Letter pubs.acs.org/macroletters
Water and the Glass Transition Temperature in a Polyelectrolyte Complex Jingcheng Fu, Rachel L. Abbett, Hadi M. Fares, and Joseph B. Schlenoff* Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *
ABSTRACT: Hydrated polyelectrolyte complexes, H-PECs, have recently started attracting renewed interest as a class of highly solvated/plasticized blends. H-PECs are observed to undergo a transition in mechanical properties close to room temperature. Whether this is a true glass transition has been questioned recently: the material has an unusually low modulus in the “glassy” state and molecular dynamics simulations have suggested temperature-induced dehydration and water structure changes are responsible for the transition. Using in situ infrared spectroscopic methods on thin films of a widely studied H-PEC we find no definitive evidence for changes in the hydration state of functional groups, the water content, or water structure on passing through Tg for stoichiometric and nonstoichiometric H-PECs. These complexes represent a promising platform for fundamental studies of the glass transition, since the coupling between chains can be modified by “doping” the material with salt, which breaks ion pairing cross-links. The Fox equation was used to estimate Tgs for paired and unpaired oppositely charged repeat units.
B
dehydration.23,24 Given the strong plasticizing effect of water,18 it is reasonable to infer that a small increase in water content would yield a large drop in modulus. In the computations of Yildirim et al., the hydrogen-bonding structure of water was also shown to change at the transition temperature.23 In the present work, we measured water content and structure in a thin film of stoichiometric PDADMA/PSS complex in situ using attenuated total internal reflectance infrared spectroscopy ATR/FTIR. This technique was introduced to the study of thin films of H-PEC by Müller et al.25 and has also been used extensively by the Sukhishvili26 and Kharlampieva27 groups. IR absorption (at 1000 cm−1) is limited to material within 0.6 μm of the germanium ATR crystal, which means, for a thicker film, only the material within the bulk of the film is probed. For a PDADMA/PSS PEC built using the layer-by-layer method in 1.0 M NaCl about 30 layers is enough to contain the IR absorption entirely within the complex3 (see Figure S1, Supporting Information). While the modulus and Tg depend on water content, it is fortunate that equilibrium hydration is achieved rapidly for thin films of PEC.18 The ion content of H-PECs controls their mechanical properties, including Tg, by decoupling polycation repeat units Pol+ and polyanion repeat units Pol− and by additional plasticization as more water enters with the ions.8 If ions are
lending of neutral polymers is driven by specific interactions between them and by their entropy of mixing. The latter parameter, weaker for polymers than for small molecules, is rarely enough to overcome even a slight preference for polymers to associate with themselves.1 Thus, polymer mixtures blended uniformly at the molecular level are the exception rather than the rule.1 In contrast, when aqueous solutions of polymers with oppositely charged repeat units are mixed the substantial entropy gain of counterion (and water) loss drives polyelectrolyte complexation,2,3 with molecular-level mixing,4 consistently and predictably for a wide variety of polymers. In a time/temperature/salt equivalence,5−7 hydrated polyelectrolyte complexes, H-PECs, soften with the addition of salt and with increased temperature, allowing them to be extruded close to room temperature.8 Scanning calorimetry9,10 and dynamic mechanical thermal analysis6 show what appears to be a distinct glass transition for a widely studied H-PEC of poly(diallyldimethylammonium), PDADMA, and poly(styrenesulfonate), PSS. However, whether this is truly a glass transition has been questioned along at least two lines. First, the modulus of the material at the “glassy” end of the temperature scale is actually a couple of orders of magnitude less (∼10 MPa) than typically observed for a glassy polymer (∼1 GPa). This is because the material already contains,11−15 and is plasticized by,16−20 a significant amount of water. In fact, dried PECs are much stiffer but do not exhibit a glass transition over accessible temperatures.21,22 Second, supported by molecular dynamics simulations it has recently been proposed that the observed transition hydrated PECs is actually driven by © XXXX American Chemical Society
Received: August 29, 2017 Accepted: September 18, 2017
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ACS Macro Letters
