Charge Effects on the Formation of Multilayers Containing Strong

DC around 68% is related to a threshold of charge reversal, which is necessary for the formation of a multilayer system. Above a DC of 75% the film th...
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J. Phys. Chem. B 2003, 107, 5273-5280

5273

Charge Effects on the Formation of Multilayers Containing Strong Polyelectrolytes Ulrike Voigt,† Werner Jaeger,‡ Gerhard H. Findenegg,† and Regine v. Klitzing*,† Stranski-Laboratorium fu¨r Physikalische und Theoretische Chemie, Technische UniVersita¨t Berlin, Strasse des 17, Juni 112, D-10623 Berlin, Germany, and Fraunhofer-Institut fu¨r Angewandte Polymerforschung, D-14476 Golm, Germany ReceiVed: February 16, 2002; In Final Form: December 19, 2002

The effect of polymer charge density, polyelectrolyte concentration, and ionic strength on the formation of multilayer films by sequential adsorption of a polyanion (poly(styrene sulfonate), PSS) and a cationic statistical copolymer poly(diallyl-dimethyl-ammoniumchloride-stat-N-methyl-N-vinylacetamide), P(DADMAC-statNMVA) is investigated. The degree of charge (DC) of the polycation is controlled through the ratio of cationic DADMAC and neutral NMVA within the copolymer. The resulting polymer films are characterized by ellipsometry and by scanning force microcopy (SFM). For the present system, a minimum DC of about 68% is needed for the formation of stable films. It is proposed that this abrupt change in adsorption behavior at a DC around 68% is related to a threshold of charge reversal, which is necessary for the formation of a multilayer system. Above a DC of 75% the film thickness decreases slightly because of changes in polymer conformation. This results in a thickness maximum the position of which is shifted with varying polymer concentration. The ionic strength has a pronounced effect on the thickness above the charge reversal threshold, that is, where multilayers are formed. With increasing ionic strength, the chains undergo a transition from an extended to a coiled conformation, which leads to an increase in film thickness. Such conformational changes of the polyelectrolytes are indicated by changes in roughness as derived from SFM images. It is concluded that multilayer formation in the present system is electrostatically driven.

1. Introduction Polymer films formed by sequential physisorption of polyanions and polycations from aqueous solution have attracted interest in the past 10 years. X-ray reflectivity and spectroscopic measurements show that the thickness of the polymer films prepared by the method proposed by Decher1 increases linearly with the number of dipping steps.2 Neutron reflectivity measurements performed on films composed of protonated and deuterated polyelectrolytes at the solid/air interface shows that adjacent layers are interdigitated to a large extent.3,4 This strong interdigitation is due to electrostatically driven formation of complexes between polycations and polyanions. Farhat et al.5 showed by FTIR spectroscopy that in the multilayer film complexes between polyanions and polycations similar to those in an aqueous polyanion/polycation solution are formed. The formation of such complexes is related to an exchange of small counterions against the charges of the counter polyion, which explains the low counterion concentration in the polyelectrolyte multilayer.6-8 Also many other groups mentioned the electrostatic attraction between surface and polyelectrolyte as the origin for multilayer formation.6,9,10 This electrostatic attraction can be controlled by the degree of polymer charge or by the ionic strength. For a given number of dipping cycles, the film thickness increases with increasing ionic strength up to more than 1 mol/L of NaCl.11-13 Generally, this is explained by a coiling of the chains, as the charges along the chain are increasingly screened.14-16 But on the other hand, the thickness increases even at a high ionic strength at which the electrostatic * To whom correspondence should be addressed. E-mail: klitzing@ chem.tu-berlin.de. Tel: ++49-30-31426774. Fax: ++49-30-31426602. † Technische Universita ¨ t Berlin. ‡ Fraunhofer-Institut fu ¨ r Angewandte Polymerforschung.

attraction between surface and polyelectrolyte should be completely suppressed. These results suggest nonelectrostatic contributions, as well as electrostatic ones. However, polymer films on solid supports may be stabilized by other interactions than electrostatics ones, such as hydrogen bonding.17,18 It has been found that polyelectrolytes can also be adsorbed onto equally charged surfaces.19-21 In all these cases, adsorption is apparently not caused by electrostatic forces. These findings pose questions about the importance of electrostatic interactions in general for the formation of polymer multilayer films. In the present work, the importance of electrostatic interactions for the stability and properties of the polymer multilayer films is investigated by combining a strong polyanion with a strong polycation in which the degree of charge can be adjusted by a statistical copolymerization of a strong cationic monomer with a neutral monomer. As a prototype of such a system, the polyanion poly(styrene sulfonate) sodium salt (PSS) and a statistical polycation poly(diallyl-dimethyl-ammoniumchloridestat-N-methyl-N-vinylacetamide) (P(DADMAC-stat-NMVA)) with different ratios between cationic DADMAC monomers and neutral NMVA monomers is used. For this copolymer containing strong cationic monomers, the degree of charge is not affected by the presence of an interface or of another polyelectrolyte. First results on this system have been presented in ref 22 in which it has been shown by X-ray reflectometry that a minimum charge is indeed necessary for the formation of a stable multilayer system. The results presented in the former paper raise the question of whether below this threshold multilayer formation takes place. The present work clarifies this point by ellipsometry and scanning force microscopy measurements. Besides the polymer charge and the ionic strength, the

10.1021/jp0256488 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/10/2003

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Voigt et al.

