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N2 separation
Journal of Membrane Science 492 (2015) 452–460
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A highly selective PEGBEM-g-POEM comb copolymer membrane for CO2/N2 separation Cheol Hun Park a, Jae Hun Lee a, Jung Pyo Jung a, Bumsuk Jung b, Jong Hak Kim a,n a b
Department of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, South Korea Department of Environmental Engineering and Energy, Myongji University, Yongin Kyeonggido 499-728, South Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 24 February 2015 Received in revised form 16 June 2015 Accepted 17 June 2015 Available online 20 June 2015
A series of amphiphilic comb copolymers composed of poly(ethylene glycol) behenyl ether methacrylate (PEGBEM) and poly(oxyethylene methacrylate) (POEM) were synthesized via an economical and facile free radical polymerization. The synthesis of comb copolymers was confirmed by gel permeation chromatography (GPC), thermogravimetric analysis (TGA), nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopy (FT-IR) spectroscopy. The microphase-separated morphology and crystalline structure of the comb copolymers were controllable by the copolymer composition, as characterized using wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), smallangle x-ray scattering (SAXS) and atomic force microscope (AFM). Due to good solubility in ethanol, the PEGBEM-g-POEM comb copolymers could be directly coated onto a microporous polysulfone support to prepare composite membranes. The maximum CO2/N2 selectivity of the PEGBEM-g-POEM membrane reached 84.7, which is much higher than that of commercially available Pebax (17.0) and is one of the highest values reported for a polymer membrane under no humidified condition. The CO2 permeance through the PEGBEM-g-POEM membrane (21.9 GPU, 1 GPU ¼10 6 cm3(STP)/(s cm2 cmHg)) was also slightly higher than that through Pebax membrane (20.5 GPU) at similar membrane thickness. & 2015 Elsevier B.V. All rights reserved.
Keywords: CO2 Gas separation Poly(ethylene glycol) Comb copolymer Membrane.
1. Introduction As considerable concern about the increasing level of carbon dioxide (CO2) in the atmosphere escalates due to expanded industrialization, a number of research groups are investigating the capture and separation of CO2. Compared to other gas separation technologies such as adsorption, absorption, and cryogenics, membrane technology has many advantages including facile preparation processes, low energy cost, and remarkable reliability [1–5]. With these promising features, membrane technology has been a significant part of the strategy for the CO2 removal industry and advanced commercial applications. In particular, CO2/N2 separation presents one of the greatest opportunities for application of CO2 membrane systems due to the greenhouse gas nature of CO2 [6–12]. However, many polymer membranes suffer from a trade-off relationship, i.e., membranes with higher permeability are less selective and vice versa [13]. Thus, it is important to develop advanced polymer membranes with higher permeability as well as higher selectivity [14–18]. Polar groups such as ether oxygens are known to have a strong affinity for CO2, and thus, there have been many studies based on n Correspondence to: Department of Chemical and Biomolecular Engineering, Yonsei University, 134 Shinchon-dong, 262 Seongsanno, Seodaemun-gu, Seoul 120749, South Korea. Tel.: þ82 2 2123 5757. E-mail address: [email protected] (J.H. Kim).
http://dx.doi.org/10.1016/j.memsci.2015.06.023 0376-7388/& 2015 Elsevier B.V. All rights reserved.
poly(ethylene oxide) (PEO) materials [19,20]. It is based on the Lewis acid–base interaction between the ether oxygen (Lewis base) and the CO2 molecule (Lewis acid). This specific interaction leads to a higher selective solubility of CO2 in PEO, which is a rubbery material with a low glass transition temperature (Tg). However, neat PEO tends to crystallize due to the helical structure of the chains, leading to low gas permeability, necessitating the modification of PEO. Poly(oxyethylene methacrylate) (POEM) is an amorphous PEO with a lack of crystallinity but with poor mechanical properties that limit its use in gas separation membranes. In order to address these issues, microphase-separated, nanostructural copolymers such as block copolymers have been applied [21– 23]. Among them, poly(amide-b-ethylene oxide), which is a commercial block copolymer and well-known as Pebax, has been intensively used for a gas separation membrane [24–27]. However, the block copolymers are typically synthesized via a living anionic polymerization, which is highly sensitive to impurity such as H2O and O2, requiring expertise of synthesis and high cost. Recently, our group reported the use of amphiphilic graft copolymers as a gas separation membrane [28–32]. It has been well known that a graft copolymer is more advantageous than a block copolymer as it is economical and has a simple synthetic method [33,34]. The graft copolymer consisting of poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM) was synthesized via atom transfer radical polymerization (ATRP)
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with a copper/ligand complex that functions as a reaction catalyst [28–32]. Interestingly, despite many reports on high performance polymer membranes, polymers that dissolve in a protic solvent such as alcohol or water are rare, which is an important consideration for membrane processing. It is because the aprotic solvent that is commonly used to dissolve the top polymer layer, e.g. tetrahydrofuran (THF), dimethylformamide (DMF) or N-methyl pyrrolidone (NMP), may also dissolve the bottom porous substrate. In order to save on materials cost, commercial membranes are typically based on composites in which a more expensive top selective layer is coated on a cheap porous support [35–37]. In this work, a series of highly CO2-philic, alcohol-soluble, comb copolymer composed of poly(ethylene glycol) behenyl ether methacrylate (PEGBEM) and poly(oxyethylene methacrylate) (POEM) was synthesized via an economical and facile free radical polymerization. Free radical polymerization is easier, cheaper and commercially more attractive than ATRP due to the simple process and components. The resultant comb copolymers were characterized using gel permeation chromatography (GPC), nuclear magnetic resonance (1H NMR), thermogravimetric analysis (TGA), atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FT-IR) spectroscopy. The microphase-separated morphology and crystalline structure of the comb copolymers were also characterized using wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC) and small-angle x-ray scattering (SAXS). The synthesized PEGBEM-g-POEM comb copolymers were directly coated onto a microporous polysulfone support to prepare composite membranes and their gas permeation properties were tested at 25 1C.
2. Experimental 2.1. Materials Poly(ethylene glycol) behenyl ether methacrylate solution (PEGBEM, Mn 1500 g/mol, containing 50 wt% in methacrylic acid/water) and poly(oxyethylene methacrylate) (POEM, Mn ¼ 500 g/mol) were purchased from Sigma-Aldrich. 2,20 -Azobis (2-methylpropionitrile) (AIBN, 98%) was purchased from Acros Organics as an initiator for free-radical polymerization, and ethyl acetate (HPLC grade) was purchased from DUKSAN, Korea, as the polymerization solvent. The polysulfone substrate membrane was kindly provided by Woongjin Chemical Co., Ltd., Korea. All of the materials were used as received without other purification. 2.2. Synthesis of PEBEM-g-POEM comb copolymer To investigate the effect of PEGBEM monomer content, a series of comb copolymers consisting of PEGBEM and POEM were synthesized with weight ratios of PEGBEM:POEM¼ 10:0, 7:3, 5:5, 3:7, and 0:10, which were designated as CP1, CP2, CP3, CP4 and CP5, respectively. The two monomers were added to 50 ml of ethyl acetate in a roundbottom flask with 0.01 g of AIBN. After purging with nitrogen gas for 1 h at room temperature, the mixture was heated in an oil bath at 70 1C for 24 h. After the free radical polymerization, the polymer solution was precipitated with the addition of an excess amount of hexane/methanol mixture. This process was repeated several times to remove side products, and the final products were completely dried in a vacuum oven overnight.