metric multilayers and solution-precipitated PECs has been frequently highlighted.32,33 The stoichiometric PDADMA/PSS film is isotropic and retains none of the layered character of the multilayer from which it was made.34 Alternatively, prolonged exposure to PDADMA can increase the nonstoichiometry, also shown in Figure 1.30 An interesting finding from Figure 1 is that the doping constant is relatively unaffected by small nonstoichiometry but the material clearly becomes easier to dope with higher levels of Pol+ versus Pol− charge mismatch. Simulations by Yildirim et al. suggested dehydration of the PSS unit represented by a 30% loss of water−PSS contacts over a 70 °C range, including a small step at 70 °C (about 30° higher than the observed Tg, see below). FTIR can be used to determine the water content of H-PEC film, which, when stoichiometric, has the same composition as macroscopic samples (made by “saloplastic” extrusion of the same polymers8) used in our prior work to measure Tg.6 Glass transitions at 1 Hz were previously observed in a PDADMA/ PSS H-PEC in 0.1, 0.5, and 1.0 M NaCl, at 37, 29, and 18 °C, respectively.6 Using a Kdop for NaCl35 of 0.30, doping levels of 0.04, 0.18, and 0.36 are calculated for the respective salt concentrations. All water peak areas were ratioed to the PSS band (−SO3−) at 1049−991 cm−1 and standard solutions of 1.00 M PSSaq were used to calibrate the relative molar amounts of water to polymer (where the −SO3− group represents one Pol+Pol− repeat unit). Figure 2 shows the ratio of water molecules to Pol+Pol− repeat units as stoichiometric films of complex were warmed and cooled in contact with salt solutions (Figure S3 presents full scale plots of styrene sulfonate and H2O peak areas and
infrared active, all the components of the complex, polymer, ions, and water, are observed in the IR spectrum.3 Ions appear in the PEC film under one of two mechanisms: if PSS and PDADMA are not present in stoichiometric amounts, additional counterions are required to balance the polyelectrolyte in excess;28 alternatively, more counterions, M+ and A−, can be doped into the complex when salt MA is added to solution. Pol+Pol−s + M+aq + A−aq → Pol+A−s + Pol−M+s
(1)
−
where Pol Pol is a paired “intrinsic” site or ion pair, and Pol−M+ and the Pol+A− are ion-compensated “extrinsic” sites. The ion population from doping under the site-specific model of eq 1 is described by a “doping level”, y, which is the fraction of Pol+Pol− converted to extrinsic sites, and 1 − y is the remaining fraction (0 ≤ y ≤ 1). Doping is under reversible, thermodynamic control given by a constant Kdop29 +
Kdop =
y2 (1 −
2 y)aMA
≈
y2 2 aMA
(as y → 0) (2)
An example of doping with MA = NaNO3 is given in Figure 1. The molar ratio of NO3− and SO3− shows the doping level.
Figure 1. Ratio of NO3−/SO3− in an H-PEC film as made (diamonds), after conversion to stoichiometric (triangles), and with (31%) excess PDADMA (circles). Kdop values (from the slopes) are 0.46, 0.45, and 2.69 for the as-made, 1:1, and excess PDADMA films, respectively. Concentrations were converted to activities using the activity coefficients listed in Table S1, Supporting Information. NO3− comes from doping plus excess PDADMA. The intercepts show the excess amount of PDADMA to be 14, 0.2, and 31%, respectively. A small amount of nonstoichiometry (≤14%) does not appear to influence Kdop. Inset shows a Pol+Pol− ion pair and a Pol+A− extrinsic site. Tg decreases as the ion content increases.6
The as-made film contains about 14% excess PDADMA over PSS. Therefore, a persistent NO3− population (14% given by the NO3−/SO3− intercept as [NaNO3] → 0) is needed to balance the excess PDADMA. Any transition is likely controlled by the balance between extrinsic and intrinsic sites. We have developed methods for producing stoichiometric or strongly nonstoichiometric quantities of PSS and PDADMA in thin films of PEC.30 Additional PSS needed to balance the excess PDADMA in as-made PEC films can be introduced, yielding stoichiometric material, as seen in Figure 1.31 Supporting Information provides more information on the way films were processed to achieve this fundamentally important material. The equivalence of stoichio-
Figure 2. Temperature ramp up (diamonds) and down (squares) for 1:1 (stoichiometric) PDADMA/PSS H-PEC film in 0.1 M NaCl (A), 0.5 M NaCl (B), and 1 M NaCl (C); for an H-PEC film with (31%) excess PDADMA in 0.1 M NaCl (D), 0.5 M NaCl (E), and 1 M NaCl (F). Data had an estimated precision of ±0.5 H2O per SO3 and an accuracy of ±1 H2O per SO3. 1115
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ACS Macro Letters their molar ratio). Although the water content increased with increasing [NaCl] (doping brings additional water into the PEC), only slight, gradual changes in water content with temperature were observed, but no transition within a detection limit of 0.5 water molecules. For comparison, the storage modulus drops about 2 orders of magnitude over the temperature range in Figure 2 (before and after Tg).6 A similar change in modulus is observed going from 0.1 to 1.0 M NaCl at room temperature, where water content increases from 8.2 to 11.5 H2O per sulfonate.8 If an increase in free volume due to an increase in water content were the reason for the transition in modulus, one might expect an additional 3 H2O, which would be clearly observed in the temperature scans in Figure 2. Temperature cycling was also performed with films that had an