Figure 1. Molecular structure of the statistic copolymer P(DADMACstat-NMVA), f being the ratio of cationic DADMAC monomers.

effect of the polyelectrolyte concentration is studied systematically. As it is already known, the polylelectrolyte conformation and the adsorbed amount are both changing with the polyelectrolyte concentration,11 which implies that the surface charge density is also changing with the concentration of the polyelectrolyte. This offers the opportunity to change the matching between the charge density of both the surface and the adsorbing chain. 2. Experimental Section 2.1. Materials and Film Preparation. The polyelectrolyte P(DADMAC-stat-NMVA) used in the present studies is a linear statistic copolymer consisting of positively charged diallyldimethyl-ammoniumchloride (DADMAC) monomers and neutral N-methyl-N-vinylacetamide (NMVA) monomers. Details about the synthesis and the characterization are described in ref 23 for PDADMAC and in ref 24 for PNMVA and the copolymers. The pure PDADMAC and the neutral PNMVA chains with the linear charge density of f ) 0.89, 0.75, 0.68, 0.53, 0.24, and 0.14 were available. The molecular weight is about 100.000 g/mol. Branched poly(ethylene imine) (PEI) and poly(styrene sulfonate) sodium salt (PSS) were obtained from Aldrich (Steinheim, Germany). The molecular weight of PEI is 750.000 g/mol, and that of PSS is 70.000 g/mol. The structure of PSS is shown in Figure 1. NaCl was purchased from Merck. The silicon wafers were provided by Wacker Siltronic AG, Burghausen (Germany) and cleaned for 30 min in 1:1 H2O2/H2SO4 mixture. The polyelectrolyte films were deposited on the silicon wafers by immersion for 20 min into aqueous solutions containing 10-2 mono mol/L (concentration of monomer units) of the respective polyelectrolyte and by rinsing with Milli-Q water after each deposition step. A complete dipping cycle in the following conforms to the sequence PSS/water/polycation/water. The first layer is a PEI layer, and after that, PSS and P(DADMAC-statNMVA) were deposited consecutively via the self-assembly technique. The outer layer was always the respective cationic copolymer. The films were dried in an air stream after completion of the multilayer assembly. 2.2. Apparatus and Measurement Procedure. 2.2.1. Ellipsometry. For a detailed description of ellipsometry, the reader is referred to the textbook by Azzam and Bashara25 and a very recent review article by Motschmann and Teppner.26 For thin films (d < 100 Å), the variation of the polarizer angle is very minor. Only the analyzer angle changes significantly, and therefore, only one parameter (thickness or refractive

Figure 2. Film thickness as a function of degree of charge of the polycation P(DADMAC-stat-NMVA) after six dipping cycles PSS/ P(DADMAC-stat-NMVA) prepared from solutions containing 10-1 mol/L NaCl: (a) at a fixed polycation concentration of 10-2 mono mol/L and at different PSS concentrations (given in mono mol/L); (b) at two different PSS/polycation ratios, 1:1 (open symbols) and 10:1 (filled symbols). The PSS/polycation concentrations are given in mono mol/ L.

index) of the coating can be determined. In the present study, the refractive index is taken from a thick film (about 1.5) under the assumption that the refractive index is the same for thin films. A Multiscope from Optrel (Berlin, Germany) has been used for the experiments presented in the following. Each type of film has been prepared at least twice. In certain cases, in which nonsystematic results have occurred (e.g., degree of charge (DC) of 14% and DC of 75% at high ionic strengths), even more samples have been prepared. The thickness has been measured at several positions on the wafer by ellipsometry (footprint around 1 × 3 mm2). The presented thickness data are average values, and the error bars are smaller than the symbol size. 2.2.2. Scanning Force Microscopy (SFM). The surface topology of the films has been investigated by a scanning probe microscope (Nanoscope III, Digital Instruments). The SFM images are recorded in the tapping mode. The surface roughness is obtained by using imaging software and is calculated by

Rrms )

xN1 ∑(z - jz)

2

i

(1)

where Rrms is the root-mean-square roughness, zi is the z value (i.e., height) for a certain pixel, jz is the average z value in the chosen area (2.5 × 2.5 µm2 in the present case), and N is the number of pixels in this area. 3. Results 3.1. Effect of Degree of Charge and Polymer Concentration. Figure 2a shows the film thickness as a function of the degree of charge of the cationic copolymer. The concentration

