pH-Dependent Thermal Transitions in Hydrated Layer-by-Layer

Nov 15, 2012 - Layer-by-layer (LbL) assemblies have remarkable potential as advanced functional materials with applications in energy and biomedical ...
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pH-Dependent Thermal Transitions in Hydrated Layer-by-Layer Assemblies Containing Weak Polyelectrolytes Ajay Vidyasagar, Choonghyun Sung, Kristen Losensky, and Jodie L. Lutkenhaus* Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: Layer-by-layer (LbL) assemblies have remarkable potential as advanced functional materials with applications in energy and biomedical related areas. However, very little is known about their thermal and viscoelastic properties owing to the inherent difficulty in their accurate measurement. Here we report on the thermal behavior of a model LbL system containing weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) as a function of assembly solution pH. Quartz crystal microbalance with dissipation (QCM-D) and modulated differential scanning calorimetry (DSC) indicate that hydrated PAH/PAA LbL assemblies undergo a thermal transition that is akin to a glass transition for most assembly pH’s investigated, with the exception being the case where both polyelectrolytes are fully charged. The nonmonotonic dependence of the glass transition temperature of the PAH/PAA LbL system with respect to assembly pH is discussed in relation to the film’s hydration, composition, film-growth mechanism (linear vs exponential), and ion-pairing density.

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INTRODUCTION Ion-pairing layer-by-layer (LbL) assembly is a versatile assembly technique based upon the alternate adsorption of oppositely charged polyions onto a surface.1 LbL films have found widespread applications in drug delivery,2−6 antifouling coatings,7,8 polymer electrolytes,9−13 batteries,9,14−16 solar cells,17,18 optics,19 and sensors.20−22 The growth and properties of LbL films can be manipulated by varying several external parameters such as assembly solution pH, 23−28 ionic strength,27,29 salt type,30−32 and temperature.33,34 One of the existing challenges for LbL systems is the characterization of their thermal properties such as glass transition temperature (Tg). Knowledge of such properties is critical for several applications because a film’s properties can be dependent on whether the film is glassy or rubbery. Also of critical interest, the film’s Tg value, as well as the number of Tg’s, can be loosely correlated with the film’s internal structure and composition. Because LbL films are so thin, there is often little material to work with, rendering thermal characterization difficult. Several thermal characterization techniques have been recently reported such as microdifferential scanning calorimetry (micro-DSC), swelling/shrinking of LbL capsules, and NMR spectroscopy.35−37 Recently, we have reported a new approach to measuring Tg’s in LbL films using quartz crystal microbalance with dissipation (QCM-D). The advantage of QCM-D as a thermoanalytical tool over several other techniques is that QCM-D is extremely sensitive to changes in viscoelasticity, which allows for the accurate detection of weak transitions. In that report, an LbL film consisting of strong polyelectrolytes poly(diallyldimethylammonium chloride)/polystyrenesulfonate (PDAC/PSS) was examined in the hydrated state. For that system, we observed that linearly growing PDAC/PSS films © 2012 American Chemical Society

(assembled without added salt) did not have an observable Tg and that exponentially growing PDAC/PSS films (assembled with added salt) did have a distinct Tg. These findings supported a hypothesis that the thermal properties of LbL films are influenced by whether the film grows linearly or exponentially.38,39 It has been proposed that films growing linearly would be rigid with limited polymer segmental mobility and would therefore have a higher T g compared to exponentially growing LbL films, which have increased polymer segmental motion. Given these recent findings with LbL films containing strong polyelectrolytes, we hypothesized that the thermal properties of LbL films containing weak polyelectrolytes may be similarly linked to linear or exponential growth mechanisms. LbL films containing weak polyelectrolytes poly(allylamine hydrochloride) and poly(acrylic acid) (PAH and PAA, respectively) are a well-studied example. PAH/PAA LbL films offer intricate control over structure and film composition via slight changes in assembly pH, and these films have found applications in antireflection coatings,40 antifogging coatings,41 drug delivery,23,42,43 ion transport media,44 and superhydrophobic coatings.45−49 Based upon work by Shiratori and Rubner24 and Bieker and Schönhoff,50 several pH regimes exhibiting distinct growth behavior have been identified for PAH/PAA LbL systems. The origin of these distinct regimes was related to the linear charge density of PAA and PAH, having pKa’s of 5.7−6.551−54 and 8− 10,52,55−58 respectively. Regime I (pH 3−4.5) produced films that were very soft and that had a large layer pair thickness; in Received: September 27, 2012 Revised: November 5, 2012 Published: November 15, 2012 9169

