液晶空盒受紫外光曝照對高介電異方性液晶介電特性的影響

Abstract

本論文以介電頻譜術為研究手段，探討將高介電異方性液晶灌入受紫外光曝照過之空盒後的液晶介電與離子特性。實驗中，我們以紫外光強度與曝照時間為變數，製作若干受紫外光曝照過的空盒。液晶材料方面，我們使用具高介電異方性之液晶（HDE）並依液晶中離子特性的差異分為新鮮液晶以及劣化液晶。在數據分析上，我們將複介電頻譜區分為三個頻率區段，它們分別由吸附在配向層上的離子（低頻）、空間中的離子遷移（中頻）以及液晶分子排列（高頻）所貢獻；此外，我們亦針對中頻段之複介電數據，透過公式擬合，進一步得到各液晶盒中的離子濃度與擴散常數。 針對劣化HDE，在固定照光時間的條件下，隨著紫外光強度的增加，高頻區之介電實部會變小，中頻區之複介電頻譜則往低頻移動，擴散常數降低且離子濃度不變；然而，這些結果皆無法在使用新鮮HDE的液晶盒中觀察到。據此，我們降低液晶盒厚度以縮短離子遷移的距離，更進一步地探討新鮮HDE在延長照光液晶空盒時間下的離子特性。我們發現，除了可觀察到與劣化液晶相同之中頻區複介電頻譜往低頻移動的現象外，在低頻區的介電鬆弛行為亦會隨著照光時間的增加而趨於平緩。根據上述結果，我們推測配向層受到紫外光的改質，使得空間中之離子擴散變慢且不易吸附在配向層上。此推論可由無配向液晶盒之介電特性不隨照光時間變化的結果得到驗證。最後，我們使用空盒具不同照光時間的新鮮HDE，測量其直流電壓相依的電容曲線，評估紫外光造成配向層改質對於液晶電學與離子特性的影響。

This thesis is aimed to investigate the dielectric and ionic properties of high-dielectric-anisotropy liquid crystal (LC) in an ultraviolet (UV)-exposed empty cell by means of dielectric anisotropy. In experiments, a number of UV-exposed empty cells, differing in UV intensity and exposure time were prepared. On the other hand, two kinds of high-dielectric-anisotropy LC (HDE) designated fresh-HDE and degraded-HDE with distinct ionic properties were used as filled materials in empty cells. For data analyses, a complex dielectric spectrum is divided into three frequency regime, attributable to ion-absorption on the alignment layer (low frequency regime), ion-transport in the cell bulk (middle frequency regime), and molecular orientation (high frequency regime), respectively. Moreover, ion density and diffusivity are deduced via curve-fitting based on complex dielectric spectrum in the middle frequency regime. For degraded-HDE-filled empty cell with fixed UV exposure time, decreased real-part dielectric permittivity in the high frequency regime, red- shifted complex dielectric spectrum in the middle frequency regime, unchanged ion density, and lowered ion diffusivity can be observed with increasing UV intensity. However, these results cannot be found for cells with fresh HDE. Accordingly, we reduce the cell gap to shorten the distance for ion-transport and further discuss the change of ionic behaviors as a function of UV exposure time for UV-exposed empty cell with fresh HDE. In addition to the red-shifted complex dielectric spectrum in the middle frequency regime, observed similar to that of cells with degraded HDE, we found that the dielectric relaxation in the low frequency regime can be suppressed by increasing the UV exposure time. The abovementioned results imply the modification of alignment layer by the UV light so that the diffusing speed of ions in the bulk become slowly and ions can hardly be absorbed on alignment layer. This speculation can simply be verified as the dielectric behaviors of alignment-layer-free fresh HDE cell are invariant with the UV exposure conditions. Finally, direct-current-voltage dependence of capacitance curves of fresh HDE cells with distinct exposure times were measured to evaluate the effect of UV-modified alignment layer on electrical and ionic properties.