Synthesis of Crosslinked PEG/IL Blend Membrane via One-Pot Thiol–Ene/Epoxy Chemistry

Poly(ethylene glycol) (PEG)-based membranes have obtained considerable attentions for CO2 separation for their promising CO2 separation performance and excellent thermal/ chemical resistance. In this work, a one-pot thiol–ene/epoxy reaction was used to prepare crosslinked PEG-based and PEG/ionic liquids (ILs) blend membranes. Four ILs of the same cation [Bmim] with different anions ([BF4] , [PF6] , [NTf2] , and [TCM]) were chosen as the additives. The chemical structure, thermal properties, hydrophilicity, and permeation performance of the resultant membranes were investigated to study the ILs’ effects. An increment in CO2 permeability (~34%) was obtained by optimizing monomer ratios and thus crosslinking network structures. Adding ILs into optimized PEG matrix shows distinct influences in CO2 separation performance depending on the anions’ types, due to the different CO2 affinities and compatibilities with PEG matrix. Among these ILs, [Bmim][NTf2] was found the most effective in enhancing CO2 transport by simultaneously increasing the solubility and diffusivity of CO2. © 2020 The Authors. Journal of Polymer Science published by Wiley Periodicals, Inc. J. Polym. Sci. 2020

INTRODUCTION Epoxy resins, or called polyepoxides, are a class of polymers formed by monomers or oligomers containing epoxy groups, 1 usually with the presence of crosslinkers, such as amines. The excellent chemical and thermal resistance, good mechanical properties, and notable adhesion of epoxy resins allow them to play important roles in various industrial applications, such as coating, reinforcement, 2,3 and adhesives. [4][5][6][7] Recently, several groups have expanded these materials to gas separation, especially as CO 2 separation membranes, thanks to the facile and rapid crosslinking, tunable chemical structure and the presence of secondary amines. [8][9][10][11] In addition, the used epoxy monomers are mainly focused on poly(ethylene glycol) (PEG)-based materials, which have been considered as one of the most promising materials to overcome the trade-off between gas permeability and selectivity of the polymeric membranes, 12 due to the high CO 2 solubility and solubility selectivity. 13,14 A series of crosslinked PEG membranes based on the epoxyamine reaction for CO 2 capture have been reported. 8,[15][16][17][18][19][20][21] Thanks to the CO 2 -philic ethylene oxide (EO) units, the CO 2 permeability of the neat crosslinked PEG membranes reaches 180 Barrer with high CO 2 /light gas selectivity. 8 Compared with the PEG membranes formed by the classical acrylate homopolymerization, membranes prepared by using the epoxy-amine reaction exhibit around 60% higher CO 2 permeability, 13 due probably to the more flexible crosslinking network. Similarly, Patil and coworkers employed diamine JEFFAMINE and a low-molecular-weight epoxy monomer (145 g mol −1 ) to prepare PEG-based membranes with promising CO 2 separation performance. 11 Further analysis found out that higher gas permeability can be obtained by decreasing crosslinking density (increased diffusivity), which can be accomplished by increasing the monomer length, 11,22 or adding low-molecular-weight CO 2 -philic additives into the crosslinked membranes. In the first case, however, due to the highly crystalline trend between long PEG chains, the maximum monomer molecular weight is suggested to be limited within~1000-1500 g mol −1 . 14,23,24 The increased monomer molecular weight seriously hinders the crosslinking reactivity, therefore higher reaction temperature or longer reaction time is required. For examples, Liu et al. used a three-stage curing method involving a precured stage at 80 C for 3 h, followed by another 2 h at 100 C and lastly 0.5 h at 120 C. 8 Even with low-molecular-weight amine functionalization crosslinkers, the preparation was still complicated compared to other crosslinking mechanisms, 16,25 like the ultraviolet (UV)induced acrylate homopolymerization.
