Control of the Water Permeability of Polyelectrolyte Multilayers by

May 6, 2004 - The diffusion of water in protected and unprotected multilayers is studied by in situ neutron reflectometry. Whereas a top layer of wax ...
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Control of the Water Permeability of Polyelectrolyte Multilayers by Deposition of Charged Paraffin Particles Karine Glinel,*,†,‡ Michelle Prevot,† Rumen Krustev,†,§ Gleb B. Sukhorukov,† Alain M. Jonas,| and Helmuth Mo¨hwald† Interfaces Department, Max Planck Institute of Colloids and Interfaces, D-14476 Golm/Potsdam, Germany, UMR 6522 CNRS, Universite´ de Rouen, F-76821 Mont-Saint-Aignan, France, Hahn-Meitner-Institute, D-14109 Berlin, Germany, and Department of Materials Science and Processing, Universite´ Catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium Received November 4, 2003. In Final Form: March 26, 2004 We present a new way to protect polyelectrolyte multilayers from water, consisting in the adsorption and subsequent fusing of charged wax particles atop a multilayer. The formation of the wax layer is demonstrated by different techniques such as ellipsometry, contact angle measurements, and atomic force microscopy. The diffusion of water in protected and unprotected multilayers is studied by in situ neutron reflectometry. Whereas a top layer of wax crystals already allows substantial reduction of the diffusion, the fusion of this top layer leads to the dominating exclusion of water from the multilayers when dipped in water. This method opens up new interesting avenues for polyelectrolyte multilayers in practical applications where permeability of water, ions, or hydrophilic drugs is an issue.

Introduction Layer-by-layer (LbL) assembly, a technique based on the alternate adsorption of oppositely charged polyelectrolytes on charged surfaces, has been extensively used to produce ultrathin films due to the versatility, the simplicity, and the flexibility of the buildup process.1-4 A variety of materials such as dyes, dendritic molecules, proteins, conductive polymers, and inorganic particles were successfully included in LbL assemblies for potential applications in biosensors, microelectronics, optics, and so forth.1,4 However, the films produced by this method are soft materials which are sensitive to atmospheric humidity5,6 and other environmental conditions such as high or low pH or high ionic strength.7-16 This environment-sensitive behavior may cause the denaturation or * Author for correspondence. E-mail address: karine.glinel@ univ-rouen.fr. Phone: ++ 33 2 35 14 65 86. Fax: ++ 33 2 35 14 67 04. † Max Planck Institute of Colloids and Interfaces. ‡ Universite ´ de Rouen. § Hahn-Meitner-Institute. | Universite ´ Catholique de Louvain. (1) Arys, X.; Jonas, A. M.; Laschewsky, A.; Legras, R. In Supramolecular Polymers; Cifferei, A., Ed.; Marcel Dekker: New York, 2000; p 505. (2) Bertrand, P.; Jonas, A. M.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (3) Decher, G. Science 1997, 277, 1232. (4) Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003. (5) Ku¨gler, R.; Schmitt, J.; Knoll, W. Macromol. Chem. Phys. 2002, 203, 413. (6) Steitz, R.; Leiner, V.; Siebrecht, R.; Klitzing, R. v. Colloids Surf., A 2000, 163, 63. (7) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. 1996, 100, 948. (8) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (9) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (10) Wang, T. C.; Cohen, R. E.; Rubner, M. F. Adv. Mater. 2002, 14, 1534. (11) Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2003, 19, 1235. (12) Kim, B. Y.; Bruening, M. L. Langmuir 2003, 19, 94. (13) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Langmuir 2002, 18, 5607. (14) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006.

