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Glassy State of Polystyrene Sulfonate/Polyallylamine Polyelectrolyte Multilayers Revealed by the Surface Force Apparatus A Ä gnes Kulcsa´r,† Jean-Claude Voegel,† Pierre Schaaf,‡ and Patrick Ke´kicheff*,‡ Institut Charles Sadron, C.N.R.S. UPR 22 6, rue Boussingault, BP 40016, 67083 - Strasbourg Cedex, France, and INSERM, Unite´ 595, Centre de Recherche Odontologique, Universite´ Louis Pasteur 11, rue Humann, 67085 - Strasbourg Cedex, France Received September 10, 2004. In Final Form: December 14, 2004 The interactions between two poly(allylamine)/poly(styrene sulfonate) multilayers made of 4.5 and 5 bilayers are investigated by the surface force apparatus (SFA). As the two surfaces approach, one reaches a threshold point where a repulsion sets in, until they become barely compressible. Repetitive load/unload cycles show that, once compressed, the films remain almost in their compressed state. This indicates that the poly(allylamine)/poly(styrene sulfonate) films are in a glassy state, in marked difference with the SFA findings on poly-(L-lysine)/poly-L-glutamic acid) multilayers. These results are discussed in the light of linearly and exponentially growing films.
* To whom correspondence should be addressed. E-mail:
[email protected]. † INSERM. ‡ Institut Charles Sadron.
of at least one of the two polyelectrolytes during each bilayer deposition step. In the case of (PGA/PLL) both polyelectrolytes diffuse in and out.8 Such films are thus expected to be much less dense than the linearly growing ones and less well structured. Insight into the interactions acting between two multilayers may be gained by surface force apparatus (SFA) measurements when two coated substrates approach and are subsequently compressed. Fundamental questions about the adhesion between two multilayers, their compressibility, and the chain relaxation within the multilayers can be explored with this technique. Despite the importance of these questions, only very few SFA studies (three to the authors’ knowledge) on polyelectrolyte multilayers have been reported up to now. The first study is due to Lowack and Helm who investigated the interactions between mica sheets covered by one PAH/PSS bilayer.9 Electrostatic repulsions appear to govern the interaction at large separations due to the net surface charge. At close approach the polyelectrolyte chains dangling into the solution lead to a steric repulsion. At even smaller separations, segment-segment attractions between polycation and polyanion chains give rise to an adhesive force during the subsequent surface separation. This adhesive force decreases when the ionic strength is increased. Blomberg et al. reached somewhat similar conclusions in their recent study of (PAH/PSS) films composed of two bilayers immersed in dilute electrolyte solution.10 Beyond adhesive forces observed after strong compression of the films, the long-range repulsion at large separations was ascribed to be of purely electrostatic origin. The present authors also used the SFA technique to investigate an exponentially growing film: (poly-Llysine)/poly-L-glutamic acid) multilayers.11 In this system, at large separations, a long-range, weak attraction is observed despite the fact that both surfaces bear charges
(1) Decher, G. Science 1997, 277, 1232-1237. (2) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309-4318. (3) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592-598. (4) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531-12535. (5) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871-8878. (6) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906.
(7) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458-4465. (8) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J.-C.; Mesini, P. J.; Schaaf, P. Macromolecules 2004, 37, 1159-1162. (9) Lowack, K.; Helm, C. A. Macromolecules 1998, 31, 823-833. (10) Blomberg, E.; Poptoshev, E.; Claesson, P. M.; Caruso, F. Langmuir 2004, 20, 5432-5438. (11) Kulcsa´r, A.; Lavalle, P.; Voegel, J. C.; Schaaf, P.; Ke´kicheff, P. Langmuir 2004, 20, 282-286.
