Atomic Force Microscopy Studies of Salt Effects on Polyelectrolyte

Received January 26, 2001. In Final Form: May 24, 2001. The morphology of multilayer films formed from polydiallyldimethylammonium chloride and ...
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Atomic Force Microscopy Studies of Salt Effects on Polyelectrolyte Multilayer Film Morphology Richard A. McAloney, Mark Sinyor, Vyacheslav Dudnik, and M. Cynthia Goh* Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6 Received January 26, 2001. In Final Form: May 24, 2001 The morphology of multilayer films formed from polydiallyldimethylammonium chloride and polystyrene sulfonic acid deposited under a range of salt concentrations (from 10-4 to 1.0 M) was investigated using atomic force microscopy (AFM). Ten-bilayer films that were deposited with less than 0.3 M added NaCl were flat and featureless, with similar characteristics to the underlying silica substrate. When formed at and above this salt concentration, a vermiculate morphology was observed. Thickness and roughness measurements were also carried out using the AFM and were found to increase with the concentration of added salt. The evolution of the vermiculate pattern was investigated by AFM studies of each layer that is deposited under high salt concentration (1.0 M NaCl). The first three bilayers were featureless and had a thickness of ∼6 nm/bilayer. A change in morphology was observed by the fourth bilayer, and the average thickness had increased to ∼46 nm/bilayer. These results may be explained in terms of a transition from an extended conformation to a more compact form that polyelectrolytes undergo as a function of the ionic strength of a solution.

Introduction Ultrathin polyelectrolyte multilayer films are composed of alternating layers of oppositely charged polyions. These multilayer films are produced with relative ease via the successive adsorption of polycations and polyanions to an initially charged surface.1 This method of preparation is appealing for many industrial applications since it is simple and versatile. Potential applications of these films include light emitting diodes,2 selective membranes,3,4 conducting layers,5 biosensors,6 nonlinear optics,7 encapsulation technology,8,9 and chemical sensors.10 The requirement for only a charged substrate and charged assembly materials allows multilayers to be composed of polyelectrolytes together with DNA,11 delaminated clay particles,12 dyes,13 and electroluminescent conjugated polymers.14 Furthermore, polyelectrolyte coatings have been investigated on a number of surfaces such as latex particles,15 colloidal metal particles,16 silica glass,17 and mica.18 * To whom correspondence should be addressed. Phone: 416978-6254.Fax: 416-978-4526.E-mail: [email protected]. (1) Decher, G. Science 1997, 277, 1232. (2) Tian, J.; Thompson, M. E.; Wu, C. C.; Sturm, J. C.; Register, R. A.; Marsella, M. J.; Swager, T. M. Abstr. Pap.sAm. Chem. Soc. 1994, 208, 128. (3) Leveslam, J. M.; McCarthy, T. J. Macromolecules 1997, 30, 1752. (4) Stroeve, P.; Vasquez, V.; Coelho, M. A. N.; Rabolt, J. F. Thin Solid Films 1996, 285, 708. (5) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985. (6) Sun, Y. P.; Zhang, X.; Sun, C. Q.; Wang, B.; Shen, J. C. Macromol. Chem. Phys. 1996, 197, 147. (7) Laschewsky, A.; Mayer, B.; Wischerhoff, E.; Arys, X.; Bertrand, P.; Delcorte, A.; Jonas, A. Thin Solid Films 1996, 285, 334. (8) Mohwald, H. Colloids Surf., A 2000, 171, 25. (9) Sukhorukov, G. B.; Donath, E.; Moya, S.; Susha, A. S.; Voigt, A.; Hartmann, J.; Mo¨hwald, H. J. Microencapsulation 2000, 17, 177. (10) Ram, M. K.; Carrara, S.; Paddeu, S.; Nicolini, C. Thin Solid Films 1997, 302, 89. (11) Lvov, Y.; Decher, G.; Sukhorukov Macromolecules 1993, 26, 5396. (12) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038. (13) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (14) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F. J. Appl. Phys. 1996, 79, 7501. (15) Donath, E.; Walther, D.; Shilov, V. N.; Knippel, E.; Budde, A.; Lowack, K.; Helm, C. A.; Mo¨hwald, H. Langmuir 1997, 13, 5294.

