PEG–imidazolium-functionalized 6FDA–durene polyimide as a novel polymeric membrane for enhanced CO₂ separation

Abstract

A series of poly(ethylene glycol)–imidazolium-functionalized 6FDA–durene polyimides (PEG–Im-PIs) with a range of PEG chain lengths was developed. The structures and properties of this series, as well as the gas separation properties of the corresponding polymer membranes, were studied. A PEG group and an imidazolium-based ionic group, both of which acted as CO₂-solubilizing groups, were incorporated onto a rigid polyimide backbone to yield a very high CO₂ solubility and, hence, excellent CO₂/CH₄ (49.2) and CO₂/N₂ (40.1) permselectivities while maintaining a good permeability of polyimide.

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Introduction

The development of CO₂ capture processes is a key technical, economical, and environmental challenge because a range of applications rely on the separation of CO₂ from other light gases, such as N₂ and CH₄. CO₂ and N₂ are flue gases produced after the combustion of fossil fuels and air (post-combustion process). Natural gas production also involves a purification step because mined natural gases (primarily CH₄) contain around 10% CO₂. This quantity of CO₂ considerably reduces the value of the mined natural gas because CO₂ is not combustible. Furthermore, CO₂ is highly corrosive, and its removal is necessary for natural gas pipeline transport.

A variety of techniques have been developed to perform these CO₂/light gas separation steps, including pressure swing adsorption, cryogenic distillation, gas–liquid absorption, and membrane-based processes. Among these techniques, polymer membrane-based gas separation approaches have gained a great deal of attention because they are flexible, easy to scale up, and compete successfully with conventional separation processes, such as absorption (amine scrubbing).^1–4 In general, polymer materials are mechanically stable, offer good separation capabilities, and can be fabricated in a range of configurations, such as hollow fibers or flat sheets.⁵ The commercially available, including polyimide (PI), poly(dimethylsiloxane) (PDMS), or cellulose acetate (CA), unfortunately do not meet several industrial requirements in that they do not afford a high CO₂ flux (i.e., their CO₂ permeability is low) or a high purity (i.e., the CO₂/light gas selectivity is poor). In fact, as dense separation materials, polymers typically exhibit a trade-off between permeability and selectivity, represented by the so-called “upper bound” in the Robeson plot.^6,7

Polymer membranes that can provide selective CO₂ separation processes may be developed through the design of polymers with a high diffusivity–selectivity and/or a high solubility–selectivity. The diffusivity–selectivity reflects the ability of a polymer matrix to select the shape and size of the penetrant molecules.⁸ Diffusivity–selectivity is mainly governed by the relative mobilities of penetrants and by structural factors, such as the polymer chain stiffness and inter-segmental polymer packing structure. In general, gas permeability can be enhanced by increasing the diffusivity–selectivity for small gas molecule separation applications involving rigid glassy polymer membranes, such as polyimides (PIs), with narrow free volume distributions.

The solubility–selectivity properties, on the other hand, are largely governed by the relative strengths of the polymer–penetrant interactions and the relative condensabilities of the penetrants. The CO₂/gas selectivity of a polymer membrane may often be improved by introducing CO₂-philic species, such as ionic liquid (ILs),⁹ amines,¹⁰ or poly(ethylene oxide)s (PEO),^11,12 all of which increase the solubility–selectivity of the material.

A handful of studies have examined CO₂/N₂ separation approaches using supported ionic liquid membranes (SILMs),^13–15 in which microporous polymers are impregnated with ILs to achieve a high permeability and selectivity. The use of SILMs in practical gas separation processes, however, is limited by stability issues (leaching of ILs from the membranes at pressure differences beyond 0.2 atm).¹³

Membranes prepared from polymeric ILs, or poly(IL)s, may offer another option for enhancing the separation of CO₂ from other gases because poly(IL)s offer a high CO₂ sorption capacity and high sorption and desorption rates compared to ILs.^16–22 Poly(IL)s are not strictly ionic liquids, but rather are polymers containing various forms of certain ionic salts. Nevertheless, poly(IL)s share similar features with ILs, such as a high CO₂ solubility. Although these poly(ionic salts) have proven to be selective in the separation of CO₂ from other gases, their utility in practical gas separation applications has been limited by their physical and chemical properties as well as by their low permeabilities (typically below 100 Barrer for CO₂). Previously examined poly(IL)s have been prepared from IL-containing reactive monomers, and the polymer structures used to prepare the poly(IL)s have largely been limited to flexible polymer backbones, such as polyacrylates or polystyrenes.

