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Influence of Charge Density and Ionic Strength on the Multilayer Formation of Strong Polyelectrolytes Roland Steitz,† Werner Jaeger,‡ and Regine v. Klitzing*,† Iwan-N.-Stranski-Institut 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 January 31, 2001. In Final Form: May 11, 2001 The influence of ionic strength and polymer charge density on the multilayer formation of strong polyelectrolytes is investigated by X-ray reflectivity. To a first approximation the adsorption behavior as a function of the degree of charge, f, is binary: For f e 50% the films are one order of magnitude thinner than those for f g 75%. This is due to a threshold of charge overcompensation after each adsorption step which seems to be at f between 50% and 75%. Above the charge reversal limit the thickness and the surface roughness increase with decreasing polymer charge. Below a charge density of 50% the film thickness cannot be changed by salt additive whereas the film thickness increases with cNaCl1/2 above a degree of charge of 75%. The density of all investigated films is quite similar.
1. Introduction Thin polymer films are of great scientific interest because they deal with many problems in basic research and because of their potential industrial applications. In the early 1990s a new preparation method was developed using alternate layer by layer deposition of anionic and cationic polyelectrolytes from aqueous solutions1,2 with the advantage that the thickness can be tuned with angstrom precision by the number of layers and by salt additives:3 Small-angle X-ray scattering and UV-vis spectroscopy experiments provided first proof for a linear increase in film thickness with the number of polyelectrolyte layers. Neutron reflectivity measurements showed that adjacent polyelectrolyte layers interpenetrate and indicated that there is no distinct layer-by-layer separation between polyelectrolytes of opposite charges.4-6 The driving force for the formation of such multilayers seems to be the electrostatic attraction between the oppositely charged polyelectrolytes. Measurements of the surface potential resulted in a change of sign in surface charge after each single adsorption step, i.e., each additional deposited polyelectrolyte layer.7,8 Radioanalytical investigations of polyelectrolyte multilayers of nearly fully charged strong polyelectrolytes showed that the film bulk does not contain any salt ions and that the excess charge resides at the surface.9 This is explained by a charge compensation between opposite charged polyelectrolytes (so-called intrinsic charge compensation) by the formation * To whom correspondence may be addressed. E-mail: klitzing@ chem.tu-berlin.de. Telephone: 49-30-31426774. Fax: 49-3031426602. † Iwan-N.-Stranski-Institut fu ¨ r Physikalische und Theoretische Chemie, Technische Universita¨t Berlin. ‡ Fraunhofer-Institut fu ¨ r Angewandte Polymerforschung. (1) Decher, G.; Hong, J. D. Makromol. Chem. 1991, 46, 321. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (3) Decher, G.; Schmitt, J. J. Prog. Colloid Polym. Sci. 1992, 89, 160. (4) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (5) Tarabia, M.; Hong, H.; Davidov, D.; Kirstein, S.; Steitz, R.; Neumann, R.; Avny, Y. J. Appl. Phys. 1998, 83, 725. (6) Hong, H.; Steitz, R.; Kirstein, S.; Davidov, D. Adv. Mater. 1998, 10, 1104. (7) Klitzing, R. v.; Mo¨hwald, H. Langmuir 1995, 11, 3554. (8) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 1999, 104, 11996. (9) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626.
of 1:1 stoichometric polyelectrolyte complexes.10 The strong stability of the films due to complexation between two adjacent layers is also predicted by theoretical models.11 By addition of salt to the aqueous solution, the thickness of the self-assembly films prepared from these solutions can be increased. The differences in film thickness are explained by different conformations of the chains: Without salt the polyelectrolyte chains are oriented flat and parallel to the substrate, with higher salt concentration of the aqueous solutions the chains form coils12,13 which are then adsorbed at the interface. Up to a salt concentration of 3 mol/L, the film thickness increases.14-16 At an ionic strength of 1 mol/L the Debye length is about 3 Å, which means that the electrostatic interaction is screened on a length scale smaller than the diameter of a sodium ion (the hydration shell included). This fact raises the question on the role of the electrostatic contribution during the adsorption process of a polyelectrolyte on an oppositely charged surface. Recent investigations of multilayer systems consisting of weak polyelectrolytes by Rubner and co-workers17,18 showed that the thickness, the interdigitation of adjacent layers, and the surface roughness are sensitive to the charge density which was varied by changing the pH of the dipping solution. The present paper is focused on the competition between surface/polyelectrolyte and polyelectrolyte/polyelectrolyte interactions. The influence of salt and of polymer charge density on the multilayer formation of strong polyelectrolytes is investigated. The use of strong polyelectrolytes gives the opportunity to distinguish between the change in charge of the adsorbing polyelectrolyte chain and the surface charge. (In the case of weak polyelectrolytes both the surface charge and the charge of the adsorbing chain (10) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, C.; Stscherbina D. In Polyelectrolytes: Formation, Characterization and Application; Hanser: Mu¨nchen, 1994. (11) Castelnovo, M.; Joanny, J.-F. Langmuir 2000, 16, 7524. (12) Schee, H. A. v. d.; Lyklema, J. J. Phys. Chem. 1984, 88, 6661. (13) Boehmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288. (14) Klitzing, R. v. Thesis, University of Mainz, 1996. (15) Schmitt, J. Thesis, University of Mainz, 1996. (16) Steitz, R.; Leiner, V.; Siebrecht, R.; Klitzing, R. v. Colloid Surf., A 2000, 163, 63. (17) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (18) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213.
