Polyelectrolyte Multilayer Films of Different Charge Density

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Polyelectrolyte Multilayer Films of Different Charge Density Copolymers with Synergistic Nonelectrostatic Interactions Prepared by the Layer-by-Layer Technique Bjoern Schoeler,† Sonja Sharpe,†,‡ T. Alan Hatton,§ and Frank Caruso*,¶ Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Golm, Germany, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Melbourne, Victoria 3010, Australia Received October 13, 2003. In Final Form: January 4, 2004 Random copolymers composed of diallyldimethylammonium chloride (DADMAC) and acrylamide with varying contents (8-100 mol %) of the cationic DADMAC component were alternated with polyanionic, fully charged poly(styrenesulfonate) to form multilayer thin films. UV-vis spectrophotometry, FTIR spectroscopy, and quartz-crystal microgravimetry (QCM) were employed to follow multilayer buildup. Atomic force microscopy was used to obtain structural information. Layer thicknesses have been determined with small-angle X-ray scattering and ellipsometry, in addition to values calculated from QCM. While in previous work,1-6 a critical charge density limit could be observed, below which no layer growth is possible; in this system, multilayer formation takes place with copolymers with charge densities as low as 8 mol %. Instead of a continuous increase of adsorbed amounts with decreasing charge density above the critical charge density, as found in previous work,2,3,6,7 similar layer thicknesses for films with 100 and 8 mol % charged polyelectrolytes and maximally adsorbed amounts for copolymers in an intermediate charge density region have been found. This adsorption behavior is explained in terms of synergistic nonelectrostatic interactions between the polyelectrolytes used.

Introduction Formation of polyelectrolyte multilayers using the layerby-layer technique8 is widely reported to be driven by electrostatic interactions between the two oppositely charged polyelectrolytes.1,9,10 Charge overcompensation by adsorbing polyelectrolytes leading to the reversal of the surface charge is considered to be a prerequisite for polyelectrolyte multilayer formation, because it enables the subsequent adsorption of oppositely charged polyelectrolytes to form the next layer.2,10-13 For that reason, the process of layer growth will be influenced by the charge density of the polyelectrolytes. An overview of the investigations on the influence of charge density on multilayer formation in the literature has been given elsewhere.3 * Author to whom correspondence should be addressed. Fax: +61 3 8344 4153. E-mail: [email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ Permanent address: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. § Massachusetts Institute of Technology. ¶ The University of Melbourne. (1) Voigt, U.; Jaeger, W.; Findenegg, G. H.; Klitzing, R. V. J. Phys. Chem. B 2003, 107, 5273. (2) Steitz, R.; Jaeger, W.; von Klitzing, R. Langmuir 2001, 17, 4471. (3) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889. (4) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J.; Bohmer, M. R. Langmuir 1996, 12, 3675. (5) Arys, X.; Jonas, A.; Laguitton, B.; Legras, R.; Laschewsky, A.; Wischerhoff, E. Prog. Org. Coat. 1998, 34, 108. (6) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408. (7) Kolarik, L.; Furlong, D. N.; Joy, H.; Struijk, C.; Rowe, R. Langmuir 1999, 15, 8265. (8) Decher, G. Science 1997, 277, 1232. (9) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 430. (10) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (11) Netz, R. R.; Joanny, J. F. Macromolecules 1999, 32, 9013. (12) Lowack, K.; Helm, C. A. Macromolecules 1998, 31, 823. (13) Castelnovo, M.; Joanny, J. F. Langmuir 2000, 16, 7524.

This paper demonstrates that nonelectrostatic interactions also affect multilayer formation with polyelectrolytes, as suggested in a number of previous studies.9,14-18 For example, Kotov showed that hydrophobic interactions can play a significant role in the formation of polyelectrolyte multilayers.17 Other groups have reported the importance of hydrogen bonding, in addition to electrostatic interactions, to form multilayer films.14-16,18 Multilayers with uncharged polymers can also be formed by using hydrogenbonding interactions.9,14-16,18,19 In particular, we show that multilayer formation can be achieved with polyelectrolytes of very low charge density (as low as 8%), with no critical minimum charge density, in contrast to previous investigations.1-6,20 Additionally, a different dependence, compared to previous studies21-24 of adsorbed amounts on charge density has been found. For the system investigated in this work, random copolymers composed of diallyldimethylammonium chloride (DADMAC) and acrylamide (AM) in different ratios, in alternation with fully charged poly(styrenesulfonate) (PSS), are used to construct multilayers. No critical charge density limit is observed. Synergistic nonelectro(14) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (15) Raposo, M.; Oliveira, O. N. Langmuir 2000, 16, 2839. (16) Pontes, R. S.; Raposo, M.; Camilo, C. S.; Dhanabalan, A.; Ferriera, A.; Oliveira, J. O. N. Phys. Status Solidi 1999, 173, 41. (17) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789. (18) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (19) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (20) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133. (21) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (22) Cochin, D.; Passmann, M.; Wilbert, G.; Zentel, R.; Wischerhoff, E.; Laschewsky, A. Macromolecules 1997, 30, 4775. (23) Fischer, P.; Laschewsky, A.; Wischerhoff, E.; Arys, X.; Jonas, A.; Legras, R. Macromol. Symp. 1999, 137, 1. (24) Schoeler, B.; Poptoschev, E.; Caruso, F. Macromolecules 2003, 36, 5258.

