Communication pubs.acs.org/cm
A 7 nm Thick Polymeric Membrane With a H2/CO2 Selectivity of 200 That Reaches the Upper Bound Minghui Wang, Song Yi, Vaclav Janout, and Steven L. Regen* Department of Chemistry, Lehigh University, Bethlehem, Pennyslvania 18015, United States S Supporting Information *
KEYWORDS: Langmuir−Blodgett, permeation selectivity, polymeric membranes
T
correlated with higher selectivities after LB bilayers were ionically cross-linked (“glued together”) with a polyelectrolyte.11−14 These facts suggested that a further increase in the number of ionic sites in the surfactant used (e.g., a polymeric surfactant) could lead to even higher H2/CO2 selectivities. The objective of the present study was to test this hypothesis. With this purpose in mind, we fabricated LB bilayers using 1 as a polymeric surfactant with a high number of ionic sites, 2 as a zero-ionic site (control) analogue, and PAA as the glue (Chart 1). The syntheses of 1 and 2 are briefly outlined in Scheme 1.
he possibility that Langmuir−Blodgett (LB) bilayers could be of use as membranes for molecular separations was first recognized by Katherine Blodgett, herself, more than 70 years ago. Specifically, she noted that “such films may be employed as sieves or filters for the segregation of previously ‘non-filterable’ substances of molecular magnitudes.”1 Since that time, considerable effort has been made to try and use LB thin films for such purposes, especially in the area of gas separations. In most cases, however, poor or modest selectivities have been reported due to the formation of film defects.2−6 In this paper, we show that single Langmuir−Blodgett (LB) bilayers made from a quaternary ammonium derivative of poly(maleic anhydride-alt-1-octadecene), 1, plus poly(acrylic acid) (PAA), which are only 7 nm in thickness, exhibit remarkable H2/CO2 selectivities of 200 that reach Robeson’s “upper bound” (Chart 1).7 Until now, obtaining significant selectivity for this particular combination of gases has proven to be especially challenging using organic polymeric membranes in general.8−10
Scheme 1
Chart 1
Thus, condensation of poly(maleic anhydride-alt-1-octadecene) (POM, average MW ca. 40 000, Polysciences) with 2dimethylaminoethylamine, followed by quaternization with CH3I, afforded 1.15 Polymer 2 was synthesized by condensing POM with 2-methoxyethylamine. Polymers 1 and 2 formed well-behaved monolayers at the air/water interface with limiting areas of 0.52 and 0.40 nm2 per repeat unit, respectively (Supporting Information). The slightly larger limiting area of 1 is a likely consequence of charge repulsion among the pendant quaternary ammonium head groups. When PAA was present in the subphase (pH 3.0), the
We were led to investigate the barrier properties of such membranes based on the following: (i) LB bilayers made from a porous calix[6]arene-containing surfactant (Calix) exhibited a H2/CO2 selectivity of 70, and (ii) a correlation appeared to exist between permeation selectivity and the number of ionic sites present in the surfactant; i.e., higher numbers of ionic sites © 2013 American Chemical Society
Received: August 21, 2013 Revised: September 16, 2013 Published: September 18, 2013 3785
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Chemistry of Materials
Communication
showed ellipsometric film thicknesses of 4.1 ± 0.1 nm in the absence of PAA. When 0.1 mM PAA was present in the subphase, the bilayer thicknesses were increased to 6.2 ± 0.2 nm at pH 3.0 and to 7.0 ± 0.2 nm at pH 2.5. Analogous LB bilayers that were made from 2 showed film thicknesses of 4.1 ± 0.1 nm in the absence of PAA. In the presence of 0.1 mM PAA, the film thicknesses of bilayers of 2 were 5.4 ± 0.1 nm at pH 3.0 and 5.8 ± 0.1 nm at pH 2.5. Examination of LB bilayers that were formed in the presence of PAA by atomic force microscopy (AFM, height images) further indicated that the relative roughnesses were 2 > Calix > 1 (Supporting Information). Confirmation of film thicknesses was made via profilometry by AFM after scratching the surface with a razor blade and determining step heights (Supporting Information). In Table 1 are shown the permeances for membranes made in the absence and in the presence of PAA.