the destruction of the film, which is disadvantageous for specific applications requiring the stability of the internal structure of the film. The diffusion of small molecules in the assembly can also be detrimental for active compounds included in the layers: One example is the diffusion of urea into myoglobin-containing multilayers reported to denature the protein.17 In this context, different routes were investigated to improve the stability of LbL assemblies: The subsequent cross-linking of polyelectrolyte chains was employed to stabilize the internal structure of the multilayers.17-22 The resulting films were shown to be resistant toward extreme environments, but they are still permeable to the small molecules. Moreover, crosslinking requires the presence of specific reactive moieties on polyelectrolyte chains, which may limit the versatility of the LbL technique. Another alternative consists of adsorbing a hydrophobic layer atop the films, presumably leading to a reduction of their sensitivity to water and to other small polar species. In addition, the ability of the films to prevent permeation of small hydrophilic molecules may be increased. In analogy to biological membranes, uniform lipid bilayers were deposited on polyelectrolyte multilayers.23-25 These lipid layers were shown to efficiently prevent the diffusion of small molecules in the (15) Wang, T. C.; Cohen, R. E.; Rubner, M. F. Adv. Mater. 2002, 14, 1534. (16) Mendelsohn, J. D.; Baret, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (17) Panchagnula, V.; Kumar, C. V.; Rusling, J. F. J. Am. Chem. Soc. 2002, 124, 12515. (18) Vuillaume, P. Y.; Jonas, A. M.; Laschewsky, A. Macromolecules 2002, 35, 5004. (19) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (20) Chen, J.; Huang, L.; Ying, L.; Luo, G.; Zhao, X.; Cao, W. Langmuir 1999, 15, 7208. (21) Dai, J.; Jensen, A. W.; Mohanty, D. K.; Erndt, J.; Bruening, M. L. Langmuir 2001, 17, 931. (22) Wang, J.; Jia, X.; Zhong, H.; Luo, Y.; Zhao, X.; Cao, W.; Li, M. Chem. Mater. 2002, 14, 2854. (23) Georgieva, R.; Moya, S.; Leporatti, S.; Neu, B.; Ba¨umler, H.; Reiche, C.; Donath, E.; Mo¨hwald, H. Langmuir 2000, 16, 7075. (24) Moya, S.; Donath, E.; Sukhorukov, G. B.; Auch, M.; Ba¨umler, H.; Lichtenfeld, H.; Mo¨hwald, H. Macromolecules 2000, 33, 4538. (25) Ku¨gler, R.; Knoll, W. Bioelectrochemistry 2002, 56, 175.

10.1021/la036078l CCC: $27.50 © 2004 American Chemical Society Published on Web 05/06/2004

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Figure 1. Schematic representation of the formation of a uniform hydrophobic barrier layer of paraffin atop the polyelectrolyte multilayer.

films.25 However, these coatings are well-known to be unstable under mechanical stress or drying. Recently, Rouse and Ferguson26 showed that the incorporation of uncharged, nonpolar polymers in multilayers from organic solutions affects strongly their swellability in a humid atmosphere. In this paper, we report on a new route of improving the stability of polyelectrolyte multilayers with respect to water diffusion. This method consists of the adsorption of negatively charged wax particles from aqueous solution atop the multilayers. The subsequent annealing of the self-assembly provides the formation of a uniform hydrophobic barrier layer (see Figure 1). The adsorption and the fusion of wax particles were tested on self-assemblies made of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS). The films were studied by ellipsometry, atomic force microscopy (AFM), and contact angle measurements. The swelling behavior of wax-free and wax-coated multilayers toward D2O was investigated by neutron reflectometry (NR). Compared to other previous methods, our scheme offers the possibility to finely tune the thickness and the structure of the barrier and is fully compatible with LbL technology. Experimental Section Materials. PAH (Mw ) 70 000 g‚mol-1) and poly(styrene sulfonic acid) sodium salt (Mw ) 70 000 g‚mol-1) were purchased from Aldrich and were used without further purification. The aqueous suspension of negatively charged wax particles (50% w/w) was purchased from Keim-Additec Surface GmbH. The melting point of the wax crystallites measured by differential scanning calorimetry was 55 °C. Light scattering measurements indicated diameters of particles ranging from ∼100 to ∼ 200 nm, with an average value of 150 nm. The water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ‚cm. The NR experiments were performed against deuterium oxide (D2O) with a purity of 99.9% from Sigma Aldrich (Munich, Germany). Preparation of Multilayers. The substrates used were oneside-polished 〈100〉 silicon wafers from Silchem Handelsgesellschaft mbH (Freiberg, Germany) cut into rectangles of 3 cm × 1 cm and two-sides-polished 8 cm × 5 cm × 1.5 cm 〈100〉 silicon blocks from Siliciumbearbeitung Andrea Holm (Tann/Ndb., Germany) for multilayer growth and NR experiments, respectively. The wafers were first cleaned by treatment in a hot piranha solution [H2O2 (30%)/H2SO4 (98%) 1:1 v/v] for 20 min (caution: piranha solution is extremely corrosive) and then thoroughly washed with pure Milli-Q water. Polyelectrolyte multilayers were self-assembled by alternately dipping the substrate in aqueous solutions of the cationic PAH (4 mg‚mL-1 in 0.5 M NaCl) and the anionic PSS (4 mg‚mL-1 in 0.5 M NaCl) for 15 min each. Note that the first layer adsorbed on the silicon wafer is PAH. After each dip, the substrate was rinsed by immersion into three beakers of pure Milli-Q water for 2 min. After the deposition of (26) Rouse, J. H.; Ferguson, G. S. Langmuir 2002, 18, 7635.

