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Hybrid Organic-Inorganic Mesostructured Membranes: Interfaces and Organization at Different Length Scales Bruno Alonso,*,† Franck Fayon,‡ Dominique Massiot,‡ Heinz Amenitsch,§ Luca Malfatti,| Tongjit Kidchob,| Stefano Costacurta,| and Plinio Innocenzi*,| Institut Charles Gerhardt de Montpellier ICGM, UMR 5253 CNRS-ENSCM-UM2-UM1, 8 rue de l’Ecole Normale, 34296 Montpellier cedex 5, France, CEMHTI, CNRS UPR 3079, 1D aV. de la Recherche Scientifique, 45071 Orle´ans cedex 2, France, and UniVersite´ d’Orle´ans, Faculte´ des Sciences, AVenue du Parc Floral, BP 6749, 45067 Orle´ans cedex 2, France, Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria, and D.A.P., Laboratorio di Scienza dei Materiali e Nanotecnologie, CR-INSTM, UniVersita` di Sassari, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (Sassari), Italy ReceiVed: February 24, 2010; ReVised Manuscript ReceiVed: May 25, 2010
The characterization of the organization of matter at different length scales is one of the main features in the field of mesostructured materials. Here, SAXS, solid-state multinuclear (29Si, 1H) MAS NMR spectra and one-dimensional 1H spin diffusion MAS NMR experiments have been used to characterize the structure and the copolymer-siloxane interfaces of monolithic hybrid organic-inorganic membranes. The membranes were prepared by cohydrolysis of organically modified alkoxysilanes (methyltriethoxysilane (MTES) and dimethyldimethoxysilane (DMDES)) with tetraethylorthosilicate (TEOS) and using a triblock copolymer PEO-PPOPEO as templating agent. After careful drying, a cubic Im3m j mesostructure is obtained. Variations in the composition of the hybrid membrane have been found to affect the adsorption of water at the copolymer-siloxane interface as well as the spatial distribution of propylene oxide (PO), ethylene oxide (EO), and siloxane domains in the final mesostructures. In that sense, the modulation of the polarity of the interfaces by simply adjusting the nature and quantity of organically modified alkoxides seems to be a promising way for the control of the final mesostructures. Introduction Mesoporous organized materials are characterized by the presence of an ordered porosity in the meso-range (2-50 nm), which can be obtained through self-assembly of templating micelles.1-9 Different types of mesostructured-mesoporous materials have been synthesized by self-assembly whose process is driven by solvent evaporation.6,10-13 In the case of thin mesostructured films the possibility to achieve organization is strictly correlated to a careful control of the processing conditions.14,15 The fast evaporation of the solvent during the deposition, for instance via dip-coating, makes the process, in fact, quite critical. In monoliths and membranes the solvent evaporates much more slowly and lower control of the process is required. Good organization of the mesophase is, therefore, generally achieved by allowing a slow solvent evaporation, and mesostructured materials that exhibit organization in a large scale, such as foams,16 bulk monoliths,17-19 and membranes20,21 (see ref 22 for a recent review), have been prepared by different groups. In a previous paper we have shown that self-assembled membranes of several centimeters, with high organization throughout the material, can be obtained.23 The structure of these mesoporous bulk objects is quite complex, they appear as “polycrystalline-like” materials of mesostructured domains. * To whom correspondence should be addressed. E-mail: bruno.alonso@ enscm.fr (B.A.) and
[email protected] (P.I.). † Institut Charles Gerhardt de Montpellier ICGM. ‡ CEMHTI and Universite´ d’Orle´ans. § Austrian Academy of Sciences. | Universita` di Sassari.
Small disorientation of the mesophase due to a gradient in the solvent evaporation and different pore organizations can be observed. In general, the possibility to control the mesophase, in terms of dimensions, pore distribution, and organization, is dependent on the choice of the surfactant and its concentration. The hybrid interface, between the micelles and the organometallic species;the “nano-bricks” whose condensation will form the interconnected solid backbone;plays a critical role. The nature of the interactions at this interface and the curvature strongly affect the mesophase organization; a significant difference is observed if ionic surfactants or block copolymers are used as the templating agents.24 Different considerations can address the choice between a surfactant and another one, but in any case controll of the organization is truly achieved only by a careful adjustment of the interactions between the organometallic species and micellar aggregates.25 The understanding of the nature of these interactions in the related hybrid interface becomes then a crucial point for further design of materials with specific textural properties. In the case of ionic surfactant, basic or acidic syntheses produce interfaces mainly based on electrostatic interactions.26-29 In the case of block copolymers,30,31 it has been observed that the hydrophilic side of the polymeric chain can be responsible of a residual microporosity within the mesoporous material32-34 due to partial penetration of this part of the surfactant within the pore walls (e.g., the ethylene oxide (EO) units can interact with the organometallic species through hydrogen bonds).35,36 When synthesizing hybrid organic-inorganic mesoporous materials37,38 using co-condensation routes between alkoxide precursors, the nature of the interactions between
10.1021/jp101652a 2010 American Chemical Society Published on Web 06/21/2010
