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Covalently Bound Polymer Multilayers for Efficient Metal Ion Sorption J. S. Major and G. J. Blanchard* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 Received October 2, 2000. In Final Form: December 21, 2000 We report on the design, synthesis, and characterization of a polymeric multilayer assembly that can act as an efficient sorbent for selected metal ions. We use a covalent layer-by-layer deposition scheme for the construction of multilayer assemblies using the alternating copolymer of 4-hydroxyphenylmaleimide and ethyl vinyl ether 2-diisopropylphosphonate. The maleimide-vinyl ether (MVE) polymer layers are reacted at the maleimide hydroxyl functionality with adipoyl chloride to form an ester linkage, and the terminal acid chloride functionality is reactive toward subsequent deposition of another MVE polymer layer. The vinyl ether isopropylphosphonates remain protected during layer growth. Once layer growth is complete, the isopropylphosphonates are deprotected using bromotrimethylsilane to activate the multilayer for metal ion sorption. The uptake of Zr4+ by the multilayer is confirmed by ellipsometry, FTIR, X-ray photoelectron spectroscopy (XPS) and quartz crystal microbalance (QCM) gravimetry. The XPS data show ∼13 atom % of the multilayer is Zr4+ after exposure to a 5 mM ethanolic solution of zirconyl chloride, and QCM data show that metal ion uptake is fast. The films also show changes in FTIR spectra and ellipsometric thickness, indicating substantial structural changes associated with metal ion uptake.
Introduction Gaining the ability to design and construct interfaces with significant control over their macroscopic properties has attracted substantial research interest. This effort has been aimed largely at optimizing molecular-level organization, with Langmuir-Blodgett films, alkanethiolgold self-assembled monolayers (SAMs),1-4 and metal phosphonate multilayers5-7 being the most widely studied examples. The array of chemical functionalities that can be introduced into the layers, as well as the range of assembly and interlayer chemistry that can be accessed, affords great versatility in the properties of the resulting materials. The promise of interfacial chemistry lies in its potential application in areas such as biosensors/biorecognition,8,9 optical second harmonic generation,10,11 chemical sensors,12,13 and separations.14,15 The ability to assemble thin films with molecular-scale control over layer thickness and uniformity necessitates the use of well-defined and * To whom correspondence should be addressed. E-mail address:
[email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Zhang, Z. J.; Hu, R. S.; Liu, Z. F. Langmuir 2000, 16, 1158. (4) Skulason, H.; Frisbie, C. D. Langmuir 1998, 14, 5834. (5) Lee, H.; Kepley, L. J.; Hong, H. G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (6) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (7) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567. (8) Yoon, H. C.; Kim, H.-S. Anal. Chem. 2000, 72, 922. (9) Delamarche, E.; Sundarababu, G.; Biebuyck, H.; Michel, B.; Gerber, C.; Sigrist, H.; Wolf, H,; Ringsdorf, H.; Xanthopoulos, N.; Mathieu, H. J.; Langmuir 1996, 12, 1997. (10) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (11) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C. H.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (12) Brousseau, L. C., III; Aurentz, D. J.; Benesi, A. J.; Mallouk, T. E. Anal. Chem. 1997, 69, 688. (13) Brousseau, L. C., III; Mallouk, T. E. Anal. Chem. 1997, 69, 679. (14) Vrancken, K. C.; VanderVoort, P.; Gillisdhamers, I.; VanSant, E. F.; Grobet, P. J. Chem. Soc., Faraday Trans. 1992, 88, 3197. (15) Pfleiderer, B.; Albert, K.; Bayer, E. J. Chromatogr. 1990, 506, 343.
