Relaxation processes molecular interpretation

Aside from the question of the precise model by which relaxation times are interpreted there is the more practical problem of isolating that part of the relaxation specifically caused by diffusion. The contributions of exchange processes (see below), spin-rotation interaction (9), and spin diffusion (9) can be identified by temperature dependences different from that which is solely the result of the motionally modulated nuclear dipolar interaction as sketched above, and corrections can be made. The molecular rotation contributions to dipolar relaxation can be removed or corrected for by (a) isotopic substitution methods (19), (b) the fact that rotation is in some cases much faster than diffusion, and its relaxation effects are shifted to much lower temperatures (7, 20), and (c) doping with paramagnetic impurities as outlined above. The last method has been used in almost all cases reported thus far, more by default than by design, because commercial zeolites are thus doped by their method of preparation this... 

The importance of intermolecular relaxation processes (Hertz, 1967) in the interpretation of proton-relaxation data necessitates a rather deep understanding of the mechanics of liquids. Thus the H nucleus, although most extensively used in work concerned with chemical shifts and spin coupling constants, was not really taken as a probe for the molecular dynamics of complex organic molecules. It is only very recently that there has been a significant increase in literature on 7", for protons in organic molecules (Hall and Preston, 1974) stimulated most likely by 13C relaxation work. The nC nucleus, however, developed to be the abundant nucleus of relaxation experiments for three main reasons ... 

The activation energy obtained by dielectric measurements for 8 -process is 28 kJ mol-1. This value is close to that obtained for PCHMA [30,38], Therefore the molecular origin of both relaxation processes should be related. This relaxation is observed in cyclohexyl compounds at temperatures below the y relaxation and, several authors [125] have related this relaxation to the motions of cyclohexyl group as a whole. In order to interpret the molecular origin of the 8 -transition, calculations... 

The interpretation of carbon T p data is complicated by the fact that spin-spin (cross-relaxation) processes as well as rotating frame spin-lattice processes may contribute to the relaxation (40). Only the latter process provides direct information on molecular motion. For the CH and CH2 carbons of PP, the Tip s do not change greatly over the temperature interval -110°C to ambient and, as opposed to the T behavior, the CH2 carbon has a shorter T p than the CH carbon. These results suggest that spin-spin processes dominate the Tip (46). However, below ca. -115°C, the Tip s for both carbons shorten and tend toward equality. McBrierty et al. (45) report a proton Ti minimum (which reflects methyl group reorientation at KHz frequencies) at -180°C. No clear minimum is observed in the data, perhaps due to an interplay of spin-spin and spin-lattice processes. Nonetheless, it is apparent that the methyl protons are responsible for the spin-lattice portion of the Tip relaxation for CH and CH2 carbons. 

Nitrogen quadrupole resonance studies have so far followed two major directions of investigation on the one hand, quadrupole coupling constants are interpreted in terms of the distribution of the bonding electrons, with many attempts to use the available molecular orbitals computed from models of various degrees of sophistication on the other hand, the effect of temperature on resonances yields information on the molecular motions and relaxation processes. 

The susceptibility minimum observed in the DS and LS relaxation spectra is a consequence of the interplay of the high-frequency tail of the a-relaxation peak and the contribution from certain fast dynamics dominating at frequencies close to but above the minimum. As was demonstrated by several experimental studies as well as by molecular dynamics simulations (cf. Fig. 13a), in addition to the vibrational contribution a fast relaxation process has to be taken into account for describing correctly the susceptibility minimum [5,9,19,55,64, 133,134,136,147]. This spectral contribution may usually be described by a power law with a positive exponent less than unity. In fact, the search for this fast relaxation process was mostly inspired by MCT, where it naturally appears in the solutions and is interpreted as rattling in the cage type of dynamics. Some authors discussed in addition a constant loss contribution [10,138,222,... 

Two different views have been adopted in the molecular interpretation of the glass transition. One view considers conditions when relaxation processes occur so slowly that the glass transition can be treated as time independent phenomenon. 

Reorientation dynamics of molecular tracers in polymers is not only important for the understanding of slow relaxation phenomena in glassy polymers but plays also a critical role in practical problems such as molecular design of nonlinear optical materials with long-term stability based on dyes/polymers complexes. We show here the reorientation dynamics of molecular tracers in glassy polymers obtained by the armealing-after-irradiation method described below. These experimental results are compared to the local relaxation processes of glassy polymers obtained by the already established measurement techniques such as dielectric relaxation and solid state NMR. Finally, the molecular interpretation of the relaxation of free-volume distribution in polymers will be discussed. 

Surface rheology Viscoelasticity of the monolayer differentiation between fluid and solid phases. Surface elasticity and viscosity in the transversal and longitudinal mode wave damping characteristics. Relaxation processes in monolayers. Mechanical stability of the monolayer. Interpretation often complicated because several molecular processes may be involved and because viscous and elastic components may both contribute. 

---