Cross-linked Sulfonated Poly(ether ether ketone) by Using Diamino

Mar 10, 2011 - Kun Si , Ryszard Wycisk , Daxuan Dong , Kevin Cooper , Marianne Rodgers , Paul Brooker , Darlene Slattery , and Morton Litt. Macromolec...
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Cross-linked Sulfonated Poly(ether ether ketone) by Using Diamino-organosilicon for Proton Exchange Fuel Cells Marie J. Kayser, Marc X. Reinholdt,† and Serge Kaliaguine* Department of Chemical Engineering, Laval University, 1065 avenue de la Medecine, Quebec, QC G1 V 0A6, Canada ABSTRACT: Fuel cells are at the battlefront to find alternate sources of energy to the highly polluting, economically and environmentally constraining fossil fuels. This work uses an organosilicon molecule presenting two amine functions, bis(3-aminopropyl)-tetramethyldisiloxane (APTMDS) with the aim of preparing cross-linked sulfonated poly(ether ether ketone) (SPEEK) based membranes. The hybrid membranes obtained at varying APTMDS loadings are characterized for their acid, proton conductivity, water uptake, and swelling properties. APTMDS may be considered as an extreme case of silica nanoparticle and is therefore most advantageously distributed within the polymeric matrix. The two amine groups can interact, via electrostatic interactions, with the sulfonic acid groups of SPEEK, resulting in a double anchoring of the molecule. The addition of a small amount of APTMDS is enhancing the mechanical and hydrolytic properties of the membranes and allows some unfolding of the polymer chains, rendering some acid sites accessible to water molecules and thus available for proton transport.

I. INTRODUCTION Proton exchange membrane fuel cells (PEMFC) are among the most promising electrochemical devices for convenient and efficient power generation. The electrolyte is an ionic polymer membrane bearing strong acid sites, generally sulfonates (SO3H)1 and is identified as the proton exchange membrane (PEM). The proton conductivity of the membranes depends on temperature, concentration, and strength of the acid sites and the membrane’s hydration.2,3 Currently, the most commonly used electrolytic membranes are perfluorinated copolymers such as Nafion,4 which have high hydrolytic and oxidative stability and excellent proton conductivity. However, the high cost of Nafion (ca. $800 U.S./m2)5 limits the widespread commercialization of PEMFC. Moreover, the constraining chemistry of fluorine is going against the requirement for environmental protection. Several nonfluorinated polymeric materials have been investigated as replacements for Nafion due to low cost and ease of synthesis.6-13 Among them, membranes based on a sulfonated aromatic poly(ether ether ketone) (SPEEK) have demonstrated to be particularly promising for fuel cell application.9,14-20 They possess good thermal stability, appropriate mechanical strength, toughness and high proton conductivity, which depend on their degree of sulfonation (DS). However, while the proton conductivity increases with the degree of sulfonation, the hydrolytic and mechanical properties of SPEEK tend to be progressively deteriorated at increasing DS.21 Above an 80% DS value, the membrane becomes excessively hydrophilic, which results in an important swelling, followed by its eventual dissolution in water. Several studies have shown that cross-linked SPEEK membranes were much less susceptible to swelling than r 2011 American Chemical Society

noncross-linked SPEEK and comparable to commercial Nafion in terms of their mechanical strength stability and proton conductivity.16,19 In a previous work,22 we have reported that in the case of a high DS (ca. 80%), the grafting of silica particles with (3aminopropyl)dimethylethoxysilane (APDMS) has a significant impact on the composite SPEEK/silica nanoparticles membranes’ water swelling. Whereas the proton conductivity remains high (>210-2 S cm-1), the grafting of the particles is obviously drastically decreasing the absorption of water, even for low particles loadings; amine-grafted particles indeed play the role of a cross-linker. Using aminopropylalkoxysilanes to graft zeolites or silica particles in acidic polymers such as polyimide is a current practice in industrial membranes production.23-25 Moreover the use of covalent linkers such as diols in modifying SPEEK polymers was already studied in our group.16,26 These studies have shown that this practice is not free of drawbacks; the chemical interaction between the acid site and the alcohol function results in some decrease in proton density and proton conductivity. Moreover cross-linking is not the result of a direct interaction of one diol molecule with two acid sites, but rather to the grafting of two diols, which give rise to further etherification. As a consequence, the thermal stability of the cross-linked materials is lower than that of noncross-linked ones. The present work deals with the use of a new cross-linker, namely an organosilicon graft presenting two amine functions in Received: August 17, 2010 Revised: December 23, 2010 Published: March 10, 2011 2916

