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Nanostructure and Transport Properties of Proton Conducting SelfAssembled Perfluorinated Surfactants: A Bottom-Up Approach toward PFSA Fuel Cell Membranes Quentin Berrod,†,∥ Sandrine Lyonnard,*,†,∥ Armel Guillermo,†,∥ Jacques Ollivier,‡ Bernhard Frick,‡ Abdelatif Manseri,§ Bruno Améduri,§ and Gérard Gébel∥,⊥ †

CNRS/CEA-INAC-SPrAM, F-38000 Grenoble, France Institut Laue Langevin, F-38002 Grenoble, France § UMR CNRS 5253, Institut Charles Gerhardt Ingenierie & Architectures Macromoleculaires, Ecole Nationale Superieure de Chimie Montpellier, 8 Rue Ecole Normale, F-34296 Montpellier, France ∥ Université Grenoble-Alpes, F-38000 Grenoble, France ⊥ CEA, LITEN, DTNM, F-38054 Grenoble, France

Downloaded by UNIV OF GLASGOW on September 14, 2015 | http://pubs.acs.org Publication Date (Web): August 27, 2015 | doi: 10.1021/acs.macromol.5b00770

‡

S Supporting Information *

ABSTRACT: A comprehensive study of commercially available and newly synthesized proton conducting perfluorinated sulfonic acid (PFSA) surfactants and polymeric systems is reported, specially designed in a bottom-up search to improve the basic understanding of PFSA polymers used as benchmark electrolytes in fuel cells. Hydration-dependent mesomorphous phases are formed by the self-assembly of these molecules in water. The impact of the hydrophobic chain length, the density of charge, the molecular architecture on the nanostructure, and the dynamics of confined water were studied by combining small-angle X-ray scattering, quasielastic neutron scattering, and pulsed-field gradient NMR. We introduce a hydration-dependent structural parameter, dw (mean size of water domains), that allows to establish the structure−transport relationship in PFSA materials. This multiscale study reveals that (i) the dynamical behavior of confined protons and water molecules are rather insensitive to the topology of the host matrix and (ii) the main parameter driving the performance of fuel cell electrolytes is the total water content required for swelling the domains above a value of 1 nm.

1. INTRODUCTION Perfluorosulfonic acid (PFSA) polymer membranes are the current benchmark materials for low temperature proton exchange membrane fuel cells (PEMFC) due to their outstanding proton conductivity and stability. These membranes are made of a Teflon-like backbone carrying perfluorinated side chains terminated by sulfonic acid groups (Figure 1). Their performances highly depend on their hydration level that determines both the hydrophilic/hydrophobic phase-separated nanostructure and the efficiency of proton transport. The water-mediated proton motions are related to complex molecular interactions, spatial confinement at nanoscale, tortuosity of the polymer structure, and connectivity of ionic nanodomains. A considerable amount of work has been devoted to the study of PFSA ionomers, e.g., Nafion from Dupont,1 Aquivion from Solvay,2 and 3M membranes.3 In particular, insights into PFSA morphology were gained by extensively scrutinizing the PFSA structure as a function of relevant parameters as hydration,4−8 chemical architecture,9−11 pretreatments,12,13 temperature,14,15 and stretching.16,17 Some general features are well-established: (i) the polymer backbone acts as a soft hydrophobic confining matrix and (ii) protons, sulfonic head groups, and water © 2015 American Chemical Society

molecules form a network of connected ionic domains that swell upon hydration. Yet, despite tremendous efforts to rationalize the multiscale organization, the size, shape, and organization of the ionic domains are still not fully elucidated. This is due to the fact that most modern scattering techniques provide spatially averaged information, and numerical simulations intrinsically fail to give a unified description of molecular-level details while grasping nanoscale phase-separated morphologies.18−22,52 Accordingly, structural models based on either spherical or cylindrical ionic clusters connected through channels,4,23 or hydrophobic aggregates embedded inside a continuous ionic medium5,6 were proposed. Hence, a comprehensive understanding of the structure−transport interplay primarily requires a more thorough description of the polymer hierarchical morphology at all relevant scales. The need for clarifying the relationship between multiscale structure and proton transport in PFSA membranes has remained hampered by the ill-defined bicontinuous PFSA membrane morphology. Therefore, to overcome the intrinsic Received: April 14, 2015 Revised: July 10, 2015 Published: August 27, 2015 6166

DOI: 10.1021/acs.macromol.5b00770 Macromolecules 2015, 48, 6166−6176

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commercially available and noncommercial PFSA materials in terms of self-assembled geometries, dilution laws, and ionic confinement sizes. The third section reports the dynamical properties of water confined in the ionic domains measured at molecular and micrometric scales by QENS and NMR, respectively. We discuss the impact of molecular architecture, nanoconfinement, and connectivity on the water and proton motions. In the concluding section, our findings have been summarized in the perspective of providing guidelines to optimize the design of efficient PFSA membranes.


