Pierre Delord

Nanostructured Silica-Based Organic–Inorganic Hybrid Materials—Evidence for Self-Organization of a Xerogel Prepared by Sol–Gel Polymerization

A hydrolytic polycondensation reaction of a rigid, rodlike nonmesomorphous precursor leads to an isotropic sol and then an anisotropic birefringent xerogel (see scheme). Optical and X-ray structural analyses demonstrate a short-range order and the possibility of a crystalline order.

Structure of silica-based organic–inorganic hybrid xerogel

Abstract The short range order in hybrid xerogels obtained by hydrolysis of (MeO)3Si–CC–C6H4–CC–Si(OMe)3 was investigated by X-ray diffraction analysis. Results were correlated to spectroscopic data and porosity measurements. Aside from the signal related to the Si–O–Si units (q=1.60 A −1 ) , the signals at q 1 =0.55 A −1 and q 2 =1.12 A −1 were attributed to the presence of organic spacers and their organization within the material. These signals were found to be independent of the porosity of the material as measured by the BET method. Thus, changing the solvent or the concentration used at the polycondensation step leads apparently to a similar structure for the materials, representative of the same local order in the material although the porosity is very different. Copolycondensation with tetramethoxysilane (TMOS) results in a shift of the q1 broad peak towards small angles and a decay of the q2 signal when increasing TMOS ratio. An interpretation by a uniaxial swelling is proposed. Elimination of the organic group by chemical treatment and thermal treatment leads to silica xerogels. While no traces of the former organization were found in the case of chemical treatment, the presence of a signal at about 0.30 A−1 indicates the possibility of a residual organization in the case of the thermal treatment.

Design of porous silica from hybrid organic–inorganic precursors

Hybrid organic-inorganic silica-based materials containing a labile bridging organic unit have been prepared. The elimination of the organic fragment is possible under very mild conditions at room temperature in the presence of a small amount (2%) of fluoride ion as catalyst. The silicas obtained after chemical treatment have been compared with those obtained by a sacrificial route (oxidation at 600°C in air). The mild cleavage of Si–C bonds eliminates the organic units and generates mesoporous silicas with a very narrow pore size distribution (45 A). In contrast, the high temperature air oxidation gave a microporous silica with a wide dispersion of pore diameters. There is a big difference between the sizes of the pores and the size of the organic molecule eliminated. A discrepancy between the sizes of the pores in silicas resulting from the chemical elimination and the sacrificial oxidation route is also observed. A rearrangement of the hybrid network to form a pure silica network takes place during the elimination of the organic moities.

Generation of porosity in a hybrid organic-inorganic xerogel by chemical treatment

A hybrid xerogel of 1,4-bis(trimethoxysilylethynyl)benzene was prepared by using a sol-gel process. Chemical treatment of this material was performed under mild conditions in order to remove the organic group using fluoride anion as a catalyst. Experimental procedures were carried out using different solvents, acidities and counter-cations in order to modify the nucleophilic power of F-. Characterization of the residues was done by elemental analysis, spectroscopy (29Si NMR), thermal analysis, pycnometry, surface area measurement and SAXS. The efficiency of the chemical treatment is related to the solvation of the F- anion. It was possible to demonstrate that elimination of the organic part occurs at the same time as a reorganization of the silica network. This reorganization is the result of two competitive F--catalyzed chemical reactions: a redistribution and a polycondensation process known in solution and which take place in the solid. These processes account for the textural characteristics of the final material and may efface the organization of the organic spacers in the hybrid xerogel. These results demonstrate the limitations of silica as a molecular imprinting material and open a new approach to porous silica with adjustable pore size.

Transformation of nanostructure of silica gels during drying

Abstract Small angle X-ray scattering (SAXS) experiments performed on silica gels dried by different methods show that for some of them, intensity oscillations occur around the Porod law. These oscillations have been analysed to give a chord length distribution of the solid network of the gels. Primary solid particles are rough in xerogels and CO2 evacuated aerogels. Aerogels obtained by methanol supercritical drying show a smooth surface with a well definited chord length distribution curve. These features are explained by silica dissolution–redeposition reactions occurring during the supercritical drying treatment in methanol.

Small angle X-ray scattering of aerogel densification

Abstract Silica aerogels were densified by isostatic compression or by thermal sintering. Small angle X-ray scattering experiments show that both types of aerogels exhibit oscillations around Porod behavior at high q values. Oscillations were modified by sintering but remain unchanged by compression. According to the Babinets principle, it is not usually possible to associate intensity change with a peculiar phase of the porous solid. However, the fact that the oscillations remain unchanged with compression, which only acts on the porous part, indicates that SAXS intensity experiments can be directly linked to the solid part. Using the Porod law [G. Porod, Kolloid Z. 124 (1951) 83; P. Debye, A.M. Bueche, J. Appl. Phys. 20 (1949) 518], the specific surface area has been estimated. Surface area of the compressed aerogels does not change with density. In contrast, the specific surface area of the sintered samples decreases markedly. The absence of shape changes of SAXS curves indicates that the mean size of the solid phase remains constant during the compression densification.

Fractal geometry change induced by compression densification

Texture change induced by an isostatic compression of low density aerogels exhibiting fractal geometry has been investigated. Fractal features are analyzed from small angle X-ray scattering experiments. It was found that at the onset of densification a decrease in the correlation length occurs while the fractal dimension increases a little. Isostatic compression leads to cluster interpenetration. This provoked solid arm entanglement modifies the fractal range of cluster. For higher pressures correlation length does not change indicating that the cluster size is constant. Further densification has been related to the collapse of pores located between clusters.

Plasticity in aerogels

When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature.The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material.The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.

Nanostrukturierte organisch-anorganische Hybridmaterialien auf Kieselsäurebasis – Nachweis der Selbstorganisation eines über Sol-Gel-Polymerisation zugänglichen Xerogels

In einer hydrolytischen Polykondensationsreaktion entsteht aus einer starren, stabchenformigen, nicht mesomorphen Vorstufe zunachst ein isotropes Sol und dann ein anisotropes, doppelbrechendes Xerogel (siehe Schema). Optische Untersuchungen und Rontgenstrukturanalysen belegen eine Nahordnung und moglicherweise auch eine kristalline Ordnung.

A low frequency Raman study of fractons in partially densified silica aerogels

Abstract We report on a low-frequency Raman scattering study of base-catalyzed silica aerogels (BCSA) and partially densified BCSA prepared by heat treatment (HT), hydrostatic pressure (HP) and uniaxial pressure (UP). Low-frequency Raman susceptibility scales with frequency, χ(ω)∼ω−φ, within a large range of frequencies whose extent decreases with densification. Changes in the spectra are correlated with densification-induced changes in structure and connectivity of BCSA. φ is a constant independent of density for all as-prepared samples, in agreement with their mutual self-similarity. φ also remains essentially constant for HT samples while it decreases for HP and UP samples. φ values are discussed in terms of fractal and spectral dimensions of the samples in the light of the theoretical predictions of Alexander, Courtens and Vacher (ACV) and recent Brillouin scattering results.