Effect of Divalent Counterions on Polyelectrolyte Multilayer Properties

Feb 19, 2016 - Joshua T. O'Neal , Ethan Y. Dai , Yanpu Zhang , Kyle B. Clark , Kathryn G. Wilcox , Ian M. George , Nandha E. Ramasamy , Daisy Enriquez...
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Effect of Divalent Counterions on Polyelectrolyte Multilayer Properties Jingjing Wei,† David A. Hoagland,‡ Guangyu Zhang,† and Zhaohui Su*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China ‡ Polymer Science and Engineering Department, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: When exposed to divalent counterion solutions, polyelectrolyte multilayer (PEM) films of poly(diallyldimethylammonium chloride) and sodium poly(styrenesulfonate) (NaPSS) prepared in the presence of monovalent salt, or equilibrated with such a salt, are physically cross-linked by divalent counterion incorporation, altering PEM properties significantly. The rapid cross-linking was monitored by the quartz crystal microbalance with dissipation (QCM-D) method, which finds PEM deswelling and rigidification after exposures to a low concentration of Cu(NO3)2; at higher concentration, deswelling is countered by increased PEM uptake of the salt, which disrupts polyelectrolyte−polyelectrolyte ion pairs. Divalent ion incorporation into PEMs has the character of ion exchange, and incorporated divalent ions are quickly and completely removed when presented with monovalent salt solution but not with water. While counterion cross-linking extends across the bulk of the PEM, the fraction of exchanged counterions remains low. Entropically driven binding of divalent ions to NaPSS in solution was studied for Cu(NO3)2 and other divalent nitrate salts by isothermal titration microcalorimetry and dynamic light scattering to support the QCM-D conclusions.

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polyelectrolyte−polyelectrolyte attractions.14 In polyelectrolyte solutions, counterions (and other small salt ions) have an additional rolescreening long-range electrostatic interactions, an effect manifested as a reduction in the Debye screening length. By the quartz crystal microbalance with dissipation (QCMD) method, Zhang et al.15 observed impacts of NaCl concentration on LbL deposition of poly(styrenesulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDDA) PEMs; shifts in resonant frequency and dissipation demonstrated, as noted in other reports,16,17 that higher NaCl concentration provides greater PEM thickness. Beyond concentration, identities of added salt ions also exert significant influence on PEM growth and properties.12,18,19 Barrett et al.,12 for example, examined counterion incorporation into PEMs grown from polyelectrolyte solutions containing the counterion series F−, Cl−, Br−, and I− and found that at equal ion concentration incorporation varied widely. Likewise, the thickness of polyelectrolyte brushes was explored during exposure to small ions of different size and valence.20,21 When counterions in a polyelectrolyte brush are exchanged with those in an overlying solution, the film swelling and hydrophilicity/hydrophobicity may change, and the polyelec-

INTRODUCTION Layer-by-layer (LbL) assembly finds application in optics,1 catalysis,2 drug delivery,3 sensors,4 and antibacterial membranes5 mainly due to the ease with which conformal thin films of controlled structure can be fabricated on solid supports. Constructed via alternating depositions of oppositely charged polymers, polyelectrolyte multilayers (PEMs), while robust in typical aqueous environments, are potentially responsive to environmental changes.1 Key PEM properties include thickness, viscoelastic character, and surface charge, reflecting choice of polyelectrolytes, pH, ionic strength, and, if pertinent, chemical modification. In applications such as chemical gating and drug delivery, response to an electrical or mechanical stimulus might be sought,3,4 but just as often, response is sought to a chemical stimulus such as a change in pH and salt concentration.6−8 Despite much technological interest, PEM construction and properties are poorly understood at a fundamental level. A key issue is PEM charge neutralization,9 which manifests the presence of a finite number of small salt ions acting as counterions to the deposited polyelectrolytes. The identity and concentration of incorporated counterions strongly influence PEM properties,10−12 and many investigations have explored the role of counterions on PEM assembly, properties, and applications.7,8,13 The electrostatic interactions between polyelectrolyte and small counterions profoundly affect all polyelectrolyte properties, effectively reducing polyelectrolyte charge according to the character of the environment, and in PEMs, counterions weaken cohesive © XXXX American Chemical Society

