Article pubs.acs.org/IECR
Effect of Fabrication and Operation Conditions on CO2 Separation Performance of PEO−PA Block Copolymer Membranes Yuanyuan Wang,†,‡ Hongyu Li,*,†,‡ Guangxi Dong,†,‡ Colin Scholes,‡,§ and Vicki Chen†,‡ †
UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia § School of Chemical & Biomolecular Engineering, The University of Melbourne, Melbourne, VIC 3010, Australia ‡ Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), The University of Melbourne, Carlton, VIC 3010, Australia S Supporting Information *
ABSTRACT: Poly(ethylene oxide)- (PEO-) based block copolymer membranes have great potential for use in CO2 separation because of their excellent selectivity and moderate permeability. Whereas numerous studies have focused on the permeation performance of such membranes, the influence of the microphase-separated structures on the gas transport is not well understood. This study examined the phase structure of PEO−polyamide (PA) (commercial name, Pebax) block copolymer membranes by scanning probe microscopy (SPM) imaging and thermal analysis. The membranes with the irregular and more disordered phase-separated structure, such as Pebax-1074 membranes, that had longer PA chains and were made using a faster sol-to-gel transition process resulted in higher CO2 permeability than the membranes with the more ordered phase structure. The CO2 solubility coefficient profile as a function of pressure in the Pebax membranes with dual-mode sorption characteristics indicated the involvement of a glassy hard phase in CO2 transport, particularly at low pressure. The effects of temperature on gas transport and separation performance for a CO2/N2 gas mixture were also investigated.
1. INTRODUCTION The separation and capture of CO2 are essential for a wide range of industrial processes, such as the purification of natural gas and biogas; the production of syngas, hydrogen, and ammonia; the capture of CO2 from flue gas, and the packing of food.1 The majority of commercially available polymeric membranes for gas separation are made of glassy polymers, for example, polyimide (PI), polyamide (PA), polycarbonate (PC), polysulfone (PSf), poly(phenylene oxide) (PPO), cellulose acetate (CA), and their derivatives.1,2 The separation of gases in glassy polymers is based on size sieving by the rigid chain structures of the polymer matrix. Although the mentioned membranes have good CO2 selectivities over larger molecular gas species, their CO2 permeation rates are not sufficient for many large-volume industrial-scale applications such as those involving syngas and flue gas in power plants. On the other hand, membranes made of rubbery polymers, such as poly(dimethylsiloxane) (PDMS), have higher CO2 permeabilities because of their flexible chain structures, but their low selectivities represent a significant drawback for many applications. To deal with this trade-off, earlier studies3−5 suggested that polymers containing CO2-philic functional groups, such as ethylene oxide (EO), which interacts strongly with CO2 molecules through dipole−quadrupole interactions, exhibit excellent CO2 selectivities over light gases, as their CO2 permeation rates are not significantly compromised by the strong interactions.6,7 Approaches for introducing poly(ethylene oxide) (PEO) segments into the polymer structure of gas membranes include physical blending,5,8−10 crosslinking,6,11−15 and synthesis of PEO-based block copolymers © XXXX American Chemical Society
such as PEO−PI (polyimide), PEO−PA (polyamide), PEO− PBT [poly(butylene terephthalate)], PEO−PU (polyurethane), PEO−PUU [poly(urethane urea)], and PEO−PE (polyester).3,4,16−22 The advantages of PEO-based block copolymers include robust mechanical strength and chemical and thermal resistance attributed to a semicrystalline hard phase. The suppression of PEO crystallization in the soft phase due to the presence of alternative block structure also contributes to better separation performance. A specific aspect of PEO-based block copolymers is the complexity of the microphases that can exist in the polymeric matrix, including two crystalline and two amorphous phases formed by rigid (hard) and flexible (soft) segments, respectively, as well as a blended composition around the interfaces of the two amorphous phases. This phase-separated microstructure makes such materials flexible and versatile for use in forming thin films, selective coatings for composite membranes,23 and blends with other materials to achieve required mechanical and separation performances. Despite many successful syntheses of PEO-based block copolymers with demonstrated superior CO2 separation performances, the influence of the phase-separated microstructure, particularly in thin films with thicknesses of less than 30 μm (relevant to thin-film composite membranes), on the gas transport is not well understood. Gas transport in PEO-based block copolymer membranes has commonly been considered Received: April 1, 2015 Revised: June 29, 2015 Accepted: July 1, 2015
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DOI: 10.1021/acs.iecr.5b01234 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
The major membrane-casting parameters (polymer concentration, solvent evaporation temperature, and membrane thickness) were systematically evaluated to correlate the fabrication conditions with the characteristics of the membrane morphology and gas permeation. Through the experimental measurement of CO2 permeability and solubility coefficient over a range of pressures, the characteristics of the contributions of the soft phase (PEO) and the hard phase (PA) to CO2 transport were assessed. Furthermore, the effects of temperature on CO2 permeability and selectivity and the permeation performance for a gas mixture were also evaluated.