Charge Effects on the Formation of Multilayers of the polycation solution has been constant at 10-2 mono mol/ L, and the concentration of the PSS solution has been varied from 5 × 10-4 to 10-1 mono mol/L. The NaCl concentration has been 0.1 mol/L. The film thickness has been measured after six deposition cycles by ellipsometry. The thickness of the SiOx layer is subtracted in Figure 2a. In general, all curves show three regimes: Up to a degree of charge of 53%, the film thickness is below 100 Å. The thinnest films are produced with PSS and the neutral PNMVA (e50 Å). Between a degree of charge of 53% and 75%, the thickness increases abruptly, and at higher charge density, the thickness decreases slightly again. A maximum in thickness occurs. The degree of charge (DC) at which the maximum occurs is called DCmax in the following. It is shifted to lower DC values with increasing PSS concentration. All curves show a systematic change. Coming from low concentration, the maximum is at 89%. At intermediate concentrations, the thicknesses at a DC of 89% and at 75% are the same, and at the highest concentration, the maximum is shifted toward a DC of 75%. In addition, the large thickness of the film at a DC of 68% indicates a multilayer formation, while at a lower PSS concentration, the film belongs to the region of thin films. This leads to the speculation that at higher PSS concentrations the maxiumum could be shifted to even lower DC. Figure 2b shows the results for two different PSS/polycation concentration ratios, 1:1 and 10:1, at different polyelectrolyte concentrations. The curves at a ratio of 1:1 (open symbols) are qualitatively similar. The maximum in thickness is at a degree of charge of 75%, and the thickness decreases monotonically toward a film with fully charged PDADMAC (100%). At a ratio of 10:1 (filled symbols) the maximum is shifted to a lower degree of charge with increasing concentration. This is in agreement with the results in Figure 2a. At a fixed concentration ratio, the film thickness increases with increasing polyelectrolyte concentration. Hence, it is remarkable that at a PSS concentration of 10-2 mono mol/L the film is thicker if it is built up from a 10-3 mono molar polycation solution than in the case of 10-2 mono molar polycation solution. On the other hand, if the PSS concentration is a factor 10 lower than the polycation concentration (10-3/10-2) the film is thinner than in the case in which both dipping solutions are 10-2 mono molar. It is remarkable that the film containing 14% charged P(DADMAC-stat-NMVA) at a PSS/polycation ratio of 10-2/ 10-3 is thicker than the film containing 24% charged polycation. Investigations of several wafers prepared under the same conditions showed that this “local maximum” is a real effect. SFM images show that the thicker 14% film has an irregular grain-like structure, while the surface of the thinner 24% film is flat and featureless. Its roughness is about a factor 8 higher than that for the 24% film (see Table 1). The maximum in thickness, which occurs between a DC of 68% and 100% (Figure 2a,b) could be explained by a change in chain conformation, which will be discussed below. In the following, it will be checked whether this probable conformational change is related to changes in surface topology, that is, surface roughness. Figure 3 shows the SFM images of the films at five different degrees of charge. The dipping solutions contained 10-2 mono mol/L of the respective polyelectrolyte and 0.1 mol/L NaCl. From 100% to 75%, the films have a more and more granular structure with decreasing degree of charge. The corresponding roughness (rms) increases from 7 Å at 100% to 38 Å at 75% (see Table 1). At a DC of 53%, the grainlike features become smaller, and at a DC of 14%, the surface is quite flat with a few

J. Phys. Chem. B, Vol. 107, No. 22, 2003 5275 TABLE 1: RMS Roughness at Different Degrees of Charge and Ionic Strengthsa degree of charge (%)

0.1 mol/L NaCl

0.5 mol/L NaCl

100 89 75

6.7 19.3 37.7 21.8 (4) 9.0 (2) 12.6 5.9 (10-3/10-2) 9.7 51.0 (10-3/10-2)

23.2 49.1

53 24 14

18.1

a Unless it is stated otherwise, the films were prepared with six dipping cycles from solutions containing 10-2 mono mol/L PSS and polycation. At a DC of 75% and an ionic strength of 0.1 mol/L, the roughness was also determined after two and four dipping cycles. At a degree of charge of 24% and 14%, the films were built up at a PSS concentration of 10-3 mono mol/L, in addition.

Figure 3. SFM images (2.5 × 2.5 µm2) of silicon wafers coated after six dipping cycles (PSS/P(DADMAC-stat-NMVA)) prepared with 10-2 mono mol/L polymer and 0.1 mol/L NaCl at different degree of charge: (a) 100%; (b) 89%; (c) 75%; (d) 53%; (e) 14%. The z range of the images is (a) 6, (b) 20, (c) 33, (d) 20, and (e) 26 nm.

grains on it. Below 75%, the roughness decreases to about 10 Å (Table 1). 3.2. Multilayer Formation. Coming back to the two regimes in film thickness at a certain PSS concentration in Figure 2, the question is whether up to a DC of 53% six thin double layers are adsorbed or whether the multilayer formation stops after the deposition of a certain number of layers. Figure 4 shows the multilayer formation at a PSS and polycation concentration of 10-2 mono mol/L at four different DCs. The films at a DC of 75% and 100% show a strictly monotonic increase in film thickness with increasing number of adsorbed double layers.

