pubs.acs.org/Langmuir © 2010 American Chemical Society
Proton Exchange Membranes for Application in Fuel Cells: Grafted Silica/SPEEK Nanocomposite Elaboration and Characterization Marc X. Reinholdt§ and Serge Kaliaguine* D epartement de G enie Chimique, Universit e Laval, 1065 avenue de la M edecine, Qu ebec, QC G1 V 0A6, Canada. § Present address: Institut Charles Gerhardt de Montpellier, UMR 5253, CNRS-UM2-ENSCM-UM1, Place Eug ene Bataillon 34095, Montpellier cedex 5, France. Received January 5, 2010. Revised Manuscript Received March 14, 2010 Hydrogen technologies and especially fuel cells are key components in the battle to find alternate sources of energy to the highly polluting and economically constraining fossil fuels in an aim to preserve the environment. The present paper shows the synthesis of surface functionalized silica nanoparticles, which are used to prepare grafted silica/SPEEK nanocomposite membranes. The nanoparticles are grafted either with hexadecylsilyl or aminopropyldimethylsilyl moieties or both. The synthesized particles are analyzed using XRD, NMR, TEM, and DLS to collect information on the nature of the particles and the functional groups, on the particle sizes, and on the hydrophilic/hydrophobic character. The composite membranes prepared using the synthesized particles and two SPEEK polymers with sulfonation degrees of 69.4% and 85.0% are characterized for their proton conductivity and water uptake properties. The corresponding curves are very similar for the composites prepared with both polymers and the nanoparticles bearing the two functional groups. The composites prepared with the nanoparticles bearing solely the aminopropyldimethylsilyl moiety exhibit lower conductivity and water uptake, possibly due to higher interaction of the polymer sulfonic acid sites with the amine groups. The composites prepared with the nanoparticles bearing solely the hexadecylsilyl moiety were not further investigated because of very high particles segregation. A study of the proton conductivity as a function of temperature was performed on selected membranes and showed that nanocomposites made with nanoparticles bearing both functional moieties have a higher conductivity at higher temperatures.
Introduction With increasing concern in our society for the preservation of the environment and the challenge to find alternative sources of energy to the highly polluting and economically constraining fossil fuels, research on hydrogen technologies is of tremendous interest both in academia and in industry.1-7 The technology of fuel cells is very attractive because of their high energy converting efficiency and their cleanliness. Among the different fuel cell technologies presently developed, proton exchange membrane (PEM) fuel cells (PEMFC) have a low operation temperature and an attractive advantage of simplicity, making them interesting devices for vehicular transportation. There are three key components in this type of fuel cell device, the electrodes, the bipolar plates, and the electrolyte, together forming a membrane electrode assembly (MEA). Research is developing on all three components, but the present work will focus on the electrolyte. In the case of PEMFC devices, the electrolyte is a membrane constituted by a polymer matrix, which presents specific properties: high proton conduction (ca. 10-1 S.cm-1 at the operation temperature of the PEMFC), low permeability to the fuel (hydrogen) and the oxidant (oxygen), enough hydration to allow good operations, and good chemical and mechanical stability for *Corresponding author. S. Kaliaguine:
[email protected], phone: 418-656-2708, fax: 418-656-3810. (1) 73. (2) (3) 181. (4) (5) (6) (7)
long-term operations. Among the various polymers used to prepare PEM membranes, perfluorosulfonated ones, especially Nafion, are the most widespread and widely studied. If this family of polymers exhibits high conductivity, it loses it at temperatures above 80 °C; they are also relatively expensive and not very environmentally friendly because of their high fluorine content. One final drawback is that they are highly permeable to methanol, limiting their utilization for direct methanol fuel cells (DMFC), which are promising for compact portable devices (laptops, cell phones, auxiliary power units, etc.). Thus, even if Nafion and Nafion-like polymers dominate the market of PEMFCs, a great deal of the research on PEM membranes is focused on finding alternate electrolytes, essentially to reduce the cost of the fuel cell, which limits its commercialization on a large scale. Most of the alternate polymers used as PEM membranes are sulfonated aromatic polymers, such as sulfonated poly(ether sulfone)s (SPES), sulfonated polybenzimidazoles (SPBI), sulfonated polyimides (SPI), sulfonated polyphosphazenes (SPP), and sulfonated poly(ether ether ketone)s (SPEEK).4,5,8 Among these polymeric compounds, SPEEKs present really interesting characteristics: low synthesis cost; good mechanical, chemical, thermal, and hydrolytic properties in certain conditions; and very low methanol permeability.1,3,4,6,9-12 However, at sulfonation degrees (DS) higher than 80%, the hydrolytic and mechanical properties of SPEEK become really poor and the membrane may eventually
Alberti, G.; Casciola, M.; Massinelli, L.; Bauer, B. J. Membr. Sci. 2001, 185, Alberti, G.; Casciola, M. Annu. Rev. Mater. Res. 2003, 33, 129. Vetter, S.; Ruffmann, B.; Buder, I.; Nunes, S. P. J. Membr. Sci. 2005, 260, Shang, X.; Tian, S.; Kong, L.; Meng, Y. J. Membr. Sci. 2005, 266, 94. Deluca, N. W.; Elabd, Y. A. J. Polym. Sci., Part B 2006, 44, 2201. Sambandam, S.; Ramani, V. J. Power Sources 2007, 170, 259. Roziere, J; Jones, D. J. Adv. Polym. Sci. 2008, 215, 219.
11184 DOI: 10.1021/la100051j
(8) Roziere, J; Jones, D. J. Annu. Rev. Mater. Res. 2003, 33, 503. (9) Zaidi, S. M. J.; Mikhailenko, S. D.; Robertson, G. P.; Guiver, M. D.; Kaliaguine, S. J. Membr. Sci. 2000, 173, 17. (10) Kaliaguine, S.; Mikhailenko, S. D.; Wang, K. P.; Xing, P.; Robertson, G.; Guiver, M. Catal. Today 2003, 82, 213. (11) Li, L.; Zhang, J.; Wang, Y. J. Membr. Sci. 2003, 226, 159. (12) Mikhailenko, S. D.; Wang, K.; Kaliaguine, S.; Xing, P.; Robertson, G. P.; Guiver, M. D. J. Membr. Sci. 2004, 233, 93.
Published on Web 06/15/2010
Langmuir 2010, 26(13), 11184–11195
Reinholdt and Kaliaguine
dissolve in water when heated. Thus, to compensate this weakness, fillers have been added to improve the mechanical and hydrolytic properties of SPEEK membranes, while only slightly altering their proton conductivity and permeability properties.2,13-22 Indeed, several attempts have been realized using clay materials,13-15 zeolites,16,17 metal oxides,18-20 or mesoporous materials,21,22 either under their pristine form or with their surface modified. Among the various studies performed, two will be discussed below. Karthikeyan et al.21 worked on MCM-41/SPEEK and laponite/SPEEK composites prepared with imidazole surface-modified particles and Carbone et al.23 worked using 3-aminopropyl functionalized silica particles to prepare silica/SPEEK composite membranes. In this work, the synthesis of surface grafted silica nanoparticles and their use in preparing grafted silica/SPEEK nanocomposite membranes were studied. The functional groups used here were either hexadecylsilyl or aminopropyldimethylsilyl moieties or both. The synthesized particles were analyzed using XRD, NMR, TEM, and DLS in the aim of obtaining information on the nature of the particles and the functional groups, on the particle sizes, and on their hydrophilic/hydrophobic character. The composite membranes prepared using the synthesized particles and two SPEEK polymers with sulfonation degrees (DS) of 69.4% and 85.0% were characterized for their proton conductivity and water uptake properties. A study of the proton conductivity as a function of temperature was performed on selected membranes.
Materials and Methods Synthesis.