to occur in the continuous amorphous PEO soft phase, whereas the semicrystalline hard phase has mostly been regarded as impermeable.21,24−26 However, the existing evidence suggests that this assumption might not be wholly appropriate in some cases. The amorphous glassy component, the irregularity of the phase architecture, and the presence of an interphase structure between the soft and glassy hard phases could also affect the overall gas separation performance.3,27 For example, Okamoto et al.3 observed differences in the CO2 separation performances of membranes made with copolymers containing similar amounts of soft segments but different hard segments and found that the hard-phase structure and the arrangement between the PEO segments (soft phase) and the polyimide segments (hard phase) played a considerable role in the permeation properties for some phase-separated block copolymers. Apart from the architecture of the hard phase of the block copolymer, gas transport can also be affected by the intersegmental arrangement of the same material. For example, the buildup of hydrogen bonding between the ether oxygen and NH groups in PEO−PA and PEO−PU block copolymer membranes could lead to penetration between the amorphous soft and hard phases and result in reduced microphase separation and a more compact polymer chain structure.21 The morphology of polymeric membranes can be significantly affected by the fabrication conditions, including polymer solution concentration, solvent composition, solvent physical properties (strength, volatility, and size), solution temperature, and desolvation and gelation kinetics.28 Many studies that evaluated the effects of fabrication parameters on the microscopic structures of membranes focused o microporous membranes for microfiltration (MF) and ultrafiltration (UF) processes29−32 and the integrally skinned membranes of glassy polymers33−35 fabricated in phase-inversion processes that involved relatively rapid solvent−nonsolvent exchange. In contrast, the fabrication of dense films involves evaporation of the solvent from the polymer solution in the top layer, in which case the polymer solution experiences a sol−gel transition prior to polymer packing and condensation. As the solvent evaporation (membrane drying) process could take much longer than the phase inversion process, polymer segment rearrangement toward ideal cross-linking could occur during the condensation process to form a more orderly structure. However, few published studies have investigated the influence of fabrication conditions on the membrane structure of dense membranes in terms of separation performance, particularly for segmented block copolymer membranes. Barbi et al.27 reported different nanostructures [examined by smallangle X-ray scattering (SAXS)] for PEO−PA and poly(tetramethylene oxide) (PTMEO)−PA block copolymer membranes prepared with different solvents (1-butanol vs cyclohexanol) that resulted in different gas-permeation properties. To further elucidate gas-transport mechanisms in PEO-based block copolymer membranes and to evaluate the influence of the microphase-separated structure on gas transport, different types of Pebax membranes were studied. [Note that Pebax is the trade name of its manufacturer, Arkema, for polyether (PE)-block-polyamide (PA) copolymer (PEBA).] Pebax membranes can be prepared using low-cost and environmentally friendly alcohol mixtures with water, making them attractive for large-scale fabrication.
2. EXPERIMENTAL SECTION 2.1. Materials. Pebax-1657 and -1074 grade polymer pellets (purchased from Arkema Australia) were used for the preparation of polymer solutions for the casting of dense membranes. Both grades contained the same PEO soft phase and linear aliphatic PA hard phase with different amounts of methylene bridges. The chemical structure of Pebax in general is shown in Figure 1. Pebax-1657 contains 60 wt % PEO and 40
Figure 1. Chemical structure of Pebax-1657 and -1074 block copolymers. The subscripts m and n represent the chain lengths of the soft and hard phases, respectively, whereas l represents the total backbone length of the block copolymer.
