Article pubs.acs.org/Langmuir
Flipped Polyelectrolyte Multilayer Films: Accessing the Buried Interface Yara E. Ghoussoub and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States S Supporting Information *
ABSTRACT: Little is known concerning the interface between a polyelectrolyte multilayer, PEMU, and its substrate. Recent models suggest that excess polymer charge, compensated by counterions, remains buried within the PEMU, especially for thicker films having a nonlinear component to their growth. We report a novel approach for making freestanding multilayers of poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS): after assembly on aluminum substrates, films were released by brief immersion in aqueous alkali. The multilayers were then flipped, allowing access to the initially buried substrate/PEMU interface. Experiments were performed to show that this method of release, one of many established for PEMUs, perturbed the surface and bulk of the film minimally. Film/solution and film/substrate interfaces were compared using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to record topography and perform nanoindentation, while XPS provided surface elemental composition. All three methods revealed data consistent with an excess of PDADMA at the buried interface. This excess PDADMA was then complexed with additional PSS to yield “nanosandwiches” of nonstoichiometric PEMU between layers of stoichiometric PEMU.
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INTRODUCTION Ultrathin polyelectrolyte films with tunable properties can be assembled on various substrates using the layer-by-layer adsorption technique.1 Polyelectrolytes of opposite charge are alternately (ad)sorbed onto the substrate, one layer at a time, to form a so-called “polyelectrolyte multilayer” (PEMU).1,2 Charge balance is conserved within the multilayer by ion pairing of polyelectrolyte repeat units (“intrinsic compensation”) and by small counterions inside the film (“extrinsic compensation”),3 where these balance an excess of polymer charge within the multilayer. It has been shown that the growth of multilayers is affected by the salt concentration,4,5 solution pH,5−9 polymer concentration,10 and temperature.11 Much is known about the external surface of PEMUs, which is usually in contact with air or aqueous solutions. This interface has been routinely examined by atomic force microscopy (AFM) to establish roughness12 and whether phase separation, often yielding nanopores, has occurred.12−14 Film growth mechanisms are inferred from such topological measurements. X-ray photoelectron spectroscopy (XPS) provides information on the composition of the first few nanometers of the external interface.15−17 Electrokinetic potential measurements yield the sign of the surface charge,18 and exchange with radiolabeled ions16 provides the fixed charge density. Far less is known about the interface between the substrate and the multilayerthe buried interface. After the first few layers of polyelectrolyte have been adsorbed this interface © 2015 American Chemical Society
becomes inaccessible to many surface characterization techniques. Because many applications, such as control of cell adhesion,19 are assumed to depend critically on the external surface, gathering information on the buried interface has not been a pressing priority. However, for a fundamental understanding of the mechanism of multilayer growth the composition throughout the film is needed. In addition, information on the composition through the film is essential to understanding and predicting separations of PEMUs used as selective membranes.20,21 Until recently it was generally assumed that polyelectrolytes were well paired within the bulk of the film and deviation from 1:1 stoichiometry of positive and negative polyelectrolyte repeat units was seen mainly at the surface.1 “Overcompensation” of surface polymer is a key feature of the growth of polyelectrolyte multilayers.1 We demonstrated that overcompensation in a popular pair of polyelectrolytes, poly(styrenesulfonate), PSS, and poly(diallyldimethylammonium), PDADMA, only occurred for the PDADMA “layer” and the PSS layer yielded a stoichiometric, glassy surface.16 After a few layers, a zone of excess PDADMA builds up within the bulk of the PEMU. A model was presented showing that this excess PDADMA extended down to the substrate, but no direct measurements were made near the Received: March 16, 2015 Revised: April 16, 2015 Published: April 30, 2015 5078
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substrate.16 Some indication of vertical composition has come from neutron reflectometry results. For example, PEMUs made from PSS and poly(allylamine) hydrochloride, PAH, have been modeled with an excess of both cations and anions,22 but each model has not been able to definitively exclude others because the fit is never perfect. In our experience, the best indicators of polyelectrolyte nonstoichiometry at the surface have been obtained with AFM and XPS. Nanoindentation with the AFM shows that, when hydrated, nonstoichiometric surfaces are much softer (due to extra hydration from the ions and lower ionic cross-linking between polyelectrolytes) than those that are stoichiometric.23 Related to the mechanical properties, PEMUs with stoichiometric surfaces were rougher when dry since highly hydrated material collapses and planarizes the surface.12 XPS measures the excess of one polyelectrolyte over another directly. We were therefore interested in removing a PEMU from a substrate, flipping it over, and employing these techniques to see whether the buried interface was stoichiometric in polyelectrolytes or whether it had an excess of polycation, as our model predicted. Most fabrication techniques yield PEMUs that are firmly adhered to their substrates. Several approaches have been developed for applications which need detached or freestanding (free, free-floating) films. Suspended membranes can be good candidates for sensors, micromechanical devices,24−26 and filtration and catalysis membranes.26−28 Methods for making free-standing layer-by-layer assembled films include dissolving the underlying substrate,28,29 direct peeling from a surface,30−33 and dissolving a sacrificial layer between the film and the underlying substrate.25,34 Sacrificial layers are composed of stimuli-responsive materials which might be disassembled by a change of salt concentration,35 pH,35−37 solvent,38−40 light,41 or temperature.42,43 We have demonstrated the release of PEMU films using a sacrificial layer of poly(acrylic acid) PAA/PDADMA, which can be selectively decomposed by a change of salt concentration or pH.35 Similarly, Mamedov et al. isolated multilayer films by dissolving an acetyl cellulose layer in acetone.38 However, these approaches potentially alter the properties of the PEMU or leave residues of the sacrificial layer on the buried interface. For example, the release of poly(L-lysine)/hyaluronic acid multilayers from polystyrene substrates in THF yields micron-sized pores in the isolated films.28 Microporous films of PAA and PAH were also obtained by a pH-induced phase separation.13 One possible mechanism for this morphology change is the breakage of ionic bonding due to their protonation at low pH. Stimuli-responsive materials were also used to control the release of various agents from multilayers by the integration of functionalities that can be degraded by hydrolysis,44,45 enzymes,46,47 or reduction.48,49 In addition to planar substrates, polyelectrolytes can also be adsorbed on particles.50,51 The controlled decomposition of template particles yields hollow polyelectrolyte microshells. Alternatively, free-standing films can be obtained without the aid of substrates at the air−water interface as exemplified by cast films52 and cross-linked Langmuir films.24 Our study of the buried interface required a method of releasing a PEMU with minimal perturbation of structure and composition. In the present work we develop an optimized release method then analyze the properties of the buried interface.
