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Preparation of Poly(ethylene glycol) Brushes on Polysulfone Membranes for Olefin/Paraffin Separation Yong Soon Park, Jongok Won,* and Yong Soo Kang* Center for Facilitated Transport Membranes, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea Received March 28, 2000. In Final Form: June 26, 2000 Thin solvent-free polymer electrolytes containing silver salts showed a facilitated transport of propylene. Poly(ethylene glycol) (PEG) was immobilized on a polysulfone (PSf) membrane surface and subsequently complexed with AgBF4, which can be used as a membrane for the separation of olefin/paraffin mixtures. Photoreactive azidophenyl group was bound to the chain terminus of PEG, and the modified PEG was immobilized onto the surface of the PSf microporous membrane by ultraviolet irradiation. The propylene permeance was increased for the PEG brush membrane complexed with silver salt. The propylene permeance was 95 gas permeance units and the propylene selectivity over propane was 12 when the theoretical mole ratio of silver to ethylene oxide unit reached 1. Excellent propylene transport performance is attributed to the thin layer of PEG as well as the high loading of silver ion, the propylene carrier, in the thin PEG matrix.
Introduction Polymer electrolytes complexed with alkali metal salts have been extensively investigated for application to solidstate batteries, fuel cells, electrochromic displays, and so on.1-4 Polymer electrolytes containing silver salts are also of particular interest because of their potential application to facilitated transport membranes for the separation of olefin/paraffin mixtures.5-12 Separation of olefin is an important and energy-intensive process in industry. Many investigators have studied olefin/paraffin separation by facilitated transport with various membranes containing carriers.5-12 In the facilitated transport of olefins, silver ions confined to the membrane medium form reversible complexes with olefin molecules. Olefin molecules donate π electrons from the occupied 2p orbitals to the empty s orbitals of silver ions to form σ-bonds. Back-donation of electrons from the occupied d orbitals of silver ions into the empty π*-2p antibonding orbitals of olefin molecules results in π-bonding.13 Because of such reversible and specific interaction of silver ions with olefin molecules, silver ions can act as olefin carriers for facilitated transport in the membrane and then lead a carrier-mediated transport in addition to a normal Fickian transport. Paraffins are unable to form complexes with silver ions * To whom correspondence should be addressed, Tel.: +82-2958-5287; fax: +82-2-958-6869; e-mail:
[email protected]. (1) Schantz, S.; Torrel, L. M.; Stevens, J. R. J. Chem. Phys. 1991, 94, 6862. (2) Armstrong, R. D.; Clarke, M. D. Solid State Ionics 1984, 11, 305. (3) Frech, R.; Manning J.; Black, B. Polymer 1989, 60, 1785. (4) Manning, J.; Frech, R.; Whang, E. Polymer 1990, 31, 2245. (5) For example, Safarik, D. J.; Eldridge, R. B. Ind. Eng. Chem. Res. 1998, 37, 2571. (6) Koval, C. A.; Spontarelli, T.; Thoen, P.; Noble, R. D. Ind. Eng. Chem. Res. 1992, 31, 1116. (7) Yoon, Y. S.; Won, J.; Kang, Y. S. Macromolecules 2000, 33, 3185. (8) Pinnau, I.; Toy, L. G.; Sunderrajan S.; Freeman, B. D. Polym. Mater. Sci. Eng. 1997, 77, 269. (9) Hong, S. U.; Won J.; Kang, Y. S. Adv. Mater. 2000, 12, 968. (10) Kim, Y. H.; Ryu, J. H.; Bae, J. Y.; Kang, Y. S.; Kim, H. S. Chem. Commun. 2000, 195. (11) Jin. J. H.; Hong, S. U.; Won, J.; Kang, Y. S. Macromolecules 2000, 33, 4932. (12) Bai, S.; Sridhar, S.; Khan, A. A. J. Membr. Sci. 1998, 147, 131. (13) Beverwijk, C. D.; Van der Kerk, G. J. M.; Leusink, A. J.; Noltes, J. G. Organomet. Chem. Rev. A 1970, 5, 215.
