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The changes in physical, chemical, and gas permeation properties of a heat-treated hydroxyl-functionalized Tröger's base-derived intrinsically microporous polyimide, 6FDA-HTB, were systematically studied. Polyimide samples were heat-treated for 30 min at a fixed temperature to form (i) polybenzoxazole (PBO) by thermal rearrangement (TR) from ∼420 to ∼440 °C, (ii) polymer-carbon transition (PCT) from ∼460 to ∼500 °C, and (iii) early-stage carbon (ESC) membranes from >500 to 600 °C. As frequently observed in previous studies, TR-derived PBO membranes showed an increase in gas permeability with a concomitant decrease in gas-pair selectivity. The intermediate PCT region represents a transition state from a PBO to a partially carbonized polymer with superior properties by showing a simultaneous boost in gas permeability and gas-pair selectivity. The ESC membranes formed between 550 and 600 °C possessed further enhanced selectivity due to tightening of the amorphous CMS structure. A 135-day aged 6FDA-HTB-600 membrane showed very high hydrogen permeability of 3544 barrer coupled with an outstanding H 2 /CH 4 selectivity of 939. The mixed-gas separation properties of selected aged samples, pristine 6FDA-HTB, PBO (6FDA-HTB-420), intermediate PCT (6FDA-HTB-480), and early stage CMS analogs (6FDA-HTB-550 and 600), were investigated using a 1:1 CO 2 /CH 4 feed up to 30 bar. In this series, early-stage CMS membranes displayed the best mixed-gas CO 2 /CH 4 separation properties with performance significantly surpassing the 2018 polymer mixed-gas upper bound. [Display omitted] • Gas transport properties of heat treated 6FDA-HTB and its thermally treated analogs. • TR-derived PBO membranes showed an increase in gas permeability and drop in gas-pair selectivity. • Intermediate PCT membranes displayed boost in gas permeability and gas-pair selectivity. • ESC membranes exhibited exceptional H 2 /CH 4 , H 2 /N 2 , He/CH 4 , and CO 2 /CH 4 performance. [ABSTRACT FROM AUTHOR]

An intrinsically microporous polyimide is synthesized in m-cresol by a one-pot high-temperature condensation reaction of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and newly designed 2,6 (7)-dihydroxy-3,7(6)-diaminotriptycene (DAT1-OH). The 6FDA-DAT1-OH polyimide is thermally stable up to 440 °C, shows excellent solubility in polar solvents, and has moderately high Brunauer-Teller-Emmett (BET) surface area of 160 m2 g−1, as determined by nitrogen adsorption at −196 °C. Hydroxyl functionalization applied to the rigid 3D triptycene-based diamine building block results in a polyimide that exhibits moderate pure-gas CO2 permeability of 70 Barrer combined with high CO2/CH4 selectivity of 50. Mixed-gas permeation studies demonstrate excellent plasticization resistance of 6FDA-DAT1-OH with impressive performance as potential membrane material for natural gas sweetening with a CO2 permeability of 50 Barrer and CO2/CH4 selectivity of 40 at a typical natural gas well partial pressure of 10 atm. [ABSTRACT FROM AUTHOR]

2,2-Bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA)-derived polyimides containing polar functional groups have been extensively studied for membrane-based gas separations and used as precursors for various solid-state thermal modifications such as thermal crosslinking of carboxyl-groups or thermal rearrangement of hydroxyl groups to form polybenzoxazoles. In this work, we studied four 6FDA-derived polyimides without polar functional groups to investigate in-situ thermal crosslinking involving partial decomposition of the hexafluoroisopropylidene (–C(CF 3) 2 –) mainchain group. A thorough examination of the 6FDA-based polyimides via thermal gravimetric analysis coupled with mass spectroscopy (TGA-MS) and energy dispersive X-ray spectroscopy (EDX) under isothermal treatment at 450 °C for up to 3 h revealed that thermal treatment triggered continuous evolution of fluorine from (–C(CF 3) 2 –) groups. The 6FDA-based polyimides were insoluble after thermal treatment for only 30 min and maintained good mechanical strength with favorable changes in gas separation properties. For example, after 3 h isothermal heat treatment, the CO 2 permeability of crosslinked 6FDA-pPDA noticeably increased threefold from 39 to 115 barrer coupled with only a small drop in CO 2 /CH 4 selectivity from 54 to 46. The crosslinked polyimides demonstrated excellent plasticization resistance when tested under high-pressure binary CO 2 /CH 4 mixed-gas conditions. [Display omitted] • Non-functionalized 6FDA-based polyimides can be crosslinked after heating to 450 °C. • Crosslinking is induced by release of fluorine from hexafluoroisopropylidene groups. • Crosslinked polyimides show higher gas permeability with only small drop in gas-pair selectivity. • Crosslinked polyimides are plasticization resistant to at least 15 bar partial CO 2 pressure. [ABSTRACT FROM AUTHOR]

