Chain Conformation and Dynamics in Spin-Assisted Weak

Article pubs.acs.org/Langmuir

Chain Conformation and Dynamics in Spin-Assisted Weak Polyelectrolyte Multilayers Aliaksandr Zhuk,† Victor Selin,† Iryna Zhuk,† Benjamin Belov,‡ John F. Ankner,§ and Svetlana A. Sukhishvili*,† †

Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States ‡ Columbia High School, Maplewood, New Jersey 07040, United States § Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: We report on the effect of the deposition technique on film layering, stability, and chain mobility in weak polyelectrolyte layer-by-layer (LbL) films. Ellipsometry and neutron reflectometry (NR) showed that shear forces arising during spin-assisted assembly lead to smaller amounts of adsorbed polyelectrolytes within LbL films, result in a higher degree of internal film order, and dramatically improve stability of assemblies in salt solutions as compared to dip-assisted LbL assemblies. The underlying flattening of polyelectrolyte chains in spin-assisted LbL films was also revealed as an increase in ionization degree of the assembled weak polyelectrolytes. As demonstrated by fluorescence recovery after photobleaching (FRAP), strong binding between spin-deposited polyelectrolytes results in a significant slowdown of chain diffusion in salt solutions as compared to dipdeposited films. Moreover, salt-induced chain intermixing in the direction perpendicular to the substrate is largely inhibited in spin-deposited films, resulting in only subdiffusional (15% of initial PEM intensity by applying moderate-intensity bleaching beams. Since the Airy distribution is very similar to a Gaussian distribution,47 we used a 3891

DOI: 10.1021/acs.langmuir.5b00401 Langmuir 2015, 31, 3889−3896

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Langmuir gamma correction factor of 1.4 that has been theoretically calculated for the case of bleaching to 20% of initial intensity. The exact value of a bleaching spot radius (0.239 μm) was determined experimentally by fluorescence correlation spectroscopy using a calibration solution of Alexa-488 Fluor (Invitrogen Co., Carlsbad, CA) with a known diffusion coefficient. The FRAP instrument was equipped with a programmable computer-controlled shutter in order to illuminate the sample at the required time intervals. The experimental cells were filled with 0.01 M phosphate/citrate buffer containing an NaCl solution of the desired concentration and then sealed with a small glass slide to avoid solution evaporation. A spot in the films was bleached for 5 s by a focused laser at 1 mW. After the bleaching process, the fluorescence intensity of multilayer films in the bleached zone was recorded every 2−60 min (depending on the observed recovery kinetics) at 1 μW. Since the recovery time required for fluorescence intensity of films in all experiments was much longer than the bleaching time (i.e., >2 h vs 5 s), the contribution of molecular motion during bleaching to the fluorescence intensity recovery profile was negligible. The diffusion coefficient in the direction parallel to the substrate, D∥, of PMAA* within the LbL films was calculated as D∥ = γR2/4t1/2 (where the shape factor γ is 1.4 for a spherical beam spot and R = 0.239 μm is the radius of the bleached spot). A lower bound of the diffusion coefficient measured with our setup with acceptable experimental scatter is 10−15 cm2/s.

much thinner (58 and 52 nm, respectively) and do not exhibit radial thickness gradients. In this high-shear regime, polyelectrolyte chains are adsorbed in strongly flattened conformations. The scaling theory predicts that the crossover between the electrostatic-dominated and shear-dominated regimes is also dependent on the Debye screening length and therefore on the ionic strength of the deposition solutions.18 In agreement with this prediction, Figure 1C shows a decrease in film thickness and disappearance of thickness gradients for films deposited at 1500 rpm from 0.1 M NaCl solutions. Moreover, Figure S1 shows that at 1500 rpm film thickness (or adsorbed amount Γ) ∝ CNaCl−0.08. The latter is only in qualitative agreement with the suggested scaling in the high shear regime, Γ ∝ CNaCl−0.5.18 Hereafter, in this work, all results are presented for 3000 rpm deposition from low-salt buffer (0.01 M phosphate buffer at pH 4.5) to achieve spatially uniform films. In these high-shear conditions, spinning spreads adsorbing polyelectrolyte chains and reduces the average bilayer thickness more than 2-fold as compared to dipdeposited films (Figure 1A). To assume a flatter conformation and spread, an adsorbing chain should increase the number of its electrostatic binding sites with previously deposited polyelectrolytes. We were seeking to explore the effects of such chain spreading and those of an increase in the number of interpolyelectrolyte ionic pairs might have on the film’s propensity to be “doped” with small ions and to disintegrate in solutions with high salt concentrations. Added ions compete with polymer chain segments involved in layer-to-layer binding and at a critical concentration ccr can “unzip” the whole polymer chain, causing film disassembly. Figure 2A shows the fraction of LbL films that