2. EXPERIMENTAL SECTION 2.1. Materials. Perfluorooctanesulfonic acid (PFOS) and perfluorohexanesulfonic acid (PHOS, potassium salt) surfactants were purchased from Sigma-Aldrich. Nafion NR212 membranes were purchased from Dupont. All reagents used in the PFSA materials synthesis were used as received unless stated otherwise. C6F13I was supplied by Elf Atochem, CRRA, Pierre Bénite, France. tert-Butylperoxypivalate (tert-butyl 2,2dimethylperoxypropanoate) in isododecane (Trigonox 25-C75, tBuOOC(O)tBu, TBPPi) (purity 75%) was kindly provided by Akzo Nobel (Compiègne, France). Vinylidene fluoride (VDF, F2C CH2) was kindly offered by Arkema. Perfluoro(4-methyl-3,6-dioxaoct7-ene sulfonyl fluoride) was purchased from Synquest Laboratories (Alachua, FL) while methanol and lithium carbonate (all of analytical grade) were supplied by Sigma-Aldrich (Saint Quentin-Fallavier, France). 1,1,1,3,3-Pentafluorobutane (C4H5F5) was given by Solvay Fluor, Hannover, and deuterated acetone for NMR characterizations was purchased from Euriso-top (Saint-Aubin, France) (purity >99.8%). The surfactants and copolymeric materials were first lyophilized and then diluted to prepare concentrations ranging from (almost) dry to 60 wt % in water. The hydration rate was controlled by weighing the added amount of liquid water to the initial freeze-dried powders. In the following, the composition of the PFSA gels is quantified by different parameters: the volume fractions of water or PFSA, Φw and ΦPFSA, respectively (Φw = 1 − ΦPFSA = mw/(mw + mPFSA/ρPFSA), where mw and mPFSA stand for the weighed amount of water and PFSA materials, respectively), and the local hydration number λ defined as the number of water molecules per sulfonic acid group. The density of PFSA surfactants and polymers is known to be ρPFSA = 2 g/cm3. 2.2. Instrumental Methods. SAXS. Small-angle X-ray scattering measurements were performed on a laboratory camera and on ID-02 and BM02 beamlines at the European Synchrotron Radiation Facility, Grenoble, France. The incoming X-rays wavelengths were 1.54, 0.995 and 1.03 Å, respectively. The sample-to-detector distances was varied on each instrument to select the Q-range, where Q is the momentum transfer: [0.02−0.7 Å−1] (lab), [0.001−0.4 Å−1] (ID2), and [0.05−0.7 Å−1] (BM02). Hermetic cells with mica windows were used, and online data corrections were applied. Isotropic 2D scattering patterns were measured due to powder averaging of PFSA locally oriented monodomains. They were radially averaged to obtain the 1D I(Q) scattering spectra. PFG-NMR. Pulsed-field gradient nuclear magnetic resonance (PFGNMR) was used to determine the self-diffusion coefficient of water, DS, as a function of the hydration rate and the molecular design of PFSA materials. The measurements were carried out at room temperature with a low field analyzer Bruker Minispec operating at 20 MHz for 1H nucleus. The NMR probe was equipped with a gradient coil that allows a magnetic field gradient up to 4 T m−1. The PFG sequence was the spin-echo sequence one.30 The water selfdiffusion coefficient, DS, was calculated from the attenuation generated by the PFG-NMR experiment of the nuclear magnetization echo according to the equation

Figure 1. PFSA materials designed for a bottom-up study: surfactants (PFHS, PFOS, PC); copolymers (Copol-1464, Copol-593) and Nafion. Atoms are colored as C (gray), F (blue), O (red), S (yellow), H (white), and I (purple). The equivalent weight (EW) is defined as the weight of dry polymer per mole of charges; the values range from 400 (highest density of charge) to 1464 g/equiv (lowest density of charge).

constraints of real commercial materials, an alternative bottomup strategy is proposed in this article. Our approach relies on the detailed investigation of the structure−transport relationship in genuine perfluorinated materials of increasing complexity. The main objective is to simplify, control, and tune the confining PFSA host matrix topology while keeping similar interactions at the hydrophobic/hydrophilic interface. The basic properties of both the hydrophobic perfluorochains and the superacid head groups define the very nature of PFSA compounds. We selected five molecules: two commercially available surfactants, e.g., perfluorooctane (PFOS) and perfluorohexane (PFHS) sulfonic acids, one surfactant mimicking the pendant chain of Nafion: F2CF-O-CF2CF(CF3)-O-C2F4-SO3H, labeled as PC, and two branched copolymers with fluorinated pendant chains similar to that of Nafion: C6F13[(CH2CF2)4.1(PC)]nI labeled as Copol-1464 and [(CH2CF2)2.33(PC)]n labeled as Copol-593. The perfluorinated sulfonic acid (PFSA) surfactants represent an interesting class of cationic surfactants because they possess a high chemical stability and outstanding properties as emulsifiers and detergents.24−26 The PFOS and PFHS, although highly toxic, bioaccumulable, and persistent,27,28 have been widely used in the textile chemistry industry, as surfactants in paints and detergents applications and as firefighting foams. Their properties originate from highly phase-separated nanomorphologies29 resulting from the superacidity of the sulfonic head groups and the high hydrophobicity of the perfluorinated tails and backbone. The PC surfactant and branched copolymers compounds were synthesized on purpose for the present study. The chemical architectures of the various materials, including that of benchmark Nafion, are schematically displayed in Figure 1. Their equivalent weights (EW), i.e., the molecular weight per sulfonic acid mole, range from 400 (PFHS) to 1464 g/equiv (Copol-1464) (Nafion’s EW is 1100 g/equiv). We report here the detailed characterization of the five “model” self-assembled systems, systematically compared to the corresponding properties of Nafion membrane, by means of small-angle X-ray scattering (SAXS), quasielastic neutron scattering (QENS), and PFG-NMR techniques. The first section describes the synthesis of Nafion pendant chain and the copolymers. The second section is devoted to the structural characterization of

E(g )/E(g = 0) = exp(− γ 2g 2δ 2Ds(Δ − δ /3))

(1)

where γ, g, and δ stand for the gyromagnetic ratio of the investigated nucleus (here 1H, γ = 2.6752 × 104 rad s−1 T−1), the intensity of the 6167


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reaction. Additionally, the hydrolysis of the −SO2F groups was confirmed by the 19F NMR spectrum in which no signal was observed at +45 ppm characteristic of the sulfonyl fluoride group. The absence of this peak indicates that the hydrolysis reaction yield was close to 100%. Synthesis of Copol-593. The same experiment as above was carried out without any chain transfer agent, as reported in refs 35−38 using 15 g (0.234 mol) of VDF, 26.12 g (0.059 mol) of PFSVE, and 0.68 g (3.91 mmol) of TBPPi in 70 mL of 1,1,1,3,3-pentafluorobutane as the solvent. After evaporation of the solvent, the viscous liquid was precipitated from cold pentane, filtered, and dried. An orange waxy poly(VDF-co-PFSVE) copolymer was obtained in 65% yield. SEC analysis of this copolymer indicated a molecular weight of 25 000 g/ mol. The 19F NMR spectrum is given by Figure S3.

