Enhanced CO₂ separation performance of P(PEGMA-co-DEAEMA-co-MMA) copolymer membrane through the synergistic effect of EO groups and amino groups

Abstract

We report a high performance and CO₂-philic, comb copolymer consisting of poly(ethylene glycol) methyl ether methacrylate (PEGMA), polymethyl methacrylate (PMMA) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) for use in composite membranes for CO₂/N₂ separation. Since the copolymers contain ethylene oxide (EO) groups and amino groups, the separation performance was expected to be enhanced by the synergistic effect of (1) the increased solubility of CO₂ by dipole–dipole interactions between CO₂ and EO units and (2) the enhanced CO₂ transport by reversible reactions between CO₂ and amino groups. The best separation performance was obtained from the PEDM5 composite membrane with 25 mol% of PEGMA, at which the CO₂ permeability was 308 barrer and CO₂/N₂ selectivity was 38, which displays a potentially promising alternative for CO₂ separation from N₂.

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Introduction

As Susumu Kitagawa pointed out, “in the 21st century, where the depletion of petroleum has become a critical concern, gases (e.g., natural gas and biogas, and even air) should play important roles-an “age of gas” is dawning”.¹ However, almost all gases are mixtures, are difficult to handle and have a low concentration under normal conditions. Therefore, in order to realize their large-scale industrialized application, essential separation and purification technologies for gases are absolutely necessary. Compared with traditional separation techniques such as cryogenic distillation or absorption, membrane-separation technology has advantages of low energy consumption, mechanical simplicity, being easy to scale up and a smaller footprint. Therefore, it becomes an effective gas separation technology, especially for CO₂/N₂ separation.^2–5 However, most of polymeric membranes suffer from a trade-off between permeability and selectivity, i.e., polymers with high gas permeability generally have low gas selectivity and vice versa.⁶ Therefore, there is an urgent and pressing need for advanced polymer membranes that both have higher permeability and selectivity.

It is well known that polar groups such as ethylene oxide (EO) groups possess an excellent affinity with CO₂ due to dipole–quadruple interaction between them, which improves CO₂/gas selectivity. Therefore, many researchers have focused on developing poly(ethylene oxide) (PEO)-based polymers to exploit CO₂ separation membranes.^7–9 On the other hand, due to reversible reactions between CO₂ and amino groups, the incorporation of amino groups into membranes can enhance CO₂ permeability.^10–14 Therefore, if the properties of EO groups and amino groups are combined together, the membranes are expected to possess both high CO₂ permeability and high CO₂/gas selectivity. Y.M. Cao groups have made a great deal of pioneering work in this field.^15–19 They successfully prepared a series of crosslinked copolymers containing ether oxygen groups and amino groups based on N,N-dimethylaminoethyl methacrylate (DMEMA) and polyethylene glycol methyl ether methyl acrylate (PEGMEMA). These copolymers exhibited good CO₂ permeability and CO₂/N₂ selectivity.¹⁵ But, the introduction of crosslinking agent in these copolymers caused densification of polymer chains, resulting in reduced CO₂ permeability.²⁰

In this work, we have firstly synthesized and characterized comb copolymer membranes comprising poly(ethylene glycol) methyl ether methacrylate (PEGMA), polymethyl methacrylate (PMMA) and poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA). A series of P(PEGMA-co-DEAEMA-co-MMA) comb copolymers was synthesized via one-step free radical polymerization. In this membrane, the PEGMA was PEO-based polymer, which was expected to improve CO₂/N₂ selectivity. The PDEAEMA was amino-rich polymer, which was aimed to enhance CO₂ permeability. And the PMMA was “hard segment”, which improved mechanical properties of copolymer membranes. We anticipated that the synergetic effect of EO groups and amino groups would improve CO₂ separation performance. These comb copolymers were characterized using Fourier transform infrared spectroscopy (FT-IR), ¹H nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC). The thermal properties and crystalline structure of comb copolymers were characterized using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and atomic force microscopy (AFM). The P(PEGMA-co-DEAEMA-co-MMA) comb copolymer was directly coated onto a microporous polyvinylidene fluoride (PVDF) support to prepare composite membranes, and the performance of the composite in CO₂/N₂ separation was tested at 25 °C. We expected that our work may provide a facile approach for the rational design of membrane materials and efficient intensification of membrane separation performance.