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piranha solution (70% H2SO4 and 30% H2O2 by volume) for 5 min followed by rinsing in 18.2 MΩ Milli-Q water. Caution: piranha solution is highly corrosive, and proper precautions must be taken while handling. The substrates were dipped in pH-adjusted PAH solution for 15 min, followed by three separate rinses with Milli-Q water at the same pH for 2, 1, and 1 min. The substrates were then dipped in pHadjusted PAA solution 15 min, followed by another series of water rinses at the same pH as the polymer solution. The process was repeated for n cycles to yield a film of n layer pairs. A film of n layer pairs assembled at pH m will be referred to as a “(PAHm/PAAm)n LbL assembly”. LbL films were dried in ambient air and stored in a desiccator until further use. The films were isolated from their Teflon or silicon substrates (Figure S1) just before modulated DSC experiments. Preparation of Polyelectrolyte Complexes. 100 mL solutions of 200 mM PAH at pH 5.5 and PAA at pH 5.5 were mixed in a beaker slowly with constant stirring, respectively. The complex was dialyzed against water at matching pH conditions for 24 h. The PAH5.5/PAA5.5 complex was later dried under vacuum at 30 °C and stored in a desiccator until further use. Dry Film Thickness Measurements. Dry film thickness for thin LbL films (200 nm were performed using profilometry (KLA - Tencor Instruments P-6). Quartz Crystal Microbalance with Dissipation (QCM-D) Measurements. QCM-D experiments were performed using the Q-sense E1 system. The gold-plated AT-cut quartz crystals with a fundamental frequency of 5 MHz were first plasma treated for 10 min followed by a 10 min immersion in a water/NH4OH/H2O2 (5:1:1) mixture at 70 °C for 10 min, dried using nitrogen, and then plasma treated as before. Caution: H2O2 is a strong oxidizing agent and highly reactive, and proper precautions must be taken while handling. The measurements were performed in a QCM-D liquid flow chamber with one side of the QCM sensor exposed to the polyelectrolyte solution. LbL assembly was then carried out by first flowing 1 mg/mL PEI solution (pH 4.0) for 15 min on the exposed side of the crystal surface at a flow rate of 200 μL/min. This initial layer was considered the zeroth layer and was used as a baseline for all QCM-D experiments. The temperatures of the flow chamber and the polyelectrolyte solutions were stabilized at 25 ± 0.2 °C for the entire LbL assembly process. Then, pH-adjusted 10 mM PAA solution was passed over the crystal for 15 min, followed by a 5 min rinse using Milli-Q water at the same pH. 10 mM PAH solution at the same pH was then passed for 15 min, followed by rinsing as before. This procedure was repeated until the desired number of layers was achieved. QCM-D measures the change in frequency and dissipation of a film on a quartz crystal. The resonant frequency of the bare crystal, F0, lowers to F upon the addition of polyelectrolyte solution owing to adsorption of polyelectrolytes to the crystal surface. If the film is uniformly distributed and relatively rigid and if the adsorbed mass is small compared to that of the crystal, then the frequency changes ΔF = F − F0 are related to the adsorbed mass per unit surface, Δm, given by the Sauerbrey equation:64,65

