Volume-recovery theory: 1. Kovacs' τeff paradox

Abstract

It is shown that there is nothing paradoxical in Kovacs' well-known τeff data. At small deviations from equilibrium (|δ| < a few times 10−4), the τeff-values are inaccurate, should be rejected, and do not allow any conclusion about the behaviour of τeff for δ → 0. Thus, there has never been any physical evidence for a ‘paradox’ or an ‘expansion gap’ at equilibrium. The reliable part of the data (|δ| > a few times 10−4) can be described, within experimental error, by the phenomenological volume-recovery theory. A dependence of τeff on the initial temperature (at constant δ) is a normal feature of linear and nonlinear systems with wide distributions of relaxation times. The dependence may even persist up to equilibrium; however, τeff then necessarily continues to increase (to ∞) with decreasing |δ| instead of approaching a finite limit as suggested by Kovacs' data. © 1997 Elsevier Science Ltd.