新與劣化向列液晶受紫外光曝照前後電學特性之比較

Abstract

本論文以介電頻譜術為研究手段，探討具不同劣化程度的單一款液晶在經過紫外燈照射之後所呈現的介電實、虛部頻譜。實驗中，我們的研究對象為2011及2014年兩瓶向列型液晶E44所購入，年份為，分別視為劣化與新液晶。首先將兩者在固定的照光時間及溫度，並改變不同的照光強度。第二將灌好的液晶盒做封口及未封口的比較經過長時間的觀察及量測30天後再照紫外燈。最後則是將灌好新液晶的液晶盒加熱至70 C，量測的介電實虛部與劣化的液晶有相同的趨勢，同樣在照射UV光後呈現出與之前新的液晶同樣變化並未像劣化的液晶有相同的趨勢變化。 實驗結果顯示，在照射紫外光的條件下，因液晶受到裂解以及由配向層注入離子的貢獻，介電頻譜會往高頻移動。相較之下，新的液晶因離子較少照射之後介電頻譜隨照光時間的變化很快地就達到了飽和狀態，而劣化的液晶因離子較多經過UV照射之後在低頻段之介電虛部值漸漸地往上升，中頻段的複介電頻譜則則往高頻位移。液晶盒封口後，新與劣化液晶主要受到框膠的影響介電曲線整體由低頻往高頻段移動，未封口之液晶盒變動不大，其介電特性主要還是受之後的UV光照射影響。新與劣化的液晶經過紫外燈的照射之後其液晶盒裡的離子濃度皆會增加隨著照射的時間越長趨於飽和的狀態，光照的強度增強離子濃度隨時間的變化也會跟著變大。根據介電量測結果，我們分析各樣品在低頻段（頻率小於 1 kHz）的介電頻譜，透過數據擬合得到直流導電率、離子濃度與擴散係數，藉上述的數據來分析討論離子效應。

This thesis investigates effect of the ultraviolet (UV)-light exposure on the dielectric properties of nematic liquid crystals (NLCs) with different degree of degradation. Here, two bottles of NLC E44 purchased in 2011 and 2014 are regarded as degraded and fresh LCs, respectively. The content of this thesis is divided into three parts. The first part is concerned with the effect of UV intensity on time-varying dielectric features of both fresh and degraded LCs at fixed temperature. In the second part, sealed and unsealed LC cells were fabricated and their time varying dielectric spectra were measured. After keeping cells undisturbed for 30 days, the dielectric and ionic properties of these cells varying with time under UV exposure are further discussed. By fixing the fresh LC cell at 70 C where the complex dielectric spectrum is similar to that of degrade LC at room temperature, the final part is to discuss the dielectric properties of the fresh LC under UV exposure at a given temperature and compare them to those of the degraded counterpart at room temperature. Under the condition of UV exposure, the dielectric spectra attributable to the space-charge polarization in the middle frequency regime of both the fresh and degraded LCs shift to high frequencies with time due to the dissociation of LC material itself and/or ion injection from the alignment layer. Because the ion density of the fresh LC is lower than that of the degraded counterpart, the saturated time in which the dielectric spectra stops shifting of the fresh LC is shorter than that of the degraded LC. Noticeably, in the low frequency regime where the complex dielectric constant is attributable to accumulation of ions near the substrate and thus the electrode polarization, the imaginary-part dielectric permittivity gets increased with increasing UV exposure time and then becomes saturated at a critical time. For cells sealed with AB glues, the shift of dielectric spectra to high frequency with time of both the fresh and degraded LCs are greater than those of the unsealed counterparts, indicating the degradation primarily dominated by the glues. Based on results of dielectric spectra, the ionic properties of the LC cells under UV exposure are further discussed in terms of the dc conductivity and ion density. It is found that the ion density of both the fresh and degraded LC cells get promoted with increasing UV exposure time.