wt % polyamide 6 (PA6) (x = 6), whereas Pebax-1074 contains 55 wt % PEO and 45 wt % polyamide 12 (PA12) (x = 12). The total molecular weight in each case is roughly 50000,23 with m = 35 and n = 9 for Pebax-1657.36 Pure N2 and CO2 gases and a CO2/N2 (20:80 mol/mol) gas mixture used for gas-permeation tests were provided by Coregas Australia. All materials were used as supplied without further purification. 2.2. Preparation of Dense Membranes. Pebax dense membranes were prepared using a solvent casting method with three distinct steps: polymer solution preparation, solution casting, and solvent evaporation. Solutions of Pebax-1657 were prepared by dissolving polymer pellets in an ethanol/water mixture (2:1 w/w) under heating at 65 °C for 2 h. Solutions of 1−6% polymer were homogeneous at room temperature without gelation. Solutions of Pebax-1074 were prepared with a protocol developed in this study using a method distinct from those used in previous studies. Typically, researchers use a single organic solvent such as 1-butanol or N-methyl-2-pyrrolidone (NMP).24,37 In this work, however, to maintain a solvent system similar to that used for Pebax-1657, Pebax-1074 pellets were dissolved in a solvent made of a 1-propanol and water mixture (2:1 w/w) under heating at 90 °C for 24 h. A concentration of 1% polymer remained in solution at room temperature without gelation, whereas Pebax-1074 solutions with higher concentrations rapidly formed gels at room temperature, indicating a higher cross-linking tendency in solution for Pebax-1074 polymer than for Pebax-1657 polymer. Casting of membranes with 1−6% Pebax-1657 and 1% Pebax-1074 was performed after the solutions had been cooled to room temperature and degassed overnight by pouring predetermined volumes of solution into clean Teflon Petri B
DOI: 10.1021/acs.iecr.5b01234 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. Constant-pressure gas-permeation measurement rig.
dishes. Membranes with Pebax-1074 polymer at concentrations of 3% and 5% were cast by maintaining the solution at 60 °C (to avoid gelation of the solution at lower temperatures) for degassing (2 h) and then pouring the solution into clean Teflon Petri dishes. The drying of the membranes was controlled by placing the partially covered Teflow Petri dishes containing Pebax solution in an enclosed cabinet with limited air flow at room temperature or in an oven with controlled air flow. The cabinet drying time at room temperature was artificially set at 7 days, whereas oven drying time at 35 °C was set at 24 h. The control of the air flow in both the cabinet and the oven were designed to ensure slow evaporation of the solvent and a sufficient time for polymer chain relaxation during the drying process to avoid segment entanglement. All dried Pebax-1657 and -1074 membranes were placed in a 30 °C vacuum oven for at least 3 days for conditioning and removal of residual solvent. To evaluate the influence of the membrane thickness on the permeation performance, membranes of different thickness were fabricated by pouring different amounts of solution into different Petri dishes for drying. The thicknesses of the dried membranes were measured with a digital micrometer (with an accuracy to 0.5 μm). Scanning electron microscopy (SEM) images of the cross-sectional areas of the membranes were also obtained to observe the cross-sectional structure and confirm the film thickness. 2.3. Membrane Characterization. The thermal properties of the membranes, in terms of their glass transition temperatures (Tg) and melting temperatures (Tm), were analyzed by differential scanning calorimetry (DSC) using a TA Instruments DSC2010 instrument. The membrane samples were dried at 35 °C in an oven overnight before being subjected to thermal analysis. The analysis involved placing a 5-mg sample (sealed in an aluminum pan) in the device oven with an empty pan used as the reference, cooling the samples to −100 °C using liquid nitrogen, and then heating the samples at 10 °C/min to 250 °C for Pebax-1657 and 200 °C for Pebax-1074. The purge gas was N2 at a flow rate of 10 mL/min. The Tg value of each membrane was determined from the midpoint of the change in the heat capacity, whereas the Tm value was estimated from the peak point in the melting endotherms. The degree of crystallinity (Xc) in both the soft and hard phases was calculated using the equation
Xc =
ΔHf Wi ΔHf*
(1)
