Neutron Reflectometry Study of Swelling of Polyelectrolyte Multilayers

Aug 27, 2009 - Water Vapors: Influence of Charge Density of the Polycation. Ralf Köhler,*,†,‡ Ingo Dönch,† Patrick Ott,§ André Laschewsky,§...
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Neutron Reflectometry Study of Swelling of Polyelectrolyte Multilayers in Water Vapors: Influence of Charge Density of the Polycation )

Ralf K€ohler,*,†,‡ Ingo D€onch,† Patrick Ott,§ Andre Laschewsky,§ Andreas Fery,^ and Rumen Krastev*,†, †

Department of Interfaces, Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany, Department of Soft Matter, Helmholtz Centre Berlin for Materials and Energy, Glienicker Strasse 100, 14109 Berlin, Germany, §Applied Polymer Chemistry, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany, ^Physical Chemistry II, University of Bayreuth, Universit€ atsstrasse 30, 95440 Bayreuth, Germany, and Department of Biomaterials, NMI Natural and Medical Sciences Institute at the University of T€ ubingen, Markwiesenstrasse 55, 72770 Reutlingen, Germany )

‡

Received April 28, 2009. Revised Manuscript Received July 16, 2009 We studied the swelling of polyelectrolyte (PE) multilayers (PEM) in water (H2O) vapors. The PEM were made from polyanion poly(styrene sulfonate) (PSS) and polycation poly(diallyldimethylammonium chloride)-N-methyl-N-vinylacetamide (pDADMAC-NMVA). While PSS is a fully charged polyanion, pDADMAC-NMVA is a random copolymer made of charged pDADMAC and uncharged NMVA monomer units. Variation of the relative amount of these two units allows for controlling the charge density of pDADMAC-NMVA. The degree ofswelling was studied as a function of the relative humidity in the experimental chamber (respectively water concentration in the gas phase) for PEM prepared from PSS and pDADMAC-NMVA with their different charge densities - 100%, 89% and 75%. The films were prepared by means of spraying technique and consisted of six PE couples-PSS/pDADMAC-NMVA. Neutron reflectometry was applied as main tool to observe the swelling process. The technique allows to obtain in a single experiment information about film thickness and amount of water in the film. The experiments were complemented with AFM measurements to obtain the thickness of the films. It was found that the film thickness increases when the charge density of the polycation decreases. The swelling of the PEM increases with the relative humidity and it depends on the charge density of pDADMAC-NMVA. The swelling behavior is 2-fold, splitting up in a charge dependent mode with relatively little volume increase, and a second mode with high volume expansion, which is independent from charge density of PEM. The “swelling transition” occurs for all samples at a relative humidity about 60% and a volume increase of ca. 20%. The results were interpreted according to the Flory-Huggins theory which assumes a phase separation in PEM network at higher water contents.

Introduction Polyelectrolyte (PE) multilayers (PEM) are a class of organic materials which consist of complexed ’layers’ of two polyions of opposite charge.1 Usually PEM are built up in a layer-by-layer (LbL) self-assembling technique2-4 using different methods for PE deposition, e.g. dip-coating or spraying of aqueous solutions of polyelectrolytes.2,5,6 In the past decade PEM have attracted great interest due to their potential for application in many fields ranging from optics, sensing and tribology7,8 up to biochemistry,9 pharmacy10 and *Corresponding authors. E-mail: (R. K€ohler) helmholtz-berlin.de; (R. Krastev) [email protected].

ralf.koehler@

(1) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831–835. (2) Decher, G. Science 1997, 277, 1232–1237. (3) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319–348. (4) Sch€onhoff, M. J. Phys.: Condens. Matter 2003, 15, R1781–R1808. (5) Schlenoff, J.; Dubas, S.; Farhat, T. Langmuir 2000, 16, 9968–9969. (6) Kolasinska, M.; Krastev, R.; Gutberlet, T.; Warszynski, P. Langmuir 2009, 25, 1224–1232. (7) Hiller, J.; Mendelsohn, J.; Rubner, M. Nat. Mater. 2002, 1, 59–63. (8) Nolte, M.; Schoeler, B.; Peyratout, C. S.; Kurth, D.; Fery, A. A. Adv. Mater. 2005, 17, 1665–1669. (9) Antipov, A.; Sukhorukov, G.; Fedutik, Y.; Hartmann, J.; Giersig, M.; M€ohwald, H. Langmuir 2002, 18, 6687. (10) Voigt, A.; Buske, N.; Sukhorukov, G.; Antipov, A.; Leporatti, S.; Lichtenfeld, H.; Baumler, H.; Donath, E.; M€ohwald, H. J. Magn. Mater. 2001, 255, 59. (11) Antipov, A.; Sukhorukov, G.; Donath, E.; M€ohwald, H. J. Phys. Chem. B 2001, 105, 2281. (12) Varde, N.; Pack, D. Expert Opin. Biol. Ther. 2004, 4, 35.

