Roughness and Salt Annealing in a Polyelectrolyte Multilayer

Article pubs.acs.org/Langmuir

Roughness and Salt Annealing in a Polyelectrolyte Multilayer Ramy A. Ghostine,† Rana M. Jisr,‡ Ali Lehaf,† and Joseph B. Schlenoff*,† †

Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States Department of Chemistry and Physical Sciences, West Virginia University Institute of Technology, Montgomery, West Virginia 25136, United States

‡

S Supporting Information *

ABSTRACT: The surface roughness of polyelectrolyte multilayers made from poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrene sulfonate), PSS, was measured as a function of film deposition conditions. For dry multilayers, the significant roughness which builds up for thicker films is much more apparent for multilayers terminated with PSS. Corresponding roughness for PDADMA-capped multilayers may be seen by imaging in situ under electrolyte. Roughness may be substantially reduced, but not eliminated, by annealing in salt. Annealing does not lead to loss of polyelectrolyte from the film, even under conditions where the salt concentration is high enough to place the film properties beyond the glass transition. Roughness does not correlate with the molecular weight of the polyelectrolyte and is thus not caused by solution or film polymer chain conformations. The wavelength of the roughness features is approximately proportional to film thickness, which supports a mechanism whereby roughness is generated by anisotropic swelling due to water and polyelectrolyte addition in a manner similar to water uptake in hydrogels. Roughness is preserved by the glassy PSS layer and probably incorporated within the film as it grows.

■

INTRODUCTION Multilayers of polyelectrolytes assembled using the layer-bylayer alternating adsorption method are now well characterized.1 A broad palette of positive and negative polyelectrolytes has been shown to yield adherent, conformal films on many substrates with architectures that are tunable over some “fuzzy” length scale.2 Although these polyelectrolyte multilayers (PEMUs) are often uniform to the eye, on a microscopic scale roughness is usually observed. For PEMUs grown with a constant thickness increment (“linear” systems or conditions) the surface roughness increases with the overall number of layers or thickness3,4 (as it does in some “exponential” systems4,5). After sufficient thickness, PEMUs appear scattering due to surface topologies on the order of the wavelength of light. While generally observed, surface roughness in PEMUs remains poorly understood. Various explanations have been provided for how the roughness is generated. For example, it has been suggested that phase separations of polyelectrolytes contribute to surface topology.6 Alternatively because roughness is on the nanometer scale, polyelectrolyte conformations (extended versus coiled, flat versus “loopy”) have been invoked as the cause.7−11 For some applications, especially those based on the optical properties of PEMUs, surface roughness is a drawback. Roughness can be largely eliminated postdeposition by “annealing” in a solution of high salt concentration.12,13 Salt enhances the mobility of polyelectrolyte chains that are otherwise “frozen” in place via numerous ion pairs cross-links. These interchain interactions are moderated by salt ions, eventually taking the thin film of polyelectrolyte complex through a glass transition at sufficiently high salt concentration.14 © 2013 American Chemical Society

In this paper we evaluate how roughness is built up using a popular duo of polyelectrolytes: poly(styrene sulfonate) and poly(diallyldimethylammonium chloride). The rate of roughening is a function of both the number of layers and the salt concentration employed for buildup. The effectiveness of annealing in salt is then probed with a view toward minimizing roughness to the lowest possible value. Atomic force microscopy imaging of wet and dry PEMUs suggests a mechanism for how roughness is generated.

■

EXPERIMENTAL SECTION

Poly(4-styrenesulfonic acid), PSS (molecular weight 75 000 g mol−1, 18 wt % in water), poly(diallyldimethylammonium chloride), PDADMAC (molecular weight 400 000−500 000 g mol−1, 20 wt % in water), sodium chloride (NaCl 99.5%), sodium sulfate (Na2SO4), sulfuric acid (H2SO4), and hydrogen peroxide (H2O2) were used as received from Sigma Aldrich. Narrow polydispersity PSS standards with Mw/Mn < 1.1 were used as received from Polymer Source. Singleside-polished Si (100) wafers with 1 in. diameter were from Silicon, Inc. All solutions were prepared using 18 MΩ deionized water (Barnstead, E-pure). Silicon wafers were first cleaned in piranha solution (70:30 H2SO4:H2O2) for 10 min, rinsed thoroughly with water, dried under a gentle stream of nitrogen, exposed to an air plasma cleaner for 1 min, rinsed with water, and dried again with nitrogen. The single-sidepolished Si wafers were mounted, face down, on shafts rotating at 300−600 rpm. Buildup of polyelectrolyte multilayers (PEMUs) on the Si wafers was carried out with the aid of a robot (StratoSequence V, Nanostrata Inc.) which allowed sequential dipping of the rotating Si wafers in the polyelectrolyte and rinsing solutions. Polyelectrolyte Received: April 29, 2013 Revised: August 2, 2013 Published: September 4, 2013 11742