Hybrid Organic-Inorganic Mesostructured Membranes organometallic species and micelles will obviously change according to the hydrophilic or hydrophobic nature of the organic functions incorporated. However, the changes are not easily proven because of the intrinsic difficulties in the characterization of the final mesostructures. One of the greatest difficulties arises from the lack of knowledge on the spatial distribution of the different chemical groups: hydrophobic and hydrophilic parts of the surfactants, and organic functions bound to the siloxane network. In this work, we have characterized at different length scales the mesostructures of mesoporous hybrid materials formed from amphiphilic block copolymer [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), 106-70106] and organosiloxane alkoxide precursors bearing zero, one, or two methyl groups. The change in composition allows tuning the polarity of the copolymer-siloxane interface and consecutively the spatial distribution at the nanometer scale of the propylene oxide (PO), ethylene oxide (EO), and siloxane domains. We have used solid-state nuclear magnetic resonance (NMR) in combination with small-angle X-ray scattering (SAXS) to investigate these complex features, and in particular to get a better insight of the micelle’s properties after formation of the mesostructure. We have also tried to disclose in detail the nature of the hybrid interfaces and the spatial distribution of the different chemical groups and domains: propylene oxide (PO), ethylene oxide (EO), and siloxane network. Experimental Section Synthesis. Transparent organic-inorganic mesoporous silica membranes were synthesized starting from two different acidic sols containing an amphiphilic block copolymer [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPOPEO), 106-70-106] (commercial name Pluronic F127) as the structure-directing agent and different (organo)siloxane sources. All the reagents were purchased from Aldrich and used as received. A precursor sol containing the siloxane source (mother solution) was prepared by adding the following in the order given: ethanol (EtOH), tetraethylorthosilicate (TEOS), an organically modified alkoxide (methyltriethoxysilane (MTES) or dimethyldiethoxysilane (DMDES)), and HCl in the molar ratios Si:EtOH:H2O:HCl ) 1: 2.78:1.04:1.43 × 10-2, where Si stands for a mixture of TEOS and an organo-alkoxysilane MTES or DMDES whose molar ratios c ) [MTES, DMDES]/[TEOS] were set to either 0.05, 0.1, 0.2, 0.3, or 0.5. This sol was stirred for 1 h at room temperature, in order to prehydrolyze the silicon alkoxides. Another solution (templating solution) was prepared dissolving Pluronic F127 in a mixture of 15 mL of EtOH and 1.5 mL of weakly acidic aqueous solution. The molar ratio [F127]/[Si] was set to 6 × 10-3. The final precursor sol, which was obtained by adding 7.7 mL of the mother solution to the templating solution, was reacted under stirring for a further 24 h (final molar proportions TEOS:MTES or DMDES:EtOH:H2O: HCl:F127 ) 1 - x:x:16.3:5.4:1.88 × 10-2:6 × 10-3, where x ) c/(1 + c)). Within these conditions, the F127 volume fraction in the dried samples will be close to 0.7. Series Mc and Dc have been named by a letter indicating the organosiloxane precursor used, MTES or DMDES, respectively. The samples have been named by adding a number to give the c molar ratio, for instance D005, D01, D02, D03, and D05 is the labeling for the samples of the Dc membrane series. The precursor solutions were poured into two Petri dishes in order to allow solvent evaporation in controlled temperature and relative humidity (RH ) 30%) conditions. After ∼30 days, transparent and self-
J. Phys. Chem. C, Vol. 114, No. 27, 2010 11731 standing membranes in the shape of disks with around 30 µm thickness and 80 mm diameter were obtained. Characterization. The membrane structure was investigated by small-angle X-ray scattering (SAXS) at the Austrian highflux beamline of the electron storage ring ELETTRA (Trieste, Italy).39 The angle of incidence of the beam (wavelength 1.54 Å) was set either to 90° (transmission mode or TSAXS) or below 3° (grazing incidence mode or GISAXS). Twodimensional diffraction patterns were recorded with a CCD detector (Photonic Science). Data analysis was performed, with the FIT2D application (A. P. Hammersley/ESRF),40 on images acquired with different exposition times in order to obtain the d-spacing values with good precision for both low-order and high-order diffraction spots. The images were corrected for flat field spatial distortion and fluctuation of primary intensity using FIT2D. CMPR software (B. Toby, NIST) was used to simulate diffraction data in order to identify the symmetry space group of the mesophase. N2 Sorption Isotherms. The sorption isotherms of N2 measured at 77 K were recorded with a Micromeritics ASAP 2010 apparatus. The as-synthesized mesostructures were calcined in flowing air at 550 °C for 8 h. Each calcined sample was outgassed at 250 °C until a stable static vacuum of 3 × 10-3 Torr was reached. Specific surface areas SBET were calculated by using the BET method. Solid-State NMR. The 29Si single-pulse MAS NMR spectra of hybrid membranes were obtained on an Avance 400 Bruker spectrometer (29Si Larmor frequency of 79.5 MHz), using a 4 mm probehead and zirconia rotors span at a spinning frequency of 10 kHz. Proton decoupling was applied during acquisition. 