Molecular structure of an aminosilane coupling agent as influenced by carbon dioxide in air, pH, and drying conditions

Abstract

The molecular structures of the hydrolyzates of γ-aminopropyl-triethoxysilane (γ-APS) were studied by Fourier transform infrared spectroscopy (FT-IR). The molecular structure of γ-APS is a function of the pH of the solution and the drying conditions. In the partially cured film, cast from a 20% by weight γ-APS aqueous solution at pH = 2, the amine group exists as a protonated amine; and at pH = 12 the amine group exists as a free NH₂. The amine group exhibits different structures subject to drying conditions when prepared at natural pH. The sample dried in nitrogen exhibited hydrogen bonding between the SiOH and the NH₂ group. Upon heating, the silanol groups condense and the amine groups no longer hydrogen bond. The increase in the availability of free amine groups was indicated by larger amounts of exchange of NH₂ to ND₂ when the sample was exposed to D₂O vapor. When the hydrolyzecl γ-APS at natural pH was dried in CO₂ or in air, a bicarbonate structure was formed, causing the appearance of bands at 1630, 1575, 1488, and 1332 cm⁻¹. These bands were previously associated with the intramolecular zwitterions of Sio⁻…NH₃ ⁺ E-glass fibers were treated with 2% γ-APS and dried under different conditions, which show the various structures of γ-APS as described above. Differences in γ-APS structures may affect the curing reactions of composite materials.