Structural studies of ideal organic-inorganic nanocomposites by high resolution diffractometry and NMR spectroscopy techniques

Abstract

Hybrid organic-inorganic materials, silica-poly(ethylene glycol) (PEG) blends, were prepared by the sol-gel process from mixtures of tetraethoxysilane and PEG of low molecular mass. The synthesis scheme (acidic [HCl] or nucleophilic [NH4F] catalysis) influences the structure of these materials and consequently their properties. Two different methods were used to investigate the structure of these blends: a) X-ray diffraction techniques; b) 29Si NMR spectroscopy. A new x-ray diffractometry technique identifies precise interference functions and radial distribution functions of these blends. The comparison of predicted radial distribution functions of the Bell and Dean's physical model refined by Gaskell with the radial distribution function obtained from this technique is implemented to identify the structure of these blends. Analysis by amorphography has identified the existence of SiO2 silica grains and provides only about the positional disorder of these grains in continuum random network. The NMR spectroscopy discriminates the different silicon sites and demonstrates the changes of the morphology and structure when the nature of the catalyst is modified. These results indicate that the structure of non-crystalline SiO2 aggregates inside nanocomposites differs from fused glass by their compositional disorders. These nanocomposites could be described as an agglomerate of SiO2 objects with the pores filled by disordered polymer chains. When these materials are obtained under acidic conditions, the polymer chains are linked to the SiO2 grains forming an ideal composite.