dx.doi.org/10.1021/la401632x | Langmuir 2013, 29, 11742−11750

Langmuir

Article

solutions were 10 mM with respect to the monomer units in NaCl. Polyelectrolyte dipping time was 5 min, and each layer was rinsed for 1 min in three consecutive beakers of water. Rinse and polymer solution volumes were approximately 50 mL each. After final rinsing, each PEMU was dried with N2 and characterized by ellipsometry, static contact angle, and atomic force microscopy (AFM). PEMUs are represented by the following notation (PDADMA/PSS)x, where the subscript “x” denotes the number of bilayers. Some PEMUs were “annealed” in solutions of 1.0 or 1.4 M NaCl at room temperature. After annealing, films were rinsed with water. Dry thicknesses of the PEMU films were measured with a Gaertner Scientific L116B Autogain ellipsometer with 632.8 nm radiation at 70° incident angle using a refractive index of 1.55. Images of the surface of PEMUs were acquired with a MFP-3D AFM (Asylum Research Inc., Santa Barbara, CA), equipped with an ARC2 controller, Igor Pro software, and silicon AC240-TS probes (Olympus, radius = 9 ± 2 nm, height = 14 ± 4 μm on Al-coated cantilevers with a spring constant of 2 N m−1). The ac mode was employed to follow morphological changes, surface roughness, and thicknesses in PEMUs with increasing number of layers built at different salt concentrations. The cantilever was tuned to 10% below its resonance frequency. The scan size was 10 × 10 μm2 (although 5 × 5 μm2 are displayed below), and the scan rate was 1.0 Hz. The roughness, defined as the rms roughness of the surface, was measured on 10 1 × 1 μm2 areas and averaged. The thickness was obtained by taking an image across a scratch in the PEMU and measuring the step height of the film. FT-IR spectra of PEMUs were acquired with a Thermo Avatar 360 equipped with a DTGS detector. PEMU films were assembled on one side of a double-side-polished Si wafer. The background was bare Si wafer, and 100 scans were averaged at 4 cm−1 resolution. All experiments were performed at room temperature (23 ± 2 °C)

forces that might impact the conformation of adsorbing polyelectrolyte, as might be the case for “spin-assisted”17,18 or sprayed19,20 deposition. Experiments showing the thickness for a 20-layer film as a function of salt concentration, Figure 2, reveal an approximately

Figure 2. Thickness of (PDADMA/PSS)10 PEMUs built from 10 mM PDADMA and PSS solutions at various salt concentration on Si wafers.

linear dependence of thickness on salt concentration to 1.0 M NaCl. In contrast with our early report on the growth on a similar system,16 a clear step is seen in growth rate thereafter. In fact, for [NaCl] greater than about 1 M this PDADMA/PSS system is beyond the glass transition, Tg, at room temperature,14 whereupon the polyelectrolytes (especially the PSS) experience enhanced mobility, which allows them to intermingle more extensively on adsorption, leading to thicker layers. For PDADMA/PSS complexes Tg is related to salt concentration and time (frequency, f) by the following empirical equation14

■

RESULTS AND DISCUSSION Buildup of PDADMA-PSS PEMUs. Polyelectrolyte multilayers assembled from PDADMA and PSS have become model systems for demonstrating buildup of PEMUs and the influence of salt on their growth. Figure 1 shows typical buildup of these

Tg = 38 + 2.3 ln f − 20[NaCl]6/5

Surface Roughness Evolution with Number of Layers. The topography of PDADMA/PSS during the course of layerby-layer buildup was evaluated by AFM for separate Si wafers built to the desired thickness. Images were collected on dry PEMUs after each wafer had been rinsed and dried. Examples of images taken of PEMUs with thicknesses of 1−16 layers grown from 0.5 M NaCl are given in the Supporting Information (Figure S1). The trend of steadily increasing roughness versus number of layers, generally seen for multilayer assembly, is shown in Figure 3. A similar increase in the roughness was observed for PEMUs built in 0.1 and 0.25 M NaCl. The ellipsometric thickness showed much less scatter in the 10 data points averaged than did the roughness values

Figure 1. Dry thickness versus number of layers for (PDADMA/ PSS)10 PEMUs deposited on Si wafer from 0.25 (+), 0.50 (△), 0.75 (○), 1.00 (◊), and 1.25 (□) M NaCl solution.

PEMUs from solutions containing salt of various concentrations. Curvature toward the beginning is observed, more so with solutions of high salt concentration, with growth approaching the linear domain after about 14 layers. Buildup is faster as salt concentration increases because salt permits more interdiffusion in the complexation/reaction zone at the film/solution interface.15 The curvature toward the beginning occurs because this reaction zone intersects with, and is limited by, the substrate. Rotating the substrate gently during deposition yields exceptionally uniform films16 but does not create high shear

Figure 3. Roughness (◆, left y-axis) and thickness (●, right axis) of dry PDADMA/PSS multilayers grown on individual Si wafers in 0.5 M NaCl. 11743


