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 micron, indicating a strong influence of the free surface on aging behavior.1 This phenomenon is an intrinsically fascinating 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. These studies showed accelerated aging rates, as compared to bulk-like films, for samples with thicknesses below one micron, and the aging rates increased with decreasing film thickness. 1 Creating defect-free films below this limit for gas permeability measurement is extremely difficult. 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. A thin coating of a highly permeable, rubbery polymer, poly(dimethylsiloxane) (PDMS), effectively blocks the convective flow through the selectivity-destroying defects in the glassy film. The resistance to mass transfer of the PDMS layer is constant, due to its equilibrium nature at the testing conditions, and accounting for this resistance enables calculation of the glassy layer properties. 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 properties of films with thicknesses down to 20 nm were monitored during physical aging at 35 °C. All films exhibited decreasing permeability and increasing pure gas selectivity with time, as expected from the densification caused by physical aging. As film thickness was decreased, the initial permeability decreased while the pure gas selectivity increased. These trends are consistent with the idea of a rapidly aging region near the surface, whereby the thinner films would exhibit a greater aging response before the first measurement could be completed, ~ 1 hour. The aging rate increases with decreasing film thickness, down to ~75 nm. Below 75 nm, the aging rate depends less on thickness and even slightly slows as film thickness decreases. It is suggested that this behavior is caused by the very rapid initial aging in the near-surface region of the film, i.e. the apparent aging rate of the ultra-thin films is reduced because the film is initially closer to the equilibrium state than the thicker films. These results are consistent with the findings of a recent variable energy positron annihilation spectroscopy study where the size of free volume elements was found to be smaller near the surface of these glassy films.

(1) Huang, Y.; Paul, D. R. Polymer 2004, 45, 8377-8393.