Desorption Behavior of Weak

Mar 25, 2013 - The water sorption and desorption of PAH/PAA LbL films are investigated using quartz crystal microbalance with dissipation (QCM-D) ...
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Integrated Study of Water Sorption/Desorption Behavior of Weak Polyelectrolyte Layer-by-Layer Films Sang-Wook Lee and Daeyeon Lee* Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: We present an integrated study of the water sorption/desorption behavior of layer-by-layer (LbL) assembled films made of two oppositely charged weak polyelectrolytes, poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). The water sorption and desorption of PAH/PAA LbL films are investigated using quartz crystal microbalance with dissipation (QCM-D) monitoring, which allows for the simultaneous determination of the swelling/ deswelling ratio and mechanical properties of the LbL films as well as the real-time monitoring of the sorption/desorption dynamics as a function of relative humidity (RH). It is shown that PAH/PAA LbL films that exhibit significant swelling/ deswelling hysteresis during water sorption and desorption have higher shear moduli than those that exhibit small hysteresis. The diffusion mechanisms during sorption and desorption, studied as a function of relative humidity (RH), also show hysteresis, which correlates with the humidity-induced swelling/deswelling hysteresis of the films. During sorption, all LbL films initially exhibit Super Case II diffusion at low RH. At high RH, the water sorption mechanism gradually transforms to anomalous diffusion in (PAH/PAA) LbL films with large hysteresis, whereas it changes to Fickian diffusion in (PAH/PAA) films with small hysteresis. Fickian diffusion indicates that the latter films become significantly plasticized at high RH, and thus, the chain mobility is significantly enhanced. During desorption, the transport mechanisms of all of the films are Super Case II diffusion at high RH (>80%). The mechanism gradually changes to anomalous diffusion in the films with large hysteresis at low RH (20−60%), whereas the transport mechanism in the films with small hysteresis changes rapidly to anomalous diffusion at relatively high RH (60−80%). Our results indicate that humidity-induced swelling/deswelling hysteretic behaviors of PAH/PAA LbL films can be attributed to impeded chain relaxation that hampers the response of the LbL films to changes in the activity of water in the gas phase. Our study provides an integrated perspective on the water sorption/desorption properties of weak polyelectrolyte LbL films, which will be useful in developing water-sensitive devices such as humidity sensors, actuators, and gas barriers using weak polyelectrolyte layer-by-layer films.

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suitable adjustment of the solution pH before LbL assembly.23 On the basis of this controllability, PAH/PAA multilayers with a wide spectrum of structures and properties have been fabricated.24,28 For many practical applications, a thorough understanding of interaction of these weak polyelectrolyte multilayer (PEM) films with water is critical. The mechanical properties of PAH/ PAA LbL films, for example, depend strongly on relative humidity, and their stiffness can vary by more than an order of magnitude depending on the hydration state of the films (e.g., dry vs fully hydrated).29 The thermal transitions in these LbL films also depend strongly on the state of hydration.30 One particularly interesting finding that was recently reported is that PAH/PAA films assembled at two different pH combinations were shown to exhibit humidity-induced swelling/deswelling hysteresis. Remarkably, such hysteresis was shown to be

INTRODUCTION Layer-by-layer (LbL) assembly is a powerful technique that enables straightforward and cost-effective fabrication of functional nanocomposite thin films with tunable architecture and properties through alternate adsorption of two species that interact with each other via electrostatic interactions1−5 or hydrogen-bonding.6−12 LbL assemblies have been explored in a variety of engineering and scientific areas including energy conversion and storage,13,14 drug delivery,15,16 tissue engineering,17,18 and functional coatings.19−22 The structure and properties of LbL assemblies can be controlled by assembly conditions such as the pH,23,24 temperature,25,26 and ionic strength27 of the polyelectrolyte solutions. In the case of LbL assemblies that contain pH-sensitive polymers (i.e., weak polyelectrolytes), the pH adjustment of each polyelectrolyte solution offers a unique opportunity to tune the charge density as well as molecular structure in polyelectrolyte multilayers (PEM). For example, the degree of ionization of weak polyelectrolytes such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) can be controlled by a © 2013 American Chemical Society

