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Highly Size-Selective Ionically Crosslinked Multilayer Polymer Films for Light Gas Separation Daejin Kim, Ping Tzeng, Kevin J. Barnett, You-Hao Yang, Benjamin A. Wilhite,* and Jaime C. Grunlan* The production of inexpensive, high purity hydrogen remains a critical challenge to improving the sustainability of fossil fuels and for realizing renewable and clean energy sources capable of displacing fossil fuels. Dramatic increases in the demand for purified hydrogen for sulfur- and nitrogen-removal from “sour” crude oils is predicted in the coming decades,[1] as traditional sources of “sweet” or low-sulfur crude oils dwindle. Biorefineries for producing “green” fuels from renewable sources are expected to further increase global demand for purified hydrogen, as current biofuels require significant upgrading to achieve combustion properties comparable to petroleum distillates.[2] Hydrogen itself promises a universal fuel derivable from any hydrocarbon resource, which may be converted to electrical energy using fuel cells at energy efficiencies surpassing those of combustion-based power systems with zero greenhouse gas emissions.[3] Industrial pressure-swing adsorption and cryogenic distillation processes for hydrogen purification are complex, energy-intensive and costly, and are incapable of satisfying purity requirements of many fuel cells.[4] In contrast, gas purification membranes offer a low-maintenance, energy-efficient alternative to these processes and these membranes are capable of achieving hydrogen purities necessary for emerging energy technologies.[5] Polymeric gas separation membranes combining low production costs with robust mechanical properties have found broad industrial application for O2 and N2 enrichment of air, upgrading of natural gas and hydrogen recovery from ammonia.[5] Dense polymeric membranes separate gas mixtures based upon disparities in individual species permeabilities, dictated by species solubility and mobility within the polymer material.[6–8] Size-selective polymer membranes rely upon differences in species mobility, which is directly related to molecular size.[8] In the absence of structure (i.e., in amorphous Dr. D. Kim, P. Tzeng, K. J. Barnett, Prof. B. A. Wilhite, Artie McFerrin Department of Chemical Engineering Texas A&M University 3122 TAMU, College Station, TX, 77843–3122, United States E-mail: [email protected] Prof. J. C. Grunlan Department of Mechanical Engineering Texas A&M University 3123 TAMU, College Station, TX, 77843–3123, United States E-mail: [email protected] Dr. Y.-H. Yang Fire Division, National Institute of standards and Technology (NIST) 100 Bureau Drive, Stop 1070, Gaithersburg, MD, 20899, United States
DOI: 10.1002/adma.201302177
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glassy or rubbery films), increasing the free volume of the polymer improves the permeability at the cost of reducing the permselectivity.[7,8] This trade-off translates to an upper bound in separation performance achievable by dense, homogeneous and amorphous polymeric films first shown by Robeson[6,7] and theoretically validated by Freeman.[8] In contrast, structured thin films assembled from molecular sieves,[9] zeolites,[10] metalorganic-frameworks (MOFs),[11] covalent-organic frameworks[12] and carbon nanotubes[13] combine nano-scale pores with high free volume in a uniform, nominally defect-free structure to avoid the trade-off between permeability and permselectivity. Several strategies have been reported for incorporating structural aspects in polymeric systems in order to overcome the “upper bound” in separation performance, while retaining the traditional advantages of polymeric membranes. Mixed matrix membranes (MMMs) combine structural aspects of molecular sieves, zeolites or MOFs with homogeneous polymers.[14,15] Polymers of intrinsic microporosity (PIMs) exploit structure and rigidity of individual macromolecules to inhibit chain packing during film synthesis, in turn realizing amorphous films with uniform pore structures.[16,17] Thermal re-arrangement of polymer segments in cast films allows introduction of tunable pore structures, combining high free volume with a “bottleneck” pore structure for maintaining high permselectivity.[18] Conversely, block co-polymers formed from two or more incompatible polymers provide templates for producing highly ordered nanopore structures via etching of a sacrificial constituent from the as-cast film.[19] Layer-by-layer (LbL) assembly has been studied extensively over the past two decades owing to its simplicity and versatility for producing functional polymeric thin films with unique structures and properties.[20,21] LbL assembly techniques have been demonstrated for integrating multiple unlike polymers,[22] incorporating nanoparticles,[23] inorganic clays,[24] and biological molecules[25] into conformal coatings for antimicrobial,[26] antireflection,[27] flame retardant,[28] and gas barrier[29] applications. Both dense and porous LbL films have been reported for use as electrolytes in fuel cells or batteries.