Structure of Membranes for Fuel Cells: SANS and SAXS Analyses of

Publication Date (Web): July 29, 2013. Copyright © 2013 ... This reorganization is attributed to an ionic domain percolation on a large scale. The th...
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Structure of Membranes for Fuel Cells: SANS and SAXS Analyses of Sulfonated PEEK Membranes and Solutions Gérard Gebel* LITEN and Laboratoire SPrAM, UMR 5819 CEA-CNRS-UJF, INAC CEA-Grenoble, 17 rue des martyrs, 38054 Grenoble cedex 9, France S Supporting Information *

ABSTRACT: The microstructure of sulfonated poly(ether ether ketone) (sPEEK) membranes was investigated by combining small-angle neutron and X-ray scattering techniques (SANS and SAXS) for large and low water contents, respectively. The ion-exchange capacity, the water content and the nature of the counterion were varied for a better understanding of the membrane microstructure. SAXS and SANS contrast variation experiments reveal a significantly more complex structure than it is commonly believed with a delicate balance of the contrasts. At low water content, the structure can be depicted by the presence of small ionic clusters and larger more or less connected core−shell domains between crystallites for low values of the sulfonation degree. The first step of the swelling process corresponds to the filling without significant structural changes of the porosity created by the solvent evaporation during the casting process. The second step is associated with a major structural reorganization induced by a large increase of the membrane water content over a small range of temperature. This reorganization is attributed to an ionic domain percolation on a large scale. The third step corresponds to the swelling of lamellar ionic domains around the crystallites as revealed by the study of the dilution laws and of the structure of sPEEK ionomer dispersions. The sizes of the ribbon-like polymer particles were determined. They do not depend linearly with the membrane ion content suggesting a nonhomogeneous distribution of the ionic groups along the polymer chain during the sulfonation process associated with the semicrystalline nature of the polymers. Finally, the effect of the degradation in oxidative media on the membrane structure is shown to correspond to an increase of the membrane water content.

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INTRODUCTION Proton exchange membrane fuel cells (PEMFC) are considered as very promising power sources for automotive, portable, and stationary applications.1 PEMFCs are stacks of elementary cells based on membrane−electrode assemblies. The standard proton exchange membranes (PEM) used as solid proton conductor electrolyte, gas separator between anodic and cathodic compartments and support for the electrodes are perfluorosulfonated ionomers such as Nafion commercialized by DuPont. In order to increase the fuel cell operating temperature from 80 to 120 °C and decrease the production costs, many efforts have been devoted to the development of alternative membranes including sulfonated aromatic polymers due to their outstanding thermomechanical stability. Sulfonated poly(ether sulfone)s,2 polyimides,3 poly(ether ketones),4,5 and polybenzimidazoles6,7 were synthesized, characterized, and investigated as PEM for fuel cells. Sulfonated polymers have been first prepared by direct sulfonation of commercial polymers5 and then by the polycondensation of sulfonated monomers multiplying the number of possible structures.8 Among high performance polymers, poly(ether ketones) are a complete set of commercial materials which structure and properties can be tuned varying the number of ether and ketone bridges in the repeat unit each of them being spaced by phenyl groups (PEK, PEEK, PEKK, PEEKK, ...). Sulfonated © 2013 American Chemical Society

poly(ether ketones) are considered as very promising materials for fuel cells and some of them are even commercially available such as the Fumapem membranes from Fumatech. The transport properties were shown to be related to the degree of sulfonation (or ion-exchange capacity, IEC), the membrane water content and the ion distribution along the polymer chains.9 As observed for most of the sulfonated arylene polymers, large values of ion content are usually necessary to attain the required level of proton conductivity (typically in the 0.1 S/cm range). However this improvement is obtained at the expense of the mechanical properties on the swollen state because of excessive water uptakes especially at elevated temperatures. The water uptake was shown to diverge at a critical temperature which occurs in the range of the fuel cell operation.4,5,10,9 This is a major drawback since the membrane dimensional stability in water is one of the critical issues for fuel cell operation over extended periods under cycling conditions. Most of the studies focused on the water sorption properties, proton conductivity, thermal stability, and fuel cell performance but, surprisingly, only limited information is available concerning the membrane microstructure despite it is Received: February 14, 2013 Revised: June 17, 2013 Published: July 29, 2013 6057