Received: January 11, 2013 Revised: March 13, 2013 Published: March 25, 2013 2793

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negligible in the single component films of PAA and PAH.31 Although a physical mechanism behind such a hysteretic behavior was not fully elucidated, a combination of molecular restructuring in response to ambient moisture and the frustration of structural relaxation by ionic cross-links within the films was proposed as the mechanism behind the observed hysteresis.31 Overall, these studies indicate that the influence of water on the properties of PAH/PAA LbL films is subtle and not fully understood. While previous reports provide useful information on the physicochemical properties of these LbL films, these studies have typically focused on characterizing a single property of LbL films under different hydration states. Revealing correlations among different physicochemical properties of hydrated LbL assemblies could provide integrated insights into the molecular mechanism behind the unique interactions between LbL films and water. In this paper, we aim to uncover correlations among humidity-induced swelling/deswelling hysteresis, mechanical properties, and water sorption/desorption dynamics in the layer-by-layer (LbL) films made of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). We use a quartz crystal microbalance with dissipation (QCM-D) monitoring, which allows us to monitor the dynamics of water uptake and also to determine the mechanical properties of the LbL films in real-time under varying humidity.32−34 In particular, humiditydependent changes in swelling/deswelling ratios, shear modulus, and sorption/desorption mechanisms are studied in PAH/PAA films assembled under different pH conditions. Although all films show humidity-induced swelling/deswelling hysteresis in water uptake during sorption and desorption, the films exhibit different extents of hysteresis, which are found to positively correlate with the stiffness of the films. The transport mechanisms studied as a function of RH during sorption/ desorption also show hysteretic behaviors, correlating with the observed swelling/deswelling hysteresis. We discuss the implications of the mechanical properties and water transport mechanism on the extent of humidity-induced swelling/ deswelling hysteresis in PAH/PAA LbL films.