We recently developed a new type of poly(IL)s in which pendant imidazolium or piperazinium-based ionic salts were introduced onto the rigid polyimide (PI) backbone as highly CO₂-selective polymer membranes.^20,21 Membranes prepared from these newly developed PI-based poly(IL)s displayed high CO₂ separation and permeation properties, together with excellent mechanical and thermal stabilities.

PEO (or PEG: poly(ethylene glycol)) membrane materials can provide excellent CO₂ separation and permeability properties because PEO (or PEG, a low molecular weight version of PEO) chains bearing polar ether oxygen units display a high affinity toward CO₂ molecules to produce a high solubility–selectivity.²³ Pure PEO (or PEG), however, tends to form a partially crystalline structure with a low permeability for gas molecules. A variety of approaches have been developed for combining the diffusivity–selective properties of glassy polymers and the solubility–selective properties of PEO in an effort to improve the CO₂-selective gas separation performance. Such polymers include PEO-containing polyimides (PEO-PI),^24–26 PEO-containing polyamides (PEO-PA),²⁷ PEO-containing polyurethane or polyurea (PEO-PU),²⁸ and PEO-containing polysulfone (PEO-PS).²⁹ Most of these PEO-containing polymer membranes unfortunately have not provided permeabilities comparable to that of rigid polymers, such as PI (with a permeability exceeding 300 Barrer for CO₂), except for a few examples.^30,31

The present work combines the benefits of the above-mentioned PEG and poly(ionic salt)s, which offer a high CO₂ solubility, with the highly permeable properties of rigid polymers by incorporating both PEG and IL onto PI, yielding a novel material for gas separation. The membranes obtained from the PEG–imidazolium-functionalized PIs (PEG–Im-PIs) displayed excellent thermal and mechanical stabilities and, most importantly, extremely high CO₂/CH₄ and CO₂/N₂ permselectivities, together with a very high CO₂ permeability of 484.8 Barrer. We also investigated the effects of the PEG chains in the PEG–imidazolium groups on the structures and properties of the polymers, as well as the gas separation properties of the corresponding polymer membranes.

Experimental

Materials

4,4′-(Hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,3,5,6-tetramethyl benzene-1,4-diamine (durene) and N-bromosuccinimide (NBS) were purchased from Tokyo Chemical Industry (TCI) Co., Ltd. (Tokyo, Japan) and used as obtained. Triethyl amine, acetic anhydride, di(ethylene glycol) monomethyl ether, tetra(ethylene glycol) monomethyl ether and poly(ethylene glycol) monomethyl ether (M_n 550 g mol⁻¹) were obtained from Sigma Aldrich. Poly(ethylene glycol) monomethyl ether (M_n 350 g mol⁻¹) were obtained from Alfa-Aesar (a Johnson Matthey Co.). 6FDA and durene were dried under vacuum at 60 °C for 24 h prior to the polymerization. All other chemicals, unless otherwise noted, were obtained from commercial sources and used as received. Synthesis of the PEG–imidazoles having various PEG chains is described, together with their ¹H NMR spectra (from Fig. S1 to S4), in the ESI.†

Characterization

¹H NMR spectra were obtained on an Agilent 400-MR (400 MHz) instrument using d₆-DMSO or CDCl₃ as a reference or internal deuterium lock. FT-IR spectra of the materials were recorded as KBr pellets using Nicolet MAGNA 560-FTIR spectrometer in the range of 4000–400 cm⁻¹. Molar masses were determined by Gel Permeation Chromatography (GPC) using two PL gel 30 cm × 5 μm mixed C columns at 30 °C running in THF and calibrated against polystyrene (M_n = 600 to 10⁶ g mol⁻¹) standards using a Knauer refractive index detector. The glass transition temperature (T_g) of each polymer was measured using a Perkin-Elmer Pyris-1 DSC from 20 °C to 300 °C with a scan rate of 10 °C min⁻¹ under nitrogen.

