and Water-Selective Membranes: Effect of Copolymer Composition

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Scalable Synthesis of Amphiphilic Copolymers for CO2- and WaterSelective Membranes: Effect of Copolymer Composition and Chain Length Faheem Hassan Akhtar,† Mahendra Kumar,† Hakkim Vovusha,‡ Rahul Shevate,† Luis Francisco Villalobos,†,§ Udo Schwingenschlögl,‡ and Klaus-Viktor Peinemann*,† Advanced Membranes and Porous Materials Center and ‡Physical Science and Engineering Division (PSE), 4700 King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia

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†

S Supporting Information *

ABSTRACT: Dehumidification is a critical energy-intensive and crucial process for several industries (e.g., air conditioning and gas dehydration). Polymeric membranes with high water vapor permeability and selectivity are needed to achieve an energy-efficient water vapor removal. Herein, we demonstrate high-performance water vapor transport membranes based on novel amphiphilic tercopolymers. A series of amphiphilic tercopolymers comprising polyacrylonitrile, poly(ethylene glycol) methyl ether methacrylate (PEGMA), and poly(N,Ndimethylamino ethyl methacrylate) (PDMAEMA) segments are synthesized via an economical and facile free radical polymerization. The water vapor permeability increases with the increase in PEGMA chain length and the content of PEGMA segments. The best performing membrane (i.e., PEGMA-9502) achieved a water vapor permeability of 174 kBarrer. By optimizing the content and chain length of the PEGMA segments, the membranes could be tuned for carbon capture applications. The optimized membranes tested for CO2 separation showed a high CO2 permeability of 47 Barrer along with CO2/N2 and CO2/CH4 selectivities of 67 and 23, respectively. This work presents a simple and economic amphiphilic tercopolymer for the fabrication of membranes with excellent gas and water vapor separation performance.

■

INTRODUCTION Water vapor exists in many gaseous streams including flue gas, natural gas, and air. The presence of water vapor in these streams is not always desirable and can cause severe problems. The removal of water vapor from specific gas streams is important in various applications like flue gas dehydration,1,2 drying of natural gas,3 air drying,4−6 foodstuff storage,7−9 and humidity control in spacecraft applications.10 Another important application is dehumidification in buildings (i.e., air conditioning).11 According to a recent report from the American Society of Heating, Refrigerating and Air-Conditioning Engineers, heating, ventilation, and air conditioning is accountable for 38% of U.S. residential and commercial buildings primary energy consumption and 89% of spacecooling energy consumption.12 During air conditioning, both the latent heat (water vapor enthalpy) and sensible heat (dry air enthalpy) are removed by the chillers with a significant fraction of latent heat.13 Therefore, it is crucial to remove water vapors before air conditioning to decrease energy consumption. Membranes have been proposed as a supplement to the conventional desiccant systems. The pressure of water vapor in air is very low; therefore, the membranes for dehumidification must have high water vapor permeability and selectivity to achieve the desired performance. To achieve this, © XXXX American Chemical Society

numerous organic, inorganic, and mixed matrix materials have been developed. Recently, we introduced different polymers for the fabrication of polymeric and mixed matrix membranes and evaluated their performance in water vapor removal applications.14−17 Polymeric membranes including both rubbery and glassy polymers present high water vapor permeability and selectivity because of the high solubility and diffusivity of water molecules. The water vapor transport mechanism through dense polymeric membranes (rubbery and glassy) is best described by the solution-diffusion mechanism and is explained in detail by Matteucci et al.18 Generally, water vapor permeability and selectivity of polymeric membranes increase simultaneously and do not exhibit the typical trade-off like permanent gas separations due to the solubility dominance.19 Polymeric membranes with adequate hydrophilicity are expected to have high water vapor permeability and selectivity due to the presence of reactive functional groups for interactions with water molecules (like hydrogen-bonding, ion−dipole, and dipole−dipole interactions).20 Thus, the use Received: March 15, 2019 Revised: June 11, 2019