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Figure 4. Film thickness as a function of dipping cycles at four different degrees of charge. The integers at the x-axis correspond to films with PDADMAC outside, and the rational numbers correspond to the films with PSS outside. Every data point corresponds to another wafer (for explanation, see text). The polyelectrolyte concentration was fixed at 10-2 mono mol/L, and the NaCl concentration was 0.1 mol/L.

The thickness of the film containing fully charged PDADMAC increases quite linearly. The thickness increment of the film containing 75% charged P(DADMAC-stat-NMVA) is larger from three double layers on. In the case of the film containing 53% charged polycation, from the second PSS layer on no further polyelectrolyte is adsorbed. The multilayer formation of a film built up with 68% charged polycation is between these two groups of films. Up to six double layers, the thickness is of the same order of magnitude as the film containing 53% charge, but the thickness increases slightly after each deposition cycle. This suggests a multilayer formation but of very thin layers at a DC of 68%. It should be mentioned that the films were dried after the last adsorption step and not in between. This means that each data point corresponds to another wafer. Therefore, the data can be directly compared to the results shown in Figure 2. This is an important point because several studies indicate that the drying procedure changes the polyelectrolyte chain conformation (e.g., refs 19 and 21). As mentioned before, the roughness changes with the degree of charge and it reaches its maximum at a degree of charge of 75%. To check whether this is caused by a simple correlation with the multilayer thickness, the roughness is determined after different dipping cycles for a film built up by 75% charged P(DADMAC-stat-NMVA) (see Table 1). The roughness increases from 9 Å after two dipping cycles to 22 Å after four dipping cycles and to 38 Å after six dipping cycles. At a fixed DC, the roughness increases with film thickness. But the film thickness is not the only important factor for the roughness. The film that contains 75% charged P(DADMAC-stat-NMVA) is about 50 Å thick (after two dipping cycles), and its roughness is similar to the 200 Å thick film of fully charged PDADMAC (after six dipping cycles). This indicates strong correlation between the surface roughness and the DC. Also the neutral PNMVA adsorbs slightly at the interface coated with PEI/PSS, but it forms a thinner layer (10-15 Å) than the charged polymers, and no further polyelectrolyte can be deposited in a next dipping step. On the other hand, only a few PSS can be deposited onto a surface previously coated with a PEI/PSS double layer. 3.3. Effect of Salt Concentration. Until now, the salt concentration was constant at 0.1 mol/L. In the experiments described in the following, the films were prepared from solutions of a fixed polymer concentration of 10-2 mono mol/L but at different salt concentrations.

Voigt et al.

Figure 5. Film thickness as a function of the counterion concentration of the polyelectrolyte solutions at different degrees of charge after six dipping cycles. The counterion concentration includes the amount of counterions of the polyelectrolyte and the salt concentration of added salt. The polyelectrolyte concentration was fixed at 10-2 mono mol/L.

Figure 6. SFM images of films after six dipping cycles at different degrees of charge of P(DADMAC-stat-NMVA) from solutions containing 10-2 mono mol/L of the respective polyelectrolyte and 0.5 mol/L salt: (a) 100%; (b) 89%; (c) 53%. The z range of the images is (a) 23, (b) 40, and (c) 28 nm.

Figure 5 shows the multilayer thickness as a function of counterion concentration (i.e., salt ions + polyelectrolyte counterions) at different degrees of polymer charge. Up to an ionic strength of 0.5 mol/L, the film thickness increases linearly with ccounterion at a DC of 75% and 89%. For a fully charged chain, the thickness increase is less-pronounced, rather with the xccounterion than with ccounterion. At a DC of 53% and lower, the films are quite thin and the ionic strength has no influence on the film thickness. Remarkable is the film behavior at a DC of 75% and a NaCl concentration of 0.5 mol/L. It was not possible to measure the thickness of this film because it is unstable: A gel-like material seems to be formed, which dewetted during the drying procedure. Waves were induced on the wafer by the gas stream. Drying in an oven instead of using a gas stream leads also to a dewetting inducing spots of different colors, that is, different thicknesses. SFM images demonstrate the change in surface topology by varying the salt concentration (Figure 6). At a DC of 75% and 100%, a kind of superstructure with deeper ridges is observed (in comparison to Figure 3). The surface roughness calculated

