Physical Aging of Perfluoropolymers in Ultrathin Film Composite Membranes and Its Interpretation Using the Free Volume Model

Thin film composite (TFC) membranes for gas separation are comprised of a selective layer of glass polymers, which are usually subjected to physical aging, i.e. permeability decreases with time. The aging behavior has been ascribed to the rearrangement of polymer chains in the thin film, resulting in changes in free volume (f) and glass transition temperature (T_g).  Our objective is to investigate the effect of physical aging on gas permeance, selectivity and T_g of the selective layer in the TFC membranes.  Two‑layer TFC membranes consisting of perfluoropolymers of Teflon^® AF or Hyflon^® AD at various film thicknesses (50 – 400 nm) on polyethersulfone ultrafiltration support were prepared.  Pure-gas permeances of CH₄, N₂, H₂ and CO₂ at 35 ^oC were measured and monitored for aging over 1000 hr.  Apparent T_g of the thin selective layer was measured over time by a new thermo-mechanical method, Nano Thermal Analysis.  In general, aging decreases gas permeances and increases T_g, and the rate of changes is more significant for thinner films.  For example, after aging for 2000 hr, N₂ permeance decreases from 1000 gpu to 550 gpu by 45% and T_g increases from 160 ^oC to 172 ^oC in a TFC membrane with 100-nm thick Teflon^® AF1600. In this talk, we will compare the aging behavior and gas permeance reduction of the TFC membranes with those of freestanding thin films.  More importantly, for the first time we have developed a correlation between the gas permeance reduction and T_g increase using the free volume model to explain the physical aging behavior.