SPEEK Nanocomposites as Proton Exchange

Jun 4, 2010 - Mashallah Rezakazemi , Abtin Ebadi Amooghin , Mohammad Mehdi Montazer-Rahmati , Ahmad Fauzi Ismail , Takeshi Matsuura. Progress in ...
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J. Phys. Chem. B 2010, 114, 8387–8395

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Amine Grafted Silica/SPEEK Nanocomposites as Proton Exchange Membranes Marie J. Kayser, Marc X. Reinholdt, and Serge Kaliaguine* Department of Chemical Engineering, LaVal UniVersity, Quebec QC G1 V 0A6, Canada ReceiVed: January 15, 2010; ReVised Manuscript ReceiVed: May 6, 2010

This work presents the elaboration of porous silica nanospheres, eventually amine functionalized, which are used as the inorganic filler in mixed matrix silica/SPEEK membranes. The surface of the silica nanoparticles is modified by grafting (3-aminopropyl)dimethylethoxysilane (APDMS). The two sets of nanocomposite membranes obtained at varying silica loadings are characterized for their proton conductivity and water uptake properties. At higher degrees of sulfonation, some cross-linking due to the interaction of the amine groups of the silica with the sulfonic acid groups of the SPEEK polymer is attested by the water uptake reduction between the composites made with amine grafted or pristine silica particles. However, even in these conditions the proton conductivity of the mixed matrix membrane is not essentially different in the two sets of nanocomposites. This indicates that the inorganic filler effect on proton conductivity is related to changes in the microstructure of the water channels in the polymer lattice. I. Introduction 1

Drying out of fossil fuels reserves and climate problems associated with greenhouse gas emissions make it critical to develop new alternative energy production means. One of the proposed solutions is the use of fuel cells. Proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are the most common. The electrolyte is an ionic polymer membrane bearing strong acid sites, generally sulfonates (SO3H),2 and is known as a 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.1 The electrolytic membranes used are generally perfluorosulfonated polymers, and among them, Nafion is the reference.3 Manufacturing Nafion membranes is, however, particularly expensive (ca. 800 U.S. $/m2),4 and the constraining chemistry of fluorine is going against the requirement for environmental protection. The current challenge is thus to find a replacement to Nafion; several potential substitutes have already been studied in different laboratories,5-8 but none reached commercial application. However, sulfonated poly(ether ether ketone) (SPEEK)9 (Scheme 1) seems to be a promising challenger. This polymer exhibits very interesting characteristics:10-13 its synthesis cost is low, and at a zero (x ) 0) degree of sulfonation (DS), it possesses good mechanical, chemical, thermal, and hydrolytic properties. The major drawback of SPEEK is, when the degree of sulfonation increases, the proton conductivity increases, but to the detriment of hydrolytic and mechanical properties. Above 80% DS, the membrane becomes excessively hydrophilic, which results in an important swelling, followed by its eventual dissolution in water. A study has shown that starting at 90 °C, the conductivity of a 47% DS SPEEK is higher than the one of Nafion.13 Consequently, if the DS of SPEEK could be increased without reducing its hydrolytic and mechanical stabilities, this would lead to a membrane exhibiting a higher conductivity than Nafion, even at room temperature. Therefore, to reduce the water * Corresponding author.

SCHEME 1: Structure of Sulfonated Poly(ether ether ketone) (SPEEK) (x Being the Degree of Sulfonation)

swelling of SPEEK, it is possible to add inorganic particles which may act as cross-linking agents. The addition of inorganic particles to SPEEK generally enhances the mechanical properties of the hydrated membranes, but at the expense of proton conductivity σ.14-18 It was, however, shown that when MCM-41 mesostructured silica particles modified by organosilanes are used as inorganic fillers, the decrease in membranes swelling is accompanied by an increase in conductivity (at MCM-41 loading lower than 10 wt %).19 The pores of MCM-41 may play the role of a water reservoir allowing a simultaneous decrease in membranes swelling and an increase in proton conductivity. The present work deals with the elaboration of porous silica nanospheres, to be dispersed into SPEEK polymer to prepare mixte matrix PEM type membranes. The surface of the silica nanoparticles was modified by grafting (3-aminopropyl)dimethylethoxysilane (APDMS), with the objective of studying the effect of this modification on conduction and water uptake properties of the SPEEK based nanocomposite membranes. II. Experimental Methods 1. Materials. Tetraethylorthosilicate (TEOS; >98 wt %), octane (C8H18; >99 wt %), L-lysine (g98 wt %), poly(oxo-1,4phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK; typical Mn ) 10 300; typical Mw ) 20 800), and N,Ndimethylacetamide (DMAc; 99 wt %) were obtained from Aldrich Chemical Corp. (3-Aminopropyl)dimethylethoxysilane (APDMS) (g99.9%) was obtained from Gelest Inc. and concentrated sulfuric acid (95-98 wt %) was from Fisher Scientific. All reagents were used without any further purification except N,N-dimethylacetamide which was filtered through a Whatman 0.02 µm inorganic membrane.

