Thermal Transformations in Extruded Saloplastic Polyelectrolyte

Dec 11, 2012 - Extruded, salt-plasticized complexes of hydrated poly(styrenesulfonate), PSS, and poly(diallyldimethylammonium), PDADMA, were analyzed ...
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Thermal Transformations in Extruded Saloplastic Polyelectrolyte Complexes Rabih F. Shamoun, Haifa H. Hariri, Ramy A. Ghostine, and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *

ABSTRACT: Extruded, salt-plasticized complexes of hydrated poly(styrenesulfonate), PSS, and poly(diallyldimethylammonium), PDADMA, were analyzed by differential scanning calorimetry and dynamic mechanical thermal analysis. Whereas the enthalpic signatures were weak, the latter technique revealed a strong transition in modulus, identified as a glass transition. The temperature of this transition, Tg, varied with deformation rate as expected from time/temperature superposition. Tg also decreased with increasing salt doping, which breaks ion pairing in the complexes, confirming the plasticizing effect of doping. Time, temperature, and salt concentration data were superposed to demonstrate the trends/equivalence of these three variables, and an empirical equation was used to connect them. Measurement time regimes were discussed with reference to the average lifetime of an ion pair.

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INTRODUCTION Polyelectrolyte complexes, PECs, are molecular blends of oppositely charged polyelectrolytes held together by a high density of ionic cross-links.1−3 PECs are found in a variety of morphologies, from ultrathin films,4,5 to coacervates6,7 to “quasisoluble” complexes.8,9 When positive and negative repeat units are matched (i.e., a 1:1 stoichiometry), PECs are in their most intractable, physically rugged form. Processing of such PECs was only feasible by dissolving/dissociating them in aggressive ternary solvents1,2 until the layer-by-layer approach was introduced to prepare ultrathin films.4,10 Methods for assembling polyelectrolyte multilayers, PEMUs, expanded to include alternating dipping, spraying,11,12 and spin-on.13,14 A recent report on the use of a laboratory extruder to process PECs opens up possibilities for large-scale production of macroscopic articles of complex.15 The addition of salt was key in facilitating extrusion and reshaping PECs in general. Salt dopes PECs according to the following equation16 Pol+Pol−S + Na +aq + Cl−aq ⇌ Pol−Na +S + Pol+Cl−S +

We were interested in exploring the thermal transitions in PECs, in particular the salt dependence. Glass transitions have been identified in PECs, mostly in their PEMU morphology, by calorimetry18−21 and/or changes in their mechanical properties.21,22 However, both the nature of the material, in particular the stoichiometry, and the level of doping and hydration have not always been well-defined in prior work. In addition, because the thermal signatures are weak, sample size, which is limited in ultrathin PEMUs, becomes an issue. In the present study, we employ large samples of stoichiometric, dense PEC in classical thermal analysis measurements. Dynamic mechanical analysis provides much clearer information than does calorimetry of thermal transitions in these PECs.

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Materials. Poly(4-styrenesulfonic acid, sodium salt) (AkzoNobel, VERSA TL130, molar mass ca. 200 000 g mol−1) and poly(diallyldimethylammonium chloride) (Ondeo-Nalco (SD 46104, molar mass ca. 400 000 g mol−1)) were used as received. All salt solutions were prepared using sodium chloride from Aldrich and deionized water (18 MΩ Barnstead, E-pure, Milli-Q). Preparation of Samples. Equimolar solutions of PSS and PDADMA were prepared at a concentration of 0.125 M with respect to their monomer units and neutralized with a few drops of 0.1 M NaOH or HCl. The concentration of salt in these solutions was increased to 0.25 M by adding NaCl. The polyelectrolyte complex (PEC) was precipitated by simultaneously mixing stoichiometric

(1)

−

where Pol Pol are paired polyelectrolyte repeat units and Pol−Na+ and Pol+Cl− are counterion-compensated repeat units (all within the PEC). Doping breaks ionic cross-links and, along with water present during extrusion, plasticizes the complex.17 The cooperative effect of salt and heat in these “saloplastic” materials was notedextrusion could be performed at lower temperatures on the addition of more salt.15 While temperature is a widely appreciated variable in polymer processing (thermoplastics) the dimension of salt doping is unique to polyelectrolytes. © XXXX American Chemical Society