Synthesis of PEG–imidazolium polyimides with various PEG chains (1)

Synthesis of 6FDA–durene polyimide (2). A general two-step synthesis such as polyamic acid formation followed by imidization was performed for the synthesis of 6FDA–durene polyimide. Into a 500 mL two-necked flask equipped with a magnetic stirrer, nitrogen inlet, and a condenser, 6FDA (13.52 g, 30.44 mmol), durene (5 g, 30.44 mmol), and DMAc (90 mL) were added. Then the reaction mixture was cooled to −5 °C (ice-bath), and allowed to stir for 12 h in order to form the corresponding polyamic acid. After this time, triethyl amine (8.91 mL, 63.9 mmol) and acetic anhydride (6.04 mL, 63.9 mmol) were added to the reaction mixture and then the temperature was raised to 110 °C (oil-bath) under vigorous stirring for 3 h to induce a complete imidization of polyamic acid to form polyimide. The viscous mixture was then cooled to r.t. and dissolved in DMAc (10 mL), followed by pouring into methanol (400 cm³). White polymer beads were collected by filtration, washed with deionized water several times and dried at 80 °C under vacuum for 48 h to give the 6FDA–durene polyimide 2 (16.3 g, 94%); δ_H (400 MHz, CDCl₃) 8.1–8.0 (2H, br signal, ArH), 8.0–7.9 (4H, br signal, ArH), and 2.14 (12H, s, CH₃); (ATR-FTIR)/cm⁻¹ 2925, 1786, 1712, 1370, 1250, 1187, 1112 and 980; GPC (DMF, RI)/Da M_n 123.82 kg mol⁻¹, M_w 174.66 kg mol⁻¹ and M_w/M_n 1.41.

Bromination of 6FDA–durene polyimide (2) to give the bromobenzylated PI (3)

The polyimide 2 (13 g, 22.7 mmol) and a catalytic amount of benzoyl peroxide (BPO) was dissolved with tetrachloroethane (60 mL) in a 250 mL two-necked flask equipped with a magnetic stirrer, nitrogen inlet, and a condenser. This was heated to 85 °C for a complete dissolution before adding N-bromosuccinimide (2.43 g, 13.6 mmol) and allowed for 12 h stirring at this temperature. The resultant red colored polymer solution was cooled to r.t. and precipitated into methanol (400 mL). The yellow-colored polymer beads were collected by filtration and washed with deionized water, and dried under vacuum at 80 °C for 48 h to give the brominated polyimide 3 (13.4 g, 90%); δ_H (400 MHz, CDCl₃) 7.9–7.6 (10H, br signal, ArH), 7.4–7.3 (4H, br signal, ArH), 7.3–7.2 (2H, br signal, ArH), 7.2–7.1 (4H, br signal, ArH), 7.15 (2H, s, ArH), 7.0–6.7 (16H, br signal, ArH), 4.51 (2H, s, ArCH₂), 2.17 (4H, s, ArH) and 1.70 (6H, s, CH₃); (KBr)/cm⁻¹ 3043, 2967, 1747, 1641, 1496, 1236, 1145, 1024, 939, 841 and 620.

Incorporation of the PEG–imidazoles to give the PEG–Im-functionalized PIs (1)

To a solution of the brominated PI 3 (5 g, 7.67 mmol) in DMF (25 cm³), the corresponding PEG–imidazoles in DMF was added dropwise. The reaction mixture was heated to 90 °C for 24 h with vigorous stirring under nitrogen. After this time, the brown solution was cooled to r.t. and precipitated into ethyl acetate (200 cm³). The brown yellow-colored polymer powders were collected by filtration, washed 3 times with ether and dried at 80 °C under vacuum to give the desired PEG–imidazolium-functionalized PIs 1;

[C₂PEG–ImPI][Br] (1a). (4.6 g, 83%); δ_H (400 MHz, d₆-DMSO) 9.0–8.9 (1H, br signal, ArH), 8.3–7.8 (6H, br signal, ArH), 7.7–7.6 (1H, br signal, ArH), 7.3–7.2 (1H, br signal, ArH), 5.7–5.4 (2H, br signal, ArCH₂), 4.4–4.2 (2H, br signal, NCH₂), 3.7–3.6 (2H, br signal, NCH₂CH₂O), 3.4–2.9 (4H, br signal, NCH₂CH₂OCH₂CH₂O), 2.5–2.4 (3H, br signal, N(CH₂CH₂O)₂CH₃) and 2.1–1.9 (9H, br signal, CH₃); (KBr)/cm⁻¹ 2925, 1795, 1710, 1560, 1485, 1450, 1360, 1270, 1190, 1100, 986 and 840.

