Self-Assembly of Bridged Silsesquioxanes - American Chemical Society

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Self-Assembly of Bridged Silsesquioxanes: Modulating Structural Evolution via Cooperative Covalent and Noncovalent Interactions Gaelle Creff,†,‡ Benoît P. Pichon,§ Christophe Blanc,† David Maurin,† Jean-Louis Sauvajol,† Carole Carcel,§ Joel̈ J. E. Moreau,§ Pascale Roy,‡ John R. Bartlett,∥ Michel Wong Chi Man,*,§ and Jean-Louis Bantignies*,† †

Laboratoire Charles Coulomb (UMR CNRS 5521), Université Montpellier 2, 34095 Montpellier Cedex 5, France Synchrotron SOLEIL St. Aubin, F-91192 Gif Sur Yvette, France § Laboratoire Architectures Moléculaires et Matériaux Nanostructurés - ICG Montpellier (UMR 5253) CNRS- UMII-ENSCM- UMI, Ecole Nationale Supérieure de Chimie de Montpellier, 8 rue de l’école normale, 34296 Montpellier Cedex 5, France ∥ Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Locked Bag 4, Maroochydore DC, Queensland, 4558 Australia ‡

ABSTRACT: The self-assembly of a bis-urea phenylene-bridged silsesquioxane precursor during sol−gel synthesis has been investigated by in situ infrared spectroscopy, optical microscopy, and light scattering. In particular, the evolution of the system as a function of processing time was correlated with covalent interactions associated with increasing polycondensation and noncovalent interactions such as hydrogen bonding. A comprehensive mechanism based on the hydrolysis of the phenylene-bridged organosilane precursor prior to the crystallization of the corresponding bridged silsesquioxane via H-bonding and subsequent irreversible polycondensation is proposed.

1. INTRODUCTION Control of the size and the shape of nanomaterials is one of the main challenges in the field of nanotechnology for the materials research community, as it is well-known that a strong correlation exists between these two factors and the key functional properties (optical, electronic, and catalytic) being sought.1,2 In the past two decades, organic−inorganic materials based on bridged silsesquioxanes3−5 have attracted significant attention owing to the tailored properties that can be achieved through careful design of the covalently bonded organic and inorganic components.6−15 These hybrids are readily obtained by the hydrolytic condensation of bis(triethoxy)silyl organics, (EtO)3Si−R−Si(OEt)3 (R = organic fragment). The resulting solids are typically amorphous when prepared by the conventional sol−gel route in “good” solvents and with a stoichiometric amount of water. In recent years, significant effort has been directed toward engineering the nanostructure of such solid materials over multiple length scales. In this context, periodic mesoporous organosilicas16−24 were first prepared via the surfactant mediated route.25,26 Subsequently, we developed a new route for preparing structured bridged silsesquioxanes by conceiving a series of molecular organosilane precursors capable of mediating the self-organization of the solid material.27,28 Additional functional groups such as urea moieties29−31 and long alkylene32−34 chains capable of interacting by hydrogen bonding and van der Waals forces, respectively, favor the self-assembling of the organic fragment. The resulting supramolecular structures could at least partly be transcribed into the hybrid materials under well-defined © 2013 American Chemical Society

synthetic conditions, leading to several shape- and structurecontrolled hybrids.35−38 Since then, the number of studies exploiting this approach has grown significantly, with the intrinsic properties of the organic fragments being modulated to produce photoresponsive or photoinduced thin films,39,40 thermally polymerizing butadienyl units,41 layered ionic-based systems,42 mechanically reinforced hybrid silicas,43 chiral solids,44 selective membranes,45 and fluorescent coatings.46 Despite this intensive effort, only a limited number of studies have been directed toward establishing the mechanistic pathways that contribute to the structuring of these solids.47 In general, it has been found that the final organized material often resulted from a solid precursor that was transformed in a heterogeneous manner into the solid hybrid material.48,49 However, we have also demonstrated that a highly crystalline hybrid bridged silsesquioxane (BS) could be obtained from a completely solubilized bis-urea phenylene-bridged precursor (P) in a mixture of water and THF (1/1 volume ratio) under acid-catalyzed hydrolysis (Scheme 1).50 We previously reported our studies on the hydrolysis− condensation of P using time-resolved X-ray scattering experiments, in which we studied the solid-phase transformation of P into the corresponding BS.51 We proposed that the formation of the supramolecular BS was based on the crystallization of hydrolyzed species followed by their Received: January 31, 2013 Revised: April 7, 2013 Published: April 11, 2013 5581