EXPERIMENTAL SECTION

Received: October 3, 2012 Revised: November 29, 2012

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into 50.00 mL of water in a thermostated cell at 25.0 ± 0.1 °C equipped with a stir bar and a four-probe conductivity electrode (Orion 3 Star, Thermo Scientific). The conductivity was measured every 30 s for 90 min, which was enough time for the conductivity readings to plateau. Standards of NaCl were used to convert conductivity to concentration. After release of NaCl, samples were dried at 110 °C to constant weight to obtain the total mass of dry, saltfree complex. The doping level is the mole ratio of salt released to PSS/PDADMA PEC. In one experiment, a 0.50 M solution of NaCl used to dope the sample was spiked with 22Na+ (supplied by PerkinElmer as 22NaCl), a γ-emitter, to yield a specific activity of 7.96 × 10−3 Ci mol−1. The sample was counted by scintillation counting on top of a piece of plastic scintillator before and after the NaCl was released from the PEC. The count rate was 123 ± 0.4 counts per second (cps) before release and 1.05 ± 0.02 cps after release by soaking in water for 90 min, showing that less than 0.8% of the NaCl remained in the ExPEC (i.e., release was 99.2% efficient).

amounts of both solutions. After stirring for 30 min, the precipitate (PEC) was decanted, rinsed in 1.0 M NaCl, and chopped into ca. 1 cm pieces. PEC pieces were soaked in 1.0 M NaCl for 24 h before extrusion. A laboratory extruder (Model LE-075 Custom Scientific Instruments) was used with the following conditions for the first extrusion: rotor temperature 90 °C; header temperature 115 °C; gap space 6.6 mm; rotor speed 110 rpm; and with a rectangular outlet 0.4 mm × 19 mm. The extruded ribbons were produced at ∼7 g min−1 and rolled on a takeup reel (Model CSI-194T) with a 3 cm diameter drum at 10 rpm. The extrudate was chopped into 5 mm lengths and soaked in 1 M NaCl for 24 h before re-extrusion. The second extrusion parameters were the same except the rotor temperature was 85 °C and the header temperature 105 °C. The second extrusion produced more uniform and compact ribbons which were stacked into 4 or 5 sheets high to form samples about 2 mm thick. Sheets were fused by annealing the stack in 1.5 M NaCl at rt for 24 h, and the stack was pressed between two microscope slides to obtain a smooth surface. Samples, each of volume ca. 0.6 cm3, were then soaked, six at a time, in 1 L of water at rt for 24 h to remove NaCl, and then reimmersed in 250 mL of NaCl solutions of various concentrations (0.1, 0.25, 0.5, 0.75, and 1.0 M) at rt for 24 h. This was sufficient time to reach equilibrium as properties did not change with further immersion in salt. The amount of salt in the beakers was about 100× the amount consumed by the PEC samples for doping. Samples were cut into discs with a 19 mm cork punch for use in dynamic mechanical tests. For differential scanning calorimetry, samples of extruded polyelectrolyte complexes, ExPECs, were annealed in 1.5 M NaCl at rt for 24 h to remove residual stress15 and then soaked in water or 1.0 M NaCl at rt for another 24 h. Some samples were dried in an oven at 120 °C for 24 h. Small cubes weighing about 10 mg were cut, excess solution shaken off, and then sealed in hermetic sample pans to prevent water loss. The quality of the hermetic sealing was monitored by ensuring weight loss was less than 1%. Methods and Equipment. Differential scanning calorimetry was performed on a Q2000 DSC from TA Instruments with a cooling rate of 20 °C min−1 from 90 to −40 °C. Mechanical analysis was performed with a temperature-controlled Bohlin Gemini 150 rheometer in a parallel plate configuration (type PP20 stainless steel plate with a diameter of 20 mm). In order to configure the instrument in a dynamic mechanical thermal analysis mode, a cylindrical Plexiglas jacket with a cover was made to fit the rheometer base and contain the salt solution. While running the experiment, each sample was soaked in ∼10 mL of NaCl solution maintained at the same level as the plate rim. The surface of the plate was roughened by sandblasting to minimize sample slip (roughness ca. 15 μm). Samples were compressed by 15%, and the temperature was gradually swept at a controlled oscillating shear stress of 200, 150, 50, 40 or 25 Pa and upper limits of 79, 76, 73, 70, or 64 °C for samples soaked in 0.1, 0.25, 0.5, 0.75, and 1.0 M NaCl, respectively. These parameters ensured samples remained in the linear viscoelastic regime in the stated salt solution. The actual sample temperature was measured using a NISTtraceable Fluke 1524 thermometer. Other parameters were 300 s thermal equilibrium, 1 s delay time, and six measurements for each data point. Dynamic oscillatory shear experiments were carried out while varying the temperature at controlled shear force over the frequency range 0.001−100 Hz; this allowed the measurement of both the storage modulus (G′) and the loss modulus (G′′) as well as the loss tangent (tan δ) of the phase difference between the shear stress and shear strain.