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average surface pore size of 0.5 μm using the RK Control coater (Model 101, Control RK Print-Coat Instruments Ltd., UK). For the preparation of Pebax membrane, poly(ether-block-amide) (Pebax 1657) with 40:60 weight ratio of polyamide and PEO was used. First, 1.5 g of Pebax was dissolved in a mixture of 3 ml of water and 7 ml of ethanol. After vigorous stirring to form homogeneous solution at 70 1C for 12 h, the polymer solution was coated on the polysulfone membrane using the RK Control coater. After drying the membranes completely overnight, the membrane was cut into 3.6 cm in diameter discs for gas permeation measurement. 2.4. Gas permeation measurement The permeation experiments were carried out using a constant pressure/variable volume apparatus provided by Airrane Co. Ltd. (Korea) according to the previously reported method. The scheme of the apparatus is shown in Fig. S1 [14,15]. The area of membrane was approximately 10.2 cm2. Monitoring the difference in the pressure between permeate and retentate and measuring the gas flow rate of component i through the permeate, the permeance, which is most commonly reported in gas permeation units (GPU) (1 GPU ¼10 6 cm3(STP)/(s cm2 cmHg), of each gas was calculated. The membrane selectivity was determined by the ratio of the permeance for each component. Six replicates of each membrane were tested and the average estimated error of permeance was approximately 75%. 2.5. Characterization The molecular weights of the comb copolymers were determined using a GPC (Yong Lin GPC 9200 system, Korea). The system consisted of a pump, two Waters GPC columns (Styragel HR 4E THF, Styragel HR 5E THF), and a RI750F refractive index detector set at a THF flow rate of 1.0 mL/min at 35 1C. The refractive index detector was calibrated with polystyrene standards with a narrow molecular weight distribution. The data were analyzed using autochro-WIN GPC software. Identification of polymerization was conducted using an Excalibur series FT-IR instrument (DIGLAS Co., Hannover, Germany) in the frequency range of 4000–600 cm-1. 1H NMR (600 MHz high-resolution, Avance 600 MHz FT NMR spectrometer, Bruker, Ettlingen, Germany) measurements were performed to analyze the copolymer composition. The surface roughness and microstructure of the copolymers were characterized using AFM (XEBIO, Park system, USA). The thermal behavior of the copolymer was analyzed using a differential scanning calorimeter (DSC 8000, Perkin Elmer, USA) operated at a heating rate of 10 1C/min in air. The polymer was heated from –70 1C to 100 1C and then recooled to – 70 1C, and the sample was again heated from –70 1C to 100 1C. The second scanning data were used to determine the thermal transition of the copolymer. TGA was conducted with a simultaneous DTA/TGA analyzer by TA instruments (USA) at a heating rate of 10 1C/min under a nitrogen atmosphere. WAXS analysis was carried out on a Rigaku 18 kW rotating anode X-ray generator with Cu-K radiation (λ ¼ 1.5405 nm) operated at 40 kV and 300 mA. The 2-theta angular region between 51 and 601 was explored at a scan rate of 41/min. SAXS data were obtained with the 4C SAXS II beamline at the Pohang Light Source (PLS), Korea. A field emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL Ltd., Japan) was used to characterize the cross-section morphology of the polymer membrane.
2.3. Preparation of gas separation membranes
3. Results and discussion
The comb copolymer solution at a 40 wt% concentration was prepared by dissolving in ethanol. Then, the solution was coated on the microporous polysulfone membrane substrate with an
Schematic structures of PEGBEM-g-POEM comb copolymers with different compositions are illustrated in Fig. 1(a). The weight average molecular weights (Mws) of PEGBEM-g-POEM comb
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Fig. 1. (a) Schematic illustrations and (b) photos of PEGBEM-g-POEM comb copolymers with different compositions.
copolymers ranged from 14,000 to 22,000 g/mol with a polydispersity index (PDI) of 1.8–2.9, as confirmed by gel permeation chromatography (GPC) (Table 1). Although the Mws of the homopolymers (CP1, CP5) were slightly smaller than those of the copolymers (CP2, CP3, CP4) due to branching effect, there was no significant difference among the comb copolymers. This structural uniformity reflects similar reactivities of the PEGBEM and POEM macromonomers in free radical polymerization and indicates that the side chains adjacent to the methacrylate unit do not significantly affect the reaction rate. The physical state of the PEGBEM-g-POEM comb copolymers was largely dependent on the composition. As shown in Fig. 1(b), the rigid solid-like state of PEGBEM-g-POEM comb copolymer was observed for CP1–CP3, while the physical state changed to a highly viscous liquid-like state for CP4 and CP5. Although the PDI of copolymers varied depending on the composition, the crystalline structure of the copolymer would not be significantly influenced by different PDI values [41,42]. As shown in Figs. S2a and S3, the PEGBEM-g-POEM comb copolymers were coated onto silicon wafer by spin coating and the roughness was measured via AFM analysis. Because of the increased crystallinity of copolymers with the PEGBEM content, the roughness gradually decreased from 5.48, 1.79 and 0.57 to
Table 1 Weight average molecular weight (Mw), polydisersity index (PDI), CO2 permeance and CO2/N2 selectivity of PEGBEM-g-POEM copolymer membranes with different compositions at 25 oC. 1 GPU ¼ 10 6 cm3(STP)/(s cm2 cmHg). Polymer
CP1 CP2 CP3 CP4 CP5 Pebax
Mw (g/mol)
16,000 20,000 22,000 21,000 14,000 –
PDI
2.0 2.9 2.2 1.8 2.4 –
Permeance (GPU) CO2
N2
1.3 15.9 21.9 38.5 117.1 20.5
0.62 0.85 0.26 2.29 67.9 1.20 7
Selectivity (CO2/N2)
2.1 18.4 84.7 16.8 1.7 17.0
0.28 nm for CP1, CP2, and CP3 to CP4, respectively. It should be also noted that the roughness of POEM homopolymer (CP5) could not be measured due to low rigidity and viscous liquid-like state. This result shows that the solid-like mechanical properties of the copolymer increased with a higher PEGBEM content, and the critical composition required for formation of a rigid solid-like film was PEGBEM:POEM ¼34:66 (CP3). The POEM chain incorporates rubbery PEO side chains nine ethylene oxide (CH2CH2O)
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units long, which is sufficiently short to avoid crystallization. However, the side chains of PEGBEM consist of 20 methylene groups (CH2) distinctly separated from 25 ethylene oxide units, which tend to crystallize due to increased regularity. Therefore, the POEM homopolymer (CP5) has an amorphous, viscous liquidlike state, while the PEGBEM homopolymer (CP1) has a crystalline solid state. This spectrum of copolymer properties demonstrates that the degree of crystallinity and physical mechanical properties, which are of pivotal importance in gas separation membranes, are tunable by adjusting the comb copolymer composition. Successful synthesis of the PEGBEM-g-POEM comb copolymer was confirmed by FT-IR and 1H NMR spectroscopy, as shown in Figs. 2 and 3, respectively. The sharp peaks at 1632 cm–1 and 1637 cm–1 for PEGBEM and POEM macromonomers, respectively, are attributed to stretching vibrations of the carbon–carbon double bond (C ¼C) of the methacrylate moiety. These peaks were not observed for PEGBEM-g-POEM comb copolymers, indicating completion of the free radical polymerization and the absence of
1098 1728
1098
CP1 1728
Absorbance
CP2
1098 1728 1098
CP3 1728
CP4
1098 1728
CP5
1090 17001632
PEGBEM
1101 1717 1637
POEM 2000
1800
1600
1400
1200
1000
800
600
-1
Wavenumber (cm ) Fig. 2. FT-IR spectra of PEGBEM macromonomer, POEM macromonomer, PEGBEM homopolymer (CP1), POEM homopolymer (CP5), and PEGBEM-g-POEM comb copolymers with different compositions (CP1, CP2, and CP3).
