Article pubs.acs.org/Langmuir
Response of Swelling Behavior of Weak Branched Poly(ethylene imine)/Poly(acrylic acid) Polyelectrolyte Multilayers to Thermal Treatment Yuanqing Gu, Emily K. Weinheimer, Xiang Ji, Clinton G. Wiener, and Nicole S. Zacharia* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States S Supporting Information *
ABSTRACT: Weak polyelectrolyte multilayers (PEMs) prepared by the layer-by-layer technique have attracted a great deal of attention as smart responsive materials for biological and other applications in aqueous medium, but their dynamic behavior as a function of exposure to a wide temperature range is still not well understood. In this work, the thermally dependent swelling behavior of PEMs consisting of branched poly(ethylenimine) and poly(acrylic acid) is studied by temperature controlled in situ spectroscopic ellipsometry. Because of diffusion and interpenetration of polyelectrolytes during film deposition, the PEMs densify with increasing bilayer number, which further affects their water uptake behavior. Upon heating to temperatures below 60 °C, the worsened solvent quality of the PEM in water causes deswelling of the PEMs. However, once heated above this critical temperature, the hydrogen bonds within the PEMs are weakened, which allows for chain rearrangement within the film upon cooling, resulting in enhanced water uptake and increased film thickness. The current work provides fundamental insight into the unique dynamic behavior of weak polyelectrolyte multilayers in water at elevated temperatures.
■
INTRODUCTION Polyelectrolyte multilayers (PEMs) are assemblies of polyelectrolytes (PEs) prepared by the layer-by-layer (LbL) technique, which is based on the entropy driven directed complexation of oppositely charged PEs at interfaces.1 The ease of preparation, finely tunable film thickness, widely available starting materials, and ability to coat 2D or 3D structured substrates have shown LbL to be a powerful method to design and fabricate versatile functional materials a range of applications.2−9 While strong PEs offer a high density of ionic binding sites for assembly with oppositely charged chains forming thin, highly cross-linked PEMs, weak PEs instead have a variable charge density determined by the surrounding environment. In response to external stimuli (e.g., pH, ion strength, temperature, etc.),2,10−12 the ionization degree of labile functional groups (e.g., carboxyl or amine groups) can rapidly change and influence the density of inter-PE ionic crosslinks in PEMs, determining properties of the final PEMs such as film thickness, porosity, mechanical strength, hydrophilicity, swelling performance, and stickiness.13,14 Such weak PE derived PEMs are especially attractive for biological applications and more generally for application in aqueous environments,15−19 due to these materials compatibility with water.19 For practical applications, one concern is the use of these materials in dynamic environments (e.g., when changes in temperature under physiological conditions or changes in pH or ionic strength might occur). In recent work, it was shown that the outermost layer of PEMs made from gelatin/poly(galacturonic © XXXX American Chemical Society
acid) disassembles at high temperature due to breaking of hydrogen bonds;20 hydrophilic−hydrophobic−hydrophilic triblock copolymer containing PEMs achieves reversible thermally induced swelling transition,21 and their transition temperature decreases by increasing hydrophobic block length.22 The behavior of PEMs in response to changes in thermal environment has been applied to controlled permeability,23 tunable fluorescence,24 and smart molecule release.25 However, most in situ measurement of swollen PEMs thickness/mass was performed within a small temperature range (typically 10−37 °C), and consequently the dynamic behavior of PEMs over a more widely varying temperature is not well-understood. In this paper, the unique swelling behavior of PEMs consisting of branched poly(ethylenimine)/poly(acrylic acid) (BPEI/PAA) multilayers in temperature controlled aqueous environment is studied. In a PEM, weak PEs interact with their neighboring counter parts as well as with water through not only electrostatics but also less stable and temperature dependent hydrogen bonds,26 making PEMs formed from weak PEs sensitive to the surrounding temperature. In previous works investigating thermal response of PEMs, thickness changes were characterized by microscopy (electron microscopy,27,28 atomic force microscopy,29,30 scanning force microscopy,31 optical microsReceived: January 20, 2016 Revised: May 12, 2016
A
DOI: 10.1021/acs.langmuir.6b00206 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir copy)32 or profilometry.33 However, these reports only describe measurement of dried samples before and after heated solution treatment. Film structure and swelling behavior at determined temperature have also been studied by neutron reflectivity,34−37 but the time-consuming measurement process limits detecting instantaneous responses to external conditions. Quartz crystal microbalance with dissipation monitoring (QCM-d) is another in situ technique used for this type of analysis. While this is a powerful method to evaluate film thickness and mass even in temperature controlled solutions,38 the commonly applied Sauerbrey expression for film thickness calculation is limited to thin film samples (typically total thickness 65 °C) induces hydrogen bond rearrangement within the film and results in an enhanced degree of swelling during the cooling process. The current work on the dynamic swelling of BPEI/PAA multilayers in aqueous solutions provides a foundation for the design and application of PEM-based devices made from weak PEs in aqueous environments.