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distinctive regimes depending on the solution pH (i.e., above and below pKa of each polyelectrolyte).24 We study four different films assembled at different pH conditions representing each growth regime mentioned above; the four films are (PAH7.5/PAA3.5)15, (PAH6.5/ PAA6.5)100, (PAH10.5/PAA10.5)12, and (PAH2.5/PAA2.5)50 films. Since these LbL films have different sorption and mechanical properties due to the difference in charge density and/or chain conformations, they provide unique model systems for investigating correlations among sorption/desorption hysteresis, mechanical properties, and transport mechanisms in weak polyelectrolyte layer-by-layer (LbL) films. The characteristics of each film is summarized as follows: During the assembly of (PAH7.5/PAA3.5) films, PAA with loopy conformations in pH 3.5 solution23 adsorbs onto a fully charged PAH layer, and then the adsorbed PAA becomes highly charged in the PAH solution (pH 7.5) and attracts highly charged PAH (about 80%)23 via electrostatic interactions. Consequently, the (PAH7.5/PAA3.5) film has a loopy conformation with a high density of ionic cross-links between charged amine groups (NH3+) in PAH and carboxylate groups (COO−) in PAA. The (PAH6.5/PAA6.5) film has a high density of ionic cross-links, similar to (PAH7.5/PAA3.5) films; however, the conformations of polyelectrolytes in these two films are different. While the (PAH7.5/PAA3.5) film has loopy chains, the (PAH6.5/PAA6.5) film has highly stretched chains.24 The (PAH10.5/ PAA10.5) film has loopy conformations due to the low ionization of PAH and thus a low density of ionic cross-links although PAA is fully charged at this pH. Similarly, in (PAH2.5/PAA2.5) films, PAA has loopy conformations owing to the low degree of ionization of PAA on the fully charged PAH layer and therefore the film has a low density of ionic cross-links between PAH and PAA. The dry thicknesses of (PAH7.5/PAA3.5)15, (PAH2.5/PAA2.5)50, (PAH6.5/PAA6.5)100, and (PAH10.5/PAA10.5)12 films determined using QCM-D are 496.4 ± 14.0, 473.6 ± 29.3, 767.7 ± 15.0, and 509.2 ± 32.4 nm, respectively. The film thicknesses are found to be larger than those measured in previous reports. We believe this difference is likely because of the holder used to support QCM sensors, which retains some residual liquid during LbL assembly, preventing full drainage when the sensors are pulled out of the solutions. Previously, it was reported that the incomplete drainage of solutions, even on planar surfaces, during layerby-layer deposition results in large film growth.36 For fabricating (PAH6.5/PAA6.5)100, the dipping times in two polymer solutions and rinsing DI water are reduced to 5 min and 1, 0.5, and 0.5 min, respectively. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). An E4 QCM-D unit (Q-Sense Inc.) is used to obtain shear modulus and thickness of PAH/PAA films by measuring shifts in the frequency and dissipation of multilayer-coated quartz crystals. The baseline frequency and dissipation are first recorded with a blank crystal by flowing dry N2 at a flow rate of 100 sccm. LbL multilayers are then formed on the crystals and dried at ambient conditions. The film on the back side of the crystals is carefully but thoroughly removed using 1.0 M NaOH and then rinsed with DI water several times. After the multilayer-coated sensors are loaded in the QCM chamber, the frequency and dissipation shifts are monitored at varying humidity by flowing a mixture of dry and humid N2. The relative humidity (RH) in N2 stream is precisely controlled by adjusting the ratio of dry (0% RH) and water-saturated (∼100% RH) N2. The total flow rate of the mixture of 0% and 100% RH N2 is fixed at 100 sccm, and the temperature in the QCM chamber is maintained at 25 °C. To monitor swelling (or deswelling), films in the QCM chambers are initially dried (or hydrated) under 0% (or 100%) RH N2 flow for about 5 h and then exposed sequentially to 20, 40, 60, 80, and 100% (or 80, 60, 40, 20, and 0%) RH for 2 h each. In all films, desorption for 2 h at each humidity is sufficient to bring the mass of films to the initial state dried at 0% RH. The measured frequency and dissipation shifts from multiple overtones are then fitted using the Voigt viscoelastic model incorporated in Q-Sense analysis software (QTools) to obtain the shear modulus of the films as well as water uptake. During the modeling, the density of LbL multilayers is assumed to be 1000 kg/m3. To validate the use of the Voigt model in extracting the thickness and modulus information, we compare the shear moduli obtained using the

MATERIALS AND METHODS

Materials. Poly(acrylic acid) (PAA) (Mw ∼ 50 000, 25% aqueous solution) and poly(allylamine hydrochloride) (PAH) (Mw ∼ 120 000− 200 000) are purchased from Polysciences. All chemicals are used as received without any further purification. Layer-by-Layer Assembly of Multilayer Films. Gold-coated quartz crystal microbalance (QCM) sensors are cleaned in H2O/ H2O2/NH4OH mixture (5:1:1 v/v/v) at 75 °C for 30 min, then rinsed with deionized (DI) water thoroughly, and finally blown dry with compressed air. Aqueous solutions of PAH and PAA are prepared in DI water (18.2 MΩ cm) to a concentration of 20 mM, based on the repeat unit molecular weight. The pH of PAH and PAA solutions are adjusted to the desired values using 1.0 M HCl or 1.0 M NaOH. A programmable slide stainer (HMS Slide Stainer, Zeiss) is used to assemble PAH/PAA multilayers on the cleaned QCM sensors. PAH/ PAA multilayers are deposited by immersing the QCM sensors into the PAH and PAA solutions (for 10 min each) with three rinsing steps in DI water (pH ∼ 5.5−6.5) for 2, 1, and 1 min between two polymer adsorption steps. Films are denoted as (PAHα/PAAβ)n, where α and β are the pH values of the PAH and PAA solutions, respectively, and n is the number of deposited bilayers. Since the pKa of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) in solution is known to be about 8−9 and 5−6,23,35 respectively, the charge density and conformation of PAA and PAH within the PAH/PAA multilayer films are strongly affected by the assembly pH. According to a previous study, the growth of PAH/PAA films via LbL assembly has four 2794

