Network stabilized liquid crystals

Polymer Network Stabilized Liquid Crystal Displays. The alignment of liquid crystals plays an important role in the operation of a display. The alignment is conventionally induced by the display cell surfaces, but by distributing the surface of a polymer network through out the bulk of the liquid crystal, new properties are possible, and the performance of conventional devices can be improved. This short section will mention some of the conventional liquid crystal display devices modified by these polymer networks. 

Polymer Network Stabilized Liquid Crystal Phase... 

Electro-optic materials can be made using liquid crystal polymer combinations. In these applications, termed polymer-stabilized liquid crystals [83,86], the hquid crystal is not removed after polymerization of the monomer and the resulting polymer network stabilizes the liquid crystal orientation. 

A modification of the oriented polymer network systems are polymer stabilized liquid crystals (PSLC) 4) being studied in detail because of their application in flat-panel displays. In these materials, photopolymerizable diacrylate monomers are usually dissolved at a concentration less than 10% in non-reactive low-molar-mass liquid crystal solvents, commonly available, along with a small concentration of photoinitiator. Typically, the addition of small amounts of monomers and photoinitiator reduces the transition temperatures of the pure low-molar-mass liquid crystals slightly, suggesting that the order in the system is not dramatically altered by the addition of monomers or initiator. In application, this solution is aligned in a particular desired state and then photopolymerized. Photopolymerization is preferred to thermal free-radical polymerization, because photopolymerizations are very fast and because the temperature of photopolymerization can be controlled more easily to optimize processing of the display. 

Figure 9 Schematic representation of the electro-optic properties of a reverse mode polymer stabilized liquid crystal as a function of the morphology of the polymer network.

Electro-Optic Properties of Polymer Stabilized Liquid Crystals. Polymer networks have been used to stabilize many of the liquid crystal display states in various types of displays quite advantageously. In this section, we present some recent work on correlating the material properties of the liquid crystal/polymer network composite to the electro-optic properties of the flat-panel displays specifically cholesteric texture displays (75) and simple nematic birefringent type displays (7(5). 

The direct correlation between the morphology of a polymer network and the observed electro-optic properties of these reverse-mode PSLC s is noteworthy. Desirable electro-optic response fi om these polymer- stabilized liquid crystals can be obtained by manipulating the structure of the polymer network inside the cells. 

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... 

The synthesis, characterization and mesomorphic behavior of diacrylates based on polymerized liquid crystals are reported. Several types of polymer stabilized liquid crystal display devices were prepared from the dispersions of low concentration of diacrylates in liquid crystals and subsequently the prepolymers were polymerizaton by ultraviolet radation toorm polymer networks. The morphology stupes show that the orientation of polymer networks induced by the surface treatment of the substrate has led to the preferential liquid crystal alignment... 

Several reactive monomers have been designed, synthesized and characterized for the polymer stabilized liquid crystals. The orientation of anisotropic polymer networks formed in different types of liquid crystals have been studied... 

Liquid crystals stabilize in several ways. The lamellar stmcture leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the Hquid crystalline phase, coalescence takes place over a thin Hquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the Hquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the Hquid crystal is highly viscous, and two droplets separated by a viscous film of Hquid crystal with only a small compressive force exhibit stabiHty against coalescence. Finally, the network of Hquid crystalline leaflets (30) hinders the free mobiHty of the emulsion droplets. 

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. 

Another consequence of the addition of fatty alcohols to cationic surfactants is the formation, under the right conditions, of liquid crystal and gel networks [41-45] that can greatly increase viscosity and confer stability upon the emulsion. Formation of such liquid crystals has been observed even at low concentrations [44,45] the ready formation of these structures, along with low cost, improved stability, and compatibility with cosmetic ingredients are important reasons why long-chain alcohols are so ubiquitous in conditioning formulations. 

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