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Scheme 1. Structure of SPEEK (x Being the Degree of Sulfonation)

Scheme 2. Schematic Representation of APTMDS

the aim of preparing cross-linked SPEEK-based membranes. Various amounts of bis(3-aminopropyl)-tetramethyldisiloxane (APTMDS) were dispersed into SPEEK polymer to prepare hybrid matrix PEM type membranes, with the objective of studying the effect of this addition on conduction, acidic, water uptake and swelling properties of the SPEEK-based membranes. APTMDS may be considered as the smallest possible cluster of amine grafted silica nanoparticle.

vacuum at 60 °C for two days, at 90 °C for two more days, and finally at 120 °C for one day. The thickness of all resulting membranes was in the range of ca. 30-40 μm. 5. Determination of the Ion-Exchange Capacity of the Membranes. The ion-exchange capacity (IEC) of pure SPEEK membranes and hybrid SPEEK/APTMDS membranes was determined by acid-base titration. Three samples of each membrane of ca. 20 mg weight were kept in 60 mL of a 10-3 M NaOH aqueous solution for 1 day; 10 mL of the solution was then back-titrated with 10-3 M HCl aqueous solution using phenolphthalein as the indicator. 6. Water Uptake and Swelling. With the aim of determining the water uptake and the swelling behavior, the membranes were first dried at 100 °C for 24 h, and their weight and their length were measured. They were then immersed in deionized water at room temperature for 24 h. Then the wet membranes were wiped out with blotting paper and quickly weighed and their length measured. The water uptake of the membranes was calculated with reference to the weight of the dry specimen ! Wwet - 1  100 water uptake ¼ Wdry

II. EXPERIMENTAL METHODS 1. Materials. Poly(oxo-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK; typical Mn = 10 300; typical Mw = 20 800) and N,N-dimethylacetamide (DMAc; 99 wt %) were obtained from Aldrich Chemical Corp. Bis(3-aminopropyl)tetramethyldisiloxane (APTMDS; g99.9%) was obtained from Gelest Inc. and sodium hydroxide and chlorhydric acid (both 10-2 N) were purchased from Laboratoire Mat (Quebec (QC), Canada). All reagents were used without any further purification except for N,N-dimethylacetamide, which was filtered through a Whatman 0.02 μm inorganic membrane. 2. PEEK Sulfonation. Typically, 20 g of PEEK was dissolved in 800 mL of concentrated (95-98%) sulfuric acid and vigorously stirred at room temperature under flowing argon. The chemical structures of the initial and SPEEK are shown in Scheme 1. The duration of the reaction was varied from 30 to 63 h to obtain the desired DS. To stop sulfonation reaction, the polymer solution was decanted into a large excess of ice cold water under continuous mechanical stirring. The suspension containing the SPEEK was then centrifuged and residual sulfuric acid was removed by dialyzing the recovered polymer in deionized water using dialyzing tubing polymer until reaching the pH of water. The resulting suspension was centrifuged again and the polymer was dried at 60-80 °C. 3. Determination of the SPEEK Polymer DS Values. The 1 H NMR spectra of the SPEEK samples were recorded using a Varian Unity Inova 400 MHz spectrometer at a resonance frequency of 399.95 MHz, using a 5 mm probe. A 45° pulse and a 2 s recycle delay were used, and 16 transients were accumulated during the acquisition. For each analysis, a 5 wt % polymer solution was prepared in deuterated dimethylsulfoxide (DMSO-d6) and the chemical shift of tetramethylsilane (TMS) was used as the internal standard reference. The DS was determined by integration of distinct aromatic signals as described previously.17,19 4. Membranes Preparation. Pure SPEEK membranes and hybrid SPEEK/APTMDS membranes were prepared by solution casting. The dry SPEEK polymer was first dissolved in dimethylacetamide (DMAc) to form a 10 wt % SPEEK solution, and various amounts of APTMDS (Scheme 2) were then added to the solution. The resulting mixture was stirred for a few minutes and then cast onto a glass plate. The cast membranes were dried under ambient conditions for two days and then cured under

