Tuning the microstructure of crosslinked Poly(ionic liquid) membranes and gels via a multicomponent reaction for improved CO2 capture performance

Journal of Membrane Science 593 (2020) 117405

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Tuning the microstructure of crosslinked Poly(ionic liquid) membranes and gels via a multicomponent reaction for improved CO2 capture performance

T

Jian Yina,b, Chenchen Zhanga, Yunfei Yua, Tingyu Haoa, Hua Wanga, Xiaoli Dinga, Jianqiang Menga,∗ a b

State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin, 300387, China Tianjin Baogang Research Institute of Rare Earths Co., Ltd., Tianjin, 300300, China

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 separation Solution-diffusion theory poly(ionic liquid) Crosslinked membrane Multicomponent reaction

Crosslinked poly(ionic liquid) (PIL) and poly(ionic liquid)-ionic liquid (PIL-IL) based membranes with tunable micromolecular structure have been synthesized in a simple way to optimize their CO2 capture performance. Amino-terminated PILs of two molecular weights (MW) were synthesized via the Debus-Radziszewski multicomponent reaction and then reacted with two commercial glycidyl ether crosslinkers, (polyoxyethylene bis (glycidyl ether) and trimethylolpropane triglycidyl ether). The effects of the PIL MW, crosslinker type and content, and IL content on CO2/N2 permeation-separation performance were systematically investigated. For the neat PIL membranes, both the utilization of ether-containing crosslinker and an increase in the crosslinker content increased the CO2 solubility and diffusivity, with the best PILs membrane exhibiting an excellent CO2 permeability of 170 Barrer and a CO2/N2 permselectivity of 36. For the PIL-IL membranes, an increase in the IL content increased the CO2 solubility and diffusivity, but also resulted in a decrease in the diffusivity selectivity. The best permeation-selectivity performance belongs to a PIL-IL membrane that showed a CO2 permeability of 2070 Barrer and a CO2/N2 permselectivity of 24.6. Among all the PIL-IL membranes reported in literature, this PIL-IL membrane demonstrated the highest permeability and also a cost-effective permselectivity, with its separation performance approaching the 2008 Robeson upper bound.

1. Introduction Carbon dioxide capture is considered a crucial strategy for reducing the concentration of atmospheric carbon dioxide (CO2) and mitigating the global warming and climate change issues. The European Union sets a target of 20% reduction in greenhouse gas emissions by 2020 compared with 1990, and reduces the cost of CO2 capture to 30–50 Euros per ton of CO2 by 2020 [1]. In order to achieve these targets, various technologies have been developed for CO2 capture. Membrane separation is one of the most promising approaches for CO2 capture due to its high energy efficiency, great stability, small footprint, and low maintenance cost [2–4]. Ionic liquids (ILs) are a class of low-temperature molten salts consisted of organic cations and organic or inorganic anions. They have been considered as one of the ideal type of materials for CO2 separation due to its low volatility, high solubility and high selectivity for CO2 [5–7]. Ionic liquid containing nanoparticles have also been used for facilitated CO2 transport [8–10]. The earliest utilization of ionic liquids for membrane separation was the supported ionic liquid membranes ∗

(SILMs), which exhibited good CO2 separation performance and high permeability. However, the SILMs suffer from poor long term stability and leaching of ILs from the support membranes at high operation pressures [11,12]. One of the solutions to address this challenge is to utilize poly(ionic liquid) (PIL) as a support membrane to fabricate PILIL composite membranes, which exhibited enhanced mechanical stability but less CO2 permeability than SILMs [13]. The CO2 permeability of the PIL-IL membranes increases with the loading content of ILs but is limited by the mechanical properties of the PIL materials. Introduction of a crosslinking structure into the PIL macrostructure is a promising strategy for achieving higher free IL loadings without sacrificing membrane mechanical stability [14]. Noble and coworkers have fabricated a series of crosslinked PIL-IL membrane for gas separation [14–20]. Moghadam and coworkers also reported a doublenetwork ion-gel membranes with higher gas separation performance [21]. These crosslinked network of PILs were fabricated via the chainaddition polymerization of vinyl monomers [14–16,21], or the stepgrowth crosslinking of epoxide and amino oligomers [17–20]. Compared with the cross-link reaction of vinyl monomers, the step-growth

Corresponding author.; E-mail addresses: [email protected], [email protected] (J. Meng).

https://doi.org/10.1016/j.memsci.2019.117405 Received 25 May 2019; Received in revised form 12 August 2019; Accepted 21 August 2019 Available online 22 August 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

Journal of Membrane Science 593 (2020) 117405

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purchased from Tianjin kermel Chemical Reagent Co., China. Glyoxal (40% in water) and 1,4-diaminobutane were purchased from Adamas Reagent, Ltd., China. Li NTf2, polyoxyethylene bis(glycidyl ether) (POBGE, Mw = 526 g/mol), trimethylolpropane triglycidyl ether (TMPGE), and 1-ethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide ([emim][NTf2]) were purchased from Aladdin Bio-Chem Techno. Co., Ltd., China. All chemicals were used as received without further purification.

(S-G) polymerization of epoxy and amino monomers does not require an inert atmosphere or produce small molecular by-products. However, the synthesis of ionic liquid oligomers for S-G polymerization often requires multistep and complicated synthesis processes. The tunability of current chemistry on the cross-link density and the microstructure of the resulted membranes is also limited. The Debus-Radziszewski reaction is a highly efficient multicomponent reaction named after Debus and Radziszewski, which was first proposed to produce imidazoles [22,23]. Recently, Lindner synthesized a series of amino terminated linear PILs via the Debus-Radziszewski reaction between diamine monomer, formaldehyde, and glyoxal [24]. This reaction provides a simple and versatile synthesis platform for the molecular design of amino PILs, including the chemical structure of the ionic liquid moiety and the molecular weight of the PILs. Furthermore, there are various commercial glycidyl ethers containing tunable content of epoxy and ether groups on the chain backbone. When reacting with the aminoterminated PILs, the cross-link density, the ether group content, and the flexibility of polymer chains can be easily tuned by the molecular weight of PIL precursor, the crosslinker type, and the molar ratio of PILto-crosslinker. In this work, we used the Debus-Radziszewski reaction to tune the chemistry and molecular weight of the amino PILs so that the microstructure of the resulting epoxy-amine crosslinking membranes and PIL-IL gels can be optimized for CO2/N2 separation. We selected 1,4-diaminobutane as the diamine monomer and lithium trifluoromethanesulfonimide (Li NTf2) as the counter anion. Two aminoterminated linear PILs with different molecular weight were synthesized, and two types of commercially available glycidyl ether crosslinkers, i. e. polyoxyethylene bis(glycidyl ether) (POBGE) and trimethylolpropane triglycidyl ether (TMPGE), were selected to form the crosslinking network structures of PILs. By varying the molecular weight of the amino PIL, type of crosslinker, and the molar ratio of PIL and crosslinker, we systematically investigated the effects of crosslink structure and ether bond structure on the CO2/N2 permeation-separation performance of the crosslinked PIL and PIL-IL membranes(see Scheme 1).

