Substrate-Dependent Physical Aging of Confined Nafion Thin Films

Feb 2, 2018 - Ion-containing polymers, like Nafion, fulfill critical roles as water purification membranes and as separators in electrochemical cells,...
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Letter Cite This: ACS Macro Lett. 2018, 7, 223−227

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Substrate-Dependent Physical Aging of Confined Nafion Thin Films Douglas I. Kushner and Michael A. Hickner* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: The humidity-induced physical aging, or structural relaxation, of spin-cast Nafion thin films on gold, carbon, and native oxide silicon (n-SiO2) substrates was examined using spectroscopic ellipsometry (SE). Physical aging rates, β, were calculated from the change in measured sample thickness, h, upon exposure to controlled humidity. Three Nafion films, h = 188, 57, and 27 nm, deposited on gold substrates demonstrated an increased β with decreasing thickness due to confinement. The Nafion film on n-SiO2, h = 165 nm, also showed a humidity-induced aging, while a Nafion film deposited on carbon, h = 190 nm, exhibited no measurable humidity-induced aging. The reported rate of aging was related to the strength of the polymer/ substrate interactions during film formation. Strong interactions between Nafion and the gold and n-SiO2 substrates anchored the thin film to the substrate during film formation, resulting in a nonequilibrium as-cast film and subsequent relaxation upon exposure to water vapor until complete plasticization. Weak interactions between the carbon substrate and Nafion resulted in fully relaxed as-cast films which displayed no relaxation upon hydration. thin films, the substrate-dependent structural relaxation of Nafion has yet to be addressed. Physical aging of molecular solids due to structural relaxations below the glass transition temperature, Tg, is responsible for time-dependent changes in material properties, including gas permeability, mechanical strength, specific volume, and optical properties.21−25 Physical aging arises after rapidly cooling a liquid to a temperature below its Tg, resulting in a glassy material that possesses excess configurational entropy (i.e., excess free volume). While in the nonequilibrium glassy state, local chain segment relaxations accommodate the driving force toward lowering configurational entropy.25 The aging rate in polymer membranes has been shown to be structure dependent, including contributions from the backbone stiffness, side groups and branching, and cohesive energy density.26−28 Nanoconfined polymers prepared either by introducing inorganic nanoparticles or by spin-casting a thin polymer film on an inorganic substrate demonstrated physical aging rates that were dependent on the confinement-induced Tg shift from the bulk polymer Tg.29 The physical aging rate in polymer thin films under confinement that exhibit an increased Tg tend to show slower aging rates, for instance nanoscopic inorganic cages in epoxy network glasses,30 while a decrease in the Tg for thin films, such as polystyrene loaded with silica nanoparticles, will accelerate the rate of aging.31

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on-containing polymers, like Nafion, fulfill critical roles as water purification membranes and as separators in electrochemical cells, in capacitors, and in sensors.1−4 These materials have received attention in energy storage and conversion devices including redox flow batteries,5,6 solar-fuel generators,7,8 and fuel cells9,10 where ionic membranes are responsible for ion transport and electrical separation of the anode and cathode. Transport across the polymer/catalyst interface in a porous electrode is integral to the operation of electrochemical devices where the ionomer structure, when confined in a thin-film geometry, can be influenced by the composition of the substrate, resulting in behaviors not observed in bulk membranes.11,12 Nafion and other perfluorosulfonic acid (PFSA) ionomers (e.g., 3M PFSA and Solvay Aquivion) are interesting because of their unique combination of ion conductivity and mechanical properties that are a direct result of their phase-separated hydrophilic and hydrophobic domains.13,14 The typical characteristic morphology of a Nafion membrane, 15−200 μm thick, is composed of 3−5 nm ionic domains distributed in a low crystallinity, semicrystalline backbone matrix, as measured using small- and wide-angle X-ray scattering,15−17 transmission electron microscopy,18 and neutron scattering.15 Thin-film confinement of this material is known to disrupt or induce anisotropy of the phase-separated morphology observed in the bulk membranes via substrate wetting interactions.12,19,20 Though there are a number of Nafion thin-film studies in the literature that are building a picture of how Nafion behaves in © XXXX American Chemical Society