magnetic field gradient, and the duration of the gradient pulse, respectively. The diffusion time, Δ, was chosen as in the 0.01−0.02 s range depending on the values of DS to be measured. Accordingly, the mean-square displacements of water molecules in PFSA systems varied between 1 and 50 μm2 depending on both water diffusivity and diffusion time. Consequently, for all measurements, the water diffusion was observed at very large spatial scale as compared to the typical size of the hydrophobic/hydrophilic phase separation. QENS. Quasielastic neutron scattering experiments were performed on the IN5 time-of-flight (ToF) spectrometer at the Institut Laue Langevin, Grenoble, France. An incoming wavelength of 8 Å was used, yielding an elastic resolution of 20 μeV and a Q-range of 0.08−1.4 Å−1. Surfactant gels were spread in between two PTFE sheets (10 μm thick) and then enclosed in a 1 mm thick aluminum flat container, which was sealed with indium wire to ensure tightness. Thin PTFE sheets were used to prevent degradation of the cell due to superacidity of the surfactants. Nafion membranes were enclosed in a cylindrical cell containing saturated solutions of salt selected to set the relative humidity (RH) in the 11−100% range. In all cases, the sample transmissions were checked to be higher than 95%, such that multiple scattering effects are negligible. The raw S(θ,t) data, where t and θ stand for the time-of-flight and the scattering angle, respectively, were transformed into S(Q,ω), where ω is the energy transfer, using the usual ILL routines. The quasielastic spectra S(Q,ω) were then corrected from sample container and detector efficiency. A flat piece of vanadium was used to measure the experimental resolution function. 2.3. Synthesis of PFSA Materials. Synthesis of Pendant Chain. The perfluoro(4-methyl-3,6-dioxaoct-7-enesulfonic acid) surfactant, labeled as pendant chain (PC), was synthesized to mimic the pendant chain of Nafion membrane. A 20 mL solution containing Li2CO3 (0.451 g, 0.0066 mol) in 25 mL of methanol was dropwise added into a solution composed of perfluoro(4-methyl-3,6-dioxaoct-7-enesulfonyl fluoride) (3.012 g, 0.0067 mol) in 30 mL of methanol. After total addition, the solution was stirred at 50 °C for 3 h. Afterward, the solution was filtered, then methanol was evaporated, and the residue was dried at 40 °C under 20 mmHg vacuum for 4 h. Then, the residue was stirred in HCl solution (3 M) at room temperature for 12 h, and after lyophilization, a yellow solid was obtained in 85% yield. Synthesis of Copol-1464. Iodine transfer copolymerization (ITP) of vinylidene fluoride (VDF) with perfluoro(4-methyl-3,6-dioxaoct-7enesulfonyl fluoride) (PFSVE) was initiated by tert-butyl peroxypivalate (TBPPi) in 1-iodoperfluorohexane and 1,1,1,3,3-pentafluorobutane (C4H5F5) as the chain transfer agent and solvent, respectively. TBPPi was shown to be an efficient initiator in radical copolymerization of VDF.31,32 The initial [VDF]0/[PFSVE]0/[C6F13I]0/[TBPPi]0 molar ratio was 14.46/3.11/1.00/0.33. The reaction was carried out at 74 °C for 6 h. After reaction and purification by precipitation, the resulting poly(VDF-co-PFSVE) copolymer was characterized by NMR spectroscopy, showing the expected normal head-to-tail VDF−VDF dyads by 1H (at 2.5−3.3 ppm, Figure S1) and 19F NMR (at −92 ppm, Figure S2) and quite a few reversed head-to-head or tail-to-tail additions, linked to ITP regarded as a controlled technique of radical polymerization,33,34 and the characteristic signals of the C6F13 endgroup. Size-exclusion chromatography (SEC) analysis of this copolymer indicated a molecular weight of 20 000 g/mol (in DMF with PMMA standards), and the yield of this ITP was 58%. The hydrolysis of the sulfonyl fluoride group of these poly(VDF-coPFSVE) copolymers in a slightly basic medium needed to be controlled to generate the sulfonic acid functions necessary for the proton conductivity. As the VDF groups are very sensitive to basic conditions, the conversion of the −SO2F groups in the PFSVE units into −SO3Li in the terpolymer was achieved using Li2CO3 (2 mol equiv for 1 mol of SO2F group) at room temperature. IR spectroscopy (not shown) was used to monitor the hydrolysis reactions and to assess the presence of sulfonic acid and unreacted fluorosulfonyl groups. The O−F frequencies (bending modes) centered at 1470 and 810 cm−1 were absent after the hydrolysis reaction while the −S(O)(O)OH stretching vibrations of the sulfonic acid groups at 3377 and 1026 cm−1 were present, indicating the successful hydrolysis

3. RESULTS AND DISCUSSION 3.1. Self-Assembly and Nanostructure. The nanostructure of acidic PFOS has been reported previously.29 The nanostructures of PFHS, PC, and copolymers Copol-1464 and Copol-593 were studied as a function of the hydration level. The 1D SAXS spectra measured at room temperature are displayed in Figure 2. All PFSA spectra show evidence of (i) a broad correlation peak at low PFSA concentrations, (ii) the presence of Bragg peaks upon dehydration, related to the

Figure 2. SAXS spectra of (a) PFHS, (b) pendant chain molecule, (c) Copol-1464, and (d) Copol-593 at different PFSA volume fractions. The spectra are shifted in intensity for clarity. 6168


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the d-spacing is associated with the mean separation distance between polymer aggregates5,41 or ionic domains.4,23 The swelling law is plotted in Figure 3 using a logarithmic

formation of highly ordered structures, and (iii) a continuous shift of the correlation peaks toward high Q values associated with the shrinking of aqueous domains when the PFSA concentration increases. PFHS surfactants form micelles below the liquid-gel transition found at a surfactant volume fraction, ΦPFSA, near 48% (Figure 2a). Upon increasing the surfactant concentration, hexagonal and lamellar self-assembled structures are successively formed, as revealed by the sequence of first-, second-, and third-order sharp Bragg peaks located at Q0, √3Q0, 2Q0, and Q, 2Q0, respectively (Figure 2a). A similar behavior was previously reported for PFOS acidic surfactant.29 The shorter hydrophobic chain of PFHS with respect to PFOS does not modify the sequence of mesomorphous phases but shifts the concentrations of the phase boundaries toward lower water concentrations (in PFOS the liquid gel transition was found at ΦPFSA > 32%). In the case of Nafion pendant chain molecule (Figure 2b), a direct transition from the micellar solution to a lamellar organization is observed, without the presence of any intermediate hexagonal or cubic phases, which are commonly formed in surfactant systems to accommodate the continuous decrease of surface curvature from elongated/spherical micelles to lamellae.39,40 The lamellar PC phase was observed over a large hydration range, from ΦPFSA = 52% up to 93%, with wellmarked first- and second-order Bragg peaks (Figure 2b). Interestingly, the Copol-1464 exhibits a more complex structure: both broad correlation peaks (Copol-1464 b) and sharp Bragg peaks (Copol-1464 s) are present in the SAXS spectra over a large range of concentrations (Figure 2c). At ΦPFSA ≤ 29%, only the broad correlation peak is present in the Q-range [0.082−0.11 Å−1], with a second-order peak observed at Q ∼ 0.22 Å−1. Above this concentration, for ΦPFSA ≥ 43%, these broad features progressively disappear while Bragg peaks typical of lamellar organization appear and shift toward high-Q values. Such a behavior could be related to the coexistence of two phases: one due to a micellar-like organization (broad peak) and the other one to the growing of a lamellar phase (Bragg peak). Finally, the SAXS spectra of the Copol-593 (Figure 2d) evolve similarly to those of the pendant chain upon water content increase, yet a much sharper micellar-like correlation peak is found at low PFSA concentration (ΦPFSA ≤ 54%), showing that the system is highly organized even at high hydration. The so-called ionomer peak identified as the signature of the nanoscale separation between ionic and hydrophobic domains of Nafion is typically recorded in the Q-range [0.07−0.2 Å−1] (for Nafion volume fraction ranging between 95% and 30%5) which is quite comparable to the first-order peak position of all the PFSA materials (Q-range [0.1−0.3 Å−1]). Thus, the mean characteristic sizes of nanodomains in the model PFSA materials evidenced by SAXS experiments are quite comparable to the well-known typical size of the nanostructure of Nafion membrane5 (2−8 nm). A more detailed description of the various PFSA nanostructures, including that of Nafion membrane, can be gained by extracting the swelling law from the SAXS data. We define the d-spacing as d = 2π/Q0, Q0 being the position of the first-order structural peak. The d-spacing corresponds to the mean correlation distance between scattering objects. These objects are likely to be micelles, cylinders, or lamellae as commonly observed in self-assembling surfactant systems. Note that in the case of the Copol-1464 two correlation distances can be defined as two sets of peaks coexist. In Nafion membrane,