[30] Extension of the LbL assembly technique to gas purification membranes promises the ability to tune overall gas permeabilities and permselectivities through varying the number of cation-anion deposition cycles, known as “bilayers,” to suit a specific need. However, efforts to realize LbL-assembled films for gas separations have focused upon producing amorphous or “fuzzy” analogues of conventional gas separation polymers, e.g., poly (allylamine hydrochloride) (PAH)/poly (sodium styrene sulfonate) (PSS). Perhaps unsurprisingly, the resulting LbL films for gas purification have to-date yielded lower selectivities than their conventional analogues at equivalent permeabilities.[31]
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comprising a 30 bilayer PEI/PAA supported on an alumina coated-PSS substrate. Figure 2b presents an optical microscopic image (x50) of the porous surface of the untreated PSS substrate. The optical microscope images of the alumina coated-PSS tube and the PEI/PAA film coated over the alumina-PSS tube are shown in Figure 2c and Figure 1. Schematic of PEI/PAA layer-by-layer gas separation membrane supported on an alu2d, respectively. As can be seen in Figure 2c, mina coated porous stainless steel tube. the bright white alumina layer formed by a vacuum assisted-coating technique fills the In the present study, a branched polyethylenimine (PEI)/poly large cavities and voids present on the surface of the untreated (acrylic acid) (PAA) system is used for investigating light gas (H2, PSS substrate. Comparing optical microscope images of the CO, CO2, O2, N2) separations using a flow-through membrane PEI/PAA film surface (Figure 2d) with that of the aluminaapparatus. Unlike other LbL systems that display linear growth coated PSS tube (Figure 2c), indicates that the conformal PEI/ due to a “ladderlike” deposition structure, PEI/PAA assembly PAA coating is free of macroscopic defects. Figure 2e shows a appears to have a “scrambled salt” structure,[32] displaying expocross-section SEM image of a 30 bilayer PEI/PAA film deposnential growth (Figure S1, Supporting Information). Titrated ited over the alumina coated-PSS tube, confirming the deposiby the oppositely charged polyelectrolyte, PEI and PAA become tion of a ∼7 μm polymer membrane over the alumina-coated more ionized and require charge overcompensation, resulting PSS substrate. in a highly interpenetrating network in both directions (from The resulting PEI/PAA coated-PSS tube was placed in a bulk film to solution, and vice versa). High modulus obtained tube-and-shell assembly (Figure S2) in which the bore of the by nanoindentation suggests high density as another evidence of PSS tube (sweep) was supplied with helium and the annulus the interpenetrated structure. This highly “ionically cross-linked” (feed) of the system with feed gases (H2, CO, CO2, O2, N2). film is believed to be denser than traditional polymer films, Both sweep and feed gases were humidified with DI water to making it an excellent candidate for gas barrier applications. 3% RH prior to entering the membrane system. Permeate gas Figure 1 presents a schematic of the membrane assembly composition was continuously monitored via mass spectromconsisting of the PEI/PAA multilayer film supported by an etry (RGA 100, Stanford Research Systems). All gas permeation alumina-coated porous stainless steel (PSS) tube. The alumina measurements were performed at room temperature (∼20 °C) layer was applied over the PSS tube to prevent defect formation with permeate and retentate volumes maintained at atmosin the LbL film arising from the porous steel surface. Individual pheric pressure. The permeance (Pi /t) of gas species (i) is calPEI/PAA bilayers were assembled by first dipping the aluminaculated from Equation (1), coated PSS substrate into the PEI solution, followed by rinsing Pi with DI water and drying with filtered air to form a positively (1) pi, f − pi,s Fi = t charged surface layer. The substrate was then dipped into the PAA solution followed by another pair of rinsing and drying where Fi is gas (i) permeation rate (mol m−2 sec−1), t is memsteps to complete assembly of the bilayer. This process was brane thickness (m), Pi is permeability (mol m−1 sec−1 Pa−1), and repeated to achieve PEI/PAA films of 10, 20 and 30 bilayers. pi,f, pi,s are partial pressure (Pa) of gas i in the feed and sweep Figure 2a shows a picture of a complete membrane assembly sides, respectively. Gas selectivity (αi/j) is given by the ratio
Figure 2. Picture of (a) a PEI/PAA-alumina-PSS tube, optical microscopic surface images (x50) of (b) a untreated PSS tube, (c) an alumina layer, (d) a PEI/PAA film, and (e) a SEM cross-section image (x1,000) of a PEI/PAA-alumina-PSS tube.
