Swelling and Thermal Transitions of Polyelectrolyte Multilayers in the

Article pubs.acs.org/Macromolecules

Swelling and Thermal Transitions of Polyelectrolyte Multilayers in the Presence of Divalent Ions Dariya K. Reid,† Alexandra Summers,† Josh O’Neal,‡ Avanti V. Kavarthapu,† and Jodie L. Lutkenhaus*,†,‡ †

Artie McFerrin Department of Chemical Engineering and ‡Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: The effects of CaCl2, MgCl2, and Na2SO4 on the swelling response and the thermal transition of poly(diallyldimethylammonium chloride)/poly(styrenesulfonate) (PDAC/PSS) layer-by-layer (LbL) assemblies are investigated. For MgCl2 and CaCl2, two swelling regimes are observed depending on the salt concentration of the overlying solution. Below 0.17 M MgCl2 or CaCl2, swelling appears to arise from electrostatic repulsion within the multilayer. Above 0.17 M MgCl2 or CaCl2, the multilayer swells linearly with the concentration of the overlying solution due to doping; the degree of swelling in this linear regime depends on the hydration shell of the doping ions. Alternatively, swelling with no concentration dependence was observed for Na2SO4. Unlike the swelling behavior, the thermal transition temperature observed in hydrated films shifts to higher temperatures with increasing concentration of Na2SO4. However, when hydrated with CaCl2 or MgCl2 solutions, the transition remains relatively constant within error. Results are discussed in terms of the salt ion’s hydration shell and Donnan exclusion.

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INTRODUCTION The effect of divalent ions on layer-by-layer (LbL) assemblies or polyelectrolyte multilayers (PEMs) is very poorly understood, yet this knowledge is critical as more LbL assemblies are proposed and deployed in environments bearing divalent ions (water purification, biomedical, self-healing films, corrosion, etc.). LbL assemblies or multilayers, prepared through alternate adsorption of oppositely charged species onto a substrate, offer a tremendous range of possibilities due to their tunable nature.1−6 Since its introduction over 20 years ago, the technique has been applied to a variety of substrates and components.7 The LbL technique has been broadly used in applications such as gas separation,8,9 optical coatings,10 sensors,11,12 drug delivery,13−15 and energy storage.16−18 Most commonly LbL assembly harnesses noncovalent interactions (electrostatic, van der Waals, hydrogen bonding) such that assembly and postassembly environment can be used to elicit responses or changes in film properties. These tuning parameters include pH,19,20 temperature,21−23 solvent type,24,25 ionic strength,26,27 and counterion type.24,28−31 The focus here is on the response of strong polyelectrolyte poly(diallyldimethylammonium chloride)/poly(styrenesulfonate) (PDAC/PSS) LbL films upon exposure to aqueous solutions bearing divalent ions, particularly Ca2+, Mg2+, and SO42−. The PDAC/PSS system is of interest as it is often treated as a model LbL system of strong polyelectrolytes. As PDAC and PSS are alternately adsorbed to a substrate, the surface charge alternates between positive and negative up to about 7 cycles or layer pairs.32 Beyond this point, the PDAC/PSS multilayer © XXXX American Chemical Society

maintains a net positive charge from extrinsically compensated PDAC sites even as growth proceeds. The general result is a well-mixed, interdigitated film as opposed to strictly stratified layers.1 The tunable surface properties of multilayers provide a unique opportunity to probe multilayer interactions with divalent ions. In recent years significant advances have been made in understanding the postassembly swelling response of LbL multilayers under a range of conditions,33−44 particularly with response to NaCl concentration,36−38 but much less so for divalent ions. PDAC-terminated PDAC/PSS LbL capsules prepared from 0.5 M NaCl expanded when placed in 1−10 mM salt solutions because of reduced charge screening and swelling dominated by electrostatic forces.37 Others have studied the effect of NaCl27,32,45 and salts containing divalent ions28,29 during LbL buildup, but this is distinguished from our present focus on postassembly behavior. Our interest regarding divalent ions extends to the thermal response of PDAC/PSS LbL films, for which salt concentration may play a role. Vidyasagar et al. demonstrated that hydrated PDAC/PSS LbL films undergo a glass transition-like thermal transition.46 It is thought that this transition consists of two steps with the first being a water−polyelectrolyte disruption and the second being polyelectrolyte chain relaxation.47−49 Modest dependence of the transition temperature on NaCl concenReceived: May 31, 2016 Revised: July 22, 2016

