Amorphous structural relaxation

The Fourier transform of this quantity, the dynamic structure factor S(q, ffi), is measured directly by experiment. The structural relaxation time, or a-relaxation time, of a liquid is generally defined as the time required for the intermediate coherent scattering function at the momentum transfer of the amorphous halo to decay to about 30% i.e., S( ah,xa) = 0.3. 

In the discussion on the dynamics in the bead-spring model, we have observed that the position of the amorphous halo marks the relevant local length scale in the melt structure, and it is also central to the MCT treatment of the dynamics. The structural relaxation time in the super-cooled melt is best defined as the time it takes density correlations of this wave number (i.e., the coherent intermediate scattering function) to decay. In simulations one typically uses the time it takes S(q, t) to decay to a value of 0.3 (or 0.1 for larger (/-values). The temperature dependence of this relaxation time scale, which is shown in Figure 20, provides us with a first assessment of the glass transition... 

A method of characterising transport mechanisms in solid ionic conductors has been proposed which involves a comparison of a structural relaxation time, t, and a conductivity relaxation time, t . This differentiates between the amorphous glass electrolyte and the amorphous polymer electrolyte, the latter being a very poor conductor below the 7. A decoupling index has been defined where... 

Photoluminescence intensity of the amorphous polymers was generally much larger than that of the more crystalline polymers. The energy level of the lowest singlet excited state Es was evaluated to be 2.5-2.7 eV for the amorphous polymer pristine films, and 2.0 eV for the more crystalline polymers. The Stokes shifts were also observed to be much larger for the amorphous polymer films compared with those of the more crystalline polymer films. This indicates a larger structural relaxation of the amorphous polymers following photoexcitation. 

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. 

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 . 

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