均勻橫向螺紋態在高分子網絡中的多相態相變點介電特性與無遲滯電光響應

Abstract

本論文針對含高分子穩定均勻橫向螺紋態之多相態膽固醇液晶，研究多相態相轉變的介電與光固化橫向螺紋態的電光效應。在實驗中，首先固定高分子濃度於3 wt%，分析在不同溫度下的高頻介電值變化（10 kHz），並以一階微分的方法觀察相變點的趨勢，然後再提高濃度至5 wt%和7 wt%並分析其相變趨勢，再透過光學紋理圖做佐證，完成光固化後的介電頻譜分析。接著以固定高分子濃度5 wt%為研究對象，探討光固化時的外加電場影響；實驗結果顯示，當光固化時外加電場的大小會改變原本二階相變的特性，同時在相變溫度TCLC-SmA的相變一階介電值會隨著電壓增加而有往下的趨勢。 光固化後的樣品在量測電光特性時，觀察不同高分子濃度的V−T%曲線，可以得到近乎無遲滯的電光效應，且遲滯電壓會隨著高分子濃度上升而有顯著的改善，同時濃度上升也影響了響應時間，在7 wt%的樣品中反應時間縮短到4 ms的。最後吾人透過改變光固化的條件，將光固化的溫度從CLC態下降至SmA相態，成功突破了ULH在SmA溫度中無法以介電驅動並留有嚴重遲滯電壓的現象；雖然在環境溫度為CLC態時遲滯電壓略大，但當溫度降到SmA態後，仍可得到極小的遲滯電壓；而在量測響應時間的實驗中，光固化於SmA相態的樣品相較於光固化於CLC相態的樣品改善了無法在低於20 C的環境溫度中測量響應時間的問題，並在環境溫度16 C時，測量到29 ms的「快速」響應。

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 (VT%) 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.