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DOI: 10.1021/acs.macromol.6b01164 Macromolecules XXXX, XXX, XXX−XXX

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(5):NH4OH (1):H2O2 (1) by volume) at 70 °C, drying using nitrogen, and then plasma-treating for an additional 10 min. For the duration of the experiment, the temperature was set to 25 °C and the flow rate was kept constant at approximately 115 μL/min. A zero baseline was established by flowing Milli-Q water (pH 4.5) over the sensor for approximately 30 min. A base layer was then deposited by passing 1 mg/mL PEI solution (pH 4.5) over the crystal for 15 min, followed by a 5 min rinse using Milli-Q water (pH 4.5) in order to increase the initial surface charge. LbL deposition was carried out by sequentially flowing solutions of 0.1 mg/mL PSS (0.5 M NaCl) and 0.1 mg/mL PDAC (0.5 M NaCl) over the crystal for 15 min, with a 5 min rinse of Milli-Q water (0.5 M NaCl) after each deposition. Following the assembly process, the films were first exposed to CaCl2, MgCl2, or Na2SO4 solution at concentrations of 0.01−1.9 M for 1−2 h, second exposed to 0.5 M NaCl solution for 1 h, and third exposed back to the initial CaCl2, MgCl2, or Na2SO4 solution for 1−2 h. The switching of solutions occurred once a plateau in the signal was reached. Salt solutions were sonicated in order to aid dissolution. Dry film thickness was taken from a parallel LbL assembly assembled on silicon wafer and measured using profilometry. Modeling of QCM-D Data. QTools modeling software (Biolin Scientific) was used to fit the changes in frequency and dissipation for the fifth, seventh, and ninth overtones according to the extended viscoelastic model. Material density (L1) was taken as 1050 kg/m3. In particular, the extended viscoelastic model was chosen over the regular viscoelastic model as it provided a significantly lower χ2 parameter for the data (Figures S1 and S2). Modulated Differential Scanning Calorimetry. Modulated DSC (TA Instruments DSC Q200) was used to assess the presence of a thermal transition in hydrated LbL assemblies. After assembly freestanding (PDAC/PSS)140 films were removed from the substrate, dried in ambient conditions overnight and then under vacuum at 115 °C for 3 h, and hydrated with CaCl2, MgCl2, or Na2SO4 solutions with concentrations ranging from 0.01 to 1.9 M (all below their solubility limit in water). The amount of hydration was kept constant as 36% of the dried film mass. The total modulated DSC sample mass (film plus solution) ranged between 7 and 11 mg. The hydrated samples were sealed inside Tzero hermetic pans and lids (TA Instruments) and allowed to rest for more than 24 h. Hydrated films were first allowed to equilibrate at 0 °C for 5 min then ramped from 0 to 115 °C at a rate of 2 °C/min with amplitude of 1.272 °C for a period of 60 s. The thermal cycle was repeated two times, and the reported thermal transition (Ttr) values were taken as the inflection point in the second heating scan.

