Abstract
Successful implementation of carbon molecular sieve (CMS) membranes in large scale chemical processes inevitably relies on fabrication of high performance integrally skinned asymmetric or thin-film composite membranes. In principle, to maximize separation efficiency the selective CMS layer should be as thin as possible which requires its lateral confinement to a supporting structure. In this work, we studied pyrolysis-induced structural development as well as ethanol vapor-induced swelling of ultrathin CMS films made from a highly aromatic polyimide of an intrinsic microporosity (PIM–PI) precursor. Utilization of a light polarization-sensitive technique, spectroscopic ellipsometry, allowed for the identification of an internal orientation within the turbostratic amorphous CMS structure driven by the laterally constraining support. Our results indicated a significant thickness dependence both in the extent of pyrolytic collapse and response to organic vapor penetrant. Thinner, substrate-confined films (∼30 nm) collapsed more extensively leading to a reduction of microporosity in comparison to their thicker (∼300 nm) as well as self-supported (∼70 μm) counterparts. The reduced microporosity in the thinner films induced changes in the balance between penetrant-induced dilation (swelling) and filling of micropores. In comparison to thicker films, the initial lower microporosity of the thinner films was accompanied by slightly enhanced organic vapor-induced swelling. The presented results are anticipated to generate the fundamental knowledge necessary to design optimized ultrathin CMS membranes. In particular, our results reinforce previous findings that excessive reduction of the selective layer thickness in amorphous microporous materials (such as PIMs or CMS) beyond several hundred nanometers may not be optimal for maximizing their fluid transport performance.
Highlights
Carbon molecular sieve (CMS) membranes represent a special class of inorganic amorphous membrane materials with a significant potential in technologically important molecular separations such as hydrogen purification, natural gas processing, air separations, or carbon capture.[1−5] Recently, carbon molecular sieve (CMS) membranes have been demonstrated to be very effective in challenging organic solvent[6] and olefin/paraffin[7−9] separations
We investigated organic-vapor-induced swelling of supported CMS thin films produced by pyrolyzing a polyimide of intrinsic microporosity (PIM−PI) precursor in two thickness ranges: ∼300 and ∼30 nm
We discovered that pyrolysis of thin precursor films led to significant orientation of the CMS structure in the direction parallel to the substrate guided by the lateral constraint
Summary
Carbon molecular sieve (CMS) membranes represent a special class of inorganic amorphous membrane materials with a significant potential in technologically important molecular separations such as hydrogen purification, natural gas processing, air separations, or carbon capture.[1−5] Recently, CMS membranes have been demonstrated to be very effective in challenging organic solvent[6] and olefin/paraffin[7−9] separations. In glassy polymers, including PIMs, the combination of nanoconfinement in the form of ultrathin films and the presence of penetrants is known to have significant consequences to their molecular separation performance.[19−23] In particular, swelling resulting from interaction with organics which may be present in the feed mixtures[24] has been shown to depend on film thickness This effect originates from a growing influence of the interfaces on the overall behavior of thin films.[20,25] While the topic of swelling in ultrathin organic polymer films has been addressed in the past 20 years, hardly any data exist on swelling of thin and ultrathin CMS films. The balance between the fraction of microporosity accessible to the penetrant and the penetrant-induced swelling showed a subtle thickness dependence
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