dx.doi.org/10.1021/la400293k | Langmuir 2013, 29, 5581−5588

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P/THF/H2O/HCl: 1/260/600/0.2. The preparation of this solution is the same for the techniques used for studying the in situ formation of BS and was adapted accordingly as described below respectively (IR, optical microscopy, and light scattering). 2.2. IR Spectroscopy. Midinfrared measurements (400−4000 cm−1) (MFTIR) were carried out on a Bruker IFS 66V spectrometer equipped with an N2-cooled MCT detector, a Globar source, and a KBr beam splitter. The spectral resolution was 2 cm−1, and 64 scans were coadded for each spectrum. Measurements were performed using an attenuated total reflectance (ATR) device equipped with a cylindrical zinc selenium (ZnSe) crystal allowing multiple reflections. The precursor was previously dissolved in THF, and the sol−gel reaction was initiated by simultaneous addition of water and acid (HCl) to the solution (volume ratio water/THF: 1/2). The first spectrum was obtained 1 min after initiating the reaction. Far-infrared measurements (400−40 cm−1) (FFTIR) were performed on the Ailes beamline at the Soleil synchrotron, using synchrotron radiation as the far-infrared source. A Bruker IFS 125 spectrometer equipped with a He-cooled bolometer detector and a 6 μm Mylar beamsplitter was used. The spectral resolution was 4 cm−1, the scanner velocity was 40 kHz, and 200 scans were coadded for each spectrum. The spectra were acquired in transmission mode using a flow cell, facilitating in situ injection and circulation of the different reagents (used in the same quantity as for MFTIR measurements). 2.3. Optical Microscopy. Optical characterization of the hybrid silica morphology during the solid formation was performed with a polarizing microscope (Leitz 12 POLS) equipped with a 1024 × 768 pixel Sony CCD camera and a Nikon D50 digital camera. The reaction was initiated in a test tube, as described above, by adding the appropriate amount of a water/HCl mixture to the precursor dissolved in THF, and the sample was rapidly homogenized with a Topmix vortex shaker. Rectangular capillaries (Vitrocom, 200 μm thick × 100 mm length) were then filled by capillarity and immediately flamesealed to prevent solvent evaporation. The nucleation and growth kinetics of the solid were followed in the capillaries placed in an Instec hot-stage (temperature regulated to within 0.1 °C) under the microscope. Time-sequenced images were automatically analyzed with IDL (Interactive Data Language) software, using standard threshold/binarization methods in order to detect the solid particles. 2.4. Light Scattering. To probe the initial stages of nucleation and growth that are inaccessible to optical microscopy, we performed dynamic and static light scattering (DLS and SLS) experiments immediately after addition of the water/HCl mixture. The P/THF solution and water/HCl mixture were individually filtered using 220 nm Millipore filters. The P/THF solution was poured into the DLS tube, its scattered intensity was measured, and then the correct amount of water/HCl mixture was added. The tube was then homogenized with the vortex shaker for a few seconds before measurements. Light scattering measurements were performed using an AMTEC goniometer equipped with a Brookhaven BI9000-AT correlator and a 514.5 nm argon-ion laser. Experiments were performed at selected angles from 20° to 150°.

Scheme 1. Hydrolysis−Polycondensation of Precursor P into Bridged Silsesquioxane BS