tration was demonstrated, where it was thought that NaCl doping lowers the transition by weakening intrinsic polyelectrolyte−polyelectrolyte pairs and increasing the extent of extrinsic polyelectrolyte−counterion pairs. Up to now, only the effect of NaCl on the thermal transition has been shown, and divalent ions have yet to be explored. A more comprehensive understanding is needed to assess structural changes and responses of polyelectrolyte multilayers exposed to multivalent ions. A recent report by Wei et al. explored the effect of divalent cation solutions on the properties of PDAC/PSS LbL films.50 Ultrathin PSS-terminated PDAC/ PSS LbL films assembled from 0.5 M NaNO3 solutions and then equilibrated with water or the rinsing solution contracted due to physical cross-linking when exposed to solutions of Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Ba(NO3)2, Zn(NO3)2, and Cu(NO3)2 at low concentration (0.01 M).50 However, the effect of other ions and a detailed concentration dependence remain relatively unexplored. This means that concentration-dependent phenomena such as swelling by electrostatic repulsion and doping have not yet been adequately captured for multivalent ions in PDAC/ PSS LbL assemblies. Here, we present the swelling and thermal properties of hydrated PDAC/PSS LbL assemblies in the presence of divalent salts (CaCl2, MgCl2, and Na2SO4) with comparison against NaCl. A broad range of concentrations are explored, ranging from 0.01 to 1.9 M. Quartz crystal microbalance with dissipation monitoring (QCM-D) is used to monitor the film thickness and mechanical properties during ion exchange between monovalent and divalent salts. The glass-transition-like thermal transition is monitored by modulated differential scanning calorimetry (DSC) of free-standing LbL assemblies in the presence of aqueous divalent salt solutions. The resulting observations are discussed in terms of electrostatic repulsion, salt doping, and Donnan exclusion.

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MATERIALS AND METHODS

Materials. Poly(diallyldimethylammonium chloride) (PDAC, Mw = 200 000−350 000 g/mol, 20 wt % solution), poly(styrenesulfonate sodium salt) (PSS, Mw = 500 000 g/mol), and linear polyethylenimine (PEI, Mw = 25 000 g/mol) were purchased from Sigma-Aldrich, Scientific Polymer Products, and Polysciences, Inc., respectively. Sodium chloride (NaCl), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), and calcium chloride (CaCl2) were all purchased from VWR. Teflon and quartz crystal substrates were purchased from McMaster Carr and Q-Sense, respectively. Preparation of Free-Standing LbL Assemblies. Free-standing LbL films were prepared using an automated slide stainer (HMS series, Carl Zeiss, Inc.) on Teflon substrates. PDAC and PSS were dissolved in Milli-Q water at a concentration of 1 mg/mL, and an ionic strength of 0.5 M NaCl was used for all solutions. The substrates were treated by sonication for 15 min in ethanol and then two rounds in 18.2 MΩ cm Milli-Q water prior to assembly. Film assembly proceeded by immersing the substrates in PDAC solution for 15 min, followed by three rinses in Milli-Q water for 2, 1, and 1 min. The process was then repeated using PSS solution. After 140 layer pairs were assembled excess salt was removed by rinsing with fresh Milli-Q water, and then the samples were dried in ambient conditions overnight followed by additional drying under vacuum at 115 °C for 3 h. The resulting LbL films are denoted as (PDAC/PSS)n where n is the number of LbL cycles or layer pairs. Preparation of LbL Assemblies Using QCM-D. QCM-D measurements were collected using the Q-Sense E1 instrument. Gold-plated AT-cut quartz crystals with a resonant frequency of 4.95 MHz were used as the substrate. Prior to film deposition, the crystals were cleaned by plasma treatment using an O2-plasma etcher for 10 min followed by a 10 min immersion in basic piranha (water

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RESULTS Figure S3 illustrates the assembly of a PEI-(PSS/PDAC)7 multilayer at 0.5 M NaCl as observed by QCM-D. By this procedure, PEI serves as the base layer, and PDAC is the terminating layer. Under these conditions, the as-assembled film possesses a mixture of intrinsic (polyelectrolyte−polyelectrolyte) and extrinsic (polyelectrolyte−small counterion) ion pairs as well as a net positive charge at the surface. Once the assembly process was complete, the LbL film was exposed to varying concentrations of NaCl. Figure 1a shows multilayer thickness upon sequential exposure to the following three solutions: (1) NaCl solution (0.01−1.9 M), (2) 0.5 M NaCl solution, and (3) repeat back to the solution at (1). As is displayed in Figure 1a, the swelling and contraction response was most extreme at the lowest and highest salt concentrations explored. When the overlying solution was changed from 0.01 M NaCl to 0.5 M NaCl, the film contracted by Δt = 189 nm. The response was similar when the film was switched from 1.9 to 0.5 M NaCl solution, exhibiting a Δt = 75 nm contraction. Changing the overlying solution from more intermediate concentrations to 0.5 M NaCl produced insignificant changes in the film thickness. The contraction thickness resulting from step 1 to 2 is shown in Figure 1b as an B


