The effects of interfacial water and SiO2 surface wettability on the adhesion properties of SiO2 in epoxy nanocomposites

In moist environment, water absorption in inorganic nanofiller reinforced polymer nanocomposite is inevitable. However, little attention has been paid to study the effects of the absorbed water on the interfacial binding behavior of the inorganic nanofiller in polymer matrix. In this study, atomistic molecular dynamics simulations have been conducted to investigate the effects of the absorbed water on the interfacial binding behavior of silica nanoparticles (SiO2 NPs) in cross-linked epoxy nanocomposites (CENs). The results show that before water absorption, strong interfacial adhesion of hydrophilic SiO2 NPs in CENs is observed originated from the strong van der Waals (vdW), electrostatic and H-bonds interactions. However, after water absorption, water molecule will transport in CEN and be adsorbed at the interface between hydrophilic SiO2 NP and CEN. These adsorbed water will weaken the interfacial binding strength of the SiO2 in CEN. In contrast, for hydrophobic SiO2, weak interfacial adhesion is obtained before water absorption, while the decrease of the binding strength after water absorption is also restrained. This work provides a molecular-level understanding of the interfacial binding behavior of inorganic NPs in CENs, and the results give some principles to improve the binding strength of inorganic NPs in CENs.