Received: September 29, 2015 Revised: February 7, 2016

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Figure 1. QCM-D frequency (Δf 3) and dissipation (ΔD3) shifts for (PDDA/PSS)3 films during assembly, subsequent exposure to 0.01 M Cu(NO3)2, and final re-equilibration with either 0.5 M NaNO3 (a) or water (b). Arrows indicate exposures to PDDA and PSS LbL solutions (orange and blue arrows, respectively), water and 0.5 M NaNO3 LbL rinses (black and gray arrows, respectively), and 0.01 M Cu(NO3)2 (red arrows). both at 0.5 M NaNO3; the flow rate was 100 μL/min. Polyelectrolyte depositions in the instrument were monitored through decreases in resonance frequency. Once the frequency stabilized after exposure to a polyelectrolyte solution, this solution was replaced by a buffer solution (NaNO3 solution or water) until a steady frequency was again recorded.29 This procedure was repeated at 25 °C until the desired number of bilayers was deposited. The LbL-constructed PEM (always capped with a PSS layer) is denoted (PDDA/PSS)N, where N corresponds to the number of bilayers. All polyelectrolyte solutions contained 0.5 M NaNO3 unless indicated otherwise. At this salt concentration the PEM thickness grows exponentially with N, and most of our data were collected for thin films (N = 3) for the convenience of the QCM-D experiments. QCM-D. QCM-D measurements (E1, Q-Sense, Gothenburg, Sweden) were performed with AT-cut quartz crystal resonators (Qsense) characterized by a 5 MHz fundamental frequency f 0. Applying a RF voltage across resonator electrodes near a resonant frequency excites shear oscillations, and with a thin elastic film attached to one resonator face, the frequency decreases, denoted Δf n for overtone n, are in proportion to the film’s mass Δm.30 Likewise, the dissipation loss ΔDn, manifesting the structure and viscoelastic properties of the deposited film, can be deduced via measurements of the resonant oscillation decay time when the RF voltage is switched off. A small loss [defined by ΔDn/(Δf n/n) < 10−7 Hz−1] indicates that a film is rigid and homogeneous.31,32 For a thin, rigid, and homogeneous film in vacuum, the linear relationship between Δf n and Δm is known as the Sauerbrey equation33

trolyte chains can even be cross-linked when a divalent ion is introduced, resulting in significant thickness reduction.21 In this work, changes to PEM mass and viscoelasticity are tracked by QCM-D during PEM exposures to overlying solutions containing various monovalent and divalent salts. The cohesive electrostatic or hydrogen bond interactions holding a PEM together are different than the chemical or physical cross-links of traditional rubbers or gels, and as a consequence of interaction weakness, PEMs are potentially reformable after damage22 or, more precisely in the current case, exposure to new, more, or different ion species.23,24 The nature of the counterion−polyelectrolyte interactions responsible for PEM responsiveness to divalent counterions is further explored here by isothermal microcalorimetry (ITC) and dynamic light scattering (DLS) experiments, which uncover thermodynamic and structural aspects, respectively. Although the preparation of PEMs in the presence of divalent counterions was reported previously,25−27 this investigation is the first to demonstrate cross-linking of an existing PEMs by divalent counterions.

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EXPERIMENTAL SECTION

Materials. Poly(diallyldimethylammonium chloride) (PDDA) (20 wt % in water; MW = 200 000−350 000 g/mol) and poly(styrenesulfonate) (PSS) (MW = 70 000 g/mol) were purchased from SigmaAldrich, while sodium nitrate (NaNO3) and cupric nitrate trihydrate (Cu(NO3)2·3H2O), both of analytical grade, were purchased from Beijing Chemical Reagents Company. All chemicals were used as received, and water for all experiments was produced by a PGeneral GWA-UN4 purification system (18.2 MΩ·cm conductivity). Substrate Preparation. A silicon wafer (1.0 × 1.5 cm) was immersed in boiling piranha solution (7:3 98% H2SO4:30% H2O2 mixture) for 30 min and rinsed afterward with copious water. (Caution: piranha solution reacts violently with organic materials and should be handled caref ully.) QCM resonators with 10 mm diameter gold electrode (Q-Sense AB, Västra Frölunda, Sweden) were sonicated in both ethanol and water before immersion for 15 min in a heated (75 °C) ammonium peroxide mixture [(5:1:1 mixture of H2O, NH3·H2O (25%), and H2O2 (30%)]. (Caution: the ammonium peroxide mixture should be used in a f ume hood with proper protection.) After rinsing with copious water, the resonators were dried in a N2 stream. PEM Fabrication. PEMs were assembled on cleaned silicon wafers and quartz slides by sequential 30 min dippings into PDDA (1.0 mg/ mL) and PSS (1.0 mg/mL) aqueous solutions, with each dipping followed by a water rinse to remove excess polymer and salt, as described previously.28 PEMs were prepared on quartz resonators prepositioned in the QCM-D flow cell by alternating exposures, via peristaltic pumping, of 1.0 mg/mL polycation and polyanion solutions,