where Wwet and Wdry represent the weight of the wet and dry membranes, respectively. The swelling was reported as a length percentage as follows ! Lwet - 1  100 swelling ¼ Ldry where Lwet and Ldry are the lengths of the wet and dry membranes, respectively. 7. Proton Conductivity Measurements. The membranes proton conductivity was measured by the AC impedance spectroscopy technique, using a system based on a Solarton 1260 gain phase analyzer, over a frequency range of 10-107 Hz. A sample of the membrane with dimensions ca. 15 mm  8 mm was clamped between two blocking stainless steel electrodes. Before the measurements, to be hydrated and stabilized the sample was placed in the measurement cell with a relative humidity of 100% during 24 h. Impedance measurements were performed at controlled temperature. The proton conductivity (σ) of the samples in the longitudinal direction was calculated from the impedance data, using the relation d R¼ RS where d and S are the distance between the electrodes and the surface area of the sample, respectively, and the resistance R of the membrane was derived from the low intersect of the high frequency semicircle on a complex impedance plane with the Re(Z) axis. 2917

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Figure 1. Room-temperature proton conductivity of the SPEEK based membranes as a function of graft content.

III. RESULTS The substitution of the PEEK backbone by sulfonic acid groups is performed in concentrated sulphuric acid and is taking place on the hydroquinone section of the polymer chain, which is activated for electrophilic substitution by the ether linkage present in the backbone. The proton conductivity of ionomeric membranes depends on the number of available acid sites and their capacity to generate a proton by dissociation in water. The amount of water present within the membrane, that is, water uptake, is significant, since the acid functionality of the polymer is dissociated by water molecules and which also participate to the proton transport. Moreover, high levels of water uptake may result in membrane fragility and dimensional changes leading to mechanical failures, eventually even leading to the solubilization of the membrane in water at elevated temperature. Thus knowing the relationship between the SPEEK polymer DS value and the membranes water uptake is critical. SPEEK membranes with a DS value lower than 45% exhibit a poor conductivity,19 typically less than 10-5 S cm-1. The 67% DS was chosen because it represents the best compromise to get a high proton conductivity while maintaining a low water uptake.18,19,27 The 80% DS was also selected because the conductivity of this polymer membrane is relatively high and also with the aim of investigating the water swelling limits of a sample that may even be soluble in water if not modified by any filler.19,28 Figure 1 reports the proton conductivity, measured at room temperature, of SPEEK/APTMDS hybrid membranes with DS values of 67 and 80%, and for APTMDS loadings up to 29.3 wt %. The conductivity is increasing with the DS value, as expected. Additionally, the evolution of the conductivity with the graft loading is similar for the two DS values. The pristine 67% DS value SPEEK membrane shows a conductivity of 7.17  10-3 S cm-1. This value is enhanced with the addition of a small amount of graft (ca. 3.1 wt %) up to 1.13  10-2 S cm-1, which is about two-third higher than the original conductivity value. Increasing the graft loading above this value contributes to a decrease of the conductivity to a value of ca. 6.78  10-3 S cm-1 for a loading of 5.7 wt %, which is near the initial conductivity, and to a value of ca. 2.15  10-3 S cm-1 for a loading of 8.8 wt %, which is less than one-third of the initial

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Figure 2. Room temperature water uptake of the SPEEK-based membranes as a function of graft content.

