Cyclic polymer liquid crystal structures.

Recent advances in liquid crystal research have included the synthesis of polymeric materials which contain liquid crystalline moieties. The work presented here concerns the study of the structure-property relationships of a particular group of liquid crystalline polymers in which the polymer backbone is cyclic, with the mesogenic moieties attached as side-chains. We have observed mesogenic phases above room temperature for materials comprising cyclic poly(dimethylsiloxane) backbones with mesogenic moieties attached by alkyl spacer units. Dielectric relaxations have been observed in the mesophases of these materials and the activation energies and extent of broadening of the relaxations have been related to the physicalstructure of the molecules. Theoretical studies have been undertaken by the use of the Metropolis Monte Carlo technique and a mean field calculation. Two models have been studied by the Monte Carlo technique in the NVT ensemble. In the first, each complete molecule was represented by a disc-like interactions potential and a tendency for the molecules to align in columns was revealed at low temperature and high density. In the second model, each mesogenic unit was represented separately, with the cyclic polymerrepresented as a constraint on the relative motions of the attached mesogens. A variety of liquid crystalline phases, from discotic nematic to calamitic nematic, were observed at low temperature as the coupling between the side-chains and the backbones was adjusted. In the mean field model energy terms were included for ring-ring interactions, mesogen-mesogen interactions and the coupling between the mesogenic moieties and the backbones. The uniaxial solution of this model also showed a shift from calamitic nematic to discoticnematic phases as the strength of the coupling was increased. Comparisons of the results of the models and the physical measurements are presented and suggestions for future work are proposed.