High-Performance Self-Cross-Linked PGP–POEM Comb Copolymer

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Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

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High-Performance Self-Cross-Linked PGP−POEM Comb Copolymer Membranes for CO2 Capture Na Un Kim,† Byeong Ju Park,† Yeji Choi,‡ Ki Bong Lee,*,‡ and Jong Hak Kim*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea Department of Chemical and Biological Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul 02841, South Korea

‡

S Supporting Information *

ABSTRACT: We report a high performance CO2 capture membrane based on the copolymerization and self-cross-linking of poly(glycidyl methacrylate-g-poly(propylene glycol))-co-poly(oxyethylene methacrylate) (PGP−POEM) comb copolymer. The epoxide−amine reaction is responsible for the self-cross-linking reaction, which takes place under mild conditions without any additional crosslinkers or catalysts. The effects of self-cross-linking on the membrane properties are investigated by comparing the copolymers with those containing a low PPG grafting density (l-PGP−POEM). Furthermore, the gas separation performance of the membranes is systematically investigated as a function of POEM content in the comb copolymer. Both the permeance and selectivity of the PGP−POEM membranes are enhanced simultaneously with increase in the POEM content up to 51.2 wt % (PGP−POEM13), at which the best performance was achieved among the membranes. The high performance results from the reduced diffusion of N2 due to the self-cross-linked structure as well as the increased CO2 solubility due to the high content of ether oxygen groups in the comb copolymer. By optimizing the membrane thickness, the performance is further improved up to a CO2 permeance of 500 GPU (1 GPU = 10−6 cm3 (STP)/(s cm2 cmHg)) and CO2/N2 selectivity of 22.4, which is close to the commercialization target area of CO2 capture membranes. This work suggests a simple and economical crosslinking method to fabricate the membranes with excellent gas separation performance.

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INTRODUCTION With global warming becoming more serious, gas separation processes based on membranes are attracting great attention as a promising technology for CO 2 capture.1 Membrane separation is considered an effective alternative to chemical absorption, which is the currently available technology. In addition, membrane separation can overcome the biggest drawbacks of the adsorption process such as high energy consumption and environmental issues.2−4 Over the past few decades, polymeric membranes have been extensively developed for CO2 separation.5−7 However, a trade-off relation has been recognized between the two factors governing the membrane performance, i.e., permeability and selectivity, which is represented by the Robeson upper bounds.8 Improving the solubility selectivity by introducing CO2-philic segments into polymers is one of the most widely used methods to overstep the upper bound limits in gas separation. Poly(ethylene oxide) (PEO) containing a large amount of ether oxygen groups in the chains has demonstrated good potential as CO2 separation membranes.9−12 The high affinity of PEO for CO2 arises from the dipole−quadrupole interactions between the oxygen atoms in the ether groups and the CO2 molecules.13 However, a high tendency for crystallinity and the poor mechanical strength of PEO-based materials have limited their industrial application in the separation process. Our group has used poly(oxyethylene methacrylate) (POEM), an amorphous © XXXX American Chemical Society

derivative of PEO, to address the above-mentioned crystallization issues. 14−17 However, it still requires further modifications such as copolymerization, physical blending, or cross-linking to overcome its poor mechanical property and low selectivity due to the intrinsic liquid-like nature of POEM. Cross-linking is an effective strategy to improve the separation performance and mechanical stability simultaneously.18−21 The interpolymer chain spacing and chain mobility can be quantitatively modified by the cross-linking method. However, cross-linking is generally achieved by UV light irradiation or high-temperature thermal treatment.22−25 Moreover, it requires the introduction of additional cross-linking agents or catalysts, which is unfavorable from an economical point of view. In order to take advantage of the cross-linking effect via a simple process under moderate conditions, the “selfcross-linking” strategy has been used in the field of anion exchange membranes (AEMs).26−29 However, the utilization of self-cross-linking reaction in gas separation membranes has not been reported until now. Herein, we report a facile and efficient process for the preparation of a self-cross-linked membrane for CO2 capturing at a modest temperature without any chemical additives. Received: September 19, 2017 Revised: November 1, 2017

