The temperature and frequency dependence of their dielectric constants have been investigated for two dimers consisting of swallow-tailed terminal groups connected via siloxane central parts of di erent length. The results have been compared with those of the respective monomer. The central part contributes in di erent ways to the static dielectric constants in the isotropic phase. From relaxation measurements, it can be concluded that the reorientation process about the short axis of the terminal groups is comparable with the respective process in monomers. The relaxation frequencies as well as intensities are almost unin¯uenced by the siloxane part which suggests a very¯exible structure of such dimers. Unfortunately, the maintenance of the antiparallel arrangement of swallow-tailed parts of the dimers could not be proved because of insu cient material for experimental work on the SA phase.
We report on 13 C NMR measurements above and below the clearing temperature of the liquid crystal 4-n-hexyloxyphenyl 4-methoxybenzoate constrained to an oriented, low concentration polymer network. The network is obtained by UV-irradiation of the reactive monomer 1,4-di-[ 4-( 6-acryloyloxyhexyloxy) benzoyloxy]-2-methylbenzene which is in admixture with the liquid crystal. The characterization of orientational order of the mixtures before UV-radiation, and hence before polymerization, reveals the high order of the components at the polymerization temperature. The chosen geometry explains the LC director orientation only by the aligned network. Above the nematic± isotropic transition a strong pretransitional order is detected. Fast molecular translational di usion averages the order over dimensions smaller than 1 mm. The Landau± de Gennes theory predicts a relation between pretransitional order and the lateral dimension of the LC regions. The experimental data are successfully explained by pore diameters of 35 and 98 nm for concentrations of 20 and 8 mol% of monomer, respectively. The results support the model of nearly cylindrical shaped liquid crystal domains surrounded by thin walls of crosslinked network.
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