Orientational enhancement

A more flexible option from an operational viewpoint is the implementation of process-oriented enhancements that intensify particle separation. This can be achieved by two different methods. In the first method, the suspension to be separated is pretreated to obtain a cake with minimal resistance. This involves the addition of filter aids, flocculants or electrolytes to the suspension. In the second method, the period during which suspensions are formed provides the opportunity to alter suspension properties or conditions that are more favorable to... 

For fibers or tubular modules, the feed is generally introduced into the inside of the tubes, or lumen, while permeate is withdrawn from the shell side. This flow orientation enhances the shear at the membrane surface for TFF operation. These modules may also be run at high conversion or NFF mode with the feed introduced on the outside of the tubes or shell side. In this case, the shell side offers greater surface area. 

Figure 3 Refractive index change associated with the orientational enhancement effect. The arrows represent the direction of the total field Ej which is the sum of the applied and space-charge fields.

The activities of many promoters in higher eukaryotes are greatly increased by another type of cis-acting element called an enhancer. Enhancers sequences have no promoter activity of their own yet can exert their stimulatory actions over distances of several thousand base pairs. They can be upstream, downstream, or even in the midst of a transcribed gene. Moreover, enhancers are effective when present on either DNA strand (equivalently, in either orientation). Enhancers in yeast are known as upstream activator sequences (UASs). 

The photorefractive performance of high-molecular-weight PVK-based composites can be enhanced by optimizing the composition of the PVK composites [73]. 2,4,7-Trinitrofluorenone is used as sensitizer material. The improved performance arises due to a large orientational enhancement effect with faster response speed in addition to samples with an appropriate glass transihon temperature. 

A fibrillar polymer-polymer composite consists of an isotropic matrix polymer with fibrils of a second polymer dispersed within it. The idea was developed by Fakirov et al. [40] with the knowledge that drawing of polymers with good molecular orientation enhances their mechanical properties. Depending on the fibril diameters, such composites are referred to as microfibrillar composites (MFCs) or nanofihrillar composites (MFCs). For simplicity, MFCs are discussed because the manufacturing process is essentially the same. One method of creating MFCs is to produce a blend of the two selected polymers in the form of a continuous wire. 

One disadvantage of SPPF is the level of residual stress and orientation remaining in the final product. This arises from stretching the sheet prior to its softening point. The presence of greater orientation enhances the clarity and stiffness of the formed article. One undesirable consequence of this, however, is the unsuitability of SPPF in the production of containers for high temperature applications. If used at elevated temperatures, e.g. in microwave oven trays, the frozen-in orientation and stresses have an opportimity to relax, causing distortion of the container. 

Because of these advantages as well as orientational enhancement effects and the superior compositional flexibility of pol5nners, the number of studies of polymers for photorefraction grew rapidly during the 1990s and into the current decade. It is now common to see a chapter on polymeric photorefraction in treatises that once emphasized only inorganic crystalline materials (for example, see References 23-27) and to see specific sections on photorefraction in overviews of nonlinear optical properties of pol5nners (for example. See references 28-31). 

Orientational Enhancement Effect. In addition to a strictly electronic and linear (Pockels) electro-optic response given by the first term on the right side of equation 1, the quasi-one-dimensional nonlinear optical chromophores incorporated into the pol5uner for their electro-optic response often exhibit large differences in the polarizability parallel versus perpendicular to the chromophore long axis (36,39-42). This so-called polarization anisotropjf (Aa a y -, where... 

Fig. 2. Oblique geometry typically used for characterizing bulk photorefracti ve polsmaers. The magnified portion illustrates the orientational enhancement effect described in the text due to the modulated poling of the electro-optic chromophore in the combined Eb + Esc fields. The beam and grating geometry applies for both four-wave mixing (beams 1-4) and two-beam coupling (beams 1 and 2).

Understanding the effects of orientational enhancements has been made more complex, however, by the need to accoimt for supramolecular interactions such as dipolar aggregation, the tendency of dipoles to adopt align antiparallel... 

Process conditions associated with the cooling rate of polymer melts, blown stretching, and film orientation enhance the polymer s natural anisotropy. Unlike low-molecular-... 

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