well-controlled reaction schemes. We have reported previously on the growth of layered polymeric interfaces using maleimide-vinyl ether (MVE) chemistry, where Zrbisphosphonate (ZP) interlayer linking chemistry is used to achieve layer-by-layer growth.16 In that work, the identity of the maleimide substituent determines the properties of the resulting multilayer interface. We report here on a different use of MVE alternating copolymers in the construction of multilayer assemblies. Our present focus is on the design, synthesis, and characterization of a covalently bonded multilayer structure where the polymer vinyl ether side groups are free to interact strongly with any metal ions they come into contact with. We construct the MVE copolymer using 4-hydroxyphenylmaleimide and ethyl vinyl ether 2-diisopropylphosphonate. The unique design aspect of this work is that we connect the polymer layers through the 4-hydroxyphenylmaleimide side groups instead of using the well-established metal ion coordination to the phosphonate side groups. We use adipoyl chloride to form covalent diester interlayer linkages. In contrast to the conventional ZP interlayer linking chemistry, where the phosphonates are sequestered by the formation of metal-bisphosphonate complexes, we use the side groups in these polymer multilayers in their deprotected form as active sites for metal ion uptake and report here on the interaction between deprotected polymer multilayers and Zr4+. To circumvent the formation of phosphoesters during layer growth, the phosphonate groups of the copolymer are protected by isopropyl functionalities. Even if these linkages form to some extent, the deprotecting agent bromotrimethylsilane (BTMS), which we use to activate the multilayers, has been shown to de-esterify phosphoesters.17 After the desired number of deposition cycles, four in this case, the phosphonate groups are deprotected using BTMS. We have chosen to demonstrate this chemistry using four layers as a matter of convenience. We have grown thicker layers and see no evidence to indicate a loss of reactivity beyond four layers. (16) Kohli, P.; Blanchard, G. J. Langmuir 1999, 15, 1418. (17) Chouinard, P. M.; Bartlett, P. A. J. Org. Chem. 1986, 51, 75.
10.1021/la0013953 CCC: $20.00 © 2001 American Chemical Society Published on Web 01/23/2001
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Scheme 1 . Monomers Used in the Synthesis of the MVE Alternating Copolymer Reported Here
Deprotection results in the facile hydrolysis of the phosphonate groups, rendering a multilayer film capable of efficient metal ion uptake. Exposure of the deprotected multilayer to ZrOCl2 solution results in rapid metal ion uptake. Ellipsometry, FTIR, and X-ray photoelectron spectroscopy (XPS) data each reveal significant changes in the multilayer structure after sorption of Zr4+. XPS measurements show that ∼13 atom % of the surface is comprised of Zr4+, consistent with the concentration of Zr4+ we would expect for saturation of the multilayer assembly. These data point to essentially irreversible uptake of the metal ion. XPS data on systems exposed to the ZrOCl2 solution for extended periods of time yielded the same concentration as those exposed for short times. Quartz crystal microbalance (QCM) data demonstrate the uptake of the Zr4+ is fast, with a steadystate loading density being achieved within seconds of exposure to metal ion solution. We report here on the design, synthesis, and characterization of these multilayer assemblies that act as metal ion “sponges”. Experimental Section Syntheses. The synthesis of maleimide-vinyl ether alternating copolymers has been described before.16,18 In this work, we use 4-hydroxyphenylmaleimide as one monomer. It is synthesized from maleic anhydride and 4-hydroxyaniline, and the reaction chemistry is the same as that described for similar, substituted maleimides.16 The vinyl ether phosphonate was prepared by reacting tri(isopropyl)phosphite with 2-chloroethyl vinyl ether according to a method described by Rabinowitz.19 This reaction was carried out under an inert argon atmosphere and refluxed at 170 °C for 5 days. The maleimide-vinyl ether alternating copolymer was prepared by radical copolymerization using azobisisobutyronitrile (AIBN) as the initiator (Scheme 1).18,20 We observe no homopolymerization of the monomers used in this work. Polymer Layer Deposition. Quartz and gold substrates, including QCMs, were cleaned using piranha solution (3:1 H2SO4/H2O2) and rinsed with ethanol and then with distilled water prior to layer deposition. The gold substrates were first exposed to a 10 mM solution of 6-mercapto-1-hexanol in a 60:40 ethanol/water solution for 1 h at 45 °C. Quartz substrates were exposed to a 5 M HCl solution for 30 min. Both substrates were rinsed with ethanol and water and then dried under a stream of nitrogen and exposed to adipoyl chloride in anhydrous acetonitrile under an argon atmosphere. The substrates were then removed and rinsed with ethyl acetate, dried under a stream (18) Kohli, P.; Scranton, A. B.; Blanchard, G. J. Macromolecules 1998, 31, 5681. (19) Rabinowitz, R. J. Org. Chem. 1961, 26, 5152. (20) Olson, K. G.; Butler, G. B. Macromolecules 1984, 17, 2480.