Experimental

Material

Poly(ethylene glycol) methyl ether methacrylate (PEGMA, M_n = 500 g mol⁻¹) and methyl methacrylate (MMA, M_n = 100 g mol⁻¹) were supplied by Aladdin. 2-(Diethylamino) ethyl methacrylate (DEAEMA, M_n = 185 g mol⁻¹) and azobisisobutyronitrile (AIBN) were supplied by Aldrich. 1,4-Dioxane, diethyl ether and chloroform were purchased from National Pharmaceutical Group Chemical Reagent Co. Ltd., China. All solvents and chemicals were reagent grade and were used as received. CO₂ and N₂ were supplied by Wuxi Xinnan Chemical Gas Co. Ltd., China, and were of at least 99.99% purity. Gases were used without further purification.

Synthesis of P(PEGMA-co-DEAEMA-co-MMA) copolymers

Various mole ratios of PEGMA : DEAEMA : MMA were used including 1 : 06 : 14, 2 : 06 : 14, 3 : 06 : 14, 4 : 06 : 14, 5 : 06 : 14 and 5 : 06 : 0, and (after polymerization) these were referred to as PEDM1, PEDM2, PEDM3, PEDM4, PEDM5 and PEDM6, respectively. Three monomers were completely dissolved in 25 mL of 1,4-dioxane, and then 1.5 mol% of AIBN out of the total mole of three monomers was added to the solution as an initiator. The mixed solutions were purged with nitrogen gas for 0.5 h and were placed in a 65 °C oil bath for 12 h. After the free-radical polymerization, all PEDM solutions were precipitated in anhydrous ether. To purify the copolymers, this precipitation process was repeated three times for each sample, and then the copolymers were dried completely in a vacuum oven.

Membrane preparation

PEDM copolymer was firstly dissolved at a concentration of 15 wt% in chloroform for 3 h at room temperature. Membranes were prepared by controlled solvent evaporation of the above-mentioned solutions. The polymer solutions were cast onto PVDF support and dried at room temperature for at least 48 h under a fume hood. In order to guarantee a complete removal of chloroform, they were further dried at vacuum drying oven at 40 °C for another 12 h. The preparation procedure was illustrated in Scheme 2.

Membrane characterization

The chemical structures of P(PEGMA-co-DEAEMA-co-MMA) were characterized by a FTLA 2000 type Fourier transform infrared (FT-IR) spectrometer with scan range of 4000–400 cm⁻¹ and resolution of 1.93 cm⁻¹ and ¹H NMR (BRUKER400, Germany). Molecular weight and molecular weight distribution were measured with gel permeation chromatography (GPC, Waters GPC System) at 35 °C using polystyrene standards and THF as the eluent. Scanning electron micrographs of PEDM composite membranes were performed on a Hitachi S4800 scanning electron microscope (SEM) instrument. All the samples were coated with a thin layer of gold to prevent charging. Thermal stability of PEDM copolymers were examined with a METTLER 1/1100SF thermogravimetric analyzer (TGA). The temperature profile was from 30 °C to 900 °C with a heating rate of 10 °C min⁻¹ and a nitrogen flow of 50 mL min⁻¹. Differential scanning calorimetry (DSC) curve of PEDM copolymers were obtained through a Perkin-Elmer (DSC 8000). All PEDM samples were subjected to a heating/cooling/heating cycle in the range from −50 °C to 170 °C at a scan rate of 10 °C min⁻¹ and the experiments were conducted using a nitrogen purge gas stream with the scavenging rate of 20 mL min⁻¹. Stretching tests of PEDM copolymer membranes were performed at room temperature using an electronic universal testing machine (Shenzhen, China) with a crosshead speed of 10 mm min⁻¹. X-ray diffraction measurements were performed using Bruker D8-Advance diffractometer with Cu-Kα radiation (λ = 0.15406 nm). The angle (2θ) of diffraction was varied from 3° to 60° to identify crystal structure and intermolecular distances (d-spacing) between the intersegment chains. The value of d-spacing can be estimated by the following equation:^21,22

The densities of prepared membranes without PVDF support were determined by specific pycnometer at 25 °C and three parallel measurements were carried out for each sample. The density was accurate to three decimal places. Prior to density measurement, the films samples were dried in vacuum oven at 25 °C for 3 days. The film density (ρ, g cm⁻³) was calculated by using the following equation:

where m₁ was the weight of pycnometer containing auxiliary liquid (g), m₂ was the dry film weight (g), and m₃ was the total weight of dry film weight and pycnometer containing auxiliary liquid (g), ρ₀ was the density of the auxiliary liquid (g cm⁻³). Anhydrous ether was used as the auxiliary liquid in our study.