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upon switching back to 0.33 M CaCl2, or from (2) to (3), consistent with an uptake of mass and softening of the film. The film displayed nearly analogous behavior when exposed to solutions of 0.33 M MgCl2 and 0.33 M Na2SO4 (Figure 2c−f). Overall, there was a strong correlation of the solid and dashed lines indicating a good fit of the extended Voigt model to the raw data. The preceding experiment was similarly conducted for a series of concentrations of CaCl2, MgCl2, and Na2SO4 solutions alternated with 0.5 M NaCl solution (Figure S5). Figure 3 shows the resulting change in thickness as the flowing solution was switched from the divalent salt solution to 0.5 M NaCl (from step 1 to 2); generally, the swollen film contracted by the quantity Δt during this procedure. At low CaCl2 or MgCl2 concentration, the film’s swollen state is controlled by electrostatic forces (regime I), and the maximum contraction observed when switching contact solutions was approximately 160 nm for both 0.01 M CaCl2 and MgCl2 solutions. At intermediate CaCl2 and MgCl2 concentrations little swelling and contraction is observed. Upon further increasing the CaCl2 and MgCl2 concentration past a certain threshold into regime II, swelling becomes controlled by doping and ion uptake, and contraction proceeds linearly with salt concentration. The linear behavior was fitted using Δt = −48.28CCaCl2 + 1.22 (R2 = 0.99) and Δt = −70.08CMgCl2 + 8.81 (R2 = 0.99) for regime II and overlying concentrations of CaCl2 and MgCl2, respectively. Notably, the absolute slope for MgCl2 was larger than CaCl2. Linear behavior was also observed in regime II for NaCl in Figure 1c. When Na2SO4 was the overlying solution for step 1, the contraction thickness Δt was constant regardless of the Na2SO4 concentration. Table 1 summarizes the parameters obtained from the linear fits of the NaCl, CaCl2, and MgCl2 data in regime II as well as the hydration numbers for the salts handled in this study. The increased absolute slope observed for MgCl2 as compared to CaCl2 potentially indicates two possibilities: (1) Mg2+ is a superior doping agent as compared to Ca2+ therefore at the same molar concentration more Mg2+ ions penetrate the film and cause a greater degree of expansion; (2) because Mg2+ has a larger hydration shell as compared to Ca2+, more water molecules enter the film during doping, causing greater expansion. A similar argument may be applied to Na+. No clear trend in the y-intercept was apparent. From viscoelastic modeling of the data presented earlier, shear modulus values as a function of exposure time to divalent and monovalent salt solutions were estimated (Figure 4). The general observation is that the film stiffens and increases in shear modulus when exposed to 0.5 M NaCl as compared to when in the presence of divalent ions. This behavior correlates well with the swelling behavior described previously. The magnitude of the change in shear modulus was highest at the lowest ion concentrations for all three investigated salts (regime I). We describe one case, in which the solution was switched from 0.01 M CaCl2 to 0.5 M NaCl and the shear modulus increased by 11 MPa. When the solutions were switched back from 0.5 M NaCl to 0.01 M CaCl2, the shear modulus dropped by 10 MPa. Similarly, the shear modulus increased by 10 MPa when the exposure solution was switched from both 0.01 M MgCl2 and 0.01 M Na2SO4 to 0.5 M NaCl. Because of the low ΔF/ΔD ratio, the QTools software had difficulty providing an acceptable fit of the viscoelastic parameters. Although the values of shear modulus presented should not be taken as absolute due to the

