Orientational relaxation

Relaxation of Interacting Dipoles.— The elementary systems considered so far may, of course, be used directly as models for the dielectric relaxation of bulk material, with more or less success. It is much more interesting to treat them as realistic models of molecular behaviour, but then the relation between the relaxing orientation of one molecule and the relaxing polarization of a bulk sample requires further attention. 

Fig. 12. Temperature dependence of the frequency of relaxation oriented parallel and perpendicular to electric field at low temperatures.

It is clear that, as each molecule rotates, all torques will vary. A minimum in the relaxation function, i , will occur when the rms of all torques in the system is at a minimum. The set of orientational parameters giving this minimum may be found empirically by a search method, or mathematically by taking first and second derivatives and determining what the relaxed orientations of molecules must be for a given set of positional parameters. 

Perform super-molecule SCF calculation using coordinates of relaxed orientations. 

Film Formation and Propoties.— Work on film preparation and subsequent processing of 2GT has been limited to studies on the transverse constriction and stresses observed during uniaxial orientation and the problems associated with the maintenance of planarity during heat relaxation. Orientation and shrinkage have been discussed in a number of papers with studies on orientational setf-ieinforcement and relaxation and its effects on orientation. The effect of the interaction of films with organic liquids has promoted studies on the structural arrangements in liquid-induced crystallization of cold-drawn 2GT films, and the effect of crystallization on adhesion and cohesion. ... 

The number of segments between entanglements, was identified above as the boundary between the wavelength of Rouse modes which could and could not rapidly relax orientation in the presence of entanglements. In real space the associated distance between entanglements is = a The Rouse theory, shown above to be consistent with rubber elasticity ideas, relates to the plateau modulus by Gjv c kT/N. ... 

FigureBl.5.16 Rotational relaxation of Coumarin 314 molecules at the air/water interface. The change in the SFI signal is recorded as a fimction of the time delay between the pump and probe pulses. Anisotropy in the orientational distribution is created by linearly polarized pump radiation in two orthogonal directions in the surface. (After [90].)...

Lahajnar, G., Zupancic, L, Rupprecht, A. Proton NMR relaxation and diffusion study of water sorbed in oriented DNA and hyaluronic acid samples. In Biophysics of Water (Franks, F., Mathias, S., eds) Wiley, New York (1982) 231-234... 

Crimp. The tow is usually relaxed at this point. Relaxation is essential because it gready reduces the tendency for fibrillation and increases the dimensional stabiUty of the fiber. Relaxation also increases fiber elongation and improves dye diffusion rates. This relaxation can be done in-line on Superba equipment or in batches in an autoclave. Generally saturated steam is used because the moisture reduces the process temperatures required. Fiber shrinkage during relaxation ranges from 10 to 40% depending on the temperature used, the polymer composition used for the fiber, and the amount of prior orientation and relaxation. The amount of relaxation is also tailored to the intended apphcation of the fiber product. 

This effect of concentration is particularly pronounced with irregularly shaped particles. A possible explanation of the variation in the specific resistance is in terms of the time available for the particles to orient themselves in the growing cake. At higher concentrations, but with the same approach velocities, less time, referred to as particle relaxation time, is available for a stable cake to form and a low resistance results. 

AppHcations of soHd-state nmr include measuring degrees of crystallinity, estimates of domain sizes and compatibiHty in mixed systems from relaxation time studies in the rotating frame, preferred orientation in Hquid crystalline domains, as weU as the opportunity to characterize samples for which suitable solvents are not available. This method is a primary tool in the study of high polymers, zeoHtes (see Molecular sieves), and other insoluble materials. 

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