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unreacted macromonomers remaining in the product. The carbonyl (C ¼O) peak position for the PEGBEM macromonomer observed at 1700 cm–1 was lower than that of the POEM macromonomer (1717 cm–1), indicating that the strength of hydrogen bonding interaction was stronger for PEGBEM [29,31]. For all of the polymers synthesized via free radical polymerization, the carbonyl peak position (1728 cm–1) was practically unchanged, indicating the smaller effect of copolymer composition on the strength of the hydrogen bonding interaction. The quantitative composition of comb copolymers was calculated upon integration of each corresponding chemical shift, i.e. –CH2– protons of PEGBEM at 1.3 ppm and OCH3 protons of POEM at 3.6 ppm [38,39]. The copolymer composition was consistent with the feed ratio of the macromonomers, indicating a strictly controlled polymerization reaction. The thermal degradation temperature of the copolymers increased gradually with crystalline PEGBEM content, and the CP3 copolymer was thermally stable at least up to 300 1C, as confirmed by TGA analysis, shown in Fig. 4. The structural changes of the homopolymers and the comb copolymers depending on the composition were investigated using WAXS analysis. The WAXS curves for the polymers are shown in Fig. 5, where the intensity of X-ray scattering is plotted against the diffraction angle, 2θ. The sharp peaks at 19.41 and 24.51 originate from the crystallite with high degree of regularity, while the broad band at 20.41 is attributed to amorphous chains with high randomness [40]. The PEGBEM homopolymer (CP1) and the copolymers (CP2 and CP3) were observed to be crystalline, while the copolymer CP4 and POEM homopolymer (CP5) had a lack of crystallinity. It is because the PEGBEM chains containing 20 methylenes and 25 ethylene oxides tend to crystallize, while the POEM chains with only nine ethylene oxides are too short to induce crystallization, as described above. The structural and morphological properties related to crystallinity were also characterized using DSC analysis, as shown in Fig. 6. Similar morphological properties were obtained from the DSC traces of the PEGBEM-g-POEM comb copolymer. The melting temperature (Tm) of PEGBEM was clearly observed at 37.6 1C for the copolymers CP1, CP2, and CP3 but was not strong for CP4 and CP5 due to a lack of crystallinity. Thus, it was concluded that (1) the solid-like, better mechanical properties of CP1 and CP2 compared to CP4 and CP5 came from the higher degree of crystallinity of the former, and (2) CP3 is an almost amorphous solid comb copolymer with a very low degree of crystallinity. SAXS analysis was also used to characterize the microstructure of the PEGBEM-g-POEM comb copolymers, but the fluidic liquid-
CP1 CP2 CP3 CP4 CP5
100
Intensity
80
Weight (%)
b
a CP2 CP3
60 40 20
CP4 0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
100
200
Fig. 3. H NMR spectra of PEGBEM-g-POEM comb copolymers with different compositions (CP2, CP3, and CP4) dissolved in CDCl3.
400
500
600
700
800
o
Chemical Shift (ppm) 1
300
Temperature ( C) Fig. 4. TGA curves compositions.
of
PEGBEM-g-POEM
comb
copolymers
with
various
Intensity (a.u)
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Intensity
456
CP1 CP2 CP3
CP1
CP2 CP3
CP4 CP5
CP4 10
20
30
40
50
60
o
Heat Flow Endo Down
CP1 CP2 CP3
CP4 CP5
-40
-20
0
20
40
o
Temperature ( C) Fig. 6. DSC curves compositions.
of
PEGBEM-g-POEM
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
q (nm )
Fig. 5. WAXS curves of PEGBEM-g-POEM comb copolymers with various compositions.
-60
0.2
-1
2 theta ( )
comb
copolymers
with
various
like POEM homopolymer (CP5) was not able to be measured (Fig. 7). The crystalline d-spacing of the polymers was calculated using the Bragg relation (d ¼2π/q) with the maximum intensity peaks of each polymer [23,28]. The strongest peaks appeared for CP1 and CP2, whose d-spacing value was approximately 9.0 nm and 9.8 nm, respectively. The increased d-spacing from 9.0 to 9.8 nm is presumably due to broadening of the crystalline lattice by the interference of amorphous POEM chains in the CP2 copolymer. For CP3 and CP4 with a very low degree of crystallinity, the strong peaks almost disappeared, which is in good agreement with the DSC and WAXS results. The microstructures of PEGBEM-g-POEM comb copolymers were further characterized using AFM measurement after O2 plasma etching, as shown in Fig. 8. In general, the hydrophilic groups in polymer are more reactive than the hydrophobic groups under the O2 plasma etching. As shown in Fig. S2, the CP5 consisting of POEM homopolymer only was significantly deformed upon O2 plasma treatment, while the CP1–CP3 with high PEGBEM contents showed lower vulnerability and higher resistance to O2 plasma. The microphase-separated morphology was observed in
Fig. 7. SAXS curves of PEGB EM-g-POEM comb copolymers with various compositions.
the CP1 due to the presence of 20 methylenes and 25 ethylene oxides. The bright and dark regions are attributed to hydrophobic and hydrophilic groups, respectively. As the POEM content increased, the dark regions formed from the etched POEM chains gradually increased and well-connected dark regions were developed. Due to good solubility in ethanol, the synthesized PEGBEM-gPOEM comb copolymers were directly coated onto a microporous polysulfone support to prepare composite membranes, which is typical for commercial membranes. Performance testing was performed for CO2/N2 separation, which presents one of the best opportunities for CO2 membrane systems due to the importance of the greenhouse gas effect of CO2. The pure gas permeation properties and CO2/N2 ideal selectivity of the membranes were measured using a constant pressure apparatus at 1–4 atm and 25 1C, as shown in Table 1. There was no significant effect of feed pressure in the range 1–4 atm on membrane performance, indicating a rigid structure and less plasticization of the membranes. A general trend was that the gas permeance increased with POEM content in the copolymer, which is due to the higher permeability of amorphous liquid-like POEM. The CO2/N2 selectivities of CP1 (PEGBEM homopolymer) and CP5 (POEM homopolymer) membranes were as low as 1–2. However, both CO2 and N2 permeated through CP5 at almost a 100-fold greater rate than through CP1, implying different mechanisms for their low selectivity. As confirmed by DSC, WAXS, and SAXS analysis, CP1 is a rigid crystalline polymer, which is less permeable to gas molecules. Structural defects and incomplete integrity of membranes might form due to the grain boundary of crystallites. However, CP5 is a liquid-like, highly viscous amorphous polymer. When CP5 was employed as the top coating layer in a composite membrane, some structural defects may form due to its weak mechanical properties, resulting in low selectivity. Interestingly, the selectivity reached 84.7 for CP3 with PEGBEM:POEM ¼34:66, mostly due to the lowest level of N2 permeance. The observed selectivity is much higher than that of membrane prepared with commercially available Pebax (17.0) and one of the highest values reported for a polymer membrane [7– 12,18–32]. The reported recently membrane performances including the CO2 permeability and the CO2/N2 selectivity are summarized in Table 2. On the other hand, the CO2 permeance through CP3 (21.9 GPU) was slightly higher than that through Pebax membrane (20.5 GPU) at similar membrane thickness. The lower
C.H. Park et al. / Journal of Membrane Science 492 (2015) 452–460
Fig. 8. AFM images of PEGBEM-g-POEM comb copolymers on silicon wafer after O2 etching (15 cm3/min and for 30 s): (a) CP1, (b) CP2, (c) CP3 and (d) CP4.