Gas permeability measurements

Gas permeability experiments were evaluated for CO₂ and N₂ using the constant pressure/variable volume method at room temperature (25 °C). Fig. 1 showed the schematic representation of the gas permeation equipment. A circular sample with 13.80 cm² effective areas (A) was cut from membranes samples and placed in the stainless steel membrane module. The gas permeability of membranes was calculated by solution-diffusion mechanism as follows.^23,24where F was the flux (cm³ (STP) per min), L was membrane thickness (cm), A was the membrane effective area (cm²), Δp was the pressure difference of membranes on both sides and P was permeability expressed in barrer.

Fig. 1 Experimental apparatus for gas permeability.

The permeability ratio for gas A and gas B which is called gas pair selectivity^25,26 was determined as follows.

where P_A and P_B were permeability of gas A and gas B, respectively. α_A/B was gas selectivity of gas A and gas B.

Results and discussion

PEDM comb copolymers were synthesized via a one-step facile free radical polymerization using AIBN as initiator at 65 °C, as shown in Scheme 1. In this comb copolymer, the PMMA segment was “hard segment”, which improved mechanical properties of copolymer membranes and provides plasticization resistance without a significant decrease in CO₂ permeance. Meanwhile, the PEGMA segment enhanced the solubility of CO₂ and the PDEAEMA segment can efficiently facilitate CO₂ transport in the membranes. Therefore, the P(PEGMA-co-DEAEMA-co-MMA) copolymer membrane combined the advantages of PEO-based membranes and FTMs, which can significantly improve gas separation performance. What's more, the PDEAEMA was CO₂-responsive polymer, which can react with CO₂ in water to form charged amidinium bicarbonate (shown in Scheme 2) and this reaction was reversible by bubbling with Ar or N₂.^27,28 This feature provided the possibility of applying this type of polymer in gas separation membrane. However, to the best of our knowledge, systematic study on the application of CO₂-responsive polymer in gas separation has rarely been reported up to now. So we sincerely hope that this work will provide some methods and references for research in this field.

Scheme 1 Synthesis of the P(PEGMA-co-DEAEMA-co-MMA) comb copolymer.

Scheme 2 Schematic illustration for the preparation of P(PEGMA-co-DEAEMA-co-MMA) composite membranes.

The successful polymerization of PEDM comb copolymers was confirmed using FT-IR spectroscopy, as shown in Fig. 2(a). A strong band at 1730 cm⁻¹ was observed in all PEDM copolymers due to the stretching vibration of carbonyl (C=O) bonds, and the position of this band did not change with changes in composition. A strong band at 1150 cm⁻¹ was observed in the PEDM comb copolymers. This band was assigned to the stretching vibration mode of the ether (C–O–C) groups. Additionally, the intensity of the PEO peaks (e.g., 2870 cm⁻¹) increased with increasing PEO weight content, as expected.²⁹

Fig. 2 FT-IR spectra (a) and ¹H NMR spectra (b) of PEDM comb copolymers with various compositions.

The composition and complete polymerization of PEDM comb copolymers were confirmed by ¹H NMR, as shown in Fig. 2(b) and S1.† The intensity of PEGMA peaks (i.e., peaks g, h and j) increased with increasing PEGMA content, indicating that the copolymerization was successfully synthesized in this study. The weight average molecular weights (M_w) and polydispersity index (PDI) of PEDM comb copolymers were tested using GPC and are summarized in Table 1. The M_w of the PEDM comb copolymers ranged from 48 000 to 52 000 g mol⁻¹ with a PDI of 1.4–1.9. The PEDM5 had the maximum M_w (52 000 g mol⁻¹). Mechanical properties of the PEDM copolymer films without PVDF support in terms of elongation and tensile strength were listed in Table 1. As PEGMA content increased from 5 to 25 mol%, tensile strength decreased while elongation increased. For example, PEDM1 had the lowest elongation but the highest tensile strength, which meant that the membrane was very robust. A flexible membrane was achieved at the 25 mol% of PEGMA (PEDM5), suggesting that the copolymers became thermoplastic elastomer at high PEGMA contents. However, because of the absence of MMA, the physical state of PEDM6 was a highly viscous liquid-like state shown in Fig. S1.† Therefore, the membrane prepared by PEDM6 had not enough mechanical stability to be handled and used in stretching tests and followed gas permeation test. The difference in mechanical properties suggested that the presence of MMA segment could improve mechanical properties of copolymer membranes.