Article
EXPERIMENTAL SECTION
Poly(4-styrenesulfonic acid) (PSS, molar mass = 7.5 × 104 g mol−1), poly(diallyldimethylammonium chloride) (PDADMAC, molar mass = 40 × 104−50 × 104 g mol−1), sodium chloride, sodium nitrate, sodium hydroxide (0.1 M and 1.0 M), hydrochloric acid (0.1 and 1 M), sulfuric acid (98%), and hydrogen peroxide (30%) were purchased from Sigma-Aldrich and used without further purification. All solutions were prepared using deionized water with a resistance of 18 MΩ (Barnstead, E-pure). Multilayer Assembly. Multilayer films of PDADMA and PSS were assembled on silicon and aluminum substrates using a robot (StratoSequence V, nanoStrata Inc.). Prior to film buildup, silicon wafers were cleaned with “piranha” (70% H2SO4 and 30% H2O2) for 10 min, rinsed extensively with water, and dried with a stream of nitrogen gas. The substrates were mounted face down on the robot shafts rotating at 300 rpm. Polyelectrolyte solutions were 10 mM with respect to the repeat unit, prepared in 1 M NaCl. The duration for each dipping step in the polymer solution was 5 min followed by three 1 min rinsing steps in water. After final rinsing, the multilayer was dried under a smooth stream of N2. The multilayers assembled on aluminum were briefly soaked in 0.1 M NaOH, which prompted the release of the films from the underlying substrate. Just prior to complete detachment, the substrate was transferred to water. After the films detached they were recovered on a clean silicon wafer either “face up” or “flipped”. In the former, the PEMU was gently lifted with the aid of tweezers and recovered “face up”. In the latter, the PEMU was turned over and recovered “flipped”, where the bottom side that was in contact with the aluminum became the upper side. Force Microscopy. The effective Young’s modulus of untreated and released PEMUs was determined by nanoindentation using an MFP-3D AFM equipped with an ARC2 controller (Asylum Research Inc., Santa Barbara, CA). The data were analyzed using Igor Pro software. The optical lever sensitivity (OLP) and the spring constant of the AC240-TS tips (around 2 N m−1) were first determined in air by the thermal fluctuation technique. After the immersion of the tip in 0.1 M NaCl, the OLP was recalibrated on a glass surface in the salt solution. Under the force mapping mode, force indentation measurements were performed on the samples immersed in NaCl, with a tip velocity of 500 nm s−1 and a scan range of 20 × 20 μm. The modulus was obtained by averaging 4 × 5 points force maps. The applied force and the resulting indentation are given by eqs 1 and 2: Fapplied = Kd = K (z − δ)
(1)
δ=z−d
(2)
where K is the spring constant of the cantilever, d is the deflection of the tip, z is the distance of the tip relative to the surface of the sample in the z-direction, and δ is the indentation of the tip into the multilayer (50 nm for PDADMA capped films and 7 nm for PSS capped films). Using Hertzian contact mechanics, force curves were analyzed based on tip geometry. Force vs indendation for a conical tip is represented by eq 3
Fcone =
Esurface 2 tan(α)(z − d)2 2 π (1 − υsurface )
(3)
where E is the effective modulus of the surface, υ is the Poisson ratio of the material, α is the half-angle of the cone (18°), and R is the radius of the indenter (10 nm). The fitted data yielded the convoluted modulus of the multilayer (EC). This value was then converted to Young’s modulus using eq 4 −1 ⎛1 − υ 2 1 − υ2 2 ⎞ 1 ⎟ + EC = ⎜ E2 ⎠ ⎝ E1
(4)
where υ1 is the Poisson ratio of the indented material set to 0.5, υ2 is the Poisson ratio of the indenter fixed at 0.27, E1 is the Young’s modulus of the indented material, and E2 is the Young’s modulus of the indenter (silicon) set to 150 GPa. 5079
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Langmuir Imaging. Topography of the films was obtained from 5 × 5 μm AFM images. NCHV tips from Veeco were used in the AC (intermittent contact) mode with a scan rate of 0.5 Hz. Roughnesses were obtained from several 5 × 5 μm areas at different positions and averaged. SEM imaging was performed using a JEOL 5900 operating at 30 kV. Contrast was obtained by detection of both secondary and backscattered electrons. Samples were sputter-coated with a thin Au film. FTIR Spectroscopy. The chemical composition of the multilayers was explored on double-side-polished Si 100 wafers (thickness = 381 ± 25 μm) using transmission FTIR spectroscopy. Spectra were acquired with a nitrogen-purged Thermo Avatar 360 FTIR equipped with a DTGS detector. For the untreated controls and the released films, silicon was used as substrate with bare silicon as a reference. The average of 100 scans was taken with a resolution of 4 cm−1. X-ray Photoelectron Spectroscopy. A PerkinElmer (PHI) 5100 XPS with a noncollimated Mg Kα X-ray source (hν = 1253.6 eV) was used for surface elemental analysis at a base pressure of 1.9 × 10−8 Torr. Data acquisition was performed at a takeoff angle of 45°, a pass energy of 89.45 eV, and a speed of 0.5 eV s−1. For each measurement, a total of 10 scans were averaged. Database 71 software (NIST) was used to determine inelastic mean free paths (IMFPs).
Scheme 1. Cartoon of (a) Al/(PDADMA/PSS)20 Multilayer, (b) (PDADMA/PSS)20 after Release “Face Up”, and (c) (PDADMA/PSS)20 after Release “Flipped”a
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RESULTS AND DISCUSSION The main purpose of the present work was to characterize the buried interface of PDADMA/PSS multilayers, that is, the substrate side of the multilayer. To this end, different approaches were tested for the release of 40-layer films. A method that we reported earlier yielded isolated films but left traces of the sacrificial layer after recovery.35 A sacrificial layer, made of PAA and PDADMA, was dissociated at pH > 6 due to the ionization of the PAA carboxylic acid units. FTIR transmission spectra showed that approximately 13% of the carboxylic acid remained in the free membrane. As an alternative approach, a thin layer of 30 nm of silicon dioxide was grown on Si wafers by heating them at 900 °C for 2 h. Multilayers deposited on these wafers were released by exposure to HF, which dissolves SiO2. However, the release was inconsistent at relatively low HF concentrations and the multilayers partially decomposed at higher HF concentrations. Another route involved the buildup of PEMUs on aluminum substrates. While the use of acid to dissolve a layer of aluminum did not yield satisfactory results, the exposure of the multilayers to alkali solutions provided the most efficient procedure for the formation of free membranes. The bubbles formed by gentle dissolution of the surface of the aluminum (2Al(s) + 2NaOH(aq) + 6H2O → 2Na +(aq) + 2[Al(OH)4]− + 3H2(g)) helped float the films off the substrate. In the following, multilayers will be denoted as substrate/ (PDADMA/PSS)n, where n is the number of bilayers. A 40layer PDADMA/PSS film assembled on an aluminum substrate ((Al/PDADMA/PSS)20) could be quickly released (within 45 s) in 0.1 M NaOHaq and recovered on a clean silicon wafer either “face up” or “flipped” (see Scheme 1). For the “flipped” multilayers, the face that was initially in contact with the substrate became the upper face, allowing direct access. The method of release was not expected to significantly affect the interactions between PDADMA and PSS because both polyelectrolytes are stable in alkaline solutions. To establish whether this assumption was valid, various techniques were used to analyze the properties of the released multilayers and their corresponding controls. The (PDADMA/ PSS)20 released “face up” was then compared to a Si/ (PDADMA/PSS)20 untreated film. The released “flipped” film, typically having excess PDADMA on top, was compared
a
The shaded area within the PEMU represents the extrinsic compensation.
to a 41-layer (Si/(PDADMA/PSS)20PDADMA) film ending with PDADMA. Bulk Composition. The untreated controls and free films recovered on silicon wafers were studied by transmission FTIR to analyze any change in their chemical composition. The relevant peaks were the distinctive PSS bands at around 1050− 990 cm−1 corresponding to the SO3− group, NO3− at 1350 cm−1, and the peak at around 1470 cm−1 contributed by PDADMA. The stability of the multilayers in NaOH solution was first verified by soaking Si/(PDADMA/PSS)20 and Si/(PDADMA/ PSS)20PDADMA control films built on silicon wafers in 0.1 M NaOH for 20 min. The multilayers maintained their integrity in the basic conditions, and the ratios of PDADMA to PSS were not significantly affected (see Supporting Information Figure S1). No additional bands were observed in the subtraction of original and base-treated PEMUs, except for a small feature from OH− ions which had exchanged with Cl−. Films released from the aluminum substrate were recovered on clean silicon wafer. These multilayers contained almost no cations. The relative anion content (extrinsic charge) was assessed by exchanging the Cl− ions with IR-active NO3− by immersing films in 10−3 M NaNO3 for 10 min. Figure 1a shows the FTIR spectra of the film released “face up” (spectrum B) and its corresponding unreleased control (spectrum A). Interactive subtraction of the two spectra, spectrum C, showed no significant changes on release. Figure 1b shows the ratio of peak areas for the PDADMA and PSS bands and includes additional information for a PDADMA-capped (41-layer) film. As expected, the 41-layer film has a slightly higher ratio of 0.80. The nitrate content labels excess PDADMA throughout the PEMU. Figure 1 also shows that the amount of excess PDADMA remains about the same when the film is released from the aluminum substrate. Note that these peak area 5080
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Figure 3. AFM 3 D images of (a) Si/(PDADMA/PSS)20, (b) (PDADMA/PSS) 20 released “face up”, (c) Si/(PDADMA/ PSS)20PDADMA, and (d) (PDADMA/PSS)20 released “flipped”. Images area is 5 × 5 μm2. All scans are recorded on dry films.
