Langmuir 2003, 19, 3947-3952
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Kinetics of Salt-Induced Annealing of a Polyelectrolyte Multilayer Film Morphology Richard A. McAloney,† Vyacheslav Dudnik, and M. Cynthia Goh* Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6 Received November 20, 2002. In Final Form: January 23, 2003 The layer-by-layer (LBL) adsorption of polyelectrolytes onto charged surfaces from aqueous solutions produces multilayered surface structures. A multilayer film of polydiallyldimethylammonium chloride (PDDA) and polystyrenesulfonic acid, sodium salt (PSS), prepared using 1.0 M NaCl in the polymer solutions, has a vermiculate morphology with roughness greater than 30 nm. Atomic force microscopy (AFM) investigations show that the morphology of the films can be annealed by the introduction of salt solutions of various concentrations. Roughness measurements taken at various times after immersion in salt solution indicate that the decrease in surface roughness is consistent with second-order kinetics. A model is proposed to explain the morphology annealing that relies on the presence of salt ions that free up polymer-polymer contacts, allowing new ones to form. The formation of new polymer-polymer contacts results in a new film morphology.
Introduction The formation of multilayer assemblies through the layer-by-layer (LBL) deposition of polyelectrolytes (PEs) or other charged material is making a significant impact as a viable fabrication route to functional materials. Reported examples of functional materials formed using the LBL technique impact many disciplines including optics,1-4 electronics,5-10 light emitting devices,11-13 sensors,14-18 and drug delivery,19-21 and the extent to which this technique can be exploited still fuels further re* To whom correspondence should be addressed. E-mail: cgoh@ alchemy.chem.utoronto.ca. Phone number: 416-978-6254. Fax number: 416-978-4526. † Current address: Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, Canada K1A 0R6. (1) Lenahan, K. M.; Wang, Y.-X.; Liu, Y.; et al. Adv. Mater. 1998, 10, 853. (2) Casson, J. L.; McBranch, D. W.; Robinson, J. M.; et al. J. Phys. Chem. B 2000, 104, 11996. (3) Koetse, M.; Laschewsky, A.; Jonas, A.; et al. Colloids Surf., A: Physicochem. Eng. Aspects 2002, 198, 275. (4) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (5) Sarkar, N.; Ram, M. K.; Sarkar, A.; et al. Nanotechnology 2000, 11, 30. (6) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; et al. J. Am. Chem. Soc. 1996, 118, 7640. (7) Cassagneau, T.; Fendler, J. H.; Mallouk, T. E. Langmuir 2000, 16, 241. (8) Liu, Y.; Wang, A.; Claus, R. J. Phys. Chem. B 1997, 101, 1385. (9) Kovtyukhova, N. I.; Martin, B. R.; Mbindyo, J. K. N.; et al. J. Phys. Chem. B 2001, 105, 8762. (10) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848. (11) DeLongchamp, D.; Hammond, P. T. Adv. Mater. 2001, 13, 1455. (12) Wu, A.; Yoo, D.; Lee, J. K.; et al. J. Am. Chem. Soc. 1999, 121, 4883. (13) Lesser, C.; Gao, M.; Kirstein, S. Mater. Sci. Eng. C: Biomimetic Supramol. Syst. 1999, 8-9, 159. (14) Caruso, F.; Niikura, K.; Furlong, D. N.; et al. Langmuir 1997, 13, 3427. (15) Yang, X.; Johnson, S.; Shi, J.; et al. Sens. Actuators B: Chem. 1997, 45, 87. (16) Onda, M.; Lvov, Y.; Ariga, K.; et al. Biotechnol. Bioeng. 1996, 51, 163. (17) Kleinfeld, E. R.; Ferguson, G. S. Chem. Mater. 1995, 7, 2327. (18) Mermut, O.; Barrett, C. J. Analyst 2001, 126, 1861. (19) Caruso, F.; Trau, D.; Mohwald, H.; et al. Langmuir 2000, 16, 1485. (20) Caruso, F.; Yang, W. J.; Trau, D.; et al. Langmuir 2000, 16, 8932. (21) Mohwald, H. Colloids Surf., A: Physicochem. Eng. Aspects 2000, 171, 25.