A

DOI: 10.1021/acs.macromol.9b00528 Macromolecules XXXX, XXX, XXX−XXX

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obtained from Sigma-Aldrich. The purified monomers were then placed into airtight glass vials and stored in a freezer. The initiator 2,2azobis(2-methylpropionitrile) (AIBN) was purchased from Pfaltz & Bauer, U.K., and was recrystallized from methanol to obtain needlelike crystals. Tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), acetonitrile (ACN), N-methyl-2-pyrrolidone (NMP), N,N-dimethyl formamide (DMF), cyclohexane (CyH), and deuterated dimethyl formamide (DMF-d7) were received from SigmaAldrich. Absolute ethanol (EtOH) and acetone were purchased from VWR Inc., U.K. 2.2. Synthesis of the Tercopolymers and Characterizations. Poly(acrylonitrile-r-PEGMA-r-N,N-dimethyl amino ethyl acrylate) [P(AN-r-PEGMA-r-DMAEMA)] tercopolymers with various compositions (weight percentages and chain lengths) were synthesized from AN, PEGMA, and DMAEMA by free radical polymerization using AIBN. The feed concentrations of AN, PEGMA, and DMAEMA monomers are provided in Table S1, Supporting Information. The typical procedure for the synthesis of tercopolymer P(AN-rPEGMA300-r-DMAEMA)1 is as follows. Initially, 2.5 g of AN was added to a round-bottom flask containing 20 mL of DMSO. After that, 1 g of PEGMA-300 and 1 g of DMAEMA were alternatively added under constant stirring at 300 rpm. Then, 1 wt % AIBN to the total weight of the monomers was further added to the reaction mixture solution at room temperature (RT). The reaction mixture flask was then purged with pure N2 gas for 45 min to eliminate traces of dissolved oxygen. The polymerization reaction was performed initially at 60 °C for 18 h under constant stirring and then at 80 °C for another 18 h to obtain a high-molecular-weight copolymer. The obtained dark yellow viscous solution was finally cooled to RT. The solution was precipitated in a mixture (70:30% v/v) of cyclohexane and absolute ethanol for 12 h to remove unreacted monomers and AIBN. The precipitated copolymer was recovered via vacuum filtration. The resulting copolymer was again dissolved in DMSO and precipitated three times to attain the purified copolymer. The tercopolymer P(AN-r-PEGMA300-r-DMAEMA)1 was obtained after drying in a vacuum oven at 50 °C overnight. In addition, other copolymers with various compositions were also synthesized following a similar protocol (Table S1, Supporting Information). The synthesized tercopolymers are presented as P(AN-r-PEGMA300-rDMAEMA) 1 , P(AN-r-PEGMA300-r-DMAEMA) 2 , P(AN-rPEGMA500-r-DMAEMA)1, P(AN-r-PEGMA500-r-DMAEMA)2, P(AN-r-PEGMA950-r-DMAEMA)1, and P(AN-r-PEGMA950-rDMAEMA)2, where subscripts 1 and 2 denote the weight (g) of PEGMA (300, 500, or 950 Da) monomers in 4.5 and 5.5 g reaction mixtures, respectively. The synthesized tercopolymers were readily soluble in aprotic solvents like DMF, dimethylacetamide (DMAc), NMP, ACN, and others (Table S2, Supporting Information). The functional groups of the synthesized copolymers were verified by recording their FTIR spectra on a Thermo Scientific attenuated total reflection-FTIR spectrometer (Nicolet iS10) in the frequency range from 400 to 4000 cm−1. The spectra were collected at 32 scans with a resolution of ±4 cm−1. 1H NMR spectra of the synthesized copolymers were measured using a Bruker Advance III 700 MHz NMR spectrometer equipped with a TCI cryoprobe. The measurements were performed by dissolving the copolymers in a deuterated DMF-d7 solvent with a concentration of 16 mg mL−1 at RT. The molecular weight (Mw) and polydispersity index (PDI) of the copolymers were determined by gel permeation chromatography (GPC) (Agilent module 1200 infinity series instrument). The GPC instrument was equipped with two PLgel 5 μm MIXED-C columns and a reflective index (RI) detector. High-performance liquid chromatography (HPLC)-grade DMF was used as an eluent at 1 mL min−1 flow rate. The RI detector was calibrated using variousmolecular-weight polystyrene standards. Polymer solutions of 1 mg mL−1 concentration were prepared by dissolving the copolymers in HPLC-grade DMF and filtered through a 5 μm poly(tetrafluoroethylene) filter before injecting into the GPC instrument. The density of the copolymers was determined with a Mettler-Toledo balance equipped with a density measurement kit based on the Archimedes principle using isooctane as the reference liquid. Thermal

of hydrophilic polymer membranes can enhance the solubility and diffusivity of water vapor in a membrane because of the strong interactions between permeating water molecules and hydrophilic groups existing on the polymer chains. Poly(ethylene glycol) (PEG)-based membranes are considered as an attractive choice, and poly(ethylene glycol) methyl ether methacrylate (PEGMA) has been used for chemical grafting of hydrophilic polymeric chains onto various types of hybrid materials to improve CO2 permeance.21−23 However, several challenges are encountered that limit the usage of PEGbased materials for practical applications. These issues include (i) the high tendency of PEG-based materials to crystallize, causing a reduction in gas permeances, (ii) the poor mechanical strength of the membranes, limiting the module fabrication, and (iii) the plasticization at high partial pressures, especially for CO2, which limits the selectivity. Approaches to overcome these challenges include copolymerization, blending, and cross-linking. Block copolymers comprising different types of hard and soft segments are an interesting class of materials since they maintain a balance between the membrane performance and mechanical strength and they have been used for water vapor removal applications.3,14,24,25 The right choice of copolymers with appropriate properties can overcome the problems mentioned before. Various poly(ethylene oxide) (PEO)based copolymers with high hydrophilicity were used in the past for CO2 separation due to dipole−quadrupole interactions between the oxygen atoms of the ether (C−O−C) groups and the CO2 molecules.23,26 In this work, a series of six high water and CO2-philic amphiphilic tercopolymers comprising poly(ethylene glycol) methyl ether methacrylate (PEGMA), polyacrylonitrile (PAN), and poly(N,N-dimethylamino ethyl methacrylate) (PDMAEMA) segments have been synthesized via an economical and facile free radical polymerization. Choosing the right segments of the synthesized copolymers can provide the required properties to the copolymer membranes. PAN segments allow the formation of a robust, mechanically stable matrix; PEG side chains of the PEGMA segment produce a continuous network of the hydrophilic channels; and PDMAEMA segments are expected to provide high water vapor and CO2 permeability due to the presence of PEG and tertiary amine groups.27 We anticipate that the synergetic effect of PEG and tertiary amine would improve the separation performance of the fabricated membranes. The resulting tercopolymers with various compositions have been characterized using gel permeation chromatography (GPC), 1H nuclear magnetic resonance (1H NMR) spectrometry, and Fourier transform infrared (FTIR) spectroscopy. Moreover, differential scanning calorimetry (DSC) was used to determine the states of water for the synthesized tercopolymers. Finally, molecular dynamics (MD) simulations and density functional theory calculations are carried out to identify the adsorption energies, polymer− water interactions, and their interaction sites.