thickness 615 nm), and (PDADMA/PSS)20 released “face up” and “flipped”. The free-floating films were transferred to a clean silicon wafer. The average surface roughness was determined to be 69 nm for PSS-ending films compared to 26 nm for PDADMA-ending films. This result is consistent with earlier reports wherein the dry surface roughness of a PSS-ending film was found to be 66 nm while that of PDAMDA-ending films was 16 nm.23 By comparison, the film released “face up” had an average roughness of 63 nm, which is significantly higher than the “flipped” film (22 nm). The roughness and topography of the released films were similar to their corresponding controls within experimental error. It can be hence concluded that no significant change to the film morphology occurred as a result of the release process. Apparent Young’s modulii were determined for the released films as well as the untreated controls using nanoindentation. Modulii were measured with samples immersed in 0.1 M NaCl, which maintains equilibrium hydration of the PEMU without doping it appreciably (doped films are softer).31 Calculated modulus values depend on the indentation rate and the thickness of the film as well as the fit model. The effective modulus decreases substantially with indentation rate due to the viscoelastic response of the polyelectrolyte complex, and it increases for thinner films due to the proximity of the silicon substrate.23,53 We have previously shown that films thicker than about 200 nm are not significantly impacted by the substrate when using the polyelectrolytes and the tip/tip geometry used here.54 All modulii should be considered effective or apparent values for the 500 nm s−1 indentation rate used here. Force curves were obtained by plotting the force vs indentation (see Figure 4 and Supporting Information Figure S2) and fitting the curve to the cone model eq 3 to obtain multilayer surface apparent modulus. Samples were immersed in 0.1 M NaCl during the force measurements. Earlier reports show that the surface modulus of a wet (i.e., immersed in aqueous solution) PDADMA/PSS PEMU is significantly influenced by the last added layer.54 Specifically, the surface of a film capped with PSS is intrinsically compensated, i.e., mostly cross-linked via ion pairs and glassy. This typically results in a minimally hydrated surface leading to a relatively high modulus. In contrast, PDADMA-terminated PEMUs exhibit high extrinsic compensation where the excess of polymer charge is neutralized by counterions leading to a softer and more hydrated surface.23 For Si/(PDADMA/PSS)20, the average wet modulus was determined to be 24 MPa, while that for Si/(PDADMA/PSS)20PDADMA was 0.21 MPa. Films released “face up” showed an average modulus of 24 MPa, which is comparable to the control film, ending with a PSS
Figure 1. (a) FTIR transmission spectra of (A) Si/(PDADMA/ PSS)20; (B) (PDADMA/PSS)20 released “face up”; (C) resulting spectrum from subtracting (A) from (B). The 1470 cm−1 peak (blue) is contributed by PDADMAC, the NO3− peak (red) is at 1350 cm−1, and the peaks at around 1050−990 cm−1 (green) correspond to PSS. (b) PDADMA/PSS and NO3−/PSS absorbance peak ratio (Q) of Si/ (PDADMA/PSS)20, Si/(PDADMA/PSS)20PDADMA, (PDADMA/ PSS)20 released “face up”, and (PDADMA/PSS)20 released “flipped”.
measurements are only comparative, and the ratio of areas Q is not the actual ratio of PDADMA/PSS. Imaging. SEM revealed a smooth surface with occasional wrinkles on the surface leading to a modest nonuniformity in the released membrane. The crumpling occurred during the transfer of the film onto the new support (see Figure 2). The
Figure 2. SEM image of (PDADMA/PSS)20 released “face up” and transferred to a silicon wafer.
fragility of the film limited the minimum thickness which could be reliably processed to about 40 layers. The slight wrinkling was not expected to significantly impact either AFM or XPS measurements. The surface topography of the PDADMA/PSS films built from 1.0 M NaCl was investigated using AFM and probes with a nominal tip radius of 10 nm. Figure 3 shows images of the untreated controls of 40 layers (Si/(PDADMA/PSS)20 thickness 560 nm) 41 layers (Si/(PDADMA/PSS)20PDADMA, 5081
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Figure 5. N(1s) and S(2p) relative surface atomic percentage of Si/ (PDADMA/PSS)20, Si/(PDADMA/PSS)20PDADMA, (PDADMA/ PSS)20 released “face up”, and (PDADMA/PSS)20 released “flipped”.
with the model of overcompensation (about 20%) by PDADMA within the bulk of the film, all the way to the buried interface, and greater overcompensation at the solution/ PEMU interface when PDADMA is on top. The buried interface probably contains a small contribution from the presumed negative surface charge on the aluminum (alumina) substrate. According to Hiemstra et al.55 at the pH of assembly (pH = ca. 6), there is (1−2) × 10−6 mol m−2 of negative charge on alumina. If the XPS is able to probe 8 nm into the film, a 20% excess PDADMA implies about 6 × 10−6 mol m−2 of excess charge within this thickness. Thus, 15−30% of the surface PDADMA charge of the buried interface may be derived from the PDADMA needed to compensate the charge on the substrate. Postprocessing: Nanosandwiches. XPS measurements on “flipped” films directly revealed the excess of PDADMA at the buried interface. Nanoindentation of this interface demonstrates a surface modulus that is between intrinsic (1:1) and highly PDADMA-overcompensated (1.6:1) material, as expected. The buried interface is now accessible to polyelectrolytes from solution. In theory, PSS should add to this interface to neutralize the excess PDADMA as illustrated in Scheme 2. The PEMU now has a band of extrinsic material (excess PDADMA) sandwiched between two layers of intrinsic PEMU. To make these “nanosandwiches”, flipped films captured on Si wafers were exposed to PSS solution in 1 M NaCl. Access to the original PEMU solution interface was blocked and PSS added to the buried interface. Addition of PSS to excess PDADMA from the buried interface was followed by AFM, which showed an increase of surface roughness (Figure 6), expected for a PSS-terminated PEMU (e.g., see Figure 3). Additional AFM images of PEMUs becoming rougher are supplied in the Supporting Information, Figure S5. The thickness increased slightly, as expected for addition of PSS to the buried interface (Figure 6c). The loss of extrinsic PDADMA from the flipped PEMU was followed with FTIR using nitrate ions. Figure 7 shows “flipped” PEMU spectra before and after exposure to PSS, as in Scheme 2. Spectral subtraction reveals the addition of PSS and the loss (negative peak) of nitrate ions from the loss of PDADMA extrinsic sites. Figure 7 also shows that the amount of PSS in the “flipped” PEMU increased more rapidly at first while the content of the nitrate ion extrinsic probe decreased.
Figure 4. (a) Force vs indentation distance curve of (PDADMA/ PSS)20 released “face up”. The cone fitting model was used with an indentation distance of up to 7 nm. (b) Apparent Young’s modulus of Si/(PDADMA/PSS)20, Si/(PDADMA/PSS)20PDADMA, (PDADMA/PSS)20 released “face up”, and (PDADMA/PSS)20 released “flipped”. All samples were tested in 0.1 M NaCl.
layer. This value is 1 order of magnitude larger than the modulus of the “flipped” film which had an average of 2.8 MPa. The fact that the surface modulus of the “buried” interface in the flipped film is between those of regular PSS- and PDADMA-ending PEMUs is consistent with surface overcompensation by PDADMA, but at a level lower than expected for a PDADMA-ending film. In other words, there appears to be excess PDADMA at the buried interface. Surface Composition. Elemental composition of film surfaces before and after release was investigated by XPS. Specifically, the N(1s) (binding energy, BE = 403.5 eV), S(2p) (BE = 168.5 eV), and Al(BE = 75.9 eV) elemental peaks were evaluated (Figure S3, Supporting Information). No trace of aluminum was detected in the XPS spectra of the released multilayers (see Supporting Information Figure S4), showing no contamination of the PEMU by the substrate and a “clean” separation. Figure 5 compares the relative surface atomic percentage in N and S (where %N + %S has been arbitrarily set to 100%) of the controls and released films. The surface composition of the control films with PSS on top and those released “face up” was approximately 51% N and 49% S. In contrast, the films ending with PDADMA showed a significant enhancement in %N. The surface of the Si/(PDADMA/PSS)20PDADMA film showed 61% N and 39% S. This result is in agreement with earlier studies by Ghostine et al., where a similar trend in the surface elemental composition (1.6:1 N:S) was obtained as a result of the addition of a PDADMA layer.16 This observation was ascribed to the asymmetric growth of a multilayer in which the additional PDADMA diffuses through the film and overcompensates the surface with an excess of positive charges. On the other hand, the surface of the “flipped” film contained 54% N and 46% S (1.2:1 N:S). These results are consistent
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CONCLUSIONS PDADMA/PSS multilayers maintained integrity during their release from aluminum substrates. Various techniques were 5082
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Langmuir Scheme 2. (a) “Flipped” (PDADMA/PSS)20 Multilayer and (b) “Nanosandwich” of PDADMA/PSSa
Figure 7. (a) FTIR transmission spectra of (A) (PDADMA/PSS)20 released “flipped”, (B) (PDADMA/PSS)20 released “flipped” after dipping in PSS for 5 min, and (C) resulting spectrum from subtracting (A) from (B) (magnified 3 times). (b) PDADMA/PSS (blue circles) and NO3−/PSS (red triangles) absorbance peak area ratio (Q) of (PDADMA/PSS)20 released “flipped” before (t = 0 min) and after dipping in 10 mM PSS (1 M NaCl) vs dipping time. a
The excess of positive charges is balanced by chloride counterions (yellow), and the added PSS compensates for the excess of polymer charge on the surface. Extrinsic compensation is represented by the shaded area.
