NMR and UV–visible spectroscopy. Polymer ( III) was neither soluble in any solvent nor fusible. Polymer ( IV) was also insoluble, but swelling was observed with many solvents and polymer ( V) was soluble in polar solvents. On slow heating, polymers IV and V also got converted to totally insoluble products. IR spectra indicated involvement of side chain unsaturation in bond formation leading to cross-linking. Threaded nematic textures were obtained when polymer ( III) was heated to below 200°C and rapidly quenched to room temperatures and observed under PLM. Otherwise, on heating polymer ( III) started decomposing near 200°C as observed by an exotherm in DSC and confirmed by TGA. Polymers ( IV) and ( V) exhibited schlieren nematic textures under PLM. Polymer ( V) gave cross-linked transparent film when cast from a solvent and cured by slow heating.