2. MATERIALS AND METHODS 2.1. Materials. Acrylonitrile (AN), N,N-dimethylamino ethyl methacrylate (DMAEMA), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) with different molecular weights (300, 500, and 950 Da) were procured from Sigma-Aldrich, Germany. All of the monomers (AN, DMAEMA, and PEGMA) were initially passed through an inhibitor remover column at 0.5 mL min−1 flow rate to remove the radical inhibitor. The inhibitor remover columns were B


ÄÅ É ÅÅ p − p ÑÑÑ Å 2Ñ ÑÑ WVTR c = WVTR mÅÅÅ 0 ÅÅ p − p ÑÑÑ ÅÅÇ 1 2Ñ ÖÑ

Macromolecules stability of the copolymers was determined by performing thermogravimetric analysis (TGA) on TA Instruments (Discovery series) under nitrogen flow. The samples were equilibrated at 100 °C for 120 min, and the temperature was further raised to 600 °C at a rate of 5 °C min−1. The states of water (bound and free water) were measured using DSC Q2000, TA Instruments. The copolymer samples were initially placed into deionized water at RT for 24 h. After that, the wet samples were taken out and mopped with tissue paper to remove surface water. The wet samples were then sealed in aluminum pans, and the analysis was performed in the temperature range of −30 to +30 °C with a scanning rate of 10 °C min−1 under an inert atmosphere. 2.3. Fabrication of Membranes. Polymer solutions were prepared by dissolving the tercopolymers in DMAc to achieve a 3 wt % solution of the individual copolymer. The copolymer solution was then cast on a Teflon plate and dried at 80 °C for 12 h under an inert atmosphere. The resulting membranes were further dried at 120 °C under vacuum for 24 h to remove traces of the residual solvent. Complete removal of the solvent was confirmed by TGA analysis. The resulting membranes are labeled as PEGMA-3001, PEGMA-3002, PEGMA-5001, PEGMA-5002, PEGMA-9501, and PEGMA-9502, explained in Table S3, Supporting Information. 2.4. Membrane Characterization. The wide-angle X-ray diffraction (WXRD) experiments were performed with a Bruker D8 Advance diffractometer to examine the change in crystalline properties of the membranes. High-pressure CO2 adsorption was performed for the membrane samples using Micromeritics ASAP 2050 at 298 K with pressure up to 0.8 MPa. Before the test, the membrane samples were cut into small pieces and activated under vacuum at 120 °C for 24 h. The water vapor sorption experiments were conducted using a VTI-SA sorption analyzer (TA Instruments) at 25 °C. Each membrane sample was equilibrated for at least 3 h at a specific humidity. Thick membrane samples of about 100 μm thickness were selected for sorption experiments to avoid any error during analysis. The samples were dried inside the sorption analyzer at 100 °C for 120 min to attain a constant weight prior to the sorption measurements. Sorption measurements were recorded at a different relative humidity (RH) in the range from 0 to 95%, and desorption measurements were also recorded over the entire range. 2.5. Membrane Performance. 2.5.1. Gas Permeation Measurements. A constant-volume/variable-pressure test method was used to measure pure gas permeation. The gas permeance through the tercopolymer membrane was calculated according to eq 1 J=

ij p − p yz V × 22.4 j 0 z zz lnjjjj f z R × T × A × t j pf − pp(t ) zz k {

Article

(3)

where p1 is the partial pressure under the membrane surface. 2.6. Computational Details. All calculations in this work were carried out using the amorphous and Forcite modules implemented in Materials Studio 8.0 with the COMPASS force field. Three chains of the P(AN-r-PEGMA2-r-DMAEMA) copolymer (each containing 10 repeat units) with various numbers of water molecules based on the relative humidity from experiment were constructed in a cubic box. We considered different molecular weights (PEGMA-300, PEGMA500, and PEGMA-950) in the calculations. For each system, the total energy was minimized by Forcite. The Forcite module was also used for performing MD simulations at 298 K. For equilibration (100 ps) and production (900 ps), we used NVT and NPT ensembles, respectively. The time step was fixed to 1 fs, and the Andersen and Berendsen methods were used for temperature and pressure control, respectively. Long-range Coulombic interactions were accounted for by the Ewald sum method, and nonbonded energies were calculated with a cut-off distance of 9.5 Å. The interaction energy of water molecules with PEGMA was calculated in the framework of density functional theory with the GAUSSIAN 09 package.

3. RESULTS AND DISCUSSION Amphiphilic tercopolymers P(AN-r-PEGMA-r-DMAEMA) with various compositions were synthesized via free radical polymerization using AIBN as an initiator in DMSO. The detailed composition of the reaction mixture for the tercopolymer synthesis is illustrated in Table S1, Supporting Information. Rigid PAN, flexible PEGMA, and hydrophilic PDMAEMA segments are distributed randomly in the amphiphilic tercopolymers of various compositions (Scheme 1). Efforts were also made to synthesize these tercopolymers in other solvents like DMF, THF, butyl acetate, and DMAc under similar experimental conditions; however, very low molecular weight tercopolymers were formed in these solvents and the fabrication of free-standing membranes was not feasible. The successful synthesis and chemical composition of the synthesized copolymers were confirmed using 1H NMR Scheme 1. Reaction Route for the Synthesis of P(AN-rPEGMA-r-DMAEMA) Tercopolymers with Different Compositions via Free Radical Polymerization

(1)

where R, T, A, and V are the ideal gas constant (m3 bar mol−1 K−1), temperature (K), membrane area (m2), and the permeate volume (m3), respectively. pf and the feed pressure at time 0 (p0) are the pressures at feed side, and pp(t) is the permeate pressure in bar. Finally, the gas permeability (thickness-normalized permeance) was calcu-

(

lated in Barrer 1 Barrer = 10−10 ×

cm3 cm 2

cm sec cmHg

).

2.5.2. Water Vapor Permeation Measurements. The water vapor permeability (Barrer) through the tercopolymer membranes was determined using a modified permeability cup test method. The detailed procedure is described in our earlier published work.14,15 In summary, water vapor permeance was calculated using eq 2 WVTR m =

dm [A × Psat × (R1 − R 2)]−1 dt

(2)

where A is the membrane area, Psat is the saturated vapor pressure at the test temperature, and R1 and R2 are the relative humidity at the source and the vapor sink. The correction factor for the actual driving force was calculated using eq 314 C


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Figure 1. (a) 1H NMR spectra of the tercopolymers with various compositions in deuterated DMF (DMF-d7) and (b) FTIR spectra of the tercopolymers with various contents and chain lengths of the PEGMA segment.

