Thermotropic side chain polyacrylates with azobenzene mesogenic groups: Synthesis and thermal behaviour

Abstract

Four side chain liquid crystalline (LC) polyacrylates containing substituted (methyl/long alkyl) azobenzene mesogens attached directly to the polymer backbone were synthesised. The polyacrylates were synthesised by free radical polymerization of the respective monomers, 4-[(4-acryloyloxyphenyl)azo]benzoic acid, 4-[(4-acryloyloxy-3-methylphenyl)azo]benzoic acid, 4-[(4-acryloyloxy-2-methylphenyl)azo]benzoic acid and 4-[(4-acryloyloxy-2-pentadecylphenyl)azo]benzoic acid. The monomers were obtained by acryloylation of the azo derivative prepared by the diazo coupling reaction between p-aminobenzoic acid and the respective phenolic derivatives with varying hydrocarbon substituents. All polymers were soluble in polar solvents. The inherent viscosities of the polymers in DMSO at 28°C were in the range of 0.55–0.62 dL/g. The monomers and their polymers were characterised by elemental analysis, IR, UV-vis, 1H and 13C NMR spectroscopies and WAXD. The thermal and phase transition behaviour of the polymers were investigated using DSC and a hot stage polarized light microscope (PLM). The polyacrylates exhibited stir opalescence and low viscosity characteristic of nematic phase on melting. However, the polymer with the long alkyl side chain (C15H31) was polymorphic with a smectic phase besides a nematic phase. The methyl substituted polymers showed only nematic phases. The effect of regioisomers on transition temperatures was significant. The carboxylic acid groups of the azophenyl mesogen are found to exist as hydrogen bonded dimers as indicated by IR, NMR and finally confirmed by WAXD. It is proposed that the long alkyl substituent might be involved in the stabilisation of the mesophase through its alignment parallel to the long axis of the molecule and might probably be supporting the formation of a smectic phase by reducing the transition temperature enough to maintain the layered arrangement of hydrogen bonded dimers.