1. Introduction

Gas separation processes based on polymeric membranes have attracted great attention due to their many technical advantages, such as high energy efficiency, easy scale-up, simple processing, and low operating costs[1-4]. Furthermore, membrane gas separation units are safe and environmentally friendly due to the absence of mechanical complexity. In particular, there have been significant developments in CO₂/N₂, CH₄/N₂, and CO₂/H₂ separation using polymeric membranes to reduce the emission of greenhouse gases[5-7]. During the last few years, composite membranes consisting of two different matrices have been developed in order to improve the performance of separation membranes [8-10]. Such membranes can combine the advantages of both materials in a single membrane.

Amphiphilic copolymers are comprised of separate hydrophilic and hydrophobic segments, enabling the chains to self-assemble at the nanometer scale[11-12]. In general, the hydrophobic segments provide good mechanical strength while the hydrophilic segments provide the functionality of the membrane. One class of amphiphilic copolymers is amphiphilic block copolymers, in which the homopolymers are clearly divided into two parts. These copolymers show a highly ordered structure and good performance. However, the synthesis of block copolymers is usually carried out via anionic polymerization, which is sensitive to impurities and the environment[13-14]. The synthesis of graft copolymers, which consist of a linear backbone of one chain and randomly connected branches of the other chain, is easier and less sensitive compared to that of block copolymers. Graft copolymers with highly ordered nanostructures have been produced via control of the polymer-solvent interaction[15-16]. Terpolymers, which are chemical compounds composed of three different chemical compounds, have rarely been utilized for gas separation membranes.

Mass transport through the polymeric membrane during gas separation is described by the solution-diffusion mechanism[17]. According to this model, permeability is the product of the solubility coefficient (thermodynamic factor) and the diffusion coefficient (kinetic parameter). The permeance of CO₂ is intrinsically higher than that of N₂ due to its high solubility (high condensation temperature) and high diffusivity (small kinetic diameter), and thus it is feasible to prepare commercially attractive polymer membranes for CO₂/N₂ separation.

Polymers based on poly (ethylene oxide)(PEO) were chosen as a candidate for CO₂ separation due to its CO₂-philicity and CO₂ separation properties[18-19]. The drawback of PEO in such membranes is its tendency to crystallize, which reduces the permeable amorphous soft volume fractions and chain mobility, leading to a decrease in gas permeability and selectivity. Lee et al. reported a comb copolymer synthesized using poly(oxyethylene methacrylate)(POEM) and poly(2-[3-(2H-benzotriazol-2-yl)-4-hydroxyphenyl] ethylmethacrylate)(PBEM). POEM is a PEO analogue with high CO₂ affinity, and has the advantage of an amorphous structure. This polymer was once considered to be of limited for use in membranes because of its liquid-like nature. However, this is compensated by the rigid, hydrophobic BEM segments in this polymer membrane[20].

In this work, we demonstrate a simple process to fabricate a composite membrane for CO₂/N₂ separation based on a terpolymer synthesized from PBEM, POEM, and methyl methacrylate (MMA) via free radical polymerization. A solution containing the as-synthesized PBEM-PMMA-POEM was coated onto a polysulfone (PSf) substrate. The PBEM-PMMA-POEM terpolymer was developed in order to increase CO₂ permeance due to its high CO₂ affinity. The structural, thermal, and morphological properties of the membrane were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) analysis, thermogravimetric analysis (TGA), and field emission scanning electron microscopy (FE-SEM). The pure gas permeance and CO₂/N₂ selectivity of the PBEM-PMMAPOEM terpolymer membrane were measured at 25°C.