(Figure 1a). The detailed 1H NMR spectrum of individual tercopolymers is provided in Figure S1a−e, Supporting Information. The characteristic peaks at 3.1 ppm are attributed to the terminal −OCH3 groups in PEGMA of the copolymers. The signals at 1.48 and 2.43 ppm belong to the −CH2 and −CH protons in the PAN segment, respectively. The signals at 2.77 and 3.31 ppm are observed because of the terminal −CH3 protons in −N(CH3)2 and the −CH2 protons located in the PDMAEMA segment of the copolymers, respectively.28 The characteristic peaks at 4.4, 3.95−3.5, 2.23, and 1.37 ppm are ascribed to −COOCH2, −CH2−O−CH2, and −CH2 protons in the PEGMA segment of the copolymers. However, the peaks for −CH2 protons in PEGMA and PDMAEMA segments are overlapped at 4.4 ppm. The intensity of the peaks obtained at 3.95 ppm for −CH2−O−CH2 is dependent on the feed mol % of the identical-molecular-weight PEGMA monomer. These results confirm the successful synthesis of the copolymers via free radical polymerization. The actual chemical composition of the copolymers was determined from the proton peak intensity at 2.43 ppm for −CH, 2.77 ppm for CH3 in −N(CH3)2, and 3.95 ppm for CH2 in −CH2− O−CH2 of the PEGMA segments (Figure 1a). The mole percentage of the PAN segment in the copolymers was determined using eq 4 PAN(%) =

I2.43 × 100 I2.43 + I3.95/2 + I2.77/2

stretching vibration of C−O−C groups in the PEGMA segment of the copolymers.30 These results reveal the successful synthesis of the copolymers. The weight-average molecular weight Mw, the numberaverage molecular weight Mn, and the polydispersity index (PDI) of the copolymers are summarized in Table S1, Supporting Information, as obtained from GPC analysis. The reported values of molecular weights are mentioned with reference to the different-molecular-weight polystyrene standards. The Mw of the copolymers is in the range from 159 to 228 kDa with PDI values in the range from 2.24 to 2.93. The Mw of the copolymers is dependent on the initial feed amount (mol %) and the molecular weight of PEGMA monomers. The Mw of the copolymers increases with the feed amount of the identical-molecular-weight PEGMA monomer. However, the Mw of the copolymers P(AN-r-PEGMA300-r-DMAEMA)1, P(AN-r-PEGMA500-r-DMAEMA)1, and P(AN-r-PEGMA950r-DMAEMA)1 declines from 193 to 159 kDa with an increase in the PEGMA molecular weight (i.e., chain length). This is ascribed to steric hindrance of the long PEG chains of the PEGMA monomer (Mw: 500 and 950 Da) and the low reactivity of the monomer in comparison with AN and DMAEMA monomers during free radical polymerization. To elucidate the water binding capacity of the tercopolymer membranes in detail, the free and bound water fractions were obtained from DSC thermograms of wet samples. The freezable water peaks were obtained in the temperature range of −5 to −2 °C for all tercopolymer membranes (Figure S2, Supporting Information). The free water fraction (φf) in the tercopolymers was estimated using eq 5

(4)

Similarly, the mole percentages of PEGMA and PDMAEMA in the synthesized copolymers were obtained, and the values are tabulated in Table S3, Supporting Information. The obtained values are close to the feed composition of the monomers used in the synthesis of copolymers with different compositions, indicating effective synthesis of desired polymers. FTIR spectra of the copolymer membranes are presented in Figure 1b. The absorption bands at ∼2930 cm−1 are assigned to the stretching vibration of −CH3 groups in the copolymers. The characteristic peaks at 2870 cm−1 are present because of an asymmetric stretching vibration of the −CH2 group in the copolymers. The absorption bands at 2780 and 1634 cm−1 demonstrate the stretching vibration of C−N in −N(CH3)3 moieties.29 The presence of −CN groups is verified by the characteristic bands at 2247, 1202, and 1035 cm−1 in the respective FTIR spectra. The strong bands at 1725 cm−1 are ascribed to the stretching vibration of −CO groups in the copolymers. The sharp peaks at 1106 cm−1 correspond to the

ϕf (%) =

ΔHm × 100 Qm

(5)

where ΔHm is the melting enthalpy for freezable water in the membrane. The bound water fraction (φb) was further determined by subtracting the free water fraction (φf) from the total water uptake (φ). The obtained φf and φb values for the tercopolymer membranes are given in Table S4, Supporting information. The φf and φb depended highly on the content and chain length of the PEGMA segment in the synthesized copolymers. The bound water fraction enhanced linearly with the increase in chain length and the content of the PEGMA segment. The noticeable increase in φb values from 24.6 to 59.69% is ascribed to the substantial water holding capacity of D


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Figure 2. Water vapor sorption isotherms of the membranes, measured at 25 °C, with different PEGMA chain lengths: (a−c) 300, 500, and 950 Da, respectively. Schematic representation of water uptake with increasing chain length: (d−f) 300, 500, and 950 Da, respectively.

compared to the one achieved for the membrane with 1.6 mol % PEGMA-950 Da. Similar results have been attained for water vapor and other condensable gases where the amorphous poly(ethylene oxide) segment changed with respect to the hydrophobic poly(butylene terephthalate) segment.34−36 The water−polymer interactions become weak at high water vapor activity, and the gain in water sorption entropy at increasing water vapor activity compensates the enthalpy loss, which results in an overall decrease of the solvation Gibbs energies of sorption.34 It can be seen from Figure 2 and Scheme 1 that the polymer structure has an enormous effect on the water vapor sorption of the P(AN-r-PEGMA-r-DMAEMA) copolymers. This is shown schematically in Figure 2d−f. The highest water uptake is observed for the copolymer comprising 2.9 mol % PEGMA950 Da, i.e., the PEGMA-9502 membrane. This increase is more prominent when the water vapor activity is higher than 0.6. Moreover, it is also known that the hydrophilicity of the membranes increases with the increase in chain length and amount of the PEGMA segment, resulting in higher water uptakes.37 The increase in the amorphous content and chain length of the PEGMA segments in the copolymers facilitates superior phase separation. A flexible polymer with lower Tg of the PEGMA segments is more likely to be permeable due to higher mobility. These two phenomena contribute to the absorption of more water.27,36,38 Water vapor sorption isotherms shown in Figure 2a−c are observed when water molecules serve as a swelling agent or form clusters of water molecules.39,40 These isotherms convex to the pressure axis at low activities can be described by a Flory−Huggins equation as given in eq 641