Figure 3. A log−log plot of swelling laws, d versus ΦPFSA, of hydrated surfactants and polymers. The mean correlation distance d is derived from the position of the first-order Bragg peak. Dashed lines represent the theoretical swelling law of cylinders (or ribbons) and solid lines the swelling law of lamellae. Data for Nafion are extracted from ref 5. d0 is the extrapolated d value at zero water content. Blue areas indicate the two main transition concentrations.

representation of d versus ΦPFSA, which has been usefully used in polymer membranes to highlight the existence of various regimes associated with the dilution of scattering objects.5 In this representation, the mean characteristic distance between scattering objects of invariant shape and size is expected to increase according to a power law upon dilution, with a slope of −1/n, with n = 1 for the 1D swelling of lamellar structures of polymer aggregates/ionic domains and n = 2 for the 2D dilution of elongated polymer objects, e.g., cylinders or ribbons in water. As seen from Figure 3, all the investigated PFSA materials exhibit the same behavior at high hydrations, i.e., the d-spacing increase upon dilution according to a power law ΦPFSA−1/2. This behavior appears to be independent of the chemical architecture. It can be related to the 2D dilution of 1D objects (such as rods or ribbons). Such a hypothesis is supported by the SAXS spectra of PFOS and PFHS, for which we observed SAXS features commonly attributed to cylinders organized into hexagonal phases (Figure 229). Moreover, such a behavior was also reported in overswollen Nafion membranes (Figure 35) and was interpreted as the continuous dilution of elongated hydrophobic aggregates. Interestingly, a transition toward a low-water content behavior is systematically observed at a hydration concentration that depends on the PFSA material. Two main transition concentrations can be noted: ΦPFSA ∼ 0.48 for the ionomer peak of Nafion membrane and the broad peaks of copolymers; ΦPFSA ∼ 0.64 for first-order peaks of PFHS, PC, and PFOS and also for the sharp peak of Copol1464. In the low water content regime, Nafion membrane and Copol-593 swelling law is linear with a slope slightly smaller and equal to −1, respectively, consistent with the existence of a local lamellar ordering (Figure 2d). The Copol-1464 is characterized by the coexistence of two characteristic lengths in the system. The two correlation peaks exhibit an opposite behavior upon dehydration: the broad one (Copol-1464 b) vanishes while the sharp one (Copol-1464 s) appears (Figure 6169


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In a hexagonal structure, dw(Φw) can be defined as the mean distance between two PFSA cylinders, calculated as (Figure S4)

2c). The larger correlation length (Copol-1464 b) is invariant upon dehydration with a mean value of d = 60 Å. It may indicate a reorganization of hydrophobic aggregates (i.e., a reduction of the larger hydrophobic aggregates (Copol-1464 b) with hydration), which could explain the invariant d-size at low water content. In contrast, the smaller length (Copol-1464 s) evolves upon dehydration, following a nice lamellar behavior characterized by a −1 slope in the dilution law. The −1 slopes indicate the 1D swelling of objects invariant in size upon hydration and continuously diluted. Accordingly, the characteristic size d0, obtained by extrapolating d at zero water content (Figure 3), corresponds to the size of the hydrophobic aggregates. In Nafion membranes, d0 has been interpreted as the mean thickness of elongated ribbon-like hydrophobic aggregates, d0,Nafion ∼ 30 Å.5 In Copol-1464 and Copol-593 d0 is slightly shorter, 26 and 23 Å, respectively. In contrast, in the genuine surfactant systems we observe that a water content reduction results in a very moderate increase of the interlamellae distance that significantly deviates from the expected −1 law, although PFOS, PFHS, and PC surfactants unambiguously form well-defined lamellar phases29 (Figures 2a and 2b). This indicates that the hydrophobic domain’s size and shape invariance is not a valid assumption for those surfactants, therefore suggesting that these domains are evolving during the hydration process. Indeed, the hydrophobic thickness might reduce upon hydration due to a tilt and/or interpenetrations of the hydrophobic surfactant tails, while water intercalates within interlamellar domains. Further insights into the nanostructures can be gained at this stage by considering the hydration-dependent mean size of aqueous domains, dw(Φw). For PFSA with invariant domain sizes, i.e., for Nafion, Copol-1464 s, and Copol-593, dw(Φw) can be calculated as dw(Φw) = d(Φw) − d0, d(Φw) being the dspacing at Φw. For PFSA showing clear signatures of lamellar or hexagonal structures, dw(Φw) can be calculated on the basis of simple topological considerations. In a lamellar structure, dw(Φw) and d(Φw) correspond to the thickness of the hydrophilic lamellae and interlamellae hydrated domains, respectively (Figure 4). The aqueous domains sizes are directly correlated to the geometry and can be calculated as follows:

d w(Φw )lam = d(Φw )Φw

d w(Φw )hex =

2d(Φw ) (1 − 3

Φpfsa )

(3)