Langmuir

Article

because the former method already averages an area of about 100 × 100 μm. Nevertheless, the significant jumps in roughness going from odd (PDADMA-terminated) to even (PSSterminated) films are a consistent feature, which will be discussed below. Surface Roughness of PEMUs Built at Different Ionic Strengths. As shown in Figure 2, construction in higher salt concentration offers an alternative path to thicker films. Thus, 15- and 16-layer PEMUs were assembled from 0.10, 0.50, 1.0, 1.5, and 2.0 M NaCl. The surface topology is compared in pairs (15,16 layers) of AFM images shown in Figure 4A−E. Roughness increases with increasing salt concentration but becomes much more pronounced (for these dry films) for PEMUs capped with PSS (16 layers). Roughness for these 15and 16-layer PEMUs is summarized in Figure 5. Annealing. Figures 4 and 5 also show the effect on topology of immersing the 15- or 16-layer PEMUs in 1.0 M NaCl for an extended period (20 h). The surfaces of the films clearly become smoother on “annealing” in salt.12,13,21−23 The role of salt in unlocking interactions between oppositely charged polyelectrolytes is well known.24−28 We observed enhanced polymer mobility at the surface of PEMUs by scanning PDADMA/PSS films in situ with the AFM as they were exposed to salt solutions.12 Material diffusing from “hills” into “valleys” provided estimates of surface (inter)diffusion coefficients. These findings were followed by studies by McAloney et al. on similar films,13 showing faster annealing for higher salt concentrations. Figure 6 demonstrates an optical method of tracking the kinetics of annealing. A (PDADMA/PSS)22 film of dry thickness 505 nm made on a quartz slide from 1.0 M NaCl was immersed in 1 M NaCl in a 1 × 1 cm silica cuvette, and the UV−vis absorption spectrum was recorded every 5 min. Initial scans showed extensive scattering from the rough surface (steady featureless increase in absorbance toward lower wavelengths). With sufficient time, scattering decreased as the film became smoother. Eventually, the film becomes smooth enough for discernible interference fringes to emerge (Figure 6) Roughness before and after annealing is also shown in Figure 5 for PEMUs grown in various salt concentrations. At a concentration of 1.0 M NaCl is able to fully anneal all PEMUs grown in [NaCl] up to 1.0 M. The residual roughness to this point is less than 3 nm. The level of roughness before annealing in the PSS-capped films is quite high, as shown in Figure 7, which depicts cross sections, including the film thickness, from the images in Figure 4. Minimum Roughness. Figures 4, 5, and 7 show that extended treatment of some as-deposited PEMUs in 1 M NaCl reduces the surface roughness below a minimum of about 3−4 nm. For PEMUs made from >1 M NaCl annealing reduced roughness but not to these minimum values. Annealing for an additional 20 h in 1 M NaCl decreased roughness only slightly (about 10%) for 16-layer films grown from 1 or 1.5 M NaCl. To find out whether the minimum roughness (postannealing) depends on the film thickness a multilayer was grown from 1.0 M NaCl beyond the usual 20-layer limit. Figure 8A shows thickness versus number of layers for every fourth layer (PSS) grown to 60 layers, and Figure 8B depicts the corresponding roughness. Annealing in 1.4 M NaCl, which at room temperature plasticizes PDADMA/PSS past the glass transition,14 proceeds much faster than annealing in 1.0 M NaCl. As seen in Figure 8B

Figure 4. AFM 3D images of (PDADMA/PSS)7PDADMA (15 layers) and (PDADMA/PSS)8 (16 layers) built in 0.10 (A), 0.50 (B), 1.0 (C), 1.5 (D), and 2.0 (E) M NaCl before (15,16) and (15′,16′) after annealing in 1.0 M NaCl for 20 h. X−Y image area 5 × 5 μm.

little additional smoothing of the PEMU is gained by annealing in 1.4 M NaCl after 1.0 M NaCl up to about layer number 40 (ca. 400 nm), after which the additional annealing step does induce additional smoothness (Figure 8B). The relationship between minimum roughness versus film thickness is shown in Figure 9. This minimum, or residual, roughness is probably 11744


Langmuir

Article

Figure 5. Dry rms roughness, before (closed symbols) and after (open symbols) annealing in 1.0 M for 20 h of (PDADMA/PSS)7PDADMA (15 layers) (●;○) and (PDADMA/PSS)8 (16 layers) (▲;△) PEMUs built from 10 mM PDADMA and PSS solutions at various salt concentrations (0.1, 0.5, 1.0, 1.5, and 2.0 M NaCl).

Figure 8. (A) Thickness as a function of the number of layers for PDADMA/PSS PEMUs deposited as multiples of 4 on Si wafers from 1.0 M NaCl salt solution. Slight bump at 300 nm was from switching from ellipsometry to AFM to measure thickness. (B) Dry surface roughness vs layer number (◆) before annealing. (▲) Roughness after annealing in 1.0 M NaCl for 24 h. (●) Roughness after additional annealing in 1.4 M NaCl for 24 h. Figure 6. Absorbance versus wavelength for a (PDADMA/PSS)22 PEMU on a quartz slide immersed in 1.0 M NaCl showing decreasing scattering versus time as the film becomes smoother. Spectra taken every 5 min. Inset suggests no loss of peak area from the PSS absorption peak (λmax at about 230 nm).

Figure 9. Minimum roughness (nm) after annealing in 1.0 and then 1.4 M NaCl (as in Figure 8B) versus film thickness.

this interesting finding we refer to our recent model15 of multilayer growth and morphology for a PEMU below Tg, especially those exhibiting curvature toward the beginning of growth (as seen in all growth curves in Figure 1). After several layers a glassy “skin” of stoichiometric complex is left on top of a growing blanket of softer nonstoichiometric complex.15 This skin should buckle under compression with a frequency and amplitude that eventually becomes independent of the underlying blanket thickness.29 Mechanism for Roughness Generation and Annealing. The mechanism by which roughness is generated is less understood than the mechanism for annealing. Here, we address the latter first. The influence of salt in plasticizing polyelectrolyte complexes, either precipitated from solution30−32 or formed into ultrathin PEMUs,33,34 has been appreciated for some time. The early work of the Moscow State group in demonstrating the role of salt in faciliting exchange between polyelectrolytes in “quasisoluble” nanoparticles of solution-dispersed PECs is noteworthy,35 as is later work by Dautzenberg.25

Figure 7. Cross sections of dry (PDADMA/PSS)7PDADMA (15 layers) and (PDADMA/PSS)8 (16 layers) PEMUs assembled from polyelectrolyte solutions in 0.1, 0.5, 1.0, 1.5, and 2.0 M NaCl (lower to upper), before (left; 15L−16L) and after annealing in 1.0 M NaCl for 20 h (right; 15L′−16L′). Topology comes from the AFM and average thickness from ellipsometry.

caused by dehydration of the PEMU by rinsing in water and losing salt from the bulk prior to drying (loss of water and salt lead to a sudden volume decrease and stresses in the film). Minimum roughness eventually becomes constant. To explain 11745


Langmuir

Article

PDADMAC was employed in all cases. Film thickness (Figure 10A) and roughness (Figure 10B) were recorded for (PDADMA/PSS)22 PEMUs.

