1. Introduction

Gas separation using membrane technology over the last 30 years has been commercialized for a variety of processes such as air separation, hydrogen separation and flue gas treatment[1-3]. Among these processes, CO₂ separation from gas mixtures is known as one of the most important topics because of its huge market and global warming by the greenhouse effect. Membrane technologies have attracted attention due to their low energy consumption, simple processing and environment- friendly properties[4]. Various materials have been used to fabricate gas separation membranes and polymer- based membrane was one of the most attractive membranes due to its low cost and easy processability [5-8]. However, in the case of a polymeric membrane, there is a problem due to a trade-off relationship between gas permeability and selectivity[9,10]. Therefore, various studies have been conducted to address such a problem. For example, there have been many studies on CO₂ separation polymeric membrane that were carried out to introduce polymers having functional groups with high affinity for CO₂[11-18].

Poly(ethylene glycol) (PEG) is a commercially available hydrophilic polymer, and soluble in various polar solvents such as water or alcohol. Therefore, PEG has been widely used as the hydrophilic moiety of amphiphilic block or graft copolymers. In addition, PEG has numerous ether (C-O-C) groups which can enhance the CO₂ solubility, thereby leading to an increase in CO₂ permeability when it is incorporated in the polymer membranes. This results from the strong Lewis acid-base interaction between the ether group acting as a Lewis base and the CO₂ molecule acting as a Lewis acid [18,19]. In this study, a low-molecular-weight PEG was introduced into the polymer matrix to increase the CO₂ solubility. As PEG has a high tendency to crystallize when its molecular weight is high, it becomes difficult to form a defect-free membrane. Therefore, we choose the PEG oligomer with a molecular weight of 200 gmol^-1 as an additive.

In this study, CO₂-philic and alcohol-soluble PBE comb copolymer was synthesized by free radical polymerization and blended with PEG at various weight ratios. Subsequently, the PBE/PEG blend solution was coated onto a poly(1-(trimethylsilyl-1-propyne))-coated polysulfone substrate to prepare a thin-film composite membrane. The CO₂/N₂ separation properties were measured at a constant pressure of 2 bar at room temperature (25 °C) according to the PEG loading in the blend membranes. The results were analyzed by using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM).

2. Experimental Section

2.1. Materials

2-[3-(2H-Benzotriazol-2-yl)-4-hydroxyphenyl] ethyl methacrylate (BEM, M_w = 323 gmol^-1), poly(oxyethylene methacrylate) (POEM, poly(ethylene glycol) methyl ether methacrylate, M_n = 500 gmol^-1), azobisisobutyronitrile (AIBN) and poly(ethylene glycol) (PEG, M_w = 200 gmol^-1) were purchased from Sigma-Aldrich. N,N-dimethylformamide (DMF), isopropyl alcohol (IPA) and methanol were obtained from J. T. Baker. Poly(1-(trimethylsilyl- 1-propyne)) (PTMSP) was purchased from Gelest. n-Hexane was obtained from Duksan. Cyclohexane was purchased from Daejung. All solvents and chemicals were used as received without further purification.

2.2. Synthesis of PBE comb copolymer

PBE copolymer was synthesized with a weight ratio of BEM : POEM = 2 : 8. First, 2 g of BEM and 8 g of POEM were completely dissolved in 100 mL of DMF by stirring for 5 h. Then 0.002 g of AIBN was added to the solution as an initiator, followed by purging with N₂ gas. The mixed solution was placed in a 90 °C oil bath for 24 h. After reaction, the PBE solution was precipitated in IPA : n-hexane (1 : 3) mixed solvent. To purify the PBE copolymer, this precipitation process was repeated three times, and then the obtained copolymer was dried completely in a vacuum oven.

