Highly Carboxylate-Functionalized Polymers of Intrinsic Microporosity

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Highly Carboxylate-Functionalized Polymers of Intrinsic Microporosity for CO2‑Selective Polymer Membranes Jun Woo Jeon,†,‡ Dong-Gyun Kim,†,§ Eun-ho Sohn,†,§ Youngjae Yoo,†,§ Yong Seok Kim,†,§ Byoung Gak Kim,*,†,§ and Jong-Chan Lee*,‡ †

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro, Yuseong-gu, Daejeon 34114, Republic of Korea ‡ School of Chemical and Biological Engineering and Institute of Chemical Processes, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea § Department of Chemical Convergence Materials, University of Science and Technology, 217 Gajeong-ro, Yuseoung-gu, Daejeon 34114, Republic of Korea ABSTRACT: Carboxylate-functionalized polymers of intrinsic microporosity (PIMs) are promising materials for gas separation application. However, highly carboxylate-functionalized PIMs (HCPIMs) have not been reported owing to overlooked intermediate products. Herein, we successfully prepared HCPIMs (∼92 mol % of carboxylic acid group) through a prolonged alkaline hydrolysis process (360 h). HCPIMs were found to be soluble in various organic solvents, such as tetrahydrofuran and dimethyl sulfoxide, and then freestanding HCPIM membranes could be prepared by the common solution casting method. The HCPIM membranes were found to have smaller interchain distances and higher CO2 affinity than original PIM-1 films. For example, small gas molecules, such as carbon dioxide, were effectively separated due to the enhanced diffusivity selectivity combined with the smaller cavity size. Further, strong interactions between carbon dioxide and the carboxylic acid groups increased solubility selectivity. These synergetic effects endowed the HCPIM membrane with a selectivity of 53.6 for CO2/N2 separation, the highest among reported chemically modified PIMs.

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INTRODUCTION The amount of CO2, the major greenhouse gas, in the earth’s atmosphere has steadily increased because of ceaseless fossil fuel consumption.1 To reduce CO2 concentrations in the atmosphere, continuous global efforts have focused on the development of specific CO2 capture technologies. As an economic and eco-friendly approach for the CO2 capture, gas separation membranes have recently gained considerable attention. Representative polymeric membranes for CO2 gas separation mainly consist of glassy polymers, such cellulose acetate and polyimide.2−4 In addition, novel glassy polymers, such as polymers of intrinsic microporosity (PIMs), have been extensively studied.5−13 Recently, various PIMs containing different functionalities, such as thioamide and tetrazole, have been also reported.5−10,14−16 As novel microporous polymers, PIMs mainly consist of polybenzodioxane structures with contorted moieties that produce awkward structures having high free volumes and high permeability behavior.17 For example, the PIM-1 membrane,5 as a representative PIM, exceeded Robeson’s 1991 upper bound for O2/N2 separation.18 However, despite high CO2 permeability, the selectivity for CO2 in CO2/N2 or CO2/CH4 mixtures remained below the upper bound. To enhance this relatively low selectivity for CO2, many efforts have focused on developing PIMs with novel CO2-philic functionalities. Thioamide-PIM-1, obtained by thionation of PIM-1, exhibits © XXXX American Chemical Society

a CO2 permeability of 150 barrer with an ideal CO2/N2 selectivity of 38.5.6 Further, the introduction of a CO2-philic pendant group in tetrazole-functionalized PIMs (TZPIMs)7 resulted in outstanding CO2/N2 separation performance with a CO2 permeabilty of 2000 barrer and a selectivity of 31, which exceed the Robeson’s 2008 upper bound.19 Thus, the development of PIM containing novel CO2-philic moieties, such as amine, thioamide, and tetrazole, is regarded as an effective strategy for preparing highly CO2-selective polymer membranes.8−10 Apart from the above-mentioned nitrogen-containing functionalities, other functional groups can also exhibit effective interactions with CO2. In particular, the affinity of carboxylic acid groups for CO2 is considered as effective as that of amine functionalities.20 Although the carboxylic acid groups are acidic, they can effectively interact with CO2 molecules. This high affinity for CO2 is explained by a strong interaction between carboxylic acid and oxygen atoms (Lewis base) of carbon dioxide or two very strong interactions, which are strong O(CO2)··· H(COOH) hydrogen bonding and electrostatic interaction between the positive partial charge on C(CO2) and a partial negative charge on the carbonyl oxygen (C(CO2)···O(COOH)).21,22 Thus, polymeric Received: June 22, 2017 Revised: September 14, 2017