10.1021/la035909k CCC: $27.50 © 2004 American Chemical Society Published on Web 03/04/2004

Polyelectrolyte Multilayer Films

Langmuir, Vol. 20, No. 7, 2004 2731

Table 1. List of Copolymers of Different Charge Densities, Including Their Molecular Weights and Notation charge density (%)a

molecular weight (g mol-1)b

notation

100 73 58 35 21 8

100 000-200 000 580 000 1 200 000 2 300 000 4 000 000 4 200 000

DAM100 DAM73 DAM58 DAM35 DAM21 DAM8

a Determined by chloride potentiometry.26 by static light scattering.26

b

MW, determined

Figure 1. Chemical structures of the polyelectrolytes used.

static interactions are herein reported to be responsible for this observation. A detailed analysis of several factors governing the layer growth for the investigated systems is given. A number of techniques that probe the amounts of adsorbed polyelectrolyte are combined with structural characterization methods to obtain insight into the assembly process. Experimental Section Materials. PSS, MW ) 70 000 g mol-1, poly(ethyleneimine) (PEI), MW ) 25 000 g mol-1, and poly(diallyldimethylammonium chloride) (PDADMAC), MW ) 100 000-200 000 g mol-1, were purchased from Aldrich. Random copolymers of DADMAC and acrylamide (DADMAC-AM) with DADMAC contents (ratio of charged monomers to all of the monomers of the polymer) of 73 mol % (DAM73), 58 mol % (DAM58), 35 mol % (DAM35), 21 mol % (DAM21), and 8 mol % (DAM8) were kindly donated by Dautzenberg (Max Planck Institute of Colloids and Interfaces, Golm, Germany) and were synthesized in the group of Jaeger (Fraunhofer Institute of Applied Polymer Research, Golm, Germany). Table 1 shows the molecular weights of the copolymers and their notation used in this paper. Information concerning the synthesis and characterization of these copolymers can be found in the work of Jaeger.25,26 Structures of the polyelectrolytes are shown in Figure 1. Poly(acrylamide)s with molecular weights of 65 000, 400 000, and 5 000 000 g mol-1 were obtained from Polysciences. Sodium chloride was purchased from Merck and used as obtained. The PSS and the DADMAC-AM copolymers were dialyzed against pure water. Quartz slides were obtained from Hellma Optik GmbH (Jena, Germany), the silicon wafers were purchased from Silchem Handelsgesellschaft mbH (Freiberg, Germany), and the quartz-crystal microbalance (QCM) electrodes were obtained from Kyushu Dentsu (Nagasaki, Japan). The RCA protocol (sonication in a 1:1 mixture of water and 2-propanol for 15 min, followed by heating at 70 °C for 10 min in a 5:1:1 mixture of water, H2O2, and a 29 vol % ammonia solution) was applied to clean the quartz slides and silicon wafers. The QCM electrodes were cleaned by treatment with a sulfuric acid/hydrogen peroxide (3:1) mixture (piranha solution). (CAUTION: Piranha solution should be handled with extreme care, and only small volumes should be prepared at any one time.)27 RCA-treated glass slides (25) Dautzenberg, H.; Gornitz, E.; Jaeger, W. Macromol. Chem. Phys. 1998, 199, 1561. (26) Brand, F.; Dautzenberg, H.; Jaeger, W.; Hahn, M. Angew. Makromol. Chem. 1997, 248, 41.

coated initially with 5 nm of chromium and then with a 120 nm thick gold layer with an Edwards Auto 306 evaporation unit operated at 1.8 × 10-6 Pa were used as substrates for FTIR measurements. Water from a three-stage USF Purelab Plus purification system with a resistivity > 18 MΩ cm was used in all of the experiments. Preparation of Multilayer Films. Polymer solutions with a concentration of 1 mg mL-1 were used for all of the experiments. For most of the experiments, the salt concentration was 0.5 M NaCl in both of the adsorption solutions. To obtain information on the influence of ionic strength, films have also been assembled using 0.01 M NaCl in both of the adsorption solutions. The substrates (six samples at a time) were dipped for 20 min in PEI solutions, followed by a washing step by immersing the samples three times in water and drying with a stream of nitrogen. The samples were then dipped in the anionic PSS solution for 20 min. The washing and drying procedure described above was repeated, and the samples were then dipped in either the cationic PDADMAC or DADMAC-AM copolymer solutions for 20 min. This procedure was repeated until 10 layers were deposited. UV-vis and QCM measurements were performed after each adsorption step to follow the layer formation. Atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and ellipsometry measurements were performed on samples after the deposition of 10 layers. In addition, AFM images were recorded after the deposition of the intermediate layers. UV-Vis Measurements. A HP8453 UV-vis spectrophotometer was used to record UV-vis spectra after the deposition of each layer. PSS has an absorption peak at 227 nm, while PDADMAC and the PDADMAC-AM copolymers show no absorption in the UV-vis region. Data were evaluated after subtracting the spectrum of the RCA-treated quartz sample (blank) from each of the measured spectra. QCM Measurements. An in-house-built QCM device with a frequency counter from Hewlett-Packard was used to determine the deposited mass after each adsorption step. The resonance frequency, F, of the QCM electrodes was ca. 9 MHz. Details of the method can be found in a previous publication.3 According to the Sauerbrey equation,28 by using a polyelectrolyte film density of 1.2 × 106 g m-3,24,29,46 the thickness d of the polyelectrolyte films can be calculated from the following equation:

∆d (nm) ) -0.019∆F (Hz)

(1)

AFM Measurements. AFM images were recorded with a Nanoscope IIIa multimode microscope (Digital Instruments Inc., Santa Barbara, CA). Measurements were performed in tapping mode using silicon tips (Nanosensors, Wetzlar) with a resonance frequency of ca. 300 kHz and a spring constant of 32-41 N m-1. Several images were recorded from different macroscopically separated areas of the air-dried films. Measurements and processing (first-order image flattening and plane fitting) of the AFM images were performed with Nanoscope 4.43r6 software. FTIR Measurements. A Bruker Equinox 55/S FTIR spectrometer operating in reflection mode was used. A modified reflection device (Harrick Co.) at an angle of 84° was used to perform the infrared-reflection absorption spectroscopy measurements. Before each measurement, nitrogen was streamed for 10 min through the measuring chamber to dry it. SAXS Measurements. Measurements were performed with a commercial θ/2θ instrument [STOE and CIE GmbH, Darmstadt, Germany, U ) 40 kV, I ) 50 mA, λ ) 0.154 nm (Cu KR)]. The divergence of the incoming beam was 0.1°, and the 2θ resolution was 0.05°. Ellipsometry. Ellipsometry measurements (null-ellipsometry mode with an angle of incidence of 70°) were performed with a Multiskop ellipsometer (Optrel) using a 632.8 nm He-Ne laser. Silicon wafers were used as the substrates for the multilayer formation and were cleaned as described earlier. The data were (27) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546. (28) Sauerbrey, G. Z. Phys. 1959, 155, 206. (29) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422.

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Figure 2. Adsorbed amounts of copolymer as a function of the adsorption time obtained from the QCM measurements for copolymer adsorption on a PEI/PSS film. The adsorption was from 0.5 M NaCl solutions for all experiments. evaluated using the software delivered with the instrument. Further details on the method and setup can be found elsewhere.30,31

Results and Discussion First, the formation of multilayers by the alternate adsorption of oppositely charged polyelectrolytes from solutions containing 0.5 M sodium chloride in both the anionic and cationic solutions was examined. These adsorption conditions are typical of the experimental protocol commonly followed for the buildup of polyelectrolyte multilayers. The multilayer formation process was followed with a wide range of techniques. Several methods were applied to determine the film thicknesses of the multilayers. The influence of drying between each adsorption step was examined, and the film formation under lower ionic strength (0.01 M sodium chloride in both of the adsorption solutions) was investigated. Further, the influence of the molecular weight of the copolymers used on the multilayer film formation is discussed. Multilayer Film Formation from Polyelectrolyte Solutions Containing 0.5 M NaCl. Before the investigation of the growth behavior of films of the DADMACAM copolymers, the following control experiment was performed. To ensure that saturation of the copolymer adsorption takes place within the adsorption time of 20 min and that the adsorbed amounts are therefore not determined by the adsorption time, the adsorbed amounts were followed as a function of the adsorption time with QCM (Figure 2). This experiment was performed for the DADMAC-AM copolymer system, because, here, the polymers of high molecular weight (for example, ca. 4 000 000 g mol-1 for DAM8; Table 1) with a low charge density are used. Therefore, it is possible that these copolymers adsorb very slowly. The adsorption of the different copolymers as a function of the adsorption time was investigated using QCM electrodes that were precoated with one PEI and one PSS layer (Figure 2). The precoated substrates were immersed four times for 5 min in the copolymer solution followed by rinsing, drying, and (30) Azzam, R. M.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, The Netherlands, 1979. (31) Harke, M.; Teppner, R.; Schulz, O.; Motschmann, H.; Orendi, H. Rev. Sci. Instrum. 1997, 68, 3130.

Figure 3. (a) UV-vis and (b) QCM measurements for the formation of multilayer films of PSS and DADMAC-AM copolymers prepared from solutions containing 0.5 M NaCl. Odd layer number ) cationic polyelectrolytes. Even layer number ) PSS.

determination of the adsorbed amount. The results, however, show that saturation of the copolymer adsorption occurs after 5 min. UV-vis and QCM experiments were conducted to follow the multilayer film formation. For the UV-vis measurements, evolution of the PSS absorption was monitored as a function of the layer number (Figure 3a) to obtain information on the adsorbed amount of PSS. QCM measurements allow for the determination of the mass of polyelectrolyte adsorbed after each adsorption step (Figure 3b). Clear differences can be observed in the UV-vis for the growth behavior between films of the six copolymers of different charge density from layer six or seven onward. In contrast to our previous work on a similar copolymer system3 and literature reports for other polyelectrolytes,1-6,20 in the current work, no critical charge density limit, below which no layer growth is possible, is observed (Figure 3a,b). The data show that layer formation is possible, even if the charge density of the copolymer is decreased to 8 mol %. This points to the influence of secondary, nonelectrostatic interactions on the multilayer film formation. Previous publications showed that nonelectrostatic interactions9 such as hydrogen bonding14-16,18 or hydrophobic17 or charge-transfer interactions32 can have (32) Shimazaki, Y.; Nakamura, R.; Ito, S.; Yamamoto, M. Langmuir 2001, 17, 953.


