Intrinsically Microporous Polyimides Derived from 2,2′-Dibromo-4,4′,5,5′-bipohenyltetracarboxylic Dianhydride for Gas Separation Membranes

This work aims to expand the structure–property relationships of bromo-containing polyimides and the influence of bromine atoms on the gas separation properties of such materials. A series of intrinsically microporous polyimides were synthesized from 2,2′-dibromo-4,4′,5,5′-bipohenyltetracarboxylic dianhydride (Br-BPDA) and five bulky diamines, (7,7′-(mesitylmethylene)bis(8-methyldibenzo[b,e][1,4]dioxin-2-amine) (MMBMA), 7,7′-(Mesitylmethylene)bis(1,8-dimethyldibenzo[b,e][1,4] dioxin-2-amine) (MMBDA), 4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-2,8-diamine (TBDA1), 4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-3,9-diamine (TBDA2), and (9R,10R)-9,10-dihydro-9,10-[1,2]benzenoanthracene-2,6-diamine (DAT). The Br-BPDA-derived polyimides exhibited excellent solubility, high thermal stability, and good mechanical properties, with their tensile strength and modulus being 59.2–109.3 MPa and 1.8–2.2 GPa, respectively. The fractional free volumes (FFVs) and surface areas (SBET) of the Br-BPDA-derived polyimides were in the range of 0.169–0.216 and 211–342 m2 g−1, following the order of MMBDA > MMBMA > TBDA2 > DAT > TBDA1, wherein the Br-BPDA-MMBDA exhibited the highest SBET and FFV and thus highest CO2 permeability of 724.5 Barrer. Moreover, Br-BPDA-DAT displayed the best gas separation performance, with CO2, H2, O2, N2, and CH4 permeabilities of 349.8, 384.4, 69.8, 16.3, and 19.7 Barrer, and H2/N2 selectivity of 21.4. This can be ascribed to the ultra-micropores (<0.7 nm) caused by the high rigidity of Br-BPDA-DAT. In addition, all the bromo-containing polymers of intrinsic microporosity membranes exhibited excellent resistance to physical ageing.


Introduction
Polyimides (PIs) have been considered as one of the most attractive and promising gas separation membrane materials due to their good film-forming properties, excellent heat and chemical resistance, good mechanical properties, and high gas perm-selectivity.Commercial polyimides for gas separation membranes include Matrimid ® 5218, P84 ® , Upilex ® , etc. [1].However, the traditional PIs exhibit dense chain packing, resulting in a low fractional free volume (FFV) and insufficient gas permeability.Thus, their gas separation performances were below the Robeson's upper bounds [2][3][4].

Polymer Synthesis
The Br-BPDA-derived polyimides were synthetized by a one-step solution polycondensation method in m-cresol.

Membrane Casting
The chloroform solutions (3 wt%) of the Br-BPDA-derived polymers were purified through 1 µm PTFE filters and cast onto glass panels.They were dried at room temperature for three days to remove chloroform.Specially, Br-BPDA-DAT was dissolved in NMP (3 wt%), and the solvent was depleted by drying at 60 • C for 5 h, 150 • C for 1 h, 200 • C for 1 h, and 250 • C for 4 h in vacuum.