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of permeability (Pi/Pj) of two gas species (i,j). Prior to depositing PEI/PAA films, the gas permeation tests were performed with the alumina-coated PSS tube to characterize gas permeation rate through the alumina-coated PSS tube in absence of the polymer coating. Figure S3a shows gas flux of H2, N2, O2, CO, and CO2 through the alumina-coated PSS tube as a function of partial pressure difference between the feed and sweep sides. Here, H2 permeance (a slope of the hydrogen flux) is 2.1 × 10−7 (mol m−2 sec−1 Pa−1) and permeance of the other gases (N2, O2, CO, CO2) is about 1.1 × 10−7 (mol m−2 sec−1 Pa−1). After depositing PEI/PAA films over the alumina-coated PSS tube, H2 permeance through the PEI/PAA films decreased by about two or three order of magnitude compared to that of the alumina coated-PSS tube (Figure S3b), with a tendency of reduction in H2 flux as the number of bilayers increase. In contrast, permeate partial pressures of all other gases studied remained below the lower detection limit of the mass spectrometer corresponding to a minimum detectable permeability of CO2 of 8.5 × 10−18 (mol m−1 sec−1 Pa−1); see Supporting Information, indicating high H2 permselectivity of the supported polymer membrane. The same permeation behavior of the PEI/ PAA films was observed using a coulometric detector system (Multi-Tran 400, Permatran C4/41, Oxtran 2/21 ML). Again, the obtained permeation rates of O2, CH4 and CO2 through the 10 bilayer PEI/PAA film were below the detection limit, while He and H2 gases exhibited appreciable permeability. Given the kinetic diameters of the gases studied [He(2.6) < H2(2.89) < CO2(3.3) < O2(3.46) < N2(3.46) < CO(3.76) < CH4(3.8)],[33] data confirms that the PEI/PAA LbL films have size-selective pore structure with a limiting diameter between 2.89 Å and 3.3 Å. The lower detection limits for CO2 and N2 were employed to estimate minimum detectable H2-CO2 and H-N2 permselectivities of 190:1 and 2350:1, respectively (see Supporting Information). While the extremely low CO2 and N2 permeabilities prevented a precise measurement of the intrinsic H2-N2 and H2-CO2 permselectivities, this minimum detectable permselectivity provides the most conservative basis for comparing the presently reported LbL film against the state-of-the-art in light gas separation polymeric materials. Figure 3a and 3b present two lines representing the upper bound limits reported in 1991and 2008 by Robeson[6,7] for (a) H2/N2 and (b) H2/CO2. Robeson’s upper bound plots were established based on data obtained from various homogeneous polymer films, excluding surface modified, mixed matrix, and carbon molecular sieving membranes. Data points in Figure 3 summarize the performance of a broad range of structured membrane materials reported in the literature. It can be seen that several heterogeneous MMMs exceed the upper bound proposed by Robeson, while previously reported LbL films do not. In contrast, the LbL-assembled PEI/PAA films studied in this work exceed the upper boundary by a margin equal to or greater than any existing polymeric gas separation membrane, given the minimum possible H2:N2 and H2:CO2 permselectivities obtained from the lower detection limit analysis (see Supporting information). When compared on the basis of selectivity (α) alone, not even structured porous inorganics can match these relatively simple, polymer-only membranes. Moreover, while existing polymeric membranes have overcome this “upper bound” by introducing heterogeneity and significant
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structural modification, our previous analysis of the LbL-assembled PEI/PAA film[29] indicates a compact, homogeneous polymer structure. Thus, this report represents the first successful use of a layer-by-layer polymer system to overcome Robeson’s “upper bound.” The compactness of this polymer film can be inferred from its mechanical behavior. Figure 4 shows the average elastic modulus and hardness of 10 bilayer PEI/PAA films, which were derived using the initial slope of force and displacement data (Figure S4). This 1μm film has an elastic modulus of 23.4 ± 5.7 GPa and hardness of 0.7 ± 0.3 GPa at 22%RH, higher than most known all-polymer[34,35] and even ultrastrong and stiff uncrosslinked polymer/clay LbL assemblies.[24] One reason for this relatively high result can be explainable by FTIR (Figure 4c). PAA has unique peaks for its neutral (–COOH at 1710 cm−1) and ionized forms (–COO− at 1560 cm−1) that were investigated to compare the ionic interaction between polycation and polyanion, which can be further quantified as “degree of ionization”. In order to derive this value, charged carboxylate and neutral carboxylic acid peaks were first assumed to have similar extinction coefficients,[36] and then the fraction of ionized PAA carboxylic groups was calculated as Abs1560/(Abs1560+Abs1710).[37] Based on the spectrum here, the portion of COO− is much higher than COOH, indicating PAA is highly ionized by PEI. Before the film assembly, both PEI (pH10) and PAA (pH4) aqueous solutions are in neutral form in which the polymer chains are more globular. As deposition proceeds, these pH sensitive polyelectrolytes become highly charged due to the alternating pH (i.e., PEI encountering a lower pH when PAA was deposited and PAA encountering a higher pH environment), resulting in a highly ionic crosslinked and interdiffused assembly. This extensive ionically linked structure might also explain why the assembly maintains a high elastic modulus at 50% RH. Rather than plasticizing, water likely occupies free volume that further densifies the film.[38] In conclusion, this study demonstrates exceptionally high hydrogen permselectivity and remarkable elastic modulus in an ionic crosslinked, layer-by-layer assembled PEI/PAA membrane, which performs beyond Robeson’s upper bound limit, despite being made of only homogenous polymer. This unique thin film overcomes the drawbacks of common polymeric membranes (i.e., low selectivity and poor mechanical properties), making it a significant advance in polymeric membranes for gas separation.