2.2. Synthesis of amino-terminated PIL precursors (PILs) Glacial acetic acid (5.05 g, 84.2 mmol), 1,4-butanediamine (3.71 g, 42.1 mmol), and deionized water (7.52 g) were mixed and stirred in a dry three-necked flask at 0 °C for 30 min. Then, a mixture of 2.3 mL formaldehyde (19.1 mol L−1) and 5 mL glyoxal (8.8 mol L−1) was added dropwise into the amine solution. In this work, we synthesized PILs with two different molecular weights: for the synthesis of the low molecular weight PIL (LPIL), the resulting solution was stirred for another 2 h at room temperature; for the high molecular weight one (HPIL), the resulting solution was stirred at 100 °C for 1 h. Subsequently, the resulting solution was diluted with 200 mL of pure water. Finally, Li NTf2 (24.17 g, 81.2 mmol) was slowly added into the resulting solution and stirred overnight. The obtained product was filtered, washed with pure water for three times, and dried in a vacuum oven at 120 °C for 12 h. 2.3. Fabrication of crosslinked neat PIL and PIL-IL composite membranes Mixtures of PILs, free ILs and crosslinkers in pre-determined molar ratios were dissolved in methanol and well stirred in three-necked flasks at room temperature for 6 h. Then, the resulting mixtures were poured into molds and vaporized at room temperature overnight to remove the solvents. Finally, the molds were heated at 60 °C in a vacuum oven for 3 h and further heated at 120 °C for 24 h. The compositions of the obtained composite membranes are listed in Table 1.

2. Experimental section

2.4. Characterization of membranes

2.1. Materials

The chemical structure of PILs were analyzed by 1H NMR on a Bruker 400 spectrometer. The samples were dissolved in the D2O with TMS as an internal reference. The Fourier transform infrared

Formaldehyde (50% in water), acetic acid and methanol were

Scheme 1. Synthesis of the crosslinked PIL membranes. 2

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Table 1 Composition of the obtained crosslinked PIL and PIL-IL membranes. Composite Membrane

Polymer(PIL)

Crosslinker

Molar ratio of PIL:Crosslinker

Mass ratio of PIL:Crosslinker

Ionic Liquid(IL)

wt% of IL

LT(1:1) LP(1:1) LT(1:2) LP(1:2) HT(1:1) HP(1:1) HT(1:2) HP(1:2) LT-x IL LP-x IL HT-x IL HP-x IL

LPIL LPIL LPIL LPIL HPIL HPIL HPIL HPIL LPIL LPIL HPIL HPIL

TMPGE POBGE TMPGE POBGE TMPGE POBGE TMPGE POBGE TMPGE POBGE TMPGE POBGE

1:1 1:1 1:2 1:2 1:1 1:1 1:2 1:2 1:2 1:2 1:2 1:2

1:0.4 1:0.7 1:0.8 1:1.4 20:1 10:1 10:1 5:1 1:0.8 1:1.4 10:1 5:1

[emim][NTf2] [emim][NTf2] [emim][NTf2] [emim][NTf2]

x = 9,23,33,41 x = 9,23,33,41 x = 9,23,33,41,66 x = 9,23,33,41

where the time-lag (θ ) was the intercept of the extrapolation of the steady-state pressure vs. time (t) plot on the t-axis. The solubility (S) of the single gas in membrane was calculated via:

spectroscopy (FT-IR) spectra of the membrane samples were recorded with a Vector-22 spectrometer (Bruker Daltonic Inc., Germany), at an incident angle of 45°, with Zinc Selenide (ZnSe) as an internal reflection element. Each spectrum was signal averaged from 32 scans at 4 cm−1 resolution. Differential scanning calorimetry (DSC) measurements were conducted on a Netzsch DSC 200 F3 Maia® instrument with the heating and cooling rate at 10 °C min−1 and nitrogen purging at 50 mL min−1. Samples were evaluated in the range of −90 to 60 °C. The XRD pattern was measured by a BRUKER D8 DISCOVER X-ray Powder Diffractometer using CuKα radiation with the wavelength of 1.5418 A. The field emission scanning electron microscopy (FE-SEM) measurement was carried out on a Zeiss Gemini SEM 500 microscope. The samples were pre-coated with gold-coating by a Leica EM ACE200 Low Vacuum sputter coater to improve the conductivity of the samples.

S=

3.1. Synthesis of PIL precursors It is reported that the chemical structure of the ionic liquid moiety and molecular weight of the PILs are determined by the diamine monomer and the reaction conditions [24]. The degree of polymerization of the PIL precursors increases with the increase of reaction time and reaction temperature. We selected 1,4-diaminobutane as the diamine monomer and synthesized low molecular weight PIL precursors (LPIL) at low temperature and short reaction time, and PILs with a high molecular weight (HPIL) by increasing the reaction time and reaction temperature. The chemical structures of the resulted PILs were characterized by 1H NMR spectroscopy (see Fig. S1 in Supporting Information). The signals for protons a, b, c, d, and e are in good agreement with Lindner's report, indicating the formation of PILs structure. The average degree of polymerization can be calculated via the peak ratio of imidazole groups (a) to the α-CH2 of the terminal amino groups (b), which is 1.2 for LPIL and 13.5 for HPIL. The number average molecular weights of obtained PILs were calculated to be about 720 g/ mol for LPIL and 5800 g/mol for HPIL. The PILs were also examined by FT-IR (Fig. 1). The broad peaks around 3420 cm−1 are attributed to the stretching vibration of the amino groups. The peak at 1645 cm−1 is attributed to the N–H stretching of the terminal amine groups. The peaks at 1384 and 1159 cm−1 are assigned to C–N and C–C stretching vibrations of the imidazolium ring and the alkyl chains, respectively. Therefore, the FTIR results confirmeded the successful preparation of PILs.

The pure gas permeabilities of the synthesized membranes were measured in the order of N2 to CO2, in order to eliminate the plasticization effect of CO2, using a custom-built permeation system at 35 °C, based on a constant-volume, variable-pressure method [25–27]. All membranes were masked as described previously and degassed overnight before measurements [28]. The upstream pressure was maintained at 0.2 atm, whereas the downstream pressure increase was recorded over time. Pure gas permeabilities were determined according to the following equation:

PA D S = ⎛ A⎞ × ⎛ A⎞ PB D B ⎝ ⎠ ⎝ SB ⎠ ⎜

⎟

⎜

⎟

(1)

where P (Barrer, 1 Barrer = 1 × 10−10 cm3 (STP) cm/(cm2 s cm Hg)) is the gas permeability, Vd is the downstream volume (cm3), l is the membrane thickness (cm), pup is the upstream pressure (cm Hg), A is the effective membrane area (cm2);

( ) dp dt

increment in the downstream (cm Hg/s);

is the steady-state pressure ss

( ) dp dt

is the leak rate of the

leak

system (cm Hg/s); T is the test temperature (K), and R is the gas constant (0.278 cm3 cm Hg/cm3(STP) K). The ideal selectivity (αA/B) for two different gases A and B, also known as permselectivity, is defined as the ratio of pure gas permeability of these two gases:

αA / B =

PA D S = ⎛ A⎞ × ⎛ A⎞ PB ⎝ DB ⎠ ⎝ SB ⎠ ⎜

DA DB

⎟

⎜

3.2. Characterization of crosslinked PIL and PIL-IL membranes The crosslinked membranes were also characterized by FT-IR (Fig. 1). The peaks at about 1100 cm−1 and 1460 cm−1 are assigned to the stretching vibration of C–O–C and the shear vibration peak of CH2. The broad peaks around 2880 cm−1 are attributed to the stretching vibration of CH. These results indicate that the glycidyl ethers, including TMPGE and POBGE, are introduced into the polymer network. Meanwhile, the decrease of the peak around 3420 cm−1 in cross-linked membranes indicates the occurrence of the crosslinking reaction between the amino groups on the PILs and the epoxy groups on the glycidyl ether crosslinkers. We calculated the conversion of epoxy groups in membranes according to the method reported in the literature [16]. The epoxy conversion rate of LT(1:1), LP(1:1), HT(1:1) and HP(1:1) are

⎟

(2) SA SB

is the diffusivity selectivity, and the is the solubility where the selectivity. The diffusivity of the single gas was calculated by the timelag parameter (θ ) and the membrane thickness (l ) as follows:

D=

l2 6θ

(4)

3. Results and discussion

2.5. Gas permeation properties

αA / B =

P D

(3) 3

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Fig. 1. FT-IR spectra of (a) LPIL cross-linked membranes and (b) HPIL cross-linked membranes.

owing to the decrease of the chain packing efficiency resulting from the crosslinking structure, while the increase of the Tg for the LT membranes is most likely attributed to the formation of the microcrytalline structure that is also confirmed by the XRD results (Fig. S4). It is also noted that there is an unusual decrease in Tg with the increase of the crosslinker content in membranes. This result is expected because the increase of crosslinker content not only decreases the chain packing efficiency, but also introduces more flexible ether bonds into membrane, leading to an compensation to the decrease of chain mobility resulted by the increase of crosslinked density, and thus, an increase in the chain flexibility and an decrease in Tg [30,31]. It is also seen that the TMPGE-crosslinked membranes have a much higher Tg than the POBGE-crosslinked membranes due to the higher crosslink density and looser PIL packing of the TMPGE-crosslinked membranes. For the PIL-IL membranes, there is a decrease in Tg when the IL content increases. For example, the Tg of LT membrane decreases from −1 °C to −36 °C with an increase of IL content from 9% to 41%. Introducing more free ILs deceases the interaction of the PIL polymer chains, resulting in increased polymer chain flexibility.

calculated as 70.9%, 67.8%, 62.9%, and 71.3%, respectively. When normalized with the epoxy group content in either POBGE or TMPGE molecules, the actual reacted molar ratios of epoxide-to-PILs in membranes are calculated as 2.1:1 for the LT(1:1) membrane, 1.4:1 for the LP(1:1) membrane, 1.9:1 for the HT(1:1) membrane and 1.4:1 for the HP(1:1) membrane. This result indicates that the TMPGE-crosslinked membranes have higher molar ratio of epoxide-to-PILs and thus higher cross-link density than the POBGE-crosslinked membranes. On the other hand, it is known that the molar contents of PIL in HT and HP membranes are much smaller than those in LT and LP membranes because of the higher molecular weight of HPIL chains; as a result, the LPIL-crosslinked membranes should have higher cross-link density than the HPIL-crosslinked membranes, i.e., LT has a higher crosslink density than HT and LP has a higher crosslink density than HP. We also examined the size of the membranes via the optic photograph (see the pictures in Fig. S2). The membranes have a size of 45 × 45 mm2 (Fig. S2) and the thickness ranging from 0.4 to 1.1 mm on the basis of the molecular weight of the PIL and the crosslinker types. The morphology of the membrane was also characterized by the scanning electron microscope (SEM) (Fig. S3). We see that both the surface and cross sections of these membranes have an uniform and dense morphology.

3.5. Gas permeation properties 3.5.1. Gas solubility of neat PIL membranes Fig. 3a displays the CO2 solubility of the neat PIL membranes as well as crosslinked PIL membranes with different molecular weights and using glycidyl ether crosslinkers with different functionality degrees. It is seen that the LT membranes have much lower CO2 solubility than other membranes. The LT(1:1) membrane has a CO2 solubility of only 0.03 × 10−2 cm3 (STP) cm−3 cm Hg−1, and the LT(1:2) membrane has a CO2 solubility of 0.06 × 10−2 cm3 (STP) cm−3 cm Hg−1.However, all the other membranes have CO2 solubility values above 0.8 × 10−2 cm3 (STP) cm−3 cm Hg−1. This is presumably related to the much higher Tg values for the LT membranes, which is ca. 19.6 °C for the LT(1:1) membrane and 15.3 °C for the LT(1:2) membrane. Therefore, these membrane were more glassy at room temperature and has a much lower free volume than other membranes. For the LP, HT and HP membranes, the solubility of the neat PIL membranes is mainly affected by the PIL molecular weight, crosslinker types, and the crosslinker content. The results show that the POBGEcrosslinked PIL membranes have a better CO2 solubility than TMPGEcrosslinked membranes at the same conditions. POBGE has a higher CO2-philic ether bond content, thus it forms a polymer network with higher CO2 favorable interactions [32], resulting in higher CO2 solubility. For example, HP(1:1) membrane has a CO2 solubility of 2.05 × 10−2 cm3 (STP) cm−3 cm Hg−1, which is almost two-fold higher than that of the HT(1:1) membrane. The results also illustrate that the molecular weight of PILs has less

3.3. XRD analysis The polymer chain morphologies of the PILs and the crosslinked membranes were examined by X-ray diffraction (XRD) (Fig. S4). We see that all the samples are amorphous except for the LT sample, which has a small sharp peak at the 2 theta of 24.5°. According to the Bragg equation, the inter-chain spacing is calculated as 3.63 Å, presumably from the formation of intermolecular hydrogen bonds between the N–H bond of PILs and the ether oxygen atoms in OEG chains [29]. 3.4. DSC analysis The thermal properties of the PILs and PIL-IL membranes were examined by differential scanning calorimetry (DSC). The DSC curves are shown in Fig. S5 and the calculated Tg values are shown in Fig. 2. As for the amino PILs, the HPIL shows a higher glass-transition temperature (Tg = 41 °C, Fig. 2a) than the LPIL (Tg = −26 °C, Fig. 2a), which should be attributed to the fact that the longer PIL chains allow for better intramolecular interaction, forming a denser polymer chain packing. For the crosslinked PIL membranes, we see that almost all the membranes show only a single Tg, which implied that the homogeneous polymers were formed for the crosslinked PIL membranes. And we also see that almost all the membranes show a lower Tg than the amino PILs, except the LT(1:1) and LT(1:2) membranes. The decrease of Tg is mainly 4

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Fig. 2. The Tg of (a) the amino PIL, the resulted PIL membranes, and (b) the PIL-IL membranes.