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RESULTS AND DISCUSSION Figure 1A illustrates the effect of the spin deposition technique on the average bilayer thickness and surface roughness of QPC/PMAA films as monitored by ellipsometry and AFM, respectively. With an increase in the spin rate from 0 to 6000 rpm, the average bilayer thickness decreases from ∼4 to ∼2 nm, while films become ∼3-fold smoother (Figure 1A). While film smoothening and improved internal structure are universally reported for spin-deposited LbL films,16,20,48,49 the effect of centrifugal forces on the thicknesses of individual monolayers is more ambiguous. For strongly bound polyelectrolyte pairs, such as PAH/PSS, the layer thicknesses are usually higher in spinassisted films as compared to their dip-assisted counterparts. Obviously, in these strongly ionically stitched systems (binding energy of per ionic pair is as high as ∼4kBT50), the shear forces generated during spinning are not sufficiently strong to overcome multisite electrostatic binding and thus to redistribute polyelectrolyte chains on the surface. On the other hand, with weaker electrostatically bound polyelectrolytes, such as LPEI/PAA20 or LPEI/PMAA,48 and with hydrogen-bonded films,21 the centrifugal forces successfully compete with intermolecular binding, resulting in chain flattening and reduced layer thicknesses. The latter case is observed in this work. Lefaux et al. have developed a scaling theory that takes into account the balance of electrostatic and shear forces applied during spin-assisted deposition of polyelectrolyte multilayers.18 This model predicts, for example, the dominance of electrostatic binding (and large adsorbed amounts of polyelectrolyte) at low shear rates and strong hydrodynamic effects on chain conformations (and lower adsorbed amounts) at high shear rates.18 Since the shear rate is linearly proportional to the distance from the center of the spinning wafer, radial thickness gradients of deposited polyelectrolytes can occur if electrostatic and shear effects are of comparable magnitude. Figure 1B shows that such a gradient is observed in our experiments when a QPC/PMAA film is deposited at 1500 rpm from low-salt buffer solutions. The 24-bilayer QPC/PMAA films exhibit significantly different thicknesses between the center and the edge of the wafer (74 and 70 nm, respectively). Films deposited at higher spin rates of 3000 and 6000 rpm are

Figure 2. Decomposition of 24-bilayer QPC/PMAA films prepared by dip- or spin-assisted deposition from 2 mg/mL solutions (solid and open symbols, respectively) in salt solutions: (A) fraction of film thickness retained after 30 min and (B) disintegration kinetics of spinassisted films at various salt concentrations. Film thicknesses were measured by ellipsometry.

remained bound to the substrate after a 30 min immersion of dip- and spin-deposited films in salt solutions. Interestingly, spin-LbL multilayer films can withstand almost twice higher salt concentrations as compared to films prepared by the dipping technique (1.4 M vs 0.8 M NaCl, respectively). Figure 2B shows that the effect is not kinetic, and after a rapid mass loss in a 2 M NaCl solution, ∼55% of the original film thickness of the spin-deposited films is retained for as long as 2.5 days. Spinassisted deposition dramatically enhanced interpolymer interactions within the entire film and had a particularly strong effect on polyelectrolyte ionic pairing in the lower portion of the LbL film, creating a strongly ionically cross-linked network in that region. Char and co-workers have reported on the surface erosion of weak polyelectrolyte multilayer films upon pH3892