Characterization
Fourier-transform infrared (FT-IR) spectra and 1 H NMR spectra were obtained on a Cary660+620 Micro FTIR instrument (Agilent, Santa Clara, CA, USA) and Bruker Advance Neo 600 spectrometer (Bruker, Rheinstetten, Germany), respectively.Number average molecular weights (M n ), weight average molecular weights (M w ), and polydispersity indices (PDI) were measured on TOSOH HLC-8420GPC (TOSOH, Tokyo, Japan) gel permeation chromatography (GPC) equipped with a refractive index detector using dimethylformamide (DMF + LiBr 0.1 wt%) as the eluent at a flow rate of 0.3 mL min −1 and polystyrene as the calibration standard at 40 • C. Wide-angle X-ray diffraction (WAXD) was performed on a Bruker D8 Advance Davinci instrument (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54 Å) and an angular range from 5 • to 50 • .The storage modulus and tan δ of the Br-BPDA-derived PIM-PIs were studied using a Q850 DMA (TA Instruments, New Castle, DE, USA) at a heating rate of 5 • C min −1 .Thermogravimetric analysis (TGA) was performed on a Q55 TGA (TA Instruments, New Castle, DE, USA) at a heating rate of 10 • C min −1 in nitrogen.Brunauer-Emmett-Teller surface areas (S BET ) were measured via N 2 adsorption at 77 K using an ASAP 2460 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA).Tensile testing was performed on a TCS-2000 electron tensile testing machine (Gotech Testing Machines Inc., Taichung City, Taiwan) at a constant displacement rate of 2 mm min −1 .Contact angles were obtained using a sessile drop water with a Dataphysics OCA-20 contact angle analyzer.The density of the Br-BPDA-derived PIM-PIs were obtained using an SQP balance (Sartorius, Gottingen, Germany) equipped with a density measurement kit.The FFV of the Br-BPDA-derived PIM-PIs was calculated using the group contribution method in the literature [10,33].The gas permeabilities of the Br-BPDA-derived PIM-PIs were measured on a PERME VAC-2 permeation system (Labthink Instruments Co., Ltd., Jinan, China) at 1 bar and 35 • C [34].