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Table 2 A comparison of membrane performances for CO2/N2 separation. Ref.
Additivea
Polymerb
CO2 permeability (Barrer)
Selectivity (CO2/N2)
This work [7] [8] [10] [12] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
– MWCNTs – – ZIF-8 (25%) Ionic liquid PEG-azide PEG dibutyl ether PDMS–PEG Glycerol triacetate CNT CNT MWCNT–NH2 MgTiO3 f-MTHS ZIF-8
PEGBEM-g-POEM Pebax1657 (crosslinked) PEO-b-PBT on PAN-PDMS PEBA2533/Tween80-65 Pebax-2533 PVC-g-PIL GPA1100-40 PEO–PBT PEO–PPO-T6T6T Pebax1657 PCZ-r-PEG PEI/PDMS/PEGBA1657/PDMS Pebax1657 Pebax1657–PEGDME Pebax1657 PVC-g-POEM PVC-g-POEM PVC-g-POEM PVC-g-POEM
43.8 17.5 90 289 1082 137.6 513 750 896 500 50.7 310 105 743 261 100 138.7 85.4 687.7
84.7 83.2 450 40.7 31.3 20.2 49 40 36 49 42.25 64 72.4 108 52 49 38.2 43.4 34.9
a MWCNTs: multi-wall carbon nanotubes, ZIF-8: zeolite imidazole framework, ionic liquid: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, PEG: poly (ethylene glycol), PDMS–PEG: poly(dimethyl siloxane)–poly(ethylene glycol), CNT: carbon nanotube, f-MTHS: functionalized mesoporous TiO2 hollow nanospheres, b PBT-b-PEO: poly(butylene terephthalate)-b-poly(ethylene glycol), PVC-g-PIL: poly(vinyl chloride)-g-(polymerized ionic liquid), PBT–PEO: poly(butylene terephthalate)b-poly(ethylene glycol), PEO–PPO-T6T6T: poly(ethylene oxide)–poly(propylene oxide)-tetra amide), PCZ-r-PEG: polycarbonate Z-r-poly(ethylene glycol), PEI: polyetherimide, Pebax1657–PEGDME: poly(amide-b-ethylene oxide)–poly(ethylene glycol) dimethylether, Pebax1657: poly(amide-b-ethylene oxide) with a weight ratio of polyamide to polyether ¼40:60. Pebax2533: poly(amide-b-ethylene oxide) with 80 wt% of poly(tetramethylene oxide) and 20 wt% Nylon-12. PVC-g-POEM: poly(vinyl chloride)-g-(poly (oxyethylene methacrylate).
Fig. 9. Cross-section SEM images of PEGBEM-g-POEM copolymer membranes coated on polysulfone substrate: (a) CP1, (b) CP2, (c) CP3, (d) CP4, (e) CP5 and (f) Pebax.
C.H. Park et al. / Journal of Membrane Science 492 (2015) 452–460
CO2/N2 selectivity reached 84.7 together with a high permeance of 21.9 GPU. This selectivity is much higher than for the membrane prepared with commercially available Pebax (17.0) and is one of the highest values reported for a polymer membrane. This comb copolymer could be combined with various inorganic porous materials such as MOF, to provide good interfacial contact in MMMs due to its sticky rubbery properties. Thus, we believe our work may provide an excellent framework for the design of a new class of polymer for CO2 capture and contribute to commercialization of gas separation membranes.
Selectivity (CO /N )
10
CP1 CP2 CP3 CP4 CP5 Pebax
10
Uppe
r Bou
nd (2
008)
459
10
10
Acknowledgments 10
10
10
10
10
10
CO permeability (Barrer) Fig. 10. Plot of CO2 permeability vs. CO2/N2 selectivity through PEGBEM-g-POEM copolymer membranes with different compositions. 1 Barrer ¼ 1 10 10 cm3 (STP) cm/cm2 s cmHg.
We acknowledge financial support from the Korea CCS R&D Center (2013M1A8A1035871),the Human Resources Program in Energy Technology (20154010200810) and the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (20122010100040).
Appendix A. Supplementary Materials N2 permeance for CP3 compared to CP1 and CP2 is because interfacial defects between crystallites would be minimized for CP3 with a reduced degree of crystallinity. The increase in CO2 permeance with POEM content is attributed to the higher affinity of ethylene oxide units for CO2, resulting in an increased solubility factor. The reduction in CO2 permeance with PEGBEM content was less dramatic compared to the reduction in N2 permeance, because PEGBEM also contains ethylene oxide moieties. Furthermore, the N2 permeance for CP3 was lower than CP4 and CP5 copolymers with lower crystallinity, which is because the highly amorphous rubbery state of POEM chains could contribute to the formation of microscale defects in polymer membrane, resulting in increased permeances of CO2 as well as N2 via the Knudsen diffusion. The gas permeability was calculated after measuring the thickness of the top selective layer using cross-sectional FE-SEM images, shown in Fig. 9. The membrane thickness slightly and gradually increased from 1.5 to 2.3 μm with PEGBEM content due to increased viscosity of the polymer solution, resulting from the higher crystallinity of PEGBEM. As shown in Fig. 10, the performance of the CP3 membrane was very close to the upper bound (2008) [13]. We believe that incorporation of more permeable inorganic materials such as mesoporous metal oxides [29,30] and a metal organic framework (MOF) [31,32] to form mixed matrix membranes (MMMs) could overcome the upper bound, and this supposition is now under investigation. Also, the membrane performance including the CO2 permeability and CO2/N2 selectivity would be enhanced significantly under the humidified condition [43], which will be investigated in the near future.