With all of these possibilities, much early work focused on studying the properties of assembled films and how to use them effectively in the plethora of roles mentioned above. It has been established that film thickness increases with an increase in concentration of added salt. However, while some evidence shows that this relationship is linear,19 other data suggest that thickness is proportional to the square of the ionic strength for different polyelectrolyte pairs.20 Moreover, it was found that when deposited from solution with high salt concentration, the thickness of the multilayer films varies over the first few layers before reaching constant incremental increases per bilayer. (A bilayer refers to the deposition of a pair of positive and negative polyelectrolyte layers.) For example, Ladam et al.21 found that the thickness increase per bilayer in the first three bilayers was different than that in the outer bilayers. Due to the inconsistent nature of these early layers, a precursor film that consists of several layers is often deposited to ensure that the outer layers are prepared on homogeneous surfaces.22,23 Such a precursor film is not needed when low salt concentrations or highly charged surfaces are used. The presence of salt in polyelectrolyte solutions drastically affects the chain configuration in bulk solution. When no salt is added to the polyelectrolyte solutions, the like charges within a single polyelectrolyte chain repel each other, such that the chain exists in an almost fully extended, rodlike configuration. In the presence of additional salt, counterions screen some of the charges allowing the polyelectrolyte chain to fold into a random coil configuration.24 If the chain retains its solution (16) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. (17) Lobo, R. F. M.; Pereira-da-Silva, M. A.; Raposo, M.; Faria, R. M.; Oliveira, O. N. Nanotechnology 1999, 10, 389. (18) Kim, D. K.; Han, S. W.; Kim, C. H.; Hong, J. D.; Kim, K. Thin Solid Films 1999, 350, 153. (19) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (20) Lvov, Y. M.; Decher, G. Crystallogr. Rep. 1994, 39, 628. (21) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249. (22) Klitzing, R. v. Langmuir 1995, 11, 3554. (23) Steitz, R.; Leiner, V.; Siebrecht, R.; von Klitzing, R. Colloids Surf., A 2000, 163, 63.

10.1021/la010136q CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001

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configuration when it adsorbs to the substrate, then the structural details of the multilayer film formed will be strongly influenced by the salt concentration. It has been reported that films produced under high salt concentrations are thicker and have a higher surface roughness.18 The influence of salt on these structural details will likely be dependent upon which polyelectrolytes are used. This study will focus on multilayer films of polydiallyldimethylammonium chloride (PDDA) and polystyrene sulfonic acid, sodium salt (PSS). The main focus of this work is to explore how the concentration of added salt influences the thickness, roughness, and morphology of a polyelectrolyte multilayer film. In addition, we examine the evolution of these properties by studying the deposition of each bilayer. Experimental Section

McAloney et al. the scanned area was removed can be verified by increasing the scan size while returning to the normal imaging parameters. The resulting image, an example of which is seen in Figure 5, was analyzed in cross section to provide the height of the film above the substrate. Films that were prepared with salt concentrations exceeding 0.01 M were immune to perturbation with the AFM probe tip, such that repeated scanning under the highest operating forces feasible with the cantilevers we used could not scrape off the material. In these cases, portions of the film were removed using the edge of a razor blade. The AFM probe tip was then scanned across the newly exposed region to verify that very little, if any, material was left. For an accurate measurement of film thickness, it was important that the film be completely removed in this region but at the same time that the razor blade not scrape off the substrate. To test the latter possibility, a fresh silica plate was scratched with the razor and examined using the AFM; no scratches or damage was visible under the conditions of the experiment. The measurement of thickness and roughness using the AFM generally requires some prior processing of the images because image bow or tilt is typically present, due either to the sample alignment or to the piezoelectric scanners employed. These bows and tilts are typically removed by fitting the image to a surface defined by a polynomial. However, it is ambiguous as to what the appropriate processing approach is, and depending on one’s choice, the measurements may give rise to different values. For all the thickness measurements in this paper, the images were fitted with a first-order plane calculated from an area of the surface that did not include the step from the substrate to the film surface. The errors we report for the film thickness are the standard deviations of all the measurements taken. These thickness values were recorded using average cross-section analysis from different areas of several independent samples. Surface Roughness Measurements. The surface roughness was obtained from the AFM images by using the imaging software of the Solver P47, which performed the calculation in the following manner:

Materials. The polycation PDDA (polydiallyldimethylammonium chloride; MW 400 000 to 500 000, Aldrich Chemical Co.) and polyanion PSS (polystyrene sulfonic acid, sodium salt; average MW ) 500 000, Polysciences) were used as received to prepare 1 mg mL-1 aqueous solutions, using purified water (Barnstead, resistivity >18 MΩ/cm; pH ∼ 5.6). The salt NaCl (99+%) was used as received (Aldrich) to prepare polyelectrolyte solutions with salt concentrations ranging from 1.0 × 10-4 to 1.00 M. The fused silica (Hareus) substrate was 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. Films were prepared by the sequential immersion of a silica substrate into the appropriate polyelectrolyte solution for 30 min. The plate was rinsed with purified water and dried with a flow of N2 between immersions. Atomic Force Microscopy (AFM). The morphology and roughness were obtained from height images collected using the Solver P47 atomic force microscope (NT-MDT, Moscow, Russia). The images were collected in intermittent contact mode with ultrasharp silicon cantilevers (Silicon-MDT, Moscow, Russia). The particular cantilever used was 90 microns in length with a resonance frequency of ∼320 kHz and a nominal tip radius of curvature of ∼10 nm. Thickness measurements were performed using the extended multimode Nanoscope IIIa (Digital Instruments, Santa Barbara, CA). All images of samples in ambient air were obtained in contact mode, using silicon nitride square pyramidal tips that have a nominal force constant of ∼0.12 N/m (Nanoprobes, Santa Barbara, CA). TappingMode in fluid images were obtained using the manufacturer supplied fluid cell, silicon nitride square pyramidal tips (nominal force constant of ∼0.12 N/m, Nanoprobes, Santa Barbara, CA), and purified water. The cantilevers were oscillated near a resonance frequency that usually occurs in the range of 5-25 kHz for these cantilevers under fluid. Scan rates employed in all imaging ranged from 2 to 3 Hz. Calibration of both instruments was performed using calibration grids (Silicon-MDT). Due to the anticipated range of film thicknesses in these experiments, the vertical, or z, dimension was calibrated around three separate regions: 22, 100, and 485 nm. Film Thickness Measurements. The AFM is well suited for the direct measurement of the heights of surface features, especially in the 50-nm-and-below regime. The polymer film thickness was measured by first creating a hole in the film so as to expose the substrate. The height of the film above the substrate was thus obtained directly. For films prepared with salt concentrations of 0.01 M or less, the AFM probe tip itself was used to remove a portion of the film. While these films can be imaged nondestructively in contact mode by choosing AFM parameters carefully (set point of ∼0 V, scan speed of ∼2 Hz), these parameters can be altered so as to cause the removal of the adsorbed material. For these samples, this was accomplished by scanning repeatedly after increasing the set point to nearly the maximum value and the scan speed to 30 Hz. That the film within

UV/Vis. PSS in aqueous solution has an absorbance spectrum with a peak at 226 nm. We used this peak to

(24) Grosberg, A. I. U.; Khokhlov, A. R. Statistical Physics of Macromolecules; AIP Press: New York, 1994.

(25) Kiely, J. D.; Bonnell, D. A. J. Vac. Sci. Technol., B 1997, 15, 1483.

Rq )

x

1

N

∑(z - z N i

2 av)

i)1

where Rq is the root-mean-square roughness, zi is the z value for a specific pixel, zav is the average of the z values in the desired scan area, and N is the number of pixels in the same scan area. The value for the surface roughness calculated thus depends on several factors including tip geometry and size, image processing, scan size, and image bow or sample tilt.25 Artifacts due to nonlinearities in the piezoelectric crystal, thermal drifts, and random tip jumps will also influence the calculation. Image bow or tilt can be removed from the images by a fitting procedure, just as for the case of thickness measurements that was described previously. There is no one best way of image processing before roughness measurements, but it is important to state what was performed and to perform the same procedure for samples that are being compared. In these studies, the AFM images were all 5 µm × 5 µm scans and were corrected for bow/tilt using a plane fit. The same tip was used for a series of measurements; while this does not eliminate tip convolution effects, it enables a fair comparison of different samples. The errors reported here were calculated from the standard deviation for all the measurements, which included at least three areas on at least two independent samples. UV/Vis Spectroscopy. 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 the reference.

Results

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Figure 2. Relative absorbance of (PDDA/PSS)n films vs the number of bilayers, n, for films deposited from different NaCl solutions. Absorbance data were taken at the maximum absorbance.