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Voigt model incorporated in Q-Sense software to the Young’s moduli of these films obtained using the strain-induced elastic buckling instability for mechanical measurement (SIEBIMM) method in another report.29 Young’s moduli (E) of (PAH7.5/PAA3.5) and (PAH2.5/PAA2.5) films reported in ref 29 were about 9.8 and 7.2 GPa at 12% RH, respectively (where Poisson’s ratio (ν) = 0.33 is used). The shear moduli (G) of (PAH7.5/PAA3.5) and (PAH2.5/PAA2.5) films obtained in this work can be converted to Young’s moduli using E = 2G(1 + ν). The estimated Young’s moduli of (PAH7.5/PAA3.5) and (PAH2.5/PAA2.5) films at 0% RH are 11.1 and 3.8 GPa, respectively (ν = 0.33 is used). Considering that these two sets of moduli are obtained using very different techniques at different frequency ranges, the fact that they are within the same order of magnitude is encouraging. The Voigt model also clearly shows that (PAH2.5/PAA2.5) films are softer than (PAH7.5/PAA3.5), consistent with the prior study.29 Also, as will be shown below, the relative thickness changes estimated based on the Voigt model in our study are in good agreement with those obtained using in situ reflectometry in a prior study.31 These results show that although the dissipation in these films is small, the Voigt model provides useful information regarding the thickness and modulus of the films. We note, however, that caution must be exercised in directly comparing the absolute values of moduli obtained in this study to those obtained using other methods.37

Figure 2. Humidity-dependent relative mass change during sorption and desorption in (a) (PAH7.5/PAA3.5)15, (b) (PAH6.5/PAA6.5)100, (c) (PAH10.5/PAA10.5)12, and (d) (PAH2.5/PAA2.5)50 films. The circles and triangles indicate relative mass change during sorption and desorption, respectively.

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RESULTS AND DISCUSSION We first study the swelling/deswelling behavior of PAH/PAA LbL films under varying relative humidity (RH). Typical frequency and dissipation shifts measured by a quartz crystal microbalance with dissipation (QCM-D) monitoring are shown in Figure 1. In this particular case, (PAH10.5/PAA10.5)12 film

film was shown to resist water sorption at 12−36% RH range possibly due to a dense hydrogen-bonding (HB) network which would resist swelling at low RH.29 With increasing humidity, the thickness of this film was shown to abruptly increase at 72−84% RH range presumably due to the disruption of hydrogen-bonding by the ionization of carboxylic acid groups.29 However, such an abrupt transition in swelling is not observed in our (PAH2.5/PAA2.5)50 films;38 the mass of the film more or less linearly increases with RH. It is interesting to note that a more recent study has shown that the swelling behavior of (PAH3.0/PAA3.0) films, of which the properties are expected to be similar to those of (PAH2.5/PAA2.5) films, exhibits no such abrupt transition at high humidity.31 This work also has shown that (PAH3.0/PAA3.0) films swell less than (PAH7.5/PAA3.5) films, which is in good agreement with our observations.31 One important aspect in the water sorption/desorption behaviors is that the mass of the films during desorption is larger than that during sorption at a given RH. As can be seen in Figure 1 and 2, the magnitude of frequency shifts, thus the relative mass change with respect to the dry state, during desorption at a given relative humidity remains larger than that obtained during swelling in all humidity ranges except 0% RH. This type of humidity-induced swelling/deswelling hysteresis is observed in all four films. A recent study has also shown that (PAH7.5/PAA3.5) and (PAH3.0/PAA3.0) LbL films exhibit humidity-induced swelling/deswelling hysteresis.31 Our results reported in relative mass change are in good agreement with previous results that were reported in relative thickness change. Since these LbL assemblies are on very rigid substrates, they most likely swell predominantly in the thickness direction; therefore, the relative changes in mass are directly proportional to those in thickness. Remarkably, the extents of hysteresis in (PAH7.5/PAA3.5)15, (PAH6.5/PAA6.5)100, and (PAH10.5/PAA10.5)12 films are quite similar. However, the sorption/desorption behavior of the (PAH2.5/PAA2.5)50 film is significantly different from those of the other three films (Figure 2d). (PAH2.5/PAA2.5)50 shows a relatively smaller extent of swelling/deswelling