Δm = −

ρq lq Δf n f0 n

= −c

Δfn n

where ρq and lq are the respective specific density and thickness of the quartz resonator. In the present case, the proportionality constant c is 17.7 ng cm−2 Hz−1. When such a film is overlaid by a viscous medium, the Sauerbrey equation must be corrected to account for fluid density and viscosity. Throughout this study, to achieve highest resolution, data are reported for n = 3. The QCM-D experiment was repeated five times for exposure to Cu2+ and three times for each of the other divalent ions. The frequency shifts were reproduced to within 2 Hz variation. Isothermal Titration Calorimetry (ITC). The heat evolved by solution complexation at 25 °C of PSS and divalent cation was measured using a Nano ITC titration microcalorimeter (TA Instruments, Waters LLC) with a cell volume of 1.4 mL. Before ITC measurements, solutions were degassed in an accessory unit (TA Instruments) by 10−15 min stirring under 500 mmHg vacuum. The thermostated sample and reference cells were filled with PSS solution (4.85 mM; 0.5 M NaNO3) and deionized water, respectively, and then, 10 mM of a M2+(NO3)2 salt solution was stepwise injected from a 250 μL syringe into the stirred sample. An initial 1 μL injection (neglected in analysis) was followed at 41 subsequent intervals of 900 s by 6 μL B

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Figure 2. QCM-D frequency (Δf 3) and dissipation (ΔD3) changes measured during ion exchange of (PDDA/PSS)3 films prepared with water rinses and followed by an equilibration with water. Arrows indicate equilibrations with water (black arrow) and (a) 0.02 M NaNO3 (gray arrow) or (b) 0.01 M Cu(NO3)2 (red arrow).

anion (NO3−), the divalent salt concentration was chosen to be less than the monovalent salt concentration used to construct the PEM. The relative mass reductions inferred from Figure 1 are modest, about 10%, as constrained by the relatively low N value. Much as for the augmented cross-linking of a hydrogel, the reductions are presumably a consequence of water expulsion. After replacing Na+ by Cu2+ to steady state, the PEMs of Figure 1 were re-equilibrated with either 0.5 M NaNO3 or water, chosen to match the preceding rinses, and the outcome for the PEM, at least over the monitored time frame, depended on the choice of equilibrating fluid. Prepared with 0.5 M NaNO3 rinses and re-equilibrated with same solution, recovery from the divalent-to-monovalent exchange was fast and complete, with the PEM reswelled to original values of Δf 3 and ΔD3 in 5−10 min. Prepared with water rinses and reequilibrated with water, however, no recovery was noted over the period monitored, about 50 min; the divalent ions stayed in place, leaving the PEM cross-linked and deswelled. These trends mirror the trends of ion exchange resins, and most importantly, monovalent-to-divalent and divalent-to-monovalent counterion exchanges are found to be reversible. Further Evidence for Cross-Linking by a Divalent Counterion. The preceding discussion hinted, but did not establish, that ion exchange for a divalent counterion can effectively cross-link PEMs. Evidence is incomplete since the accompanying monovalent counterion concentration (i.e., that of NO3−) was not fixed between the two experiments. Therefore, a modified pair of ion exchange experiments was pursued. Here, two (PDDA/PSS)3 PEMs made with water rinses were equilibrated against water until Δf 3 stabilized, and at this stage, Δf 3 and ΔD3 were assigned zero values. The PEMs were then presented with monovalent and divalent counterion solutions of equal anion concentration (i.e., approximately equal anion activity), 0.02 M NaNO3 and 0.01 M Cu(NO3)2, and after a re-equilibration, Δf 3 and ΔD3 for the two cases were compared, as illustrated in Figure 2. In Figure 2a, which reports response to 0.02 M NaNO3, Δf 3 drops and ΔD3 rises, with both parameters regaining new steady states in less than 5 min. These trends establish that the film swelled and softened, behaviors inconsistent with cross-linking alone. Such swelling can possibly be linked to the chaotropic placement of NO3− in the Hofmeister series, which suggests that the counterion’s incorporation into a PEM is likely to be

