Surface modification of nano-silica with amides and imides for use in polyester nanocomposites

Surface modified nano-silica was synthesized through a condensation reaction of the silanol groups on the silica surface with 3-aminopropyltrimethoxysilane (APS). This primary amine offers a wide range of derivatization options and acts as a linker between a silica surface and many organic species. Quantitation of the results indicated the presence of 3–4 reactive APS groups per nm2 of silica. This implies that the silanol groups present on the surface of the silica have reacted to a high extent. APS-treated silica was further treated with a series of acid anhydrides to yield more stable amidoalkyl and imidoalkyl silica. The modified surfaces were analysed by thermogravimetric analysis (TGA), Fourier-transform infra-red (FT-IR) and solid-state 13C NMR spectroscopy techniques to study the organic content and functional composition. Results indicate that the silica particles functionalized with imidoalkylsiloxane (IASi) have much higher thermal stabilities compared to aminoalkyl functionalized silica. It was also observed that thermal stability of the IASi does not depend on the acid anhydride used for the derivatization of the APS treated silica. Furthermore we used this IASi for the synthesis of poly(ethylene terephthalate) (PET)–silica nanocomposites. Dynamic mechanical analysis revealed an enhancement of the storage modulus with increasing silica content at ambient temperature as well as above the glass-transition temperature.