Nanomechanical properties of polysiloxane-oxide interphases measured by interfacial force microscopy

Using an interfacial force microscope (IFM), we have measured the elastic n anoindentation modulus of thin films of poly[(aminopropyl)siloxane] spin-co ated on Nai-containing glass and native SiO2. The films were prepared by th e hydrolytic polycondensation of gamma-(aminopropyl)triethoxysilane (gamma -APS). The Elastic moduli of 500 Angstrom thick films deposited on Naf-cont aining glass and native SiO2 are 8 +/- 2 and 35 +/- 3 G Pa, respectively. I FM data were complemented with atomic force microscopy and infrared and X-r ay photoelectron spectroscopy. We propose that the significantly smaller mo dulus of gamma -APS on glass is related to the incorporation of Na+ ions fr om the glass into the siloxane network of the film. These incorporated Naions act as Lewis acids and catalyze the depolymerization of the Si-O-Si ne twork, resulting in a less rigid polysiloxane framework and a lower elastic modulus. A clue to the films' structural and chemical difference is provid ed by the observation of time-dependent morphological changes for gamma -AP S on glass but not for gamma -APS on SiO2.