injections; 900 s was sufficient for the heat signal to return to baseline after each injection.29 Offering the enthalpy difference ΔH associated with solution mixing,34 ITC is often used to track the binding of small molecules to larger macromolecules.35 Adopting a binding model, analysis provides the binding affinity K and the number of binding sites m of the macromolecule. From these parameters, the Gibbs free energy change ΔG and the entropy change ΔS are trivially calculated. All ITC experiments were performed in triplicate. Dynamic Light Scattering (DLS). A DLS instrument (Nanosize 90, Malvern, England) provided the z-average hydrodynamic radii RH of dilute polyelectrolytes dissolved in fixed ionic strength solutions of various salts. Multiple decay modes were not observed at conditions (salt and polymer concentrations) of the experiments.

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RESULTS AND DISCUSSION PEM Cross-Linking by a Divalent Counterion. Figure 1 shows that Δf 3 drops with each PDDA or PSS LbL deposition during N = 3 PEM assembly, consistent with the expected incremental addition of mass to the film. The subsequent rinses with 0.5 M NaNO3 or water usually cause Δf 3 to increase slightly, reflecting a slight loss of the deposited polyelectrolyte, as noted during LbL assembly of other PEMs.36 (Impacts of rinsing on Δf 3 during LbL assembly are generally small and somewhat obscured by transients associated with the displacement of one liquid by another in the flow cell.) For PDDA or PSS injections at N > 1 and rinses with 0.5 M NaNO3, ΔDn rises with each polyelectrolyte injection, presumably due to the attachment of “loose” polymer at the film’s top surface, and after the subsequent rinse, ΔDn falls due to desorption of a portion of this polymer. Assuming that the Sauerbrey equation is valid and that the average PEM density is 1.0 g/cm3, the final steady state value of Δf 3 corresponds to PEMs of thickness 25.6 and 34.7 nm when the rinses are respectively with 0.5 M NaNO3 and water. Irrespective of the rinse fluid used in LbL assembly, upon exposure of the final PEM to 0.01 M Cu(NO3)2, Δf 3 rose and ΔD3 fell, the former suggesting a decrease in PEM mass (and thickness) and the latter suggesting an increase in PEM rigidity. The two behaviors suggest that ion exchange, replacement of monovalent for divalent counterion, effectively cross-links a PEM. In this context, cross-linking is viewed as the creation of long-lived segment−segment physical attractions analogous to those of a physical gel. To focus on impact of divalent cation, for PEMs with a near constant background of monovalent C