Reagents. The reagents were used as received,
except N,N-dimethylacetamide and the solvents used for dynamic light scattering measurements, which were filtered through a Whatman 0.02 μm inorganic membrane. The following chemicals were purchased from Sigma-Aldrich: tetrapropylammonium hydroxide 1 M (ca. 20 wt %) aqueous solution, tetraethoxysilane (98 wt %), poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl1,4-phenylene) (PEEK; typical Mn = 10 300; typical Mw = 20 800), 1-butanol (99.9 wt %), and N,N-dimethylacetamide (99 wt %). Cyclohexane (99þ wt %), toluene (99.5 wt %), and concentrated sulfuric acid (95-98 wt %) were obtained from Fisher Scientific and anhydrous ethanol from Commercial Alcohols. Gelest provided hexadecyltriethoxysilane (95 wt %) and 3-aminopropyldimethylethoxysilane (>95 wt %).
Synthesis of Grafted Silica Particles. Synthesis of C16 Grafted Particles. The synthesis of the silica particles was per-
formed using a three-step method adapted from Huang et al.24 for the first step and Vuong and Do for the other two.25 In the first step, two solutions were prepared and matured separately. With the aim of obtaining a clear gel solution, 3.5 g of tetrapropylammonium hydroxide (TPAOH, 1 M) aqueous solution was added (13) Chang, J.-H.; Park, J. H.; Park, G.-G.; Kim, C.-S.; Park, O. O. J. Power Sources 2003, 124, 18. (14) Gaowen, Z.; Zenthao, Z. J. Membr. Sci. 2005, 261, 107. (15) Gosalawit, R.; Chirachanchai, S.; Shishatskiy, S.; Nunes, S. P. J. Membr. Sci. 2008, 323, 337. (16) Ahmad, M. I.; Zaidi, S. M. J.; Rahman, S. U. Desalination 2006, 193, 387. (17) Sengul, E.; Erdener, H.; Akay, R. G.; Yucel, H.; Bac, N.; Eroglu, I. Int. J. Hydrogen Energy 2009, 34, 4645. (18) Dou, Z.; Zhong, S.; Zhao, C.; Li, X.; Fu, T.; Na, H. J. Appl. Polym. Sci. 2008, 109, 1057. (19) Mecheri, B.; D’Epifanio, A.; Traversa, E.; Licoccia, S. J. Power Sources 2008, 178, 554. (20) Gao, Q.; Wang, Y.; Xu, L.; Wei, G.; Wang, Z. Chin. J. Chem. Eng. 2009, 17, 207. (21) Karthikeyan, C. S.; Nunes, S. P.; Prado, L. A. S. A.; Ponce, M. L.; Silva, H.; Ruffmann, B.; Schulte, K. J. Membr. Sci. 2005, 254, 139. (22) Bello, M.; Zaidi, S. M. J.; Rahman, S. U. J. Membr. Sci. 2008, 322, 218. (23) Carbone, A.; Pedicini, R.; Sacca, A.; Gatto, I.; Passalacqua, E. J. Power Sources 2008, 178, 661. (24) Huang, Q.; Vinh-Thang, H.; Malekian, A.; Eic, M.; Do, T.-O.; Kaliaguine, S. Microporous Mesoporous Mater. 2006, 87, 224. (25) Vuong, G. T.; Do, T.-O. J. Am. Chem. Soc. 2007, 129, 3810.
Langmuir 2010, 26(13), 11184–11195
Article to 2.25 g of tetraethoxysilane (TEOS) with continuous stirring for 4 h (solution A). 1.5 g of 1 M TPAOH solution was added to 44 g of deionized water (solution B) and submitted to 1 h stirring before adding it slowly to solution A under vigorous stirring, followed by continuous stirring overnight. The molar composition of the gel was SiO2-0.47 TPAOH-251.72 H2O. The clear gel subsequently obtained was transferred into a 60 mL polypropylene (PP) bottle and autoclaved at 80 °C for 6 h under autogenous pressure. After this precrystallization period, the bottle was cooled down to room temperature in air. In the second pregrafting step, the previously cooled suspension was added to about 500 mL of a N,Ndimethylacetamide(DMAc)/1-butanol(ButOH) mixture (30 wt % in alcohol) containing the required amount (10 mol %/SiO2, 0.41 g) of hexadecyltriethoxysilane (C16(EtO)3Si). The suspension was heated for ca. 15 h at 60 °C in a boiling flask equipped with a condenser and a thermometer, and under continuous stirring. Afterward, the suspension resulting from the pregrafting stage was transferred, for the third step, into an autoclave for further hydrothermal treatment at 170 °C during 120 h. The resulting suspension was centrifuged after cooling to room temperature and washed three times with ethanol (EtOH). Following this washing process, all solid parts were suspended in EtOH and then heated to dryness at 60-80 °C. The powder was subsequently ground with an agate mortar. The sample grafted with hexadecyltriethoxysilane will be designated below as sample A. The calcined form, at 550 °C for 10 h, of this sample will be designated as sample D. Synthesis of Amino-Functionalized Particles. The C16 grafted particles, either as made sample A or as calcined sample D, were further grafted with an amine functional group. Amino-Functionalization of the C16 Grafted Particles. In a boiling flask, 0.5 g of the grafted silica powder (sample A) was added to 200 mL of toluene containing 0.5 g of 3-aminopropyldimethylethoxysilane (NH2-Pr(Me)2EtOSi). The mixture is heated up to 60 °C for about 15 h under stirring. The resulting suspension was centrifuged after cooling down to room temperature, washed with EtOH, and then centrifuged. The process was repeated twice. After washing, all solid parts were suspended in EtOH and then heated to dryness at 60-80 °C. The dried powder was subsequently ground in an agate mortar. The sample grafted with hexadecyltriethoxysilane and 3-aminopropyldimethylethoxysilane will be designated below as sample B. Amino-Functionalization of the Calcined Particles. This second process of functionalization was adapted from a method described by Kulkarni et al.26 and uses the form calcined at 550 °C for 5 h of the previously synthesized C16-grafted particles (sample D). In a beaker, 0.4 g of the calcined silica powder was dispersed into 50 mL of anhydrous EtOH containing 0.43 g of NH2-Pr(Me)2EtOSi. The mixture was sonicated for about 35 min and transferred into a 250 mL three-neck flask. About 50 mL of anhydrous EtOH was then added and the flask was heated up to 50 °C for 4 h, under stirring. The resulting suspension was centrifuged after cooling to room temperature and washed three times with EtOH. All solids were suspended in EtOH and then heated to dryness at 60-80 °C. The dried powder was subsequently ground in an agate mortar. The sample solely grafted with 3-aminopropyldimethylethoxysilane will be designated as sample C. Sulfonation of the Polymer PEEK. Typically, 20 g of PEEK was dissolved in 800 mL of concentrated sulfuric acid in a 1 L Erlenmeyer flask equipped with mechanical stirring and tubing to perform the reaction under a flow of argon. The reaction was conducted during the time required to obtain the desired sulfonation degree (DS). However, since the reaction was very sensitive to the experimental equipment and conditions the straight line giving DS as a function of synthesis duration was established every time a parameter or a piece of equipment was modified. After the desired time of reaction, the acidic viscous liquid was transferred into a large excess of ice-cooled deionized water under continuous (26) Kulkarni, S. S.; Hasse, D. J.; Corbin, D. R.; Patel, A. N. U.S. Patent 6,508,860 B1, Jan. 21, 2003.
DOI: 10.1021/la100051j
11185
Article stirring to precipitate the sulfonated polymer (SPEEK). The suspension was then centrifuged, and the polymer was recovered for further dialysis in deionized water until reaching the pH of water. The resulting suspension was centrifuged again and the polymer dried at 60-80 °C. Preparation of the Silica/SPEEK Membranes. Suspensions of about 10 wt % of material in N,N-dimethylacetamide (DMAc) were prepared by adding the required amounts of SPEEK polymer and silica particles to the solvent. The suspensions were then cast onto a glass plate in the form of 7 cm2 membranes which were allowed to dry for two days under gentle air flow. The resulting membranes were then cured in a vacuum oven for 48 h at 60 °C, then 48 h at 90 °C, and finally 24 h at 120 °C. The resulting cured membranes were cooled down to room temperature and stuck off the plate by using deionized water, then eventually warmed. The membranes were then allowed to dry for several days at room temperature before being characterized.