Figure 5 shows the evolution of dw versus Φw for all investigated PFSA materials, namely polymers (Figure 5a)

Figure 5. Aqueous domain sizes, dw(Φw), calculated for (a) Nafion, Copol-1464 s, and Copol-593 and (b) PFOS, PFHS, and PC. dw(Φw) is calculated with (i) d − d0 for polymers (invariant size of hydrophobic domains with hydration), (ii) eq 2 or eq 3 for lamellar and hexagonal phases of PFSA, respectively.

and surfactants (Figure 5b). A general finding is that dw increases continuously from few angstroms to several nanometers over the investigated hydration range. In the case of Nafion, at low hydration (Φw ≤ 0.45, Figure 5a), it is noted that d(Φw) − d0 = d(Φw) × Φw. This observation strongly supports the existence of well-defined hydrophobic ribbon-like aggregates forming flat hydrophobic/hydrophilic interfaces and locally arranged into semilamellar domains at the nanoscale.5,6 The same behavior is observed in both copolymers (Copol1464 s, and Copol-593) that show very similar dw, despite the important differences in EW. Nafion has the highest dw as compared to all other PFSA, highlighting the important swelling of the benchmark membrane, as also observed in Figure 3. Interestingly, the aqueous domain sizes of all surfactants display very similar water content dependence with very near absolute values (Figure 5b). Hence, the typical extension of ionic channels in PFSA self-assembled phases is found to be rather independent of the chemical details of the main chains. The total water (or PFSA) concentration primarily drives the self-assembly process as due to similar local interactions in the studied compounds. The ionic domains are found to be similar in size to those in PFSA polymers for Φw < 0.3 and smaller at higher hydrations. Therefore, at this stage, we have defined and

(2)

Figure 4. Schematic representation of the lamellar structure of PFSA materials evidenced at low hydration (Φw < 0.4). At a given hydration rate Φw, the d-spacing defined as d(Φw) = 2π/Q0, Q0 being the firstorder structural peak, corresponds to the mean thickness of hydrophobic lamellae (dPFSA) plus the ionic interlamellae thickness (dw). 6170


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Macromolecules synthesized a nice palette of PFSA materials with wellcontrolled long-range order morphologies. The ionic domains size can be set easily below or above a nanometer by adjusting the total water volume fraction. As a consequence, the PFSA molecules, which act as soft host hydrophobic confining matrixes, are well suited for further investigations of the dynamics of the confined protons and water molecules in between charged walls. 3.2. Water Dynamics at Molecular Scale. The local dynamics of hydrogen atoms (protons and water molecules) have been investigated by quasielastic neutron scattering (QENS) in the various self-assembled and polymeric PFSA materials. The dynamical structure factors S(Q,ω) were measured over an extended hydration range at room temperature. In earlier QENS studies, a deviation of water dynamics with respect to bulk water had been reported for Nafion membranes42−44 due to nanoscale confinement. Various diffusion models were employed to analyze the QENS spectra, e.g., single-Lorentzian analysis,44,45 the diffusion within a sphere model,42,46 and the Gaussian model for restricted translational motions.43,47 The Gaussian-model-based QENS study emphasized the complexity of the dynamics which was shown to develop over an extended time scale [0.1−500 ps]. The purpose of the present work is to compare the dynamics in a number of PFSA materials and to correlate the water diffusive properties to the nature and nanostructure of the host matrix. Here we focus on the time scale where restricted translational motions are expected, i.e., 0.1−50 ps, using ToF experiments. In that case, the one-Lorentzian approach is relevant as it already grasps the most prominent features of the dynamical properties of the solvent. ToF data were fitted in an energy window limited to −0.5 to 0.5 meV applying the following theoretical S(Q,ω): S(Q , ω) = DW[A(Q )L(ω) + P(Q ) + B] ⊗ Re(Q , ω)

Figure 6. Quasielastic neutron scattering (QENS) spectra S(Q,ω) of (a) Nafion membrane and (b) PFOS surfactant at hydration Φw = 0.22 and Φw = 0.18, respectively. The data are well reproduced over the Q range using the diffusion model of eq 4 (red line). The quasielastic Lorentzian component is highlighted (light blue area).

(4)

where DW, L(ω), A(Q), P(Q), B, and Re(Q,ω) stand for the Debye−Waller factor, the quasielastic Lorentzian component of half-width at half maximum (hwhm) Γ(Q), the quasielastic structure factor, the elastic contribution, a flat background, and the instrumental resolution, respectively. P(Q) contains contributions from (i) the coherently scattering atoms which are too slow for the time scale of the experiment (0.1−50 ps), i.e., the perfluorinated chains, (ii) the incoherent scattering of too slow molecular entities, and (iii) possibly the elastic incoherent structure factor associated with confined motions defined as 1 − A(Q). A flat background was considered to account for the presence of broad quasielastic signals of several meV arising from fast local motions of water molecules or for vibrations, an approximation which is justified within the narrow analyzed energy window. The Q dependence of typical spectra recorded on hydrated PFOS and Nafion is displayed in Figure 6. The QENS spectra recorded on surfactants and polymers exhibit similar shapes at same Q values, showing that the local proton dynamics are occurring on the same time scale. A broadening of the quasielastic line with Q is systematically evidenced and clearly indicates the existence of proton diffusive motions. The experimental S(Q,ω) data are satisfactorily reproduced by eq 4 over the available Q range, for all PFSA materials at all investigated concentrations (Figure 6). Typical variation of the HWHM, Γ, versus Q2 is plotted in Figure 7 (for PFOS and Nafion). The values of Γ are found to vary between

Figure 7. Half-width at half-maximum Γ of the quasielastic Lorentzian function as versus Q2, for different hydrations, Φw, in (a) Nafion membrane and (b) PFOS surfactant. At high Q, the Q dependence of hwhm is fitted with the random jump diffusion model (gray line). 6171