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RESULTS AND DISCUSSION

The materials used here are compact, stoichiometric blends of polyelectrolytes. The ExPEC is essentially nonporous, except for a few pores of dimension less than 10 μm making up less than 5% of the volume.15 Extrusion is particularly helpful in this respect, as the shear forces in the extruder minimize porosity. As an additional benefit of polyelectrolyte complexation, impurities, such as unreacted monomers and other small molecules, are left behind in the precipitation. Thus, crude, or technical grade starting polyelectrolyte solutions yielded precipitates of equivalent purity to those starting from “reagent” grade monomers. An NMR comparison of PEC from PSS and PDADMA obtained in bulk (55 gallons) quantity directly from the manufacturer, versus a higher purity grade from SigmaAldrich, is shown in the Supporting Information. Although multilayers have been employed as films and capsules to evaluate thermal transitions and water content, there is increasing evidence that PEMUs are heterogeneous. For example, if thin multilayers are prepared, a substantial amount of the polymer resides in a layer near the surface known to be in nonstoichiometric excess.4 In contrast, for thick multilayers, there may be a population of excess polymer, such as PDADMA when PSS/PDADMA PEMUs are deposited from 1 M NaCl.16,23 The origin of thermal transitions or changes may thus be uncertain. In the present work, it was possible to employ NMR measurements on dissolved complexes to prove they were stoichiometric. In addition, when they were rinsed in water, radiotracer analysis using 22Na+ revealed >99% release of NaCl, showing the charged polyelectrolyte segments are able to come together and pair, expelling NaCl, as was concluded previously with thermal gravimetric analysis.15 To make thicker samples of PEC, thin tapes of ExPEC were stacked and fused by exposing them to high salt concentration. This fusing technique worked well: no voids were seen, and the strips could not be pulled apart. Stacking was needed to provide sufficiently thick samples for traditional rheology with a plate/ plate geometry. The water content in these ExPECs as a function of [NaCl], reported in our previous work, ranged from 37 to 41 wt % H2O over the range 0.10−1.0 M.15 The equilibrium amounts of NaCl within the ExPEC are shown in Figure 1, where y is the fraction of PEC in the counterion-compensated (extrinsic) form (i.e., the right-hand side of eq 1). y is also the mole ratio of NaCl to polyelectrolyte. Differential Scanning Calorimetry. Qualitative observations based on the softening of PECs1 or PEMUs24−26 by salt

tan(δ) = G″ /G′ The dynamic modulus (G*) is calculated from G′ and G″ as |G*| = |G′2 + G″2 |1/2 The equilibrium salt doping level as a function of solution [NaCl] was measured using 1.3 mm diameter ExPEC rods15 extruded and treated in the same way as the tapes. 1 cm long pieces of ExPEC rod were immersed in 50 mL of the appropriate [NaCl] at rt for 24 h. They were individually removed, dabbed dry with a wipe, and dropped B

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observe a Tg. With samples of 1−10 mg of ExPEC it was possible to increase the heat flow signal in DSC experiments. The samples were fully hydrated and sealed in hermetic pans. The temperature of hydrated ExPEC samples was maintained below 100 °C to avoid rupture of the pan. A large endotherm (exotherm) was observed for water melting (freezing) due to the aqueous solution wetting samples. Since the size of the melting peak made it impossible to record heating curves, cooling curves are presented in Figure 2.