Physically blending with low-molecular-weight additives is another widely used approach to enhance the gas transport properties of PEG-based membranes. With the presence of the liquid additives, the concentration of reactive functional groups inside monomer mixture is further diluted, which increases the difficulty to complete the reaction. To address this issue, Shao and coworkers employed a two-step method to incorporate free-PEG into the crosslinked PEG membranes, where the PEG membranes were prepared via a thermalinduced epoxy-amine reaction, then the membranes were immersed into aqueous PEG solution to obtain the PEGimbedded membranes. 15,25 A CO 2 permeability of up to 1301 Barrer has been reported accompanied with a CO 2 /H 2 selectivity of 13 after imbedding free PEG into the crosslinked PEG membranes, which makes it very promising for various CO 2 separation applications. However, the multistage membrane preparation procedure is rather time-consuming: at least 4 days was required to obtain one batch of membranes. Besides, the amount of free liquid additives inside membranes may not be controlled precisely, making it challenging for the membrane upscaling and industrial applications. Moreover, fast membrane preparation method is of great importance to promote the industrial potential of the PEG-based membranes.
Thiol-epoxy is well known for its more controllable crosslinking density, high reaction rate, and moderate reaction condition. 26,27 In this work, a one-pot synthesis method using thiol-epoxy reaction was developed to fabricate crosslinked PEG-based membranes with the presence of ionic liquids (ILs) as liquid additives. Another classical thiol-click reaction, thiolene, was also used to study the effects of the crosslinking reactions. ILs, the salt containing organic cation and inorganic/organic anion, which usually have a melting point lower than 100 C, have been well accepted as membrane additives to improve the CO 2 permeation properties for their high CO 2 solubility. Various studies have incorporated highly CO 2 -philic "free" ILs into polymeric membranes, and the results shows that the presence of ILs greatly enhances the CO 2 separation performance of highly crystallized polymers or crosslinked polymers, such as poly(ILs) 10,28 or PEG-based polymers. 24,[29][30][31][32] In addition, the numberless combinations of various anions and cations and ILs' negligible vapor pressure offer the great opportunity for further improvement and excellent long-term stability. 33,34 Therefore, the addition of ILs was taken as an approach to improve the CO 2 permeation in the crosslinked membranes. In order to study the effect of the ILs of different anions, four ILs in a series with the same cation [Bmim] + , 1-butyl-3-methylimidazolium), but different anions were selected and incorporated into the optimized crosslinked PEG-based polymeric matrix. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and water uptake experiments were conducted to study the chemical structure and evaluate the thermal properties and hydrophilicity of the resultant membranes. The CO 2 and N 2 transport properties of the aforementioned membranes were tested to investigate the effects of the crosslinked network structure and the addition of ILs on the CO 2 separation performance.

Membrane Preparation
Membranes were fabricated by a crosslinking method similar with those reported in literature 13,25 (Fig. 2). A brief description is presented below to the readers' convenience: PEGDA, PEGDGE, equivalent crosslinker 3T (mole ratio), a calculated amount of ILs, and 0.01-0.1 wt % HCPK were mixed in a glass vial for a few minutes. The content of ILs w IL (%) is calculated based on the mass ratio of IL to the total membrane mass in eq 1: The well-mixed monomer mixture was transferred into a vacuum oven to remove the bubbles. After that, the mixture was sandwiched between two quartz plates separated by a spacer to control the thickness of membranes (~200 μm). UV irradiation with a wavelength of 365 nm (UVLS-28; Ultra-Violet Products Ltd., Cambridge, UK) was then applied to these mixture. To ensure a complete reaction, the time of UV crosslinking is set as In this work, all membranes without ILs are systematically named according to the mole ratio between three monomers. For example, the "1-0.25-1.25" presents the membrane with 1 mol PEGDA acrylate group, 0.25 mol epoxy group and 1.25 mol thiol group, which needs 0.4745 g PEGDGE and 0.1786 g 3T per gram PEGDA. Then, 1.6531 g IL is required to prepare this membrane with 50 wt% ILs.