PEGMA and PDMAEMA segments in the synthesized copolymers. TGA data of the tercopolymer membranes are depicted in Figure S3, Supporting Information. Two-step thermal degradation is observed for all of the copolymers with different compositions. The first degradation step induced beyond 220 °C is due to the thermal decomposition of the PEGMA chains of the copolymers. The second-stage degradation initiates at around 420 °C because of the thermal decomposition of PDMAEMA and the main chains of the copolymers.31 With the increase in the PEGMA chain length and content, thermal stability of these copolymers increases up to 250 °C.31,32 Moreover, the broad 2θ peaks observed near 18° in the WXRD patterns of all of the copolymers (Figure S4, Supporting Information) confirm the amorphous nature of the synthesized copolymers. These results represent a sufficiently high thermal stability for gas separation and dehumidification applications. 3.1. Water Vapor Sorption and Diffusivity Measurements. 3.1.1. Sorption. Figure 2a−c shows the water vapor sorption isotherms of the membranes at various relative humidity levels measured at 25 °C. The relaxation phenomenon does not occur, and no hysteresis is observed. The equilibrium isotherms show a substantial upturn at high relative humidity. The copolymers used in this study are rubbery polymers, and these types of sorption isotherms are characteristic of rubbery polymers.14,18,33 There may be localized interactions between water and the PAN segments of the copolymers through hydrogen bonds; however, these interactions are not very strong as compared to those with the flexible PEGMA segments of the copolymers. The water uptake increases with the increase in content and the chain length of the PEGMA segment due to strong polymer−water interactions. In the case of PEGMA-3001, 5001, 9501 membranes, the highest water uptake is found for the copolymer containing 950 Da PEGMA. With the increase in feed amount of PEGMA (i.e., PEGMA-3002, 5002, and 9502 membranes), the water uptake increases sharply, and the highest water uptake of 31% is obtained for the membrane having 2.9 mol % PEGMA-950 Da. This is 33% higher

ji p zy lnjjjj zzzz = ln φ + (1 − φ) + χ (1 − φ)2 jp z k 0{

(6)

where χ is the Flory−Huggins interaction parameter and can be calculated with the help of an empirical power series expression given in eq 7.33 The best fit of these parameters (χ0, χ1, χ2) gives rise to the interaction parameter. E


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Figure 3. Interaction parameters for the water vapor measured at 25 °C at various water vapor activities when the PEGMA content is (a) 1 g and (b) 2 g.

Figure 4. Diffusion coefficients of water vapor measured at 25 °C as a function of (a, b) water vapor activity; and (c, d) the chain length of PEGMA segment.

χ = χ0 + χ1 (1 − φ) + χ2 (1 − φ)2

calculated from the best fit of eq 6 and using eq 7 are shown in Figure 3. For PEGMA-950 Da (1.6 and 2.9 mol %), there is a distinct decrease in χ with increasing water vapor activity, indicating that the sorption becomes more favorable with increasing water vapor activity (Figure 3). The interaction parameter depends on the chain length of the PEGMA segments in the copolymers. It decreases from 3.2 to 2 with an increase in water vapor activity from 0.1 to 0.95 for the membranes having PEGMA chain lengths of 500 and 300 Da with the PEGMA content of 2.7 and 4.5 mol %, respectively. Considering the largest chain length of the PEGMA segment, i.e., 950 Da (1.6 mol % PEGMA content), a 50% decrease in the interaction parameter is observed with an increase in water vapor activity

(7)

The volume fraction of the penetrant absorbed by the polymer (φ) can be calculated using eq 833 φ=

C × (V /22 414) 1 + C × (V /22 414)

(8) 3

−3

where C is the concentration (cm STP cm polymer at STP) and V is the molar volume of water at 25 °C (18.02 g mol−1 divided by 0.9941 g cm−3). 3.1.2. Interaction Parameter χ. The interaction parameter (χ) is usually concentration-dependent; the polymer−water vapor interactions are small when χ > 2 and they become very strong when χ ≤ 0.5.18,33 The interaction parameters F


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Figure 5. Visual representation of MD simulations and the proposed interactions of water molecules with (a) PEGMA-3002, (b) PEGMA-5002, and (c) PEGMA-9502 membranes.

3.1.3. Diffusivity. The kinetic sorption data of the tercopolymers is modeled for the diffusion coefficient using Fick’s second law (eq 9) by considering the water sorption as a result of one-dimensional, isothermal penetrant diffusion into a thin slab of the copolymer and using Fick’s second law.43

from 0.1 to 0.95, indicating strong interactions between the copolymers and the water molecules, ultimately resulting in high water uptake. When the feed amount of the PEGMA segment is doubled, the decline in χ with increasing water vapor activity is more pronounced for similar chain lengths of the PEGMA segments. For the PEGMA-9502 membrane, the interaction parameter has the lowest value of 1.26 when the water vapor activity is 0.95. This can also be confirmed from the highest water uptake compared to that for other copolymers at similar water vapor activity. This means that a slight variation in the PEGMA content while maintaining the chain length has a large impact on the interaction parameter (Figure S5, Supporting Information). MD simulation studies in the next sections explain this phenomenon as a result of the increase in hydrogen bond interactions with increasing water vapor activity due to strong binding of the water vapor molecules with the swollen matrix of the copolymer membrane. However, the exact physical background of the decline in the interaction parameter with increasing water vapor activity is still difficult to explain because χ contains both the enthalpic (interaction) and entropic (swelling) contributions.42 These two phenomena can be distinguished by different behaviors of the diffusion coefficient. Swelling leads to an increase in the diffusion coefficient with activity or concentration due to an increased segmental polymer mobility caused by the penetrant molecules.