10.1021/jp100430h  2010 American Chemical Society Published on Web 06/04/2010

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2. PEEK Sulfonation. Typically, 20 g of PEEK was dissolved in 800 mL of concentrated sulfuric acid at room temperature under vigorous stirring and under an argon flow. The chemical structures of the initial and sulfonated PEEK (SPEEK) are shown in Scheme 1. The duration of the reaction was varied from 30 to 63 h to obtain the desired degree of sulfonation (DS). The sulfonation reaction was terminated by pouring the polymer solution into a large excess of ice-cold water under continuous mechanical stirring to precipitate the sulfonated polymer (SPEEK). The suspension containing SPEEK was then centrifuged and residual sulfuric acid was removed by dialyzing the recovered polymer in deionized water until the pH reached neutrality. The resulting suspension was centrifuged again and the polymer dried at 60-80 °C. 3. Silica Nanoparticles Synthesis. The molar composition of the reaction mixture was as follows:

1 SiO2:0.025 L-lysine:1,3 C8H8:154,4 H2O Typically, 0.185 g of L-lysine was dissolved under vigorous mechanical stirring in a 500 mL polypropylene bottle containing 139.05 g of water and 7.43 g of octane. A 10.43 g sample of TEOS was then added, and the reaction mixture was stirred at room temperature for 20 h. The mixture was then heated to 100 °C for 20 h. Finally, the aqueous phase was evaporated at 100 °C, and the obtained white powder underwent a calcination cycle reaching 600 °C. 4. Grafting of the Silica Particles. Typically, 1 g of silica particles was dispersed in 20 g of a 2% solution of (3aminopropyl)dimethylethoxysilane (APDMS) in 95/5 w/w ethanol/water mixture using an ultrasonic probe. The dispersion was heated to 50 °C and held at that temperature for 1.5 h before being centrifuged (Beckman Avanti centrifuge; conditions: 20 000 rpm, 1 h, 20 °C). To remove the residual grafts, the recovered particles were washed with ethanol and centrifuged again, twice. The particles were finally dried at 100 °C. 5. Membrane Preparation. Pure SPEEK membranes and composite SPEEK/silica nanoparticles 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 particles, grafted or not, were then added to the solution. The resulting mixture was stirred for several hours until complete dispersion of the particles and then cast onto a glass plate. The cast membranes were dried under ambient conditions for 2 days and then cured under vacuum at 60 °C for 2 days, at 90 °C for 2 more days, and finally at 120 °C for 1 day. The thickness of all resulting membranes was in the range ca. 30-40 µm. 6. Determination of DS of SPEEK Polymer. 1H 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 sample, a 5 wt % polymer solution in deuterated dimethyl sulfoxide (DMSO-d6) was prepared to perform the analysis. Chemical shifts were referenced at 0 ppm to external tetramethylsilane (TMS). The DS was determined by integration of specific aromatic signals as described previously.12,20 7. Characterization of the Silica Particles. X-ray diffractograms were recorded on a Bruker Discover D8 diffractometer, using the KR1 ray of copper (λ ) 1.5406 Å) and a monochromator constituted by a Ge monocrystal. The diffractogramm was recorded in Debye-Scherrer mode with a 2D HI-STAR

Kayser et al. detector and a rotary anode. The 2θ-angular domain was chosen between 0.4 and 6° and the step size was 0.02°/point. The morphology of the silica particles was investigated using a transmission electron microscope (JEOL 1230) equipped with a LaB6 filament and working at an excitation voltage of 80 kV. Nitrogen adsorption isotherms at 77 K were recorded using a Quantachrome Autosorb-1 apparatus. Prior to the measurements, the samples were outgassed under a vacuum during 12 h at 200 °C for nongrafted particles and at 80 °C for grafted particles. Measurements were carried out on about 20 mg of particles. The BET (Brunauer-Emmett-Teller) specific surface area was calculated from the data collected at 0.05 < P/P0 < 0.1, and the pore size distribution was determined using the DFT (density functional theory) method.21 29 Si MAS NMR spectra of the samples were recorded on a Bruker Avance 300 MHz spectrometer at a resonance frequency of 59.601 56 MHz, using a 4 mm probe. The experiments were performed at a MAS frequency of 10 kHz, with a contact time of 1000 µs, a relaxation delay of 60 s, and a pulse length of 3 µs. A total of 720 transients were recorded, and the chemical shifts were referenced to external TMS at 0 ppm. 8. Characterization of the Grafted Materials. FT-IR spectroscopy was used to confirm the presence of amine functional groups in the grafted chains of the silica particles. Measurements were performed using a Digilab FTS-60 spectrometer, and spectra were recorded in transmission mode after preparing KBr pellets with the powders. Thermogravimetric (TG) analysis was employed to assess the quantity of grafts on the silica particles surface. TG measurements were achieved with a Seiko Intruments Inc. EXSTAR 6000 thermogravimetric analyzer in an air atmosphere (60 mL/ min), with a heating rate of 5 °C/min over the temperature range 25-800 °C. About 20 mg of grafted particles was introduced in a Pt-crucible in a heated enclosure. 1 H-13C cross-polarization (CP) MAS NMR spectra were recorded at a frequency of 75.4426 MHz, using the same spectrometer and probe as for the 29Si nucleus. The experiments were performed at a MAS spinning frequency of 10 kHz, a contact time of 1000 µs, a relaxation delay of 4 s, and a 90° pulse of 3.9 µs. A total of 10 000 transients were recorded, and the chemical shifts were referenced to the methylene group of external adamantine at 38.56 ppm. During the acquisition, protons were decoupled at a frequency of 299.9994 MHz and with a decoupling width of ca. 65.5 kHz. 9. Characterization of the Composite Membranes. The membrane proton conductivity was measured by ac impedance spectroscopy using a Solarton 1260 gain phase analyzer, over a frequency range of 10-107 Hz. A 15 mm × 8 mm sample of the membrane was clamped between two blocking stainless steel electrodes. Before the measurements, the sample must be hydrated and stabilized. It was therefore placed in the measurement cell with a relative humidity of 100% during 24 h before measurement. All impedance measurements were performed at room temperature. The proton conductivity (σ) of the samples in the longitudinal direction was calculated from the impedance data with the relationship:

σ ) d/RS where d and S are the distance between the electrodes and the surface area of the sample, respectively, and R was derived from the low intersect of the high frequency semicircle on a complex impedance plane with the Re(Z) axis.

Amine Grafted Silica/SPEEK Nanocomposites

Figure 1. TEM pictures of a sample of (a) calcined and (b) amine grafted silica particles.

For determining the water uptake of the membranes, they were dried at 100 °C for 24 h, weighed, and immersed in deionized water at room temperature for 24 h. The wet membranes were wiped out with blotting paper and quickly weighed. The water uptake of membranes is reported as a weight percentage as follows:

water uptake ) [(Wwet - Wdry)/Wdry] × 100 where Wwet and Wdry are the weights of the wet and dry membranes, respectively. III. Results and Discussion 1. Silica Nanoparticles. The pKa values of R-COOH, R-NH3+, and R-(CH2)4NH3+ in L-lysine are estimated to be 2.18, 8.90, and 10.28, respectively, and the isoelectric point of the L-lysine is 9.74.22 Thus, about 92% of R-(CH2)4NH3 and 33% of R-NH2 would be protonated under the synthesis conditions, i.e., pH ) 9.2. In this case, two kinds of interactions can occur: the electrostatic interactions between anionic silicates (tSiO-) and protonated amino groups in L-lysine, and hydrogen-bonding between L-lysine molecules. During the particles growing

J. Phys. Chem. B, Vol. 114, No. 25, 2010 8389 process, the L-lysine molecules can cover the nanosphere surface through the interaction of their protonated amino groups with the silicates, leading to the control of the silica sphere size. Finally, the uniform-sized spheres and the hydrogen-bonding interaction between L-lysine molecules lead to a well-packed 3D-structure,23 but the nanoparticles will hopefully be redispersed using ultrasounds in a second step. 2. Role of L-Lysine during the Synthesis Reaction. L-Lysine has a triple role during the synthesis reaction: catalyzing the hydrolysis of TEOS, controlling the size of the silica nanospheres, and finally imposing a periodical compact arrangement to the silica nanospheres, probably with the help of the hydrogen bonds existing between the L-lysine molecules well connected (or coupled) to the spheres.23 TEM pictures of the calcined form of the synthesized samples are presented in Figure 1a. One can observe spherical particles, with homogeneous sizes, agglomerating into larger irregular masses. The diameter of these spherical particles is about 9 nm, which is significantly smaller than the silica particles made by the similarly modified Sto¨ber technique.24-26 Most often these have sizes on the micrometer scale. It has been shown that the 3D arrangement of the silica nanospheres exhibit the cubic close packing (ccp) structure from the Fm3jm space group.23 Typical X-ray diffractogram of calcined silica particles is presented as Figure 2a. Three diffraction peaks are observed in the 2θ region 0.5-2.0°, corresponding to d ) 14.7, 10.2, and 5.0 nm, respectively. Considering the ccp structure, these peaks correspond to the following hkl reflections planes: 110, 111, and 331, respectively.23 A typical 29Si MAS NMR spectrum obtained for calcined silica particles is shown in Figure 3a, and the deconvolution results are given in Table 1. The spectrum is characteristic of a well condensed material: the peak observed at ca. -111 ppm is assigned to Q4 type silicons (a silicon atom surrounded by four other silicons linked via bridging oxygen bonds) and represents the majority of the silicon environments (ca. 80%); the shoulder observed at ca. -102 ppm corresponds to Q3 type silicons (a silicon atom surrounded by three other silicon atoms linked via bridging oxygen bonds and one hydroxyl group) and represents about 13% of the Si; finally, a small peak observed at ca. -96 ppm is assigned to Q2 type silicons (a silicon atom surrounded by two other silicon atoms linked via bridging oxygen bonds and two hydroxyl groups) and represents about 7% of the silicon environments.27-29 Q3 and Q2 silicon atoms are essentially observed at the surface of the material. The high

Figure 2. X-ray diffraction pattern of (a) a sample of calcined silica particles and (b) a sample of grafted silica particles.

