Electrical properties, thermotropic

Electrical properties have been reported on numerous carbon fiber-reinforced polymers, including carbon nanoflber-modified thermotropic liquid crystalline polymers [53], low-density polyethylene [54], ethylene vinyl acetate [55], wire coating varnishes [56], polydimethyl siloxane polypyrrole composites [50], polyacrylonitrile [59], polycarbonate [58], polyacrylonitrile-polycarbonate composites [58], modified chrome polymers [59], lithium trifluoromethane sulfonamide-doped polystyrene-block copolymer [60], boron-containing polyvinyl alcohols [71], lanthanum tetrafluoride complexed ethylene oxide [151, 72, 73], polycarbonate-acrylonitrile diene [44], polyethylene deoxythiophe-nel, blends of polystyrene sulfonate, polyvinyl chloride and polyethylene oxide [43], poly-pyrrole [61], polypyrrole-polypropylene-montmorillonite composites [62], polydimethyl siloxane-polypyrrole composites [63], polyaniline [46], epoxy resin-polyaniline dodecyl benzene sulfonic acid blends [64], and polyaniline-polyamide 6 composites [49]. 

Unique materials in themselves, thermotropic LCPs are opening up many non-traditional design and functional possibilities. As will be discussed in more detail, the trend towards miniaturization in electrical/ electronic components places additional demands on the polymer s processability into complex, thin-walled forms, and is opening up a wide variety of applications. Coupled with good electrical properties and flame retardance, tailorable CTEs and excellent dimensional stabilities, thermotropic LCPs are being used in a diversity of final product shapes. 

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. 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

Aromatic liquid crystalline (LC) copolyesters of commercial importance are made by processes which produce compositionally heterogeneous chemical sequences along the polymer chain. Such chemical heterogeneity is responsible for a number of interesting properties of thermotropic LC polymers. These properties include a small amount of 3Current address General Electric Plastics, Mount Vernon, IN 47620... 

The most uncommon molecular structure known so far to produce lyotropic mesophases is that of the chiral, dinuclear chromium(II) complex of phenanthroline and tartaric acid, (323). The complex does not possess an amphiphilic structure, i.e., no polar headgroup nor aliphatic tail, does not belong to the columnar type, and does not show thermotropic properties. Thus, the phenanthroline ligands must play the role of the hydrophobic part, while the central tartrate link must act as the hydrophilic one. A room-temperature, lyotropic mesophase was induced at low concentration in water (0.006 mol dm ). Moreover, in the concentration range of 0.06 to 0.1 mol dm the mesophase was still observed and remained stable up to 100 °C. The self-aggregation of the complex into a birefringent mesophase was proved by the use of several techniques including polarized optical microscopy, viscosity, and electrical conductivity measurements, as well as Na... 

Polymer/surfactant complexes may exhibit thermotropic mesomorphism as well as the lyotropic one discussed above. Among conducting polymers, PANi has been of considerable scientific interest because of its unique electrical and optical properties and environmental stability. Besides the covalent alkyl substitution di.scussed above, a significant approach has been to use functionalized acids, which usually serve as surfactants. Dodedcylbenzenesulfonic acid (DBSA) has been widely used for protonation of PANi and found to improve the solubility and processibility of the conducting polymer. 

The industrial challenge ahead will be to define and develop applications which take advantage of the unique properties of aromatic thermotropic polyesters. Already, commercial demand is growing in the fiber optic, chemical process, electrical/electronic, automotive and houseware markets, and new areas of potential application continue to emerge. 

Up to now the most extensively investigated chiral lyotropics are the chiral nematics (the historical but not appropriate term cholesteric is avoided in this chapter). The first evidence on this kind of mesophase has been reported by Radley and Saupe in 1978 [9]. Two studies on a lyotropic phase proposed to be analogous to the thermotropic Blue Phase have been published [10], one has been retracted [11] if there is any analogy, it is small. Blinov et al. reported on a nonaqueous chiral lamellar phase with ferroelectric properties experimental evidence for ferroelectricity in lyotropics is difficult to gain because of high electric conductivity and mostly nonuniform sample orientation. Nevertheless, the existence of piezoelectricity was interpreted as a manifestation of ferroelectricity [12]. 

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