Comb-branch liquid crystalline polymers: the synthesis and characterisation of both chiral and non-chiral structures

Abstract

Various comb-branch, thermotropic liquid crystalline copolymers having both chiral and non-chiral structures have been prepared and studied by differential scanning calorimetry and polarising microscopy. These copolymers were found to display either smectic, nematic or chiral nematic phases, or a combination of them.

The novel smectogenic monomer di[6-(4-methoxy-4'- oxybiphenyl)hexyl]-2-methylene butane-1,4-dioate (DMBI-6) and the nematogenic monomer 4-cyanophenyl-4-(6- acryloyloxyhexyloxy)-benzoate (CPBA-6) were prepared and characterised. DMBI-6 was copolymerised with both achiral and chiral non-mesogenic monomers; the resultant copolymers were observed to form both smectic A and B phases. CPBA-6 was copolymerised with a chiral, non-mesogenic monomer and phases which appeared to be nematic in nature were detected. These copolymers showed a tendency to form crosslinked gels at high conversions. One of these materials was partially characterised and its elastomeric-liquid crystalline behaviour studied.

Two novel cholesteric monomers, namely 1-[1-(cholesteryloxy)carbonyl]-10-decanyl]-2-methylene-4-mefhyl butane-1,4-dioate (ChMI-10) and di[l-[(cholesteryloxy)carbonyl]- 10-decanyl]-2-methylene butane-1,4-dioate (DChI-10) were synthesised. Copolymers of one of these, ChMI-10, with CPBA-6 was found to form the cholesteric phase predominantly. The attempted polymerisation of DChI-10 was studied.

Three novel, chiral mesogenic monomers bearing the chiral centre in the flexible spacer were synthesised. Two of these monomers, namely (S)-l-(4-methoxy-4’-oxybiphenyl)-2- methacryloyloxypropane (MBM*) and (S)-l-(4-cyano-4‘- oxybiphenyl)-2-methacryloyloxy propane (CBM*) were copolymerised with DMBI-6 and both smectic and chiral nematic phases were seen to result. Copolymers of CBM* and CPBA-6 yielded only chiral nematic phases. Copolymers prepared from CPBA-6 and (S)-l-(4-methoxy-4'-oxybiphenyl)-2-(2- methacryloyloxy-1-ethoxy)propane (XMBM*) formed either chiral nematic or crystalline phases.

An attempt has been made to rationalise the temperature and composition dependent optical properties of the chiral nematic phases formed in terms of the hindered rotation of the chiral mesogens around their long molecular axis. The results were found to compare favourably with previously reported results and theories.