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Competition of Hydrogen-Bonding and Electrostatic Interactions within Hybrid Polymer Multilayers Eugenia Kharlampieva and Svetlana A. Sukhishvili* Department of Chemistry and Chemical Biology, Stevens Institute of Technology, Hoboken, New Jersey 07030 Received June 24, 2004. In Final Form: September 8, 2004 Using a layer-by-layer sequential adsorption technique, we report the construction of hybrid films in which layers of hydrogen-bonded polymers are embedded within electrostatically associated polyelectrolytes. The components of the hybrid film include a neutral hydrogen-bonding polymer, a weak polycarboxylic acid, and a strong polycation. Depending on the pH value used for the deposition of the electrostatic film, we found two distinctive regimes of film growth. At pHs lower than a critical value, deposition of electrostatic layers occurred on top of hydrogen-bonded stacks to produce hybrid, three-component films. At pHs higher than a critical value, neutral, hydrogen-bonded chains were displaced by the adsorbing chains of the polycation, producing two-component films. The property of the hydrogen-bonded stacks of hybrid films to be selectively dissolved by exposing them to a high pH makes these films promising candidates for producing free polyelectrolyte films.
Introduction Layer-by-layer sequential adsorption of polymers on solid substrates is a simple and versatile technique for producing ultrathin coatings.1-8 In these films, polymer layering is templated by a substrate, and though interlayer boundaries are not sharp, densities of the film constituents show distinct oscillation along the surface normal.2 The surface structures produced by such a layer-by-layer deposition technique are usually referred to as polyelectrolyte multilayers (PEMs), since the majority of studies involve charged polymers. The film formation in this case is controlled by the requirement of charge neutralization, as polymers of opposite charge self-assemble within a film. The use of weak polyelectrolytes as film constituents represents a simple way to engineer responsive PEMs whose properties can be tuned by both changes in pH values during self-assembly as well as by variation of pH values after self-assembly.9,10 Multilayer growth based on hydrogen-bonding interactions is also possible.11-13 As has been recently shown by one of us, pairs of water-soluble uncharged molecules showing intermolecular hydrogen-bonding interactions, such as polycarboxylic acids at low pH values and poly(ethylene oxide) or poly(vinylpyrrolidone), can also be self* To whom correspondence should be addressed. (1) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (2) Decher, G. Science 1997, 277, 1232. (3) Lvov, Yu.; Haas, H.; Decher, G.; Mo¨hwald, H.; Mikhailov, A.; Mtchedlishvily, B.; Morgunova, E.; Vainshtein, B. Langmuir 1994, 10, 4232. (4) Cheung, J. H.; Stockton, W. B.; Rubner M. F. Macromolecules 1997, 30, 2712. (5) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (6) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (7) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (8) Graul, W. G.; Li, M.; Schlenoff, J. B. J. Phys. Chem. B 1999, 103, 2718. (9) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (10) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (11) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (12) Wang, L. Y.; Fu, Y.; Wang, Z. Q.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360. (13) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550.
assembled at surfaces.13,14 We have also demonstrated that hydrogen-bonded self-assembly could be performed on particulate substrates.15 The latter opened the way to explore polymeric hydrogen-bonded self-assembly as an effective means of encapsulating various chemicals for controlled delivery applications. The distinctive feature of hydrogen-bonded self-assembly involving polycarboxylic acids is that, unless crosslinked, they can be controllably dissolved at a higher pH value, when ionization of a polyacid increases.11-13 Several routes for cross-linking of hydrogen-bonded self-assembly have been reported. Recently, Rubner et al. developed thermal and photochemical techniques for stabilizing hydrogen-bonded multilayers.16 More recently, we described cross-linking of hydrogen-bonded capsules in solution using carbodiimide chemistry.15 Note that earlier Serizawa et al. reported on the stabilization of electrostatically assembled multilayers by means of chemical cross-linking using carbodiimide chemistry.17 Building polymer hybrid films containing stacks of hydrogen-bonded and electrostatically adhering polymers presents an attractive means to regulate the film deconstruction. This feature is useful for drug delivery applications. Another promising utility of hybrid hydrogenbonding/electrostatic self-assembly is for preparing freestanding polyelectrolyte membranes. In this scenario, hydrogen-bonded stacks act as release layers that could be dissolved in an aqueous environment, when the pH is changed from an acidic to a more neutral value. Recently, using polymers that bind primarily through hydrogenbonding, Caruso et al. showed that the deconstruction properties of hydrogen-bonded multilayers sandwiched between electrostatically assembled stacks were dependent upon the number of layers deposited within the hydrogen-bonded stacks.18 Electrostatically assembled hybrid films that can be selectively decomposed in response (14) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (15) Kozlovskaya, V.; Ok, S.; Sousa, A.; Libera, M.; Sukhishvili, S. A. Macromolecules 2003, 36, 8590. (16) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (17) Serizawa, T.; Nanameki, K.; Yamamoto, K.; Akashi, M. Macromolecules 2002, 35, 2184. (18) Cho, J.; Caruso, F. Macromolecules 2003, 36, 2845.
