Photoresponsive liquid crystal physical gels are formed from a hydrogen-bonded gelator containing photochromic azobenzene moieties and nematic or discotic liquid crystals. The bistable gel structures based on the trans-azobenzene gelator could be achieved by combining the trans-cis photoisomerization of the azobenzene moieties and thermal treat-ment. Upon UV irradiation, the trans-cis photoisomerization causes the transition from the initial gel states to the liquid crystal sol states. The cis-trans back-isomerization causes reaggregation of the trans-gelator in the liquid crystals. This leads to the formation of the second gel states which have the structures reflecting the liquid crystal order. The initial gel states can be reversibly changed to the reformed gel states by photoirradiation and thermal treatments. The photo-induced reversible structural changes of the anisotropic physical gels are applied to rewritable information recordings.
Light scattering electrooptical switching has been achieved for liquid crystal physical gels consisting of a room temperature nematic liquid crystal and a hydrogen-bonded gelator. The use of the nematic liquid crystal exhibiting higher isotropization temperature and the gelator with four hydrogen-bonded moieties leads to the formation of thermally more stable liquid crystal gels. The liquid crystal gels show significant electrooptical properties for light scattering display materials. We have examined the effects of the concentration of the gelator and the cell thickness on the electrooptical behavior.
A nematic liquid crystal, 4-(trans-4- pentylcyclohexyl)benzonitrile, has been physically gelled by hydrogen-bonded network formation of low molecular weight additives, that is, three amide compounds. Electro-optic measurements in twisted nematic (TN) cells have been performed for the resultant gels that exhibit liquid-crystalline gel states at room temperature. Each of the anisotropic gels formed by the three gelling agents, exhibiting microphase- separated structures, shows different electro-optic responses. One of the gels responds to an electric field more than twice as fast as the single liquid crystal component. Network aggregates with different morphologies are observed for each of the gelling agents. More finely dispersed fibrous networks would contribute to the faster electro-optic response. Thermal transition behavior and composite structures of the anisotropic physical gels have been examined by polarizing optical microscope observation and differential scanning calorimetry.
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