[C₄PEG–ImPI][Br] (1b). (4.1 g, 81%); δ_H (400 MHz, d₆-DMSO) 9.0–8.9 (1H, br signal, ArH), 8.3–7.8 (6H, br signal, ArH), 7.8–7.7 (1H, br signal, ArH), 7.3–7.2 (1H, br signal, ArH), 5.7–5.5 (2H, br signal, ArCH₂), 4.4–4.3 (2H, br signal, NCH₂), 3.8–3.6 (2H, br signal, NCH₂CH₂O), 3.5–3.3 (10H, br signal, NCH₂CH₂(OCH₂CH₂O)₂CH₂), 3.2–3.1 (2H, br signal, N(CH₂CH₂O)₃CH₂CH₂O), 2.5–2.4 (3H, br signal, N(CH₂CH₂O)₄CH₃) and 2.1–1.9 (9H, br signal, CH₃); (KBr)/cm⁻¹ 2910, 1795, 1710, 1560, 1485, 1450, 1360, 1270, 1190, 1100, 986 and 841.

[C₈PEG–ImPI][Br] (1c). (4.2 g, 86%); δ_H (400 MHz, d₆-DMSO) 9.0–8.8 (1H, br signal, ArH), 8.3–7.8 (6H, br signal, ArH), 7.7–7.6 (1H, br signal, ArH), 7.3–7.2 (1H, br signal, ArH), 5.6–5.4 (2H, br signal, ArCH₂), 4.3–4.2 (2H, br signal, NCH₂), 3.7–3.6 (2H, br signal, NCH₂CH₂O), 3.5–3.3 (26H, br signal, NCH₂CH₂(OCH₂CH₂O)₆CH₂), 3.2–3.1 (2H, br signal, N(CH₂CH₂O)₇CH₂CH₂O), 2.5–2.4 (3H, br signal, N(CH₂CH₂O)₈CH₃) and 2.1–1.9 (9H, br signal, CH₃); (KBr)/cm⁻¹ 2920, 1795, 1710, 1560, 1480, 1440, 1360, 1270, 1190, 1100, 986 and 840.

[C₁₂PEG–ImPI][Br] (1d). (4.1 g, 87%); δ_H (400 MHz, d₆-DMSO) 9.0–8.8 (1H, br signal, ArH), 8.3–7.8 (6H, br signal, ArH), 7.7–7.6 (1H, br signal, ArH), 7.3–7.2 (1H, br signal, ArH), 5.7–5.5 (2H, br signal, ArCH₂), 4.4–4.2 (2H, br signal, NCH₂), 3.8–3.6 (2H, br signal, NCH₂CH₂O), 3.5–3.3 (42H, br signal, NCH₂CH₂(OCH₂CH₂O)₁₀CH₂), 3.2–3.1 (2H, br signal, N(CH₂CH₂O)₁₁CH₂CH₂O), 2.5–2.4 (3H, br signal, N(CH₂CH₂O)₁₂CH₃) and 2.1–1.8 (9H, br signal, CH₃); (KBr)/cm⁻¹ 2923, 1795, 1710, 1555, 1480, 1445, 1360, 1270, 1190, 1100, 986 and 850.

Membrane preparation

All the PEG–Im-functionalized PIs (1) and pristine PI (6FDA–durene, 2) membranes were prepared in a DMF solution of the corresponding polymers using the solution-casting method. The corresponding polymers 1 (or 3) (1.0 g) were dissolved in 5.0 cm³ of dry DMF and stirred at r.t. overnight. The solutions were poured onto glass plates after a thorough filtration through a plug of cotton. The plates were then placed in an oven, covered with aluminum foils having small holes and allowed to slow solvent evaporation at 70 °C for 48 h and further dried at 90 °C for 16 h in a vacuum oven. After becoming completely dried, the resulting membranes were cooled to r.t. and peeled off from the glass plate, and then being dried at the ambient temperature. The membrane thickness was controlled to be 70 to 90 μm.

Membrane characterization

The densities of the membranes (g cm⁻³) were determined experimentally using a top-loading electronic Mettler Toledo balance (XP205, Mettler-Toledo, Switzerland) coupled with a density kit based on Archimedes' principle. The samples were weighed in air and a known-density liquid, high purity water. The measurement was performed at room temperature by the buoyancy method and the density was calculated as follows
where, W₀ and W₁ are the membrane weights in air and water respectively. The water sorption of the PEG–ImPI membranes was not considered due to their extremely low water uptake property.

The X-ray diffraction patterns of the membranes were measured using a Rigaku DMAX-2200H diffractometer by employing a scanning rate of 4° min⁻¹ in a 2θ range from 5° to 30° with a Cu Kα1 X-ray (λ = 0.1540598). The d-spacings were calculated using the Bragg's law (d = λ/2 sin θ).