polycondensation in the solid state. However, noncovalent interactions, such as hydrogen bonding, would be expected to compete with covalent processes during sol−gel processing. Furthermore, the condensation step leading to the formation of Si−O−Si bonds would be expected to impose structural constraints which might consequently promote disorganization, as previously shown in the solid state.51 It thus appears important to attempt to elucidate the mechanisms that might control the evolution of the organized solid from such sol−gel processing: self-assembly, hydrolysis, polycondensation of precursor molecules, and nucleation and growth of the hybrid solid in solution. In this work we have combined in situ infrared spectroscopy (IR), polarizing optical microscopy, and light scattering to monitor the different stages of the selforganization process during the sol−gel reaction. While the two last experimental methods indicate the nature and the state of advancement of the solid formation, the local changes in intra/intermolecular covalent and hydrogen bonding induced by hydrolysis and polycondensation have been followed by IR. Our results clearly indicate the respective roles of weak interactions (mainly H-Bond formation) and polycondensation in the evolution of the material’s structure.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Hybrid Precursor and Material.50 Synthesis of 1,4-Bis(triethoxysilyl)propylureidobenzene, P. In a Schlenk tube 1,4-diaminobenzene (560 mg, 5.18 mol) was dissolved in 40 mL of freshly distilled CH 2 Cl 2 under a nitrogen atmosphere. (3Isocyanatopropyl)triethoxysilane (2.69 g, 10.9 mmol) was then added dropwise with a syringe, and the mixture was stirred overnight at room temperature. The solvent was removed in vacuo, and the white solid residue was washed with pentane to remove the excess of (3-isocyanatopropyl)triethoxysilane. The recovered solid was then dried then recrystallized in THF. Yield 88% (2.75 mg, 4.56 mmol); mp 190−192 °C. IR (KBr, cm−1): 1576 (δNH), 1636 (νCO), 3321 (νNH). 1 H NMR (200 MHz, CDCl3): δ ppm 0.63 (t, 4H, CH2−Si), 1.20 (t, 18H, CH3), 1062 (qt, 4H, CH2), 3.13 (m, 4H, NCH2), 3.80 (qd, 12H, OCH2), 6.6 (s, 4H, Har), 6.7 (m, 2H, NH), 7.6 (m, 2H, NH). 13C NMR (50 MHz, CDCl3): δ ppm 7.8 (CH2−Si), 18.3 (CH3), 23.8 (CH2), 42.9 (NCH2), 58.3 (OCH2), 98.5 (CHar), 134 (Car), 157.9 (CO). 29Si NMR (50 MHz, CDCl3): δ ppm −45.4. Mass spectrum (FAB): m/z 602 (25%). Elemental analysis: Calcd for C26H50N4O8Si2 (%): C 51.80, H 8.37, N 8.82. Found: C 51.51, H 8.16, N 8.82. Synthesis of Hybrid Material, BS. P (125 mg, 0.21 mmol) was dissolved in THF (4.4 mL) by gentle heating. On cooling the solution to room temperature, water (2.2 mL) was added under stirring. Then a 1 M HCl solution (41.5 μL) was added to this clear solution again under stirring. The resulting mixture corresponds to a molar ratio of

3. RESULTS AND DISCUSSION 3.1. Morphology: From Isolated Precursor Molecules to Crystalline Material. In situ optical microscopy reveals that the sol−gel synthesis of BS is characterized by the nucleation and growth of rectangular needles with typical sizes of around 10 μm × 200 μm at the end of the reaction (Figure 1). Under crossed polarizers (Figure 1a), these needles display a strong and uniform birefringence with a perfect optical extinction when the long axis is parallel to the polarizer or to the analyzer (Figure 1b). This simple observation indicates that the needles are single crystals. The reaction kinetics were explored by acquiring optical micrographs as a function of reaction time and extracting the time evolution of the crystals’ projected area as shown in Figure 1c. Three different domains, referred to as I, II, and III below, 5582


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Figure 2. Evolution of the background of scattered light intensity at 90° of the P solution during the sol−gel synthesis. Inset: Full scale intensity. Frontiers of the domains are determined from the optical microscopy results.

structured solid during the sol−gel process is primarily controlled by the molecular-scale attachment of isolated precursor molecules at the crystal/solution interface. However, these experiments do not indicate the extent of hydrolysis of the attached molecules and when polycondensation occurs. 3.2. Microstructure: Competition between Noncovalent and Covalent Interaction. 3.2.1. Hydrolysis/Polycondensation. Direct information on the progress of the hydrolysis and polycondensation reactions during the sol−gel process can be obtained via MFTIR spectroscopy in the range 1300 to 900 cm−1 by probing the Si−O antisymmetric stretching modes (νas(Si−O)) and the methyl rocking vibrations (ρ(CH3)) of the SiOCH2CH3 groups.52−54 νas(Si−O) provides insights into the progress of the polycondensation while ρ(CH3) enables the extent of hydrolysis to be probed. The time dependence of the IR spectra of the BS/P system in the region of νas(Si−O) and ρ(CH 3 ) vibrations is shown in Figure 3, while the corresponding integrated intensities are illustrated in Figure 4. These data clearly reveal that increasing intensity in the νas(Si−O) region is accompanied by a corresponding decrease in the intensity of the ρ(CH3) modes, illustrating the expected correlation between hydrolysis and condensation reactions. If we correlate the spectroscopic data with the time domains introduced above (i.e., domains I, II, and III), then it is evident that in domain I the decrease in the intensity of ρ(CH3) commences essentially immediately after the addition of the acidic aqueous solution to the reaction medium, while essentially no polycondensation is evident during the initial 10 min. At the beginning of domain II after around 10 min (and after the intensity of ρ(CH3) has decreased to around 50% of its initial value), the rate of hydrolysis decreases significantly concomitantly to the crystallization. It indicates that almost all nonhydrolyzed ethoxy groups are rapidly trapped in the growing microcrystal. At this stage, polycondensation commences (as reflected by the emergence of the νas(Si−O) mode around 1016 cm−1). The intensity of the νas(Si−O) mode increases very rapidly during domain II, and it is evident (Figure 4a,b) that nucleation and growth are closely coupled with polycondensation, as expected. During domain III (which commences around 20 min after initiating the sol−gel reaction), water liberated within the microcrystals during polycondensation in domain II promotes additional hydrolysis (albeit at a slower rate), together with relatively slow polycondensation. Note that there is essentially no increase in the size of the crystallites observed via optical microscopy in