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physical depiction of PEM cross-linking by multivalent counterions; the number of polyelectrolyte−Na+ ion pairs, replaced by binary polyelectrolyte−Cu2+ ion pairs, or crosslinks, is small, and the latter are certainly transient, making the cross-linking physical. Somewhat analogous multivalent ion cross-linking of aqueous polyelectrolyte solutions is a wellknown route to stable hydrogels.37,38 Cross-Linking vs Salt Uptake. To assess the impact of multivalent ion concentration on PEM cross-linking, two (PDDA/PSS)3 PEMs as just described were presented with Cu(NO3)2 solutions at higher concentration, 0.03 and 0.1 M, and afterward, again re-equilibrated with water. Figure 3 shows the resulting Δf 3 and ΔD3 responses. While the Δf 3 responses to the two exposures were similar, both ran counter to the Δf 3 response at lower Cu(NO3)2 concentration; i.e., swelling is manifested rather than deswelling. And, just as at 0.01 M, ΔD3 drops with the 0.03 M exposure, but with the 0.1 M exposure, ΔD3 instead rises, the two responses reflecting stiffening and softening, respectively. The final re-equilibrations with water did not lead to PEM recovery, indicating that the PEM-bound Cu2+ counterions were again resistant to release. These complicated trends can be explained by the following speculation: Cu2+ ions are able to access, and thereby ion exchange to form cross-links, only a limited fraction of PEM sites. Given the experimental protocol, this hypothesis predicts that the steady state density of cross-links is independent of the incorporated Cu(NO3)2 concentration when this concentration is high enough. Nevertheless, as the Cu(NO3)2 concentration in the overlying solution increases, more salt (and accompanying water) incorporates into the PEM, causing swelling; this salt is in the form of Cu2+ and NO3−, both hydrated. These ions presumably replace polyelectrolyte−polyelectrolyte ion pairs with counterion−polyelectrolyte ion pairs, lowering PEM cohesive interactions. Voids or other heterogeneous PEM features pose an alternative mechanism for the swelling, but AFM showed that such features are absent (Figure S2, Supporting Information). The swelling of PEMs after an increase in salt concentration has been reported before.39 Somewhere between 0.01 and 0.03 M Cu(NO3)2 lies a crossover between two effectsCu2+ cross-linking (favoring deswelling) and salt uptake (favoring swelling). Impacts of a Divalent Counterion on PEMs of Different Thickness. For low and modest salt levels, the mass increment of deposited polyelectrolyte during LbL assembly grows with the concentration of salt in the dipping

accompanied by water incorporation. After the equilibration with 0.02 M NaNO3, the swollen PEM was re-equilibrated with water to assess recovery, which, as shown by Figure 2a, is essentially complete for both Δf 3 and ΔD3. The recovery was more rapid than the response to the original challenge, probably because the film was swollen at this stage, enhancing ion and water mobilities. Figure 2b shows that PEM responses to 0.01 M Cu(NO3)2 are strikingly different; indeed, shifts in Δf 3 and ΔD3 are reversed in direction, demonstrating deswelling and rigidification, trends consistent with counterion-induced crosslinking. The time scale for the response to Cu(NO3)2 was about the same as with NaNO3, but once deswollen, the film showed no recovery when re-exposed to water, as neither Δf 3 nor ΔD3 changed in a meaningful way. Analysis of the PEM by X-ray photoelectron spectroscopy (XPS) also revealed that the Na+ ions in the PEM are replaced by Cu2+ upon exposure to the Cu(NO3)2 solution, and the Cu2+ ions are retained in the film after rinses with water (Figure S1, Supporting Information), consistent with the QCM-D result. Exchange of the Na+ counterions on PSS in a PEM for another cation is well established in the literature.5,28 Scheme 1 gives a simplified Scheme 1. PEM Cross-Linking Induced by Divalent Copper Counterions

Figure 3. Δf 3 and ΔD3 for water-equilibrated (PDDA/PSS)3 PEMs exposed to Cu(NO3)2 at (a) 0.03 M and (b) 0.1 M. After reaching a new steady state, each PEM was re-equilibrated with water. D

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Table 1. Steady State Δf 3, ΔD3, and Thickness Shifts after 0.01 M Counterion Exposure

solutions, and beyond a crossover salt concentration, LbL growth become exponential, with the mass increment proportional to the previously deposited PEM mass.40,41 The larger mass increment can be rationalized in terms of the “loosening” of PEM structure as a greater portion of polyelectrolyte charges are compensated by small counterions, an argument buttressed by the higher levels of salt found in PEMs assembled at higher salt concentration.6 To study the impacts of divalent counterion on thicker PEMs, the previous and relatively thin PEMs were replaced by (PDDA/PSS)5 PEMs assembled from polyelectrolyte solutions of the previous NaNO3 concentration, 0.5 M; LbL growth at this condition is exponential. Thicker PEMs can also be more precisely characterized by QCM-D. With the thicker film, whether the limited divalent counterion crosslinking is localized at/near the PEM surface or, alternatively, distributed across the PEM thickness can be addressed. Figure 4 presents Δf 3 and ΔD3 shifts observed when a waterequilibrated (PDDA/PSS)5 PEM is first presented with 0.01 M

PEM

ion

Δf 3 (Hz)

(PDDA/PSS)3 (PDDA/PSS)3 (PDDA/PSS)5

Na+ Cu2+ Cu2+

−54 37 167

ΔD3 (×106)

original thickness (nm)

thickness change (nm)

