LINEAR AND NONLINEAR STRUCTURAL RELAXATION

The widely applied Tool-Narayanaswamy (TN) model for structural relaxation associated with the glass transition is discussed and critiqued. The TN model accounts for the non-exponential character of the structural relaxation by effectively invoking a distribution of relaxation times and for the non-linear character by allowing the relaxation times to depend both on temperature and structure. The model successfully explains qualitatively all of the at first glance unusual features of experimental structural relaxation results and usually also gives a good quantitative fit to such data, provided that the system is not extremely far out of equilibrium. Analysis of some rate cooling/rate heating enthalpy structural relaxation experiments, however, reveals some deficiencies in the TN model. In particular, the model appears to fail a critical test in which the TN analysis of non-linear relaxation experiments on glycerol cannot be reconciled with the results of linear ac calorimetry relaxation experiments on the same material.