Figure 1. UV/vis absorbance spectra for each bilayer of the multilayer film prepared from (a) 0.001 M NaCl and (b) 1.0 M NaCl. The peak in (a) is at ∼217 nm, whereas the peak in (b) is at 226 nm.

track the amount of material deposited per bilayer by UV/vis spectroscopy. Since PDDA has no absorbance peak in the region of interest, measurements were conducted and reported in terms of bilayers (i.e., a PSS/PDDA pair) instead of individual layers. In Figure 1 is shown a collection of spectra taken after each bilayer of material was deposited on the substrate. For low salt concentrations (e.g., 0.001 M NaCl shown in Figure 1a), the absorbance peaks at ∼217 nm. With increasing salt concentration, the peak gradually red-shifts, and a strong signal below 210 nm begins to appear, as shown in Figure 1b for the 1.0 M NaCl solution. The PSS solution phase absorbance peak of 226 nm was reached for a 0.3 M NaCl solution (data not shown). This spectral shift introduces a complication when trying to quantify the amount of material using UV/vis spectroscopy. A further complication is introduced with the appearance of a strong peak below 210 nm at salt concentrations greater than 0.1 M NaCl. The red-shift of the absorption peak may indicate changes in the PSS chromophore environment with the salt concentration or may be a result of the appearance of this strong signal. In any case, these features complicate the use of UV/vis spectroscopy for quantification purposes, and one has to be careful in analyzing such results. In this paper, we simply use the UV/vis data for qualitative purposes and to corroborate results obtained for the morphology and thickness studies by the AFM. In Figure 2 is plotted the measured absorbance at 226 nm per bilayer for films prepared from solutions with NaCl concentrations ranging from 0.00010 to 1.00 M. For the same number of bilayers, the absorbance increased monotonically with the salt concentration except for one case: below 5 bilayers, the absorbance of the 0.7 M sample is less than that of the 0.5 M sample. While it may be possible that this is due to small errors in the baseline

signal (which may be ascribed to changes in the reference substrate used), we do not believe this is so: the crossing of the curves in Figure 2 for the 0.5 and 0.7 M NaCl was reproducible for different samples and was within the estimated errors of the experiment. The absorbance of a sample is proportional to the amount of absorbers. Thus, in the formation of multilayer assemblies, it is expected that there be a constant increment in absorbance for each deposited layer of absorbers, resulting in a linear relationship between intensity and number of layers. Alternatively, if this relationship is not linear, it means that either the number of absorbers per layer is not constant, or the environment is changing, or both. Our results, summarized in Figure 2, show a linear relationship between intensity and number of deposited bilayers for solutions with salt concentrations below 0.3 M and a nonlinear relationship for concentrations above that. The nonlinearity is most pronounced for the highest concentration investigated (1.00 M), the data for which can be fitted by an exponential. Due to the complications in the spectra noted previously, these data cannot be quantified in depth. However, the difference between the behavior at low and high salt concentrations is drastically different, such that it is safe to conclude that for low salt concentrations, each subsequent layer contains the same amount of material as the previous one, while for high salt concentrations, this is no longer true. The breakpoint appears to be around 0.3 M. The solution referred to as “no added salt” does in fact contain ions due to the polyelectrolyte itself. In these studies, we used a constant polyelectrolyte concentration that gives rise to an ion concentration of 3 × 10-3 M. Thus, when the concentration of added salt is too low, one cannot neglect this contribution to the total concentration of ions in solution. This is in accord with what we see in Figure 2, where the data for 0.00010 and 0.0010 M films almost overlap with each other but are slightly higher than for the solution with no added salt. Film Morphology. Atomic force microscopy was used to examine the films produced after 10 bilayers have been deposited in the presence of additional salt ranging from 0.000 10 to 1.0 M NaCl. We were particularly interested in the regime where the UV/vis data changed over from a linear to a nonlinear addition of material per layer, that is, ∼0.3 M, to see if there is in fact a morphological observable that would account for this changeover.

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Figure 4. The root-mean-square roughness of 10-bilayer films of PDDA/PSS deposited from different NaCl concentrations. The error bars represent standard deviations.

Figure 3. AFM topographic images of the morphology of 10bilayer films of PDDA/PSS deposited from 0.1 M (a), 0.3 M (b), and 1.0 M (c) NaCl solutions. Images are 5 µm × 5 µm, and the z scales are as shown.

