(660i) Physical Aging of Perfluoropolymers in Ultrathin Film Composite Membranes
Industrial membranes for gas separation and water purification are comprised of a selective layer as thin as 100 nm or below on top of a porous support. The selective layer is often made of glassy polymers, which are subjective to physical aging (i.e., gas permeability decreases with time). The aging behavior has been ascribed by the rearrangement of polymer chains in the thin films, resulting in changes in the free volume and glass transition temperature (Tg). The goal of this study is to evaluate the thin film aging in the form of membranes tracked by the in-situ measurement of gas transport properties (including permeance and selectivity) and glass transition temperature. Amorphous perfluoropolymers (such as Teflon AF1600 and Hyflon AD80) are chosen as the selective layer materials, and fabricated into thin film composite membranes on polyethersulfone support. Pure-gas permeance decreases with time and pure-gas selectivity increases with time in these thin film composite membranes, and thinner layer shows more rapid aging behavior. The Tg of the thin film was determined using NanoTA, and monitored as a function of time. It is found that the Tg increases with time and the increase is more substantial for thinner films. This presentation will also discuss the correlation between the decrease in the gas permeance and the Tg increase for the thin films of perfluoropolymes in industrial membranes.