紫外光照射對於高介電異方性液晶盒溫變介電與光學特性之影響

Abstract

本論文以介電頻譜術為手段，研究具高介電異方性液晶盒在紫外光照射下，其分子排列與介電特性的變化。實驗中，紫外光的強度固定為20 mW/cm2並在不同溫度條件下，即時量測液晶盒隨紫外光曝照時間的複介電頻譜。我們將介電頻譜分為高頻(f > 1 kHz)以及低頻(f < 1 kHz)區段，分別探討液晶盒的分子排列與離子特性。此外，我們以預傾角與穿透光譜量測以及DiMOS模擬，進一步驗證高介電異方性液晶分子排列受紫外光的影響。 在溫度為10 C的條件下，實驗結果顯示，高頻區的介電實部隨著紫外光曝照時間的增加首先會變小，此時介電虛部曲線會呈現紅移(即往低頻移動)。當照光時間大於一特定值(tc)時，高頻區的介電實部隨時間的下降曲線趨於平緩，同時，介電虛部曲線則開始藍移(即往高頻移動)。隨著溫度上升，tc值下降，當溫度為50 C時，tc~0 min，代表在此溫度下，介電實部在高頻區的變化趨於常數，而介電虛部曲線隨溫度變化的曲線為藍移。因本研究之液晶盒為水平配向且所使用的液晶具有高的正介電異向性，我們直觀的認為高頻區介電實部的下降代表液晶傾角的降低。此結論更進一步地以預傾角與光譜量測以及DiMOS模擬驗證。 更進一步地，我們利用適當的公式作數據擬合，得到直流導電率與擴散係數來探討液晶盒在紫外光照射下的時變離子行為。在溫度為10 C與20 C時，液晶盒之直流導電率與擴散係數並不會隨照光時間變化。當溫度大於20 C時，兩者皆會隨著照光時間的增加而上升。由於離子的傳輸會隨著溫度的增加而提升，我們認為離子在高溫時可以較輕易的從紫外光的照射獲得足夠的能量進而擴散。 最後，與前述實驗手段不同，先將空盒以紫外光照射後再灌注液晶，利用介電頻譜術觀察，發現離子行為往低頻區移動；亦利用穿透光譜、晶體旋轉法量測與DiMOS光學模擬，其結果與上述相同。此實驗值得後續更深入的探討，但本實驗並未往下繼續琢磨。

In this thesis, the molecular orientation and dielectric characteristics of high dielectric anisotropy nematic liquid crystal (NLC) cells under ultraviolet (UV) light exposure are investigated by means of dielectric spectroscopy. In experiments, time-varying complex dielectric spectra of NLC cells under UV exposure were measured with fixed UV intensity of 20 mW/cm2 and varied temperatures (T), ranging from 10 to 70 C. The dielectric spectrum in the high-frequency (f > 1 kHz) and low-frequency (f > 1 kHz) are used to discuss the molecular and ionic properties of a NLC cell, respectively. Moreover, pretilted angle and transmission spectra measurements and DiMOS simulations were performed to further confirm the effect of UV exposure on the molecular alignment of the high dielectric anisotropy NLC. In the case of T = 10 C, the real-part dielectric permittivities (ε’) in the high frequency regime firstly get increased and the imaginary-part dielectric spectrum (ε”(f)) red-shifts (i.e., shifting to low frequencies) with raising UV exposure time. In contrast, when the UV exposure time is higher than a critical value (tc), the change of ε’ in the high frequency as a function of the UV exposure time becomes gentle but ε”(f) starts blue-shifting (i.e., shifting to high frequencies). Because the surface alignment of used cells is planar and the NLC exhibit positive dielectric anisotropy the decreased value of ε’ can directly be attributable to the decrease in the tilted angle of NLC molecules as confirmed further by pretilted angle and transmission spectra measurements. Furthermore, by fitting complex dielectric spectra with appreciated formula, the time-varying ionic behavior of the NLC cell under UV exposure are clarified in terms of the dc conductivity and the diffusivity. It is suggested that both the dc conductivity and diffusivity preserve as constants with time at the temperature conditions of T = 10 C and 20 C but they start increasing with raising UV exposure time when the temperature is higher than 20 C. Since the ion-transport gets promoted with increasing temperature in a LC cell, it is implied that ions can readily receive enough energy from the UV light to diffuse within the cell at high temperature. Finally, an individual study concerning the illumination of UV light to the cell before LC injection was preliminarily investigated. As confirmed by transmission spectra, pretilted angle measurement and DiMOS simulation, similar result to the decrease in the tilted angle of LC molecules can be found in this experiment as well. Further experiments are worth investigating separately in the future.