Introduction The alternate deposition of polyanions and polycations on a charged solid surface leads to the formation of films, called polyelectrolyte multilayers, whose thickness and mass increase with the number of deposition steps.1,2 The motor for the continuous growth of multilayers is provided by the charge overcompensation that appears on top of the films at the end of each polyelectrolyte deposition.3 This allows the subsequent interaction with polyelectrolytes of opposite charge. Due to their widespread applications, polyelectrolyte multilayers have received considerable attention. Two kinds of multilayers were reported: There are films whose thickness increases linearly with the number of deposition steps1 and others that grow exponentially.4 Polystyrene sulfonate (PSS)/polyallylamine (PAH) constitutes a prominent example of linearly growing films.5 The growth process and the structure of (PSS/PAH) multilayers have been investigated in great detail. During each deposition step, the polyelectrolyte chains from the bulk solution interact only with the outer polyelectrolyte layers of the film. Once deposited the polyelectrolyte chains do not diffuse within the film. Thus the film exhibits a regular and periodic structure along the substrate normal, as signaled by the presence of Bragg reflection peaks in contrast variation neutron experiments when some of the deposition steps used deuterated polyelectrolytes.6 Poly-(L-glutamic acid) (PGA)/poly-(Llysine) (PLL) on the other hand constitutes a system whose multilayers grow exponentially with the number of deposition steps.7 It was shown that such a growth process is related to the diffusion, in and out of the entire film,
10.1021/la047737c CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005
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of the same sign. The interaction turns into repulsion as the two opposite multilayers interpenetrate when the surface separation is reduced. This is attributed to steric effects. Finally, after full compression, formation of complexes between the oppositely charged chains makes the opposite films become highly adhesive. But SFA experiments also allow access to film relaxation processes through consecutive load/unload cycles. Such experiments, performed on poly-(L-lysine)/poly-(L-glutamic acid) multilayers, revealed that, once compressed, the films relax toward their initial state when the constraints are removed.11 This must be due to the high mobility of polyelectrolyte chains inside the exponentially growing multilayers. Such a mobility is not expected on linearly growing films such as PAH/PSS. The present paper is mainly devoted to investigating the behavior of PAH/PSS films under consecutive load/unload cycles. Materials and Methods All solutions are freshly prepared and filtered (0.22 µm) before use. Water is purified by a commercial MilliQ Gradient system and degassed for 30 min. The MES-Tris buffer solution at pH 7.4 consists of 100 mM/L NaNO3 (Sigma, purity >99.99%), 25 mM/L 2-(N-morpholino)ethanesulfonic acid (MES) (Sigma, purity >99.5%), and 25 mM/L tris(hydroxymethyl)aminomethane (Tris) (Sigma, purity >99.9%). The cationic poly(allylamine) (CAS: 71550-12-4, MW ) 70000) and the anionic poly(styrene sulfonate) (CAS: 25704-18-1, MW ) 70000), both from Sigma, are dissolved at 1 mg/mL in filtered MES-Tris buffer solution. As the pH of 7.4 is kept constant, the polyelectrolytes remain ionized; this caution prevents collapse or swelling changes in the initial film structure that would otherwise be due to charge variations upon the subsequent deposition of additional layers.12 Film deposition on mica sheets is carried out following the procedure described previously.11 In short, after thickness calibration of the mica sheets glued silvered back onto the curved silica disks (radius ≈ 2 cm) of the surface force apparatus, the surfaces are transferred from the apparatus into a sealed flow chamber to construct, in situ, the polyelectrolyte multilayer on the mica surfaces. Solutions of alternatively positively (PAH) and negatively (PSS) charged polyelectrolytes are run through. The first deposited layer is the cationic polyallylamine (PAH), since mica is negatively charged in aqueous solution. To ensure homogeneous coated films, at least four (PAH/PSS) bilayers were deposited onto mica surfaces in electrolyte solutions. The method, described in detail elsewhere,11 yields homogeneous coverage of the mica surface, as demonstrated by atomic force microscopy. All experiments were performed at 25.0 °C with mica surfaces coated with (PAH/PSS)5 films, where 5 stands for the number of deposited layer pairs, or with (PAH/PSS)4-PAH films. The films were studied at the same ionic strength used during their buildup and were never dried: this preserves the native structural organization of the polyelectrolyte multilayers which could otherwise collapse in the usual drying and washing process. Force-distance profiles were measured using a homemade device based on the initial version of the Tabor-Israelachvili surface force apparatus.13 As described in detail elsewhere,14 the instrument allows the force F between two mica surfaces (of mean radius of curvature R) to be measured to within 0.1 µN as a function of the determined surface separation D, which can be measured to a typical accuracy of 0.2 nm, using multiple beam interferometry.15 The normalized force F/R can be detected to within 0.005 mN/m, while the maximum reliably measurable force will depend on the mechanical compressibility of the entire system. Typically surface deformations occur for applied loads larger than 8-15 mN/m and F/R becomes meaningless due to the deformation of the glue beneath the mica sheet; for that (12) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249-1255. (13) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975-1001. (14) Ke´kicheff, P.; Iss, J.; Courtier, F.; Lambour, C. Rev. Sci. Instrum., to be submitted. (15) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259-272.
