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Langmuir 2001, 17, 8236-8241
Formation of Ultrathin, Defect-Free Membranes by Grafting of Poly(acrylic acid) onto Layered Polyelectrolyte Films Kang Ping Xiao,† Jeremy J. Harris, Alexander Park, Christina M. Martin, Vinod Pradeep, and Merlin L. Bruening* Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824 Received July 16, 2001. In Final Form: October 10, 2001 Deposition of hyperbranched poly(acrylic acid) (PAA) films on porous supports is an attractive method for synthesizing derivatizable, ultrathin composite membrane skins. We previously synthesized these films by sputtering a thin gold layer onto a porous alumina support and then grafting several layers of PAA to a self-assembled monolayer of mercaptoundecanoic acid on the gold. This paper demonstrates grafting of PAA onto PAA/poly(allylamine hydrochloride) (PAH) films that were prepared by alternating polyelectrolyte deposition. This procedure overcomes the inconvenience associated with sputtering of gold and allows synthesis of defect-free membranes using only one grafted PAA layer. Membrane skins consisting of 2.5 bilayers of PAA/PAH plus 1 layer of grafted PAA (total thickness of ∼14 nm) effectively cover the underlying pores of porous alumina supports without filling them, as shown by field-emission scanning electron microscopy (FESEM) images. When derivatized with H2NCH2(CF2)6CF3, these composite membranes show an ideal selectivity of 2.4 for O2 over N2, demonstrating the absence of defects in this system. The chemistry developed for the grafting process also allows modest derivatization of PAA/PAH films with H2NCH2(CF2)6CF3 to increase hydrophobicity.
Introduction Composite membranes with ultrathin skins are attractive for many separations applications because the minimal thickness of the skin layer can result in high flux along with selectivity.1,2 Because of the mechanical weakness of ultrathin films, however, they must be deposited on highly porous supports to form practical membranes.3 Current methods for preparing ultrathin films on porous supports include plasma grafting,3 interfacial polymerization,4-7 film casting,8 and phaseinversion processes.9-14 Recently, Langmuir-Blodgett * To whom correspondence should be addressed. E-mail: bruening@ cem.msu.edu. Tel: (517) 355-9715 ext 237. Fax: (517) 353-1793. † Current address: Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109-1055. (1) Loeb, S.; Sorirajan, S. In Saline Water Conversion-II; Gould, R. F., Ed.; Advances in Chemistry Series 38; American Chemical Society: Washington, DC, 1963; pp 117-132. (2) Prasad, R.; Notaro, F.; Thompson, D. R. J. Membr. Sci. 1994, 94, 225-248. (3) Kesting, R. E.; Fritzsche, A. K. Polymeric Gas Separation Membranes; John Wiley & Sons: New York, 1993. (4) Liu, C.; Martin, C. R. Nature 1991, 352, 50-52. (5) Petersen, J.; Peinemann, K.-V. J. Appl. Polym. Sci. 1996, 63, 1557-1563. (6) Cadotte, J. In Materials Science of Synthetic Membranes; Lloyd, D. R., Ed.; American Chemical Society: Washington, DC, 1985; pp 273294. (7) Chern, Y.-T.; Bae-Shyang, W. J. Appl. Polym. Sci. 1997, 63, 693701. (8) Le Roux, J. D.; Paul, D. R. J. Membr. Sci. 1992, 74, 233-252. (9) Pinnau, I.; Koros, W. J. Ind. Eng. Chem. Res. 1991, 30, 18371840. (10) Gantzel, P. K.; Merten, U. Ind. Eng. Chem. Process Des. Dev. 1970, 9, 331. (11) Suda, H.; Haraya, K. In Membrane Formation and Modification; Pinnau, I., Freeman, B. D., Eds.; American Chemical Society: Washington, DC, 2000; pp 295-313. (12) Shieh, J.-J.; Chung, T.-S. Ind. Eng. Chem. Res. 1999, 38, 26502658. (13) Gagne´, S.; Chowdhury, G.; Matsuura, T.; Laverty, B. J. Appl. Polym. Sci. 1999, 72, 1601-1610.