4. Conclusion Herein, an approach to enhance gas separation performance with amphiphilic comb copolymers was demonstrated. The use of crystalline PEGBEM and amorphous POEM macromonomers resulted in the formation of a highly CO2-philic, alcohol soluble, amorphous solid polymer, as confirmed by GPC, FT-IR, TGA and NMR spectroscopy. WAXS, DSC, SAXS and AFM analysis revealed that the solid-like, better mechanical properties of CP1 and CP2 compared to CP4 and CP5 came from the higher degree of crystallinity of the former. Furthermore, the CP3 with PEGBEM: POEM ¼34:66 was an almost amorphous solid comb copolymer with a very low degree of crystallinity. For the CP3 copolymer, the
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
A highly selective PEGBEM-g-POEM comb copolymer membrane for CO2/N2 separation Cheol Hun Park a, Jae Hun Lee a, Jung Pyo Jung a, Bumsuk Jung b, Jong Hak Kim a,n a b
Department of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, South Korea Department of Environmental Engineering and Energy, Myongji University, Yongin Kyeonggido 499-728, South Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 24 February 2015 Received in revised form 16 June 2015 Accepted 17 June 2015 Available online 20 June 2015
A series of amphiphilic comb copolymers composed of poly(ethylene glycol) behenyl ether methacrylate (PEGBEM) and poly(oxyethylene methacrylate) (POEM) were synthesized via an economical and facile free radical polymerization. The synthesis of comb copolymers was confirmed by gel permeation chromatography (GPC), thermogravimetric analysis (TGA), nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopy (FT-IR) spectroscopy. The microphase-separated morphology and crystalline structure of the comb copolymers were controllable by the copolymer composition, as characterized using wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), smallangle x-ray scattering (SAXS) and atomic force microscope (AFM). Due to good solubility in ethanol, the PEGBEM-g-POEM comb copolymers could be directly coated onto a microporous polysulfone support to prepare composite membranes. The maximum CO2/N2 selectivity of the PEGBEM-g-POEM membrane reached 84.7, which is much higher than that of commercially available Pebax (17.0) and is one of the highest values reported for a polymer membrane under no humidified condition. The CO2 permeance through the PEGBEM-g-POEM membrane (21.9 GPU, 1 GPU ¼10 6 cm3(STP)/(s cm2 cmHg)) was also slightly higher than that through Pebax membrane (20.5 GPU) at similar membrane thickness. & 2015 Elsevier B.V. All rights reserved.
Keywords: CO2 Gas separation Poly(ethylene glycol) Comb copolymer Membrane.
1. Introduction As considerable concern about the increasing level of carbon dioxide (CO2) in the atmosphere escalates due to expanded industrialization, a number of research groups are investigating the capture and separation of CO2. Compared to other gas separation technologies such as adsorption, absorption, and cryogenics, membrane technology has many advantages including facile preparation processes, low energy cost, and remarkable reliability [1–5]. With these promising features, membrane technology has been a significant part of the strategy for the CO2 removal industry and advanced commercial applications. In particular, CO2/N2 separation presents one of the greatest opportunities for application of CO2 membrane systems due to the greenhouse gas nature of CO2 [6–12]. However, many polymer membranes suffer from a trade-off relationship, i.e., membranes with higher permeability are less selective and vice versa [13]. Thus, it is important to develop advanced polymer membranes with higher permeability as well as higher selectivity [14–18]. Polar groups such as ether oxygens are known to have a strong affinity for CO2, and thus, there have been many studies based on n Correspondence to: Department of Chemical and Biomolecular Engineering, Yonsei University, 134 Shinchon-dong, 262 Seongsanno, Seodaemun-gu, Seoul 120749, South Korea. Tel.: þ82 2 2123 5757. E-mail address: [email protected] (J.H. Kim).
http://dx.doi.org/10.1016/j.memsci.2015.06.023 0376-7388/& 2015 Elsevier B.V. All rights reserved.
poly(ethylene oxide) (PEO) materials [19,20]. It is based on the Lewis acid–base interaction between the ether oxygen (Lewis base) and the CO2 molecule (Lewis acid). This specific interaction leads to a higher selective solubility of CO2 in PEO, which is a rubbery material with a low glass transition temperature (Tg). However, neat PEO tends to crystallize due to the helical structure of the chains, leading to low gas permeability, necessitating the modification of PEO. Poly(oxyethylene methacrylate) (POEM) is an amorphous PEO with a lack of crystallinity but with poor mechanical properties that limit its use in gas separation membranes. In order to address these issues, microphase-separated, nanostructural copolymers such as block copolymers have been applied [21– 23]. Among them, poly(amide-b-ethylene oxide), which is a commercial block copolymer and well-known as Pebax, has been intensively used for a gas separation membrane [24–27]. However, the block copolymers are typically synthesized via a living anionic polymerization, which is highly sensitive to impurity such as H2O and O2, requiring expertise of synthesis and high cost. Recently, our group reported the use of amphiphilic graft copolymers as a gas separation membrane [28–32]. It has been well known that a graft copolymer is more advantageous than a block copolymer as it is economical and has a simple synthetic method [33,34]. The graft copolymer consisting of poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM) was synthesized via atom transfer radical polymerization (ATRP)
C.H. Park et al. / Journal of Membrane Science 492 (2015) 452–460
with a copper/ligand complex that functions as a reaction catalyst [28–32]. Interestingly, despite many reports on high performance polymer membranes, polymers that dissolve in a protic solvent such as alcohol or water are rare, which is an important consideration for membrane processing. It is because the aprotic solvent that is commonly used to dissolve the top polymer layer, e.g. tetrahydrofuran (THF), dimethylformamide (DMF) or N-methyl pyrrolidone (NMP), may also dissolve the bottom porous substrate. In order to save on materials cost, commercial membranes are typically based on composites in which a more expensive top selective layer is coated on a cheap porous support [35–37]. In this work, a series of highly CO2-philic, alcohol-soluble, comb copolymer composed of poly(ethylene glycol) behenyl ether methacrylate (PEGBEM) and poly(oxyethylene methacrylate) (POEM) was synthesized via an economical and facile free radical polymerization. Free radical polymerization is easier, cheaper and commercially more attractive than ATRP due to the simple process and components. The resultant comb copolymers were characterized using gel permeation chromatography (GPC), nuclear magnetic resonance (1H NMR), thermogravimetric analysis (TGA), atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FT-IR) spectroscopy. The microphase-separated morphology and crystalline structure of the comb copolymers were also characterized using wide-angle x-ray scattering (WAXS), differential scanning calorimetry (DSC) and small-angle x-ray scattering (SAXS). The synthesized PEGBEM-g-POEM comb copolymers were directly coated onto a microporous polysulfone support to prepare composite membranes and their gas permeation properties were tested at 25 1C.
2. Experimental 2.1. Materials Poly(ethylene glycol) behenyl ether methacrylate solution (PEGBEM, Mn 1500 g/mol, containing 50 wt% in methacrylic acid/water) and poly(oxyethylene methacrylate) (POEM, Mn ¼ 500 g/mol) were purchased from Sigma-Aldrich. 2,20 -Azobis (2-methylpropionitrile) (AIBN, 98%) was purchased from Acros Organics as an initiator for free-radical polymerization, and ethyl acetate (HPLC grade) was purchased from DUKSAN, Korea, as the polymerization solvent. The polysulfone substrate membrane was kindly provided by Woongjin Chemical Co., Ltd., Korea. All of the materials were used as received without other purification. 2.2. Synthesis of PEBEM-g-POEM comb copolymer To investigate the effect of PEGBEM monomer content, a series of comb copolymers consisting of PEGBEM and POEM were synthesized with weight ratios of PEGBEM:POEM¼ 10:0, 7:3, 5:5, 3:7, and 0:10, which were designated as CP1, CP2, CP3, CP4 and CP5, respectively. The two monomers were added to 50 ml of ethyl acetate in a roundbottom flask with 0.01 g of AIBN. After purging with nitrogen gas for 1 h at room temperature, the mixture was heated in an oil bath at 70 1C for 24 h. After the free radical polymerization, the polymer solution was precipitated with the addition of an excess amount of hexane/methanol mixture. This process was repeated several times to remove side products, and the final products were completely dried in a vacuum oven overnight.