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Langmuir induced disintegration of spin-assisted LbL films, but the effect they observed was kinetic.48 Studies of salt-induced disintegration of weak polyelectrolyte multilayers prepared with fully ionized polyacids of different molecular weights also showed that molecular weight, i.e., the overall number of ionic pairs per chain, does not impact the critical salt concentration ccr for film disintegration.27 Similarly, the total number of hydrogen bonds per polymer chain (and therefore the polyelectrolyte molecular weight) does not affect the critical pH of disintegration of hydrogen-bonded multilayers.51,52 These findings reflect the fact that dissociation of polyelectrolyte complexes is controlled by segments, i.e., cooperative sequences of associated functional groups, with the molecular weight of assembled polyelectrolytes having only a kinetic effect. Figure 2 shows that film processing significantly alters segmental interactions within multilayers, drastically strengthening cooperative association between functional groups in spin-assisted films. We hypothesize that such strengthening is a result of a shear-induced increase in the density of associating functional groups and is related to shearinduced chain flattening. Therefore, we seek to directly observe such an effect in studies of the ionization of assembled PMAA. Figure 3 compares FTIR spectra of 60-bilayer QPC/PMAA films deposited on IR-transparent Si wafers using dip-assisted

S2) revealed that the content of polyacid within spin-deposited films was ∼3 times lower than that in dipped films (ratios of 1500−1800 cm−1 FTIR intensities to AFM-measured thicknesses were 0.021 and 0.007 abs units/nm for dip- and spinassisted films, respectively). The latter result is consistent with the expectation of flatter chain conformations and higher ionization in spin-assisted LbL assemblies. In this work, we also aim to study, to the best of our knowledge for the first time, the effect of LbL deposition technique on polyelectrolyte chain dynamics. To induce molecular motions in ionically cross-linked polyelectrolyte multilayers, films were “doped” with small ions by immersing in NaCl solutions. For FRAP measurements of the lateral diffusion of polyelectrolyte chains, a fluorescently labeled marker layer (PMAA with covalently attached Alexa-488, PMAA*) was incorporated into the LbL film during deposition. Figure S3 illustrates that repeated FRAP measurements with three different samples showed little experimental scatter. Earlier studies revealed that the deposition depth of PMAA* within LbL films has a significant effect on the mobility of polymer chains, with polyelectrolytes assembled within closeto-the-substrate layers moving slower and those residing within film top layers faster than polyelectrolytes at an intermediate depth within the film.33,36,42 To avoid these surface and substrate effects, PMAA* was sandwiched between two threebilayer QPC/PMAA stacks resulting in a QPC/PMAA)3/ (QPC/PMAA*)1/QPC/PMAA)3 film architecture. Moreover, to demonstrate the robustness of our results and their independence from the location of the marker layer within the central region of the film, we have also performed control experiments with three and five PMAA* marker layers deposited within the middle region. The control experiments summarized in Figure S4 show that the results were very similar to those obtained using a single PMAA* marker layer. Therefore, all further experiments were performed with a single marker PMAA*. Previous studies in our group demonstrated that exposure of dip-assisted QPC/PMAA films to 0.2 M NaCl solutions was sufficient to induce motions of assembled PMAA* chains with a diffusion coefficient of 1 × 10−16 cm2/s.42 Here, we focus on the effect of the deposition technique on D∥ when films are exposed to salt concentrations in annealing solutions varying from 0.2 to 1.0 M NaCl. Figure 4 summarizes the data for films

Figure 3. Representative FTIR spectra of 60 bilayer (QPC/PMAA)60 films prepared by dip-LbL (left) and spin-LbL (right) techniques.

or spin-assisted sequential adsorption. Two major bands associated with stretching vibrations of uncharged carboxylic groups (ν, −CO) at 1720 cm−1 and a band at 1552 cm−1 associated with asymmetric stretching vibrations of the carboxylate groups (νas, −COO−) allow one to evaluate the ionization degree of the assembled PMAA. The ionization degree of PMAA within assembled multilayers was calculated as the ratio of the area of −COO− to the sum of −CO and −COO− absorbances, assuming equal extinction coefficients for vibrations associated with these bands.28 Note that an additional peak at 1671 cm−1 was identified within the −COOH band. This peak, earlier assigned to hydrogen-bonded dimers of protonated carboxylic groups, has been added to the 1720 cm−1 band intensity. Similarly, a small peak at ∼1610 cm−1 in the spin-LbL spectrum has been added to the 1552 cm−1 band intensity. The 1610 cm−1 peak was previously observed for carboxylic groups bound with metal ions.53,54 The FTIR study revealed a significantly higher ionization degree of carboxylic groups (25% for spin vs 10% for dip-assisted assemblies) and therefore a larger number of ionic cross-links per PMAA chain in spin-assisted films. Finally, shear-induced spreading of polymer chains during spin deposition is expected and has been reported to decrease the concentration of a weak polyelectrolyte within the film.20 In this work, comparison of the FTIR intensities of PMAA carboxylic group stretching vibrations with the film thicknesses measured by AFM (Figure