where the melting enthalpy (ΔHf) was estimated from the area of the melting peak in the DSC curves, Wi is the weight percentage of the soft or hard phase in the copolymer, and the enthalpies of the pure crystalline phases (ΔHf*) were taken from the literature (PEO = 166.4 J/g, PA6 = 230 J/g, and PA12 = 246 J/g38,39). A membrane phase image was obtained by scanning probe microscopy (SPM) (Bruker Dimension Icon SPM). The membrane surface was raster scanned with a silicon nitride probe under a peak force in tapping mode with a ScanAsyst automatic control system. A phase image with a size of 1 μm2 was obtained at a scan rate of 0.977 Hz. Scanning electron microscopy (SEM) imaging of the membrane cross sections was conducted with a Hitachi SEM 3400 instrument. The membrane samples were coated with gold before scanning. The density of each membrane was estimated by measuring the weight of a 2.5 × 2.5 cm2 sample with a microbalance and estimating the volume of the sample with the measured thickness (average of three measurements at different locations on the sample). 2.4. Sorption Measurement. The measurement of CO2 solubility in Pebax-1657 and -1074 membranes was conducted by the gravimetric sorption method using a GTI Instruments gravimetric sorption analyzer equipped with a Cahn 2000D balance. The measurement procedure involved drying approximately 50−100 mg of the membrane at 35 °C in a vacuum oven for 30 min before measuring the initial weight; exposing the sample to CO2 gas at stepwise preset pressures of 15, 30, 60, 100, and 200 psi; and maintaining each pressure for 6 h to obtain the equilibrium gas amount absorbed in the membrane. The weight of the sample was continuously monitored by an electronic balance, and the buoyancy was corrected by measuring the sorption of helium gas. The corrected weight difference in the equilibrium state gave the CO2 weight absorbed in the membrane. The CO2 solubility in the membrane (on a volume basis) was estimated as the ratio of the volume of CO2 absorbed in the membrane, obtained by converting the gas weight into a volume at standard conditions (14.5 psi and 273.15 K), and the C
DOI: 10.1021/acs.iecr.5b01234 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 1. Thermal Properties of Pebax-1657 and -1074 Dense Membranes Fabricated under Different Conditions and Comparison with the Literature soft phase material Pebax-1657 film 1%, ambient dry 3%, ambient dry 6%, 35 °C oven dry 3%, 30 °C dry Pebax-1657 pellets Pebax-1074 film 1%, 35 °C oven dry 3%, 35 °C oven dry Pebax-1074 polymer Pebax-1074 pellets
hard phase
thickness of film (μm)
density of film (g/cm3)
Tg (°C)
Tm (°C)
Tm (°C)
crystallinity (wt %)
41 ± 0.5 28 ± 0.5 98 ± 0.5 60−100
1.07 ± 0.013 1.05 ± 0.019 1.03 ± 0.005 − 1.147
−57 −57 −53 −53
12 11 13 7
206 206 206 209
43 44 36 38
0.95 ± 0.009 0.92 ± 0.024 1.09 1.064
−58 −61 −55
53 ± 0.5 19 ± 0.5
8 6 11
155 152 156
33 42 40
ref this this this 40 this
work work work work
this work this work 38 this work
Figure 3. (a,b) Phase images and (c,d) SEM cross-sectional images of (a,c) Pebax-1657 membrane made with 3% solution dried at room temperature and (b,d) Pebax-1074 membrane made with 1% solution dried in a 35 °C oven.
to ensure that the stage cut (i.e., the ratio of the permeate flow rate to the feed flow rate) was less than 1% to avoid concentration polarization. The flow rate of gas permeate was measured with a microbubble flow meter at atmospheric pressure. In the tests with mixed gas, the composition of the permeate was analyzed with a gas chromatograph (Shimadzu GC2400). For each test, the permeation rig was purged a few times with the feed gas at a pressure of less than 10 psi before the required pressure was set. The permeate flow rate was recorded at least 1 h after a stable value had been achieved. Gas permeability [P, with units of Barrer, where 1 Barrer = 10−10 (cm3·cm)/(cm3·cm2·s·cmHg)] was estimated from the measured permeate flux and transmembrane pressure difference using the equation
volume of the membrane, obtained using the membrane densities of Pebax-1657 and Pebax-1074 measured in this study. 2.5. Permeation Measurements. The measurement of gas permeability was performed using a constant-pressure permeation rig as shown in Figure 2. Gas from the cylinder was fed to the membrane cell from a Millipore high-pressure stainless steel filter holder with an effective membrane area of 9.6 cm2. The membrane cell was placed in a temperaturecontrolled oven to maintain a constant temperature during the permeation tests. The feed pressure was controlled with a gas regulator, monitored with a pressure transducer, and recorded with a Labview data-logging program. In the tests with pure gases, the retentate outlet of the membrane cell was closed after purging, whereas in the tests with mixed gases, a sufficient retentate flow rate was maintained D
DOI: 10.1021/acs.iecr.5b01234 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. Relationships of CO2 permeability and CO2/N2 selectivity with the polymer solution concentration and drying conditions for Pebax membranes: (a) Pebax-1657, (b)Pebax-1074.
