Molecular Origin of Adhesion at Epoxy Resin-Titania Composite Interfaces: Crystal Phase and Hydroxylation Effects

Polymer nanocomposites composed of polymer matrices and inorganic fillers are widely used in structural materials, coatings, and electronics. Bisphenol A epoxy resins are commonly employed as matrix materials, and the incorporation of various inorganic nanoparticles has been investigated to impart additional functionalities. Titanium dioxide (TiO₂) with anatase (A), brookite (B), and rutile (R) crystal phases has attracted attention as a promising nanofiller, but molecular understanding of the interfacial adhesion mechanism with epoxy resins remains limited. In this study, we performed density functional theory (DFT) calculations to analyze the interfacial interactions between a fragment model of bisphenol A epoxy resin and three TiO₂ crystal surfaces. Both pristine surfaces and hydroxylated surfaces, considering the chemisorption of atmospheric moisture, were examined. The results revealed that adhesion performance depends on the crystal structure. The pristine R-TiO₂ surface exhibited the highest adhesive strength, attributed to the highly active titanium atoms exposed on the surface. On hydroxylated surfaces, A-TiO₂ also showed strong adhesion, which was explained by the formation of multiple hydrogen bonds induced by its sawtoothed surface structure. In contrast, B-TiO₂ exhibited low adhesion performance for both pristine and hydroxylated surfaces. These findings provide molecular insights into the interfacial adhesion mechanism between epoxy resins and TiO₂ nanoparticles, offering theoretical guidelines for the design of nanocomposites with adjusted interfacial properties.