the required number of PAH/PSS bilayers, the sample with a PAH end layer was dipped in the suspension of negatively charged wax particles (3% w/w in water) for 15 min, then rinsed three times in pure Milli-Q water, and dried with a nitrogen stream. Such self-assemblies will be designated as (PAH/PSS)n/wax bilayers in the following. The subscript n refers to the number of PAH/PSS bilayers deposited on the silicon substrate. Analytical Techniques. Ellipsometry. The thickness of the film was determined in air with a null ellipsometer (Multiskop from Optrel, Berlin, Germany) at a fixed incident angle of 70° and fixed wavelength of 5320 Å. A film refractive index of 1.54 was assumed to determine the thickness of the films. Because the refractive index of wax is about 1.48, the total film thickness determined by ellipsometry is slightly underestimated when a layer of wax is present atop the polyelectrolyte multilayer. Contact Angle Measurements. Static water contact angle values were determined with a Kru¨ss contact angle measuring system (G10). AFM. Topographic AFM images of the multilayers were recorded in air at room temperature using a Nanoscope III Multimode SFM (Digital Instruments, Inc., Santa Barbara, U.S.A.) with a 100 µm × 100 µm scanner. Silicon nitride (Si3N4) cantilevers with a spring constant of 42 N/m (Digital Instruments) were used for tapping mode measurements. AFM images were processed by using the Nanoscope software (version 4.43b). NR. The NR method is based on the variation of the specular reflection of neutrons at the interface between two phases as a function of the vertical projection of the wave vector kz0 ) 2π sin θ/λ, where θ is the angle of incidence of the incoming beam and λ is the wavelength of the beam. The scattering length density (SLD) F ) Nb in the case of neutrons depends on the number density N in units of 1/Å3 and the scattering length b in units of Å. The reflectivity R(kz0) is related to the SLD profile in the direction normal to the surface, F(z). A perfectly smooth interface between two half-spaces, each with constant F(z) up to the interface, yields a monotonically decaying R(kz0) curve, while the presence of a layer with a different SLD adjacent to the interface causes an undulation of the reflectivity curve. The period of this undulation is related to as the layer thickness, while its amplitude is related to the SLD of the layer. NR measurements were performed in the solid/liquid experimental setup on the V6-reflectometer at BENSC (HMI, Berlin, Germany) in θ/2θ geometry from kz0 ) 0.0023-0.06 Å-1. A detailed description of the instrument is given elsewhere.27 The resolution was set by the slit system to ∆kz0 ) 0.0005 Å-1 for kz0 e 0.0259 Å-1 and ∆kz0 ) 0.001 Å-1 otherwise. The background signal was collected simultaneously with a counter offset from the specular position by 0.44° toward larger scattering angle. The reflectivity data were footprint-corrected for the varying flux on the sample as Q increased and normalized to the measured incident intensity to obtain the reflectivity R(kz0). For each sample, the NR measurements were performed at room temperature according to the following steps: (i) measurements in the dried state in air; (ii) measurements in a closed solid/liquid experimental cell in D2O; (iii) removal of D2O, drying with nitrogen gas, and annealing at 60 °C for 45 min; (iv) measurements in the dried state in air; and (v) measurement in D2O. (27) Meizei, F.; Goloub, R.; Klose, F.; Toews, H. Physica B 1995, 213/214, 898.

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Figure 2. Ellipsometric thickness (circles) and contact angle values (triangles) for the (PAH/PSS)6 (1), (PAH/PSS)7.5 (2), and (PAH/PSS)7.5/wax (3) samples. Lines are guides for the eyes. All experiments were performed at equilibrium conditions. The experiments started approximately 1 h after the Si block with the polyelectrolyte multilayer on top was exposed to the surroundings: air or D2O. At least two consecutive scans were performed on a sample at the specified conditions, each taking about 8 h. No difference between the scans was observed, which is proof that the equilibrium was reached. This is of high importance when samples covered with wax particles were exposed to D2O. In this case, the wax layer can reduce the rate of D2O penetration and extend the process to many hours. The reflectivity curves for each experiment were fitted by a model-free routine which discretizes the SLD profiles into slabs of adjustable height and constant thickness d [fixed by the largest scattering vector of the experiment (kz0,max): d ) π/(2kz0,max)]. To avoid noise amplification, a Tikhonov’s regularization procedure was applied. Details of the analytical routine used are reported elsewhere.28

Figure 3. Topographic AFM images (5 µm × 5 µm) of the surface of the (PAH/PSS)7.5/wax sample before (a) and after (b) annealing at 60 °C.