Figure 1. Typical frequency and dissipation shifts in a PAH/PAA film. (PAH10.5/PAA10.5)12 film is initially dried at 0% RH and is sequentially exposed to 20, 40 60, 80, and 100% RH at 0, 2, 4, 6, and 8 h, respectively, and then 80, 60, 40, 20, and 0% at 10, 12, 14, 16, and 18 h, respectively.

is initially dried at 0% RH, and humidity is changed every 2 h in 20% increments. The frequency decreases with increasing humidity during sorption measurement (0−10 h), indicating that the mass of the film increases. The frequency increases during desorption and returns to the original dry state value (dashed line) at 0% (10−20 h). Since frequency shifts in a blank crystal upon increasing humidity from 0 to 100% are significantly smaller than those observed for crystals with LbL films (∼0.15%), the effect of crystal surface on the water sorption behavior of the LbL films is negligible (Figure S2, Supporting Information). The relative mass of (PAH7.5/ PAA3.5)15, (PAH6.5/PAA6.5)100, and (PAH10.5/PAA10.5)12 films, determined using QCM-D, increases about 1.35 times their dry mass at 100% RH as seen in Figure 2. However, the overall swelling ratio of (PAH2.5/PAA2.5)50 is considerably lower than the other films. Previously, a (PAH2.5/PAA2.5)50 2795

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hysteresis compared to the other films, especially at low humidity. For example, the differences in the relative mass of the film between swelling and deswelling curves at 20 and 40% RH in the (PAH2.5/PAA2.5)50 film are 0.014 and 0.036, respectively, whereas the differences are 0.072 and 0.089 at 20 and 40% RH, respectively, in the (PAH7.5/PAA3.5)15 film. A similar difference in the extent of swelling/deswelling hysteresis between (PAH3.0/PAA3.0) and (PAH7.5/PAA3.5) films under humid air was also observed previously; however, the physical mechanism behind the observed difference was not proposed.31 We note that our study used nitrogen as the carrier gas, whereas ref 31 used air. Nevertheless, no significant difference between our work and the previous report indicates that the effect of O2 and/or CO2 in air on the water sorption/ desorption properties of PAH/PAA films is not significant. Interestingly, the stiffness of PAH/PAA LbL films is positively correlated with the extent of swelling/deswelling hysteresis (Figure 3). The three films that exhibit a relatively

is investigated (Figure 4). The concentration of water in a swollen film can be calculated using the following relationship:

Figure 4. Normalized shear modulus as a function of water concentration in PAH/PAA films.