Figure 4. Log−log plots of lateral diffusion coefficients, D∥, versus NaCl concentration for dip-coated (red circles) and spin-coated (blue triangles) (QPC/PMAA)3/(QPC/PMAA*)1/(QPC/PMAA)3 LbL films containing 35 kDa PMAA*. Inset shows representative FRAP recovery curves for PMAA* assembled within dip- and spin-assisted films in 0.4 M NaCl. 3893

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Langmuir deposited in low salt conditions and exposed to solutions with high salt concentrations. Significantly slower diffusion is observed within spin-assisted multilayers as compared to dipassisted films. This observation reflects shear-induced chain stretching and tighter binding of PMAA chains to QPC in spinassisted assemblies. A 3−5-fold slower chain dynamics in spinassisted films persisted over a wide range of salt concentrations. Note that measurements of dip-deposited films in salt solutions containing >0.6 M NaCl were not possible because of the instability of these films at these salt concentrations. Within experimental scatter, the data could be fitted by either a power law dependence D∥ ∝ [NaCl]x shown in Figure 4 (where x is the slope of a log−log plot of the data) or by an exponential function D∥ ∝ eb[NaCl] (not shown), where b is the slope of a semilogarithmic plot of the data. The quality of the fit was better, however, for the power law dependence of chain mobility on salt concentration, giving values for x of ∼0.58 and 0.78 with R2 of 0.9622 and 0.9798 for dip- and spin-deposited films, respectively (Figure 4). Power law dependencies typically follow from scaling arguments, and such a dependence of polyelectrolyte diffusion within multilayer films has been predicted by Schlenoff and co-workers.45 If one assumes that x in the D∥ = D0 × [NaCl]x dependence is correlated to the number of cooperative sequences participating in the diffusional motion, then one concludes that the size of these units must be larger within spin-assisted multilayers. Even for the highest salt concentration of 1 M NaCl used with spin-deposited films, PMAA* chains diffused significantly more slowly than in dipassisted films at lower salt concentrations, presumably as a result of flatter chain conformations and a larger number of interchain contacts within spin-assisted films. Next, we directly assess the chain conformations that give rise to these differences in polymer chain dynamics and also explore the coupling of chain dynamics in directions parallel and perpendicular to the substrate. Figure 5 shows neutron reflectivity data from 24-bilayer QPC/PMAA films deposited in low-salt solutions by dip- and spin-assisted techniques. The dry film thickness determined by NR was in good agreement with ellipsometry and AFM data. To enhance the scattering features, the data are presented as neutron reflectivity multiplied by momentum transfer to the fourth (RQ4) plotted as a function of Q. The NR data were fitted by diffuse marker layers exhibiting significant layer intermixing. The scattering densities, layer thicknesses, and internal roughnesses within the multilayer films in Figure 5 are summarized in Tables S1−S4 of the Supporting Information. The overall film thicknesses obtained from NR were 60 and 102 nm, in good agreement with ellipsometry data. For both films, in order to fit the NR data, the thicknesses of dPMAA marker layers significantly exceeded their nominal values determined from the average ellipsometric film thickness. Figure 5 clearly shows sharper and better developed Bragg peaks and Kiessig fringes for the spin-assisted multilayers, indicating a higher degree of layering in these films. The average fitted dPMAA layer thickness d and dPMAA interfacial roughness σint were twice lower for spin-assisted films as compared to dip-deposited multilayers. Thus, the dPMAA thickness values were 31 and 60 Å, whereas interfacial roughnesses σint were 26 and 39 Å for spin- and dip-assisted multilayers, respectively. These numbers indicate that while dPMAA chains intermix only with single neighboring bilayers during spin-assisted deposition, chains diffuse much larger distances in the absence of shear forces. Interpenetration of