employed to compare the roughness, modulus, and composition of both faces of the free films. All results were consistent with an excess of PDADMA through the PEMU down to the substrate. This excess charge is likely to be more prevalent for thicker PEMUs which include a nonlinear layer-by-layer growth pattern somewhere in their assembly (at the beginning or throughout growth). Though buried, nonstoichiometry controls many properties of PEMUs, such as permeability and bulk modulus. Thus, surface probes such as XPS and surface nanoindentation to shallow depths may not reflect properties of the entire film. Released films were treated with additional polyelectrolyte which compensated the excess charge of the buried interface.
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ASSOCIATED CONTENT
S Supporting Information *
Control FTIR spectra, XPS spectra, force vs indentation distance curves, and AFM images of nanosandwiches after soaking in PSS for different times. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00975.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (J.B.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Qifeng Wang for helpful discussions and Dr. Eric Lochner for help with XPS. This work was supported by Grant DMR-1207188 from the National Science Foundation.
Figure 6. (a) 3D images of (PDADMA/PSS)20 released “flipped” before and after dipping in PSS for 5 min. (b) Roughness of (PDADMA/PSS)20 released “flipped” vs dipping time in PSS. (c) Thickness of (PDADMA/PSS)20 released “flipped” vs dipping time in PSS.
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REFERENCES
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Langmuir (2) Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Decher, G.; Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2012. (3) Schlenoff, J. B.; Ly, H.; Li, M. Charge and Mass Balance in Polyelectrolyte Multilayers. J. Am. Chem. Soc. 1998, 120 (30), 7626− 7634. (4) Dubas, S. T.; Schlenoff, J. B. Polyelectrolyte Multilayers Containing a Weak Polyacid: Construction and Deconstruction. Macromolecules 2001, 34 (11), 3736−3740. (5) Rmaile, H. H.; Schlenoff, J. B. “Internal pK(a)’s” in Polyelectrolyte Multilayers: Coupling Protons and Salt. Langmuir 2002, 18 (22), 8263−8265. (6) Shiratori, S. S.; Rubner, M. F. pH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak Polyelectrolytes. Macromolecules 2000, 33 (11), 4213−4219. (7) Sukhishvili, S. A.; Granick, S. Layered, Erasable, Ultrathin Polymer Films. J. Am. Chem. Soc. 2000, 122 (39), 9550−9551. (8) Sukhishvili, S. A.; Granick, S. Layered, Erasable Polymer Multilayers Formed by Hydrogen-Bonded Sequential Self-Assembly. Macromolecules 2002, 35 (1), 301−310. (9) Sui, Z. J.; Schlenoff, J. B. Phase Separations in pH-Responsive Polyelectrolyte Multilayers: Charge Extrusion versus Charge Expulsion. Langmuir 2004, 20 (14), 6026−6031. (10) Shen, L. Y.; Chaudouet, P.; Ji, J. A.; Picart, C. pH-Amplified Multilayer Films Based on Hyaluronan: Influence of HA Molecular Weight and Concentration on Film Growth and Stability. Biomacromolecules 2011, 12 (4), 1322−1331. (11) Tan, H. L.; McMurdo, M. J.; Pan, G. Q.; Van Patten, P. G. Temperature Dependence of Polyelectrolyte Multilayer Assembly. Langmuir 2003, 19 (22), 9311−9314. (12) Ghostine, R. A.; Jisr, R. M.; Lehaf, A.; Schlenoff, J. B. Roughness and Salt Annealing in a Polyelectrolyte Multilayer. Langmuir 2013, 29 (37), 11742−11750. (13) 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 (11), 5017−5023. (14) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Reversibly Erasable Nanoporous Anti-Reflection Coatings from Polyelectrolyte Multilayers. Nat. Mater. 2002, 1 (1), 59−63. (15) Gilbert, J. B.; Rubner, M. F.; Cohen, R. E. Depth-Profiling X-ray Photoelectron Spectroscopy (XPS) Analysis of Interlayer Diffusion in Polyelectrolyte Multilayers. Proc. Natl. Acad. Sci. U. S. A. 2013, 110 (17), 6651−6656. (16) Ghostine, R. A.; Markarian, M. Z.; Schlenoff, J. B. Asymmetric Growth in Polyelectrolyte Multilayers. J. Am. Chem. Soc. 2013, 135 (20), 7636−7646. (17) Delcorte, A.; Bertrand, P.; Arys, X.; Jonas, A.; Wischerhoff, E.; Mayer, B.; Laschewsky, A. ToF-SIMS Study of Alternate Polyelectrolyte Thin Films: Chemical Surface Characterization and Molecular Secondary Ions Sampling Depth. Surf. Sci. 1996, 366 (1), 149−165. (18) Adamczyk, Z.; Zembala, M.; Warszyński, P.; Jachimska, B. Characterization of Polyelectrolyte Multilayers by the Streaming Potential Method. Langmuir 2004, 20 (24), 10517−10525. (19) Boudou, T.; Crouzier, T.; Ren, K.; Blin, G.; Picart, C. Multiple Functionalities of Polyelectrolyte Multilayer Films: New Biomedical Applications. Adv. Mater. 2010, 22 (4), 441−467. (20) Cheng, C.; Yaroshchuk, A.; Bruening, M. L. Fundamentals of Selective Ion Transport through Multilayer Polyelectrolyte Membranes. Langmuir 2013, 29 (6), 1885−1892. (21) Krasemann, L.; Tieke, B. Selective Ion Transport across SelfAssembled Alternating Multilayers of Cationic and Anionic Polyelectrolytes. Langmuir 2000, 16 (2), 287−290. (22) Schmitt, J.; Grünewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lösche, M. Internal Structure of Layer-by-Layer Adsorbed Polyelectrolyte Films - a Neutron and X-Ray Reflectivity Study. Macromolecules 1993, 26 (25), 7058−7063. (23) 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 (15), 6348−6355. (24) Mallwitz, F.; Goedel, W. A. Physically Cross-linked Ultrathin Elastomeric Membranes. Angew. Chem., Int. Ed. 2001, 40 (14), 2645− 2647. (25) Jiang, C. Y.; Markutsya, S.; Pikus, Y.; Tsukruk, V. V. Freely Suspended Nanocomposite Membranes as Highly Sensitive Sensors. Nat. Mater. 2004, 3 (10), 721−728. (26) Cheng, W. L.; Campolongo, M. J.; Tan, S. J.; Luo, D. Freestanding Ultrathin Nano-Membranes via Self-Assembly. Nano Today 2009, 4 (6), 482−493. (27) Ulbricht, M. Advanced Functional Polymer Membranes. Polymer 2006, 47 (7), 2217−2262. (28) Lavalle, P.; Boulmedais, F.; Ball, V.; Mutterer, J.; Schaaf, P.; Voegel, J. C. Free Standing Membranes Made of Biocompatible Polyelectrolytes using the Layer by Layer Method. J. Membr. Sci. 2005, 253 (1−2), 49−56. (29) Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nanostructured Artificial Nacre. Nat. Mater. 