(Ms − Md)/Ms, where Ms is the mass of the swollen film and Md is the mass of the dry film. The normalized shear modulus decreases more or less linearly as the water concentration increases in PAH/PAA films, illustrating the role of water as a plasticizer. A similar trend of a linear decrease in Young’s modulus as a function of plasticizer concentration has been previously observed in nonionic polymers such as polystyrene.9,43 The change in the normalized shear modulus of (PAH2.5/PAA2.5) films again significantly deviates from that of the other films, which indicates that (PAH2.5/PAA2.5) films are more easily plasticized by a relatively small amount of water in the films. In summary, (PAH2.5/PAA2.5) films have the lowest shear modulus (Figure 3) and the most significant decrease in the normalized shear modulus with an increase in water uptake among the four LbL films studied here. Taken together, these observations suggest that the chain mobility in PAH/PAA films, as indicated by their mechanical properties, plays a significant role in determining the extent of the sorption/desorption hysteresis. To further verify the importance of chain mobility in determining the extent of the swelling/deswelling hysteresis in PAH/PAA LbL films, the dynamics of water transport under varying water activity is studied by increasing the relative humidity in 20% increments from 0 to 100%. The swelling and deswelling curves measured in each RH range (Figure 1) are fitted to an empirical equation: Mt/M∞ = ktn,44−46 where Mt is the amount of water absorbed at time t, M∞ is the amount of water absorbed at saturation, k is a constant, and n is the exponent that describes the mechanism of diffusion.45 For n = 0.5, the transport mechanism follows Fickian (or Case I) diffusion. Such a transport mode is often observed in solvent diffusion through rubbery polymers, in which the time scale of chain relaxation (τR) is much shorter than that of solvent diffusion (τD). For n = 1, the transport is associated with Case II diffusion, in which a solvated layer is separated from a dry layer by a sharp front, and the front moves linearly with time. In the case of n > 1, the transport mechanism is called Super Case II in which the marching velocity of the sharp front between swollen and dry layers increases with time. Case II and Super Case II diffusions are typically observed in solvent diffusion in glassy polymers where τR > τD. An intermediate exponent, 0.5 < n < 1, indicates anomalous diffusion, in which τR ∼ τD. Figure 5 shows the exponent obtained in each humidity interval during sorption and desorption in PAH/PAA films. Interestingly, the transport mechanisms in (PAH7.5/ PAA3.5)15, (PAH6.5/PAA6.5)100, and (PAH10.5/PAA10.5)12

Figure 3. Humidity-dependent shear modulus of (a) (PAH7.5/ PAA3.5)15, (b) (PAH6.5/PAA6.5)100, (c) (PAH10.5/PAA10.5)12, and (d) (PAH2.5/PAA2.5)50 films. The shear moduli are determined based on the data obtained in the sorption process.

large extent of swelling/deswelling hysteresis have higher shear moduli than the (PAH2.5/PAA2.5)50 film at a given relative humidity. This finding suggests that the stiffness of PAH/PAA films is a strong indicator for the hysteretic swelling/deswelling behaviors of these films. It is interesting to note that significant hysteresis in solvent sorption/desorption has been observed in nonionic amorphous polymers such as poly(methyl methacrylate) (PMMA)39 and poly(vinyl chloride) (PVC).40 It was proposed that the impeded chain relaxation, thus significantly large relaxation time in glassy polymers, plays an important role in the observed sorption/desorption hysteresis.39 The hysteretic swelling/deswelling behavior in the PAH/ PAA LbL films, we believe, can be understood in a similar context. We believe, during sorption and desorption, the chain rearrangements necessary for the diffusion of water are hampered due to low chain mobility as indicated by the high stiffness, and in turn, the glassy nature of the films.41,42 Therefore, the diminished swelling/deswelling hysteresis in (PAH2.5/PAA2.5)50 films can be understood in terms of their relatively high chain mobility, compared to that in the other films, as indicated by the low stiffness of (PAH2.5/PAA2.5) films, which in turn allows for the facile diffusion of water molecules. To more clearly elucidate the plasticizing effect of water in the four LbL films under study, the change in the normalized shear modulus of each film as a function of water concentration 2796