1 H and 29Si nutation frequencies were about 50 kHz. A flip angle of 30° and a recycle delay of 30 s which ensured effective longitudinal relaxation were used. The siloxane units were noted Qi, Ti, and Di for tetra-, tri-, and bifunctional units, respectively, where i stands for the number of siloxane bonds. The siloxane condensation degree C is calculated from the percentages of each kind of unit, % Xi, using the general formula C ) {∑(i · % Xi)}/{imax · ∑(% Xi)}, where X stands for Q, T, or D. All 1H and 13C NMR spectra were recorded on an Avance 750 Bruker spectrometer (1H and 13C Larmor frequencies of 750.2 and 188.6 MHz, respectively), using 2.5 mm zirconia rotors span at a spinning frequency of 30 kHz. 1H and 13C nutation frequencies were about 50 kHz (except 75 kHz for the 1 H double quantum-single quantum correlation experiment). 1 H single-pulse spectra were obtained by using a flip angle of 30°, a recycle delay of 5, and 16 scans. The 2D 13C{1H} INEPT41,42 MAS NMR spectra were recorded with recycling delays of 1-3 s, a pumping delay ∆1 ) 1.5 ms, and a refocusing delay ∆2 ) 1 ms, close to the parameters previously used for other mesostructured materials.43 1H longitudinal relaxation time (T1) measurements were done by using a saturation-recovery sequence with 12 geometric increments from 0.05 to 20000 ms for the variable interpulse delay. 1H transverse dephasing time (T2′) measurements were done by using a Hahn spin-echo pulse sequence with 64 constant time increment from 0.13 to 33.73 ms for the variable interpulse delay. The 1H-1H doublequantum-single-quantum (DQ-SQ) correlation MAS NMR spectra were recorded with the BaBa pulse sequence.44 The DQ coherences excitation and reconversion blocks were set to 67 µs corresponding to two rotor periods. Modeling of all NMR spectra was done with the freely available DmFit software.45 The 1H, 13C, and 29Si chemical shifts were referenced toward external neat TMS. Assignment of 1H
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SCHEME 1: Pulse Sequence of the 1D Longitudinal Magnetization Exchange MAS NMR Experiment and the Corresponding Coherence Pathwaya
Alonso et al. TABLE 1: Longitudinal Relaxation Rate T1, Transverse Dephasing Time T2′, and Diffusivity Constants D, of 1H Signals for D03 and M03 Samples sample
a The pathway was selected using the following phase list;Φ1: 16{+X}; Φ2: 4{+X, -X, +Y, -Y}; Φ3: 16{+X}; Φ4: 4{+X}, 4{+Y}, 4{-X}, 4{-Y}; Φrec: 2{+X}, 2{-X}, 2{+Y}, 2{-Y}, 2{-X}, 2{+X}, 2{-Y}, 2{+Y}.
resonances is discussed in the Results and Discussion section and more deeply in the Supporting Information, SI 5. To rapidly and efficiently monitor proton magnetization exchange we implemented a specific pulse sequence that involves the selection of a single resonance and the direct measurement of its magnetization exchange with other resonances in a 1D experiment (Scheme 1). This sequence consists of a selective Hahn spin-echo (90°-τ-180°-τ) involving a 180° selective Gaussian pulse (at the frequency of the selected resonance) followed by a longitudinal magnetization exchange block (90°-mixing time τm-90°). A presaturation loop was also employed to avoid saturation effect and to reduce the overall acquisition time of the experiment. The length of the 180° Gaussian shaped selective pulse (i.e., 2 ms) and the corresponding 1H nutation frequency (i.e., ∼200 Hz), as well as the delay τ in the spin-echo sequence (i.e., 2 ms) were optimized in order to acquire exclusively the signal of the methyl groups of polypropylene oxide (PO) blocks (δiso ) 1.3 ppm) when τm ) 0 s. When increasing τm, the other signals will appear as expected in a typical exchange experiment (see for example Figure 6a). The advantages of this method compared to standard 2D exchange experiments is the reduction of the overall experimental time and the easier modeling of the 1D spectra. In turn, this allows obtaining more complete spin diffusion curves (typically 30 spectra can be recorded in 12 h). The series of spectra are modeled by using Voigt line shapes. The resulting peak areas vary as expected as a function of τm: a monotonic decrease for the methyl resonance in PO units, and a corresponding increase followed by a decrease for the other groups (Supporting Information, SI 1). This latter decrease is due to longitudinal T1 relaxation as noticed by the variations of the sum of all peak areas as a function of τm (Supporting Information, SI 2). Because we observe very similar T1 values for all 1H resonances (Table 1) we can directly report the relative intensities of the exchanged magnetization and further use them to quantify the obtained results. The spin diffusion curves correspond here to the normalized exchanged magnetization (M - M∞)/(M0 - M∞) as a function of τm1/2, where M is the magnetization measured at the current τm, M0 the magnetization at τm ) 0, and M∞ the magnetization at equilibrium (τm f ∞).46 Alternatively, we also plotted the normalized exchanged magnetization as a function of (Deff · τm)1/ 2, where Deff is an effective diffusivity constant. This second way of plotting the spin diffusion curves gives a better view of the spatial proximities for protons having very different diffu-
proton
T1 (s)a
T2′ (ms)a,b (0.95 ( 0.02) (4.17 ( 0.10) 1.69 ( 0.08 22.48 ( 0.09 17.63 ( 0.28 18.31 ( 0.39 10.35 ( 0.13 (0.96 ( 0.02) (1.14 ( 0.02) (6.26 ( 0.26) 1.68 ( 0.08 22.66 ( 0.12 19.52 ( 0.28 21.07 ( 0.44 5.76 ( 0.06 (0.43 ( 0.03)
D03
CH3-Si
0.93 ( 0.02
M03
[CH2-CH(CH3)-O] [CH2-CH(CH3)-O) [CH2-CH(CH3)-O] [CH2-CH2-O] -OH CH3-Si
0.85 ( 0.01 0.81 ( 0.02 0.73 ( 0.04 0.85 ( 0.01 0.86 ( 0.02 0.74 ( 0.01
[CH2-CH(CH3)-O] [CH2-CH(CH3)-O) [CH2-CH(CH3)-O] [CH2-CH2-O] -OH
0.63 ( 0.12 0.61 ( 0.02 0.64 ( 0.04 0.72 ( 0.02 0.71 ( 0.03
D (nm2/ms)
0.125 0.009 0.011 0.010 0.015
0.126 0.009 0.010 0.010 0.026
a Error bars were those obtained by simulation (least-squares fit) of experimental data points. b T2′ values in parentheses were obtained from simulation but not considered for the estimation of D. See text for details.