The results are summarized in Figure 3. Films prepared from solutions with salt concentrations of 0.1 M or less

were flat and featureless with only a slight granular texture, as illustrated in Figure 3a for 0.1 M NaCl. These features were indistinguishable from those of the underlying silica substrate. When the films were prepared from a salt concentration of 0.3 M NaCl, a major change in morphology was observed (Figure 3b). A wormlike or vermiculate pattern emerges, making the surface rough. The bumps were about 15 nm in depth and about 150 nm wide, although these numbers may be an overestimate because of AFM tip convolution effects.26 This type of pattern was seen for all higher salt concentrations, although the pattern became more pronounced: the ridges got deeper, and wormlike features wider. Figure 3c shows the image of a film deposited from 1.00 M NaCl solution; the ridges in this case were ∼50 nm in depth, and the characteristic wormlike features were ∼200 nm wide. The vermiculate morphology was quite robust and did not change with further drying or heating. AFM images of the films were taken as the temperature was increased up to 140 °C, and no change in morphology was seen. The films also retained their morphology after immersion in water or dilute salt solution followed by subsequent drying. Film Roughness. The observed changes in morphology are expected to be reflected in the surface roughness, which we used for quantification. Figure 4 shows a plot of the surface roughness as a function of the concentration of added salt for 10-bilayer films. For low salt concentrations, the roughness remained constant at a small value that is indistinguishable from that of the underlying silica substrate. Films prepared from solutions above 0.010 M NaCl were noticeably rougher, with values increasing almost linearly with salt concentration. Recall that the vermiculate pattern appears at 0.3 M NaCl; the increased surface roughness is a measure of the presence of ridges associated with this structure. Film Thickness. Figure 5a-d shows how the film thickness was measured using the AFM. The darker region in the image corresponds to the substrate, where the film was removed either by a probe tip (Figure 5a) or by a razor blade (Figure 5c). The black rectangular box is where the average cross section was taken, from which one can obtain the step height that corresponds to the film thickness. The average cross section differs from a simple linear cross section in that every point in the cross section is an average of several points within the selected box. Note the presence of high ridges by the step edges. These were the buildup of material that was removed from the (26) Markiewicz, P.; Goh, M. C. Langmuir 1994, 10, 5.

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Figure 5. Thickness measurements are obtained by scratching to remove all the polyelectrolyte multilayer film from an area. These scratches are represented by dark valleys in the AFM height images in (a) and (c). The image in (a), made in 0.1 M salt solution, has a 1 µm2 area cleared by repeated scanning with a contact mode AFM probe. The image in (c), made in 1.0 M salt solution, has an area scratched by hand with a razor blade. In both images, a black border encloses the area where the average cross section was taken. The average cross sections are shown in (b) and (d). Thickness measurements are made at several points. Examples are indicated by black arrows. The high peaks adjacent to the scratched area in (b) and (d) represent the buildup of the polyelectrolyte displaced during scratching. These appear as bright areas in (a) and (c).

substrate, and they should be ignored in the measurement of thickness. Neither the film surface nor the scraped region is perfectly flat, as would be expected; the thickness measurement was thus conducted by taking the average of the measured heights and performed for several regions and samples. This is particularly important for high salt concentrations, when the surface roughness is large (Figure 5c). The results of the thickness measurements for 10bilayer films of PDDA/PSS deposited from various salt concentrations are plotted in Figure 6. As in the case of the surface roughness, the film thickness remains small and approximately constant for salt concentrations below 0.1 M NaCl. The inset graph indicates that the film thickness grows linearly with the concentration of added salt. Layer-by-Layer Morphology. We have noted that under high salt concentrations, a vermiculate pattern exists for 10-bilayer films. The underlying silica substrate, however, is featureless. We thus carried out studies of the thickness and morphology after the deposition of each bilayer to gain a picture of how the vermiculate morphology evolved. Figure 7 shows thickness of the film as a function of the number of bilayers for a salt concentration of 1.0 M. Note that after the third bilayer, there is approximately a linear relationship between thickness and number of layers. This

Figure 6. The thickness of 10-bilayer films of PDDA/PSS made from different NaCl solutions. Error bars are the standard deviation of all the measurements. The logarithmic scale is used to show all data clearly; the inset graph shows the same data on a linear scale.

result is consistent with the view that there is a difference between the first few layers, which have been referred to as “precursor layers”, and later ones. In the linear regime, the slope was 46 nm/bilayer, indicating a bilayer thickness of ∼46 nm. Below the linear regime, that is, for 3 bilayers or less, each bilayer had an average thickness of ∼6 nm.