Figure 3. Room temperature swelling of the SPEEK-based membranes as a function of graft content.

conductivity. Finally, above 17.1 wt % of graft the proton conductivity is close to zero (e6.34  10-6 S cm-1). As in the case of membranes prepared with the 67% DS SPEEK, the addition of a small amount of graft to a membrane prepared with the 80% DS SPEEK is leading to an initial increase of the conductivity, which is then decreasing with further addition. However, this decrease is more pronounced in the case of the 80% DS. Thus, the original conductivity of 3.91  10-2 S cm-1 observed for the pristine SPEEK is enhanced to 4.81  10-2 S cm-1 for an APTMDS loading of ca. 3.0 wt %, which is about one-third higher than the original conductivity value. Then, for a graft loading of ca. 5.0 wt %, the membrane exhibits a proton conductivity of 3.18  10-2 S cm-1, which is slightly under the initial conductivity, and then for a graft loading of ca. 8.7 wt %, the conductivity value is about 1.78  10-2 S cm-1, that is, less than half of the original conductivity value. Finally, above a graft loading of 21.95 wt %, the proton conductivity is close to zero (e1.02  10-6 S cm-1). Figures 2 and 3 show, respectively, the water uptake and the swelling of the same SPEEK/graft hybrid membranes, obtained at room temperature. Both variables are intrinsically correlated, and thus the curves on both sets of figures are behaving similarly. 2918

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The Journal of Physical Chemistry B In general, one can easily observe that the increase in graft loading leads to a decrease of the membranes water uptake and swelling, whatever the DS value; however, the phenomenon seems to be more pronounced in the case of the highest DS value, i.e., 80%. As expected, the membrane water uptake and swelling increase also with the DS value. In the case of the membranes elaborated with the 67% DS SPEEK, the pristine polymer membrane is capable of absorbing about 31% of its weight in water, and the resulting swelling length is about 10.6% higher than the initial value. The addition of the APTMDS graft is considerably modifying this absorption behavior. Indeed, a 13.3 wt % loading is decreasing the initial absorption of ca. 60% (water uptake decreasing from ca. 31 to ca. 12.5 wt %), and the initial swelling of ca. 64% (from ca. 10.6 to ca. 3.8 lg %). For a 29.3 wt % graft addition, the water uptake is decreasing of ca. 77% (from ca. 31 to ca. 7 wt %) and the swelling is decreasing of ca. 81% (from ca. 10.6 to ca. 2 lg %). In the case of the 80% DS SPEEK membranes, as in the case of the 67% ones, the addition of a small amount of graft is enough to drastically decrease the absorption of water. Initially, a pristine SPEEK membrane is capable of absorbing ca. 60 wt % of its own weight in water and its swelling is about 21.6 lg %, which is about twice that of a 67% DS membrane. It is considered that beyond a water uptake of 30-35 wt %, the mechanical and hydrolytic properties of the membranes are strongly affected.19,29 Already for an APTMDS loading of 12 wt %, the membranes water uptake is about 29 wt %, which is under the acceptable value, and the resulting swelling is about 10.4 lg %, which is the half of the initial value. Finally, a loading of 25.8 wt % is decreasing the initial water uptake and swelling of about 82-83% (from ca. 60 to ca. 11 wt %, and from ca. 10.6 to ca. 2 lg %, respectively), and these values are comparable to those obtained with the 67% DS SPEEK. The water uptake as a function of swelling graphs, for the 67 and the 80% DS values, are represented in Figure 4. The obtained straight line, going through the origin of the graph, verifies that swelling is intrinsically related to water uptake. Figure 5 represents the progression of the proton conductivity as a function of temperature, obtained for several membranes elaborated with the 80% DS SPEEK polymer. The membranes selected for this study were a pristine SPEEK membrane and three APTMDS/SPEEK hybrid membranes (3.0, 5.0, and 8.7 wt % APTMDS loadings). The evolution of the four proton conductivity curves measured as a function of temperature is similar. Indeed, the conductivity increases with temperature, as expected, but below ca. 60 °C this effect is not so pronounced. Moreover, as expected after comparison with Figure 1, obtained at room temperature, the proton conductivity of the pristine SPEEK membrane is lower than the 3.0 wt % APTMDS/SPEEK membrane but higher than the 5.0 wt % APTMDS/SPEEK membrane one. The 8.7 wt % APTMDS/SPEEK hybrid membrane is showing the lowest conductivity. Deterioration of the membrane is observed at 81 °C for the pristine SPEEK membrane, whereas the 3.0 wt % APTMDS/ SPEEK membrane is deteriorating beyond 93 °C. The 5.0 and 8.7 wt % APTMDS/SPEEK membranes are mechanically stable until 90 and 86 °C, respectively. Figure 6 represents the proton conductivity as a function of temperature, for various membranes elaborated with the 67% DS SPEEK polymer. The membranes selected for this study were a pristine SPEEK membrane, a 3.1 wt % APTMDS/SPEEK

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Figure 4. Room temperature water uptake of the 67 and 80% DS SPEEK-based membranes as a function of swelling.