Langmuir, Vol. 20, No. 7, 2004 2733 Table 2. Necessity of Charge Overcompensationa charge density (%) [Mmon]/x

100 126

73 152

58 177

35 285

21 393

8 943

a The relative amounts of copolymer that are to be adsorbed to compensate a fixed surface charge are shown. X ) number of charged monomers/number of all monomers of a PE chain; [Mmon] ) xMDADMAC + (1 - x)MAM ) average mass of one monomer unit (see text).

Table 3. Water Content of the 10-Layer PSS/ DADMAC-AM Films Assembled with 0.5 M NaCl in Both of the Adsorption Solutions As Determined by QCMa

Figure 4. Adsorbed amounts obtained from the UV-vis and QCM measurements for a 10-layer film as a function of the charge density (adsorption from 0.5 M NaCl solutions).

a significant influence on the multilayer formation or even enable it. The amide groups of the acrylamide monomers of the copolymers used in this work can form hydrogen bonds with the other copolymer chains and therefore are likely to influence multilayer growth.33 This could be an explanation for the observation that multilayer formation is possible for the DADMAC-AM system with only 8 mol % charged copolymer. Furthermore, an interesting observation is that similar adsorbed amounts are found for films with 100 and 8 mol % charged polyelectrolytes, while the thickest layers are obtained for intermediate charge densities. The QCM data are in good agreement with the UV-vis data (Figure 3). For a better overview, the adsorbed amounts obtained from the UV-vis and QCM measurements are plotted versus the charge density in Figure 4. Two of the maxima appearing for intermediate charge densities point out that several of the parameters govern the multilayer film formation in this system. As reported in several previous publications, the adsorbed amounts upon multilayer formation are determined by the conformation of the polyelectrolytes in solution34-36 and by the necessity of charge overcompensation upon adsorption on a surface.2,10-13 With a decreasing charge density, the polyelectrolytes have a more coiled conformation in solution, an effect that is similar to the salt-induced screening of charges for a fully charged polyelectrolyte.7,37 This more coiled conformation is also kept upon adsorption on a surface, leading to thicker layers.38 A higher film thickness with a decreasing charge density of the adsorbing polyelectrolytes can also be explained with the necessity of charge overcompensation upon adsorption on a surface.2,11 With a decreasing charge density of the adsorbing polyelectrolytes (increasing mass per charge unit), more polyelectrolyte chains have to adsorb on the surface to achieve charge reversal. Table 2 shows the calculated relative amounts of DADMAC-AM copolymer that have to be adsorbed to compensate for the surface charge with the assumption that charge overcompensation takes place for (33) Garces, F. O.; Sivadasan, K.; Somasundaran, P.; Turro, N. J. Macromolecules 1994, 27, 272. (34) Rojas, O. J.; Ernstsson, M.; Neuman, R. D.; Claesson, P. M. Langmuir 2002, 18, 1604. (35) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (36) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237. (37) Reed, W. F. In Macroion Characterization; Schmitz, K., Ed.; ACS Symposium Series 548; American Chemical Society: Washington, DC, 1994; pp 297-314. (38) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481.

charge density (%)

∆F(dry/wet) (Hz)

∆F(dry/wet)/∆Ffilm (%)

100 73 58 35 21 8

262 429 602 833 1288 nab

19.2 18.8 19.0 19.9 20.2 nab

a ∆F (dry/wet) is the frequency change between the QCM crystals dried over silica gel and QCM crystals exposed to a water-vapor atmosphere. ∆Ffilm is the frequency change between the uncoated QCM crystals and QCM crystals coated with a 10-layer PSS/ DADMAC-AM copolymer film, which were measured in a watervapor atmosphere. b na ) not available.