Experimental Details Substrates: Porous stainless steel (PSS) tubes (0.5 μm grade, OD: 0.5”, porous length: 2”, Mott Corporation) were used as supports for a PEI/PAA assembly. PSS supports were pretreated by immersion in an alkaline solution (sodium hydroxide, organic detergent, DI water) at 60 °C for 1 hour, followed by rinsing thoroughly with DI water and then drying at 120 °C for 2 hours. The pretreated PSS tubes were coated with nanopowder alumina (Sigma-Aldrich) by a vacuum pump which was connected to one end of the tube immersed in a nanopowder alumina solution (nanopowder alumina: alumina sol (∼20%, Alfa-Aser), DI water (wt.% ratio: 1:7:0.1)), and the other end of the tube was plugged with a rubber stopper. After being annealed at 450 °C for 4 hours with a
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COMMUNICATION Figure 3. Robeson’s upper bound plots ((a) H2/N2, (b) H2/CO2) of 10, 20, and 30 bilayer PEI/PAA polymer films and various other polymer and inorganic (or mixed matrix) membranes (the full term for each abbreviation listed here is presented in the Supporting Information). heating and cooling rate of 3 °C/min, the PSS tubes were airbrushed with alumina gel (dissolved alumina in nitric acid (Mallinckrodt Baker), followed by pH titration to near-neutral using ammonium hydroxide (Mallinckrodt Baker)) and annealed at the same condition described above. Materials: Branched polyethylenimine (Aldrich, St. Louis, MO) (MW ∼ 25,000 g mol−1) was dissolved into deionized water (18.2 MΩ) for making solution (0.1 wt%). The pH was adjusted from its unaltered value (∼10.5) to 10 by adding hydrochloric acid (HCl) (1.0 M). Poly(acrylic acid) (Aldrich) (MW ∼ 100,000 g mol−1) solution (0.2 wt%) was prepared with deionized water (18.2 MΩ). The pH of PAA was adjusted from its unaltered value (∼3.1) to 4 by adding NaOH (1.0 M).
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LbL deposition: The alumina-coated PSS tube was first dipped into the polycation solution (PEI) for 5 minutes, followed by rinsing with deionized water for 30 seconds and drying with a stream of filtered air. After the first positively-charged layer was adsorbed, the substrate was dipped into PAA solution for another 5 minutes, followed by another rinsing and drying cycle. One deposition cycle was defined as one ‘bilayer’. Starting from the second deposition cycle, the remaining numbers of layers were created using one minute dip times. This process was carried out using home-built robot systems.[68,69] Characterization: Gas permeation testing was performed by MOCON (Minneapolis, MN) in accordance with ASTM D-3985,[70] using Oxtran 2/21 ML for oxygen, Permatran-C 4/41 ML for carbon dioxide and Multi-Tran 400 ML for hydrogen, helium and methane at 23 °C and
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Acknowledgements The authors would like to thank the Artie McFerrin Department of Chemical Engineering at Texas A&M University for financial support of this work. Received: May 14, 2013 Revised: October 15, 2013 Published online: December 2, 2013
Figure 4. (a) Average elastic modulus and (b) hardness of 10 bilayer PEI/ PAA films under different environmental conditions (error bars represent standard deviation), and (c) FTIR spectra of 10 bilayer PEI/PAA film. “Special Offer”.
0% RH. A Hysitron TI 950 TriboIndenterTM was used to measure mechanical properties of 10 bilayer PEI/PAA film. The modulus and the hardness of the sample were measured in two different environments; 38 °C with 50% relative humidity (RH) and 25 °C with 22% RH. FTIR spectra of LbL films were measured with a Bruker Optics ALPHA-P 10098–4 spectrometer in ATR mode. PAA peaks in its covalent (COOH) and ionic form (COO−) are of interest in our study to compare the ionic interaction between polycation and polyanion, or so called ‘degree of ionization’.
Supporting Information Supporting Information is available from the Wiley Online Library or From the author.
