A surface anisotropy has been shown previously to be induced in thin films of photoreactive coumarin side-chain polymers by polarized UV illumination. Consequently, the resultant cross-linked polymer layers can be used as photoalignment layers for liquid crystal displays. Homogeneous alignment of a nematic liquid crystal in contact with a layer of a model coumarin side chain polymer is obtained with the director parallel or perpendicular to the UV polarization axis depending on the incident fluence. Spectroscopic analysis of the alignment layer now confirms that both photodegradation and cross-linking occur with different dependencies on fluence. Low UV fluences give parallel photoalignment and high cross-linking reactivity. However, the residual, unreacted polymer side chains show negligible anisotropy because of their freedom to move in an isotropic fashion. Hence, parallel liquid crystal alignment is attributed to a steric interaction between the liquid crystal and syndimerized side chains of the cross-linked polymer. A switch of the photoalignment direction accompanies the subsequent development of anisotropy of the intact, unreacted polymer side chains. The side-chain anisotropy and hence perpendicular liquid crystal alignment is ascribed to photodegradation rather than cross-linking.
Lateral substitution in the peripheral phenyl groups of the triphenylen-2,3,6,7,10,11 -hexayl hexakis(4-alkoxybenzoate)~ was investigated via substitution of methyl, ethyl, isopropyl and tert-butyl groups ortho to either the peripheral alkoxy chain or the internal ester linking group. The discotic mesogenic compounds were prepared by the esterification of 2,3,6,7,10,1l-hexahydroxytriphenylene (HHTP) with the respective 3-alkyl-or 2-alkyl-4-decyloxybenzoyl chlorides (alkyl= methyl, ethyl, isopropyl or tert-butyl) in the presence of pyridine. Thermal polarized light microscopy and differential scanning calorimetry were used to investigate the liquid-crystalline behaviour of the materials prepared. Suppression of columnar mesophases was observed for both series. A general increase in the clearing temperatures was observed for the outer-substituted series when the size of the lateral substituent was increased, but a depression of the clearing points was found for the inner-substituted series. These observations were investigated through molecular modelling.
Ill this paper, a range of polyrnethacrylate derivatives of hydroxycoumarin are investigated as photomduced alignment layers for liquid crystals, and their performance is compared with poiynyi cinnamate. For all the coumarin-containing polymers, the liquid crystal alignment direction is parallel to the polarization direction of the incident UV light at low fluences. At a critical fluence threshold, a sharp change to perpenthcular alignment is found. Molecular models and spectroscopy are used to explain this phenomenon as well as the fact that only perpendicular alignment is observed for polyvinyl cinnarnate. The azimuthal anchoring energies ofthe alignment layers are measured and values greater than 6 x io-5 j in2 are found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. Exposure of the coumarin-containing alignment layers at oblique incidence gives pretilted alignment with pretilt angles up to 5.1°.
A series of hexa-n-alkylcyclohexanoates of triphenylene were prepared by the esterification of hexahydroxytriphenylene with a variety of 4-n-alkylcyclohexylcarbonyl chlorides in the presence of pyridine. The liquidcrystalline phase behaviour of the materials synthesized was investigated by a combination of thermal polarized light microscopy, differential scanning calorimetry and X-ray diffraction. All of the compounds were found to exhibit a single discotic mesophase that had hexagonal symmetry. The hexagonal columnar phase was found to occur over much wider temperature ranges in this series than was observed for the aromatic analogues.
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