Results Deposition of Wax Particles onto Polyelectrolyte Films. Thin polyelecrolyte films were prepared by sequential adsorption of PAH and PSS from aqueous solutions on silicon substrates. The deposition of negatively charged particles was first tested on the (PAH/PSS)7.5 sample whose outer surface is positively charged because of PAH. As shown in Figure 2, the immersion of the polyelectrolyte film in the wax suspension causes a strong increase of the ellipsometric thickness of the film, with a parallel large increase of the static water contact angle. The thickness increment measured after wax adsorption is less than the expected value obtained by considering the deposition of a continuous layer of particles of limited size distributed and average diameter of 150 nm, which may be due to the presence of voids in the top layer or to particle deformation upon adsorption and drying. The inspection of the morphology of the wax-coated films by AFM (Figure 3a) shows that the wax particles are well and randomly distributed over the surface but with various sizes and shapes. The average root-mean-square (rms) roughness of the film (measured over a 5 × 5 µm2 surface) was determined to be 27 nm. Influence of Annealing. The fusion of wax particles into a continuous layer was followed by measuring the variation of the thickness of the (PAH/PSS)7.5/wax sample with annealing time (tann) at 60 °C. Figure 4 indicates a continuous decrease of the total thickness of the film with tann. In contrast, the thickness of the wax-free (PAH/PSS)7.5 remains unchanged for an identical treatment (Figure 4). Images of the surface of annealed (PAH/PSS)7.5/wax films (Figure 3b) appear continuous and homogeneous with an (28) Arys, X.; Laschewsky, A.; Jonas, A. M. Macromolecules 2001, 34, 3318.

Figure 4. Variation of the ellipsometric thickness of the (PAH/ PSS)6 (circles) and (PAH/PSS)7.5/wax (triangles) samples with annealing time. Lines are guides for the eyes.

average rms roughness of 6.5 nm (measured over a 5 × 5 µm2 surface). Swelling Behavior. We used specular NR to study the D2O distribution within the multilayers. As a result of the sensitivity of NR to deuterium, a direct view of the laterally averaged concentration in heavy water along a line perpendicular to the film surface is provided. The reflectivity curves of the (PAH/PSS)9.5 and (PAH/PSS)9.5/ wax multilayers before and after annealing, measured in the dry state, are shown in Figure 5a. The SLD profiles computed from the best fits of the experimental data (Figure 5a) are shown in Figure 5b. From these measurements performed in air, no significant difference was observed between the wax-coated and wax-free films. This is due to the low SLD of the wax (r ≈ -4 × 10-7 Å-2), which is close to that of the air;29 consequently, the wax layer does not discriminate against air, and the profiles of wax-free and wax-coated films are very similar.30 From these profiles, the average thickness of the (PAH/PSS)9.5 part of the film was determined to be about 26 nm. This value is slightly lower than expected from previous (29) Sears, V. F. Neutron News 1992, 3, 26. (30) X-ray reflectivity measurements on the same samples confirmed the presence and large roughness of the wax layer on the samples.

Permeability of Polyelectrolyte Multilayers

Figure 5. (a) Neutron reflectivity of multilayers measured in air: (PAH/PSS)9.5 unannealed (1), (PAH/PSS)9.5 annealed at 60 °C (2), (PAH/PSS)9.5/wax unannealed (3), and (PAH/PSS)9.5/ wax annealed at 60 °C (4). Dots are experimental data, and lines are fits using the SLD profiles shown in part b. For clarity, curves have been displaced vertically. (b) The SLD profiles of (PAH/PSS)9.5 unannealed (continuous line), (PAH/PSS)9.5 annealed at 60 °C (dotted line), (PAH/PSS)9.5/wax unannealed (dotted and dashed line), and (PAH/PSS)9.5/wax annealed at 60 °C (dashed line).