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acidified water (pH = 4−5) for 2 h. After vacuum filtration, each one was washed with water until pH adjusted to neutral and dried at 80 °C in the vacuum oven. The dried powder was dissolved in THF, filtered, and precipitated in water. The precipitate was filtered, rinsed with water, and dried in a vacuum oven at 80 °C. Finally, products were refluxed in MeOH, and they were designated as PIM-COOH-X, where X indicates the reaction time. The resulting PIM-COOH-360h powder was dried in a vacuum oven for 3 days. Yield: 7.0 (70%). Molar mass (determined from Mn,LS): Mn = 25 700, Mw = 27 500, PDI = 1.07. Anal. Calcd for C29H22O8 (wt %): C, 69.87; H, 4.45; O 25.68. Found: C, 67.7; H, 5.8, N, 0.4; O, 22.5%. BET surface area = 284 m2 g−1; total pore volume = 0.17 cm3 g−1 at (p/p0 = 0.99), Td 5% of PIM-COOH360h = 323 °C. PIM-1 Membrane Fabrication. Dense PIM-1 membranes for gas permeability tests were prepared by the solution casting method using 2 wt % PIM-1 chloroform solution filtered through 0.45 μm PTFE filters. The solution was cast into glass Petri dish and allowed to be evaporated slowly for 3 days. The membrane was soaked in methanol and dried in a vacuum oven at 60 °C for 24 h. The resulting membranes have thickness of 62 μm. PIM-COOH Membrane Fabrication. Dense PIM-COOH membranes for gas permeability tests were prepared by solution casting method from 2 wt % PIM-COOH-360h THF solution filtered through 5 μm PTFE filters. The solution was casted into glass Petri dish and allowed to be evaporated slowly for 3 days. The membranes were dried in vacuum oven at 60 °C for 24 h. The resulting membranes with thickness have thickness of 48 μm. Characterization Methods. 1H and 13C nuclear magnetic resonance (NMR) spectra of PIM-1 and its hydrolyzed polymers were recorded on a Bruker AVANCE 500 MHz spectrometer using CDCl3 (for PIM-1) or dimethyl sulfoxide-d6 (for carboxylatefunctionalized PIMs) as a solvent. Number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of PIM-1 and PIM-COOH-360h were measured by size exclusion chromatography (SEC) in THF, using Shodex columns (KF-800 series) operating at 40 °C. Signals were detected with Wyatt WREX-06 (differential refractive index, RI) and Wyatt Dawn 8+ (five-angle light scattering, LS) detectors. The specific refractive index increment (dn/dc) for PIM-1 and PIM-COOH-360h at 25 °C and 658 nm was measured using a Wyatt WREX-06; the values are 0.2381 and 0.2197 mL g−1 for PIM-1 and PIM-COOH-360h, respectively. Infrared spectra of PIM-1 and hydrolyzed PIMs were recorded as KBr pellet using a Bruker ALPHA-T spectrometer. Each sample was scanned 128 times at a resolution of 4 cm−1 in the range 4000−500 cm−1. Elemental analysis was obtained on a Thermo Scientific FLASH EA-2000 Organic Elemental Analyzer. Thermogravimetric analysis (TGA) was performed on Pyris 1 TGA thermogravimetry analyzer. Each sample was heated from ambient temperature to 1100 °C under nitrogen flow at heating rate of 10 °C min−1. The X-ray diffraction (XRD) pattern of each polymer was collected on a Rigaku Ultima IV diffractometer, equipped with a graphite monochromator and using

