Conduction anisotropy

Mathis, N.E., Measurement of Thermal Conductivity Anisotropy in Polymer Materials, PhD thesis, Chemical Engineering Department, University of New Brunswick, Fredericton, N.B., Canada, 1996. 

Although the dominant means to create a space-charge field within the interference pattern in liquid crystals is given by Eq. (2), it has been shown that there are other mechanisms to create a space-charge field. One is derived from the conductivity anisotropy and is known as the Carr-Helfrich effect [43,49] ... 

The electrical conductance shows a weaker concentration dependence above than below the CMC corresponding to a decrease in the equivalent conductance (Fig. 2.10). The transport number of the surfactant ion rises sharply at the CMC while that of the counterion may become negative. This as well as electrophoretic mobilities may yield information on micellar charge. At high concentrations, conductance anisotropies have been observed for flowing systems. This, as well as flow birefringence, is useful for the demonstration of nonspherical micelle shape. 

Rheological properties are particularly sensitive and for some solubilizates, such as a-methylnaphthalene, the solutions may become viscoelastic the appearence of viscoelasticity often depends on subtle effects in chemical structure29). Certain spectroscopic features are strongly influenced. Thus the H and 13C NMR line widths show large increases, the 81 Br quadrupole relaxation may be strongly affected and there may be the appearance of linear dichroism, birefringence and conductance anisotropy for flowing systems. 

The aim of this article is to show that the new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 [hereafter called (BEDO)CsHg] (BEDO-TTF - bis-(ethylenedioxy)tetrathiafulvalene) which contains closed and open orbits displays rather complicated oscillatory spectra associated with magnetic breakdown (MB) and quantum interference (QI) effects. Tight binding band structure calculations for this compound are proposed to characterise its Fermi surface. The aim of the article includes also an investigation of the optical conductivity anisotropy with polarized infrared reflectance spectra. 

Figure 12 (a) Temperature dependence of the longitudinal resistivity of (TMTSF)2PF6 obtained with the clamped contact technique. (From W. Kang, private communication, and O. Traetteberg, Thesis, Univ. Orsay, 1993.) (b) The conductivity anisotropy is temperature independent as long as the transverse motion remains incoherent. Data for (TMTSF)2PF6 under 12 kbar. (After Ref. 6.)... 

The method has also been applied to PAni [53]. The plasticizer is a solvent of the polymer N-methylpyrrolidone (NMP), and the film to be stretched is not, or is very poorly, crystalline and may contain up to 20 wt% NMP. Tg decreases as the NMP content increases, down to 110°C heating and stretching swollen EB films above that Tg value produces X > 6, increased crystallinity (Fig. 11), and sizable orientation and anisotropic properties [27,99,100]. Such films, doped with HC1, show a conductivity anisotropy = 25 [99-101]. Contrary to what is observed with PPV, the mechanical properties of stretched PAni remain modest the Young s modulus is below 0.5 GPa [53]. 

Fig. 22 Schematic self-assembled transverse-witlzin-parallel aligned structure of PS-6-P4VP(TSA)i.o(PDP)i.o supramolecules and the direct-current conductivity as a fimction of temperature. Presented also is the isotropic conductivity of a nonaligned sample (4). In the aligned case (3) the conductivity is increased, probably due to fewer domain boimd-aries, and the nanoscale conductivity anisotropy is manifestly present [155]...

Single crystals are the most valuable subjects for investigating the bulk properties of materials. In particular, they clearly demonstrate the conduction anisotropy (if any), and are free from grain-boundary effects. For technical utilization, however, they are usually impractical due to their unsuitable shape and size, and high cost of production. 

The VTF dependence was strongly suggestive of coupling between high-am-plitude segmental motion and long-range cation transport in these composites. The conductivity anisotropy arose from the variation in the a0 term. 

IV. FLOW-INDUCED CONDUCTANCE ANISOTROPY IN POLYELECTROLYTE SOLUTIONS... 

A measure of the intrinsic conductivity anisotropy is obtained from the... 

Polymeric sulfur nitride is a highly anisotropic crystalline solid in which the measured conductivity anisotropy is of... 

The most useful experimental configuration for measuring the conductivity anisotropy was with the current flow perpendicular to the b-axis. The transverse magnetoresistance could then be measured for H//b and Hib where the greatest anisotropy is expected. The angular dependence of the observed transverse magnetoresistance follows a... 

Experiments and theoretical considerations have shown that the key parameters are the symmetry of the system (planar or homeotropic boundary conditions), the dielectric and the conductivity anisotropies. It is therefore convenient to categorize the various combinations of parameters as listed in Table 1. In the last column the structures predicted and/or observed are summarized and will be discussed below systematically. 

Case A planar alignment, Ca <0 0. This is the most studied, classical case, since the conductivity anisotropy of usual nematics (substances without a smectic phase) is typically positive. As for , there is a wide range of materials with negative dielectric anisotropy. 

The important role played by the electrical conductivity anisotropy a a is evident if Ga decreases, U q,f) increases and diverges at a small positive value of a a when the two terms in the denominator of Eq.(7) cancel each other. If a a approaches zero or becomes negative (case F), the CH term vanishes or acts as a stabihzing term, respectively thus convection is not expected. The role of a is somewhat different an EC transition exists for vanishing and even for positive (case E). 

Fig. 3.10.7. Charge segregation in an applied field caused by a bend fluctuation in a nematic of positive conductivity anisotropy. The resulting transverse field is E. ...

J. C. Hutchison, R. Bissessur, and D. F. Shriver, Conductivity anisotropy of polyphosphazene-montmorillonite composite electrolytes, Chem. Mater., 8, 1597-1599 (1996). 

Some of the early Hopkins conductivity data for the series TMTTF-TCNQ, TTF-TCNQ (for whom the conductivity anisotropy is 500 5 <1), TMTSF-TCNQ, and HMTSF-TCNQ are shown in Fig. 10 [25, 26, 112-115]. From other (mainly magnetic) data it is surmised that... 

IBr2, Aul2 etc.), the structure approximates much more closely to two-dimensional, since the anions are segregated quite clearly into layers, separated by blocks of the organic material. The conductivity anisotropy, parallel and perpendicular to these blocks,is of the order of 500. 

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