Tensile strength and elongation at break of the membranes were measured on a Shimadzu EZ-TEST E2-L instrument benchtop tensile tester using a crosshead speed of 1 mm min⁻¹ at 25 °C under 50% relative humidity. Engineering stress was calculated from the initial cross sectional area of the sample and Young's modulus (E) was determined from the initial slope of the stress–strain curve. The membrane samples were cut into a rectangular shape with 80 mm × 8 mm (total) and 80 mm × 4 mm (test area), and five specimens were used for the measurements.

Gas permeation procedure

The pure gas permeation measurements were performed using a high-vacuum time lag measurement unit based on constant-volume/variable-pressure method. All the experiments were performed at a feed pressure of 2 atm and a feed temperature of 35 °C. Before the gas permeation measurements, both the feed and the permeate sides were thoroughly evacuated to below 10⁻⁵ Torr until the readout showed zero values to remove any residual gases. The downstream volume was calibrated using a Kapton membrane and was found to be 50 cm³. The upstream and downstream pressures were measured using a Baraton transducer (MKS; model no. 626B02TBE) with a full scale of 10 000 and 2 Torr, respectively. The pressure rise versus time transient of the permeate side, equipped with a pressure transducer, was recorded and passed to a desktop computer through a shield data cable. The permeability coefficient was determined from the linear slope of the downstream pressure rise versus time plot (dp/dt) according to the following equation:where P is the permeability expressed in Barrer (1 Barrer = 10⁻¹⁰ cm³ (STP) cm cm⁻² s⁻¹ cm⁻¹ Hg⁻¹); V (cm³) is the downstream volume; l (cm) is the membrane thickness; A (cm²) is the effective area of the membrane; T (K) is the temperature of measurement; p₀ (Torr) is the pressure of the feed gas in the upstream chamber and dp/dt is the rate of the pressure rise under the steady state. The gas permeation tests were repeated more than three times for all the gases, and the standard deviation from the mean values of permeabilities was within ca. ±3%. Sample to sample reproducibility was high and within ±3%. The effective membrane areas were 15.9 cm². The ideal permselectivity, α_A/B, of the membrane for a pair of gases (A and B) is defined as the ratio of the individual gas permeability coefficients as

The diffusivity and solubility were obtained from the time-lag (θ) value according to the following equations:

where D (cm² S⁻¹) is the diffusivity coefficient, l is the membrane thickness (cm) and θ is the time lag (s), obtained from the intercept of the linear steady state part of downstream pressure rise versus time plot. Solubility, S, was calculated from eqn (4) with permeability and diffusivity obtained from eqn (1) and (3).

Results and discussion

Synthesis of the PEG–imidazolium-functionalized polyimides having a variety of PEG chains

The synthesis of PEG–imidazolium-functionalized polyimides (PEG–Im-PI)s 1 is summarized in Scheme 1. The 6FDA–durene polyimide 2 was first synthesized by a polycondensation reaction between durene and 6FDA, following the literature procedures.^32,33 This step was followed by a selective bromination at the benzylic position and the incorporation of a PEG–imidazolium group.

Scheme 1 Schematic representation of the preparation of the PEG–imidazolium-functionalized PI membranes with various PEG chain lengths.

The polyimide 2 was found to have a very high molecular weight (M_w = 174.66 kDa, as confirmed by GPC), and similarly high molecular weights were also reported.^32,33 The selective bromination of the ArCH₃ unit of PI 2 was conducted in tetrachloroethane solvent using 0.6 equiv. NBS to produce the bromobenzylated PI 3. Selective bromination was confirmed using a comparative ¹H NMR spectroscopic analysis of 2 and 3 (Fig. S5 in the ESI†). Although the intensity of the benzylic proton (H_a) decreased and a new bromobenzyl proton peak (H_b) appeared at 4.5 ppm, no significant changes were observed among the other aromatic peaks, indicating the selective bromination of the benzyl group in 2. The degree of bromination in 3 was estimated to be 95%, based on the ratio of the integral of the bromobenzyl proton (H_b) in 3 relative to that of the benzylic proton (H_a) in 2.

Finally, PEG–imidazolium functionalization was carried out by treating a DMF solution of 3 with PEG–imidazole groups having a variety of ethylene glycol chain lengths, with C₂, C₄, C₈, and C₁₂, to give the corresponding PEG–Im-functionalized PIs (1) with bromide anions [C₂PEG–Im-PI][Br], [C₄PEG–Im-PI][Br], [C₈PEG–Im-PI][Br], and [C₁₂PEG–Im-PI][Br] (1a, 1b, 1c, and 1d), respectively.