Charge Effects on the Formation of Multilayers from these SFM images increases with increasing salt concentration (Table 1). In the case of the film containing 53% charged P(DADMACstat-NMVA), the influence of the ionic strength is lesspronounced, which is confirmed by the values of roughness. The SFM images look quite different from SFM images of PSS/ PDADMAC films in the same regime of ionic strength in ref 16. There, a rather worm-like structure instead of a grain-like one is observed. This difference could be caused by the degree of charge. In addition the PDADMAC in ref 16 is a commercial one from Aldrich, which is usually branched, while the PDADMAC of the present study is a homemade linear one. 4. Discussion The results presented above clearly show that the thickness of the polyelectrolyte multilayer films is strongly influenced by the degree of polymer charge density, the polyelectrolyte concentration, and the ionic strength of the dipping solutions. As discussed later, the density of the film is quite independent of these parameters. This implies that the film thickness is proportional to the adsorbed amount. 4.1. Influence of Charge Density. Depending on the degree of charge of the cationic copolymer, two regimes can be distinguished. Region of Low Charge Density. Only thin films up to the second PSS layer (PEI/PSS/polycation/PSS) are formed in this region (see Figure 4). The charge overcompensation after adsorption of the second PSS layer seems to be not high enough for the adsorption of further polyelectrolyte layers, irrespective of the charge density of the polycation (between 14% and 53%, Figure 2). In the case of the uncharged PNMVA, the film formation does not proceed beyond the adsorption of the first PNMVA layer. Although the PNMVA is neutral, it adsorbs onto the negatively charged PSS layer. The adsorption of PNMVA on top of a dried PEI/PSS layer is assumed to be caused by the interdigitation of PEI and PSS. Hydrogen bonding between the amide group of PNMVA (acceptor) and the amine group of PEI (donor) is suggested. For further adsorption of PSS, the interdigitation by PEI is apparently not large enough, and a direct binding between PNMVA and PSS does not occur because PNMVA is a neutral hydrogen-bonding donor and PSS is a nonhydrogen-bonding polyelectrolyte. The fact that polyelectrolytes can be adsorbed on top of a polyelectrolyte of equal charge is already known from the literature:19,21 Decher et al.19 showed that PSS films up to 55 Å thick can be formed on top of a PSS surface by several adsorption steps, between which the film was dried. But on the other hand, in aqueous environment without drying the adsorption did not continue after the first adsorption step of PSS. This leads to the conclusion that the drying procedure changes the chain conformation. The formerly adsorbed oppositely charged polyelectrolyte layer (PAH in ref 19) could interdigitate the PSS layer at the surface. By several adsorption steps, the interdigitation could be extended up to 100 Å.19 A conformational change after drying was also assumed to be the main reason for the adsorption of two different quaternized polycations on top of a substrate previously coated with a polyanion.21 Hydrogen bonding is believed to be decisive for the formation of multilayers consisting of poly(aniline) and neutral polymers17 and of films consisting of poly(acrylic acid) (hydrogen-bonding donor) and poly(4-vinylpyridine) (hydrogen-bonding acceptor).18 It has been shown that multilayer formation via hydrogen-bond interactions requires polymers that are capable of forming very strong hydrogen bonds. On the other hand, fabrication of

J. Phys. Chem. B, Vol. 107, No. 22, 2003 5277 multilayer films based on hydrogen-bonding polymers and either polycations or polyanions is not possible. Region of High Charge Density. The formation of multilayers of PSS and PDADMAC must be caused by electrostatic interactions because hydrogen bonds between the two components cannot be formed. The present results show that for a multilayer formation a minimum degree of charge of P(DADMAC-stat-NMVA) between 53% and 75% is necessary under the present experimental conditions (see Figure 2). An explanation for the existence of such a threshold value of the DC is likely to be the charge reversal after each adsorption step, which seems to be necessary for the electrostatically driven formation of multilayers. At a concentration of 10-2 mono mol/L (of both PSS and P(DADMAC-stat-NMVA), thick films can be formed at a DC of 75% and slightly thinner films as the DC is increased beyond that value of degree of charge. The existence of a maximum in film thickness as a function of DC is attributed to the transition from a more coiled to a flat conformation of the adsorbed chain because of the increasing electrostatic repulsion between the charges along the chain. Such a change in conformation is indicated by the decreasing roughness from 38 Å (75%) to 7 Å (100%) (see Table 1). The larger roughness at 75% DC is attributed to chain loops at the surface, whereas chains with many trains lead to a molecularly smooth surface.27 This difference in roughness could explain the observation of a larger increase in film thickness with the number of deposition cycles in the case of the 75% DC film due to a larger surface area compared to the films containing fully charged PDADMAC. An increase in roughness after each deposition step could also explain the observed increasing increment in film thickness after each deposition cycle at a DC of 75%, which is also observed by Schoeler et al.28 and Glinel et al.29 For the fully charged PDADMAC chain and PSS, the thickness increment per double layer increases over the first three steps and is constant afterward. A similar effect has been also observed by other groups for the (PSS/PDADMAC)n system.16,30 For other polyelectrolyte pairs, Hoogveen et al. observed a minimum charge density of 20% to form stable multilayers.31 This means that the threshold can be different for different systems. A possible explanation could be additional hydrogen bonding or steric interactions due to differences in chain stiffness and charge distances. For weakly charged polyelectrolytes, a sharp maximum in film thickness at intermediate charge density is observed;32,33 the film thickness decreases toward lower as well as higher pH. A similar dependence on the pH is predicted by theoretical models.34 This sharp maximum at a specific DC is not found for the present system containing PSS and PDADMAC derivatives, which are strong polyelectrolytes. This difference between weak and strong polyelectrolytes is because weak anionic polyelectrolytes are charged in the basic regime but uncharged in the acidic regime and vice versa for polycations.35 Hence, if a weak polyelectrolyte is adsorbed under its optimum pH conditions, the surface charge of the oppositely charged weak polyelectrolyte adsorbed in the preceding step will vanish. Therefore, in the case of weak polyelectrolytes, an intermediate pH, at which both polyanions and polycations carry an intermediate amount of charges is favorable for the formation of a multilayer system. 4.2. Effect of Polyelectrolyte Concentration. As shown in Figure 2, the overall adsorbed amount of polyelectrolyte increases with increasing polyelectrolyte concentration of the dipping solution. This is in agreement with earlier studies in which the adsorbed amount was measured by total internal