■
number of BPEI/PAA bilayers (denoted as (BPEI/PAA)n hereafter) were deposited on the silicon wafer. These PEM films were dried at room temperature (∼25 °C) and atmospheric relative humidity (RH, ∼ 70%) for 24 h prior to characterization. For comparison, various (BPEI/PAA)6 films were fabricated by using BPEI solution with pH 8.0 and 8.5, respectively. Characterization. The dry film thickness of the as-prepared (BPEI/PAA)n films was measured by the average value from five different positions of the film using a stylus profilometer (P6, KLA Tencor Instruments, CA, USA). Fourier transform infrared (FTIR) spectra were collected by an FTIR spectrometer (Alpha-P, Bruker Optics, MA, USA) using transmission mode. The samples were tested immediately at room temperature (∼25 °C) with the atmospheric RH (∼70%) after vacuum drying overnight except denoted elsewhere. To check the presence of PE component released from the PEMs into water, the solution used to soak PEMs (10 mL) was drop casted onto silicon wafer (1 cm × 1 cm), dried in air, and further vacuum-dried overnight for FTIR and energy dispersive spectroscopy analysis (EDS, acceleration voltage 10 keV). FTIR analysis at different temperatures was carried out by applying FTIR spectrometer equipped with Praying Mantis and thermal controller. The samples were equilibrated at fixed temperature for 15 min and then tested in reflectance mode. For in situ characterization of the PEM films to define their thicknesses and optical constants in both dry and swollen states, a variable angle spectroscopic ellipsometer (VASE, M-2000 UV− visible−NIR [240−1700 nm] J. A. Woollam Co., Inc., Lincoln, NE, USA) equipped with a temperature controlled liquid cell was used (accuracy ±0.1 °C, SI, Scheme S1b). The cell geometry dictated the angle of incidence to be 75°. Because of absorption in the ultraviolet and near-infrared light region by the solvents, a limited wavelength range from 250 to 1050 nm was used for the recursive fits. Before each measurement, the sample was aligned, and standard window correction function of Complete EASE (J. A. Woollam, Co., Inc., Lincoln, NE, USA) was run with silicon wafer with thermal oxide (25.0 nm thick) as the reference using Si with thermal oxide model (silicon substrate layer, a fixed 1.0 nm Si−SiO2 interface layer, and a thermal oxide layer).39,40 To fit the ellipsometry data of each sample, a fourlayer model consisting of a silicon substrate layer (temperature library), a fixed 1.0 nm Si−SiO2 interface layer, a thermal oxide layer, and an effective medium approximations (EMA) layer coupled with Cauchy model and H2O (temperature library) model.41,42 All in situ measurements were started in dry state at a fixed temperature of 25 °C. After 1 min, DI water was carefully charged to the cell by using a syringe to avoid generating possible air bubbles, and ambient index with H2O (temperature library) model was meanwhile applied to fit the ellipsometric data. The liquid cell was then heated at a heating rate of 2 °C/min, and the sample was maintained at selected temperatures until reaching equilibrium. Temperature data from parameter log were used during data fitting in all cases to reflect actual temperature effect on film thickness. X-ray photoelectron spectrometry (XPS) depth profiling test of the as-prepared PEM films were proceeded by applying a PHI VersaProbe II X-ray photoelectron spectrometer with a scanning monochromated Al Kα source (E = 1486.6 eV; 50 W; spot size, 100 μm). The C60+ ion source of the equipment (10 kV, 10 nA) was set at an angle of 70°, and it rastered over a 2 mm × 2 mm area. Sputtering was carried out at 1 min intervals, and the sample was moved through concentric Zalar rotation at 1 rpm. Atomic composition was defined by the photoelectron peak areas as well as the relative sensitivity factors given in PHI’s MultiPak processing software. All data were corrected based on carbon−carbon bond (binding energy 284.6 eV) after background subtraction. The silicon surface was determined as the point that the atomic concentration of silicon reached 5% in the depthprofiling data.