Given the similarity of composition between solutionprecipitated complexes and PEMUs,36 the possibility of loss of material from the surface of a PEMU during formation or afterward has been raised.33,37 PEMUs from some combinations of polyelectrolytes, such as poly(acrylic acid) and PDADMA, can be decomposed in solutions of sufficient salt concentration.38 The thickness of other combinations of polyelectrolytes can be “etched” down by salt exposure.39 This raises the question of whether smoothing is due to etching and loss of surface material or whether no polymer is lost and the existing PEMU is truly “annealed” by salt. To verify that polymer was not lost on salt exposure, individual PEMUs were assembled on Si wafers and their ellipsometric thicknesses measured before and after salt annealing. Figure S3, Supporting Information, shows corresponding pre- and postannealing thicknesses for each film thickness up to layer 14 for PEMUs built from 0.10, 0.25, and 0.50 M NaCl. Only a slight loss of measured thickness is observed, which we attribute to loss of a few residual ions and water within the PEMU with annealing, causing small changes in thickness and refractive index. The question of PEC loss was pursued with thicker films made from 1.0 M NaCl and AFM to measure thickness, eliminating refractive index variations as a source of error. In addition, the molecular weight of PSS was systemmatically varied using narrow molecular weight standards of PSS to verify whether molecular weight mismatch, observed previously as a cause of polyelectrolyte loss from the surface of a PEMU,40 might lead to film etching in salt solution. Table S1, Supporting Information, shows that (PDADMA/PSS)22 with PSS molecular weights between 58 × 103 and 2260 × 103 exhibited minimal loss in thickness, even after annealing in 1.4 M NaCl. We were somewhat surprised at the stability of the PEMUs in 1.4 M NaCl (beyond Tg). As a final check, the amounts of each polyelectrolyte were compared with FTIR before and after annealing a 15-layer PEMU. Figure S4 (Supporting Information) shows, within error, no detectable loss of either PDADMA or PSS. We conclude that the thickness changes are slight, probably due to loss of small amounts of ions and water, possibly even smaller amounts of polymer, but that these changes are nowhere near enough to account for the smoothing of features on the PEMU surface. For roughness on the order of the dimensions of a polyelectrolyte molecule in solution it is reasonable to attempt to relate roughness to polyelectrolyte conformation. This approach has been somewhat successful with PEMUs made from polyelectrolytes or conditions which promote thin films, such as low salt concentration. However, there are several pieces of evidence that point away from projecting a solution molecular conformation onto a surface roughness. First, even for PEMUs having roughness on the scale of a molecular coil the roughness increases with film thickness, suggesting a film property, rather than a solution size, is the cause. Second, the scale of roughness features in the x−y plane of the film is much greater than the scale in the z direction (perpendicular). For example, the x−y scale in Figure 4 is compressed over 10 times relative to the z scale. This creates an impression of a mountainous landscape, whereas the features are actually gently undulating on a length scale that is much larger than the dimensions of a polymer coil. To probe the relationship between roughness and polymer dimensions PEMUs were constructed from a series of lowpolydispersity PSS standards. The same broad polydispersity

Figure 10. Upper panel; Thickness versus log Mw for the buildup of (PDADMA-PSS)22 PEMUs from 1.0 M NaCl using PDADMA 400− 500 kDa and varying the PSS molecular weight: 58, 65, 127, 263, 505.1, and 2260 kDa. Lower panel; Surface roughness before (◆) and after annealing in 1.0 (▲) and 1.4 M (●) NaCl versus log Mw of PSS for (PDADMA-PSS)22 PEMUs . Samples were annealed for 24 h.

Film thickness decreased with PSS molecular weight, in contrast to the reported constant, or slight increase in, thickness for a PDADMA/PSS system where the PDADMA molecular weight was varied.41 These results are consistent with a mechanism for polyelectrolyte complexation in a reaction zone near the surface of the PEMU where PSS is the diffusionlimited reagent.15 The plateau at higher MW correlates with a thickness for each layer of about 2 nm, closer to the dimensions of a polyelectrolyte repeat unit than the solution coil size. At this limit it is likely that minimal interdiffusion of polyelectrolytes occurs as PSS forms a stoichiometric, glassy complex with PDADMA at the very surface of the PEMU.42 Such behavior is also inferred with systems that yield thin PEMUs over a wide salt concentration, such as PAH/PSS.43 In Figure 10B the roughness of as-deposited PEMUs follows the opposite trend to that expected if it were generated by solution molecular conformations. Roughness steadily decreases with PSS molecular weight. In addition, the magnitude of the roughness is much greater than the solution molecular dimensions for the lower MW PSS. Annealing in 1.0 M NaCl substantially decreases roughness for all the MW PSS (Figure 10B). For the initially rougher films, additional annealing in 1.4 M NaCl leads to slight further smoothing. Roughness for the higher MW PSS is significantly lower than the solution coil dimensions. Heavily screened in 1.0 M NaCl at room temperature, PSS is somewhat more expanded than it would be in a θ solvent (4.17 M NaCl at 16 °C44). For 263, 505, and 2260 kDa PSS the respective rms radii of gyration (Rg) would be about 8.9, 12.3, and 26 nm in a θ solvent,44 which may be compared with postannealing roughnesses of 4.7, 11746