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Figure 7. Model of polyelectrolyte conformation (a) at a low PSS concentration and adsorbing highly charged PDADMAC chain (89%100%) and (b) at a high PSS concentration and medium (68%-75%) charged P(DADMAC-stat-NMVA). PSS is marked in black, and the polycation is marked in gray.

reflection fluorescence (TIRF),11 as well as with reports from other groups.18,30 This may be explained by a change in conformation: At low polyelectrolyte concentration, each polyelectrolyte chain will find many binding sites. Therefore, the conformation will be rather flat and is determined by trains. Conversely, at high polyelectrolyte concentrations, many polyelectrolyte chains interact with the interface at the same time and each can adsorb on just a few binding sites, which results in short trains and long tails, oriented toward the solution.36 In addition, the concentration of counterions increases with increasing polyelectrolyte concentrations, causing a coiling of the polymer chains due to electrostatic screening of the charges along the chain.37 Another effect of the polyelectrolyte concentration is that the DCmax is shifted to lower DC values as the PSS concentration increases. This might be explained by a change in conformation of the adsorbed PSS chains. A model of chain conformations is shown in Figure 7: At a low PSS concentration, the conformation is rather flat and stiffer polycation chains (i.e., those with higher charge density) are preferred to be adsorbed because they adsorb also in a flatter conformation as shown in Figure 7. At a high PSS concentration, the adsorbed PSS chains again will show longer tails. Highly charged polycations are too stiff to fold between the tails. Therefore, a more coiled structure is preferred to be adsorbed. This means that the position of the maximum adsorbed amount is caused by steric effects. This effect will be of importance if polycation and polyanion concentrations are different from each other. These arguments lead to the conclusion that in each adsorption step, a chain conformation of the adsorbing chain is preferred that is similar to the conformation of the chain adsorbed in the previous deposition step. In such a case, the charge distances of adsorbed and adsorbing chains would fit in a better way, and more charges would be compensated than in the case in which adsorbed and adsorbing chains have different conformations. The shift of DCmax to lower DC with increasing polyelectrolyte concentration is connected to a shift of the charge reversal threshold to lower DC (see Figure 2). This could mean that also the charge (over)compensation is related to steric conditions. 4.3. Influence of Ionic Strength. With increasing salt concentration, the charges along the polyelectrolyte chain and between different chains are more and more screened. This leads to an enhanced coiling of the chains and an increase in adsorbed

