The τ-effective paradox: new measurements towards a resolution

New volume recovery measurements have been made on an epoxy glass for temperature down- and up-jumps from 1.0 to 4.0 K to investigate the τ-effective paradox. Due to the sensitivity of the measurements and to the fact that data points are obtained linearly in time, we are able to accurately measure the effective relaxation time at much smaller departures from equilibrium than those examined originally by Kovacs and subsequently by other researchers. The paradox is observed for the largest (2.0 and 4.0 K) up-jumps performed at the highest aging temperature. The magnitude of the expansion gap disappears as the departure from equilibrium approaches zero indicating a resolution of the paradox.