EXPERIMENTAL SECTION
Materials. Branched polyethylenimine (BPEI, Mw = 25,000), poly(diallyldimethylammonium chloride) (PDAC, average M w 100000−200000, 20 wt % in H2O), sodium poly(styrenesulfonate) (SPS, average Mw ≈ 70000), sodium hydroxide, hydrochloric acid, sulfuric acid, and hydrogen peroxide were purchased from SigmaAldrich. Polyacrylic acid (PAA, Mw = 50000, 25 wt % solution) was purchased from Polysciences. Deionized (DI) water used in all experiments from a Milli-Q DQ-3 system (Millipore, Bedford, MA, USA) with resistivity of 18.2 MΩ cm. All materials were used as received without further purification. Piranha solution was prepared by mixing 98% sulfuric acid with 30% hydrogen peroxide with a volume ratio of v/v = 7/3. BPEI solution was prepared at 80 mM with respect to the amine group of BPEI in DI water. The solution was stirred overnight, and pH was adjusted to 9.5 by adding 0.1 M hydrochloride solution. Similarly, PAA solution was prepared at 60 mM with respect to the carboxyl group of PAA in DI water, stirred overnight, and subsequently pH was adjusted to 4.5 by adding 0.1 M sodium hydroxide solution. All PE solutions were filtered prior to use. Film Assembly. PEMs were LbL assembled on silicon wafers as described previously (Supporting Information (SI), Figure S1).2,13 Typically, silicon wafers were first immersed in a freshly prepared piranha solution at room temperature for 2 h, rinsed with excess DI water until neutral, dried by nitrogen flow, and then plasma treated for 5 min prior to use. The LbL assembly of PEMs was carried out sequentially at room temperature employing a StratoSequence VI dipper (NanoStrata Inc., USA). In a typical procedure, dried silicon wafers were first immersed in the BPEI solution for 10 min and then washed by three separate DI water rinse baths of 1 min each. Subsequently, these substrates were exposed to the PAA solution for 10 min and washed with DI water in a same manner. Thus, a thin BPEI/PAA bilayer was deposited on each silicon wafer surface. By repeating the BPEI/PAA bilayer assembly step for n cycles, a desired
■
RESULTS AND DISCUSSION BPEI/PAA multilayer films were deposited onto silicon wafers by the LbL technique using BPEI and PAA solutions (SI, B
DOI: 10.1021/acs.langmuir.6b00206 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Scheme S1a).2,14 Polished silicon wafers were used because the flat surface and high refractive index enable collection of high resolution ellipsometric spectra with ease of data fitting.42 BPEI and PAA are weak PEs, and the pKa of the primary, secondary, and tertiary amine groups in BPEI and of the carboxyl acid group in PAA are 4.5, 6.7, 11.6, and 6.5, respectively.43,44 Both PE chains were partially charged during assembly through adjusting pH 9.5 for the BPEI solution and 4.5 for the PAA solution, realizing construction of relatively thick PEMs. By repeating BPEI/PAA bilayer assembly cycle n times, (BPEI/ PAA)n films with desired thicknesses were formed on silicon wafers. The average film thicknesses measured by ellipsometry are 39, 264, 758, 1353, 1888, 2854, 3631, 4771, and 5582 nm for 5−13 BPEI/PAA bilayers, which match well with the stylus profilometry results for the corresponding samples (Figure 1a),
(Figure S1, see SI for data and experimental description). To investigate the composition of the BPEI/PAA multilayers, a C60+ ion beam was used to etch the films and analyzed by XPS technique.48 The resulting XPS depth profiling data reveals constant atomic ratio of C, O, and N elements throughout the deposition direction of the (BPEI/PAA)n films (Figure S2, SI). Both of these experiments confirm that there is no gradient in film composition throughtout. This then means that the density of (BPEI/PAA)n films can be calculated by the Lorentz−Lorenz equation,49 which correlates film density with corresponding the refractive index with the following equation:50 rd =
n2 − 1 n2 + 2
(1)
where r is specific refractivity of PEs, n refers to the film refractive index, and d represents the density. As the specific refractivity is almost independent of chain aggregation, the r value remains the same for (BPEI/PAA)n films under identical condition.49,50 The ratio in densities between two different (BPEI/PAA)n films can then be described as d1 (n 2 − 1) × (n22 + 2) = 12 d2 (n1 + 2) × (n22 − 1)
(2)
On the basis of the density of a first film, which is taken as dry (BPEI/PAA)6, calculated from previous in situ ellipsometry combined with QCM-D (ca. 1.05 g/cm3 as d1),13 it is possible to obtain the density of a second (BPEI/PAA)n film (d2) from the related refractive indices (n1 and n2) at the same temperature (25 °C). Starting from 0.97 g/cm3 for dry (BPEI/PAA)5 film, the film density increases with the addition of each bilayer and it reaches a nearly stable value after 8 BPEI/ PAA bilayers (∼1.08 g/cm3, Figure 1b). Although the density does change with number of bilayers, these values are never very far off from the densities of bulk, dry BPEI and PAA (∼1.03 g/cm3 according to the chemical suppliers). That is to say, the film density is seen to increase over the portion of film growth that is exponential and plateaus at the point that growth becomes linear. For exponential LbL assembly of partially charged weak PEs, chains penetrate into neighboring bilayers, diffusing into the previously deposited film.46 These interpenetrating PE chains occupy free spaces within the film and densify the PEM. At some point, further interpenetration is limited as the film becomes too dense to allow for further interpenetration, and it is at this point that the growth becomes linear again. The swelling behavior of (BPEI/PAA)n multilayer films was monitored by in situ spectroscopic ellipsometry, and the swelling ratio of the films is calculated based on the change in film thickness in water:14
Figure 1. (a) Average dry film thickness of different BPEI/PAA bilayers measured by ellipsometry and profilometry. (b) Refractive index and density of dry (BPEI/PAA)n films.
