Letter pubs.acs.org/Langmuir
Polyelectrolyte Multilayers on PTMSP as Asymmetric Membranes for Gas Separations: Langmuir−Blodgett versus Self-Assembly Methods of Anchoring Cen Lin, Qibin Chen, Song Yi, Minghui Wang, and Steven L. Regen* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: Polyelectrolyte multilayers derived from poly(diallyldimethylamonium chloride) and poly(sodium 4-styrenesulfonate) have been deposited onto poly[1-(trimethylsilyl)-1-propyne] (PTMSP) with anchoring layers formed by Langmuir−Blodgett and self-assembly methods. Using gas permeation selectivity as a basis for judging the efficacy of each anchoring method, we have found that similar CO2/N2 selectivities (ranging from 110 to 140) could be achieved by both methods and that their permeances were also similar. Although LB anchors require fewer layers of polyelectrolyte to reach this level of selectivity, the greater ease associated with self-assembly and its applicability to curved, high-surface-area supports (e.g., PTMSP-coated hollow fibers) encourage its use with PTMSP in creating new membrane materials for the practical separation of gases.
■
INTRODUCTION Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is unique among all known organic polymers in terms of its exceptionally high gas permeability (Chart 1).1−6 For this reason, considerable
noteworthy about these membranes is that their permeationselective layers were only 7 nm in thickness and capable of reaching the “upper bound” (i.e., an empirical boundary for CO2/N2 that is based on the barrier properties of a large number of organic polymers that have been reported in the literature8,9). In addition to LB bilayers, we have begun to explore polyelectrolyte multilayers (PEMs) as gas-permeation-selective membranes that are prepared via the layer-by-layer (LbL) deposition method.10−14 Because PTMSP is strongly hydrophobic, we have used glued LB monolayers as anchors to increase its surface hydrophilicity and to aid the LbL deposition process. For example, we have used an LB monolayer of a calix[6]arene-based surfactant that was ionically cross-linked with poly(sodium 4-styrenesulfonate) (PSS) as an anchor layer for multilayers derived from poly(diallyldimethylamonium chloride) (PDADMAC) and PSS.15 Remarkably, PEMs made from these polyelectrolytes exhibited CO2/N2 selectivites ranging from 100 to 150 at room temperature under dry conditions. This level of selectivity is, in fact, higher than what has been reported for all known organic polymers.8 We have also observed similar selectivities for analgous PEMs made from poly(allylamine) and PSS.16 In sharp contrast, it has been reported that PEMs made from poly(allylamine) and PSS, deposited on untreated poly(dimethylsiloxane) supports (a solid, rubbery organic polymer that also has high gas permeability), exhibited a CO2/N2 selectivity of only 6.2 at
Chart 1
effort has been made to exploit PTMSP as a membrane material for the separation of gases. Despite such effort, low permeation selectivity has proven to be a limiting factor. Our own interest in PTMSP has focused on its use as a support material for hyperthin (i.e., APEI > ASA), these differences were essentially eliminated after depositing ca. 3 bilayers of [PDADMAC/PSS] onto them. Gas Permeation Properties. To judge the barrier properties of these PEMs using ALB, APEI, and ASA as anchor layers, we employed ca. 30-μm-thick PTMSP films as the substrates that were supported on metal O-rings for LB and LbL depositions and for gas permeation measurements. Specific procedures that were used for these depositions and measurements were similar to those previously reported.4 Our principal findings are summarized in Table 1. When ALB was used as the anchor, the permeances, P/l, for CO2 and N2 dropped dramatically after the deposition of four bilayers