films have even been utilized to prepare ultrathin membranes.15-17 Although much success has been achieved in this area of research, deposition of selective, ultrathin (0.16 cm3(STP)/(cm2 s) at 10 psi) through 2.5 bilayers of PAA/PAH, the fluxes through 1-g-PAA + 2.5 PAA/PAH films are very small (3.2 × 10-3 cm3(STP)/(cm2 s) at 10 psi, see Figure 3). This could be due to the onset of full surface coverage (with small defects) or the presence of large amounts of polymer in the substrate pores. The latter possibility, however, is inconsistent with the FESEM image in Figure 2d. Derivatization of 1-g-PAA + 2.5 PAA/PAH films with H2NCH2(CF2)6CF3 increases both flux and selectivity through these membranes. After fluorination, the permeability of all tested gases increases, and gas selectivities no longer depend solely on molar mass. In a few cases, e.g., He and CO2, flux increases dramatically (Figure 4), and thus the selectivity ratios for several gas pairs are
P ) Fl/∆p
(1)
where F is the gas flux, ∆p is the pressure gradient across the membrane, and l is the membrane thickness. We assumed a film thickness of 140 Å for an underivatized membrane on the basis of the thickness of 1-g-PAA + 2 PAH/PAA on Au-coated Si wafers. The permeability coefficients for the various gases with a membrane pressure drop of 40 psi are as follows (units are in Barrers, which are 1010 cm3(STP) cm/(cm2 s cmHg)): He (1.0 ( 0.4), CH4 (0.6 ( 0.3), N2 (0.4 ( 0.2), O2 (0.4 ( 0.20), CO2 (0.3 ( 0.2), and SF6 (0.3 ( 0.2). Over the range of 10-60 psi, the permeability coefficients of these membranes differed by less than 30% from the value at 40 psi. These coefficients are a factor of 3-4 lower than those for hyperbranched PAA and may reflect tighter film packing. In calculating the permeability coefficients of fluorinated films, we used a thickness of 280 Å because film thickness doubles upon derivatization with H2NCH2(CF2)6CF3.42 This thickness value is also consistent with the FESEM image in Figure 2d. After fluorination, the permeability coefficients increase for all gases and are as follows (pressure drop of 40 psi): He (18 ( 6), CH4 (2.5 ( 0.4), N2 (2.0 ( 0.4), O2 (4.8 ( 1.5), CO2 (20 ( 6), and SF6 (0.5 ( 0.3). These values are comparable to those of fluorinated hyperbranched PAA films deposited on a self-assembled monolayer, showing that grafting PAA onto multilayered polyelectrolyte films yields defect-free, ultrathin membrane skins without cumbersome pretreatments such as gold coating. Additionally, only one layer of PAA needs to be grafted to the surface to enhance selectivity. Arnold and co-workers recently reported the synthesis of membranes composed of poly(1,1′-dihydroperfluorooctyl acrylate), and these materials present a very useful comparison for the membranes discussed in this manuscript.47 The O2/N2 selectivity for poly(1,1′-dihydroperfluorooctyl acrylate) membranes is 2.4, essentially the same as the value for 1-g-PAA + 2.5 PAA/PAH derivatized with H2NCH2(CF2)6CF3. However, the permeability coefficients of poly(1,1′-dihydroperfluorooctyl acrylate) are (47) Arnold, M. E.; Nagai, K.; Freeman, B. D.; Spontak, R. J.; Betts, D. E.; DeSimone, J. M.; Pinnau, I. Macromolecules 2001, 34, 56115619.
Formation of Ultrathin, Defect-Free Membranes
about 25 times higher than those for 1-g-PAA + 2.5 PAA/ PAH derivatized with H2NCH2(CF2)6CF3. We suspect that this stems in large part from the fact that derivatization of -COOH groups in PAA films occurs in only 50% yield. When we reactivate fluorinated membranes with ethyl chloroformate and derivatize again with H2NCH2(CF2)6CF3, gas flux through these membranes increases by about 50%. Previous studies showed that a second reaction cycle can increase the fraction of amidated -COOH groups in PAA films from 55% to 67%.21 Thus, the increase in flux after a second fluorination gives support to the fact that flux is somewhat limited by residual -COOH groups. Such groups may form hydrogen bonds and create a tightly packed membrane structure. Other studies also show an effect of polymer structure on the permeability of fluorinated acrylates. Arnold reported a 4-fold decrease in permeability on going from poly(1,1′-dihydroperfluorooctyl acrylate) to poly(1,1′-dihydroperfluorooctyl methacrylate) membranes.47 The selectivities of fluorinated PAA membranes definitely show that even one layer of derivatized PAA grafted onto layered polyelectrolytes can be defect free. Such defect-free membranes may prove interesting for other applications because they can be widely derivatized. We are currently studying the possible use of PAA films and membranes in purification of biological molecules. Direct Derivatization of PAH/PAA Films with H2NCH2(CF2)6CF3. PAH/PAA films on gold can also be directly derivatized with H2NCH2(CF2)6CF3 using procedures similar to those for grafting of PTBA. The process simply involves activation with ethyl chloroformate followed by immersion in a 0.1 M solution of H2NCH2(CF2)6CF3 in DMF for 1 h. Reflectance FTIR spectra indicate that only a small percentage (