Table 1 Molecular weight, d-spacing, density and mechanical properties of PEDM comb copolymers

Sample	PEDM1	PEDM2	PEDM3	PEDM4	PEDM5	PEDM6
PEGMA : DEAEMA : MMA (mol)	1 : 06 : 14	2 : 06 : 14	3 : 06 : 14	4 : 06 : 14	5 : 06 : 14	5 : 06 : 0
Density (g cm^−3) at 25 °C	1.252	1.233	1.227	1.202	1.193	—
d-Spacing (Å)	5.62	5.54	5.39	5.34	5.28	—
Mn (g mol^−1)	23 000	34 000	35 000	35 000	36 000	13 000
Mw (g mol^−1)	48 000	49 000	49 000	48 000	52 000	36 000
PDI (Mw/Mn)	1.9	1.4	1.4	1.4	1.5	2.7
Tensile strength (MPa)	8.7	6.4	5.4	3.8	3.6	—
Elongation (%)	73	338	436	512	550	—

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The thermal stability of a polymeric material is always a key issue and needs careful consideration.³⁰ Fig. 3(A) showed the thermal stability of PEDM comb copolymers. For all the copolymers, the three discrete thermal degradation steps were observed. The first slightly thermal degradation began at 100 °C, which attributed to volatilization of solvent (e.g. chloroform and water). The second stage thermal degradation started at around 250 °C. This indicated that PEO chains and PMMA chains thermally decomposed.^31,32 The last stage thermal degradation started at around 420 °C. This indicated that PDEAEMA chains thermally decomposed.³³ In a N₂ atmosphere, all the copolymers were stable up to 300 °C, indicating that the polymers had good thermal stability. Surface hydrophilicity of the copolymer membranes was analyzed via contact angle measurements, as shown in Fig. 3(B), contact angle continuously decreased with the amount of PEGMA component increasing from 5 to 25 mol%, indicating gradually increased surface hydrophilicity.

Fig. 3 TGA curves (A), contact angle (B) and XRD (C) of PEDM comb copolymers with various compositions (TGA: (a) PEDM1, (b) PEDM2, (c) PEDM3 (d) PEDM4 and (e) PEDM5).

As the crystallinity of membrane is known to affect membrane performance, the crystal structures of the comb copolymers depending on the composition were investigated using XRD analysis. The XRD curves for the copolymers were shown in Fig. 3(C), where the intensity of X-ray scattering was plotted against the diffraction angle, 2θ. It can be clearly seen that the diffraction patterns of all copolymers were very similar except for the reflection peak angles having slightly shift to right. It has been reported that the XRD pattern of pure PEO had sharp characteristic peaks at 2θ values around 17–19° and 22–24° for crystals.³⁴ But, there were no sharp crystalline peaks around these angles observed in all comb copolymers, indicating that all copolymers were the amorphous nature at room temperature. These results were in accordance with the DSC analysis shown in Fig. S3.† This is good for gas transport in membrane, as the higher the crystallinity in the polymer matrix, the more difficult the penetrant diffusion is.³⁵ Generally, the broad peak center on XRD represents the average intersegmental distance of polymer chains.³⁶ The d-spacing has been widely used to characterize the average inter-segmental distance directly in polymers. The value of d-spacing can be calculated from eqn (1). The values of d-spacing were also given in Table 1. It indicated that increasing the content of PEGMA decreased the d-spacing and the average inter-segmental distance. The similar phenomena were reported by Kim et al.²⁰ when they studied CO₂/N₂ separation performance of PBEM-g-POEM comb copolymer membranes and Park et al.³⁷ when they investigated gas diffusivity, solubility and permeability of polysulfone–poly(ethylene oxide) random copolymer membranes. The decreased d-spacing may restrict the diffusion of larger molecules and favor the diffusion of small molecules with less resistance, improving permselectivity for small penetrants.³⁸

Cross-sectional SEM images of PEDM composite membranes were observed using SEM, as shown in Fig. 4. The thickness of the PEDM upper layer was between 30 and 40 μm for PEDM1 to PEDM5 membranes, and the upper layer slightly permeated into the microporous support, indicating good interface compatibility between copolymer layer and PVDF layer. This good interface compatibility could prevent mechanical separation between the two layers and could also enhance the long-term stability and durability of the membrane during operation.