Mt =1− M∞

∞

∑ n=0

{

8 t exp −D(2n + 1)2 π 2 2 (2n + 1)2 π 2 l

} (9)

For early time analysis, when Mt/M∞ ≤ 0.6, Mt/M∞ the equation can be expressed as follows 1/2 Mt i Dt y = 4jjj 2 zzz M∞ k πl {

(10)

Figure 4 shows the diffusion coefficients calculated using eqs 9 and 10 for complete sorption and early time analysis, respectively. The diffusion coefficient decreases with the increase in water vapor activity for 2 g PEGMA content. However, in the case of 1 g PEGMA content, the diffusivity increases until the water vapor activity reaches about 0.6. When a > 0.6, the diffusivity decreases with an increase in chain length of the PEGMA segment from 300 to 950 Da. This proves that the exponential increase in water uptake for a ≥ 0.6 is predominantly due to self-aggregation of the water molecules as a result of cluster formation. Similar results have been reported for other G


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Figure 6. Interactions of water molecules with the copolymer and formation of water clusters at relative humidity of (a) 30%, (b) 40%, (c) 60%, (d) 70%, and (e) 90% for a PEGMA-9502 membrane.

copolymers in which the hydrophilic content increased in a series of rubbery polymers.2,25,44 In the case of 2 g PEGMA content, the diffusivity decreases over the entire range of water

vapor activity, suggesting the cluster formation over the whole activity range. This phenomenon can be ascribed to the high content of the PEGMA segment. The results of the state of H


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Figure 7. Relaxed structures and interaction energies of water molecules with the tercopolymers (a) P(AN-r-PEGMA3002-r-DMAEMA), (b) P(AN-r-PEGMA5002-r-DMAEMA), and (c) P(AN-r-PEGMA9502-r-DMAEMA).

The P(AN-r-PEGMA2-r-DMAEMA) tercopolymer has more than one hydrophilic segments, and the water molecules are expected to interact with them. More details of the water interaction with the P(AN-r-PEGMA2-r-DMAEMA) copolymer can be obtained from the interaction energy. We consider various adsorption sites for water molecules such as −CN, −(CH3)N, −CO, −C−O−C−, and alkyl parts of P(AN-rPEGMA3002-r-DMAEMA), P(AN-r-PEGMA5002-r-DMAEMA), and P(AN-r-PEGMA9502-r-DMAEMA). Initially, water molecules are placed at different random positions, and the obtained final conformations are presented in Figure 7. Movement of water molecules toward the final positions at sites with strong interactions is evident from Videos S1−S3, Supporting Information. The interaction energies of the different groups with water molecules are calculated, and the one with the highest adsorption energy in each case is shown in Figure 7. The calculated interaction energies of the water molecules w ith P ( A N-r-PEGMA300 2 -r-DMA EMA), P (AN-rPEGMA500 2 -r-DMAEMA), and P(AN-r-PEGMA950 2 -rDMAEMA) are −10.19, −10.20, and −10.24 kcal mol−1, respectively. It is clear that water has a stronger affinity to the PEO segments and −N(CH3)2 groups present in tercopolymers than with other parts of the polymer, facilitating a fast water vapor transport. 3.3. Separation Performance of the Membranes. 3.3.1. Gas Separation. Table 1 lists the single-gas permeation

water and their quantification in the copolymers corroborate our findings where the amount of bound water increases with an increasing chain length of the PEGMA segment due to the availability of more adsorption sites (Figure S2 and Table S4, Supporting Information). Moreover, the increase in PEGMA chain length presents more free water, especially for the membranes fabricated from 2 g PEGMA content (i.e., PEGMA-3002, PEGMA-5002, and PEGMA-9502). Therefore, the cluster formation is predominant in these membranes with even a minor amount of free water (Figure 4d). This ultimately increases the tendency to enhance the cluster size of the water molecules due to self-hydrogen-bonding interactions (Figure 4c,d). The decrease in diffusivity with increasing water vapor activity verifies the cluster formation, and similar behavior is observed for other types of hydrophilic polymers.25,45−48 3.2. Simulation Studies. MD simulation snapshots of PEGMA-3002, PEGMA-5002, and PEGMA-9502 with 95% relative humidity are plotted in Figure 5. Considering the various water-interacting groups in the synthesized copolymers, the proposed interactions between the interacting sites of the copolymer segments and water molecules are highlighted in the magnified regions. It can be seen that the induced δ+−δ− (dipole−dipole) interactions dominate the interaction with water molecules. Water molecules interact with the cyano, ether, carbonyl, and trimethylamine groups of the P(AN-rPEGMA-r-DMAEMA) copolymers. Complexes of P(AN-r-PEGMA2-r-DMAEMA) and water molecules are stabilized by hydrogen bonds and van der Waals interactions. As seen from Figure 4, these interactions either lead to water clusters or swell the membranes. Further evidence of the formation of water clusters at various relative humidity levels is shown in Figure 6. A linear chain of water molecules is formed when RH < 60% due to membrane swelling. Small linear water clusters containing three to four molecules with a linear structure may also be due to the polymer confinement.49,50 In the case of 60, 70, and 90% relative humidity, we observed 3-cyclic, 4cyclic, 5-cyclic, and 6-cyclic structures, and it is evident from Figure 6 that the size of the water clusters increases with increasing relative humidity. Increasing the cluster size decreases the water diffusivity, as observed in Figure 4a,b. The main driving force for the growth of the cluster size at high relative humidity is believed to be the increase in hydrogen bonds. To support this assumption, we extract the number of hydrogen bonds from the MD simulations. Figure S6, Supporting Information, shows a strong increase in the number of hydrogen bonds with increasing relative humidity. This trend is in agreement with the increase in water vapor sorption (Figure 2).