Figure 1. (a) PEI-(PSS/PDAC)7 film thickness as a function of time as the overlying solution is switched between 0.01 and 1.9 M NaCl solution (steps 1 and 3) and 0.5 M NaCl solution (step 2). (b) Change in thickness Δt upon switching from a desired NaCl concentration (xaxis) to 0.5 M NaCl (i.e., from step 1 to 2).

inverted U-shaped curve, where the x-axis is the concentration of NaCl in step 1. From the observations shown in Figure 1, we denote two regimes that capture the varied response of the film. The first regime (regime I) consists of salt concentrations below 0.17 M in which electrostatic forces drive the swelling response of the LbL film.51 Electrostatic forces dominate when extrinsically compensated polyelectrolytes in the LbL film experience selfrepulsion due to low ionic strength and insufficient charge screening. The second regime (regime II) consists of salt concentrations above 0.17 M in which doping and salt uptake drive the swelling response of the LbL film.38 Excessive doping can lead to disruption of intrinsic ion pairing and eventual deconstruction of the film, although this was not observed here. Having established the basic behavior of NaCl, we next turn to the swelling behavior of PDAC/PSS LbL films assembled at 0.5 M NaCl and then later exposed to overlying CaCl2, MgCl2, and Na2SO4 solutions. These salts were chosen because they possess a divalent cation or anion complemented by monovalent Na+ or Cl−, allowing for comparison with the NaCl data shown in Figure 1. Figure 2 shows the observed changes in frequency and dissipation upon ion exchange, as denoted by the following nomenclature: (1, 3, 5) exposure to 0.33 M CaCl2, MgCl2, or Na2SO4 solution; (2, 4) exposure to 0.5 M NaCl solution. Figure S4 shows the initial response of the LbL film as it is first exposed to divalent salt solution directly after assembly. Figure 2a shows an increase in the frequency from (1) to (2) as the flowing solution was switched from 0.33 M CaCl2 to 0.5 M NaCl. This step change indicates a reduction in film thickness and hydrated mass. The change is also mirrored by a sharp reduction in dissipation, indicating that the LbL film becomes more compressed and rigid (Figure 2b). The behavior reverses C


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Figure 2. Frequency (a, c, e) and dissipation (b, d, f) changes of a PEI-(PSS/PDAC)7 LbL film exposed to solutions in the following order: (1) 0.33 M CaCl2, MgCl2, or Na2SO4, (2) 0.5 M NaCl, (3) 0.33 M CaCl2, MgCl2, or Na2SO4, (4) 0.5 M NaCl, and (5) 0.33 M CaCl2, MgCl2, or Na2SO4. Panels (a) and (b) show switching between CaCl2 and NaCl, (c) and (d) show switching between MgCl2 and NaCl, and (e) and (f) show switching between Na2SO4 and NaCl. Solid lines indicate raw data. Dashed lines indicate the fit provided by QTools software. Data for the 5th, 7th, and 9th overtones are shown.

Figure 3. Net change in thickness observed during the ion exchange from (a) CaCl2, (b) MgCl2, and (c) Na2SO4 of varying concentrations (x-axis) to 0.5 M NaCl. The solid lines are drawn to help guide the eye and represent a linear fit of Δt = −48.28CCaCl2 + 1.22 (R2 = 0.99) and Δt = −70.08CMgCl2 + 8.81 (R2 = 0.99) for the data falling in the range of regime II (≥0.17 M).