P=
Jl AΔp
glassy transition temperature of the PA phase, as well as the estimated crystallinity of the hard phase, are presented in Table 1. Comparing the Tm and Tg values of the dense membranes fabricated in this study with the reported properties of the polymer and dense film for the same grade of Pebax-1657, shown in Table 1, the membranes made with solutions with low concentrations of 1% and 3% and with smaller thicknesses exhibited higher crystallinity of the hard phase than the thick membrane made with the higher-concentration solution. The PA6 crystallinity of the thick film made with 6% Pebax-1657 solution was very similar to that reported in a previous study,40 also estimated for a dense film. On the other hand, the density of the dense film decreased slightly with increasing polymer solution concentration, and the ratio of the film density to the polymer pellet density for Pebax-1657 (at 0.92) was higher than that for Pebax-1074 (at 0.89). The presence of a microphase-separated nanostructure can also be demonstrated by the phase images obtained by SPM scanning for block copolymer membranes. As the SPM images of the membranes made of the same grade of Pebax pellets under different fabrication conditions are very similar, images of Pebax-1657 and -1074 membranes of similar thicknesses
(2)
where J denotes the permeate flow rate (cm /s); Δp is the transmembrane pressure difference (cmHg); and l and A are the membrane thickness (μm) and area (cm2), respectively. The selectivity (α) between binary gases was estimated as 3
αi / j =
Pi Pj
(3)
where i and j represent the different gas species.
3. RESULTS AND DISCUSSION 3.1. Thermal Properties and Microscopic Morphology. The thermal properties of Pebax block copolymer membranes were characterized by two melting points (Tm), indicating a microphase-separated structure with soft domains dominated by PEO (lower Tm) and hard domains dominated by PA (higher Tm). The glass transition temperature (Tg) of the soft phase was observed in the DSC analysis, whereas the glassy transition endotherm of the hard phase (PA6 in Pebax-1657 and PA12 in Pebax-1074) was too weak to be detected in our tests. Both the melting points of the PEO soft phase and the E
DOI: 10.1021/acs.iecr.5b01234 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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The effects of the polymer concentration and the rate of solvent evaporation on the gas permeability can be related to the difference in the microphase architecture of the block copolymer membranes. In the dense-membrane formation process, with the evaporation of the solvent from the top layer of the solution, the polymers in the top skin first experience a transition from a relaxed free polymer to a gel polymer network with cross-linked polymer chains. As expected, the top layers of the 5% and 6% Pebax-1657 solutions reached the gel point faster than those of the 1% and 3% solutions at the same temperature. Thus, a faster sol-to-gel process was experienced at the top layer of the film, which could lead to more randomly cross-linked gels and imperfection of the polymer packing at the surface. In contrast, slow evaporation allows polymer rearrangement in the solution phase under the surface to form more a regularly structured film in the bulk of the film. Similarly, rapid solvent evaporation could also lead to a more rapid sol-to-gel transition at the top surface, resulting in a more disordered phase structure at the top of the resulting membrane. Higher permeabilities of both N2 and CO2 were observed with the Pebax-1074 membrane (which has a more irregular structure, as shown in Figure 3) compared to Pebax-1657, even though the former contained a lower proportion of PEO soft phase. This could also be related to the faster sol−gel transition in Pebax-1074 solution caused by the longer PA12 chain (as opposed to the PA6 chain in Pebax-1657) and the higher evaporation rate of 1-propanol compared to ethanol. The polymer chain rearrangement in Pebax-1074 solution was also more restrictive because of the longer PA12 chain, which could lead to more random chain packing in the bulk film and, thus, a less regular microstructure. The membranes made of Pebax1074 also had a higher density ratio (density of membrane/ density of pellets ≈ 0.92) than the Pebax-1657 membranes (∼0.89), implying a higher free volume in the Pebax-1074 membranes than in the Pebax-1657 membranes. 