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DOI: 10.1021/acs.macromol.7b02024 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis and Self-Cross-Linking Reaction of PGP−POEM Comb Copolymer

glycol) methyl ether methacrylate, Mn = 500 g mol−1) were purchased from Sigma-Aldrich. Poly(1-(trimehthylsilyl-1-propyne)) (PTMSP) (SSP-070, >95% purity) was purchased from Gelest. 2,2′-Azobis(2methylpropionitrile) (AIBN, 98%) as an initiator for free-radical polymerization was purchased from Acros Organic, and ethyl acetate (HPLC grade) as a polymerization solvent was purchased from J.T. Baker. The polysulfone (PSf) support membrane was provided by Toray Chemical Korea Inc., which showed a CO2 and N2 permeance of approximately 21 000 and 24 000 GPU, respectively. All the chemicals were used without further purification. Synthesis of PGP−POEM Comb Copolymer. A series of PGP− POEM comb copolymer containing different weight ratios of GMA:POEM (1:0, 1:1, 1:3, and 1:5) were synthesized. The synthesized copolymers were denoted as PGP−POEM10, PGP− POEM11, PGP−POEM13, and PGP−POEM15, respectively. First, equimolar amounts of GMA (1 g) and am-PPG (4.2 g) were added to ethyl acetate with 0.02 g of AIBN. Different amounts of POEM (0, 1, 3, and 5 g) were added to the 50 wt % solution, which was then placed in an oil bath at 70 °C for 18 h. After polymerization, the obtained yellow, viscous solution was precipitated into an excess amount of nhexane with vigorous stirring thrice to remove the unreacted monomers completely. The produced copolymer was immediately dissolved in an ethanol/water mixed solvent (70/30 wt %) to prevent further self-cross-linking. For comparison, homopolymers of GMA (i.e., PGMA) and POEM (i.e., PPOEM) were synthesized by the same process. The yield of all synthesized polymers was about 50%. PGP− POEM comb copolymers with a low PPG graft density were synthesized following the same procedure except for the amount of ethyl acetate. To inhibit the reaction between GMA and am-PPG, an excess amount of ethyl acetate was added to obtain 20 wt % polymer solution. The series of copolymers with low PPG content is denoted as l-PGP−POEM1X, where X represents the content of POEM. Preparation of Self-Cross-Linked PGP−POEM Comb Copolymer Membranes. The PGP−POEM comb copolymer solution was prepared by dissolving in ethanol/water mixed solvent (70/30 wt %) at 10 wt % concentration. The comb copolymer solution was then coated onto a PTMSP-coated PSf support membrane using an RK Control coater (Model 101, Control RK Print-Coat Instruments Ltd.,

The epoxide−amine reaction is considered a promising option for self-cross-linking methods because it can occur at room temperature in the absence of catalysts with high yields.30,31 It is a well-known type of click chemistry, where the highly strained and reactive epoxy rings react with the nucleophilic amine groups. Some copolymers containing pendant epoxide groups have shown self-cross-linking properties under certain conditions.32−34 In this work, we demonstrate a simple process to fabricate self-cross-linked composite membranes based on a poly(glycidyl methacrylate-g-poly(propylene glycol))-co-poly(oxyethylene methacrylate)) (PGP−POEM) comb copolymer. O-(2-Aminopropyl)-O′-(2-methoxyethyl)poly(propylene glycol) (am-PPG) and glycidyl methacrylate (GMA) were used as the sources of the epoxide−amine cross-linking reaction. The PGP−POEM comb copolymer was synthesized via a facile onepot free-radical polymerization, and the solution containing the as-synthesized PGP−POEM copolymer was coated onto a polysulfone (PSf) support to prepare the composite membranes. Importantly, the self-cross-linking of the membrane was induced by a simple drying process at ambient temperature without any post-treatment. The compositional, structural, and thermal properties of the PGP−POEM comb copolymers were investigated by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (1H NMR) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). The CO 2 /N 2 separation performances of the PGP−POEM comb copolymer membranes were measured at 25 °C.

