彎曲與棒狀分子混合膽固醇液晶的電光響應及結構切換特性

Electro-Optical Responses and Structural Switching Properties of Bent-Core/Rod-Shaped Cholesteric Liquid Crystals

魏子喬(Tzu-Chiao Wei)

指導教授 ： 李偉

國立交通大學/光電學院/影像與生醫光電研究所/碩士(2019年)

摘要

本實驗為摻雜彎曲型液晶於正型膽固醇液晶內，有效提升樣品在各種結構的光電特性。膽固醇液晶擁有平面態、焦錐態、指紋態以及垂直態四態，而此研究著力於平面態、焦錐態以及垂直態的三態切換。利用調控手性分子使布拉格反射峰於近紅外波段，有效反射紅外波段抵禦熱源。在施加電場後，使結構轉變為焦錐態，運用焦錐態之多疇域散射機制來阻隔入射光穿透，此結構雖稱之為穩態。但由於布拉格反射設計於紅外波段，使得螺距較長，自組裝螺旋力下降，導致焦錐態結構在移除電場後十分不穩定，甚至直接回復至平面態。但在摻雜入彎曲型液晶後，大大的改善此現象，讓焦錐態即使在較弱的自組裝螺旋力下，移除電場後仍然可以維持良好的結構穩定度及強度。而將散射態施加高電場調控樣品至垂直態，此高穿透態能使光與熱都能穿越，而垂直態移除電場回到平面態的過程中，未摻雜之膽固醇液晶，其回復過程只由孱弱的自組裝螺旋力及配向膜之錨定力主導，使得回復時之平面態有較多的油絲狀缺陷，且響應時間較慢。但在摻雜入彎曲型液晶後，獲得了快速的響應時間。不僅於此，回復平面態後，光學紋理圖上幾乎沒有任何油絲狀缺陷。綜合以上的結論，使此元件的弱項得到大大補償。故彎曲型液晶在螺旋結構上的輔助特性，使得可以結合長波段之膽固醇液晶優點及改善其短處，使此元件在爾後有更多元的組合及應用。

關鍵字

膽固醇型液晶 ； 相態切換特性 ； 彎曲型液晶 ； 智慧窗

並列摘要

In this study, the capability of electro-optical properties in cholesteric liquid crystals is effectively promoted by doping optimized concentration of bent-core liquid crystal (LC) dimer CB7CB into rod-like cholesteric liquid crystal (CLC) with positive dielectric anisotropy. There are four states in CLC, including planar (P) state, focal conic (FC) state, fingerprint state and homeotropic (H) state. Here we focus on the tristate-switching of P state, FC state and H state. With the appropriate concentration of chiral dopant in nematic liquid crystal, the pitch can be selected such that the Bragg reflection is located at the near infrared (NIR) regime to provide a remarkably thermal insulation. Besides, with the applied electric field, the multi-domains of the reoriented LC molecules are caused in the FC texture. Because the designated pitch is longer and the self-assembled helical twisting power is reduced, the FC state is unstable after removing the electric field.
In this study, the capability of electro-optical properties in cholesteric liquid crystals is effectively promoted by doping optimized concentration of bent-core liquid crystal (LC) dimer CB7CB into rod-like cholesteric liquid crystal (CLC) with positive dielectric anisotropy. There are four states in CLC, including planar (P) state, focal conic (FC) state, fingerprint state and homeotropic (H) state. Here we focus on the tristate-switching of P state, FC state and H state. With the appropriate concentration of chiral dopant in nematic liquid crystal, the pitch can be selected such that the Bragg reflection is located at the near infrared (NIR) regime to provide a remarkably thermal insulation. Besides, with the applied electric field, the multi-domains of the reoriented LC molecules are caused in the FC texture. Because the designated pitch is longer and the self-assembled helical twisting power is reduced, the FC state is unstable after removing the electric field.
In this study, the capability of electro-optical properties in cholesteric liquid crystals is effectively promoted by doping optimized concentration of bent-core liquid crystal (LC) dimer CB7CB into rod-like cholesteric liquid crystal (CLC) with positive dielectric anisotropy. There are four states in CLC, including planar (P) state, focal conic (FC) state, fingerprint state and homeotropic (H) state. Here we focus on the tristate-switching of P state, FC state and H state. With the appropriate concentration of chiral dopant in nematic liquid crystal, the pitch can be selected such that the Bragg reflection is located at the near infrared (NIR) regime to provide a remarkably thermal insulation. Besides, with the applied electric field, the multi-domains of the reoriented LC molecules are caused in the FC texture. Because the designated pitch is longer and the self-assembled helical twisting power is reduced, the FC state is unstable after removing the electric field.
However, after doping CB7CB in CLC, a good structural ability can be maintained without the electric field even if the LC molecules of FC state is under a weak self-assembled helical twisting power. When applying the high voltage from the FC state to H state, its high transmittance enables the light pass through as usual. Notably, after turning the voltage off, the response time of the recovery from H state to P state is much faster than that of undoped-CLC, meanwhile, there is almost no oily streaks defects. In conclusion, the bent-core mesogen (CB7CB) not only make a stable structure in long pitch CLC but also shorten the response time during CLC state transition (H to P). Therefore, our work provides a potential and promising application of photonic device in the future.

並列關鍵字

Cholesteric Liquid Crystals ； Structural Switching Properties ； Bent-Core liquid crystal ； Smart window

參考文獻

[1] F. Reinitzer, “Beiträge zur kenntnis des cholesterins,” Monatshefte für Chemie 9, 421–441 (1888).

[2] H. Kawamoto, “The history of liquid crystal display,” Proceedings of the IEEE 90, 460–500 (2002).

[3] O. Lehmann, “Über fliessende Krystalle,” Zeitschrift für Physikalische Chemie 4, 462–468 (1889).

[4] 田民波，《TFT液晶顯示原理與技術》，五南出版社，台北，民國97年。

[5] G. Chen, L. Wang, Q. Wang, J. Sun, P. Song, X. Chen, X. Liu, S. Guan, X. Zhang, L. Wang, H. Yang, and H. Yu, “Photoinduced hyper-reflective laminated liquid crystal film with simultaneous multicolor reflection,” ACS Applied Materials Interfaces 6(3), 13801384 (2014).