As shown in Fig. 1, the ¹H NMR spectra of the PEG–imidazolium-functionalized PIs display peaks characteristic of imidazolium protons (H_d) at 9.0 ppm and benzylic protons (H_c) of PEG–imidazoles at 5.6 ppm, indicating the successful incorporation of the PEG–imidazolium groups. The degree of functionalization was found to be almost 100% based on a calculation of the integral ratio of the H_b protons in 3 to the H_c protons in 1. FT-IR spectroscopy was performed and further confirmed the structures of the PEG–Im-functionalized PIs based on the peaks present at 1460 cm⁻¹, 1480 cm⁻¹, and 1560 cm⁻¹, which corresponded to the vibrational modes of the benzyl imidazolium cations (Fig. S6 in ESI†).³⁴

Fig. 1 ¹H NMR spectra of [C₂PEG–Im-PI][Br] (a), [C₄PEG–Im-PI][Br] (b), [C₈PEG–Im-PI][Br] (c), and [C₁₂PEG–Im-PI][Br] (d).

Preparation of the PEG–Im-functionalized PI membranes

The PEG–Im-functionalized PIs (1) displayed a high solubility in common organic solvents, such as CHCl₃, DCM, DMF, DMSO, and DMAc. The corresponding membranes were prepared by casting a DMF solution of polymer 1, followed by vacuum drying to give dense, transparent, and flexible membranes (Fig. 2).

Fig. 2 Photographs of the PEG–Im-PI membranes.

Physical properties

DSC analysis was carried out to investigate the changes that occurred at the glass transition temperature (T_g) as a function of the PEG chain length in the PEG–imidazolium groups of the PEG–Im-PIs 1. All four PEG–Im-PIs ([C₂PEG–Im-PI][Br], [C₄PEG–Im-PI][Br], [C₈PEG–Im-PI][Br], and [C₁₂PEG–Im-PI][Br]) exhibited a high T_g (>300 °C, Table 1 and Fig. 3), even in the presence of the pendant PEG–imidazolium groups. These T_g values were much higher than those obtained from poly(IL)s or other typical glassy polymers, demonstrating that these newly developed materials (PEG–Im-PIs 1) were glassy in nature. The T_g values increased with the PEG chain length in the PEG–imidazolium cation, possibly due to the increased crystallization of the PEG component.

Table 1 Physical parameters that characterized the PEG–Im-PI membranes

Membrane	Tg (°C)	d-Spacing (Å)	Density
6FDA–durene	424 (ref. 14)	6.7	1.33
[C2PEG–Im-PI][Br]	320	6.4	1.37
[C4PEG–Im-PI][Br]	325	6.3	1.38
[C8PEG–Im-PI][Br]	335	6.1	1.41
[C12PEG–Im-PI][Br]	340	5.9	1.45

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Fig. 3 DSC curves of the PEG–Im-PI membranes.

Wide-angle X-ray scattering (WAXS) results obtained from the PEG–Im-PIs revealed that the intersegmental (d-) spacings between the polymer chains in the membranes decreased as the PEG chain length increased (Table 1 and Fig. 4). These decreases in the d-spacings were associated with an increase in the densities of the PEG–Im-PIs. [C₁₂PEG–Im-PI][Br], characterized by the longest PEG chain length, yielded the densest film among the four polymers tested (Table 1). The high density of the PEG–Im-PIs with longer PEG chain lengths were expected to enhance the resistance of the PEG–Im-PI membrane substructure to gas transport by reducing the diffusivity coefficients. A reduced permeability was expected to accompany an increase in the PEG chain length (vide infra).

Fig. 4 Wide-angle X-ray diffraction plots obtained from the PEG–Im-PI membranes.

Mechanical properties

The mechanical properties of the four PEG–Im-PI membranes at 50% RH showed excellent tensile strengths up to 73.2 MPa, with Young's moduli as high as 2.1 GPa (Fig. S7 in ESI† and Table 2). The mechanical strengths of the PEG–imidazolium-functionalized PIs indicated that these membranes were appropriate for gas separation measurements.

