Filler-matrix interaction in sodium montmorillonite-organosilica nanocomposite coatings for corrosion protection

Sodium montmorillonite/organosilica sol-gel coatings were obtained from a hydrolysed solution of 3-glycidoxypropyltrimethoxysilane (GPTMS), methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS) with 2 wt% of sonicated sodium montmorillonite (Mt). The effect of different sonication times (0, 1, 3, 12 h) on the structure of both Mt and sol-gel clay-organosilica nanocomposites was investigated by means of 29Si and 27Al solid state nuclear magnetic resonance (NMR) and X-ray diffraction (XRD). Mt underwent relevant structural changes with the sonication treatment and the effect appeared time-dependent, although not linear. The structural changes induced on clay by the sonication process were kept in the clay-organosilica nanocomposite, according to 29Si NMR. Mt addition to the hybrid sol-gel network led to decreasing the amount of completely condensed silicon units by formation of new Si-O-Al hetero-metallic bonds, as shown by 27Al NMR results. Moreover, organosilica matrix and Mt appeared to interact even in the absence of the sonication treatment of the filler. The NMR study of GPTMS/MTES/Mt and TEOS/Mt samples clearly proves the interaction of Mt with the silsesquioxane (T) species in the obtained composites.