10.1021/la0484321 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/21/2004
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to pH and ionic-strength variations have also been reported by Dubas et al.19 In this paper, we study, in a wide range of experimental conditions, the construction of hybrid multilayers containing stacks of decomposable hydrogen-bonded and electrostatically adhering polymers. We found that, depending on the external pH, there are two distinct regimes for multilayer deposition. In the first regime, when the ionization of self-assembled polyacid is low, hydrogenbonded stacks are stable and hybrid multilayers can be constructed by depositing electrostatic layers on top of the film. In the second regime, the deposition of electrostatically adhering polymers displaces neutral polymer from the hydrogen-bonded stacks, producing purely electrostatically assembled films. This study is specifically focused on understanding different routes of film response and provides an essential step toward the rational design of hybrid multilayers. While hybrid films can be successfully constructed in the first regime, the exchange of polymer chains within multilayers occurring in the second regime presents an easy, one-step way to readjust the chemical composition of the film after the film is already constructed. Experimental Section Materials. The cationic polymer polymethacryloxyethyltrimethylammonium bromide (PMAETM) with a Mw of 200 000 was obtained from Polysciences, Inc. Poly(vinylpyrrolidone) (PVPON) with a Mw of 360 000 and poly(methacrylic acid) (PMAA) with a Mw of 150 000 were purchased from Scientific Polymer Products, Inc. To eliminate possible traces of residual monomer, the samples were dialyzed against deionized water using tubing with a molar mass cutoff of 3000 Da. The dialysis was continued for 3 days (with the water bath frequently changed to purified water during that time), and after that, all the samples were freeze-dried. Poly(4-vinylpyridine) (PVP) with a Mw of 200 000 (Scientific Polymer Products, Inc.) was quaternized with ethyl bromide in ethanol solution using well-established methods20 to obtain a polymer of 98% pyridinium units (Q98) as determined by infrared spectroscopy. In these experiments, the solvent was D2O rather than water. This eliminated overlap of the IR spectra of polymers in the 1700-1500 cm-1 region with the strong water band. D2O with 99.9% isotope content was purchased from Cambridge Isotope Laboratories and was used as received. To control pH and ionic strength, concentrated HCl and the inorganic salts Na2HPO4 and NaH2PO4 (General Storage, pure grade) were used as received. The H2O used for glassware cleaning was deionized and further purified by passage through a Milli-Q system (Millipore). Deposition of the Multilayers. Multilayers were prepared on the surface of an oxidized attenuated total reflection (ATR) silicon crystal by the sequential deposition of a polybase and a polyacid from 0.1 mg/mL buffered solutions in deuterium oxide. PMAA, a weak polyacid with a solution pKa of ∼6,14 was a common constituent of both hydrogen-bonded and electrostatic stacks. To produce hydrogen-bonded self-assembly, PMAA was sequentially deposited with PVPON. For electrostatic self-assembly, a polycation with a permanent charge density, PMAETM, was used. The silicon crystal was installed within a flow-through stainless steel cell. Prior to multilayer deposition, the surface of the silicon crystal was pretreated with a 0.1 mg/mL solution of 98%quaternized poly-N-ethyl-4-vinylpyridinium bromide at pH 9.2. This first layer was deposited to increase the adhesion of the multilayer to the surface and in a majority of experiments was taken as a background. The amounts of polymers adsorbed and the degree of ionization of the carboxylic groups were quantified (19) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368. (20) Margolin, A. L.; Izumrudov, V. A.; Sˇ vedas, V. K.; Zezin, A. B.; Kabanov, V. A.; Berezin, I. V. Biochim. Biophys. Acta 1981, 660, 359.