Figure 1. Fraction of PSS/PDADMA PEC compensated by NaCl, y, or doping level, versus solution salt concentration at room temperature. For example, at y = 0.5, 50% of the Pol+Pol− ion pairs are broken.

lead to the idea of a glass transition, or at least plasticization, induced by salt. DSC is widely employed to detect thermal transitions in polymers.27 Of course, in order for these transitions to be revealed, there must be a change in net heat output or input. There have been various reports on DSC measurements of the individual polyelectrolyte components (PSS and PDADMAC, which is PDADMA with a chloride counterion) and on their PECs. The Tg for dry PDADMAC was reported by Yeo and Eisenberg to be 68 °C,28 and Imre et al. determined a Tg of 180 °C19 for dry PSS. However, Vidyasagar et al.21 reported a much higher Tg of PDADMAC homopolymer of 166 °C, and M’Bareck et al. estimated the Tg for PSS to be 211 °C using the Fox equation.29 Much of the disparity between reported Tgs can be attributed to the differences in water content. For example, Yeo and Eisenberg reported a drop in the Tg value of PDADMAC by 31 and 44 °C for water contents of 10% and 25%, respectively.28 Turning to the complex, Imre et al. showed thermograms for several molar ratios of dry PSS/PDADMA with Tg values between 90 and 143 °C, where Tg for the stoichiometric PEC was between 90 and 100 °C.19 However, it has been known since the pioneering work of Michaels in the 1960s1,2 that water significantly plasticizes PECs. In our experience, water content between dry and fully saturated (i.e., immersed in an aqueous solution) is difficult to maintain in PEC of any morphology. Because a large share of applications for polyelectrolyte complexes has them immersed in, and presumably in equilibrium with, aqueous solutions, we focus here on ExPECs in contact, and in equilibrium, with aqueous solutions. Shrinkage of PSS/PDADMA microcapsules was cited as evidence for a Tg in the range 35−40 °C by Köhler et al.22 In a following micro-DSC study on the same system these authors reported a thermal transition at this temperature.18 Phase transitions of similar system in the same temperature range were subsequently reported by Fortier-McGill and Reven using nano-DSC of PEMU-coated silica nanoparticles.20 Recently, Vidyasagar et al. published slightly higher Tgs using DSC for PSS/PDADMA multilayers in various salt concentrations.21 In prior work, we were unable to detect thermal transitions in a hydrated PSS/PDADMA multilayer that had been scraped off a silicon wafer.17 It is likely that we lacked the sensitivity, a combination of instrument performance and sample size, to

Figure 2. DSC thermograms: (A) ExPEC fully hydrated by immersion in water, (B) ExPEC hydrated and doped in 1 M NaCl, (C) ExPEC dried after equilibrating in 1.0 M NaCl, and (D) ExPEC dried after immersion in water. Two consecutive thermograms on the same sample are shown. The solid lines represent the first cooling run, and the dashed lines represent the second run. The cooling rate was 20 °C min−1.

Scans A and B in Figure 2 present two consecutive cooling runs each at 20 °C min−1 for ExPEC in water and 1.0 M NaCl, respectively. A faint transition of about 20−50 mW g−1 was observed at about 30 °C for the sample in water and about 10 °C for the sample in NaCl. Transitions at about 51 °C were seen by Vidrasagar et al.21 for PSS/PDADMA PEMUs of about 1/10th the magnitude and 1/10th the cooling rate. The transition temperature is probably higher than ours due to the lower water content in their samples (12%) compared to ours (37−41%). No transitions between −10 and 80 °C were seen for dry ExPEC, whether it contained salt or not (Figure 2). Additional DSC runs were performed on dry polyelectrolytes and PEC (Supporting Information) up to 220 °C. Dry ExPEC and PSS showed no distinctive transitions up to this temperature, while PDADMAC showed transitions above 150 °C that were not repeatable run-to-run and were thus not pursued. While the DSC data yield thermal transitions that are reasonably consistent with the literature, they are less than definitive. In fact, as will be discussed below, the virtually athermal nature of the transition is a particularly interesting feature. Huglin et al.30 were unable to detect a Tg of a PEC between poly(4-vinylpyridinium chloride) and poly[sodium (2acrylamido-2-methylpropanesulfonate)] by DSC and so concluded there was no glass transition. Dynamic Mechanical Testing. Materials going through Tg are expected to show a significant decrease in modulus (i.e., going from a “glassy” to a “rubbery” state). Since the Tg from DMTA is close to that from DSC,31 mechanical testing may provide data more definitive than, yet comparable to, calorimetry. Using PSS/PDADMA capsules, the Möhwald group, in addition to observing shrinkage at about 34 °C,22 C