Langmuir

Article

3.3, and 1.5 nm in Figure 10B. In fact, the solution dimensions of PSS in 1.0 M NaCl are likely to be closer to those in 0.50 M NaCl, determined by Yashiro and Norisuye45 as Rg ≈ 0.01Mw0.6, for respective Rg’s of 18, 26, and 65 nm. The size of the polyelectrolyte coil within the PEMU may be a more relevant parameter. Using neutron scattering we found that the PSS dimension (Rg) within well-annealed stoichiometric PDADMA/PSS complex was slightly more than that under θ conditions46 and given approximately (using only 2 molecular weights) by Rg ≈ 2.6 × 10−3Mw0.72 nm, which gives respective sizes of 21, 33, and 98 nm for the three PSS standards. In typical “antipolyelectrolyte” behavior, these compact coils expanded slightly with additional salt.46 In any case, the Rg’s of these larger PSS molecules are smaller, or considerably smaller, than the roughness. In fact, roughness related to polyelectrolyte chain dimension should increase on annealing as PSS chains adsorbing like flattened “pancakes” expand as they relax into random coils. The fact that the surface remains smooth suggests that the polymer coils at the surface are able to distort to accomplish 1:1 stoichiometry and minimize the interfacial energy that drives smoothing. Comparison of Surface Roughness for Wet and Dry PEMUs. A significant piece of information on the possible mechanism for roughness generation comes from comparison of wet and dry PEMUs. Figure 11 shows AFM images for 30-

and 31-layer PDADMA/PSS grown in 1.0 M NaCl dry and in an aqueous solution (containing 10 mM NaCl). We performed such a comparison previously in attempting to account for substantial differences in surface modulus between PSS- and PDADMA-capped PEMUs when fully hydrated.42 As seen in Figure 11, there is a much greater difference in roughness between wet and dry PDADMA-terminated films than for those terminated with PSS. This result is fully consistent with the widely observed property that such multilayers are more hydrated and softer if they are capped with PDADMA rather than PSS.34,47−49 In a recent study on the counterion content of PDADMA/ PSS multilayers we found that the reason for this odd−even alternation in hydration and (wet) modulus was that surfaces of PEMUs capped with PSS were stoichiometric and glassy (intrinsic compensation) whereas those ending with PDADMA were nonstoichmetric and thus contained counterions (extrinsic compensation) which made the material softer and more hydrated.42 Mechanism of Roughness Generation. Spinodal decomposition, or other types of phase separation, at the surface of PDADMA/PSS multilayers was ruled out when force mapping showed the surface composition, i.e., the ratio of PSS to PDADMA, to be uniform even on rough surfaces.42 A survey of whether roughness evolves with number of layers or with increasing salt concentration shows a general trend: roughness correlates to film thickness, and thicker films tend to be rougher. In other words, the entire film, or film history, rather than just the surface, contributes to the roughness. In addition, the stresses within the film must be anisotropic, since the PEMU is stuck to the substrate and allowed to grow in the z direction only without expanding in the x−y (surface) plane. There are a couple of sources of volume expansion which must be accommodated during multilayer buildup. First, additional polyelectrolyte adds to a surface layer. PSS and PDADMA do not add symmetrically: PDADMA overcompensates the surface, whereas PSS compensates almost exactly the excess PDADMA toward the surface but leaves some excess PDADMA behind in the bulk of the PEMU.42 Second, water cycles in and out of the multilayer accompanying the additional counterions (Cl−) which overcompensating PDADMA requires for electrical neutrality. The additional water brought in by PDADMA is clearly seen in the highly swollen (compared to dry) features in Figure 11. The full magnitude of wet PDADMA roughness is not observed when the PEMU is dried because the swollen layer collapses as it loses water. Addition of a PSS layer (11A, wet) appears to have preserved the large PDADMA features. Because the PSScapped film is less hydrated and more glassy the roughness remains when the film is dried. The PDADMA-terminated surfaces are more hydrated and gel-like.47 Drying and rehydration of traditional hydrogels has been a topic of research for many years. As gels swell, authors have noted surface deformations variously reported as buckling, wrinkling, or creasing: all relieve internal stresses from solvent swelling on constrained, cross-linked polymers.50 Tanaka et al.51 analyzed the patterns on swelling polyacrylamide gels to which acrylate functionality had been added to boost water uptake. Films of acrylamide/acrylate gel were explored by Trujillo et al.,52 and formation of surface instabilities was found to follow theoretical predictions. Ultrathin films of poly(Nisopropylacrylamide) gels exhibited instabilities on swelling with wavelengths that were proportional to film thickness.53

Figure 11. (A) Dry and wet AFM 5 × 5 μm images of (PDADMA/ PSS)15, with 3-D representation, respective roughnesses 86 and 98 nm. (B) Dry and wet AFM image of (PDADMA/PSS)15PDADMA, with 3D representation, respective roughnesses 25 and 83 nm. These films were prepared in 1.0 M NaCl and imaged in 10 mM NaCl. Topographies on the PSS-capped film appear similar, wet or dry, whereas those on the dry PDADMA-capped film appear collapsed in comparison to their wet counterparts. 11747


Langmuir

Article

responds well to added salt, whereas the stronger binding PSS/PAH pair12 does not, producing thinner films. In addition, the PDADMA/PSS system is strongly asymmetric with respect to the overcompensation of PSS and PDADMA.42 Second, systems which provide thick “exponentially” growing films due to internal polyelectrolyte mobility may reach a limit where they are able to flow during assembly, producing smooth films,56 in contrast to other exponential films.4,5 In other words, the time scale for assembly must be shorter than that for relaxation of roughness, which, for example, permits observation of annealing by 1.0 M NaCl of roughness of PEMUs produced in 1.0 M NaCl. Exponential films, and the ones in Figure 4 grown in 1.5 or 2.0 M NaCl, past the glass transition, are still able to produce roughness, even though long-term annealing makes them smoother, because the time allowed for each layer is only a few minutes. Third, there is a clear subset of responsive multilayers, sensitive to other environmental stimuli such as temperature or pH change, which experience phase separation57 or dewetting58 following the stimulus.