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Highly Size-Selective Ionically Crosslinked Multilayer Polymer Films for Light Gas Separation Daejin Kim, Ping Tzeng, Kevin J. Barnett, You-Hao Yang, Benjamin A. Wilhite,* and Jaime C. Grunlan* The production of inexpensive, high purity hydrogen remains a critical challenge to improving the sustainability of fossil fuels and for realizing renewable and clean energy sources capable of displacing fossil fuels. Dramatic increases in the demand for purified hydrogen for sulfur- and nitrogen-removal from “sour” crude oils is predicted in the coming decades,[1] as traditional sources of “sweet” or low-sulfur crude oils dwindle. Biorefineries for producing “green” fuels from renewable sources are expected to further increase global demand for purified hydrogen, as current biofuels require significant upgrading to achieve combustion properties comparable to petroleum distillates.[2] Hydrogen itself promises a universal fuel derivable from any hydrocarbon resource, which may be converted to electrical energy using fuel cells at energy efficiencies surpassing those of combustion-based power systems with zero greenhouse gas emissions.[3] Industrial pressure-swing adsorption and cryogenic distillation processes for hydrogen purification are complex, energy-intensive and costly, and are incapable of satisfying purity requirements of many fuel cells.[4] In contrast, gas purification membranes offer a low-maintenance, energy-efficient alternative to these processes and these membranes are capable of achieving hydrogen purities necessary for emerging energy technologies.[5] Polymeric gas separation membranes combining low production costs with robust mechanical properties have found broad industrial application for O2 and N2 enrichment of air, upgrading of natural gas and hydrogen recovery from ammonia.[5] Dense polymeric membranes separate gas mixtures based upon disparities in individual species permeabilities, dictated by species solubility and mobility within the polymer material.[6–8] Size-selective polymer membranes rely upon differences in species mobility, which is directly related to molecular size.[8] In the absence of structure (i.e., in amorphous Dr. D. Kim, P. Tzeng, K. J. Barnett, Prof. B. A. Wilhite, Artie McFerrin Department of Chemical Engineering Texas A&M University 3122 TAMU, College Station, TX, 77843–3122, United States E-mail: [email protected] Prof. J. C. Grunlan Department of Mechanical Engineering Texas A&M University 3123 TAMU, College Station, TX, 77843–3123, United States E-mail: [email protected] Dr. Y.-H. Yang Fire Division, National Institute of standards and Technology (NIST) 100 Bureau Drive, Stop 1070, Gaithersburg, MD, 20899, United States
DOI: 10.1002/adma.201302177
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glassy or rubbery films), increasing the free volume of the polymer improves the permeability at the cost of reducing the permselectivity.[7,8] This trade-off translates to an upper bound in separation performance achievable by dense, homogeneous and amorphous polymeric films first shown by Robeson[6,7] and theoretically validated by Freeman.[8] In contrast, structured thin films assembled from molecular sieves,[9] zeolites,[10] metalorganic-frameworks (MOFs),[11] covalent-organic frameworks[12] and carbon nanotubes[13] combine nano-scale pores with high free volume in a uniform, nominally defect-free structure to avoid the trade-off between permeability and permselectivity. Several strategies have been reported for incorporating structural aspects in polymeric systems in order to overcome the “upper bound” in separation performance, while retaining the traditional advantages of polymeric membranes. Mixed matrix membranes (MMMs) combine structural aspects of molecular sieves, zeolites or MOFs with homogeneous polymers.[14,15] Polymers of intrinsic microporosity (PIMs) exploit structure and rigidity of individual macromolecules to inhibit chain packing during film synthesis, in turn realizing amorphous films with uniform pore structures.[16,17] Thermal re-arrangement of polymer segments in cast films allows introduction of tunable pore structures, combining high free volume with a “bottleneck” pore structure for maintaining high permselectivity.[18] Conversely, block co-polymers formed from two or more incompatible polymers provide templates for producing highly ordered nanopore structures via etching of a sacrificial constituent from the as-cast film.[19] Layer-by-layer (LbL) assembly has been studied extensively over the past two decades owing to its simplicity and versatility for producing functional polymeric thin films with unique structures and properties.[20,21] LbL assembly techniques have been demonstrated for integrating multiple unlike polymers,[22] incorporating nanoparticles,[23] inorganic clays,[24] and biological molecules[25] into conformal coatings for antimicrobial,[26] antireflection,[27] flame retardant,[28] and gas barrier[29] applications. Both dense and porous LbL films have been reported for use as electrolytes in fuel cells or batteries.