and permeate only through Fickian transport. This results in high olefin/paraffin separation. In general, the facilitated transport phenomena have been typically observed in supported liquid membranes or ion-exchange membranes containing silver ion carrier together with water;5,6 however, this technology has not been applied to industrial applications because of the evaporation of the liquid media in such liquid membranes. Recently, solvent-free polymer electrolyte membranes containing silver ions showed high performance in olefin/ paraffin separation.7-9,11,12 Poly(ethylene oxide) (PEO),8 poly(2-ethyl-2-oxazoline), poly(vinyl pyrrolidone),7,9 and poly(acrylamide) containing polar groups in their chains were used as a polymer matrix to dissolve the low-latticeenergy silver salts for the olefin-facilitated transport. Gas permeation can be improved through the preparation of a composite membrane, where a thin dense top layer is supported by a porous sublayer. The thickness of the top layer determines the efficiency of the permeation. Because the typical thickness of the top layer obtained from coating processes was on the order of micrometers, alternative processes that reduce the top layer thickness are desirable. The grafting technique onto the surface of an asymmetric microporous support is an alternative to make a thin polymer layer. In this respect, surface modification with the highly selective materials is considered of great importance to improve the gas transport properties. A polymer brush is a model structure of such an ultrathin layer onto a support membrane that can be readily obtainable by photochemical immobilization.14 In the present work, poly(ethylene glycol) (PEG) brushes are prepared by photochemical immobilization of PEG on to a microporous polysulfone (PSf) membrane and complexed with AgBF4 to make facilitated transport membranes. The facilitated transport behavior of propylene through PEG brush membranes with silver salts has been investigated. Experimental Section Materials. O-(2-Aminoethyl)-O′-methylpoly(ethylene glycol) (PEG-NH2, Mw ) 5000), silver tetrafluoroborate (AgBF4, purity ) 99.0%), and 1,3-(3-dimethylaminopropyl)ethylcarbodiimide (14) Park, Y. S.; Ito Y.; Imanishi, Y. Langmuir 1998, 14, 910.
10.1021/la000466v CCC: $19.00 © 2000 American Chemical Society Published on Web 10/12/2000
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Figure 2. ATR FT-IR spectra of (a) PSf and (b) PEGimmobilized PSf membranes. and the permeated pressure was atmospheric. All the membranes were evacuated for 30 min before the gas permeation experiments. The distribution of silver atoms for cross-section of the prepared membrane was characterized by the scanning electron microscopy-wavelength dispersive spectrometer (SEM-WDS, JEOL JXA-8600).
Figure 1. (a) Synthetic scheme of photoreactive PEG. (b) Schematic presentation of surface immobilization of azidophenyl-derivatized PEG on microporous PSf membrane. hydrochloride (water-soluble carbodiimide, WSC) were purchased from Aldrich (Milwaukee, WI) and used without further purification. 4-Azidobenzoic acid was purchased from Tokyo Kasei (Tokyo, Japan). PSf microporous membrane15 as a support was kindly provided by Saehan Inc. (Seoul, Korea). Preparation of Azidophenyl-Derivatized PEG. PEG carrying an azidophenyl group at the chain terminus (PEG-Az) was synthesized as follows (Figure 1a): PEG-NH2 (1.0 g, 0.2 mmol), 4-azidobenzoic acid (326 mg, 2.0 mmol), and WSC (2.0 mg/mL) were dissolved in distilled water (50 mL, pH ) 7.0) and the solution was stirred for 3 h at 4 °C and left standing for 24 h at room temperature in the dark. After the reaction, the solution was subjected to dialysis (cutoff greater than Mw ) 3000) for 2 days. Surface Immobilization of Azidophenyl-Derivatized PEG. The surface immobilization of PEG-Az on the PSf membrane was carried out as shown in Figure 1b. An aqueous solution containing PEG-Az was cast on the porous PSf membrane. After drying under N2 atmosphere at 40 °C, the membrane was irradiated with ultraviolet (UV) light (power ) 8 W, λmax ) 365 nm) for 20 min and washed with distilled water (pH ) 9 and 7) until the 280-nm absorption due to the polymer released in the washing liquid became undetectable. Then, a desired concentration of AgBF4 solution was loaded on the PEGimmobilized membrane surface and the membrane was dried at 35 °C under N2 atmosphere for 12 h. Then, the membrane was dried in a vacuum at room temperature for 24 h and stored in a vacuum before measurement. Membrane Characterization. Fourier transform infrared (FT-IR) measurements were performed on a 6030 Galaxy Series FT-IR spectrometer (Mattson Inst.); 256 scans were signalaveraged at a resolution of 4 cm-1. The attenuated total reflection (ATR) spectra were obtained using a KRS-5 prism with an incident angle of 45°. The dry and pure gases propane (99%) and propylene (99%) were used for the permeation experiments. The gas permeation properties were measured by a soap-bubble flow meter. The unit of the gas permeance is GPU, where 1 GPU ) 1 × 10-6 cm3(STP)/cm2 s cmHg. The feed pressure was 276 kPa (15) The gas permeance of the PSf support membrane was in the order of 104 GPU (1 GPU ) 1 × 10-6 cm3 (STP)/cm2 s cmHg) with no selectivity. Average and maximum pore sizes were estimated ca. 7 and 20 nm, respectively, by the liquid displacement method.