Received: December 27, 2017 Accepted: January 28, 2018

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Figure 1. Thickness profiles for a (a) 188 nm thick film on gold and (b) 162 nm thick film on n-SiO2. Mass uptake profiles for a (c) 140 nm thick film on a gold-coated quartz crystal and (d) 72 nm thick film on a SiO2-coated quartz crystal. The RH is labeled with the blue dashes marking the RH steps.

plots, water is an integral contributor to the physical aging phenomenon observed in the complex structure of Nafion. The film thickness rapidly increases due to water sorption when the humidity is incrementally increased from 0% RH to 95% RH, followed by an immediate relaxation to a lower thickness. The observed rapid increase in thickness is due to the quick water sorption kinetics followed by a slow mechanical relaxation.35 Water sorption in the film approaches a thermodynamic equilibrium, which determines the total water content of the material but does not determine the final film thickness after the structural relaxation of the matrix. As water swells the hydrophilic domains and plasticizes the polymer chains, there is relaxation of the hydrophobic backbone, which contains little to no water, and the side chain. This plasticization and relaxation is likely due to the weakening of the Coulombic forces between the ionic groups in the material when shielded by water. The magnitude of the thickness reduction shows that the strength of interactions between the substrate, gold, or n-SiO2 and the polymer dictates the conformational entropy of the polymer during initial film formation. The relaxation was not observed in films that have been thermally annealed (Figure S1) and were not studied further. The Coulombic forces of the ionic groups in thermally annealed membranes were overcome by temperature which increases the mobility of the chains at the interface, permitting structural relaxation of the film. In order to rule out water desorption causing the film thickness decrease (e.g., high water uptake followed by desorption of water throughout the constant humidity hold) the water mass uptake was measured using a quartz crystal microbalance. Figure 1c and Figure 1d show the mass uptake for freshly prepared Nafion films on gold and SiO2-coated quartz crystals, respectively. Fresh samples were necessary as a result of the relaxation event only occurring during the initial RH increase cycle. The mass continually increased, while the thickness decreased throughout the constant humidity hold,

Traditionally, physical aging studies are conducted by heating an amorphous polymer above Tg followed by quenching at Tage, where the driving force is determined by Tage − Tg = ΔTage. In this work, we have adopted the physical aging formalism to describe the structural relaxation observed in spin-cast Nafion thin films upon exposure to water vapor using in situ spectroscopic ellipsometry (SE) by monitoring the thickness change of the sample as a function of time. The humidified environment was monitored using a relative humidity (RH) sensor and reported as the % RH. In order to generate high configurational entropy in thin films, Nafion was quenched from solution to solid by rapidly evaporating the solvent: a typical consequence of spin-cast thin films. Nafion thin films were deposited on gold-coated, carbon-coated, and native oxide-coated silicon wafers, dried at 40 °C, and exposed to humidified air. The swelling measurements provided insights into the strength of the effect that substrate composition has on Nafion properties via a humidity-induced structural relaxation when using a strongly interacting metal substrate32 (gold), interacting nonmetallic substrate via hydrogen bonding of the silanol groups33 (n-SiO2), and a hydrophobic substrate (carbon). The change in the Nafion thin-film thickness with time was monitored at 25% RH, 50% RH, 75% RH, and 95% RH. Here we show that Nafion deposited on gold and n-SiO2 substrates exhibits a water-plasticized structural relaxation as the humidity was increased. The Nafion film deposited on the hydrophobic carbon substrate exhibited no observable relaxation when exposed to humidity indicating that the film was fully relaxed prior to the swelling measurements, likely due to increased mobility at the interface.34 This analysis provides insight into the compositional influence of the substrate with respect to confined Nafion thin films during film formation. Figure 1 shows the measured thickness profile of 188 and 162 nm thick as-cast Nafion films deposited on gold and nSiO2, respectively. Upon closer examination of the thickness 224