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medicine.11,12 Furthermore, the physicochemical complexity of PEM makes them an interesting field of fundamental research of (self-assembling) processes on nanometer scale (see refs 13 and 14 and references cited therein). The thickness of the PEM, their swelling in ambient gaseous and liquid solvents and the amount of solvent uptake were extensively studied15-18 as a function of the used PE, of the salt concentration during film preparation and of the postpreparation treatment,19,20 and the thermal handling.21-23 Some (13) Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J., Eds.; Wiley-VCH: Chichester, U.K., 2003. (14) Klitzing, R.; Steitz, R. Structure of Polyelectrolyte Multilayers. In Handbook of Polyelectrolytes; Triphaty, S., Nalva, H., Eds.; American Scientific Publishers: Valencia, CA, 2002. (15) L€osche, M.; Schmitt, J.; Decher, G.; Bouwman, W.; Kjaer, K. Macromolecules 1998, 31, 8893–8906. (16) Wong, J.; Rehfeldt, F.; Hanni, P.; Tanaka, M.; Klitzing, R. v. Macromolecules 2004, 37, 7285–7289. (17) K€ugler, R.; Schmitt, J.; Knoll, W. Macromol. Chem. Phys. 2002, 203, 413– 419. (18) Ivanova, O.; Soltwedel, O.; Gopinadhan, M.; K€ohler, R.; Steitz, R.; Helm, C. A. Macromolecules 2008, 41, 7179–7185. (19) Dubas, S.; Schlenoff, J. Langmuir 2001, 17, 7725–7727. (20) K€ohler, K.; Biesheuvel, P.; Weinkamer, R.; M€ohwald, H.; Sukhorukov, G. Phys. Rev. Lett. 2006, 97, 188301. (21) Steitz, R.; Leiner, V.; Tauer, K.; Khrenov, V.; v.; Klitzing, R. Appl. Phys. A: Mater. Sci. Process. 2002, 74, s519–s521. (22) K€ohler, K.; Shchukin, D.; M€ohwald, H.; Sukhorukov, G. J. Phys. Chem. B 2005, 109, 18250–18259. (23) Mueller, R.; K€ohler, K.; Weinkamer, R.; Sukhorukov, G.; Fery, A. Macromolecules 2005, 38, 9766–9771.

Published on Web 08/27/2009

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Figure 1. Scheme of polyelectrolyte multilayer system.

recent reviews summarized the most of these results known until now.13,24,25 Other reports focus more on the mechanical properties (e.g., elastic module) of free-standing and supported PEM and their potential to be tuned23,26-30 what is one of the key features for future application. One characteristic of PEM is their distribution of charges along the chain, but although the importance of Coulomb forces for the internal interactions in PEM is obvious, up to now only a few reports address this topic. The present study investigates the impact of the amount of involved charges along the PE chain on the internal interactions of the PEM by means of swelling experiments. Swelling experiments mirror the response of a (polymeric) network on external physicochemical stimuli. They can examine the internal forces with respect to specific interactions of the incorporated molecule (e.g., solvent or salt ions),17,19 and can give hint on buried substructure in thin films. For instance it was found that PEM of intermediate thickness consist in slabs of different density,31 on which the top layer rules the solvent uptake to a great extend, as the “odd-even effect” of swelling observed for hydrophobic or hydrophilic PE as outermost layers shows.16 Recent studies with polymers and PEM present gradually decaying water distribution profiles toward the substrate interface.32-34 A suitable and powerful technique used in numerous studies of PEM15,21,33,34 is neutron reflectivity (NR). This technique, in contrast to other methods, allows for probing volume change and uptake of solvent independently and simultaneously. NR measures the film thicknesses in the z-direction normal to the film (24) Klitzing, R. Phys. Chem. Chem. Phys. 2006, 8, 5012–5033. (25) Sch€onhoff, M.; Ball, V.; Bausch, A.; Dejugnat, C.; Delorme, N.; Glinel, K.; Klitzing, R.; Steitz, R. Colloids Surf., A 2007, 303, 14–29. (26) Gao, C.; Leporatti, S.; Moya, S.; Donath, E.; M€ohwald, H. Langmuir 2001, 17, 3491–3495. (27) Vinogradova, O. I.; Andrienko, D.; Lulevich, V. V.; Nordschild, S.; Sukhorukov, G. Macromolecules 2004, 37, 1113–1117. (28) Stafford, C.; Harrison, C.; Beers, K.; Karim, A.; Amis, E.; Vanlandingham, M.; Kim, H.; Volksen, W.; Miller, R.; Simonyi, E. Nat. Mater. 2004, 3, 545–550. (29) Nolte, A.; Treat, N.; Cohen, R.; Rubner, M. Macromolecules 2008, 41, 5793–5798. (30) Dubreuil, F.; Elsner, N.; Fery, A. Europhys. J. E 2003, 12, 215–221. (31) Steitz, R.; Leiner, V.; Siebrecht, R.; Klitzing, R. Colloids Surf. A 2000, 163, 63. (32) Krasteva, N.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Langmuir 2003, 19, 7754–7760. (33) Mukherjee, M.; Singh, A.; Daillant, J.; Menelle, A.; Cousin, F. Macromolecules 2007, 40, 1073–1080. (34) Tanchak, O. M.; Yager, K. G.; Fritzsche, H.; Harroun, T.; Katsaras, J.; Barrett, C. J. J. Chem. Phys. 2008, 129, 084901.

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surface up to a length scale in the range of about 200-300 nm, with a resolution of 0.2 nm. The relatively weak interaction of neutrons with most materials gives possibility to study buried interfaces and bulk properties, as well as interfacial roughness. Since neutrons do not produce irradiation damage, what is the prerequisite for long irradiation times, investigation of samples of organic and even of biochemical origin is possible. NR exploits the variant of specular reflection, R as a function of the wave vector transfer q=(4π/λ) sin θ, (θ is the angle of incidence of the neutron beam, and λ is the neutron wavelength) at the interface between phases of different scattering length densities SLD, F(z)= Σnibi. Here, ni stands for the number density of the element i, and bi is its scattering length. Incorporation of additional material into the film gives rise to change of SLD and thus it is detectable by the NR experiments. We report results of swelling experiments performed with PEM supported on quartz glass slides in water vapor of different saturation (i.e., relative humidity). We studied the water uptake in the PEM and the thickness change (i.e., volume change) evoked by it which is a result of variation of surrounding chemical potential. The films were prepared from deuterated poly(styrene sulfonate) (dPSS) and poly(diallyldimethylammonium chloride)N-methyl-N-vinylacetamide (pDADMAC-NMVA) using spraying technique.5 pDADMAC-NMVA is a random copolymer made of charged pDADMAC and the uncharged but hydrophilic NMVA monomer units. Variation of the relative amount of these two units allows for controlling the mean charge density (ChD) of pDADMAC-NMVA over the length of the molecule. The alteration of the averaged line charge density allows for examination of the role of the Coulomb interactions in the process of film formation and PE network swelling. The general finding was a nonlinear increase of volume and water content of the PEM during increase of ambient moisture. The water incorporation into the polymer network is discussed as a solvation process in terms of Flory-Huggins theory. The theory states a different solubility for the differently charged PE’s and predicts a phase separation during swelling for all three PEM systems.

