STRUCTURE OF POLYMER NETWORKS IN LIQUID-CRYSTALS STUDIED BY C-13 NMR

We report on C-13 NMR measurements above and below the clearing temper ature of the liquid crystal 4-n-hexyloxyphenyl 4-methoxybenzoate const rained to an oriented, low concentration polymer network. The network is obtained by UV-irradiation of the reactive monomer (6-acryloyloxyhe xyloxy)benzoyloxy]-2-methylbenzene which is in admixture with the liqu id crystal. The characterization of orientational order of the mixture s before UV-radiation, and hence before polymerization, reveals the hi gh order of the components at the polymerization temperature. The chos en geometry explains the LC director orientation only by the aligned n etwork. Above the nematic-isotropic transition a strong pretransitiona l order is detected. Fast molecular translational diffusion averages t he order over dimensions smaller than 1 mu m. The Landau-de Gennes the ory predicts a relation between pretransitional order and the lateral dimension of the LC regions. The experimental data are successfully ex plained by pore diameters of 35 and 98 nm for concentrations of 20 and 8 mol% of monomer, respectively. The results support the model of nea rly cylindrical shaped liquid crystal domains surrounded by thin walls of crosslinked network.