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multilayers. The free-standing films are assumed to have a well intermixed structure and net positive charge due to the large number of layer pairs and the nature of PDAC/PSS growth.32 A second-order thermal transition resembling a glass transition in PDAC/PSS multilayers in contact with NaCl solution was previously observed.46 The data presented here expand previous work by looking at the effect of ion type and ion concentration on the thermal transition. Considering the strong swelling and contraction behavior displayed in Figures 1−4, it is expected that the transition temperature would similarly vary. Modulated DSC thermograms in Figure 6 show a weak transition similar to that observed previously for NaCl contacting solutions. For films exposed to CaCl2 and MgCl2, the thermal transition could not be detected for concentrations above 0.33 M (Figure 6a,b). No dissolution of the assembled films was observed over the range of concentrations used in the study, so the reason for this remains unclear. It is possible that at high ionic strengths in regime II, CaCl2 and MgCl2 strongly disrupt intrinsic ion pairing to a point at which the transition is unobservable, yet further evidence for this is needed. On the other hand, the thermal transition was observed consistently in films in contact with Na2SO4 solutions ranging in concentration from 0.01 to 1.8 M (Figure 6c). Figure 7 plots the transition temperature as a function of salt type and concentration. The transition for CaCl2 and MgCl2 solutions remained constant at about 44−51 °C, within standard deviation for the full range of concentrations considered in this work. As shown in Figure 7c, samples hydrated using Na2SO4 solutions up to a concentration of 0.17 M displayed a constant thermal transition at approximately 45 °C. However, at higher concentrations the thermal transition began to increase linearly with increasing concentration. This linear dependence can be written as Ttr = 12.44CNa2SO4 + 43.53 with an R2 = 0.95, yet it

Table 1. Summary of Linear Fits in Regime II for Δt = mCsalt + ba ions

total hydration numberb,52

slope

NaCl CaCl2 MgCl2 Na2SO4

5.5 11.2 14 10.1

−46.6 −48.3 −70.1 N/A

a

Csalt is the concentration of the overlying solution in step 1. bTotal hydration number (Nhyd) = nNhyd,cation + nNhyd,anion.

uncertainty presented by the model, the trend evident in the data can be used to speculate qualitatively on general mechanical behavior. To probe whether swelling and contraction occurred throughout the film or within a finite volume, the response was investigated as a function of the number of layer pairs (i.e., film thickness). Figure 5 shows the contraction thickness and the percent change of LbL films of 5, 7, and 9 layer pairs prepared from 0.5 M NaCl solutions upon exposure from 0.5 M NaCl to 0.01 M CaCl2 and 0.17 M CaCl2. These two concentrations were chosen in order to probe the overall thickness effect in both regime I and regime II. Overall, the absolute contraction thickness increased with layer pair number, and no trend in percent change with respect to layer pair number was observed. If contraction/expansion had occurred in a fixed volume, the percent change would decrease as the number of layer pairs increased, which is contrary to our observation. Figure 5c illustrates the two cases of uniform vs finite volume swelling and contraction. These observations indicate that the contraction and expansion of the film upon ion exchange occur uniformly throughout the film’s thickness. We next turn to the thermal behavior of the LbL films in contact with CaCl2, MgCl2, and Na2SO4 solutions analyzed using modulated DSC of thick free-standing PDAC/PSS

Figure 4. Shear modulus as a function of exposure time for PEI-(PSS/PDAC)7 multilayers as a function of time as the overlying solution is switched between 0.01 and 1.9 M (a) CaCl2, (b) MgCl2, and (c) Na2SO4 solution (steps 1 and 3) and 0.5 M NaCl solution (step 2). E


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Figure 5. Change in thickness and percent change in thickness upon contraction due to switching from (a) 0.01 M CaCl2 to 0.5 M NaCl and (b) 0.17 M CaCl2 to 0.5 M NaCl as a function of layer pair number n for PEI-(PSS/PDAC)n LbL films. The error bars represent the standard deviation taken from an average of three samples. (c) Schematic of nonuniform and uniform swelling. Results here indicate uniform swelling.

Figure 6. Effect of divalent ions on the thermal transition of (PDAC/PSS)140 LbL films. modulated DSC measurements of free-standing LbL films hydrated using solutions of (a) CaCl2, (b) MgCl2, and (c) Na2SO4 of varying concentrations. Data were shifted along the y-axis for clarity. Heating at 2 °C/min, amplitude of 1.272 °C for a period of 60 s. Second heating scans are shown.

distinct behaviors (swelling caused by electrostatic repulsion or by doping), which both result in an expansion of the LbL film at low and high salt concentrations, respectively. This general behavior mirrors a previous study of NaCl-induced swelling in PDAC/PSS LbL films, in which the molar fraction of water molecules in the film relative to the sulfonate groups was monitored using Fourier transform infrared (FTIR) spectroscopy in PSS-terminated PDAC/PSS LbL films, where water

should be noted that there is large error within individual data points.