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EXPERIMENTAL SECTION

Materials. Glycidyl methacrylate (GMA), O-(2-aminopropyl)-O′(2-methoxyethyl)poly(propylene glycol) (am-PPG, average Mn = 600 g mol−1), and poly(oxyethylene methacrylate) (POEM, poly(ethylene B

DOI: 10.1021/acs.macromol.7b02024 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules UK). The PSf support membrane with an average surface pore size of 10 nm was precoated with 0.5 wt % PTMSP solution in cyclohexane to minimize the effect of porous structure of the support and the penetration of the coating materials. The resulting membrane was dried at room temperature overnight, followed by further drying at 50 °C in a vacuum oven to remove the residual solvent completely. For a reference sample, a commercial block copolymer Pebax 1657 (40 wt % of polyamide and 60 wt % of PEO) membrane was prepared. A solution of Pebax (10 wt %) in ethanol/water mixed solvent (70/30 wt %) was coated on the PTMSP-coated PSf membrane and dried following the same procedure. Gas Permeation Measurements. The gas separation performances of the membranes were measured using a constant pressure/ variable volume apparatus (Airrane Co. Ltd., Korea) with pure gases. The permeation experiments were carried out by a flat-sheet permeation cell with an effective area of approximately 10.2 cm2 at 25 °C. A gas permeation unit (GPU) (1 GPU = 10−6 cm3 (STP)/(s cm2 cmHg) was used to express the permeance of each gas. The selectivity of the membranes was determined by the ratio of the permeance for each component. The membranes were tested for 2 h with single gas to obtain the performance in steady state after the drying process in a vacuum oven. Characterization. The functional groups of the synthesized comb copolymers were identified by FT-IR (Spectrum 100, PerkinElmer, USA) in the frequency range 4000−500 cm−1. 1H NMR (AVANCE III HD 400, Bruker, Germany) measurements were conducted using deuterated chloroform (CDCl3) as a solvent to analyze the compositions of the comb copolymers. The thermal stabilities of the comb copolymers were analyzed by TGA (TGA/DSC1, Mettler Toledo, Korea) at a heating rate of 20 °C min−1 in an air atmosphere. The glass transition temperature (Tg) of the comb copolymer was evaluated by DSC performed (Discovery DSC, TA Instruments, USA) at a heating rate of 20 °C min−1 under a nitrogen atmosphere in the temperature range −75 to 150 °C. The second heating profile was used. The cross-sectional morphology of the composite membrane was characterized using an FE-SEM (JSM-7001F, JEOL Ltd., Japan). The XRD patterns of the comb copolymers were collected using a high-resolution X-ray diffractometer (SmartLab, Rigaku, Japan) using Cu Kα radiation (λ = 0.154 nm) operated at 45 kV and 200 mA at a scanning speed of 2° min−1 in the 2θ range of 5°−60°. CO2 uptake was measured using a thermogravimetric analyzer (TGA, Q50, TA Instruments). Prior to the experiment, the polymers were heated to 100 °C and maintained at this temperature for 1 h under N2 flow. After the pretreatment, the temperature was decreased to 30 °C, and the changes in the sorption amount were recorded under CO2 flow for 2 h. The CO2 sorption capacities of polymers were measured at 1 atm.

The GMA and POEM chains were randomly arranged during the polymerization process. At the same time, am-PPG was grafted onto the GMA chains via a ring-opening reaction between the primary amine group of am-PPG and the epoxide group of GMA, resulting in the formation of secondary amine and hydroxyl groups. A certain amount of GMA chains might remain unreacted with am-PPG due to the interference of solvent, which could be the source of self-cross-linking. After polymerization, the as-synthesized copolymers were immediately dissolved in the ethanol/water mixed solvent (70/30 wt %) to suppress the rapid cross-linking reaction. The self-cross-linking reaction occurred during the drying process at room temperature after the comb copolymer solution was coated onto the PSf support. As the solvent evaporated, the distance between the polymer chains might decrease and the self-cross-linking reaction could take place.26,27 The reaction occurred between the unreacted epoxide segments and the secondary amine groups in the PPG-grafted GMA chains (Scheme 1b). The proposed reactions for the cross-linking of epoxide and amines are shown in Figure S1.30,36 1 H NMR characterization was conducted to verify the successful synthesis and analyze the actual composition of the comb copolymer series after polymerization (Figure 1). The comb copolymer was dissolved in deuterated chloroform immediately after the precipitation process in n-hexane. Thus, the solution contained a small amount of n-hexane, which resulted in the appearance of the signals (∗) at 1.1 and 0.7 ppm.37 The signal (a) at around 0.9 ppm is assigned to the