although ellipsometric data gives slightly higher values than stylus profilometry due to the pressure of the stylus pushing down on the film surface during measurement.14 As expected, film thickness increases with increasing number of bilayers. The film thickness grows exponentially until about eight bilayers, and then increases linearly after nine bilayers, which is in fair agreement with the typical film growth for this system.45 In systems like this with regimes of exponential growth, a good deal of interdiffusion can take place during the deposition process.46 X-ray reflectivity has shown that systems with at least one strong polyelectrolyte such as poly(dimethyldiallylammonium chloride) and poly(allylamine hydrochloride) (PDAC/PAH) has something of a stratified structure. In this case, individual layers exist but are interdigitated.47 For BPEI/PAA, neutron reflectivity shows no inner structure, consistent with the idea that chains are continually rearranging during the film build up
swelling ratio % =
H − H0 × 100% H0
(3)
where H represents the swollen film thickness in water and H0 refers to the initial dry film thickness. As summarized in Figure 2a, when the as-prepared (BPEI/PAA)5 films were immersed in DI water at room temperature, a five bilayer film swells to a thickness which is 5.02% higher than that in the dry state, and the degree of swelling increases with higher bilayer numbers, reaching a peak value of 58.18% for eight bilayers. For even thicker films, the swelling ratio decreases from the peak swelling value of the eight bilayer system. This ascending-to-descending trend is consistent with a previous QCM-d study of PEMs C
DOI: 10.1021/acs.langmuir.6b00206 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 2. Swelling ratio and fitted water content for BPEI/PAA films with different numbers of bilayers soaked in DI water at room temperature.
consisting of weak PEs, in which swelling degree reaches maximum at 7.5 bilayers.51,52 BPEI/PAA multilayer films were also prepared by varied pH condition of BPEI solution (8.0− 8.5). In spite of small differences in initial charge densities with these BPEI solutions of different pH values,2 similar swelling ratios (ca. 37%) as the (BPEI/PAA)6 films made with pH 9.5 BPEI so long as they have similar film thicknesses. FTIR spectra show that changing the BPEI pH over this range (8.0− 9.5) have a similar degree of ionization of PAA (54−59%, Figure S3, SI), which was calculated from the ratio of stretching vibration bands of COOH (1714 cm−1) and COO− (1560 cm−1).2,14,45 From these data, it can be seen that these small changes in charge density will not strongly influence the swelling but rather the film thickness is more determinative of swelling. This is consistent with what has been shown in another study of the swelling of BPEI/PAA multilayers in water and organic solvents.53 The water fraction in the swollen PEMs was fit from ellipsometric data by applying the EMA model coupled with the Cauchy model and H2O material, and the data were well-fit with acceptable error for both dry and swollen states at different temperatures (mean square error 65 °C), this phenomenon changes. During the heating process, the film continues to deswell, but the deswelling rate slows down (Figure 4a). Once cooled after reaching 75 °C, the swollen film thickness increases and the film thickness reaches 3 nm higher than the original value at 25 °C. In subsequent heating cycles, the film is thicker at increased temperature as well; that is, after once having been heated to 75 °C and returned to room temperature, the second time the film is heated to 35, 45, or 55 °C and it is thicker than it was during the first heating. The increased thickness at room temperature remains so for at least 2 h. Upon applying further heating/ cooling cycles, a similar decrease/increase in swelling degree E
DOI: 10.1021/acs.langmuir.6b00206 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir occurs (Figure 4b). Although a slight increase in film roughness is accompanied by the swelling, the maximum roughness remains within 0.6 nm, which is below 0.2% compared with the total film thickness, meaning that the reporting values of changes in film thickness and swelling are meaningful (Figure S8 (SI)). The final swelling degree at 25 °C in increased throughout repeated heating/cooling cycles, the film thickness growing by ∼14 nm from ∼353 to ∼367 nm after three cycles (Figure 4c). Figure 4d presents FTIR spectra of a (BPEI/PAA)6 film at different temperatures. While no additional new peaks form, the peaks associated with the carboxyl group change upon heating (∼1714 cm−1). Starting from a single peak at 25 °C, it splits into doublets centered at ∼1740 and ∼1710 cm−1 at 45 °C, which are respectively ascribed to intermolecular hydrogen bonding and intramolecular hydrogen bonding of PAA.60 These doublets grow with rising temperature. During the cooling process after having first reached 75 °C, the intensity of these doublets decreases when compared with the COO− peak at ∼1560 cm−134 and then increases below 45 °C. This shows that the carboxyl groups have dissociated and hydrogen bonds have been weakened or broken by heating.60,61 With the increase