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Figure 7. Thickness of a (PDDA/PSS)n multilayer film deposited from a 1.0 M NaCl salt solution vs the number of bilayers, n. The error bars represent standard deviations of all the measurements. The dashed line is a fit to the points after n ) 3.

The morphology of these layers were examined using the AFM, and the results are summarized in the series of images in Figure 8. The first and second bilayers are flat and featureless, with a slight granular texture (Figure 8a,b). For the third bilayer, small pores as well as some larger globular features were observed (Figure 8c). By the fourth bilayer, clearly defined structures were evident (Figure 8d). By the fifth bilayer (Figure 8e), the vermiculate morphology seen in Figure 3c was apparent. This morphology remained constant for subsequent layers.

McAloney et al.

longer constant, implying differences from layer to layer, although it is difficult to quantify these results due to the previously mentioned complications in the spectra. The AFM results, however, are unequivocal. The thickness increment as each bilayer is deposited changes drastically around the third bilayer (Figure 7). Moreover, this is accompanied by a major change in the morphology, as seen in the AFM images in Figure 8a-e. For solutions deposited at low salt concentrations, we did not observe a similar transition. It would thus appear that for the precursor layers, which in our studies correspond to ∼3 bilayers, the substrate has a strong effect in the way the polyelectrolytes adsorb. The thickness measurements indicate that the polymer is more or less flattened onto the surface, and the featureless morphology of the film supports this view. For the subsequent layers, we can try to account for the differences in film morphology between those deposited at low and at high salt concentrations by considering the behavior of polyelectrolytes in solution. The conformation of the polyelectrolyte chain in solution is strongly influenced by the presence of other ions, and a transition from an extended conformation to a more compact, globular form is expected. There are many papers discussing solution properties28-30 of polyelectrolytes based on the scaling theory of de Gennes et al.31 We can estimate the salt concentration at which this transition takes place using a simple picture as follows. The distances between monomers, b, in the case of strong polyelectrolytes PSS and PDDA are 2.6 and 5.2 Å, respectively. These are both less than the Bjerrum radius, rB, which is the distance between charges where the electrostatic energy is equal to the thermal energy. In the case of monovalent ions in water at room temperature (295 K),

Discussion In these studies, we have addressed the influence of the salt concentration on the deposition of polyelectrolyte multilayer films. Previous work1,19,23,27 has mentioned that there are precursor layers, which are largely influenced by the substrate and which behave differently than subsequent layers. That there should be differences between the first few and later layers is to be expected: after all, the first layer deposits directly on the substrate and its adsorption is thus governed by the substratepolymer interaction, while a much later layer will not “see” the substrate at all, but only a polymer surface. Where the precursor layer ends is not clear. One possibility is that the surface charge per layer, which drives the adsorption of the next layer, changes eventually from that of the substrate to a “steady-state” value.19 We are, at present, unable to measure surface charges of the layers; instead, we address this issue by conducting morphological and thickness studies as each layer is deposited. Our investigations showed clear evidence for the existence of precursor layers, but only in the presence of high salt concentrations. The UV/vis spectra show a linear relationship between the absorbance intensity and the number of layers for films deposited from solutions of low salt concentration (Figure 2). This means that each layer contains the same amount of material, and therefore the polyelectrolyte layers form uniformly from the first layer onward. On the other hand, for solutions of high (above 0.1 M) salt concentrations, the increment per layer is no (27) Tsukruk, V. V.; Bliznyuk, V. N.; Visser, D.; Campbell, A. L.; Bunning, T. J.; Adams, W. W. Macromolecules 1997, 30, 6615.

