Directionally oriented structure

Kawabata S, Warp-knitted direct oriented structures for pre-shaped composites, J Text Inst, 81, 432-447, 1990 Also in Andersson CH, Eng K, Zah W, Stahl J-E, Proceedings, TexComp-2, Leuven, May 17-19, 1994. 

A further increase in extension leads to irreversible changes which immediately precede the transition of the polymer into the oriented state. During this transition, the spherulites undergo considerable structural changes and are thus converted qualitatively into different structural elements i.e. macrofibrils4). After a certain critical elongation has been attained, the initial crystallites collapse and melt and a new oriented structure is formed in which the c axes of crystals are oriented in the direction of extension. 

Fairly recently, another method for obtaining polymer materials with uniaxial orientation has been developed. It is the directed polymerization i.e. the synthesis of polymers under conditions at which the material attains instanteneously the oriented structure. The formation of crystals from the macromolecules in an extended conformation occurs in those polymerizing systems simultaneously with polymerization22. ... 

It is possible, therefore, to use the GL(r), normally for L = 2 and possibly 4, to explore the extent of the local structure determined by the direct orientational correlations. An alternative and more detailed way in which to investigate the... 

The rate of the active transport of sodium ion across frog skin depends both on the electrochemical potential difference between the two sides of this complex membrane (or, more exactly, membrane system) and also on the affinity of the chemical reaction occurring in the membrane. This combination of material flux, a vector, and chemical flux (see Eq. 2.3.26), which is scalar in nature, is possible according to the Curie principle only when the medium in which the chemical reaction occurs is not homogeneous but anisotropic (i.e. has an oriented structure in the direction perpendicular to the surface of the membrane or, as is sometimes stated, has a vectorial character). 

Analyzing orientational structures of adsorbates, assume that the molecular centers of mass are rigidly fixed by an adsorption potential to form a two-dimensional lattice, molecular orientations being either unrestricted (in the limit of a weak angular dependence of the adsorption potential) or reduced to several symmetric (equivalent) directions in the absence of lateral interactions. In turn, lateral interactions should be substantially anisotropic. 

An inference of fundamental importance follows from Eqs. (2.3.9) and (2.3.11) When long axes of nonpolar molecules deviate from the surface-normal direction slightly enough, their azimuthal orientational behavior is accounted for by much the same Hamiltonian as that for a two-dimensional dipole system. Indeed, at sin<9 1 the main nonlocal contribution to Eq. (2.3.9) is provided by a term quadratic in which contains the interaction tensor V 2 (r) of much the same structure as dipole-dipole interaction tensor 2B3 > 0, B4 < 0, only differing in values 2B3 and B4. For dipole-dipole interactions, 2B3 = D = flic (p is the dipole moment) and B4 = -3D, whereas, e.g., purely quadrupole-quadrupole interactions are characterized by 2B3 = 3U, B4 = - SU (see Table 2.2). Evidently, it is for this reason that the dipole model applied to the system CO/NaCl(100), with rather small values 0(6 25°), provided an adequate picture for the ground-state orientational structure.81 A contradiction arose only in the estimation of the temperature Tc of the observed orientational phase transition For the experimental value Tc = 25 K to be reproduced, the dipole moment should have been set n = 1.3D, which is ten times as large as the corresponding value n in a gas phase. Section 2.4 will be devoted to a detailed consideration of orientational states and excitation spectra of a model system on a square lattice described by relations (2.3.9)-(2.3.11). 

Fig. 2.17. Azimuthal angles p, (a) and energies (b) for orientational structures of adsorbed molecules on a square lattice with quadrupole-quadrupole interactions plotted versus the inclination angle 6 reckoned from the surface normal direction. Sublattices j of the structures described are labelled by the numbers 1,2,3, and the boundaries of the orientational phases under consideration are designated by the letters A, B, C, D, E, F, G. Regular and bold lines refer to the limiting cases AUt = 0 and Al/4 kBT(AU4 < 0).

These investigations agree well with experiments by Trillat [129] who found that the X-ray diagram of colloidal nitrocellulose showing a non-oriented structure of crystallites (interference rings), altered after being stretched to indicate a fibrous structure in which the crystallites were oriented along the axis in the direction in which the film was stretched. 

The most striking conformational variant observed for a DNA double helix with Watson-Crick base pairing is referred to as the Z form. In Z DNA the backbone is twisted in the left-handed (counterclockwise) direction. This structure was first detected by Alex Rich and his co-workers (fig. 25.8). The Z form is a considerably slimmer helix than the B form and contains 12 bp/turn rather than 10. In the Z form, the planes of the base pairs are rotated approximately 180° with respect to the helix axis from their orientation in the B form (fig. 25.9). 

Coiled coils are bundles of a-helices that are wound into superhelical structures (Fig. 1). Most commonly, they consist of two, three, or four helices, running in the same (parallel) or in opposite (antiparallel) directions, but structures with five and more helices have been determined. They are usually oligomers either of the same (homo) or of different chains (hetero), but on occasion consist of consecutive helices from the same polypeptide chain, which in that case almost always have an antiparallel orientation. 

The forces at the surface of a solid will depend on the number of atoms per unit area in the surface layer, on the arrangement of the atoms with respect to each other in the surface layer, and on the number and arrangement of atoms in at least two layers below the surface. The term arrangement includes both the distance between the atoms and the directional orientation of the position of one atom with respect to that of another. Also, the number of steps and kinks in a surface is determined by its crystallographic orientation. All these structural factors are included in the use of the term crystal face, and this term will be generally used in this sense in this paper. In addition, other factors, such as edges between faces and imperfections of various kinds, must also be taken into account in considering the activity of a surface. 

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