membranes with carboxylic acid groups are expected to be highly CO2-philic. Du et al. first introduced carboxylatefunctionalized PIMs,11 but Satilmis et al. pointed out that they overlooked the formation of amide-functionalized intermediates.12 Based on Satilmis’s study, the carboxylate-functionalized PIMs reported by Du et al. are regarded as PIM-COOH, with less than 20% carboxylic acid group functionalization. Even though Satilmis et al. reported improved functionalization of PIMs with carboxylic acid groups, only 51% carboxylation was achieved, and the gas permeation properties of this polymer were not investigated. Consequently, fully carboxylate-functionalized PIM membranes have not been reported until now, and their potential as CO2 separation membranes remains unknown. Herein, highly carboxylate-functionalized PIMs (HCPIMs) are synthesized to prepare highly CO2-philic gas separation membranes. Recently, we present that HCPIMs could be prepared using the same reagent used for the hydrolysis reaction in previous studies while using much longer reaction times because the conversion of amides to carboxylic acids needs harsher reaction conditions than that of nitriles to amides because the amide ion (−NH2) is a very poor leaving group.23 The chemical conversion of nitrile groups to carboxylic acids was found to simultaneously decrease interchain distances and improve CO2 affinity of the gas separation membranes based on PIMs. This synergetic effect has a positive impact on the selectivity for CO2, allowing the HCPIM membrane to realize the highest selectivity among reported postmodified PIM-1.

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EXPERIMENTAL SECTION

Materials. Sodium hydroxide (>98%, Sigma-Aldrich), chloroform (Sigma-Aldrich), potassium carbonate (99.99%), dimethylformamide (DMF, Burdick & Jackson), tetrahydrofuran (THF, J.T. Baker), and ethanol (95%, SAMCHEN Chemicals) were used as purchased. 5,5′,6,6′-Tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI, >97%, TCI) was recrystallized by addition of methylene chloride to clear TTSBI solution dissolved in MeOH. Tetrafluoroterephthalonitrile (TFTPN, >98%, Matrix Scientific) was purified by sublimation at 150 °C under low pressure. Synthesis of Polymers of Intrinsic Microporosity (PIM-1). PIM-1 was synthesized as described in our previous paper.24 All the glassware was dried in an oven prior to use. Under a nitrogen atmosphere, a mixture of TTSBI (10.21 g, 30 mmol), TFTPN (6.00 g, 30 mmol), and anhydrous K2CO3 (8.29 g, 60 mmol) was dissolved in DMF (210 mL) in a reaction flask, and then it was put into preheated oil bath at 55 °C and maintained for 3 days. The mixture was then cooled, and THF was added to the flask for removing low molecular fraction. The resulting yellow polymer was dissolved in THF and reprecipitated in methanol and H2O, respectively. The synthesized PIM-1 powder was dried in a vacuum oven for 3 days. Yield: 10.3 g (74.6%). 1H NMR (500 MHz, CDCl3): δH (ppm) = 6.80 (2H, s), 6.41 (2H, s), 2.46−2.04 (4 H, dd), 1.45−1.16 (12H, br). Molar mass (determined from Mn,LS): Mn = 60 900, Mw = 96 200, PDI = 1.58. Anal. Calcd for C29H20N2O4 (wt %): C, 75.64; H, 4.38; N, 6.08; O 13.90. Found: C, 74.20; H, 4.40, N, 6.20; O, 14.10%. BET surface area = 735 m2 g−1; total pore volume = 0.57 cm3 g−1 at (p/p0 = 0.99), Td 5% = 519 °C. Hydrolysis of PIM-1. Hydrolyzed PIM-1 was prepared via alkaline hydrolysis in an aqueous sodium hydroxide solution.11 PIM-1 powder (10.00 g) was added to 20% NaOH solution (H2O/ethanol = 1/1 (w/w)). The mixture was magnetically stirred and refluxed at 125 °C. To obtain highly carboxylate-functionalized PIM, the alkaline hydrolysis of PIM was undertaken for 360 h (15 days). To observe hydrolysis process, hydrolyzed samples were collected periodically by using glass pipet. For the purification, each sample was boiled in slightly

Figure 1. FT-IR spectra of hydrolyzed PIMs at different hydrolysis times. B

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“time-lag” method. The downstream pressure was measured using a manometer (MKS Baratron 626B). P was calculated by using the equation