10.1021/la010168d CCC: $20.00 © 2001 American Chemical Society Published on Web 06/26/2001
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Figure 1. Molecular structure of the statistic copolymer P(DADMAC(f)-NMVA(100 - f)) with f as ratio of cationic DADMAC monomers in %.
are modified simultaneously by changing the pH of the dipping solution.) 2. Experimental Section 2.1. Materials and Film Preparation. The polyelectrolyte P(DADMAC-NMVA) used in the present studies is a linear statistic copolymer consisting of positively charged diallyldimethylammonium chloride (DADMAC) monomers and neutral N-methyl-N-vinylacetamide (NMVA) monomers. Details about the synthesis and the characterization are described in ref 19 for P(DADMAC) and in ref 20 for P(NMVA) and the copolymers. Beside the pure P(DADMAC) and the neutral P(NMVA), chains with the linear charge density of f ) 89%, 75%, 50%, 24%, and 14% were available. The structure is shown in Figure 1. The molecular weight is about 100 000. Branched polyethylenimine (PEI) and poly(styrenesulfonate) sodiumsalt (PSS) were obtained from Aldrich (Steinheim, Germany). The molecular weight of PEI is 750 000 and 70 000 in the case of PSS. NaCl was purchased from Merck. The silicon wafers were provided by Wacker Siltronic AG, Burghausen (Germany), and cleaned for 30 min in a 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 monomol/L (concentration of monomer units) of the respective polyelectrolyte and by rinsing with MilliQ-water after each deposition step. The first layer is one PEI layer, and after that PSS and P(DADMAC-NMVA) 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. Two sets of wafers were prepared: (1) at different charge densities of P(DADMAC-NMVA) (cNaCl ) 0.1 mol/L); (2) at different salt concentrations (0, 0.5, and 1.0 mol/L). 2.2. Apparatus and Measurement Procedure. The X-ray reflectivity experiments were performed on a triple axis diffractometer built at the HMI Berlin. The primary beam of this instrument is defined by the line focus of a sealed X-ray tube (0.04 mm × 8 mm, Cu anode, 30 kV acceleration voltage, 30 mA anode current) and a diaphragm (0.2 mm × 8 mm) at a distance of 500 mm. The reflected beam is monochromatized by a pyrolythic graphite crystal and the pulse height discriminator of the scintillation detector. The graphite crystal is set to reflect the Cu KR doublet (1.541 Å). For measurements at high intensities, e.g., primary beam and region of total reflection, a remote controlled Ni absorber was inserted in the reflected beam. All data were fitted by applying the optical matrix method.21,22 Besides thickness and density, the fits gave also information about the root mean square roughness of the film and substrate surface. Additionally, the surface roughness of the films was measured with a Scanning Probe Microscope (Nanoscope III, Digital Instruments).
3. Results Figure 2 shows the X-ray reflectivity spectra of two different wafers which were coated with 10 double layers (PSS/P(DADMAC(89)-NMVA(11))) and 10 double layers (PSS/P(DADMAC(14)-NMVA(86))), respectively. Both spectra show Kiessig oscillations,23 which yield direct (19) Dautzenberg, H.; Go¨rnitz, E.; Jaeger, W. Macromol. Chem. Phys. 1998, 199, 1561. (20) Ruppelt, D.; Ko¨tz, J.; Jaeger, W.; Friberg, S. E.; Mackay, R. A. Langmuir 1997, 13, 3316. (21) Parrat, L. G. Phys. Rev. 1954, 95, 359. (22) Russel, T. P. Mater. Sci. Rep. 1990, 5, 171.