Figure 1. (a) Polarized optical micrograph of growing needles. (b) Complete extinction occurs upon rotation between crossed polarizers, indicating that the solid needles are single crystals. (c) Time dependence of the apparent projected area of the crystals in the observation zone. Inset: polarized optical micrographs at the beginning (9 min) and end (22 min) of crystal growth.

are observed. Domain I corresponds to a 10 min latency period after the water/HCl addition, where no aggregates are detected on optical length scales. Most of the microcrystals then nucleate at the beginning of domain II and grow freely. The growth slows considerably after 20 min, and the shape of the crystals remains essentially unchanged throughout domain III. The single-crystal nature of the needles suggests that they grow by attachment of individual molecular species. We nevertheless checked for the possible presence of any submicrometer aggregates during domain I by performing DLS measurements on the solution immediately after initiating the reaction. The intensity autocorrelation of scattered light, measured for a delay range of 25 ns to 0.5 s, revealed no noticeable index fluctuations in the sample. Since a relaxation time of 1 μs at 90° would correspond to a typical hydrodynamic size of 1 nm for aggregates in the low-viscosity THF solvent, we anticipate that only molecular species are present in solution. The static light scattering intensity (see the example reported in Figure 2) remains particularly low, with a magnitude similar to that of the P/THF solution, also indicating that essentially no molecular aggregation occurs during domain I. However, the decrease in the static intensity of scattered light evident at the beginning of the domain reveals a change in the “composition” of the molecular species in solution. The decrease is most likely due to hydrolysis. During domain II, the presence of isolated macroscopic crystals floating in solution is revealed by an increasing number of intensity bursts (as the crystals traverse through the laser beam), but the background Rayleigh scattering originating from the solution remains at the same level. The crystals present at the end of the reaction in the light scattering cells were similar to those obtained in capillaries. The combination of optical microscopy and light scattering observations strongly suggests that the formation of the 5583


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Figure 3. Time dependence of the IR spectra of the BS/P system: (a) νas(Si−O) and (b) ρ(CH3) regions. Spectra are vertically shifted for clarity. Figure 4. (a) Time dependence of the crystallite size of BS monocrystals. (b) Time dependence of the IR band intensities of the BS/P system in the νas(Si−O) and ρ(CH3) regions. (c) Time dependence of the IR band intensities of the BS/P system in the amide 1 region.

domain III, despite an increase of around 20% in the intensity of the νas(Si−O) mode during this phase of the reaction. 3.2.2. Noncovalent Interaction between Organic Moieties (MFTIR). The noncovalent interactions between organic species, and particularly the strength of H-bonding interactions between urea groups in such materials, can be studied by IR measurements in the intermediate wavenumber range from 1350 to 1750 cm−1 or in the 3000 to 3700 cm−1 spectral region where intramolecular ν(NH) and ν(OH) vibrations are localized.55 In the high-wavenumber domain, the OH and NH stretching vibrations are superimposed, and hence it is more reliable to interpret only the intermediate domain. Several specific intramolecular vibrations sensitive to noncovalent interactions between organic moieties can be studied: (1) the CO stretching vibration ν(CO)cycle of the aromatic cycle; (2) N−H deformation (δ(NH)cycle); (3) C−C stretching ν(CC)cycle; (4) urea amide-1 modes arising mainly from ν(CO); and (5) amide-2 modes coming predominantly from NH inplane bending (δ(NH)).53,54 Typically, urea-mediated selfassembly of bridged silsesquioxanes bearing urea groups leads to the presence of H-bonding between these groups in the final material (BS),55 and the wavenumber of the amide-1 mode is a sensitive indicator of the strength of intermolecular H-bonding in such hybrid systems.50,53,54 For example, a significant decrease in the wavenumber of the amide-1 band was observed upon cooling, consistent with an increase in the strength of intermolecular H-bonds and an associated decrease in the intramolecular bond strength.55 The separation of the amide-1 and amide-2 modes (ΔNH) is also sensitive to the strength of the H-bonding interactions, with strongly H-bonded systems exhibiting ΔNH values of around 40 cm−1 and non-H-bonded systems exhibiting values in excess of 100 cm−1.56−58