2.3. Preparation of PBE/PEG thin-film composite membranes

First, 30 wt% of PEG and PBE solutions were prepared respectively by using methanol as a solvent. Then these solutions were mixed together with varying the PBE : PEG ratios of 9 : 1, 8 : 2 and 7 : 3 and stirred for 5 h. Besides, 1 wt% PTMSP solution in cyclohexane was prepared, and coated onto the microporous polysulfone support (Toray Chemical Inc.) as a gutter layer using a RK Control coater (Model 101, Control RK Print-Coat Instruments Ltd., UK). After that, the PBE/PEG mixed solutions (10 : 0, 9 : 1, 8 : 2 and 7 : 3 weight ratio) were coated onto the PTMSP-coated polysulfone support. After coating, the membranes were dried in a vacuum oven overnight to remove the solvent completely. As a control group, the neat PBE membrane without a gutter layer was also prepared.

2.4. Gas permeation measurements

The gas permeance of the thin-film composite membranes was measured by using gas separation equipment provided by Airrane Co. Ltd. (Korea). Pure gas permeability (CO₂ and N₂) was measured in gas permeation units (GPU) (1 GPU = 10^-6 cm³ (STP) / (s cm² cmHg)), and the selectivity was determined by the ratio of the permeance for each component. (CO₂ permeance/ N₂ permeance) Five thin-film composite membranes were tested, and the average estimated error in the gas permeation measurement was approximately.

2.5. Characterization

FT-IR spectra of the polymer blends were obtained with a Spectrum 100 spectrometer (PerkinElmer, USA) over the wavenumber range of 4,000~500 cm^-1. XRD patterns were measured by using a SmartLab (Rigaku, Japan). The morphologies of the polymer blends were characterized by using a JEOL-7610F-Plus field-emission scanning electron microscope (FE-SEM, JEOL Ltd., Japan).

3. Results and Discussion

3.1. Synthesis and spectroscopic characterization

The neat polymers (PBE, PEG) were mixed in three different ratios (i.e. 9 : 1, 8 : 2, 7 : 3). The PBE comb copolymer was synthesized via a one-step free radical polymerization using AIBN as the initiator, as shown in Scheme 1. This polymerization involves a relatively facile reaction. PBE chains have three tertiary amine groups in triazole which can improve the capacity for CO₂ loading through specific interactions between the basic amines and acidic CO₂. PEG has polar etheric oxygen atoms in their main chains which favorably interact with CO₂ molecules[18]. For these reasons, this polymer blends are expected to have high affinity toward CO₂, resulting in higher CO₂ solubility in the membranes.

FT-IR spectra were obtained to confirm the chemical structure and interactions of the polymer blends. The FT-IR spectra of neat PBE, PEG and PBE/PEG blends with various ratios are shown in Fig. 1. It was confirmed that the PBE comb copolymer synthesis was carried out successfully because the C=C band that appearing near 1,638 cm^-1 was not observed in the spectrum of the PBE. The O-H stretching vibration band in the PBE (3,499 cm^-1) was observed at higher wavenumber than that of PEG (3,411 cm^-1). With increasing PEG loading, the O-H band appeared at a gradually lower wavenumber and its intensity also increased. This is because the PBE comb copolymer and the PEG oligomer contain identical functional groups in their structures, therefore the PBE and PEG blend is expected to be a miscible and interacting system. There were no band shifts at 1,727 and 1,097 cm^-1 corresponding to the carbonyl (C=O) groups and the ether (C-O-C) groups, respectively. It indicates that there was no hydrogen bonding between the O-H groups and the carbonyl groups nor between the O-H groups and the ether groups. The intensity of small C-C stretching vibration (in-ring) bands at 1,517 cm^-1 and sharp C-H aromatic stretching bands at 750 cm^-1 in PBE comb copolymer gradually decreased with increasing PEG contents, which confirms the incorporation of PEG into the PBE comb copolymer matrix.

3.2. Structural and morphological properties

The microstructures of neat PBE, PEG and their blends were characterized using XRD, as shown in Fig. 2. There was no sharp crystalline peaks, which indicates that these polymer blends have amorphous natures. The neat PBE comb copolymer exhibited one broad halo centered at 2θ = 19.4°, and it shifted to a higher 2θ value with increasing PEG loading. In the case of PBE/PEG (7 : 3) blend, 2θ was centered at 19.9°. According to the Bragg’s law (2 dsinθ = nλ), the increased 2θ causes lower d-spacing value. Therefore, the shift indicates that the d-spacing decreased with increasing PEG composition. In neat PBE, the d-spacing was determined to be 4.58 Å and that of PBE/PEG (7 : 3) blend was 4.46 Å. This phenomenon is because both POEM in PBE and PEG are hydrophilic chains, and possibly interacts with each other. These XRD results indicate that PEG oligomer interacts with the hydrophilic groups in PBE (POEM).