Major and Blanchard of nitrogen, and exposed to a 10 mM solution of the MVE polymer in DMSO for 1 h at 45 °C. The resulting polymer layer was reacted with adipoyl chloride and then immersed in the polymer solution, and the process was repeated as required to deposit the desired number of layers (Chart 1). Deprotection/Hydrolysis of the Diisopropylphosphonate Groups. Once the multilayer synthesis is completed, the polymer layers, which contain diisopropylphosphonate side groups, were exposed to bromotrimethylsilane (BTMS) in anhydrous acetonitrile under an argon atmosphere for 2 h. The substrates were then removed from the BTMS solution and immersed in acetonitrile for 30 min, rinsed with ethyl acetate, and dried under a stream of nitrogen. After the substrates were rinsed, ellipsometric measurements were taken, followed by the exposure of the substrates to a 5 mM ethanolic ZrOCl2 solution for at least 1 h. Optical Null Ellipsometry. Ellipsometric thickness measurements of the layers deposited on gold were made with a Rudolph Auto-EL II optical null ellipsometer operating at 632.8 nm. Rudolph DAFIBM software was used for data collection and processing. For all films, the refractive index was taken to be n ) 1.54 + 0i. Infrared Spectroscopy. For the multilayer assemblies on gold, FTIR spectra were acquired following each individual deposition cycle. IR spectra were also collected prior to multilayer reaction with BTMS and ZrOCl2. For all measurements, a Nicolet Magna 750 FTIR spectrometer was used, and for all data presented here, the instrumental resolution was set to 4 cm-1. The data were acquired using an external reflectance sample mount, set to an incidence angle of 80°. UV-Visible Spectrophotometry. UV-visible absorption data were acquired for samples grown on quartz substrates. Spectra were acquired after each deposition cycle using a Cary 300 UV-visible spectrophotometer. The wavelength range scanned was 190-500 nm, at a scan rate of 600 nm/min. The collected data were plotted using Microcal Origin 6.0 software. X-ray Photoelectron Spectroscopy (XPS). XPS data were acquired using a Perkin-Elmer Physical Electronics PHI 5400 X-ray photoelectron spectrometer. This system was equipped with a Mg X-ray source operated at 300 W (15 kV, 20 mA). The carbon (C 1s) line at 284.6 eV was used as a reference in determining the binding energies of the various species. Quartz Crystal Microbalance (QCM). Time-resolved solution phase QCM data were acquired using a Hewlett-Packard 53131A 225-MHz universal frequency counter with data acquisition programmed using National Instruments LabVIEW programming language. The in-house-built QCM holder was connected to a resonant oscillator circuit (Maxtek, Inc.) powered by a 5-V dc supply. A Neslab RTE-111 refrigerated circulating water bath was used for temperature control at 303 ( 0.01 K. The mounted QCM is immersed in a jacketed beaker containing 100 mL of mixed hexanes at 30 °C. The solution is stirred vigorously. Once a baseline oscillation frequency is established, 1.0 mL of a 5 mM solution of zirconyl chloride in 60:40 ethanol/water is introduced. Data collection is terminated after a new steadystate frequency had been established, typically within a minute.