Roughness appears and grows in PEMUs regardless of whether the films are dried after each layer. As mentioned before, PDADMA addition is the main cause of swelling. A general finding with swelling-induced gel film deformations is that the average separation between surface features is proportional to film thickness.51 Several measurements of feature separations (peak to peak) were made on the images in Figure 4 (zoom-ins, not shown, were employed for the thinner films). Figure 12 shows the relation, as a log−log plot, between

■

CONCLUSIONS Roughness in polyelectrolyte multilayers is another manifestation of the “fuzzy” distribution of their components. Roughness is not a severe drawback for many applications because it exists mainly on the nanometer scale and does not disrupt the film, coating, or barrier properties of PEMUs. The periodicity of surface features in PDADMA/PSS multilayers, thought to be generated by swelling of anisotropically constrained films, approximately follows the same proportionality to film thickness observed for (the more elastic) crosslinked hydrogels. Unlike hydrogels, internal stress in PEMUs can be relieved by annealing in NaCl, diminishing roughness.

Figure 12. Average distance (nm) between neighboring surface features on the 16-layer PEMUs from Figure 4 vs film thickness (nm).

such separations and the film thickness. The slope (0.93) is close to linear (1.00). We were not necessarily expecting close agreement to the predicted linear scaling, as the mechanical behavior of PEMUs does not match a true hydrogel, where a strong, frequency-independent elastic response (constant storage modulus, G′) is typically observed with a much weaker loss modulus (G″ corresponding to a viscous response). A significant viscous component is, in fact, observed in PEMUs from frequency-dependent modulii54 or nanoindentation at different velocities.42 At longer times viscous flow should vanish, leaving a time-independent or “equilibrium” modulus. The cross-sectional symmetry (Figure 7) does not have the shape of a cusp or crease seen in hydrogels,52 being rather more undulating in form. From the viscous response of polyelectrolyte complexes the surface features are possibly better described as “damped” creases, where a small amount of flow is allowed in the most stressed regions. Positive and negative layers should have different roles in propagating surface roughness. While roughness is created by swelling on PDADMA adsorption, it is preserved (for imaging when dry) by addition of PSS, which yields a glassy material. The PSS layer is relatively immobile, yielding fuzzy strata2 within the PEMU,21 but the internal roughness between strata increases41,55 as a consequence of the external roughness, which is translated into the bulk as the multilayer builds. A couple of additional features of interest are seen in the AFM images. First, the dry PDADMA layer (Figure 4) is not featureless, a residual pattern of cusps on the surface may still be discerned. Second, in situ images of swollen PDADMA show evidence of a prior history, with smaller features on top of larger ones. Images with more detail are presented in the Supporting Information (Figure S5). Generality. To what extent do the results and interpretations above for PDADMA/PSS apply to other polyelectrolyte multilayers? This question must be answered taking into consideration at least three points. First, polyelectrolyte combinations or conditions which promote thin, glassy films should yield less roughness. The PDADMA/PSS system

■

ASSOCIATED CONTENT

* Supporting Information S

AFM images showing increasing roughness as a function of number of layers and scratch over one film; thickness of PEMUs deposited from 0.10, 0.25, and 0.5 M NaCl before and after annealing; thickness of 44-layer PEMUs after annealing in NaCl; FTIR of 15-layer PEMU before and after annealing; zoom-in of AFM of 15-layer surface under electrolyte. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by grants DMR-0939850 and DMR1207188 from the National Science Foundation. REFERENCES

(1) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials, 2nd ed.; Wiley-VCH: Weinheim, 2012. (2) Decher, G. Fuzzy nanoassemblies: Toward layered polymeric multicomposites. Science 1997, 277, 1232−1237. (3) Schmitt, J.; Grünewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lösche, M. Internal Structure of Layer-by-Layer Adsorbed Polyelec11748