Analytical Methods. Characterization of the Silica Particles. Powder XRD patterns were recorded for all samples from
1° to 60° 2θ with a step size of 0.02°/point and a rate of 1°.min-1 using a Siemens D5000 diffractometer employing Cu KR radiation (λ = 1.540 59 A˚) and working at 40 kV and 30 mA. Samples were pressed on plastic holders for analysis. 29 Si NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer at a frequency of 59.6016 MHz, using a Bruker 4 mm MAS probe. The experiments were performed using a Hahn echo pulse sequence (π/2 pulse - τ - π pulse - τ - acquisition) with a 90° pulse of 3 μs and a recycle delay of 60 s. The calculated value of τ is correlated to the pulse width and synchronized with the spinning frequency. 720 transients were recorded at a spinning speed of 10 kHz, and the chemical shifts were referenced at 0 ppm to external tetramethylsilane (TMS). 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 29Si. The experiments were performed at a MAS frequency of 10 kHz, with a contact time of 1000 μs, a relaxation delay of 4 s, and a 90° pulse of 3.9 μs. 10 000 transients were recorded and the chemical shifts were referenced to the methylene group of external adamantane 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. For both 29Si MAS and 1H-13C CP-MAS, rotors were made of zirconia and the caps were made of Kel-f polymer. After their acquisition, the free induction decay (FID) signals were treated following standard procedures and the 29Si MAS spectra were simulated using the NMR Utility Transform software (NUTS, Acorn NMR software). Silicon elemental analysis was performed on a Perkin-Elmer 1100B atomic absorption spectrophotometer. Typically about 20-30 mg of sample was dissolved in a mixture of 25 mL of HCl (10 wt %) and 1 mL of HF (40 wt %). The digestion of the sample is performed under rotation during 6 h at about 60 °C. The solution is then adequately diluted and analyzed. CHN elemental analyses were performed on a Perkin-Elmer CHNS/O analyzer, series II - 2004. The samples were examined by TEM using a Jeol JEM 1230 microscope equipped with a LaB6 filament and working at an excitation voltage of 80 kV. Prior to analysis, a few milligrams of sample were dispersed in methanol, toluene, or cyclohexane, and a few droplets of the suspension were left to evaporate on a tiny nickel grid covered with a Formvar film. Dynamic light scattering measurements were made on a Malvern Zetasizer Nano ZS (Malvern Instruments Ltd.) with a detection angle of 173°. All measurements were taken at a temperature of 20 °C and 5 to 10 repeat measurements were taken for each sample. The Nano ZS uses a 4 mW He-Ne laser operating at a wavelength of 633 nm. The intensity size distributions were obtained from analysis of the correlation functions using the Multiple Narrow Modes algorithm in the instrument software (Dispersion Technology Software, Malvern Instruments 11186 DOI: 10.1021/la100051j
Reinholdt and Kaliaguine Ltd.). Suspensions of 0.5-2.0 g 3 L-1 of the grafted silicate samples dispersed in filtered cyclohexane, toluene, DMAc, or various other solvents were submitted to 2-3 h of sonication before performing the analysis. Characterization of the SPEEK Polymer. 1H NMR spectra of the SPEEK samples were recorded on a Varian Unity Inova 400 MHz spectrometer at a frequency of 399.95 MHz, using a 5 mm probe. A 45° pulse and a 2 s recycle delay were used for the acquisition and 16 transients were accumulated for each spectrum. The chemical shifts were referenced at 0 ppm to external tetramethylsilane (TMS). A solution of ca. 5 wt % of SPEEK in deuterated dimethyl sulfoxide (DMSO-d6) was prepared for the analysis. The DS was determined by integration of specific aromatic signals as described previously.9,27 Characterization of the Composite Membranes. The proton conductivities of the membranes were measured by AC impedance spectroscopy, using a Solartron 1260 gain phase analyzer, over the frequency range 10-107 Hz with controlled voltage. The membrane strips, ca. 15 mm 8 mm, were clamped between two pairs of electrodes and placed in a chamber where the temperature was controlled, the relative humidity maintained at 100%, and the pressure equal to the atmospheric pressure. The membranes where placed in the cell for 24 h before performing the analysis. The conductivity was calculated from the impedance data which were measured in the longitudinal direction of the membrane strip. The resistance R of the membrane was derived from the low intersection of the high-frequency semicircle on a complex impedance plane with the real Re(Z) axis. The conductivity is calculated using the relationship σ = d/(R S), where σ is the conductivity in S.cm-1, R the measured resistance of the membrane in Ω, d the distance between the two electrodes in cm, and S = w t, where w is the width and t the thickness of the membrane strip, both in cm. The σ values reported in this work were the average of at least three measurements performed on each membrane. The water uptake was determined by weighing membrane pieces of ca. 10 mm 10 mm after immersion in deionized water for 24 h, and after drying in an oven at 60-70 °C for 24 h. Before measuring the weight of the soaked membrane, the water was removed from its surface by blotting it with a paper towel. The weighing of the hydrated films was performed three times, and the average value used as the wet mass. The water content was calculated from the equation Wc = [(Ww - Wd)/Wd] 100, where Wc is the water content, Ww and Wd the wet and the dry mass, respectively.
Results and Discussion Synthesis and Characterization of the Functionalized Silica Particles. Since the purpose of this work is to use as small as possible grafted silica particles, the particle size was investigated at different stages of the synthesis procedure including the final particles used to prepare the composite membranes. This was eventually performed by dispersing the particles in various solvents. First, the size of the subcolloidal particles obtained at the end of the first synthesis step of precrystallization (vide supra) was determined right before performing the second step of pregrafting. DLS measurements performed on a sample of the clear gel suspension are presented in Figure 1. Before discussing these results, some explanations on the DLS principle must be given. This technique allows determination of the size of colloidal suspensions by measuring the intensity of laser light scattered by a suspension of particles undergoing Brownian motion.28 The fluctuations of scattered light are time-dependent and correlated (27) Xing, P.; Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Wang, K.; Kaliaguine, S. J. Membr. Sci. 2004, 229, 95. (28) Kaszuba, M.; McKnight, D.; Connah, M. T.; McNeil-Watson, F. K.; Nobbmann, U. J. Nanopart. Res. 2008, 10, 823.
Langmuir 2010, 26(13), 11184–11195
Reinholdt and Kaliaguine
Article
Figure 2. XRD pattern of sample A silica particles.
Figure 1. Average particle size distributions recorded by DLS on the clear gel suspension obtained at the end of the first step of synthesis, prior to the grafting process. Distribution observed by (a) intensity of scattered light, (b) volume, and (c) number of particles.
to the diffusion of the particles being measured. The measurement of the speed of the Brownian motion provides a translational diffusion coefficient, which can be converted into a hydrodynamic diameter. The distribution of the diameters can be observed in different modes: intensity of scattered light, volume, and number of particles, which are presented as Figure 1a,b,c, respectively. The first mode, intensity of scattered light, is actually the signal recorded; the volume and number of particles modes are calculated from the former using the Mie theory.29 Since the particle scattering intensity is proportional to the square of the molecular weight, a small number of larger particles or aggregates can dominate the distribution and lead to a misinterpretation of the data. In some applications, a volume or number distribution is more informative than the intensity distribution, though the calculation using Mie theory may introduce errors, related to the theory assumptions (spherical particles, homogeneous and equivalent density of the particles, known optical properties), which will increase from the volume to the number distributions. Thus, it is highly advised to use the volume and number (29) Mie, G. Ann. Physik 1908, 4, 377. (30) Calculating Vol. Distributions From DLS Data, Malvern Instruments Ltd., available online at http://www.malvern.com/malvern/kbase.nsf/allbyno/KB000775 (registration required).