sivity constants (see Results and Discussion). Following SchmidtRohr and Spiess,47 Deff is defined by Deff1/2 ) 2(DA · DB)1/2/(DA1/2 + DB1/2), where DA and DB are the diffusivity constants of the exchanging protons. They correspond here to D values of the methyl protons in PO units and D values of the other protons under study (see Table 1 for the values considered). D values are estimated from T2′ values following the empirical relationship previously proposed for mobile segments in polymers analyzed in static conditions.48 We have considered T2′ values measured under the same experimental conditions as that used for the 1H magnetization exchange experiments, e.g. samples rotating at a 30 kHz frequency. Therefore, the resulting D values incorporate the possible attenuation effect of MAS frequencies on spin diffusion rates.49,50 Results and Discussion The mesostructured membranes are formed by a hybrid organic-inorganic siloxane interconnected network templated by self-assembled PEO-PPO-PEO copolymers. In addition to a typical silica alkoxide precursor (TEOS) we have employed two types of organically modified alkoxides (DMDES, MTES) to modulate the polarity of the copolymer-siloxane interfaces. Texture. Figure 1 shows the GISAXS patterns of the asprepared membranes with c ) [MTES or DMDES]/[TEOS] ) 0.2. These patterns were attributed to the cubic Im3m j symmetry group and the indexation was done according to this symmetry.23 In particular, the cubic unit cell shows a preferential orientation, with the (110) family plane aligned parallel to the membrane surface; the cell constants of the ordered mesostructure were calculated according to the formula ahkl ) dhkl(h2 + k2 + l2)1/2. The d-spacing values were calculated for each spot in the GISAXS patterns and then averaged to obtain the estimated cell parameters: a ) 17.4 ( 0.8 nm for D02 (Figure 1a) and 18.1 ( 0.5 nm for M02 (Figure 1b) membranes. Increasing c up to 0.5, the preferential orientation of the mesophase is progressively lost (widening of the diffraction spots), but the symmetry remains Im3m j . SAXS data obtained in transmission mode agree with the Im3m j space group assignment, and reveal the existence of planar disorder (Supporting Information, SI 3). The Im3m j symmetry is also consistent with previous work on mesoporous samples obtained from F127 and TEOS alone with similar F127/ Si molar ratios.51
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Figure 1. GISAXS diffraction pattern of hybrid membranes: (a) D02 and (b) M02 samples. Full indexation is shown in Figure 1a.
Figure 2. 29Si{1H} single-pulse MAS NMR spectra of hybrid membranes: (a) D03 and (b) M03.
Complementary information about the texture was obtained on calcined samples. Calcination burns out the organic groups and liberates the porosity, but also leads to a global contraction due to further condensation and densification of the siloxane network. From N2 adsorption-desorption isotherms obtained on calcined samples D03 and M03 (Supporting Information, SI 4), we have estimated similar specific surface areas (90-110 m2 · g-1 range) and found smaller pores for calcined D03 rather than for M03. This suggests the presence before calcination of smaller copolymer aggregates in the Dc series compared to the Mc series. Chemical Structure. The 29Si single-pulse MAS NMR spectra of samples D03 and M03 are shown in Figure 2. As observed in the spectra and estimated from spectrum modeling, the siloxane condensation degrees, C, of D and T units (0.96 ( 0.05 and 0.90 ( 0.05, respectively) are higher than that of Q units (0.81 ( 0.02). This is consistent with previous works on hydrolysis and condensation of silicon alkoxides in acid media52 and appears also correlated to the different degrees of hydrophobicity of the membranes (vide infra). Lastly, D/Q and T/Q molar ratios estimated from 29Si peak areas are roughly close ((20%) to the initial values used in the sol preparation (parameter c). The 1H single-pulse spectra recorded at high magnetic field under fast MAS are presented in Figure 3 (samples D03 and M03) and display a spectral resolution allowing the identification of the main proton resonances. The 1H signals related to organic
Figure 3. 1H single-pulse MAS NMR spectra of hybrid membranes: (a) D03 and (b) M03.