Voigt et al. amount both at noninterpenetrable surfaces14,15 and at surfaces previously coated with polyelectrolytes. The chain coiling leads to a rougher film surface. This has been observed, for instance, by McAloney et al.16 by SFM for polyelectrolyte multilayers. The formation of complexes between oppositely charged polyelectrolytes of adjacent layers leads to a stronger interdigitation and higher internal roughness, which has been observed by neutron reflectometry.41 Below a DC of 53%, the ionic strength has no influence on the film thickness. On the other hand, at higher DC of PDADMAC, the film thickness changes dramatically with the salt concentration (see Figure 5). This means that the film thickness can be only influenced if a multilayer is formed. The dependence of the film thickness on the ionic strength is discussed controversily in the literature. While some results suggest a linear relationship, others indicate that the thickness is proportional to the square root of the ionic strength. In refs 38 and 11, it is shown that at low salt concentration (below 0.5 mol/L), the thickness increases with the ionic strength as xI. Such a xI dependence is also reported for films investigated by X-ray reflectometry under ambient conditions and for solvent-swollen polyelectrolyte films of (PSS/PAH) at the solid/ liquid interface against D2O studied by neutron reflectometry12,13 and also for PSS/PAH multilayer system at the air/liquid interface, studied by ellipsometry.39,40 Lo¨sche and co-workers41 showed by neutron reflectivity that the thickness of a fully hydrated layer pair (PSS/PAH) at the solid/air interface (100% relative humidity) varied linearly with the ionic strength of the dipping solutions in a concentration range between 0.5 and 3 M NaCl additive. A linear dependence of the thickness on the ionic strength is also reported for the (PSS/PDADMAC)n system.16,30 This result does not agree with the result for the fully charged PDADMAC chain shown in this study and in ref 22. The main difference between those experiments and the experiments described in the present paper is that the PDADMAC used in the former studies16,30 was branched, while the PDADMAC used in our labratory is linear, as already mentioned above. Because the charge density is higher for branched polyelectrolytes than for linear ones, the counterion contribution and therefore the effect of additional salt ions will be different. At a lower DC (75%, 89%), we observe a linear dependence of the film thickness on the ionic strength. In this case, the chain conformation is suggested to be loopy even in the absence of salt, which could lead to a different influence of ionic strength on the additional coiling due to addition of salt. The exponents of 0.5 or 1.0 do not match the exponents ranging from 0.05 to 0.15 as determined by scanning angle reflectometry (SAR) experiments.43 However, the dependence of the thickness on the ionic strength with its different exponents is not clear at all, and it is still under discussion. All of these measurements on the effect of degree of charge and the influence of ionic strength seem to lead to a paradox, explained in the following: (a) Above a salt concentration of 1 mol/L, the thickness increase is less-pronounced, but nevertheless, the film becomes thicker up to a concentration of 3 mol/ L.12,13 No plateau or decrease in thickness is observed up to the highest experimental salt concentration. While the increase in thickness with increasing salt concentration is due to a change in chain conformation (see arguments given above), at the same time the electrostatic attraction between film surface and polyelectrolyte in solution decreases. This counteracts the net increase in film thickness due to the coiling effects. Therefore, one would expect a plateau or even a decrease in film thickness

Charge Effects on the Formation of Multilayers at a certain ionic strength. At a salt concentration of 1 mol/L, the screening length as estimated from Gouy-Chapman theory is about 3 Å. This screening length is smaller than the diameter of a hydrated counterion and, as a consequence, electrostatic attraction should completely vanish. But, at even higher salt concentration, the film thickness increases. (b) On the other hand, the experiments at different charge densities show that electrostatic attraction plays an important role for the adsorption of polyelectrolytes onto an oppositely charged polymer surface. An explanation of this paradox could be that mean field theories such as the Gouy-Chapman theory are insufficient to describe the electrostatic field in front of the polyelectrolyte surface for our purpose. In addition, density fluctuations in the cloud of counterions, which make surface charges “visible” for a polyelectrolyte in front of the film surface, seem to be also important. The task to correlate the film density at different ionic strengths to former transport experiments leads to a further paradox: The electron density of the investigated films, that is, at different charge densities and different ionic strength, is quite similar, between 0.374 and 0.393 Å-3 determined by X-ray reflectivity.22 On the other hand, in transport experiments through polyelectrolyte multilayer systems,44 the addition of salt to the dipping solution reduces the diffusion coeffiecient. This was explained by a closer packing of the polymer chains or a reduction of defects in the film or both. X-ray reflectivity measurements are not sensitive to an irregular distribution of defects, but they give an average density. Hence, the mismatch of the results produced by transport studies and X-ray reflectivity is explained by a change in size distribution of defects from several large defects to many small ones in a way that the average density remains the same. 5. Conclusions The present results lead to the conclusion that electrostatically driven multilayer formation needs a minimum charge density of both the surface and the oppositely charged polyelectrolyte chain. This causes two different situations of film formation. At a DC below the charge reversal threshold (about 68%), the films are thin, but thick films are obtained at DC above that threshold value. In the regime of thick films, a second effect becomes important: with increasing charge density, the polymer chains become stiffer, and the chains adsorb in a flatter conformation, which in turn leads to a thinner film. This change in conformation is related to a decreasing roughness of the films. This results in a weak maximum in film thickness, the position of which depends on the polyelectrolyte concentration because of conformational changes of the adsorbed chain. If the charge overcompensation is insufficient for the formation of the multilayer system, the salt has no influence on the film thickness. Above the charge overcompensation threshold, the thickness can be fine-tuned by adding salt to the dipping solution. The ionic strength of the dipping solutions enhances the adsorbed amount because of charge screening, even at high ionic strength at which the screening length is smaller than the diameter of the counterions. At this point, mean-field descriptions such as the Gouy-Chapman model are not sufficient, and local charge fluctuations have to be taken into account, in addition. These results raise the question of whether the average charge density of the polyelectrolyte chain is the decisive factor for the multilayer formation or whether the charge distribution along the chain also has an important effect.45 Another open question is whether the charge reversal has an absolute value or whether it depends on the charge of the oppositely charged polyelectrolyte. Experiments aimed to solve this question are underway.