2003, 2 (6), 413−U8. (30) Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. T. Elastomeric Flexible Free-Standing Hydrogen-Bonded Nanoscale Assemblies. J. Am. Chem. Soc. 2005, 127 (49), 17228−17234. (31) Jaber, J. A.; Schlenoff, J. B. Mechanical Properties of Reversibly Cross-Linked Ultrathin Polyelectrolyte Complexes. J. Am. Chem. Soc. 2006, 128 (9), 2940−2947. (32) Larkin, A. L.; Davis, R. M.; Rajagopalan, P. Biocompatible, Detachable, and Free-Standing Polyelectrolyte Multilayer Films. Biomacromolecules 2010, 11 (10), 2788−2796. (33) Caridade, S. G.; Monge, C.; Gilde, F.; Boudou, T.; Mano, J. F.; Picart, C. Free-Standing Polyelectrolyte Membranes Made of Chitosan and Alginate. Biomacromolecules 2013, 14 (5), 1653−1660. (34) Jiang, C. Y.; Markutsya, S.; Tsukruk, V. V. Compliant, Robust, and Truly Nanoscale Free-Standing Multilayer Films Fabricated Using Spin-Assisted Layer-by-Layer Assembly. Adv. Mater. 2004, 16 (2), 157. (35) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. Multiple Membranes from “True” Polyelectrolyte Multilayers. J. Am. Chem. Soc. 2001, 123 (22), 5368−5369. (36) Gui, Z. L.; Qian, J. W.; Du, B. Y.; Yin, M. J.; An, Q. F. Fabrication of Free-Standing Polyelectrolyte Multilayer Films: A Method Using Polysulfobetaine-Containing Films as Sacrificial Layers. J. Colloid Interface Sci. 2009, 340 (1), 35−41. (37) Zhang, J. W.; Yuan, B.; Wang, Z. Q.; Chen, T. Fabrication of Free-Standing Multilayer Films by Using pH-Responsive Microgels as Sacrificial Layers. Colloid Polym. Sci. 2014, 292 (5), 1235−1240. (38) Mamedov, A. A.; Kotov, N. A. Free-Standing Layer-by-Layer Assembled Films of Magnetite Nanoparticles. Langmuir 2000, 16 (13), 5530−5533. (39) Vendamme, R.; Onoue, S. Y.; Nakao, A.; Kunitake, T. Robust Free-Standing Nanomembranes of Organic/Inorganic Interpenetrating Networks. Nat. Mater. 2006, 5 (6), 494−501. (40) Zimnitsky, D.; Shevchenko, V. V.; Tsukruk, V. V. Perforated, Freely Suspended Layer-by-Layer Nanoscale Membranes. Langmuir 2008, 24 (12), 5996−6006. (41) Pennakalathil, J.; Hong, J. D. Self-Standing Polyelectrolyte Multilayer Films Based on Light-Triggered Disassembly of a Sacrificial Layer. ACS Nano 2011, 5 (11), 9232−9237. (42) Okano, T.; Yamada, N.; Okuhara, M.; Sakai, H.; Sakurai, Y. Mechanism of Cell Detachment from Temperature-Modulated, Hydrophilic-Hydrophobic Polymer Surfaces. Biomaterials 1995, 16 (4), 297−303. (43) Zhuk, A.; Pavlukhina, S.; Sukhishvili, S. A. Hydrogen-Bonded Layer-by-Layer Temperature-Triggered Release Films. Langmuir 2009, 25 (24), 14025−14029. (44) Zhang, J. T.; Montanez, S. I.; Jewell, C. M.; Lynn, D. M. Multilayered Films Fabricated from Plasmid DNA and a Side-Chain Functionalized Poly(beta-amino ester): Surface-Type Erosion and Sequential Release of Multiple Plasmid Constructs from Surfaces. Langmuir 2007, 23 (22), 11139−11146. 5084
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Langmuir (45) Zhang, J. T.; Chua, L. S.; Lynn, D. M. Multilayered Thin Films that Sustain the Release of Functional DNA under Physiological Conditions. Langmuir 2004, 20 (19), 8015−8021. (46) Ren, K. F.; Ji, J.; Shen, J. C. Construction and Enzymatic Degradation of Multilayered Poly-L-lysine/DNA Films. Biomaterials 2006, 27 (7), 1152−1159. (47) Picart, C.; Schneider, A.; Etienne, O.; Mutterer, J.; Schaaf, P.; Egles, C.; Jessel, N.; Voegel, J. C. Controlled Degradability of Polysaccharide Multilayer Films in Vitro and in Vivo. Adv. Funct. Mater. 2005, 15 (11), 1771−1780. (48) Blacklock, J.; Handa, H.; Manickam, D. S.; Mao, G. Z.; Mukhopadhyay, A.; Oupicky, D. Disassembly of Layer-by-Layer Films of Plasmid DNA and Reducible TAT Polypeptide. Biomaterials 2007, 28 (1), 117−124. (49) Chen, J.; Huang, S. W.; Lin, W. H.; Zhuo, R. X. Tunable Film Degradation and Sustained Release of Plasmid DNA from Cleavable Polycation/Plasmid DNA Multilayers under Reductive Conditions. Small 2007, 3 (4), 636−643. (50) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Möhwald, H. Novel Hollow Polymer Shells by Colloid-Templated Assembly of Polyelectrolytes. Angew. Chem., Int. Ed. 1998, 37 (16), 2202−2205. (51) Sukhorukov, G. B. Designed Nano-Engineered Polymer Films on Colloidal Particles and Capsules. Stud. Interface Sci. 2001, 11, 383− 414. (52) Lim, M. H.; Ast, D. G. Free-Standing Thin Films Containing Hexagonally Organized Silver Nanocrystals in a Polymer Matrix. Adv. Mater. 2001, 13 (10), 718−721. (53) Domke, J.; Radmacher, M. Measuring the Elastic Properties of Thin Polymer Films with the Atomic Force Microscope. Langmuir 1998, 14 (12), 3320−3325. (54) Lehaf, A. M.; Moussallem, M. D.; Schlenoff, J. B. Correlating the Compliance and Permeability of Photo-Cross-Linked Polyelectrolyte Multilayers. Langmuir 2011, 27 (8), 4756−4763. (55) Hiemstra, T.; Yong, H.; Van Riemsdijk, W. H. Interfacial Charging Phenomena of Aluminum (Hydr)oxides. Langmuir 1999, 15 (18), 5942−5955.
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Flipped Polyelectrolyte Multilayer Films: Accessing the Buried Interface Yara E. Ghoussoub and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States S Supporting Information *
ABSTRACT: Little is known concerning the interface between a polyelectrolyte multilayer, PEMU, and its substrate. Recent models suggest that excess polymer charge, compensated by counterions, remains buried within the PEMU, especially for thicker films having a nonlinear component to their growth. We report a novel approach for making freestanding multilayers of poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS): after assembly on aluminum substrates, films were released by brief immersion in aqueous alkali. The multilayers were then flipped, allowing access to the initially buried substrate/PEMU interface. Experiments were performed to show that this method of release, one of many established for PEMUs, perturbed the surface and bulk of the film minimally. Film/solution and film/substrate interfaces were compared using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to record topography and perform nanoindentation, while XPS provided surface elemental composition. All three methods revealed data consistent with an excess of PDADMA at the buried interface. This excess PDADMA was then complexed with additional PSS to yield “nanosandwiches” of nonstoichiometric PEMU between layers of stoichiometric PEMU.