Characterization
A Thermo Nicolet Nexus FTIR spectrometer (NicoletTM iSTM 50; Thermo Fisher, Oslo, Norway) with a smart endurance reflection cell was used to study the chemical structure of the resultant membranes and reactants in this work. All membranes or reactants were scanned in the range of 650-4000 cm −1 .
Thermal stabilities of these investigated membranes were tested by a TGA (TG 209 F1 Libra; NETZSCH, Selb, Germany). Around 10-20 mg samples were used in a typical TGA test, with a temperature range of 25-700 C and a heating rate of 10 C min −1 . N 2 is employed to prevent oxidation of samples. In addition, a DSC (DSC 214 Polyma; NETZSCH) was used to investigate the glass transition temperature of the resultant membranes and reactant. Samples of around 10 mg were collected in a standard aluminum pan covered by a proper lid and heated at the rate of 10 C min −1 under N 2 atmosphere.
Water uptake tests of the membrane samples were employed to evaluate the hydrophilicity of membranes. To completely remove the moisture, all membrane samples were vacuumed at 40 C overnight. Dried samples with known weights were placed in a water desiccator at room temperature. The weights of these membrane samples were measured with a certain interval until they were stabilized. The water uptake (w H2O ) was calculated using eq 2: where w f and w 0 are the weight of the wet sample and dried sample, respectively.

Gas Permeation Test
Gas permeation tests were conducted at room temperature with a feed pressure of 2 bar to evaluate the CO 2 and N 2 transport properties by the constant-volume-variablepressure method. The permeability (P) of CO 2 or N 2 was calculated by eq 3.
where P is the permeability, p d and p u represent the downstream and upstream pressure, respectively, t refers to time, V d is the downstream volume, A means the effective permeation area of the membrane, R and T are the ideal gas constant and temperature, respectively, and l is the thickness of the membrane. The leakage rate dp d =dt ð Þ leak was measured by isolating the membrane cell at vacuum condition with air for a certain period. The thicknesses of all membranes were measured by an ABS Digimatic Indicator from Mitutoyo (Suzhou, China). The average thicknesses were given based on more than 10 measurements for each membrane. The permeability was the average values obtained based on, at least, two samples with a relative error of less than 10%. The ideal selectivity α CO2=N2 was calculated using eq 4: Furthermore, the diffusivity (D) was determined by the timelag (θ) method from the single gas permeation test, and the calculation equation is shown below: The solubility (S) was calculated by the permeability (P) and the diffusivity (D) based on the solution-diffusion mechanism:

Material Properties
The FTIR analysis was employed to confirm the thiol-ene and thiol-epoxy reactions between PEGDA, PEGDGE, and the crosslinker. The spectra of crosslinker, PEGDA, and the resultant membrane (1-0-1) are presented in Figure 4(A). The reactive thiol (2570 cm −1 for S─H) and acrylate groups (1630 cm −1 for C═C, 1410 and 830 cm −1 for H─C═) are clear in the spectra of crosslinker and PEGDA, respectively, but all of them disappear in the spectra of the resultant membranes.
After the reaction, the peaks assigned to C─S bond shifts to the right side (1144-1100 cm −1 ), which is in agreement to the literature. 35 In addition, the other peak of PEGDA at around 850 cm −1 becomes boarder and bigger in the resulted membranes, which may be overlapped with the peak related with the newly formed C─S bond. 36 These results suggest that the acrylate reacts with thiol and forms C─S groups under experiment conditions, confirming the completion of the thiol-ene reaction. On the other hand, in addition to the change of acrylate and thiol groups, the peak for epoxy (~904 cm −1 ) has also disappeared in all PEGDA-PEGDGE-3T membranes, suggesting that the epoxy groups are consumed by the thiol groups, which is the only possible reactant in the system. Considering that the newly formed bonds in thiol-ene and thiol-epoxy are exactly the same, to distinguish between them via FTIR spectra is impossible and unnecessary. Hence, the membranes with different PEGDGE/PEGDA ratios have identical spectra, as present in Figure 4(C).
Since three different ILs have been embedded into the crosslinked PEG membranes, the states of ILs inside the membranes may be different, which influence the membrane properties and separation performance. The membranes with ILs were characterized by FTIR analysis, taking membranes containing 50% IL as samples, as shown in Figure 4(D). The FTIR The thermal stabilities of all resultant membranes in this work were studied by TGA, as shown in Figure 5. The PEGDA-3T (1-0-1) membrane shows a one-stage decomposition behavior, starting at 320 C, while the membranes containing PEGDGE decompose at a lower temperature, and the more PEGDGE it contains, the more easily it decomposes. For the membrane containing the highest amount of PEGDGE (1-0.75-1.75), a two-stage decomposition behavior is observed; the first decomposition begins at 155 C and the second one appears at around 300 C, as can be seen in the insert in Figure 5(A). The weight loss during the first stage is up to 5 wt %, which may be a small amount of unreacted PEGDGE due to the less reactive thiol-epoxy under experimental conditions in this work.
The thermal stabilities of the optimized crosslinked PEG membranes (represented by 1-0.25-1.25), three IL-containing blend membranes and the corresponding ILs are displayed in Figure 5(B-D  show that the membranes containing PEGDGE may not be suitable for a high-temperature process (≥150 C). Generally, most CO 2 separation membrane processes are operated at a temperature range of up to 80 C, so these resultant membranes have satisfactory thermal stabilities for this application.
As the glass transition temperature of polymers (T g ) is the temperature where polymer chains change from "restricted" to "flexible," for gas separation, a lower T g of a membrane generally indicates a higher gas permeation property. To investigate the effects of incorporating epoxy monomers and ILs on T g of the resultant membranes, a DSC analysis was conducted.
The obtained curves are shown in Figure 6, and the T g s are presented in Table 1 The water uptake experiments were conducted at room temperature for all resultant membranes. The weight gains of the membrane samples with time are presented in Figure 7. It is clear that all crosslinked PEGDA-PEGDGE-3T membranes have water uptakes of around 40 wt. %, and the PEGDGE/ PEGDA ratio seems to have little effect on the material hydrophilicity. Considering that both monomers are PEG-based materials, it is reasonable to assume that the resultant networks have similar hydrophilicity.
However, the blend membranes containing different ILs present different water uptake and generally, decreases with the increasing IL amount, especially at the low IL loading range. Figure 7(B-D) presents the water uptake of various IL contents in the crosslinked membrane (1-0.25-1.25). As it can be seen, when the ILs amount is over 37.5 wt %, the water uptake is almost unchanged. Since the ILs used in this work are less hydrophilic compared with the PEG-based materials, the initial trend of declining in water uptake is expected. When the loading of ILs inside membranes increases to a certain level (>37.5 wt %), the apparent hydrophilicities of the membranes decrease to a value close to the intrinsic hydrophilicity of the ILs; hence, the water uptake of the membranes remains nearly constant with the further increase in the IL amount.
Based on the chemical structures, the hydrophilicities of the studied ILs should be different and follow the order: The water uptake of the blend membranes follows the same order as the neat ILs, as can be seen in Figure 7.