[30] Extension of the LbL assembly technique to gas purification membranes promises the ability to tune overall gas permeabilities and permselectivities through varying the number of cation-anion deposition cycles, known as “bilayers,” to suit a specific need. However, efforts to realize LbL-assembled films for gas separations have focused upon producing amorphous or “fuzzy” analogues of conventional gas separation polymers, e.g., poly (allylamine hydrochloride) (PAH)/poly (sodium styrene sulfonate) (PSS). Perhaps unsurprisingly, the resulting LbL films for gas purification have to-date yielded lower selectivities than their conventional analogues at equivalent permeabilities.[31]
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comprising a 30 bilayer PEI/PAA supported on an alumina coated-PSS substrate. Figure 2b presents an optical microscopic image (x50) of the porous surface of the untreated PSS substrate. The optical microscope images of the alumina coated-PSS tube and the PEI/PAA film coated over the alumina-PSS tube are shown in Figure 2c and Figure 1. Schematic of PEI/PAA layer-by-layer gas separation membrane supported on an alu2d, respectively. As can be seen in Figure 2c, mina coated porous stainless steel tube. the bright white alumina layer formed by a vacuum assisted-coating technique fills the In the present study, a branched polyethylenimine (PEI)/poly large cavities and voids present on the surface of the untreated (acrylic acid) (PAA) system is used for investigating light gas (H2, PSS substrate. Comparing optical microscope images of the CO, CO2, O2, N2) separations using a flow-through membrane PEI/PAA film surface (Figure 2d) with that of the aluminaapparatus. Unlike other LbL systems that display linear growth coated PSS tube (Figure 2c), indicates that the conformal PEI/ due to a “ladderlike” deposition structure, PEI/PAA assembly PAA coating is free of macroscopic defects. Figure 2e shows a appears to have a “scrambled salt” structure,[32] displaying expocross-section SEM image of a 30 bilayer PEI/PAA film deposnential growth (Figure S1, Supporting Information). Titrated ited over the alumina coated-PSS tube, confirming the deposiby the oppositely charged polyelectrolyte, PEI and PAA become tion of a ∼7 μm polymer membrane over the alumina-coated more ionized and require charge overcompensation, resulting PSS substrate. in a highly interpenetrating network in both directions (from The resulting PEI/PAA coated-PSS tube was placed in a bulk film to solution, and vice versa). High modulus obtained tube-and-shell assembly (Figure S2) in which the bore of the by nanoindentation suggests high density as another evidence of PSS tube (sweep) was supplied with helium and the annulus the interpenetrated structure. This highly “ionically cross-linked” (feed) of the system with feed gases (H2, CO, CO2, O2, N2). film is believed to be denser than traditional polymer films, Both sweep and feed gases were humidified with DI water to making it an excellent candidate for gas barrier applications. 3% RH prior to entering the membrane system. Permeate gas Figure 1 presents a schematic of the membrane assembly composition was continuously monitored via mass spectromconsisting of the PEI/PAA multilayer film supported by an etry (RGA 100, Stanford Research Systems). All gas permeation alumina-coated porous stainless steel (PSS) tube. The alumina measurements were performed at room temperature (∼20 °C) layer was applied over the PSS tube to prevent defect formation with permeate and retentate volumes maintained at atmosin the LbL film arising from the porous steel surface. Individual pheric pressure. The permeance (Pi /t) of gas species (i) is calPEI/PAA bilayers were assembled by first dipping the aluminaculated from Equation (1), coated PSS substrate into the PEI solution, followed by rinsing Pi with DI water and drying with filtered air to form a positively (1) pi, f − pi,s Fi = t charged surface layer. The substrate was then dipped into the PAA solution followed by another pair of rinsing and drying where Fi is gas (i) permeation rate (mol m−2 sec−1), t is memsteps to complete assembly of the bilayer. This process was brane thickness (m), Pi is permeability (mol m−1 sec−1 Pa−1), and repeated to achieve PEI/PAA films of 10, 20 and 30 bilayers. pi,f, pi,s are partial pressure (Pa) of gas i in the feed and sweep Figure 2a shows a picture of a complete membrane assembly sides, respectively. Gas selectivity (αi/j) is given by the ratio
Figure 2. Picture of (a) a PEI/PAA-alumina-PSS tube, optical microscopic surface images (x50) of (b) a untreated PSS tube, (c) an alumina layer, (d) a PEI/PAA film, and (e) a SEM cross-section image (x1,000) of a PEI/PAA-alumina-PSS tube.