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INTRODUCTION Ultrathin polyelectrolyte films with tunable properties can be assembled on various substrates using the layer-by-layer adsorption technique.1 Polyelectrolytes of opposite charge are alternately (ad)sorbed onto the substrate, one layer at a time, to form a so-called “polyelectrolyte multilayer” (PEMU).1,2 Charge balance is conserved within the multilayer by ion pairing of polyelectrolyte repeat units (“intrinsic compensation”) and by small counterions inside the film (“extrinsic compensation”),3 where these balance an excess of polymer charge within the multilayer. It has been shown that the growth of multilayers is affected by the salt concentration,4,5 solution pH,5−9 polymer concentration,10 and temperature.11 Much is known about the external surface of PEMUs, which is usually in contact with air or aqueous solutions. This interface has been routinely examined by atomic force microscopy (AFM) to establish roughness12 and whether phase separation, often yielding nanopores, has occurred.12−14 Film growth mechanisms are inferred from such topological measurements. X-ray photoelectron spectroscopy (XPS) provides information on the composition of the first few nanometers of the external interface.15−17 Electrokinetic potential measurements yield the sign of the surface charge,18 and exchange with radiolabeled ions16 provides the fixed charge density. Far less is known about the interface between the substrate and the multilayerthe buried interface. After the first few layers of polyelectrolyte have been adsorbed this interface © 2015 American Chemical Society
becomes inaccessible to many surface characterization techniques. Because many applications, such as control of cell adhesion,19 are assumed to depend critically on the external surface, gathering information on the buried interface has not been a pressing priority. However, for a fundamental understanding of the mechanism of multilayer growth the composition throughout the film is needed. In addition, information on the composition through the film is essential to understanding and predicting separations of PEMUs used as selective membranes.20,21 Until recently it was generally assumed that polyelectrolytes were well paired within the bulk of the film and deviation from 1:1 stoichiometry of positive and negative polyelectrolyte repeat units was seen mainly at the surface.1 “Overcompensation” of surface polymer is a key feature of the growth of polyelectrolyte multilayers.1 We demonstrated that overcompensation in a popular pair of polyelectrolytes, poly(styrenesulfonate), PSS, and poly(diallyldimethylammonium), PDADMA, only occurred for the PDADMA “layer” and the PSS layer yielded a stoichiometric, glassy surface.16 After a few layers, a zone of excess PDADMA builds up within the bulk of the PEMU. A model was presented showing that this excess PDADMA extended down to the substrate, but no direct measurements were made near the Received: March 16, 2015 Revised: April 16, 2015 Published: April 30, 2015 5078
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substrate.16 Some indication of vertical composition has come from neutron reflectometry results. For example, PEMUs made from PSS and poly(allylamine) hydrochloride, PAH, have been modeled with an excess of both cations and anions,22 but each model has not been able to definitively exclude others because the fit is never perfect. In our experience, the best indicators of polyelectrolyte nonstoichiometry at the surface have been obtained with AFM and XPS. Nanoindentation with the AFM shows that, when hydrated, nonstoichiometric surfaces are much softer (due to extra hydration from the ions and lower ionic cross-linking between polyelectrolytes) than those that are stoichiometric.23 Related to the mechanical properties, PEMUs with stoichiometric surfaces were rougher when dry since highly hydrated material collapses and planarizes the surface.12 XPS measures the excess of one polyelectrolyte over another directly. We were therefore interested in removing a PEMU from a substrate, flipping it over, and employing these techniques to see whether the buried interface was stoichiometric in polyelectrolytes or whether it had an excess of polycation, as our model predicted. Most fabrication techniques yield PEMUs that are firmly adhered to their substrates. Several approaches have been developed for applications which need detached or freestanding (free, free-floating) films. Suspended membranes can be good candidates for sensors, micromechanical devices,24−26 and filtration and catalysis membranes.26−28 Methods for making free-standing layer-by-layer assembled films include dissolving the underlying substrate,28,29 direct peeling from a surface,30−33 and dissolving a sacrificial layer between the film and the underlying substrate.25,34 Sacrificial layers are composed of stimuli-responsive materials which might be disassembled by a change of salt concentration,35 pH,35−37 solvent,38−40 light,41 or temperature.42,43 We have demonstrated the release of PEMU films using a sacrificial layer of poly(acrylic acid) PAA/PDADMA, which can be selectively decomposed by a change of salt concentration or pH.35 Similarly, Mamedov et al. isolated multilayer films by dissolving an acetyl cellulose layer in acetone.38 However, these approaches potentially alter the properties of the PEMU or leave residues of the sacrificial layer on the buried interface. For example, the release of poly(L-lysine)/hyaluronic acid multilayers from polystyrene substrates in THF yields micron-sized pores in the isolated films.28 Microporous films of PAA and PAH were also obtained by a pH-induced phase separation.13 One possible mechanism for this morphology change is the breakage of ionic bonding due to their protonation at low pH. Stimuli-responsive materials were also used to control the release of various agents from multilayers by the integration of functionalities that can be degraded by hydrolysis,44,45 enzymes,46,47 or reduction.48,49 In addition to planar substrates, polyelectrolytes can also be adsorbed on particles.50,51 The controlled decomposition of template particles yields hollow polyelectrolyte microshells. Alternatively, free-standing films can be obtained without the aid of substrates at the air−water interface as exemplified by cast films52 and cross-linked Langmuir films.24 Our study of the buried interface required a method of releasing a PEMU with minimal perturbation of structure and composition. In the present work we develop an optimized release method then analyze the properties of the buried interface.
Article
EXPERIMENTAL SECTION
Poly(4-styrenesulfonic acid) (PSS, molar mass = 7.5 × 104 g mol−1), poly(diallyldimethylammonium chloride) (PDADMAC, molar mass = 40 × 104−50 × 104 g mol−1), sodium chloride, sodium nitrate, sodium hydroxide (0.1 M and 1.0 M), hydrochloric acid (0.1 and 1 M), sulfuric acid (98%), and hydrogen peroxide (30%) were purchased from Sigma-Aldrich and used without further purification. All solutions were prepared using deionized water with a resistance of 18 MΩ (Barnstead, E-pure). Multilayer Assembly. Multilayer films of PDADMA and PSS were assembled on silicon and aluminum substrates using a robot (StratoSequence V, nanoStrata Inc.). Prior to film buildup, silicon wafers were cleaned with “piranha” (70% H2SO4 and 30% H2O2) for 10 min, rinsed extensively with water, and dried with a stream of nitrogen gas. The substrates were mounted face down on the robot shafts rotating at 300 rpm. Polyelectrolyte solutions were 10 mM with respect to the repeat unit, prepared in 1 M NaCl. The duration for each dipping step in the polymer solution was 5 min followed by three 1 min rinsing steps in water. After final rinsing, the multilayer was dried under a smooth stream of N2. The multilayers assembled on aluminum were briefly soaked in 0.1 M NaOH, which prompted the release of the films from the underlying substrate. Just prior to complete detachment, the substrate was transferred to water. After the films detached they were recovered on a clean silicon wafer either “face up” or “flipped”. In the former, the PEMU was gently lifted with the aid of tweezers and recovered “face up”. In the latter, the PEMU was turned over and recovered “flipped”, where the bottom side that was in contact with the aluminum became the upper side. Force Microscopy. The effective Young’s modulus of untreated and released PEMUs was determined by nanoindentation using an MFP-3D AFM equipped with an ARC2 controller (Asylum Research Inc., Santa Barbara, CA). The data were analyzed using Igor Pro software. The optical lever sensitivity (OLP) and the spring constant of the AC240-TS tips (around 2 N m−1) were first determined in air by the thermal fluctuation technique. After the immersion of the tip in 0.1 M NaCl, the OLP was recalibrated on a glass surface in the salt solution. Under the force mapping mode, force indentation measurements were performed on the samples immersed in NaCl, with a tip velocity of 500 nm s−1 and a scan range of 20 × 20 μm. The modulus was obtained by averaging 4 × 5 points force maps. The applied force and the resulting indentation are given by eqs 1 and 2: Fapplied = Kd = K (z − δ)
(1)
δ=z−d
(2)
where K is the spring constant of the cantilever, d is the deflection of the tip, z is the distance of the tip relative to the surface of the sample in the z-direction, and δ is the indentation of the tip into the multilayer (50 nm for PDADMA capped films and 7 nm for PSS capped films). Using Hertzian contact mechanics, force curves were analyzed based on tip geometry. Force vs indendation for a conical tip is represented by eq 3
Fcone =
Esurface 2 tan(α)(z − d)2 2 π (1 − υsurface )
(3)
where E is the effective modulus of the surface, υ is the Poisson ratio of the material, α is the half-angle of the cone (18°), and R is the radius of the indenter (10 nm). The fitted data yielded the convoluted modulus of the multilayer (EC). This value was then converted to Young’s modulus using eq 4 −1 ⎛1 − υ 2 1 − υ2 2 ⎞ 1 ⎟ + EC = ⎜ E2 ⎠ ⎝ E1
(4)
where υ1 is the Poisson ratio of the indented material set to 0.5, υ2 is the Poisson ratio of the indenter fixed at 0.27, E1 is the Young’s modulus of the indented material, and E2 is the Young’s modulus of the indenter (silicon) set to 150 GPa. 5079
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Langmuir Imaging. Topography of the films was obtained from 5 × 5 μm AFM images. NCHV tips from Veeco were used in the AC (intermittent contact) mode with a scan rate of 0.5 Hz. Roughnesses were obtained from several 5 × 5 μm areas at different positions and averaged. SEM imaging was performed using a JEOL 5900 operating at 30 kV. Contrast was obtained by detection of both secondary and backscattered electrons. Samples were sputter-coated with a thin Au film. FTIR Spectroscopy. The chemical composition of the multilayers was explored on double-side-polished Si 100 wafers (thickness = 381 ± 25 μm) using transmission FTIR spectroscopy. Spectra were acquired with a nitrogen-purged Thermo Avatar 360 FTIR equipped with a DTGS detector. For the untreated controls and the released films, silicon was used as substrate with bare silicon as a reference. The average of 100 scans was taken with a resolution of 4 cm−1. X-ray Photoelectron Spectroscopy. A PerkinElmer (PHI) 5100 XPS with a noncollimated Mg Kα X-ray source (hν = 1253.6 eV) was used for surface elemental analysis at a base pressure of 1.9 × 10−8 Torr. Data acquisition was performed at a takeoff angle of 45°, a pass energy of 89.45 eV, and a speed of 0.5 eV s−1. For each measurement, a total of 10 scans were averaged. Database 71 software (NIST) was used to determine inelastic mean free paths (IMFPs).