of [PDADMAC/PSS]. In addition, a large increase in the CO2/N2 permeation selectivity was observed. A further increase in this selectivity was found when five bilayers of [PDADMAC/PSS] were added. However, a sixth bilayer of these polyelectrolytes did not increase this selectivity. On the basis of these results, we conclude that asymmetric membranes composed of a PTMSP support, an ALB anchor layer, and five bilayers of [PDADMAC/PSS] are optimal. When APEI and ASA were used as anchors, the optimal numbers of bilayers of [PDADMAC/PSS] were six and seven, respectively. It should be noted that the maximum CO2/N2 permeation selectivity that was reached for each membrane type and also their permeances were essentially the same. These results clearly show that the anchor layer does not play a critical role in controlling the permeation properties of these membranes. Surface Morphologies. In Figure S-2 are shown the surface morphologies of PTMSP before and after deposition of the three different anchors. Also included in this figure are the surface morphologies of the optimized PEMs. In brief, the
P-ASA-Xn P-ASA-Xn P-ASA-Xn P-ASA-Xn P-ASA-Xn
nc 0 0 0 0 4 4 5 5 6 6 0 0 3 3 4 4 5 5 6 6 7 7 0 0 3 3 4 4 5 5 6 6 7 7 8 8
CO2 1.8 1.7 1.6 1.6 13 19 13 11 8.1 8.5 1.4 1.4 3.8 4.0 46 40 24 24 12 13 9.0 8.5 1.3 1.4 1.2 1.2 77 1.2 31 38 21 15 14 16 13 13
× × × ×
× × × ×
× × × ×
103 103 103 103
103 103 102 102
103 103 103 103
× 102
N2 3.1 × 3.0 × 3.5 × 2.9 × 0.25 0.31 0.096 0.080 0.069 0.066 2.8 × 2.7 × 63 73 1.4 1.0 0.29 0.26 0.096 0.11 0.095 0.069 2.4 × 2.7 × 2.2 × 2.4 × 1.6 2.9 0.53 0.65 0.20 0.16 0.13 0.12 0.098 0.097
CO2/N2 102 102 102 102
102 102
102 102 102 102
5.8 5.7 4.7 5.5 53 60 1.4 1.4 1.2 1.3 5.1 5.3 6.0 5.5 34 40 83 92 1.2 1.2 95 1.2 5.4 5.2 5.5 5.0 48 41 58 58 1.1 94 1.1 1.3 1.3 1.3
× × × ×
102 102 102 102
× 102 × 102 × 102
× 102 × × × ×
102 102 102 102
Permeances at ambient temperature, 106P/l (cm3/cm2·s·cmHg), were calculated by dividing the observed flow rate by the area of the membrane (9.36 cm2) and the pressure gradient (40 psi) employed, using ca. 30-μm-thick PTMSP (P) supports. All measurements were made at ambient temperature. Average values were obtained from 5 to 10 independent measurements of the same sample; the error in each case was ±5%. Duplicate membranes are reported in all cases. bX = [PDADMAC/PSS]. cThe number of bilayers of polyelectrolyte, X, (not including the anchor layers). In all cases, PEMs were formed using polyelectrolyte solutions that were 15 mM in repeat units plus 100 mM NaCl. a
deposition onto ALB led to a slight smoothing of the surface, and the deposition of ASA resulted in a slight roughening. In the case of APEI, a large increase in surface roughness was observed. The relative roughnesses of the optimized PEMs using these anchors were APEI > ALB ≈ ASA. Film Thicknesses. To determine the thickness of the optimized PEMs, we first measured the thickness of the PTMSP layer itself and then subtracted this value from the combined thickness of the PTMSP + anchor + PEM layers. Because of the extreme thinness of the anchor layers relative to the PTMSP support, no effort was made to determine their thickness as a result of inaccuracies in such estimates. 689
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A summary of the surface roughnesses, wettabilities, and film thicknesses is shown in Table S-2. Although no significant differences in wettability were found for the three different PEMs and only slight differences in surface roughness could be detected, the differences in film thickness were significant. In particular, the relative thicknesses for PEMs deposited on these anchor layers were ASA > APEI > ALB. Performance of PEMs as Permeation-Selective Layers. To place the performance of these PEMs into perspective, they have been included in an “upper bound” plot for CO2/N2.8,9 Here, the straight line in this plot of CO2/N2 selectivity versus PCO2 represents the combination of the highest permeation selectivities and highest permeation coefficients that have been reported for homopolymer-based membranes as of 2008.8 As can be seen in Figure 3, the intrinsic permeation properties for each PEM have similar positions in the upper