Variations in the positions and intensities of the amide-1 and amide-2 modes during the sol−gel processing of P are shown in Figure 5. Depending on the extent of reaction, two amide-1 bands are observed at 1635 and 1686 cm−1. The high wavenumber component is assigned to the amide-1 mode of free amide species (associated with urea groups that are not engaged in intermolecular H-bonding with adjacent urea moieties), while the lower-energy component is assigned to the corresponding mode of H-bonded species.50 Variations in the integrated intensities of these modes with reaction time in the BS/P systems are shown in Figure 4c. In domain I, only free amide species are evident, consistent with the absence of significant intermolecular H-bonding during the initial hydrolysis step. The full width at half-maximum of this band (Figure 5) is indicative of the range of different interactions between the solvent molecules and BS/P. The integrated intensity of the mode remains essentially constant during domain I but begins to decrease during domain II where polycondensation commences. The decreasing intensity of the free amide-1 mode is accompanied by a corresponding increase in the integrated intensity of the amide-1 mode associated with H-bonded species, reflecting a gradual conversion of free urea groups to H-bonded urea during domain II and associated polycondensation. The intensity variations in the amide-2 region are consistent with this interpretation, with the emergence of a band at around 1590 cm−1 when significant 5584


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Figure 5. Time dependence of the IR spectra of BS/P system: (a) amide 2 and (b) amide 1 regions. The selected spectra presented are vertically shifted for clarity.

intermolecular H bonding appears.50,55 In the domain III, after 50 min, we observe a slight downshift of the H bonded CO of 4 cm−1 and a narrowing as well as an increasing intensity of the peak with increasing reaction time. This behavior indicates a strengthening of H-bonding while polycondensation still proceeds and is consistent with further local rearrangement or intermolecular ordering in the system. Vibrational spectroscopies and particularly far-infrared are valuable tools for studying intermolecular interactions between organic moieties,59 although to the best of our knowledge, this technique has not previously been used to probe intermolecular interactions such as H-bonding in BS systems. Intermolecular H-bonding vibrations of urea groups can be probed in the farinfrared between 120 and 180 cm−1,60 and as shown in Figure 6, a new feature emerges at around 132 cm−1 with increasing

disordered (as reflected by the broad, ill-defined amide-1 peak centered at around 1685 cm−1). As expected for such a system, no nanosized objects were detected by DLS, and the SLS revealed only weak scattering consistent with the presence of small molecular species (P). The processes occurring during evolution of the needle-like crystals are illustrated schematically in Figure 7. At the

Figure 7. Structural evolution in the BS/P system with reaction time.

beginning of domain I, the sol−gel reactions are initiated by addition of an acidic aqueous solution to P. Hydrolysis and the associated conversion of ≡Si−OEt species to ≡Si−OH species are evidenced by the decrease in the intensity of the ρ(CH3) mode at 1165 cm−1, although essentially no siloxane species are formed. Consistent with these data, the static light scattering intensity changes during domain I while DLS indicates that no aggregation has occurred at this stage. The static light scattering intensity decreases to around background levels at the end of domain I, when around 50% of the alkoxy species have been hydrolyzed. The lack of any discernible condensation during the relatively short duration of domain I (about 10 min of reaction) is not unexpected, given the prevailing acidic conditions. At the beginning of domain II, supersaturation of the relatively insoluble hydrolyzed monomers occurs, and welldefined, needle-like single crystals begin to form in the system (optical microscopy, Figure 1), which continue to evolve until the end of domain II. During this time, no increase in the static light scattering intensity of the solution phase is evident, although “flashes” corresponding to the passage of crystallites through the laser beam are observed as expected. This suggests that there are no large polymeric species present in solution during domain II (their formation in solution is presumably

Figure 6. Time dependence of the far-IR spectra of BS/P system in the region of intermolecular H-bonding. The selected spectra presented are vertically shifted for clarity.

