Structural Features of Surface Layers of Bifunctional Polysiloxane Xerogels Containing 3-Aminopropyl Groups and 3-Mercaptopropyl Groups

Abstract

Polysiloxane xerogels containing both -SH and -NH2 functions have been prepared using the sol-gel method with alkoxysilane precursors [such as Si(OEt)4, (MeO)3Si(CH2)3SH and (EtO)3Si(CH2)3NH2]. Molar ratios of 3.2:1, 1.1:1 and 0.4:1 for the two functions -SH and -NH2 were employed. Xerogels which contain only one of the two functions were also synthesized for comparative purposes.It was shown that the use of stannic compounds as catalysts for xerogels with 3-mercaptopropyl groups resulted in a product which possessed a non-porous structure, while the use of fluoride ions allowed the synthesis of a sample with a porous structure (Ssp = 216 m2/g, Vs = 0.152 cm3/g, d = 3.5 nm). Xerogels with a bifunctional surface layer possess a bimodal structure. Infrared, Raman and multinuclear solid-state NMR (1H, 13C, 29Si) spectroscopy data showed: (i) the absence of deprotonation for the 3-mercaptopropyl groups in the bifunctional surface layer of the xerogels; and (ii) that 3-aminopropyl groups form hydrogen bonds in...