search.22 With this said, it is interesting that there is still a rather limited understanding of the film structure and formation, and the factors that influence them during fabrication and postfabrication. This paper explores the postmodification of polyelectrolyte multilayer (PEM) films through contact with solutions containing salt. The morphology and thickness of PEM films depend on the nature of the PEs and on the experimental parameters used in their fabrication. Major changes in film structure are typically induced by the addition of salt to the PE solution and by changes in pH for pH sensitive PEs. Usually, it is only the thickness which can be controlled with a reasonable degree of accuracy. This imposes a limitation on the number of distinct structures that can be fabricated directly. Postmodification of multilayer films may result in novel structures or advantageous structural changes in the film. In one such example, Mendelsohn et al.23 exposed films of poly(acrylic acid) and poly(allylamine) to a low pH bath, inducing an irreversible structural change in the film. The resulting microporous film had a thickness three times that of the original film. Functionally, the films exhibited promising characteristics for microelectronic or biomaterial applications that were not present in the original film. Using the same polyelectrolyte system, Fery et al.24 produced new film morphologies by changing the salt concentration of the rinsing solution both during and after film preparation. This produced lateral structural features (pores) in the films, thereby affecting the permeability of the film. In these experiments, the film morphology was examined in the dry state after the films had been prepared and modified. Differences between the dry state and wet state of PEM films have been documented. For example, in the wet state PEM films are swollen in the presence of salt solution and in some cases in water. Therefore, not only will in-situ studies provide information about structural transitions in a film as they happen, but also they may shed light on the structure of the wet state of PEM films. We have previously observed vermiculated morphology in 10-bilayer films of polydiallyldimethylamonium chloride (PDDA)/polystyrenesulfonic acid, sodium salt (PSS) sys(22) Decher, G. Science 1997, 277, 1232. (23) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; et al. Langmuir 2000, 16, 5017. (24) Fery, A.; Scholer, B.; Cassagneau, T.; et al. Langmuir 2001, 17, 3779.
10.1021/la026882s CCC: $25.00 © 2003 American Chemical Society Published on Web 03/14/2003
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tems deposited using 0.1 M NaCl. We observed that the film morphology was retained when immersed in pure water25 but not when in contact with salt.26 This latter observation was also made by Dubas and Schlenoff,27 who demonstrated that the rough morphology becomes smooth and the film changes thickness upon immersion into solutions of various ionic strengths of NaCl. They interpret the smoothing kinetics of smoothing of the film in terms of polyelectrolyte interdiffusion or the movement of a charged polymer against an oppositely charged polymer. In this article, we explore the annealing of the film leading to destruction of the vermiculite morphology insitu. Since the film morphology is retained upon immersion in water,25 we can stop the annealing at various times to examine how the morphology evolves with time. We also introduce a one-parameter theory that models the kinetics of the morphological change. Materials and Methods Materials. The polycation PDDA (polydiallyldimethylammonium chloride; MW 400 000 to 500 000, Aldrich Chemical Co.) and polyanion PSS (polystyrenesulfonic acid, sodium salt; average MW ) 500 000, Polysciences) were used as received, to prepare 1 mg mL-1 aqueous solutions, containing 1.0 M NaCl (Aldrich), using purified water (Barnstead, resistivity >18 MΩ/cm; pH ∼ 5.6). Fused silica (Hareus) substrates were cleaned in hot nitric acid (concentrated) for a minimum of 5 h, rinsed with copious amounts of purified water, and blown dry with a stream of N2. Film Preparation. Ten-bilayer films were prepared by the sequential immersion of a silica substrate into the appropriate polyelectrolyte solution for 30 min. The plates were rinsed with purified water and dried with a flow of N2 between immersions. The postmodifications of the films in the NaCl salt solutions, in particular the immersion times and salt concentrations used, are indicated in the text. UV/Vis. UV/vis spectra were collected with a Chem2000 spectrometer (Ocean Optics, Dunedin, FL). A clean silica plate of the same thickness as the substrate was used as a reference. Atomic Force Microscopy. Imaging under Fluid. Tapping mode AFM images under fluid were obtained using an extended multimode Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) with the manufacturer supplied fluid cell. Silicon nitride square pyramidal cantilevers (nominal force constant of ∼0.12 N/m, Nanoprobes, Santa Barbara, CA) were oscillated near their resonance frequency, usually in the range 5-25 kHz for these cantilevers under fluid. The scan rate employed in all imaging ranged from 2 to 5 Hz. Imaging in Air. Images under ambient conditions were obtained using the Solver P47 atomic force microscope (NT-MDT, Moscow, Russia). Intermittent contact mode images were obtained with cantilevers 90 µm in length with a resonance frequency of ∼320 kHz and a nominal tip radius of curvature of ∼10 nm. Surface roughness was obtained from the images using the imaging software of the NT-MDT instrument. Thickness measurements were obtained via a method previously reported.25 Morphology Destruction. Prior to imaging in fluid, the samples were first imaged in air, and roughness and thickness measurements were obtained. The 10-bilayer films were then placed in an AFM fluid cell and brought into contact with the appropriate solution using the flow-through capabilities of the cell. Imaging was then performed under fluid. The films were then removed from the fluid cell, rinsed with water, and blown dry with N2, and again their roughness and thickness were determined in air.
Results UV/Vis. UV/vis spectra for all the films used in this work were identical to those obtained in the previous work (25) McAloney, R. A.; Sinyor, M.; Dudnik, V.; et al. Langmuir 2000, 17, 6655. (26) McAloney, R. A. Ph.D. Thesis, Department of Chemistry, University of Toronto, Toronto, Canada, 2002. (27) Dubas, S. B.; Schlenoff, J. B. Langmuir 2001, 17, 7725.
Figure 1. Typical images obtained for 10-bilayer films of PDDA/PSS prepared from 1 M NaCl in (a) air, (b) pure water, and (c) 1.0 M NaCl. Images are 5 µm a side with the vertical scale shown. Note the slightly different vertical scale for the image in air.
for 10-bilayer films of PDDA/PSS from 1 M NaCl.25 As previously seen, the curves become linear after the third or fourth bilayer. The spectra for all the films used in this study differed in absorbance by no more than 5%. This is a well-known characteristic of the layer-by-layer fabrication technique. Morphology. Atomic force microscopy was used to examine the 10-bilayer films. Prior to imaging under fluid, the films were characterized in air. A typical scan of a 10-bilayer film imaged in air is seen in Figure 1a. The root-mean-square (RMS) roughness of all films prior to treatment was on the order of 34 nm, and the thickness was 373 ( 34 nm. After the dry film was characterized, it was placed in the fluid cell and imaged under water. Figure 1b shows an image of a 10-bilayer film under water. The vermiculate morphology is clearly present, indicating that the film retains its morphology in water. A closer look reveals a
Polyelectrolyte Multilayer Film Morphology
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Figure 2. Image in 3.5 M NaCl of a 10-bilayer film of PDDA/PSS prepared from 1 M NaCl. The film likely appears flat and featureless because it is easily deformable by the AFM tip at these salt concentrations.