Cu Kα radiation (λ = 1.5406 Å). The 2θ scanning range was 10°−70° with a scanning speed of 1°/min. The surface area and total pore volume of PIM-1 and PIM-COOH-360h were calculated with Micromeritics ASAP 2020 instrument. Each sample was dried at 60 °C in a vacuum oven and then degassed for 30 min at 90 °C and for 24 h at 120 °C before analysis. Carbon sorption isotherms were carried out on a Micromeritics 3Flex instrument at 273 and 298 K, ranging from 0.01 to 1.1 bar. Permeability coefficients (P) of N2, O2, H2, and CO2 were determined at 25 °C at a feed pressure of 760 cmHg using the constant-volume/variable-pressure method, the so-called as

P=

22.4 × 10−3 V l dP A RT P dt

where dP/dt is the increase rate of the downstream pressure and T is the operation temperature (K). The membrane effective area was 4.0 cm2.

Figure 2. (a) 1H NMR spectra of hydrolyzed PIMs at different hydrolysis times. (b) The content of amide and carboxylic acid per repeating unit with reaction time. (c) Chemical structure of PIM-1 and hydrolyzed PIM. (d) Thermogravimetric analysis of PIM-1 and PIM-COOH-360h. (e) 1H NMR spectrum of PIM-COOH-360h. (f) 13C NMR spectrum of PIM-COOH-360h.

Table 1. Content of Amide and Carboxylic Acid in Polymers Calculated from 1H NMR Spectra of Hydrolyzed PIMs reaction time

aromatic Ha

aliphatic Ha

amide Ha

carboxylic acid Ha

mol % amideb

mol % carboxylic acidb

PIM-1 24 h 72 h 168 h 264 h 360 h

1.00 1.00 1.00 1.00 1.00 1.00

4.01 3.91 4.06 4.16 4.19 3.99

0 0.54 0.33 0.12 0.06 0.02

0 0.09 0.25 0.37 0.42 0.46

54 33 12 6 2

0 18 50 74 84 92

The relative number of hydrogens present at each signal was integrated after setting the aromatic protons at δ 6.0−7.0 as the reference. bThe percentage of each functional group (ρfunc) was calculated by the equation ρfunc = NH,quan × 100/(NH,func × 2), where NH,quan is the quantitative number of hydrogens for the functional group in a repeating unit and NH,func is the number of hydrogens in the functional group. NH,func was multiplied by 2 due to two functional groups in a repeating unit. a

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(1715 cm−1), CO stretching from amide (1678 cm−1), and NH bending from amide (1605 cm−1). In the early stage of the reaction, the peak intensities of CO stretching of amide and NH bending of amide are large, implying the formation of amide. However, as the hydrolysis reaction progresses, the intensity of the CO band from amides decreases, whereas the intensity of CO band from carboxylic acid steadily increases. Consequently, nitriles in PIM-1 are first converted to amide groups, and then these intermediates are gradually transformed to carboxylic acid. These findings are consistent with Satilmis’s results and are similar to the observation during alkaline hydrolysis of polyacrylonitrile (PAN).25−27 For quantitative analysis of the functionality changes in the polymer with time, 1H NMR spectra of PIM-1 and the corresponding hydrolyzed polymers were recorded, as shown in Figure 2. Especially, the intensity changes of the proton peak from carboxylic acid in the 13.0−14.0 ppm region and those from amide in the 7.0−8.0 ppm region could be used to estimate the conversion.28 As shown in Figure 2 and Table 1, the content of carboxylic acid in the hydrolyzed polymer steadily increases with reaction time, whereas the amide content decreases correspondingly, which is consistent with the FT-IR results. The contents of these functional groups with different hydrolysis time are plotted in Figure 2b. The highest content of carboxylic acid in the PIM-COOH series was found to be 92 mol % for PIM-COOH-360h. Although we extended the hydrolysis time to 600 h, the content of carboxylic acid groups remained nearly unchanged. The observed retardation of PIM-1 hydrolysis might be attributed to electrostatic