Letter
CONCLUSIONS Detailed comparisons that have been been made for optimized PEMs derived from PDADMAC and PSS on PTMSP surfaces bearing ALB, APEI, and ASA anchor layers have revealed similar CO2/N2 permeation selectivities, similar permeances, similar wettabilities, and similar morphologies. These similarities show that the PEM layer itself is mainly responsible for the barrier properties of these asymmetric membranes and that the anchoring layer plays only a minor role. Although the PEMs that were formed using the Langmuir− Blodgett-based anchors required fewer depositions of polyelectrolyte for optimization than those formed via selfassembled anchors, the greater experimental ease associated with the latter and its applicability to curved, high-surface-area supports strongly encourage its use in exploring other combinations of PEMs and PTMSP as membrane materials for gas separations.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures used for spin coating and the deposition of the anchor layers and the PEMs. Comparison of AFM film thickness measurements with those determined by ellipsometry and surface morphology. Summary of surface roughnesses, wettabilities, and film thicknesses. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. C.L. and Q.C. contributed equally. Figure 3. Upper bound plot for CO2/N2 selectivity versus CO2 permeability, P(CO2). The data shown in red are for homopolymers that have been reported to date (Figure adapted by authors from reference8). Also shown are data for (□) P-ALBX5, ( ) P-APEI-X6, and ( ) P-ASA-X7. Here, X=[PDADMAC/PSS].
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to our colleague, Dr. Vaclav Janout, for supplying a sample of POMTMA for this investigation and for valuable comments. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award DE-FG02-05ER15720.
bound plot. The slightly higher permeability of the PEM that was deposited on the ASA anchor reflects the fact that it was thicker than the PEMs deposited onto the ALB and APEI anchors. Although the PEMs appear below the upper bound, their high CO2/N2 permeation selectivity, in combination with their extreme thinness, makes them attractive as permeationselective layers for achieving high selectivity and high flux. In preliminary studies, we have also measured the H2/CO2 permeation selectivity of P-ASA-X7. Similar to what we have previously found for related PEMs on PTMSP, the selectivity of P-ASA-X7 was relatively low (i.e., H2/CO2 = 4.9) in comparison to its CO2/N2 permeation selectivity.15 Because the size difference between CO2 (0.33 nm) and N2 (0.36 nm) is similar to the size difference between H2 (0.29 nm) and CO2, this difference in selectivity implies that it is the solubility of CO2 that is largely responsible for the high CO2/N2 selectivity. In essence, the permeation of the smaller CO2 molecule is strongly favored over that of N2 because of favored differences in diffusivity and solubilty. However, the favored diffusivity of the smaller H2 molecules is offset by the favored solubility of the CO2 molecule, which results in relatively low selectivity.
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REFERENCES