Materials and Methods Polymers and Chemicals. The highly branched polymer poly(ethylene imine), PEI, (25 kDa) and the fully charged pDADMAC (100-200 kDa) were obtained from Sigma-Aldrich. Fully deuterated poly(sodium styrene) sulfonate, dPSS, (86.5 kDa) DOI: 10.1021/la901508w

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was from PSStandards (Mainz, Germany). pDADMAC was synthesized as described earlier.35,36 The various charge densities were obtained by random attachment of uncharged spacer molecules N-methyl-N-vinylacetamide, NMVA, during chain synthesis (for the structural formula see Figure 1). NMVA and DADMAC are comparable in size (accordingly, the bond length in the monomer units molecular sizes of 4.3 A˚ and 5.5 A˚ can be roughly estimated) and show equal copolymerization reactivity, thus during molecule formation, a statistical assembly of these monomers in the polymer chain is obtained. dDADMAC-derivatives have the following charge densities: 89% (107 kDa), 75% (98 kDa), and 70% (92 kDa). NaCl was from Merck, Germany. All PE and other chemicals were applied without further purification. Always ultra pure water with Milli-Q quality was used (Milli-Q Plus 185 water generation system, Millipore, resistivity >18.2 MΩ cm). Substrate. The polyelectrolyte multilayers were build up on quartz glass slides (Schott, Germany), previously cleaned with alkalic RCA-procedure:37 Glass slides were heated for 10 min in solution of 1:1:5 parts of NH4OH (25%), H2O2 (30%) and pure water at 70 C. (Caution! Cleaning solution is highly oxidizing!) Film Preparation. The PEM formation was performed using the spraying option of the LbL deposition techniques.5,38 The PEM were prepared from PE solutions with a concentration of 0.001 monomol/dm3 in presence of 0.5 M NaCl. The solution of PEI was salt free. The cleaned quartz glass slides were mounted perpendicular in a dust reduced extractor hood and the solutions of the either the polyanion or the polycation were sprayed on the substrate surface for 2 s. PEI was used always as first layer to ensure stable contact between the film and the support.6 The surface was treated for a second time for 2 s with the same polymer solution after 10 s break. This time is necessary for formation of uniform layers. The sample was sprayed with pure water for 10 s to finalize each PE deposition step. The procedure was continuously repeated until the desired amount of layers was deposited. The preparation finished always by drying the samples in a nitrogen stream. All samples had the structure quartz glass/ PEI/(dPSS/pDADMAC-NMVA)6. The following abbreviations will be used for sake of simplicity: PEM-100, PEM-089, PEM-075, and PEM-070 for PEM prepared with pDADMACNMVA with charge density of 100%, 89%, 75%, or 70% respectivly. The same convention will be applied for the cationic PE and the pDADMAC-NMVA-derivatives themselves: PE100, PE-089, PE-075, and PE-070. Experimental Techniques. Atomic Force Microscopy. AFM was used for determining thicknesses and roughnesses of films. All AFM experiments were performed in tapping mode with a Nanoscope instrument (Veeco Instruments, USA) with a 100 μm scanner. The sample and the measuring head were tightened with plastic foil to ensure constant humidity around the probe. The ambient water vapor of different humidity was established with a continuous H2O-vapor stream during the experiments. The PEM thickness was measured at an edge in the film scratched with tweezers (Figure 2). The film roughnesses is presented as a root-mean-square value (rms) of an area of 10 10 μm2 size. Time for film equilibration of several minutes was allowed for swelling process after each humidity change step. (35) Dautzenberg, H.; G€ornitz, E.; Jaeger, W. Macromol. Chem. Phys. 1998, 199, 1561. (36) Ruppelt, D.; K€otz, J.; Jaeger, W.; Friberg, S.; Mackay, R. Langmuir 1997, 13, 3316. (37) Kern, W. J. Electrochem. Soc. 1990, 137, 6. (38) Izquierdo, A.; Ono, S.; Voegel, J.-C.; Schaaf, P.; Decher, G. Langmuir 2005, 21, 7558–7567.

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Figure 2. AFM images of scratched PEM surfaces with different charge densities (ChD): roughness (rms) increases with decreasing ChD (30 bilayer PEM at 30-40% RH, (scale bar 5 μm, hight scale 600 nm for thickness data see Supporting Information.).

Neutron Reflectometry. All experiments were performed at the V6 reflectometer in Helmholtz Centre for Materials and Energy GmbH, Berlin. The intensity of the incoming neutron beam and the angular resolution of the instrument allow for detection of Kiessig fringes of films with a thickness of about 200 to 2000 A˚. The samples were mounted in a homemade gas-flow box. Dry samples were obtained by dehumidifying in a dry nitrogen stream. This relatively simple method appears suitable, since, as shown from Delajon et al.,39 exchanging light water with heavy water in surrounding vapor followed by consecutive drying steps, demonstrated that SLD of a dry sample was the same: i.e., no removable water, neither H2O nor D2O remains in PEM after an drying time of around 30 min, and furthermore, no hydrogen/deuterium exchange takes place. Thus, the water content at zero relative humidity (RH) was set as zero. Dry nitrogen was led to humidify through H2O-filled gas washer bottles and passed afterward the sample box in a steady flow. The relative humidity in the gas stream was regulated by varying the quantity of the floating gas (i.e., gas velocity in washer bottles). Humidity was monitored by a digital hygrometer (HygroClip SP05, Rotronic) with an accuracy better than 0.2%. The stability of the relative humidity in the experimental box was checked several times during the ca. 5-7 h long scans. Deviations did generaly not exceed 2% RH. Temperature at experimental site was ca. 20 C. Times allowed for equilibration was at least 60 min. The information that can be extracted in a single NR experiment includes the film thickness, d, the scattering length density profile, F(z), across the film and the interfacial roughnesses, σ, between the different layers. The experimentally obtained reflectivity curves were analyzed by applying the standard fitting routine Parratt32.40 It determines the optical reflectivity of neutrons from planar surfaces with a calculation based on Parratt’s recursion scheme for stratified media.41 The film is modeled as consisting of layers of specific thickness, scattering length density and roughness, which are the fitting parameters. The model reflectivity curve is calculated and compared to the measured one. Then the model is adjusted by a change in the (39) Delajon, C.; Krastev, R.; Teichert, A. BENSC Experimental Report, 2006. (40) Braun, C. Parratt32 program (ver 1.6); HMI: Berlin, 2000. (41) Parratt, L. G. Phys. Rev. 1954, 95, 359–369.

