APPLICATION OF LAYERED INORGANIC MATERIALS IN RECTIFICATION OF ELECTRO-OPTICAL PROPERTIES OF NEMATIC LIQUID CRYSTALS AND POLYMER-ENCAPSULATED LIQUID CRYSTALS

抄録

The electro-optical (EO) properties were measured for cells composed of layered inorganic materials; i.e., either the smectite sodium montmorillonite (MMT) or layered double hydroxides (LDHs), as an additive in the organic nematic liquid crystal (NLC) E7 or blends of E7 and a photoinduced polymer. For a twisted NLC (TNLC) device driven by an applied voltage, the smectite hybridized into the LC was found to suppress the formation of electric bilayers and the field-screening effect, significantly reducing the ion-charge concentration down to as low as 〜1% for a TNLC-clay cell containing 〜0.5wt% MMT. Similar results were also observed from the LC-clay nanocomposite confined in a homogeneously-aligned cell. On account of the clay platelets and NLC molecules being aligned by the externally applied voltage and the nanoplatelets homogeneously dispersed in the NLC hydrosol, the voltage-transmittance curve of the hybrid cell exhibited increasing contrast ratio of the LC photonic device. Besides, the voltage-capacitance characterization indicated that the layered inorganic clay in E7, depending strongly on the proper charge properties such as charge affinity of the layered materials itself, was able to dramatically decrease the threshold voltage for cells of TNLC-clay composites operated by dc electric field. Consequently, adding an appropriate amount of clay substantially rectified the EO switching properties of a typical low-resistivity NLC in the TNLC-clay hybrid materials system. On the other hand, holographic polymer-dispersed LCs (H-PDLCs) consisting of E7 and layered inorganic materials in photo-polymerized macromolecular matrices were investigated. The H-PDLC were fabricated from a monomer hybrid solution including dipentaerythritol pentaacrylate (SR399), cross-linking N-vinyl-pyrrollidone (NVP), photo-initiator dye Rose Bengal (RB), co-initiator N-phenylglycine (NPG) and E7 by wave mixing of two coherent argon-ion laser beams at the 514.5nm line. Due to the photo-induced polym-erization phase separation, the NLC-polymer blends were separated into mesophase and polymer phase and the role of clay as a dopant in the phase separation processes could be established by observing the morphologies of the composite films and the resulting performance of the cells. The effect of MMT doping in the NLC-polymer systems was first revealed by the wide-angle X-ray diffraction pattern, indicating that the liquid-crystalline signal was enhanced by doping various clays at adequate contents. Further evidence was manifested by the increases in both the range of the size distribution and the maximum size of the NLC droplets with increasing clay content up to 3wt%, as examined with a transmission electron microscope as well as a field-emission scanning electron microscope after the cell substrates were taken apart and E7 droplets was removed from the PDLCs. We found the PDLC comprising 3wt% clay with higher cation exchange capacity (168meq per 100g of clay in this study) to be dramatic in particular to lower the driving voltage, switching time and threshold voltage simultaneously and to suffer less serious off-axis haze at large angles of incidence in general. For H-PDLCs, self-diffraction experiments in the Raman-Nath regime showed that the first-order diffraction efficiency was enhanced in those consisting of pristine MMT nanoparticles and yet depressed by the incorporation of organophilic MMT. Holographic nature of the nanocomposites with polymeric matrices was confirmed by optical polarizing microscopy. Without the use of an applied voltage, the 5-phr-clay-hybridized and 1-phr-LDH-doped nanocomposites yielded the improved first-order diffraction efficiencies of 6 times and 4 times higher than that of the pristine H-PDLC counterpart, respectively. In contrast, the diffraction efficiency of the phase gratings recorded in the cells of E7 hybridized with 1wt% MMT clay was found to be only three

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