Tumbling rotational

Lorentzian line shapes are expected in magnetic resonance spectra whenever the Bloch phenomenological model is applicable, i.e., when the loss of magnetization phase coherence in the xy-plane is a first-order process. As we have seen, a chemical reaction meets this criterion, but so do several other line broadening mechanisms such as averaging of the g- and hyperfine matrix anisotropies through molecular tumbling (rotational diffusion) in solution. 

Some calculations have also been made for a loose model (Table II, complex 6) in which the four bendings above are replaced by four tumbling rotations of the ethyls, f... 

Given that phase space for highly excited colllnear OCS Is divided. It Is not surprising that energy relaxation would be complicated In planar OCS, where the molecule Is allowed to bend and rotate In a plane but no tumbling rotations are allowed. This expectation Is borne out In Figure 3 which shows an energy relaxation plot for OCS... 

Several processes, both within the molecule (intramolecular) and between molecules (intermole-cular), contribute to spin-lattice relaxation. The principal contributor is dipole-dipole interaction. The spin of an excited nucleus interacts with the spins of other magnetic nuclei that are in the same molecule or in nearby molecules. These interactions can induce nuclear spin transitions and exchanges. Eventually the system relaxes back to the Boltzmaim equilibrium. This mechanism is especially effective if there are hydrogen atoms nearby. For carbon nuclei, relaxation is fastest if hydrogen atoms are directly bonded, as in CH, CH2, and CH3 groups. Spin-lattice relaxation is also most effective in larger molecules, which tumble (rotate) slowly, and it is very inefficient in small molecules, which tumble faster. 

For extrusion compounding, if the components have different densities (e.g. polymer versus fillers) or different shapes (pellets versus regrind flakes) then prefer to use the extruder for mixing. That is, operate the extruder with starved feeding conditions with the components metered separately into the extruder, see Section 9.11. On the other hand, for the blending of polymer feedstock for extrusion, if the components have very similar properties, then use tumble, rotating drum or ribbon blenders or rotor-stator blenders of the feed upstream of the extruder and use flood feeding of the mixture to the extruder. 

In polymeric materials the rigid limit can be obtained at ambient temperatures if the polymer is below the glass-transition temperature, Tg otherwise the sample must be cooled. For rotational correlation times between the two limits, the line shape depends strongly on Xq. For imrestricted, isotropic tumbling, rotational correlation times can be determined from the extreme separation 2Azz (Fig. 2c). On the basis of the relative anisotropy defined in equation 6,... 

Zinc dust is used in the sherardizing process where work pieces are tumbled with zinc dust in rotating steel dmms which are heated electrically or by gas to 370—420°C (149). The steel parts are uniformly coated with zinc. In the chemical and metallurgical industries, zinc dust is used as a reducing agent, in the manufacture of hydrosulfite compounds for the textile and paper industries, and to enhance the physical properties of plastics and lubricants (2). 

Vacuum or Pressure Most tumbling mixers can have provision for vacuum or pressure. Mixers which cannot be adapted to these conditions are iTuulers with rotating pans. Continuous mixers introduce problems of sealing the charge and discharge ends. 

Particle Motion and Scale-Up Veiy little fundamental information is published on centrifugal granulators. Qualitatively, good operation rehes on maintaining a smoothly rotating stable rope of tumbling... 

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