Figure 5. Neutron reflectivity data (plotted as RQ4 to enhance small features) (A, C) and corresponding fitted scattering length density profiles (B, D) for dry [(QPC/PMAA)4/(QPC/dPMAA)]4/(QPC/ PMAA)4 PEMs prepared by dip-assisted (A, B) and spin-assisted assembly (C, D), after annealing for 24 and 200 h in 0.01 M phosphate/citrate buffer with 0.4 and 1 M NaCl solutions for dip- and spin-coated samples. Top and bottom curves in each panel correspond to films before and after annealing in salt, respectively.

layers within dip-assisted LbL assemblies is a well-known phenomenon which was reported in other PE multilayer systems34,36,41,42,48 and hydrogen-bonded6 films. In the case of multilayers prepared by the spin-assisted method, a highly ordered internal structure far superior to the structure of dipLbL has been reported.13,14,19 The degree of interdiffusion of macromolecules during spin-assisted deposition can be controlled by the salt concentration.19 How do differences in initial film ordering affect the propensity of these films to intermix in salt solutions? The diffusion coefficient in the direction perpendicular to the substrate, D⊥, was estimated from NR measurements of films annealed in salt solutions using the equation Δσint2 = 2D⊥Δt, where Δσint is the interfacial roughness change of the deuterated layers and Δt is the salt annealing time.35 In the case of dip-assisted assemblies exposed to 0.4 M NaCl, the thickness and interfacial roughness of the dPMAA layers increased with time, suggesting subdiffusional motion (Figure 5A) with D⊥ of dPMAA calculated to be 9.7 × 10−19 cm2/s. Within spin-assembled films, these motions were almost completely inhibited, as shown by the absence of significant changes in film internal roughness (≤1 Å) after 200 h annealing in 1 M solutions (Figure 5B). This significant difference in the mobility of polyelectrolytes within spin- and dip-assisted assemblies reflects the higher density of ionic pairs and stronger binding of polyelectrolyte chains within spin-assisted films. This result also means that chain motions are more anisotropic when the spin assembly technique is chosen over traditional dipping. The strength of interpolyelectrolyte binding and the anisotropy in film structure are introduced into these assemblies during film deposition, and these characteristics dictate the magnitude and anisotropy of chain dynamics when motions are later “unfrozen” in salt solutions. Even in the dipdeposited films, this structural templating results in an ∼104 faster diffusion of polyelectrolyte chains in the direction lateralto-the-substrate as compared to that in the normal-to-thesubstrate direction.42 In this work, we show that this anisotropy 3894