TMPGE-crosslinked membranes. The CO2 diffusivity of LP(1:1) membrane is almost three times higher than that of the LT(1:1) membrane, and the LP(1:2) membrane has a CO2 diffusivity which is also almost three times of the LT(1:2) membrane. The high Tg of the TMPGEcrosslinked membranes (shown in Fig. 2a) implies a lower chain flexibility, and thus decreased CO2 diffusivity. The CO2 diffusivity can also be increased by increasing the crosslinker content in LP and LT membranes. It is noted that the CO2 diffusivities of LT(1:2) and LP(1:2) membranes have increased by 105% and 91%, respectively, compared to those of the LT(1:1) and LP(1:1) membranes. This result is also attributed to the decrease of Tg and the increase of chain flexibility (shown in Fig. 2a). In comparison, the variation tendency of CO2 diffusivity in HT and HP membrane are more complicated. When the TMPEG content is low, the HT(1:1) membrane has a CO2 diffusivity similar to that of the HP (1:1) membrane. When doubling the crosslinker content, the HP(1:2) membrane has a higher CO2 diffusivity than the HT(1:2) membrane. It is known that both the polymer chain flexibility and the polymer-penetrant specific interactions have effects on the CO2 diffusivity [32]. We believe that the polymer-CO2 specific interactions dominate when the TMPEG content is low, in which case the CO2 diffusion is mainly inhibited by the sorption of HPIL chains. When the TMPEG content is high, the polymer chain flexibility prevails. The HT(1:2) membrane has a lower crosslink density and thus a lower CO2 diffusivity. As for the crosslinker content, we see that the CO2 diffusivity decreased with an increase in TMPGE content, but keep unchanged with an increase in the POBGE content. This result is mainly due to a compromise between the inhibition from crosslink structure and the inhibition of the membrane-CO2 specific interactions. On one hand, the introduction of more ether bonds can decrease the Tg of membrane and increase the chain flexibility, even with the decrease of chain mobility as mentioned, which totally increases the gas diffusivity [32]. On the other hand, the increase of the ether bond content also can increase the membrane-CO2 adsorption and inhibit the gas diffusion [35,36]. As a

effect on the CO2 solubility of the membranes. For example, the LP(1:1) membrane has a CO2 solubility of 2.05 × 10−2 cm3 (STP) cm−3 cm Hg−1, which is closed to the HP(1:1) membrane, and the LP(1:2) membrane also has a similar solubility to the HP(1:2) membrane. It is evident that the CO2-philic group content has a more noted effect on CO2 solubility than the molecular weight (MW) of PIL membrane. As for the crosslinker content, the results clearly indicate that an increase in ether bonds contained in the crosslinkers results in an increase in the membrane CO2 solubility. When doubling the TMPGE content in membranes, the CO2 solubility of LT(1:2) and HT(1:2) membranes both increased by 100%. The CO2 solubility of LP(1:2) and HP(1:2) membranes also increased by 38% and 81%, respectively, compared to LP(1:1) and HP(1:1) membranes. This observation is different from those reported in the literatures [15,33]. In Lin's work, a PEG-based polymer film was prepared from a di-functional crosslinker PEGDA. The crosslinker content was found to be irrelevant to the CO2 solubility of membranes [33]. Carlisle and coworkers also utilized a crosslinker containing ether bond to fabricate crosslinked PIL membranes. Their results also indicated that the CO2 solubility was irrelevant to the crosslinker content [15]. We attribute the increase of solubility with the crosslinker content to two reasons. Firstly, an increase in crosslinker content introduces more CO2-philic ether bonds into the polymer network, which increases the polymer-CO2 favorable interactions and thus the gas solubility [32]. Secondly, an increase in the crosslinker content increases the crosslink density of membranes as well as decreases the glass transition temperature (shown in Fig. 2a), bringing the membranes higher flexibility and thus higher gas solubility [32,34]. 3.5.2. Gas diffusivity of neat PIL membranes Fig. 3b shows the CO2 diffusivity of the PIL membranes. The CO2 diffusivity is affected by the PIL molecular weight, crosslinker types, and the crosslinker content. By comparing LT and LP, it is seen that the POBGE-crosslinked membranes have a higher CO2 diffusivity than

Fig. 3. The CO2 solubility (a) and diffusivity (b) of the crosslinked PIL membranes. 5

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Fig. 4. The CO2/N2 solubility selectivity (a) and diffusivity selectivity (b) of the neat PILs.

selectivity of 2.69, which is higher than the diffusivity selectivity of the LP(1:1) membrane (0.52). These results are presumably owing to the fact that the TMPGE-crosslinked network has a high crosslinking density, suppressing intra-segmental mobility, resulting in a higher diffusivity selectivity [37,38]. For the TMPGE-crosslinked membranes, the results indicate that the diffusivity selectivity decreases with the increase of crosslinker content. The diffusivity selectivity of the LT membrane decreased from 2.69 to 0.56 with an increase in the crosslinker content; and the diffusivity selectivity of the HT membrane also decreased from 1.26 to 0.90 when the TMPGE content doubled. We attribute this trend to the increased chain flexibility which is beneficial to the intra segmental mobility and the N2 diffusivity [37,38]. For the POBGE-crosslinked membranes, the variation tendency of the diffusivity selectivity with the POBGE content is different from that of the TMPGE. We see the diffusivity selectivity increased in LP membranes, but decreased in HP membranes when the POBGE content increases. This result may be owing to the fact that CO2 has a higher diffusion coefficient than N2 between the amorphous ether-containing polymer chains [39]. On the other hand, the CO2 diffusivity in HP membranes is further inhibited by the membrane-CO2 adsorption, while the N2 diffusion is facilitated by an increase in chain flexibility. As a result, the diffusivity selectivity in HP(1:2) membrane decreased.

result, the decrease of CO2 diffusivity with the TMPGE content can be attributed to the fact that the inhibition of the membrane-CO2 adsorption is stronger than the promotion from the increased chain flexibility. Compared with the HP(1:1) membrane, the HP(1:2) membrane has a higher flexibility, which offsets the effect of membrane-CO2 specific interactions on CO2 diffusion, and thus resulted in an unchanged CO2 diffusivity. 3.5.3. The solubility selectivity and diffusivity selectivity of neat PIL membranes Fig. 4 displays the solubility selectivity and diffusivity selectivity of the neat PIL membranes. The results indicate that the POBGE-crosslinked PIL membranes have a higher CO2 solubility selectivity than the TMPGE-crosslinked membranes (Fig. 4a). For example, the LP(1:1) membrane has the solubility selectivity of 24.7, which is higher than that of the LT(1:1) membrane (0.3); the HP(1:1) membrane also has a higher CO2 solubility selectivity (19.7) than the HT(1:1) membrane (13.77). The results also indicate that the solubility selectivity increases with increasing crosslinker content. For example, the LP(1:2) membrane has a solubility selectivity of 39.3, which is 59% higher than the solubility selectivity of the LP(1:1) membrane; the HT(1:2) membrane also has a higher solubility selectivity (16.1) than HT(1:1) membrane (13.8). These results are likely attributed to the fact that both the utilization of the ether-containing compounds as crosslinker and the increase of crosslinker content both increase the membrane CO2 solubility, thus resulting in an increase in solubility selectivity. Regarding the diffusivity selectivity of membranes, it is seen that the TMPGE-crosslinked membranes have a higher diffusivity selectivity than the POBGE-crosslinked membranes at the PILs-to-crosslinker molar ratio of 1:1 (Fig. 4b). For example, the HT(1:1) membrane has a diffusivity selectivity of 1.26, which is higher than that of the HP(1:1) membrane (0.83); and the LT(1:1) membrane has a diffusivity

3.5.4. The permeability and selectivity of neat PIL membranes Fig. 5a shows the Robeson plot for CO2/N2 separation performance of the neat crosslinked PIL membranes investigated in the current work. The CO2/N2 permselectivity is plotted versus the CO2 permeability, and the solid red line represents the empirical 2008 upper bound for the CO2/N2 gas pair. It is seen that the LT membranes have much poorer permeability and permselectivity than other membranes due to the inhibition of the CO2 solution and diffusion by a semicrystalline and