this regime PAA was partially charged, and PAH was fully charged. Regime II (pH 4.5−6) gave films that were less soft but had layer pair thicknesses larger than that of regime I; here PAA and PAH were both partially charged. For regime III (pH 6.5−8), both PAH and PAA were fully charged, and the resulting films were rigid and extremely thin. Finally, for regime IV (pH 8−10), the films were somewhat soft, and the layer pair thickness was intermediate; PAH was partially charged, and PAA was fully charged. Predominantly linear growth was observed for regimes I and III and exponential growth for regimes II and IV for hydrated PAH/PAA films. It is often mistaken that LbL films are stratified structures with discrete layers. However, numerous studies have demonstrated that LbL films are highly interpenetrated structures with fuzzy boundaries between adjacent layers and can essentially be treated as miscible blends.1,59,60 Prior work has shown that PAH and PAA homopolymers have distinct Tg’s at 190−223 °C and at 80−128 °C, respectively.47,61,62 Based on this reasoning, PAH/PAA LbL films can be expected to have a single Tg between that of PAH and PAA homopolymers. Using modulated DSC of dry PAH/PAA LbL films, we observed two endothermic peaks upon heating, which were a result of anhydride formation and amidation upon heating; however, no Tg was observed.47 In another study, the amidation temperature was shown to decrease with decreasing film thickness as a result of catalytic hydroxyl groups on the substrate’s surface.63 In spite of the above volume of work, key questions still remain unanswered as to whether the addition of water, linear or exponential growth, and film composition have a role in tuning thermal properties. The thermal properties of hydrated LbL assemblies containing weak polyelectrolytes are presently unknown, and such information could yield valuable insight into the internal structure of LbL assemblies. In this paper, we investigate the thermal properties of hydrated PAH/PAA LbL films using QCM-D and modulated DSC. The PAH/PAA system was chosen as a model system based on the ease with which the growth and viscoelastic properties can be manipulated by changing assembly pH conditions. Hydrated ultrathin PAH/PAA LbL films of thickness ranging from 22 to 142 nm assembled from varying assembly pH values were studied using temperature-controlled QCM-D, while hydrated bulk PAH/PAA LbL films of thickness 0.7−2.2 μm were investigated using modulated DSC. We have demonstrated for the first time that hydrated PAH/PAA LbL films have a well-defined Tg. The effect of the outermost layer and the aging time is also presented.

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EXPERIMENTAL SECTION

Materials. Poly(allyamime hydrochloride) (PAH, Mw = 120−200 000 g mol−1) and poly(acrylic acid) (PAA, Mw = 100 000 g mol−1) were obtained from Sigma-Aldrich. Polyethylenimine (PEI, Mw = 25 000 g mol−1) was obtained from Polysciences, Inc. Teflon and quartz crystal substrates were purchased from McMaster Carr and Q-sense, respectively. Fabrication of Free-Standing Layer-by-Layer Assemblies. PAH and PAA solutions were made from their respective homopolymers and 18.2 MΩ Milli-Q water at a concentration of 10−2 M based on the molar mass of the repeat unit. LbL assemblies were constructed using an automated slide stainer (HMS series, Carl Zeiss, Inc.). For the preparation of free-standing LbL films (PAH/PAA at pH 5.5 and 9.0), Teflon substrates, cleaned using sequential sonication for 15 min in ethanol and 15 min in deionized water, were used. For PAH/PAA LbL films assembled at pH 3.5, silicon substrates were used, where the substrates were prepared by immersion in

Δm = −

C ΔF n

(1)

where C is a sensitivity constant and n is the overtone number of the oscillating frequency (n = 1, 3, 5, 7, 9, 11, 13). The mass deposited on the QCM-D crystal is the coupled mass (i.e., the mass of the adsorbed polymer including water). For viscoelastic films, complex models such as the Voigt model must be employed to capture changes in both frequency and dissipation.66 The changes in dissipation of a viscoelastic film are determined by monitoring the decay of the crystal’s oscillation after an initial excitation close to the resonant frequency. The decay rate is dependent on the crystal and the viscoelastic properties of the contacting liquid. The dissipation is given by64,65 9170

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Figure 1. (a, b) Hydrated thickness as a function of layer pairs for PEI/(PAA/PAH) LbL films assembled from pH 3.5 to 9.0. The initial PEI layer is taken as the 0th layer. The hydrated thickness was estimated from applying the Voigt model to QCM-D data. (c, d) Dry thickness as a function of layer pairs for PAH/PAA LbL films assembled from pH 3.5 to 9.0, where thickness was measured using ellipsometry and profilometry. The error was taken as the standard deviation of three measurements.

D=

then dried at 40 °C to promote evaporation until a water content of 18 wt % was achieved. The pans were subsequently sealed with hermetic lids and left at room temperature for 24 h before modulated DSC. Hydrated LbL films and complexes were ramped from 0 to 115 °C at a rate of 2 °C min−1 with an amplitude of 1.272 °C and a period of 60 s. All modulated DSC thermograms are shown in “exotherm down” format. The Tg was taken as the inflection point.