Results and Discussion The focus of this work is on a novel covalent growth strategy for MVE polymers that allows their use as efficient metal ion sorbent materials. In this work, the ZP chemistry usually employed in the connection of layers is used instead as a chemically reactive site for metal ion complexation. The novel aspect of the work we report here lies in our ability to assemble these systems in a discrete manner such that the phosphonate groups are available at a later time for metal ion sequestration. The maleimide monomer we use in polymer synthesis possesses a terminal hydroxyl functionality which we react with adipoyl chloride to form interlayer linkages. Because we use a protected phosphonate-containing vinyl ether, formation of a phosphoester functionality does not compete efficiently with the formation of the diester. With this chemistry, the formation of porous multilayers is facile and subse-
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Chart 1. Idealized Schematic of the MVE Polymer System Showing Two Layers Connected by a Diester Linkage
quent reaction of the protected phosphonates with BTMS results in essentially complete deprotection. Several groups have explored the use of covalent ester and amide interlayer linking chemistry21-24 for the growth of thin films. The method we report here uses diester formation for layer adhesion, resulting in rapid growth of robust layers. By use of this chemistry, the assembly of a fourlayered system takes about 8 h. Optical null ellipsometry and FTIR and UV-visible spectroscopies all confirm the deposition of the polymer layers. The ellipsometry data (Figure 1) demonstrate a linear dependence of interface thickness on number of layers, with a slope of 16 ( 1 Å per adipoyl chloride-MVE (21) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 4655. (22) Beyer, C.; Bohanon, T. M.; Knoll, W.; Ringsdorf, H. Langmuir 1996, 12, 2514. (23) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (24) Van Ryswyk, H. B.; Turtle, E. D.; Watson-Clark, R.; Tanzer, T. A.; Herman, T. K.; Chong, P. Y.; Waller, P. J.; Taurog, A. L.; Wagner, C. E. Langmuir 1996, 12, 6143.
polymer layer growth cycle. Each ellipsometric data point is the average of 40 individual determinations at different locations on the sample surface. Semiempirical calculations for the layer unit suggest a thickness of 24 Å for the fully extended structure. The fact that we recover 16 Å/layer experimentally, which is the same as seen for layers of phenyl-substituted maleimide-containing polymer,16 is likely the result of the substantial lack of order in these layers. The ellipsometric measurements represent the average of the thickness distribution for the sampled area, and there is likely heterogeneity over length scales smaller than those sampled by the ellipsometric experiment. We have also used FTIR to monitor the layer-bylayer deposition of the polymer and present these data in Figure 2a. We monitor the carbonyl stretching region between 1770 and 1650 cm-1 because both the maleimide monomer and diester interlayer linkages contain these functionalities. Unfortunately, these bands are not resolved. The CdO absorbance depends linearly on the number of layers deposited (Figure 2b).
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Figure 1. Ellipsometric data for a four-layered MVE polymer system. Each layer consists of the MVE polymer and the adipoyl chloride underlayer. The slope of the best-fit line through the data is 16 ( 1 Å/layer.
Figure 2. (a) FTIR absorbance data for layers 1-4. The spectra are offset for clarity of presentation. (b) Carbonyl stretching band absorbance as a function of number of layers.
The UV-visible spectroscopic data were collected for each layer deposited and they also yielded a linear response per layer (Figure 3). The dominant absorption band is associated with the hydroxyphenyl succinimide moiety in the polymer backbone and its maximum is centered at 231 nm. We are not aware of any reports of the extinction coefficient for this chromophore, precluding our ability to estimate the loading density of the polymer layers on the surface. We use both ellipsometry and FTIR to monitor the deprotection/hydrolysis of the isopropylphosphonate groups. After the deprotection reaction, the ellipsometric thickness of the four-layer interface decreased from 83 ( 6 Å to 44 ( 1 Å (the uncertainties are (1σ). We believe that this decrease in thickness is the result of replacing the relatively bulky isopropyl groups with the smaller hydroxyl groups. This reduction in thickness can be
Major and Blanchard
Figure 3. UV-visible absorbance spectra of MVE polymer layers as a function of number of layers. Inset: Dependence of the 231 nm absorption intensity as a function of number of layers.