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Macromolecules typically 20 and 100 μeV in both PFSA materials, and a very similar Q dependence is found at similar hydrations. In Figure 7 two prominent features are highlighted: the presence of a lowQ plateau and the clear broadening effect at high Q, significantly enhanced upon increasing hydration. The Qdependent behavior in the high Q range indicates the presence of a diffusive process that is accelerated upon hydration. The constant width at low Q is the fingerprint of confined motions. The threshold value, Q*, between both regimes is shifted to high Q values when decreasing the water content. This plateau extension toward high Q relates to the confinement of protons in smaller regions.46,47 In the framework of the diffusion into a sphere or the Gaussian model, the mean values of Γ at low Q relate to the average time needed for exploring one confining domain. Here, the low-Q plateau level is found to be poorly hydration-dependent, with mean values of Γ ∼ 24 μeV corresponding to an average time of ∼30 ps. Therefore, when the water content is increased in the PFSA materials, it was observed that water molecules move faster in water droplets of increasing sizes, but the total residence time within a confining domain is not much hydration dependent and found to be ∼30 ps. Quantitative parameters can be extracted by analyzing the high-Q behavior using an appropriate diffusive model. Here, the Q dependence of Γ was analyzed with the random jump diffusion model48 which accounts for the high Q deviation to the standard Fick’s law: Γ(Q ) = D locQ 2/(1 + D locQ 2τ )

(5)

where Dloc is the diffusion coefficient and τ the mean jump time, associated with elementary proton hopping and/or water molecules reorientationnal events.49 The high-Q part of the hwhm Q-dependent profile is correctly reproduced using such a model in all PFSA, as exemplified in Figure 7 for Nafion and PFOS. The hydration dependence of τ and Dloc was obtained for all PFSA materials (Figures 8a,c). Beside the high-Q analysis, we are also interested in evaluating the confined motions (low-Q part), in particular the spatial extent or confinement size which is usually obtained from the so-called elastic incoherent structure factor (EISF). For incoherent scattering only and if all scatterers participate in the motion, the EISF is determined from the elastic intensity normalized by the total of elastic and quasielastic intensity. This is not convenient in our case due to non-negligible coherent scattering from the polymer, as already described above. We rather consider the integrated intensity of the quasielastic line, Iqel = A(Q) × DW, where A(Q) = 1 − EISF. A number of EISF functions has been reported in the literature to describe complex jump-like motions, diffusion on segments, circles, spheres, and/or reorientations of molecules. As previously shown in Nafion, the Gaussian model is adapted to a soft confinement situation; i.e., the molecular motions are not restricted to domains with impermeable boundaries. Therefore, the integrated intensities, Iqel, of the Lorentzian function were analyzed with the Gaussian EISF A(Q)gaus47 (Figure S5):

Figure 8. (a) Elementary jump time τ, (b) confinement size 2σ, and (c) diffusion coefficient Dloc obtained from the fits of the QENS spectra, plotted versus the confinement sizes dw. Dloc and τ of bulk water are represented for comparison. Dashed lines are guides for the eyes to highlight the general trends along hydration. The shaded areas highlight the low hydration regions characterized by steep variations and severe confinement.

and the theoretical one using the Gaussian EISF is obtained (Figure S5), confirming the relevance of the “soft” confinement Gaussian model and yielding the hydration-dependent values of 2σ (Figure 8b). After the simultaneous analysis of the QENS hwhm and Iqel, a full set of dynamical parameters (τ, 2σ, Dloc) was obtained for all the PFSA materials over the full range of concentrations. For a thorough comparison of surfactant and polymer behaviors, we adopt a representation of these parameters versus the nanoscale variable dw (Figure 8) to correlate the nanostructure to the dynamical properties. Data are also plotted vs the usual variables to quantify the water content, i.e., the water volume fractions Φw and the hydration number λ (Figure S6).

2 2

A(Q )gaus = 1 − e−Q σ

(6)

where σ represents the variance of the random Gaussian variable that defines the proton displacement. Typically, 2σ corresponds to the mean size of the soft confinement domain where motions are restricted at the time scale of the neutron experiment. A good agreement between the experimental Iqel 6172


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is noticed that (i) the general behavior upon hydration is recovered, but (ii) at high hydration the diffusion coefficient values of Nafion and PFSA are found to be comparable at comparable total water content. The usefulness of using either dw or Φw as the adequate parameter to extract general behaviors will be further discussed in the next section. 3.3. Water Dynamics at Micron Scale. To evaluate the impact of the soft confining matrix features (geometry, tortuosity, connectivity) on the water dynamics at a more macroscopic length and time scales, the self-diffusion coefficient of water molecules, DS, was determined by PFG-NMR in all PFSA materials over the full hydration range. The PFG-NMR technique probes the long-range diffusion of the species on typically the millisecond time scale and with a micronic resolution. Only one decaying exponential was measured in the free induction decay (FID) of all the samples, therefore showing that there is only one type of diffusive species, e.g., one type of water molecules. There was no noticeable change in the measured DS values when tuning the experimental sequence of the NMR, and accordingly, no dependence of the diffusion coefficient with the spatial resolution. Similar to the QENS results, an unconventional representation of DS was adopted to highlight its hydration dependence and the structure−transport correlation in PFSA compounds. In this view, the variations of DS versus the confinement sizes, dw, was first plotted (Figure 9a) and compared with the local diffusion coefficient Dloc. As for the QENS dynamical parameters, a similar trend is found for all the surfactants and the PFSA membranes: DS increases continuously with the water content, toward an almost bulklike behavior (DS ∼ 2 × 10−5 cm2/s at the highest hydration). At low hydration, all DS values concentrate along a unique welldefined master curve until a threshold of dw ∼ 6 Å. This confinement value is close to that corresponding to the appearance of a plateau-like behavior in the local jump time and diffusion coefficients determined by QENS. The local and the long-range diffusion coefficient variations directly highlight the detrimental impact of subnanometer confinement on the dynamics, although the effect is much more dramatic at the scale of the micron (DS is decreased by 2 orders of magnitude, whereas Dloc was only reduced by a factor 2, Figure 9a). These observations point out two features (i) the morphology of the PFSA matrix (nanostructure, ionic domains sizes, tortuosity, connectivity), which highly depends on the water content, plays a major role at low hydration in controlling the water dynamics, and (ii) the connectivity between ionic domains, and thus the micronic diffusion coefficient, is mostly driven by local confinement and more or less independent of the nature of the hydrophobic backbone and the local topology. For a given high dw, Φw is much lower in Nafion than in all other PFSA, and the resulting DS is lower. This observation suggests that at high water content Φw drives the micronic diffusion rather than the confinement sizes. To emphasize these findings, the same DS data versus the total PFSA content, ΦPFSA, was plotted (Figure 9b). This is a suitable parameter considering that PFG-NMR probes the global diffusion of water through the interconnected network of nanodomains. At low PFSA content, below typically ΦPFSA = 0.7, DS values for all PFSA samples nicely superimpose, as highlighted using a linear representation (inset in Figure 9b). The reduction of DS during the dehydration process is induced by the continuous replacement of water by PFSA surfactants or polymers. This effect, which is strikingly independent of the local geometry, mainly arises from the tortuosity imposed by

