Changes in the Aggregation States and Physical Properties at the Adhered Interfaces of Epoxy Resins Under Hygrothermal Conditions

A better understanding of the hygrothermal aging behavior of an epoxy resin at adhesive interfaces is crucial for improving adhesive strength and long-term durability. Since hygrothermal aging involves water molecules segregated at the interface, the water segregation should be studied as a first benchmark. In this study, the water distribution near interfaces with different surface chemistries was examined by neutron reflectivity (NR) measurements, where an incident neutron beam introduced from the substrate side. Hygrothermal treatment and NR measurements under water vapor were repeated for fully cured epoxy films on OH and H-terminated Si wafers, representing hydrophilic and hydrophobic surfaces, respectively. As a result, the amount of water near the SiOH interface increased over the course of hygrothermal treatment and exceeded that in the bulk. On the other hand, the amount of water near the SiH interface was almost the same as that in the bulk and remained constant regardless of the treatment time. These differences likely reflect the different extents of deterioration upon hygrothermal treatment. In our presentation, the relationship between interfacial deterioration and adhesive behavior will be discussed.