accounted for only in part by the change in physical size of the phosphonate terminal functionalities. For the deprotected phosphonic acid functionalities, the hydroxyl groups are capable of participating in hydrogen bonding. Hydrogen bonding between the hydroxyl groups would naturally result in a more compact structure, leading to a decrease in the thickness, as we observe. A key issue relative to the structural freedom within the polymer layers is the ability of the deprotected phosphonate groups to come into close proximity with phosphonate groups from adjacent layers. This is because of the ability of deprotected phosphonates to H-bond. Interlayer H-bonding can affect the thickness of the films substantially, while not affecting the availability of the phosphonate groups for metal ion complexation. The dramatic change in thickness upon deprotection argues for significant structural freedom within the polymer matrix. The FTIR data also confirm the deprotection/hydrolysis reaction based on the significant changes seen in the organophosphorus spectral region (Figure 4a). Unambiguous assignment of the features in this spectral region is not possible due to extensive overlap in the ∼14151085 cm-1 region, but it is known that the position of these resonances is sensitive to the nature of the substituents attached to these groups. A significant difference is observed in the spectrum after deprotection and exposure to the ionic solution (Figure 4a). The -OH stretching region at ∼3600 cm-1 exhibits a pronounced change upon exposure to Zr4+. The interfacial assembly we report here changes substantially upon exposure to Zr4+. After immersion in a 5 mM ethanolic ZrOCl2 solution, the polymer multilayer increases to 137 ( 23 Å, an increase of more than three times the original thickness of the deprotected polymer film suggesting substantial structural disruption upon metal ion sequestration. This increase in thickness resulting from Zr4+ uptake is likely related to the incorporation of nonstoichiometric waters of hydration in the multilayer matrix. The FTIR spectrum of the deprotected and zirconated polymer multilayer is shown in Figure 4b. After rinsing the zirconated multilayer with 100% ethanol, then with distilled water, we see a decrease in the thickness of the interface to 110 Å, which we attribute to the removal of nonstoichiometric waters of hydration incorporated into the multilayer structure during Zr4+ uptake. An FTIR spectrum taken after the
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Figure 5. XPS survey scan of a four-layer system following exposure to Zr4+.
Figure 4. (a) FTIR absorbance data for a four-layered system prior to deprotection of the phosphonate groups with BTMS (spectrum I), after deprotection with BTMS followed by hydrolysis (spectrum II), and after exposure to ZrOCl2 (spectrum III). The spectra are offset for clarity of presentation. (b) Comparison of the four-layered system, after exposure to ZrOCl2 (spectrum III) and after rinsing with absolute ethanol and then water (spectrum IV). These spectra are offset for clarity of presentation.
ethanol/water rinse confirms the loss of water, as seen by the decrease in the ∼3400 and ∼1500-1600 cm-1 regions (Figure 4b). Once this decrease has occurred, however, no further thickness change was observed, even after heating the interfacial assembly at 50 °C in 100% ethanol for 1 h. FTIR spectra taken before and after heating in 100% ethanol were identical. One issue of potential concern is whether the polymer swells upon exposure to ethanolic solution. We measured the ellipsometric thickness before and after immersion of the sample in a 60:40 ethanol/water solution and observed no thickness change to within the experimental uncertainty, indicating that these polymers do not swell upon exposure to the solvent system when metal ions are not present. When these same substrates are exposed to a solution of ZrOCl2 in 60:40 ethanol/water, the 100 Å thickness increase for the four-layer interface is reproduced readily. Examination of the interface FTIR spectrum after Zr4+ uptake revealed significant changes in the hydroxyl stretch region of the spectrum. One significant change is the shift in the peak maximum from ∼3200 to ∼3400 cm-1 (Figure 4b) with processing. The reason for the prominence of this band in spectrum III of Figure 4b is the presence of liquid water in the polymer multilayer after exposure to metal ions. When rinsed with ethanol, we observe a decrease in this peak (spectrum IV, Figure 4b), coincident with a decrease in the ellipsometric thickness, consistent with the removal of nonstoichiometric water from the polymer film. After the sample was heated to 50 °C in 100% ethanol
for 1 h, no further change was observed in the FTIR spectrum, again consistent with the ellipsometric data. We have studied the uptake of Zr4+ in the polymer multilayers using both XPS and quartz crystal microbalance gravimetry. We consider the XPS data first and present a survey scan of a zirconated four-layer assembly in Figure 5. For the area sampled, approximately 12.9 atom % of the interface is Zr4+.25 This finding suggests that most of the Zr4+ is complexed by one phosphonate group. XPS data also confirm the presence of Cl in the multilayer assembly in some of the samples, and the meaning of its presence is not completely clear. We posit two possible explanations for this observation. First, the presence of Cl may be consistent with the maintenance of macroscopic charge neutrality within the polymer matrix. We note that the observed Cl concentration