The Effects of Network Structure on the Resistance of Silane Coupling Agent Layers to Water-Assisted Crack Growth

Abstract

Silane adhesion promoters are commonly used to improve the adhesion, durability, and corrosion resistance of polymer−oxide interfaces. The current study investigates a model interface consisting of the natural oxide of 〈100〉 Si and an epoxy cured from diglycidyl ether of bisphenol A (DGEBA) and triethylenetetraamine (TETA). The thickness of (3-glycidoxypropyl)trimethoxysilane (GPS) films placed between the two materials provided the structural variable. Five surface treatments were investigated: a bare interface, a rough monolayer film, a smooth monolayer film, a 5 nm thick film, and a 10 nm thick film. Previous neutron reflection experiments revealed large extension ratios (>2) when the 5 and 10 nm thick GPS films were exposed to deuterated nitrobenzene vapor. Despite the larger extension ratio for the 5 nm thick film, the epoxy/Si fracture energy (G_c) was equal to that of the 10 nm thick film under ambient conditions. Even the smooth monolayer exhibited the same G_c. Only when the monolayer included a significant number of agglomerates did the G_c drop to levels closer to that of the bare interface. When immersed in water at room temperature for 1 week, the threshold energy release rate (G_th) was nearly equal to G_c for the smooth monolayer, 5 nm thick film, and 10 nm thick film. While the G_th for all three films decreased with increasing water temperature, the G_th of the smooth monolayer decreased more rapidly. The bare interface was similarly sensitive to temperature; however, the G_th of the rough monolayer did not change significantly as the temperature was raised. Despite the influence of pH on hydrolysis, the G_th was insensitive to the pH of the water for all surface treatments.