Ultrathin films for gas separation and their physical aging behavior
Brandon Rowe, Benny Freeman, and Don Paul
Most gas separation membranes are formed from glassy polymers because of their exceptional permeability-selectivity properties. However, glassy polymers are non-equilibrium materials that will spontaneously, but usually slowly, change over time towards an equilibrium state by a process known as physical aging. The physical aging rate becomes orders of magnitude more rapid when film thickness is decreased below one micrometer, indicating a strong influence of the free surface on aging behavior. This phenomenon is an intrinsically interesting scientific issue, and understanding physical aging has broad impacts in several technologies including gas separation materials, sensors, and microelectronics.
Previous studies have investigated the aging behavior of free-standing films with thicknesses down to approximately 400 nm. A coating technique has been developed in this work to enable the study of gas transport properties in glassy films with thicknesses down to 20 nm. This thickness is similar to, if not below, the thickness of the selective skin in state-of-the-art hollow fiber membranes. This is the first report, to our knowledge, on the aging behavior of ultra-thin (<100 nm) glassy polymer films monitored by gas permeability. The gas transport results are highly consistent with the findings from our recent variable energy positron annihilation spectroscopy study where the size of free volume elements was studied as a function of distance from the surface of the film. A direct comparison of the findings from these two studies will be presented.