This comprehensive study explored the aging process of PIM-1, a ladder polymer of intrinsic microporosity (PIM), by applying molecular dynamics simulations for the first time. Through detailed analysis, our work illustrates the evolution of the polymer structure from a loosely packed, less dense state of the pristine polymer to a more tightly packed configuration due to physical aging. For this purpose, a novel molecular dynamics (MD) methodology was employed in the process toward equilibration of PIM-1. This structural transition was quantitatively captured by measuring key parameters such as density, fractional free volume (FFV), cohesive energy density (CED), d -spacing, surface area, and gas permeabilities. The simulations demonstrate a noticeable increase in density by approximately 7 % in aged PIM-1 compared to a fresh sample. This increase in density is accompanied by a corresponding decrease in FFV, suggesting a more compact molecular arrangement. The impact of these structural changes is evident in the gas transport properties. Permeabilities of all gases tested, He, H 2 , O 2 , N 2 , CO 2 and CH 4 , decreased by 33 %–80 %. Moreover, the selectivity of gas pairs like CO 2 /CH 4 and O 2 /N 2 exhibited increasing trends due to aging, as previously reported in experimental work. Structural analysis performed on the fresh and aged structures indicated collapse of free volume over aging, by disappearance of pores larger than ∼6.5 Å. Furthermore, no intrachain rearrangement was observed during physical aging in the ladder PIM-1 structure; rather, the aging resulted in increased interchain packing efficiency. Our methodology can be employed to other PIM architectures, such as polyimides of intrinsic microporosity (PIM-PIs) as well as low-free volume glassy polymers. [Display omitted] • Physical aging in PIM-1 was successfully simulated by MD. • Simulated fresh and aged pure-gas transport properties matched experimental data. • Effect of aging on density, d-spacing, FFV, PSD was analyzed. • Aging resulted in more efficient interchain packing without significantly affecting intrachain packing. • Our methodology was extended to other high-free-volume glassy polymers. [ABSTRACT FROM AUTHOR]

Tröger's base (TB) has been utilized as an important building block in designing ladder polymers of intrinsic microporosity (PIMs) and microporous polyimides (PIM-PIs) for membrane-based gas separations due to its unique V-shaped bicyclic structure and versatile molecular chemistry. Nearly a century after its discovery, Hünlich's base (HB) was recently reintroduced as a valuable diamine derivative of TB made by a single-step reaction of 2,4-diaminotoluene and formaldehyde, spurring use in molecular devices such as molecular tweezers and photo-switches. Unlike TB, HB has not been explored as a building block of PIMs and PIM-PIs for membrane-based gas separations. In this study, we synthesized two soluble PIM-PIs for the first time by reaction of HB as diamine and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) or 9,10-diisopropyltriptycene tetracarboxylic dianhydride (TDA i 3), respectively. 6FDA-HB exhibited high Brunauer-Teller-Emmett (BET) surface area of 415 m2 g−1 and fractional free volume (FFV) of 0.26. The gas separation performance of the 6FDA-HB polymer was similar to other 6FDA-based dimethyl-substituted Tröger's base polyimide analogues, exhibiting O 2 , CO 2 and H 2 permeability of 62, 286, and 391 Barrer with O 2 /N 2 , CO 2 /CH 4 and H 2 /CH 4 selectivity of 4.4, 26 and 36, respectively. Compared to 6FDA-HB, the triptycene-containing Hünlich's base polyimide (TDA i 3-HB) displayed a higher BET surface area (501 m2 g−1) owing to the presence of the rigid bridged tricyclic triptycene contortion site, resulting in about two-fold increase in O 2 permeability to 188 Barrer coupled with slightly compromised O 2 /N 2 selectivity of 4.1. Beside the merit of facile synthesis, the Hünlich's base-derived polyimides experienced relatively modest effects of physical aging on gas permeation properties. Image 1 • Hünlich's base, a diamine analogue of Tröger's base, was synthesized. • Facile synthesis of two new polyimides with 6FDA and a triptycene dianhydride. • Polyimides showed microporous features with high BET surface area. • Polyimides displayed modest effects of aging on gas permeability and selectivity. [ABSTRACT FROM AUTHOR]