groups have been assigned by using previous data and complementary experiments (Supporting Information, SI 5). We also observe a broad peak which location varies from 4.8 to 5.5 ppm within the series of samples. These chemical shifts are in the range of that observed in different series of porous silica samples for hydroxyl groups coming from water molecules and silanol groups in interaction.35,53-58 Here, the Lorentzian line shape of this broader peak (in contrast with a Gaussian line shape expected for a random chemical shift distribution) and the similarity between the defocusing time constant T2* (determined from the full line width at half-maximum fwhm, not shown) and dephasing time constant T2′ (corresponding to the homogeneous broadening) are characteristic of mobile -OH groups rather than a distribution of static -OH groups at the siloxane surface. In addition, we should mention that, when fast MAS is maintained over a long period of time (several hours), some modifications in line width and in intensity are observed for this resonance as a partial drying of the samples might occur. Therefore, the peaks at 4.8-5.5 ppm can likely be weighted averages of different hydroxy signals caused by fast chemical exchange between water and silanols or by water surface hopping in agreement with previous studies.55 Water Adsorption. The polarity of the siloxane network can be tuned by adjusting the initial proportion of organosiloxane precursors T (MTES) or D (DMDES) in the respective series Mc and Dc. As shown in Figure 4a, we observe a decrease in the relative intensity of the 1H peaks of the -OH groups as a function of the c parameter related to the initial T/Q or D/Q
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1
Figure 4. Variations of (a) relative percentage areas and (b) H isotropic chemical shifts of -OH groups as a function of the initial ratio c for samples of series Dc ([) and Mc (0). All lines are only guides for the eyes.
ratios. As c increases, the final siloxane surface becomes more hydrophobic. The effect depends on the nature of the organosiloxane units incorporated: with one (Mc) or two methyl groups (Dc). Concerning the 1H isotropic chemical shifts δ, either they vary gradually with c (case of -OH (Figure 4b), -CH2- in EO, CH3-Si), or they do not vary with c (case of (-CH-, -CH2- and CH3-) in PO). To gain more information on these aspects, we have plotted in Figure 5 the variations of δ as a function of the molar ratio nOH/nEO of -OH groups over EO units for the resonances of -OH, -CH2- in EO, and CH3- in PO. The ratio nOH/nEO is obtained from 1H peak areas (fresh samples) and is indicative of the number of silanol groups and water molecules in the samples. In the case of -OH signals (Figure 5a), we found that variations of δ are correlated to the nOH/nEO ratio. This can be due to variations in the relative proportions of the chemical species under exchange (silanols, water). For -CH2- in EO and CH3- in PO, we have estimated the difference δdiff ) (δ - δF127) between the 1H isotropic chemical shift measured on a membrane sample, δ, and that measured for the neat block copolymer, δF127. This allows comparing the variations in chemical shift as a function of nOH/ nEO for both groups (Figure 5b). In the case of -CH2- in EO, δdiff values are within 0.1 ppm and are roughly proportional to nOH/nEO. There is here no chemical exchange between protons in EO and in -OH and the variations in δdiff can be driven by various effects such as intermolecular interactions (e.g., hydro-
Figure 5. Variations of 1H isotropic chemical shifts as a function of the molar ratio nOH/nEO (-OH groups over EO units) for Dc ((, )) and Mc (9, 0) samples: (a) case of -OH signal ([, 9), (b) case of the signals: -CH2- in EO ([, 9), and CH3- in PO (], 0). Here, δ is replaced by δdelta ) (δ - δF127) for easier comparison. All lines are only guides for the eyes.
