Evaluation of transport and transport stability in glassy polymer membranes
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Both novel membrane materials with better separation characteristics and a better fundamental understanding of membrane transport stability are needed to improve the competitiveness of commercial membrane separations. In this work, the effect of a novel moiety, hexafluoroalcohol, on the gas transport properties of an aromatic polyimide membrane are evaluated. The hexafluoroalcohol group increases the membrane’s fractional free volume, which increases the membrane’s permeability to all gases. Additionally, the HFA-containing polyimide shows resistance to plasticization by carbon dioxide. However, ideal selectivity for several gas pairs is unchanged by the inclusion of hexafluoroalcohol and the increase in the polymer’s fractional free volume. This lack of selectivity loss with increasing free volume is attributed to hydrogen bonding between the hexafluoroalcohol and imide groups, which reduces chain mobility. The ethanol dehydration characteristics of a so-called “TR” polymer are also evaluated in this work. TR polymers are heterocyclic, aromatic polymers synthesized by a solid-state, high temperature condensation from ortho-functional polyimides. Pervaporation studies on a representative TR polymer film demonstrate that the material has separation properties that exceed those of a commercial ethanol dehydration membrane. The transport properties of the TR film, combined with high thermal and chemical stability characteristic of these materials, make TR polymers promising materials for high-temperature, high-water content ethanol dehydration. Finally, the physical aging and plasticization of cellulose triacetate, the dominant natural gas purification membrane, is presented. Although this material has been used industrially for over 30 years, the physical aging and plasticization of the material, particularly in sub-micron films, has never been studied. Although cellulose triacetate does show physical aging behavior, as observed by permeability decreases over time, cellulose triacetate thin films do not show accelerated aging. Furthermore, the plasticization of thin cellulose triacetate films is reduced, rather than increased as seen in other polymers. The unusual transport stability of thin cellulose triacetate films may be due to their complex, semi-crystalline morphology, which, due to the thermal instability of the material, may not be thermally controlled.