Langmuir

Article

trolyte Films - a Neutron and X-Ray Reflectivity Study. Macromolecules 1993, 26, 7058−7063. (4) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Comparison of the Structure of Polyelectrolyte Multilayer Films Exhibiting a Linear and an Exponential Growth Regime: An in Situ Atomic Force Microscopy Study. Macromolecules 2002, 35, 4458−4465. (5) Kujawa, P.; Moraille, P.; Sanchez, J.; Badia, A.; Winnik, F. M. Effect of molecular weight on the exponential growth and morphology of hyaluronan/chitosan multilayers: A surface plasmon resonance spectroscopy and atomic force microscopy investigation. J. Am. Chem. Soc. 2005, 127, 9224−9234. (6) Cornelsen, M.; Helm, C. A.; Block, S. Destabilization of Polyelectrolyte Multilayers Formed at Different Temperatures and Ion Concentrations. Macromolecules 2010, 43, 4300−4309. (7) Kim, D. K.; Han, S. W.; Kim, C. H.; Hong, J. D.; Kim, K. Morphology of multilayers assembled by electrostatic attraction of oppositely charged model polyelectrolytes. Thin Solid Films 1999, 350, 153−160. (8) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Atomic force microscopy studies of salt effects on polyelectrolyte multilayer film morphology. Langmuir 2001, 17, 6655−6663. (9) Shiratori, S. S.; Rubner, M. F. pH-dependent thickness behavior of sequentially adsorbed layers of weak polyelectrolytes. Macromolecules 2000, 33, 4213−4219. (10) Wong, J. E.; Zastrow, H.; Jaeger, W.; von Klitzing, R. Specific Ion versus Electrostatic Effects on the Construction of Polyelectrolyte Multilayers. Langmuir 2009, 25, 14061−14070. (11) Buron, C. C.; Filiâtre, C.; Membrey, F.; Bainier, C.; Buisson, L.; Charraut, D.; Foissy, A. Surface morphology and thickness of a multilayer film composed of strong and weak polyelectrolytes: Effect of the number of adsorbed layers, concentration and type of salts. Thin Solid Films 2009, 517, 2611−2617. (12) Dubas, S. T.; Schlenoff, J. B. Swelling and smoothing of polyelectrolyte multilayers by salt. Langmuir 2001, 17, 7725−7727. (13) McAloney, R. A.; Dudnik, V.; Goh, M. C. Kinetics of saltinduced annealing of a polyelectrolyte multilayer film morphology. Langmuir 2003, 19, 3947−3952. (14) Shamoun, R. F.; Hariri, H. H.; Ghostine, R. A.; Schlenoff, J. B. Thermal Transformations in Extruded Saloplastic Polyelectrolyte Complexes. Macromolecules 2012, 45, 9759−9767. (15) Ghostine, R. A.; Markarian, M. Z.; Schlenoff, J. B. Asymmetric Growth in Polyelectrolyte Multilayers. J. Am. Chem. Soc. 2013, 135, 7636−7646. (16) Dubas, S. T.; Schlenoff, J. B. Factors controlling the growth of polyelectrolyte multilayers. Macromolecules 1999, 32, 8153−8160. (17) Cho, J.; Char, K.; Hong, J. D.; Lee, K. B. Fabrication of Highly Ordered Multilayer Films Using a Spin Self-Assembly Method. Adv. Mater. 2001, 13, 1076−1078. (18) Chiarelli, P. A.; Johal, M. S.; Casson, J. L.; Roberts, J. B.; Robinson, J. M.; Wang, H. L. Controlled Fabrication of Polyelectrolyte Multilayer Thin Films Using Spin-Assembly. Adv. Mater. 2001, 13, 1167−1171. (19) Schlenoff, J. B.; Dubas, S. T.; Farhat, T. Sprayed polyelectrolyte multilayers. Langmuir 2000, 16, 9968−9969. (20) Kolasinska, M.; Krastev, R.; Gutberlet, T.; Warszynski, P. Layerby-Layer Deposition of Polyelectrolytes. Dipping versus Spraying. Langmuir 2009, 25, 1224−1232. (21) Jomaa, H. W.; Schlenoff, J. B. Salt-induced polyelectrolyte interdiffusion in multilayered films: A neutron reflectivity study. Macromolecules 2005, 38, 8473−8480. (22) Krebs, T.; Tan, H. L.; Andersson, G.; Morgner, H.; Gregory Van Patten, P. Increased layer interdiffusion in polyelectrolyte films upon annealing in water and aqueous salt solutions. Phys. Chem. Chem. Phys. 2006, 8, 5462−5468. (23) Nolte, A. J.; Chung, J. Y.; Walker, M. L.; Stafford, C. M. In situ Adhesion Measurements Utilizing Layer-by-Layer Functionalized Surfaces. ACS Appl. Mater. Interfaces 2009, 1, 373−380.

(24) Dautzenberg, H.; Jaeger, W.; Kötz, J.; Philipp, B.; Seidel, C. H.; Stscherbina, D. Polyelectrolytes: Formation, Characterization and Applications; Hanser: Munich, 1994. (25) Dautzenberg, H. Polyelectrolyte complex formation in highly aggregating systems. 1. Effect of salt: Polyelectrolyte complex formation in the presence of NaCl. Macromolecules 1997, 30, 7810− 7815. (26) Dautzenberg, H.; Jaeger, W. Effect of charge density on the formation and salt stability of polyelectrolyte complexes. Macromol. Chem. Phys. 2002, 203, 2095−2102. (27) Izumrudov, V.; Kharlampieva, E.; Sukhishvili, S. A. Salt-induced multilayer growth: Correlation with phase separation in solution. Macromolecules 2004, 37, 8400−8406. (28) Guzmán, E.; Ritacco, H.; Rubio, J. E. F.; Rubio, R. G.; Ortega, F. Salt-induced changes in the growth of polyelectrolyte layers of poly(diallyl-dimethylammonium chloride) and poly(4-styrene sulfonate of sodium). Soft Matter 2009, 5, 2130−2142. (29) Nolte, A. J.; Rubner, M. F.; Cohen, R. E. Determining the Young’s Modulus of Polyelectrolyte Multilayer Films via StressInduced Mechanical Buckling Instabilities. Macromolecules 2005, 38, 5367−5370. (30) Michaels, A. S. Polyelectrolyte complexes. J. Ind. Eng. Chem. 1965, 57, 32−40. (31) Bixler, H. J.; Michaels, A. S. Polyelectrolyte complexes. Encycl. Polym. Sci. Technol. 1969, 10, 765−80. (32) Yano, O.; Wada, Y. J. Appl. Polym. Sci. 1980, 25, 1723. (33) Kovačević, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Kinetics of Formation and Dissolution of Weak Polyelectrolyte Multilayers: Role of Salt and Free Polyions. Langmuir 2002, 18, 5607− 5612. (34) Salomäki, M.; Laiho, T.; Kankare, J. Counteranion-Controlled Properties of Polyelectrolyte Multilayers. Macromolecules 2004, 37, 9585−9590. (35) Bakeev, K. N.; Izumrudov, V. A.; Kuchanov, S. I.; Zezin, A. B.; Kabanov, V. A. Kinetics and mechanism of interpolyelectrolyte exchange and addition-reactions. Macromolecules 1992, 25, 4249− 4254. (36) Sukhishvili, S. A.; Kharlampieva, E.; Izumrudov, V. Where polyelectrolyte multilayers and polyelectrolyte complexes meet. Macromolecules 2006, 39, 8873−8881. (37) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J.; Böhmer, M. R. Formation and stability of multilayers of polyelectrolytes. Langmuir 1996, 12, 3675−3681. (38) Dubas, S. T.; Schlenoff, J. B. Polyelectrolyte multilayers containing a weak polyacid: Construction and deconstruction. Macromolecules 2001, 34, 3736−3740. (39) Nolte, A. J.; Takane, N.; Hindman, E.; Gaynor, W.; Rubner, M. F.; Cohen, R. E. Thin film thickness gradients and spatial patterning via salt etching of polyelectrolyte multilayers. Macromolecules 2007, 40, 5479−5486. (40) Sui, Z. J.; Salloum, D.; Schlenoff, J. B. Effect of molecular weight on the construction of polyelectrolyte multilayers: Stripping versus sticking. Langmuir 2003, 19, 2491−2495. (41) Soltwedel, O.; Nestler, P.; Neunnann, H. G.; Paßvogel, M.; Köhler, R.; Helm, C. A. Influence of Polycation (PDADMAC) Weight on Vertical Diffusion within Polyelectrolyte Multilayers during Film Formation and Postpreparation Treatment. Macromolecules 2012, 45, 7995−8004. (42) Lehaf, A. M.; Hariri, H. H.; Schlenoff, J. B. Homogeneity, Modulus, and Viscoelasticity of Polyelectrolyte Multilayers by Nanoindentation: Refining the Buildup Mechanism. Langmuir 2012, 28, 6348−6355. (43) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. In situ determination of the structural properties of initially deposited polyelectrolyte multilayers. Langmuir 2000, 16, 1249−1255. (44) Hirose, E.; Iwamoto, Y.; Norisuye, T. Chain Stiffness and Excluded-Volume Effects in Sodium Poly(styrenesulfonate) Solutions at High Ionic Strength. Macromolecules 1999, 32, 8629−8634. 11749