Figure 1. Reversibility of the attractive normalized forcedistance profile measured at large separations (region C, see text) for mica surfaces, one coated with a film of (PAH/PSS)4PAH multilayers and the other with a (PAH/PSS)5 film. The direction of the surface displacement was reversed before the repulsive regime was attained. Solid symbols correspond to the force on approach of the surfaces, while the open symbols are the data measured on separation. The reversibility observed in the force profile suggests that the two opposite multilayers have not yet interpenetrated and that the attraction results from electrostatic interactions between the outer charges of opposite sign borne by the two multilayers. reason, data are only reported for smaller loads, where the measured values of F/R correspond to the free energy E per unit area of two equivalent flat surfaces as given by the Derjaguin approximation (F/R ) 2πE).
Results and Discussion Two experimental configurations were investigated: the symmetric configuration, corresponding to the interaction of two alike (PAH/PSS)5 films that thus carry the same outer negative charge, and the converse situation, called the asymmetric case, where a (PAH/PSS)5 film interacts with an oppositely charged (PAH/PSS)4-PAH multilayer. Force-distance profiles were measured directly after film buildup and monitored over time. They can roughly be divided into three separation regions. Let us start at a large separation distance corresponding to the region marked C in Figure 1. In the asymmetric configuration, as the separation distance is diminished, one observes an attraction at large separation, starting at around 300 nm up to 150 nm (Figure 1). This attraction is weak. It spans over about 100 nm along a shallow well (maximum about 0.02 mN/m). Furthermore the force regime is almost fully reversible as the surfaces approach or separate along this separation range. When the experiment is performed with two similar films, thus exhibiting a similar surface charge, of the same sign, these attractive forces are absent and eventual repulsive forces were not detected. These results are different from those of Blomberg et al. who found strong forces at large separations, attractive in the asymmetric case and repulsive in the symmetric case.10 Note that the attraction magnitude was 50 times larger than the one reported here. They attributed their forces to the electrostatic component of a Derjaguin-Landau-VerweyOverbeek (DLVO) type interaction. The large difference between their results and our observations can be explained by the much higher ionic strength used in our experiments, which should strongly screen any electrostatic interactions. For the asymmetric case, the relative long attractive interaction range (of the order of 100 nm) compared to the Debye length (of the order of 1 nm) must, in our case, be due to oppositely charged polyelectrolyte
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Figure 2. Effect on subsequent approaches on the normalized force-distance profile for mica surfaces in the asymmetric configuration of multilayers carrying outer charges of opposite sign (a (PAH/PSS)5 film in interaction with a (PAH/PSS)4PAH film). Upon compression (solid symbols), the local structure of the coated films is irreversibly affected, as indicated by both the hysteresis observed upon separation (empty symbols) and the changes in the magnitude and range upon subsequent approaches. The direction of the surface displacement was reversed at different separations. Note that upon a subsequent approach the force profile follows the path of the force measured on the preceding unload of the surfaces. Squares correspond to the first load-unload cycle, diamonds to the second cycle, circles to the third cycle, and triangles to the fourth cycle. The first load/unload cycle (turning point ≈ 84 nm for the direction reversal of the surface displacement) is also represented in a linear scale in the insert (squares). Note that the separation curve catches up the attractive part measured during the first approach when one comes from very large separations. This is compatible with the dangling out of the chains from the outer part of the films. Labels A-C represent the different separation distance regions defined in the main text.