Edissipated 2ΠEstored

(2)

The thickness and mass calculated from the Sauerbrey equation was used as an initial guess to model LbL film growth using the Voigt viscoelastic model.66 We have found that the Sauerbrey and Voigt models deviate from each other by less than 3% for all overtones. All calculations for the Voigt model were performed by taking all overtones (n = 3−13) into consideration using QSoft software (Qsense). Temperature-controlled QCM-D experiments were performed on LbL films assembled on the crystal surface after hydrating the films for 24 h by flowing water at the same pH as the assembled LbL film. The high temperature flow cell (QHTC 101) was ramped at a rate of 1 °C/ min from 30 to 60 °C, while simultaneously monitoring changes in both frequency and dissipation. Because both density and viscosity of water are functions of temperature,67 the frequency and dissipation changes of the LbL film must be corrected by subtracting the frequency and dissipation values of the bare crystal submerged in water. As the temperature is increased, a sudden step increase in dissipation is seen for some samples, indicative of an increase in viscoelasticity of the film. The Tg was taken as the temperature at which this sharp step increase in dissipation occurred. QCM-D probes a film at varying depths, depending on the penetration depth of the different overtones. For instance, the third overtone probes deep into the film, whereas the 13th overtone probes the film closer to the crystal−film interface. Considering that most of the LbL films in this study had low ΔD values, there were no drastic variations among the overtones. We therefore limit our analysis only to the third overtone for this study. Modulated Differential Scanning Calorimetry (Modulated DSC) Measurements. Modulated DSC was performed on hydrated samples using a heat−cool−heat−cool cycle. The sample weight ranged between 3 and 12 mg, depending on sample availability. Tzero hermetic pans and lids were used for hydrated samples. Hydrated samples were first prepared by weighing the dried LbL film in the DSC pans and by adding water at matching assembly pH. The samples were

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RESULTS We performed QCM-D experiments on PEI/(PAA/PAH)9/ PAA LbL films, which were assembled in situ from solutions adjusted to pH = 3.5, 5.5, 7.0, and 9.0, each a representative of regimes I−IV. Polyethylenimine (PEI) was added as an initial layer to promote adsorption. Our motivation was to detect and fine-tune thermal transitions in linear and exponentially growing PAH/PAA LbL films based on the pH of the assembly solutions. Matching previously reported results,50 hydrated PEI/(PAA/PAH)9/PAA LbL films assembled from pH 3.5 and 7.0 showed predominantly linear growth, whereas those assembled from pH 5.5 and 9.0 showed exponential growth. From the difference between hydrated and dry thicknesses, the percentage swelling can be estimated using % swelling =

t − t0 t0

(3)

where t is the hydrated thickness and t0 is the dry thickness. As assembly pH increased from pH 3.5, the percentage swelling decreased from 95% to 22% and then increased for more basic assembly pH (Figure 2). The swelling of the film also correlated with the viscoelastic properties of the hydrated film measured using QCM-D. An increase in ΔD signifies an increase in viscoelasticity, and a decrease in ΔF indicates an increase in mass. We found that 9171

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bare crystal immersed in water at the same pH as that of the LbL films were subtracted from the LbL assemblies resulting in Figures 3a,b.

Figure 2. Swelling percentage and changes in dissipation as a function of assembly pH of PAH/PAA LbL films at 25 °C. Error bars are taken as the standard deviation of three or more measurements.