Results and Discussion PEG Brush Membranes. Azidophenyl derivation of PEG was carried out by the condensation reaction of a chain-terminal amine group of PEG with a carboxyl group of 4-azidobenzoic acid in a buffer solution. The derivation of PEG was confirmed from the characteristic IR absorption peak of the azido group at ca. 2115 cm-1. The amount of incorporated azidophenyl group per unit weight of PEG derivatives was estimated by measuring the UV absorption at 280 nm. As a result, 75% of the terminal amine group of PEG was capped with the azidophenyl group. The introduced azidophenyl group causes the facile elimination of molecular nitrogen from the azido group upon UV irradiation and the formation of phenyl nitrene.16 The photochemically generated phenyl nitrene, which is a highly reactive species in a triplet state, undergoes the formation of a covalent bond with neighboring atoms. Therefore, the azidophenyl-derivatized PEG can be photochemically immobilized on the membrane surface upon loading and subsequent UV irradiation to make PEG brush membranes. Figure 2 shows the ATR FT-IR spectra of a PSf membrane and a PEG-immobilized PSf membrane. After the immobilization of PEG, the IR spectrum of the PEGimmobilized membrane shows new peaks at 2873, 1456, 1080, and 948 cm-1, which were ascribed to the CH2 symmetric stretch, CH2 scissor, C-O and C-C stretch, and CH2 rock and C-C stretch of PEG, respectively. These observations imply the introduction of PEG molecules on the membrane surface and the formation of PEG brush membranes. The branched PEG brush would be obtained because the azidophenyl group at a terminal end can react with not only the membrane surface but also adjacent other PEG backbone. The formation of a branched polymer brush is a general trend in a photoreactive polymer having an azidophenyl group at a chain terminus.17 The amount of immobilized PEG on the membrane calculated by gravimetric change of membrane before and after immobilization was 2.5 µg/cm2. Therefore the thickness of (16) Matsuda, T.; Sugawara, T. Langmuir 1995, 11, 2272. (17) Devanand, K.; Selser, J. C. Macromolecules 1991, 24, 5943.
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Park et al. Table 1. Pure-Gas Permeation Properties of PEG Brush Membranes Incorporated with Ag Ions
[EG]:[Ag+] mole ratio pristine PSf PSf/Aga PEG PEG/Ag 0.2 PEG/Ag 1 PEG/Ag 2
1:0 1:0.2 1:1 1:2
C3H8
C3H6
ideal separation factor (C3H6/C3H8)
∼21 700 2190 18 200 80 7.9 6.1
∼20 000 2170 19 000 150 95 107
0.92 1.0 1.0 2.0 12.0 17.5
pure-gas permeance (GPU)
a 1.0 mg/cm2 were simply loaded on the surface of the PSf support membrane.
Figure 3. The SEM-WDS photographs of the cross-section of membranes with various mole ratios of EO units to silver ions (a)1:0.02, (b)1:1, and (c)1:2.
the immobilized PEG layer, which was calculated from the amount of immobilized PEG and the density of PEG (1.204 g/cm3), is 20.8 nm. The difference of surface morphologies between the PSf membrane and the PEG-immobilized membrane is hardly seen, implying that the thickness of the PEG was too thin to be detected simply by the SEM photographs. Because the estimated hydrodynamic radius of gyration of PEG is 1.9 nm,17 which is smaller than the surface pore size of the membrane () 20 nm), it is possible that some part of the PEG chains penetrate into the pores and are attached inside of the pores while PEG was immobilized onto the surface of the membrane. The WDS photographs in Figure 3 display the crosssection of the PEG brush membranes complexed with AgBF4. The white solid-line denotes the distribution of silver across the membrane. When the small amount of silver salt was loaded on the PEG-brushed membrane surface (Figure 3a), silver atoms were distributed mostly at the membrane surface. However, silver atoms were distributed to the inside of the membrane with increasing loading amount of silver salt. This indicates that the silver salt not only incorporated in the PEG layer but also penetrated into the pores of the support membrane because of the existence of incomplete coverage of pores by PEG branches (Figure 3b, c). Therefore it is considerable that the amount of incorporated silver salt complexed with
PEG is smaller than the amount of the loaded silver salt, that is, the actual mole ratio of ethylene oxide (EO) unit to silver ion would be lower than the theoretical mole ratio.18 Gas Transport Properties. Propylene permeation of PEO-based polymer electrolyte composite membranes containing AgBF4 is ca. 25 GPU when the mole ratio of EO to silver is 1, and the selectivity of propylene over propane is ca. 250.19 This value is comparable with the value in ref 8. The silver salt dissolved in PEO through the interaction of silver ions with the oxygen atom in PEO would be active for the olefin complexation. Table 1 shows the propylene and propane permeation properties through the PEG brush membranes containing silver salt. A pristine PSf membrane showed high permeance for both gases showing no selectivity. It is intriguing that the gas permeance decreased as an order of magnitude through the AgBF4 loaded on the PSf membrane without the PEG matrix. This result means