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RESULTS AND DISCUSSION Synthesis and Self-Cross-Linking Mechanism of PGP− POEM Comb Copolymer. The synthesis of comb copolymers and the self-cross-linking mechanism are illustrated in Scheme 1. A series of PGP−POEM comb copolymer with different ratios of GMA to POEM were synthesized via a facile one-pot free radical polymerization. We carefully designed the selfcross-linkable PGP−POEM comb copolymer with the expectation of enhancing the CO2 separation performance and the thermal stability simultaneously due to the following reasons. First, the self-cross-linked network formed by the epoxide− amine reaction may increase the gas selectivity as well as the thermal stability. Second, the am-PPG chains offering the amine groups as the reactive site for self-cross-linking also provides a number of propylene oxide groups with high CO2 affinity,35 which would enhance the CO2/N2 solubility selectivity of the membrane. Third, the introduction of POEM possessing ethylene oxide segments with high chain mobility could not only prevent the excessive densification of the polymer matrix but also further improve the CO2 solubility in the membranes.

Figure 1. (a) 1H NMR spectra and (b) proton assignment of PGP− POEM comb copolymers (before cross-linking) with different compositions in CDCl3. C

DOI: 10.1021/acs.macromol.7b02024 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Compositions of PGP−POEM Comb Copolymers Calculated from 1H NMR Spectra PGP−POEM10 PGP−POEM11 PGP−POEM13 PGP−POEM15

feed weight ratio (GMA:PPG:POEM)

resultant mole ratio (x:y:z)

resultant weight ratio (GMA:PPG:POEM)

POEM content (wt %)

1:4.2:0 1:4.2:1 1:4.2:3 1:4.2:5

1:0.31:0 1:0.23:0.23 1:0.24:0.67 1:0.24:1.08

1:1.00:0 1:0.79:0.66 1:0.81:1.90 1:0.83:3.08

0 26.9 51.2 62.8

primary amine protons in am-PPG, which completely disappeared after polymerization. The signals (c) at 3.6 ppm and (d) at 3.3 ppm correspond to the protons of −OCH2− and −OCH3, respectively, which exist in both the POEM and amPPG moiety. The signal (b) at 1.0 ppm is attributed to the −CH3 protons of PPG moiety, which is the characteristic signal of the PPG-grafted GMA chains. The remaining unreacted epoxide was confirmed by the signals (e) at 2.6−2.7 ppm. Multiple signals in the range of 3.3−3.5 ppm are assigned to −CH and −CH2 protons of am-PPG. In addition, the peaks at 1.9 and 4.1 ppm were from methylene protons of the polymer backbone (−CH2−) and attached to the oxygen atoms of mathacrylate groups (−OCH2−), respectively. The mole fraction of each segment (x:y:z) and the POEM content were quantified by integrating signals (b), (d), and (e). The calculated values are tabulated in Table 1. The grafting ratio of PPG remained almost unchanged, while the POEM content gradually increased with the monomer ratio of POEM. Besides, only 20−25 wt % of am-PPG was grafted during the polymerization process, which can be explained by the interference of solvent. It is worth noting that the calculated mole ratio of the chains represents the one before cross-linking. The result reveals that a large number of unreacted epoxide segments actually existed after polymerization, thus confirming the possibility of the assumed self-cross-linking mechanism presented in Scheme 1. In order to support the successful synthesis of PGP−POEM comb copolymers, FT-IR analysis was conducted (Figure 2a). After polymerization, the CC stretching vibration bands of the GMA and POEM monomers at 1637 cm−1 completely disappeared. In addition, a shift in the carbonyl bands (CO) of GMA and POEM from 1717 cm−1 to a higher wavenumber at 1729 cm−1 was observed in all the synthesized comb copolymers, which is attributed to the interaction between the adjacent carbonyl groups in the long polymer chains.15,38 A shoulder band at 1155 cm−1 represented the stretching vibration mode of the C−O group in GMA. The strong absorption bands at 1095 and 1099 cm−1 were attributed to the C−O−C stretching in POEM and am-PPG, respectively. This band gradually shifted from 1095 to 1099 cm−1 with increase in the POEM content. Furthermore, the absorption bands at 1373 and 2971 cm−1 corresponding to the −CH3 symmetrical bending and stretching in am-PPG were also observed in all the comb copolymers. This indicates that am-PPG was successfully grafted onto the comb copolymer chain during the polymerization process. The cleavage of the epoxide ring of GMA and the formation of −OH groups were confirmed from the FT-IR spectra measured during polymerization, which are shown in Figure S2. These results demonstrate that the free-radical polymerization of GMA and POEM along with the epoxide− amine ring-opening reaction between GMA and am-PPG occurred successfully at the same time. Thermal and Structural Properties of Self-CrossLinked PGP−POEM Copolymer. The thermal stabilities of the self-cross-linked PGP−POEM comb copolymers were

