Physical aging of glassy perfluoropolymers in thin film composite membranes. Part I. Gas transport properties

Physical aging of thin film glassy polymers continuously decreases gas permeability, presenting a great challenge in designing membrane systems for long-term gas separation. Most studies on the effect of physical aging on membrane applications use freestanding thin films, which are often annealed above the polymer glass transition temperature (T_g) before gas permeability is determined. However, industrial membranes are often thin film composites (TFCs) comprising the thin film on top of porous polymeric supports, and they may not be annealed above the T_g. The objective of this study is to investigate the effect of physical aging of the selective layer on gas permeance and selectivity of TFC membranes to establish industrial relevance. Two-layer TFC membranes consisting of perfluoropolymers (including Teflon^®AF1600 and Hyflon^®AD) at various film thicknesses (50 – 400 nm) on polyethersulfone porous support were prepared and determined for permeances of CH₄, N₂, H₂and CO₂at 35 °C for over 1000 h. Gas permeances decrease with time, and the decrease is more significant for larger penetrants and for membranes with thinner selective layers. For example, CH₄permeance decreases by 54% and 27% after aging for about 1400 h in TFC membranes comprising 50-nm- and 370-nm-thick Teflon AF1600, respectively. The decrease of gas permeances over time in these TFC membranes is compared with that of freestanding films. This study is one of only a few to present the results of physical aging in industrial TFC membranes and to provide useful insights for practical membrane applications.