1 Example of PDLC architectural window

2 Polymer Dispersed Liquid Crystal (PDLC)

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4 Chapter 2. Polymer Liquid Crystals Mesophase: a phase lying between solid (crystal) and isotropic (liquid) states. Liquid crystals: fluid (l) but also show birefringence (c); have properties associated with both crystals and liquids. Thermotropic: liquid crystalline phase is formed when the pure compound is heated. Lyotropic: liquid crystalline phase forms when the molecules are mixed with a solvent (solution) Liquid Crystalline Phases

5 (a)(b) Fig 9.3 The structure of (a) nematic and (b) Columnar discotic phases. Fig 9.2 The structure of molecules which form (a) calamitic and (b) discotic liquid crystalline SOLIDMESOPHASE ISOTROPIC LIQUID 3D order 1or 2D order NONE Mesogen Smectogen Nematogen Lamellar Non-lamellar

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8 Molecular structure: rigid, long lathe-like shapes with a high length to breadth ratio, or disc shapes. Chemical structure: composed of a central core comprising aromatic or cycloaliphatic units by rigid links and having either polar or flexible alkyl and alkoxy terminal groups. Molecular design of liquid crystalline polymers (1) Incorporation of mesogens into chain-like structures by linking them together through both terminal units to form main-chain liquid crystalline polymers. (2) Attachment of mesogens through one terminal unit to a polymer backbone to produce a side chain comb-branch structure. (3) A combination of both main and side chain structures.

9 Thermotropic LC Molecule

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11 The mesogenic units in a polymer can form ordered structures characterized by long-range orientational order, with the long axes of the mesogenic groups arranged in one preferred direction of alignment called the director. The lateral forces between the molecules in the smectic phases are stronger than the forces between the layers. In the nematic phase, while the directional ordering of the mesogen long axes is maintained, the centers of gravity are no longer confined in layers, but are distributed randomly in the phase. A chiral center imparts a twist to each successive layer in the cholesteric phase where the orientation of the director changes regularly from layer to layer, forming a helical arrangement of the directors in three dimensional space.

12 In main chain liquid crystal polymers, there is usually a transition from crystal to a mesophase, whereas in more amorphous systems when a glass transition is present. The mesophase may appear after this transition has occurred. In multiple transition thermotropic systems, the increase in temperature leads changes from the most ordered to the least ordered states. k: crystal, g: glass, i: isotropic S: smectic (S*: chiral S), N: nematic (N*: chiral N) Ch: cholesteric 2.2 Identification of the Mesophases By polarized optical microscope (POM)  Nematic phases: e.g. Schlieien texture, threaded texture, and marbled texture.

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14  Smectic phases: e.g. focal conic and fan texture, mosaic texture, and broken focal conic texture.  Cholesteric phases: e.g. Grandjear texture. By differential scanning calorimeter(DSC) Widely used for detecting temperatures of thermotropic mesophase transitions. First-order transitions. By X-ray diffraction (XRD) For the least ordered phases, one diffuse halo is seen at large diffraction angles. At smaller diffraction angles, a diffuse inner ring is formed in nematic samples. When the smectic phases are present, one or more sharp inner rings, arising from the more ordered lamellar structure, are seen.

15 Nematic Schlieren texture Mosaic texture of a hexatic SmB phase SmA fan-shaped textureSmA focal conic texture SmC Schlieren texture and focal conic Nematic Phase (threaded) Nematic Phase (marbled)

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