PEI/(PAA/PAH)9PAA LbL films assembled from pH 3.5 were very soft with a ΔD value of 40 × 10−6 units, which was far larger compared to films assembled from pH 5.5, 7.0, and 9.0 with ΔD values ranging from 0.4 to 1.5 × 10−6 units, correlating well with Bieker and Schönhoff’s recent report.50 As the water content within the film increases, so does the percentage swelling and the viscoelasticity, leading to an increase in ΔD. From the changes in frequency observed using QCM-D during LbL assembly, the mass of adsorbed PAH and PAA can be estimated as well as the percentage of PAA within the film, assuming that PAH and PAA adsorb at the same levels of hydration, Table 1. However, because PAA is more hydrophilic than PAH,24 PAA will likely adsorb to the surface along with a greater number of water molecules than PAH. This could result in an overestimation of the percentage of PAA within the LbL film compared to dry PAH/PAA films.24 Also, it has been reported that the adsorbed mass of the polyelectrolytes for a flow process like QCM-D is usually lower than that from dipping processes without flow.68 Nevertheless, we see a trend in %PAA with assembly pH that matches reports by Beiker,50 except for PAH3.5/PAA3.5 LbL films, perhaps owing to the differences in molecular weight of PAH (120−200 000 g mol−1) used in our study compared to PAH with Mw = 56 000 g mol−1 used in the previous study.24 After the LbL films were assembled, temperature-controlled QCM-D was performed in situ to observe changes in frequency and dissipation for PEI/(PAA/PAH) 9 /PAA LbL films assembled from solutions pH-adjusted to 3.5, 5.5, 7.0, or 9.0. During the experiment, LbL films were submerged in water of pH matching their assembly conditions, and samples were ramped from 30 to 60 °C at 1 °C/min. It is known that both frequency and dissipation are dependent on temperature for a bare crystal immersed in water because the density and viscosity of water are also a function of temperature. In order to exclude the effect of the bare crystal and water on the LbL assemblies, the changes in frequency and dissipation for the

Figure 3. Corrected temperature dependence of (a) ΔF and (b) ΔD for PEI/(PAA/PAH)9/PAA LbL films with PAA as the outermost layer assembled from pH 3.5, 5.5, 7.0, and 9.0 solutions. The corrected ΔF and ΔD are from the 3rd overtone. Heating was performed at a rate of 1 °C/min (2nd heating cycle shown). The curves have been shifted vertically along the y-axis for clarity.

With the exception of LbL films assembled at pH 7.0, ΔF increases slightly as temperature increases, suggesting a decrease in the film’s water content (Figure 3a). As temperature further increases, there is an abrupt decrease in ΔF, signifying an abrupt influx of water into the LbL film. This step change was also observed in the ΔD−temperature curve (Figure 3b) for PEI/(PAA/PAH)9/PAA LbL films assembled from pH 3.5, 5.5, and 9.0 at 51 ± 1, 40 ± 5, and 56 ± 3 °C, respectively; ΔD abruptly increased, signifying an increase in the film’s viscoelasticity. Excluding, the step change, the overall trend in ΔD with temperature can be examined to elucidate general changes in the films’ viscoelasticity. Because of the rigid nature of the LbL films assembled from pH 5.5, 7.0, and 9.0 solutions, ΔD remained more or less constant (with the exception of any step changes) as temperature increased. On the other hand, ΔD

Table 1. Properties of Hydrated (PAH/PAA)9 LbL Films Assembled at Varying pH’s PAH/PAA pH pH pH pH a

3.5 5.5 7.0 9.0

wt % PAA 51 29 43 15

± ± ± ±

3 2 1 2

% swelling 100 28 20 40

± ± ± ±

hydrated thickness at 9.5 layer pairs (nm)

20 5 10 20

124 142 25 31

± ± ± ±

5 4 2 4

QCM-D Tg (°C)

MDSC Tg (°C)

51 ± 1a (53 ± 1)b 40 ± 5a (44 ± 1)b no Tga (no Tg)b 56 ± 3a (55 ± 3)b

62 ± 1a 51 ± 6a N/Aa 62 ± 10a

PAA was the outermost layer. bPAH was the outermost layer. 9172

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decreased significantly for films assembled from pH 3.5 solutions, signifying that the film was becoming relatively more rigid. Considering the high level of hydration for films assembled from pH 3.5 solutions, it is reasonable that these films are more sensitive to temperature and fluctuations in hydration. Step changes were reproducible upon repeated heating and cooling cycles (Figure S2). On the other hand, films assembled at pH 7.0 show no apparent changes in frequency, suggesting very rigid films. No such step change in ΔF or ΔD was observed for PEI/(PAA/ PAH)9/PAA LbL films assembled from pH 7.0. Recent studies have highlighted that the outermost layer of an LbL assembly can influence the film’s properties.35,36,69 We performed QCM-D on PEI/(PAA/PAH)10 LbL films, where PAH was the outermost layer (Figure 4). As before, LbL films