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DISCUSSION In brief summary, the structural changes observed herein are expansion of the LbL film upon exposure to divalent salt solutions and contraction of the film upon exposure to 0.5 M NaCl (i.e., the original assembly condition). We observed two F


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Figure 7. Concentration dependence of the thermal transition of LbL multilayer films in contact with (a) CaCl2, (b) MgCl2, and (c) Na2SO4 solutions. The solid line is drawn to help guide the eye and represents a linear fit of Ttr = 12.44CNa2SO4 + 43.53 (R2 = 0.95). The error bars represent the standard deviation over at least three samples.

Figure 8. Schematic representation of the contraction and expansion of an LbL film during ion exchange.

uptake consistent with regimes I and II was observed.51 One important difference to consider with the study herein is that our work focuses upon polycation PDAC-terminated films. While the results are somewhat similar, the prior study did not examine divalent ions; thus, effects of the outermost layer cannot be compared. We attempted to evaluate PSS-terminated films, but the resultant data were inconsistent and more investigation is warranted. Whereas the general trends in regime I appear to be largely similar regardless of salt type, the trends in regime II depend heavily on salt type, as evidenced by the difference in slopes of the linear fits. A possible reason for this can be found by comparing the hydration shells of the various ions and salts (Table 1). For example, in the literature the hydration number for calcium is about 9,53,54 and for sodium it is about 455 at room temperature. As is illustrated in Figure 8, the observed change in thickness can be attributed to the associated water molecules entering and leaving the film in addition to the ion itself. As hydrated ions enter the film, they may simply increase the free volume of the film and induce swelling; else, they may disrupt intrinsic polyelectrolyte−polyelectrolyte ion pairs, also causing swelling. The former is more likely at intermediate salt

concentrations, whereas the latter possibility increases at higher salt concentrations. Divalent ions in principle could cause bridging, a form of physical cross-linking, between the polyelectrolyte chains, which has been reported in a number of polyelectrolyte multilayer systems.50,56−58 In a different multilayer system as explored here, Huang et al. observed that divalent ions caused an increase in Young’s modulus and hardness because of bridging.58 For PDAC/PSS multilayers assembled from NaNO3 with PSS as the topmost layer, physical cross-linking was observed upon exposure to divalent cations (including Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, and Cu2+) at low concentrations.50 Here in this work, bridging is unlikely for the polyelectrolyte system, as this would have caused a contraction of the films in the presence of divalent ions, contrary to our observations. This may be because our films were terminated in PDAC and assembled from NaCl, different from ref 50. It is curious that the Na2SO4 case exhibited constant swelling regardless of concentration, such that no clear boundary between regimes or I or II could be defined. This could be caused by Donnan inclusion and exclusion, which has been previously examined in nanofiltration membranes made by LbL G


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CONCLUSION The effect of CaCl2, MgCl2, or Na2SO4 on the swelling and thermal behavior of PDAC/PSS LbL assemblies was investigated. A contraction and stiffening of the films was observed using QCM-D when the solution in contact with the film was switched from CaCl2, MgCl2, or Na2SO4 to 0.5 M NaCl. The structural change was reversible and repeatable as the film largely regained its original thickness upon the reverse ion exchange process. For CaCl2 and MgCl2, swelling was most extreme at ultralow or ultrahigh concentrations, which we attributed to swelling induced by electrostatic repulsion (regime I) and ion doping (regime II), respectively. In regime II, changes in thickness were found to be linearly dependent on CaCl2 or MgCl2 concentration and correlated to the hydration shells of the ions. However, no remarkable trend in swelling with regard to Na2SO4 concentration was observed. The detected structural changes were interpreted on the basis of Donnan exclusion/ inclusion principles and the differences in the hydration shells of ions in contact with the polyelectrolyte multilayers. Reduced shear moduli in the presence of these divalent salts is consistent with plasticization effects from water molecules penetrating into the film. A thermal transition was observed when PDAC/PSS assemblies were hydrated using CaCl2, MgCl2, and Na2SO4 solutions of varying concentrations. The position of the thermal transition shifted to higher temperatures with increasing Na2SO4 concentration, while the CaCl2 or MgCl2 had no effect. These findings provide some fundamental insight into the effects of multivalent ions on PDAC/PSS multilayers, which have hitherto been lacking in the field of LbL assembly. In future work, we will systematically study a broader collection of ions bridging the kosmotropic and chaotropic regimes.