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commonly believed that it controls most of the relevant membrane properties. The understanding of the microstructure of proton conducting membranes and its evolution as a function of water and ion content is considered of primary interest for the development of efficient alternative membranes to Nafion. The ionic groups embedded in the hydrophobic polymer matrix phase separate on a nanoscale to form ionic domains usually called clusters. This nanophase separation is evidenced by the observation of the so-called “ionomer peak” in the small-angle X-ray and neutron scattering spectra (SANS and SAXS, respectively). Despite three decades of intensive research, the structure of Nafion membranes is still subject to debate. The ionic domains first depicted as isolated spherical clusters were then modeled by a network of spherical domains connected by small channels in order to explain the high values of the ionic conductivity at low water contents.11 The SAXS and SANS analyses over an extended angular range,12 the structural study of Nafion dispersions13 and of Nafion membranes under deformation14,15 revealed a locally highly anisotropic structure interpreted either by parallel cylindrical ionic domains16 or polymer ribbons.12 With regard to the thorough studies on Nafion, the structural study of alternative membranes appears as very limited. SAXS spectra were only recorded as a function of one of the synthesis parameters (degree of sulfonation, nature of the monomers, cross-linking, and addition of inorganic species). The ionomer peak position was then compared to the data obtained for Nafion membranes relating its position and width to the size of the ionic domains and to the quality of the phase separation. In the case of sulfonated poly(ether ketones), this approach mainly lead to confusing conclusions. Most of the authors observed a broad ionomer peak10,17−21 while some others either do not observe any maximum,22−24 two peaks25 and more recently three different maxima.26 Indeed it can be argued that the results were obtained with different polymers (sPEEK or sPEEKK) prepared either by postsulfonation or direct synthesis based on sulfonated monomers, with various ion distributions along the polymer chain (random or block copolymers), different membrane preparation conditions and different counterions (acid or stained by cesium cations). In order to better understand the structure-transport properties relationships in sulfonated polyaromatic membranes, a more complete study of the sPEEK membrane microstructure is still necessary. In the present work, the structure of sPEEK membranes prepared by postsulfonation will be investigated by SANS and SAXS. The main parameters will be varied (sulfonation degree, water content and nature of the counterion). The structural evolution will be studied over an extended range of water content up to solutions. Contrast variation either by SANS or SAXS will be used to better characterize the scattering curves. Additional results on the structure in solution and the effect of oxidative attack in hydroperoxide solutions will be presented.