First of all, the dynamical behavior of protons and water molecules confined within the five model PFSA systems and Nafion membrane appear to be comparable, with values of Dloc, 2σ, and τ in the same range (Figure 8). In particular, the dynamical parameters extracted in lamellar surfactants and in low water content Nafion are very close, as shown previously in the case of PFOS by a multiscale QENS study.29 Next, it is observed that the general trend upon hydration is the same among all materials: as the water content increased, protons and water molecules are shown to be less confined and to move faster. The confinement size 2σ increases from typically 3 to 5.5 Å. Dloc typically increases from 1 × 10−5 to 2.3 × 10−5 cm2/s while the mean jump time τ decreases from 14 to 2 ps. At the highest hydration, Dloc and τ values are close to those of bulk water.50 The increase in Dloc upon hydration was noted to be moderate (factor 2) while τ decreases by a factor of 7. The characteristic time τ relates to elementary events as proton hopping and H-bonding reorientations which may significantly affect local rotational motions. The combination of several events as large angle jumps might be required to produce a mass diffusion of the water molecules according to a complex cooperative process within the H-bond network.49 Therefore, it is not straightforward to establish a direct relationship between the observed variables τ and Dloc. Interestingly, the three dynamical parameters reflect here a continuous and similar evolution for all PFSA materials, with a low hydration regime at dw ≤ 6 Å, characterized by steeper variations and severe confinement, and a high hydration plateau-like regime at dw ≥ 10 Å (mostly observable from the jump time profile). The sizes 2σ of dynamical confining domains, i.e., water droplets where proton diffusion is restricted, are comparable to dw at low water content, showing that the local mobility is highly correlated to the spatial nanoconfinement in this hydration region. After the threshold, the water droplets size saturate independently of the swelling of ionic nanodomains. It has to be underlined that dw is a timeindependent structural size while 2σ is a dynamical parameter that relates to the range of water−wall interactions. The presence of the charged surface effectively limits proton local mobility in the vicinity of charged headgroups, typically in the first hydration layer (below 5 Å). Strong effective interactions between sulfonates, hydronium ions, and water molecules must dominate at low Φw, as demonstrated in the case of Nafion51 and surfactants.52 Hence, efficient fast long-range diffusion is hampered by the presence of slow interfacial molecules formed when the ionic channel size is reduced. Clearly, it can be concluded that the local water dynamics is significantly reduced when the PFSA structures are forced to adopt a lamellar ordering (Figure 5). The local mobility of the species is highly restricted by subnanometer spatial confinement in between locally flat objects formed by the packing of perfluoro surfactants or perfluoro polymeric aggregates. Above the hydration threshold at dw ≥ 10 Å, the dynamical behavior reflects the higher ability of water molecules to diffuse within the well-connected network of swollen ionic domains. In the plateau-like regime, it was noted that the diffusion coefficient in PFOS is higher than that of Nafion at the same dw. Yet, a typical high-hydration dw is obtained for distinct total water contents, Φw, in these PFSA materials, suggesting that at high hydration not only the microscopic variable, e.g., confinement size dw should be considered for a thorough comparison of the PFSA behavior but also the macroscopic variable Φw. If we represent the local dynamical variables as a function of Φw (Figure S6), it 6173


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and the hydration number λ. Here, we introduce an additional hydration-dependent parameter: the mean size of water domains, dw. This new structural nanoscale parameter proved to be extremely useful to compare the structure−transport interplay in different PFSA materials as self-assembling surfactants, oligomers, and polymers. Indeed, as dw defines the water volume explored by water independently of the soft hydrophobic confining matrix topology, it is well-suited to explore the dynamical trends under severe confinement, i.e., at low water content. Though available hydration variables (Φw, ΦPFSA, λ, dw) are clearly related, it has to be underlined that comparing the various representations of the dynamical parameters (in particular DS) provides unique clues on the origin of restricted mobility of water inside ionic nanodomains. Some general behaviors were evidenced through the observation of two master curves at low and high hydration content. This result could only be assessed after comparing the dynamical parameters evolutions versus the local structural dw and the macroscopic ΦPFSA variables. In other words, there is no master curve that develops along the whole hydration range, as a consequence of the multiscale nature of transport mechanisms in the PFSA materials. Therefore, the introduction of the nanoscale structural variable dw is combined with standard Φw (or ΦPFSA) macroscopic quantification of the water content to gain a thorough understanding of the complex dynamical behaviors and the underlying driving forces. It was found that the ionic domains, which contain sulfonic acid side groups, protons, and water, are increasing in size upon hydration and typically expand over few nanometers in all PFSA compounds. Surfactants self-assemble in water forming a sequence of mesomorphous phases, including lamellar phases at high surfactant concentration. The presence of a local lamellar ordering was also evidenced for the synthesized copolymer model compounds. A detailed comparison of the swelling laws of the various materials with that of a Nafion membrane allowed us to draw sound conclusions about the microstructure of the membrane, which is still a highly debated issue. The results showed that Nafion, as all PFSA materials at low water content, swells according to a dilution law which is characteristic for lamellar structures. This supports the existence of flat hydrophobic/hydrophilic interfaces, as postulated earlier.5,6 The lamellar-like organization of concentrated perfluorosulfonic materials seems to be a very robust property, notwithstanding the obvious differences that polymer with side chains display with respect to short linear surfactants. This finding strongly indicates that Nafion structuration is driven by the same interplay of hydrophobic/hydrophilic and electrostatic forces as those driving the self-assembling process of surfactants in water. Our results support the dilution of elongated hydrophobic aggregates forming locally flat interfaces, with the ionic phase expanded within the interaggregate space. Therefore, this study strongly supports the ribbon-like model,5 in agreement with NMR relaxometry data suggesting a lamellar organization at low hydration.53 Moreover, we observed that the hydrophobic aggregates in Nafion do not significantly evolve in size and shape during the swelling process, in contrast to those of the surfactants. This can be ascribed to the constraints that the polymer backbone structure imposes to the side-chain interactions and organization compared to the less constrained internal chain mobility of surfactant systems optimizing the hydrophobic packing and minimizing the interfacial energy during water incorporation.

