The origin of the faster mechanism of partial enthalpy recovery deep in the glassy state of polymers

Abstract

A novel finding made by Cangialosi and coworkers in the physical aging of several polymers way below the glass transition temperature T_g is that equilibrium recovery occurs by reaching a plateau in the enthalpy with partial enthalpy recovery. This observation points to the existence of a much faster mechanism capable of partial equilibrium recovery deep in the glassy state. A similar phenomenon was found in different glassy materials. The generality of the phenomenon indicates that the faster mechanism of equilibrium recovery is universal and fundamental. In this paper the faster mechanism is identified to be the universal JG β-relaxation having dynamic and thermodynamic properties analogous to the α-relaxation, and thus capable of effecting enthalpy and volume recovery far below T_g in several high-T_g polymers. The JG β-relaxation is also the mechanism responsible for the first step of two steps in the approach to equilibrium found in another polymer with much lower T_g. The Coupling Model is used to explain why the first step transpires far below T_g in some polymers but much closer to T_g in another polymer.