Figure 5. Proton conductivity of various 80% DS SPEEK-based membranes as a function of temperature.

membrane, and two other membranes resulting from our previous work22 and having the following composition: 2.5 wt % nongrafted silica nanoparticles/SPEEK membrane and 2.5 wt % APDMS grafted silica nanoparticles/SPEEK membrane (where APDMS is (3-aminopropyl)dimethylethoxysilane (Scheme 3), and silica particles are porous silica nanospheres with a 9 nm diameter and an estimated porosity of about 415 m2 g-1 in the case of the nongrafted particles, and about 310 m2 g-1 in the case of the APDMS grafted ones). The ion-exchange capacity (IEC) values of these composite membranes were determined and are given in Table 1. They all exhibit a value around 1.6-1.7 meq/g, giving them an acid content relatively comparable. The shape of the four proton conductivity curves measured as a function of temperature is similar. Indeed, until a limit 2919

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Table 1. IEC of the 67% DS SPEEK-Based Membranes Used for the Measurements of Proton Conductivity as a Function of Temperature type of membrane

ion-exchange capacity (meq/g)

pristine SPEEK membrane

1.61

3.1 wt % APTMDS/SPEEK membrane

1.66

2.5 wt % nongrafted silica

1.73

nanoparticles/SPEEK membrane 2.5 wt % APDMS grafted silica nanoparticles/SPEEK membrane

1.57

Figure 6. Proton conductivity of various 67% DS SPEEK-based membranes as a function of temperature.

Scheme 3. Schematic Representation of APDMS

temperature is reached, which is different for each membrane, the proton conductivity does not change much with increasing temperature. Then, beyond this limit temperature, the conductivity increases abruptly. For temperatures below 45 °C, the four membranes present conductivities of the same order of magnitude, comprised between 8.2  10-3 and 1.2  10-2 S cm-1. For the pristine SPEEK membrane, the highest conductivity value is reached at 2.5  10-1 S cm-1 for a temperature of 84 °C. Above this temperature, the deterioration of the membrane is observed. In the case of the 3.1 wt % APTMDS/SPEEK hybrid membrane, the rupture of the membrane is observed at 85 °C, whereas in the case of the two composite membranes (2.5 wt % nongrafted silica nanoparticles/SPEEK and 2.5 wt % APDMS grafted silica nanoparticles/SPEEK membranes), the breakdown occurs at about 77 and 82 °C, respectively. Figure 7 depicts the IEC of APTMDS/SPEEK hydrid membranes as a function of graft content for SPEEK DS values of 67 and 80%. In both cases, the IEC behavior with increasing APTMDS loading is similar. Indeed, the IEC decreases with the increasing graft content, and this decrease is linear with a -2 slope. For the two DS values, only the first point, which is obtained at zero APTMDS content, is found out of the straight line. In all cases, for a given APTMDS content the IEC is more elevated in the case of the highest DS value, that is, 80%. Initially, the IEC of the pristine SPEEK membrane is ca. 1.6 meq/g for

Figure 7. IEC of APTMDS/SPEEK membranes as a function of graft content.