Langmuir 2010, 26(13), 11184–11195
distribution only for comparison purposes and for estimating the relative amounts of multimodal samples (multiple size peaks).30 As a consequence for this work, all the discussion concerning DLS will be based on the examination of both intensity and volume distributions. From the above statements, one can easily analyze the results shown in Figure 1, which are the average particle size distributions deduced from the multiple scans recorded on the clear gel suspension and shown as Figure S1 in the Supporting Information. A main population of particles with an average size of 8.9 ( 2.5 nm is clearly dominating in the clear gel solution, though some larger particles are observed (Figure 1a). Their instability observed when recording the various scans (Supporting Information Figure S1a) demonstrates that these are agglomerates undergoing assembly and dispersion cycles rather than real particles. This clear gel solution was then used to pursue the functionalization process with the aim of conserving the nanosize of the silica particles. Indeed, further heating of the clear gel led to growth of the particles. It was virtually impossible to recover the nanoparticles by centrifugation at this stage of the process. However, it was possible to recover the particles after the next C16 grafting step, and these particles (samples A and D), as well as the subsequent ones (samples B and C), were analyzed using XRD, NMR, TEM, and DLS. At this point, we shall highlight the role of TPAþ cations during the synthesis process. A sample was synthesized at the same conditions as described previously for the first precrystallization step of the C16 grafting process, though instead of using TPAOH, a 1 M NaOH solution was used to adjust the pH of the mixture at the same value. Here, again, the size of the subcolloidal particles obtained at the end of the precrystallization step was investigated by DLS. The measurements show that the clear gel contains only a small amount of nanoparticles with a size of ca. 30 nm and a much larger amount of instable particles with a size larger than 1 μm, probably agglomerates (Supporting Information Figure S2). These observations demonstrate that TPAþ cations are playing an important role for maintaining the smaller size of the particles during the growing process. These cations are most likely covering the surface of the particles limiting their growth and probably giving them a regular shape. The cation coverage is also probably increasing the positive charge density at the particle surface, increasing the electrostatic repulsion between particles, and thus limiting agglomeration. Samples A to D were analyzed using XRD and their patterns are typical of an amorphous phase. The diffractogram of sample A is shown in Figure 2 as an example. The different particles have thus been further investigated using 29Si MAS NMR. The spectra of the particles are presented DOI: 10.1021/la100051j
11187
Article
Figure 3.
Reinholdt and Kaliaguine
29
Si MAS NMR spectra of the various silica particles: (a) sample A, (b) sample B, (c) sample C, and (d) sample D. Table 1. 29Si Chemical Shifts and Their Attribution for the Different Silica Particles Used in This Work silica particles
sample ID
chemical shift (ppm)
-111.9 -106.8 -100.1 -68.0 -63.9 B -112.4 C16(EtO)3Si þ NH2-Pr(Me)2EtOSi grafting -107.4 -100.0 -68.3 -64.7 -22.5 11.0 C -111.5 NH2-Pr(Me)2EtOSi grafting -106.0 -99.6 12.0 Calcined at 550 °C during 10 h D -111.5 -105.2 -99.6 a Peak relative areas are for a given group of resonances, such as Qn or Tn. C16(EtO)3Si grafting
A
in Figure 3 and the NMR data are given in Table 1. All spectra exhibit resonances in the Qn region.31-37 The spectrum of sample A (Figure 3a, Table 1) shows a resonance at -111.9 ppm corresponding to silicon atoms in the Q4 environment, i.e., a silicon atom linked (31) Engelhardt, G.; Michel, D. High Resolution Solid-State NMR of Silicates and Zeolites; John Wiley & Sons: New York, 1987. (32) Kimura, T.; Saeki, S.; Sugahara, Y.; Kuroda, K. Langmuir 1999, 15, 2794. (33) Daehler, A.; Boskovic, S.; Gee, M. L.; Separovic, F.; Stevens, G. W.; O’Connor, A. J. J. Phys. Chem. B 2005, 109, 16263. (34) de Monredon, S.; Pottier, A.; Maquet, J.; Babonneau, F.; Sanchez, C. New J. Chem. 2006, 30, 797. (35) Shylesh, S.; Singh, A. P. J. Catal. 2006, 244, 52. (36) Garcia, N.; Benito, E.; Guzman, J.; Tiemblo, P. J. Am. Chem. Soc. 2007, 129, 5052. (37) Garcia, N.; Benito, E.; Guzman, J.; Tiemblo, P.; Morales, V.; Garcia, R. A. Microporous Mesoporous Mater. 2007, 106, 129.
11188 DOI: 10.1021/la100051j
attribution
fwhm (Hz)
peak relative area (%)a
Q4 Q3 Q2 T3 T2 Q4 Q3 Q2 T3 T2 M Q4 Q3 Q2 M Q4 Q3 Q2
510 580 510 400 400 510 490 370 420 410 610 560 400 620 480 460
61 28 11 61 39 65 26 9 68 32 67 25 8 72 19 9
via oxygen bridges to four other silicon atoms. A shoulder is visible at -106.8 ppm on the left side of the peak and corresponds to the Q3 environment, i.e., a silicon atom linked via oxygen bridges to three other silicon atoms. The presence of a Q2 resonance, i.e., a silicon atom linked via oxygen bridges to two other silicon atoms, at -100.1 ppm shows that the sample has a low crystallinity or contains many defects. A resonance appears also at -68.0 ppm and corresponds to a T3 environment, i.e., a silicon atom linked via oxygen bridges to three other silicon atoms, the fourth bond being Si-C, which confirms that the surface of the particles has been grafted by hexadecyltriethoxysilane.34-37 A resonance at -63.9 ppm corresponds to T2 environments, i.e., a silicon atom linked via oxygen bridges to two other silicon atoms, the third and fourth bonds being Si-C and Si-OH bonds. The spectrum of Langmuir 2010, 26(13), 11184–11195
Reinholdt and Kaliaguine
Figure 4. 1H-13C CP-MAS NMR spectra of the various silica particles: (a) sample A, (b) sample B, and (c) sample C.
sample B (Figure 3b, Table 1) exhibits not only the resonances corresponding to the Q4, Q3, Q2, T3, and T2 environments, but also some additional features at -22.5 and 11.0 ppm. The latter is associated to an M environment, i.e., a silicon atom linked via one oxygen bridge to another silicon atom and three Si-C or Si-OH bonds, characteristic of the grafting of the NH2-Pr(Me)2Si- moiety at the surface of the particle.32,33,36,37 The resonance at -22.5 ppm may correspond to an alkyl chain grafted to the silica surface by two Si-O bonds,38 i.e., a geminal G2 moiety, though since NH2-Pr(Me)2EtOSi has only one alkoxy functionality this is most improbable. A more probable possibility is that some NH2-Pr(Me)2EtOSi has reacted with the few available T2 moieties, either C16(EtO)Sid or C16(HO)Sid, to form a more complex polycondensed moiety. Such polycondensations between moieties is known to show resonances between -10 and -20 ppm.38,39 The spectrum of sample C (Figure 3c, Table 1) displays the resonances corresponding to the (38) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 3767. (39) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1981, 103, 4263.
