Directional property oriented

The representation of molecular properties on molecular surfaces is only possible with values based on scalar fields. If vector fields, such as the electric fields of molecules, or potential directions of hydrogen bridge bonding, need to be visualized, other methods of representation must be applied. Generally, directed properties are displayed by spatially oriented cones or by field lines. 

Then, in the early 1980s the concept of OSB was realized in the constmction and operation of large-size mills. OSB is a panel product made from wood strands and somewhat like plywood in that the strands on the two faces are oriented in the long direction of the panel and the core strands are oriented in the cross-panel direction. The use of orientation yields panels having exceUent directional properties, much like plywood, and thus an exceUent and economical stmctural sheathing material is created. 

Extmsion of polyethylene and some polypropylenes is usually through a circular die into a tubular form, which is cut and collapsed into flat film. Extmsion through a linear slot onto chilled rollers is called casting and is often used for polypropylene, polyester, and other resins. Cast, as well as some blown, films may be further heated and stretched in the machine or in transverse directions to orient the polymer within the film and improve physical properties such as tensile strength, stiffness, and low temperature resistance. 

The grain-oriented steels, containing ca 3.25% siUcon, are used in the highest efficiency distribution and power transformers and in large turbine generators. They are processed in a proprietary way and have directional properties related to orientation of the large crystals in a preferred direction. 

Orientation sometimes leads to lower permeabiHty values (better barrier properties). Orientation can iacrease packing density, which lowers the diffusion coefficient D it can also iacrease the difficulty of hopping or diffusiag ia a direction perpendicular to the film. In the latter case, movement ia general may be fast, but movement through the film is limited. However, mere stretching does not always lead to orientation of the molecular chains. In fact, stretching can lead to void formation, which iacreases permeabiHty. 

The concepts of directed valence and orbital hybridization were developed by Linus Pauling soon after the description of the hydrogen molecule by the valence bond theory. These concepts were applied to an issue of specific concern to organic chemistry, the tetrahedral orientation of the bonds to tetracoordinate carbon. Pauling reasoned that because covalent bonds require mutual overlap of orbitals, stronger bonds would result from better overlap. Orbitals that possess directional properties, such as p orbitals, should therefore be more effective than spherically symmetric 5 orbitals. 

The reinforcement type and form chosen (woven, braided, chopped, etc.) will depend on the performance requirements and the method of processing the RP (Fig. 6-15). Fibers can be oriented in many different patterns to provide the directional properties desired. Depending on their packing arrangement, different reinforcement-to-plastic ratios are obtained (Appendix A. PLASTICS TOOLBOX). 

Another important orienting fabricating procedure concerns applying directional properties to reinforced plastics. This subject is reviewed in Chapter 3, DESIGN CONCEPT, Reinforced Plastic Directional Property. 

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... 

Fig. 8-55 Overview of RPs directional properties (a) polar directional, (b) different fiber orientations and tensile fracture characteristics, and (c) stress vs. strain diagrams of RPs.

Most tests will be made on standard test pieces which may be pieces cut from a component or a sheet, or they may have been moulded separately from the same material. Where test pieces or sheet are produced for the trials it is important that they are produced in as near as possible the same way as the product and that the processing conditions are recorded. Different results can be expected from compression and injection moulding or from extrusion (where a choice is possible). Directional properties can result from the conditions of flowing and cooling in a mould. For example, in a study at ERA, the creep strain of unfilled HDPE, either individually moulded or cut from square plaques, varies by up to a factor of two depending on the orientation of flow [40]. This difference becomes even more marked with short fibre reinforcement. 

For those complexes where the chromophore is not coordinated to the metal center directly, the orientation of the chromophore is important to ensure efficient energy transfer. The series of ligands L29-L32 were investigated for correlations between structural parameters found in the solid state (see, for example, Fig. 12) and solution (by NMR spectroscopy) and photophysical properties (69,70). It was found that both chromophore-metal separation and the angle of orientation have a direct influence on the quantum yield of the europium complexes. For example, the difference in quantum yield between [Eu(L29)]3+ and [Eu(L30)]3+ (0.06 and 0.02, respectively) cannot be attributed solely to the chromophore-metal separation, so may also depend on the better orientation of the chromophore in the L29 system as measured by the angle a between the metal center, the amide nitrogen atom, and the center of the phenyl ring. 

Because of such orientation the products may be anisotropic in behaviour, with mechanical properties differing if measured in different directions (tensile strength will be higher in the direction of orientation, and impact strength also will be affected—fracture can take place more easily parallel to the direction of orientation). 

Specihc crystallographic orientations and symmetry-dependent properties may also be exploited in synthetic schemes in which solids are used as starting materials. Properties and phenomena are facilitated along, or may even be restricted to, specihc crystallographic directions or orientations. Examples of anisotropy so utilized include ... 

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