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Figure 1. Representative FTIR-ATR spectra of a nine-layer PVPON/PMAA film with a PMAETM layer deposited on top at pH 5.5. The deposition time of the PMAETM layer was 2 min (top panel) and 70 min (bottom panel). The PVPON/PMAA multilayer (total amount adsorbed 30 mg‚m-2) was deposited at pH 2. The measurement was obtained in D2O solution using 0.01 M phosphate buffer. The two spectral integration and curvefitting intervals, 1775-1590 and 1590-1505 cm-1, are shown by arrows. by in situ Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR) using experimental protocols and calibration methods that were described earlier.21,22 Representative spectra of the hybrid multilayers are shown in Figure 1 and discussed in the next section. The adsorbed amounts of PMAA and PVPON were quantified from the calibrated intensities of the vibrational bands of -COO- and -COOH and the CON stretch region of the pyrrolidone ring, as described ealier.14 The calibration constant for the adsorbed amount of PMAETM was determined in this work using the 1723 cm-1 PMAETM carbonyl band which was shifted to higher energies compared to the PMAA and PVPON carbonyl vibrational peaks. This value was 0.035 abs.units m2‚mg-1.
Results and Discussion Nine PVPON/PMAA layers were first deposited at pH 2 on the surface of the precursor-modified silicon crystal. The multilayer growth was linear, with constant amounts of polymers deposited at each step, in good agreement with our earlier report,13 and the amounts of PVPON and PMAA adsorbed at each step of the layer-by-layer deposition were 2 and 3.8 mg‚m-2, respectively. The hydrogenbonded multilayers contained PMAA as the outermost layer. Electrostatic PMAETM/PMAA multilayers were then deposited on top of the hydrogen-bonded film, starting with PMAETM, at a higher and constant pH ranging from 3 to 5.5. The infrared spectra of a nine-layer PVPON/ PMAA film with a top PMAETM layer deposited at pH 5.5 are shown in Figure 1. The deposition time of the PMAETM layer was 2 min (top panel) and 70 min (bottom panel). Both spectra show three major peaks: two broad absorption bands at 1710 and 1650 cm-1 and a band at 1552 cm-1. The peak intensities of the spectra were integrated by curve-fitting using Galactic Grams/32 software in two spectral regions: 1775-1590 cm-1 (region 1) and 1590-1505 cm-1 (region 2). In each series of (21) Sperline, R. R.; Maralidharan, S.; Feiser, H. Langmuir 1987, 3, 198. (22) Azzopardi, M. J.; Arribart, H. J. Adhes. 1994, 46, 103.
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Figure 2. The response of a nine-layer PVPON/PMAA film to adsorption of PMAETM at pH 5 (top panel) and pH 5.5 (bottom panel). The amount of PMAETM adsorbed (open circles) and mass loss of PVPON (filled circles) are plotted as a function of time. The PVPON/PMAA multilayer (total amount adsorbed 30 mg‚m-2) was deposited at pH 2. The experiments were done in D2O, which contained 0.01 M phosphate buffer.