chains that dangle out of each surface. As the separation between the mica surfaces is further reduced, the interaction becomes repulsive at a distance of the order of 150 nm and increases steadily. We enter here in the separation region marked B in Figure 2 that spans roughly from 150 to 40 nm. As soon as we enter into this separation region, the behaviors of both symmetric and asymmetric systems are similar, not only from a qualitative but also from a quantitative point of view. For this reason the following results are only given for the asymmetric case. One can expect that the threshold separation corresponds to the distance where the films begin to interpenetrate. The separation distance at which this repulsive interaction sets in is much larger than the 50-60 nm expected for two juxtaposed (PAH/PSS)4-PAH and (PAH/PSS)5 films as measured by optical techniques under similar ionic strength conditions.12 Such a great difference between the onset of the repulsive forces and the thickness of the film predicted from optical techniques was also observed by Blomberg et al.10 It clearly indicates that the homogeneous monolayer model characterized by one refractive index and one film thickness does not correctly describe a multilayer that is rather characterized by a refractive index profile that extends from the core of the film into the solution. Such profile was originally proposed by Ladam et al. who denoted the zone extending into the solution as zone III of the film.12 The interaction increases steadily as the separation between the mica surfaces is further reduced between 130 and 50 nm (region B) as can be seen in the semilogarithmic plot of the force versus the separation distance of Figure 2. One can notice that it is not a truly exponential repulsion as observed in the same
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intermediate separation distance region for the exponentially growing (poly-L-lysine)/poly-L-glutamic acid) multilayers.11 An electrical double-layer interaction cannot be at the origin of the repulsion for several reasons. First, the measured decay lengths lie in the 20-30 nm range, more than an order of magnitude larger than the Debye screening length expected at the ionic strength of the electrolyte solution. Second, reversible force profiles are not observed upon compression and on separation (see below). The origin of this repulsive regime observed for both linearly and exponentially growing films is still in debate: one may invoke a steric hindrance or alternatively an increase of the osmotic pressure of the monomer and/ or the counterions present in the film. When the films are further compressed, one reaches the separation region marked A in Figure 2. This regime starts roughly at separations of the order of 50 nm and ends at 30 nm with the occurrence of a steeper repulsive wall. This wall is the likely signature of a state where the multilayers are barely compressible. The thickness value of 30 nm, when the multilayers appear fully compressed, is smaller than the 50-60 nm expected from optical technique measurements. The refractive index (nfilm) for PAH/PSS multilayers constructed under similar conditions as used here and measured with optical techniques is of the order of 1.48. The optical mass, given by (nfilm - nsolution)d, where nsolution ≈ 1.33 is the refractive index of the solution and d is the thickness of two juxtaposed films (one on each mica sheet), is thus of the order of 9 nm. If one assumes that the optical mass remains constant during the compression of the films, the refractive index of the fully compressed film would be of the order of 1.63. This is close to the value of 1.60 found for crystallized proteins16 and about the actual experimental value of 1.59 ( 0.03 extracted from the wavelength measurement of the fringes of equal chromatic order set in the optical cavity of the SFA.15 To gain insight into the dynamic behavior of the multilayers, subsequent approach/separation cycles are performed on the same contact area. The direction of the surface displacement is reversed at different separations during the approach of the two opposite films. As already mentioned, one observes full reversibility of the forcedistance profile upon approach or separation of the surfaces as long as one remains at separation distances corresponding to the regime of attractive forces corresponding to region C (see Figure 1). However, below the threshold separation where repulsive forces set in (region B) and where the films are expected to interpenetrate, the force profile no longer follows a reversible path when the direction of the surface displacement is reversed. Much larger compliances are observed upon the unload than upon loading (Figure 2). In addition, a slight adhesive force arises during the retraction. The strength of this adhesive force increases for compression/separation cycles with increasingly smaller turning points in separation (Table 1). For small turning point separations, the adhesion is large enough and the two surfaces jump apart due to the mechanical instability of the cantilever. This adhesive force can have two origins: the disentanglements of the polyelectrolyte chains of the two interacting multilayers as well as the electrostatic interactions of PSS chains from one multilayer with PAH chains of the other film and vice versa. The fact that the value of this rupture force increases with compression must be due to stronger interpenetration of the two films when compressed. The adhesive forces are, however, rather small. For compari(16) Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S. Langmuir 1998, 14, 5636-5648.