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(5) Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Detailed Structure of Molecularly Thin Polyelectrolyte Multilayer Films on Solid Substrates as Revealed by Neutron Reflectometry. Macromolecules 1998, 31, 8893−8906. (6) Kharlampieva, E.; Kozlovskaya, V.; Ankner, J. F.; Sukhishvili, S. A. Hydrogen-Bonded Polymer Multilayers Probed by Neutron Reflectivity. Langmuir 2008, 24, 11346−11349. (7) Salomaki, M.; Laiho, T.; Kankare, J. Counteranion-Controlled Properties of Polyelectrolyte Multilayers. Macromolecules 2004, 37, 9585−9590. (8) Dubas, S. T.; Schlenoff, J. B. Factors Controlling the Growth of Polyelectrolyte Multilayers. Macromolecules 1999, 32, 8153−8160. (9) Itano, K.; Choi, J. Y.; Rubner, M. F. Mechanism of the pHInduced Discontinuous Swelling/Deswelling Transitions of Poly(allylamine Gydrochloride)-Containing Polyelectrolyte Multilayer Films. Macromolecules 2005, 38, 3450−3460. (10) Shiratori, S. S.; Rubner, M. F. pH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak Polyelectrolytes. Macromolecules 2000, 33, 4213−4219. (11) Salomaki, M.; Vinokurov, I. A.; Kankare, J. Effect of Temperature on the Buildup of Polyelectrolyte Multilayers. Langmuir 2005, 21, 11232−11240. (12) Sun, B.; Jewell, C. M.; Fredin, N. J.; Lynn, D. M. Assembly of Multilayered Films Using Well-Defined, End-Labeled Poly(acrylic Acid): Influence of Molecular Weight on Exponential Growth in a Synthetic Weak Polyelectrolyte System. Langmuir 2007, 23, 8452− 8459. (13) Jain, M. K. The Flow of a Non-Newtonian Liquid Near a Rotating Disk. Appl. Sci. Res. 1961, 10, 410−418. (14) 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. (15) Lee, S.-S.; Hong, J.-D.; Kim, C. H.; Kim, K.; Koo, J. P.; Lee, K.B. Layer-by-Layer Deposited Multilayer Assemblies of Ionene-Type Polyelectrolytes Based on the Spin-Coating Method. Macromolecules 2001, 34, 5358−5360. (16) 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. (17) Chiarelli, P. A.; Johal, M. S.; Holmes, D. J.; Casson, J. L.; Robinson, J. M.; Wang, H.-L. Polyelectrolyte Spin-Assembly. Langmuir 2001, 18, 168−173. (18) Lefaux, C. J.; Zimberlin, J. A.; Dobrynin, A. V.; Mather, P. T. Polyelectrolyte Spin Assembly: Influence of Ionic Strength on the Growth of Multilayered Thin Films. J. Polym. Sci., Polym. Phys. 2004, 42, 3654−3666. (19) Kharlampieva, E.; Kozlovskaya, V.; Chan, J.; Ankner, J. F.; Tsukruk, V. V. Spin-Assisted Layer-by-Layer Assembly: Variation of Stratification as Studied with Neutron Reflectivity. Langmuir 2009, 25, 14017−14024. (20) Lee, Y. M.; Park, D. K.; Choe, W. S.; Cho, S. M.; Han, G. Y.; Park, J.; Yoo, P. J. Spin-Assembled Layer-by-Layer Films of Weakly Charged Polyelectrolyte Multilayer. J. Nanosci. Nanotechnol. 2009, 9, 7467−7472. (21) Seo, J.; Lutkenhaus, J. L.; Kim, J.; Hammond, P. T.; Char, K. Effect of the Layer-by-Layer (LbL) Deposition Method on the Surface Morphology and Wetting Behavior of Mydrophobically Modified PEO and PAA LbL Films. Langmuir 2008, 24, 7995−8000. (22) Kozlovskaya, V.; Zavgorodnya, O.; Wang, Y.; Ankner, J. F.; Kharlampieva, E. Tailoring Architecture of Nanothin Hydrogels: Effect of Layering on pH-Triggered Swelling. ACS Macro Lett. 2013, 2, 226− 229. (23) Lee, S.-S.; Lee, K.-B.; Hong, J.-D. Evidence for Spin Coating Electrostatic Self-Assembly of Polyelectrolytes. Langmuir 2003, 19, 7592−7596. (24) An, M. S.; Hong, J. D. Consecutively Spin-Sssembled Layered Nanoarchitectures of Poly(sodium 4-styrene sulfonate) and Poly(allylamine hydrochloride). Thin Solid Films 2006, 500, 74−77.

is further enhanced as a result of chain stretching and stronger layering in the film caused by shear forces arising during spinassisted deposition.

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CONCLUSIONS In summation, we have studied the effect of deposition technique on stability, internal structure, and dynamics of polyelectrolyte chains within LbL. Deposition of polyelectrolytes within multilayer films using the spin-assisted technique results in increased ionic pairing and enhanced stability of the films in salt solutions. Significant flattening of weak polyelectrolyte chains by centrifugal forces leads to slower diffusion and higher anisotropy of chain motions as compared to films prepared by the dip-assisted technique. While with dipLbL multilayers, diffusion was anisotropic with ∼104 difference in diffusion coefficients in directions lateral and perpendicular to the substrate, spin-LbL films demonstrated an even greater anisotropy of chain motions. Specifically, there is almost complete “freezing” of chain mobility in the direction normal to the substrate for long annealing times in salt solutions, while diffusion in the direction parallel to the substrate is much less suppressed as compared to dip-deposited films. Such a finding is highly important in applications where the interdiffusion of coating constituents should be minimized, such as in multidrug delivery systems and structured optical coatings.