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Figure 3.

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Si MAS NMR spectra of (a) a sample of nongrafted silica particles and (b) a sample of APDMS grafted silica particles.

TABLE 1: Results of the Deconvolution of the 29Si MAS-NMR Spectra of the Silica Particles Grafted with APDMS or Not silica particles MKA021 grafted

MKA021 calcined

chemical shift fwhm peak relative (ppm) attribution (Hz) area (%) -111.3 -101.4 -96.1 10.8 -111.1 -101.7 -95.8

Q4 Q3 Q2 M Q4 Q3 Q2

680 520 790 420 720 480 710

78.5 12.0 3.7 5.8 80.1 13.0 6.9

relative intensity of the Q4 peak suggests a highly condensed silica possibly associated with some inter nanospheres silanol condensation. The N2 adsorption-desorption isotherm and the corresponding pore size distribution of the calcined silica nanospheres are represented in Figure 4a. The isotherm is of the type IV, characteristic of materials showing mesoporosity. The BET specific surface area is ca. 415 m2 g-1, a value larger than the one obtained by Tatsumi et al.,23 which is 228 m2 g-1. This difference is probably due to the smaller size of the particles prepared by us compared to the size of the particles prepared by Tatsumi et al. (ca. 9 nm vs ca. 13 nm). The difference in silica particle size is probably due to the presence of a larger amount of L-lysine in the reaction mixture in our study. The pore size distribution obtained using the DFT method reveals the presence of two populations of pores. The first one possesses a pore diameter varying from 41 to 57 Å with a maximum at 55 Å and is attributed to the intrinsic porosity of the silica. The second population of pores has a diameter between 65 and 135 Å with a maximum at 82 Å diameter. This second population is attributed to the interstitial space between the regular-sized silica nanospheres. The total pore volume is ca. 0.62 cm3.g-1. 3. Amine Grafted Silica Particles. The grafting of the nanospheres is performed with the aim of increasing the crosslinking capacity of the particles when used as fillers in the composite membranes.19,30,31 Proper selection of the grafted functional groups should allow us to limit the water swelling of the composite membrane. In our case, the grafted molecule selected is an organosilane bearing a terminal amine group: (3aminopropyl)dimethylethoxysilane (APDMS) (Scheme 2). The amine function will react with the sulfonic acid function of SPEEK and consequently will create cross-linking of polymer chains during the membranes elaboration. The grafting of the APDMS molecules at the surface of the silica nanoparticles

results from the condensation of the surface hydroxyl groups with the Si-OEt moieties from APDMS. The nucleophile attack from the surface hydroxyl onto the silicon atom of APDMS results in an ethanol molecule leaving and the creation of a Si-O-Si bridge (Scheme 3). A typical X-ray diffractogram of the grafted silica particles is shown in Figure 2b. It allows verifying that the regular arrangement of the particles has been retained during the grafting process. Three diffraction peaks are observed in the 2θ region 0.5-2.0°, as previously observed on the diffractogram of the calcined particles. The first and the third peaks retained their position at d ) 14.7 ((110) plane) and 5.0 nm ((331) plane), respectively. The second peak, assigned to the (111) plane, is, however, slightly shifted and is now located at d ) 11.0 nm. It is, however, believed that this shift is not significant, suggesting that the regular ccp arrangement of the nanoparticles is not significantly affected by the grafting operation. The change in relative intensities of the (110) and (111) reflections may, however, be associated with amine groups as new diffusers on the silica surface. TEM pictures of grafted silica particles are presented in Figure 1b. Spherical particles with homogeneous sizes (9-10 nm in diameter), agglomerated into larger irregular masses are observed, as for the calcined form of the particles, before grafting (compare Figure 2a,b). The grafting seems not to affect significantly the morphology; the size and the tridimensional arrangement of the particles are in good agreement with the XRD data. N2 adsorption-desorption measurements have been performed on the grafted silica particles, and the isotherm and the corresponding pore size distribution are represented in Figure 4b. As for the nanospheres before grafting, the isotherm is of the type IV, characteristic of materials showing mesoporosity. The BET specific surface area is about 310 m2 g-1 of sample, which can be reformulated as ca. 330 m2 g-1 of silica particles (by taking into account the thermogravimetric analysis, shown below, which indicates that for ca. 100 g of grafted sample, about 93.8 g come from the silica spheres). This value is lower than the one obtained for the nongrafted silica spheres (415 m2 g-1), which may be explained by considering that part of the surface is occupied by the grafts. Just as previously with the calcined, but not grafted, silica spheres, the pore size distribution obtained using the DFT method shows the existence of two populations of pores. The first one possesses a pore diameter varying from 44 to 57 Å with a maximum at 55 Å, and, here again, is attributed to the intrinsic porosity of the silica. The

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Figure 4. BET adsorption-desorption isotherms and corresponding pore size distributions obtained by the DFT method, for nongrafted (a) and amine grafted (b) calcined silica particles.