2. Experimental Section

2.1. Materials

2-[3-(2H-Benzotriazol-2-yl)-4-hydroxyphenyl] ethylmethacrylate (BEM, M_w = 323 g/mol), poly (oxyethylene methacrylate) (POEM), poly (ethyleneglycol)methylethermethacrylate, M_n = 500 g/mol), methyl methacrylate (MMA, 99%), azobisisobutyronitrile (AIBN), and poly[1-(trimethylsilyl)-1-propyne] (PTMSP) were purchased from Sigma Aldrich. N-hexane, cyclohexane, N,N-dimethylformamide (DMF), isopropyl alcohol (IPA), acetonitrile, methanol (MeOH, 99.9%), ethanol (EtOH, 99.9%), and dimethyl sulfoxide (DMSO, 99.9%) were obtained from J. T. Baker. The polysulfone (PSf) substrate was provided by Toray Chemical Korea Inc. All solvents and chemicals were of reagent grade and were used as received without any treatment.

2.2. Synthesis of the terpolymer PBEMPMMA- POEM

For the synthesis of the terpolymer, a weight ratio of 1 : 1 : 8 (PBEM : MMA : POEM) was used. First, 1 g of the BEM monomer was completely dissolved in 70 mL of DMF under stirring for 1 h, and then 1 mL MMA and 8 mL POEM were added to the solution. Subsequently, 0.002 g of AIBN was added to the solution to initiate the free-radical polymerization. After purging the mixed solution with nitrogen gas for an hour, it was placed in a 90°C oil bath for 20 h to allow the free radical polymerization to occur. The viscosity of the solution increased during this time. The resultant solution was precipitated in an n-hexane:IPA (8:2) mixture. To eliminate impurities and residual monomers, this dissolution-precipitation process was repeated three times, and the resulting polymer was dried completely in a vacuum oven. The final weight of polymer obtained was 4.22 g.

2.3. Preparation of the PBEM-PMMA-POEM composite membrane

First, a 0.5% solution of PTMSP in cyclohexane was coated onto the microporous PSf support (average pore size = 0.5 μm, Toray Chemical Inc.) as a gutter layer using an RK-coater (Model 101, Control RK Print-Coat Instruments Ltd., UK) to minimize the effect of the porous structure of the support and the penetration of the coating solution. Then, a 20% solution of PBEM-PMMA-POEM in EtOH was coated onto the PTMSP-coated PSF support using the RK-coater. After the coating process, the membrane was dried in a vacuum oven overnight to remove the solvent completely.

2.4. Characterizations

FT-IR spectra of the synthesized copolymer were obtained in the frequency range from 4000 to 500 cm^-1 with an Excalibur series FT-IR instrument (DIGLAB CO., Hannover, Germany) to confirm the successful polymerization. XRD patterns of the sample were collected at 30 kV and 15 mA for structural analysis. The morphologies of the fabricated membranes were characterized using a field emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL Ltd., Japan). The thermal stability of the terpolymer was investigated by TGA (TGA/DSC1, Mettler Toledo, Korea) at heating rate of 20°C min^-1 in an air atmosphere.

2.5. Gas permeation measurements

Gas permeance tests were conducted at 25°C with the constant pressure/variable volume method using gas separation equipment from Airrane Co. Ltd (Korea). The permeance was measured in gas permeation units (GPU); 1 GPU equal 10^-6 cm³(STP)/s⋅cm²⋅cmHg. The selectivity of the membrane was calculated by the ratio of the permeances of the gases, in this case, the permeability of the more permeable gas, CO₂, was divided by that of the less permeable gas, N₂. The gas permeance of the membrane was measured for 2 h using the individual gases to determine their steady-state performance, and the flowrate measurements were taken under pressures of 1 to 5 bar to investigate the effect of changes in pressure on the permeability and selectivity of the membrane.

3. Results and Discussion

3.1. Synthesis and properties of the terpolymer PBEM-PMMA-POEM terpolymer

The terpolymer PBEM-PMMA-POEM was synthesized via a facile one-step free radical polymerization using AIBN as an initiator at 90°C, as shown in Fig. 1. The PBEM-PMMA-POEM terpolymer was designed to exhibit a microphase-separated structure composed of hydrophobic PBEM and PMMA domains and hydrophilic POEM domains. The hydrophobic PBEM and PMMA domains contributed rigid properties to the terpolymer, while the hydrophilic POEM domains imbued rubbery characteristics. Moreover, the three tertiary amines in the triazole units of the PBEM chains have a high CO₂ uptake, which compensates for the poor mechanical properties of the POEM chains. As a result, the fabricated membrane was expected to exhibit high CO₂ permeance and plasticization resistance without a drastic decrease in CO₂ permeance at high pressure.

