Browsing by Subject "Polybenzimidazole"

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Effects of elevated temperature on the physical aging and gas transport of sub-micron polybenzimidazole gas separation membranes
(2020-07-16) Merrick, Melanie Mae; Freeman, B. D. (Benny D.); Paul, Donald R; Sanchez, Isaac C; Riffle, Judy S; Lynd, Nathaniel A
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.
Impact of humidity and polymer blending on the gas transport properties of polybenzimidazoles
(2019-07-17) Moon, Joshua David; Freeman, B. D. (Benny D.); Paul, Donald R.; Sanchez, Isaac C; Riffle, Judy S; Lynd, Nathaniel A
Polybenzimidazoles (PBIs) are attractive polymers for gas separation membranes due to their high chemical and thermal stability and rigid, size-selective molecular structures. Opportunities exist for using PBIs for high temperature H₂/CO₂ separation, among other separations, where significant amounts of water are often present. However, PBIs are uniquely hydrophilic glassy polymers, and the impact of humidity on PBI gas transport properties is not well understood. Highly sorbing penetrants like water are often considered to affect molecular transport in polymers through phenomena such as competitive sorption, antiplasticization, and plasticization, but greater fundamental understanding is needed to relate these phenomena to other key concepts in polymer transport like free volume. Additionally, opportunities exist to improve low PBI gas permeabilities through material modification. This study investigates fundamentals of water sorption, dilation, and diffusion in PBIs to develop a systematic understanding of how water uptake affects molecular transport in hydrophilic glassy polymers. Water vapor sorption and swelling in PBIs were experimentally measured, which enabled direct evaluation of polymer free volume changes arising from water uptake. Gas transport properties were measured across a range of humidities using a custom experimental apparatus and correlated with humidity-induced free volume changes. This analysis enabled unique insight into the tradeoff between competitive sorption, antiplasticization, and plasticization effects of water sorption on PBI transport properties. Similar analysis could be used to investigate fundamentals of mixed penetrant sorption and diffusion in other polymers. Finally, a method of improving PBI gas separation properties by blending PBIs with a more permeable polymer was investigated. Commercial PBI was blended with an ortho-functional polyimide capable of undergoing thermal rearrangement at high temperatures. Films of PBI blended with a small fraction of polyimide exhibited matrix-droplet morphologies that enabled synergistic combination of PBI and polyimide gas separation properties. Heat treatment caused thermal rearrangement of the polyimide phase, increasing blend H₂ permeabilities, while also increasing structural order in the PBI phase, increasing blend H₂/CO₂ selectivities. The net result of heat treatment was simultaneous improvement in both H₂ permeability and H₂/CO₂ selectivity at ambient temperatures, surpassing the 2008 H₂/CO₂ upper bound