experiments, the same curve-fitting file with fixed band centers and widths was applied to all spectra for consistency. The following peaks were introduced in the curvefitting: (a) peak at 1723 cm-1 with width fixed at 24 cm-1, associated with PMAETM carbonyl group vibration; (b) peaks at 1701 and 1671 cm-1 with widths fixed at 40 cm-1, associated with stretch vibrations of uncharged carboxylic groups (ν, CdO); (c) peaks at 1659 and 1641 cm-1 with widths of 23 cm-1, assigned to carbonyl stretch vibrations of the pyrrolidone ring which do not participate in binding with the polyacid, and a weaker peak at 1622 cm-1 of a 29 cm-1 width, corresponding to hydrogen-bonded pyrrolidone groups; and (d) a band at 1552 cm-1, associated with asymmetric stretching vibrations of carboxylate groups (νa, COO-). It is seen that, with an increase in adsorption time, the contribution of the PMAETM peak increases and drastically affects the contribution from the other peaks in these spectral regions. Figure 2 contrasts the deposition of the first PMAETM layer onto a predeposited nine-layer PVPON/PMAA film at pH values of 5 and 5.5. Note that PMAA brought to the surface by hydrogen-bonded self-assembly always remained bound at the surface. However, as Figure 2 shows, the retention of PVPON within the films was drastically dependent on the pH value used in the deposition of electrostatically self-assembled stacks. A striking observation in Figure 2 is that a relatively small increase in the pH from 5 to 5.5 resulted in a completely different response of the hydrogen-bonded film to PMAETM adsorption. It is seen that, while almost all PVPON was retained within the film at pH 5, complete release of PVPON from the film occurred at pH 5.5, suggesting the replacement of PVPON chains with adsorbing PMAETM chains within the multilayer. A similar exchange of polymer chains within a multilayer film has been recently observed by Voegel, Schaaf et al. for the case of charged polymer chains.23 In contrast to these studies, in the PVPON/PMAA/PMAETM system, the competition occurred between neutral, hy-
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drogen-bonded and charged, electrostatically associated macromolecules. Note that the difference in the mechanism of the film response at pH 5 and 5.5 was also evident in distinctively different kinetics of the PMAETM adsorption that is shown in Figure 2. For the same PMAETM solution concentration, the characteristic time required for binding half of the saturation amount of PMAETM was much smaller, ∼10 min, at pH 5, compared to 50 min at pH 5.5. This difference reflects the fact that, at pH 5, the hydrogen-bonded film did not change its composition in response to PMAETM adsorption and the rate-limiting step was probably diffusion of PMAETM to the surface and conformational changes of PMAETM within the top adsorbed layer. At pH 5.5, in addition to binding to the film surface, PMAETM chains also replaced PVPON, within the whole film, from its complex with PMAA. This slower, rate-limiting exchange of polymer chains explains the slower kinetics of PMAETM binding at pH 5.5. To rationalize the fact that competitive binding of PMAETM and PVPON with PMAA strongly depends on the external acidity of the solution, consider the competition between PVPON/PMAA and PMAETM/PMAA units. Scheme 1 illustrates that, at high acidity, hydrogenbonding interactions are favored, while, at higher pH values, electrostatic pairing is preferred. Experimentally observed sharpness, in the pH scale, of the transition from hydrogen-bonding to electrostatic pairing originates from the intrinsically highly cooperative nature of polymerpolymer association. The experimentally observed pH-dependent transition between hydrogen-bonding and electrostatic association regimes is in good agreement with earlier data on selective pH-dependent binding of polycarboxylic acids with various polymer chains in solution.24 In this work, we have measured the average ionization of self-assembled PMAA in situ and correlated this value with the propensity of PMAETM to replace PVPON within the film. Figure 3A shows, for several pH values, the evolution of the amount of PVPON released from a nine-layer film as a function of the average ionization of PMAA as PMAETM was allowed to adsorb on the top of the hydrogen-bonded stratum. The data are obtained in the experiments illustrated in Figure 2. Figure 3B shows that an increase in average ionization of polyacid was caused by the adsorption of PMAETM and that the correlation between PMAA ionization and the amount of polycation bound to the film was linear. At small ionization of self-assembled PMAA (and, correspondingly, at small amounts of PMAETM adsorbed), hydrogen-bonded self-assembly was stable and no PVPON was released from the film. This regime is achievable either by shortening the time allowed for PMAETM adsorption or by lowering the pH of polycation deposition (see data in Figures 2 and 3). In each scenario, no release of PVPON occurs unless PMAA ionization exceeds ∼25%. This critical value was defined as the PMAA ionization at which ∼5% of PVPON was released from the film. Note that the ionization of a single PMAA layer could not be determined in our experiment and the reported value of the ionization is averaged over all layers of self-assembled PMAA. In the low-ionization region, the hydrogen-bonding stratum remains intact upon deposition of polycations, and hybrid films are formed. Using an analogy with the exchange of polyelectrolyte chains in solution, this regime corresponds to the formation of triple complexes in solution, when the PMAA chain is (23) Boulmedais, F.; Bozonnet, M.; Schwinte´, P.; Voegel, J.-C.; Schaaf, P. Langmuir 2003, 19, 9873. (24) Abe, K.; Koide, M.; Tsuchida, E. Macromolecules 1977, 10, 1259.