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Table 1. Adhesion between Two Opposite Polyelectrolyte Multilayered Films as a Function of Their Interpenetration Extent thicknessa (nm) 14
25
42
50
adhesion 0.330(0.010 0.060(0.008 0.022(0.005 0.002(0.001 energyb 2 (mJ/m ) a Considered as half the separation at which the direction of the surface movement has been reversed. b Determined from the pulloff force (F/R)0 required to wrench the surfaces apart using the Johnson-Kendall-Roberts theory (ref 18) rather than the Derjaguin-Muller-Toporov theory as a limiting case. The adhesion energy E is related to (F/R)0 by E ) (F/R)0/3π. Note, however, that the values extracted can be regarded only as estimates. The error bars reflect the range of calculated values from different experiments.
son, Lowack and Helm measured an adhesion of the order of 1 mJ/m2 for a PSS layer interacting with a bare mica surface in a 1 mM NaCl solution.9 This is an order of magnitude larger than the value measured in our case which ranges between 0.002 and 0.3 mJ/m2. The interactions between the chains of different multilayers must thus be rather small and limited. A subsequent reload sequence gives the following features. The attractive force that was observed at large separations during the first approach is no longer observed when the surfaces are brought together a second time. Even long annealing times (days) fail to recover the pristine situation. This indicates the absence of excess charge, initially present, on top of the films. Furthermore, the local structure of the multilayers adsorbed on the two mica surfaces is also irreversibly affected once interpenetrated: upon a second reload, the starting point of the repulsive regime is about the separation at which the initial unload has left the surfaces free of interaction. This is the separation at which the polyelectrolyte chains had disentangled from the films that were interpenetrated upon the first compression. Remarkably, the films interact by following the same upward path upon reloading as followed previously downward upon the unload (Figure 2). Steep compliance is maintained as the separation is decreased down to the former turning point. If the direction of the surface displacement is reversed once more at that turning point, the same path will be followed again upon the new unload (data not shown). Conversely, if the turning point is passed upon further reduction of the surface separation, the steep compliance is abandoned there, and the force-distance profile catches up with the original force-distance repulsion profile as if no unload/load cycle had taken place in between. The same features take place whatever the separation distance at which the turning point is positioned. Similar results were observed for the symmetric system where the two films bear the same surface charge. These results suggest two remarks: (1) PAH/PSS polyelectrolyte multilayers appear frozen and behave as glassy materials, and (2) the disruption that occurs during the unload path should only affect the outer part of the film. Due to the small disruption forces, it is expected that the entanglements and the interactions of the polyelectrolyte chains of the two films only concern the two outer layers of the films. However, even if the entanglements concern only the outer layers of the film, the compression should take place rather homogeneously over the whole film. The structural modifications induced by a primary compression over a given range are irreversible. During the unload process, the preconditioned film relaxes only
Figure 3. Schematic representation of the force-separation curves during consecutive load-unload cycles (a) for the PAH/ PSS linearly growing film system and (b) for the PLL/PGA exponentially growing film system. The numbers 1-3 next to the profiles represent the cycle number. The dashed lines indicate that the surface jumps upon separation. For the (PAH/ PSS) case, the force-separation curve follows the same path during the reload than during the preceding unload as long as the turning point where the direction of the surfaces is reversed is not passed. Conversely for the (PLL/PGA) case, the reload path 2 is almost identical to the loading during the first path, while subsequent load cycles would induce an interaction of shorter range.