Scheme 1. Cartoon of (a) Al/(PDADMA/PSS)20 Multilayer, (b) (PDADMA/PSS)20 after Release “Face Up”, and (c) (PDADMA/PSS)20 after Release “Flipped”a
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RESULTS AND DISCUSSION The main purpose of the present work was to characterize the buried interface of PDADMA/PSS multilayers, that is, the substrate side of the multilayer. To this end, different approaches were tested for the release of 40-layer films. A method that we reported earlier yielded isolated films but left traces of the sacrificial layer after recovery.35 A sacrificial layer, made of PAA and PDADMA, was dissociated at pH > 6 due to the ionization of the PAA carboxylic acid units. FTIR transmission spectra showed that approximately 13% of the carboxylic acid remained in the free membrane. As an alternative approach, a thin layer of 30 nm of silicon dioxide was grown on Si wafers by heating them at 900 °C for 2 h. Multilayers deposited on these wafers were released by exposure to HF, which dissolves SiO2. However, the release was inconsistent at relatively low HF concentrations and the multilayers partially decomposed at higher HF concentrations. Another route involved the buildup of PEMUs on aluminum substrates. While the use of acid to dissolve a layer of aluminum did not yield satisfactory results, the exposure of the multilayers to alkali solutions provided the most efficient procedure for the formation of free membranes. The bubbles formed by gentle dissolution of the surface of the aluminum (2Al(s) + 2NaOH(aq) + 6H2O → 2Na +(aq) + 2[Al(OH)4]− + 3H2(g)) helped float the films off the substrate. In the following, multilayers will be denoted as substrate/ (PDADMA/PSS)n, where n is the number of bilayers. A 40layer PDADMA/PSS film assembled on an aluminum substrate ((Al/PDADMA/PSS)20) could be quickly released (within 45 s) in 0.1 M NaOHaq and recovered on a clean silicon wafer either “face up” or “flipped” (see Scheme 1). For the “flipped” multilayers, the face that was initially in contact with the substrate became the upper face, allowing direct access. The method of release was not expected to significantly affect the interactions between PDADMA and PSS because both polyelectrolytes are stable in alkaline solutions. To establish whether this assumption was valid, various techniques were used to analyze the properties of the released multilayers and their corresponding controls. The (PDADMA/ PSS)20 released “face up” was then compared to a Si/ (PDADMA/PSS)20 untreated film. The released “flipped” film, typically having excess PDADMA on top, was compared
a
The shaded area within the PEMU represents the extrinsic compensation.
to a 41-layer (Si/(PDADMA/PSS)20PDADMA) film ending with PDADMA. Bulk Composition. The untreated controls and free films recovered on silicon wafers were studied by transmission FTIR to analyze any change in their chemical composition. The relevant peaks were the distinctive PSS bands at around 1050− 990 cm−1 corresponding to the SO3− group, NO3− at 1350 cm−1, and the peak at around 1470 cm−1 contributed by PDADMA. The stability of the multilayers in NaOH solution was first verified by soaking Si/(PDADMA/PSS)20 and Si/(PDADMA/ PSS)20PDADMA control films built on silicon wafers in 0.1 M NaOH for 20 min. The multilayers maintained their integrity in the basic conditions, and the ratios of PDADMA to PSS were not significantly affected (see Supporting Information Figure S1). No additional bands were observed in the subtraction of original and base-treated PEMUs, except for a small feature from OH− ions which had exchanged with Cl−. Films released from the aluminum substrate were recovered on clean silicon wafer. These multilayers contained almost no cations. The relative anion content (extrinsic charge) was assessed by exchanging the Cl− ions with IR-active NO3− by immersing films in 10−3 M NaNO3 for 10 min. Figure 1a shows the FTIR spectra of the film released “face up” (spectrum B) and its corresponding unreleased control (spectrum A). Interactive subtraction of the two spectra, spectrum C, showed no significant changes on release. Figure 1b shows the ratio of peak areas for the PDADMA and PSS bands and includes additional information for a PDADMA-capped (41-layer) film. As expected, the 41-layer film has a slightly higher ratio of 0.80. The nitrate content labels excess PDADMA throughout the PEMU. Figure 1 also shows that the amount of excess PDADMA remains about the same when the film is released from the aluminum substrate. Note that these peak area 5080
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Figure 3. AFM 3 D images of (a) Si/(PDADMA/PSS)20, (b) (PDADMA/PSS) 20 released “face up”, (c) Si/(PDADMA/ PSS)20PDADMA, and (d) (PDADMA/PSS)20 released “flipped”. Images area is 5 × 5 μm2. All scans are recorded on dry films.
thickness 615 nm), and (PDADMA/PSS)20 released “face up” and “flipped”. The free-floating films were transferred to a clean silicon wafer. The average surface roughness was determined to be 69 nm for PSS-ending films compared to 26 nm for PDADMA-ending films. This result is consistent with earlier reports wherein the dry surface roughness of a PSS-ending film was found to be 66 nm while that of PDAMDA-ending films was 16 nm.23 By comparison, the film released “face up” had an average roughness of 63 nm, which is significantly higher than the “flipped” film (22 nm). The roughness and topography of the released films were similar to their corresponding controls within experimental error. It can be hence concluded that no significant change to the film morphology occurred as a result of the release process. Apparent Young’s modulii were determined for the released films as well as the untreated controls using nanoindentation. Modulii were measured with samples immersed in 0.1 M NaCl, which maintains equilibrium hydration of the PEMU without doping it appreciably (doped films are softer).31 Calculated modulus values depend on the indentation rate and the thickness of the film as well as the fit model. The effective modulus decreases substantially with indentation rate due to the viscoelastic response of the polyelectrolyte complex, and it increases for thinner films due to the proximity of the silicon substrate.23,53 We have previously shown that films thicker than about 200 nm are not significantly impacted by the substrate when using the polyelectrolytes and the tip/tip geometry used here.54 All modulii should be considered effective or apparent values for the 500 nm s−1 indentation rate used here. Force curves were obtained by plotting the force vs indentation (see Figure 4 and Supporting Information Figure S2) and fitting the curve to the cone model eq 3 to obtain multilayer surface apparent modulus. Samples were immersed in 0.1 M NaCl during the force measurements. Earlier reports show that the surface modulus of a wet (i.e., immersed in aqueous solution) PDADMA/PSS PEMU is significantly influenced by the last added layer.54 Specifically, the surface of a film capped with PSS is intrinsically compensated, i.e., mostly cross-linked via ion pairs and glassy. This typically results in a minimally hydrated surface leading to a relatively high modulus. In contrast, PDADMA-terminated PEMUs exhibit high extrinsic compensation where the excess of polymer charge is neutralized by counterions leading to a softer and more hydrated surface.23 For Si/(PDADMA/PSS)20, the average wet modulus was determined to be 24 MPa, while that for Si/(PDADMA/PSS)20PDADMA was 0.21 MPa. Films released “face up” showed an average modulus of 24 MPa, which is comparable to the control film, ending with a PSS
Figure 1. (a) FTIR transmission spectra of (A) Si/(PDADMA/ PSS)20; (B) (PDADMA/PSS)20 released “face up”; (C) resulting spectrum from subtracting (A) from (B). The 1470 cm−1 peak (blue) is contributed by PDADMAC, the NO3− peak (red) is at 1350 cm−1, and the peaks at around 1050−990 cm−1 (green) correspond to PSS. (b) PDADMA/PSS and NO3−/PSS absorbance peak ratio (Q) of Si/ (PDADMA/PSS)20, Si/(PDADMA/PSS)20PDADMA, (PDADMA/ PSS)20 released “face up”, and (PDADMA/PSS)20 released “flipped”.
