Structural relaxation time dispersion correlation with

Accordingly, since the dispersion and xa are obtained independently as separate and unrelated predictions, in such models the dispersion (or the time/frequency dependence) of the structural relaxation bears no relation to the structural relaxation time. This means it cannot govern the dynamic properties. As have been shown before [2], and will be further discussed in this chapter, several general properties of the dynamics are well known to be governed by or correlated with the dispersion. Therefore, neglect of the dispersion means a model of the glass transition cannot be consistent with the important and general properties of the phenomenon. The present situation makes clear the need to develop a theory that connects in a fundamental way the dispersion of relaxation times to xa and the various experimental properties. 

The general experimental fact of constant frequency dispersion (or time dependence of the correlation function) of the a-relaxation at constant Ta for different combinations of T and P has an immense impact on glass transition. Although the data were mostly obtained by dielectric relaxation, the same effect was found in some glass-formers by photon correlation spectroscopy. The primary concern of most theories, including those mentioned in the NY Times article, is to explain the temperature and pressure dependences of the structural relaxation time Tq.. In these theories, the dispersion of the structural relaxation is either not addressed, or else considered separately with additional input not involved in arriving at r . Consequently, the frequency dispersion is unrelated to the relaxation time of the structural a-relaxation in these theories, and they are unlikely to be consistent with the T, / -superpositioning property by happenstance. 

Figure IV-10 illustrates how F may vary with film pressure in a very complicated way although the v-a plots are relatively unstructured. The results correlated more with variations in film elasticity than with its viscosity and were explained qualitatively in terms of successive film structures with varying degrees of hydrogen bonding to the water substrate and varying degrees of structural regularity. Note the sensitivity of k to frequency a detailed study of the dispersion of k should give information about the characteristic relaxation times of various film structures.

Another approach to determining the viscoelastic properties of dense microemulsions at high frequencies is to conduct ultrasonic absorption experiments. In such experiments it has been found that the percolation process is correlated to a shift of the ultrasonic dynamics from a single relaxation time to a distribution of relaxation times [121]. Other experiments showed an increase in the hypersonic velocity for samples at and beyond the percolation threshold. The complex longitudinal modulus deduced from such experiments is also correlated with the occurrence of the percolation phenomenon, which suggests that the velocity dispersion is clearly correlated with structural transformations [122]. 

An interesting use of GVS, for amorphous dispersions, is detection and quantification of crystallinity. Due to higher hygroscopicity of hydrophilic polymers and amorphous API of ASD, the exposure of a sample to elevated humidity environment triggers the sorption of water molecules by polar/hydrophilic functional groups at the air-solid interface. The a-relaxation time of amorphous material often correlates with its water sorption potential (Bhardwaj and Suryanarayanan 2013). The gravimetric vapor sorption (GVS)-desorption led to the dissimilar structurally reversal of annealed amorphous trehalose when compared to that obtained by heating beyond Tg (Saxena et al. 2013). The sorbed water molecules Tg = -137°C) increases... 

The structural information derived from relaxation enhancement studies depends somewhat on the model chosen to describe the interaction of solvent protons with the protein molecules. For example even if the experiments indicated that the dispersion of Tfpr were essentially determined by the correlation time for rotational tumbling of the protein the tumbling of the hydration waters would not necessarily have to be restricted to that of the entire hydrated protein. Evidence was found that fast intramolecular tumbling about an axis fixed with respect to the surface of the hydrated species reduced the proton and O17 nuclear relaxation rates even in extremely stable aquocomplexes of Al3+ and other metal ions (Connick and Wiithrich (21)). The occurrence of similar... 

---