以介電頻譜術測量形成高分子分散型液晶的聚合行為

Using Dielectric Spectroscopy to Monitor the Photopolymerization Behaviors of Polymer-Dispersed Liquid Crystals

郭宏銘(Hong-Ming Kuo)

指導教授 ： 李偉

國立交通大學/光電學院/光電科技學程/碩士(2019年)

摘要

本論文的研究對象為高分子分散液晶，研究主軸係以介電頻譜術探討利用紫外光誘導光聚合相分離所需的最佳時間。在此，我們選用向列型液晶E7與高分子單體NOA65在特定比例混合下製作液晶/單體前驅體並在溫度為30 C的條件下，照射特定參數的紫外光，藉此形成高分子分散液晶。
我們首先在照射紫外光的過程中，量測在特定頻率下之介電實部（ε’）隨時間（tUV）的變化，探討高分子化過程對於介電特性的影響。在頻率為100 Hz的條件下，我們依介電實部隨照光時間的變化，定義出兩個關鍵時間點，t1和t2（t1 < t2）。隨著照光時間上升，介電實部在tUV < t1時，會隨之下降，在t1< tUV < t2時，趨於一常數，在tUV > t2時，隨之上升。
在上述基礎下，我們選定在特定照光時間條件下製作高分子分散液晶並量測其電學特性隨直流與交流電場大小以及穿透率隨交流電場大小的變化。實驗結果顯示， 我們確認形成高分子分散液晶所需的最短時間為t1。當照光時間大於t1時，在固定照光強度下，介電特性隨交流與直流電壓的變化會隨著照光時間的增加而劣化。另外，根據光電特性的量測結果，我們認為在t1 和 t2間的時間為最佳照光時間區域，因此時PDLC具有較低的臨界與飽和電壓以及較高的對比度。

關鍵字

高分子分散型液晶 ； 臨界電壓 ； 液晶滴

並列摘要

In this thesis, we focus on the polymer dispersed liquid crystals (PDLCs) and purpose to determine the optimized time for the photo-induced phase separation by the ultraviolet (UV) light exposure. Here, we adopt the nematic LC host E7 mixed with monomer NOA65 in specific concentrations to prepare the LC/monomer precursors. Various PDLC samples were then formed by illuminating certain condtions of UV light at the temperature of 30 C.
Firstly, we measured the change of the real-part dielectric permittivity (ε’) of samples as a function of the time (tUV) during the UV exposure to discuss the impact of photopolymerization process on the dielectric characterisitics. According to the ε’– tUV under the condition of f at 100 Hz, two critical times, namely t1 and t2 (> t1) can be defined. With increasing UV exposure time, our results indicated that ε’ is gradually decreased in the time range of tUV < t1, preserved as a constant in t1< tUV < t2, and progressively increased as the time is higher than t2 (tUV > t2).
Based on the abovemntioned definitions, PDLC samples, differing in the UV exposure times, were fabricated and their electrical properties under DC and AC voltages applied and the voltage-dependent transmission (V–T) curves measured. Our results implied that the shortest time for forming PDLC via UV exposure is t1. As tUV is higer than t1, the dielectric response to either the AC or DC voltage would get degarded. Moreover, results based on (V–T) curves suggested that the time regime, varying in between t1 and t2 is optimal for the PDLC as the electro-optical characteristics exhibit lower threshold and saturated voltages and higher contrast ratio.

並列關鍵字

Polymer-Dispersed Liquid Crystals ； Threshold voltage ； Liquid crystal drop

參考文獻

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