DISTRIBUTION OF ACTIVATION ENTHALPIES IN VISCOELASTIC RELAXATIONS

The distribution of relaxation times in viscoelastic relaxations can be expressed as the corresponding distribution of activation enthalpies, assuming that the activation entropy is constant. When this is done, the earlier finding that certain simple, noncooperative motions have activation entropies close to zero is confirmed. The widths of the distributions of activation enthalpy for many relaxations are quite similar, even though the distribution of relaxation times is much broader for secondary relaxations, which are observed at low temperatures, than for glass transitions. This is shown to be a natural property of the formalism. When the frequency-temperature behavior of local-mode relaxations is analyzed, it is found that the data on the low-temperature side of the loss peak correspond to an activation entropy close to zero while those on the high-temperature side give much larger activation energies and entropies. © 1990, American Chemical Society. All rights reserved.