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Figure 1. Chemical structure of sPEEK membranes. standard procedure adjusting the IEC value by the reaction time and temperature.4,5 The IEC values 1.1, 1.3, 1.6, and 1.9 mequiv/g correspond 35%, 42%, 53% and 64.5% as degree of sulfonation, respectively (in other words a monomer is sulfonated on average every 2.9, 2.4, 1.9, and 1.55 monomers along the polymer chains). The samples were reacidified by soaking the membrane in a 1 M HCl solution at room temperature for at least 12 h. Small-angle neutron scattering (SANS) experiments were performed at the Laboratoire Léon Brillouin (LLB, Saclay, France) on the PAXE spectrometer. Two configurations were used with a 1.5 and 5 m as sample-to detector distances was and 5 and 10 Å as incident wavelength, respectively to cover a q range from 0.01 to 0.5 Å−1 with q = 4π(sin θ)/λ where θ is the scattering angle and λ the neutron incident wavelength. The detector was tilted with respect to the incident axis to increase the accessible angular range. Cells with quartz windows and spacers were used for the study of swollen membranes. The usual corrections were applied to SANS data (detector and incident flux normalization, background subtraction, transmission correction, membrane thickness, ...). Small-angle X-ray scattering (SAXS) experiments were performed using the European synchrotron source (ESRF, Grenoble, France) on the CRG D2AM, ID2 “high brilliance” and ID13 “microfocus” beamlines. The incident wavelength was monochromatized at 13 keV. The beam size was adjusted to 500 × 500 μm2 full width at halfmaximum for the experiments performed on the D2AM, 50 × 50 μm2 on the ID2 beamlines and to 1 × 1 μm2 on the ID13 microfocus beamline. For classical SAXS experiments, the samples were studied in cells with Kapton windows with an excess of water to avoid variation of the membrane hydration during the experiments. The microfocus beamline was used to investigate the in-plane membrane structure analysis. A 1 mm wide and 2 cm long slice of membrane was cut and stuck between two small blocks to be parallel to the incident beam. The samples were then studied at room humidity (RH≈40%). For SANS and SAXS contrast variation experiments, the sPEEK membranes were neutralized with different salts by immersing the samples in a concentrated (>1 M) chloride solution of the desired cation (sodium, potassium, cesium, protonated or deuterated tetramethylammonium) for at least 3 h followed by a careful rinsing with a large amount of pure water (18.2 MΩ). Water swollen samples were obtained immersing the membrane samples (typically 10 mg) in water at various temperatures for at least 12 h. The membrane water uptakes were determined gravimetrically weighing the samples before and after immersion in hot water. For temperatures larger than 100 °C, the swelling experiments were conducted in an autoclave. The water uptakes were calculated with respect to the dry membrane weight. sPEEK membranes were aged in concentrated hydroperoxide solutions. Membrane samples were soaked in a 30% H2O2 at 80 °C. This aging process is analogous to the famous Fenton’s test27 except that the hydroperoxide is thermally decomposed instead of adding some iron salts. For an aging time longer than 12 h, the membranes became very brittle and could not be handled anymore.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION The water uptakes of sPEEK membranes in acidic form and presenting various IECs were first measured as a function of the swelling temperature (Figure 2). As observed for Nafion,12 the water uptakes obtained at elevated temperatures remain at room temperature due to an irreversible deformation of the polymer matrix. For very high levels of sulfonation degree (i.e., 1.9 mequiv/ g), the sPEEK samples easily dissolve at temperatures larger

sPEEK membranes have the generic chemical formulas given in Figure 1 with a succession of sulfonated and nonsulfonated repeat unit units (in Figure 1 only a sulfonated repeat unit is presented). sPEEK membranes are primarily characterized by their ion exchange capacity (IEC). The 1.1, 1.6, and 1.9 mequiv/g sPEEK membranes were kindly provided by the AIME laboratory (Montpellier, France) and the 1.3 and new 1.6 mequiv/g sPEEK were provided by Fumatech Company. These polymers were obtained by direct sulfonation in concentric sulfuric acid according to the 6058

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(□),

Figure 3. SANS spectra of SPEEK membranes immersed in liquid water at room temperature for various ion exchange capacities.

1.6 (○), and 1.3 mequiv/g (Δ)

Figure 2. Water uptakes of 1.9 SPEEK membranes immersed in liquid water at different temperatures.

respectively. PEEK polymers and oligomers are known to be semicrystalline polymers.35,36 The crystallinity index was known to decrease as the sulfonation degree increases. Above a sulfonation degree of 50% (IEC > 1.5 mequiv/g), the resulting membranes are then amorphous as revealed by DSC.37 The absence of both the low angle maximum and crystallinity in the 1.6 and 1.9 sPEEK polymers supports the attribution of the low angle scattering peak to a long period between crystallites. Both WAXS and DSC experiments were performed with 1.1, 1.3, and 1.6 mequiv/g sPEEK samples and the data confirm that the polymer crystallinity decreases as the ion content increases (see Supporting Information). The membranes were then studied by SANS equilibrated in liquid water at different temperatures. The results are presented for the 1.3 mequiv/g sPEEK membrane (Figure 4).