Figure 5. (a) Modulated DSC thermograms of hydrated PAH/PAA LbL films assembled from pH 5.5. (b) Reversing heat flows for hydrated PAH/PAA LbL films assembled from pH 3.5, 5.5, and 9.0 solutions. Curves in (b) have been shifted along the y-axis for clarity. Heating at 2 °C min−1, amplitude of 1.272 °C, and period of 60 s (2nd heating cycle shown).

assembled from 5.5 and 9.0 were assembled on Teflon substrates, while PAH/PAA LbL films assembled from pH 3.5 were assembled on silicon substrates owing to delamination when assembled on Teflon. LbL films assembled from pH 7.0 were too thin and had insufficient mass for modulated DSC experiments. Modulated DSC is ideal for analyzing weak thermal transitions in polymeric systems. A modulated temperature ramp allows for the separation of the total heat flow into reversing and nonreversing heat flows. The reversing heat flow captures thermal events such as Tg and changes in Cp at time scales shorter than the modulation period, whereas the nonreversible heat flow captures events such as aging and reactions at longer time scales. Although there was some aging (enthalpic relaxation) present in nonreversible heat flows, we focus our attention primarily on the reversible heat flows to analyze Tg’s unless otherwise noted. Unlike dry, brittle PAH/PAA LbL films (Figure S1), which are devoid of any Tg,47 hydrated PAH/PAA LbL films show distinct Tg’s. For example, in Figure 5a, a distinct glass transition at 55 °C was observed in the reversing curve, accompanied by enthalpic relaxation associated with physical aging observed in the nonreversing curve. Figure 5b shows modulated DSC thermograms for hydrated PAH/PAA LbL films assembled from various pH-adjusted solutions, where PAA was the terminal layer. There is a clear Tg, characterized by a sigmoidal shape in the reversing heat flow, present in PAH/ PAA LbL films assembled from pH 3.5, 5.5, and 9.0 at 62 ± 1, 51 ± 6, and 62 ± 10 °C, respectively. Table 1 summarizes findings from both QCM-D and MDSC experiments. The Tg’s

Figure 4. Corrected temperature dependence of (a) ΔF and (b) ΔD for PEI/(PAA/PAH)10 LbL films with PAH as the outermost layer assembled from solutions of varying pH. The corrected ΔF and ΔD are from the 3rd overtone. Heating was performed at a rate of 1 °C/ min, and the second cycle is shown. The curves have been shifted vertically along the y-axis for clarity.

assembled at pH 7.0 did not have a Tg. LbL films assembled from pH 3.5, 5.5, and 9.0 show Tg’s at 53 ± 1, 44 ± 1, and 55 ± 3 °C, respectively. Considering that these differences are within error of measurements of films where PAA was the outermost layer, our results indicate that the Tg is independent of the outermost layer for the PAH/PAA LbL system. This is in contrast to the PDAC/PSS LbL system, which is comprised of strong polyelectrolytes. The LbL film expanded upon heating through the Tg when PDAC was the outermost layer but shrank when PSS is the outermost layer. To corroborate our QCM-D results, we performed modulated DSC on PAH/PAA LbL films assembled from pH 3.5, 5.5, and 9.0 solutions (Figure 5). PAH/PAA LbL films 9173

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COO− group and in the formation of amide bonds,70−73 which perhaps could have formed under prolonged exposure to the desiccators’ dry environment.

measured using modulated DSC were consistently higher than those measured using QCM-D. A possible reason for this difference could be related to the dissimilar way in which the films were made. For modulated DSC, films were assembled using simple immersion; for QCM-D, films were assembled under flow using slightly lower concentrations. Indeed, this might result in lower adsorbed amounts of the polyelectrolyte giving rise to films of dissimilar composition compared to films assembled using the dip process.50,68 Surprisingly, we found no evidence of a glass transition in PAH/PAA LbL films that had been stored for 3 months in a desiccator (Figure 6). Both reversing and nonreversing heat