is not present in a stoichiometric amount. Second, the Cl could be due to the presence of unreacted adipoyl chloride used as an interlayer linker. We note that hydrolysis of the acid chloride functionality is known to be very efficient, making this explanation unlikely. In some cases, Cl was observed prior to exposure to ZrOCl2 and in other cases no Cl was detected even after exposure to the metal ion solution, making a conclusive explanation impossible. We note the apparent absence of a P resonance in these data. XPS is relatively insensitive to P; however, 31P NMR data of polymer multilayers bound to silica and suspended in solution26 (not shown) reveal the presence of P in these multilayers. We note that the XPS indication of 1:1 complexation is consistent with the increase in layer thickness upon metal ion sorption. If 2:1 complexation dominated, it would cross-link the polymer matrix, precluding an increase in thickness. We consider the QCM data next. This measurement records the change in quartz crystal microbalance resonant frequency as a function of time. The injection of an aliquot of Zr4+ causes a change in the resonant frequency of a polymer-coated QCM suspended in solution, and we present these data in Figure 6. The response of a QCM to changes in mass is typically quantitative in the gas phase but not in solution due to the dielectric properties of the surrounding medium. Our interest in these data is not in terms of the quantitative mass uptake (this information is available from the XPS data) but rather in the time course of the QCM response. The kinetics of mass uptake for these systems can be understood in terms of the BET adsorption isotherm and thus we can obtain valuable information on the interaction of the metal ions (25) The concentration of Zr4+ was calculated using the raw experimental data and the sensitivity factors for the elements found. The corrected data indicate that 12.9% of the atoms detected by XPS are Zr4+ in the four-layer films. (26) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 695.
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Figure 6. QCM data showing rapid uptake of Zr4+ in a fourlayer film.
with the polymer matrix. We provide the QCM data in this paper only as a demonstration of rapid metal ion uptake and will treat these data in greater detail in a subsequent paper. We have found that the uptake of Zr4+ and Ca2+ (not shown) is very fast for this polymer multilayer structure. We note that among the complexities associated with extracting quantitative information from these data is the change in the thickness of the polymer layer as a result of metal ion complexation. Detailed modeling of this effect will need to be considered before useful kinetic information can be extracted from the data. Conclusions We have reported on two key points in this paper. First, we have demonstrated facile covalent layer-by-layer growth for MVE alternating copolymers. Second, we have demonstrated that these materials can be used as highly efficient metal ion sorbents based on the XPS loading data and the QCM kinetic data. We provide ellipsometric, spectroscopic (FTIR, UV-visible, and XPS), and gravimetric (QCM) data to support facile covalent layer construction and metal ion uptake. The ellipsometric, FTIR, and UV-visible spectroscopic data demonstrate linear layer growth and the XPS and QCM data provide
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the mass and kinetic uptake information for Zr4+ sorption from solution. The XPS are consistent with predominantly 1:1 PO32-/Zr4+ complexation, with the presence of Cl supporting this assertion. We observe a large ellipsometric thickness increase upon uptake of Zr4+ and speculate that the basis for this thickness change is (1) structural rearrangement within the polymer matrix to accommodate the presence of the metal ions and (2) the presence of nonstoichiometric water that is somehow associated with the incorporation of the metal ion. These data are not consistent with polymer swelling upon exposure to the solvent system we use. A substantial fraction of the thickness change can be eliminated by washing of the metal-containing polymer multilayer with ethanol, supporting the assertion that nonstoichiometric water is associated with the initial thickness increase on exposure to metal ions. Several issues remain to be explored for these polymer multilayers. One important consideration is that, for four polymer layers, the polymer matrix appears to be sufficiently open to allow ready access by metal ions. How the polymer matrix permeability will change with the growth of additional layers will ultimately determine the use of these materials for large-scale rapid metal ion sorption applications. A second vital issue that needs to be addressed is a detailed understanding of the metal ion uptake kinetics. Gaining this understanding will be complicated by the changes in the polymer matrix associated with the incorporation of the metal ions. Understanding the forward and reverse kinetic processes will allow us to determine the thermodynamic properties of these layers and thereby understand the extent to which metal ion uptake is reversible. These issues are under investigation. Acknowledgment. We are grateful to the U.S. Department of Energy for support of this research through Grant DEFG029ER15001. We thank Professor M. L. Bruening for the use of his FTIR, Dr. P Askeland for his assistance in acquisition and interpretation of the XPS data, and Dr. P. Kohli for his suggestions, insight, and encouragement. LA0013953