a 67% DS, and ca. 2.0 meq/g for an 80% DS. For an APTMDS loading higher than 20 wt % (i.e., higher than 0.8 mmol/g), the IEC of the membrane is lower or equal to 0.5 meq/g for both DS values. Figure 8 represents the Arrhenius plots of the proton conductivity of several membranes elaborated with the 80% DS SPEEK polymer. The membranes selected for this study were the same as represented in Figure 5. These diagrams are divided into two parts at about 55-60 °C. At temperatures below this limit, the curves can be fitted by straight lines the slope of which yield the activation energy (Ea) value. These values are on the order of 4-6 and ca. 10 kJ 3 mol-1 for the hybrid and pure SPEEK membranes, respectively. At temperatures higher than 60 °C, all curves exhibit a similar behavior with same activation energy value of 74 kJ 3 mol-1. Figure 9 represents the Arrhenius diagrams of the proton conductivity of two 67% DS SPEEK-based membranes. Here again the curves show different parts below and above 50-60 °C. Below this value, the hybrid and pristine SPEEK membranes show a similar behavior with straight lines yielding activation energy values of 4.4 kJ 3 mol-1. Above the limit of 60 °C temperature, the straight lines exhibit different slopes conducting to activation energy values of 65 and 164 kJ 3 mol-1 for the pristine and hybrid membrane, respectively. Above 75 °C, the hybrid membrane shows a lower activation energy closer to the 65 kJ 3 mol-1 of the pristine SPEEK membrane. 2920

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Figure 8. Arrhenius plots of the proton conductivity of 80% DS SPEEK-based membranes.

IV. DISCUSSION Both proton conductivity and water sorption increase with the sulfonic acid group content,that is, DS value, because of the strong hydrophilicity of the sulfonic acid group. Earlier studies demonstrated that the microstructuration of SPEEK is composed of two distinct regions: ionic domains, where the ions form hydrophilic clusters by aggregating together instead of being evenly dispersed, and a matrix, where the hydrocarbon polymer backbone forms nonionic hydrophobic regions.30-32 The proton conductivity and water uptake of the polymer are principally due to the hydrophilic ion clusters. These are constituted by narrow channels, which are highly branched, and contain many dead ends. Depending on the loading, we can postulate that the addition of fillers can modify the intrinsic microstructuration of the entangled polymer matrix and thus the volume and connectivity of the water channels. Without a doubt, the APTMDS grafts can interact through their amine groups via electrostatic interactions with the sulfonic acid groups of SPEEK (because both groups are in their form with one positive, i.e., NH3þ, or negative, i.e., SO3-, charge), therefore increasing the cohesion (cross-linking) within the membrane. A greater cohesion in the membrane reduces the channel opening and increases the number of dead-end channels. Both water uptake and conductivity are consequently reduced, as several studies have shown previously [SPEEK/ZrO2,33-35 SPEEK/(n-propylamine modified ZrPh with polybenzimidazole),33,36 SPEEK/montmorillonite,37 SPEEK/(SiO2 with N-(3-(triethoxysilyl)propyl)-4,5-dihydroimidazole].38 Figures 2 and 3 show that water uptake and swelling are reduced by half with respect to their pristine values for a loading of ca. 11.0-11.5 wt % of APTMDS in the membrane. This corresponds to a content of ca. 0.90 mmol of -NH2 groups per gram of membrane. Now, the concentration of -SO3H groups in the same hybrid membrane is about 2.0 mmol per gram of membrane in the case of the 80% DS and of about 1.7 mmol per gram of membrane in the case of the 67% DS, which is about twice the content of -NH2 groups. This seems to show that the reduction of both water uptake and swelling by half is due to the neutralization of half of the -SO3H groups present within the membrane, by reaction of the two -NH2 groups of the

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Figure 9. Arrhenius plots of the proton conductivity of 67% DS SPEEK-based membranes.

