Conducting liquid crystals

Conducting liquid crystals. Several approaches have been tried, more or less successfully, based also on a 1-D stacking. One of the firsts has been to prepare a liquid crystal based on a phthalocyanine as an intrinsic semi-conductor [83]. One other attempt has been to use discotic liquid crystals, based on the triphenylene molecule oxidized by bromine [84] or by aluminium trichloride [8] in the latter, the conductivity changes by six orders of magnitude (from 10" cm- to 10 cm-i), while the mesomorphic character is conserved. 

While metallation-cross-coupling strategies dominate the construction of materials for the investigation of conduction, liquid crystal, transistor, fluorescence, electroluminescence, ion receptor, cluster catalysis, among other properties, the application of the DoM connection has not been widely tested. As evident from the discussion concerned with the synthesis of bioactive molecules and natural products (see above), DoM-cross-coupling synthetic design may lead to the provision of new molecules with unusual, and perhaps useful, physical and chemical properties because of the derived electronic and steric factors. 

King, J. A., Barton, R. L., Hauser, R. A. et al. 2008. Synergistic effects of carbon fillers in electrically and thermally conductive liquid crystal polymer based resins. Polymer Composites 29 421-428. 

The other two instabilities shown in Fig. 28 may be observed only in liquid crystals (nematic, cholesteric, and smectic C). The first is the Carr-Helfrich instability, which is caused by a low-frequency electric field and occurs in the form of elongated vortices with their axis perpendicular to the original director alignment. The vortices cause a distortion of the director orientation, which is observed optically as a one-dimensional periodic pattern (Kapustin-Williams domains). The other anisotropic mode is observed only in highly conductive liquid crystals. For its interpretation the inertial term dvidt for the fluid velocity must be taken into account, which is why this mode may be called inertial mode. 

Fig. 3.2 (a) Schematic of the cells used for the chemical doping studies of conducting liquid crystals (b) the frequency dependence of the conductivity of HAT6 doped with 0.5 mol% of aluminium trichloride in the Coli, phase (parallel and perpendicular to the axis of the column)... 

Within the family of organic electronic materials, liquid crystals are relative newcomers. The first electronically conducting liquid crystals were only reported in 1988, but already a substantial literature has developed. The potential advantage... 

A very high, price and performance family of polymers called liquid crystal polymers (LCPs) exhibit extremely high mechanical and thermal properties. As their ease of processing and price improve, they may find appHcation in thin-waH, high strength parts such as nails, bolts, and fasteners where metal parts cannot be used for reasons of conductivity, electromagnetic characteristics, or corrosion. 

However, conductive elastomers have only ca <10 of the conductivity of soHd metals. Also, the contact resistance of elastomers changes with time when they are compressed. Therefore, elastomers are not used where significant currents must be carried or when low or stable resistance is required. Typical apphcations, which require a high density of contacts and easy disassembly for servicing, include connection between Hquid crystal display panels (see Liquid crystals) and between printed circuit boards in watches. Another type of elastomeric contact has a nonconducting silicone mbber core around which is wrapped metalized contacts that are separated from each other by insulating areas (25). A newer material has closely spaced strings of small spherical metal particles in contact, or fine soHd wires, which are oriented in the elastomer so that electrical conduction occurs only in the Z direction (26). 

Because STM measures a quantum-mechanical tunneling current, the tip must be within a few A of a conducting surface. Therefore any surface oxide or other contaminant will complicate operation under ambient conditions. Nevertheless, a great deal of work has been done in air, liquid, or at low temperatures on inert surfaces. Studies of adsorbed molecules on these surfaces (for example, liquid crystals on highly oriented, pyrolytic graphite ) have shown that STM is capable of even atomic resolution on organic materials. 

As with all branches of science, polymer chemistry is continually advancing. New topics in polymer chemistry which involve new concepts, new polymers or novel uses for existing materials are being studied in research laboratories throughout the world. In this chapter, some of the more important of these developments are described including the use of polymers in medicine, electronically conducting polymers, and polymer liquid crystals. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

Liquid crystal display technology, 15 113 Liquid crystalline cellulose, 5 384-386 cellulose esters, 5 418 Liquid crystalline conducting polymers (LCCPs), 7 523-524 Liquid crystalline compounds, 15 118 central linkages found in, 15 103 Liquid crystalline materials, 15 81-120 applications of, 15 113-117 availability and safety of, 15 118 in biological systems, 15 111-113 blue phases of, 15 96 bond orientational order of, 15 85 columnar phase of, 15 96 lyotropic liquid crystals, 15 98-101 orientational distribution function and order parameter of, 15 82-85 polymer liquid crystals, 15 107-111 polymorphism in, 15 101-102 positional distribution function and order parameter of, 15 85 structure-property relations in,... 

As mentioned earlier, much attention was being given to the formation of ion-radical conductors in the appropriate crystalline form. Meanwhile, Ziolkovskiy et al. (2004) reported data on high conductivity at 77-300 K of the methyl-TCNQ anion-radical salts with A-alkylpyridinium cations that keep their conductivity after crystallization from the melted forms. The melting temperatures of the salts described are rather low and the melting proceeds without salt destruction. This feature opens a possibility to create definite, much essential constructive elements directly from the liquid phase. Importantly, these salts also possess affinity to metals due to the metal-nitrogen coordina-tive ability. The authors notice that such ion-radical salts are promising for use in electronics and microelectronics. 

The driving force in polymer synthesis is the search for new polymers with improved properties to replace other materials of construction. Polymers are lightweight and can be processed easily and economically into a wide range of shapes and forms. The major synthetic efforts at present are aimed at polymers with high temperature, liquid crystal, conducting, and nonlinear optical properties [Maier et al., 2001 Sillion, 1999]. There is an interrelationship between these efforts as will become apparent. 

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

---