Effects of elevated temperature on the physical aging and gas transport of sub-micron polybenzimidazole gas separation membranes

The promising potential of polybenzimidazole (PBI) membranes for high temperature (~200 °C), hydrogen-selective gas separations has been reported for many membrane geometries (e.g., bulk, composite, and hollow fiber), but never for sub-micron, spin-cast membranes. Numerous studies have shown that the performance of spin-cast membranes, which simulate commercially relevant thicknesses, declines more quickly with time than that of thick membranes due to accelerated physical aging. However, because most existing membranes are used near ambient temperature, the physical aging of sub-micron, spin-cast membranes has never been studied at temperatures above 55 °C. Because physical aging is dependent on both thickness and temperature, emerging high temperature membrane applications make it both intellectually and practically imperative to characterize spin-cast membranes in this new temperature regime. For the first time, physical aging studies of spin-cast, sub-micron membranes have been extended to elevated temperatures. PBI membranes, cast from commercial-grade Celazole®, were aged in a high-temperature permeation system while the gas permeabilities were periodically measured over more than 1500 hours. When aging at 190 °C, membrane gas permeabilities decreased rapidly then plateaued after 300 hours of aging. The observation of a plateau (i.e., equilibration) had never before been seen for a membrane, nor, to our knowledge, for any polymer ~250 °C below its glass transition temperature. Decreases in membrane permeability were accompanied by increases in selectivity for H₂, which are traditionally represented by Robeson upper bound plots. These shifts were consistent with previous membrane physical aging studies and indicate membrane size-sieving ability improves with aging. Celazole®’s permeability reductions at lower aging temperatures (e.g., 175 °C) were qualitatively similar to those at 190 °C, but occurred over a longer time period. When graphed vs. the logarithm of aging time, the permeability reductions at various temperatures could be superimposed via time-temperature superposition, which is a hallmark of physical aging. A thorough review of physical aging studies in the polymer physics literature is presented to give context for this unexpectedly short equilibration time far below the glass transition. Comparisons are then made between the current study and previous aging studies in the polymer physics field. Overall, the observation of a plateau at short aging times for a polymer deep in the glassy state casts doubt on our understanding of physical aging’s temperature-dependence and our ability to predict membranes’ long-term stability in elevated temperature applications.