than 60 °C. For lower IECs, the water uptakes present quasi linear behaviors when plotted on a semilogarithmic representation with two discontinuities at 85 and 95 °C. The first discontinuity suggests the occurrence of a structural first order transition. sPEEK membranes are known to exhibit a very high glass transition temperature which increases with the sulfonation degree (typically around 200 °C).4 However, it has been shown28 for sulfonated aromatic polymers that the plasticizing effect induced by water sorption can significantly decrease the glass transition temperature (e.g., down to 100 °C for sulfonated polysulfones). Carbone et al.29 have evidenced a transition around 95 °C on a second heating scan by differential scanning calorimetry (DSC) attributed to the occurrence of irreversible swelling. A DSC experiment was run for a 1.6sPEEK membrane in the presence of an excess of water (data not shown) and the thermogram appears completely flat in the region of interest (50−200 °C). The swelling transition could originate from a structural reorganization of the ionic domains induced by the volume increase consecutive to water uptake. The second transition at higher temperatures appears as a hindering of the water uptake that refrain membrane dissolution. The system then looks like a cross-linked polymer gel. A chemical cross-linking could have been generated by the direct sulfonation process on the aromatic rings in concentrated acids inducing a partial polymer degradation and interchain cross-links.30 For very large water uptakes, the dry weight of the samples was also measured after the experiments and no significant weight loss compared to the dry weight prior to the immersion in water was noticed confirming the absence of partial membrane dissolution at the contrary to the data measured for Nafion.31 It supports the hypothesis of the occurrence of chemical cross-linking in sPEEK membranes during the sulfonation process. These cross-links should mostly take place in the ionic domains limiting the water uptake since the membranes can be dissolved in organic solvents. The SANS spectra of membranes on acidic form equilibrated in liquid water at room temperature are shown in Figure 3 as a function of the ion exchange capacity varying from 1.1 to 1.9 mequiv/g. The SANS spectra clearly exhibit two scattering maxima for the 1.1 and 1.3 mequiv/g membranes while only one is observed for the 1.6 and 1.9 mequiv/g SPEEK membranes. Analogously to the data obtained with Nafion membranes,32−34 the large angle maximum around 0.2 Å−1 (30 Å) and the low angle one around 0.04−0.05 Å−1 (125−150 Å) could be attributed to the ionic and crystalline domain distribution,

Figure 4. SANS spectra of 1.3 mequiv/g sPEEK membranes equilibrated in water at different temperatures.

The effect of the increase of the membrane water content is a shift toward smaller angles of both the ionomer peak and the crystalline component revealing a continuous increase of the distance between the scattering objects. The simultaneous shift of the two peaks increasing the membrane water content has already been observed for Nafion extruded membranes with a variation very close to be proportional to the polymer volume fraction.12 The peak positions were thus plotted as a function of the polymer volume fraction (ϕp) calculated with a polymer density of 1.4 g/cm3 (Figure 5). The polymer density was determined using a gas pycnometer on swollen samples assuming the volume additivity of the aqueous and polymer phases (see Supporting Information). For large water contents (i.e., membranes equilibrated at temperatures larger than 80 °C), the results are similar to the ones observed for Nafion membranes with a strong correlation of the two peaks and a linear behavior when plotted as a double logarithm with a slope 6059