Langmuir

Article

(45) Yashiro, J.; Norisuye, T. Excluded-volume effects on the chain dimensions and transport coefficients of sodium poly(styrene sulfonate) in aqueous sodium chloride. J. Polym. Sci. B: Polym. Phys. 2002, 40, 2728−2735. (46) Markarian, M. Z.; Hariri, H. H.; Reisch, A.; Urban, V. S.; Schlenoff, J. B. A Small-Angle Neutron Scattering Study of the Equilibrium Conformation of Polyelectrolytes in Stoichiometric Saloplastic Polyelectrolyte Complexes. Macromolecules 2012, 45, 1016−1024. (47) Schönhoff, M.; Ball, V.; Bausch, A. R.; Dejugnat, C.; Delorme, N.; Glinel, K.; Von Klitzing, R.; Steitz, R. Hydration and internal properties of polyelectrolyte multilayers. Colloids Surf. A 2007, 303, 14−29. (48) Ramos, J. J. I.; Stahl, S.; Richter, R. P.; Moya, S. E. Water Content and Buildup of Poly(diallyldimethylammonium chloride)/ Poly(sodium 4-styrenesulfonate) and Poly(allylamine hydrochloride)/ Poly(sodium 4-styrenesulfonate) Polyelectrolyte Multilayers Studied by an in Situ Combination of a Quartz Crystal Microbalance with Dissipation Monitoring and Spectroscopic Ellipsometry. Macromolecules 2010, 43, 9063−9070. (49) Schwarz, B.; Schönhoff, M. Surface Potential Driven Swelling of Polyelectrolyte Multilayers. Langmuir 2002, 18, 2964−2966. (50) Cerda, E.; Mahadevan, L., Geometry and physics of wrinkling. Phys. Rev. Lett. 2003, 90 (7), 074302. (51) Tanaka, T.; Sun, S. T.; Hirokawa, Y.; Katayama, S.; Kucera, J.; Hirose, Y.; Amiya, T. Mechanical Instability of Gels at the PhaseTransition. Nature 1987, 325, 796−798. (52) Trujillo, V.; Kim, J.; Hayward, R. C. Creasing instability of surface-attached hydrogels. Soft Matter 2008, 4, 564−569. (53) Ortiz, O.; Vidyasagar, A.; Wang, J.; Toomey, R. Surface Instabilities in Ultrathin, Cross-Linked Poly(N-isopropylacrylamide) Coatings. Langmuir 2010, 26, 17489−17494. (54) Jaber, J. A.; Schlenoff, J. B. Dynamic viscoelasticity in polyelectrolyte multilayers: Nanodamping. Chem. Mater. 2006, 18, 5768−5773. (55) Soltwedel, O.; Ivanova, O.; Nestler, P.; Müller, M.; Köhler, R.; Helm, C. A. Interdiffusion in Polyelectrolyte Multi layers. Macromolecules 2010, 43, 7288−7293. (56) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C. Buildup Mechanism for Poly(l-lysine)/ Hyaluronic Acid Films onto a Solid Surface. Langmuir 2001, 17, 7414−7424. (57) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Fabrication of microporous thin films from polyelectrolyte multilayers. Langmuir 2000, 16, 5017−5023. (58) Zhang, L.; Zheng, M.; Liu, X.; Sun, J. Layer-by-Layer Assembly of Salt-Containing Polyelectrolyte Complexes for the Fabrication of Dewetting-Induced Porous Coatings. Langmuir 2010, 27, 1346−1352.

11750


Roughness and Salt Annealing in a Polyelectrolyte Multilayer

Recommend Documents