11 −16 −104

25 25 76

3 −2 −10

ions into semidilute solutions of NaPSS. While the ITC environment differs from that in a PEM, the interaction between the two species is much the same. Figures 5a and 5b plot raw and integrated ITC curves for Cu(NO3)2, and the ion exchange reaction is immediately seen as endothermic for all ratios of counterion to polyelectrolyte, revealing that a positive entropy of mixing must underscore the attraction between polyelectrolyte and divalent counterion. During the exchange, two polyelectrolyte-bound Na+ counterions are replaced by one polyelectrolyte-bound Cu2+ counterion, as illustrated in Scheme 2. This scheme shows two polyelectrolytes involved in the exchange, thereby forming an interpolyelectrolyte cross-link, but of course, some exchanges will involve just one polyelectrolyte, forming an intrapolyelectrolyte cross-link. PEM swelling/deswelling will be affected about equally by both cases. The replacement can raise entropy in two distinct ways: first, one free counterion is replaced by two free counterions (“counterion release” mechanism) and, second, as the charged species rearrange during exchange, the total number of their associated waters of hydration could diminish.42,43 Figure 5b shows that the binding of divalent counterions to polyelectrolytes saturates as the bulk molar ratio of divalent counterions to polyelectrolyte repeat units reaches order unity and that more additions of salt provided no further enthalpy change. Applying the independent binding site model,44 ITC data from three runs of Cu(NO3)2 titrated into NaPSS were averaged to generate intrinsic binding constant K = 1.1 × 104, binding enthalpy ΔH = 1.6 kJ mol−1, and stoichiometric binding number m = 0.51 (defined for each NaPSS repeat unit offering one binding site). From K and ΔH values, ΔG = −23.1 kJ mol−1 and ΔS = 82.8 J mol−1 K−1 are directly calculated. The fitted m, as expected, supports the binding of Cu2+ to −SO3− at a stoichiometric ratio of 1:2. The magnitudes of the interactions per repeat unit are not high. PEM Impacts of Other Divalent Counterions. To determine whether the PEM impacts described for exposure to Cu2+ extend to exposures to other divalent metal cations, analogous ITC and QCM-D PEM experiments were performed for various M(NO3)2 salts, where M is a divalent cation from the list Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, and Cu2+. Figure 6 plots the resulting ITC curves, and Table 2 lists the interaction parameters calculated by fitting these curves. The same table gives the Δf 3 and ΔD3 shifts of water-equilibrated PEM after presentations with same list of divalent counterions, all at 0.01 M. Variations in PEM response among the counterions are remarkably small, although minor differences are discernible. The PEM impacts described for Cu2+ therefore extend little changed to a broad spectrum of divalent cations. Impact of a Divalent Ion on Dissolved PSS Coil Size. Cu2+-induced cross-linking of PSS in a PEM is surely mimicked by Cu2+-induced cross-linking of PSS in a dilute solution, where the cross-linking can be expected to be dominantly intramolecular. Intramolecular cross-linking reduces dissolved coil

Figure 4. Δf 3 and ΔD3 shifts for a water-equilibrated (PDDA/PSS)5 PEM presented with 0.01 M Cu(NO3)2. After reaching a new steady state, the PEM was re-equilibrated with water.

Cu(NO3)2 and later re-equilibrated with water. The steady state Δf 3 shift after the exposure, about +150 Hz, is several times larger than the +30−50 Hz Δf 3 shifts observed after similar exposures to thinner (PDDA/PSS)3 PEMs (Figures 1a and 2b). While the steady state Δf 3 shift is clearly not proportional to N, only to be expected if PEM properties were N-independent, proximity to the substrate makes properties N-dependent when N is small. With more bound segments restricting chain configurational rearrangements, the bilayers closest to the surface seem less affected by divalent counterion cross-linking. Nevertheless, the relative magnitudes of the steady Δf 3 shifts are consistent with cross-linking across the bulk of the PEM, not just at/near the top surface. The Δf 3 shift for the thicker PEM is likewise larger, and although the physical interpretation is more difficult, this feature only reinforces the argument for cross-linking across the PEM bulk. Lastly, the 5−10 min time scale for reaching steady state after the exposure is noticeably longer than for the thinner PEM, a change likely reflecting the greater diffusion length of a thicker PEM. Steady state values of Δf 3 and ΔD3 after the Cu(NO3)2 exposure are given in Table 1 alongside values for PEM thickness and thickness change calculated from the Sauerbrey equation. The latter is much larger for the thicker PEM. Thermodynamic Interaction Parameters by ITC. To quantify the binding/exchange of multivalent counterions to PSS, thermodynamic parameters characterizing their interaction were extracted from ITC experiments that titrated multivalent E

