Synthesis of PVA-g-POEM graft copolymers and their use in highly permeable thin film composite membranes
Graphical abstract
Introduction
Since the phenomenon of global warming became widely known in the late 20th century, a plethora of studies to reduce greenhouse gas emissions have emerged, with their number further accelerated by the 2015 Paris climate accord [1]. Among many viable solutions to reduce carbon dioxide (CO2) emissions for the prevention of global warming, renewable energy sources for diminishing reliance on fossil fuels and gas separation technologies for reducing generated CO2 predominate over other solutions [2], [3], [4], [5], [6]. As a key component of the above two fields, membrane system has drawn much attention, and it has been utilized in various fields. In the former case, membranes consisting of amorphous polymers with high ionic conductivity play a critical role in facilitating electrochemical reaction in renewable energy production and storage system such as supercapacitors [7], dye-sensitized solar cells [8] and secondary batteries [9]. In the latter case, membranes effectively separate greenhouse gas emissions from gas mixtures, mitigating global warming.
Membrane-based gas separation processes have great potential due to their simple and flexible operation, low energy consumption, and low capital cost [10], [11], [12]. In particular, polymeric membranes have attracted much attention because of their notable benefits such as low cost, versatile applications, and eco-friendly processes [13]. However, most polymer membranes suffer from distinct trade-offs involving permeability and selectivity; that is, selectivity diminishes at high permeability and vice versa, which clearly has an adverse effect on membrane performance [14]. Thus, it is imperative to address this drawback of polymer membranes by preparing highly permeable membranes with high selectivity.
Glassy polymers have been considered as a commercial source of membranes for gas separations because they have outstanding gas selectivity and excellent mechanical strength. In practice, some glassy polymers, including polysulfone, polyimides, and cellulose acetate, are used to separate gases in industrial membrane processes: polysulfone for H2 separation [15], polyimide for CO2/CH4 separation [16], and cellulose acetate for CO2 separation [17], [18]. Poly(vinyl alcohol) (PVA), a representative glassy polymer, is beneficial for the preparation of polymer-based membranes due to its desirable benefits such as good film-forming properties, good compatibility with inorganic and organic compounds, environmental friendliness, non-toxicity, and its water-soluble nature [19]. However, despite these advantages, PVA has rarely been applied to gas separation technologies due to its intrinsic barrier properties in dry conditions [20]. This undesirable property is caused by its high crystallinity, which does not allow gases such as H2, O2, N2, and CO2 to penetrate through dry PVA films.
To overcome the impermeability of neat PVA, PVA membranes with high humidity have been fabricated and their properties have been measured under humidified conditions [21], [22], [23]. On the condition that the relative humidity is above 50%, the barrier properties of PVA declined and the gas permeability, especially for CO2 and O2, increased abruptly [20]. Saeed et al. controlled the swelling degree of PVA membranes by adjusting the solution pH and by using carbon nanotubes [21]. This membrane showed a CO2 permeance of 0.44 GPU with a CO2/N2 selectivity of 60 at 1.2 bar. Additionally, various methods such as blending, cross-linking, and use of additives have been applied to fabricate PVA-based gas separation membranes. Deng et al. reported a polyvinyl amine (PVAm)/polyvinyl alcohol (PVA) blended membrane with a CO2 permeance of 210 GPU and a CO2/N2 selectivity of 174 at 2 bar [24]. Francisco et al. fabricated PVA-based membranes with various amine carriers for CO2/N2 separation; in this case, a PVA/diethanolamine (DEA) membrane showed a high selectivity of 92 with a CO2 permeance of 6 GPU [25]. Duan et al. prepared cross-linked PVA membranes containing poly(amidoamine) (PAMAM) dendrimers for CO2/H2 separation. The selectivity and CO2 permeance of the freestanding PVA membrane with 63.3 wt% of PAMAM was 42 and 1.6 GPU, respectively, at a CO2 partial pressure of 560 kPa [26]. Mondal et al. also studied a CO2/N2 separation membrane using cross-linked PVA/polyvinylpyrrolidone (PVP) blended membranes doped with a pentaethylenehexamine (PEHA) amine carrier. The membrane showed a high selectivity of 362 with a CO2 permeance of 26.3 GPU at 2.8 atm [23]. Notwithstanding these efforts, most PVA-based membranes have showed a low CO2 permeance and the studies were performed under humidified conditions.
Poly(ethylene oxide) (PEO) is a representative rubbery polymer that possesses polar ether groups, which are advantageous to increase CO2 solubility due to their high affinity for CO2 [27]. However, pristine PEO is difficult to apply to gas separation membranes because of its high crystallinity. This crystalline nature limits overall chain mobility, leading to low separation performance, and also often results in the formation of structural defects through which all gases pass easily without selectivity [28]. Moreover, the weak mechanical properties of PEO impede industrial applications; thus, modification of PEO is required. To this end, poly(oxyethylene methacrylate) (POEM), which is analogous to PEO with amorphous nature, has been suggested [29], [30]. Although its lack of crystallinity is beneficial to CO2 permeability and separation performance, POEM has poor liquid-like mechanical properties, which has hitherto hampered its application to gas separation membranes.
Here, we report the synthesis of graft copolymers consisting of PVA and POEM and their use in highly CO2 permeable thin film composite membranes. To address the aforementioned drawbacks of PVA and POEM, the PVA-g-POEM graft copolymers were synthesized via free radical polymerization. The CO2-philic PVA-g-POEM graft copolymers were characterized by Fourier transform infrared (FT-IR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, and thermogravimetric analysis (TGA). The crystalline behavior of the PVA-g-POEM graft copolymers was characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Additionally, a series of PVA-g-POEM graft copolymers were coated on a microporous polysulfone (PSf) support to prepare thin film composite membranes. The gas permeation properties of the resultant membranes were tested at 25 °C to investigate the change in CO2 permeation with POEM content.
Section snippets
Materials
PVA (Mw = 85,000–124,000 g/mol, 99+ % hydrolyzed), POEM (Mn = 500 g/mol, containing 900 ppm monomethyl ether hydroquinone as an inhibitor), and ceric ammonium nitrate (CAN, ≥98.5%) were purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO, 99.9%) as a polymerization solvent was obtained from Duksan, Korea. Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) was purchased from Gelest, Inc. The sponge-like PSf substrate was obtained from Toray. All materials were used as received without any
Synthesis of the PVA-g-POEM graft copolymers
The PVA-g-POEM graft copolymers with different weight ratios were synthesized by free radical polymerization, as shown in Scheme 1. To verify that polymerization occurred, the FT-IR spectra of neat PVA, the POEM monomer, and the PVA-g-POEM graft copolymers were measured (Fig. 1). The main characteristic bands of neat PVA and POEM were observed for all PVA-g-POEM copolymers; a broad band at 3273 cm−1 resulting from the OH stretching vibration from neat PVA and a sharp band at 1717 cm−1 resulting
Conclusions
A highly CO2-permeable thin film composite membrane for CO2/N2 separation was fabricated based on an eco-friendly, CO2-philic, water-soluble, PVA-g-POEM graft copolymer. To surmount a major drawback of neat PVA (i.e., its gas barrier property on account of its high crystalline nature and strong hydrogen bonding network), a series of PVA-g-POEM graft copolymers with different POEM contents was synthesized via free radical polymerization. The successful synthesis of the PVA-g-POEM graft
Acknowledgments
This work was supported by a National Research Foundation (NRF) grant funded by the Ministry of Science ICT and Future Planning (NRF-2017R1A4A1014569, NRF-2017M1A2A2043448, NRF-2017K1A3A1A16069486).
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