Kovacs’ experimental procedure on polyvinyl acetate consisted of two main stages. In the first step, the sample is instantaneously quenched from a high initial temperature T 0 to a low reference temperature T r , under constant pressure. The time-dependent volume of the system in T r , V ( t ) | T r , is recorded, until the time t e q when the system is considered to be at equilibrium. The volume at t e q is defined as the equilibrium volume of the system at temperature T r :