assembly for ion separations. Poly(allylamine hydrochloride) (PAH)/PSS multilayers terminated with negatively charged PSS demonstrated preferential rejection of the divalent anions because of electrostatic repulsion and Donnan exclusion.59 Similarly, PDAC/PSS multilayers demonstrated different Na+/ Mg2+ selectivities based upon the terminating layer’s charge.60 In our work, we propose that hydrated SO42− ions experience Donnan inclusion because our PDAC/PSS multilayers are terminated with positively charged PDAC. This would allow SO42− ions to penetrate and fully saturate the LbL film. The favorable electrostatic interactions would also result in a lack of concentration dependence even at low Na2SO4 concentrations. Also, the kosmotropic nature of SO42− probably contributes to the swelling behavior. We next discuss the mechanical properties of the LbL films exposed to various salts (Figure 5). The results demonstrate a reduction in shear modulus when the film is in contact with CaCl2, MgCl2, and Na2SO4 solutions, which have a higher number of associated water molecules as compared to NaCl. In other words, as the water content within the films increases, the observed shear modulus decreases. From previous studies, water molecules have been shown to serve as plasticizers. Stiffer PDAC/PSS complexes and multilayers have been observed after the discharge of water molecules upon exposure to an osmotic stressor.61 Dynamic mechanical analysis of PDAC/PSS LbL films immersed in NaCl solutions of varying concentrations indicated a decreased storage modulus as the ionic strength was increased from 0 to 1 M, with a general range on the order of 1− 25 MPa.62 As for the thermal transition, only the Na2SO4 case exhibited a slight linear dependence on the transition temperature with Na2SO4 concentration was detected. It has been previously claimed that anions have a stronger effect on water structures than cations,63 which possibly explains why Na2SO4 shows stronger trends than CaCl2 and MgCl2. This may be attributed to the kosmotropic nature of the SO42− ion, where kosmotropes are referred to as “structure making” due to their ability to stabilize water structures and interactions. On the other hand, chaotropes are known as “structure breaking” because of their tendency to disturb water structures.64 SO42−, Ca2+, and Mg2+ are more kosmotropic compared to Cl− and Na+.65 As a kosmotrope, SO42− could have stronger interactions with water molecules as compared to the sulfonate group of PSS. This could lead to two outcomes. First, with increasing concentration of Na2SO4 fewer PSS−H2O interactions would form, and more energy would be necessary to induce the transition, leading to an elevated thermal transition temperature. Second, another outcome could also be that the apparent or effective water content as experienced by the multilayer could be reduced because the kosmotropic nature of SO42−. As for CaCl2 and MgCl2, the transition was observed only below 0.33 M. It is possible that the transition disappears because the amount of doping induces such a great disruption of the PSS−H2O interactions that the transition can no longer be observed or else intrinsic polyelectrolyte−polyelectrolyte ion pairs are excessively disrupted (although film deconstruction was not observed). Also in parallel work, we have observed a strong dependence on hydration level for the thermal transition temperature in PAH/PAA LbL assemblies,49 but it should be noted that here the hydration level was carefully controlled at 36% of the dry film mass, so the effects observed here should be attributed to the salts themselves.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01164. Observed changes in thickness and χ2 of a PDAC/PSS LbL film assembled at 0.5 M NaCl as it is exposed to solutions of different ions. QCM-D during the LbL assembly process of a PDAC/PSS film; mass and thickness changes upon exposure 0.5 M NaCl from varying concentrations of salts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.L.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. 1049706.

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REFERENCES

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