SCHEME 2: Schematic Representation of (3-Aminopropyl)dimethylethoxysilane (APDMS)

second pore population has diameters between 63 and 130 Å, the maximum being around 81 Å. This second population is attributed here also to the interstitial space between silica nanospheres. The pore size seems not to be affected considerably by the grafting of the organosilane. Since an APDMS graft is about 6-7 Å long, one would expect to observe a decrease of the pore diameter after grafting. But one may postulate that the particle arrangement is less dense than previously since the graft exhibits some flexibility at the materials surface, and the effect due to this less dense arrangement of the particles tends to balance the steric effect introduced by the presence of the graft. The total pore volume has here been reduced to ca. 0.46 cm3 g-1. It is also interesting to note that the C constant from the BET equation is divided by a factor of 2 compared to its initial value obtained for nongrafted particles, which confirms the surface modification (Table 2). Indeed, if we refer to the BET theory, the C constant is related to the enthalpy of adsorption,32 and a

decrease of its value reflects a decrease of the interactions between the surface and the nitrogen molecules. Therefore, there was a modification of the surface state of the silica particles due to grafting. The examination of the 29Si MAS NMR spectrum as well as the one of infrared absorption allows evidencing the functionalization. Indeed, the 29Si MAS NMR spectrum, shown in Figure 3b, exhibits a peak at ca. -111 ppm corresponding to Q4 silicon atoms, a shoulder at ca. -101 ppm corresponding to Q3 environments, and a small peak at ca. -96 ppm corresponding to Q2 type silicon, as previously observed with the nongrafted material. An additional peak is, however, observed at ca. 11 ppm and is assigned to the monodentate graft.28,29,33,34 The results of the deconvolution of the 29Si MAS NMR spectra (Table 1) show that if the proportions of the Q4 and Q3 type silicons are essentially the same for the grafted and nongrafted particles, ca. 79-80% and ca. 12-13%, respectively, the proportion of the Q2 silicons is decreasing with the grafting, ca. 4% for the grafted and ca. 7% for the nongrafted particles. Indeed, the anchoring of the APDMS grafts is occurring at the surface of the particles via a surface hydroxyl, thus decreasing the density of OH rich Q2. The formation of Si-O-Si bonds (M type silicons)28,29,33,34 at the surface of the material results in a decrease of the number of Q2 type silicon atoms. About

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SCHEME 3: Schematic Representation of the Grafting Process of (3-Aminopropyl)dimethylethoxysilane (APDMS) onto the Silica Particles

TABLE 2: BET Specific Surface Area and C Constant of the Grafted and Nongrafted Silica Particles particles

BET specific surface area (m2/g)

C constant

nongrafted grafted

415 310

62 27

6% of the silicon atoms of the grafted particles come from the grafting and thus are M type silicon atoms. The silica particles grafted with APDMS have also been further investigated using 1H-13C CP-MAS NMR, and a corresponding typical spectrum is presented in Figure 5. Resonances at 43.9 and 25.1 ppm are assigned to carbon atoms near a -NH2 group, -CH2sNH2, and tSisCH2s*CH2s CH2sNH2 (signal of the carbon marked with an asterisk) environments, respectively.35-37 Resonances at +14.5 and -3.0 ppm are assigned to -CH2sSit and tSisCH3 carbons, respectively.35-37 All these carbons are observed in the APDMS graft, thus confirming the functionalization of the silica particles. Infrared spectroscopy confirms these results (Figure 6). The O-H stretching bands at 3477 and 3401 cm-1, associated to the Si-OH groups and the surface adsorbed water,38,39 appear as negative bands (after subtraction of the background from the nongrafted silica particles), which implies that condensation of the Si-OH occurred and that the surface hydrophilic character decreased. Moreover, the presence of the absorption bands characteristic of NH2 (δNH2 at 1577 cm-1),40 CH2 (νas at 2933

and νs at 2880 cm-1),40,41 and the one associated with Si-O-Si (840 cm-1) confirms that the graft has been anchored at the materials surface.42-44 A typical thermogravimetric curve of the amine grafted sample is represented in Figure 7, with an estimated water weight loss of 2% (from 20 to 100 °C), and an estimated organic weight loss of ca. 4.2% (from 100 to 800 °C). From the water and organic weight losses deduced from this analysis, the BET specific surface obtained from nitrogen adsorption-desorption, and the particles size obtained from TEM microscopy, it is possible to calculate the number of grafts per silica particle (see the detailed calculation in the Supporting Information). In this

Figure 6. Difference FTIR spectrum (grafted minus nongrafted) of silica particles. Negative νOH lines indicate OH consumption upon grafting.