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Figure 3. Reflectivity curves and fits (a) and scattering length profiles (b) for PE multilayer with different charge density in dry state, ChD 100%, 89%, and 75%, respectively (reflectivity curves are shifted vertically for better visibility). Table 1. Overall Film Thickness, Box Thickness and Scattering Length Density (SLD) Data for Dry Film of dPSS/pDADMAC Derivates, Theoretical Scattering Length (SL) Values for the Polyions SLPE, and Mass Densities of Compact Film (Box) of PEM GPEM45,46 PEM ChD

dry film d [A˚]

box thickness d1 [A˚]

SLDPEM [10-6 A˚-2]

SLPE(theor) [fm]

FPEM [g/cm3]

PEM-100 PEM-089 PEM-075 dPSS

195 ( 2 249 ( 4 297 ( 6

190.22 231.98 274.55

2.501 ( 0.02 2.446 ( 0.09 2.554 ( 0.09

2.745 4.570 7.667 120.162

1.07 ( 0.01 1.07 ( 0.04 1.09 ( 0.04

fitting parameters to best fit the data. For sufficiently large Q values, the layer thickness, d, can be estimated from the spacing of the maximum of two neighboring interference fringes, ΔQ, by d ≈ 2π/ΔQ.

Results AFM Measurements. The samples were characterized with AFM measurements to determine sample thickness and roughness for ensuring the feasibility of reflectometry techniques. We found that stable and smooth films can only be formed with PE-075, PE-089, and PE-100. Roughness increases when charge density (ChD) decreases, until finally, AFM-scan shows for pDADMAC with charge density of 70% very rough, and inhomogeneous films, not stable and thus, not suitable for reflectometry studies (see Figure 2). This is similar to earlier results,42,43 where a continuous layer assembly by means of dipping technique was not possible for material with charge density e70%. Adsorption stopped after one bilayer, due to missing charge overcompensation, resulting in thin but flat polymer film. Due to its high surface roughness, the 70%-ChD sample was not included in this reflectometry study furthermore. AFM measurements showed a largely linear increase of total film thickness with the number of bilayers from 3 until 50 (see Supporting Information). With this results the suitable number of bilayers for NR experiments was set to be six. The expected thicknesses for swollen and dry films of 6 bilayers of all three PEM’s lay in the high sensitive resolution range of the used NR instrument. NR Measurements: Dry Film. In Figure 3a, the reflectivity curves for the dry samples are presented. Kiessig fringes in reflectivity data shift to the left with decreasing charge density. This means an increase of film thickness. The associated SLD profiles at Figure 3b were obtained by fitting with a one-box model; i.e. the film is assumed to consist of a compact layer of constant density in between two interfaces with Gaussian density transition to the bordering semi-spaces, here, air and quartz (42) Voigt, U.; Khrenov, V.; Tauer, K.; Hahn, M.; Jaeger, W.; Klitzing, R. J. Phys.: Condens. Matter 2003, 15, S213–S218. (43) Steitz, R.; Jaeger, W.; Klitzing, R. Langmuir 2001, 17, 4471–4474.

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substrate. Density transitions are commonly interpreted as interfacial roughnesses but can originate in internal SLD variations as well. With a one-box model fit, one gets the following measures: the external or surface roughness, σ1, the thickness of compact film core, d1, equivalent to the box-thickness in the one-box fitting model, and the internal roughness of PEM/glass interface, σ2 (see Supporting Information). In a separate NR experiment the roughness of the pure quartz substrate, was measured to be zero with an error of less than 5 A˚, i.e. the density change at the interface of the substrate does not contribute significantly neither on thickness, nor on internal roughness of PEM film (σ2 scatters nonsystematic from 0 until ca. 23 A˚). Thus, the obtained “inner roughness” of PEM mainly if not completely represents a feature of the PEM. Although this contribution is of minor importance for the whole film, it suggests to introduce a “mediating layer” of thickness σ2 between the film box of thickness, d1, and the substrate interface. The overall thickness of the PEM film, d, then results in d=d1 þ σ2. Again, the box thickness, d1, is the distance between the midpoints of the slopes of the interfacial model functions44 in each case connecting layers with constant SLD’s. For all further reported results only the behavior of the inner compact film slab, d1 was analyzed. To test for a possible improvement of the quality of the fits, additionally, we investigated multiple-box models and models with free parametrized curves, but it shows that the single box model was sufficient to get reasonable fits. The overall film thicknesses, d, for 6 bilayer film in dry state are as follows: 195 ( 2, 249 ( 4, and 297 ( 6 A˚. This yields average thickness for one bilayer of each of this samples of 32.5, 41.4, and 49.5 A˚ for PEM-100, PEM-089, and PEM-075 respectively. The film roughnesses also rises when the ChD decreases. Compared with the internal roughness between substrate and PEM 5, 17, and 23 A˚, the values of surface roughnesses of all three PE combinations, 5, 9, and 16 A˚ (PEM-100, PEM-089, PEM-075), are of the same order of size and show the same trend (Figure 3). To confirm (44) The model functions of the interfaces are shaped like Gaussian error functions. The width of the “transformation zone” is about 2 times the full width at half-maximum (FHWM).40

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Figure 4. Reflectivity curves and fits (a) and scattering length density curves (b) for PE multilayer with 100%, 89%, and 75% charge density in different ambient water vapors (reflectivity curves shifted vertically for better visibility).