all of the samples to the same extent. The mass per charge unit values (mass of the polyelectrolyte divided by the number of its charges) increase with the decreasing charge density of the polyelectrolytes. This is the case for the DADMAC-AM copolymers, though the average monomer mass of the copolymers decreases with the decreasing charge density because the acrylamide monomers have a noticeably lower mass than the DADMAC monomers. Both the values in Table 2 and consideration of the polyelectrolyte conformation can only explain the continuously increasing adsorbed amounts with the decreasing charge density. However, to explain the appearance of two maxima for the system investigated here, additional effects need to be considered. Secondary nonelectrostatic interactions, as already mentioned, may play a role. Relevant to this is that the films of copolymers of intermediate charge densities were visually observed to form gel-like structures after eight layers. Gel-like structures have also been observed for similar copolymer systems.1,3,39 Formation of the “gel-like” structures was most apparent for films prepared with DAM21; however, it did not occur in the cases of 100 and 8 mol % charged copolymers. Furthermore, these thick films reveal no linear growth but reveal more of an exponential-like growth. To investigate this gelling behavior and to obtain information on the water content of the films, QCM measurements were performed after drying the coated QCM crystals for 12 h over silica gel (in addition to drying with a stream of nitrogen). Afterward, the QCM crystals were exposed to a water-vapor atmosphere by placing them at the top of a beaker, which contained a small amount of saturated sodium chloride solution, to ensure a constant water-vapor pressure even after a part of the water evaporated. The frequency of the QCM crystals exposed to a water-vapor atmosphere was determined after 30 min. The frequency shifts of the QCM crystals with films formed with copolymers of different charge densities dried over silica gel and those exposed to a water-vapor atmosphere are shown in Table 3. The data show an increase in the absolute water content with decreasing charge density of the copolymers used to construct the (39) Serizawa, T.; Kawanishi, N.; Akashi, M. Macromolecules 2003, 36, 1967.

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layers. This result is in accordance with the observed gelling of the films, which is strongest for films with 21 mol % charged copolymers. The relative water content for the different films, that is, the frequency shift for the water content divided by the total frequency shift for the adsorbed film measured in a water-vapor atmosphere is similar (∼20%) for all of the samples investigated. The high water contents in the films with DADMACAM copolymers have a significant influence on the structure and layer thickness of the multilayers. For that reason, layer thicknesses of the multilayer films in water and air were determined with surface plasmon resonance spectroscopy (SPRS).24 SPRS measurements show noticeably higher layer thicknesses for the films in water compared with those determined in air (22 nm versus 16 nm for six layers with DAM73). However, for the measurements performed in water, only the layer thickness of films with DAM73 is larger than that of films with DAM100. The films with copolymers of lower charge density show noticeably lower layer thicknesses. This can be explained by the significant dissolution of the films of low charge density copolymers upon exposure to water. UV-vis measurements (not shown) confirm such dissolution of the films. Partial desorption of the polyelectrolytes for film buildup with copolymers of low charge density upon the adsorption of the following layer has also been shown by FTIR measurements. FTIR spectroscopy allows simultaneous monitoring of the amounts of both PSS and the adsorbed DADMAC-AM copolymers. Part a of Figure 5 shows FTIR spectra of films with PSS/DAM21 after deposition of 1-12 layers. Spectra were evaluated using the absorbance of the sulfonate group from PSS (1037 cm-1) and absorbance of the carbonyl group from the acrylamide components of the copolymers (1682 cm-1). In Figure 5b, the absorbances of these groups in the PSS/DAM21 films and, for comparison, absorbance of the sulfonate group of PSS in films with DAM100 are plotted as a function of the polyelectrolyte layer number. Regular layer growth and a linear increase of the absorbance with each added bilayer is observed for PSS/DAM100 films, in accordance with previous work,3,40 while films with DAM21 show a nonmonotonic increase of the adsorbed amounts per layer. Furthermore, the alternating increase and decrease of the absorbance for the sulfonate group indicates that the adsorption of copolymer causes partial removal of the previous PSS layer, and accordingly data for the carbonyl group indicate partial removal of the copolymer upon adsorption of PSS. Because more PSS adsorbs in the following adsorption step than is removed in the previous step, multilayer growth can take place. An effect of the molecular orientation within the film on the FTIR absorbances using polarized radiation can be ruled out because we observe linear growth for the DAM100 film, in agreement with UV-vis and QCM data for this film. UV-vis measurements also show a slight removal of material for films formed with DAM21 and PSS. Such a removal of material for films with lowly charged polyelectrolytes has also been observed for similar systems3,4,24,39 and can be explained by the formation of polyelectrolyte complexes in solution, which are entropically favorable.41,42 Formation of soluble polyelectrolyte complexes is further favored when the two polyelectrolytes have very different chain lengths,42 which is the case for the DAM21/PSS pair used here (Table 1). Cohen-Stuart

et al. also found that dissolution of polyelectrolyte complexes occurs when multilayers are formed with weakly charged polyelectrolytes in the presence of salt and excess polyelectrolyte in the adsorption solution.43 This behavior was explained by the existence of an equilibrium between a solid polyelectrolyte complex phase (multilayer) and the soluble complexes in solution. Above a critical salt concentration, a transition from a “glassy state” of the solid complex phase to a “liquid state” with very mobile chains takes place. Because this “liquidlike” layer can equilibrate with the surrounding solution, dissolution of the solid phase takes place.43,44 Because multilayer films composed of DADMAC-AM copolymers of low charge density show this tendency to desorb, lower layer thicknesses for films with copolymers of decreasing charge density are expected. This desorption has to be taken into consideration to explain the observed adsorbed amounts of the films as a function of the charge density. Weaker binding of polyelectrolytes to a surface resulting in desorption can be attributed to charge mismatching of the anionic and cationic polyelectrolytes used to build up

(40) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621. (41) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (42) Sui, Z. J.; Salloum, D.; Schlenoff, J. B. Langmuir 2003, 19, 2491.