Figure 5. 1H-13C CP-MAS NMR spectrum of a sample of APDMS grafted silica particles.

Figure 7. Thermal analysis (TGA) of a grafted silica sample.

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Figure 8. Proton conductivity of silica/SPEEK nanocomposite membranes as a function of the silica content.

Figure 9. Water uptake of silica/SPEEK nanocomposite membranes as a function of the silica content.

study, the concentration of grafts at the surface is about 0.66 graft/nm2 of particle, corresponding to ca. 167 grafts per silica particle. This value may be compared with the expected density of surface OH’s, which is believed to be below 1.0 OH/nm2 for silica samples calcined at 600 °C.45,46 4. Composites Membranes. The electrophilic substitution of PEEK by sulfonic acid groups is occurring on the hydroquinone segment of the polymer chain, which is activated for electrophilic substitution by the ether linkage present in the backbone and is performed in concentrated sulfuric acid. The proton conductivity in ionomeric membranes such as SPEEK depends on the number of available acid groups and their capability of dissociation in water, accompanied by proton generation. Because the acid functionality of the polymer is dissociated by water molecules and also because they participate in the proton transport, the amount of water present within the membrane, i.e., water uptake, is of significance. Actually, high levels of water uptake may result in membrane fragility and dimensional changes leading to mechanical failures, and in some cases even to the solubilization of the membrane in water at elevated temperature. As a consequence, knowing the relationship between the SPEEK polymer DS value and the membranes water uptake is critical. Sulfonation of PEEK up to 45% DS yields membranes with poor conductivity,20 typically less than 10-5 S/cm. The 67% DS was chosen because it represents the best compromise to get the highest proton conductivity while maintaining the lowest possible water uptake for SPEEK membranes.13,20,47 The 80% DS was also selected because the conductivity of the polymer is relatively high at this value and also to investigate the possibilities of limiting the water swelling of a polymer sample, which may even be soluble in water if not modified by any filler.20,48 Figure 8 depicts the proton conductivity of SPEEK/particles (grafted or not) composite membranes, obtained at room temperature, for DS values of 67 and 80%, and for particle loadings up to 30 wt %. The conductivity behaviors with increasing particle loading are different for the two DS values, but for a given DS, the evolutions of the conductivity with the particle loading are similar, whether the particles are grafted or not. Additionally, the conductivity is increasing with the DS, as expected. All membranes prepared with the 67% DS SPEEK, containing either grafted or nongrafted particles, exhibit proton conductivity values between 4.34 × 10-3 and 1.59 × 10-2 S cm-1. Though, for loadings between 2.5 and 15 wt %, the conductivity observed for composites containing particles grafted with APDMS is

higher than the one observed for equivalent loadings of nongrafted particles. Beyond the latter loading, the grafting seems not to affect significantly the conductivity. Pristine 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 particles (ca. 2.5 wt %) up to 1.28 × 10-2 and 1.59 × 10-2 S cm-1 with nongrafted and grafted particles, respectively, which is more than twice the original value for the latter. Increasing the particle loading above this value contributes to a decrease of the conductivity to a value of ca. 4.40 × 10-3 S cm-1 for a loading of 30 wt %, either for grafted or for nongrafted particles, which is less than one-third of the original conductivity value. With the 80% DS SPEEK polymer, the addition of a small amount of particles is leading to an initial decrease of the conductivity, which is then increased to reach a maximum at a loading of ca. 10 wt %, before decreasing again with further addition. Thus, the original conductivity of 3.91 × 10-2 S cm-1 observed for the pristine SPEEK is reduced to 2.49 × 10-2 S cm-1 for a loading of ca. 5 wt % and to 2.26 × 10-2 S cm-1 for a loading of ca. 7.5 wt %, in the case of the nongrafted and grafted particles, respectively, which is about one-third of the initial conductivity. Then, for a particle loading of ca. 10 wt %, the conductivity increases to the values of 3.12 × 10-2 and 3.56 × 10-2 S cm-1 for the nongrafted and grafted particles, respectively, which remains below the conductivity of the pristine SPEEK. Finally, beyond a particle loading of 10 wt %, the conductivity decreases to (1.40-1.60) × 10-2 S cm-1. Figure 9 depicts the water uptake of the same SPEEK/ particles composite membranes, obtained at room temperature. In general, one can easily observe that the increase in particle loading leads to a decrease of the membranes water uptake, 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 increases with DS. In the case of the membranes elaborated with the 67% DS SPEEK, the curves of the composites elaborated with grafted and nongrafted particles are very similar. Initially, the pristine SPEEK membrane is capable of adsorbing about 31% of its weight in water, but the addition of particles is only slightly modifying this absorption. In fact, a 30 wt % particle loading is only decreasing the initial absorption of ca. 13%, i.e., water uptake decreasing from ca. 31 to ca. 27 wt %. In the case of the 80% DS SPEEK membranes, the addition of particles or the fact that they are grafted or not has a significant impact on the membranes’ water swelling. Indeed,