Fig. 4 Cross-section SEM images of PEDM composite membranes (a and b) PEDM1 (c and d) PEDM3 and (e and f) PEDM5 (HM was defined as the SEM images of membranes obtained at high magnification).

The surface morphology of copolymer membranes were imaged with AFM. AFM phase images along with the height images of the PEDM copolymer membranes with various compositions were shown in Fig. 5. As shown in the AFM phase images (top row in Fig. 5), no obvious phase-separated pattern can be observed in the phase images for all copolymer membranes, indicating that the hard and soft segments were randomly distributed throughout the membrane. This observation was in accordance with the DSC results, where only one glass transition temperature was detected, and no melting/crystallization peaks were observed for all PEDM copolymer membranes. AFM height images (bottom row in Fig. 5) showed that all films had smooth surfaces with roughness values of less than 5 nm (film thickness of ∼60 μm).

Fig. 5 AFM phase images (the top row) and height images (the bottom row) of PEDM comb membranes (a) PEDM1, (b) PEDM3, and (c) PEDM5.

The pure gas permeability and CO₂/N₂ ideal selectivity of PEDM/PVDF composite membranes were shown in Fig. 6 as a function of comb copolymer composition. For all composite membranes, the CO₂ permeability increased rapidly while the N₂ permeability increased slowly as the PEGMA content increased. The enhancement of CO₂ permeability was due to follow four reasons. Firstly, it is well known that FFV of polymers can be related with their densities. Decreased density will increased FFV of copolymer, causing loose structure, which is good for gas diffusion.³⁹ As shown in Table 1, the density of PEDM composite membrane was decreased with the increase of PEGMA content, suggesting that the composite membrane had bigger FFV and more loose structure and consequently facilitating CO₂ diffusion. Secondly, according to the DSC results in Fig. S3,† increased PEGMA content lead to lower T_g values of PEDM comb copolymers, favoring CO₂ permeance.^40–42 Last but not least, the soft PEGMA segment of the PEDM comb copolymer contains a large number of ethylene oxide (EO) groups which had high affinity for CO₂ molecules due to dipole–quadrupole interactions, increasing the CO₂ solubility in the membrane.^23,43,44 At last, amine groups of PEDM comb copolymer had reversible reactions with CO₂, enhancing the CO₂ transport through the membrane.^45–47 Although the first two reasons also improved the N₂ permeability, this increase was smaller than that of CO₂ permeability due to larger kinetic diameter of N₂ (3.6 Å) than that of CO₂ (3.3 Å). Additionally, the last two reasons only selectively enhanced the CO₂ solubility. As a result, a more profound increase in CO₂ permeability was observed when compared with the N₂. Accordingly, the ideal CO₂/N₂ selectivity increase almost monotonically with PEGMA content due to the same reasons. The best separation performance was obtained from the PEDM5 composite membrane with 25 mol% of PEGMA, at which the CO₂ permeability was 308 barrer and CO₂/N₂ selectivity was 38. These values were higher than that of reported polymer membranes shown in Table 2, which was attributed to synergistic effect of EO groups and amino groups that the EO groups enhanced the solubility of CO₂ and the amino groups can efficiently facilitate CO₂ transport in the membranes.

Fig. 6 Gas separation properties of PEDM composite membranes at 25 °C.

Table 2 A comparison of membrane performances for CO₂/N₂ separation

Ref.	Membrane	CO2 permeability (barrer)	Selectivity (CO2/N2)	Type^a
a EO presented PEO-based membranes, FTM represented facilitated transport membranes, EO + FTM represented the membranes containing ethylene oxide groups and reactive carriers simultaneously.
15	PDMAEME-PEGMEMA-90	112.77	31.02	EO + FT
54	Ps-b-PEO/M1	214.6	1.46	EO + FT
55	PDMAEMA/Psf	30	53	FT
56	PEIE	231.3	70	FT
57	PVAm	73.8	70	FT
29	Pent-PI-PEO2000 (60%)	39	46	EO
20	PBEM-g-POEM	58.6	73.3	EO
8	Psf-PEG6000-6	6.27	48.5	EO
50	PEGBEM-g-POEM	43.8	84.7	EO
58	PEO-PBT/PDMS/PAN	90	750	EO
51	PCZ-r-PEG	50.7	42.2	EO
59	PVC-g-POEM	100	49	EO
This work	PEDM5	308	38	EO + FT