limiting area of 1 increased by ca. 20%. While PAA had a similar effect on the limiting area for Calix, it had no effect on the limiting area of 2. The slight increase in the limiting area for 1 and Calix in the presence of PAA is a likely result of stronger association and partial penetration of PAA into these cationic monolayers. In Figure 1 are shown the relative surface viscosities for monolayers of 1, 2, and Calix over pure water and also over
Table 1. Permeances of LB Bilayersa LB layer -b -b Calixc Calixc 1c 1c 2c 2c Calix/PAA-3.0d Calix/PAA-3.0d 1/PAA-3.0d 1/PAA-3.0d 2/PAA-3.0d 2/PAA-3.0d Calix/PAA-2.5e Calix/PAA-2.5e 1/PAA-2.5e 1/PAA-2.5e 2/PAA-2.5e 2/PAA-2.5e
Figure 1. Relative surface viscosities of 1 (a) and Calix (b) over 0.1 mM PAA, pH 3.0; 1 (c) and Calix (d) over 0.1 mM PAA, pH 2.5; 1 (e) and Calix (f) over 0.1 mM PAA, pH 2.0; (g−k) 1, 2, and Calix over pure water and 2 over 0.1 mM PAA at pH 3.0 and 2.5.
aqueous subphases containing 0.1 mM PAA (repeat unit concentration) at pH 3.0, 2.5, and 2.0. These viscosities reflect their relative monolayer cohesiveness at the air/water interface. Thus, after compressing each monolayer to 30 mN/m and exposing it to a slit opening (6 mm) of a canal viscometer, the decrease in surface pressure was monitored as a function of time. As is evident, negligible viscosities were observed for all monolayers over pure water. For 1, significant surface viscosities were observed at both pH 3.0 and 2.5 when PAA was included in the subphase. For Calix, similar strength of viscosity was observed only at pH 3.0. In the case of 2, adding PAA in the subphase did not increase its surface viscosity at all. It should be also noted that further increases in pH above 3.0 did not enhance the surface viscosity of Calix or 1 (Supporting Information). At each pH tested, the relative monolayer viscosities were 1 > Calix ≫ 2, which follows the same ordering as number of ionic sites that are present in each surfactant. To determine the barrier properties of membranes made from 1, 2, Calix, and PAA, we deposited single LB bilayers onto cast film made from poly[1-(trimethylsilyl)-1-propyne] (PTMSP). In all cases, transfer ratios were 1.0 ± 0.1. Experimental procedures that were used were similar to those previously described.11 Permeances (P/l) were calculated by dividing the observed flux (J) by the pressure gradient (Δp) (eq 1). Here, l is the thickness of the membrane. J P (cm 3/(cm 2·s·cm Hg)) = I Δp
H2 7.3 7.2 6.4 6.2 5.8 4.7 5.6 5.5 4.0 5.7 5.9 5.6 5.7 5.6 3.0 4.5 2.9 2.6 5.7 6.1
× × × × × × × ×
CO2 102 102 102 102 102 102 102 102
× 102 × 102
× 102 × 102
1.7 × 1.7 × 1.4 × 1.3 × 1.3 × 1.1 × 1.3 × 1.3 × 0.054 0.081 0.062 0.049 1.2 × 1.2 × 0.19 0.66 0.012 0.013 1.2 × 1.2 ×
103 103 103 103 103 103 103 103
103 103
103 103
H2/CO2 0.43 0.42 0.46 0.48 0.45 0.43 0.43 0.42 74 70 95 1.1 × 102 0.47 0.47 16 6.8 2.4 × 102 2.0 × 102 0.48 0.51
Permeances at ambient temperature, 106P/l (cm3/(cm2·s·cm Hg)), 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 supports. All measurements were made at ambient temperatures. Average values were obtained from 5−10 independent measurements of the same sample; the error in each case was ±5%. Each membrane listed was prepared, independently. bBare PTMSP support. cPure water subphase used at ambient pH. dpH 3.0. e pH 2.5. a
In the absence of PAA, all three LB bilayers showed negligible barrier properties. In sharp contrast, LB bilayers made from Calix that were ionically cross-linked with PAA at pH 3.0 exhibited substantial barrier properties and high H2/ CO2 permeation selectivities, similar to what we have previously reported.11 Analogous PAA-glued LB bilayers of 1 that were formed at this same pH showed even higher selectivities. When fabricated at pH 2.5, the H2/CO2 selectivities of PAA-glued bilayers of 1 further increased to ca. 200. In contrast, PAA-glued bilayers of Calix showed reduced selectivities when fabricated at pH 2.5. Thus, optimal pH values for fabricating single PAA-glued bilayers of 1 and Calix
(1)
Similar LB bilayers of 1, which were deposited onto silicon wafers that were silylated with n-octadecyltrichlorosilane, 3786
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Chemistry of Materials
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optimized 1/PAA membranes toward N2 is only ca. three times lower than toward CO2 despite a size difference between CO2 (0.33 nm) and N2 (0.36 nm) that is similar to that between CO2 and H2 (0.29 nm) (Supporting Information).16 Currently, we speculate that ionic cross-linking between 1 and PAA and hydrogen bonding within PAA lead to a glassy membrane with gaps that are large enough for H2 (but not CO2 and N2) to readily traverse. Finally, it should be noted that replacement of dry gaseous permeants with humid streams resulted in a substantial loss of permeation selectivity. Studies that are ongoing in our laboratories are aimed at gaining further insight into PAA-glued LB bilayers and creating other classes of all-polymer-based LB bilayers.