As shown in Fig. 2, the FT-IR spectrum confirmed the successful synthesis of the PBEM-PMMA- POEM terpolymer. FT-IR spectra of the three monomers (BEM, MMA, POEM) and of the solvent DMF were also recorded. The absorption bands at 1715 cm^-1 for BEM and 1717 cm^-1 in the POEM spectrum were assigned to the stretching vibrations of the C=O double bond. This vibrational peak shifted to a higher wavenumber of 1728 cm^-1 in PBEM-PMMA-POEM, which indicated an increase in the strength of the C=O double bond. The BEM, MMA, and POEM monomers all exhibited a weak band at 1637 cm^-1, which originated from the C=C stretching vibration of the methacrylate moiety. This band was not observed in the spectrum of the PBEM-PMMA-POEM terpolymer after polymerization, which indicated that the free radical polymerization was completed and that no residual monomer was present in the resulting terpolymer. The PBEMPMMA- POEM terpolymer showed a significant broad peak at 2865 cm^-1, which originated from the O-H stretching vibrations of the POEM domains. The absorption band at 1516 cm^-1 in the spectrum of the monomer BEM was associated with the stretching vibrational mode of the aromatic ring, and was significantly decreased in the spectrum of the PBEMPMMA- POEM copolymer. The strong absorption band of the PBEM-PMMA-POEM copolymer at 1100 cm^-1 was due to the stretching vibration mode of the ether (C-O-C) groups resulting from the POEM domains. Finally, the absorption band at 750 cm^-1 in the PBEM-PMMA-POEM terpolymer spectrum was assigned to the bending mode of the aromatic C-H ring, and appeared at slightly higher wavenumber compared to the BEM aromatic C- H ring. No DMF solvent peaks were observed in the spectrum of the PBEMPMMA- POEM terpolymer, suggesting that DMF was completely removed.

3.2. Structural, thermal, and morphological properties

The results of the TGA analysis are shown in Fig. 3. As shown in the TGA curve, the PBEM-PMMAPOEM terpolymer demonstrated thermal stability up to 250°C; degradation was observed upon heating above this temperature. This result indicated that the terpolymer had good thermal stability. The TGA curves of POEM and PBEM measured by Lee et al. showed thermal degradation at approximately 150°C, and those of other POEM-POEM copolymers showed degradation at approximately 250°C, which did not differ significantly from the PBEM-PMMA-POEM copolymer [20]. The improved thermal stabilities of the polymers compared to the individual components may have resulted from the increased molecular weight and intermolecular forces after polymerization.

Fig. 4 shows the XRD patterns of the PBEMPMMA- POEM terpolymer. The absence of a sharp crystalline peak demonstrates that this terpolymer has an amorphous structure. A broad main peak was observed at 2θ = 20. 5°, and a shoulder peak was observed at 2θ = 32°. This XRD pattern was compared to those reported for other similar polymers in the literature. The POEM and PBEM homopolymer patterns reported by Lee et al. showed peaks at 2θ = 20.2° and 11.7°, respectively. Islam et al. reported that the broad peak of the PMMA homopolymer was located at 2θ = 30°, which is in good agreement with our results[20,21]. The d-spacing values calculated using Bragg’s law for these peaks were 4.4 Å and 3.0 Å. For PBEM-PMMA-POEM terpolymer, the peak at 2θ = 20.5° is believed to correspond to correspond to the peak of POEM at 20.2°, and the peak at 32° was thought to be related to the PMMA peak at 30°. These XRD results suggest that overall, the copolymerization was successful based on the evidence of POEM and PMMA nanostructures in the terpolymer; the PBEM peak was not significantly expressed because of the low ratio of PBEM used in the synthesis.