Figure 2. X-ray reflectivity patterns of two different silicon wafers coated with (PSS/P(DADMAC(89)-NMVA(11)))10 (solid squares) and (PSS/P(DADMAC(14)-NMVA(86)))10 (open triangles), respectively. The solid lines are fits to the experimental reflectivity curves. The arrows indicate the critical Q value of total external reflection at the air/polymer interface (1) and at the air/silicon interface. Inset: The scattering length density perpendicular to the substrate and the film surface for (PSS/ P(DADMAC(89)-NMVA(11)))10 (solid line) and for (PSS/ P(DADMAC(14)-NMVA(86)))10 (dashed line). Note: The reduced intensity in the q-regime between the two arrows in the case of the thicker film is due to the absorption of X-rays in the film.
Figure 3. Thickness of a (PSS/P(DADMAC(f)-NMVA(1 - f)))10 film as a function of the degree of polymer charge f. The aqueous solutions the films were adsorbed from contained 10-2 monomol/L and 0.1 mol/L NaCl. The error bars are of symbol size.
information on the total thickness of the films. The minima positions of the Kiessig fringes in the spectra of the film built with 89% charged P(DADMAC-NMVA) are shifted toward lower Q, and the distance, ∆Q, between adjacent minima is smaller than in the case of the film of the less charged P(DADMAC-NMVA). This indicates a thicker film from the higher charged P(DADMACNMVA). The solid lines are fits to the data based on a one-box model. The resulting density profiles perpendicular to the interface are shown in the inset. The film thickness of (PSS/P(DADMAC(89)-NMVA(11)))10 is about 600 Å, and the film thickness of (PSS/P(DADMAC(14)NMVA(86)))10 is about 77 Å. The scattering length density F is the product of the electron density Fel and the Thomson radius (r0 ) 2.82 × 10-5 Å) and is about 1.05 × 10-5 Å-2 for both systems. Figure 3 shows the film thickness as a function of the degree of charge of the cationic copolymer. There is a maximum in thickness of 660 Å at a degree of charge of (23) Kiessig, H. Ann. Phys. (Germany) 1931, 10, 769.
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4. Discussion
Figure 4. Thickness of a (PSS/P(DADMAC(f)-NMVA(1 - f)))10 film as a function of the root-mean-square of the salt concentration of the polyelectrolyte solutions. The salt concentration also includes the amount of counterions of the polyelectrolyte.
75%. At a higher charge density the thickness decreases slightly to 500 Å for the fully charged PDADMAC chain. Below a degree of charge of 50%, the thickness of the films is about 75 Å and almost independent of the charge density. Only in the case of a neutral PNMVA chain is the film thinner (50 Å). In further X-ray experiments the film thickness was checked after each dipping step. The results show that below a degree of charge of 50%, no multilayer formation is possible. Figure 4 shows the thickness of the PSS/P(DADMACNMVA) multilayer system as a function of the salt (NaCl) concentration of the polymer dipping solution. For a certain film both the PSS and the P(DADMAC-NMVA) solution contained the same amount of NaCl. In the case of the fully charged PDADMAC chain, the film thickness increases linearly with cNaCl1/2. At the highest concentration studied in the present work, the data indicate a decreasing influence of the salt concentration. This implies that the fit of film thickness by a power law results in an exponent lower than 0.5 for concentrations higher than 0.5 mol/L NaCl. For a degree of charge of 50% and 14%, the ionic strength has no influence on the film thickness. The films are between 70 and 75 Å thick. All 14 investigated films had a scattering length density between 1.055 and 1.11 × 10-5 Å-2, which corresponds to an electron density between 0.374 and 0.393 Å-3. On the assumption that the charged DADMAC monomers form complexes with the opposite charges of PSS by exchange of counterions, the number of electrons per DADMAC and NMVA monomer is quite similar. This means that the monomer density is similar in the different systems. Below a degree of polycation charge of 50%, the Gaussian roughness of the air/film interface is between 10 and 15 Å. Above a degree of charge of 75% the roughness depends on the charge density. At 75% the roughness is about 40 Å and decreases to 15 Å for a fully charged PDADMAC chain. The roughness values determined by the fit to the X-ray reflectivity results are confirmed by scanning force microscopy measurements. During preparation we observed that the coated wafers became more and more hydrophobic after dipping in the solution with the polycation with decreasing P(DADMACNMVA) charge. After the wafers were dipped in the PSS solution, they were hydrophilic again. This is a proof that the outermost layer controls the wettability, which was also found for other multilayer systems.17