(1) Masuda, T.; Isobe, E.; Higashimura, T.; Takada, K. Poly[1trimethylsilyl)-1-propyne]: a new high polymer synthesized with transition metal catalysts and characterized by extremely high gas permeability. J. Am. Chem. Soc. 1983, 105, 7473−7474. (2) Masuda, T.; Isobe, E.; Higashimura, T. Polymerization of 1(trimethylylsilyl)-1-propyne by halides of niobium (V) and tantalum (V) and polymer properties. Macromolecules 1985, 18, 841−845. (3) Nakagawa, T.; Saito, T.; Asakawa, S.; Saito, Y. Polyacetylene derivatives as membranes for gas separations. Gas. Sep. Purif. 1988, 2, 3−8. (4) Hendel, R. A.; Nomura, E.; Janout, V.; Regen, S. L. Assembly and disassembly of Langmuir−Blodgett films on poly[1-(trimethylsilyl)-1propyne]: the uniqueness of calix[6]arene multilayers as permeationselective membranes. J. Am. Chem. Soc. 1997, 119, 6909−6918. (5) Nagai, K.; Masuda, T.; Nakagawa, T.; Freeman, B. D.; Pinnau, I. Poly[1-(trimethylsilyl)-1-propyne] and related polymers: synthesis, properties and functions. Prog. Polym. Sci. 2001, 26, 721−798. 690
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(6) Kelman, S. D.; Rowe, B. W.; Bielawski, C. W.; Pas, S. J.; Hill, A. J.; Paul, D. R.; Freeman, B. D. Crosslinking of poly[1-(trimethylsilyl)-1propyne] and its effect on physical stability. J. Membr. Sci. 2008, 320, 123−134. (7) Wang, M.; Yi, S.; Janout, V.; Regen, S. L. A 7 nm thick polymeric membrane with H2/CO2 selectivity of 200 that reaches the upper bound. Chem. Mater. 2013, 25, 3785−3787. (8) Robeson, L. M. The upper bound revisited. J. Membr. Sci. 2008, 320, 390−400. (9) Freeman, B. D. Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes. Macromolecules 1999, 32, 375−380. (10) Iler, R. K. Multilayers of colloidal particles. J. Colloid Interface Sci. 1966, 21, 569−594. (11) Decher, G.; Hong, J.-D. Buildup of ultrathin multilayer films by a self-assembly process: I. Consecutive adsorption of anionic and cationic bipolar amphiphiles. Macromol. Chem. Macromol. Symp. 1991, 46, 321−327. (12) Schanze, K. S.; Shelton, A. H. Functional polyelectrolytes. Langmuir 2009, 25, 13698−13702. (13) Schlenoff, J. B. Retrospective on the future of polyelectrolyte multilayers. Langmuir 2009, 25, 14007−14010. (14) Ghostine, R. A.; Markarian, M. Z.; Schlenoff, J. B. Asymmetric growth in polyelectrolyte multilayers. J. Am. Chem. Soc. 2013, 135, 7636−7646. (15) Wang, M.; Janout, V.; Regen, S. L. Unexpected barrier properties of structurally matched and unmatched polyelectrolyte multilayers. Chem. Commun. 2013, 49, 3576−3578. (16) Wang, Y.; Stedronsky, E.; Regen, S. L. Defects in a polyelectrolyte multilayer: the inside story. J. Am. Chem. Soc. 2008, 130, 16510−16511. (17) Stroeve, P.; Vasquez, V.; Coelho, M. A. N.; Rabolt, J. F. Gas transfer in supported films made from molecular self-assembly of ionic polymers. Thin Solid Films 1996, 284−285, 708−712. (18) Park, J.; Hammond, P. T. Polyelectrolyte multilayer formation on neutral hydrophobic surfaces. Macromolecules 2005, 38, 10542− 10550. (19) Yang, Y.-H.; Bolling, L.; Priolo, M. A.; Grunlan, J. C. Super gas barrier and selectivity of grapheme oxide-polymer multilayer thin films. Adv. Mater. 2013, 25, 503−508. (20) Chen, W.; McCarthy, T. J. Layer-by-layer deposition: a tool for polymer surface modification. Macromolecules 1997, 30, 78−86. (21) Kleinfeld, E.; Ferguson, G. S. Healing of defects in the stepwise formation of polymer/silicate multilayer films. Chem. Mater. 1996, 8, 1575−1578. (22) Rouse, J. H.; Ferguson, G. D. Stepwise incorporation of nonpolar polymers within polyelectrolyte multilayers. Langmuir 2002, 18, 7635−7640. (23) Chen, W.; McCarthy, J. T. Layer-by-layer deposition: a tool for polymer surface modification. Macromolecules 1997, 30, 78−86. (24) Levasalmi, J.-M.; McCarthy, T. J. Poly(4-methyl-1-pentene)supported polyelectrolyte multilayer films: preparation and gas permeability. Macromolecules 1997, 30, 1752−1757. (25) Wang, M.; Janout, V.; Regen, S. L. Hyperthin organic membranes with exceptional H 2 /CO2 permeation selectivity: importance of ionic crosslinking and self-healing. Chem. Commun. 2011, 47, 2387−2389.