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ASSOCIATED CONTENT

S Supporting Information *

Fitting parameters of neutron reflectivity, atomic force microscopy pictures, and time recovery of fluorescence intensity. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.A.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Award DMR-0906474. The neutron reflectometry measurements were performed at the Spallation Neutron Source at the Oak Ridge National Laboratory, managed by UTBattelle, LLC, for the DOE under Contract DE-AC0500OR22725.

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REFERENCES

(1) Decher, G.; Lvov, Y.; Schmitt, J. Proof of Multilayer Structural Organization in Self-Assembled Polycation Polyanion Molecular Films. Thin Solid Films 1994, 244, 772−777. (2) Decher, G., Schlenoff, J. B., Eds.; Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials, 2nd ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2012. (3) Kellogg, G. J.; Mayes, A. M.; Stockton, W. B.; Ferreira, M.; Rubner, M. F.; Satija, S. K. Neutron Reflectivity Investigations of SelfAssembled Conjugated Polyion Multilayers. Langmuir 1996, 12, 5109−5113. (4) Korneev, D.; Lvov, Y.; Decher, G.; Schmitt, J.; Yaradaikin, S. Neutron Reflectivity Analysis of Self-Assembled Film Superlattices with Alternate Layers of Deuterated and Hydrogenated Polysterenesulfonate and Polyallylamine. Physica B 1995, 213, 954−956. 3895

DOI: 10.1021/acs.langmuir.5b00401 Langmuir 2015, 31, 3889−3896

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Langmuir (25) Patel, P. A.; Dobrynin, A. V.; Mather, P. T. Combined Effect of Spin Speed and Ionic Strength on Polyelectrolyte Spin Assembly. Langmuir 2007, 23, 12589−12597. (26) Cho, J.; Lee, S. H.; Kang, H. M.; Char, K.; Koo, J.; Seung, B. H.; Lee, K. B. Quantitative Analysis on the Adsorbed Amount and Structural Characteristics of Spin Self-Assembled Multilayer Films. Polymer 2003, 44, 5455−5459. (27) Dubas, S. T.; Schlenoff, J. B. Swelling and Smoothing of Polyelectrolyte Multilayers by Salt. Langmuir 2001, 17, 7725−7727. (28) Sukhishvili, S. A.; Granick, S. Layered, Erasable Polymer Multilayers Formed by Hydrogen-Bonded Sequential Self-Assembly. Macromolecules 2002, 35, 301−310. (29) Izumrudov, V.; Sukhishvili, S. A. Ionization-Controlled Stability of Polyelectrolyte Multilayers in Salt Solutions. Langmuir 2003, 19, 5188−5191. (30) Kharlampieva, E.; Sukhishvili, S. A. Ionization and pH Stability of Multilayers Formed by Self-Assembly of Weak Polyelectrolytes. Langmuir 2003, 19, 1235−1243. (31) McAloney, R. A.; Dudnik, V.; Goh, M. C. Kinetics of SaltInduced Annealing of a Polyelectrolyte Multilayer Film Morphology. Langmuir 2003, 19, 3947−3952. (32) Jourdainne, L.; Lecuyer, S.; Arntz, Y.; Picart, C.; Schaaf, P.; Senger, B.; Voegel, J. C.; Lavalle, P.; Charitat, T. Dynamics of Poly(LLysine) in Hyaluronic Acid/Poly(L-Lysine)Multilayer Films Studied by Fluorescence Recovery After Pattern Photobleaching. Langmuir 2008, 24, 7842−7847. (33) Nazaran, P.; Bosio, V.; Jaeger, W.; Anghel, D. F.; von Klitzing, R. Lateral Mobility of Polyelectrolyte Chains in Multilayers. J. Phys. Chem. B 2007, 111, 8572−8581. (34) Xu, L.; Ankner, J. F.; Sukhishvili, S. A. Steric Effects in Ionic Pairing and Polyelectrolyte Interdiffusion within Multilayered Films: A Neutron Reflectometry Study. Macromolecules 2011, 44, 6518−6524. (35) Soltwedel, O.; Ivanova, O.; Nestler, P.; Müller, M.; Köhler, R.; Helm, C. A. Interdiffusion in Polyelectrolyte Multilayers. Macromolecules 2010, 43, 7288−7293. (36) Xu, L.; Pristinski, D.; Zhuk, A.; Stoddart, C.; Ankner, J. F.; Sukhishvili, S. A. Linear versus Exponential Growth of Weak Polyelectrolyte Multilayers: Correlation with Polyelectrolyte Complexes. Macromolecules 2012, 45, 3892−3901. (37) Kharlampieva, E.; Ankner, J. F.; Rubinstein, M.; Sukhishvili, S. A. pH-Induced Release of Polyanions from Multilayer Films. Phys. Rev. Lett. 2008, 100, 128303. (38) 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. (39) Jang, Y.; Akgun, B.; Kim, H.; Satija, S.; Char, K. Controlled Release from Model Blend Multilayer Films Containing Mixtures of Strong and Weak Polyelectrolytes. Macromolecules 2012, 45, 3542− 3549. (40) Soltwedel, O.; Nestler, P.; Neunnann, H. G.; Passvogel, M.; Kohler, 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. (41) Xu, L.; Selin, V.; Zhuk, A.; Ankner, J. F.; Sukhishvili, S. A. Molecular Weight Dependence of Polymer Chain Mobility within Multilayer Films. ACS Macro Lett. 2013, 2, 865−868. (42) Xu, L.; Kozlovskaya, V.; Kharlampieva, E.; Ankner, J. F.; Sukhishvili, S. A. Anisotropic Diffusion of Polyelectrolyte Chains within Multilayer Films. ACS Macro Lett. 2012, 1, 127−130. (43) Zhang, X.; Xia, J. H.; Gaynor, S. G.; Matyjaszewski, K. Atom Transfer Radical Polymerization Using Functional Initiators Containing Carboxylic Acid Group. Abstr. Pap. Am. Chem. Soc. 1998, 216, U872−U872. (44) Pristinski, D.; Kozlovskaya, V.; Sukhishvili, S. A. Fluorescence Correlation Spectroscopy Studies of Diffusion of a Weak Polyelectrolyte in Aqueous Solutions. J. Chem. Phys. 2005, 122, 014907.

