Moisture Diffusion and Solid-State NMR Investigations on the Physical Aging Processes in Network Epoxy Glasses

A polymer does not, in general, reach thermodynamic equilibrium when quenched through its glass-transition temperature Tg because the motion of the polymer chains is impaired owing to the increase in viscosity. Thus, the polymer cannot reach its equilibrium conformation or degree of packing. One characteristic of the quenched glassy state is an excess of trapped free volume, and a consequence is a lowering of the bulk polymer density. This change in free volume also alters the molecular mobility and hence influences the segmental relaxation times.