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average surface pore size of 0.5 μm using the RK Control coater (Model 101, Control RK Print-Coat Instruments Ltd., UK). For the preparation of Pebax membrane, poly(ether-block-amide) (Pebax 1657) with 40:60 weight ratio of polyamide and PEO was used. First, 1.5 g of Pebax was dissolved in a mixture of 3 ml of water and 7 ml of ethanol. After vigorous stirring to form homogeneous solution at 70 1C for 12 h, the polymer solution was coated on the polysulfone membrane using the RK Control coater. After drying the membranes completely overnight, the membrane was cut into 3.6 cm in diameter discs for gas permeation measurement. 2.4. Gas permeation measurement The permeation experiments were carried out using a constant pressure/variable volume apparatus provided by Airrane Co. Ltd. (Korea) according to the previously reported method. The scheme of the apparatus is shown in Fig. S1 [14,15]. The area of membrane was approximately 10.2 cm2. Monitoring the difference in the pressure between permeate and retentate and measuring the gas flow rate of component i through the permeate, the permeance, which is most commonly reported in gas permeation units (GPU) (1 GPU ¼10 6 cm3(STP)/(s cm2 cmHg), of each gas was calculated. The membrane selectivity was determined by the ratio of the permeance for each component. Six replicates of each membrane were tested and the average estimated error of permeance was approximately 75%. 2.5. Characterization The molecular weights of the comb copolymers were determined using a GPC (Yong Lin GPC 9200 system, Korea). The system consisted of a pump, two Waters GPC columns (Styragel HR 4E THF, Styragel HR 5E THF), and a RI750F refractive index detector set at a THF flow rate of 1.0 mL/min at 35 1C. The refractive index detector was calibrated with polystyrene standards with a narrow molecular weight distribution. The data were analyzed using autochro-WIN GPC software. Identification of polymerization was conducted using an Excalibur series FT-IR instrument (DIGLAS Co., Hannover, Germany) in the frequency range of 4000–600 cm-1. 1H NMR (600 MHz high-resolution, Avance 600 MHz FT NMR spectrometer, Bruker, Ettlingen, Germany) measurements were performed to analyze the copolymer composition. The surface roughness and microstructure of the copolymers were characterized using AFM (XEBIO, Park system, USA). The thermal behavior of the copolymer was analyzed using a differential scanning calorimeter (DSC 8000, Perkin Elmer, USA) operated at a heating rate of 10 1C/min in air. The polymer was heated from –70 1C to 100 1C and then recooled to – 70 1C, and the sample was again heated from –70 1C to 100 1C. The second scanning data were used to determine the thermal transition of the copolymer. TGA was conducted with a simultaneous DTA/TGA analyzer by TA instruments (USA) at a heating rate of 10 1C/min under a nitrogen atmosphere. WAXS analysis was carried out on a Rigaku 18 kW rotating anode X-ray generator with Cu-K radiation (λ ¼ 1.5405 nm) operated at 40 kV and 300 mA. The 2-theta angular region between 51 and 601 was explored at a scan rate of 41/min. SAXS data were obtained with the 4C SAXS II beamline at the Pohang Light Source (PLS), Korea. A field emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL Ltd., Japan) was used to characterize the cross-section morphology of the polymer membrane.
2.3. Preparation of gas separation membranes
3. Results and discussion
The comb copolymer solution at a 40 wt% concentration was prepared by dissolving in ethanol. Then, the solution was coated on the microporous polysulfone membrane substrate with an
Schematic structures of PEGBEM-g-POEM comb copolymers with different compositions are illustrated in Fig. 1(a). The weight average molecular weights (Mws) of PEGBEM-g-POEM comb
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Fig. 1. (a) Schematic illustrations and (b) photos of PEGBEM-g-POEM comb copolymers with different compositions.
copolymers ranged from 14,000 to 22,000 g/mol with a polydispersity index (PDI) of 1.8–2.9, as confirmed by gel permeation chromatography (GPC) (Table 1). Although the Mws of the homopolymers (CP1, CP5) were slightly smaller than those of the copolymers (CP2, CP3, CP4) due to branching effect, there was no significant difference among the comb copolymers. This structural uniformity reflects similar reactivities of the PEGBEM and POEM macromonomers in free radical polymerization and indicates that the side chains adjacent to the methacrylate unit do not significantly affect the reaction rate. The physical state of the PEGBEM-g-POEM comb copolymers was largely dependent on the composition. As shown in Fig. 1(b), the rigid solid-like state of PEGBEM-g-POEM comb copolymer was observed for CP1–CP3, while the physical state changed to a highly viscous liquid-like state for CP4 and CP5. Although the PDI of copolymers varied depending on the composition, the crystalline structure of the copolymer would not be significantly influenced by different PDI values [41,42]. As shown in Figs. S2a and S3, the PEGBEM-g-POEM comb copolymers were coated onto silicon wafer by spin coating and the roughness was measured via AFM analysis. Because of the increased crystallinity of copolymers with the PEGBEM content, the roughness gradually decreased from 5.48, 1.79 and 0.57 to
Table 1 Weight average molecular weight (Mw), polydisersity index (PDI), CO2 permeance and CO2/N2 selectivity of PEGBEM-g-POEM copolymer membranes with different compositions at 25 oC. 1 GPU ¼ 10 6 cm3(STP)/(s cm2 cmHg). Polymer
CP1 CP2 CP3 CP4 CP5 Pebax
Mw (g/mol)
16,000 20,000 22,000 21,000 14,000 –
PDI
2.0 2.9 2.2 1.8 2.4 –
Permeance (GPU) CO2
N2
1.3 15.9 21.9 38.5 117.1 20.5
0.62 0.85 0.26 2.29 67.9 1.20 7
Selectivity (CO2/N2)
2.1 18.4 84.7 16.8 1.7 17.0
0.28 nm for CP1, CP2, and CP3 to CP4, respectively. It should be also noted that the roughness of POEM homopolymer (CP5) could not be measured due to low rigidity and viscous liquid-like state. This result shows that the solid-like mechanical properties of the copolymer increased with a higher PEGBEM content, and the critical composition required for formation of a rigid solid-like film was PEGBEM:POEM ¼34:66 (CP3). The POEM chain incorporates rubbery PEO side chains nine ethylene oxide (CH2CH2O)
C.H. Park et al. / Journal of Membrane Science 492 (2015) 452–460
units long, which is sufficiently short to avoid crystallization. However, the side chains of PEGBEM consist of 20 methylene groups (CH2) distinctly separated from 25 ethylene oxide units, which tend to crystallize due to increased regularity. Therefore, the POEM homopolymer (CP5) has an amorphous, viscous liquidlike state, while the PEGBEM homopolymer (CP1) has a crystalline solid state. This spectrum of copolymer properties demonstrates that the degree of crystallinity and physical mechanical properties, which are of pivotal importance in gas separation membranes, are tunable by adjusting the comb copolymer composition. Successful synthesis of the PEGBEM-g-POEM comb copolymer was confirmed by FT-IR and 1H NMR spectroscopy, as shown in Figs. 2 and 3, respectively. The sharp peaks at 1632 cm–1 and 1637 cm–1 for PEGBEM and POEM macromonomers, respectively, are attributed to stretching vibrations of the carbon–carbon double bond (C ¼C) of the methacrylate moiety. These peaks were not observed for PEGBEM-g-POEM comb copolymers, indicating completion of the free radical polymerization and the absence of
1098 1728
1098
CP1 1728
Absorbance
CP2
1098 1728 1098
CP3 1728
CP4
1098 1728
CP5
1090 17001632
PEGBEM
1101 1717 1637
POEM 2000
1800
1600
1400
1200
1000
800
600
-1
Wavenumber (cm ) Fig. 2. FT-IR spectra of PEGBEM macromonomer, POEM macromonomer, PEGBEM homopolymer (CP1), POEM homopolymer (CP5), and PEGBEM-g-POEM comb copolymers with different compositions (CP1, CP2, and CP3).