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desorption. As in the case of the three films above, the sorption mechanism in (PAH2.5/PAA2.5)50 films initially shows Case II diffusion, suggesting that the films are in a glassy state at this low humidity range (0−20%). However, at high RH, the transport mechanism gradually converts to Fickian diffusion (n = 0.5), indicating that the films have been significantly plasticized compared to the other films at high RH. This observation is consistent with the rapid decrease in shear modulus as water concentration increases in the film (Figure 4). During desorption of water in the (PAH2.5/PAA2.5)50 films, a noticeable difference from the other films is observed in the 80−60% RH step (when RH is varied from 80 to 60%). The mechanism transforms rapidly to anomalous diffusion (0.5 < n < 1) from Super Case II of the first desorption step (100− 80% RH). Such a rapid transition is an indication of the facilitated chain relaxation in this film during desorption. To gain further insights into the effect of water on the chain relaxation in PAH/PAA LbL films, the exponent (n) is plotted as a function of water concentration in the LbL films during sorption and desorption as shown in Figure 6a and b,

Figure 5. Variation of transport mechanism in each humidity interval during sorption and desorption in (a) (PAH7.5/PAA3.5)15, (b) (PAH6.5/PAA6.5)100, (c) (PAH10.5/PAA10.5)12, and (d) (PAH2.5/ PAA2.5)50 films. RH1 − RH2% (labels in x-axis) denotes that humidity is changed from RH1 to RH2. The circles and triangles indicate the exponent (n) in Mt/M∞ = ktn during sorption and desorption, respectively. n = 0.5 and 1 indicate Fickian and Case II diffusion, respectively.

films again show a qualitatively similar hysteretic behavior during sorption and desorption. When RH is increased from 0 to 20%, the transport mechanism is Super Case II (n > 1) and gradually changes to anomalous diffusion (0.5 < n < 1) in high humidity ranges. This result indicates that dry films are initially in a glassy state at low humidity and is only slightly plasticized by water at high RH. Our observation indicates that at low humidity, the chain relaxation is extremely slow (Super Case II, τR > τD) and as water molecules diffuse into the dense polymer matrix at high humidity, the time scale of chain relaxation becomes comparable to that of water diffusion (anomalous diffusion, τR ∼ τD). However, the transport mechanism fails to become fully Fickian (i.e., n = 0.5), indicating that the chain mobility in these LbL films is still quite constrained at high RH. One important finding in the desorption behavior is that during the first desorption step (when RH is decreased from 100 to 80%), the transport mechanism is found to be Super Case II (n > 1) rather than anomalous diffusion (0.5 < n < 1). Although the exact mechanism for this observation is not clear, it suggests that the water diffusion and the chain relaxation in the 100− 80% RH step during desorption are not simply the reverse of those in the 80−100% RH step during sorption. On the basis of the results obtained using QCM-D, it is challenging to accurately reveal the molecular scale mechanism behind the difference in the transport modes during sorption and desorption. The interactions between water molecules and functional groups present in these LbL films (i.e., free carboxylic acid and amine groups as well as carboxylate-charged amine complexes) likely play an important role in influencing the mode of transport and the hysteresis observed. As RH is further lowered, the transport mechanism eventually becomes anomalous diffusion (0.5 < n < 1), which indicates that the diffusion of water molecules and the chain rearrangements occur at a similar time scale (τR ∼ τD). The transport mechanism in the (PAH2.5/PAA2.5)50 film deviates from that of the other three films during sorption and

Figure 6. Variation of transport mechanism during (a) sorption and (b) desorption as a function of water concentration in PAH/PAA films. n = 0.5 and 1 indicate Fickian and Case II diffusion, respectively.