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Scheme 1. pH-Dependent Chain Association in the PVPON/PMAA/PMAETM System
Figure 3. Correlation between the amount of PVPON released and the average ionization of self-assembled PMAA (part A) and between the average ionization of PMAA and PMAETM adsorption (part B) during PMAETM deposition at pH 5.0 (circles), pH 5.3 (triangles), and pH 5.5 (squares), from the ninelayer PVPON/PMAA film. The conditions are the same as those in Figure 1, except that an additional experiment at pH 5.3 is shown.
associated with both PVPON and PMAETM chains. In the case of the multilayer film, this triple complex is formed at the film surface. We have shown in our earlier work that purely hydrogen-bonded PVPON/PMAA self-assembly disintegrates when the critical ionization of PMAA exceeds ∼15%.14 The high polyacid ionization of 25% required for release of PVPON chains in the PVPON/ PMAA/PMAETM system reflects the stabilizing effect of the polycation, which compensates for unfavorable electrostatic repulsions between negative charges emerging within the PVPON/PMAA stratum as a result of the increased external pH and adsorption of the polycation. An increase in the ionization of PMAA was accompanied by a decrease in the number of hydrogen bonds within the PVPON system, and the chain exchange occurred when the number of PVPON/PMAA hydrogen bonds decreased below a certain critical value. Using a procedure described earlier,14 and assuming that an extinction coefficient of PVPON segments is not influenced by hydrogen bonding, we have estimated the fraction of hydrogen-bonded PVPON units. We found that displacement of the PVPON chains occurred after the average fraction of hydrogen bonds per PVPON molecule decreased, during polycation
Figure 4. (top panel) Amount of PMAETM deposited (filled squares) and average PMAA ionization before (filled circles) and after (open circles) PMAETM deposition as a function of pH. (bottom panel) Amount of PVPON retained within multilayers as a function of pH. Except for the wider pH range studied here, the conditions are the same as those in Figure 2.
adsorption, from ∼25% in purely hydrogen-bonded PVPON/PMAA multilayers at pH 5.5 to ∼14%. Comparison of the critical PMAA ionization (25%) with the critical fraction of hydrogen-bonded PVPON units (14%) suggests comparable energetics of electrostatic and hydrogen bonds in this system. To further investigate the driving force for the binding of PMAETM with hydrogen-bonded film, we studied the adsorption of the first PMAETM layer in a wide range of pHs. The top panel in Figure 4 (filled squares) shows the amounts of PMAETM deposited within the first electrostatically adhering layer on top of a nine-layer PVPON/ PMAA film as a function of pH. The deposition increased with pH, and especially large amounts of PMAETM were deposited at pH 5-5.5. However, ionization of the hydrogen-bonded film before the polycation adsorption (filled circles in the top panel of Figure 4) did not show such a strong dependence on pH and changed only from 0.5 to 3% in the pH range from 3.0 to 5.5. These values are much lower than the ionization of free PMAA chains in solution at these pH values. The difference is explained by strong hydrogen bonding of protonated carboxylic groups with PVPON units, which decreases the dissociation constant of -COOH groups. However, after PMAETM adsorption, PMAA ionization within the PVPON/PMAA film was enhanced (open circles in Figure 4) and showed good correlation with the amount of adsorbed polycation. A simple calculation of charge balance showed that the charge-to-charge ratio was about unity during the poly-
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Table 1. Growth of PMAETM/PMAA Electrostatic Films on top of a Nine-Layer PVPON/PMAA Hydrogen-Bonded Film as Determined by in situ FTIR-ATR growth conditionsa
amount of PMAETM deposited per layer (ΓPMAETM), mg‚m-2
amount of PMAA deposited per layer (ΓPMAA), mg‚m-2
stability of the hydrogen-bonded stratum
pH 3 pH 4.5 pH 5.3 pH 5.5
0.28 2.28 9.4 16.8
1.88 5.14 9.14
stable partial stability (20% of PVPON released) unstable
a Hydrogen-bonded film was deposited from 0.1 mg/mL D O solutions at pH 2, adjusted with hydrochloric acid. The amount of PMAETM 2 deposited within the first layer is not taken into account.
