Abstract
In this thesis, the electro-optical properties including threshold voltage (Vth), transient currents and photorefractive effect of nematic liquid crystal (E7) hybridized with montmorillonite clay were studied. The optical transmittance and the electrical capacitance as a function of the externally applied dc voltage are obtained for a twisted nematic cell composed of inorganic clay; i.e., sodium montmorillonite, as an additive to the organic liquid crystal E7. Higher loaded nematic, with smectite clay content of over 3 phr, exhibits wider voltage–transmittance and voltage–capacitance hysteresis, indicating that such cells suffer from more serious ion-charge effects and the orientation barrier of the liquid crystal. Experimental evidence suggests that the addition of merely 0.5–1phr of clay substantially rectifies the electro-optical characteristics of the pristine nematic, leading to an apparent lowering of the dc threshold voltage. Transient currents induced by the polarity reversal of an applied dc voltage in nematic liquid crystal hybridized with montmorillonite clay have been studied. Comparative values of the ion-charge concentrations deduced from the experimental data suggest that the clay hybridized into liquid crystal has a great impact on the ion-charge concentration, which can be dramatically reduced to as low as to ∼1% for a nematic cell containing merely 0.5 phr clay. Such hybridization suppresses the formation of electric bilayers and the charge-screening effect. Due to the clay and liquid-crystal molecules being aligned to the direction of the field, a higher mobility is observed in hybridized cells. The orientational photorefractive effect was observed in an organic–inorganic nanocomposite of nematic liquid crystal hybridized with montmorillonite clay. Both the self-diffraction and beam-coupling effects were evaluated in a two-wave-mixing experiment in conjunction with an externally applied dc field. The experimental results indicate that photoinduced generation was enhanced by the addition of smectite clay with adequate concentration. Physically, the drifting ion charges were trapped by clay layers and separated by interlayer cations, creating an internal, spatially modulated space-charge field, which led to nematic molecular orientation and, then, refractive-index modulation via the electro-optical response. The diffraction efficiency as well as the beam-coupling ratio of the phase gratings recorded in the cells of the nematic liquid crystal hybridized with montmorillonite clay was found to be two to three times higher than that in the pristine nematic cell. It is our interest to study the physical properties of the system of holographic polymer-dispersed liquid crystals doped with clay. The doped clay in PDLC slightly retarded photo polymerization rate, but since it is similar to a nucleus medium, separation of the liquid crystal and the polymer is enhanced. In the meantime, the PDLC doped clay has a larger droplet size than pristine PDLC, suggesting a smaller wall surface anchoring, which results in reorientational modulation and higher grating diffraction efficiency of the liquid crystal within the droplet by electro-optical field. This study demonstrates that montmorillonite clay hybridized into liquid crystal yields many advantages to improve the electro-optical properties of the display or storage device.