Bimodal relaxation

Except for the rotationless system (p = °o) all the other systems exhibit a bimodal relaxation, with a fast relaxation component that accounts for most of the solvation energy. The relaxation of the rotationless solvent is exponential (a fit to exp(-t/x) yields x= 2.2 ps). 

Dyn ic mechanical analysis was discussed in terms of the nodular morphology concept in crossllnked structures. Beta relaxations in all the cured resins were bimodal in appearance. But, vAiile MPD-cured resins shewed a maximum at 25 C with a smaller shoulder at -40 C, TDA and DAEB-cured resins had maxima at -40 C with a less significant peak at 25 C. For DAIPB and DATBB-cured resins the two peaks were approximately equal in magnitude. The two overlapping peaks at -40 and 25 C were attributed to crankshaft motions in the matrix and nodules. 

Computation, using the dynamic RIS model, of the relaxation times for POM helices. The bimodal distribution of relaxation times is rationalized with a simple model. 

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

Figure 4 depicts the imaginary part of the frequency-dependent viscosity which clearly demonstrates the bimodality of the viscoelastic response. In the same figure the prediction from the Maxwell s relation have also been plotted. In the latter the relaxation time xs is calculated by the well-known... 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

Bulky crosslinks or side-groups in the network chains, e.g., dendritic wedges [73], may also influence molecular mobility and viscoelastic properties of polymer networks. For example, UV curing of difunctional acrylates results in the formation of zip-like network junctions, which may be regarded as extreme cases of bimodal networks [52], Results obtained with the NMR T2 relaxation method agree well with those of mechanical tests... 

Recently NOESY MAS was used to study molecular motions in technically relevant materials such as rubbers [46, 47]. For the evaluation of these parameters, it is necessary to understand the cross-relaxation process in the presence of anisotropic motions and under sample spinning. Such a treatment is provided in [47] and the cross-relaxation rates were found to weakly depend on fast motions in the Larmor-frequency range and strongly on slow motions of the order of the spinning frequency vR. Explicit expressions for the vR dependent cross-relaxation rates were derived for different motional models. Examples explicitly discussed were based on a heterogeneous distribution of correlation times [1,8,48] or on a multi-step process in the most simple case assuming a bimodal distribution of correlation times [49-51]. 

Figure 8 Temperature-dependent Mossbauer spectra of metallic iron nanoparticles in zeolite NaX (a). The superparamagnetic blocking temperature Ti is aroimd 40 K. The solid lines have been calculated by a relaxation formalism assuming a hyperfine field distribution as shown in (b) with a bimodal size distribution of metallic iron particles. (Reprinted from Schiinemann, Winkler, Butzlaff and Trautwein. With kind permission from Springer Science Business Media)...

Thus the anomalous relaxation in a double-well potential is effectively determined by the bimodal approximation, Eq. (159) the characteristic times of the normal diffusion process—namely, the inverse of the smallest nonvanishing eigenvalue, the integral, and effective relaxation times—appear as time... 

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