Synthesis of Polymers
According to previous reports, polyimides derived from 2,2 ′ -disubstituted BPDA exhibit better solubility than those based on 4,4 ′ -BPDA [24,25].And, given the close V w values of Br-BPDA and 6FDA (161.12 vs. 184.75cm 3 mol −1 ) [35], it could be anticipated that that polyimides from Br-BPDA have a similar FFV to those based on 6FDA.Meanwhile, MMBMA, MMBDA, TBDA1, TBDA2, and DAT have been extensively used for the preparation of PIM-PIs with a high FFV [12,31,32].In this work, PIM-PIs were obtained by combining Br-BPDA with five typical rigid and twisted diamines through hightemperature solution polymerization (Scheme 1).The M w and PDI values of these polymers were 18.0-45.0kg mol −1 and 1.8-3.8(Table 1), respectively.Br-BPDA-MMBMA exhibited the highest molecular weight, which may be attributed to the high reactivity of MMBMA.The FT-IR spectra of the Br-BPDA-derived polymers are shown in Figure S1, and the successful synthesis of the imine ring structure are demonstrated by the characteristic peaks at ~1770 cm −1 (asymmetric C=O stretching), ~1730 cm −1 (symmetric C=O stretching), and ~1360 cm −1 (C-N stretching).Moreover, the absorption peaks of amide or amino groups, located at ~3350 cm −1 (asymmetric N-H stretching vibration), ~3170 cm −1 (symmetric N-H stretching), ~1650 cm −1 (N-H bending) completely disappear, indicating the complete reaction of diamine with no residual diamine molecules in the polymer chain.And, in the 1 H NMR spectra (Figure S2), the peaks at 8.3 ppm and 7.8 ppm represent the aromatic ring protons in the Br-BPDA residues.The absence of peaks for -NH 2 and -COOH groups also proves the successful preparation of polyimides.Additionally, the chemical structure of Br-BPDA-DAT is further confirmed by 13 C NMR (Figure S4), where the features are fully assigned.These results confirm the successful synthesis of Br-BPDA-derived PIM-PIs.All the Br-BPDA-derived PIs exhibited good solubility in high-boiling-point solvents due to the introduction of bromine substituents (Table S1).In addition, the PIs with TB and dibenzodioxane segments were soluble in low-boiling-point solvents like CHCl3.In contrast, Br-BPDA-DAT exhibited relatively poor solubility in the overall series perhaps due to the unique Trip structure in DAT leading to a higher aromatic ring content.Due to the lower chain packing density (Table 2), Br-BPDA-MMBDA and Br-BPDA-MMBMA Scheme 1. Synthesis of Br-BPDA-derived polymers.All the Br-BPDA-derived PIs exhibited good solubility in high-boiling-point solvents due to the introduction of bromine substituents (Table S1).In addition, the PIs with TB and dibenzodioxane segments were soluble in low-boiling-point solvents like CHCl 3 .In contrast, Br-BPDA-DAT exhibited relatively poor solubility in the overall series perhaps due to the unique Trip structure in DAT leading to a higher aromatic ring content.Due to the lower chain packing density (Table 2), Br-BPDA-MMBDA and Br-BPDA-MMBMA showed the best solubility.These polymers showed high modulus (59.2-109.3MPa) and good strength (1.8-2.2GPa), indicating that they have outstanding mechanical properties (Table 1 and Figure S4).The glass transition temperatures (T g ) of these polymers were obtained by DMA testing (Figure 1), and all T g values were higher than 400 • C. In particular, the T g of Br-BPDA-DAT was higher than 500 • C. The T g values of these PIM-PIs followed the order of Br-BPDA-MMBMA < Br-BPDA-TBDA1 < Br-BPDA-TBDA2 < Br-BPDA-MMBDA < Br-BPDA-DAT.Br-BPDA-MMBMA exhibited the lowest T g due to its relatively low chain rigidity, resulting from the absence of ortho-positioned methyl substituents.All the Br-BPDA-derived PIs exhibited good solubility in high-boiling-point solvents due to the introduction of bromine substituents (Table S1).In addition, the PIs with TB and dibenzodioxane segments were soluble in low-boiling-point solvents like CHCl3.In contrast, Br-BPDA-DAT exhibited relatively poor solubility in the overall series perhaps due to the unique Trip structure in DAT leading to a higher aromatic ring content.Due to the lower chain packing density (Table 2), Br-BPDA-MMBDA and Br-BPDA-MMBMA showed the best solubility.These polymers showed high modulus (59.2-109.3MPa) and good strength (1.8-2.2GPa), indicating that they have outstanding mechanical properties (Table 1 and Figure S4).The glass transition temperatures (Tg) of these polymers were obtained by DMA testing (Figure 1), and all Tg values were higher than 400 °C.In particular, the Tg of Br-BPDA-DAT was higher than 500 °C.The Tg values of these PIM-PIs followed the order of Br-BPDA-MMBMA < Br-BPDA-TBDA1 < Br-BPDA-TBDA2 < Br-BPDA-MMBDA < Br-BPDA-DAT.Br-BPDA-MMBMA exhibited the lowest Tg due to its relatively low chain rigidity, resulting from the absence of ortho-positioned methyl substituents.The contact angles of the Br-BPDA-derived PIM-PIs were 95.0-103.9• (Figure S5).The hydrophobicity of these polymers can be explained by their relatively lower imide contents, as well as the methyl substituents for some cases.

Microstructural Properties
It can be observed that there were significant hysteresis loops and a higher nitrogen absorption at a relatively low pressure in the nitrogen adsorption desorption isotherm, both of which were obvious characteristics of intrinsic microporous polymers (Figure 2a) [11].The values of S BET ranged from 211 to 342 m 2 g −1 , with the order of Br-BPDA-TBDA1 < Br-BPDA-DAT < Br-BPDA-TBDA2 < Br-BPDA-MMBMA < Br-BPDA-MMBDA.And, the value of S BET for Br-BPDA-MMBDA was 21% higher than that of Br-BPDA-MMBMA, and the value of S BET for Br-BPDA-TBDA2 was 64% higher than that of Br-BPDA-TBDA1, which is also consistent with the value of the FFV.It can be deduced that the introduction of bulky bromine atoms and ortho-positioned methyl group will restrain the chain densification of PI, open the polymer backbone, and induce a larger free volume [23].
The contact angles of the Br-BPDA-derived PIM-PIs were 95.0-103.9°(Figure S5).The hydrophobicity of these polymers can be explained by their relatively lower imide contents, as well as the methyl substituents for some cases.