reaction time. This peak is assigned to intermolecular H-bonds formed between adjacent urea groups during polycondensation and growth of the crystals shown in Figure 4a. 3.3. Self-Structuring Mechanism of BS. Optical microscopy, light scattering, and vibrational spectroscopy provide a powerful and complementary suite of techniques for investigating synergistic effects and competition between covalent and noncovalent interactions during the growth of the crystalline materials produced in this study (Figures 1−6). The precursor solution is characterized by the absence of siloxane bonds (as confirmed by MFTIR) and is essentially 5585


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Scheme 2. Representation of the Mechanistic Pathways from P in Solution to BS

reactions in mediating crystal growth. The dramatic spectroscopic changes during this domain may be attributed to local reorganization arising from mechanical stress associated with increasing polycondensation within the needles. The final size of the crystals is around 100 μm in length. A tentative representation of the self-structuring mechanism of BS starting from initially completely solubilized in solution P is given in Scheme 2.

suppressed by the prevailing acidic conditions) and that the growth of the needle-like crystals occurs via the direct addition of hydrolyzed monomers to the evolving single crystals. The similar time evolution of the abundances of siloxane species (νas(SiO)) and H-bonds (ν(CO)bonded, Figure 4) suggests that crystal growth is mediated by a cooperative process involving siloxane bond and H-bond formation. Within this context, a key question is whether the evolution of the single crystals is controlled by H-bonding interactions between adjacent urea groups (and subsequent condensation of the hydrolyzed silane moieties) or the reverse. To the best of our knowledge, such crystal formation has not been reported in the case of precursors that do not contain a functionality capable of directing self-assembly (such as urea), emphasizing the important role of the noncovalent interactions in mediating the formation of the needle-like crystals observed in this work. In addition, siloxane bonds typically formed in sol−gel systems are disordered and randomly oriented. These observations suggest that the cooperative process leading to crystal growth involves the initial attachment of the hydrolyzed monomers to the growing crystal faces, followed by condensation and associated formation of siloxane bonds. Neither H-bonding nor condensation is complete at the end of domain II (around 22 min), as clearly shown by the dramatic increases in the intensities of ν(CO)bonded (Figure 5) and νas(SiO) after 20 min of reaction. At the beginning of domain III, water generated during the condensation reactions promotes further hydrolysis of the remaining alkoxy groups, and further condensation occurs within the crystals. After 60 min, around 50% of the silanols have been condensed, with these reactions continuing for several hours. The abundance of siloxane species increases during this interval, reflecting the continuing growth of the structured, needle-like crystals. However, the most dramatic changes are evident in ν(CO)bonded (Figure 5) with time from 20 min to 60 h, with increasing reaction time leading to a corresponding increase in the intensity of the peak and a decrease in its fwhm. Over the same time scale, the integrated intensity of νas(SiO) also increases more than 2-fold, again reflecting the cooperative role of the noncovalent and covalent

4. CONCLUSIONS The self-assembly of a bis-urea phenylene-bridged silsesquioxane precursor (P) during sol−gel processing and the subsequent formation of a self-assembled organic−inorganic nanohybrid have been investigated by in situ optical microscopy, infrared spectroscopy, and light scattering. Competition between covalent (hydrolysis/condensation of SiOx(OR)y(OH)(4−y−2x) sites) and H-bonding interactions (urea moieties) following addition of water (pH 2) led to the formation of organized needle-like crystals (100 μm in length), which exhibited uniform birefringence under crossed polarizers, with complete optical extinction when the long axis of the crystal was aligned parallel to the polarizer or analyzer. This observation indicates that the needles are single crystals. A mechanism describing the formation of the organized nanohybrids is proposed, based on the in situ data: • Hydrolysis of P proceeds without significant condensation until around 50% of the available alkoxy ligands are consumed. • When the critical supersaturation concentration of the hydrolyzed precursor molecules is reached, formation of the organized hybrid commences via a concerted process involving intermolecular H-bonding of urea groups and condensation of adjacent silanol/alkoxy species to form siloxane species. Subsequent growth of the organized material is mediated by direct “attachment” of precursor molecules at the solid/solution interface. • During subsequent aging, local reorganization arising from mechanical stress associated with increasing 5586


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polycondensation within the needles occurs. The final size of the crystals is around 100 μm in length.

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AUTHOR INFORMATION

Corresponding Author

*Tel +33 467 147 219, Fax +33 467 147 212, e-mail michel. [email protected] (M.W.C.M.); Tel +33 467 144 639, Fax +33 467 144 637, e-mail [email protected] (J.-L.B.). Notes

The authors declare no competing financial interest.

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REFERENCES

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