Parratt-fitting results, the thickness values of dry and swollen PEM were additionally calculated from the differences in Q values of the normalized maximum values of the Kiessig fringes of reflectivity curves via the formula of the interference condition nλ=2πD sin θ, with n the order of maximum, D the film thickness, and θ the reflection angle, and the wavelength λ = 4.66 A˚. The thickness data calculated from Kiessig fringes are systematically shifted to higher values compared with Parratt-fit data for the compact film, i.e., the box size d1. The deviations can be traced back to the thickness contribution of the rough interfaces of the film. The average deviation is 6 ( 5%. The highest differences, ranging between 8 and 13%, arose for four data pairs, only. This finding allows to treat the inner compact film slap, d1, as basis of our calculations and to ignore the inferior influence of the interfacial regions. The presented results are based on the Parratt-fit data. The full set of data is shown in the Supporting Information. (45) Sears, V. Neutron News 1992, 3, 29–37. (46) http://www.ncnr.nist.gov/resources/sldcalc.html.

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The charge density of the used PE’s shows no systematic effect on the scattering length densities of PEM in dry state. The deviations from average SLD do not exceed 2% (Table 1). The deutered PSS has high positive scattering length (SL). The different pDADMAC-derivatives show, due to their hydrogen content, significant lower values for SL with respect to one charged unit (Table 1).45 Variations caused by different contents of spacer units are obvious. NR Measurements: Film in H2O Vapor. The reflectivity curves of the three samples exposed to ambient water vapor show the similar response (Figure 4a). Kiessig fringes shift to the left and their frequency increases with rise of ambient humidity. This points to a rise of film thickness due to the water adsorption from the ambient gaseous phase. Following the measurements all three PEM have the same systematic behavior. Their thickness increases with the rise of relative humidity (RH) in ambient water vapor, whereby PEM-089 and PEM-075 exhibit obviously nonlinear course (Figure 5). The thickness changes due to the water uptake are reversible and can be repeated for two times at least. The film remains stable and shows same thickness values. Basic Langmuir 2009, 25(19), 11576–11585

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Figure 5. Overall film thickness, d, of PEM in surrounding water vapors of varied relative humidity (full symbols). AFM data at ca. 40% RH (empty symbols).

swelling characteristic could be confirmed with AFM measurements. A spot AFM check for a 6 bilayer film is included in Figure 5. The external roughness of the PEM surface was computed for NR data (see SLD profiles in Figure 4b). The roughness for all three samples increases with relative humidity (Figure 6). It was found that the roughness of PEM with less ChD shows higher values. AFM measurements yield to values around (12 ( 2)A˚ for samples PEM-089 and PEM-100, with no significant ChD dependence at RH ≈ 40%. These findings coincide with the results for the both PEM from NR which show roughnesses arround 11 and 6 A˚ at RH 30%. Due to the strong scattering of the data, the humidity dependent behavior of the “internal roughness” is difficult to interpret. While the “internal roughness” of charge diluted PEM decreases with increasing ambient vapor pressure: 23-14 ( 6 A˚ for PEM075, and 17-9 ( 8 A˚ for PEM-089, the fully charged PEM-100 does not show changes 5 ( 4 A˚ (Supporting Information).

Discussion Dry PEM Films. The observed trend of increasing thicknesses with decreasing ChD was already reported for films of the same material but prepared by dipping technique. Steitz et al. found average bilayer thicknesses of ca. 50, 60, and 66 A˚ for PEM of 10 bilayers in total.43 The systematical shift toward higher thickness values compared with our data (33, 41, and 50 A˚) can traced back on the different preparation techniques. Different authors have reported lower film thicknesses for spraying deposition compared with dipping deposition technique, whereby the surface roughness proved to be unaffected from the chosen preparation method.6,38 The thicknesses of dry films increase systematically with decrease of charge density (Table 1). This can be explained as follows. The intramolecular repelling Coulomb forces are reduced with increased distance of charged sites at the polyion molecule, under sufficient screening with small counterions in solution.4 Therefore, pDADMAC-NMVA molecules get more flexible, and molecules adsorb in more coiled shape when the ChD decreases. This interpretation is supported by the systematic increase of the external roughness for charge diluted PEM. Besides this, the ability for interpenetration increases with rising chain flexibility. Thus, finally, the layers can become thicker. Langmuir 2009, 25(19), 11576–11585

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Figure 6. Surface roughness, σ1, of PEM with different charge densities in water vapors of varied relative humidity (RH). (Lines guide the eye: from the top down, PEM-075, PEM-089, and PEM100.).

For the contributions of the different involved PE’s in the PEM we assume that during PEM formation a complete charge compensation of the polyions occurs and that only a small amount of inorganic counterions remains in the films.47 Thus, full complexation of the polyanions with identical amount of polycationic units means a stoichiometric ratio of 1:1 for all three PE combinations. With a monomer volume of 200 A˚3 for dPSS and the calculated SL of ca. 120 fm one gets a SLDdpss =6.01 10-6 A˚-2. Assuming a similar monomer volume for the polycations gives SLD’s of 1.35, 2.3, and 3.8510-7 A˚-2 for the cations PE-100, PE-089, and PE-075 respectively. Then the estimate values of SLD for charge compensated PEM would be 3.07, 3.12, and 3.2010-6 A˚-2 (PEM-100, PEM-089, and PEM-075). The difference to the measured values is about 25% (Table 1). On one hand, this deviation could be accepted as resulting from the rough estimate for the monomer volume, on the other hand, the result could match to either ≈ 25% void volume (cavities or empty space within the PEM film) or ≈ 15% bound water (water which is not removed in dry nitrogen atmosphere) volume in the PEM or a combination of both in according ratio. Each of this three options would reduce the SLD of the PEM. But since the 25% deviation is not ChD dependent and does not show effect during swelling we trace it back to the rough estimate. When one recalls that SLD ¼ Fmass 3 Σ

bi mi

it is possible to calculate the mass density of the film from the experimentally known values of SLD.46 These are, FPEM, of 1.07, 1.07, and 1.09 g/cm3 respectively for PEM-100, PEM-089, and PEM-075 (Table 1). The result shows that within the error of e4% the charge density does not effect the mass density of PEM. Other authors found for a PSS/PAH system density values of 1.13 and 1.1 g/cm3.15,21 PSS/PAH and PSS/PDADMAC are chemically different, but both PEM systems consist of strong PE. The similarity of the FPEM values show the plausibility of the obtained results. PEM Films in Water Vapor. In equilibrium state the swollen film and the ambient water have the same chemical potential, which is proportional to the relative humidity in the sample (47) Dubas, S.; Schlenoff, J. Macromolecules 1999, 32, 8153–8160.