with maximum H2/CO2 selectivities were 2.5 and 3.0, respectively (Supporting Information). All Langmuir−Blodgett bilayers that were fabricated from 2 plus PAA showed negligible H2/CO2 selectivities using a pH of 3.0 and 2.5 in the subphase. The higher H2/CO2 selectivity found with PAA-glued bilayers of 1 at pH 2.5 relative to 3.0 was unexpected since these monolayers are less cohesive. It appears, therefore, that monolayer cohesiveness (as judged by measurements at the air/ water interface) is not the only factor that is controlling the barrier properties of these membranes. One possibility is that increased membrane thickness and enhanced hydrogen bonding within the PAA layer more than compensate for the reduction in the degree of ionic cross-linking in the assembly, when the pH is reduced from 3.0 to 2.5. It should be noted, however, that the PAA, by itself, does not represent a major barrier for gas transport. This is clearly evident by the fact that while polymer 2 is capable of “picking up” a thin layer of PAA, its barrier properties are negligible. To place their barrier properties into perspective, optimized PAA-glued bilayers of 1 and Calix have been included in an upper bound plot for H2/CO2 (Figure 2).7 As is evident, the
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ASSOCIATED CONTENT
S Supporting Information *
Experimental procedures used for all physical measurements and syntheses of 1 and 2. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-FG02-05ER15720.
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REFERENCES
(1) Blodgett, K. B. Film Structure and Method of Preparation. U.S. Patent 2,220,860, 1940. (2) Rose, G. D.; Quinn, J. A. Science 1968, 636−637. (3) Albrecht, O.; Laschewsky, A.; Ringsdorf, H. Macromolecules 1984, 17, 937−940. (4) Stroeve, P.; Coelho, M. A. N.; Dong, S.; Lam, P.; Coleman, L. B.; Fiske, T. G.; Ringsdorf, H.; Schneider, J. Thin Solid Films 1989, 180, 241−248. (5) Stroeve, P.; Bruinsma, P. J. Thin Solid Films 1994, 244, 958−961. (6) Riedl, T.; Nitsch, W.; Michel, T. Thin Solid Films 2000, 379, 240−252. (7) Robeson, L. M. J. Membr. Sci. 2008, 320, 390−400. (8) Yampolskii, Y. Macromolecules 2012, 45, 3298−3311. (9) Osterloh, F. E. Chem. Soc. Rev. 2013, 42, 2294−2320. (10) Xu, J. G.; Froment, G. F. AIChE J. 1989, 35, 88−96. (11) Wang, M. H.; Janout, V.; Regen, S. L. Chem. Commun. 2011, 47, 2387−2389. (12) McCullough, D. H.; Janout, V.; Li, J. W.; Hsu, J. T.; Truong, Q.; Wilusz, E.; Regen, S. L. . J. Am. Chem. Soc. 2004, 126, 9916−9917. (13) Li, J. W.; Janout, V.; Regen, S. L. Langmuir 2005, 21, 1676− 1678. (14) Wang, M. H.; Janout, V.; Regen, S. L. Langmuir 2010, 26, 12988−12993. (15) Wang, Y.; Janout, V.; Regen, S. L. Macromolecules 2008, 41, 497−500. (16) Freeman, B. D. Macromolecules 1999, 32, 375−380.
Figure 2. Upper bound plot for H2/CO2 selectivity versus H2 permeability, PH2. Data in red are for homopolymers previously reported (figure adapted by authors from ref 7). Also included are the barrier properties for LB bilayers (calculated by use of the series resistance model) made from 1 + PAA, pH 2.5 (7 nm, open blue squares); 1 + PAA, pH 3.0 (6 nm, open black squares); Calix + PAA, pH 3.0 (6 nm, open green circles).
intrinsic barrier properties of the LB bilayers made from 1 are exceptional, falling right on the upper bound (pH 2.5). To our knowledge, H2/CO2 selectivities of ca. 200 for an all-polymerbased membrane are without precedent. The virtual absence of significant barrier properties for single LB bilayers of 2 and the exceptional performance of the glued bilayer of 1 highlight the need for high numbers of ionic sites per surfactant and ionic cross-linking for achieving high permeation selectivity. In a broader context, these results also clearly demonstrate that the use of porous surfactants is not necessary to achieve high H2/CO2 permeation selectivity. The basis for H2/CO2 selectivities of 200 is not presently clear. However, preliminary results suggest that molecular sieving may be operating. Thus, the permeability of these 3787
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