| Summary: | 碩士 === 國立交通大學 === 照明與能源光電研究所 === 105 === We demonstrate the morphological and electro-optical properties of the polymerstabilized uniform lying helix (PS-ULH) structure in chiral-doped liquid crystal (CDLC) cells. In this study, the mixture was doped with two types of monomers (RM257 and TMPTA) at 3 wt%, 5 wt% and 7 wt% to create monomer/CDLC precursors. The PS-ULH structure in each sample was created by firstly forming stable ULH at zero voltage in the CLC phase, followed by the illumination of ultraviolet light for an hour to accomplish the photopolymerization process of the precursor. The PS-ULH state in the cell after UV exposure revealed excellent stability after cycles of temperature tests from the isotropic to the crystal phases and the electric field treatment.
By means of temperature-dependent dielectric spectroscopy (10 kHz) and observations of optical tetures, our results show that the PS-ULH structure can be obtained in the temperature range covering both the CLC and the SmA phases. We then investigate the influence of external electric field (VUV) while curing the cell made by 5 wt% monomer concentration. The result shows that the external electric field will change the properties of second-order phase transition. At the same time, the value of first-order differential dielectric spectrum at TCLC-SmA will decrease with increasing the VUV. While the ULH is regarded as a uniaxial crystal in the CLC phase, the voltage-dependent transmission (VT%) curve of the cell under polarizers exhibits hysteresis-free electro-optical responses, suggesting the ability of linear gray-levels. Noticeabliy, the switching time between the PS-ULH and the homeotropic states measured 4 ms while increasing the concentration of the monomer to 7 wt%. Consequently, the proposed PS-ULH structure, with high contrast ratio, hysteresis-free responses and millisecond order response time, is potemtially applicable for developments of electro-optical devices, such as phase modulators and displays.
In addition, we present a method to decrease the hysteresis voltage in SmA phase. After forming ULH texture, we decreased the temperature and cured the cell in SmA phase. It is found that this method make the PS-ULH operable while the temperature cooling to SmA. Furthermore, the new method improve the response time in CLC state comparing with the same concentration of monomer curing in CLC state. While the temperature controlled at 16 C, we can measure the response time which is 29 ms. Also, the sample shows the properties of fast response at room temperature.
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