The morphologies of PBE/PEG blend membranes were observed using SEM, as shown in Fig. 3. According to the cross-sectional SEM images, both neat PBE and PBE/PEG (9 : 1) blend had a dense structure and there were no obvious structural defects through which gas molecules pass without selectivity. This confirms the good compatibility and miscibility between the PBE comb copolymer and PEG oligomer. The thicknesses of the selective layer were approximately 400~500 nm. The surface SEM images are shown in Fig. 4. The membranes showed typical smooth dense polymer structures, and there was no significant difference between the neat PBE and PBE/PEG blends. It indicates that the PBE comb copolymer has high affinity for PEG oligomer.

3.3. Gas separation performances

First, the performance of neat PBE membrane without PTMSP gutter layer was measured using gas separation equipment. This membrane exhibited very low CO₂/N₂ selectivity due to the formation of structural defects. It is attributed to the penetration of PBE chains into the pores of polysulfone supports. Therefore, PTMSP with a high permeability was used as a gutter layer. When PTMSP was coated onto polysulfone support, the membranes exhibited significantly increased CO₂/N₂ selectivity. It is possible that the PTMSP gutter layer effectively prevents the PBE/PEG coating solution from penetrating through the pores of polysulfone support. It is noteworthy that PTMSP was coated with a dilute solution of 1 wt% concentration, therefore it could have little effect on the permeance of the membranes.

The pure gas permeance and CO₂/N₂ selectivity through the PBE/PEG blend membranes are shown in Table 1 and Fig. 5. The permeance of each pure gas was measured at a constant pressure of 2 bar at room temperature (25 °C). As the content of PEG increased, the CO₂ permeance decreased gradually while the CO₂/ N₂ selectivity increased significantly. This might be due to the reduced distance between polymer chains, which was proved by XRD analysis. From XRD results, it was confirmed that the d-spacing decreased with increasing PEG content, which indicates that the free volume which gas molecules can pass through was reduced. Therefore, the N₂ permeance decreased at a greater rate than CO₂ permeance due to its larger kinetic diameter than CO₂[20]. The PBE/PEG (7 : 3) blend membrane showed the highest CO₂/N₂ selectivity up to 100.3 with a moderate CO₂ permeance of 170.5 GPU. This performance represents high values reported for polymer blend membranes without inorganic filler such as metal organic frameworks, zeolites, porous carbons or mesoporous metal oxides[21-27]. The PBE/ PEG (7 : 3) blend membrane was compared with other polymer blend membranes reported in the literatures as shown in Fig. 6.

4. Conclusions

In this study, PBE/PEG blend membranes were prepared by incorporating a commercial PEG oligomer into the CO₂-philic PBE comb copolymer. The CO₂/N₂ separation properties of the membranes were analyzed according to the PEG contents. The PBE comb copolymer was synthesized via free radical polymerization, which is economical and facile reaction. Specific interactions between the PBE comb copolymer and PEG oligomer were confirmed by FT-IR analysis. Upon the incorporation of PEG into PBE comb copolymer, no significant shifts were observed in the FT-IR spectra. Furthermore, from the XRD analysis, it was confirmed that the POEM chain showed a more compact structure as the PEG loading increased. The change in polymer structure influenced the gas permeance of the PBE/PEG blend membranes. As the PEG content increased, the gas permeability gradually decreased while the selectivity significantly increased. For the PBE/PEG (7 : 3) membrane, the CO₂ permeance decreased from 279.4 GPU to 170.5 GPU while the CO₂/N₂ selectivity increased from 28.1 to 100.3, when compared with the neat PBE.