Liquid Crystals. Iam-Choon Khoo

Liquid Crystals - Iam-Choon Khoo


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from the nematic to the smectic phase. The cell preparation methods for surfaced‐stabilized FLC (SSFLC) operation is more complicated as it involves surface stabilization [28, 29]. On the other hand, Sm‐A* cells for soft‐mode (SM‐FLC) operation are easier to prepare using the above methods [30].

      1.5.2. Cholesteric Liquid Crystal Cell Assembly

      Recent studies [31–33] have shown that the so‐called field assisted self‐assembly (FASA) technique could produce very stable and well‐aligned CLC cells with thicknesses approaching 1 mm, with a period number N as large as 3000. The technique works by using nematic liquid crystals with negative dielectric anisotropy and a strong AC field during the sample preparation stage to enforce the required planar alignment of the LC molecules and therefore prevent the director axis rotation helix from deviation into a focal conic structure, as illustrated in Figure 1.20b and c. The empty cell is first filled with CLC mixture in the isotropic phase (see Figure 1.20b); then, as the sample is allowed to cool down slowly to room temperature, an AC electric (~2500 V; 1 kHz) is applied across the cell window for an extended period while the mixture sits at room temperature (Figure 1.20c and d).

Schematic illustration of (a) CLC with focal conic structure as a result of director axis deviating from planar alignment. (b) Random orientation of director axis in the isotropic liquid phase; AC voltage on. (c) Sample cooling down to ordered phase; strong AC field enforces planar alignment. (d) AC field removed after an extended period. Schematic illustration of the logo obstructed by a thin CLC cell with focal conic alignment.

      1.5.3. Blue‐phase Liquid Crystal Cell Assembly

      In order to extend the working temperature range, a standard practice is polymer stabilization of the BPLC lattice [34–38]. Polymer‐stabilized blue‐phase liquid crystals (PS‐BPLC) are obtained by blending the chiral nematic material used to synthesize BPLC with photocurable prepolymers and polymerize the blend with UV or visible light depending on the type of photocurable prepolymer used. Typical phase sequence of the resulting PS‐BPLC in [34] is: Iso‐(56.2 °C)‐BP‐(<0 °C)‐N, i.e. the temperature range is ~56 °C.

      As a function of the temperature, pristine BPLC with the constituents described in [34] typically exhibits the following phase sequence: transparent isotropic liquid phase for T > 31 °C; bluish color BPII phase (at T ~ 31–28 °C); greenish color BPI (at T ~ 28–23 °C), and cholesteric focal conic phase N* (below 23 °C). The BPLC temperature range is ~8 °C. The defect (disclination) lines are discontinuous in BPI phase but form a continuous network in BPII phase with body‐centered cubic (BCC) or simple cubic symmetries depicted in Figure 1.22. The lattice constants (dimension of these unit cells) are on the order of optical wavelength, and thus BPLCs typically exhibit photonic crystalline properties such as bandgaps and selective reflections in the blue‐green region [34–39] but are otherwise optically isotropic at a wavelength outside the bandgap. In [38], it is shown that with the so‐called RAF (repetitively applied field) technique, one could reconfigure the network into other non‐cubic crystalline symmetries.

      In general, specific alignment on the cell windows is not needed for preparing BPLC sample, owing to the random orientations of the director axis of the defect lines or double‐twist cylinders on the boundary surfaces. In most studies, the cell windows are coated with an alignment layer to impart specific crystalline plane orientations.


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