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even if for high particle loadings, i.e., 30 wt %, the two water uptake curves tend to join together, the grafting of the particles is obviously drastically decreasing the absorption of water, even for low particles loadings. The diminution of the water uptake is still important but less pronounced in the case of the nongrafted particles. Initially, a pristine SPEEK membrane is capable of adsorbing ca. 60 wt % of its own weight in water, 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.20,49 In the case of grafted particles, a 5 wt % loading is sufficient to reduce the water absorption of about 22% (from ca. 60 to ca. 47 wt %), whereas in the case of nongrafted particles, a 20 wt % loading only decreases this absorption by about 18% (from ca. 60 to ca. 49 wt %). For a comparable loading, the grafted particles allow us to reduce the water uptake by about 37% (from ca. 60 to ca. 38 wt %). Finally, for a loading of 30 wt %, the membranes water uptake is about 37-38 wt %, which is relatively close to the acceptable value. Proton conductivity and water sorption both increased with sulfonic acid group content due to the strong hydrophilicity of the sulfonic acid groups. Previous studies indicated that the microstructure of SPEEK consists of two regions: ionic domains, where the ions are not evenly dispersed but aggregate together to form hydrophilic ion clusters; and a matrix, where the hydrocarbon polymer backbone forms nonionic hydrophobic regions.50-52 The hydrophilic ion clusters are principally responsible for the proton conductivity and the water uptake of the polymer. These clusters are constituted by narrow channels, which are highly branched, and contain many dead ends. Depending on the loading, the addition of inorganic particles modifies the intrinsic microstructuration of the entangled polymer matrix and consequently the volume and connectivity of the water channels. Indeed, the silica particles can interact, through their surface silanol groups or the amine groups in the case of the grafted particles, with the sulfonic acid groups of SPEEK, via hydrogen or van der Waals bonding, thus increasing the cohesion (cross-linking) within the membrane. A greater cohesion in the membrane increases the number of dead-end channels and reduces the channel opening. The consequence is a reduction of the water uptake but also of the conductivity, as several studies have shown [SPEEK/ZrO2,15,53,54 SPEEK/(npropylamine modified ZrPh with PBI),15,16 SPEEK/MMT,17 SPEEK/(SiO2 with N-(3-(triethoxysilyl)propyl)-4,5-dihydroimidazole].18 If the added particles exhibit porosity, like in the case of the MCM-41 particles used by Karthikeyan et al.,19 the decrease in water uptake resulting from a greater cohesion in the membrane and leading to a reduction of the conductivity, might be eventually counterbalanced by the remaining water entrapped in the pores. This will help to transport the charge carriers, due to the good ordering of the water channels contained within the pores network. Thus, Karthikeyan et al. have shown that SPEEK composite membranes containing about 10 wt % of particles exhibit enhanced proton conductivity, while the overall water uptake is reduced. Moreover, the composites in this study were prepared using particles having a surface modified with imidazole functionalities, which is also improving the silica intrinsic conductivity. Additionally, Carbone et al. worked using 3-aminopropyl functionalized silica particles (Sigma-Aldrich, product reference 364258, particles size range 40-63 µm) to prepare silica/SPEEK composite membranes.31 They observed that the membranes conductivity is decreasing regularly with the silica content for

Kayser et al. a 35% DS but is much less affected in the case of a 52% DS polymer. The introduction of the amino-grafted silica particles also greatly reduced the water uptake and the swelling of the membranes, especially for the highest DS value. This second phenomenon was attributed either to the hydrophilic character of the grafted particles, meaning that an appropriate amount of water is retained in the sulfonated polymer without much alteration of its conductivity, or to a greater interaction of the amine moieties with the sulfonic acid groups, reducing the interaction of the latter with water molecules. To understand the results reported in Figures 8 and 9, different possible effects of adding nanoparticles in the SPEEK membranes should be discussed: (1) Silica being a nonconducting material should decrease the proton conductivity of the mixed membrane. Moreover, since the pore volume of the silica particles is 0.62 cm3 g-1, its maximum intrinsic water retention should be 0.62 g g-1. Because the water uptakes of the pure membranes at 80% and 67% DS are 0.60 and 0.31 g g-1, it is expected that this blending effect should increase the water uptake only in the 67% DS polymer membranes; (2) Both the silica surface OH’s and grafted amine groups may interact with the sulfonated acid groups of the polymer. These interactions are, however, quite different, the OH’s yielding weak hydrogen bridges whereas the electrostatic interactions of the quaternary ammonium formed by reaction with the proton are much stronger.55 Both should therefore decrease proton mobility but to different extents, the amine grafted silica is therefore expected to decrease proton conductivity more; (3) Introducing solid particles in the SPEEK membrane should result in a change of the microstructure of the hydrophilic/hydrophobic regions and therefore the geometry of the water channels. These changes may take different forms: (a) The particles may increase the interchain gaps in the polymer depending on their size.50-52 (b) The hydrophilic silica particles may create water channelling at the polymer/particle interface.50-52 This should be drastically decreased by the strong bonding of the amine grafts. (c) The porous silica may introduce new water channels not contributing to swelling. Prior to discussing the data in Figures 8 and 9, it should be recalled that the two measurements of proton conductivity and water uptake were not performed in the same conditions. Water uptake was measured after soaking the membranes in deionized water for 24 h. Proton conductivity was established for membranes saturated at RH 100% at room temperature also for 24 h. Thus the volumes of the water channels are believed to be higher in the first compared to the latter case. In Figure 8, it is observed that at both 80% and 67% DS, the changes in conductivity are indeed very similar for the grafted and nongrafted nanoparticles. This suggests that, contrary to expectations, the binding of the particles to the polymer is not the major factor in controlling conductivity. From Figure 9, however, it is found that at least at 80% DS, the grafted particles yield a significantly lower water uptake. This is obviously indicating that the amine grafted particles indeed play the role of a cross-linker. This cross-linking effect is, however, not a major factor of conductivity. Focusing on the 80% DS membranes, the complex evolution of conductivity at increasing silica loading indicates that several of the above-discussed factors act simultaneously. On both