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superimpose. An almost flat “glassy” plateau at lower temperatures is evident with a transition to a somewhat less clear “rubbery” plateau. Strong peaks in tan(δ), coincident with the midpoint of the G′ and G″ transition, are observed, with close peak positions on heating and cooling. The glass transition temperature was taken to be the average peak position in tan(δ) for heating and cooling ramps. A summary of Tgs determined for various salt concentrations and two frequencies is shown in Figure 4. Tg decreases by about

observed a strong drop in Young’s modulus from about 100 MPa to about 1 MPa using nanodeformation with an atomic force microscope.32 Recently, Vidyasagar et al., using an innovative quartz crystal microbalance approach, reported a Tg in the range of 49−56 °C for PSS/PDADMA multilayers at different salt concentrations.21 In the present case, the sample had to be immersed in a temperature-controlled solution of fixed salt concentration for extended periods, so the rheometer was fitted with a small solution bath. For the DMTA measurements, samples were compressed by about 15%, and the temperature was increased at fixed increments and allowed to stabilize before the rheometer plates started oscillating at constant frequency. Though temperature ramps were rather slow, with an effective rate less than 0.5 °C min−1, the sample was at a uniform temperature. Sample temperature was decreased using the same procedure. Salt concentrations employed were 0.1, 0.5, and 1.0 M. The upper temperature limit was set to the point where G′ fell to about 104 Pa, after which samples tended to deform past the linear viscoelastic limit and slip between the plates. To counteract sample slippage, it proved essential to roughen the plates via bead blasting. As seen in Figure 3, G′, G″, and tan(δ) all show remarkably clear evidence for a strong transition in mechanical properties. A decrease in G′ and G″ is observed near room temperature. The transition temperature decreases with increasing salt concentration. Data on cooling and heating cycles almost

Figure 4. Tg vs [NaCl] for ExPECs at frequencies of 1 Hz (◆) and 0.1 Hz (●). Solid lines are a fit to Tg = 38 + 2.3 ln f − 20[NaCl]6/5.

20 °C over the range of salt concentration employed. Figure 1 shows that the salt content (doping level) of the ExPEC is proportional to solution salt concentration, as also seen in PEMUs.16,23 Significantly, the doping level over this temperature range is constant for constant salt concentration,33 so the temperature response is not a function of a change in salt content of the PEC. A decrease in Tg is a classical signature of a plasticizer.34 A decrease in Tg may be understood to result from breaking of PEC ion pair cross-links upon doping by salt. Increased cross-link density typically leads to higher Tgs, as seen, for example, in postcuring of vinyl ester resins.35 Tg also increases with increased interpenetration of polymer networks, such as those between polyurethane and poly(methyl methacrylate).36 Using both DMTA and DSC, Lutkenhaus et al. reported a decrease in Tg with increasing pH for a hydrogen bonded poly(ethylene oxide)/poly(acrylic acid) multilayer, due to breaking hydrogen bonds between the two polymers on ionization of poly(acrylic acid).37 The dependence of Tg on frequency also followed the expected trends. Figure 5 shows Tg as a function of frequency from 0.001 to 100 Hz for three salt concentrations. Tg increases approximately linearly with log f. The change in Tg over the frequency range is on the order of 25 °C. Time (frequency) and temperature are often interrelated using time/temperature superposition (TTS) where data collected at different temperatures can be reduced to a master curve using a shift factor (aT).38,39 The dynamic modulus G*, calculated from the data measured in DMTA experiments at different temperatures, was plotted versus time (reciprocal of frequency). A shift factor (aT) was applied to correct for the horizontal difference on the time axis for the curves at different temperatures. The WLF equation27 was used to calculate (aT):