Langmuir 2010, 26(13), 11184–11195
Article
Q4, Q3, Q2, and M environments, the latter confirming the grafting of the NH2-Pr(Me)2Si- moiety at the surface of the particles. The spectrum of the calcined particles, sample D (Figure 3d, Table 1), shows only Q4, Q3, and Q2 resonances. The various samples have also been further investigated using 1 H-13C CP-MAS NMR. The spectra of the particles are presented in Figure 4, and the chemical shifts and corresponding attributions are given in Table 2. The resonances at ca. 34, 33, and 31 ppm, observed in the spectra of samples A and B, are assigned to the carbons of the grafted hexadecyl chain, in either trans or gauche conformations for the first two and the latter resonances, respectively.40-42 The resonances at ca. 63, 60, and 14.5 ppm are assigned to -CH2- and -CH3 carbons present in alkoxy silane moieties, which demonstrates that some of the ethoxy silane groups of hexadecyltriethoxysilane have not reacted with the silica surface.37 This is in good agreement with the presence of T2 resonances in the 29Si MAS NMR spectra (vide supra). Resonances at 23.4 and 13.3 ppm, observed in A and B samples, are associated with groups which are present in either C16(EtO)3Si and NH2-Pr(Me)2EtOSi grafting molecules.37,43-45 Sample B is the only one for which the spectrum exhibits resonances at ca. 44 and 0.5 ppm, which are assigned to -CH2-NH2 and tSi-CH3 carbons, only present in the NH2-Pr(Me)2Si- graft.32,33,43,45,46 This confirms that sample B has been grafted by two different alkyl chains, as shown by 29Si NMR (vide supra), one of them bearing an amine group. The spectrum of sample C shows resonances which are readily assigned to the carbons present in the NH2-Pr(Me)2Si- graft.32,33,43,45,46 The splitting of the resonance corresponding to the -CH2- in the middle of the propyl chain is probably due to the fact that this carbon is at a comparable distance between a silicon atom and an amine group. Attempts have been made to measure specific surface area for samples A to D. The results indicated an almost complete lack of microporosity and nitrogen BET specific surface area not measurable likely due to too low values of the C constant. The results of the elemental analysis performed on the various samples used in this study are given in Table 3. One can easily observe that the samples grafted with the hexadecylsilyl moiety exhibit very high carbon contents (samples A and B). Additionally, the two samples containing aminopropyldimethylsilyl moieties do contain nitrogen (samples B and C), while the sample bearing solely hexadecylsilyl moiety does not (sample A). Although it is not possible to determine the amount of aminopropyldimethylsilyl contained in sample B because of the complexity of the system containing two types of grafts, it is however possible to have an estimation of the number of amino graft contained in sample C. From the nitrogen content of this sample, it is easy to estimate that it contains about 4.4 10-4 mol of graft per gram of sample, leading to about 4.57 10-4 mol of graft per gram of silica, corresponding to about 2.75 1020 grafts per gram of silica. This number shall be compared to about 1.014 1022 atoms of silicon per gram of silica, leading to an estimation of 1 graft for 37 atoms of silicon. TEM observations of samples A to C show that all samples are constituted of nanoparticles with a size ranging mostly between 8 and 20 nm, with a few larger particles having a size up to ca. 50 nm (40) Venkataraman, N. V.; Vasudevan, S. J. Phys. Chem. B 2001, 105, 7639. (41) Kubies, D.; Jerome, R.; Grandjean, J. Langmuir 2002, 18, 6159. (42) Shimojima, A.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Kuroda, K. J. Am. Chem. Soc. 2005, 127, 14108. (43) Lippmaa, E. T.; Alla, M. A.; Pehk, T. J.; Engelhardt, G. J. Am. Chem. Soc. 1978, 100, 1929. (44) Toury, B.; Babonneau, F. J. Eur. Ceram. Soc. 2005, 25, 265. (45) Calmettes, S.; Albela, B.; Hamelin, O.; Menage, S.; Miomandre, F.; Bonneviot, L. New J. Chem. 2008, 32, 727. (46) Kallury, K. M. R.; Macdonald, P. M.; Thompson, M. Langmuir 1994, 10, 492.
DOI: 10.1021/la100051j
11189
Article
Reinholdt and Kaliaguine Table 2. 13C Chemical Shifts and Their Attribution for the Different Silica Particles Used in This Work silica particles
sample ID
chemical shift (ppm)
attribution
C16(EtO)3Si grafting
A
C16(EtO)3Si þ NH2-Pr(Me)2EtOSi grafting
B
NH2-Pr(Me)2EtOSi grafting
C
ca. 63 59.9 ca. 34 32.6 30.7 23.4 14.5 13.3 ca. 63 59.9 ca. 44 ca. 34 32.8 30.8 23.4 14.5 13.3 0.5 43.8 25.5 22.4 14.5 -3.0
-CH2- in Et-O-Sit -CH2- in Et-O-Sit carbons C3 to C15 in hexadecyl c in trans conformation carbons C3 to C15 in hexadecyl chain in trans conformation carbons C3 to C15 in hexadecyl chain in gauche conformation -*CH2-CH2-Sit a -CH3 from Et-O- group and hexadecyl chain -CH2-Sit -CH2- in Et-O-Sit -CH2- in Et-O-Sit -CH2-NH2 carbons C3 to C15 in hexadecyl chain in trans conformation carbons C3 to C15 in hexadecyl chain in trans conformation carbons C3 to C15 in hexadecyl chain in gauche conformation -*CH2-CH2-Sit a -CH3 from Et-O- group and hexadecyl chain -CH2-Sit tSi-CH3 -CH2-NH2 tSi-CH2-*CH2-CH2-NH2a tSi-CH2-*CH2-CH2-NH2a -CH2-Sit tSi-CH3
a
Signal of the carbon marked with an asterisk.
Table 3. Elemental Analysis Data Obtained for the Different Silica Particles Used in This Work content (wt%) silica particles
sample ID
Si
C
N
C16(EtO)3Si grafting C16(EtO)3Si þ NH2-Pr(Me)2EtOSi grafting NH2-Pr(Me)2EtOSi grafting
A B
42.0 34.3
19.36 21.10
nda 0.40
C
45.5
2.59
0.62
a
nd: none detected.
(Figure 5). The observation of a large number of very small particles, i.e., below 20 nm, in sample A, which is grafted with a long alkyl chain of 16 carbons, is in good agreement with the size of the particles, measured using DLS, observed at the previous stage of precrystallization (i.e., ca. 8 nm, vide supra). This also demonstrates that the grafting process allows maintaining a smaller size of the nanoparticles. The TEM pictures also show that samples B and C retain their particle sizes. However, since the surfaces of the different samples are modified by either a long alkyl hydrophobic chain (sample A) or a shorter, eventually charged, hydrophilic moiety (sample C), or both (sample B), their behavior when dispersed in various solvents and their electrostatic properties are expected to be very different. Thus, we further analyzed the behavior of the various nanoparticles by performing DLS measurements on suspensions in different solvents, including the casting solvent used to prepare the nanocomposite membranes, i.e., DMAc. Hydrophobic solvents, such as toluene, cyclohexane, heptane, octane, and hexadecane, and hydrophilic solvents (i.e., alcohols), such as methanol, ethanol, and isopropanol, were used. Surprisingly, in the case of the alcohols, none of the samples exhibited a stable population of particles having a size below several hundreds of nanometers or even 1 μm (results not shown), showing that all particles tend to agglomerate in these solvents. Among all hydrophobic solvents, suspensions of samples A and B in toluene or cyclohexane exhibited the smallest agglomerates. A suspension of sample C in any of these two solvents resulted in the flocculation of the particles, which then settled down to the bottom of the analysis cell. Thus, sample A clearly shows a hydrophobic charac11190 DOI: 10.1021/la100051j
Figure 5. TEM pictures, taken at lower and higher magnifications from left to right, of the silica particles (top) sample A dispersed in cyclohexane, (middle) sample B, and (bottom) sample C, both dispersed in methanol.