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dx.doi.org/10.1021/la404660f | Langmuir 2014, 30, 687−691
Polyelectrolyte Multilayers on PTMSP as Asymmetric Membranes for Gas Separations: Langmuir−Blodgett versus Self-Assembly Methods of Anchoring Cen Lin, Qibin Chen, Song Yi, Minghui Wang, and Steven L. Regen* Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
ABSTRACT: Polyelectrolyte multilayers derived from poly(diallyldimethylamonium chloride) and poly(sodium 4-styrenesulfonate) have been deposited onto poly[1-(trimethylsilyl)-1-propyne] (PTMSP) with anchoring layers formed by Langmuir−Blodgett and self-assembly methods. Using gas permeation selectivity as a basis for judging the efficacy of each anchoring method, we have found that similar CO2/N2 selectivities (ranging from 110 to 140) could be achieved by both methods and that their permeances were also similar. Although LB anchors require fewer layers of polyelectrolyte to reach this level of selectivity, the greater ease associated with self-assembly and its applicability to curved, high-surface-area supports (e.g., PTMSP-coated hollow fibers) encourage its use with PTMSP in creating new membrane materials for the practical separation of gases.
■
INTRODUCTION Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is unique among all known organic polymers in terms of its exceptionally high gas permeability (Chart 1).1−6 For this reason, considerable
noteworthy about these membranes is that their permeationselective layers were only 7 nm in thickness and capable of reaching the “upper bound” (i.e., an empirical boundary for CO2/N2 that is based on the barrier properties of a large number of organic polymers that have been reported in the literature8,9). In addition to LB bilayers, we have begun to explore polyelectrolyte multilayers (PEMs) as gas-permeation-selective membranes that are prepared via the layer-by-layer (LbL) deposition method.10−14 Because PTMSP is strongly hydrophobic, we have used glued LB monolayers as anchors to increase its surface hydrophilicity and to aid the LbL deposition process. For example, we have used an LB monolayer of a calix[6]arene-based surfactant that was ionically cross-linked with poly(sodium 4-styrenesulfonate) (PSS) as an anchor layer for multilayers derived from poly(diallyldimethylamonium chloride) (PDADMAC) and PSS.15 Remarkably, PEMs made from these polyelectrolytes exhibited CO2/N2 selectivites ranging from 100 to 150 at room temperature under dry conditions. This level of selectivity is, in fact, higher than what has been reported for all known organic polymers.8 We have also observed similar selectivities for analgous PEMs made from poly(allylamine) and PSS.16 In sharp contrast, it has been reported that PEMs made from poly(allylamine) and PSS, deposited on untreated poly(dimethylsiloxane) supports (a solid, rubbery organic polymer that also has high gas permeability), exhibited a CO2/N2 selectivity of only 6.2 at
Chart 1
effort has been made to exploit PTMSP as a membrane material for the separation of gases. Despite such effort, low permeation selectivity has proven to be a limiting factor. Our own interest in PTMSP has focused on its use as a support material for hyperthin (i.e., APEI > ASA), these differences were essentially eliminated after depositing ca. 3 bilayers of [PDADMAC/PSS] onto them. Gas Permeation Properties. To judge the barrier properties of these PEMs using ALB, APEI, and ASA as anchor layers, we employed ca. 30-μm-thick PTMSP films as the substrates that were supported on metal O-rings for LB and LbL depositions and for gas permeation measurements. Specific procedures that were used for these depositions and measurements were similar to those previously reported.4 Our principal findings are summarized in Table 1. When ALB was used as the anchor, the permeances, P/l, for CO2 and N2 dropped dramatically after the deposition of four bilayers of [PDADMAC/PSS]. In addition, a large increase in the CO2/N2 permeation selectivity was observed. A further increase in this selectivity was found when five bilayers of [PDADMAC/PSS] were added. However, a sixth bilayer of these polyelectrolytes did not increase this selectivity. On the basis of these results, we conclude that asymmetric membranes composed of a PTMSP support, an ALB anchor layer, and five bilayers of [PDADMAC/PSS] are optimal. When APEI and ASA were used as anchors, the optimal numbers of bilayers of [PDADMAC/PSS] were six and seven, respectively. It should be noted that the maximum CO2/N2 permeation selectivity that was reached for each membrane type and also their permeances were essentially the same. These results clearly show that the anchor layer does not play a critical role in controlling the permeation properties of these membranes. Surface Morphologies. In Figure S-2 are shown the surface morphologies of PTMSP before and after deposition of the three different anchors. Also included in this figure are the surface morphologies of the optimized PEMs. In brief, the
