Moisture effect on the mechanical and interfacial properties of epoxybonded material system: An atomistic and experimental investigation

This paper uses molecular dynamics simulations and experimental methods to investigate the moisture effect on the mechanical and interfacial properties of epoxy-bonded material systems consisting of SU-8 photoresist and silica substrate. In the simulation, the density (ρ) and Young's modulus (E) of cross-linked SU-8 epoxy network are determined. The ρ shows a monotonic increase with increasing moisture contents, whereas the E displays an initial increase followed by a decrease, with the largest deterioration of 5.5% compared with the dry case. Furthermore, a SU-8/silica interface is modeled for the investigation of interfacial integrity. A 77.1% reduction of surface energy at the interface is observed in the presence of moisture. In the experiment, the SU-8 photoresist coated on the silica substrate was conditioned in a water bath under different durations. The SU-8 photoresist after 2-week moisture conditioning became milky and black particle-liked material texture was observed under optical microscope, which may imply a change of material structure. The E of SU-8 photoresist was measured as a function of moisture conditioning duration by using nanoindentation, and it was not deteriorated with pronounced moisture ingress, which agrees well with the simulation observations. Moreover, the fracture energy (Γ_i) of the SU- 8/silica interface is quantified and the reduction of Γ_i under presence of moisture corresponds to the simulation observations. Based on both the computational and experimental approaches, it is found that moisture has insignificant effect on the epoxy mechanical properties, but deteriorates the interfacial adhesion dramatically. The dramatic interfacial deterioration should be carefully considered in epoxy-bonded material systems when subjected to a moisture condition.