Figure 2. (a) FT-IR spectra of monomers and self-cross-linked PGP− POEM comb copolymers with different compositions. (b) TGA curves of PGMA and P(POEM) homopolymers and a series of self-crosslinked PGP−POEM comb copolymers.

determined by TGA, as shown in Figure 2b. The P(POEM) and PGMA homopolymers show thermal degradation temperatures (defined as the temperature at 10% weight loss) of 200 and 220 °C, respectively, which are consistent with the previously reported values.16,39 The series of self-cross-linked PGP−POEM comb copolymers exhibited no weight loss up to approximately 290 °C, which represents a sufficiently high thermal stability for gas separation applications. It is apparent that the thermal stabilities of the comb copolymers were significantly enhanced compared to those of homopolymers due to the self-cross-linking properties. The Tg’s of the PGP−POEM comb copolymers were identified by DSC, and the results are shown in Figure 3a. No endothermic peak was observed in all the comb copolymers, indicating that they existed in amorphous states. In addition, the presence of a single Tg demonstrates that all the comb copolymers show no phase separation and exist in one D

DOI: 10.1021/acs.macromol.7b02024 Macromolecules XXXX, XXX, XXX−XXX

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state. With increasing POEM content, the Tg of the PGP− POEM comb copolymers gradually decreased from −43.5 to −57.7 °C, which can be explained by the increased segmental motion of the POEM side chains. A similar decrease in the Tg with increasing in POEM content was reported in our previous research.15 It has been reported that a polymer with low Tg is more likely to have a high gas permeability.42 However, the gas permeability of polymer is also affected by the interchain spacing in the polymer matrix. The XRD patterns of the self-cross-linked PGP−POEM comb copolymers are shown in Figure 3b. The absence of a sharp crystalline peak demonstrates that all the comb copolymers have amorphous structures, which is well consistent with the above DSC results. The XRD patterns can be used to determine the d-spacing, the intersegmental distance in polymers, which enables the diffusion of small gas molecules through the polymer membrane.43−45 The value of d-spacing can be calculated by Bragg’s law: d = nλ/2 sin θ. In Figure S4, the PGMA homopolymer showed three broad peaks at 2θ values of 18.4, 29.2, and 41.4, while am-PPG showed a single amorphous peak at 19.6, which corresponds to previously reported data.35,46−48 To evaluate the microstructure of PGP−POEM comb copolymers ascribed to the self-cross-linking properties, a series of copolymers with a lower am-PPG graft density (thus, lower cross-linking density) were prepared as the control group and characterized (represented as l-PGP−POEM). The low PPG content in the comb copolymers was confirmed by FT-IR, as shown in Figure S5. The X-ray diffractograms for the lPGP−POEM comb copolymers are also illustrated in Figure S6. Interestingly, another broad shoulder peak was observed in the XRD patterns of the PGP−POEM series, while only a single amorphous peak was observed for those of the l-PGP− POEM series. These new shoulder peaks might be related to the formation of cross-linked networks. The PGP−POEM10 exhibited two distinct d-spacing values of 6.1 Å (2θ = 14.5) and 5.0 Å (2θ = 17.7), which are assigned to the distance within the cross-linked networks and the distance between the side chains, respectively. The more introduction of POEM chains led to a decrease in the side chain distance from 5.0 to 4.52 Å (2θ value shifted from 17.7 to 19.6), which is because the domain of POEM branches within