Microstructural Properties
It can be observed that there were significant hysteresis loops and a higher nitrogen absorption at a relatively low pressure in the nitrogen adsorption desorption isotherm, both of which were obvious characteristics of intrinsic microporous polymers (Figure 2a) [11].The values of SBET ranged from 211 to 342 m 2 g −1 , with the order of Br-BPDA-TBDA1 < Br-BPDA-DAT < Br-BPDA-TBDA2 < Br-BPDA-MMBMA < Br-BPDA-MMBDA.And, the value of SBET for was 21% higher than that of Br-BPDA-MMBMA, and the value of SBET for Br-BPDA-TBDA2 was 64% higher than that of Br-BPDA-TBDA1, which is also consistent with the value of the FFV.It can be deduced that the introduction of bulky bromine atoms and ortho-positioned methyl group will restrain the chain densification of PI, open the polymer backbone, and induce a larger free volume [23].The CO2 uptake is affected by the CO2 affinity and specific surface areas of polymers [2].For these Br-BPDA-derived PIM-PIs, at 273 K and P/P0 = 0.029, the CO2 uptake ranged from 26.1 to 30.4 cm 3 g −1 (Figure 2b and Table 2).Among them, Br-BPDA-TBDA1/TBDA2 exhibited a higher CO2 uptake because of the dipole-quadrupole interaction between the The CO 2 uptake is affected by the CO 2 affinity and specific surface areas of polymers [2].For these Br-BPDA-derived PIM-PIs, at 273 K and P/P 0 = 0.029, the CO 2 uptake ranged from 26.1 to 30.4 cm 3 g −1 (Figure 2b and Table 2).Among them, Br-BPDA-TBDA1/TBDA2 exhibited a higher CO 2 uptake because of the dipole-quadrupole interaction between the tertiary amine in the TB skeleton and the polarized CO 2 molecule [12,[36][37][38].This result also demonstrates that ortho-positioned methyl groups have a similar effect on the surface areas of polymers, like Br-BPDA-TBDA1(29.3 cm 3 g −1 ) < Br-BPDA-TBDA2 (30.4 cm 3 g −1 ) and Br-BPDA-MMBMA (26.1 cm 3 g −1 ) < Br-BPDA-MMBDA (28.1 cm 3 g −1 ).This indicates that ortho-substituted groups (-Br and -CH 3 ) hinder rotational chemical bonds and have an impact on the surface, which is consistent with the FFV and S BET results.
The cumulative volume of micropores (V M ) for these polymers spanned a range of 0.030-0.045cm 3 g −1 .Br-BPDA-MMBDA/MMBMA showed a relatively lower V M value, owing to the relatively low rigidity of dibenzodioxane moieties relative to TB or Trip segments.In addition, the V M values of Br-BPDA-TBDA1 were lower than Br-BPDA-TBDA2, and the V M values of Br-BPDA-MMBMA were lower than Br-BPDA-MMBDA, respectively, due to the absence of an ortho substituents methyl group.
Regarding density, according to Bondi's group contribution method [35,39], the values of the FFV of these polyimides, ranged from 0.169 to 0.216 (Table 2).And, similar tendencies were observed for the FFV and S BET of these polymers.The chain packing profiles of the Br-BPDA-derived PIM-PIs were characterized by WAXD measurements (Figure 3 and Table 2).The diffraction peaks were mainly located in the range of 10-30 • , indicating their amorphous features.The average distances between different molecular chains of the corresponding PIM-PIs were in the range of 4.92-5.88and 3.75-3.94Å, respectively.Two major peaks were fitted in these diffraction peaks (labelled as d A and d B ) [40,41], where d A and d B were respectively assigned to the interchain distances and the π-π stacking interactions.The effect of ortho substituents on d-spacing was also observed in WAXD curves, the d A value of Br-BPDA-MMBMA < Br-BPDA-MMBDA, and the d A value of Br-BPDA-TBDA1 < Br-BPDA-TBDA2.Additionally, a shoulder peak was also observed at around 10 • , which corresponded to the larger micropores in the PIM-PIs.The result is beneficial for improving gas permeability.The amorphous feature was a typical characteristic of the PIM-PI that could be applied to gas separation.This tendency was consistent with FFV and S BET of the polymers.
owing to the relatively low rigidity of dibenzodioxane moieties relative to TB or Trip segments.In addition, the VM values of Br-BPDA-TBDA1 were lower than Br-BPDA-TBDA2, and the VM values of Br-BPDA-MMBMA were lower than Br-BPDA-MMBDA, respectively, due to the absence of an ortho substituents methyl group.
Regarding density, according to Bondi's group contribution method [35,39], the values of the FFV of these polyimides, ranged from 0.169 to 0.216 (Table 2).And, similar tendencies were observed for the FFV and SBET of these polymers.The chain packing profiles of the Br-BPDA-derived PIM-PIs were characterized by WAXD measurements (Figure 3 and Table 2).The diffraction peaks were mainly located in the range of 10°-30°, indicating their amorphous features.The average distances between different molecular chains of the corresponding PIM-PIs were in the range of 4.92-5.88and 3.75-3.94Å, respectively.Two major peaks were fitted in these diffraction peaks (labelled as dA and dB) [40,41], where dA and dB were respectively assigned to the interchain distances and the ππ stacking interactions.The effect of ortho substituents on d-spacing was also observed in WAXD curves, the dA value of Br-BPDA-MMBMA < Br-BPDA-MMBDA, and the dA value of Br-BPDA-TBDA1 < Br-BPDA-TBDA2.Additionally, a shoulder peak was also observed at around 10°, which corresponded to the larger micropores in the PIM-PIs.The result is beneficial for improving gas permeability.The amorphous feature was a typical characteristic of the PIM-PI that could be applied to gas separation.This tendency was consistent with the FFV and SBET of the polymers.