gen bonds), changes in chain conformation, and local variations in magnetic or electric fields. Whatever their exact origin, these variations reveal a gradual modification of the environment of the EO units induced by the presence of increased numbers of -OH groups. Therefore, EO units and -OH groups (silanols, water molecules) can be considered to be spatially close in the samples. Similar conclusions can be made for protons of CH3-Si (D and T units) from the variations of their chemical shifts as a function of nOH/nEO (not shown). On the contrary, for the protons of the PO units δdiff is close to 0 ppm and does not vary with nOH/nEO (as illustrated in Figure 5b for the CH3in PO units). This suggests that the hybrid membranes possess at least two separated spatial areas: one area made of silanols, water molecules, EO units, and also methyl moieties, and another area made of PO units. This general result is consistent with previous studies of other mesostructured silica materials obtained through self-assembly of block copolymers.17,36 Here, the introduction of apolar methyl groups leads to an increase in the hydrophobicity of the siloxane network (decrease in -OH content and modification of the environment of EO units), but a priori not to a break in the general mesostructure of the materials (same Im3m j space group). In addition, from the experimental data of Figure 5b, we found again that Dc samples are less polar than Mc samples. Residual Mobility. We have measured longitudinal relaxation rate T1 and transverse dephasing time T2′ on some selected
Hybrid Organic-Inorganic Mesostructured Membranes samples under MAS conditions (νMAS ) 30 kHz). The results are presented in Table 1. The obtained T1 values are very similar and vary between 0.6 and 1.0 s. For each sample, the range of T1 values is even more restricted: 0.73-0.93 s for D03, and 0.63-0.74 s for M03. In case of -CH2- in EO and -OH groups, the values are almost identical. This can be an effect of significant residual 1H homonuclear dipolar couplings under magic angle spinning leading for example to a spin diffusion process during T1 measurement.59 The T2′ transverse dephasing times are here related to the spin-echo decay recorded under MAS condition which can be fitted by a monoexponential decay for all the 1H resonances except that of the CH3-Si groups. In this latter case, it is necessary to introduce a second T2′ value in order to fit precisely the spin-echo decay. The presence of spatial areas with different mobility may account for this result, or more likely for a wide distribution in T2′. For the sake of simplicity we use afterward a single T2′ obtained from a monoexponential fit. The monoexponential decay observed for all the other resonances does not give evidence of the presence of units (EO or PO) with contrasted mobility as found for F127-silica mesostructures by T1F (longitudinal relaxation rate in the rotating frame) NMR measurements35 or for P123-silica mesostructures by EPR analysis of spin-labeled block copolymers.36 Concerning -OH groups, T2′ values are disregarded because of the chemical exchange between water molecules and silanol groups (vide supra). It should be noted that the T2′ values measured for the various groups are significantly different, indicating variations in their degree of mobility. The more mobile groups appear to be here the hydrophobic PO units [CH2-CH(CH3)-O] with T2′ values varying between 17 and 23 ms. They are followed by the hydrophilic EO units [CH2-CH2-O] (range 5-11 ms), then by the methyl groups attached to the siloxane network (range 1-7 ms). The higher mobility of middle block PO units compared to the pending block EO units might be unexpected. It reflects here the higher interactions and constraints experienced by the EO units at the vicinity of the -OH groups and the dense siloxane network. We also observe that, for each kind of group, the T2′ values vary from one sample to another. The mobility of PO units follows the order M03 > D03 that is the order of cell parameters, whereas that of EO units follows the order D03 > M03 that is the order of decreased polarity found above. Indeed, for the more hydrophobic siloxane networks, the EO are in interaction with less -OH groups; they are therefore more mobile. The differences in the mobility of the copolymer units are also strongly related to the differences in the spatial organization of the polymers and the siloxane network as discussed below. Spatial Organization of the Chemical Groups through 1H Exchange Experiments. There are different possible NMR experiments that allow exploring the spatial organization of mesostructured hybrid organic-inorganic materials.24,60 One of them consists of looking at the 1H-1H spatial proximities using 2D double-quantum-single-quantum (DQ-SQ) MAS correlation experiments based on 1H homonuclear dipolar couplings.44 However, this approach is useless here because of the high mobility of the protonated groups (Supporting Information, SI 6). An alternative approach consists of using longitudinal magnetization exchange experiments61,62 based on 1H spin diffusion processes. It allows exploring larger 1H-1H distances up to 200 nm in favorable cases.47,63 These experiments have recently been used for studying spatial proximities in organized hybrid
J. Phys. Chem. C, Vol. 114, No. 27, 2010 11735 materials.60,64-70 The choice of the pulse sequence depends on the spectral resolution achievable in the 1H spectrum, using if needed homonuclear decoupling schemes, high magnetic field, and/or fast MAS.71-75 In our case, experiments based on X nucleus indirect detection scheme are not required since the 1H spectra obtained at high magnetic field and fast MAS have enough resolution. Therefore, we have used a 1D exchange experiment based on the selective irradiation of one 1H resonance. The advantage of this method with regard to the standard 2D experiment is obviously the reduction of the overall experiment time, which allows recording larger series of experiments with incremented values of mixing time τm resulting in more accurate spin diffusion curves. The pulse sequence, which incorporates a 180° Gaussian selective pulse and a longitudinal magnetization exchange block, is depicted in Scheme 1 and described in the Experimental Section. We have focused our attention on the signal of CH3- in the PO units as it can be easily selected by optimizing the length of the 180° selective pulse and the spin-echo delay. The residual mobility of these methyl groups, and therefore the related long T2′ values, helps the experiment to succeed. In addition, this signal is related to the more hydrophobic part of the block copolymer, which forms the core of the micelle aggregates. Following the magnetization exchanges between this signal and the resonances of the other chemical groups allows exploring their spatial distribution from the core of the aggregates toward their periphery. Figure 6a presents some of the spectra obtained for sample M03 at different mixing times τm. The signal of CH3- in PO is clearly selected at τm ) 0 ms, and other signals grow in amplitude as a function of τm. For the two samples M03 and D03, we have recorded 30 spectra for increased values of τm. We obtained, after spectrum modeling and normalization (see the Experimental Section for details), complete diffusion curves (M - M∞)/(M0 - M∞) ) f(τm1/2) like those presented in Figure 6b for sample M03. Two kinds of curves can be distinguished as schematized in Figure 6c. The first kind I corresponds to curves that strongly decrease from the beginning and pass through a minimum at negative values of (M - M∞)/(M0 - M∞). They are related here to the internal diffusion in the PO blocks from CH3- to -CH- or -CH2-. The magnetization from CH3- in PO is here first transferred to the protons of the same domain, yielding for them to M > M∞ and thus negative values of (M - M∞)/(M0 - M∞). Afterward the magnetization is transferred to the chemical groups of the other domains. Similar kinds of curves have already been observed or modeled for other mesoporous materials,76 for lipid membranes,77,78 and for organic crystals.79 The second kind II corresponds to curves that do not vary at small mixing time and then decrease almost linearly as a function of τm1/2 before reaching equilibrium. They are related to the diffusion toward protons outside the PO domains. The presence of extended interfaces between different domains (PO, EO, siloxane network) can also contribute to the observed initial delay46 or at least to a slower spin diffusion at small τm values.59,80 The initial slope after the delay is indicative of the spatial distribution of the protonated groups from the PO domain but depends on various parameters such as the dimensionality of the domains, the volume fractions of each domain, and the diffusivity constants.46,47 However, we have tentatively determined the domain sizes of the micellar aggregates’ cores made of the hydrophobic PO domains. We used the initial rate method47 and assumed a twophase system consisting of spherical PO domains included in an external phase comprising the other domains (EO and
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Figure 6. (a) Typical 1H spectra obtained by the 1D magnetization exchange experiment for different mixing times τm (membrane M03). (b) Related normalized spin diffusion curves. (c) Types of spin diffusion curves obtained.