RESULTS AND DISCUSSION Highly carboxylate-functionalized PIMs (HCPIMs) were prepared using the same reagents as reported before.11 Satilmis et al. reported that the hydrolysis time of 5 h is not sufficient for the preparation of HCPIMs, and the prolonged reaction time of 43 h was found to produce HCPIM with 51% conversion.12 This result indicates harsher reaction conditions or longer reaction times are required to prepare HCPIMs. In this study the conversion of PIM-1 to PIM-COOH with hydrolysis time was thoroughly characterized by Fourier transform infrared (FT-IR), elemental analysis (EA), and 1H NMR spectroscopy. Figure 1 shows the change of the nitrile group in PIM-1 to carboxylic acid group in HCPIM with hydrolysis time especially in the three regions of the FT-IR spectra. First, the absorption band corresponding to nitrile (2240 cm−1) disappears during the initial reaction period, indicating that nitriles can be easily hydrolyzed as reported by Satilmis et al.12 In the shorter wavelength region of 2400−3400 cm−1, the second change was observed, where the absorption bands corresponding to the NH of amide (3000−3400 cm−1) and the OH of carboxylic acids (2400−3400 cm−1) overlap. As the reaction time increases, the intensity of amide NH peak gradually decreases, whereas the OH peak from carboxylic acid becomes broader, indicating that amide in the intermediate product is converted to carboxylic acid. It should be noted that the resulting carboxylic acid can interact with another carboxylic acid through strong hydrogen bonding. Finally, the intensities of three major peaks in the 1600−1700 cm−1 region change with the hydrolysis time; they are CO stretching from carboxylic acid

Figure 3. (a) Argon adsorption desorption isotherms for PIM-1 and PIM-COOH-360h. (b) XRD data for PIM-1 and PIM-COOH-360h.

Figure 4. CO2 adsorption isotherms at (a) 273 K and (b) 298 K for PIM-1 and PIM-COOH-360h. D

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Macromolecules repulsion between the COO− groups and the −OH ions.27 Throughout the entire alkaline hydrolysis process, three types of repeating units can be found in hydrolyzed PIMs, as depicted in Figure 2c. During the initial stage of hydrolysis, hydrolyzed PIM mainly contains amide, whereas after 360 h, PIM-COOH has 92 mol % of carboxylic acid. The 1H NMR and 13C NMR of the final product, PIM-COOH-360h, are depicted in Figures 2e and 2f, respectively.28 To analyze the surface areas of PIM-1 and PIM-COOH360h, gas sorption analyses were conducted at 87 K with argon as the probe molecule. Although nitrogen is more commonly used as a probe molecule to determine the surface areas of porous materials, argon is more effective for analyzing the

surface areas of polymers containing polar units because, unlike nitrogen molecules, argon has no quadrupole interaction with polar functional groups, such as hydroxyl or carboxylic acid groups.29 As shown in the argon adsorption−desorption isotherms in Figure 3a, both PIM-1 and PIM-COOH-360h exhibit type I isotherms, indicating that most pores in these materials are of micropores.30 Compared with the surface area of PIM-1 (735 m2 g−1), PIM-COOH-360h has smaller surface area of 284 m2/g, only one-third of the total pore volume of PIM-1. These results can be explained by strong intermolecular hydrogen bonding between adjacent carboxylic acid groups. Similar reductions of the surface area were observed for other chemically modified PIMs.6,7 Since intra- and intermolecular interactions through the hydrogen bonding between carboxylic acid groups may change the arrangement of polymer chains in PIMs, the interstitial space of PIM powders before and after alkaline hydrolysis was studied by XRD. As shown in Figure 3b, both PIM-1 and PIMCOOH-360h are amorphous. The average interchain distances (d-spacing) were calculated by Bragg’s law (d = λ/(2 sin θ)) using the maximum 2θ values of the broad peaks. In PIM-1, major three bands corresponding to d-spacing of 3.8, 4.9, and 6.6 Å are shown.31 The d-spacing of 3.8 Å is typical for aromatic systems, whereas that of 4.9 Å represents the chain-to-chain distance of efficiently arranged polymer chains. The broad band at 6.6 Å is attributed to more loosely packed polymer chains, consistent with the presence of micropores.13,31−33 After the