rB )

e2 ) 7.2 Å 4π0kT

where 0 is the vacuum dielectric constant, e is the electron charge,  ) 80 is the dielectric constant of water, and T is the temperature. In strong polyelectrolytes, the average distance between ions, l, increases to the value rB due to ion condensation.32 Consider a single polyelectrolyte chain in solution surrounded by ions. The potential falls off with distance due to screening by counterions. The Debye shielding radius, rD, is the distance at which this potential is 1/e of its original value, and is given by

rD )

x

kT

∑i zi2ni

e2

where zi and ni are the charge and number of ions of type i, respectively, and the summation is over the different types of ions present. The Debye radius depends on the concentration of ions. At low salt concentrations, in particular, when rD g l ) rB, the polyelectrolyte charges repel each other causing an extended conformation. When the ion concentration increases such that rD < l ) rB, the charges have been screened by the counterions. We assume (28) Khokhlov, A. R. J. Phys. Chem. A 1980, 13, 979. (29) Muthukumar, M. J. J. Chem. Phys. 1987, 86, 7230. (30) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. Macromolecules 1995, 28, 1859. (31) de Gennes, P.-G. d.; Pincus, P.; Velasco, R. M.; Brochard, F. J. J. Phys. (Paris) 1976, 37, 1461. (32) Manning, G. S. J. Chem. Phys. 1969, 51, 924.

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Figure 8. AFM images of PDDA/PSS films deposited from 1.0 M NaCl solution at different number of bilayers deposited: (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5 bilayers. Images are 5 µm × 5 µm, and the z scales are as shown.

that the collapse of the polyelectrolyte occurs at this point, when the Debye radius (rD) is comparable to the Bjerrum

radius (rB); the salt concentration at which this happens can be estimated. In the case of a monovalent salt solution

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in water, rB ) 7.2 Å, rD ) 3.04/xC Å, where C is the molar salt concentration, the transition will occur at C ) 0.2 M. This rough estimate can account for our observations provided that the polyelectrolyte conformation in solution is more or less retained during its adsorption (for other than the precursor layers), such that changes in the morphology of the adsorbed layer as a function of salt concentration are correlated with the transition from extended to collapsed conformation. Thus, in this picture, the polyelectrolytes will adsorb as coils under conditions of high salt concentration and as extended chains otherwise, resulting in thicker and rougher films for the former in comparison with the latter. Note that within this view, only at high salt concentrations can we observe a difference between the precursor and subsequent layers; under conditions of low salt, the polyelectrolyte will adsorb with extended configurations regardless of layer number. Thus, the fact that we observe no evidence for precursor layers under low salt conditions does not necessarily imply they do not exist but implies simply that measurements of morphology, thickness, or roughness cannot detect differences between precursor and subsequent layers, if any. Assuming that the polyelectrolytes adsorb as coils under high salt conditions, we can proceed to estimate thickness of the layers by looking at the size of these coils. In this regime of small Debye radius, the charges are highly screened, and the polyelectrolyte chain can be considered as similar to an uncharged polymer in good solvent. The radius of gyration, Rg, of polymer coils is given by

Rg )N3/5b/61/2 where N is the number of monomer units and b is the distance between monomers.24 For a PSS chain of molecular weight 500 000, there are ∼2800 monomers/chain, and the segment-to-segment distance is 2.6 Å (estimated using Hyperchem), yielding an estimated radius of ∼12 nm. Similarly, for PDDA (MW ) 500 000), there are 3600 monomer units/chain and a segment-to-segment distance of 5.2 Å (estimated using Hyperchem), for an estimated coil radius of ∼29 nm. A bilayer made up of a stack of PSS and PDDA coils would thus have a maximum thickness of ∼81 nm; it is expected that the thickness will be reduced due to interpenetration between bilayers. Our experimental observation of 46 nm/bilayer (for bilayers beyond the third one) correlates well with this simple estimate. We should point out that our measured bilayer thickness differs from that reported by Dubas and Schlenoff,19 who obtained a result of 27 nm/bilayer for the same system, by using ellipsometry. There are several differences between their experiment and ours that could account for this discrepancy. They use a rotating silicon substrate with shorter dipping times than ours. However, they did show that the rotating disk method produced the same results as the 30 min immersion method. The silicon substrate will offer a different interaction with the polyelectrolyte than the silica we used, but this effect is highly unlikely to affect more than the precursor layers. It is most likely that the difference in our results stems from the way we measure thickness. The AFM measures the total topographic dimension of the film, regardless of the degree of packing. Note that in these thick films, the roughness is substantial. It is also likely that the layers have defects. In both situations, the density and refractive index of the multilayer film may be lower than expected; this would be expected to result in a lower estimate for the film thickness in an ellipsometric measurement.

McAloney et al.