Table 2 Tensile properties of the PEG–Im-PI membranes

Membrane	Maximum tensile strength, MPa	Elongation at break, %	Young's modulus, GPa
6FDA–durene	76.7	10.4	1.7
[C2PEG–Im-PI][Br]	73.2	7.9	2.1
[C4PEG–Im-PI][Br]	71.7	7.5	2.0
[C8PEG–Im-PI][Br]	66.6	5.5	1.9
[C12PEG–Im-PI][Br]	57.1	5.2	1.8

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Gas separation properties

The pure gas permeabilities and permselectivities of the PEG–Im-PIs 1 were measured at 2 atm and 30 °C using the constant-volume/variable-pressure method, and the results are summarized in Table 3. The results were compared with those obtained from the pristine PI 1 (6FDA–durene PI). The permeabilities of the PEG–Im-PI membranes to all gases decreased dramatically as the PEG chain length increased (Table 3). This trend corresponded to a significant decrease in the diffusivity coefficients (Fig. 5a and Table 4) caused by a decrease in the interchain spacing (lower d-spacings) with increasing PEG chain length (Table 1, vide supra). In other words, the presence of longer PEG chains in the PEG–Im-PIs further reduced the free volume of the polymers, possibly due to an increase in the crystallinity of the PEG (higher T_g values were observed for the PIs with longer PEG chains), which decreased the gas permeability of these materials. In fact, the diffusivity coefficients of the PEG–imidazolium-functionalized PI were much lower than the value obtained from the pristine 6FDA–durene PI 1. For example, the CO₂ diffusivities of the PEG–Im-PIs decreased by factors of 2, 3, 6, and 10 for [C₂PEG–Im-PI][Br], [C₄PEG–Im-PI][Br], [C₈PEG–Im-PI][Br], [C₁₂PEG–Im-PI][Br], respectively, compared to the 6FDA–durene PI 1 (Table 4).

Table 3 Gas permeability (P) and permselectivity (α) of the pristine PI and PEG–Im-PI membranes at 2 atm and 35 °C^a

Membrane	PCO2	PN2	PCH4	αCO2/N2	αCO2/CH4
a P in Barrers, where 1 Barrer = 10^−10 [cm^3 (STP) cm] (cm^−2 s^−1 cm^−1 Hg^−1).
6FDA–durene	495	41.1	37.3	12.1	13.2
[C2PEG–Im-PI][Br]	484.8	18.1	13.2	26.7	36.7
[C4PEG–Im-PI][Br]	284.3	8.7	6.3	32.4	45.1
[C8PEG–Im-PI][Br]	107.9	3.0	2.2	35.1	48.4
[C12PEG–Im-PI][Br]	54.1	1.3	1.1	40.1	49.2

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Fig. 5 Diffusivity coefficients (a) and solubility coefficients (b) of the PEG–Im-PI membranes as a function of the PEG chain length.

Table 4 Gas diffusivity coefficients^a and solubility coefficients^b at 2 atm and 35 °C

Membrane	DCO2	DN2	DCH4	SCO2	SN2	SCH4	SCO2/N2	SCO2/CH4
a Diffusivity coefficient (10^−8 cm^2 s^−1).b Solubility coefficient (cm^3 (STP) cm^−3 cm^−1 Hg^−1).
6FDA–durene	29.1	18.1	6.57	0.17	0.022	0.057	7.49	2.98
[C2PEG–Im-PI][Br]	15.9	9.03	3.31	0.31	0.020	0.039	15.5	7.79
[C4PEG–Im-PI][Br]	9.87	5.11	1.72	0.29	0.017	0.036	17.0	7.90
[C8PEG–Im-PI][Br]	5.14	3.24	1.25	0.21	0.009	0.018	23.3	11.6
[C12PEG–Im-PI][Br]	2.80	1.84	0.84	0.19	0.007	0.013	27.5	14.8

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On the other hand, all PEG–Im-PIs showed much higher solubilities toward CO₂ than to other gases (CH₄ and N₂, Fig. 5b and Table 4). These behaviors were attributed to the effects of both the polar PEG and imidazolium-based IL groups. The extremely high CO₂ solubility of the PEG–imidazolium-functionalized PI yielded a high CO₂ permeability to these newly developed PEG–Im-PI membranes 1 (Table 3). The solubility (in addition to the diffusivity) is another contributor to the permeability. In fact, a CO₂ permeability as high as 484.8 Barrer was obtained for [C₂PEG–Im-PI][Br] due to its extremely high CO₂ solubility (0.31, Table 4). This permeability was much higher than the corresponding values obtained from other PEO-based copolymers or poly(IL)s and approached the value measured from pristine 6FDA–durene PI 1 (495 Barrer, Table 4). The solubility toward CO₂ was the highest for [C₂PEG–Im-PI][Br]. This value decreased rapidly as the PEG chain length increased. This trend was thought to originate from a decrease in the free volume within the polymer matrix as the PEG chain length increased because the free volume can affect the diffusivity as well as the solubility.³⁵ Similarly, a gradual decrease in the solubility toward other gases (N₂ and CH₄) was observed as the PEG chain length increased (Table 4 and Fig. 5b).