in temperature, the energy barrier for hydrogen bond reconfiguration is reduced.62 This decreases the probability of hydrogen bond formation,63 and it is likely that heating to 75 °C promotes the breakage of hydrogen bonds between PE chains. When the film is heated to 75 °C and quickly cooled, reformation of broken hydrogen bonds is inhibited, and the film has fewer hydrogen bonds then after initial film formation. When a (BPEI/PAA)6 film is equilibrated in 75 °C DI water and then is rapidly quenched to 25 °C in 1 min, the film thickness suddenly drops instead of undergoing the afore described swelling (Figure S9a, SI), and the collapsed film thickness is ∼1 nm lower than the deswollen film thickness at 50 °C. It takes the collapsed film more than 1 h to slowly swell back to that original film thickness, which is quite different than the stepwise heating/cooling case. This film collapse is due to hydrogen bond rearrangement. As discussed above, upon heating to 75 °C, the initial hydrogen bonds in the film, allowing for greater polyelectrolyte chain mobility. As solvent quality of water improves by cooling from 75 to 25 °C, cooled water diffuses into the film, causing it to swell. Cooling also promotes reformation of hydrogen bonds, which once again reduces chain mobility. Eventually an equilibrium thickness is reached as these two processes are balanced. Films of (BPEI/PAA)6 were equilibrated in DI water at 25 °C after rapid heating/quenching cycles at different temperatures. The film thickness changes compared with the initial (BPEI/PAA)6 film thickness when swollen in DI water at 25 °C are summarized in Figure S9b, SI. The equilibrated film thickness at 25 °C after cycling keeps decreasing which each cycle when treated at 55 °C, but the after heat cycling thickness remains stable for the 60 °C case and it actually increases when the maximum temperature in the heating cycle is 65 °C. For 75 °C heating/quenching cycles, the swollen film thickness continues increasing by much as ∼8 nm through the first five cycles, and then it deswells after further heating/quenching treatment. The final change in film thickness is ca. 4 nm, which is the same for 65 and 75 °C cases. It therefore seems that 60 °C is the critical temperature to trigger hydrogen bond rearrangement, which agrees with other reports.64 The higher the temperature applied for heating/quenching treatment, the more the film swells and increases in thickness but the internal
electrostatic interactions within the BPEI/PAA, but eventually an equilibrium value in thickness is reached. The formation/breakage of hydrogen bonds plays a key role in the deswelling/swelling behavior of the BPEI/PAA multilayer system. When heated, the PE chains are brought closer which promotes intra- and interpolymer chain interactions, fixing the deswollen BPEI/PAA multilayers. Once cooled, the hydrogen bonds weaken and the polyelectrolyte chains are released and rearrange within the multilayer, allowing for the uptake of more water and increased swelling. It requires being heated to above a critical temperature (60 °C) to trigger this swelling enhancement, as sufficient energy is needed to overcome energy barrier to break and reform hydrogen bonds. Below this critical temperature, hydrogen bonds are fixed, although there is still some hysteresis in the recovery of film swelling. Swelling behavior with temperature has not been previously examined in this particular system; similar swelling hysteresis as a function of pH has been reported in weak polyelectrolytes systems,66 and changes in swelling as a function of aging are also known.65,66
■
CONCLUSIONS In this work, dynamic hydrogen bonding within LbL assembled (BPEI/PAA)n films is shown to enable a temperature responsive swelling and annealing behavior in DI water. The physical properties of the multilayer system are dependent on the number of bilayers. For example, at low bilayer numbers, increasing the number of BPEI/PAA bilayers results in a denser film. Low density films are able to swell more, while increased film density limits water uptake, realizing an ascending-todescending trend in swelling ratio, with eight BPEI/PAA bilayer films exhibiting the highest swelling degree. Increasing the water temperature decreases the solvent quality and deswells the PEM. Heating up to 60 °C and then cooling PEM again improves water quality, which causes a reswelling but with some hysteresis. This hysteresis is explained by the combination of ionic and hydrogen bonds restricting the polyelectrolytes’ ability to rearrange. Once exposed to water of a critical temperature (60 °C), the hydrogen bonds within the film are weakened. Then, during the cooling process, subsequent polyelectrolyte chain rearrangement, now possible due to the weakened secondary interactions within the film, allows for an increase in film swelling. The response in swelling behavior to exposure to this wide range of temperature provides a better understanding of the unique behaviors of weak PE derived PEMs in aqueous environments, which may be potentially useful for using thermal treatment on PEM based materials for actuators66 and biological applications such as smart drug release in aqueous condition.