455
unreacted macromonomers remaining in the product. The carbonyl (C ¼O) peak position for the PEGBEM macromonomer observed at 1700 cm–1 was lower than that of the POEM macromonomer (1717 cm–1), indicating that the strength of hydrogen bonding interaction was stronger for PEGBEM [29,31]. For all of the polymers synthesized via free radical polymerization, the carbonyl peak position (1728 cm–1) was practically unchanged, indicating the smaller effect of copolymer composition on the strength of the hydrogen bonding interaction. The quantitative composition of comb copolymers was calculated upon integration of each corresponding chemical shift, i.e. –CH2– protons of PEGBEM at 1.3 ppm and OCH3 protons of POEM at 3.6 ppm [38,39]. The copolymer composition was consistent with the feed ratio of the macromonomers, indicating a strictly controlled polymerization reaction. The thermal degradation temperature of the copolymers increased gradually with crystalline PEGBEM content, and the CP3 copolymer was thermally stable at least up to 300 1C, as confirmed by TGA analysis, shown in Fig. 4. The structural changes of the homopolymers and the comb copolymers depending on the composition were investigated using WAXS analysis. The WAXS curves for the polymers are shown in Fig. 5, where the intensity of X-ray scattering is plotted against the diffraction angle, 2θ. The sharp peaks at 19.41 and 24.51 originate from the crystallite with high degree of regularity, while the broad band at 20.41 is attributed to amorphous chains with high randomness [40]. The PEGBEM homopolymer (CP1) and the copolymers (CP2 and CP3) were observed to be crystalline, while the copolymer CP4 and POEM homopolymer (CP5) had a lack of crystallinity. It is because the PEGBEM chains containing 20 methylenes and 25 ethylene oxides tend to crystallize, while the POEM chains with only nine ethylene oxides are too short to induce crystallization, as described above. The structural and morphological properties related to crystallinity were also characterized using DSC analysis, as shown in Fig. 6. Similar morphological properties were obtained from the DSC traces of the PEGBEM-g-POEM comb copolymer. The melting temperature (Tm) of PEGBEM was clearly observed at 37.6 1C for the copolymers CP1, CP2, and CP3 but was not strong for CP4 and CP5 due to a lack of crystallinity. Thus, it was concluded that (1) the solid-like, better mechanical properties of CP1 and CP2 compared to CP4 and CP5 came from the higher degree of crystallinity of the former, and (2) CP3 is an almost amorphous solid comb copolymer with a very low degree of crystallinity. SAXS analysis was also used to characterize the microstructure of the PEGBEM-g-POEM comb copolymers, but the fluidic liquid-
CP1 CP2 CP3 CP4 CP5
100
Intensity
80
Weight (%)
b
a CP2 CP3
60 40 20
CP4 0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
100
200
Fig. 3. H NMR spectra of PEGBEM-g-POEM comb copolymers with different compositions (CP2, CP3, and CP4) dissolved in CDCl3.
400
500
600
700
800
o
Chemical Shift (ppm) 1
300
Temperature ( C) Fig. 4. TGA curves compositions.
of
PEGBEM-g-POEM
comb
copolymers
with
various
Intensity (a.u)
C.H. Park et al. / Journal of Membrane Science 492 (2015) 452–460
Intensity
456
CP1 CP2 CP3
CP1
CP2 CP3
CP4 CP5
CP4 10
20
30
40
50
60
o
Heat Flow Endo Down
CP1 CP2 CP3
CP4 CP5
-40
-20
0
20
40
o
Temperature ( C) Fig. 6. DSC curves compositions.
of
PEGBEM-g-POEM
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
q (nm )
Fig. 5. WAXS curves of PEGBEM-g-POEM comb copolymers with various compositions.
-60
0.2
-1
2 theta ( )
comb
copolymers
with
various
like POEM homopolymer (CP5) was not able to be measured (Fig. 7). The crystalline d-spacing of the polymers was calculated using the Bragg relation (d ¼2π/q) with the maximum intensity peaks of each polymer [23,28]. The strongest peaks appeared for CP1 and CP2, whose d-spacing value was approximately 9.0 nm and 9.8 nm, respectively. The increased d-spacing from 9.0 to 9.8 nm is presumably due to broadening of the crystalline lattice by the interference of amorphous POEM chains in the CP2 copolymer. For CP3 and CP4 with a very low degree of crystallinity, the strong peaks almost disappeared, which is in good agreement with the DSC and WAXS results. The microstructures of PEGBEM-g-POEM comb copolymers were further characterized using AFM measurement after O2 plasma etching, as shown in Fig. 8. In general, the hydrophilic groups in polymer are more reactive than the hydrophobic groups under the O2 plasma etching. As shown in Fig. S2, the CP5 consisting of POEM homopolymer only was significantly deformed upon O2 plasma treatment, while the CP1–CP3 with high PEGBEM contents showed lower vulnerability and higher resistance to O2 plasma. The microphase-separated morphology was observed in
Fig. 7. SAXS curves of PEGB EM-g-POEM comb copolymers with various compositions.
the CP1 due to the presence of 20 methylenes and 25 ethylene oxides. The bright and dark regions are attributed to hydrophobic and hydrophilic groups, respectively. As the POEM content increased, the dark regions formed from the etched POEM chains gradually increased and well-connected dark regions were developed. Due to good solubility in ethanol, the synthesized PEGBEM-gPOEM comb copolymers were directly coated onto a microporous polysulfone support to prepare composite membranes, which is typical for commercial membranes. Performance testing was performed for CO2/N2 separation, which presents one of the best opportunities for CO2 membrane systems due to the importance of the greenhouse gas effect of CO2. The pure gas permeation properties and CO2/N2 ideal selectivity of the membranes were measured using a constant pressure apparatus at 1–4 atm and 25 1C, as shown in Table 1. There was no significant effect of feed pressure in the range 1–4 atm on membrane performance, indicating a rigid structure and less plasticization of the membranes. A general trend was that the gas permeance increased with POEM content in the copolymer, which is due to the higher permeability of amorphous liquid-like POEM. The CO2/N2 selectivities of CP1 (PEGBEM homopolymer) and CP5 (POEM homopolymer) membranes were as low as 1–2. However, both CO2 and N2 permeated through CP5 at almost a 100-fold greater rate than through CP1, implying different mechanisms for their low selectivity. As confirmed by DSC, WAXS, and SAXS analysis, CP1 is a rigid crystalline polymer, which is less permeable to gas molecules. Structural defects and incomplete integrity of membranes might form due to the grain boundary of crystallites. However, CP5 is a liquid-like, highly viscous amorphous polymer. When CP5 was employed as the top coating layer in a composite membrane, some structural defects may form due to its weak mechanical properties, resulting in low selectivity. Interestingly, the selectivity reached 84.7 for CP3 with PEGBEM:POEM ¼34:66, mostly due to the lowest level of N2 permeance. The observed selectivity is much higher than that of membrane prepared with commercially available Pebax (17.0) and one of the highest values reported for a polymer membrane [7– 12,18–32]. The reported recently membrane performances including the CO2 permeability and the CO2/N2 selectivity are summarized in Table 2. On the other hand, the CO2 permeance through CP3 (21.9 GPU) was slightly higher than that through Pebax membrane (20.5 GPU) at similar membrane thickness. The lower
C.H. Park et al. / Journal of Membrane Science 492 (2015) 452–460
Fig. 8. AFM images of PEGBEM-g-POEM comb copolymers on silicon wafer after O2 etching (15 cm3/min and for 30 s): (a) CP1, (b) CP2, (c) CP3 and (d) CP4.
