Ionic relaxation mechanism

Ratner, M.A. (2000), Polymer Electrolytes Ionic Transport Mechanisms and Relaxation Coupling, MRS Bull. 25(3), 31. 

A detailed description of the relaxation mechanism associated with an excitation transfer based on a recursive (regular) fractal model was introduced earlier [47], where it was applied for the cooperative relaxation of ionic microemulsions at percolation. 

In NMR work, spin-lattice relaxation measurements indicated a non-exponential nature of the ionic relaxation.10,11 While this conclusion is in harmony with results from electrical and mechanical relaxation studies, the latter techniques yielded larger activation energies for the ion dynamics than spin-lattice relaxation analysis. Possible origins of these deviations were discussed in detail.10,193 196 The crucial point of spin-lattice relaxation studies is the choice of an appropriate correlation function of the fluctuating local fields, which in turn reflect ion dynamics. Here, we refrain from further reviewing NMR relaxation studies, but focus on recent applications of multidimensional NMR on solid-ion conductors, where well defined correlation functions can be directly measured. 

Experimentally, the activation energy, Ea, of the nuclear spin relaxation has been studied systematically to evaluate different theoretical models [71]. Reproducing activation energies constitute a crucial test for MD simulations of the relaxation mechanism. It has been studied in MD simulations for both inert and ionic solutes [62,66]. For Ne, Kr, and Xe in acetonitrile [66], it was difficult to relate the Ea for the relaxation and those for individual molecular processes. This reflects the general problem of rationalizing Ea for collective processes. 

Relaxation mechanisms in ionic poljoners (other than dispersions due to motion of ions or small ionic groups) have been investigated only very briefly. 

In most non-crystalline linear polymers described to date, the relaxation mechanism (in the absence of such extraneous factors as degradation) is the simple molecular flow, or the a mechanism. Exceptions have been found, for instance in the case of the polysulfides (4,54,56,57) or pol5mrethanes (57) in which far above the gla transition temperature a ixmd interchange mechanism was observed. For a number of reasons (which will be described below), it is of interest to study viscoelasticity of polymers which are subject to both mechanisms, i. e., a and (as bond-interchange will be called due to the intrinsically chemical nature of the reaction), particularly if both mechanisms occur with comparable relaxation times. Among the benefits of such a study, particularly in the case of the ionic inorganic polymers would be ... 

This paper has reviewed several studies, particularly those performed on the phosphate polymers, in which ionic forces were imjx>rtant in determining the properties or behavior of the material. Only very few properties were investigate among these were 1) the glass transitions as a function of the molecular we ht, the nature of the terminal group, and the nature of the counterion, 2) the viscoelastic properties as a function of the counterion and 3) the viscoelastic relaxation mechanism, with specific emphasis on the separation of the a and mechanisms. In this treatment, a deliberate attempt was made to present pertinent theories, insofar as they exist, but it seems evident that here as much work remains to be done as on the experimental level, if not more. 

While ionomers of many types have been made and characterized [1,2,3], there is little work on the overall relaxation mechanisms. For polymers with low ionic concentrations, there is general agreement on the fundamental relaxation step. The stress relaxes by detachment of an ion pair from one cluster and reattachment to another. For the styrene/methacrylic acid Na salt (ST/-MAA-Na) system, there is a secondary plateau in the relaxation modulus which depends on the ionic content and can be described as a rubbery modulus [4], While a rubbery modulus with stress relaxation due to ionic interchange has been invoked earlier, it does not adequately describe the relaxation curves. A different approach is taken here. 

The primary objective of this paper is to review the mechanical behavior of dry and hydrated Nafions with an emphasis on the mechanical relaxations. The tentative assignments of the relaxation mechanisms underlying the three mechanical relaxations are discussed in connection with the structure of the ionic aggregates... 