measurements are only comparative, and the ratio of areas Q is not the actual ratio of PDADMA/PSS. Imaging. SEM revealed a smooth surface with occasional wrinkles on the surface leading to a modest nonuniformity in the released membrane. The crumpling occurred during the transfer of the film onto the new support (see Figure 2). The
Figure 2. SEM image of (PDADMA/PSS)20 released “face up” and transferred to a silicon wafer.
fragility of the film limited the minimum thickness which could be reliably processed to about 40 layers. The slight wrinkling was not expected to significantly impact either AFM or XPS measurements. The surface topography of the PDADMA/PSS films built from 1.0 M NaCl was investigated using AFM and probes with a nominal tip radius of 10 nm. Figure 3 shows images of the untreated controls of 40 layers (Si/(PDADMA/PSS)20 thickness 560 nm) 41 layers (Si/(PDADMA/PSS)20PDADMA, 5081
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Figure 5. N(1s) and S(2p) relative surface atomic percentage of Si/ (PDADMA/PSS)20, Si/(PDADMA/PSS)20PDADMA, (PDADMA/ PSS)20 released “face up”, and (PDADMA/PSS)20 released “flipped”.
with the model of overcompensation (about 20%) by PDADMA within the bulk of the film, all the way to the buried interface, and greater overcompensation at the solution/ PEMU interface when PDADMA is on top. The buried interface probably contains a small contribution from the presumed negative surface charge on the aluminum (alumina) substrate. According to Hiemstra et al.55 at the pH of assembly (pH = ca. 6), there is (1−2) × 10−6 mol m−2 of negative charge on alumina. If the XPS is able to probe 8 nm into the film, a 20% excess PDADMA implies about 6 × 10−6 mol m−2 of excess charge within this thickness. Thus, 15−30% of the surface PDADMA charge of the buried interface may be derived from the PDADMA needed to compensate the charge on the substrate. Postprocessing: Nanosandwiches. XPS measurements on “flipped” films directly revealed the excess of PDADMA at the buried interface. Nanoindentation of this interface demonstrates a surface modulus that is between intrinsic (1:1) and highly PDADMA-overcompensated (1.6:1) material, as expected. The buried interface is now accessible to polyelectrolytes from solution. In theory, PSS should add to this interface to neutralize the excess PDADMA as illustrated in Scheme 2. The PEMU now has a band of extrinsic material (excess PDADMA) sandwiched between two layers of intrinsic PEMU. To make these “nanosandwiches”, flipped films captured on Si wafers were exposed to PSS solution in 1 M NaCl. Access to the original PEMU solution interface was blocked and PSS added to the buried interface. Addition of PSS to excess PDADMA from the buried interface was followed by AFM, which showed an increase of surface roughness (Figure 6), expected for a PSS-terminated PEMU (e.g., see Figure 3). Additional AFM images of PEMUs becoming rougher are supplied in the Supporting Information, Figure S5. The thickness increased slightly, as expected for addition of PSS to the buried interface (Figure 6c). The loss of extrinsic PDADMA from the flipped PEMU was followed with FTIR using nitrate ions. Figure 7 shows “flipped” PEMU spectra before and after exposure to PSS, as in Scheme 2. Spectral subtraction reveals the addition of PSS and the loss (negative peak) of nitrate ions from the loss of PDADMA extrinsic sites. Figure 7 also shows that the amount of PSS in the “flipped” PEMU increased more rapidly at first while the content of the nitrate ion extrinsic probe decreased.
Figure 4. (a) Force vs indentation distance curve of (PDADMA/ PSS)20 released “face up”. The cone fitting model was used with an indentation distance of up to 7 nm. (b) Apparent Young’s modulus of Si/(PDADMA/PSS)20, Si/(PDADMA/PSS)20PDADMA, (PDADMA/PSS)20 released “face up”, and (PDADMA/PSS)20 released “flipped”. All samples were tested in 0.1 M NaCl.
layer. This value is 1 order of magnitude larger than the modulus of the “flipped” film which had an average of 2.8 MPa. The fact that the surface modulus of the “buried” interface in the flipped film is between those of regular PSS- and PDADMA-ending PEMUs is consistent with surface overcompensation by PDADMA, but at a level lower than expected for a PDADMA-ending film. In other words, there appears to be excess PDADMA at the buried interface. Surface Composition. Elemental composition of film surfaces before and after release was investigated by XPS. Specifically, the N(1s) (binding energy, BE = 403.5 eV), S(2p) (BE = 168.5 eV), and Al(BE = 75.9 eV) elemental peaks were evaluated (Figure S3, Supporting Information). No trace of aluminum was detected in the XPS spectra of the released multilayers (see Supporting Information Figure S4), showing no contamination of the PEMU by the substrate and a “clean” separation. Figure 5 compares the relative surface atomic percentage in N and S (where %N + %S has been arbitrarily set to 100%) of the controls and released films. The surface composition of the control films with PSS on top and those released “face up” was approximately 51% N and 49% S. In contrast, the films ending with PDADMA showed a significant enhancement in %N. The surface of the Si/(PDADMA/PSS)20PDADMA film showed 61% N and 39% S. This result is in agreement with earlier studies by Ghostine et al., where a similar trend in the surface elemental composition (1.6:1 N:S) was obtained as a result of the addition of a PDADMA layer.16 This observation was ascribed to the asymmetric growth of a multilayer in which the additional PDADMA diffuses through the film and overcompensates the surface with an excess of positive charges. On the other hand, the surface of the “flipped” film contained 54% N and 46% S (1.2:1 N:S). These results are consistent
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CONCLUSIONS PDADMA/PSS multilayers maintained integrity during their release from aluminum substrates. Various techniques were 5082
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Langmuir Scheme 2. (a) “Flipped” (PDADMA/PSS)20 Multilayer and (b) “Nanosandwich” of PDADMA/PSSa
Figure 7. (a) FTIR transmission spectra of (A) (PDADMA/PSS)20 released “flipped”, (B) (PDADMA/PSS)20 released “flipped” after dipping in PSS for 5 min, and (C) resulting spectrum from subtracting (A) from (B) (magnified 3 times). (b) PDADMA/PSS (blue circles) and NO3−/PSS (red triangles) absorbance peak area ratio (Q) of (PDADMA/PSS)20 released “flipped” before (t = 0 min) and after dipping in 10 mM PSS (1 M NaCl) vs dipping time. a
The excess of positive charges is balanced by chloride counterions (yellow), and the added PSS compensates for the excess of polymer charge on the surface. Extrinsic compensation is represented by the shaded area.
employed to compare the roughness, modulus, and composition of both faces of the free films. All results were consistent with an excess of PDADMA through the PEMU down to the substrate. This excess charge is likely to be more prevalent for thicker PEMUs which include a nonlinear layer-by-layer growth pattern somewhere in their assembly (at the beginning or throughout growth). Though buried, nonstoichiometry controls many properties of PEMUs, such as permeability and bulk modulus. Thus, surface probes such as XPS and surface nanoindentation to shallow depths may not reflect properties of the entire film. Released films were treated with additional polyelectrolyte which compensated the excess charge of the buried interface.
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ASSOCIATED CONTENT
S Supporting Information *
Control FTIR spectra, XPS spectra, force vs indentation distance curves, and AFM images of nanosandwiches after soaking in PSS for different times. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00975.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (J.B.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Qifeng Wang for helpful discussions and Dr. Eric Lochner for help with XPS. This work was supported by Grant DMR-1207188 from the National Science Foundation.
Figure 6. (a) 3D images of (PDADMA/PSS)20 released “flipped” before and after dipping in PSS for 5 min. (b) Roughness of (PDADMA/PSS)20 released “flipped” vs dipping time in PSS. (c) Thickness of (PDADMA/PSS)20 released “flipped” vs dipping time in PSS.
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