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of permeability (Pi/Pj) of two gas species (i,j). Prior to depositing PEI/PAA films, the gas permeation tests were performed with the alumina-coated PSS tube to characterize gas permeation rate through the alumina-coated PSS tube in absence of the polymer coating. Figure S3a shows gas flux of H2, N2, O2, CO, and CO2 through the alumina-coated PSS tube as a function of partial pressure difference between the feed and sweep sides. Here, H2 permeance (a slope of the hydrogen flux) is 2.1 × 10−7 (mol m−2 sec−1 Pa−1) and permeance of the other gases (N2, O2, CO, CO2) is about 1.1 × 10−7 (mol m−2 sec−1 Pa−1). After depositing PEI/PAA films over the alumina-coated PSS tube, H2 permeance through the PEI/PAA films decreased by about two or three order of magnitude compared to that of the alumina coated-PSS tube (Figure S3b), with a tendency of reduction in H2 flux as the number of bilayers increase. In contrast, permeate partial pressures of all other gases studied remained below the lower detection limit of the mass spectrometer corresponding to a minimum detectable permeability of CO2 of 8.5 × 10−18 (mol m−1 sec−1 Pa−1); see Supporting Information, indicating high H2 permselectivity of the supported polymer membrane. The same permeation behavior of the PEI/ PAA films was observed using a coulometric detector system (Multi-Tran 400, Permatran C4/41, Oxtran 2/21 ML). Again, the obtained permeation rates of O2, CH4 and CO2 through the 10 bilayer PEI/PAA film were below the detection limit, while He and H2 gases exhibited appreciable permeability. Given the kinetic diameters of the gases studied [He(2.6) < H2(2.89) < CO2(3.3) < O2(3.46) < N2(3.46) < CO(3.76) < CH4(3.8)],[33] data confirms that the PEI/PAA LbL films have size-selective pore structure with a limiting diameter between 2.89 Å and 3.3 Å. The lower detection limits for CO2 and N2 were employed to estimate minimum detectable H2-CO2 and H-N2 permselectivities of 190:1 and 2350:1, respectively (see Supporting Information). While the extremely low CO2 and N2 permeabilities prevented a precise measurement of the intrinsic H2-N2 and H2-CO2 permselectivities, this minimum detectable permselectivity provides the most conservative basis for comparing the presently reported LbL film against the state-of-the-art in light gas separation polymeric materials. Figure 3a and 3b present two lines representing the upper bound limits reported in 1991and 2008 by Robeson[6,7] for (a) H2/N2 and (b) H2/CO2. Robeson’s upper bound plots were established based on data obtained from various homogeneous polymer films, excluding surface modified, mixed matrix, and carbon molecular sieving membranes. Data points in Figure 3 summarize the performance of a broad range of structured membrane materials reported in the literature. It can be seen that several heterogeneous MMMs exceed the upper bound proposed by Robeson, while previously reported LbL films do not. In contrast, the LbL-assembled PEI/PAA films studied in this work exceed the upper boundary by a margin equal to or greater than any existing polymeric gas separation membrane, given the minimum possible H2:N2 and H2:CO2 permselectivities obtained from the lower detection limit analysis (see Supporting information). When compared on the basis of selectivity (α) alone, not even structured porous inorganics can match these relatively simple, polymer-only membranes. Moreover, while existing polymeric membranes have overcome this “upper bound” by introducing heterogeneity and significant
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structural modification, our previous analysis of the LbL-assembled PEI/PAA film[29] indicates a compact, homogeneous polymer structure. Thus, this report represents the first successful use of a layer-by-layer polymer system to overcome Robeson’s “upper bound.” The compactness of this polymer film can be inferred from its mechanical behavior. Figure 4 shows the average elastic modulus and hardness of 10 bilayer PEI/PAA films, which were derived using the initial slope of force and displacement data (Figure S4). This 1μm film has an elastic modulus of 23.4 ± 5.7 GPa and hardness of 0.7 ± 0.3 GPa at 22%RH, higher than most known all-polymer[34,35] and even ultrastrong and stiff uncrosslinked polymer/clay LbL assemblies.[24] One reason for this relatively high result can be explainable by FTIR (Figure 4c). PAA has unique peaks for its neutral (–COOH at 1710 cm−1) and ionized forms (–COO− at 1560 cm−1) that were investigated to compare the ionic interaction between polycation and polyanion, which can be further quantified as “degree of ionization”. In order to derive this value, charged carboxylate and neutral carboxylic acid peaks were first assumed to have similar extinction coefficients,[36] and then the fraction of ionized PAA carboxylic groups was calculated as Abs1560/(Abs1560+Abs1710).[37] Based on the spectrum here, the portion of COO− is much higher than COOH, indicating PAA is highly ionized by PEI. Before the film assembly, both PEI (pH10) and PAA (pH4) aqueous solutions are in neutral form in which the polymer chains are more globular. As deposition proceeds, these pH sensitive polyelectrolytes become highly charged due to the alternating pH (i.e., PEI encountering a lower pH when PAA was deposited and PAA encountering a higher pH environment), resulting in a highly ionic crosslinked and interdiffused assembly. This extensive ionically linked structure might also explain why the assembly maintains a high elastic modulus at 50% RH. Rather than plasticizing, water likely occupies free volume that further densifies the film.[38] In conclusion, this study demonstrates exceptionally high hydrogen permselectivity and remarkable elastic modulus in an ionic crosslinked, layer-by-layer assembled PEI/PAA membrane, which performs beyond Robeson’s upper bound limit, despite being made of only homogenous polymer. This unique thin film overcomes the drawbacks of common polymeric membranes (i.e., low selectivity and poor mechanical properties), making it a significant advance in polymeric membranes for gas separation.
