Directional property isotropic

The wide choice available in plastics makes it necessary to select not only between TPs, TSs, reinforced plastics (RPs), and elastomers, but also between individual materials within each family of plastic types (Chapters 6 and 7). This selection requires having data suitable for making comparisons which, apart from the availability of data, depends on defining and recognizing the relevant plastics behavior characteristics. There can be, for instance, isotropic (homogeneous) plastics and plastics that can have different directional properties that run from the isotropic to anisotropic. Here, as an example, certain... 

Orientation of reinforcement The behavior of RPs is dominated by the arrangement and the interaction of the stiff, strong fibers with the less stiff, weaker plastic matrix. The features of the structure and the construction determine the behavior of RPs that is important to the designer. A major advantage is the fact that directional properties can be maximized in the plane of the sheet. As shown in Fig. 8-55 they can be isotropic, orthotropic, etc. Basic design theories of combining actions of plastics and reinforcements... 

In solids, the chemical shift is a directional property and varies with the crystal s orientation in the magnetic field. This orientation dependence, the chemical shift anisotropy (CSA), can be described by a symmetric second-rank tensor that can be diagonalized and reduced to three principal values 5n, 822,833, where the isotropic chemical shift, 8i, is the average of these values 8i = l/3-(8n + >22 + 833). For a single crystal, or any particular nucleus in a powdered sample, the frequency varies according to ... 

In addition to its variation with wavelength, the monochromatic emissivity of many surfaces is not isotropic and has directional properties. Experimental data on these properties, however, are scarce. Frequently used mean values are 6/e = 1.2 for polished metallic surfaces and e/e = 0.96 for insulators. Here e denotes the average hemispherical emissivity and en the emissivity normal to the surface. 

Basically, birefringence is the contribution to the total birefringence of two-phase materials, due to deformation of the electric field associated with a propagating ray of light at anisotropically shaped phase boundaries. The effect may also occur with isotropic particles in an isotropic medium if they dispersed with a preferred orientation. The magnitude of the effect depends on the refractive index difference between the two phases and the shape of the dispersed particles. In thermoplastic systems the two phases may be crystalline and amorphous regions, plastic matrix and microvoids, or plastic and filler. See amorphous plastic coefficient of optical stress compact disc crystalline plastic directional property, anisotropic ... 

An isotropic material has the same properties in all directions. Properties such as refractive index and Young s modulus are independent of direction, and if we wish to refer the properties to a set of rectangular cartesian co-ordinates, we can rotate the axes to be in any orientation without any preferoice. For an anisotropic material, where the properties differ with direction, it is usually convenient to choose coordinate systems which coincide with axes of S3rmmetry if this is possible. The material is then described by its properties referred to these principal directions, which affords considerable simplification. 

Polymer composites are plastics within which fibres are embedded. The plastic is known as the matrix (resin) and the fibres dispersed witbin it are known as the reinforcement Thermosetting matrix materials include polyester, vinyl ester and epoxy resins. For higher temperature and extreme environments, bismaleimlde, polyimide and phenolic resins are used. Composites can be used to replace metal parts but care must be taken during design. Most engineering materials have similar properties in any direction (called isotropic) where composites have not This can however be offset by arranging the reinforcement layers in varying directions. 

Reaction gas-phase collisions should generate Newton spheres with isotropic distributions. However, very often these surface patterns are anisotropic (as the one shown in the figure) due to the existence of some directionality in the process. In photodissociation, this is often due to the use of a linearly polarized laser, which acts as the reference axis. In a bimolecular collision (see below), carried out in a crossed-beam experiment, the relative velocity vector introduces a reference axis to which the directional properties of the reaction products are referenced. 

Certain pitches can be spun directly into isotropic pitch fibers with only minor devolitization. Carbonized isotropic pitch fibers cannot be graphitized and develop mechanical and thermal properties that are substantially inferior to those produced by other precursors. However, isotropic pitch fibers are very inexpensive and have found commercial applications in areas that do not require the exceptional mechanical and thermal properties of mesophase pitch-based carbon fibers. Isotropic pitch-based carbon fibers are used in filters, brake pads, activated carbons, and as substrates for chemical vapor deposition (23). Table 3 summarizes properties of isotropic pitch-based carbon fibers. 

In chemical crystallography, these two antonymic terms refer to the directional dependence of a physical property. Isotropic properties or features are directionally independent, while anisotropic properties or features vary according to direction. Properties to which these terms can apply include dimensions, thermal expansion, flexibility, transmission of light (see birefringence and pleochromism), tensile strength, second harmonic generation, and so on. 

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