ter, as expected, sample B seems to be also more hydrophobic than hydrophilic, and sample C seems to be more hydrophilic, but tends to agglomerate in alcohols. The size of the agglomerates in cyclohexane and DMAc, the membrane casting solvent, is further discussed below. Sample A dispersed in cyclohexane shows three populations of agglomerates, which are continuously undergoing assembly and disassembly processes (Supporting Information Figures S3a and S4a). However, the DLS volume distribution Langmuir 2010, 26(13), 11184–11195
Reinholdt and Kaliaguine
Article
Figure 6. Average particle size distributions by volume of particles recorded by DLS on suspensions of the different silica particles: sample A in (a) cyclohexane or (b) DMAc; sample B in (c) cyclohexane or (d) DMAc; and (e) sample C in DMAc. Table 4. Volume Distribution Results Obtained by DLS Measurements on the Different Grafted Silica Particles Dispersed in Cyclohexane or DMAc silica particles C16(EtO)3Si grafting
sample ID
solvent
peak
diameter (nm)
volume (%)
width (nm)
A
cyclohexane
1 2 3 1 2 1 2 3 1 1
24 94 842 73 269 32 103 382 190 336
75.4 13.4 11.2 38.6 61.4 55.7 12.2 32.1 100 100
7 32 298 12 38 8 31 138 28 57
DMAc C16(EtO)3Si þ NH2-Pr(Me)2EtOSi grafting
B
cyclohexane
NH2-Pr(Me)2EtOSi grafting
C
DMAc DMAc
shows that the main population of agglomerates represents ca. 75% of the total volume of particles and has an average size of 24 ( 7 nm (Figure 6a, Table 4). The same sample shows a more stable behavior in DMAc (Supporting Information Figures S3b and S4b), and two populations of agglomerates are observed at 73 ( 12 and 269 ( 38 nm representing ca. 39% and 61% of the total volume of particles, respectively (Figure 6b, Table 4). Sample B dispersed in cyclohexane shows three populations of agglomerates, which are continuously undergoing assembly and disassembly processes, as for sample A, but the instability seems to be much higher (Supporting Information Figures S3c and S4c). The DLS volume distribution shows that the main population of agglomerates represents ca. 56% of the total volume of particles and has an average size of 32 ( 8 nm and that there is also a major Langmuir 2010, 26(13), 11184–11195
contribution of ca. 32% of the agglomerates with a size of 382 ( 138 nm (Figure 6c, Table 4). Here again, the dispersion in DMAc seems to be more stable and shows a single distribution at 190 ( 28 nm (Figure 6d, Supporting Information Figures S3d and S4d, Table 4). As mentioned above, suspension of sample C in cyclohexane resulted in the flocculation of the particles. The dispersion of sample C in DMAc conducted to a single distribution of agglomerates with a size of 336 ( 57 nm (Figure 6e, Supporting Information Figures S3e and S4e, Table 4). All these DLS results demonstrate that sample A is highly hydrophobic, but it can be dispersed in DMAc and produce two relatively stable populations of agglomerates. Sample B is still hydrophobic, even though partially hydrophilic; i.e., its main agglomerates in cyclohexane are larger than for sample A and it exhibits one single agglomerate DOI: 10.1021/la100051j
11191
Article
Reinholdt and Kaliaguine
Figure 8. Proton conductivity of sample B silica/SPEEK nanocomposite membranes as a function of the particles content.
Figure 7. 1H NMR spectra of sulfonated poly(ether ether ketone) (SPEEK) polymers with a sulfonation degree of (a) 69.4% and (b) 85.0%.
distribution in DMAc, the size of which is intermediate between the two sizes observed for sample A. Surprisingly, sample C exhibits the largest agglomerates in DMAc, even if it is a hydrophilic sample and the solvent is polar and protic, i.e., it can give or receive hydrogen bonds. This discrepancy may arise from the creation of a hydrogenbonding network between the amino-grafted silica and DMAc, allowing a stabilization of larger agglomerates. These observations suggest that sample B might be a better compromise between hydrophobic particles (sample A) and charged particles (sample C). However, we shall take into account that these observations are indicative, made in the absence of SPEEK, and thus do not necessarily reflect the phenomena occurring in the drying process of the nanocomposite membranes, during which the concentrations of particles and polymer are varying. Characterization of the SPEEK Polymer. The 1H NMR spectra of the two polymers used here are shown in Figure 7. The spectra exhibit characteristic resonances of sulfonated PEEK polymers.9,10,12,27 More specifically, the signals at 7.4-7.5 ppm are associated with the proton of the carbon at the R position of the carbon bearing sulfonate group in the aromatic ring. Thus, the integration of these resonances with respect to all other peak surfaces allows determination of the DS of the polymer by using the following formula: DS/(12 - 2DS) = IR/It, where IR is the surface of the resonance corresponding to the proton at the R position and It is the total area of all remaining resonances. Integration of the spectra and calculation give DS values of 69.4% and 85.0% for the two polymers used in this work. Characterization of the Composite Membranes. A set of composite membranes with silica loading varying from ca. 2.5 to ca. 20 wt % was prepared for both DS values with the nanoparticles of sample B, which are grafted by amino functionalized and hexadecyl groups. Membranes with a silica nanoparticles loading of ca. 10 wt % were prepared using nanoparticles from samples A and C, and this for both DS values. Because of the lack of nanoparticles, fewer loadings were investigated for the set of 85% DS membranes than for the 69.4% DS one. 11192 DOI: 10.1021/la100051j
Aspect of the Membranes. The membranes prepared with the pristine SPEEK polymer are transparent and yellowish for both 69.4% and 85.0% DS values, the latter membranes being more brownish than the former. For the set of membranes prepared with various loadings of nanoparticles of sample B, all membranes are opaque for both DS, yellowish for the 69.4% DS membranes and brown-yellowish for the 85.0% DS membranes. An obvious segregation of the particles starts to appear for loadings equal to or above 15 wt %, and this is the reason why higher loadings than 20 wt % were not investigated in this work. For lower loadings, sample B nanoparticles are welldistributed and embedded in the membrane. For very low loadings, i.e., ca. 2.5 and ca. 5 wt %, a network of opaque and clear channels appear in the membrane, with the clearer region containing obviously much fewer particles. If this network is relatively limited for the 5 wt % loading, it is much more widespread for the 2.5 wt % one. This is probably due to a lack of particles in the membranes, which then migrate to preferential areas yielding microstructuration of the polymer in hydrophobic and hydrophilic regions.47-49 Such a network is much more extended in the membranes prepared with the nanoparticles of sample A, i.e., grafted with the hydrophobic hexadecyl chain, even at loadings up to ca. 10 wt %. The membranes prepared with sample A were therefore not investigated further. Finally, the nanocomposite membranes prepared with nanoparticles from sample C, i.e., grafted only with amino silane moieties, exhibit a partial segregation of the particles in areas with higher and lower concentrations. This segregation of the nanoparticles seems to create a network of rays emerging from a center point close to the center of the membrane. Since it seems that there is no area without particles, these membranes will be further investigated. All membranes have a thickness between 40 and 100 μm. Proton Conductivity. The proton conductivity of the two sets of composite membranes as a function of loading of sample B particles is represented in Figure 8. One can easily observe that the composites for both DS, i.e., 69.4% and 85.0%, exhibit a similar behavior at increasing particles loading, although with slight differences. First, the membranes prepared with the polymer having a 69.4% DS all exhibit a conductivity between 12 and 33 mS.cm-1. The conductivity of the membrane of pure SPEEK is ca. 31 mS. cm-1, and then when a small amount of grafted silica particles is added, ca. 2.4 wt %, the conductivity is almost divided by a factor of 2 down to 15.7 mS.cm-1. The conductivity seems to somehow (47) Kreuer, K.-D. J. Membr. Sci. 2001, 185, 29. (48) Paddison, S. J. Annu. Rev. Mater. Res. 2003, 33, 289. (49) Kreuer, K.-D.; Paddison, S. J.; Spohr, E.; Schuster, M. Chem. Rev. 2004, 104, 4637.