Fig. 5. CO2/N2 separation performance of the neat PIL membranes plotted on Robeson plots: (a) the neat PIL membranes; (b) the comparison results of the LP(1:2) membrane with other neat PIL membranes in literature tested under ideal (i.e., single-gas) conditions [41,42,44,46]. 6

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implied that the LP membrane already has a polymer network with sufficiently high flexibility. The results also indicate that both the crosslinker type and the molecular weight of PILs have some effects on the membrane CO2 solubility. The POBGE-crosslinked PIL-IL membranes have a higher CO2 solubility than the TMPGE-crosslinked membranes at the same IL content due to the fact that the POBGEformed polymer network has a lower crosslinking density and a higher flexibility [32,34]. Regarding the molecular weight of PILs, it is seen that the LPILcontained membranes have a higher CO2 solubility than the HPILcontained membranes at same conditions. For example, the CO2 solubility of LT-41 IL membrane is higher than that of the HT-41 IL membrane; and the CO2 solubility of LP-31 IL membrane is also higher than that of the HP-31 IL membrane. This result is attributed to the fact that shorter PIL chains are likely to form a lessly packed molecular structure, resulting in higher flexibility and thus higher CO2 solubility [48]. The results also show that LT-IL membranes with the ILs content ranged from 9%-23% have a lower CO2 solubility than the HT-IL membranes with the same IL content, which can be attributed to the much higher Tg (Fig. 2b) and the more crosslinked network of the LT-IL membranes than the HT-IL membranes.

stiff structure. For the HT and HP membranes, the permeability of the membranes increased with increasing crosslinker content, which is mainly attributed to an increase in CO2 solubility. But the permselectivities of these membranes keep unchanged when the crosslinker content increases, this is because the decrease of diffusivity selectivity offsets the increase of solubility selectivity. Only the LP membranes showed increase in both permeability and permselectivity when increasing the crosslinker content. This can be attributed to the increase of the CO2 solubility, CO2 diffusivity, solubility selectivity and diffusivity selectivity. Overall, the LP(1:2) membrane showed the best gas separation performance, with the CO2 permeability of 170 Barrer and CO2/N2 selectivity of 36, approaching the 2008 Robeson upper bound. The LP(1:2) membrane is compared with other neat PIL membranes in the literature (Fig. 5b) [14,40–47]. With the well-known polystyrenebased PILs and polyacrylate-based PILs as examples, five PILs terminated with different alkyl-groups were synthesized by the Noble group [40]. Among those crosslinked membranes, the best CO2 permeability and CO2/N2 permselectivity reached 22 Barrer and 31.7 Barrer, respectively. In comparison, the LP(1:2) membrane has a much higher CO2 permeability as well as an excellent CO2/N2 permselectivity (marked as a green star in Fig. 5b). When compared with other neat PILs, the LP(1:2) membrane demonstrated the highest gas permeability with a similar permselectivity.

3.5.6. Gas diffusivity of PIL-IL membranes The CO2 diffusivity of PIL-IL membranes are presented in Fig. 6b. Increasing the IL loading resulted in an increase in the CO2 diffusivity. The CO2 diffusivity of HT-IL membranes increased from 34.7 to 137 (10−8 cm2 s−1) with the IL content varied from 9% to 41%, because the free ILs increase the chain flexibility and facilitate the CO2 diffusivity. It is also observed the crosslinker type also has some effects on the CO2 diffusivity. TMPGE-crosslinked PIL-IL membranes are more sensitive to the variation of the free IL content than the PEG-crosslinked membranes. The CO2 diffusivity of LT-41 IL membrane is 313% higher than that of the LT-9 IL membrane, while the CO2 diffusivity of LP-41 IL membrane is 104% higher than that of the LP-9 IL membrane. The CO2

3.5.5. Gas solubility of PIL-IL membranes In order to further improve the permeability of membrane, we further investigated the gas separation performance of crosslinked PILIL membranes. The CO2 permeability and CO2/N2 selectivity of the cross-linked PIL-IL membranes are presented in Table S1 and Fig. 6. The CO2 solubility of almost all the PIL-IL membranes increased with an increase in the IL loading (shown in Fig. 6a), as the free ILs increase the membrane CO2 adsorption, except for the LP-IL membranes. We see that the solubilities of the LP-IL membranes are quite close and also much higher than those of other membranes, which may

Fig. 6. The CO2 solubility (a), diffusivity (b), solubility selectivity (c), and diffusivity selectivity (d) of the PIL-IL membranes. 7

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permselectivity of the PIL-IL membranes are mainly determined by the solubility selectivity of the membranes. The permselectivity of the TMPGE-crosslinked membranes increased with the IL content, due to the increase of solubility. Therefore, the gas permeation-separation performance of the LT-IL and HT-IL membranes are moving towards the Robeson bound, when increasing the IL content. On the other hand, for the POBGE-crosslinked membrane, the permselectivity reaches the maximum and then decreases with the increase of IL content, which is mainly affected by the solubility selectivity. When using the upper bound as a benchmark, we see the neat LP (1:2) membrane has the best performance, but its LP-IL membranes are far away from the upper bound. This result also implies that the fabrication of the high-performance gas separation membrane requires an optimal global structure design. The free ILs content in crosslinked PILIL membrane is not “the more the better”. When the crosslink density is a major factor on the CO2 transport resistance, an increase in IL content can facilitate the gas separation performance via increasing the chain flexibility. However, when the CO2 transport is mainly inhibited by the membrane-CO2 interactions, further increase in IL content would continue to increase the permeability but also decrease the permselectivity, pushing the gas permeation-separation performance away from the upper bound. We also examined the relationship between the Tg and the CO2 permeability and that between the Tg and the selectivity of the PIL-IL membranes (see Fig. S7). We see the increase of CO2 permeability with the increase of IL contents, as corresponds well with the decrease of Tg and should be due to the increase of free volume. The results also show that the relationship between the Tg and the gas selectivity is complex and varies with the crosslinker types. The TMPGE-crosslinked membranes show the tendency of increased selectivity with the decrease of Tg, while the POBGE-crosslinked membranes show an opposite trend. We believe that the evolution of the CO2/N2 selectivity with Tg depends on which factor dominates the gas transport. For example, the evolution of the selectivity of the HT-IL membranes with Tg shows the same tendency with the CO2 permeability. The reason should be due to the high crosslink density of the HT-IL membranes. The polymer chain mobility and free volume factors, which is reflected by Tg, should dominate the gas transport. The membrane with high Tg shows low gas permeability but high selectivity. On the other hand, for the looser LPIL membranes, the membrane-CO2 interaction dominates and the increase in the IL content (decrease Tg) would continue to increase the N2 permeability but decrease the selectivity. Based on all the results we obtained so far, it was inferred that the TMPGE-crosslinked PIL membranes with highly crosslink density should be preferred as the base PIL membranes. Therefore, we selected the HT(1:2) membrane as the base PIL membrane, and further increased the IL content from 41 wt% to 66 wt%. This membrane design strategy was validated by our experiment results. The CO2 permeability was found further increased from 366 Barrer to 2070 Barrer, while the selectivity only decreased slightly, from 30 to 24.6. The ideal permeation-separation performance of the resulted membranes are also compared with other type of crosslinked PIL-IL membranes [15,16], including the SILMs [49,50] and the linear PIL-IL membranes [43,45,47,51,52] reported in the literatures (shown in Fig. S8). Take the “curable PIL” membrane as an example [16], the crosslinked membrane with ≥80 wt% [emim][NTf2] IL has a CO2 permeability of 500 Barrer and a selectivity of 24. In comparison, the HT-66 IL membrane in the current work, which carries less free ILs, has an almost 4-fold higher CO2 permeability and a similar selectivity. Among the SILMs containing [emim][NTf2], the best performance reported is a CO2 permeability of 1000 Barrer and a CO2/N2 selectivity of 22, which is also much lower than those of the HT-66 IL membrane [50]. The membranes in this work hold an excellent promise for industrial applications. According to the CO2 capture target of the US Department of Energy (DOE) (10$ per ton of CO2 captured by 2023–2035) [53], the membranes with a CO2/N2 selectivity of ≥20 and a CO2 permeability

