Anisotropic properties Orientation

Molecules of nematic Hquid crystals also are aligned in flow fields which results in a viscosity that is lower than that of the isotropic Hquid the rod-shaped molecules easily stream past one another when oriented. Flow may be impeded if an electric or magnetic field is appHed to counter the flow orientation the viscosity then becomes an anisotropic property. 

Flory, in 1956, predicted that solutions of rodlike polymers could also exhibit LC behavior. The initial synthetic polymers found to exhibit LC behavior were concentrated solutions of poly(gamma-benzyl glutamate) and poly(gamma-methyl glutamate). These polymers exist in a helical form that can be oriented in one direction into ordered groupings, giving materials with anisotropic properties. 

Various other instances of hydrodynamic and electrohydrodynamic instabilities in nematic and, to a lesser extent, smectic liquid crystals have been investigated. No attempt is made here to review this work. For the present discussion, it is sufficient to note that (a) most of the work has dealt with oriented layers having anisotropic properties, and (b) some interesting instabilities arise in oriented layers which do not occur for isotropic materials. An example of the latter is cellular convection in a fluid layer confined between horizontal plates maintained at different temperatures. With an isotropic fluid, convection can arise only if the lower plate is hotter than the upper plate. Then, fluid near the lower plate is less dense and tends to rise while fluid near the upper plate is denser and tends to sink. With an oriented layer, however, convection can arise even when the upper plate is hotter if the anisotropy of thermal conduction properties is of a particular type (8). 

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]. 

The fibres generally used for polymer reinforcement are glass, carbon and less often PTFE or metals. For use in thermosets, glass or carbon fibre may be used in the form of a pressed mat, or bundles of fibres may be laid in a preferred orientation. This enables anisotropic properties to be obtained where this is beneficial. The alternative is to disperse chopped fibres randomly in the polymer matrix. This gives... 

From the simulations, we conclude that two hydrogen bonding force constants are a basic requirement for reproducing the measured spectrum. If a water-water potential generates sufficiently large force constant differences for the different proton configurations (or the different relative dipole-dipole orientations in water or ice), it should produce the same effect as seen in the LR model. The anisotropic properties of the classic potentials are a result of charge interaction and this anisotropy should increase in the polarisable potentials and hence they produce a broad optic peak. This broad peak indicates that the orientational variation of the potential function has been increased considerably but it may still be less than the critical value of 1.5 as we indicated in the section 6.1. One would, therefore, expect that a better polarisable potential would, eventually, be able to reproduce the split optic peaks in the measured INS spectrum. 

First we review some typical materials parameters obtained from measurements on randomly oriented ceramics (6-7). Since the YBaCuO structure (5) and electronic properties 81 are highly anisotropic, the orientationally-averaged values obtained from studies of ceramics are only an initial indication until more complete experimental results on single crystals and oriented films and ceramics become available. For material with a resistivity just above the transition of 400 /xficm, a Hall carrier density of 4xl021cm , and dHc2/dT of 2 T/K (6-7). one deduces a BCS coherence length (0) of 1-4 nm, a London penetration depth A(0) of 200 nm, a mean free path t of 1.2 nm, a thermodynamic critical field Hc(0) of 1 T (10000 Oe) and an upper critical... 

Define a locally anisotropic property as a property whose value is highly sensitive to whether each atom or bond is in the chain backbone or in a side group. By contrast, while the value of a globally anisotropic property may be very sensitive to the overall orientation of the polymer chains, it is somewhat less sensitive to the precise location of any given atom or bond in the structure. For example, the refractive index is a globally anisotropic property. The stress-optic coefficient, whose value depends both on the refractive index and on the difference in the polarizability of a polymer chain segment parallel and perpendicular to the chain, is a locally... 

Anisotropic interactions, orientational dependence, 26f Anisotropic rotation, polyformal spin relaxation, local motion, 70,78t Anisotropy, chemical shift (CSA), solid sample NMR, 22 Antiphase components, undesirable INEPT properties, 106 Areas, integrated, quantitative NMR studies, 1371... 

Molded samples of LCP s often form a "skin-core" structure. The phenomenon is depicted by a significant dependence of the anisotropic properties of molded parts on part thickness ( 5.). Scanning electron photomicrographs of the cross-section of a molded part reveal a highly oriented "skin" layer surrounding a less ordered inner "core." Apparently, the fraction of disordered core material diminishes as the sample thickness decreases. 

For valence band excitations, however, a different method has to be used to study the anisotropic dielectric properties due to the smaller q ( rv- 0.02 A 1 for 20 eV energy loss with 100 kV incoming beam energy), which means that the momentum transfer in the low loss region is mostly due to q. This requires the sample to be oriented in a certain direction relative to the direction of the electron beam to study the dielectric response in a particular orientation. Because of the small beam size available in cFEG TEM, a specific orientation of a thin section with millimeter size area can be easily obtained by ion beam thinning techniques to directly measure the anisotropic properties of the material. 

Crosslinkable oxadiazole monomers with pending acrylic units have been described. The s)mthesis is shown in Figure 10.2. The molecules can be oriented and this orientation can be frozen by a polymerization or crosslinking process, yielding a material with anisotropic properties. 

If the medium is isotropic, is a scalar, i.e. the relationship between E and Plinear IS independent of the direction of the field vector E and the polarization is parallel to E. Many polymers possess amorphous structures and their optical properties are isotropic. However, electro-optic polymeric systems containing polar moieties can be made anisotropic by orienting these moieties, for example, by electric field-induced or optical alignment. In this case, the polarization is not necessarily parallel to the direction of E and its component in one direction is related to the field components in all three directions ... 

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