Langmuir 2010, 26(13), 11184–11195
Reinholdt and Kaliaguine
Article
Figure 9. Water uptake of sample B silica/SPEEK nanocomposite membranes as a function of the particle content. Table 5. Properties of the Nanocomposite Membranes Prepared with Sample C (Silica Particles Content ca. 10 wt %) SPEEK DS (%)
silica content (wt %)
Thickness ( μm)
conductivity (mS.cm-1)
water uptake (%)
69.4 85.0
9.9 10.0
50 50
10.8 44.4
33.7 49.9
reach a lower plateau at a little less than 13 mS.cm-1 for 5.1 and 7.6 wt % loadings. Then, when increasing the content of particles, the conductivity is gradually increasing to reach a maximum of ca. 32.6 mS.cm-1 at a particle content of ca. 14.6 wt %. Finally, the conductivity decreases again to a value of ca. 27.6 mS.cm-1 for a particle content of ca. 19.8 wt %. Second, the membranes prepared with the polymer having 85.0% DS all exhibit conductivity between 23 and 48 mS.cm-1. The conductivity of the pure SPEEK membrane is on the order of 46.3 mS.cm-1. Then, when a small amount of particles is added, the conductivity decreases but not as fast as for the 69.4% DS. The conductivity reaches a minimum at ca. 23 mS.cm-1 for a particle content of ca. 9.7 wt %. The conductivity is then increasing with further addition of particles, to become slightly higher than that of the pristine SPEEK at ca. 47.3 mS.cm-1 for a silica content of ca. 14.8 wt %. We shall notice here that the maximum conductivity is not observed for the pristine SPEEK polymer and this for both DS. The composite membranes prepared with ca. 10 wt % content of particles of sample C show much lower conductivity with the 69.4% DS and a much higher one with the 85.0% DS, with respect to their counterparts prepared with sample B (Table 5, Figure 8). At this point, one can postulate that this difference is due either to the fact that for every system the extremes do not appear at the same position or to the fact that the proton transport process is strongly affected by the segregation of the particles in the membranes as discussed above. Water Uptake. The water uptake of the two sets of composite membranes as a function of sample B particle loading is represented in Figure 9. In the case of the 69.4% DS polymer, the water uptake exhibits comparable behavior to that of the conductivity at an increasing loading of nanoparticles up to 10 wt % (Figure 8), showing a plateau at ca. 32 ( 2% between 2.4 and 7.6 wt % silica content. Then, at higher content the uptake decreases slowly to reach a value of ca. 31% at a loading of ca. 19.8 wt %. In the case of the 85.0% DS polymer, the water uptake curve is reminiscent of the conductivity curve on the whole silica content range, showing a minimum at ca. 59% between ca. 5 and 9.9 wt % loadings. For both DS, the pristine SPEEK polymer shows the highest uptake at ca. 42% and 91% for DS 69.4% and 85.0%, respectively. Langmuir 2010, 26(13), 11184–11195
The composite membranes prepared with a ca. 10 wt % content of particles from sample C show much lower water uptakes for both DS values, with respect to their counterparts prepared with sample B (Table 5, Figure 9). This lower water content would be in good agreement with the lower conductivity observed with the 69.4% DS membrane but does not fit with the higher one observed for the 85.0% DS membrane (vide supra). Discussion. Proton conductivity and water uptake properties are related to the microstructure of the polymeric chains in hydrophobic and hydrophilic channels within the membrane.47-49 Despite the fact that both measurements are performed in slightly different conditions, i.e., 100% RH for the conductivity and equilibrium with liquid water for the uptake, both at room temperature, these two intrinsic properties of the membranes are related, i.e., water being a proton carrier, and may therefore be influenced similarly to conductivity. Silica particles are much less conductive than SPEEK, even if their surface has been modified with some charged moieties, and their intrinsic water retention is also clearly lower even at small size. Additionally, SPEEK is constituted of relatively narrow channels between polymer walls in which water molecules and protonic charge carriers are flowing.47 There are also many dead ends, whereas the channels are highly branched and sulfonate groups are relatively separated from each other. This microstructure of the polymer contributes significantly to the membranes intrinsic properties. Thus, the initial decrease of both conductivity and water uptake with the nanoparticle loading, up to ca. 3 wt %, is likely associated with the reduced contributions inherent to the particle nature. It is also most probable that the amount of particles is too low to impact the structure of the aqueous phase in the entangled polymer matrix, i.e., the channel network. At intermediate loadings between ca. 3 and ca. 8 wt %, the attenuated decrease of the membrane properties is probably associated with the silica nanoparticles affecting the volume and connectivity of the channels. These effects are obviously different depending on the DS of the polymer and depend undoubtedly on the nature and properties of the particles. This modification of the microstructure of the polymer when adding more filler even improves the properties of the membrane until some point, ca. 10-15 wt % of nanoparticles depending on the DS, above which the conductivity drops again. This final change might be either due to the presence of too many particles within the channels creating more obstacles for the transport of the charge carriers or simply because the membrane contains more and more less-conductive matter. In most cases, the addition of a surface modified filler improves the conductivity of the composite membranes even at low loadings because of an increase of the concentration of the charge carriers20,50 or an increase of the water retention capabilities of the particles.21 When observed, a decrease of conductivity at higher silica content was attributed to an increase of the obstacles for proton conductivity,21,51 i.e., reorganization of the water channel structure. In our case, the segregation of the nanoparticles for loadings higher than 15 wt % favors this assumption. Karthikeyan et al. worked on MCM-41/SPEEK and laponite/ SPEEK composites prepared with particles, the surface of which had been modified by imidazole functionalities. This greatly improved the intrinsic conductivity of the silicate.21 The authors observed that the addition of small amounts of particles (up to 10 wt %) increased the conductivity of the composites for MCM-41 and laponite, both modified, and further addition of particles reduced the conductivity. The first improvement was attributed to (50) Pezzin, S. H.; Stock, N.; Shishatskiy, S.; Nunes, S. P. J. Membr. Sci. 2008, 325, 559. (51) Libby, B.; Smyrl, W. H.; Cussler, E. L. AIChE J. 2003, 49, 991.
DOI: 10.1021/la100051j
11193
Article
the capability of the particles to retain water, but the decrease at higher silicate content was attributed to an increase of the obstacles for proton conductivity. This particular study using surface modified silicate particles supports our observations concerning the reorganization of the water channel structure within the membrane at higher silica contents. Carbone et al. worked using 3-aminopropyl functionalized silica particles, provided by Sigma-Aldrich, to prepare silica/ SPEEK composite membranes.23 They observed that the membrane conductivity decreases regularly with the silica content for a 35% DS, at 100 and 120 °C, and for different RH values. A similar study on membranes prepared with a 52% DS SPEEK shows that the conductivity is much less affected by the silica content, especially for the lower RH values. The authors also observed that the introduction of the amino-grafted silica greatly reduced the water uptake and the swelling of the membranes, especially for the highest DS value. They attributed this second phenomenon either to the fact that the modified silica particles are hydrophilic, thus meaning an appropriate amount of water in the sulfonated polymer without altering its conductivity much, or to the interaction between sulfonic-aminic groups, reducing their interaction with water molecules. The observed behavioral discrepancy between higher and lower DS value SPEEK polymers is in good agreement with the discrepancy observed in our own systems containing sample C nanoparticles (vide supra); thus, the reduction of both conductivity and water uptake in our case may come from a higher interaction of the sulfonic acid sites with the amine group of the grafted silica particles. This is supported by the observations made by us in another work concerning the use of solely amino grafted nanoparticles to prepare SPEEK composite membranes.52 Indeed, in the present paper, the nanoparticles used are highly hydrophobic and aprotic making any hydrogen bonding readily impossible. A consequence is that both conductivity and water uptake evolve together with the particle loading. In the previously cited work of our group, the nanoparticles are grafted with a propylamine functionality allowing an electrostatic interaction, stronger than potential hydrogen bonding, between the positively charged ammonium of the functionality and the negatively charged sulfonate groups of SPEEK. Indeed, in this work the proton conductivity was still rising with particle loading while the water uptake was reduced, as observed above for membranes prepared with particles of sample C, due to greater cohesion, i.e., cross-linking, within the membrane. However, it is difficult to give a definitive assessment because of the presence of particle segregation in our system. Additionally, the particles used by Carbone et al. have a size in the range 40-63 μm,53 whereas our particles are on the nanometer scale, and this size difference may have a great impact, as observed by us in a yet-unpublished work using zeolite Silicalite-1 particles having various sizes.54 Proton Conductivity as a Function of Temperature. With the aim of comparing the nanocomposite membranes prepared with the particles from samples B and C, the conductivities of some membranes with 69.4% DS are shown in Figure 10 as a function of temperature, together with the conductivity of pristine SPEEK. All membranes exhibit relatively steady conductivity for temperatures between 30 and 50 °C and lower than 50 mS.cm-1. Starting at ca. 60 °C, the conductivity starts to rise abruptly for pristine SPEEK, less abruptly for the composites made with sample B particles (for both DS values), and even less abruptly for the composite made with sample C particles. Pristine SPEEK (52) Kayser, M. J.; Reinholdt, M. X.; Kaliaguine, S. J. Phys. Chem. B, in press. (53) Information available online at http://wsigm/sigma-aldrich/home.html; Product reference 364258. (54) Reinholdt, M. X.; Kaliaguine, S., submitted.