excess of PDADMA. Figure 2 shows that there is more water in these films (because of the additional chloride ions and a lower extent of physical cross-linking) and that the water content is not as stable with heating and cooling in 1.0 M NaCl. Although this system is not as stable, perhaps due to loss of complex or film delamination, there is no defined, reversible transition in water content over the temperature range studied. The frequency of the sharp symmetric stretching vibration of the sulfonate group (ca. 1035 cm−1) depends on its state of hydration.36,37 Levy et al. reported shifts in the PSS−SO3− band of several wavenumbers over the range of 1−9 water molecules of hydration per PSS.36 High resolution FTIR spectra of the position of this SO3− band, shown in Supporting Information, Figure S4, indicate no change in the state of hydration of PSS over the temperature range studied (see Figure S4, Supporting Information). The simulations of Yildirim et al. also showed changes in the structure of water.23 The signature for a disruption in the hydrogen-bonding network of water is typically a change in the spectral features of the O−H stretching region from 3000 to 4000 cm−1.38 The symmetric H-bonded network at 3244 cm−1 is particularly sensitive to water structure perturbation.39 Thus, direct evidence for changes in water structure was sought by comparing IR absorptions of water within the H-PEC at various temperatures. An example is shown in Figure 3, which presents the O−H stretch region at temperatures below and above Tg
for H-PEC in 1.0 M NaCl. Small changes in absolute intensities were observed, possibly due to changes in refractive index, but when the mode at 3244 cm−1 (more sensitive to H-bonding40) was compared to that at 3412 cm−1 (less sensitive to Hbonding) no differences (within experimental error) were seen below and above Tg. As a final check into the nature of water within the PEC, we evaluated the claim that there are different kinds of water in a polyelectrolyte complex in the form of a multilayer: exchangeable and nonexchangeable.14 To study the degree and rate of water exchange, D2O with 0.1 M NaCl was pumped through the ATR cell to replace 0.1 M NaCl in H2O. The exchange of O−H by O−D was followed, as shown in Figure 4 using the corresponding IR stretches at 2993−3932 and 2194− 2755 cm−1. Within 30 s all H2O had been replaced by D2O (Figure 4B). Differences in PEC film water content measured by ellipsometry (containing H2O) and neutron reflectometry
Figure 4. (A) Time resolved spectra on pumping 0.1 M NaCl in D2O over an H-PEC film doped with 0.1 M NaCl in H2O, from 0 to 30 s with 5 s intervals. (B) Kinetics for the exchange of H2O (circles) by D2O (diamonds) in the PEC film, after introducing 0.1 M NaCl in D2O. (C) Equilibrium molar ratio of H2O/D2O in the PEC vs the ratio of H2O/D2O in the contacting water (diamonds). Dashed line is no preference for H2O over D2O in the PEC.
Figure 3. Water O−H stretching bands (3000−3600 cm−1) for 1:1 stoichiometric H-PEC film in 1.0 M NaCl at 10 °C (solid line), 20 °C (dashed line), 40 °C (dash and dot), and 60 °C (dotted). 100 scans at 1 cm−1 resolution. Point A is 3412 cm−1; Point B is 3244 cm−1. Spectra are offset by 0.01 AU for clarity. Supporting Information, Figure S5, shows the entire IR spectrum collected for these samples. 1116
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ACS Macro Letters (film contains D2O) have been ascribed to isotopic effects.13 Any possible preference for one isotopic form of water over another was evaluated by injecting various molar ratios of H2O to D2O into the ATR cell and measuring the corresponding ratio taken up by the H-PEC (Figure 4C). The slope in Figure 4C indicates a slight preference (7%) for H2O over D2O by the PEC, but deuterated water is a good surrogate for H2O. The coupling strength between chains in H-PECs is directly and reversibly related to the doping level. An extrinsic site, created by breaking a Pol+Pol− ion pair, a physical cross-link, should have greater mobility than an intrinsic one. Using the experimental dependence of y on NaCl activity: y = 0.55aNaCl,35 and adapting prior data on Tgs for PDADMA/PSS H-PEC,6 when Tg is plotted as a function of the doping level for frequencies of 0.1 and 1.0 Hz (Figure 5), the intercepts indicate
solution temperature (LCST)23 would be characterized as a phase transition occurring at a fixed temperature independent of the measurement frequency/time.
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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/acsmacrolett.7b00668. Details on experimental procedure, film buildup, creating stoichiometric films, and additional doping curves. Full FTIR spectra of heated H-PECs. High resolution FTIR of water bands (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hadi M. Fares: 0000-0003-4009-2037 Joseph B. Schlenoff: 0000-0001-5588-1253 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grant DMR1506824 from the National Science Foundation. REFERENCES
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