Amine Grafted Silica/SPEEK Nanocomposites curves an initial conductivity decrease is followed by some increase between 5 and 10% silica content followed by a continuous decrease up to 30% loading. Since the two decreasing branches are not associated with the decrease in free proton concentration, they must be related to changes in the microstructure that are associated with a decreased volume of the water channels, as also indicated by the water uptake data in Figure 9. The conductivity increase observed at silica loading 5-10% is thus also related to some differences in the geometry of the water channels. These differences are likely associated with the channelling at the polymer/particle interface, which creates more open channels than the interchains gaps and more direct pathways for the protons. For some reason a similar effect is observed at 0-2.5% loading in 67% DS membranes. IV. Conclusion Nanospherical silica particles prepared by a modification of the Sto¨ber26 method proposed recently by Tatsumi23 were amine functionalized by grafting APDMS on their surface. Both functionalized and nonfunctionalized nanospheres were fully characterized before being used as the inorganic filler in mixed matrix silica/SPEEK membranes. Comparing the two sets of membranes obtained at varying silica loadings in terms of their proton conductivities and water uptakes allowed us to illustrate the complexity of the effects of introducing a filler on the behavior of these polymer electrolytes. Some cross-linking effect of the polymer chains owing to the interaction of the amine groups of the silica with the sulfonic acid groups of the SPEEK polymer is mostly observed at a higher degree of sulfonation but even in these conditions the proton conductivity of the mix matrix membrane is not essentially different for the amine grafted or pristine silica particles. This indicates that the main effect of the inorganic filler on proton conductivity is related to changes in the microstructure of the water channels in the wet polymer related to the distribution of nanoparticles in their lattice. Acknowledgment. The financial support of the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. Supporting Information Available: Calculation of the number of grafts/nm2 from the organic matter weight loss observed on thermogravimetric analysis (TGA). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) http://www.annso.freesurf.fr. (2) Higuchi, M.; Minoura, N.; Kinoshita, T. Chem. Lett. 1994, 2, 227. (3) Shoesmith, J. P.; Collins, R. D.; Oakley, M. J.; Stevenson, D. K. J. Power Sources 1994, 49, 129. (4) http://www.senat.fr/rap/r05-125/r05-12541.html. (5) Lufrano, F.; Squadrito, G.; Patti, A.; Passalacqua, E. J. Appl. Polym. Sci. 2000, 77, 1250. (6) Nolte, R.; Ledjeff, K.; Bauer, M.; Mu¨lhaupt, R. J. Membr. Sci. 1993, 83, 211. (7) Yamada, O.; Yin, Y.; Tanaka, K.; Kita, H.; Okamamoto, K.-I. Electrochim. Acta 2005, 50, 2655. (8) Woo, Y.; Oh, S. Y.; Kang, Y. S.; Yung, B. J. Membr. Sci. 2003, 220, 31. (9) Kaliaguine, S.; Mikhailenko, S.; Zaidi, S. M. J. U.S. Patent 6 716 548 2004. (10) Kaliaguine, S.; Mikhailenko, S. D.; Wang, K. P.; Xing, P.; Robertson, G.; Guiver, M. Catal. Today 2003, 82, 213. (11) Mikhailenko, S. D.; Wang, K.; Kaliaguine, S.; Xing, P.; Robertson, G. P.; Guiver, M. D. J. Membr. Sci. 2004, 233, 93. (12) Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Wang, K.; Kaliaguine, S. J. Membr. Sci. 2004, 229, 95. (13) Li, L.; Zhang, J.; Wang, Y. J. Membr. Sci. 2003, 226, 159.

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