(43) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen-Stuart, M. A. Langmuir 2002, 18, 5607. (44) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen-Stuart, M. A. J. Phys. Chem. B 2003, 107, 7998.

Figure 5. (a) Multilayer formation with PSS and DAM21 copolymer followed by FTIR spectroscopy (adsorption from 0.5 M NaCl solutions). From bottom to top, the layer number increases (from 1 to 12). Absorbance of the SO3 group of PSS ) 1037 cm-1 . Absorbance of the CdO group of acrylamide ) 1682 cm-1. (b) Absorbance of the SO3 group of PSS and the CdO group of acrylamide as a function of the layer number. Odd layer number ) cationic polyelectrolytes. Even layer number ) PSS.


Langmuir, Vol. 20, No. 7, 2004 2735

Figure 6. AFM images of the 10-layer PSS/DADMAC-AM copolymer films prepared on Si substrates from 0.5 M NaCl adsorption solutions. Films with (a) DAM100, (b) DAM73, (c) DAM58, (d) DAM35, (e) DAM21, and (f) DAM8 (from top left to bottom right). The image area is 1 µm2. The vertical scale bar for all of the images is 40 nm.

the films.21,23 In this context, Dautzenberg et al. investigated the formation of complexes between PSS and DADMAC-AM copolymers of different charge densities in solution.45 A decreasing packing density of the complexes with a decreasing charge density of the copolymers was observed and was explained with an increasing incompatibility of charge-charge distances along the chains between the fully charged PSS and copolymers when their charge densities are decreased. While the charge distances along the PSS chain are around 0.25 nm, along the copolymer chains, charge distances are 0.55, 0.64, 0.73, 1.49, and 3.43 nm for DAM100, DAM73, DAM58, DAM21, and DAM8, respectively.45 Dautzenberg et al. also found an increased swelling of the polyelectrolyte complexes formed with a decreasing charge density. This swelling behavior, attributed to a looser packing of the electrostatically associated polyelectrolytes, can also explain the thicker films obtained in the intermediate charge density region and the formation of a gel-like structure for these films in the systems investigated here.39 The formation of a gel-like structure is enabled by the amide groups of poly(acrylamide) that allow inclusion of water molecules inside the films. With decreasing charge density of the copolymers, the polyelectrolytes within the multilayers have a higher flexibility and are less densely packed. This facilitates the inclusion of water molecules. This explains the observed gel-like structure for polyelectrolyte multilayer films with the copolymers of lower charge densities and is supported by the QCM measurements, which show higher water contents for polyelectrolyte films with lower charge density copolymers. For characterization of the surfaces of the multilayer films, AFM measurements were performed after deposition of 10 layers. Clear differences in the surface morphology for the six samples with films composed of copolymers with different charge densities can be observed (Figure 6). For films with DAM100, DAM73, and DAM58, the surface roughness and grain sizes increase with a decreasing charge density. This is in accordance with an increasing coiled conformation of the copolymers with a decreasing charge density.1,7,37 Multilayers with DAM35 and DAM21 show smoother surfaces. However, these films show the strongest gelling, and the high water content of these samples might influence the AFM measurements. Films built up with the 8% charged polyelectrolyte show (45) Dautzenberg, H.; Hartmann, J.; Grunewald, S.; Brand, F. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1024.

Figure 7. AFM images of the PSS/DADMAC-AM copolymer films prepared on Si substrates for different charge densities and layer numbers (adsorption from 0.5 M NaCl solutions). From left to right, the charge densities decrease ) 100, 73, and 8 mol %. From top to bottom, the layer number increases ) 1, 2, 3, 6, 8, and 10 layers. The image area is 1 µm2. The vertical scale bar for all of the images is 40 nm.

a smoother surface as compared with films with DAM58; however, they show a rougher surface in comparison to films prepared with DAM100. To follow the development of surface morphology during film growth, AFM measurements were performed after deposition of 1, 2, 3, 6, 8, and 10 layers for films prepared with DAM100, DAM73, and DAM8 (Figure 7). In accordance with UV-vis and QCM measurements, no significant differences for films formed with polyelectrolytes of different charge densities can be observed for films composed of less than about six layers. Starting with the sixth layer, an increasing surface roughness with a decreasing charge density is observed. The surface roughness also increases with an increasing layer number for all three charge densities. This surface roughness increase offers an explanation for the exponential-like growth behavior observed with UV-vis and QCM measurements.24 Layer thicknesses were determined from the AFM images by making a scratch inside the multilayer films

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Table 4. Layer Thicknesses Determined by Different Methods for the 10-Layer PSS/DADMAC-AM Copolymer Films on Silicon Wafers (Si) and QCM Crystals (Au) charge density (%) 100 73 58 35 21 8

SAXS/ ellipsometry/ (Si)a d (nm) (Si)b d (nm) 32 52 42 26 nae 32

30 54 41-54 84-116 36-117 31

AFM/ QCM/ (Si)c d (nm) (Au)d d (nm) 27 42 53 27-83 46-58 33-37

27 61 39 49 114 32

a,b The error in these values is estimated as (10%; this is derived from variations in the assumed refractive index values of the films. c The error is estimated as (5%. d The error is estimated as (10%; this is due to the variations in the frequency values and errors in the assumed polyelectrolyte film density. e na ) not available.

