Polymer network stabilized liquid crystal phase

6 Polymer Network Stabilized Liquid Crystal Phase 

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Good physical stability can, however, be obtained by developing a viscoelastic network in the continuous phase. The elastic component acts as a net that prevents the droplets from settling or creaming. Viscoelastic networks can be obtained with high-molecular-weight water-soluble polymers or lyotropic liquid crystals. 

Polymer stabilized liquid crystals are formed when a small amount of monomer is dissolved in the liquid crystal solvent and photopolymerized in the liquid crystal phase. The resultant polymer network exhibits order, bearing an imprint of the LC template. After photopolymerization, these networks in turn can be used to align the liquid crystals. This aligning effect is a pseudo-bulk effect which is sometimes more effective than conventional surface alignment. Several characterization techniques... 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

To widen BP temperature range, several approaches have been proposed [15-18] Here, we focus on the blue phases induced by incorporating chiral dopants into a nematic LC host. To make a polymer-stabilized blue phase liquid crystal, a small fraction of monomers (-8%) and photoinitiator (-0.5%) is added to the blue phase system. Figure 14.4 shows some exemplary nematic LC compounds, chiral dopants, and monomers [19]. Then we control the temperature within the narrow blue phase range to conduct UV curing. After UV irradiation, monomers are polymerized to form a polymer network, which stabilizes the blue phase lattice stmctures. 

Sato H, Fujikake H, Lino Y, Kawakita M, Kikuchi H (2002) Flexible grayscale ferroelectric liquid crystal device containing polymer walls and networks. Jpn J Appl Phys 41 5302-5306 Sato H, Fujikake H, Kikuchi H, Kurita T (2003) Rollable polymer stabilized ferroelectric liquid crystal device using thin plastic substrates. Opt Rev 10(5) 352-356 Schrader DM, Jean YC (1988) Positron and positronium chemistry. Elsevier, Amsterdam Shinkawa K, Takahashi H, Fume H (2008) Ferroelectric liquid crystal cell with phase separated composite organic film. Ferroelectrics 364 107-112 Simha R, Somcynsky T (1969) On the statistical thermodynamics of spherical and chain molecule fluids. Macromolecules 2 342-350... 

In PSBP system polymer network plays important role in controlling material properties. They concentrate on the core in a random coil conformation in the lattice structure leading to stabilization of the blue phase (Kikuchi et al. 2002). Different monomers used collectively stabilize the blue phase and these are also needed to expand the temperature range. However it should be noted that the polymer obtained through polymerization of the monomer, should not readily mix with the liquid crystal. Otherwise there will be no stabilization effect (Kikuchi et al. 2002). Polymer network also influences electrooptic properties and increase in monomer concentration leads to higher operation voltage and smaller hysteresis. 

The continuous phase can be a solid or have some characteristics of a solid. This implies that the structural elements are immobilized, which considerably enhances physical stability of the system. When making such a dispersion, the continuous phase is always liquid, but it can solidify afterwards, e.g., by lowering the temperature or by evaporating the solvent. The liquid can become crystallized, form a glass (Section 16.1), or turn into a gel. Especially the last named situation is frequently encountered in foods. Also the solvent, generally an aqueous solution, in the continuous phase then is more or less immobilized (Section 5.3). If the gel is a classical polymer gel (Section 17.2.2), the polymer molecules provide a continuous network, but do not make up a continuous phase the polymer strands cannot be seen as a structure in which other molecules can be present and diffuse. 

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