Figure 3. Dynamic moduli G′ (◆), G″ (■), and loss tangent δ (▲) at 0.1 Hz, measured at increasing (solid symbols) and decreasing (open symbols) temperature for the ExPECs soaked in (A) 0.1 M NaCl, (B) 0.5 M NaCl, and (C) 1.0 M NaCl.

log(a)T = D

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master graphs of (A) 0.1, (B) 0.5, and (C) 1.0 M exhibit an 18 order time axis after shift factor correction, and each curve has a glassy plateau, transition, and indications of a rubbery plateau. All master curves show a roughly constant G* of ca. 107 Pa in the glassy plateau and a drop of about 103 Pa. As a cross-check with the temperature scan DMTA experiments, variable frequency measurements were performed on ExPECs immersed in 0.1, 0.5, and 1.0 M NaCl at constant temperature (30 °C). Suggestions of a glassy plateau in G′ and G″ at high frequency in 0.1 M NaCl and a rubbery plateau at low frequency in 1.0 M NaCl are evident in Figure 7. Data for Figure 5. Tg vs frequency for ExPECs soaked in solutions of different [NaCl]: 0.1 M (◆), 0.5 M (●), and 1.0 M (▲). Solid lines are fits to Tg = 38 + 2.3 ln f − 20[NaCl]6/5.

The reference temperature (Tr) was set equal to the Tg value, in which case, C1 = C1g and C2 = C2g, at 1 Hz at 37, 29, and 19 °C for different salt concentrations of 0.1, 0.5, and 1.0 M, respectively. Both constants C1 and C2 were determined empirically, and the best superposition was obtained for C1g = 10 °C and C2g = 70 °C. Values of constants vary from one polymer to another, but they may become universal when Tr = Tg within a specific range.40−42 Constants are typically found to be between 10 and 15 °C for C1g and 50 and 60 °C for C2g as discussed by Hiemenz and Lodge.41 In Figure 6, the three

Figure 7. Frequency sweep of G′ (blue ◆), G″ (red ■), and loss tan(δ) (green ▲) for ExPECs at 30 °C equilibrated in (A) 0.1 M NaCl, (B) 0.5 M NaCl, and (C) 1.0 M NaCl.

the intermediate salt concentration show indications of both plateaus. Tan(δ) shows Tg marching toward higher frequencies with increasing salt plasticization. The trends in Tg for the frequency sweep data in Figure 6 correlate well with the DMTA data. At the temperature employed for Figure 7 (30 °C) Tg occurs at 0.025, 1.2, and 70 Hz for 0.1, 0.5, and 1.0 M NaCl, respectively. Reading from the DMTA data in Figure 5, frequencies for Tg at 30 °C in the respective salt solutions would be 0.034, 1.6, and 100 Hz. Although we have previously referred (loosely) to PECs as hydrogels43,44 because of their water content,45,46 Figure 7 shows behavior that is not typical of highly hydrated gels with a lighter degree of chemical cross-links. Hydrogels, in the stricter sense, exhibit G′ independent of frequency over a wide range and with G′ typically an order of magnitude or so larger than

Figure 6. Master curves of dynamic modulus versus reduced time for ExPECs soaked in (A) 0.1 M NaCl, (B) 0.5 M NaCl, and 1.0 M NaCl. Legend shows symbols for various temperatures. E