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Table 2 A comparison of membrane performances for CO2/N2 separation. Ref.
Additivea
Polymerb
CO2 permeability (Barrer)
Selectivity (CO2/N2)
This work [7] [8] [10] [12] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
– MWCNTs – – ZIF-8 (25%) Ionic liquid PEG-azide PEG dibutyl ether PDMS–PEG Glycerol triacetate CNT CNT MWCNT–NH2 MgTiO3 f-MTHS ZIF-8
PEGBEM-g-POEM Pebax1657 (crosslinked) PEO-b-PBT on PAN-PDMS PEBA2533/Tween80-65 Pebax-2533 PVC-g-PIL GPA1100-40 PEO–PBT PEO–PPO-T6T6T Pebax1657 PCZ-r-PEG PEI/PDMS/PEGBA1657/PDMS Pebax1657 Pebax1657–PEGDME Pebax1657 PVC-g-POEM PVC-g-POEM PVC-g-POEM PVC-g-POEM
43.8 17.5 90 289 1082 137.6 513 750 896 500 50.7 310 105 743 261 100 138.7 85.4 687.7
84.7 83.2 450 40.7 31.3 20.2 49 40 36 49 42.25 64 72.4 108 52 49 38.2 43.4 34.9
a MWCNTs: multi-wall carbon nanotubes, ZIF-8: zeolite imidazole framework, ionic liquid: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, PEG: poly (ethylene glycol), PDMS–PEG: poly(dimethyl siloxane)–poly(ethylene glycol), CNT: carbon nanotube, f-MTHS: functionalized mesoporous TiO2 hollow nanospheres, b PBT-b-PEO: poly(butylene terephthalate)-b-poly(ethylene glycol), PVC-g-PIL: poly(vinyl chloride)-g-(polymerized ionic liquid), PBT–PEO: poly(butylene terephthalate)b-poly(ethylene glycol), PEO–PPO-T6T6T: poly(ethylene oxide)–poly(propylene oxide)-tetra amide), PCZ-r-PEG: polycarbonate Z-r-poly(ethylene glycol), PEI: polyetherimide, Pebax1657–PEGDME: poly(amide-b-ethylene oxide)–poly(ethylene glycol) dimethylether, Pebax1657: poly(amide-b-ethylene oxide) with a weight ratio of polyamide to polyether ¼40:60. Pebax2533: poly(amide-b-ethylene oxide) with 80 wt% of poly(tetramethylene oxide) and 20 wt% Nylon-12. PVC-g-POEM: poly(vinyl chloride)-g-(poly (oxyethylene methacrylate).
Fig. 9. Cross-section SEM images of PEGBEM-g-POEM copolymer membranes coated on polysulfone substrate: (a) CP1, (b) CP2, (c) CP3, (d) CP4, (e) CP5 and (f) Pebax.
C.H. Park et al. / Journal of Membrane Science 492 (2015) 452–460
CO2/N2 selectivity reached 84.7 together with a high permeance of 21.9 GPU. This selectivity is much higher than for the membrane prepared with commercially available Pebax (17.0) and is one of the highest values reported for a polymer membrane. This comb copolymer could be combined with various inorganic porous materials such as MOF, to provide good interfacial contact in MMMs due to its sticky rubbery properties. Thus, we believe our work may provide an excellent framework for the design of a new class of polymer for CO2 capture and contribute to commercialization of gas separation membranes.
Selectivity (CO /N )
10
CP1 CP2 CP3 CP4 CP5 Pebax
10
Uppe
r Bou
nd (2
008)
459
10
10
Acknowledgments 10
10
10
10
10
10
CO permeability (Barrer) Fig. 10. Plot of CO2 permeability vs. CO2/N2 selectivity through PEGBEM-g-POEM copolymer membranes with different compositions. 1 Barrer ¼ 1 10 10 cm3 (STP) cm/cm2 s cmHg.
We acknowledge financial support from the Korea CCS R&D Center (2013M1A8A1035871),the Human Resources Program in Energy Technology (20154010200810) and the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (20122010100040).
Appendix A. Supplementary Materials N2 permeance for CP3 compared to CP1 and CP2 is because interfacial defects between crystallites would be minimized for CP3 with a reduced degree of crystallinity. The increase in CO2 permeance with POEM content is attributed to the higher affinity of ethylene oxide units for CO2, resulting in an increased solubility factor. The reduction in CO2 permeance with PEGBEM content was less dramatic compared to the reduction in N2 permeance, because PEGBEM also contains ethylene oxide moieties. Furthermore, the N2 permeance for CP3 was lower than CP4 and CP5 copolymers with lower crystallinity, which is because the highly amorphous rubbery state of POEM chains could contribute to the formation of microscale defects in polymer membrane, resulting in increased permeances of CO2 as well as N2 via the Knudsen diffusion. The gas permeability was calculated after measuring the thickness of the top selective layer using cross-sectional FE-SEM images, shown in Fig. 9. The membrane thickness slightly and gradually increased from 1.5 to 2.3 μm with PEGBEM content due to increased viscosity of the polymer solution, resulting from the higher crystallinity of PEGBEM. As shown in Fig. 10, the performance of the CP3 membrane was very close to the upper bound (2008) [13]. We believe that incorporation of more permeable inorganic materials such as mesoporous metal oxides [29,30] and a metal organic framework (MOF) [31,32] to form mixed matrix membranes (MMMs) could overcome the upper bound, and this supposition is now under investigation. Also, the membrane performance including the CO2 permeability and CO2/N2 selectivity would be enhanced significantly under the humidified condition [43], which will be investigated in the near future.
4. Conclusion Herein, an approach to enhance gas separation performance with amphiphilic comb copolymers was demonstrated. The use of crystalline PEGBEM and amorphous POEM macromonomers resulted in the formation of a highly CO2-philic, alcohol soluble, amorphous solid polymer, as confirmed by GPC, FT-IR, TGA and NMR spectroscopy. WAXS, DSC, SAXS and AFM analysis revealed that the solid-like, better mechanical properties of CP1 and CP2 compared to CP4 and CP5 came from the higher degree of crystallinity of the former. Furthermore, the CP3 with PEGBEM: POEM ¼34:66 was an almost amorphous solid comb copolymer with a very low degree of crystallinity. For the CP3 copolymer, the
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