Gas Separation Performance
The water and CO 2 in the air can cause the film to plasticize, increasing the FFV and reducing the gas selectivity of the polymers [42].So, before conducting gas separation tests, it was necessary to soak the fresh membranes in methanol for one day, which was beneficial for improving the gas separation performance [43], and then dry them under vacuum at 100 • C to remove methanol and H 2 O from the membranes.The pure gas transport properties of these PIs were investigated with gases including H 2 , CO 2 , O 2 , N 2 , and CH 4 .Their gas permeabilities and selectivities are shown in Table 3.For the given polymers, the gas permeabilities of these polymers were in the order of P CO 2 > P H 2 > P O 2 > P CH 4 > P N 2 .However, after ageing for 300 or 900 days, the order between P CH 4 and P N 2 was reversed.For instance, the P H2 , P CO 2 , P O2 , P N 2 , and P CH 4 of Br-BPDA-MMBDA (FFV = 0.216) were 576.5, 724.5, 143.2, 42.9, and 61.7 Barrer, respectively, which were 1.6-3.1 times greater than those of Br-BPDA-DAT (FFV = 0.189).In addition, Br-BPDA-TBDA1 (FFV = 0.169), Br-BPDA-TBDA2 (FFV = 0.177), and Br-BPDA-DAT (FFV = 0.189) showed moderate gas permeabilities despite their smaller FFV because of the Trip and TB groups, leading to higher rigidity.However, compared with Br-BPDA-TBDA1, Br-BPDA-TBDA2 was more permeable due to the presence of ortho-substituted methyl groups, leading to insufficient space for the free rotation of C-N bonds in the imide rings.Br-BPDA-MMBDA exhibited the lowest selectivity due to its moderate FFV but a low cumulative micropore volume (Figure 2c).In contrary, Br-BPDA-TBDA1 displayed the highest selectivity because of the lowest d-spacing (d A = 4.92 Å) and FFV.In these PIs containing TB or Trip structures, the unique rigid structure gave the polymer a higher cumulative volume of ultramicropores, while strong π-π stacking interactions gave the polymer a higher gas sieving ability [44].The synergistic effect of these two factors maintained a good balance between permeability and selectivity.The gas separation performance of the Br-BPDA-derived PIM-PIs with other commercial membranes was also compared according to Robeson's upper bounds (Figure 4).The Br-BPDA-derived PIM-PIs exhibited better performances than most commercial polymers, such as Matrimid ® 5218, P84 ® [3, [44][45][46][47][48][49][50][51][52][53].However, the Br-BPDA-derived polymers showed a slightly inferior or similar performance regarding gas separation compared to their counterparts based on 6FDA due to the slightly lower V w of Br-BPDA than 6FDA (161.12 vs. 184.75cm 3 mol −1 ) [12,39,42,45].
The physical aging characteristics of the Br-BPDA-derived PIM-PIs were systematically assessed.The gas separation performances of fresh membranes and those aged membranes were shown in Figure 4 and Table 3.The permeabilities of all the PIM-PIs decreased, and the selectivities increased due to the collapse of larger micropores and the decrease in FFV.The loss in gas permeability was most pronounced in gases with larger kinetic diameters, resulting in the improved selectivity of H 2 /N 2 , H 2 /CH 4 , and CO 2 /CH 4 .Zhao et al. [28,29] investigated the effect of -Br groups on the physical aging of polymer membranes and found that the bromo groups could effectively retard the speed of physical aging by the interference with the nearby carbonyl groups and restricted rotation.Similar trends were also observed in this work.The H 2 permeability of all the Br-BPDA-derived PIM-PIs decreased by only 26.7-41.5% after aging for 300 days and 45.2-68.6% for 900 days (Figure 5).In addition, the gas separation performance of all the aged membranes was well retained or even improved compared to the fresh membranes, indicating their excellent resistance to physical ageing.compared to their counterparts based on 6FDA due to the slightly lower Vw of Br-BPDA than 6FDA (161.12 vs. 184.75cm 3 mol −1 ) [12,39,42,45].The physical aging characteristics of the Br-BPDA-derived PIM-PIs were systematically assessed.The gas separation performances of fresh membranes and those aged membranes were shown in Figure 4 and Table 3.The permeabilities of all the PIM-PIs decreased, and the selectivities increased due to the collapse of larger micropores and the decrease in FFV.The loss in gas permeability was most pronounced in gases with larger kinetic diameters, resulting in the improved selectivity of H2/N2, H2/CH4, and CO2/CH4.Zhao et al. [28,29] investigated the effect of -Br groups on the physical aging of polymer membranes and found that the bromo groups could effectively retard the speed of physical aging by the interference with the nearby carbonyl groups and restricted rotation.Similar trends were also observed in this work.The H2 permeability of all the Br-BPDA-derived PIM-PIs decreased by only 26.7-41.5% after aging for 300 days and 45.2-68.6% for 900 days (Figure 5).In addition, the gas separation performance of all the aged membranes was well retained or even improved compared to the fresh membranes, indicating their excellent resistance to physical ageing.

Conclusions
Five high-molecular weight PIM-PIs were prepared from Br-BPDA and different diamines (MMBMA, MMBDA, TBDA1, TBDA2, and DAT) via the "one-step" method in m-cresol.These Br-BPDA-derived PIM-PIs exhibited extremely high Tg (>400 °C) because of their twisted structure from diamines and ortho-substituted groups (-Br and -CH3) hinder rotational chemical bonds.And, the Br-BPDA-derived PIM-PIs showed favorable mechanical properties for gas separation application, tensile strength > 59.2 MPa, modulus > 1.8 GPa, and elongation at break > 3.4%.The markedly high d-spacing values of the polymers (>0.492 nm) were observed due to the incorporation of large bromine substituents and a twisted diamine monomer.Br-BPDA-MMBDA displayed the highest

Figure 3 .
Figure 3. WAXD patterns of Br-BPDA-derived PIM-PIs (Black line: Experimental data; Red line: Linear fitting curve of experimental data; Red point: The highest peak at dA).

Figure 3 .
Figure 3. WAXD patterns of Br-BPDA-derived PIM-PIs (Black line: Experimental data; Red line: Linear fitting curve of experimental data; Red point: The highest peak at d A ).