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When exposed to water vapors, the film thickness increases; i.e., the film swells, and thus the polymer network is stretched and the polymer density is reduced. On the other hand, water is incorporated in the film, which decreases the overall SLD of the film by its small and negative SLD (Figure 4b). The amount of absorbed water can be estimated either from the volume change of the film or using the data for the SLD of the film obtained in the same experiment. Under the assumption, that the incorporated solvent displaces film material of exactly the same volume corresponding to complete “volume conservation” during swelling, this water content from volume change, wS, can be directly calculated from thickness data of swollen film, dS, and dry film, d0; i.e., wS can be expressed in terms of swelling, S=ds/d0: wS ¼ Figure 7. Isotherms: Water content in PEM according to relative humidity. Full symbols depict wSLD computed from SLD data, empty symbols show water fraction, wS, deduced from volume change. (The line is guiding the eye.)

ds -d0 ¼ 1 -S -1 ds

ð1Þ

A second measure of water content, wSLD, can be computed from the SLD values obtained by fitting the experimental data. The SLD of the film in presence of water is a sum of the SLD of all individual components multiplied with their volume fraction Fexp PEM ¼ wSLD  FH2 O þð1 -wSLD ÞF0

ð2Þ

where Fexp PEM stands for the SLD for swollen film, F0 is the SLD of the dry PEM, and FH2O =-0.56 A˚-2 is the SLD for water. The change in the film density, F0, caused by the film swelling has to be considered with the factor 1/S. Thus, the SLD calculated water content can be obtained as follows: wSLD ¼

Figure 8. Water fraction in PEM: Swelling vs wS (empty symbols) and water content wSLD (full symbols). The similar slope of the second part of the swelling curves demonstrate “displacement-like” swelling characteristics.

environment. Figure 5 shows that the linear increase of RH yields, at least for the charge diluted PEM-075 and PEM-089, to a nonlinear volume change. At a RH value of ca. 60% the swelling rate increases distinctly. This points to a change in the internal interactions of the PEM due to the uptake of water and the mechanical elongation of the polymer network which will be discussed below. The humidity dependent behavior of the external roughness is difficult to interpret because of the strong scattering of the roughness data (Figure 6). In a very rough estimate, the values of the surface roughness for PEM-075 are in the range of a third of the bilayer thickness of PEM-075; for PEM-100, it is the fifth. The increase of the external roughness with RH might be a consequence of the adsorption of water molecules. This could lead to an increasing mobility of the outer noncomplexed polyelectrolyte molecules due to partial break of hydrogen bondings. The slope appears to be rather moderate and uniform than pronounced nonlinear or bend, as it was observed for the thickness data. However, again, the strong scattering of the data allows no final conclusions. Water Content in the Film. The change of humidity in the vicinity of the film affects the thickness and the SLD of the film. 11582 DOI: 10.1021/la901508w

Fexp PEM -F0 =S FH2 O -F0 =S

ð3Þ

The water content wS and the real water content wSLD are presented in Figure 7 as a function of the relative humidity, i.e., as water absorption isotherms. The value wS scales almost linear with relative humidity, without showing significant dependence on ChD. Only for humidity around 100% a minor scattering of water fraction is visible with water contents of 35%, 38%, and 30% for PEM-075, PEM-089, and PEM-100, respectively. The water content, wSLD, at small RH (25-30%) shows no influence of charge density on water uptake. All samples contain the same amount of water in the limits of the experimental error. But beginning at intermediate RH (ca. 60%) ChD dependence becomes visible. The data split up, and the films with lower ChD exhibit systematically higher water content. The graph shows that the volume increase is higher than expected from simple water adsorption (pure displacementlike mechanism of swelling) at low RH. This behavior is followed by the PEM-100 sample even at high RH while the PEM-089 and PEM-075 are able to collect more water as their volume increase indicates at the middle range of the curve (RH ≈ 60%). We conclude that only a part of incorporated water causes the volume change of charge reduced PEM the other part is stored at active or empty sites of the PE network. Active sites are those places at the polymer chains which easily adsorb water molecules (e.g., ionic or polar sites) while empty sites represent voids or cavities in PEM where water clusters could be formed.16,24 A further increase of partial water pressure in sample surrounding causes/forces additional water uptake which is combined with a relaxation (i.e., expansion) of the PEM network. Langmuir 2009, 25(19), 11576–11585