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nearly identical compared to the wet sample despite a significant difference in water content. The absence of change in the peak position and the increasing level of scattered intensity can be interpreted as the filling of an existing porosity by water molecules without any significant structural modification. The presence of porosity can be attributed to empty spaces generated by the solvent evaporation during the solution-casting process and to the low polymer chain mobility at moderate temperatures hindering the polymer matrix relaxation. Similar behaviors were observed for the different sulfonation degrees. The presence of porosity was confirmed by the measurement of the membrane volume change by gas pycnometry (see Supporting Information). At the contrary to Nafion membranes, the water desorption by dry helium in the gas pycnometer is not accompanied of a significant volume change. These pores which are likely to correspond to the empty cores of the ionic clusters are not large enough to be evidenced by electron microscopy. Despite a low number of data points, the dilution law obtained with the 1.9sPEEK membranes appears also linear as a function of the polymer volume fraction similarly to the 1.3sPEEK membrane. In addition, an almost q−2 power law of the scattered intensity is observed at low angles (Figure 3). Despite the low angular range and number of data points on which this behavior is observed, the combination with the dilution law suggests a locally lamellar structure. Therefore, the lamellar structure is apparently intrinsic to sPEEK polymers and not a direct consequence of the semicrystalline nature of the polymer since the 1.9sPEEK membrane is fully amorphous. However, the natural tendency of these polymers to crystallize probably generates some polymer chain alignments that favor the formation of lamellar structures and the crystallinity at least at low ion contents. The membrane becomes soluble at temperatures higher than 65 °C. A membrane sample was dissolved in water at 75 °C under stirring to ensure a complete and homogeneous dissolution. A series of dilution by a factor of 2 was then applied to the stock solution. The SANS spectra were recorded and plotted after normalization by the polymer volume fraction (Figure 6). As usually observed for ionomer solutions in water,38 the SANS spectra exhibit a scattering maximum when plotted on a linear scale (see Supporting Information) which position and intensity reveal the presence of colloidal particles. The position scales as a −1/3 power law with the polymer volume fraction similarly to polyethylene comethacrylique acid (Surlyn) ionomer dispersions which were shown to form elongated polymer particles with a finite length.38 At the contrary, Nafion solutions exhibit a dilution law with a −1/2 exponent.39 This difference is mainly due to the length of the elongated particles which appear very long in Nafion solutions.39−41 The average distance between the sulfonic groups randomly distributed along the polymer chain supposed to be fully extended is 23 Å for 1.9 mequiv/g sPEEK (sulfonation degree equal to 64.5%). Since the base polymer is not soluble in water, the polymer chains form aggregates and at least one of the particle dimensions should be close to 23 Å. The largest dimension cannot exceed the interparticle distance since a −1/3 dilution law is obtained (the polymeric aggregates should freely rotate around all their main axes). This maximum dimension is then 150 Å. It should be noted that a SAXS analysis of sPEEK solutions was already published by Carbone et al.,29 they concluded to the presence of random coils but the sPEEK was dissolved in DMAC which is a polar solvent that

Figure 5. Evolution of the ionomer and crystalline peaks as a function of the polymer volume fraction in 1.3 mequiv/g sPEEK membranes swollen in liquid water at different temperatures.

very close one characteristic of a lamellar structure. As expected, the second transition at 95 °C attributed to a limitation of membrane swelling due to the presence of cross-links has no visible effect on a structural point of view. At intermediate water contents (i.e., in the range 15 to 20% v/v), the evolution of the peak position is also a quasi linear behavior with a slope around 12. Such a slope has to be interpreted as the occurrence of significant polymer reorganization upon swelling corresponding to a percolation process of neighboring ionic domains to form larger ones. The first swelling transition (Figure 2) thus appears as a structural change from clusters to lamellae as a results of a percolation process. It follows that the structure of sPEEK membranes strongly depend on the swelling history. For large IEC values (1.9 mequiv/g), the ionomer peak for a membrane equilibrated in liquid water at room temperature is more intense and located a lower q values compared to the data obtained with the other sulfonation degrees (Figure 3). This is certainly attributable to the presence of a larger amount of water in the membrane (Figure 2). A dry sample was analyzed but the SANS signal in the absence of water was too low to observe a structural signature due to a lack of contrast (Figure 6). A sample equilibrated at room humidity (typically RH = 50%) presents an ionomer peak with an intermediate intensity between the dry and wet samples. Surprisingly, its position is

Figure 6. SANS spectra of 1.9 mequiv/g sPEEK membranes at different water contents and solutions obtained by dilution of a membrane dissolved at 75 °C. The solid line is the calculated spectrum of a finite length cylinder of 13.5 Å as radius and 45 Å as semilength. 6060