APTMDS molecule with two -SO3H groups of the SPEEK matrix. These observations demonstrate unambiguously that the APTMDS plays indeed the role of a cross-linker. Figure 7 supports this hypothesis. Indeed, whatever the DS value, the straight lines obtained for the equation IEC = f(APTMDS content), for not nil APTMDS contents, exhibit a -2 slope. This signifies that each mole of APTMDS introduced in the SPEEK matrix reacts with two moles of -SO3H acid sites. Consequently, there is undeniably double anchoring of the APTMDS molecules within the hybrid membrane. On Figure 1, it is observed that for both 80 and 67% DS values, at ambient temperature, the maximum conductivity is obtained for the addition of about 3 wt % of APTMDS graft. Since the APTMDS molecule is at the origin of two electrostatic bonds with the acid sites of the SPEEK matrix, it is expected that the addition of APTMDS in the SPEEK matrix results in a decrease of the number of acid sites available for the protons transport within the membrane and thus in a decrease in conductivity. This aberration may be explained by supposing that the initial addition of APTMDS molecules might increase the interchain gaps in the polymer. Thus, some acid sites, initially inaccessible because of the polymer chain entanglement, may now be reachable by water molecules and available for proton transport. This hypothesis is sustained by the fact that the increase of conductivity, when adding about 3 wt % of APTMDS, is more pronounced in the case of the 80% DS SPEEK. The acid sites initially blocked by the polymer chain entanglement, next becoming accessible to protons transport, shall be more numerous than in the case of the 67% DS SPEEK. Moreover, in Figure 7, the two straight lines obtained by extrapolation of the data do not go through the origin point, that is, the IEC of the pristine SPEEK membrane. This point being lower than the theoretical value also suggests the nonaccessibility of some acid sites in the pristine SPEEK polymer. Then, for a loading of APTMDS higher than 3 wt % the advantage of the chains unfolding is counterbalanced by the cross-linking generated by the graft, and the number of available acid sites for the protons transport decreases with the APTMDS loading increase, leading to a decrease of the conductivity. 2921

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The Journal of Physical Chemistry B The effect of the addition of APTMDS molecules to a SPEEK polymer matrix on the mechanical properties of the membranes can be discussed based on the data presented in Figure 5. One can observe that for APTMDS loadings below 9 wt %, the hydrothermal stability of the membrane is higher than the one of the pristine SPEEK membrane. This suggests that the APTMDS spacer consolidates the membrane by the creation of electrostatic bonds, rendering it mechanically and thermally more stable. Besides, consistently with what was postulated above, the conductivity reached at the rupture point of the membrane is higher in the case of the 3.0 wt % APTMDS/SPEEK hybrid membrane than in the case of the pristine SPEEK one. On the contrary, as expected, the two other hybrid membranes, with higher APTMDS loadings (5.0 and 8.7 wt %), present a conductivity at the rupture point less elevated than the pristine SPEEK membrane. Figure 5 also allows observing that the degradation of the hybrid membrane happens at lower temperature as the APTMDS content is increased. At higher content, the APTMDS spacer, by rigidifying the SPEEK matrix, is rendering it more brittle. It is also observed in Figure 5 that at 3.0 wt % APTMDS loading the low temperature part of the curve is above the pristine SPEEK curve as discussed above in relation with Figure 1. Above 60 °C, this order is reversed and the pristine SPEEK membrane conductivity is now higher than the 3.0 wt % APTMDS crosslinked membrane. This suggests that the increased accessibility of some of the acid sites due to APTMDS spacer is not observed anymore, which might be due to the increased chain mobility of SPEEK at higher temperature. Figure 6 allows a comparison between the best hybrid membrane with a DS of 67% obtained in the present work (3.1 wt % APTMDS/SPEEK membrane) with the finest composite membranes with a DS of 67% obtained during our previous work (2.5 wt % APDMS grafted and nongrafted silica nanoparticles/ SPEEK membranes). The protonic conductivity of the APTMDS/ SPEEK hybrid membrane seems to be lower than the ones of the composite membranes for temperatures below 70 °C, but seems to be comparable for temperatures above 70 °C. On the opposite, the APTMDS/SPEEK hybrid membrane exhibits a temperature of rupture and a conductivity at this temperature more elevated than the corresponding ones for the composite membranes. This suggests that the mechanical properties of the APTMDS/SPEEK membrane are slightly better than the ones of the nanocomposite membranes, and that might come from a better distribution of the filler within the membrane. It may be recalled here that in our previous study, it was demonstrated that silica nanospheres form aggregates/clusters of variable morphologies and sizes within the polymeric membrane.22 Consequently, the distribution of these nanospheres aggregates within the polymeric membrane can only be more heterogeneous than the one of the APTMDS molecules. Figure 8 highlights the presence of two different processes of proton conduction within the membranes. Indeed, for the 80% DS polymer, temperatures below 55-60 °C, and whatever the APTMDS content, the activation energy Ea is quite low. This suggests that the proton transport process does not comprise the rupture of a strong bond and consequently that a Grotthuss mechanism30,39 is taking place. Furthermore, the addition of APTMDS molecules lowers the activation energy. As demonstrated above, the addition of a small amount of APTMDS renders accessible some acid sites, initially blocked by the polymeric chains entanglement. It was postulated that the addition of the filler modifies the intrinsic microstructure of