ellipsometry measurements as a result of the uncertainties in the refractive index of the film and presence of the native oxide layer at the surface of the silicon wafer. The annealing does not lead to noticeable variations of the structure of the PAH/PSS part of the films. The reflectivity curves and SLD profiles of samples measured in D2O are presented in Figure 6a,b, respectively. Kiessig fringes, which correspond to interferences resulting from multiple reflections at the interfaces, are more visible for wax-free films than for wax-coated films as a result of the considerable increase of the roughness of the surface after wax adsorption. For (PAH/PSS)n films lacking the final wax coating, the value of the SLD increases after immersion in D2O because of the sorption of D2O molecules in the multilayer [compare Figures 5b and 6b (curve 1)]. This sorption results in the swelling of the polyelectrolyte film, whose thickness increases from 26 to 37 nm; from the average SLD of the film, the amount of D2O in the film was estimated to be about 50%. In addition, for such films, no significant difference could be detected before and after annealing (Figure 6b, compare curves 1 and 2). These results confirm that the annealing process itself does not affect the swelling of the film by water, in agreement with previous reports by others.31 In contrast, the amount of D2O incorporated in the waxcovered unannealed sample is much lower and decreases with increasing proximity to the silicon substrate (Figure 6b, curve 3). This is more pronounced after annealing (Figure 6b, curve 4): the fusion of the wax particles now (31) Kurth, D. G.; Volkmer, D.; Klitzing, R. v. In Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003, p 393.

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Figure 6. (a) Neutron reflectivity of multilayers measured in D2O: (PAH/PSS)9.5 unannealed (1), (PAH/PSS)9.5 annealed at 60 °C (2), (PAH/PSS)9.5/wax unannealed (3), and (PAH/PSS)9.5/ wax annealed at 60 °C (4). Dots are experimental data, and lines are fits using the SLD profiles shown in the right side. For clarity, curves have been displaced vertically. (b) Corresponding SLD profiles.

almost completely prevents the sorption of water in the film. Interestingly, for all films, water seems to be more excluded with closer proximity to the silicon substrate, possibly due to a difference in the structure of the film as postulated by others.32,33 Discussion The increase of the thickness as well as the large increase of the water contact angle obtained after immersion of the (PAH/PSS)7.5 sample in the wax suspension provides strong evidence for the deposition of the hydrophobic particles atop the film assembly. The presence of wax particles is also confirmed by the inspection of the surface by AFM. The successful deposition of wax particles onto the PAH-ended multilayer most probably results from the electrostatic attraction between negatively charged particles and the positively charged polyelectrolyte surface, although hydrophobic interactions may also play a role. We found that the deposited wax layer was left untouched after immersion in a water bath for several weeks, indicating its stability. Consequently, subsequent deposition of PAH/PSS bilayers could even be performed on wax-coated multilayers (result not shown), which allowed the preparation of composite films. It is relevant to note here that the protocol used for the deposition of this stable hydrophobic layer is only based on aqueous solutions, which differs from previously reported methods requiring the presence of organic solvents.26 As such, the wax particles already form a barrier, decreasing water sorption in the films. However, voids in the adsorbed wax layers still allow some penetration of D2O molecules in the underlying polyelectrolyte film. To get complete prevention of water diffusion, fusion of the wax particles into a homogeneous smooth layer is required. This can be effectively performed by a gentle heating, which does not affect the structure and properties of PAH/PSS assemblies (32) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249. (33) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058.

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but results in the formation of a protective impermeable external barrier. The fusion of the particles was demonstrated by a decrease of the apparent ellipsometric thickness and the AFM roughness of the films upon annealing, as well as by the neutron scattering results, and provides an easy and versatile way to protect polyelectrolyte multilayers from unwanted moisture effects. Conclusion In this paper, we proposed on a new technique limiting efficiently the water diffusion in polyelectrolyte multilayers. The method consists of adsorbing wax particles atop the film using electrostatic assembly followed by the subsequent melting of the particles at the raised temperature. The efficiency of this stable hydrophobic capping layer was demonstrated by studying the swelling behavior of the self-assemblies by in situ neutron reflectivity. This

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method shows promise for the fabrication of devices requiring protection against environmental moisture. In addition, it may also used to reduce the permeation of hydrophilic compounds such as drugs, peptides, or ions through the film, which is of relevance for many controlledrelease applications. Acknowledgment. This research was financially supported by Sofja Kovaleskaja Program of the Alexander von Humboldt Foundation and by the German Ministry of Science and Education and by the French-German Network “Complex Fluids from 3 to 2 Dimensions”. K.G. thanks the EU and the French Network “Mate´riaux Polyme`res, Plasturgie” for supporting the travel costs. BENSC (HMI, Berlin) is acknowledged for allocation of beam time (Project No. MAT-04-775). LA036078L