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In this work, we present (i) the dilation and refractive index variation associated with changes in film density and (ii) gas uptake of pure CO2and CH4, as well as their equimolar mixture in thin films of two polymers of intrinsic microporosity (PIMs), that is, PIM-1 and poly(trimethylsilyl)propyne (PTMSP). A conventional low-free-volume glassy polymer, cellulose triacetate, was also investigated as the reference material. All experiments were performed with ∼50 and ∼500 nm-thick films up to partial pressures of 25 bar using in situinterference-enhanced spectroscopic ellipsometry. In all cases, film thickness reduction promoted the collapse of the frozen-in free volume. Particularly for thin PIM-1 and PTMSP films, the CO2and CH4pure-gas uptakes were generally lower than in bulk samples. In the most extreme case of the ultrathin ∼50 nm PTMSP film, we could detect a strikingly similar qualitative behavior to the penetrant partial molar volume and dilation in rubbery polymers. Remarkably, in PIM-1, the collapse of the frozen-in free volume seemed to be opposed by its ultra-micropores (<7 Å), which was not the case in PTMSP with larger micropores (>10 Å). In mixed-gas experiments, the refractive index response of all investigated films closely followed the trend observed during CO2pure-gas sorption. In both thickness ranges and throughout the entire pressure range, the samples dilated less in the multicomponent environment than under the corresponding ideal pure-gas conditions. We found this phenomenon consistent with the pure- and mixed-gas uptake behavior of PIM-1 and PTMSP bulk films reported in the literature.

A novel trimethyl-substituted carboxyl-containing polyimide was synthesized via a one-pot high-temperature polycondensation reaction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,5-diamino-2,4,6-trimethylbenzoic acid (TrMCA). The polyimide (6FDA-TrMCA) displayed a Brunauer–Emmett–Teller surface area of 260 m2g–1, demonstrating intrinsic microporosity, in contrast to the related low-free volume COOH-functionalized polyimide 6FDA-DABA. Compared to the nonfunctionalized 6FDA polyimide analogue made from 2,4,6-trimethyl-m-phenylenediamine (TrMPD)—also known as 6FDA-DAM—carboxyl functionalization in 6FDA-TrMCA resulted in reduced surface area, lower fractional free volume, and tighter average chain spacing. Gas permeabilities of 6FDA-TrMCA were typical of functionalized polyimides of intrinsic microporosity (PIM–PIs). For example, at 2 atm and 35 °C, 6FDA-TrMCA showed pure-gas H2and CO2permeability of 193 and 144 barrer, coupled with H2/CH4and CO2/CH4selectivity of 61 and 45, respectively. Notably, in mixed-gas permeation tests with an equimolar CO2–CH4mixture at a CO2partial pressure of 12 atm, 6FDA-TrMCA demonstrated performance located on the 2018 mixed-gas upper bound with a CO2permeability of ∼98 barrer and CO2/CH4permselectivity of 38. As the first reported COOH-functionalized PIM–PI homopolymer, 6FDA-TrMCA revealed excellent resistance against CO2-induced plasticization at least up to a CO2partial pressure of 15 atm, covering the range of typical wellhead CO2partial pressures (5–10 atm).