Synthesis and characterisation of side chain liquid crystal copolymers containing sulfonic acid groups

A series of novel side chain liquid crystal copolymers, the poly[10-(4-methoxy-4′-oxy-azobenzene) decyl methacrylate]–co-poly[2-acrylamido-2-methyl-1-propanesulfonic acid]s (10-MeOAzB/AMPS-x/y) have been synthesised by free radical copolymerisation. The mol fraction of the sulfonic acid-based group, AMPS, has been varied over a broad range of composition in the copolymers, and these have been characterised using differential scanning calorimetry (DSC), polarised light microscopy (PLM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD). The liquid crystal homopolymer and copolymers containing up to 0.54 mol fraction AMPS-based units, all exhibit a smectic A phase, while at higher mol fraction AMPS-based groups, liquid crystallinity is extinguished and the copolymers are amorphous. The local packing arrangements within the smectic A phase varies with composition attributed, in part, to phase separation between acid rich domains and liquid crystal regions. The dependence of the transitional properties on copolymer composition may be interpreted in terms of the packing efficiency of the mesogenic side chains and hydrogen bonding between the sulfonic acid groups, as a consequence of increasing the concentration of acid-based groups.