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100 °C that the ionomer peak is no more visible. The SANS spectra can then be reproduced assuming a finite length cylindrical shape with a radius of 21 Å and a semilength of 300 Å revealing a huge increase of both the radius and the length compared to the 1.9-sPEEK. A similar analysis was not possible with the highly swollen 1.3-sPEEK membrane due to the presence of the two scattering maxima even for a sample swollen at 150 °C (Figure 4). However, the shape of the underlying scattering curve is compatible with elongated particles with a radius around 30−40 Å. The large increase of the particle size decreasing the IEC value should be understood as a nonhomogeneous sulfonation along the polymer chains due to the postsulfonation procedure. The probability to sulfonate a monomer is larger in a close vicinity of an already functionalized monomer which favors the penetration of the sulfonating agent within the hydrophobic bulk polymer. This effect should be enhanced by the presence of crystallites in which the sulfonating agent can hardly penetrate. It is likely to mainly occur for short sulfonation times (i.e., small sulfonation degrees). Superimposable ionomer peaks are surprisingly obtained for the 1.1, 1.3, and 1.6 mequiv/g sPEEK membranes when plotted on an absolute scale of intensity (Figure 3). In first approximation, the scattered intensity can be considered as the product of a contrast term, the number of scattering objects, the square of their volume, a form factor relative to the shape and size of individual scattering objects and finally an interference term relative to their spatial distribution within the polymer matrix. Since the three first terms are constant as a function of the scattering angle, a scattering maximum can be generated either by the shape of the objects (form factor) or by their spatial distribution (interference term). The hypothesis of a complex shape generating a maximum in the form factor was often considered in the ionomer field.43 However, this hypothesis assumes the presence of isolated ionic domains dispersed in the polymer matrix,43,44 which obviously contradict the outstanding membrane conducting properties requiring a full percolation of the ionic domains.39 The interference origin modulated by the form factor is now commonly preferred for PEM membranes. A similar position, width, and intensity for the ionomer peak suggest a similar spatial distribution of similar objects. It follows that the microstructure cannot be fully homogeneous. The structure could present either anisotropic features along the membrane thickness as previously observed for sulfonated rigid polyimide membranes45 or a heterogeneous structure with some ion-rich and ion-poor phases consecutive to a nonhomogeneous sulfonation process as often claimed in ionomer systems.46 It is difficult to conclude definitively and additional experiments were conducted on partially humidified samples in order to study samples with a similar number of water molecules per SO3 groups (λ values). Indeed, sulfonated polyaromatic membranes with various ion content usually present identical water sorption isotherms when expressed as λ values.3 SAXS was preferred to SANS because of the low signal observed in SANS with partially hydrated samples. SAXS experiments were conducted with cesium neutralized samples in order to increase the contrast between the ionic domains and the polymer matrix. The experiments were performed on the microfocus beamline of ESRF (ID13) with a very small ( H+ > K+) confirm the delicate balance of the contrast. The most probable interpretation is then the presence of an ionic layer at the interface between the polymer matrix and the water pools. A small but clearly visible shift of the ionomer peak toward small angles is observed increasing the electron density of the counterion (Figure 9 and 10). The origin of this shift cannot be an effect of water content since the water uptake slightly decreases as the size of the counterion increases. The counterions are most likely condensed at the polymer−water interface of the ionic domains instead of being homogeneously distributed within the ionic domains. This effect known as the counterion condensation originates from the electrostatic attraction between the negatively charged sulfonate anions and the positively charged cations. It follows that the cations have the propensity to be mainly located very close to the anion. It is also enhanced by the mutual electrostatic repulsions between the negatively charged sulfonate anions all located at the polymer interface and the positively charged cations dispersed in the ionic domains. These electrostatic repulsions are screened by the counterion condensation which finally minimizes the total energy of the system. The apparent form factor describing the shape of the ionic domains underlying the interference term is then modified due to the contrast variation induced by the change in the electron density of the counterion layer. This unusual observation is probably due to the delicate

Figure 9. SAXS spectra obtained with a 1.3 mequiv/g membrane neutralized with sodium, potassium, and cesium ions equilibrated in liquid water. The straight line corresponds to a q−2 power law.

The cesium neutralized membrane equilibrated with liquid water exhibits only one scattering maximum corresponding to the ionomer peak at q = 0.21 Å−1 (30 Å as interobject distance). At the opposite, the spectrum obtained with the Na + neutralized membrane exhibit a well-defined crystalline peak and a hardly detectable ionomer peak. As expected, the two peaks are clearly visible for the membrane on potassium form but with an intermediate level of scattered intensity. The increased contrast for the cesium form enhances the ionomer peak signal and matches the signal from the crystalline peak indicating the crystalline component present the same scattering length density than the ionic domains in this particular configuration. In such a case, the small-angle upturn should not be influenced by the presence of a strong crystalline component and can be analyzed to extract information on the shape of the ionic domains. The scaling law of the small-angle upturn in intensity is q−2 which strongly suggests a lamellar structure.47 In the presence of Na+ ions, the swollen ionic domains almost match the amorphous polymer. These results clearly indicate that the analysis of SAXS spectra obtained for PEM membranes should be carefully conducted and contrast variation experiments varying the counterion or the water content appear necessary to propose reliable conclusions. A new series of experiments was conducted on the ID2 beamline at ESRF with membranes equilibrated in liquid water varying the ion exchange capacity and the counterion (Figure 10). 6063