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the entangled polymer matrix and thus the volume and connectivity of the water channels. This indicates that for temperatures below 55-60 °C, the process limiting the proton conduction depends on the shape of the water channels. Beyond 60 °C, both the conductivity and its energy of activation increase abruptly and seem to be of the same order of magnitude whatever the amount of APTMDS. In this case, it may postulated that the mobility of the polymeric chains is suddenly changed, which affects proton conductivity in a different way. At lower DS, the polymeric chains show a reduced mobility.29 Thus the large difference in activation energy between pristine 67% DS SPEEK and the 3.1% APTMDS membranes observed in Figure 9 is reflecting a limitation in increased chain mobility as temperature is raised. This limitation is observed at higher temperature in the hybrid membrane having indeed lower free sulfonic acid group contents. This suggests that the energy necessary to move the polymeric chains is largely higher in the SPEEK/APTMDS membrane, and this phenomenon is explained by the presence of the strong electrostatic interactions rigidifying the membrane. In the case of humidified pristine SPEEK, the glass transition temperature (Tg) may be estimated using Fox Equation29 1 WA WB ¼ þ ð1Þ Tg TgA TgB where A and B represent the polymer and water, respectively. W stands for weight fraction. In ref 17, the Tg values of SPEEK polymers with DS ranging from 59 to 79% were measured between 217 and 224 °C. Thus the Tg values of 67 and 80% DS SPEEK may be estimated to be 220 and 224 °C, respectively. The approximate Tg value for water is -132 °C.29 Thus from eq 1 the Tg values for the humidified pristine SPEEKs used in this study may be calculated assuming WB to be on the order of magnitude of 20% in both 67 and 80% DS, respectively. The respective Tg estimates so calculated are found to be 56 and 57 °C. It seems therefore that the transition observed in Figures 8 and 9 corresponds approximatively to the glass transition which would be in full agreement with the above interpretation of the proton transport process.

V. CONCLUSION In our previous work,22 we have studied silica nanoparticles/ SPEEK membranes, where nanospherical silica particles were amine functionalized by grafting APDMS on their surface. In the present work, APTMDS graft may be considered as a limit case of silica nanoparticles, despite its main organic character, where the amine density per silicon atom is much higher than in the amine grafted silica particles. The size of the silica particles having been reduced at a maximum, their distribution within the polymeric matrix is optimal; cross-linking is also improved. The conductivity of the membranes remains at the same order of magnitude as compared to the grafted nanoparticles/SPEEK membranes, but mechanical and hydrothermal properties are improved. The proton conductivity Arrhenius plots show that below a temperature limit of 55-60 °C the Grotthuss-like displacement of the protons is strongly dependent on the shape of the water channels present in the entangled polymer matrix. Indeed the addition of a small amount of APTMDS is unfolding the polymer chains, thus rendering some acid sites accessible to water 2922

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The Journal of Physical Chemistry B molecules and available for proton transport. These were initially unreachable because of the polymer chain entanglement. To our knowledge, this is the first time that such assessment is sustained by measurable data in the case of SPEEK hybrid or composite membranes. Above the 55-60 °C temperature value the humidified SPEEK membranes undergo glass transition. The enhanced mobility of the polymer chains induces a large increase in proton conductivity yielding also a large increase in conductivity apparent activation energy. In 67% DS pristine SPEEK membranes the latter increase is limited above 60 °C and this new transition is only observed around 75 °C in the hybrid membrane (Figure 9). Finally, the main originality of this work resides in the use of a commercially available coupling agent, favoring the PEM cost reduction, which is neither a polymer nor an inorganic filler. This agent will obviously be of universal application in cross-linking of other acidic polymers. Present Addresses †

Institut Charles Gerhardt de Montpellier, UMR 5253, CNRSUM2-ENSCM-UM1, Place Eugene Bataillon 34095, Montpellier cedex 5, France.

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