Figure 3. (a) DSC curves of self-cross-linked PGP−POEM comb copolymers with different compositions. (b) XRD patterns of selfcross-linked PGP−POEM comb copolymers with different compositions.

phase due to the cross-linked structure. As shown in Figure S3, the Tg of the PGMA homopolymer was determined as 56.8 °C,40 indicating that PGMA existed in a glassy state. In the case of am-PPG, the Tg was not clearly observed possibly due to its low molecular weight. It is known that the Tg of pure PPG is −70 °C.41 Thus, the introduction of am-PPG chains resulted in a significant decrease in the Tg of PGMA, whereas PGP− POEM10 exhibited a Tg of −43.5 °C, revealing its rubbery

Figure 4. Cross-sectional SEM images of PGP−POEM comb copolymer membranes on PSf support: (a) PGP−POEM10, (b) PGP−POEM11, (c) PGP−POEM13, (d) PGP−POEM15, (e) P(POEM), and (f) Pebax. E

DOI: 10.1021/acs.macromol.7b02024 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the comb copolymer becomes dominant. Thus, the copolymer is more likely to exhibit a d-spacing similar to that of P(POEM) (2θ = 20.0, d = 4.43 Å). On the other hand, the d-spacing associated with the cross-linked structure decreased as the POEM content increased up to PGP−POEM13 and then increased for PGP−POEM15. The initial decrease in the dspacing value may arise from the secondary bonding interaction among the POEM chains and the amine or hydroxyl groups formed by self-cross-linking. In the case of PGP−POEM15, it is possible that an excessive number of POEM chains hindered the self-cross-linking reaction, resulting in the slightly increased interchain spacing. These results are intimately linked to the gas separation properties of the comb copolymer membranes, which will be discussed in the next section. Gas Separation Performance of Self-Cross-Linked PGP−POEM Membranes. Self-cross-linked PGP−POEM comb copolymer membranes were prepared by coating 10 wt % copolymer solution onto the PTMSP-coated PSf support layer using a RK control coater. The porous polysulfone substrates had finger-like pore structures with high porosity and average surface pore size of 10 nm (Figure S7). The PTMSP gutter layer was applied to serve as a flat and smooth surface for the coating to minimize intrusion of the polymer solution into the porous support and to allow for fair comparison with the performance of a commercially available Pebax block copolymer. The PTMSP gutter layer on PSf membrane was less than 20 nm thick (Figure S7) and showed the CO2 permeance of 12 180 GPU and CO2/N2 selectivity of 1.3, exhibiting slightly lower selectivity than the previously reported data.49,50 This is likely due to possible small defects resulting from its extremely thin layer,51 which could be tightly filled with PGP−POEM polymer solution. Thus, only 0.5 wt % of PTMSP solution was sufficient to prevent the undesired defects of selective layer, not contributing to the gas separation performance of the membranes due to its ultrasmall thickness and intrinsic high permeability. Self-cross-linking began to occur as the solvent evaporated at room temperature and continued during the vacuum drying at 50 °C until the residual solvent was completely removed. As illustrated in Figure 4, a thin selective layer of the PGP− POEM comb copolymer with a thickness of around 300 nm showed intimate contact with the PTMSP-coated PSf support, without defects or voids. The Pebax membrane had a thickness of 340 nm, which is similar to that of PGP−POEM membranes. Thus, the separation properties of PGP−POEM comb copolymers can be directly compared to those of the commercial block copolymer membrane. The gas separation performance of the PGP−POEM membranes was evaluated by single gas permeation test at 25 °C with a feed pressure of 1 bar. The pure gas permeance and selectivity of these membranes as a function of POEM content are shown in Figure 5 and Table S1, where the POEM content is calculated from NMR analysis (Table 1). The performance of Pebax membrane as the control group was also measured; the results are presented in Table S1. Figure 5a shows a linear increase in the CO2 permeance of PGP−POEM comb copolymer membranes with increasing POEM content, which can be attributed to the enhanced CO2 solubility in the membranes. Figure 6 shows the CO2 uptake of a series of PGP−POEM comb copolymers in comparison with that of Pebax. The quantity of CO2 adsorbed onto the comb copolymer was measured by TGA apparatus, in millimoles per gram of polymer. All the PGP−POEM comb copolymers

Figure 5. (a) Pure gas permeance and (b) CO2/N2 selectivity of PGP−POEM comb polymer membranes with various POEM contents at 25 °C and 1 bar.