Table 2. Solubility Tests for PIM-1 and Hydrolyzed PIMs sample

CHCl3

DMSO

THF

PIM-1 PIM-COOH-5h PIM-COOH-10h PIM-COOH-24h PIM-COOH-51h PIM-COOH-72h PIM-COOH-120h PIM-COOH-168h PIM-COOH-216h PIM-COOH-264h PIM-COOH-360h

soluble insoluble insoluble insoluble insoluble insoluble insoluble insoluble insoluble insoluble insoluble

insoluble partially soluble partially soluble soluble soluble soluble soluble soluble soluble soluble soluble

soluble insoluble insoluble insoluble insoluble partially soluble soluble soluble soluble soluble soluble

Figure 5. (a) Schematic diagram of PIM-1 and HCPIM (PIM-COOH-360h) membranes. (b) Optical image of a free-standing PIM-COOH-360h film.

Table 3. Gas Permeation Properties of PIM-1 and PIM-COOH-360h Membranes at 30 °C permeability (barrer)a

a

selectivity (α)b

sample

H2

CO2

O2

N2

CH4

CO2/CH4

CO2/N2

O2/N2

H2/N2

PIM-1 PIM-COOH-360h

3511 90.53

3934 96.43

1049 9.80

269 1.80

366 3.82

11.0 25.2

14.6 53.6

3.9 5.4

13.0 50.3

1 barrer = 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1. bIdeal selectivity = Pi/Pj. E

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a D is measured by constant volume time-lag method. bS is calculated from P = DS. cDiffusivity selectivity and solubilty selectivity were derived from the equation αij = Di/Dj (αD) × Si/Sj (αS), where αij is selectivity for gas pair, Di/Dj (αD) is diffusivity selectivity, and Si/Sj (αS) is solubility selectivity.

0.17 0.21 1.19 1.25. 10.26 21.83 3.82 5.32 77.69 242.82 3.29 4.34 1.43 2.45 2.89 4.74 9.61 6.36 3.58 1.55 4.24 1.94 36.72 33.84 0.60 0.32 37.1 0.6 75.1 1.16 247 5.04 107 2.85 5800 282 PIM-1 PIM-COOH-360h

O2/N2 CO2/N2 H2

CO2

O2

N2

CH4

H2

CO2

O2

N2

CH4

CO2/CH4

CO2/N2

O2/N2

H2/N2

CO2/CH4

αSc αDc Sb (10−2 cm3/cm3 cmHg−1) Da (10−8 cm2/s)

Table 4. Diffusion Coefficient (D), Solubility Coefficient (S), Diffusivity Selectivity (αD), and Solubility Selectivity (αS) for Different Gases for PIM-1 and PIM-COOH-360h Membranes