The thickness of the polyelectrolyte multilayer is strongly influenced by the degree of polymer charge and the ionic strength of the polyelectrolyte solution during preparation. Influence of Charge Density. Two regimes can be distinguished: f g 75%. Thick films are built up and they become slightly thinner with increasing degree of charge. This is due to the transition from a more coiled to a flat chain conformation of the adsorbed chain because of the increasing electrostatic repulsion between the charges along the chain. This change in conformation is also indicated by the decreasing roughness from 40 Å (75%) to 15 Å (100%). The larger roughness is due to chain loops at the surface whereas chains with flat segments lead to a molecularly smooth surface. f e 50%. Only thin films are built-up. In this regime the change in surface charge after adsorption of the polycation seems not to be high enough for the formation of a multilayer system. The film thickness of about 77 Å suggests the adsorption of a PEI/PSS/P(DADMACNMVA) layer only.24 This fact is almost independent of the charge density (between 14% and 50%). The charge overcompensation after adsorption of the P(DADMACNMVA) is not high enough for the adsorption of the following PSS layer. In the case of the neutral PNMVA chain the film is even thinner (about 50 Å). This could mean that the neutral chain cannot adsorb onto the previously adsorbed PSS layer and the film is just a PEI/ PSS layer. This, in turn, leads to the conclusion that at least a low polymer charge (between 0 and 14%) is necessary for the adsorption of a polymer chain onto a charged surface. A maximum in film thickness at intermediate charge density as found for multilayer systems of weakly charged polyelectrolytes25 was not observed in the present case. Influence of Ionic Strength. In the case of the fully charged PDADMAC, the film thickness changes dramatically with the salt concentration. The observed change in layer thickness is attributed to the increasing screening of charges along the polyelectrolyte chain with increasing ionic strength which induces an increasing coiling of the polyelectrolyte chains. The characteristic dependence of the thickness on the square root of the ionic strength was also observed for the multilayer system (PSS/PAH)n against water14,16 and air15 and can be considered as a general property of this kind of polyelectrolyte film. Below a charge density of 50% the ionic strength has no influence on the film thickness. These results lead to the conclusion that a minimum charge of both the surface and the oppositely charged polyelectrolyte chain is necessary for the adsorption of the polyelectrolyte. 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 salt additive in the dipping solution. The polymer density of all investigated films is quite similar. This is not consistent with results of studies on dye transport through a PSS/PAH system (PSS, poly(styrenesulfonate); PAH, poly(allylamine hydrochloride)).26 The addition of salt to the dipping solution reduces (24) Klitzing, R. v.; Steitz, R. Unpublished results: One PEI layer with the included SiO2 layer is around 30 Å thick. (25) Park, S. Y.; Barrett, C. J.; Rubner, M. F.; Mayes, A. M. Macromolecules 2001, 34, 3384. (26) Klitzing, R. v.; Mo¨hwald, H. Macromolecules 1996, 29, 6901.
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the diffusion coefficient measured by TIRF (total internal reflection fluorescence), which was explained by a closer packing of the polymer chains and/or a reduction of defects in the film. X-ray reflectivity measurements are not sensitive to an irregular distribution of defects. Hence, the mismatch of the results produced by TIRF and X-ray reflectivity can be explained by the reduction of defects with increasing ionic strength. 5. Conclusions Charge overcompensation is the stringent condition for the formation of polyelectrolyte multilayers. In the present case the charge overcompensation threshold is between 50% and 75%. Further, the adsorbing polymer must have a minimum charge (between 0 and 14% for P(DADMACNMVA)). These findings are to say that the surface/
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polyelectrolyte interactions play the most important role for the polyelectrolyte adsorption. In the present case the segment/segment interactions (influenced by ionic strength and also by charge density) affect only the film thickness but they do not decide whether an adsorption takes place or not. Acknowledgment. W.J. and R.v.K. wish to 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, Tobias Fu¨tterer for the introduction into the experimental work with the scanning probe microscope, and H. Mo¨hwald and co-workers for fruitful discussions. LA010168D