DOI: 10.1021/acsmacrolett.7b01004 ACS Macro Lett. 2018, 7, 223−227

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Figure 2. Time-dependent normalized thickness at 25% RH (black ■), 50% RH (red ●), 75% RH (blue ▲), and 95% RH (pink ▼) for (a) 188 nm Nafion on gold, (b) 162 nm Nafion on n-SiO2, and (c) 190 nm Nafion on carbon.

with β calculated from the slope of the time-dependent thickness. It is evident from Figure 2 that the time-dependent normalized thickness is dependent upon the substrate; the measured β values are tabulated in Table 1. Structural

indicating that the observed thickness reduction was the direct result of polymer relaxation. The mass uptake (Figure 1d) shows a large mass increase at low humidity compared to the Nafion sample on the gold substrate which was attributed to water sorption in the porous SiO2 structure; silicon wafers consist of a dense native oxide layer and do not exhibit this phenomenon.36,37 Although aging was observed in films prepared on gold and n-SiO2, the Nafion thin-film sample on the gold substrate demonstrated the greatest thickness relaxation when humidified. As shown in Figure 1a, the initial film thickness of Nafion on the gold substrate was 188 nm prior to humidification followed by a 4.5% reduction to a relaxed dry thickness of 180 nm, whereas Nafion on n-SiO2 experienced a 1% decrease in the initial thickness from 162 nm to a relaxed dry thickness of 160.5 nm. The degree of relaxation suggests that the influence of the polymer/gold interactions persisted furthest into the film thickness, which slowed aging of the film prior to the humidityinduced plasticization. The aging rate, β, can be calculated based on a number of dimensional measurements involving volume and thickness changes. Originally observed by Kovacs, Struik defined the physical aging rate as the change in the time-dependent specific volume, dV/d(log ta), shown in eq 1.22,38 β=−

1 dV V∞ d(log ta)

Table 1. Measured β Values for Nafion Thin Films Deposited on Gold and n-SiO2 at a Given % RH Aging Rate (× 102)

1 dh h∞ d(log ta)

substrate

25% RH

50% RH

75% RH

95% RH

165 188 55 27

SiO2 gold gold gold

0.6 0.5 1.5 5.9

0.5 1.5 1.9 3.5

0.47 1.1 2 4

0.7 1.2 1.5 4.2

relaxations were observed in the Nafion thin-film samples on gold and n-SiO2 substrates, while no reduction in thickness during the humidity hold was detected for the Nafion thin film on the carbon substrate. The relaxation, or decrease in thickness during the constant humidity hold, was a direct result of the influence of the substrate composition on the polymer behavior. The attractive interactions between the sulfonate group of Nafion and gold and n-SiO2 substrates have been shown to drive the preferential alignment of the Nafion backbone parallel to the substrate in ultrathin films, which likely reduced the polymer mobility and slowed the relaxation of Nafion prior to humidification.40 The fact that no observable humidity-induced structural relaxation was detected for Nafion deposited on a carbon substrate suggested that the weak interactions between Nafion and carbon permitted the polymer chains to relax rather than remaining trapped during film formation. The aging or relaxation rate for Nafion films on gold substrates with different thicknesses (Figure 3) was examined in order to reveal the persistence of the polymer/gold interactions from the substrate into the polymer film. Measured aging rates are tabulated in Table 1 for the 27, 55, and 188 nm thick samples on the gold substrate, from the fitted regions shown in Figure S2, and the 165 nm thick Nafion film on n-SiO2 at each humidity set point. For each humidity value, the β increased as the thickness decreased. Moreover, β increased as the thickness decreased suggesting that the thinnest films are frozen further from equilibrium due to thin-film confinement, resulting in a greater relaxation rate during hydration. The aging rates for a given sample thickness showed little dependence on humidity where the β was similar at 25, 50, 75, and 95% RH compared to the obvious increase in β with decreasing thickness.

