2018 Volume 67 Issue 2 Pages 229-234
In this study, the grafting modes of the silane coupling agent (3-aminopropyltriethoxysilane; APTMS) that modifies the silica surface were analyzed by considering the chemical adsorption onto the surface and the self-condensation of the APTMS molecules. First-principles electronic state calculations were performed to evaluate the difference in energy among a variety of molecular forms of APTMS, which were supposed to arise from the surface adsorption of an APTMS molecule and the oligomer formation of APTMS molecules. The APTMS monomer exhibited an energetically stable structure via strong interactions of its OH groups and NH2 group with the OH groups on the silica surface. Furthermore, there was a possibility that an APTMS molecule can be covalently bonded by mono- and di-grafting with the silica substrate. In addition, we found that the APTMS undergoes condensation reaction between the molecules with forming an extended-chain type di- to tetramer, a cyclic tetramer, and a larger branched oligomer. Such oligomer formation was energetically favorable compared to the chemisorption onto the silica surface. The finding provides an insight for understanding the atomic details of network structures of silane coupling agent molecules in the interfacial region of composite materials.