Figure 9. Influence of drying/not drying between the adsorption steps on the polyelectrolyte multilayer formation of the PSS/DADMAC-AM films investigated with UV-vis and QCM. Adsorbed amounts are plotted as a function of the charge density. 0.5 M NaCl was used in all of the adsorption solutions.

Figure 8. Film thickness determination using SAXS. SAXS spectra of the 10-layer PSS/DADMAC-AM copolymer films prepared on Si substrates with charge densities of the copolymers of 100 and 8 mol % (adsorption from 0.5 M NaCl solutions).

followed by scanning with the AFM tip over the scratched region. Using this method, local layer thicknesses were obtained from several macroscopically separated areas on the surface. The results are shown in Table 4. The large variation of the values determined from different areas of the same surface of samples with copolymers of intermediate charge densities shows that these samples are very inhomogeneous. This is not a result of the scratching procedure, because AFM images of the unscratched surfaces show a high inhomogeneity. For this reason, a range of layer thickness is specified for these samples. While this method allows for the determination of local layer thicknesses, average film thicknesses were determined using SAXS, ellipsometry, and QCM data. For the calculation of thicknesses from QCM data, a density for the polyelectrolyte multilayers of 1.2 g cm-3 was assumed.3,29,46 Two SAXS spectra for 10-layer films with DAM100 and DAM8 are shown in Figure 8. Layer thicknesses determined with SAXS from the distances of the Kiessig fringes are given in Table 4. For the samples with copolymers of intermediate charge densities, Kiessig fringes were less pronounced, indicating rougher surfaces. For this reason, it was not possible to measure the sample with PSS/DAM21 films with SAXS. Layer thicknesses calculated from QCM data and thicknesses determined by ellipsometry are also shown in Table 4. For films with DAM100, DAM73, and DAM8, the thickness values (46) Because the multilayers that formed with polyelectrolytes of different charge densities show significant differences in the surface roughness as observed by AFM and significant differences in water contents, an accurate assumption of the film density is difficult. However, the maximum error is estimated at 10%.

obtained by the different experimental methods are in good agreement, while for samples with copolymers in the intermediate charge density region (DAM58, DAM35, and DAM21), significant differences are obtained because of the heterogeneity and large roughness of these samples. However, the layer thicknesses determined with several methods show a dependence on the charge density of the copolymers, similar to that of the adsorbed amounts derived from the UV-vis measurements. Influence of Drying on Multilayer Film Formation. For all previous experiments, films were dried with nitrogen after each adsorption step and rinsed. To determine if there is an influence of drying on the film formation behavior, UV-vis and QCM measurements were performed. Figure 9 shows the adsorbed amounts as a function of the charge density for 10-layer films. The data shown are for films obtained by drying after each layer and those dried only after deposition of a total of 10 layers. For most of the samples, the adsorbed amounts are significantly higher when films have been dried after each deposition step. However, a similar dependence of the adsorbed amounts on the charge density is obtained for the two different procedures of multilayer formation. In both cases, similar amounts are adsorbed for films with DAM100 and DAM8, while larger adsorbed amounts are obtained for intermediate charge densities. AFM measurements support these observations. Comparison of the AFM images of the 10-layer films prepared with and without drying after each adsorption step shows slightly higher surface roughnesses for the former case, which is in agreement with the previous work in the literature.47 However, a similar dependence of the surface roughness on the charge density of the copolymers used can be observed for both of the preparation methods of the polyelectrolyte multilayer films. Multilayer Film Formation from Polyelectrolyte Solutions Containing 0.01 M NaCl. Several investigations have shown that the ionic strength of the adsorption solution significantly influences the polyelectrolyte conformation in solution and therefore also influences the multilayer formation.35,36,48,49 In Figure 10, absorbance values obtained from the UV-vis measurements, which (47) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146, 337. (48) van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984, 88, 6661.


Figure 10. UV-vis measurements for the formation of the multilayer films of PSS and DADMAC-AM copolymers prepared from solutions containing 0.01 M NaCl. Odd layer number ) cationic polyelectrolytes. Even layer number ) PSS.