hydrolysis, the 2θ values of PIM-COOH are shifted to higher values, and the corresponding d-spacing decreases. In particular, the chain-to-chain distances of the loosely packed polymer chains noticeably decreases from 6.76 to 6.01 Å, indicating tightening of the interstitial distances between polymer chains. Further, the d-spacing of the closely packed polymer chains decreased from 4.95 to 4.80 Å because of strong hydrogen bonding between carboxylic acids after the alkaline hydrolysis. Although d-spacings cannot be taken as real interchain distances, d-spacing changes serve as an indicator of the room available for small molecules to penetrate through polymer membranes.34 Consequently, the introduction of carboxylic acid groups onto the PIM backbone reduces the interstitial space, which is helpful for separating small molecules from gas mixtures with different sized molecules.35 Alkaline hydrolysis of PIM-1 was found to increase the interactions with CO2 gas. It is well-known that the solubility of penetrant increases when there are favorable interactions between the penetrant and polymer.36 While carbon dioxide is slightly acidic, the oxygen atoms of CO2 act as a Lewis base because of their lone pair electrons. Thus, as the carboxylic acid in HCPIM is a Lewis acid, effective interactions with CO2 is possible by Lewis acid−base interactions.21 This strong affinity for CO2 is also described by two very strong interactions, which are strong O(CO2)···H(COOH) hydrogen bonding and electrostatic interaction (C(CO2)···O(COOH)).22 To verify the increased CO2 affinity of HCPIM, CO2 sorption isotherms were obtained for PIM-1 and PIM-COOH-360h at 273 and 298 K, as shown in Figure 4, and the isosteric heat of adsorption (Qst) of each polymer was calculated. The Qst values for PIM-1 and PIM-COOH-360h are 27.7 and 30.4 kJ mol−1, respectively. Such increased affinity for CO2 gas is directly related to the favorable interaction between CO2 and the polymer matrix, resulting in increased solubility for gas separation. The good solubility of polymers in common organic solvents is very important for the applications of the polymers for their applications as the membranes for various applications including gas separation.36 Although covalent organic frameworks (COFs)37,38 and conjugated microporous polymers (CMPs)39,40 have a high affinity for a certain gas such as CO2, they have not been widely used for the membrane application because of their poor solution processability. In the case of PIMs, the non-cross-linked structure endows these polymers with solubility in common solvents. In addition, they are easily fabricated as free-standing films for gas separation. However, the superior film formation ability of PIMs sometimes disappears after chemical modification owing to changed polarity, which can restrict the use of chemically modified PIMs as gas separation membranes.14 Thus, the good solubility in common solvents for the easy fabrication of membranes is one of the important prerequisites for the application in gas separation process. The solubility of the PIM samples depends on the hydrolysis time (Table 2). PIM-1 is soluble in CHCl3 and THF, while the solubility of the hydrolyzed PIM sample changes by alkaline hydrolysis. Partially hydrolyzed PIMs are insoluble in THF and chlorinated solvents, such as CH3Cl, while they show partial solubility in aprotic solvents, such as NMP, DMSO, and DMF. This solubility in polar solvents can be attributed to strong hydrogen-bonding interactions between functional groups, such as amide, with the polar solvents and is consistent with solubility of PIMCONH2.14 After half the functional group is converted to

H2/N2

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pairs are listed in Table 3. PIM-COOH-360h shows a high CO2/N2 selectivity of 53.6, which is much larger than that of the commercial membrane, Matrimid (33.47), and is the largest value reported for a PIM.50 In the solution-diffusion model, selectivity (αA/B) is defined as the product of the solubility selectivity (SA/SB) and diffusivity selectivity (DA/DB). Diffusivity selectivity increases by controlling cavity size or increasing chain rigidity,7,34 while solubility selectivity increases by introducing functionalities that allow specific interactions.36 Carboxylation of PIM-1 positively affects both the diffusion selectivity and solution selectivity. First, PIM-COOH-360h has a higher diffusion selectivity than PIM-1 because of reduced intermolecular distances. In other words, the introduction of substantial amounts of carboxylic acid groups in the HCPIM allowed interactions with adjacent carboxylic acid groups through strong hydrogen bonding, which shortened the interchain distances. Consequently, small CO2 molecules (3.3 Å) can be effectively separated from slightly larger N2 (3.64 Å) or CH4 (3.8 Å) owing to the sieving effect.35 Moreover, similar effects were observed for other gas pairs. For example, the high selectivity of H2 or O2 over N2 can also be explained by the sieving effect because H2 (2.89 Å) or O2 (3.46 Å) is smaller than N2 (3.64 Å). Second, the high CO2 affinity of the carboxylic acid groups on the polymer backbones improves the solution selectivity. Carboxylic acid groups in the polymeric matrix can interact with CO2 through Lewis acid−base interactions or two strong interactions by hydrogen bonding and electrostatic interaction. As shown in Table 4, the solution selectivity of PIM-COOH-360h (21.83) is twice that of PIM-1 (10.26) for CO2/N2. Consequently, the product of the increased diffusivity and solubility selectivity gives the high selectivity value of 53.6 for CO2/N2.