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The water content wSLD at the highest relative humidities (RH ≈100%) depends on the charge density of the used polymers. It decreases when charge density increases in the order - PEM-075 (wSLD = 0.33), PEM-089 (wSLD = 0.27), and PEM-100 (wSLD = 0.20). The latter value is lower than reported values of other authors, e.g. 0.35 water content for the system (PSS/PAH)6 (PAH = poly(allylamine hydrochloride))31 or 0.50 for the same system in liquid D2O.48 A different chemical composition and differences in the preparation (dipping/spraying) of the films might be the reasons for this discrepancies. Film Swelling. Using the parameter swelling, S, for relative thickness changes makes the thickness changes of the different films comparable despite different initial film thickness. In Figure 8 swelling, S, is plotted against the two different water content measures, wS and wSLD, discussed in the previous section. Empty symbols depict swelling according to the displacement model using the theoretical value wS for the adsorbed water. The full symbols display swelling versus the water content wSLD calculated from SLD. S(wS) rises with wS monotonically with continuously increasing slope. S(wSLD) also increases with wSLD, but the behavior is more complex. Each curve splits in two parts with strongly diverging slopes indicating two different modes of swelling. The shape of the curves depends on the charge density. Figure 8 shows for the three S(wSLD) curves the same characteristics: In the beginning, a gentle increase of thickness is registered. Its slope depends on the ChD of the sample (higher slope for higher ChD). After a “swelling transition” at roughly the same extend of swelling, S ≈ 1.2, a section with similar slopes for the three samples, but about four times higher than in the first section appears. A 2-fold swelling behavior was already reported by Wong for the system PAH/ PSS16 and by Nolte for the systems PSS/PAH and PAH/PAA (PAA=poly(acrylic acid)).29 Apparently, the swelling mechanism for all three samples is the same in the first state, and it depends on the ChD. PEM-089 shows a maximum volume increase of 61%, PEM-100 and PEM075 swell by 44% and 54%, respectively. However, the relatively great error bars do not permit an unambiguous ranking of the effect on the maximum swelling. The incorporated maximum amount of water differs between the samples. The least charged PEM-075 incorporates more water (ca. 70%) than the fully charged PEM-100. The comparison of the SLD-water content data with those for the displacement model delivers deeper insight into the swelling mechanism. The different ChD’s in PEM do not turn out to be the main reason for water incorporation. Though higher charge density causes faster achievement of the transition point, the total amount of water is higher for the less charged materials on the end of the first phase of swelling. At this point the shown swelling of PEM-089 and PEM-075 is lower compared to the predicted values from displacement model, this means, the water is stored in the network without yielding to adequate expansion of PEM. This may be considered as a strong hint for the existence of voids or cavities in the film. The swelling behavior changes after the filling of the voids. Although in PEM-100 the presence of voids cannot be excluded, they must be smaller or less numerous as in PEM-089 and PEM-075, and thus their filling is always connected with a volume increase according to the displacement model. The cutting across of the S(wS) curve and the S(wSLD) curve when water fraction increases indicates, that the densely packed water phase has expanded to a less dense packed one. The PEM (48) Glinel, K.; Prevot, M.; Krustev, R.; Sukhorukov, G.; Jonas, A.; M€ohwald, H. Langmuir 2000, 4, 4898–4902.

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Figure 9. Estimation of Flory-Huggins solvation parameter χ for three polyelectrolyte compounds.

has extended more than necessary for the integration of the additional solvent molecules. This could be the case if the mobility of polymer chains changed and/or the water has switched over from a state with water cluster to a liquid state. Physically this can be interpreted as a kind of percolation transition, at which an almost dry polymer network, with low molecular mobility, transforms to water filled rather gel-like structure, where molecules surrounded by water have higher mobility. Such hypothesis was proposed by Nolte et al.29 who suggests that hydrophilic cites in PEM structure attract water molecules until this aggregates coalesce, forming a continuous water phase. This concept can describe our findings to a great extend. Whereas the highly charged PEM-100 needs the lowest amount of water to hydrate the charged sites, PEM-089 and PEM-075 systematically allow for hydration shells of higher order to fill their more extended voids and increase chain mobility, thus the water content at the transition point becomes indirectly dependent from ChD. Nevertheless, it remains questionable, whether the hydration and the entropy gain due to increased chain mobility is sufficient to explain the mechanism of swelling in the second phase. Ongoing swelling will reduce chain entropy from the point when the network elongation starts to restrict the number of chain conformations. Thus, phase separation processes as described later might give a better explanation. An alternative model for water incorporation, suggested by Tanchak et al.,34,49 holds micropores responsible for water transportation and accumulation of water in the PE films. The authors have reported a tunable transport velocity in PAH/PAA multilayers, which depends from the internal interplay of hydrophobic interactions of the PAH and PAA polymer backbone and water clusters partially pinned at hydrophilic side groups of PAA. This way a hydrogen bonding-network is formed which finally hinders or blocks a further diffusion of water into the deeper film. The outcome is an inhomogeneous water distribution with a significant higher water content toward the surface of the film and a lower water fraction close to the substrate. Although, the occurrence of such a mechanism in PSS/PDADMAC cannot be neglected, it seems not to be relevant in our case as an asymmetric water distribution was not found in our significant thinner films. The Flory-Huggins solvation theory based on a lattice model with mean field approximation for internal interactions can (49) Tanchak, O. M.; Yager, K. G.; Fritzsche, H.; Harroun, T.; Katsaras, J.; Barrett, C. J. Langmuir 2006, 22, 5137–5143.

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Figure 10. Model of 2-fold swelling behavior: (a) dry sample, (b) low water content (water and polymer molecules are homogeneously distributed), and (c) high water content (water and PE chains are phase separated: inhomogeneous mixture). The swelling transition occurs between states b and c. (Water molecules are symbolized by small circles.)

deliver a quantitative description of interactions in polymer solutions and networks. Thus, it can be used to describe the swelling behavior of a PEM. The external vapor pressure and swollen PEM have the same chemical potentials in equilibrium. Then in the Flory-Huggins equation the relative humidity RH can be directly related to the relative volume change of polymer films,50 the first two terms reflect the entropy of mixing, the third term corresponds with the enthalpy of mixing:   p ¼ ln wS þð1 -wS Þþχð1 -wS Þ2 ln p0

ð4Þ

Substituting solvent volume fraction with swelling wS ¼

ds -d0 ¼ 1 -S -1 ds

The lines in Figure 9 show the best fits to the experimental data using the Flory-Huggins equation with only one fitting parameter χ. All χ values are distinctly larger than 0.5: this is 0.85 ( 0.15 (PEM-100), 0.78 ( 0.08 (PEM-089), and 0.77 ( 0.15 (PEM-075). The result for PEM-100 is close to that already reported χ=0.91 ( 0.05 for a PSS/PAH system.17 The obtained χ values suggest a high tendency for phase separation during water uptake and swelling, i.e. up to a certain water content a homogeneous mixture of PEM and water is formed, but further increase of vapor pressure forces the appearance of a second homogeneously mixed phase with higher solvent content. From this point, both phases coexist and can reversely be transformed into each other. The polymers with reduced ChD have lower interaction parameters than PEM-100. Due to the large error bars of the swelling data of PEM-089 and PEM-075, no clear trend of the solvation parameter according ChD could be found. However, an increase of χ with increasing ChD would be likely.

