Browsing by Author "Leaw, Wai Loon"

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    4-CYANO-4’-n-pentylbiphenyl liquid crystal physical gel for twisted nematic and dynamic scattering electro-optical applications
    (Universiti Teknologi Malaysia, 2017) Leaw, Wai Loon
    Liquid crystal physical gels were invented to improve the electro-optical performance of the liquid crystal displays. The characteristic of liquid crystal physical gel is greatly dependent on the physicochemical properties of the gel networks. Most of the conventional gelators produce only fibrous gel networks and their morphology is solely dependent on the order of the sol-gel transition temperature (Tsol-gel) of the gelator as well as the isotropic-anisotropic transition temperature (Tiso-lc) of the liquid crystal. Therefore, the optimisation of electro-optical performance is restricted by the gel structure that cannot be tuned substantially. In the present study, a type of gelator that anomalously possesses mesomorphic state was explored. Resulting liquid crystal physical gels showed electro-optical switching on twisted nematic and light scattering modes. Remarkably, a sample with 48.0 wt% cholesteryl stearate/4-cyano-4’-pentylbiphenyl was able to create extraordinary light scattering effect. The light scattering effect is produced by the randomized liquid crystal director orientation as induced by the plate-like aggregates of the gel network. During the gelation, the chirality of the cholesteric type gelator altered the nematic phase of the mixture into chiral nematic phase, and hence different gel structure was produced. The size of the gel aggregates was also optimised by cryogenic cooling using liquid nitrogen. The excessively high cooling rate had prevented the secondary aggregation. As revealed by polarized optical microscopy (POM) and scanning electron microscopy (SEM), the gel network was composed of smaller gel aggregates which assembled to form identical spherical pores of micron size. As a result, a contrast ratio (CR) of 4.2 for the light-scattering display was achieved. In addition, the threshold and saturation voltages, as well as rise time were retained as low as5.0 V, 29.0 V and 3.7 µs, respectively. This cholesteryl stearate gelator was suitable for the preparation of liquid crystal physical gel since the liquid crystal molecule was not strongly entangled despite the high randomization power