The use of multinuclear solid state NMR for the characterization of siloxane-oxide hybrid nanocomposites
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The use of multinuclear solid state NMR for the characterization of siloxane-oxide hybrid nanocomposites Christel Gervais1, Beatriz Julián1,2, Eloisa Cordoncillo2, Purificación Escribano2, Mark E. Smith3, Florence Babonneau1, and Clément Sanchez1 1 Chimie de la Matière Condensée, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France 2 Departamento de Química Inorgánica y Orgánica, Campus de Riu Sec, Universitat Jaume I, 12071, Castellón, Spain 3 Department of Physics, University of Warwick, Coventry, CV4 7AL, U.K ABSTRACT The objective of this paper is to review various solid state NMR techniques that have been, or can be used for the structural characterization of siloxane–oxide hybrid systems prepared by sol-gel process, and to discuss the type of information they provide, as well as their limitations. More precisely, this paper focuses on NMR techniques to probe and quantify the different types of oxo-bridges (M−O−M of oxide network, Si−O−Si of siloxane chains and Si−O−M siloxane−MxOy interface) in siloxane-oxide nanocomposites prepared through hydrolysis and condensation of organosilanes and M(OR)n alkoxides. In addition the influence of the functionality of the silicon alkoxides and the nature of the MxOy oxides on the extent of the Si-O-M interface will be examined. INTRODUCTION Properties of hybrid materials in which organic and inorganic components are combined in the nanometer range [1] are usually partially controlled by the extent and the nature of the interface between the constituents. The degree of homogeneity of the final materials depends on the ability to favour co-condensation reactions between the precursors, despite their intrinsically different reactivities to hydrolysis. The siloxane component can be easily characterized through vibrational spectroscopy and 29 Si NMR, and the size or the average distance between metal-oxo nanodomains deduced from small angle X-ray scattering (SAXS) analysis, but the characterization of the interface is not an easy task. When the siloxane component presents a high elasticity like for instance poly(dimethylsiloxane), 29Si MAS and CP-MAS NMR experiments can distinguish and quantify mobile siloxane units from rigid ones directly linked to the metal atoms [2]. 17 O solid-state NMR experiments appear as a very straightforward tool to characterize the structure of these functional hybrids by probing directly and quantifying the different types of oxo-bridges (M−O−M of oxide network, Si−O−Si of siloxane chains and Si−O−M siloxane−MxOy interface) of the nanocomposites. However, the main drawback of this technique is its poor sensitivity due to the low natural abundance of the 17O isotope. This problem can be overcome using 17O-enriched water for the hydrolysis step of the precursors. The evolution of the different types of oxo-species during the formation of the xerogel and the extent of the Si-OM interface in the final structure of the hybrid material can be investigated to determine the influence of the functionalization of the silic
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