one gets the Flory-Huggins equation as function of S: p ¼ ð1 -S -1 Þ 3 expðS -1 þχS -2 Þ p0

Conclusion ð5Þ

Here p/p0 = RH/100% is the ratio of vapor pressure p and vapor saturation pressure p0. The only free parameter in Equation 5 is χ, the Flory-Huggins solvation parameter. In a simple form, this phenomenologic parameter unites the lattice constant, z (z=6 for ideal cubic lattice, solvent and polymer units have same size) and the sum of internal interaction energies: polymer-solvent, g12, polymer-polymer, g11, and, solvent-solvent, g22: χ ¼

  ðz -2Þ 1 g12 - ðg11 þg22 Þ kT 2

At χ < 0 the polymer-solvent interaction, g12, is stronger than the polymer-polymer, g11, and solvent-solvent interaction, g22, Usually χ > 0 is the more often case which shows that equal units like themselves more than each other. χ = 0 stands for ideal mixing, where interchain forces and solvent-polymer forces cancel out. In solutions of polymer and low-molar mass solvent the entropic terms become significant and shift the solvation parameter for ideal mixing to χ=0.5 (Θ-solvent condition).51-53 (50) Naylor, T. Permeation Properties. In Comprehensive Polymer Physics; Allen, G., B. J., Ed.; Pergamon Press: Oxford, U.K., 1989; Chapter 20. (51) Young, R.; Lovell, P. Introduction to Polymers; Stanlay Thornes Ltd: Cheltenham, U.K., 2000. (52) Casassa, E.; Berry, G. Polymer Solutions. In Comprehensive Polymer Physics; Allen G., B. J., Ed.; Pergamon Press: Oxford, U.K., 1989; Chapter 3. (53) Rubinstein, M.; Colby, R. Polymer Physics; Oxford University Press: Oxford, U.K., 2004.

11584 DOI: 10.1021/la901508w

We studied the swelling in water (H2O) vapors of polyelectrolyte multilayers prepared from PSS and PDADMACNMVA with different charge densities. pDADMAC derivatives with a charge density of 100%, 89%, and 75% were used. The films were prepared by spraying technique and consisted of six pairs of PE’s. Neutron reflectivity was used as a tool to follow the build-up and the swelling process. The experiments were complemented with AFM experiments which provided information on topology and the degree of swelling of the films. It was found, that a lower limit for charge density exist, below which no continuous film formation can take place with spraying LbL-technique. Down to this value the thickness of dry films increases continuously with falling ChD of polyanion, without significantly altering the mass density of the multilayer, F ≈ 1.1 g/cm3. The films swell monotonically when exposed to water of different relative humidity, the process is charge density dependent. The swelling behavior is 2-fold, splitting up in a charge dependent mode with relatively little volume increase, and a second mode with high volume expansion, which is independent from the charge density of PEM. The “swelling transition” occurs for all samples at a relative humidity about 60% and a volume increase of ca. 20%. The results were interpreted according to the Flory-Huggins theory assuming a phase separation in PEM network at higher water contents. Figure 10 illustrates the molecular picture of the two-step swelling model. Water accumulation in PEM would be facilitated with ascending partial vapor pressure, as suggested by the Flory-Huggins theory. Water uptake appears to be moderated by the presence of (hydrophilic) spacer units, Langmuir 2009, 25(19), 11576–11585

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which allow the network to enlarge, and thus, provide space for additional water storage. The reason might be that in contrast to the ionic units the interactions of NMVA are only short-range and leave the unit a higher flexibility and mobility. Generally it was found, that strong PEM can only be redissolved in salt solutions of concentrations in molar range, thus the state of maximum swelling can be seen as being similar to the adsorption step after washing, i.e. after removal of the small counterions. The finding of an imperfect adsorption, i.e. complexation, of the PE-070 molecule in the AFM measurements, supports this assumption. The high solubility of PE-070, i.e. its high entropy of mixing in solution, can be only fragmentary compensated from the increase in entropy due to counterion release during the adsorption process. Since basically the intermolecular interactions in solution and PEM are the same (ionic, and hydrogen bonding), the gain in intermolecular energy during adsorption should be negligible. But the entropic contribution increases in PE with higher ChD compared to the entropy in solution, and should thus be responsible for the adsorption. Concluding, it can be stated that the presence of uncharged, but relatively hydrophilic, spacer units in the chains of the PDADMAC-derivatives is the key feature for the swelling process. While the spacer molecules enhance the chain mobility during the

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swelling of the PEM, the ionic sites hinder mobility and serve rather as “cross-linkers”. The rise of the Flory-Huggins solvation parameter with increase of ChD demonstrates this. It is noteworthy that the charged monomers are statistically distributed along the molecule chain, and thus, chain sections with different characteristics can coexist in pDADMAC-derivatives according to their local charge density distribution, and furthermore, that interfaces are involved, thus confinement effects and interfacial interactions can additionally modulate or even govern the intermolecular behavior in PEM. Acknowledgment. The fruitful discussions with R. Steitz, M. Sch€onhoff, C. Helm, and H. M€ohwald were essential for the preparation of the present work. BENSC at Helmholtz Center Berlin for Material and Energy is acknowledge for beamtime allocated at the neutron reflectometer V6. The work was partially supported by the Deutsche Forschungsgemeinschaft (Grants FE 600/4 and LA 611/5). Supporting Information Available: Figure showing a plot of the thickness data and a table of data obtained from the reflectivity. This material is available free of charge via the Internet at http://pubs.acs.org.

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