cation adsorption at each specific pH value (data not shown). These data suggest that the polycation adsorption is largely controlled by electrostatic interactions between PMAETM and the PVPON/PMAA stratum, and by the requirements for charge neutralization within the selfassembled film. A large increase in the ionization of PMAA after the deposition of the first PMAETM layer is explained by the local electrostatic effects of the polycation on the PMAA ionization. In related studies, high acidity of weak polyacids sandwiched between polycations within multilayers has been reported for dry multilayer films9,25 and for films in contact with solution.26-28 The increase of ionization is due to a strong electric charge created by the adsorbed polycation that causes the adjacent PMAA molecules to ionize. The cooperativity of the formation of polymer-polymer sequences enhances this effect. In our earlier publication, we showed that the charge adjustment is not restricted to the outermost layer but occurs within a few polymer layers from the film surface.27 A likely scenario is that the adsorption of PMAETM induces the ionization of PMAA within several top layers of the film. Depending on the external pH, the induced ionization could be relatively low so that PMAETM remains bound to the film surface, or high enough to trigger the replacement of PVPON chains with the polycation chains within the film. As shown in the bottom panel of Figure 4, polycation-triggered release of PVPON chains from the film sets on at pH > 5.3. It is interesting that, at pH 5.5, PVPON release goes to completion until all PVPON chains included in a nine-layer hydrogen-bonded stratum are released into solution. It is likely that the chain exchange starts from the surface, where polycation-induced ionization of the polyacid is the largest, and then propagates within the film interior. The result strongly suggests that PMAETM chains penetrate the self-assembled film. The observation of polyelectrolyte chain exchange within polymer multilayers is an intriguing phenomenon that clearly shows certain similarities with the exchange of polyelectrolyte chains when they constitute water-soluble polyelectrolyte complexes.24 This result is also consistent with the picture of multilayers as swollen structures with enough free space to permit transport of macromolecules through the films. Electrostatic stacks were then deposited on top of the hydrogen-bonded film in a wide range of pHs, but these hybrid multilayers were only produced at pH e 5.3. The multilayer growth was robust, with the same amounts of PMAA or PMAETM deposited at each step, starting from a second electrostatically deposited layer. At a low pH value of the external solution (pH 3), the amount of the polycation binding to the PVPON/PMAA was very small, (25) Mendelsohn, J. D.; Barrett, C. J.; Chan, V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (26) Rmaile, H. H.; Schlenoff, J. B. Langmuir 2002, 18, 8263. (27) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 1235. (28) Petrov, A. I.; Antipov, A. A.; Sukhorukov, G. B. Macromolecules 2003, 36, 10079.
∼0.1 mg‚m-2, to ensure monolayer coverage. For the first PMAETM layer, large amounts of PMAETM deposited at pH 5.5 reflect the inclusion of PMAETM chains within the film; these amounts are controlled by the requirement that PMAETM binds with all PMAA chains available within the film. At later deposition steps, when no polymer exchange occurs within the film, the amounts of polymers are smaller and regulated by charge neutralization at the film surface. The incremental amounts of PMAA and PMAETM deposited within electrostatic self-assembly at various pH values are shown in Table 1. One can see that larger amounts of polymers are deposited within the film at higher pH values. Control experiments showed that the same amounts of polymers were deposited within the PMAETM/PMAA stratum at pH 4.5 and 5.5 on a silicon wafer substrate free of hydrogen-bonded layers. Also seen in Table 1 is that the ratio of PMAA to PMAETM amounts is larger at lower pH values, when the ionization of the polyacid is decreased. This is in good agreement with experiments on the adsorption within a single adsorption layer29,30 as well as with experiments with multilayers composed of weak polyacids.9,31,32 This effect is consistent with an electrostatic mechanism or charge-exchange mechanism of adsorption, where the number of charged units, not the total mass adsorbed, is the important parameter. It is also intriguing to consider the local ionization of PMAA self-assembled within the hybrid film. Figure 5 shows the average ionization of PMAA in the hybrid film as a function of the number of layers deposited. One can see that, up to the 10th layer, when hydrogen-bonded film was deposited at pH 2, PMAA was not ionized. It is also seen that after a nine-layer PVPON/PMAA film was exposed to a buffer solution at pH 4.5, there was only a slight increase in PMAA ionization from 0 to 1%. The ionization of PMAA within the hydrogen-bonded stratum was denoted as RHB. The value of RHB at pH 4.5 was significantly lower than the 10% ionization of free PMAA chains in solution. This difference reflects the harder dissociation of protons from carboxylic groups when included within a hydrogen bond. The deposition of the first PMAETM layer resulted in a sharp increase of the average PMAA ionization up to 7%, as shown in Figure 5. Further construction of the electrostatic stratum was accompanied by growth of the average ionization of the polyacid, as is expected for the growing contribution of electrostatically associated PMAA into the total absorbance. Ionization of PMAA within the electrostatic stratum was directly measured in this work, when an 11-layer film (nine layers of PVPON/PMAA and two “insulating” (29) van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538 and references therein. (30) Bo¨hmer M. R.; Heesterbeek, W. H. A.; Deratani, A.; Renard, E. Colloids Surf., A 1995, 99, 53. (31) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (32) Chen, K. M.; Jiang, X. P.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825.