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DISCUSSION Temperature-controlled QCM-D in tandem with modulated DSC forms a potent suite of tools for characterizing thermal transitions in LbL assemblies. Together, the two complementary techniques were used to elucidate the role of film internal structure, hydration, and composition on the Tg’s of PAH/PAA LbL films. As discussed by Bieker50 and Shiratori,24 PAH/PAA LbL films have several distinct growth regimes based upon their assembly pH. Here, we generally observe that hydration plays a critical role in the viscoelasticity of the film as evidenced by a comparison of the swelling percentage to ΔD. In regime I (pH 3−4.5), the films are thick and have a high dissipation. Similarly, the PAH3.5/PAA3.5 LbL film swelled with water by 100%, which led to a very large ΔD. At pH 3.5, PAH is fully charged and assembles into compact layers, and PAA is partially charged and assembles into thicker layers with loops and trains.50 In regime II (pH 4.5−6), the films are the thickest of those studied but are far more rigid as compared with regime I. Here, PAH is nearly fully charged and PAA is partially charged. Similarly, the PAH5.5/PAA5.5 LbL film consisted of 28% water, far less than the film from regime I. Regime IV (pH 8−10) was analogously similar to regime II, where both polyelectrolytes are partially charged, and PAH9.0/PAA9.0 LbL films were relatively rigid. Finally, in regime III (pH 6.5−7.5), both polyelectrolytes are fully charged, leading to tightly bound rigid layers. Our PAH7.0/PAA7.0 LbL film was the thinnest of all those studied and had the lowest swelling percentage (22%). Upon comparing hydration and the observed Tg’s, no clear correlation can be obtained. Although water is essential for the transition to occur, its relative abundance does not affect the Tg value. On the other hand, it has been proposed that the growth mechanism can influence the relative mobility of polyelectrolytes within an LbL film. Exponential growth is characterized by the “in and out” diffusion of polyelectrolytes, which requires a good deal of segmental mobility within the LbL film.59 If this were true, then it is expected that exponentially growing films would have a lower Tg than linearly growing films. We have found supporting evidence for PDAC/PSS LbL films,74 but the results presented herein are not directly supportive. For instance, PAH/PAA LbL films assembled in regimes II and IV grow exponentially24,50 and have Tg’s of 40 and 56 °C, respectively; PAH/PAA films assembled in regimes I and III grow linearly and only the film from regime I has an observable Tg at 51 °C. Because the Tg of a linearly growing film (regime I) is lower than that of an exponentially growing film (regime IV), we are not able to conclusively determine a direct correlation between growth mechanism and Tg value for PAH/ PAA LbL films. Assuming the PAH/PAA LbL film can be treated as a miscible blend, the composition may play an important role in the film’s Tg. However, a comparison of composition and Tg’s from Table 1 shows no clear correlation. For instance, PAH3.5/ PAA3.5 LbL films have more PAA present than PAH5.5/PAA5.5 LbL films, but the film assembled at pH 3.5 has a higher Tg. Hydrated (18 wt % H2O) PAH and PAA homopolymers did not exhibit a Tg, meaning that the values could be above or below our measurement window (5−110 °C) (Figure S3).

Figure 6. Modulated DSC thermogram for a 3 month old hydrated (18%) (PAH9.0/PAA9.0)150 LbL film.

flows were featureless for these samples. To determine any structural changes that might have taken place over this time period, FTIR spectroscopy was performed on dry (PAH9.0/ PAA9.0)150 LbL films both freshly prepared and stored for 3 months (Figure 7). Two distinct peaks unique to PAH/PAA

Figure 7. FTIR spectra for freshly prepared and 3 month old dry (PAH9.0/PAA9.0)150 LbL films.

LbL assemblies were of note. First, the freshly prepared film had a peak at 1625 cm−1, which was assigned in the region of the asymmetric −NH3+ stretch (1624−1627 cm−1), representative of PAH.52 Second, a peak at 1552 cm−1 was present, which was representative of asymmetric stretching of PAA’s ionized carboxylate (COO−) groups, (1565−1542 cm−1).52 The 3 month old (PAH9.0/PAA9.0)150 dry LbL film exhibited most of the same peaks as the freshly prepared LbL film, except that two distinct peaks at 1578 and 1518 cm−1 appeared. These peaks are likely attributed to a shift in environment of the 9174