diffusivity of HT-41 IL membrane is also 295% higher than that of the HT-9 IL membrane, and the CO2 diffusivity of HP-41 IL membrane is 28% higher than that of the HP-9 IL membrane. All these results are attributed to the fact that an increase in the free IL content leads to improved membrane-CO2 interactions and inhibited CO2 diffusion. Compared with POBGE-crosslinked membranes, the TMPGE-crosslinked membranes have lower flexibility, with the CO2 diffusion being mainly affected by chain flexibility. The free ILs can significantly swell the three-dimensional polymer network, which largely increases the chain flexibility and facilitates the CO2 transport. The DSC results indeed confirm the effects of crosslinker type on chain flexibility in membranes. The difference of Tg between the LT-9 IL membrane and LT-41 IL membrane can reach ca. 36 °C, while the Tg of the LP-9 IL membrane and LP-41 IL membrane only varied from −32.7 °C to −39.3 °C. On the other hand, the TMPGE-crosslinked PIL-IL membrane has a higher CO2 diffusivity than the PEG-crosslinked membrane. For example, the LT-41IL membrane has a CO2 diffusivity of 110 (10−8cm2s−1), which is higher than the LP-41 IL membrane (72.3 × 10−8 cm2s−1); and the CO2 diffusivity of HT-41 IL membrane is ca. 137 10−8cm2s−1, which is also higher than that of the HP-41 IL membrane (59 × 10−8 cm2s−1). As the POBGE chains have a higher ether group content than TMPGE, they provide stronger CO2-philicity and more inhibition to CO2 transport. 3.5.7. Gas solubility selectivity and diffusivity selectivity of PIL-IL membranes Fig. 6c shows the solubility selectivity of PIL-IL membranes. For the TMPGE-crosslinked membrane, it is clear that the solubility selectivity increases with increasing IL content, presumably due to the increased flexibility when the IL content increases. The results also show that the HT-IL membranes have higher solubility selectivity than the LT-IL membranes at the same IL content. The main reason for this is that the HT-IL membranes have lower Tg and higher flexibility than the LT membranes carried with same IL content (shown in Fig. 2b). However, for the PEG-crosslinked PIL-IL membranes, the solubility selectivity reached the maximum first, then decreased with an increase in IL content. For example, the HP-23 IL membrane has a maximal solubility selectivity of 61.2, and the solubility selectivity of the HP-41 IL is only 21. This result can be explained by the fact that the increase of flexibility also facilitated the N2 dissolution in membrane, thus resulting in adecrease in solubility selectivity. In terms of the diffusivity selectivity of PIL-IL, we see that almost all membranes exhibited a decrease of diffusivity selectivity with an increase in IL content. An increase of IL content can improve the membrane-CO2 interactions and inhibit the CO2 diffusion. On the other hand, the increase of IL content can also swell the membrane and facilitate the transport of N2, resulting in the decrease of CO2 diffusivity selectivity. The results also show that the TMPGE-crosslinked PIL-IL membranes have higher diffusivity selectivity than the PEG-crosslinked membranes at the same IL content. For example, the LT-33 IL membrane has a diffusivity selectivity of 1.12, higher than that that of the LP-33 IL membrane (0.37); the HT-41 IL membrane has a diffusivity selectivity of 0.52, which is also higher than that of the HP-41 IL membrane (0.16). This result is due to the fact that the PEG-formed network has higher ether bond content, which provided a stronger CO2philicity than the TMPGE-crosslinked network and also inhibited the CO2 diffusion. 3.5.8. The permeability and selectivity of the PIL-IL membranes Fig. S6shows the CO2/N2 separation performance of the cross-linked PIL-IL membranes, and compares these results with the Robeson upper bond. We see that the CO2 permeability increases with the increase of IL content, which can be attributed to the increase of both CO2 solubility and diffusivity. As for the permselectivity, we see that the 8

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excellent CO2 permeability of 2070 Barrer with a selectivity of 24.6. The permselectivity of this membrane meets the 2013 US DOE CO2 capture target and the membrane also has highest permeability than the other PILs/PIL-IL membranes reported in the literature. The present study used a simple method to fabricate the crosslinked PIL and PIL-IL membranes with tunable network structure and systematically investigated the effects of network structure on CO2/N2 separation performance, which renders the method highly promising for practical industrial use.

as high as possible are required [12]. For the HT-66 IL membrane, the membrane has an acceptable CO2/N2 selectivity and almost the highest permeability reported in the literature for PIL-IL membranes. When used as the active functional layer, the thickness of this type of membrane would only need to be ca. 100–2000 nm to achieve the desired CO2 permeance (≥1000 GPU), which is thicker than what can be achieved by the current membrane fabrication techniques. In the process of preparing this manuscript, Moghadam and coworkers have reported a double-network ion-gel membranes that exhibited higher gas separation performance [21]. With the decrease of crosslinker content from 4 to 0.5 mol.%, the CO2 permeability of the membranes increased from 2254 to 7569 Barrer, and the CO2/N2 selectivity also increased from 130 to 210. However, as mentioned above, the fabrication of these membranes required an inert atmosphere during the radical polymerization process. We also see that the Marrucho's group have reported that the counter anions of PIL and ILs had a great effect on membrane gas separation performance [52]. The PIL-IL membrane performance can be further significantly improved by replacing [NTf2] with cyano-functionalized anions. For example, the PIL C(CN)3–60C(CN)3 membrane has the permeability of 439.3 Barrer, and the selectivity of 64.4, which surpassed the 2008 Robeson upper bound for CO2/N2 separation and is actually the only case for PIL membranes surpassing the upper bound, as far as we know. This result is mainly because the cyano-functionalized anions, such as [C(CN)3]-, increase the CO2 solubility. However, in spite of their excellent performance, the cyano-functionalized ILs are also the most expensive among the commercially available ILs [48,52]. For example, the price of the [emim] [C(CN)3] obtained from the Sigma-Aldrich website is about 47.5 USD/ g, which is almost 10-fold higher than the [emim][NTf2] with the price of 3.7 USD/g. The high fabrication cost of membranes may somewhat limit its potential for industrial use.

