Morphology and Dynamic Interaction of Defects in Polymer Liquid Crystals
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MRS BULLETIN/SEPTEMBER 1995
exhibited over a range of temperatures, or lyotropic, where nonmesogenic solvent molecules are present in addition to the mesogens, and liquid crystallinity is observed over a range of concentrations as well. Solvents are often added to facilitate polymer processing, and thus, ly-
Figure 1. Mesogenic units are covalently stitched together as part of the main chain (a) or as side groups on a polymer backbone (b).23
Figure 2. A semiflexible, mainchain, liquid crystal polyether that exhibits a melting temperature of 148°C and a nematic to isotropic transition temperature of 183°C. This crystallizable polymer has a glass-transition temperature of 70°C, and thus, its director field may be lamellar decorated.
otropic liquid crystal polymers are more common than their small molecule counterparts. The longchain nature of polymer liquid crystals introduces new and interesting liquid crystal properties and textural defects. For an LCP, one must consider the polydispersity and rigidity of the entire molecule, in addition to the properties of the mesogen. The influence of molecular weight on the physical behavior is unique to polymers and affords numerous property and processing advantages since it is possible, for example, to shape sidechain LCPs via thermoprocessing and then to quench the polymer backbone below its glass-transition temperature to freeze in a desired superstructure while maintaining the fast response of the still highly mobile mesogenic sidechain units to optical switching by electric fields. Liquid crystals are commonly used in display devices. Polymer liquid crystals are also useful as high performance polymer composites, particularly hightemperature composites, and as highstrength, high modulus fibers, for example, DuPont's Kevlar. The elevated transition temperatures and outstanding mechanical properties of LCPs allow for additional applications for which small molecule liquid crystals are not appropriate. Another example unique to polymer liquid crystals is the recently developed liquid crystal elastomers, which exhibit reversible, very large strain, mechano-optical properties.3 As with all other types of materials, a better understanding of polymer liquid crystals may be obtained through a better understanding of the type and nature of their defects. Moreover, as is often the case, certain applications call for the complete elimination of all defects: For low-molecular-weight liquid crystals, the current 10 in. X 10 in. flat panel displays must be defect free over an enormous area and must remain defect free during their switching operations. As liquid crystal displays grow ever larger, understanding and control of defects will be of critical importance. The patterns of the director field provide information on the physics of the defects. As is usual in materials science, a classification scheme of 0, 1, and 2 dimensionality is adopted for defects. Defects in polymer liquid crystals are often more stable against external forces than are defects in small molecule liquid cr
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