siloxane). In addition, the effective diffusivity constant is calculated by assuming that the initial diffusion rate depends to a first approximation on the magnetization exchanges between PO and EO domains. With these hypotheses, the estimated domain sizes for the PO domains are 13.1 and 9.4 nm for M03 and D03, respectively (see the Supporting Information, SI 7, for details). These values are in accord with those found by SAXS. In addition, the difference between them indicates a variation in the aggregation number of F127 copolymers. Less polar dimethylsiloxane units lead to smaller F127 aggregates in agreement with other results found for D03, namely the smaller pores (in calcined samples), the smaller mobility of PO units, and the higher proximity between PO and EO domains (vide infra). As discussed above, there is a significant contrast in the degrees of mobility for the different protons in the different domains and samples. As a consequence, the 1H homonuclear dipolar couplings and the diffusivity constants D are also different and a direct comparison of the curves might lead to
erroneous conclusions about spatial proximities. To overcome somehow this problem, we have estimated for each group D from T2′ values (Table 1) and effective diffusivities Deff from D values, and we have replotted the normalized magnetization as a function of (Deff · τm)1/2 instead of τm1/2 (see the Experimental Section for more details). Using this rescaling procedure, (Deff · τm)1/2 has directly the dimension of length (expressed in nm), but it still does not incorporate the dimensionality of the spatial organization. However, samples have identical mesostructure (Im3m j cubic phase), and the rescaled curves can be used safely for comparison. We have compared the rescaled diffusion curves obtained for the organic groups -CH2- in EO units, and CH3-Si (Figure 7). In the case of sample D03, the diffusion curves of -CH2in EO and CH3-Si are very similar (Figure 7a). This would mean that both groups of EO and siloxane domains are equally distant, on average, from the PO domain. Therefore, the EO and siloxane domains do not have a highly separated and defined interface. In the case of sample M03 (Figure 7b), the diffusion
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Figure 7. Rescaled diffusion curves from CH3- in PO to -CH2- in EO units (9) and CH3-Si (O) for each membrane: (a) D03, and (b) M03. Onsets are given for comparison of initial delays and slopes.
from CH3- in PO to CH3-Si is more delayed than that to -CH2- in EO. For this sample, we can state to a first approximation that domains of PO are surrounded by domains of EO and not directly in contact with the CH3-Si groups of the siloxane network. We have also compared the diffusion curves obtained for a given group in different samples. The diffusion curves from CH3- in PO to -CH2- in EO are presented in Figure 8a. The initial delays are identical. On the contrary, the slope for sample D03 is slightly higher (in scalar values) than that for M03 samples. Therefore, we can state that PO and EO units are on average closer in D03 rather in M03 samples. This observation might correspond to smaller PO and EO domains for D03 (lower aggregation number) or to more homogeneous EO domains (lower content in -OH groups). The diffusion curves from CH3- in PO to CH3-Si groups are compared in Figure 8b. For sample D03, the initial delay is shorter and the first slope is higher (in scalar values) than that for sample M03. Siloxane domains are thus closer to PO domains in D03 in accordance with the above remarks. We also observe a significant change of slope in the curve of sample D03. This could correspond to the existence of two populations of CH3-Si, or at least to a wide spatial distribution of these groups46 in agreement with T2′ analysis. The rescaling procedure is meaningless for -OH groups, because the measured T2′ values cannot be considered (vide supra). However, we should mention that their “non-rescaled” diffusion curves are close to that of -CH2- in EO or CH3-Si. This is in agreement with variations in chemical shifts and somehow with DQ-SQ MAS correlation spectra showing correlation peaks between these groups (Supporting Information, SI 6). Therefore, the -OH groups may be spatially distributed within the EO domains and the siloxane network in agreement with δ variations (vide supra).