(45) Jomaa, H. W.; Schlenoff, J. B. Salt-Induced Polyelectrolyte Interdiffusion in Multilayered Films: A Neutron Reflectivity Study. Macromolecules 2005, 38, 8473−8480. (46) Crank, J. The Mathematics of Diffusion, 2nd ed.; Clarendon Press: Oxford, England, 1975; p 414. (47) Axelrod, D.; Koppel, D. E.; Schlessinger, J.; Elson, E.; Webb, W. W. Mobility Measurement by Analysis of Fluorescence Photobleaching Recovery Kinetics. Biophys. J. 1976, 16, 1055−1069. (48) Jang, Y.; Seo, J.; Akgun, B.; Satija, S.; Char, K. Molecular Weight Dependence on the Disintegration of Spin-Assisted Weak Polyelectrolyte Multilayer Films. Macromolecules 2013, 46, 4580−4588. (49) Jiang, C.; 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, 157− 161. (50) Jaber, J. A.; Schlenoff, J. B. Counterions and Water in Polyelectrolyte Multilayers: A Tale of Two Polycations. Langmuir 2006, 23, 896−901. (51) Kharlampieva, E.; Sukhishvili, S. A. Hydrogen-Bonded Layer-byLayer Polymer Films. J. Macromol. Sci., Part C 2006, 46, 377−395. (52) Lee, H.; Mensire, R.; Cohen, R. E.; Rubner, M. F. Strategies for Hydrogen Bonding Based Layer-by-Layer Assembly of Poly(vinyl alcohol) with Weak Polyacids. Macromolecules 2011, 45, 347−355. (53) Piccolo, A.; Stevenson, F. J. Infrared-Spectra of Cu2+, Pb2+, and Ca2+ Complexes of Soil Humic Substances. Geoderma 1982, 27, 195− 208. (54) Pathak, A.; Kumar, S. A Novel Multiscale Model of Cell Adhesion and Migration on Defined Extracellular Matrices. Biophys. J. 2010, 98, 729a.

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DOI: 10.1021/acs.langmuir.5b00401 Langmuir 2015, 31, 3889−3896