Nano-Confinement Effects on Structural Development and Organic Solvent-Induced Swelling of Ultrathin Carbon Molecular Sieve Films

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.

Gas separation performance and physical aging of tubular thin-film composite carbon molecular sieve membranes based on a polyimide of intrinsic microporosity precursor

“…This effect reveals a profound fundamental similarity in the way the intrinsic microporosity collapses as a result of prolonged annealing and mere thickness reduction. This is again in line with our previous findings for PIMs and thin CMS films 25,27 . While the permeances decreased several fold between 15 min and 10 h soak times, the ideal selectivities increased only to a moderate extent and it seems that the shortest soak time of 15 min presented the best combination of overall gas separation properties.…”

“…Based on these results, it could be postulated that the microporosity of the thin-film PIM-PI precursor seems to, at least to some extent, collapse and tighten to favor more efficient penetrant size-exclusion. Such a microporosity collapse is in a general agreement with our previous sorption results for thin films of other microporous polymers (PIM-1 and PTMSP) as well as CMS films 25,27 . Table 1.…”

“…It is also plausible to suspect that even thicker films, in the range of several µm, would continue this trend and collapse even less than the 500 nm films. We believe that the remarkably fast pyrolysis-induced thickness reduction of the 100 nm film is further evidence of a rapid, undesirable collapse of the free volume that is accelerated in ultra-thin, submicron microporous films and manifests itself as severe losses in membrane permeance and sorption capacity 10,25,26 . Unfortunately, because of the limitations of the maximum temperature of the employed thermal ellipsometry setup (~560 °C), we could not observe the pyrolysis in the thin PIM-6FDA-OH films directly.…”

“…To obtain the extinction coefficient during the thermal rearrangement and initial phases of pyrolysis in the mostly absorbing wavelength range of 200 -1000 nm the dielectric function was extracted using a Kramers-Kronig consistent B-Spline optical model 30 (node resolution 0.3 eV, forcing ε2 > 0, and allowing for depolarization-correction when necessary). The data for the Si wafer support was taken from a temperature dependent in-built library 25 . For simplification, and to limit the number of fit parameters, the films were assumed isotropic.…”

“…The resulting membranes are characterized by high permeances without the need for ultra-thin selective layers and maintain favorable permeance/selectivity combinations over long periods of time (at least 90 days). Based on the data presented in this work and our previous studies 10, [25][26][27] , we investigate whether or not excessive reductions of the selective layer thickness (significantly below ~1 µm) for solution-processed amorphous microporous materials (such as CMS and PIMs) are actually counterproductive to obtaining highly efficient separations. The avoidance of ultra-thin selective layers would reduce membrane fabrication complexity and limit the probability of non-selective defects.…”