Figure 9. Water self-diffusion coefficient, DS, measured by PFG-NMR versus (a) dw and (b) ΦPFSA (inset: DS is represented on a linear scale; the black dashed line highlights the linear dependence of DS at high water content). DS of bulk water is indicated by the gray dashed line. The shaded areas highlight the two regions where similar DS values are found for all PFSA materials, using either dw (a) or ΦPFSA (b) as the relevant hydration variable.

charged obstacles. In other words, when the mean size of hydrated ionic domains exceeds the nanometer, the presence of charged walls primarily restricts the water diffusion, independently from the internal structure of the self-assembled hydrophobic objects embedded in the ionic medium. In turn, at high ΦPFSA (typically at ΦPFSA > 0.7), some variations are found and the master curve character is lost. In particular, Nafion exhibits higher DS value than surfactants do. This discrepancy arises from the mean size of the ionic domains, which are larger in Nafion than in PFSA surfactants for a given ΦPFSA (Figure 5), in particular at high hydration. Additionally, the sharp decrease in DS values is observed at ΦPFSA ∼ 0.77 and 0.83 for surfactants and Nafion, respectively. These concentration values correspond to the same mean size of ionic channels, dw ∼ 6 Å, confirming the prominent effect of subnanoscale confinement on the transport properties in the low hydration regime.

4. DISCUSSION The dynamical properties of water confined in PFSA materials strongly depend on their hydration level, which imposes the confining matrix topology and the mean size of water domains. The structure−transport interplay can be monitored using the usual hydration variables, i.e., the total water volume fraction Φw (or, equivalently, the total PFSA volume fraction, ΦPFSA) 6174


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the possible impact of chemical architecture, chain length, chain complexity, and flexibility, while keeping the prominent features driving the overall self-assembly process in water, phase separation, and interactions, i.e., the same highly hydrophobic backbone and the same superacid head groups. The geometry of the confining matrices, the mean ionic channel size, and the dilution laws were determined from SAXS experiments performed over an extended range of PFSA concentrations. The water mobility was quantified by combining on a molecular level QENS experiments and on a micrometer scale PFG-NMR measurements. The structure−transport correlations in both self-assembled genuine surfactant phases and in more complex polymer hydrated microstructures were elaborated by carefully analyzing the variations of dynamical parameters like diffusion coefficients, relaxation times, and confinement domains as a function of hydration. The effect of hydration was scrutinized at both the macroscopic and the microscopic scales by systematically analyzing the water dynamics as a function of either global quantities (water or PFSA volume fractions) or nanostructural variables which account for the mean size of generic ionic domains. This study provides the first comprehensive correlation between nanostructure and transport properties using a multiscale bottom-up approach. The local dynamics of water molecules are shown to be affected by the global water content and the mean size of ionic domains, the latter being definitely controlled by the former. High proton mobility is achieved, independently from the chemical details of main-chain and side-chain backbones, as soon as the confinement overcame the value of 1 nm, i.e., three water layers. New advanced polymer electrolyte membranes should be designed to fulfill such primary condition to optimize proton conductivity, while finding the best compromise in terms of swelling and mechanical properties.

The similarity between Nafion and perfluorinated model compounds was further highlighted by the dynamics of the confined fluid. The transport properties and the water dynamics were determined by a subtle balance between geometrical restriction to diffusion and interactions with functional groups. In this work, the PFG-NMR measurements revealed that the water mobility is driven by the total volume fraction of water at high water content (Φw ≥ 0.3). Details of the PFSA molecular organization, as confinement geometry and mean separation distances, do not matter in this regime, therefore indicating that highly hydrated PFSA materials can be considered as an assembly of shapeless hydrophobic obstacles embedded in a continuous ionic phase. In contrast, at low Φw, typically below the 0.3 threshold value, the diffusive properties of water at all relevant scales are fully determined by the local confinement size. When the ionic domains width remains below the nanometer, all the PFSA show strikingly similar variations of both the local molecular-level water diffusion coefficient, Dloc, and the micronic water self-diffusion coefficient, DS. Such a severe confinement in the lamellar ordering region induces a significant reduction of mobility witnessed through the steep decrease of diffusion coefficients. The local diffusion coefficient of water molecules is reduced by roughly a factor 2 on the whole hydration range, and DS is drastically reduced by 2 orders of magnitude. This discrepancy originates from the distinct length scale probed by QENS on the one hand and PFG-NMR on the other. Clearly, the local diffusion coefficient quantifies the mean mobility at a subnanometer scale. Therefore, it is defined after probing short trajectories likely to develop in the vicinity of ionic groups. In contrast, DS determined over several microns represents an average value accounting for the diffusion inside one domain, from a domain to a neighboring one, from one locally oriented domain to another one connected via ill-defined grain boundary regions. Overall, the existence of ionic domains larger than a nanometer appears to be the most stringent condition for enhanced fluid mobility and therefore efficient proton conductivity. Important variations of the local density of charges decorating the hydrophobic/hydrophilic interfaces, from moderate (Copol-1464) to average (Nafion) up to high (PFHS), do not apparently affect the local motions, therefore suggesting that charge interactions are not primarily piloting the dynamics. Similarly, it can be noted that the degree of freedom associated with chain dynamics (surfactants vs polymer side chains) does not significantly help in providing facilitated pathways for diffusion within soft domains. As a consequence, the main parameter to carefully consider for an optimized dynamical behavior is the total water content required for swelling the domains above a value of 1 nm.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information on the instrumental methods (NMR analysis); experimental descriptions of the syntheses of PFSA and Copol-1464 and their NMR characterizations; schematic representation of the hexagonal structure of PFSA material; assessment of confinement size dw and calculation of the radius of the cylinders and the molecules lengths in the different surfactant and copolymers of this study; water dynamics at molecular scale and quasielastic integral, Iqel, of Nafion at 3 hydration levels (Figures S1−S7). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00770.

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5. CONCLUSIONS Selected perfluorosulfonic acid materials were used as welldefined soft confining matrices mimicking the phase-separated morphology of state-of-the art PFSA ionomer membranes used as electrolytes in fuel cells. Because of the extreme complexity of commercial Nafion-like membranes, this bottom-up approach advantageously provides a set of model nanostructures suited for a systematic investigation of the dynamics of water molecules confined within ionic nanodomains intercalated between perfluorosulfonic aggregates. The structural and dynamical properties of both polymeric (including commercially available Nafion membrane) and nonpolymeric (surfactants) materials were systematically compared to disentangle

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Akzo Nobel (Compiègne, France), Arkema, and Solvay Fluor for supplying some starting materials. We thank the ILL and ESRF for beamtime allocation. Q.B. was funded by ILL-PhD and UJF Grant. 6175


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