Experimental Details Substrates: Porous stainless steel (PSS) tubes (0.5 μm grade, OD: 0.5”, porous length: 2”, Mott Corporation) were used as supports for a PEI/PAA assembly. PSS supports were pretreated by immersion in an alkaline solution (sodium hydroxide, organic detergent, DI water) at 60 °C for 1 hour, followed by rinsing thoroughly with DI water and then drying at 120 °C for 2 hours. The pretreated PSS tubes were coated with nanopowder alumina (Sigma-Aldrich) by a vacuum pump which was connected to one end of the tube immersed in a nanopowder alumina solution (nanopowder alumina: alumina sol (∼20%, Alfa-Aser), DI water (wt.% ratio: 1:7:0.1)), and the other end of the tube was plugged with a rubber stopper. After being annealed at 450 °C for 4 hours with a
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COMMUNICATION Figure 3. Robeson’s upper bound plots ((a) H2/N2, (b) H2/CO2) of 10, 20, and 30 bilayer PEI/PAA polymer films and various other polymer and inorganic (or mixed matrix) membranes (the full term for each abbreviation listed here is presented in the Supporting Information). heating and cooling rate of 3 °C/min, the PSS tubes were airbrushed with alumina gel (dissolved alumina in nitric acid (Mallinckrodt Baker), followed by pH titration to near-neutral using ammonium hydroxide (Mallinckrodt Baker)) and annealed at the same condition described above. Materials: Branched polyethylenimine (Aldrich, St. Louis, MO) (MW ∼ 25,000 g mol−1) was dissolved into deionized water (18.2 MΩ) for making solution (0.1 wt%). The pH was adjusted from its unaltered value (∼10.5) to 10 by adding hydrochloric acid (HCl) (1.0 M). Poly(acrylic acid) (Aldrich) (MW ∼ 100,000 g mol−1) solution (0.2 wt%) was prepared with deionized water (18.2 MΩ). The pH of PAA was adjusted from its unaltered value (∼3.1) to 4 by adding NaOH (1.0 M).
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LbL deposition: The alumina-coated PSS tube was first dipped into the polycation solution (PEI) for 5 minutes, followed by rinsing with deionized water for 30 seconds and drying with a stream of filtered air. After the first positively-charged layer was adsorbed, the substrate was dipped into PAA solution for another 5 minutes, followed by another rinsing and drying cycle. One deposition cycle was defined as one ‘bilayer’. Starting from the second deposition cycle, the remaining numbers of layers were created using one minute dip times. This process was carried out using home-built robot systems.[68,69] Characterization: Gas permeation testing was performed by MOCON (Minneapolis, MN) in accordance with ASTM D-3985,[70] using Oxtran 2/21 ML for oxygen, Permatran-C 4/41 ML for carbon dioxide and Multi-Tran 400 ML for hydrogen, helium and methane at 23 °C and
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Acknowledgements The authors would like to thank the Artie McFerrin Department of Chemical Engineering at Texas A&M University for financial support of this work. Received: May 14, 2013 Revised: October 15, 2013 Published online: December 2, 2013
Figure 4. (a) Average elastic modulus and (b) hardness of 10 bilayer PEI/ PAA films under different environmental conditions (error bars represent standard deviation), and (c) FTIR spectra of 10 bilayer PEI/PAA film. “Special Offer”.
0% RH. A Hysitron TI 950 TriboIndenterTM was used to measure mechanical properties of 10 bilayer PEI/PAA film. The modulus and the hardness of the sample were measured in two different environments; 38 °C with 50% relative humidity (RH) and 25 °C with 22% RH. FTIR spectra of LbL films were measured with a Bruker Optics ALPHA-P 10098–4 spectrometer in ATR mode. PAA peaks in its covalent (COOH) and ionic form (COO−) are of interest in our study to compare the ionic interaction between polycation and polyanion, or so called ‘degree of ionization’.
Supporting Information Supporting Information is available from the Wiley Online Library or From the author.
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