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balance of the contrast between the different phases that enhance the effect of the counterion condensation. Additional experiments were then performed by SANS on tetramethylammonium (TMA) neutralized membranes with solvent contrast variation. Such experiments have been shown to be very valuable to extract some structural information studying Nafion membranes,48 sulfonated trifluorostyrene membranes (called BAM3G from Ballard advanced materials)49 and lamellar phases of perfluorocarboxylic surfactant.50 sPEEK membranes were neutralized with protonated or deuterated TMA cations and the SANS spectra were recorded in D2O, H2O, and mixtures of D2O and H2O. A careful examination of the SANS spectra for the 1.9 mequiv/g sPEEK neutralized with protonated TMA and swollen in light and heavy water (Figure 11) suggests that the scattering maxima is less marked and

Figure 12. Schematic representation of the three swelling steps for the 1.3 mequiv/g sPEEK membrane: (a) step at low water content and low temperature, (b) percolation transition in the 85−95 °C range of temperature, and (c) step for very large water content and temperatures.

created by the solvent evaporation during the casting procedure. The large values of the polymer matrix glass transition hinder the chain relaxation upon drying. The SAXS data with membranes on cesium form suggest the coexistence of very small ionic domains (typically 5 Å) and larger ones with a core−shell structure with a typical size around 25 Å. The small domains are not observed by SANS due to a lack of contrast. The increase of water content is limited to an increase of the scattering intensity without any shift of the ionomer and matrix peaks. (ii) When increasing temperature up to 80 °C, the water content rapidly increases and a percolation transition occurs between all the ionic domains. The ionomer and matrix peaks are shifting very fast toward small angles (typically with an exponent larger than 10 for the dilution law, peak position as a function of the polymer volume fraction). It is also likely that the crystallinity index is altered due to the disruption of some crystallites during this transition since a significant shift of the crystalline component also observed. (iii) For temperatures larger than 80 °C, the water content increases to very high values and the polymer structure appears as connected polymer lamellae (or ribbon) phase surrounded by water. At very large water content, the membrane then looks like a cross-linked polymer gel formed of connected ribbons. The ribbon thickness is 22, 35, and 50 Å for the 1.9, 1.6, and 1.3 mequiv/g sPEEK membranes, respectively. It confirms the nonhomogeneous ion distribution along the polymer chains as the ion content decreases and the ribbon length also significantly increases. Finally, a series of 1.6 mequiv/g sPEEK samples were aged in an oxidative media (H2 O 2 30%) at 80 °C. At such temperatures, the H2O2 decomposition is thermally activated and it was not necessary to use some iron salt to catalyze this decomposition as in the famous Fenton test.27 Indeed, the presence of multivalent counterions (Fe2+ and Fe3+) should significantly modify the membrane swelling properties and the contrast between the ionic domains and the polymer matrix. The high concentration of H2O2 (30% instead of the usual 3%51) was chosen to accelerate the degradation in such a way that the global concentration of oxidizing agent can be considered as constant during the aging process limited to few hours. The membranes became very brittle and almost impossible to handle after 12 h of this harsh treatment. The effect of aging on SANS spectra appears clearly as a shift of the

Figure 11. SANS spectra of 1.9 mequiv/g sample with protonated TMA in H2O and D2O.