are a mean for the amounts of adsorbed PSS, are plotted as a function of the layer number for the different copolymers adsorbed from 0.01 M NaCl solutions. The absorbance values are a factor of ∼4 times lower than those for the films prepared from solutions containing 0.5 M NaCl. The trend in the dependence of the adsorbed amounts on the charge density, however, is similar to that obtained for multilayer formation from 0.5 M NaCl solutions. Thicker layers are obtained for films with intermediate charge density copolymers (DAM35 and DAM21), while layers with DAM100 form the thinnest films. A further interesting observation concerns the stability of the layers. While regular layer growth was observed for nearly all charge densities of the copolymers for the adsorption from 0.5 M NaCl solutions, irregular adsorption takes place with the desorption of PSS upon the copolymer adsorption step when 0.01 M NaCl is used in both adsorption solutions. While, in the first case (0.5 M NaCl), desorption takes place only starting with around the eighth layer, upon using 0.01 M NaCl in both adsorption solutions, desorption takes place at lower layer numbers. Desorption of PSS is most pronounced for multilayer films with DAM21 and DAM35. This reduced stability of the films composed of the copolymers of lower charge density that have been assembled at low salt concentration has also been found for the DADMAC-Nmethyl-N-vinylacetamide copolymer system.3 AFM measurements show that, in comparison with films assembled from 0.5 M NaCl, a smoother surface morphology is observed for all samples (not shown). For films with DAM100, DAM73, and DAM58, an increase in the surface roughness and grain size with a decreasing charge density is also observed, while films with DAM35 and DAM21 show smoother surfaces, which may be attributed to an influence of the high water content of these films on the AFM measurements. Influence of the Molecular Weight on Multilayer Film Formation. As shown in Table 1, there is a large variation in the molecular weight of the copolymer samples used. With a decreasing charge density, the molecular weight of the copolymer chains increases. DAM8 has a 20 times larger molecular weight compared with that of DAM100. For films prepared with fully charged strong polyelectrolytes from solutions with a low ionic strength, (49) Boehmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288.

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no significant influence of the molecular weight on the film formation has been reported.21,32,41,50-52 In contrast, films formed from solutions containing higher amounts of salt or for polyelectrolytes of lower charge density, where the polyelectrolyte chains are in a more coiled conformation, a significant influence of the molecular weight has been found.32,41,42,50 Higher chain lengths of the polymers can enable more attachment points with the surface, resulting in a higher stability of the layers. For that reason, an influence of the molecular weight on the multilayer film formation cannot be ruled out for the systems investigated here. DADMAC-AM copolymers with molecular weights in a narrow range were not available. Also, copolymers of a fixed charge density and varying in molecular weight were not available, which would have enabled an investigation of the influence of the molecular weight on the multilayer film formation. Poly(acrylamide). A possible explanation for the observed growth behavior of the multilayer films as a function of the charge density is the presence of further synergistic nonelectrostatic interactions. The amide groups of the poly(acrylamide) monomer units in the chain offer the possibility of forming hydrogen bonds.33 The formation of hydrogen bonds within the film could explain why multilayer formation is possible with copolymers of a charge density of 8 mol % and why thinner films are formed in this case. Experiments where poly(acrylamide) is adsorbed in alternation with PSS show that no multilayer growth takes place. Using poly(acrylamide) of different molecular weights (65 000, 400 000, and 5 000 000 g mol-1) also results in no layer growth. These results show that multilayer formation for the DADMAC-AM copolymer/ PSS system is not possible in the complete absence of electrostatic interactions. Conclusion The influence of the linear charge density of a polyelectrolyte chain on the formation of multilayers has been investigated. Random copolymers of varying charge density that associate as a result of electrostatic and also nonelectrostatic interactions and a fully charged anionic polyelectrolyte were alternately adsorbed. In contrast to previous publications, no critical charge density limit, below which no layer growth is possible, was observed. Instead, multilayer film formation with copolymers of charge densities as low as 8 mol % was possible, and these films have thicknesses similar to those of films with 100 mol % charged polyelectrolytes. Furthermore, for the adsorbed amounts as a function of the charge density, two maxima were observed for films with copolymers in an intermediate charge density region. This shows that for the systems investigated here a number of effects have to be taken into account. Here, synergistic nonelectrostatic interactions allow the growth of multilayers with poly(50) Rehmet, R.; Killmann, E. Colloids Surf., A 1999, 149, 323. (51) Loesche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (52) The 100% charged polyelectrolyte PDADMAC could be obtained with four different molecular weights: 20 000, 100 000-200 000, 200 000-350 000, and 400 000-500 000 g mol-1. Multilayers have been built up with these polyelectrolytes in alternation with PSS, using 0.5 M NaCl in both of the adsorption solutions. QCM measurements reveal no significant differences in the growth behavior of the films with polyelectrolytes of different molecular weights. AFM measurements show no differences in the surface morphology and also confirm that the molecular weight has no influence on the film formation of these samples. However, in these experiments, multilayer formation has only been investigated with the 100 mol % charged polyelectrolytes. For this reason, it cannot be ruled out that polyelectrolytes of lower charge density will show a different behavior.

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electrolytes of a very low charge density. Furthermore, an effect of the molecular weight of the copolymers used cannot be ruled out. In the systems investigated, the molecular weight of the copolymers increases with a decreasing charge density, which may support multilayer formation with polyelectrolytes of a low charge density due to more adsorption points.42 The significant differences in the surface roughness and packing density of the films with copolymers of different charge densities are of interest for permeability and elasticity studies and offer possible applications in membrane technology or drug delivery.

Schoeler et al.

Acknowledgment. C. Pilz is thanked for technical assistance, H. Krass, for help with the FTIR measurements, R. Teppner, for support with the ellipsometry measurements, and H. Schollmeyer, for help with the SAXS experiments. H. Mo¨hwald, R. von Klitzing, and H. Dautzenberg are also acknowledged for stimulating discussions. W. Jaeger is thanked for the synthesis of the copolymers used. The BMBF and Australian Research Council are acknowledged for support of this work. LA035909K

Polyelectrolyte Multilayer Films of Different Charge Density

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