J. Phys. Chem. B, Vol. 107, No. 22, 2003 5279 Acknowledgment. W.J. and R.v.K. thank the Deutsche Forschungsgemeinschaft for financial support (Schwerpunktprogramm “Polyelektrolyte mit definierter Moleku¨larchitektur”). Further, the authors are indebted to Wacker Siltronic AG for providing the silicon wafers and Tobias Fu¨tterer for the introduction to the experimental work with the scanning force microscope. R.v.K. thanks Karine Glinel, Bjo¨rn Scho¨ler, and Helmuth Mo¨hwald for fruitful discussions. References and Notes (1) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (2) Decher, G.; Schmitt, J. J. Prog. Colloid Polym. Sci. 1992, 89, 160. (3) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (4) Tarabia, M.; Hong, H.; Davidov, D.; Kirstein, S.; Steitz, R.; Neumann, R.; Avny, Y. J. Appl. Phys. 1998, 83, 725. (5) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621. (6) Klitzing, R. v.; Mo¨hwald, H. Langmuir 1995, 11, 3554. (7) Farhat, T.; Schlenoff, J. B. Langmuir 2001, 17, 1184. (8) Klitzing, R. v.; Steitz, R. In Handbook of Polyelectrolytes and Their Applications; Tripathy, S. K., Kumar, J., Nalwa, H. S., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2002; Chapter 12. (9) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 1999, 104, 11996. (10) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319 and references therein. (11) Klitzing, R. v., Ph.D. Thesis, University of Mainz, Mainz, Germany, 1996. (12) Steitz, R.; Leiner, V.; Siebrecht, R.; Klitzing, R. v. Colloids Surf., A 2000, 163 63. (13) Klitzing, R. v.; Leiner, V.; Steitz, R. ILL Millenium Symp. Eur. User Meet., Proc. 2001, 73. (14) Van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984, 88, 6661. (15) Boehmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288. (16) McAloney, R. A.; Sinoyr, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655. (17) Stockton, W.; Rubner, M. Macromolecules 1997, 30, 2717. (18) Wang, L.; Fu, Y.; Wang, Z.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360. (19) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (20) Lowack, K.; Helm, C. Macromolecules 1998, 31, 823. (21) Fischer, P.; Laschewsky, A. Macromolecules 2000, 33, 1100. (22) Steitz, R.; Jaeger, W.; Klitzing, R. v. Langmuir 2001, 17, 4471. (23) Dautzenberg, H.; Go¨rnitz, E.; Jaeger, W. Macromol. Chem. Phys. 1998, 199, 1561. (24) Ruppelt, D.; Ko¨tz, J.; Jaeger, W.; Friberg, S. E.; Mackay, R. A. Langmuir 1997, 13, 3316. (25) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light, Elsevier: Amsterdam, 1987. (26) Motschmann, H.; Teppner, R. Ellipsometry in Interface Science. In NoVel Methods to Study Interfacial Layers; Mo¨bius, D., Miller, R., Eds.; Studies in Interface Science 11; Elsevier: Amsterdam, 2001. (27) Trains are adsorbed with all of their segments at the substrate, loops consists of nonadsorbed segments and connect two trains, and the free chain ends are called tails. (28) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889. (29) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408. (30) Dubas, S.; Schlenoff, J. Macromolecules 1999, 32, 8153. (31) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. Langmuir 1996, 12, 3675. (32) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (33) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (34) Park, S. Y.; Barrett, C. J.; Rubner, M. F.; Mayes, A. M. Macromolecules 2001, 34, 3384. (35) Hodak, J.; Etchenique, R.; Calvo, E. J.; Singhal, K.; Bartlett, P. N. Langmuir 1997, 13, 2708. (36) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: Cambridge, U.K., 1993. (37) Klitzing, R. v.; Kolaric´, B.; Jaeger, W.; Brandt, A. Phys. Chem. Chem. Phys. 2002, 4, 1907.

5280 J. Phys. Chem. B, Vol. 107, No. 22, 2003 (38) Schmitt, J. Ph.D. Thesis, University of Mainz, Mainz, Germany, 1996. (39) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871. (40) Essler, F. Ph.D. Thesis, University of Mainz, Mainz, Germany, 1998. (41) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893.

Voigt et al. (42) Decher, G. In ComprehensiVe Supramolecular Chemistry Sauvage, J.-P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, U.K., 1996; Vol. 9, p 507. (43) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249. (44) Klitzing, R. v.; Mo¨hwald, H. Macromolecules 1996, 29, 6901. (45) Voigt, U.; Khrenov, V.; Tauer, K.; Hahn, M.; Jaeger, W.; Klitzing, R. v. J. Phys. Condens. Matter 2003, 15, S213-S218.