Figure 6. CO2 uptakes of PGP−POEM comb copolymers with different compositions and Pebax.

showed higher CO2 uptake than Pebax (0.01 mmol/g), which is likely due to the large number of ether oxide groups included in the POEM and PPG moieties. It is noteworthy that polar ether oxide (EO) groups have high affinity for CO2 molecules due to the dipole−quadrupole interactions.13 As the ratio of POEM chains containing CO2-philic EO units increased, the amount of CO2 uptake by the copolymer increased from 0.029 to 0.044 mmol/g. These results suggest that the increased CO2 solubility is responsible for the linear increment in CO2 F

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physical aging of PTMSP gutter layer, as reported in the literature.52,53 For practical application of the composite membrane to the postcombustion separation process, high CO2 permeance with moderate CO2/N2 selectivity is required.54 To maximize the CO2 permeance of the PGP−POEM comb copolymer membranes, the membrane thickness was controlled by reducing the concentration of coating solution. The thickness of the membranes prepared with different concentrations of the coating solution was measured by cross-sectional SEM images, as presented in Figure S9. As the concentration of the coating solution was decreased, both the N2 and CO2 permeance increased, with a slight decrease in the CO2/N2 selectivity (Table 3). It is worth mentioning that no severe defect was

permeance with the POEM content. Meanwhile, almost no increment was observed in the N2 permeance up to approximately 50 wt % of POEM content (PGP−POEM13). As the POEM content was further increased to 62.8 wt %, the N2 permeance started to increase, leading to a decrease in selectivity. This trend can be explained by the variation in the free volume of the polymer matrix, which is possibly associated with its self-cross-linking properties. For a better understanding of the self-cross-linking effect on the gas separation performance, the permselectivity of the lPGP−POEM comb copolymers was also measured. As illustrated in Table 2, the overall gas selectivity of l-PGP− Table 2. Gas Separation Performance of l-PGP−POEM Comb Copolymer Membranes

Table 3. Gas Separation Performances of PGP−POEM13 Comb Copolymer Membrane Prepared with Different Concentrations of Coating Solution

permeance (GPU) l-PGP−POEM11 l-PGP−POEM13 l-PGP−POEM15

N2

CO2

selectivity (CO2/N2)

8.3 13.2 31.9

106 233 441

12.8 17.6 13.8

permeance (GPU)

POEM membranes was much lower than that of the PGP− POEM membranes with high PPG graft density. Furthermore, in the case of the l-PGP−POEM membranes, a notable increase in the N2 permeance with the POEM content was observed. These phenomena may be due to the following aspects. At low PPG graft density, the self-cross-linking reaction is limited due to the lack of secondary amine groups in the comb copolymer. Without the cross-linking effect, the diffusivity of gas molecules is directly affected by the mobility of the polymer chain segment in the membrane. As confirmed by DSC analysis, the flexibility of the comb copolymers increases significantly as the POEM content increases. This results in a significant increase in N2 permeance of the l-PGP−POEM membranes from 8.3 to 31.9 GPU. It would be reasonable to say that the lower diffusivity and higher selectivity of the PGP−POEM comb copolymer membranes are attributed to the self-cross-linking reaction. The decrease in the polymer free volume resulting from self-cross-linking would restrict gas diffusion into the polymer matrix up to 51.2 wt % of POEM content, as confirmed by the XRD results. The effect of reduction in the free volume would offset that of the increased chain flexibility, leading to a negligible increase in N2 permeance up to 51.2 wt % of POEM content (from 10.5 to 11.7 GPU). Additionally, the small increase in d-spacing at higher POEM content would be the reason for the increase in N2 permeance from 11.7 to 15.4 GPU, since self-cross-linking can be restrained by an excess of POEM chains. Consequently, up to 51.2 wt % of POEM in the copolymer, both the CO2 permeance and ideal CO2/N2 selectivity were enhanced simultaneously with the POEM content. This is because the solubility of CO2 in the membranes is enhanced with a large amount of EO groups in POEM, while the increase in diffusivity is limited by the crosslinking effect. The PGP−POEM13 comb copolymer membrane showed the highest performance, i.e., CO2 permeance of 281 GPU and CO2/N2 selectivity of 24.1, exhibiting 2.5 times higher permeance than Pebax membranes with comparable selectivity. Furthermore, to evaluate the time dependence of membrane performance, the best performing PGP−POEM13 membrane was tested over a week (Figure S8). The performance did not significantly change over time, showing only slight decrease in permeance. This can be due to the