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Figure 5. Representative (a) raw and (b) integrated ITC curves for titration of 0.01 M Cu(NO3)2 into 1 mg/mL PSS. Molar ratio is the concentration of divalent cations divided by the concentration of NaPSS repeat units.

Scheme 2. Illustration of Exchange of Na+ for Cu2+ Counterion

Table 2. Summary of ITC and QCM-D Data for Different Divalent Counterions ITC dataa counterion 2+

Mg Ca2+ Sr2+ Ba2+ Zn2+ Cu2+

−1

QCM-D datab −4

m

ΔH (kJ mol )

K (×10 )

0.50 0.51 0.50 0.50 0.50 0.50

1.7 1.6 1.9 1.5 1.6 1.6

1.2 1.1 1.4 1.0 1.1 1.1

Δf 3 (Hz) ΔD3 (×106) 36 38 43 37 40 37

−18 −19 −22 −18 −20 −16

a

Titration of 0.01 M M(NO3)2, where M is a divalent counterion listed, into 1 mg/mL PSS. bResponse of the (PDDA/PSS)3 PEM to 0.01 M M(NO3)2.

size should be attributed to the greater fraction of intramolecular cross-links in a dilute solution. Because of the faster dynamics in solution, “transient association” might be a better term than “cross-link”.

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CONCLUSIONS

PEMs prepared in the presence of monovalent salts are rapidly cross-linked when presented with an overlying solution containing divalent ions, with the mechanism of cross-linking simply the exchange of monovalent for divalent counterions, which can neutralize the charge of more than one polyelectrolyte chain. This physical cross-linking is (i) not specific to particular divalent ion, (ii) reversible to subsequent exposure to monovalent salt solution, and (iii) entropically driven. A key consequence of the cross-linking is PEM thinning, but the relative thickness reduction is typically less than observed when polyelectrolyte chains in dilute solution are exposed to divalent counterions. While not documented here, divalent counterion cross-linking has profound effects on various PEM properties, including stiffness, wettability, chemical and solvent degradation, and cell adhesion, and thus the new multivalent counterion cross-linking treatment can be regarded as another convenient tool for modifying PEM properties to suit technological needs. Although the preparation of PEMs cross-linked by covalent bonds was reported previously,45 this investigation is the first to demonstrate cross-linking of existing PEMs by divalent counterions.

Figure 6. Integrated ITC curves for titration of 0.01 M M(NO3)2, where M is a divalent counterion defined in the legend, into 1 mg/mL PSS. Molar ratio is the concentration of divalent cations divided by the concentration of NaPSS repeat units.

size. Whether this reduction carries over, at least in qualitative fashion, to the reduction of PEM thickness described earlier can be addressed by DLS study of PSS solutions. The DLS-derived z-average hydrodynamic radius for this PSS sample in 0.5 M Na(NO3) is 37 nm and the same radius in 0.25 M Cu(NO3)2 is 17 nm, and so, the monovalent-to-divalent reduction of dissolved coil size is much greater than the analogous reduction of PEM thickness. While the solution salt concentrations are greater than those in the PEM experiments, they again matched the NO3− concentrations. Beyond the obvious physical differences between PEM and solution environments, there are equal differences for electrostatics, perhaps most notably, in dielectric constant and density of free ions. However, we suggest that the larger impact of divalent ions on dissolved coil F

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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.5b02151. XPS spectra and AFM topography images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone +86-431-85262854; Fax +86-431-85262126; e-mail [email protected] (Z.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (21174145). D.A.H. thanks the NSFfunded University of Massachusetts Materials Research Science and Engineering Center for support.

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REFERENCES

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DOI: 10.1021/acs.macromol.5b02151 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02151 Macromolecules XXXX, XXX, XXX−XXX