The ability of the PEG–Im-PIs 1 to separate other nonpolar gases (N₂ and CH₄), appeared to be normal, unlike the CO₂ separation trends, and these materials yielded excellent CO₂/N₂ and CO₂/CH₄ permselectivities. As a result, [C₂PEG–Im-PI][Br] showed high selectivities of 26.7 and 36.7 for CO₂/N₂ and CO₂/CH₄ while maintaining a high CO₂ permeability of 484.8 Barrer. The high permeability and high selectivity of [C₂PEG–Im-PI][Br] was ascribed to a combination of the enhanced solubility of CO₂ in this membrane and the relatively small loss in CO₂ diffusivity compared to the pristine PI (Fig. 5a and b), which in turn is caused by our structure being unique in having a pendant PEG and IL, both as a CO₂-philic group, onto the diffusivity selective rigid PI backbone.

The CO₂/N₂ and CO₂/CH₄ permselectivities increased as the PEG chain length increased due to an enhancement in the solubility selectivities (Table 4). The solubility selectivity increased whereas the diffusivity selectivity remained almost constant as the PEG chain length increased (Fig. 6a and b).

Fig. 6 Diffusivity selectivity (a) and solubility selectivity (b) of the PEG–Im-PI membranes as a function of the PEG chain length.

Permeability vs. selectivity among the PEG–ImPIs

The permeability–selectivity tradeoff results obtained for the CO₂/CH₄ (Fig. 7a) and CO₂/N₂ (Fig. 7b) separations in the membranes prepared from PEG–Im-PI with different PEG chains were compared by plotting a Robeson plot for each membrane.^6,7 Data obtained from the SILMs (supported ionic liquid membranes)¹⁴ and poly(IL)s^16,17,36 are included for comparison.

Fig. 7 “Robeson upper bond 2008” plot for comparing the CO₂/CH₄ (left) and CO₂/N₂ (right) separation performances of the PEG–Im-PIs ([C₂PEG–Im-PI][Br] (a), [C₄PEG–Im-PI][Br] (b), [C₈PEG–Im-PI][Br] (c), and [C₁₂PEG–Im-PI][Br] (d)) with other previously reported SILMs and poly(IL)s. Data were taken from [6, 7, 14, 16, 17, 32].

All PEG–Im-PI membranes exhibited outstanding CO₂/CH₄ performances with curves positioned above the Robeson upper bound of 1991. Moreover, the [C₂PEG–Im-PI][Br] and [C₄PEG–Im-PI][Br] membrane curves appeared above even the 2008 upper bound. Although all PEG–Im-PI membranes fell below the upper bound line for CO₂/N₂, their performances were intermediate between those of the SILMs and the poly(IL)s, and they outperformed the poly(IL)s. These results indicated that the PEG–imidazolium-functionalized PI polymers displayed remarkable capacities for enhanced selective separation of CO₂.

Conclusions

We prepared a series of novel PEG–imidazolium-functionalized polyimides and successfully demonstrated the utility of the corresponding membranes for CO₂ gas separation. Imidazolium-based IL and polar PEG groups, which are highly CO₂-solubilizing groups, have previously been incorporated into highly permeable rigid polymers, such as PI, as a strategy for increasing the CO₂ selectivity of polymer-membrane-based gas separation membranes. This is the first example of the incorporation of both a PEG and an ionic group onto the rigid PI polymer unit to serve as a CO₂-solubilizing group. We investigated the effects of the PEG chain length within the PEG–imidazolium groups on the structures and properties of the polymers, as well as the gas separation properties of the corresponding polymer membranes. The PEG–imidazolium-functionalized PI (PEG–Im-PI) membranes showed higher solubilities toward CO₂ than to other gases (CH₄ and N₂), leading to a high CO₂ selectivity while maintaining a good permeability. Excellent mechanical and thermal properties were measured for these newly developed polymers. Overall, these materials were found to be useful for preparing excellent low-cost highly energy efficient polymer membranes for CO₂ separation.

Acknowledgements

This work was supported by the Korea Carbon Capture and Sequestration R&D Center under the Korea CCS2020 Program of the Ministry of Education and Science and Technology, Republic of Korea.

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Footnote

† Electronic supplementary information (ESI) available: Details of the characterizations and PEG–imidazoles preparation. See DOI: 10.1039/c6ra00735j

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