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The lack of correlations between Tg value and composition, hydration, or growth mechanism suggests that other factors may explain the trends in Tg for PAH/PAA LbL films. For instance, the density of ion pairs can strongly control the Tg because each ion pair acts as a cross-link that restricts segmental mobility. On the other hand, free volume can play a role, where films with greater free volume may possess lower Tg’s. As the density of ion pairing decreases, polyelectrolytes can assume conformations of “loops and trains” that increases the film’s free volume. An example of such phenomena is present in regime III, where there were so many ion pairs that no Tg could be observed, and the film’s free volume is likely very low. In contrast, regime II is likely to have the highest free volume because both PAH and PAA are partially charged, which leads to a film possessing many segment loops and trains; indeed, the film assembled in regime II had the lowest Tg of all regimes investigated. Complexes and LbL assemblies are often compared to one another because they are formed using the same interactions. Our results, however, indicated that the two are not necessarily one in the same. The Tg’s of complexes and analogous PAH5.5/ PAA5.5 LbL films were compared using modulated DSC. The complexes yielded a Tg of 35 °C, which was 16 °C lower than that of the corresponding LbL films (Figure S4). The difference in Tg can be explained by the dissimilar composition of the complexes (about 1:1 by repeat unit) and the films (3.3:1 by PAH:PAA repeat units), where increasing PAH content should increase the Tg. Unlike PDAC/PSS LbL assemblies, which have a strong “odd−even” effect,35,36,69,74 no such phenomena were observed for PAH/PAA LbL films. Results vary among groups, where Notley et al.75 have shown that the viscoelasticity of PAH/PAA LbL films was dependent on the outermost layer. However, Shiratori et al.24 and Yoo et al.25 have shown that advancing contact angle measurements were independent of the outermost layer for PAH/PAA LbL films. In our experiments, PAH/ PAA LbL films had similar Tg values irrespective of whether PAA or PAH was the outermost layer and is in good agreement with the work of Shiratori et al.24 Only in two distinct cases were the Tg’s outside of the measurement range. In the first case, films freshly assembled at pH 7 (regime III) bore no observable Tg. This film was expected to be highly ion-paired because both PAH and PAA were nearly fully charged. The high extent of ion-pairing likely inhibited segmental motion and elevated the films’ Tg beyond our measurement range. In the second case, a film assembled at pH 9.0 and stored under dry conditions for 3 months also bore no observable Tg. In contrast, the freshly prepared analogue had a calorimetric Tg at 62 °C. FTIR spectra showed that aging in dry conditions led to the formation of amide bonds, which likely inhibited segmental motion and significantly increased Tg. In both cases, cross-linkseither noncovalent or covalentled to elevated Tg’s beyond the measurement range of our present techniques (80 °C for QCM-D and 105 °C for modulated DSC). These findings also indicate that one must consider the effects of aging and storage environment in the investigation of PAH/PAA LbL assemblies.

be limited to linear and exponential growth alone. We demonstrate distinct Tg’s for exponentially growing PAH/ PAA LbL films assembled from pH 5.5 and 9.0 and for linearly growing PAH/PAA LbL films assembled from pH 3.5. However, there was no detectable Tg for linearly growing PAH/PAA LbL films assembled from pH 7.0 in the temperature range under investigation. Also, thermal properties did not depend on the outermost layer for PAH/PAA LbL films as has been previously reported for PDAC/PSS LbL assemblies. QCM-D and modulated DSC results indicate that the thermal properties of PAH/PAA LbL films have a complex dependence on film composition, hydration, and linear or exponential growth, and free volume. QCM-D in particular forms a potent tool for studying thermal transitions in thin films by sensitively monitoring fluctuations in film hydration and viscoelasticity. Future work will entail a detail study of thermal properties in ultrathin neutral polymers and polymeric systems containing nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

Digital images of free-standing (PAH9.0/PAA9.0)150 LbL films isolated from Teflon substrates, temperature-controlled QCMD cooling cycle experiments for PEI/(PAA/PAH)10 LbL films with PAA as the outermost layer, and modulated DSC thermograms for hydrated PAH and PAA homopolymers and hydrated PAH5.5/PAA5.5 complex. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (NSF Grant 1049706) and Texas A&M University for support. We thank Dr. Nichole Zacharia for profilometer access. We thank Dr. Archana Jaiswal, Dr. Matthew Dixon, and Dr. Mark Poggi from Q-Sense for help with QCM-D data analysis.

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REFERENCES

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CONCLUSION The influence of assembly pH on the Tg of PAH/PAA LbL assemblies was investigated using QCM-D and modulated DSC. Our results indicate that there are several factors that determine thermal properties in LbL films and analysis cannot 9175

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Macromolecules

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