Hydrolytic aging of polysiloxane networks modeling the glass fiber epoxy‐amine interphase

Abstract It has previously been shown that polycondensates of trialkoxysilanes (AlkO)₃SiR could chemically simulate coupling agent layers located at the interphase of amine‐crosslinked epoxy/glass fiber composites (1) and that a humid environment modifies the degree of condensation of the network. Although it is generally believed that water will inevitably hydrolyze the polysiloxane structure and destroy the interphase (2), the authors have demonstrated that the siloxane links of the network evolve toward an equilibrium state. This state depends on the chemical structure of the organic chain that can react with the matrix. For example, in the case of aminopropyltriethoxysilane, the siloxane equilibrium concentration is low enough to allow total hydrolysis of the polymer. Conversely, propylsilane network stability could be explained by a very high siloxane equilibrium concentration. In this article, one of the previously studied systems has been selected: GPS (glycidylpropylsilane), in which the coupling function is an epoxide group (glycidyl). In an epoxy/glass fiber composite, this group is expected to react with an amine group belonging to the epoxy network. Aniline has been used here to model the GPS‐epoxy network bonding. This reaction modifies the chemical nature of the organic chain branched on the silicon and then potentially displaces the siloxane equilibrium. A gravimetric method, size exclusion chromatography (SEC), Fourier transform infrared spectroscopy (FTIR), ²⁹Si nuclear magnetic resonance (NMR), and ¹³C NMR have been used. The results are that, when exposed to hot water, the GPS and the GPS‐aniline networks evolve contradictorily. Although GPS tends to hydrolysis, GPS‐aniline tends to condensation. This article analyzes the compatibility of the different behaviors with the simple kinetic model reported in a previous paper (1) and the importance of this phenomenon concerning the aging of the glass/matrix interphase of composite materials.