Thickness dependent physical aging and supercritical carbon dioxide conditioning effects on crosslinkable polyimide membranes for natural gas purification
Kratochvil, Adam Michal
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Membrane separations are rapidly growing alternatives to traditionally expensive gas separation processes. For natural gas purification, membranes are used to remove carbon dioxide to prevent pipeline corrosion and increase the heating value of the natural gas. The robust chemical and physical properties of polyimide membranes make them ideal for the numerous components and high pressures associated with natural gas production. Typically, the performance of membranes changes over time as a result of physical aging of the polymer. Previous work shows that the thin selective layer of an asymmetric hollow fiber membrane, the morphology of choice for gas separations, ages differently than a thick dense film of the same material. Also, carbon dioxide, which is highly soluble in most polymers, can actively swell and plasticize polymer membranes at higher pressures. In this work, free acid groups present in the model polyimide are covalently crosslinked to stabilize the matrix against plasticization. Physical aging of two different crosslinked derivatives are compared to the free acid polyimide through gas permeation, gas sorption, and refractive index measurements. Thick (~50 m) and thin (~650 nm) films are examined to determine the effects of sample dimension on physical aging. The crosslinking mechanism employs diol substituents to form ester linkages through the free acid group. However, the annealing treatment, above the glass transition temperature, used to "reset" the thermal history of the films is found to form a new crosslinked polymer. Characterization of this new crosslinking mechanism reveals a high-temperature decarboxylation of the free acid creates free-radical phenyl groups which form covalent crosslinks through other portions of the polymer structure. Since ester crosslinks may be vulnerable to hydrolysis in aggressive gas feed streams, this new mechanism of crosslinking may create a more robust membrane for aggressive separations. In addition to the physical aging study, supercritical carbon dioxide conditioning of the two glycol crosslinked polyimides is compared to the free acid polymer. In this case, the free acid polymer is not crosslinked since the esterification crosslinking reaction occurs at much lower temperature than the decarboxylation mechanism. The free acid polymer displays an atypical permeation response under supercritical carbon dioxide conditions which suggests a structural reorganization of the polymer occurs. The crosslinked polymers do not exhibit this type of response. Mixed gas permeation confirms a substantial decrease in the productivity of the free acid polyimide and reveals the enhanced stability of the crosslinked polyimides following the supercritical carbon dioxide conditioning. Finally, examination of structurally similar fluorine-containing polyimides following approximately 18 years of aging allows the study of polymer structure on physical aging. A 6FDA-based polyimide is compared to a BPDA-based polyimide to understand the effects of bulky, CF3 groups on physical aging, and polyimides with diamine isomers reveal the effects of structural symmetry on physical aging.