Influence of previous history on physical aging in thin glassy polymer films as gas separation membranes

The physical aging behavior of thin glassy polysulfone (PSF) films (∼125 nm) with different previous histories was tracked using gas permeability measurements. The initial states of these materials were modulated by thermal annealing at fixed temperatures below the glass transition or by exposure to high pressure (800 psig (56.2 bara)) CO2 for various times. Regardless of the previous history, the nature of the aging response in these samples was consistent with the aging behavior of an untreated film that was freshly quenched from above Tg, i.e., permeability decreased and pure gas selectivity increased with aging time. However, the extent of aging-induced changes in transport properties of these materials depended strongly on previous history. The aging behavior was described using Struik's aging model by allowing the initial conditions to depend on each sample's previous history.