Relaxation processes hypersonic

From the experimental point of view, the method of Brillouin spectroscopy (BS) has played the central rote for the identification of the T transition. - In Section II we present an extension of this method which provides further indication for the existence of the transition and is a sensitive probe for identifying obscure hypersonic relaxation processes. 

The frequencies (70 and / °A (jj have to be determined for equivalent symmetry sound propagation directions as well as for the same polarization. So far the application of eq. (4) has not been restricted to isotropic materials. If hypersonic relaxation processes are active, two different cases have to be taken into account ... 

Fig. 21. Example of a hypersonic relaxation process obscuring a possible T transition. A T kink could be suspected near 360 K. (a) The D-fiinction compared with the refractive index clearly indicates a hypersonic relaxation (compare Figs. 2 and 3). (b) Sound frequency measured in (he 90A> geometry versus temperature. The formula of the oligomer is depicted in die inset of (a).

While a broader distribution of relaxation times would provide a better fit to the experimental data, a reasonable description of the hypersonic loss process is of a major relaxation with t approximately equal to 2.5 X 10 sec and a secondary process with t = 5 X 10 sec at 320 K. 

Insufficient ultrasonic data are available for an analysis of the relaxation in PIB, nonetheless the raw data (Figures 2a and 2b) indicate a similar process. The behavior of PDMS is more complicated. However, at least two relaxations appear to be associated with the major hypersonic loss process. 

In viscous fluids, however, a coupling occurs between internal and external modes of freedom and translational motions. This structural relaxation process, characterized by a frequency >e, influences both the central line as well as the Brillouin doublet, depending on the frequency of this relaxation relative to the frequency of the hypersonic waves. In principal three different cases must be considered ... 

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]. 

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