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The effect of operating pressure on gas separation performance was shown in Fig. 7. The CO₂ permeability decreased with the increase of feed gas pressure. Unlike the CO₂ permeation behavior with pressure, the N₂ permeability was only slightly changed with increasing feed pressure. The similar phenomenon was reported by Wu et al. when they studied the gas separation performance of PEG–PEI–GO doped mixed matrix membranes.⁴⁷ They thought that difference in the permeability of CO₂ and N₂ implied that transport of CO₂ followed both facilitated transport mechanism and solution-diffusion mechanism, while the permeability of nonpolar N₂ only followed the solution-diffusion mechanism.⁴⁷ Generally, if a gas permeates through the membrane following the ideal solution-diffusion mechanism, its permeability is independent of concentration driving force across the membrane.⁴⁸ CO₂/N₂ selectivity show a decreased trend mainly due to the decrease of CO₂ permeability.

Fig. 7 Effect of pressure on gas separation properties of PEDM5 composite membranes at 25 °C.

The effect of temperature on CO₂ permeability, N₂ permeability and CO₂/N₂ selectivity was also investigated as shown in Fig. 8. The temperature used for the effect of temperature ranged from 25 °C to 65 °C. Both the CO₂ and N₂ permeabilities increased with the temperature. As the temperature rised, the chain mobility of polymer increased, resulting in reduced gas transport resistance and then accelerating CO₂ and N₂ transports through polymer membrane. The decreased CO₂/N₂ selectivity was mainly due to faster permeability of N₂ than that of CO₂, which was in good agreement with the previous results.^20,47,49 Even though the gas CO₂/N₂ selectivity was 30.0 with the CO₂ permeability being 370 barrer at 65 °C, they were still higher than the values of many reported polymeric membranes.^{20,29,50–53}

Fig. 8 Effect of temperature on gas separation properties for PEDM5 membrane.

Separation performance of PEDM composite membranes was investigated by plotting the obtained data on Robeson upper bound graphs,⁶⁰ as shown in Fig. 9. The PEDM5 composite membrane showed the highest combinations of permeability and selectivity for CO₂/N₂, with a CO₂ permeability of 308 barrer, and CO₂/N₂ selectivity of 38, respectively, which was very close to the upper bound (2008). This also implied that our strategy based on synergetic effect of PEO and amino group was very effective to obtain polymer membrane with high gas separation performance. In this work, for all PEDM composite membranes we only changed the PEGMA content while keeping other contents unchanged. Therefore, we believe that relatively increasing the DEAMA content will prompt prepared composite membranes to overcome the upper bound while increasing the PEGMA content.

Fig. 9 Robeson plot for CO₂/N₂ for PEDM copolymer membranes with different compositions.

Conclusion

High performance gas separation membranes were prepared based on CO₂-philic and CO₂-responsive, PEDM comb copolymers. The PEDM comb copolymers consisted of rigid hydrophobic PMMA segments, flexible hydrophilic PEGMA segments and DEAMA segments that had unique response to CO₂. The successful synthesis of comb copolymer was confirmed by GPC, FT-IR and ¹H NMR spectroscopy. The PEDM copolymer membranes were completely amorphous without noticeable phase separation as revealed by DSC, XRD and AFM. EO units and amino groups of PEDM copolymers exerted a favorable synergistic effect on the gas separation performance. The EO units enhanced the solubility of CO₂ in the membranes. Meanwhile, the amino groups can efficiently facilitate CO₂ transport in the membranes due to reversible reactions between CO₂ and amine groups. For the PEDM5 composite membrane, the CO₂/N₂ selectivity reached 38 and showed a high permeability of 308 barrer, which was very close to the upper bound (2008). These values were higher than many reported polymeric membranes. Therefore, we believe our work is useful for rational design of a new class of polymeric membrane for CO₂ separation.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 51173072, No. 21571084 and No. 21576114), the Fundamental Research Funds for the Central Universities (JUSRP51408B), and MOE & SAFEA for the 111 Project (B13025).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10475d

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