that silver salts alone cannot induce the facilitation effect for olefin transport and act as a barrier for the gas transport. The immobilization of PEG on the PSf membrane caused ca. 30% decrease in gas permeance of propylene and propane compared with that of a pristine PSf membrane, which is a general trend for the case of the surface-modified membranes. For the PEG-immobilized brush membranes, both propylene and propane permeances decrease. Both propylene and propane permeances decrease with the loading amount of AgBF4. Decrease of gas permeability through the polymer electrolyte films can be explained as follows: (a) The chain mobility will be reduced when silver ions coordinate with the oxygen atoms of PEO, which was confirmed by the increasing glass transition temperature of the PEO-AgBF4 system. The reduced mobility would be due to the possibility of the role of anion as a cross-link point while cations were coordinated with functional groups of the polymer.20 Therefore decreasing mobility of the chain would affect the gas transport behavior with increasing loading amount of salt. (b) At higher salt concentrations, it is well-known that there are free ions, contact ion pairs, and higher ion aggregates in a number of polymer electrolytes at higher salt concentrations.21-26 Excess amount of salts loaded would decrease gas per(18) Although the silver ion dissolved in water would coordinate with PEG during the evaporation of solvent, we expected that the actual mole ratio, which would depend on the evaporation conditions as well as the interaction between silver ion and solvent, would be lower than the theoretical mole ratio. Therefore excess amount of silver salts were loaded in this experiment. (19) Preliminary results in this lab of PEO (linear PEO, Mw ) 1 × 106) composite membrane coated on PSf support. We assumed the permeance of propane to be 0.1 GPU because it was not detected below the practical lower limit of the bubble flow meter.; see also ref 8. (20) Kim, J. Y.; Hong, S. U.; Won, J.; Kang, Y. S. Macromolecules 2000, 33, 3161.
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meance and act as a barrier for the gas transport, as shown in Table 1. However, the interaction of silver ions with propylene molecules is specific and reversible; silver ions can act as propylene carriers for facilitated transport in the membrane and then lead a carrier-mediated transport in addition to a normal Fickian transport, which results in propylene/propane separation. The basic requirement in the fabrication of practical membranes is obtaining a minimum resistance to flow through the membrane coupled with maximum selectivity. According to the resistance model,27,28 the surface porosity is a significant factor to determine the selectivity. For example, when the surface porosity exceeds the value of 10-4 for PSf microporous membranes coated by silicone, the selectivity of the membrane approaches close to unity.27,28 From the comparison of the sizes between the PEG chains and the surface pores, it is reasonable to consider that the pores are not covered completely, (21) Teeters, D.; Frech, R. Solid State Ionics 1986, 18/19, 271. (22) Frech, R.; Manning, J.; Black, B. Solid State Ionics 1988, 28/30, 954. (23) Dissanayake, M. A. K. L.; Frech, R. Macromolecules 1995, 28, 5312. (24) Papke, B. L.; Ratner, M. A.; Shriver, D. F. J. Phys. Chem. 1981, 42, 493. (25) Huang, W.; Frech, R.; Wheeler, R. A. J. Phys. Chem. 1994, 98, 100. (26) Papke, B. L.; Dupon, R.; Ratner, M. A.; Shriver, D. F. Solid State Ionics 1981, 5, 685. (27) Henis, J. M. S.; Tripodi, M. K. J. Membr. Sci. 1981, 8, 233. (28) Mulder, M. Basic Principles of Membrane Technology, 2nd ed.; Kluwer Academic Publishers: Dordrecht, 1996; Chapter 6.
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resulting in low selectivity. However, improvement of the selectivity of silver-impregnated PEG brush membranes implies the role of silver carrier for olefin. It is also possible that the impregnation of silver salt would help the PEG chains extend and cover the pores because of the interaction between the silver ion and the EO unit. Whereas silver-PEG complexes facilitated the propylene transportation, propane could penetrate through the incompletely covered pores, resulting in low to moderate selectivity. Conclusions PEG brushes on PSF support membranes were prepared and the facilitated transport of propylene through the PEG brush membranes complexed with silver salt has been investigated. The photoreactive PEG-Az was immobilized by UV irradiation and the ultrathin film as a selective layer was formed on the PSf microporous membrane surface forming PEG brush membranes. The estimated thickness of the PEG brush was ca. 20.8 nm. The selectivity of propylene/propane also increased with increased loading amount of silver salt. The high propylene permeance as well as the high selectivity came from the facilitation action of the propylene carrier, that is, silver complexes in thin PEG matrix. Acknowledgment. We gratefully acknowledge financial support from the Ministry of Science and Technology of Korea through the Creative Research Initiative program. LA000466V