Table 1. Gas Permeation and Ideal Selectivity of the Tercopolymer Membranes permeability (Barrer)

selectivity (α)

membrane

N2

CH4

CO2

CO2/N2

CO2/CH4

PEGMA-3001 PEGMA-3002 PEGMA-5001 PEGMA-5002 PEGMA-9501 PEGMA-9502 PEGMA-9503

0.096 0.125 0.104 0.3 0.13 0.355 0.7

0.077 0.21 0.14 0.63 0.24 0.95 2.08

1.2 5.8 2.7 15.3 5.5 23 47

12.5 46.4 25.9 51 42.1 64.8 67.2

15.6 27.6 19.3 24.3 22.9 24.2 22.6

results of the tercopolymer membranes measured at 25 °C. The gas permeability is the highest for CO2 followed by CH4 and N2 for all of the fabricated membranes. The observed decrease in permeability for inert gases compared to CO2 is comparable to other PEG-based copolymers reported in the literature.31,32,51,52 The higher CO2 permeability compared to other gases is due to a combination of a higher CO2 diffusivity and solubility coefficient. Moreover, the copolymers contain PEGMA segments with ethylene oxide and PDMAEA I


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Figure 8. CO2 solubility coefficient and diffusivity of the membranes as a function of PEGMA chain length with varying feed contents of the PEGMA segment: (a) 1 g and (b) 2 g. Performance of the tercopolymer membranes relative to the 2008 Robeson permeability/selectivity upper bound for (c) CO2/N2 separation and (d) CO2/CH4 separation.

As seen from Table 1 and Figure S7c, Supporting Information, for all of the fabricated membranes, the CO2 gas permeability increases rapidly with the PEGMA content compared to other gases. At a low molecular weight of PEGMA (300 Da), the CO2 gas permeability increases 5 times with the increase of the content of PEGMA-300 from 4.5 to 8 mol %. This trend becomes more prominent with an increase in the chain length of the PEGMA segment, and the highest CO2 permeability is obtained for the PEGMA-9503 membrane. This can be attributed to the increase in fractional free volume (FFV) of the PEGMA-9503 membranes. It is well known that the FFV increases with the decline in the density of the polymers.26,53 As seen from Table S5, Supporting Information, the density of various-composition tercopolymers is not changed much when the content of different-molecular-weight PEGMA segment is 1 g. However, it decreases with the increase in PEGMA chain length when the PEGMA content is 2 g. This can cause structure loosening, which increases the diffusivity of molecules through the membranes.36,55 Another important reason is that the increase in the number of ethylene glycol groups (known to have high affinity for CO2 molecules due to dipole−quadrupole interactions) with the increase in content and chain length of PEGMA segments increases the solubility coefficient of CO2 molecules in the membranes. We conducted high-pressure CO2 sorption measurements, and the obtained data are presented in Figure S8, Supporting Information. The CO2 sorption increases with pressure with a linear increase at pressure above about 4 bar. The CO2 solubility coefficient does not depend on the content of PEGMA segments; the fluctuation shown in Figure 8a is within the experimental error. However, the solubility coefficient rises with increasing content of the PEGMA segment. The similar

segments with tertiary amine groups, which have a high affinity for CO2 molecules due to dipole−quadrupole interactions between the oxygen atoms of the ether groups (C−O−C) and CO2 molecules.53 As seen from Figure S7, Supporting Information, there is an increase in N2 and CH4 gas permeability with the increase in chain length of the PEGMA segment. Such increase is more prominent for the tercopolymer membranes comprising more than 1 g content of the PEGMA segment. For PEGMA-950 Da there is an almost 3 times rise in N2 gas permeability with increasing content of PEGMA from 1.6 to 2.9 mol %. As expected, the addition of flexible polymer chains like PEGMA increased the permeability. The chain flexibility can be related to inter/intrachain spacing available to gas molecules moving through the membranes. In general, the polymers with increasing content of PEGMA segments are prone to lower Tg values and they are ultimately more permeable to gas molecules.27,32,35,38,54,55 We also prepared membranes with increasing content of PEGMA-950 Da up to 3 g. Although the membrane prepared with 3 g was mechanically not robust compared with other membranes, it was stable enough to measure the gas permeation. The nitrogen permeability for the PEGMA-9503 membrane is more than 5-fold the permeability for the PEGMA-9501 membrane with a similar chain length of the PEGMA segments. In addition, a similar trend is obtained for the methane permeability with increasing PEGMA content. Single-gas permeability for CH4 is higher compared to N2. Although the kinetic diameter of CH4 is larger (i.e., lower diffusivity) than N2, its higher critical temperature (190.6 K) causes a higher solubility, which leads to a higher permeability. This increase in permeability becomes more significant with the increase in PEGMA content and chain length. J


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Macromolecules

Figure 9. Water vapor permeability and selectivity of the membranes as a function of PEGMA chain length when the contents of PEGMA segment are (a) 1 g and (b) 2 g. (c) Comparison of water vapor permeability and selectivity with other reported membranes.

illustrated in Figure 8c,d. The gas separation performance of the PEGMA-9503 membrane approaches the upper bound especially for CO2/N2 separation. This proves that our tercopolymer-based strategy that uses the synergetic effect of incorporating PEGMA and PDMAEMA segments is effective to obtain a novel membrane type with high gas separation performance compared to that of the standard industrial membrane materials. 3.3.2. Water Vapor Permeability and Selectivity. The water vapor permeability and selectivity of the membranes with different contents of the PEGMA segments are shown in Figure 9. The water vapor permeability increases monotonically with the increase in chain length of the PEGMA segments. For 1 g content of PEGMA, the water vapor permeability increases from 48 ± 3 to 132 ± 21 kBarrer on increasing chain length from 300 to 950 Da with PEGMA contents of 4.5 and 1.6 mol %, respectively. However, in the case of 2 g loading content of the PEGMA segment, the water vapor permeability is even higher, and an increase from 85 ± 7 to 174 ± 27 kBarrer is achieved with the same increase in chain length (i.e., 300−950 Da; PEGMA contents of 8 and 2.9 mol %, respectively). As seen from Figure 2, the water uptake increases with an increase in chain length of the PEGMA segment. The highest water uptake and ultimately the water vapor permeability are seen for the PEGMA-9502 membrane. The increase in water uptake is also reported in the literature for other PEG- and PEO-based membranes with increasing content of PEG diglycidyl ether.58,59 Husken and Gaymans found a similar trend in water vapor transport with the increasing content of PEO in PEO-based block copolymer membranes.59 Here also, the high PEGMA content leads to a very high water vapor permeability of the PEGMA-9502 membrane compared to that of other membranes. Although the water diffusivity of the PEGMA-9502 membrane is less than that of other membranes prepared in this study, this