over short distances and behaves elastically over a small length scale. However, it does not recover its initial state over larger length scales and there behaves as being in a frozen state. This again shows that the polyelectrolyte chains in the (PAH/PSS) multilayers do not rearrange over large distances and explains why these films keep a periodic structure after deposition and give rise to Bragg peaks in neutron reflectivity experiments. The glassy state of these systems is also supported by the fact that all the systems described here retain their features over a weeklong period. The absence of differences in the force profiles as the films are stored, not only qualitatively but also in terms of magnitude and range, indicates that the native structures of the multilayers do not evolve significantly over this period of time. Also, once disrupted, the films do not evolve over time: even long annealing times (days) do not allow recovery of the initial situation. This indicates that the local structure of the multilayers adsorbed on both mica surfaces is irreversibly affected once compressed. Furthermore, the measured adhesion, which also remains unchanged over a week-long period, does not present a dependence on the resting time at contact under an applied load (up to a few hours) nor on the number of cycles (load-unload) previously performed at the same contact position. This additional observation supports the conclusion that polyelectrolyte chains do not undergo significant rearrangement or diffusion within the PAH/ PSS multilayers once the film structure is built or disrupted. Let us now compare our results to those of Blomberg et al.10 and of Lowack and Helm.9 The first authors found
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that, when not fully compressed, PAH/PSS films follow a perfectly reversible path during load and unload. Their results do thus not support the conclusion that PAH/PSS films are in a frozen state. However, their results were obtained on multilayers of only two or fewer bilayers. It is known that film homogeneity is not attained for such a low number of layer deposition. Moreover, their films were constructed at 0.5 M KBr whereas the measurements were conducted in 10-4 M KBr solutions. A change in ionic strength is known to lead to strong film restructurations which could also favor the dynamics of polyelectrolyte chains within the film.12 Lowack and Helm, on the other hand, only worked on single bilayer films. Moreover, these authors did not investigate the reversibility of moderately compressed films but found, in agreement with our findings, an adhesive force when the films are fully compressed. The “glassy state” behavior that we observe here is in marked difference with the observations made on the exponentially growing (PLL/PGA) multilayers.11 Here again, larger compliances were observed during unloading than during loading. However, the adhesive forces, corresponding to the forces measured before rupture, were higher for the (PLL/PGA) films (of the order of 0.4-0.7 mJ/m2) compared to the 0.002-0.33 mJ/m2 measured for the PAH/PSS system. In addition, in the (PLL/PGA) case, the rupture seems to take place first in small irregular jumps before the pull-off force is sufficient to wrench the surfaces apart. However, as the surfaces approach for a second time, a similar force-separation curve was followed as during the first approach cycle. It is only during a third approach that the threshold distance at which the repulsive force sets in started to decrease slightly and that the loading forces were somewhat smaller than during the first two approaches. These differences between the behavior of (PAH/PSS) and (PLL/PGA) films are schematically summarized in Figure 3. Finally, in
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contrary to PAH/PSS films, PLL/PGA multilayers evolved with time. The films swelled significantly during the first day. All these features point toward a more dynamic structure of the (PLL/PGA) system with multiple relaxation paths and are consistent with a gel-like structure of exponentially growing films. The comparison between the behavior of the two systems, PAH/PSS and PLL/PGA, starts also to give a clue for predicting the linear or exponential nature of the polyanion/polycation multilayers. The present findings and those of ref 11 support the suggestion made by Cohen Stuart and co-workers17 that linearly growing films should correspond to multilayers in which the polyelectrolytes are in a glassy state whereas exponentially growing films rather correspond to systems that are above the glass transition and are thus more gel-like. This statement must, however, be confirmed on other systems. We are currently working in this direction. If it is confirmed, the next step toward a better prediction of the behavior of polyanion/ polycation systems, as far as multilayer buildup is concerned, would be to know what physical and chemical properties lead to glassy states or to gel-like states. Acknowledgment. This work was supported by CNRS and INSERM, by a Marie Curie Individual Fellowship, Fellowship of the European Community program “Quality of Life and Management of Living Resources” under contract number QLK3-CT-2001-50965, and by the Fond National de la Science (Project No. NR204, Biofilm Multifonction). Jean Iss is gratefully acknowledged for technical help in the SFA measurements. LA047737C (17) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. J. Phys. Chem. B 2003, 107, 7998-8002. (18) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301.