Acknowledgements We gratefully acknowledge the support from the National Natural Science Foundation of China (Grant No. 21875162, 21574100) and the Program for Innovative Research Team in Universities of Tianjin (No. TD 13-5044). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117405. References [1] IPCC, Climate Change 2013: the Physical Science Basis.Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013. [2] S.E. Kentish, C.A. Scholes, G.W. Stevens, Carbon dioxide separation through polymeric membrane systems for flue gas applications, Recent Pat. Chem. Eng. 1 (2008) 52–66. [3] S. Pacala, R. Socolow, Stabilization wedges: solving the climate problem for the next 50 Years with current technologies, Science 305 (2004) 968–972. [4] R. Socolow, R. Hotinski, J.B. Greenblatt, S. Pacala, Solving the climate problem: technologies available to curb CO2 emissions, Environment 46 (2004) 8–19. [5] N.V. Plechkova, K.R. Seddon, Applications of ionic liquids in the chemical industry, Chem. Soc. Rev. 37 (2008) 123–150. [6] M.J. Earle, J.M.S.S. Esperança, M.A. Gilea, J.N. Canongia Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, The distillation and volatility of ionic liquids, Nature 439 (2006) 831. [7] Y.-F. Hu, Z.-C. Liu, C.-M. Xu, X.-M. Zhang, The molecular characteristics dominating the solubility of gases in ionic liquids, Chem. Soc. Rev. 40 (2011) 3802–3823. [8] J.H. Lee, I.S. Chae, D. Song, Y.S. Kang, S.W. Kang, Metallic copper incorporated ionic liquids toward maximizing CO2 separation properties, Separ. Purif. Technol. 112 (2013) 49–53. [9] J. Yuan, M. Fan, F. Zhang, Y. Xu, H. Tang, C. Huang, H. Zhang, Amine-functionalized poly(ionic liquid) brushes for carbon dioxide adsorption, Chem. Eng. J. 316 (2017) 903–910. [10] K.W. Yoon, H. Kim, Y.S. Kang, S.W. Kang, 1-Butyl-3-methylimidazolium tetrafluoroborate/zinc oxide composite membrane for high CO2 separation performance, Chem. Eng. J. 320 (2017) 50–54. [11] P.K. Parhi, Supported liquid membrane principle and its practices: a short review, J. Chem. 2013 (2013) 1–11. [12] M.G. Cowan, D.L. Gin, R.D. Noble, Poly(ionic liquid)/ionic liquid ion-gels with high "free" ionic liquid content: platform membrane materials for CO2/light gas separations, Acc. Chem. Res. 49 (2016) 724–732. [13] J.E. Bara, E.S. Hatakeyama, D.L. Gin, R.D. Noble, Improving CO2 permeability in polymerized room-temperature ionic liquid gas separation membranes through the formation of a solid composite with a room-temperature ionic liquid, Polym. Adv. Technol. 19 (2008) 1415–1420. [14] J.E. Bara, E.S. Hatakeyama, C.J. Gabriel, X. Zeng, S. Lessmann, D.L. Gin, R.D. Noble, Synthesis and light gas separations in cross-linked gemini room temperature ionic liquid polymer membranes, J. Membr. Sci. 316 (2008) 186–191. [15] T.K. Carlisle, G.D. Nicodemus, D.L. Gin, R.D. Noble, CO2/light gas separation performance of cross-linked poly(vinylimidazolium) gel membranes as a function of ionic liquid loading and cross-linker content, J. Membr. Sci. 397–398 (2012) 24–37. [16] T.K. Carlisle, W.M. McDanel, M.G. Cowan, R.D. Noble, D.L. Gin, Vinyl-functionalized poly(imidazolium)s: a curable polymer platform for cross-linked ionic liquid gel synthesis, Chem. Mater. 26 (2014) 1294–1296. [17] W.M. McDanel, M.G. Cowan, T.K. Carlisle, A.K. Swanson, R.D. Noble, D.L. Gin, Cross-linked ionic resins and gels from epoxide-functionalized imidazolium ionic liquid monomers, Polymer 55 (2014) 3305–3313. [18] W.M. McDanel, M.G. Cowan, J.A. Barton, D.L. Gin, R.D. Noble, Effect of monomer structure on curing behavior, CO2 solubility, and gas permeability of ionic liquidbased epoxy–amine resins and ion-gels, Ind. Eng. Chem. Res. 54 (2014) 4396–4406. [19] W.M. McDanel, M.G. Cowan, N.O. Chisholm, D.L. Gin, R.D. Noble, Fixed-site-carrier facilitated transport of carbon dioxide through ionic-liquid-based epoxy-amine ion gel membranes, J. Membr. Sci. 492 (2015) 303–311.

4. Conclusion A series of crosslinked PIL membranes and PIL-IL composite membranes were fabricated via the reactions between amino-terminated PILs and commercial glycidyl ether crosslinkers. Thanks to the high efficiency of the Debus-Radziszewski reaction and its tunability on PIL chemistry and molecular weight, we were able to tune the microstructure of the PIL and PIL-IL membranes and systematically investigated its effect on the CO2 capture performance. As for the neat crosslinked PIL membranes, we see that both the utilization of ether-containing crosslinker and the increase of crosslinker content can increase the CO2 solubility and diffusivity, owing to the increase of gas permeability and improved solubility selectivity of PIL membranes. The molecular weight of the PILs mainly influences the CO2 diffusivity and diffusivity selectivity of membranes. The inhibition of CO2 diffusion was observed in HPIL-contained membranes due to the strong interaction between CO2 and PILs, resulting in the decrease of diffusivity selectivity in HT and HP membranes. Consequently, the resulted neat LP(1:2) membrane has the best gas permeation-separation performance with a CO2 permeability of 170 Barrer and a selectivity of 36. Compared with other neat PIL membranes with similar permselectivity, the LP(1:2) membranes exhibited the highest permeability. As for the crosslinked PIL-IL membrane, we see that both the CO2 solubility and diffusivity increased with the increase of IL content, resulting in increase in CO2 permeability. However, the increase in IL content can increase the membrane-CO2 interaction as well as facilitate the N2 solubility and diffusivity by swelling the crosslinked PIL, leading to a decrease in solubility selectivity and diffusivity selectivity. Meanwhile, the solubility selectivity is also affected by the crosslinker types. The solubility selectivity of TMPGE-crosslinked membranes increased with an increase in the IL content because of the inhibition of N2 diffusion by the TMPGE-formed rigid molecular structure. As a result, we selected the HT(1:2) membrane as the base PIL membrane, and increased the IL content to 66 wt%. The resulted membrane has an 9