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G″ over this range.47 For PECs it is seen that G″ approaches G′ more closely, the latter with increasing frequency. Clearly, viscous losses remain significant over the whole frequency range, which is why PECs have such efficient damping properties.48 Implications for Complexes and Multilayers: Is There a Time/Temperature/Salt Superposition? Several intriguing concepts are illustrated by the experimental results shown here. First, the transition, labeled as a glass transition, is largely athermal, making it extremely difficult to locate via calorimetry. On the other hand, the transition in mechanical properties is abundantly clear with DMTA. The concept and nature of a glass transition continues to be strongly debated.49 It is understood to be a kinetic transition, permitting time/ temperature superposition. However, there is no a priori way to quantify the enthalpy change, if any, expected for a given material. A purely kinetic transition, such as a jamming transition,50,51 might be expected to yield no thermal signature. Second, while the appropriate changes in tan(δ) are observed, the modulus at the glassy extreme, about 10 MPa, is considerably lower than for polymers that are typically called “glassy” (GPa range), pointing out the dangers of using an absolute value to label glassy states. The PEC material used here, and in other morphologies in contact with aqueous solution, is already plasticized by watera fact known for decades.1,52 The water content does not change with temperature (at least for PSS/PDADMA in NaCl53), providing a constant “baseline” of plasticization which is enhanced by salt doping. It is perhaps because of this “preplasticization” that the TTS treatment shown in Figure 6 works both above and below Tg (usually, TTS is applied above Tg). Third, ion diffusion should rapidly accelerate as the material goes through Tg. We recently observed a strong increase in ion (ferricyanide) diffusion coefficient as a function of temperature for a PSS/PDADMA multilayer in 0.6 M NaCl.53 The temperature range for this increase was from 15 to 45 °C. From the 0.5 M NaCl data in Figure 3, this encompasses the width of the transition. Our previous conclusion, that we could not observe a Tg, turns out to have been attributable to the temperature range for that experiment, which was too narrow to capture a Tg about 30 °C wide. In fact, the strong increase in ion mobility is consistent with a glass transition. Fourth, both ion and polymer segment mobility increase through the Tg. Much is known of the response of PSS/ PDADMA PEMUs to experimental variables. For example, this system has been shown to transition from a “linear” to an “exponential” growth mode on increasing salt concentration beyond 1.0 M.54 According to the accepted mechanism for exponential growth, at least one polyelectrolyte exhibits high mobility.55 From Figure 3C, it is clear that above 1.0 M NaCl the glass transition, above which polymer mobility is significantly enhanced, is almost complete by room temperature. Thus, it appears that exponentially growing multilayers are at conditions above their Tg. Finally, it is clear that increasing the doping level pushes the Tg in the same direction as decreasing the frequency. An intriguing question arises as to whether there is a principle of time/temperature/salt superposition, represented by Scheme 1. A salt−temperature relationship was posited by Tan et al.56 when a strong dependence of the rate of PEMU growth on both temperature and salt was observed. Recently, Spruijt et al.57 analyzed the response of polyelectrolyte complexes with a quantitative time−salt superposition using the molecular weight

Scheme 1. Time/Temperature/Salt Superposition

dependence predicted by the dynamic associative interaction model of Rubinstein and Semenov.58 Such a triple superposition as in Scheme 1 could be considered only if same molecular motions were activated in each case. For time/temperature superposition, temperature, putting more energy into the “α” motions of a polymer backbone is understood to be equivalent to waiting more time for the same motions to occur. Because doping breaks ion pair cross-links, it is possible to imagine these same backbone modes occurring more easily (i.e., with greater amplitude) on doping and faster.57 The concept is illustrated in Scheme 2. The context for Scheme 1 is the reptation model where polymers wriggle through evanescent tubes full of entanglements.27 The model is appropriate because the interactions between polyelectrolytes are described both by classical geometrical entanglements between long chains and by reversible physical interactions from ion pairing. These physical cross-links are dynamic, and doping effectively accelerates the kinetics of making/breaking cross-links (frequency) as well as increasing the distance between cross-link junctions (amplitude). Both of these effects enhance the activity of α-modes. At the highest salt concentration employed (1.0 M), about 35% of the PEC ion pairs are broken (doping level of 35%) and 65% remain. Thus, chains are highly associated, highly entangled, and well above the overlap limit below which Rouse behavior dominates. Scheme 2 provides an extreme scenario for “sticky reptation”.58 If ion pairs are to be considered cross-links for the purpose of analysis using classical theories of rubber elasticity, they must last long enough to act as cross-links. In a recent analysis of PSS/PDADMA PEMU mechanical properties we did, in fact, assume an ion pair was a persistent cross-link in our elasticity analysis.17 However, the time allowed for stress relaxation to an “equilibrium” modulus was of order tens of ms; which, as seen in Figure 7, is still in the pre-Tg glassy frequency response regime. Later, in a similar analysis of extruded saloplastic PECs,15 stress was allowed to relax for ca. 100 s, which is either well into, or past, the Tg (Figure 7). The difference in measurement time leads to increasingly diverging modulus, shown in Figure 8, adapted from our previous work. Dynamic ion pair making/breaking must occur on the 100 ms time scale, making them appear quasi-static in a fast relaxation measurement, but dynamic in a 100 s experiment. Using NMR measurements, the molecular motions of the polymer repeat unit were estimated to be of order 10 ms at rt.60 Assuming defect hopping from site to site (Scheme 2) to be coupled to polymer repeat unit motions, like electrons coupled to phonons, 10 ms would represent the minimum lifetime for an ion pair. The WLF equation correlates only two variables: time and temperature. To include a third variable, salt concentration, we used the empirical relationship Tr = 38 − 20[NaCl]6/5, which relates salt concentration to the temperature. When the three F

