Amorphous structural relaxation process

The main feature of the nonequilibrium behavior of solutions dnring cryocrystallization is the appearance of amorphous solids. Generally vitrification of the liquid system depends on the rate of structural relaxation processes, which are substantially determined by the viscosity of the solution. At higher cooling rates and reduced temperatures, the cluster structure of the solution cannot follow the changes, predetermined by the equilibrium behavior of the system, so that even after solidification, the structure of the amorphous solid is very similar to the structure of the solution at low temperatnres. According to modem concepts, the amorphous state can be considered as a kind of snpercooled liqnid with an extremely high viscosity coefficient. 

With a simple change of notation the present theory may be used to set the Gilroy and Philips model [82] of structural relaxation processes in amorphous materials and Dyre and Olsen s minimal model for beta relaxation in viscous liquids [83] in the framework of the general theory of stochastic processes. Moreover, the formulation of the theory in terms of kinetic equations as the... 

Clear evidence of L-L transitions has been found only in /-Si modeled by the SW potential [269]. Sastry and Angell [288] performed MD simulations of supercooled /-Si using the SW potential. After cooling at ambient pressure, the liquid (HDL) was transformed to LDL at 1060 K. The Nc in LDL is almost 4, and the diffusivity is low compared with that in HDL. The structural properties of LDL, such as g(r) and Nc, are very close to those of LDA, which indicates that this HDL-LDL transition is a manifestation of the multiple amorphous forms (LDA and HDA) of Si. McMillan et al. [264] and Morishita [289] have also found structural fluctuations between LDL-like and HDL-like forms in their MD calculations for /-Si at 1100 K. Morishita has demonstrated that such a structural fluctuation induces spatial and temporal dynamical heterogeneity, and this heterogeneity accounts for the non-Debye relaxation process that becomes noticeable in the supercooled state [289]. 

The problems associated with freeze drying of peptides and proteins for therapeutic use have also received calorimetric attention recently - particularly, attempts to understand and interpret the dynamics of amorphous solids. Structural relaxation time is a measure of molecular mobility involved in enthalpy relaxation and thus is a measure of the dynamics of amorphous (glassy) solids. These dynamics are important in interpretation of the physicochemical properties and reactivities of drugs in amorphous formulations. The authors conclude that microcalorimetry may provide data useful for rational development of stable peptide and protein formulations and for control of their processing . 

In a series of publications by Ishii et al. [54-57], effects of the electric field on structural changes in the amorphous regions, accompanied by an additional relaxation process, were discussed. These effects are reflected in the angular dependence of the second moment at different temperatures. The separation of any orientational effects due to poling from stretching effects were made by the preparation of different sample types. The complications for such a separation arose from the facts that (i) mechanically induced effects on chain orientations are much larger than that of the (electric) dipole reorientation and (ii) after poling only a small irreversible electric polarisation remains. 

Disaccommodation phenomena (a decrease in initial permeability as a function of time, see Section 4.4.2) also occur in metallic glasses, at practically any temperature between 4 K and T. Disaccommodation is related to atomic disorder it is affected by structural relaxation and by differences in the degree of atomic disorder and is introduced by varying the quenching rate during preparation (Allia Vinai, 1987). Diffusion processes involved in disaccommodation of amorphous materials are not fully understood. 

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