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and diamines for gas separation applications, J. Membr. Sci. 50 (1990) 285–297. [39] H. Lin, B.D. Freeman, Gas solubility, diffusivity and permeability in poly(ethylene oxide), J. Membr. Sci. 239 (2004) 105–117. [40] J.E. Bara, S. Lessmann, C.J. Gabriel, E.S. Hatakeyama, R.D. Noble, D.L. Gin, Synthesis and performance of polymerizable room-temperature ionic liquids as gas separation membranes, Ind. Eng. Chem. Res. 46 (2007) 5397–5404. [41] J.E. Bara, C.J. Gabriel, E.S. Hatakeyama, T.K. Carlisle, S. Lessmann, R.D. Noble, D.L. Gin, Improving CO2 selectivity in polymerized room-temperature ionic liquid gas separation membranes through incorporation of polar substituents, J. Membr. Sci. 321 (2008) 3–7. [42] W. Jeffrey Horne, M.A. Andrews, M.S. Shannon, K.L. Terrill, J.D. Moon, S.S. Hayward, J.E. Bara, Effect of branched and cycloalkyl functionalities on CO2 separation performance of poly(IL) membranes, Separ. Purif. Technol. 155 (2015) 89–95. [43] T.K. Carlisle, J.E. Bara, A.L. Lafrate, D.L. Gin, R.D. Noble, Main-chain imidazolium polymer membranes for CO2 separations: an initial study of a new ionic liquidinspired platform, J. Membr. Sci. 359 (2010) 37–43. [44] T.K. Carlisle, E.F. Wiesenauer, G.D. Nicodemus, D.L. Gin, R.D. Noble, Ideal CO2/ light gas separation performance of poly(vinylimidazolium) membranes and poly (vinylimidazolium)-ionic liquid composite films, Ind. Eng. Chem. Res. 52 (2013) 1023–1032. [45] P. Li, D.R. Paul, T.-S. Chung, High performance membranes based on ionic liquid polymers for CO2 separation from the flue gas, Green Chem. 14 (2012) 1052–1063. [46] M.G. Cowan, M. Masuda, W.M. McDanel, Y. Kohno, D.L. Gin, R.D. Noble, Phosphonium-based poly(Ionic liquid) membranes: the effect of cation alkyl chain length on light gas separation properties and Ionic conductivity, J. Membr. Sci. 498 (2016) 408–413. [47] L.C. Tomé, D. Mecerreyes, C.S.R. Freire, L.P.N. Rebelo, I.M. Marrucho, Pyrrolidinium-based polymeric ionic liquid materials: new perspectives for CO2 separation membranes, J. Membr. Sci. 428 (2013) 260–266. [48] L.C. Tomé, D.C. Guerreiro, R.M. Teodoro, V.D. Alves, I.M. Marrucho, Effect of polymer molecular weight on the physical properties and CO2/N2 separation of pyrrolidinium-based poly(ionic liquid) membranes, J. Membr. Sci. 549 (2018) 267–274. [49] L.C. Tomé, D.J.S. Patinha, C.S.R. Freire, L.P.N. Rebelo, I.M. Marrucho, CO2 separation applying ionic liquid mixtures: the effect of mixing different anions on gas permeation through supported ionic liquid membranes, RSC Adv. 3 (2013) 12220–12229. [50] P. Scovazzo, J. Kieft, D.A. Finan, C. Koval, D. DuBois, R. Noble, Gas separations using non-hexafluorophosphate [PF6]− anion supported ionic liquid membranes, J. Membr. Sci. 238 (2004) 57–63. [51] L.C. Tomé, A.S.L. Gouveia, C.S.R. Freire, D. Mecerreyes, I.M. Marrucho, Polymeric ionic liquid-based membranes: influence of polycation variation on gas transport and CO2 selectivity properties, J. Membr. Sci. 486 (2015) 40–48. [52] R.M. Teodoro, L.C. Tomé, D. Mantione, D. Mecerreyes, I.M. Marrucho, Mixing poly (ionic liquid)s and ionic liquids with different cyano anions: membrane forming ability and CO2/N2 separation properties, J. Membr. Sci. 552 (2018) 341–348. [53] U.S. DOE National Energy and Technology Laboratory, Carbon Capture Technology Program Plan, (2013).

[20] K. Friess, M. Lanč, K. Pilnáček, V. Fíla, O. Vopička, Z. Sedláková, M.G. Cowan, W.M. McDanel, R.D. Noble, D.L. Gin, P. Izak, CO2/CH4 separation performance of ionic-liquid-based epoxy-amine ion gel membranes under mixed feed conditions relevant to biogas processing, J. Membr. Sci. 528 (2017) 64–71. [21] F. Moghadam, E. Kamio, T. Yoshioka, H. Matsuyama, New approach for the fabrication of double-network ion-gel membranes with high CO2/N2 separation performance based on facilitated transport, J. Membr. Sci. 530 (2017) 166–175. [22] B. Radziszewski, Ueber die Constitution des Lophins und verwandter Verbindungen, Ber. Dtsch. Chem. Ges. 15 (1882) 1493–1496. [23] H. Debus, Ueber die Einwirkung des Ammoniaks auf Glyoxal, Justus Liebigs Ann. Chem. 107 (1858) 199–208. [24] J.-P. Lindner, Imidazolium-based polymers via the poly-radziszewski reaction, Macromolecules 49 (2016) 2046–2053. [25] W.J. Koros, A.H. Chan, D.R. Paul, Sorption and transport of various gases in polycarbonate, J. Membr. Sci. 2 (1977) 165–190. [26] W.-H. Lin, R.H. Vora, T.-S. Chung, Gas transport properties of 6FDA-durene/1,4phenylenediamine (pPDA) copolyimides, J. Polym. Sci., Part B: Polym. Phys. 38 (2000) 2703–2713. [27] N.P. Patel, A.C. Miller, R.J. Spontak, Highly CO2-permeable and -selective membranes derived from crosslinked poly(ethylene glycol) and its nanocomposites, Adv. Funct. Mater. 14 (2004) 699–707. [28] H. Czichos, T. Saito, L. Smith, Springer Handbook of Materials Measurement Methods, (2006). [29] B.G. Oliveira, M.C.A. Lima, I.R. Pitta, S.L. Galdino, M.Z. Hernandes, A theoretical study of red-shifting and blue-shifting hydrogen bonds occurring between imidazolidine derivatives and PEG/PVP polymers, J. Mol. Model. 16 (2010) 119–127. [30] V.A. Bershtein, L.M. Egorova, P.N. Yakushev, P. Sysel, R. Hobzova, J. Kotek, P. Pissis, S. Kripotou, P. Maroulas, Hyperbranched polyimides crosslinked with ethylene glycol diglycidyl ether: glass transition dynamics and permeability, Polymer 47 (2006) 6765–6772. [31] P. Maroulas, S. Kripotou, P. Sysel, R. Hobzova, J. Kotek, P. Pissis, Molecular dynamics in hyperbranched polyimides cross-linked with ethylene glycol diglycidyl ether, J. Non-Cryst. Solids 352 (2006) 4800–4803. [32] H. Lin, B.D. Freeman, Materials selection guidelines for membranes that remove CO2 from gas mixtures, J. Mol. Struct. 739 (2005) 57–74. [33] H. Lin, E.V. Wagner, J.S. Swinnea, B.D. Freeman, S.J. Pas, A.J. Hill, S. Kalakkunnath, D.S. Kalika, Transport and structural characteristics of crosslinked poly(ethylene oxide) rubbers, J. Membr. Sci. 276 (2006) 145–161. [34] H. Lin, B.D. Freeman, Gas and vapor solubility in cross-linked poly(ethylene glycol diacrylate), Macromolecules 38 (2005) 8394–8407. [35] S.G. Kazarian, M.F. Vincent, F.V. Bright, C.L. Liotta, C.A. Eckert, Specific intermolecular interaction of carbon dioxide with polymers, J. Am. Chem. Soc. 118 (1996) 1729–1736. [36] S.P. Nalawade, F. Picchioni, J.H. Marsman, L.P.B.M. Janssen, The FT-IR studies of the interactions of CO2 and polymers having different chain groups, J. Supercrit. Fluids 36 (2006) 236–244. [37] T.-H. Kim, W.J. Koros, G.R. Husk, Advanced gas separation membrane materials: rigid aromatic polyimides, Separ. Sci. Technol. 23 (1988) 1611–1626. [38] M.R. Coleman, W.J. Koros, Isomeric polyimides based on fluorinated dianhydrides

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