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00206. Neutron reflectivity spectra of BPEI/PAA multilayers, depth profiling XPS plots of (BPEI/PAA)n films, FTIR spectra of (BPEI/PAA)6 films obtained from different pH BPEI solutions, FTIR and EDS spectra of aqueous solutions applied to soak (BPEI/PAA)6 films, ionization degree of (BPEI/PAA)n films after heating to 50 °C, representative original and fitted ellipsometry data, film F
DOI: 10.1021/acs.langmuir.6b00206 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
■
(15) Tong, W.; Gao, C. Multilayer microcapsules with tailored structures for bio-related applications. J. Mater. Chem. 2008, 18, 3799− 3812. (16) De Koker, S.; Hoogenboom, R.; De Geest, B. G. Polymeric multilayer capsules for drug delivery. Chem. Soc. Rev. 2012, 41, 2867− 2884. (17) Deshmukh, P. K.; Ramani, K. P.; Singh, S. S.; Tekade, A. R.; Chatap, V. K.; Patil, G. B.; Bari, S. B. Stimuli-sensitive layer-bylayer(LbL) self-assembly systems: Targeting and biosensory applications. J. Controlled Release 2013, 166, 294−306. (18) DeMuth, P. C.; Moon, J. J.; Suh, H.; Hammond, P. T.; Irvine, D. J. Releasable layer-by-layer assembly of stabilized lipid nanocapsules on microneedles for enhanced transcutaneous vaccine delivery. ACS Nano 2012, 6, 8041−8051. (19) Costa, R. R.; Mano, J. F. Polyelectrolyte multilayered assemblies in biomedical technologies. Chem. Soc. Rev. 2014, 43, 3453−3479. (20) Westwood, M.; Gunning, A. P.; Parker, R. Temperaturedependent growth of gelatin poly(galacturonic acid) multilayer films and their responsiveness to temperature, pH, and NaCl. Macromolecules 2010, 43, 10582−10593. (21) Tan, W. S.; Cohen, R. E.; Rubner, M. F.; Sukhishvili, S. A. Temperature-induced, reversible swelling transitions in multilayers of a cationic triblock copolymer and a polyacid. Macromolecules 2010, 43, 1950−1957. (22) Tan, W. S.; Zhu, Z.; Sukhishvili, S. A.; Rubner, M. F.; Cohen, R. E. Effect of block copolymer architecture on the thermally induced swelling of micelle-containing multilayer thin films. Macromolecules 2011, 44, 7767−7774. (23) Jaber, J. A.; Schlenoff, J. B. Polyelectrolyte multilayers with reversible thermal responsivity. Macromolecules 2005, 38, 1300−1306. (24) Tong, L.; Ma, N.; Tang, F.; Qiu, D.; Cui, Q.; Li, L. pH and thermoresponsive Ag/polyelectrolyte hybrid thin films for tunable metal-enhanced fluorescence. J. Mater. Chem. 2012, 22, 8988−8993. (25) Zhu, Z.; Sukhishvili, S. A. Temperature-induced swelling and small molecule release with hydrogen bonded multilayers of block copolymer micelles. ACS Nano 2009, 3, 3595−3605. (26) Yang, S. Y.; Rubner, M. F. Micropatterning of polymer thin films with pH-sensitive and cross-linkable hydrogen-bonded polyelectrolyte multilayers. J. Am. Chem. Soc. 2002, 124, 2100−2101. (27) Gu, Y.; Huang, J. Ultrathin cellulose film coating of porous alumina membranes for adsorption of superoxide dismutase. J. Mater. Chem. B 2013, 1, 5636−5643. (28) Gu, Y.; Niu, T.; Huang, J. Functional polymeric hybrid nanotubular materials derived from natural cellulose substances. J. Mater. Chem. 2010, 20, 10217−10223. (29) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Atomic force microscopy studies of salt effects on polyelectrolyte multilayer film morphology. Langmuir 2001, 17, 6655−6663. (30) Lobo, R. F. M.; Pereira-da-Silva, M. A.; Raposo, M.; Faria, R. M.; Oliveira, O. N., Jr.; Pereira-da-Silva, M. A.; Faria, R. M. In situ thickness measurements of ultra-thin multilayer polymer films by atomic force microscopy. Nanotechnology 1999, 10, 389. (31) Leporatti, S.; Gao, C.; Voigt, A.; Donath, E.; Möhwald, H. Shrinking of ultrathin polyelectrolyte multilayer capsules upon annealing: A confocal laser scanning microscopy and scanning force microscopy study. Eur. Phys. J. E: Soft Matter Biol. Phys. 2001, 5, 13− 20. (32) Heuvingh, J.; Zappa, M.; Fery, A. Salt softening of polyelectrolyte multilayer capsules. Langmuir 2005, 21, 3165−3171. (33) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Fabrication of microporous thin films from polyelectrolyte multilayers. Langmuir 2000, 16, 5017−5023. (34) Kharlampieva, e.; Kozlovskaya, V.; Ankner, J. F.; Sukhishvili, S. A. Hydrogen-bonded polymer multilayers probed by neutron reflectivity. Langmuir 2008, 24, 11346−11349. (35) Gopinadhan, M.; Ivanova, O.; Ahrens, H.; Günther, J.-U.; Steitz, R.; Helm, C. A. The Influence of secondary interactions during the formation of polyelectrolyte multilayers: Layer thickness, bound water and layer Interpenetration. J. Phys. Chem. B 2007, 111, 8426−8434.
thickness change in BPEI/PAA multilayers through heating/cooling cycles (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Prof. Bryan D. Vogt for kindly providing ellipsometer for in situ study of film swelling behavior. This work was supported by NSF grant 1425187 and Department of Polymer Engineering at University of Akron. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work.