respectively. One discernible observation is that the transport mechanism in (PAH2.5/PAA2.5)50 film shows a rapid transition from Super Case II to Fickian diffusion within a relatively small range of water concentration (∼15%), whereas the other films undergo a gradual shift from Super Case II to anomalous diffusion over a wide concentration range (∼25% or higher). It is interesting to note that the transition in transport mechanism shares a common feature with the normalized shear modulus of PAH/PAA films; transport mechanisms and normalized shear modulus show a similar (rapid or gradual) transition as a function of water concentration in the films. All of our observations strongly indicate that the diminished humidity swelling/deswelling hysteresis in the (PAH2.5/ PAA2.5)50 film can be explained by more facilitated chain mobility in these films, as indicated by the mechanical properties and water sorption dynamics results, compared to the other PAH/PAA films studied in this work. The difference in relative mass, shear moduli, and diffusion mechanisms of 2797

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Table 1. Relative Mass, Shear Moduli, and Diffusion Mechanism of PAH/PAA Filmsa

a n is the exponent in Mt/M∞ = ktn, where Mt is the amount of water absorbed at time t, M∞ is the amount of water absorbed at saturation, and k is a constant.

PAH/PAA films under different humidity conditions for the four LbL films are summarized in Table 1.

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chain mobility due to plasticization of the polymer network upon water sorption. The observed correlations among sorption/desorption hysteresis, mechanical properties, and transport mechanisms indicate large relaxation times in glassy LbL films significantly hamper their response to the change in the activity of water in the gas phase and, in turn, the water diffusion during sorption and desorption. These phenomena underlie the observed hysteretic sorption/desorption behaviors in the PAH/PAA LbL films. Another important implication of our study is that the swelling/deswelling hysteretic behavior of weak polyelectrolyte LbL films, and thus their chain mobility under different relative humidity, cannot simply be predicted based on the assembly pH of the films. While one may expect that (PAH2.5/PAA2.5) and (PAH10.5/PAA10.5) films would behave similarly because these films are assembled both with fully charged and partially charged polyelectrolytes, they show very different humidity-induced swelling/deswelling behaviors. The interactions between water molecules and functional groups present in the LbL films likely play an important role in determining the swelling/deswelling hysteresis and the

CONCLUSIONS

We have studied the correlations among humidity-induced sorption/desorption hysteresis, mechanical properties, and sorption/desorption dynamics in poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) multilayer films assembled under four different assembly conditions. We have revealed that (PAH7.5/PAA3.5)15, (PAH6.5/PAA6.5)100, and (PAH10.5/PAA10.5)12 films exhibit a greater extent of sorption/desorption hysteresis than (PAH2.5/PAA2.5)50 films and that the extent of hysteresis is positively correlated with the stiffness of these films. The transport mechanism studied by monitoring the dynamics of water sorption under varying relative humidity also shows a strong correlation with the humidity-induced swelling/deswelling hysteresis. The (PAH2.5/PAA2.5) LbL film, which has the smallest extent of humidity swelling/deswelling hysteresis, exhibits transport mechanisms that are consistent with significantly enhanced 2798

dx.doi.org/10.1021/ma400076d | Macromolecules 2013, 46, 2793−2799

Macromolecules

Article

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mechanism of water transport. We believe that the elucidation of the molecular scale mechanism behind these observations warrants future investigation using spectroscopic methods such as attenuated total reflectance Fourier-transform infrared spectroscopy (ATR FT-IR),47 which allows for the direct examination of the interactions between water and functional groups in the films. Our study provides an integrated perspective on the correlations among seemingly independent properties of LbL films and thus a unique insight into how LbL films are interacting with water in humid air. We anticipate that our findings and approach will be beneficial for developing water-sensitive devices using weak polyelectrolyte LbL films such as humidity sensors, actuators, and gas barriers.48,49

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ASSOCIATED CONTENT

S Supporting Information *

Relative mass change in (PAH2.5/PAA2.5)50 films containing PAH from different sources and frequency shifts in a blank crystal under varying relative humidity. These materials are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported primarily by a NSF CAREER Award (DMR-1055594) and partly by the PENN MRSEC (DMR1120901).

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