Figure 9. AFM image of a 10-bilayer PDDA/PSS film deposited from 1.0 M NaCl solution, taken while in contact with water using TappingMode.

These thick films generated at high salt concentrations have a very distinct morphology, with intricate wormlike features. This vermiculate morphology has been reported only in the weak polyelectrolyte pair of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH), under conditions where PAA was weakly charged and PAH was nearly fully charged.33 This is entirely different from the situation of our experiments, where such morphology is observed when the PSS and PDDA charges are highly screened. It is possible that the morphology is a result of drying: the thick film collapses when the water evaporates, and it is possible that such collapse can produce a crumpled surface morphology that would appear vermiculated. However, it is also possible the observed pattern can be due to the polyelectrolytes adsorbing not as extended chains but either as coils or as aggregates, presumably having formed such structure from solution. Adsorption of such large structures will give rise to a rough surface consisting of domains with characteristic length scale related to the size of the adsorbing entities. While it is expected that there is interpenetration between these entities after adsorption, total conformational relaxation within the film presumably does not occur to a large extent: note that the glass transition temperatures are quite high, and in addition we observe that the vermiculate surface morphology is still robust at a temperature of 140 °C. To verify that the vermiculate morphology is not simply a result of drying, the film was rehydrated, and AFM imaging was performed with the film in contact with the water. Due to the difficulties with imaging such a soft sample under fluid, the resolution is not as clear as on the dried sample; however, it was clear that the film is far from featureless, as can be seen in Figure 9. We also carried out studies on 10-bilayer films that were prepared without drying between immersions and found that these also possessed a vermiculate morphology upon drying at the very end of the immersion cycles. Thus, we believe that the complex surface morphology does not arise solely as a result of drying. However, the process of drying may serve to highlight and enhance the surface morphology: it is likely that the film while wet has a looser packing, and perhaps traps more water within it, and that upon drying, the heterogeneities become more pronounced as the polymer contracts to expel the water. (33) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017.

Salt Effects on Multilayer Film Morphology

Due to the finite width of the AFM probe tip, we are unable to obtain more accurate measurements of the depth and width of the wormlike domains, but it is clear that they depend on the ionic strength of the solution over the range covered by this work (Figure 3). If we consider the adsorbing entity as the random coils discussed previously, the characteristic length scale should be related to the coil diameter, which, in our simple picture, does not vary with the ionic strength. However, there are two possible scenarios we can consider. First, the adsorbing species could be aggregates of polymer, the size (or aggregation number) of which increases with salt concentration due to screening. Alternatively, the adsorbing species could be a mixture of coils, semi-coils, and extended chains. This latter picture may occur either because the rod-to-coil transition is not very sharp, or because the adsorption process itself (that is, the interaction between surface and adsorbing material) is enough to disrupt some of the coils, flattening them out slightly, or both. In either case, the domains and surface roughness will increase as one gets further away from the transition, that is, with increasing salt concentration. The above arguments show that it is possible to account for our observations in terms of a simple picture of the polyelectrolyte conformation in solution being affected by the concentration of ions and that this conformation is more or less retained upon adsorption (except for the precursor layers, which are strongly influenced by the substrate). A recent paper has proposed a different mechanism for the growth of polyelectrolyte multilayer films, emphasizing the role of charge overcompensation

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and penetration.34 Unfortunately, we are not able to provide a test for such proposed model using our experimental observations of morphological changes. Summary and Conclusions We have utilized atomic force microscopy to study the morphology, thickness, and roughness of polyelectrolyte multilayer films deposited with varying salt concentrations. Significant differences were observed between films formed under low salt concentrations and those produced under high concentrations. These differences can be accounted for by considering a conformational transition from extended rod to globular coil in the bulk polyelectrolyte solution, and we estimate the salt concentration at which this should occur. We can thus interpret our results by assuming that the bulk conformation is retained during the adsorption. At low salt concentrations, the extended rod configuration of the polyelectrolyte adsorbs, producing a thin, flat film. At high salt concentration, the polyelectrolyte adsorbs in the globular coil conformation, producing a vermiculated morphology. Acknowledgment. The authors acknowledge support from the Natural Science and Engineering Research Council of Canada (NSERC). R.M. thanks the Ontario government for an Ontario Graduate Scholarship (OGS). R. Rakhit is also acknowledged for assistance with the experiments. LA010136Q (34) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592.