P-ASA-Xn P-ASA-Xn P-ASA-Xn P-ASA-Xn P-ASA-Xn
nc 0 0 0 0 4 4 5 5 6 6 0 0 3 3 4 4 5 5 6 6 7 7 0 0 3 3 4 4 5 5 6 6 7 7 8 8
CO2 1.8 1.7 1.6 1.6 13 19 13 11 8.1 8.5 1.4 1.4 3.8 4.0 46 40 24 24 12 13 9.0 8.5 1.3 1.4 1.2 1.2 77 1.2 31 38 21 15 14 16 13 13
× × × ×
× × × ×
× × × ×
103 103 103 103
103 103 102 102
103 103 103 103
× 102
N2 3.1 × 3.0 × 3.5 × 2.9 × 0.25 0.31 0.096 0.080 0.069 0.066 2.8 × 2.7 × 63 73 1.4 1.0 0.29 0.26 0.096 0.11 0.095 0.069 2.4 × 2.7 × 2.2 × 2.4 × 1.6 2.9 0.53 0.65 0.20 0.16 0.13 0.12 0.098 0.097
CO2/N2 102 102 102 102
102 102
102 102 102 102
5.8 5.7 4.7 5.5 53 60 1.4 1.4 1.2 1.3 5.1 5.3 6.0 5.5 34 40 83 92 1.2 1.2 95 1.2 5.4 5.2 5.5 5.0 48 41 58 58 1.1 94 1.1 1.3 1.3 1.3
× × × ×
102 102 102 102
× 102 × 102 × 102
× 102 × × × ×
102 102 102 102
Permeances at ambient temperature, 106P/l (cm3/cm2·s·cmHg), were calculated by dividing the observed flow rate by the area of the membrane (9.36 cm2) and the pressure gradient (40 psi) employed, using ca. 30-μm-thick PTMSP (P) supports. All measurements were made at ambient temperature. Average values were obtained from 5 to 10 independent measurements of the same sample; the error in each case was ±5%. Duplicate membranes are reported in all cases. bX = [PDADMAC/PSS]. cThe number of bilayers of polyelectrolyte, X, (not including the anchor layers). In all cases, PEMs were formed using polyelectrolyte solutions that were 15 mM in repeat units plus 100 mM NaCl. a
deposition onto ALB led to a slight smoothing of the surface, and the deposition of ASA resulted in a slight roughening. In the case of APEI, a large increase in surface roughness was observed. The relative roughnesses of the optimized PEMs using these anchors were APEI > ALB ≈ ASA. Film Thicknesses. To determine the thickness of the optimized PEMs, we first measured the thickness of the PTMSP layer itself and then subtracted this value from the combined thickness of the PTMSP + anchor + PEM layers. Because of the extreme thinness of the anchor layers relative to the PTMSP support, no effort was made to determine their thickness as a result of inaccuracies in such estimates. 689
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A summary of the surface roughnesses, wettabilities, and film thicknesses is shown in Table S-2. Although no significant differences in wettability were found for the three different PEMs and only slight differences in surface roughness could be detected, the differences in film thickness were significant. In particular, the relative thicknesses for PEMs deposited on these anchor layers were ASA > APEI > ALB. Performance of PEMs as Permeation-Selective Layers. To place the performance of these PEMs into perspective, they have been included in an “upper bound” plot for CO2/N2.8,9 Here, the straight line in this plot of CO2/N2 selectivity versus PCO2 represents the combination of the highest permeation selectivities and highest permeation coefficients that have been reported for homopolymer-based membranes as of 2008.8 As can be seen in Figure 3, the intrinsic permeation properties for each PEM have similar positions in the upper
Letter
CONCLUSIONS Detailed comparisons that have been been made for optimized PEMs derived from PDADMAC and PSS on PTMSP surfaces bearing ALB, APEI, and ASA anchor layers have revealed similar CO2/N2 permeation selectivities, similar permeances, similar wettabilities, and similar morphologies. These similarities show that the PEM layer itself is mainly responsible for the barrier properties of these asymmetric membranes and that the anchoring layer plays only a minor role. Although the PEMs that were formed using the Langmuir− Blodgett-based anchors required fewer depositions of polyelectrolyte for optimization than those formed via selfassembled anchors, the greater experimental ease associated with the latter and its applicability to curved, high-surface-area supports strongly encourage its use in exploring other combinations of PEMs and PTMSP as membrane materials for gas separations.