3.3. Effect of Pressure on CO2 Permeability, Solubility, Diffusivity Coefficient, and Selectivity in Pure Gases. The CO2 permeabilities of both Pebax-1657 (made with 3% solution) and Pebax-1074 (made with 1% solution) roomdried membranes were evaluated in the pressure range of 15− 300 psi (approximately 1−20 bar). These membranes were selected on the basis of the robustness of the membranes for the gas-permeation tests under pressure; the evenness of the membrane thickness was also a factor in the selection of membranes for pressure and temperature tests. A relatively constant CO2 permeability was observed as a function of pressure for the Pebax-1657 membrane, whereas a slight increase was observed for the Pebax-1074 membrane, as shown in Figure 5. The stable performance of the Pebax-1657 membrane as a function of pressure is likely the result of the negative influence of the membrane compaction at higher pressure balanced by the positive influence of membrane plasticization. On the other hand, the increase in CO2 permeability with pressure for the Pebax-1074 membrane is likely due to the smaller influence of compaction (as the majority of the PA12 was amorphous at the gas-permeation testing temperature of 35 °C, as Tg for PA12 was reported as 36 °C in the literature42) overtaken by the influence of plasticization. The change in solubility of CO2 in the membrane at increased pressure might also make a small contribution. CO2 sorption in Pebax-1657 and -1074 dense membranes was measured with stepwise increasing pressures, and the CO2
(shown in Figure 3c,d) are presented in panels a and b, respectively, of Figure 3. The fibril segments (in dark color) correspond to the crystalline hard domains, whereas the remaining parts (in light color) mainly represent the amorphous soft phase filling the spaces between the hard domains. The phase images of the Pebax-1657 and -1074 membranes obtained in this study are similar to those reported by Yave et al. for PEO−PBT block copolymer membranes.41 For membranes of similar thicknesses, the hard domains in the Pebax-1657 films (Figure 3a) are longer and more regular than those in the Pebax-1074 films (Figure 3b). The relatively irregular microstructure of Pebax-1074 was also confirmed by Barbi et al.27 using SAXS analysis to study the microdomain structure of block copolymers. 3.2. Effects of Fabrication Conditions. The main parameters for dense-membrane fabrication using solution casting include the solvent type and solvent composition, polymer solution concentration, solvent evaporation rate, and membrane thickness. For Pebax polymer, an alcohol/water complex solvent system was selected because it allowed gelation to be circumvented at room temperature as a result of the high polarity (high dielectric constant) of water40 (except for Pebax-1074 at polymer concentrations above 2%). This solvent system simplifies the membrane fabrication process. Three parameters, namely, polymer concentration, solvent evaporation rate, and membrane thickness, were evaluated against the gas-permeation performance in this study. Pebax-1657 and -1074 dense membranes of different thicknesses (15−60 μm) were fabricated by pouring different amounts of solution into different Petri dish and drying them according to the same process. For solutions of the same concentration and same drying process, the sol−gel transition of the top layer and the microstructure of the top layer should be very similar. Given the controlled slow evaporation process in this study under the controlled slow air flow in the cabinet or oven where the Petri dishes were placed, the solution phase under the top layer should have sufficient time for rearrangement to reach a more stable structure. Therefore, the microstructures of the films of different thickness might not be very different, so that similar gas separation performances were observed (results provided in the Supporting Information). The effects of the polymer solution concentration on the permeation properties of the Pebax membranes were evaluated by comparing CO2 permeability and CO2/N2 selectivity. As shown in Figure 4, membranes made with higher polymer concentrations (5% and 6%) were found to have higher CO2 permeabilities than membranes made with lower concentrations (1% and 3%). The CO2 permeability of the Pebax-1657 membrane cast with 6% solution was more than twice that of the membrane cast with 1% solution, whereas the change in selectivity was much smaller. Similarly, the CO2 permeability of the Pebax-1074 membrane made with 5% solution at 263 Barrer was also much higher than the 170 Barrer achieved for the membranes made with 1% and 3% polymer solutions. Meanwhile, more rapid solvent evaporation resulted in membranes with higher CO2 permeabilities. The membranes cast by evaporating the solvent at 35 °C exhibited approximately 15−45% higher CO2 permeabilities than those cast at room temperature (