Gas Permeation Properties
Effect of PEGDGE Addition The CO 2 and N 2 permeation properties of the PEGDGE-PEGDA-3T membranes were tested by single gas permeation tests at a feed pressure of 2 bar and room temperature to study the effect of PEGDGE addition. The CO 2 permeability, ideal CO 2 /N 2 selectivity, the gas diffusivity and solubility of the PEGDGE-PEGDA-3T membranes with different PEGDGE/ PEGDA ratios are presented in Figure 8.
The CO 2 permeability increases from 100.3 to 134.2 Barrer with a nearly unchanged CO 2 /N 2 selectivity of around 67 by adding a small amount of PEGDGE (1-0.25-1.25), which may be explained by the disturbance of the crosslinked PEG network due to the addition of the PEGDGE, leading to a rearranged crosslinking network and enhanced gas transport properties. The increase in diffusivities of CO 2 and N 2 [as shown in Fig. 8(B) ] also suggests the increase in the free volume of the crosslinked PEG membranes. However, the CO 2 permeabilities and gas diffusivities decrease with the further increasing PEGDGE/PEGDA ratio. This trend is believed to be caused by the reduced overall chain length between the crosslinked sites since PEGDGE has shorter polymer chains (around eight EO units) compared to those of PEGDA (around 13 EO units). On the other hand, the solubility of CO 2 and N 2 in the PEGDGE-PEGDA-3T membranes is rising with the increasing PEGDGE/PEGDA ratio, which can be explained by the increased EO unit content inside these membranes. Therefore, the crosslinked networks with higher PEGDGE/PEGDA ratio become more rigid (less diffusive) with slightly increased CO 2 solubility. The CO 2 /N 2 selectivity slightly The T g of Membranes Presented in Figure 6 Membranes T g ( C) decreases with the increasing amount of PEGDGE in membranes, but the value (56-66) is still reasonably high and within the range of the inherent selectivity of typical PEGbased membranes. 8 Since the membrane containing the lowest amount of PEGDGE (1-0.25-1.25) holds the highest CO 2 permeability and CO 2 /N 2 selectivity, it is chosen as the optimized polymeric matrix for further study.

Effects of IL Addition
Gas permeation tests of the crosslinked PEG membranes (1-0.25-1.25) containing different ILs were performed to evaluate the effects of IL addition, as presented in Figure 9. It is clearly seen that ILs with different anions show distinguishing effects on the gas permeation properties.  43 compared with PEG-based materials (around 40-60). 8 Another possible reason is that ILs may act as the plasticizers in crosslinked PEG membranes, which could lower CO 2 selectivity. 42 In addition, different anions have distinct effects on CO 2 /N 2 selectivity. Generally speaking, in this work, the order of CO 2 /N 2 selectivity with a low IL amount (≤37.5%) in membranes is

CONCLUSIONS
In the present work, crosslinked PEG-based membranes and PEG/ILs blend membranes were fabricated using one-pot thiol-ene/epoxy click chemistry. The crosslinking reactions were confirmed by FTIR analysis. The crosslinked PEG membrane was systematically evaluated using various characterization methods. The monomer ratios were optimized to enhance gas transport properties. Up to 50 wt % of ILs can be incorporated into the PEG/ILs blend membranes. The anions in ILs have significant effects on membrane structure and hence the gas separation performance. PEG membranes containing [Bmim][NTf 2 ] shows the highest CO 2 permeability due to the high CO 2 affinity of [NTf 2 ] − anion and the possible micro-separation in the membranes. On the other hand, the addition of [Bmim][TCM] is more beneficial for increasing CO 2 solubility than diffusivity, while the [Bmim][PF 6 ] has a negative influence on both CO 2 solubility and diffusivity.
The fabrication of the crosslinked PEG/IL membranes via the one-pot thiol-ene/epoxy click chemistry is a facile and fast method. The pairing of ILs and polymer has significant influences on the membrane material properties as well as the CO 2 separation performance. The properties of final materials are tunable by various PEG monomers and ILs, which may be practical and advantageous for feasible optimization of the blend membranes. Even the performance in this work was not greatly improved by the addition of physical-absorption ILs in this work, the addition of more CO 2 -selective ILs (e.g., a chemical-absorption type of IL with CO 2 /N 2 selectivity of >40) may be a promising approach worthy of further research to promote the CO 2 permeability and the selectivity of PEGbased membranes simultaneously.