■
REFERENCES
(1) Ariga, K.; Hill, J. P.; Ji, Q. Layer-by-layer assembly as a versatile bottom-up nanofabrication technique for exploratory research and realistic application. Phys. Chem. Chem. Phys. 2007, 9, 2319−2340. (2) Huang, X.; Chrisman, J. D.; Zacharia, N. S. Omniphobic slippery coatings based on lubricant-infused porous polyelectrolyte multilayers. ACS Macro Lett. 2013, 2, 826−829. (3) Xiang, Y.; Lu, S.; Jiang, S. P. Layer-by-layer self-assembly in the development of electrochemical energy conversion and storage devices from fuel cells to supercapacitors. Chem. Soc. Rev. 2012, 41, 7291− 7321. (4) Li, Y.; Chen, S.; Wu, M.; Sun, J. Polyelectrolyte multilayers impart healability to highly electrically conductive films. Adv. Mater. 2012, 24, 4578−4582. (5) Lutkenhaus, J. L.; Hammond, P. T. Electrochemically enabled polyelectrolyte multilayer devices: from fuel cells to sensors. Soft Matter 2007, 3, 804−816. (6) Tokarev, I.; Minko, S. Stimuli-responsive hydrogel thin films. Soft Matter 2009, 5, 511−524. (7) Gong, X.; Han, L.; Gao, J.; Gao, C. Stability of polyelectrolyte multilayer micropatterns in response to post-treatments. Colloids Surf., A 2012, 396, 299−304. (8) Joseph, N.; Ahmadiannamini, P.; Hoogenboom, R.; Vankelecom, I. F. J. Layer-by-layer preparation of polyelectrolyte multilayer membranes for separation. Polym. Chem. 2014, 5, 1817−1831. (9) Yan, Y.; Björnmalm, M.; Caruso, F. Assembly of layer-by-layer particles and their interactions with biological systems. Chem. Mater. 2014, 26, 452−460. (10) Huang, H.-C.; Zacharia, N. S. Polyelectrolyte multilayers and complexes to modify secondary interactions in ethylene-co-methacrylic acid ionomers. ACS Macro Lett. 2012, 1, 209−212. (11) Gao, C.; Leporatti, S.; Moya, S.; Donath, E.; Möhwald, H. Swelling and shrinking of polyelectrolyte microcapsules in response to changes in temperature and ionic strength. Chem. - Eur. J. 2003, 9, 915−920. (12) Köhler, K.; Shchukin, D. G.; Möhwald, H.; Sukhorukov, G. B. Thermal behavior of polyelectrolyte multilayer microcapsules. 1. The effect of odd and even layer number. J. Phys. Chem. B 2005, 109, 18250−18259. (13) Gu, Y.; Huang, X.; Wiener, C. G.; Vogt, B. D.; Zacharia, N. S. Large-scale solvent driven actuation of polyelectrolyte multilayers based on modulation of dynamic secondary interactions. ACS Appl. Mater. Interfaces 2015, 7, 1848−1858. (14) Huang, J.; Gu, Y. Self-assembly of various guest substrates in natural cellulose substances to functional nanostructured materials. Curr. Opin. Colloid Interface Sci. 2011, 16, 470−481. G
DOI: 10.1021/acs.langmuir.6b00206 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir (36) Schönhoff, M.; Ball, V.; Bausch, A. R.; Dejugnat, C.; Delorme, N.; Glinel, K.; Klitzing, R. v.; Steitz, R. Hydration and internal properties of polyelectrolyte multilayers. Colloids Surf., A 2007, 303, 14−29. (37) Schönhoff, M. Self-assembled polyelectrolyte multilayers. Curr. Opin. Colloid Interface Sci. 2003, 8, 86−95. (38) Lee, S.-W.; Lee, D. Integrated study of water sorption/ desorption behavior of weak polyelectrolyte layer-by-layer films. Macromolecules 2013, 46, 2793−2799. (39) Johs, B. D.; Herzinger, C. M. Methods for uncorrelated evaluation of parameters in parameterized mathematical model equations for window retardence, in ellipsometer and polarimeter systems. U.S. Patent 6,034,777 A, Sep 29, 1998. (40) Johs, B. D.; Hale, J.; Ianno, N. J.; Herzinger, C. M.; Tiwald, T. E.; Woollam, J. A. Recent developments in spectroscopic ellipsometry for in-situ applications. Proc. SPIE 2001, 4449, 41−57. (41) Herzinger, C.; Johs, B.; McGahan, W. A.; Woollam, J. A.; Paulson, W. Ellipsometric determination of optical constants for silicon and thermally grown silicon dioxide via a multi-sample, multiwavelength, multi-angle investigation. J. Appl. Phys. 1998, 83, 3323− 3336. (42) Baszkin, A.; Norde, W. Physical Chemistry of Biological Interfaces. In Ellipsometry; Arwin, H., Ed.; CRC Press: New York, 1999; Chapter 17, pp 577−608. (43) Willner, I.; Eichen, Y.; Frank, A. J.; Fox, M. A. J. Phys. Chem. 1993, 97, 7264−7271. (44) Choi, J.; Rubner, M. F. Influence of the degree of ionization on weak polyelectrolyte multilayer assembly. Macromolecules 2005, 38, 116−124. (45) Li, Y.; Wang, X.; Sun, J. Layer-by-layer assembly for rapid fabrication of thick polymeric films. Chem. Soc. Rev. 2012, 41, 5998− 6009. (46) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Molecular basis for the explanation of the exponential growth of polyelectrolyte multilayers. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 12531−12535. (47) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (48) 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, 6651−6656. (49) Kawaguchi, M.; Tohyama, M.; Takahashi, A. Ellipsometric study of polymer monolayers spread at the air-water interface. 2. Adsorbed amount of polymers. Langmuir 1988, 4, 411−413. (50) Schael, G. W. Determination of polyolefin film properties from refractive index measurements. J. Appl. Polym. Sci. 1964, 8, 2717− 2722. (51) Schmidt, D. J.; Min, Y.; Hammond, P. T. Mechanomutable and reversibly swellable polyelectrolyte multilayer thin films controlled by electrochemically induced pH gradients. Soft Matter 2011, 7, 6637− 6647. (52) Gu, Y.; Ma, Y.; Vogt, B. D.; Zacharia, N. S. Contraction of weak polyelectrolyte multilayers in response to organic solvents. Soft Matter 2016, 12, 1859−1867. (53) Carrière, D.; Krastev, R.; Schönhoff, M. Oscillations in solvent fraction of polyelectrolyte multilayers driven by the charge of the terminating layer. Langmuir 2004, 20, 11465−11472. (54) Dodoo, S.; Balzer, B. N.; Hugel, T.; Laschewsky, A.; Klitzing, R. V. Effect of ionic strength and layer number on swelling of polyelectrolyte multilayers in water vapour. Soft Mater. 2013, 11, 157−164. (55) Schlenoff, J. B.; Dubas, S. T. Mechanism of polyelectrolyte multilayer growth: charge overcompensation and distribution. Macromolecules 2001, 34, 592−598. (56) Schonoff, M. Self-assembled polyelectrolyte multilayers. Curr. Opin. Colloid Interface Sci. 2003, 8, 86−95.
(57) Galvin, C. J.; Dimitriou, M. D.; Satija, S. K.; Genzer, J. Swelling of polyelectrolyte and polyzwitterion brushes by humid vapors. J. Am. Chem. Soc. 2014, 136, 12737−12745. (58) Gu, Y.; Zacharia, N. S. Self-healing actuating adhesive based on polyelectrolyte multilayers. Adv. Funct. Mater. 2015, 25, 3785. (59) Alexandridis, P.; Andersson, K. Effect of solvent quality on reverse micelle formation and water solubilization by poly(ethylene oxide)/poly(propylene oxide) and poly(ethylene oxide)/poly(butylene oxide) block copolymers in xylene. J. Colloid Interface Sci. 1997, 194, 166−173. (60) Lutkenhaus, J. L.; McEnnis, K.; Hammond, P. T. Tuning the glass transition of and ion transport within hydrogen-bonded layer-bylayer assemblies. Macromolecules 2007, 40, 8367−8373. (61) Meaurio, E.; Velada, J. L.; Cesteros, L. C.; Katime, I. Blends and complexes of poly(monomethyl itaconate) with polybases poly(N,Ndimethylacrylamide) and poly(ethyloxazoline). Association and thermal behavior. Macromolecules 1996, 29, 4598−4604. (62) Nicodemus, R. A.; Ramasesha, K.; Roberts, S. T.; Tokmakoff, A. Hydrogen bond rearrangements in water probed with temperaturedependent 2D IR. J. Phys. Chem. Lett. 2010, 1, 1068−1072. (63) Ohmine, I.; Tanaka, H. Fluctuation, relaxations, and hydration in liquid water. Hydrogen-bond rearrangement dynamics. Chem. Rev. 1993, 93, 2545−2566. (64) Yildirim, E.; Zhang, Y.; Lutkenhaus, J. L.; Sammalkorpi, M. Thermal transitions in polyelectrolyte assemblies occur via a dehydration mechanism. ACS Macro Lett. 2015, 4, 1017−1021. (65) Itano, K.; Choi, J.; Rubner, M. F. Mechanism of the pH-induced discontinuous swelling/deswelling transitions of poly(allylamine hydrochloride)-containing polyelectrolyte multilayer films. Macromolecules 2005, 38, 3450−3460. (66) Lee, S. W.; Prosser, J. H.; Purohit, P. K.; Lee, D. Bioinspired hygromorphic actuator exhibiting controlled locomotion. ACS Macro Lett. 2013, 2, 960−965.
H
DOI: 10.1021/acs.langmuir.6b00206 Langmuir XXXX, XXX, XXX−XXX