carboxylic acid, hydrolyzed PIMs show solubility in THF. PIMCOOH-360h shows good solubility in THF and membranes could be easily fabricated using the solvent casting method (Figure 5a). As seen in Figure 5b, the prepared PIM-COOH360h membrane is defect-free, free-standing, and flexible enough to bend. As the unmodified PIMs having good solution processability is the platform materials for various applications such as gas separation,5,41 gas storage materials,42 fuel cell catalyst support,43 and solvent filtration,44 HCPIM should also be suitable as the platform materials. Furthermore, the properties of PIM-COOH-360h having high content of carboxylic acid should be different from those reported for PIM-COOH with low content of carboxylic acid for applications such as gas separation. The properties of PIMs modified by decarboxylation,45 ionomer formation,46,47 and polymer blending48,49 are known to be affected by the increased content of carboxylic acid. The pure gas permeation properties of PIM-1 and HCPIM are summarized in Table 3. For PIM-COOH-360h, the permeation rates decrease in the order of CO2 > H2 > He > O2 > CH4 > N2, which is the same as those observed for other PIMs, such as PIM-1 and thioamide-PIM-1.6 After the hydrolysis, the gas permeability of PIM-COOH-360h for every gas decreases significantly, which should be related to the decreases of total pore volume and pore size. However, notably, the CO2 permeability of this membrane is 10 times higher than that of the commercial polyimide membrane, Matrimid, which has a CO2 permeability of 5−10 barrer.50 Compared with another commercial polyimide membrane, Torlon, having a CO2 permeability of ≤0.5 barrer, the CO2 permeability of PIM-COOH360h is approximately 100 times higher.51,52 The selectivities of the PIM-1 and PIM-COOH-360h for industrially important gas

Figure 6. Double-logarithmic plots for (a) CO2/N2, (b) CO2/CH4, (c) H2/N2, and (d) O2/N2 separation: (A) PIM-1 (this work), (B) PIMCOOH-360h (this work), (C) HCPIM (120 °C_5 h),11 (D) MTZ100-PIM,10 (E) TZPIM-3,7 (F) thioamide-PIM-1, and (G) thioamide-PIM-1 (EtOH treated).6 G

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The gas permeabilities and selectivities of PIM-1, PIM-COOH360h, and various other PIMs are logarithmically plotted in Figure 6. As suggested by Robeson in 1991 and 2008, there is trade-off relationship, known as Robeson’s upper bound, between permeability and selectivity in polymeric materials. In other words, membranes with higher selectivity have lower permeability, and this relationship is often used as a reference for gas separation data.18,19 As shown in Figure 6, most PIMs are located very close to Robeson’s upper bound and some even surpass this line, representing excellent gas separation properties. Therefore, PIM-1 and its chemically modified derivatives have been frequently investigated as promising gas separation membrane.6,7,10,11 Among the various postmodified PIMs, carboxylate functionalization of PIMs has attracted considerable attention, while the effect of carboxylation on CO2 selectivity has been underestimated until now because of highly carboxylate-functionalized PIM membranes have not been prepared yet.11,12,27 This time we report that alkaline hydrolysis of PIM-1 is very effective for preparing CO2 selective membrane because of the synergetic effect of the increase of affinity and the decrease of interchain distances. The highly carboxylate-functionalized PIM, PIM-COOH-360h, membrane prepared in this study has the highest selectivity reported to date for postmodified PIMs.

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CONCLUSIONS HCPIMs with up to 92% carboxylation were successfully prepared by alkaline hydrolysis of PIM-1 with an extended reaction time of 360 h. The hydrolysis process was confirmed by FT-IR and 1H NMR analyses. During alkaline hydrolysis of PIM-1, the nitrile groups of the polymer were first converted to amide intermediates and then subsequently hydrolyzed to carboxylic acids. The conversion of the nitrile to carboxylic acid groups was found to decrease the interchain distances and increased CO2 affinity for the PIM-based membranes, which in turn increased the diffusivity selectivity and solubility selectivity, respectively. Therefore, the HCPIM membrane prepared in this study shows the highest CO2 selectivity value of 53.6 for CO2/ N2 mixtures among chemically modified PIMs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (B.G.K.). *E-mail [email protected] (J.-C.L.). ORCID

Dong-Gyun Kim: 0000-0003-0384-0067 Byoung Gak Kim: 0000-0003-0688-2852 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Research Institute of Chemical Technology (KRICT) core project (KK1702-D00 and SKO1707C02).

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