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Scheme 2. Representation of the Modes Enhanced on Doping PEC with Salta

a

The black arrows represent long-range motions (α-modes) thought to be important at the glass transition. Some short-range, local relaxation (γmodes), suggested by the white arrows, are probably minimally perturbed by doping. PECs are completely amorphous, with consecutive ion pairs as either “ladders” (as shown in the scheme) or cross-links between different polymer chains (not shown).17 This mutually interpenetrating network is in the so-called “scrambled salt” morphology.3,59 Dynamic ion hopping, shown on the right side, enhances the rate at which chain/chain entanglements are released in the sticky reptation model. Water molecules hydrating extrinsic and intrinsic charge are not shown.

superposition for combinations of time, temperature, and salt. Since doping depends not only on salt concentration but also on the type of salt, those salts with a higher equilibrium doping constant,23,61 which order according to a Hofmeister series,62 would push the curves in Figure 4 to the left and those in Figure 5 downward. To illustrate time/temperature/salt superposition in Figures 4 and 5, the data points were fit according to the empirical equation Tg = 38 + 2.3 ln f − 20[NaCl]6/5 which includes the term for salt/temperature equivalence. It should be emphasized that we have no theoretical basis for the scaling factors or the constants in this equation, except for the intercept (38 °C) which is a reference temperature. The PEC system described here could certainly be refined, for example, by using more precise molecular weight fractions. Although the Tg is generally not influenced by molecular weight (at least, for sufficiently high MW), scaling relationships between moduli with mass are key diagnostics for dynamic models.58,63 For frequency sweep data such as in Figure 7 the rubbery plateau might be better developed and the width of the transition might be narrower.

Figure 8. Room temperature equilibrium modulus of ExPEC fiber (●),15 taken after 150 s relaxation, and of PSS/PDADMA PEMU (■),17 taken after 70 ms relaxation time. In both cases a pseudoplateau in the stress relaxation data was observed.

TTS master plots in Figure 6 were replotted using such an equivalence, Figure 9 was obtained, showing reasonably good

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CONCLUSIONS Thermal transitions in equilibrium hydrated polyelectrolyte complexes were revealed more clearly by dynamic mechanical thermal analysis than by calorimetry. For the PSS/PDADMA system a Tg was observed near room temperature. As expected from time/temperature superposition, the location of this Tg depends on the deformation rate, but it also depends on the salt doping level. Hydrated PECs have several ideal attributes for polymeric materials: they are blended at the molecular level;64 they are interpenetrating networks and are completely amorphous. Interactions between molecules are entirely physical: classical entanglements and ion pair interactions, the latter under reversible control by salt doping. Equilibrium plasticization by water creates a more tractable material toward the glassy end of the frequency response. Since the transition is

Figure 9. Combined curve of the three master curves at 0.1 M NaCl (●), 0.5 M NaCl (■), and 1.0 M NaCl (◆). The solid line is to guide the eye. G

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“kinetically pure”, having little enthalpic component, as seen from the DSC results, the PEC system may be a rewarding platform for theoretical treatments of polymer mobility with interaction energies continually adjustable via the level of doping.

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ASSOCIATED CONTENT

S Supporting Information *

Solution NMR of dissolved individual and complexed polyelectrolytes showing removal of impurities; DSC thermograms of individual polyelectrolytes and dried PEC; modulus versus temperature at 1.0 Hz. 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 The authors gratefully acknowledge grants DMR-0939850 and DMR-1207188 from the National Science Foundation for supporting this work. We thank Prof. S. Ramakrishnan for help with rheometry and Prof. R. Alamo and Dr. J. Lopez-Majada for assistance in DSC experiments.

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