3.3. Gas separation performance of the PBEMPMMA- POEM/PSf composite membrane

The PBEM-PMMA-POEM terpolymer composite membrane was prepared by coating a 20 wt% solution of the terpolymer onto the PTMSP-coated PSF supporting layer using an RK control coater. The PTMSP gutter layer was used to ensure good coating of the terpolymer, and prevented intrusion of the coating solution into the porous PSf supporting layer. While this gutter layer did not affect the gas separation performance of the membrane due to its thinness, it helped to prevent undesirable defects in the coating layer.

The cross-sectional and surface morphologies of the PBEM-PMMA-POEM/PSf composite membrane were observed using SEM, as shown in Fig. 5. A thin se- lective layer of the PBEM-PMMA-POEM terpolymer with a thickness of 1.845 μm was coated onto the PSMSP-coated PSf supporting layer. The intimate contact between the terpolymer coating layer and the PTMSP-coated PSf supporting layer was expected to ensure the durability and long-term stability of the composite membrane during operation. Fig. 4(c) and 4(d) show the uniformity of the surface, on which no defects or voids are observed.

The gas separation performance of the PBEMPMMA- POEM membrane was first evaluated using single gas permeation tests at 25°C with feed pressures from 1 to 5 bar. The pure gas permeances and the ideal CO₂/N₂ selectivity of the PBEM-PMMA-POEM membrane with increasing feed pressure are shown in Fig. 6 and 7, respectively. As seen in Fig. 6, the pure CO₂ permeance was always higher than the pure N₂ permeance, regardless of the feed pressure. This was due to the higher diffusivity and the higher solubility of CO₂ as a result of its smaller kinetic diameter (3.30 Å) and higher condensability (195 K) compared to N₂ (3.64 Å and 71 K)[22,23]. The PBEM-PMMA-POEM terpolymer contains a polar group (the ether oxygen) in the flexible hydrophilic POEM chains that has high affinity for CO₂ molecules via dipole-quadrupole interactions.

To investigate the plasticization phenomena, which could influence the gas separation performance of the membrane, the feed pressure was increased up to 5 bar. As the pressure increased, the permeance of both CO₂ and N₂ increased, except in the case of CO₂ at 1-3 bar. Increased pressure caused plasticization of the membranes, in which the polymer chains swell and the free volume of the membrane is increased. This swelling led to an increase in N₂ permeance, resulting in decreased CO₂/N₂ selectivity, as shown in Fig. 7 and Table 1. As a result, the CO₂/N₂ selectivity decreased from 30.2 to 23.6 as the pressure increased from 1 to 5 bar.

4. Conclusions

In conclusion, we fabricated a high-performance gas separation membrane based on the CO₂-philic terpolymer PBEM-PMMA-POEM, which was synthesized via free radical polymerization. The PBEM-PMMAPOEM terpolymer was composed of rigid hydrophobic BEM and MMA segments and flexible hydrophilic POEM segments. FT-IR spectroscopy and XRD analysis confirmed the successful synthesis of the targeted polymer and its amorphous structure. The thermal stability of the terpolymer at temperatures up to approximately 250°C was verified via TGA. The surface and cross-sectional morphologies of the PBEM-PMMAPOEM/ PSf composite membrane were observed using SEM. Gas permeation tests of the membrane showed its high selectivity for CO₂ over N₂. The presence of polar groups (ether oxygens) that have high affinity for CO₂ molecules via dipole-quadrupole interactions in the flexible hydrophilic POEM chains enhanced the CO₂ permeance. The CO₂/N₂ selectivity of the PBEMPMMA- POEM membrane reached 30.2 at a CO₂ permeance of 57.4 GPU. An additional advantage of the PBEM-PMMA-POEM terpolymer reported here is the low cost of its synthesis due to the utilization of free radical polymerization rather than the anionic polymerization techniques that are typically used for conventional block copolymers.