■
ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures used for spin coating and the deposition of the anchor layers and the PEMs. Comparison of AFM film thickness measurements with those determined by ellipsometry and surface morphology. Summary of surface roughnesses, wettabilities, and film thicknesses. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. C.L. and Q.C. contributed equally. Figure 3. Upper bound plot for CO2/N2 selectivity versus CO2 permeability, P(CO2). The data shown in red are for homopolymers that have been reported to date (Figure adapted by authors from reference8). Also shown are data for (□) P-ALBX5, ( ) P-APEI-X6, and ( ) P-ASA-X7. Here, X=[PDADMAC/PSS].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We are grateful to our colleague, Dr. Vaclav Janout, for supplying a sample of POMTMA for this investigation and for valuable comments. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award DE-FG02-05ER15720.
bound plot. The slightly higher permeability of the PEM that was deposited on the ASA anchor reflects the fact that it was thicker than the PEMs deposited onto the ALB and APEI anchors. Although the PEMs appear below the upper bound, their high CO2/N2 permeation selectivity, in combination with their extreme thinness, makes them attractive as permeationselective layers for achieving high selectivity and high flux. In preliminary studies, we have also measured the H2/CO2 permeation selectivity of P-ASA-X7. Similar to what we have previously found for related PEMs on PTMSP, the selectivity of P-ASA-X7 was relatively low (i.e., H2/CO2 = 4.9) in comparison to its CO2/N2 permeation selectivity.15 Because the size difference between CO2 (0.33 nm) and N2 (0.36 nm) is similar to the size difference between H2 (0.29 nm) and CO2, this difference in selectivity implies that it is the solubility of CO2 that is largely responsible for the high CO2/N2 selectivity. In essence, the permeation of the smaller CO2 molecule is strongly favored over that of N2 because of favored differences in diffusivity and solubilty. However, the favored diffusivity of the smaller H2 molecules is offset by the favored solubility of the CO2 molecule, which results in relatively low selectivity.
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REFERENCES
(1) Masuda, T.; Isobe, E.; Higashimura, T.; Takada, K. Poly[1trimethylsilyl)-1-propyne]: a new high polymer synthesized with transition metal catalysts and characterized by extremely high gas permeability. J. Am. Chem. Soc. 1983, 105, 7473−7474. (2) Masuda, T.; Isobe, E.; Higashimura, T. Polymerization of 1(trimethylylsilyl)-1-propyne by halides of niobium (V) and tantalum (V) and polymer properties. Macromolecules 1985, 18, 841−845. (3) Nakagawa, T.; Saito, T.; Asakawa, S.; Saito, Y. Polyacetylene derivatives as membranes for gas separations. Gas. Sep. Purif. 1988, 2, 3−8. (4) Hendel, R. A.; Nomura, E.; Janout, V.; Regen, S. L. Assembly and disassembly of Langmuir−Blodgett films on poly[1-(trimethylsilyl)-1propyne]: the uniqueness of calix[6]arene multilayers as permeationselective membranes. J. Am. Chem. Soc. 1997, 119, 6909−6918. (5) Nagai, K.; Masuda, T.; Nakagawa, T.; Freeman, B. D.; Pinnau, I. Poly[1-(trimethylsilyl)-1-propyne] and related polymers: synthesis, properties and functions. Prog. Polym. Sci. 2001, 26, 721−798. 690
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Langmuir
Letter
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dx.doi.org/10.1021/la404660f | Langmuir 2014, 30, 687−691