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Figure 5. The average ionization of self-assembled PMAA plotted against the number of polymer layers deposited. The vertical line separates hydrogen-bonded layers from the electrostatically assembled stratum. The dashed horizontal line shows PMAA ionization in solution at pH 4.5. The experimental uncertainty in determining the degree of PMAA ionization was within the symbol size.
layers of PTMEA/PMAA) was taken as a background during spectra collection, while additional PTMEA/PMAA layers were deposited. The ionization of PMAA for the film containing PMAETM (RELp) and PMAA (RELa) in an outmost layer was 38 and 30%, respectively. Oscillations of ionization with layer number are also seen in Figure 5, with higher ionization of the polyacid at the step of PMAETM deposition. This observation is consistent with earlier reports on the construction of multilayers composed of weak polyelectrolytes26-29 and reflects enhancement of PMAA ionization by positively charged PMAETM chains. It is also interesting to study the stratification of the ionization of PMAA molecules which are included within hydrogen-bonded and electrostatically assembled stacks of the hybrid film. Note that the ionization of PMAA within the electrostatic stratum was much higher than the average PMAA ionization (see Figure 5) measured from the whole hybrid film. The simplest assumption is that PMAA ionization within the film is bimodal and that the ionization of PMAA molecules embedded within hydrogenbonded stacks (except those PMAA molecules that interface the electrostatic stratum) is not affected by the
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deposition of the PMAETM/PMAA stack. Since it is not experimentally feasible to directly measure the ionization of PMAA with the hydrogen-bonded stack embedded within the electrostatic stratum, we checked the validity of this assumption by making a simple calculation of the average ionization of PMAA expected for the hybrid film in this case (Rav). From the known amounts of PMAA deposited within each stratum and measured values of PMAA ionization within hydrogen-bonded and electrostatic stacks (RHB ) 1%, RELp ) 38%, and RELa ) 30%), we calculated the average PMAA ionization for hybrid films using the following simple additivity rule: Rav ) mRHB + nREL, where m and n are the mass fractions of PMAA deposited within hydrogen-bonded and electrostatic stacks, respectively. The calculated values were in good agreement (with a maximum deviation in PMAA ionization of 2%) with the experimental data shown in Figure 5. This comparison confirms that the ionization of PMAA within hybrid films is highly stratified, in the direction normal to the surface. At the same external pH, local ionization of a self-assembled polyacid greatly depends on the type of intermolecular adhesion. Within hydrogen-bonded stacks, the ionization of the polyacid is greatly suppressed, but it is increased, compared to the solution value, within the electrostatically assembled strata. In summary, we found different regimes, in the scale of external pH, of construction of hybrid multilayers composed of hydrogen-bonded and electrostatic strata, in which a weak polyacid is included in both strata. At moderately acidic pH values, hybrid layers could be successfully constructed. At neutral pH values, hybrid films were not formed, but polyelectrolyte exchange occurred within the film, with release of neutral polymer and formation of a two-component film. On the basis of the known property of hydrogen-bonded strata to dissolve, in a controlled way, when exposed to high pH values,13,14 we suggest that hydrogen-bonding/electrostatically assembled hybrid films are promising candidates for producing free polyelectrolyte films and membranes. Acknowledgment. This work was supported by the National Science Foundation under Award DMR-0209439. LA0484321