coating solution (wt %)

thickness (nm)

N2

CO2

selectivity (CO2/N2)

10 5 2.5 1

340 150 120 70

11.7 22.3 36.9 44.3

281 500 637 682

24.1 22.4 17.2 15.4

observed with a low concentration of the coating solution, revealing the good film forming ability. At 5% concentration of coating solution, the CO2 permeance reaches to 500 GPU maintaining a reasonable selectivity of 22.4. Figure 7 shows the CO 2 permeance versus CO 2 /N 2 selectivity plot, which compares the performance of the PGP−POEM13 comb copolymer membrane with that of various composite membranes reported in the literature.21,55−59 The colored target region is proposed by Merkel et al.,54 which represents the desired gas separation performance (i.e., CO2 permeance >1000 and CO2/N2 selectivity >20) for the postcombustion capture of CO2. The gas separation performance of the PGP−POEM13 comb copolymer membranes is close to the target area and is comparable to that of commercial natural gas membranes and Pebax 2533. It should be pointed out that the CO2 and N2 permeances of the PSf support used in our study are approximately 21 000 and 24 000 GPU, respectively, which belongs to medium porous support membrane.60 This implies that the permeance of our membranes can be further improved by using a more permeable polymer support (on the order of 105 GPU), which will be part of future research.

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CONCLUSION In this work, high-performance, self-cross-linked PGP−POEM comb copolymer membranes were successfully prepared via epoxide−amine click chemistry without any additional crosslinkers or catalysts. The self-cross-linking reaction occurred during the membrane drying process without high-temperature thermal treatment. The successful synthesis of PGP−POEM comb copolymer was confirmed by FT-IR spectroscopy, and the actual composition of the copolymer was determined by 1H NMR analysis. The enhanced thermal stability resulting from the cross-linked structure was verified by TGA analysis. The DSC curves showed that the comb copolymers existed in an G

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Macromolecules

Figure 7. CO2 permeance vs CO2/N2 selectivity plot, comparing the performance of PGP−POEM13 membranes with that of various composite membranes reported in the literature.21,55−59 The target region is proposed by Merkel et al. for the postcombustion capture of CO2.54

2017M1A2A2043448, NRF-2017K1A3A1A16069486) and the Human Resources Program in Energy Technology (20154010200810) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP).

amorphous state without phase separation. From the XRD patterns, two distinct d-spacing values were observed for the PGP−POEM comb copolymers due to the cross-linked polymer network. The PGP−POEM comb copolymer membrane showed a thickness of approximately 300 nm without apparent defects or voids, indicating its excellent film forming capability. Both the CO2 permeance and CO2/N2 selectivity of the PGP−POEM membranes were improved simultaneously with increasing POEM content up to 51.2 wt %. This is because the cross-linking could effectively restrain the increase in diffusivity while the large amount of ether groups in PPG and POEM moieties could improve the CO2 solubility, resulting in increased selectivity. The PGP−POEM13 membrane with 150 nm thickness exhibited the best performance, showing high CO2 permeance of 500 GPU and CO2/N2 selectivity of 22.4. These values are close to the target performance region for the postcombustion process, indicating the great potential for the practical application of this membrane in CO2 capture. The proposed method could be one of the easiest strategies to improve both the thermal stability and performance of gas separation membranes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02024. Figures S1−S9 and Table S1 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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

Ki Bong Lee: 0000-0001-9020-8646 Jong Hak Kim: 0000-0002-5858-1747 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation (NRF) grant funded by the Ministry of Science, ICT, and Future Planning (NRF-2017R1A4A1014569, NRFH

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