Modeled Spatial Organization of the Hybrid Mesostructured Membranes. Two simple models describing the organization of the micelles in domains in Mc and Dc mesostructured membranes are presented in Scheme 2. The different domains are formed by PO (dark gray) and EO (clear gray), -OH groups on the pore surface are indicated by black dots, and the siloxane network modified by CH3-Si groups in D or T units is represented by the external white part of the scheme. The models are simplistic representations but give a direct visual comparison in agreement with the experimental data. Silanols and adsorbed water molecules (the so-called -OH groups) are represented at the interface between EO and siloxane domains, although the interfaces between these domains are not so well-defined (from observations of 1H chemical shifts variations and 1H spin diffusion curves). In the case of samples made with Q and T (MeSiO1.5) units (series Mc), there is a slight increase in the hydrophobicity of the siloxane network but the number of silanols and water molecules remains significant (lower condensation degree from 29Si NMR, higher number of -OH groups from 1H NMR). Therefore, the EO units have a significant interaction with the siloxane network (variations in 1H chemical shifts, lower mobility from T2′{1H}) leading to well-separated PO domains and possibly higher aggregation numbers for F127 copolymers (larger distances CH3-Si T PO from 1H spin diffusion, larger PO domains from initial rate estimations, higher mobility of PO units from T2′{1H}, higher cell parameters from SAXS). For the samples made with Q and D (Me2SiO) units (series Dc), the hydrophobicity is higher (higher condensation degree from 29Si NMR, lower number of -OH groups from 1H NMR). Therefore, the EO units are less interacting and their environment becomes closer to that in neat F127 (lower variations in 1H chemical shifts, higher mobility from T2′{1H}). PO units are not so well separated from the siloxane network and may share a common interface (distances CH3-Si T PO
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Figure 8. Rescaled diffusion curves for membranes D03 ([) and M03 (0) from CH3- in PO to (a) -CH2- in EO units and (b) CH3-Si. Onsets are given for comparison of initial delays and slopes.
SCHEME 2: Modeled Spatial Organizations of the Hybrid Membranes
of the siloxane network by choosing simple organosiloxane precursors allows tuning the interaction of the siloxane species with EO or PO units. This interaction affects the final mesostructure properties: penetrating EO segments (SBA-15), separated EO and siloxane domains (series Mc), and poorly defined siloxane-EO interfaces (series Dc). Conclusions
similar to EO T PO from 1H spin diffusion, lower mobility of PO from T2′{1H}). The spatial distribution of domains in these hybrid organicinorganic mesostructures slightly differs from that of SBA-15 mesoporous silica.31 In particular, we have no strong evidence of occluded and immobilized copolymers or EO segments in the siloxane network.35,36 From our analysis we deduce the presence of a poorly defined interface between EO and siloxane domains for the more hydrophobic network (series Dc). This can be understood considering the formation of these mesostructured samples. Solubilization of the more hydrophobic D units inside the micelles’ core will be more favored than that of T units. In addition, D units lead to less dense and more flexible networks (alone D units only form linear or cyclic species) that can be more easily accommodated with a deformed interface. In the case of Mc samples, the favorable interaction between EO units and silanols, which leads to occluded EO segments in SBA-15, is compensated by a moderate degree of hydrophobicity. In other terms, modulation of the hydrophobicity
Mesostructured membranes prepared by using triblock copolymer PEO-PPO-PEO, as template, and different types of organically modified alkoxysilane, as siloxane precursors, have been prepared. From SAXS measurements, the hybrid membranes show a cubic Im3m j mesostructure with a significant degree of order. When increasing the concentration of the organosiloxane precursors, the order is progressively lost but the local symmetry is maintained. The mesostructured phase appears, therefore, not dependent on the type of organosiloxane precursor while the relative composition affects the order. On the other hand, characterization of the membranes at different length scales using 1H solid-state NMR has pointed out that different organosiloxane units give rise to differently distributed PO, EO, and siloxane domains. The proportion of these organosiloxane units as well as the number of organic groups in these units (one or two methyl) influences the degree of hydrophobicity of the siloxane network. This is reflected in a different interaction at the micelle-siloxane interface that affects the mobility of the EO and PO units as well as their spatial organization with respect to the siloxane network. In these hybrid mesostructured samples, the NMR data give no clear evidence of formation of occluded and immobilized copolymers or EO segments into the siloxane pore walls but rather indicate
Hybrid Organic-Inorganic Mesostructured Membranes the presence of more or less defined siloxane-EO interfaces depending on the hydrophobicty at the copolymer-siloxane interface. In that sense, the modulation of the degree of hydrophobicity by simply adjusting the nature and quantity of organically modified alkoxides seems to be a promising path for the control of the organization and spatial distribution of domains in the final mesostructured materials. Acknowledgment. EU, CNRS, and Re´gion Centre (France) are acknowledged for funding the high magnetic field (17.6 T) NMR spectrometer. Supporting Information Available: Solid state MAS NMR: H exchange experiments; 1H NMR signal assignment; 1H-1H 2D DQ-SQ correlation spectra; SAXS patterns in transmission mode; and N2 sorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org. 1
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