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

Liquid crystalline (LC) polymers with cross-linked network structures containing azobenzene mesogens were synthesised from cardanol, the unsaturated Cls 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, H-1 and C-13 NMR and UV-visible spectroscopy. Polymer (LU) was neither soluble in any solvent nor fusible. Polymer (TV) 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 degrees C and rapidly quenched to room temperatures and observed under PLM. Otherwise, on heating polymer (III) started decomposing near 200 degrees 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. (C) 2000 Elsevier Science Ltd. All rights reserved.