(1)

where V∞ is the specific volume at equilibrium; V is the measured volume; and ta is the time of aging. Since polymers confined to a substrate experience the swelling and contraction in a single dimension normal to the substrate, e.g., 1D swelling, the aging rate relation can be written using eq 2

β=−

thickness (nm)

(2)

where h∞ is the equilibrium film thickness and h is the measured thickness. Physical aging occurs in three regimes including a time-independent regime at the beginning, an intermediate regime, and a terminal aging regime.23 Baker et al. showed that the thickness corresponding to an aging time of 10 min was sufficient to approximate the aging rate without knowledge of h∞, if the intermediate aging regime was achieved.39 We have adopted the approach of calculating β by normalizing h to the film thickness corresponding to an aging time for 10 min (h0); however, it is important to note that the inflection tangent may lead to large deviations in relaxation rates when compared to very long aging rates.27 This procedure was sufficient when examining the intermediate aging regime in order to describe the humidity-induced relaxation of Nafion 225

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elucidate how the influence of the substrate dictates the polymer structure.11,19,41,42

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b01004. Detailed description of experimental methods; annealing effect; and fitted regimes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Douglas I. Kushner: 0000-0002-3020-7737 Michael A. Hickner: 0000-0002-2252-7626 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of the US Department of Energy, the Office of Energy Efficiency and Renewable Energy, the Fuel Cells Technology Program through a subcontract from General Motors Corporation under grant DE-EE0000470. Additional support for this work was provided by the Office of Naval Research, Grant N00014-10-1-0875. Infrastructure support was provided by The Pennsylvania State University Materials Research Institute and the Institutes of Energy & the Environment. M.A.H. acknowledges the Corning Foundation and the Corning Faculty Fellowship in Materials Science and Engineering for support.

Figure 3. Time-dependent thickness at 25% RH (black ■), 50% RH (red ●), 75% RH (blue ▲), and 95% RH (pink ▼) for (a) 55 nm and (b) 27 nm thick Nafion films on a gold-coated silicon wafer.

In conclusion, humidity-induced physical aging or structural relaxation measurements were performed on Nafion thin films deposited on gold, carbon, and n-SiO2 substrates to study the influence of the substrate composition on the behavior of the polymer film. Physical aging or structural relaxation, as demonstrated by a reduction in film thickness over time during periods of constant RH, was observed for samples deposited on gold and n-SiO2 substrates but was undetectable on carbon substrates. By considering the influence of the substrate composition on the behavior of thin Nafion polymer films, we determined that the interactions between Nafion and gold slowed humidity-induced structural relaxations in as-cast samples prior to hydration. In order for the rapid structural relaxation to occur, water was required to plasticize the polymer. Aging was observed in Nafion films prepared on nSiO2 but exhibited a slower aging rate and smaller overall changes in thickness between the unrelaxed thickness and the near-equilibrium thickness during the desorption cycle when compared to gold, suggesting that the Nafion structure on nSiO2 was in a more relaxed state than Nafion on gold substrates. Nafion thin films coated on gold also exhibited a clear thickness dependence with increasing aging rates as the film thickness decreased. Attractive interactions between the polymer and the substrate froze Nafion in a state of high configurational entropy as the film formed during spin casting. The hydrophobic carbon substrates exhibited little influence on Nafion during the spin-casting process as observed by the lack of any detectable relaxation upon humidification. It is important to note that anchoring the polymer to the substrate during film formation may result in increased residual stresses in the film. Humidity may simply relax this stress as opposed to the typical structural recovery in glasses where vitrification of the polymer provides the driving force. Overall, it is important to understand the behavior of Nafion during its structural evolution toward morphologies observed in small-angle X-ray scattering, neutron reflectometry, and FTIR studies in order to

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