Back Cover: Triptycene is a unique building block for high‐performance polymers due to its symmetric D3h structure containing fixed internal free‐volume elements. A novel triptycene diamine (2,6(7)‐dihydroxy‐3,7(6)‐diaminotriptycene) (DAT1‐OH) for the synthesis of an intrinsically microporous polyimide (6FDA‐DAT1‐OH) is described by Ingo Pinnau and co‐workers in article number 1700303. As illustrated in the cover design, it exhibits excellent performance as membrane material for gas separation applications, specifically for the removal of CO2 from natural gas. [ABSTRACT FROM AUTHOR]

A hydroxyl-functionalized triptycene-based polyimide of intrinsic microporosity (TDA1-APAF) was converted to a polybenzoxazole (PBO) by heat treatment at 460 °C under nitrogen atmosphere. TDA1-APAF treated for 15 min (TR 460) resulted in a PBO conversion of 95% based on a theoretical weight loss of 11.7 wt% of the polyimide precursor. The BET surface area of the TR 460 (680 m 2 g −1 ) was significantly higher than that of the TDA1-APAF polyimide (260 m 2 g −1 ) as determined by nitrogen adsorption at −196 °C. Heating TDA1-APAF for 30 min (TRC 460) resulted in a weight loss of 13.5 wt%, indicating full conversion to PBO and partial main-chain degradation. The TR 460 membrane displayed excellent O 2 permeability of 311 Barrer coupled with an O 2 /N 2 selectivity of 5.4 and CO 2 permeability of 1328 Barrer with a CO 2 /CH 4 selectivity of 27. Interestingly, physical aging over 150 days resulted in enhanced O 2 /N 2 selectivity of 6.3 with an O 2 permeability of 185 Barrer. The novel triptycene-based TR 460 PBO outperformed all previously reported APAF-polyimide-based PBOs with gas permeation performance close to recently reported polymers located on the 2015 O 2 /N 2 upper bound. Based on this study, thermally-rearranged membranes from hydroxyl-functionalized triptycene-based polyimides are promising candidate membrane materials for air separation, specifically in applications where space and weight of membrane systems are of utmost importance such as nitrogen production for inert atmospheres in fuel lines and tanks on aircrafts and off-shore oil- or natural gas platforms. Mixed-gas permeation experiments also demonstrated good performance of the TR 460 membrane for natural gas sweetening with a CO 2 permeability of ∼1000 Barrer and CO 2 /CH 4 selectivity of 22 at a typical CO 2 wellhead partial pressure of 10 bar. [ABSTRACT FROM AUTHOR]

An intrinsically microporous hydroxyl-functionalized polyimide (PIM-PI) made from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,6(7)-dihydroxy-3,7(6)-diaminotriptycene (DAT1-OH), was thermally converted to polybenzoxazole (PBO). The thermal rearrangement of the PIM-PI to PBO significantly increased the free volume, which was reflected by a boost in its microporosity as indicated by enhanced Brunauer-Emmett-Teller (BET) surface area from 167 to 405 m2 g−1. The increase in free volume noticeably improved the gas permeability but also resulted in reduced gas-pair selectivity. The fresh PBO membrane made by thermal treatment at 460 °C for 30 min (TRIP-TR-460-30) with a PBO conversion of 98% displayed a 20-fold higher CO 2 permeability of 840 barrer than the initial value of 43 barrer for the 6FDA-DAT1-OH polyimide at the expense of ~ 60% decrease in pure-gas CO 2 /CH 4 selectivity from 52 to 21. The TRIP-TR-460-30 PBO showed good performance for propylene/propane separation with pure-gas C 3 H 6 permeability of 21 barrer and C 3 H 6 /C 3 H 8 selectivity of 16 for a 28-days aged sample. When tested under mixed-gas conditions C 3 H 6 permeability dropped to 12.8 barrer and C 3 H 6 /C 3 H 8 selectivity of 8. TRIP-TR-460-30 PBO displayed mechanical properties comparable some rigid polyimides with tensile strength, Young's modulus and elongation at break of 58 MPa, 1.83 GPa and 4.3%, respectively. Image 1 • TR-derived PBO membranes made from 6FDA/triptycene OH-based diamine polyimide. • Fully converted PBO exhibited improved gas permeability but reduced selectivity. • Physical aging had large effect on gas permeability but small effect on selectivity. • PBO demonstrated performance located on the propylene/propane upper bound. [ABSTRACT FROM AUTHOR]