Effects of controllable defects on the bistable properties of cholesteric liquid crystal materials and the potential application
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摘要: 通過彎曲分子的摻雜實現了膽甾相液晶平面態油絲缺陷的調控和焦錐形貌的穩定，研究了各因素對雙穩態穩定性的影響，并展示了雙穩態的潛在應用。偏光顯微鏡觀察和透過率?電壓曲線測試結果表明，彎曲分子的引入可以調控甚至消除平面態的油絲缺陷，實現透過率增加，同時調節焦錐態的形貌，使扇形疇更細小破碎，有利于增強焦錐態的散射和穩定性，實現雙穩態效果。這是因為彎曲分子具有很小的彎曲彈性常數，進而對膽甾相液晶的彎曲彈性常數和扭曲彈性常數相對大小進行調節。平面取向層的錨定越強，對雙穩態(主要是焦錐態)的穩定越不利，但錨定作用太弱，不利于平面態的完美排列。可以通過取向作用強弱調節、盒厚調整來達到較優的效果。少量聚合物的引入有利于實現雙穩態的穩定。最后，基于缺陷調制后的雙穩態特性制備了無色和彩色的調光膜，展示了雙穩態的潛在應用。Abstract: The worldwide energy crisis necessitates the urgent need for energy conservation, especially in buildings whose energy consumption has already surpassed that of transportation and industrial sectors. Most of the energy loss in buildings is related to windows because their heat transfer coefficient is different from other building fabrics. Therefore, smart window technologies are proposed to eliminate energy usage, among which cholesteric liquid crystals show great potential due to their electrically controlled bistable properties. Cholesteric liquid crystals are characterized by two stable states: (1) the planar state that shows selective reflection or high light transmittance and (2) focal conic state that scatters incident light and shows opaque appearance. To realize a bistable state in which both the planar state and the focal conic state could be maintained without an external field is of vital importance for the application, since it could save considerable energy. In this study, oily streak defects and stability of the focal conic state are controlled by introducing a type of bent molecular materials. Effects of various factors on the bistable states are investigated, and the potential applications are explored. Results show that the bent molecule CB7CB could diminish the oily streak defects in the planar state and reduce the domain size of the focal conic state because it has a smaller bent elastic constant. The electro-optical test implies that bistable states are acquired in the composite containing CB7CB. Factors such as the anchoring strength of the alignment layer and cell thickness are then investigated and results reveal that strong anchoring and thinner cell thickness are not favored by the focal conic state. Moreover, bistable states could be stabilized by introducing a small amount of polymer followed by polymerization. Based on the above guidelines, colorless and colorful bistable light shutters are demonstrated, which could maintain both transparent and opaque states without any external electric field. The light shutter saves more energy than the one based on polymer-dispersed liquid crystals. Other potential applications, such as bistable displays and electronic label could also be realized based on the bistable property.
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圖 3 （a）具有較大伯格斯矢量（b）的位錯分裂為兩個向錯的示意圖[29]；（b）在平行且等間距的體系中，具有半圓末端的油絲缺陷層結構示意圖[29]；（c）圓環形焦錐疇[29]
Figure 3. (a) Splitting of the core of a large Burgers vector (b) dislocation into two disclinations[29]; (b) an oily streak with a semicircular end in a system of parallel and equidistant layers[29]; (c) toroidal focal conic domain with a circular base smoothly embedded in the set of horizontal flat layers[29]
圖 4 膽甾相液晶解螺旋和螺旋回復過程中的分子排列示意圖[30]。（a）平面態；（b）垂直取向態；（c）錐狀螺旋態；（d）過渡平面態；（e）從過渡平面態到平面態的轉變
Figure 4. Schematic showing the unwinding and restoring of the helical structure in cholesteric liquid crystals[30]: (a) planar state; (b) homeotropic state; (c) conic helical state; (d) transient planar state; (e) transition from the transient planar state to the intrinsic planar state
圖 5 不同CB7CB單體含量的膽甾相液晶平面態（上）和焦錐態（下）的偏光顯微鏡照片。（a）A1 （0%）；（b）A2（10%）；（c）A3（20%）；（d）A4（28.5%）
Figure 5. Planar (upper) and focal conic (lower) texture of samples with different concentrations of CB7CB: (a) A1 (0%); (b) A2 (10%); (c) A3 (20%); (d) A4 (28.5%)
圖 6 （a）CB7CB含量對透過率性能曲線的影響；（b）樣品A1和（c）樣品A4的平面態?焦錐態轉變電光性能曲線
Figure 6. (a) Transmittance curve of samples containing different concentrations of CB7CB; transmittance as a function of voltage in (b) sample A1 and (c) sample A4
圖 7 焦錐態隨時間的變化。（a）偏光織構；（b）透過率
Figure 7. Time evolution of the focal conic state: (a) POM texture; (b) transmittance
圖 11 無色及彩色樣品的雙穩態效果照片。（a）無色；（b）中心反射波長550 nm，彩色；（c）中心反射波長650 nm，彩色
Figure 11. Photos of colorless and colored bistable samples: (a) colorless sample; (b) colored sample with reflective wavelength centered at 550 nm; (c) colored sample with reflective wavelength centered at 650 nm
表 1 樣品組分及質量分數
Table 1. Components and mass fraction of samples
% Sample No. BL038 CB7CB CB15 A1 84 0 16 A2 74 10 16 A3 64 20 16 A4 55.5 28.5 16 
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表 2 聚合物穩定樣品的透過率測試數據
Table 2. Transmittance result of polymer-stabilized samples
Polymer mass fraction/% Curing state Transmittance before polymerization Transmittance after polymerization TP/% TFC/% CR TP/% TFC/% CR 0.5 P 87.2 1.20 72.7 89.3 1.06 84.2 Fc 88.3 1.35 65.4 88.9 1.13 78.7 FcV 85.5 1.27 67.3 88.3 1.19 74.2 1.0 P 85.5 1.56 54.8 87.1 1.14 76.4 Fc 87.0 1.81 48.1 87 1.11 78.4 FcV 84.2 1.61 52.3 85 1.38 61.6
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