slight roughening of the ridges. This roughening is too small to be characterized by an RMS roughness increase, but it is clear from the image that the ridges definitely have some fine structure. This roughening of the surface morphology is not seen when the films are immersed in water immediately after preparation, but it becomes apparent after the films have been stored in air for several days. As suggested by Dubas and Schlenoff,27 when films are not used immediately, the aggregation of hydrophobic domains may occur. It is possible that these proposed aggregated hydrophobic domains could be the fine structure that is seen in Figure 1b. The unfavorable interaction between the hydrophobic domains and water could give rise to the observed structure. Salt solution was then introduced into the fluid cell, and imaging was performed while the salt solution was present in the cell. Figure 1c shows an image of a 10bilayer film after incubation in 1.0 M NaCl for approximately 15 min. The vermiculate morphology is still present; hence, the film is robust to 1.0 M NaCl even after 15 min of immersion, although there is a noticeable smoothing of the ridged structures compared to the case of Figure 1b. Salt ions may have penetrated into the film, causing it to swell and appear smoother than the image in water. A slight swelling ( c0, then eq 11 transforms to an even simpler form,
N + ) N- ) N N+/- ) Ntot - 2N
k)
(9)
where c ) NAv/V is the salt ion concentration. Similarly, the probability of finding n ions in n different volume elements, V0, is
Pn ) [1 - exp(-V0/V)]n ) [1 - exp(-c/c0)]n (10) If the chain reconfiguration time, or time to form new polymer contacts, depends only on these probabilities, then the rate constant will be some time constant multiplied by the probability of finding n ions in n volume elements (eq 10). This can be expressed as (30) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (31) Laurent, D.; Schlenoff, J. B. Langmuir 1997, 13, 1552. (32) Grosberg, A. I. U.; Khokhlov, A. R. Statistical Physics of Macromolecules; AIP Press: New York, 1994.
(12)
This indicates how an exponential dependence of the half-life on salt concentration may arise. A linear fit to the data in Figure 6 gives c0 ) 0.14 M, k0 ) 2.2 × 10-7 s-1, and τ ) 1260 h. The physical meaning of c0 is that it is the amount of salt ions that equals the charges on all polyelectrolyte chain monomers. Assuming that each monomer carries one charge, we can estimate c0 from the monomer concentration, which in turn can be derived using the polyelectrolyte parameters: for PSS, the number of monomers Nm ∼ 2500, and the distance between monomers a ∼ 2.7 Å.25 Hence, the monomer concentration using the polymer in a good solvent scaling model is
cm )
Nm 3
a Nm1.8NAv
)
1 ) 0.16 M a3N0.8 m NAv
(13)
which is quite close to the best fit c0 value that we obtain experimentally. There is no doubt that the above is a gross oversimplification of the real system; however, it is comforting that the numbers are in the same range. Summary and Conclusions The AFM was utilized to investigate the morphology of 10-bilayer films of PDDA/PSS (prepared from 1.0 M NaCl) in the presence of water and solutions of various salts and salt concentrations. The initial vermiculate morphology of the film was retained upon immersion into pure water. When the films were immersed in salt solutions above a certain concentration, the vermiculate morphology was immediately lost, and a flat featureless surface morphology was obtained. The control of surface morphology, and hence surface roughness, allows for the tailoring of the surface properties of the film for a given application. It was observed that the imaging process (tapping mode) removed the polymer film when the imaging was performed in the presence of a 3.5 M NaCl solution. This phenomenon could possibly be exploited as a possible route for the lateral patterning of PEM films. The kinetics of the morphology destruction was investigated by monitoring the decrease in surface roughness of the film with immersion time in various concentrations of NaCl. The roughness decreased in a fashion consistent with second-order kinetics. We propose a model in which the annealing arises from the formation of new polymerpolymer contacts, which are formed due to the presence of salt ions. Salt ions free up polymer segments, allowing them to form new polymer-polymer contacts resulting in a new film morphology. Acknowledgment. The authors acknowledge support from the Natural Science, Engineering Research Council of Canada (NSERC), and the Materials and Manufacturing Ontario (EMK). R.M. thanks the Ontario government for an Ontario Graduate Scholarship (OGS). LA026882S