slightly shifted in D2O. It reveals a nonhomogeneous distribution of the ionic species within the ionic domain as already observed by SAXS varying the nature of the counterion. In the 1:1 D2O and H2O mixtures, no signal was observed over the entire angular range disclosing that this mixture corresponds to the match point where the contrast is zero. PEEK polymers are known to be semicrystalline polymers with lamellar polymer crystals35,36 similarly with Nafion and at the contrary to most of the hydrocarbon membranes such as sulfonated polysulfones which are purely amorphous. It is likely that the crystallinity is not the actual driving force properties for defining the membrane microstructure since the semicrystalline 1.3 and amorphous 1.9 mequiv/g sPEEK membranes exhibit similar local structures. The semicrystalline nature of the polymer backbone just traduces the natural propensity of the polymer chains to assemble and form polymer particles excluding the ionic groups. It is also possible that the presence of crystallinity impacts the homogeneity of the sulfonation process. Indeed, it is expected that the sulfonation should mainly take place in the amorphous zones of the polymer. The evolution of the water uptake as a function of temperature combined with the associated SAXS and SANS data suggests that the membrane swelling can be divided in three successive steps (Figure 12). (i) The first step of water uptake from the dry to the liquid water-equilibrated sample corresponds to the filling of existing porosity within the membrane without major structural modifications. This porosity is likely to be 6064

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of water content and in solutions is also very informative. The structure in solution and at large water contents was described by elongated objects which dimensions depends nonlinearly on the ion exchange capacity suggesting a nonhomogeneous sulfonation along the polymer chains. These elongated particles are thought to be polymer ribbons as previously proposed for Nafion. The contrast variation by SANS with TMA neutralized membranes and by SAXS using different counterions suggests a nonhomogeneous counterion distribution in the ionic domains but the low values of the signal when varying the contrast do not allow quantitive analysis. Finally, the oxidative degradation in hydroperoxide solutions has major effects on the SANS spectra revealed an increase of the membrane swelling at a local scale.

ionomer peak and an increase of its intensity (Figure 13). This behavior is very similar to an increase of the membrane water

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ASSOCIATED CONTENT

S Supporting Information *

Density measurements, wide-angle X-ray scattering, differential scanning calorimetry, and small-angle scattering data. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 13. SANS spectra of the 1.6 mequiv/g sPEEK membrane aged in 30% H2O2 at 80 °C.

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content by a hydrothermal treatment. Indeed, the spectrum after 2 h treatment is very close to the one obtained in pure water at 80 °C, revealing that the effect of the oxidative attack is mainly limited to the membrane water swelling at 80 °C. At the contrary, after 6 h treatment, the SANS spectrum appears similar to the one obtained for a membrane swollen 12 h at 90 °C. The oxidizing solution penetrates the ionic domains and induces polymer chain scissions.51,52 The resistance to swelling of the polymer matrix then decreases and the water content increases. However, it becomes very brittle after an oxidizing degradation as a consequence of numerous polymer chain scissions while the membrane looks like a cross-linked polymer gel after a hydrothermal treatment at high temperature. As previously indicated only scarce structural studies of hydrocarbon membranes have been published and up-to-date only one structural study refer to membrane degradation.3 Sulfonated polyimide membranes are sensitive to hydrolyis inducing polymer chain scissions in the ionic domains. However, the combined SANS and SAXS analysis does not reveal major structural evolution and particularly no evolution of the ionomer peak position was observed. The degradation is then a decrease of the SAXS signal due to the elution of the ionic species out from the ionic domains and an increase of the SANS signal since these ionic species were replaced by water molecule increasing the contrast.3 The significant difference between the effect of aging in sulfonated polyimide and sPEEK materials can be attributed to the specific structure of both membranes and especially the fact that polymides are block copolymers.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author would like to thank the LLB and the ESRF for the beam time allocation and José Teixeira, Richard Davies and Cyril Rochas for their help as local contact. The author is also indebted to Jacques Rozière for kindly providing sPEEK samples and Olivier Diat, Manuel Maréchal, Govind K. Prajapati and Stéphanie Pouget for their help in the different measurements.

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REFERENCES

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CONCLUSIONS The combined SAXS and SANS studies of sPEEK membranes as a function of the water and ion content reveal a complex microstructure with a delicate balance of contrast. The signals not only depend on the nature of the radiation but they are also highly sensitive to the nature of the counterion and the membrane water content. It mainly explains some discrepancies in the literature about the structure of sPEEK membranes. Some possible misinterpretations can be avoided combining different sets of experiments. It provides new insights on the complex structure of these membranes. As previously shown with Nafion membranes the structural study over a wide range 6065

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