solubility coefficient of these membranes with increasing PEGMA chain length can be explained by the comparable CO2 affinity.56 The diffusion coefficient of CO2 gas rises sharply with the increase in chain length of the PEGMA segments, as shown in Figure 8a,b. CO2 diffusivity is always higher for 2 g content of PEGMA compared to 1 g. A 5-fold increase in diffusivity is attained when the chain length of the PEGMA segment increases from 300 to 950 Da with PEGMA contents of 4.5 and 1.6 mol %, respectively (Figure 8a). However, in the case of 2 g PEGMA content (Figure 8b), the gain in CO2 diffusivity is even higher, and a 6-fold increase in diffusivity is observed when the PEGMA chain length increases from 300 to 950 Da with the PEGMA contents of 8 and 2.9 mol %, respectively. The decline in density of the tercopolymers especially for PEGMA-950 Da causes an increase in the diffusivity due to a looser structure.26,36,54 This leads to a high CO2/N2 selectivity and CO 2 permeability of 47 Barrer for PEGMA-950 3 membranes. The CO2/N2 selectivity rises linearly with the increase in chain length as well as with the content of PEGMA segments due to the sharper increase in CO2 permeability compared to the increase in N2 permeability. However, in the case of CO2/CH4, the selectivity decreases with the increase in chain length as well as with the content of PEGMA segments due to an increase in CH4 permeability. The highest CO2/CH4 selectivity (27.6) is achieved for PEGMA-3002 membranes with a low CO2 permeability of 5.8 Barrer. The membranes with the highest CO2 permeability (PEGMA-9503) present a relatively low CO2/CH4 selectivity of 22.6. It means that the increase in diffusivity is more favorable to increase the CO2 permeability and ultimately the CO2/N2 and CO2/CH4 selectivities.36,57 Therefore, the increase in CO2 permeability is mainly attributed to the increase in diffusivity. The performance of these membranes for CO2/N2 and CO2/CH4 relative to the 2008 permeability/selectivity upper bounds is K


Article

Macromolecules decrease is small compared to the exponential increase in solubility. The water vapor permeability of the developed membranes is high due to the high solubility and diffusivity. The consequence is a very high water vapor/N2 selectivity. However, the selectivity decreases with the increase in chain length as well as with the PEGMA content. As discussed in the earlier section, the N2 gas permeability increases with the chain length of the PEGMA segment due to an increase in gas mobility.60 The results shown in Figure 9c present a combination of very high water vapor permeability and water vapor/N2 selectivity. These results indicate the potential of the tercopolymer-based amphiphilic membranes for dehydration applications like dehumidification and flue gas dehydration.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

CONCLUSIONS In this work, we synthesized a series of PEGMA-based amphiphilic tercopolymers with various chemical compositions via a facile and simple free radical polymerization. Freestanding membranes were solution-cast and investigated for water vapor removal and CO2 separation at ambient conditions. By optimizing the content and chain length of the each of the segments, the membranes could be tuned for water vapor removal or for carbon capture applications. The optimized membranes showed an impressive water vapor permeability of 174 kBarrer and a high CO2 permeability of 47 Barrer along with CO2/N2 and CO2/CH4 selectivities of 67 and 23, respectively. Our findings demonstrate that the content of PEGMA segments and chain length significantly enhanced the water vapor permeability of the copolymer membranes, and a 4-fold increase in water vapor permeability is achieved for the 2.9 mol % PEGMA-950 membrane compared to the 4.5 mol % PEGMA-300 membrane. The amount of sorbed water vapor depends on the polymer structure: high content and the larger chain length of PEGMA segments result in higher solubility and lower diffusivity. MD simulation studies and interaction parameters calculated from sorption isotherms reveal that the interaction between the water vapor and the polymer becomes stronger at higher water uptake. The presence of the PDMAEMA segment not only benefits the intrinsic water vapor permeability but also provides mechanical strength for standalone membranes. An increase in the PEGMA segment content induces the chain flexibility of the copolymer, thereby causing a high CO2 permeability. The simple strategy introduced here to synthesize the tercopolymers through free radical polymerization and the resulting combination of very high water vapor permeability with good selectivity show the potential of P(AN-r-PEGMA-r-DMAEMA) copolymers for dehydration (e.g., air dehumidification and gas dehydration) and CO2 separation.

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tercopolymers; comparison of the interaction parameters; single-gas permeability as a function of chain length; high-pressure sorption isotherms of CO2 (PDF) Structural conformation of water molecules at the highest adsorption energy site for the P(AN-rPEGMA3002-r-DMAEMA) tercopolymer (MP4) Structural conformation of water molecules at the highest adsorption energy site for the P(AN-rPEGMA5002-r-DMAEMA) tercopolymer (MP4) Structural conformation of water molecules at the highest adsorption energy site for the P(AN-rPEGMA9502-r-DMAEMA) tercopolymer (MP4)

ORCID

Faheem Hassan Akhtar: 0000-0001-6367-5011 Luis Francisco Villalobos: 0000-0002-0745-4246 Udo Schwingenschlögl: 0000-0003-4179-7231 Klaus-Viktor Peinemann: 0000-0003-0309-9598 Present Address

Laboratory of Advanced Séparations (LAS), É cole Polytechnique Fédérale de Lausanne (EPFL), EPFL Valais Wallis, Rue de l’Industrie 17, Sion 1950, Switzerland (L.F.V.). §

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research reported in this publication was supported by the funding from the King Abdullah University of Science and Technology (KAUST), Kingdom of Saudi Arabia. The table of contents figure and schematic in Figure 2 were created by Heno Hwang, scientific illustrator at KAUST.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00528. Composition of the reaction mixture; solubility of the tercopolymers; actual compositions of the reactants; states of water (free and bound water) in tercopolymer mermbranes; 1H NMR spectra, DSC heating thermograms, TGA analysis data, and WXRD patterns of L


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