Counterions. 1. Sodium-23 Alkali metal MIR is a sensitive >robe of the immediate chemical environment and mobility of alkali metal ions in aqueous and nonaqueous solvents (7, 8). The chemical shifts of alkali metal nuclei will respond to" electronic changes only in the immediate environment of the cation since alkali metals rarely participate in covalent bonding (7). All alkali metal nuclei have spins greater than 1/2 and hence have quadrupole moments. The interaction of these moments with electric field gradients, produced by asymmetries in the electronic environment, is modulated by translation and rotational diffusive motions in the liquid. It is via this relaxation mechanism that the resonance line width is a sensitive probe of ionic mobility. 

Above 140 K, the situation is quite different. Ionic relaxation, either in the form of Frenkel pair generation adjacent to the impurity or silver ion diffusion from the space-charge layer, provides an alternative decay pathway for the shallowly trapped electron state. The resultant Agj+ is reduced by electron transfer from the impurity trap, a process that has been followed by EPR in particular detail in silver chloride crystals and emulsions. For Pb2 + in AgCl, this ionic decay mechanism has an activation energy of 0.36 + 0.05 eV [85,105], approximately the sum of the formation and diffusion energies for a silver ion interstitial in AgCl [42]. This result supports the Frenkel pair mechanism for the annihilation of the electron shallowly trapped at Pb2+. Similar results have been obtained from EPR studies of Cd2 +-doped AgCl. 

From Figure 10 it appears that a dipolar relaxation labeled a is superimposed on the phenomenon we have just discussed. The behavior of this a peak correlates well with the behavior of the dynamic mechanical a relaxation since it increases in magnitude and decreases in temperature with increasing sulfonation. The presence of this peak in the dielectric spectra of these materials and its behavior as a function of sulfonate concentration are consistent with the assignment of the mechanical a relaxation to an ionic-phase mechanism. However, it is not possible to cite this dielectric peak as proof of the mechanical assignment the known presence of ionic impurities in these systems and the unknown origin of the large increases in tan 8 and c dictate that the dielectric results be interpreted with caution. 

The a relaxation observed by dynamic mechanical techniques is attributable in part to an ionic-phase mechanism. The existence of this relaxation is suggested as evidence for the presence of phase-separated clusters in these materials. The temperature at which the fi relaxation occurs is found to result from the complex interaction of crystallinity and ionic group concentration. 

Accelerated particles (ions, photons, electrons) interact with condensed matter and produce secondary species molecular ions, electrons, photons, radicals, and ionic species (Chapiro 1974, Sanche 2003). Figure 16.1 summarizes the interactions of radiation with a hypothetical diatomic molecule AB. As the primary radiation can ionize (1) or excite (2) the molecule, several relaxation mechanisms can occur to stabilize the molecule and return it to its ground state (i.e., AB AB+E,... 

There have been no widespread studies of solvent effects on relaxation rates, although various workers have reported isolated cases. In correlating solvent effects it is necessary to be sure that the same species and relaxation mechanisms are involved. For example with ionic solutes a change to a solvent of lower dielectric constant may produce significant ion association, while on changing to a solvent with small molecules some spin-rotational relaxation of the solvent may occur. 

Carper and co-workers have performed a detailed analysis of the relaxation times of both [C4mim] [25] and [Cgmim]" [26] with [PF0]", which was later extended with more detail to deconvolute the relative contributions of the various relaxation mechanisms [27]. They found that the contribution of CSA to the experimentally observed relaxation time was about half of the contribution from dipolar relaxation. This work raises doubts about the applicability of isotropic relaxation models to ionic liquids. It is important to note that the and measurements of ionic liquids in the literature show different behaviour when attached to the same ion. The random Brownian motion that occurs in most liquids leads to rapid spin diffusion between nuclei bonded to a common ion or molecule, causing them to all exhibit the same T. The lack of such behaviour is a clear indication that the dynamics of ionic liquids are... 

Relaxation mechanisms of dipoles located in dissimilar environments, or originating from complex forms of molecular or ionic motion, usually exhibit curved Arrhenius diagrams. This curvature is usually interpreted in terms of the semiempirical Williams-Landel-Ferry (WLF) equation (Williams et al. 1955)... 

---