11194 DOI: 10.1021/la100051j
Reinholdt and Kaliaguine
Figure 10. Proton conductivity of sample B silica/SPEEK, sample C silica/SPEEK nanocomposite, and pristine SPEEK membranes as a function of temperature.
reaches a maximum of ca. 260 mS.cm-1 at ca. 66 °C, and then the conductivity drops due to a rupture of the membrane at higher temperatures. The membranes prepared with a sample B content of ca. 10 wt % exhibit a similar behavior and show a maximum of ca. 123 mS.cm-1 at ca. 67 °C. Both the membranes prepared with a sample B content of 5 wt % and sample C content of 10 wt % show a regular increase in conductivity at increasing temperature, up to ca. 165 mS.cm-1 at ca. 76 °C for the former sample and up to ca. 81 mS.cm-1 at ca. 79 °C for the latter one. The conductivity of the pristine SPEEK membrane is always the highest except at temperatures above ca. 70 °C. The conductivity of the composite membrane prepared with sample C nanoparticles is always the lowest. The conductivity of the membranes prepared with the nanoparticles of sample B are in between, with the conductivity of the 5 wt % content membrane essentially above or almost equal to the conductivity of the 10 wt % content membrane for temperatures above 60 °C, and the order reverted at lower temperatures. Both Karthikeyan et al. and Carbone et al. observed an increase of the conductivity with the temperature increase for their composites prepared with surface modified particles.21,23 Our results of conductivity as a function of temperature show clearly that the use of nanoparticles grafted with aminopropyl groups does not improve the conductivity of these composites compared with composites made with nanoparticles bearing both hexadecyl and aminopropyl moieties, at least for a loading of ca. 10 wt % of particles. This conductivity decrease accompanies a water uptake decrease. These decreasing properties are probably due to a stronger electrostatic interaction of the sulfonic acid groups of the polymer with the amine group of the grafted particles, reducing the inherent interaction with water molecules and their availability for proton transport, as observed in another work.52 Additionally, the inversion of the conductivity curves observed with increasing temperature of the membranes prepared with two different loadings of the same particles sample, i.e., the conductivity of one composite being lower than that of the other at lower temperatures and the inverse at higher temperatures, demonstrates clearly that preparation and working conditions of both membranes are critical and often not investigated thoroughly enough.
Summary In this work, the synthesis of silica nanoparticles grafted solely with hexadecylsilyl moieties (sample A), solely with 3-aminopropyldimethylsilyl moieties (sample C), and with both 3-aminopropyldimethylsilyl and hexadecylsilyl moieties (sample B) has been achieved. XRD showed that all samples are amorphous in nature and 29Si MAS NMR and 1H-13C CP-MAS NMR demonstrated the nature of the moieties grafted on the various nanoparticles. Langmuir 2010, 26(13), 11184–11195
Reinholdt and Kaliaguine
The TEM investigation of the samples exhibits that they are all essentially constituted of nanoparticles with a size ranging between 8 and 20 nm, thus demonstrating that the grafting process allows the smaller size of the nanoparticles to be maintained. The hydrophilic/hydrophobic character of the various nanoparticles was investigated by dispersing the samples in various solvents and performing DLS measurements. Sample A is highly hydrophobic, but it can be dispersed in DMAc, a polar and protic solvent used for membrane casting, and producing two relatively stable populations of agglomerates at 73 ( 12 and 269 ( 38 nm. Sample B is more hydrophobic than hydrophilic and exhibits one single agglomerate distribution at 190 ( 28 nm in DMAc. Surprisingly, sample C exhibits the largest agglomerates in DMAc, even though it is a hydrophilic sample and the solvent is polar and protic. This discrepancy may arise from the creation of a hydrogen-bonding network between the amino-grafted silica and DMAc, allowing a stabilization of larger agglomerates. This suggests that sample B might be a better compromise, to prepare proton exchange membranes, compared to hydrophobic particles (sample A) and charged hydrophilic particles (sample C). These various nanoparticles were used to prepare grafted silica/ SPEEK nanocomposites, and their aspect, proton conductivity, and water uptake properties were investigated. In the case of sample B, an obvious segregation of the particles appears in the membrane for loadings equal to or above 15 wt %, and a network of opaque and clear channels, obviously containing much fewer particles, appears in the membrane for loadings equal to or below 5 wt %. For intermediate loadings, the nanoparticles are welldistributed and embedded in the membrane. The membranes prepared with sample A also exhibit a network of opaque and clear areas, but this network is much more extended and the membranes were not further investigated. Finally, the membranes prepared with sample C exhibit a partial segregation of the particles with areas having higher and lower concentrations, forming a network of rays emerging from a center point. These membranes were further investigated because no particle free area was observed. Concerning proton conductivity, the membranes prepared with sample B and the two different SPEEK DS values exhibit a complex similar behavior represented in Figure 8. The nanocomposite membranes exhibit conductivity values between 12-33 and 23-48 mS.cm-1 for the SPEEK DS values of 69.4% and 85.0%, respectively. For both DS, the maximum of conductivity is not observed for the pristine SPEEK. The composite membranes prepared with nanoparticles from sample C show much different conductivity values than their counterparts prepared with sample B. This difference either comes from limits appearing at different positions for each system or is because the proton transport process is strongly affected by the segregation of the particles observed in the case of the nanocomposites prepared with sample C. In the case of sample B, the water uptake curves for both DS values are reminiscent of the curves observed for the conductivity,
Langmuir 2010, 26(13), 11184–11195
Article
but here the pristine polymers exhibit the highest uptake. The composites prepared with sample C show lower water uptake than their counterparts prepared with sample B. By comparing our results with those in the literature, we have been able to give a possible interpretation of the observed phenomena. In the case of the nanocomposites prepared with sample B, the initial decrease of both conductivity and water uptake with the nanoparticle loading is associated with the addition of less conductive and less hygroscopic material; however, this amount is too low to impact the structure of the channels network in the entangled polymer matrix. At intermediate loadings, the decrease of these membrane properties is probably attenuated because the silica nanoparticles are affecting the structure of the channels, and up to some point, these two properties are improved with the loading increase. The final reduction of proton conductivity and water uptake may result from the presence of too many obstacles, i.e., particles, for the transport of the charge carriers, and/or because the membrane contains too much less-conductive material. In the case of the nanocomposites prepared with sample C, the lower conductivity and water uptake, compared to the composites prepared with sample B, may result from a stronger electrostatic interaction of the sulfonic acid sites with the amine group of the grafted silica particles, as observed by us in another work.52 However, the proton transport process may also be strongly affected by the segregation of the particles observed in these membranes. The study of proton conductivity as a function of temperature for some membranes prepared with samples B and C and the pristine SPEEK, shows that the conductivity at higher temperatures of the latter is superior to that observed for the composites and that composites made with nanoparticles bearing both hexadecyl and aminopropyl moieties (sample B) have a higher conductivity than those made with nanoparticles bearing solely aminopropyl groups (sample C). This difference in conductivity is probably due to a stronger electrostatic interaction of the sulfonic acid groups of the polymer with the amine group of the grafted particles, as observed somewhere else,52 reducing the proton transport and thus favoring sample B. The composites prepared with the nanoparticles bearing both hexadecyl and aminopropyl moieties seem to be promising materials for application in PEM fuel cells, because for some given particle loadings, they exhibit higher proton conductivity and lower water uptake than the pristine SPEEK polymer. The composites also seem to be more stable mechanically at higher temperatures than the pristine polymer. Acknowledgment. Natural Sciences and Engineering Research Council of Canada and SiM Composites Ltd are gratefully acknowledged for financial support of this research. Supporting Information Available: Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la100051j
11195
