Adhesion Strength of Block Copolymer Toughened Epoxy on Aluminum

The effect of poly(ethylene-alt-propylene)-b-poly(ethylene oxide) block copolymer modifiers on the adhesion strength of epoxies was evaluated. Three block copolymer modifiers of varying block weight fractions were synthesized and mixed into the epoxy precursors and cured, forming well-dispersed nanoscale structures in the epoxy matrix. The symmetric block copolymers formed spherical micelles, and the asymmetric block copolymer with shorter poly(ethylene oxide) blocks formed small bilayer vesicles in the epoxy matrix. For both block copolymer nanostructure morphologies, compact tension tests of bulk specimens showed that modified epoxies exhibited a significant improvement in critical strain energy release rate, GIc, over the neat epoxy with no reduction in the elastic modulus. When spherical micelle modified epoxies were used as adhesives for aluminum, single-lap-joint shear tests showed an apparent adhesion strength enhancement of up to ∼46%, with increased cohesive failure. Electron micrographs of the topological features on the fracture surfaces of these materials indicate that micelle cavitation plays a role in both the toughening and adhesion strength enhancement of these epoxies. By contrast, a 28% reduction in adhesion strength was observed in the bilayer vesicle modified epoxies. Possible mechanisms for the disparate adhesion strength results for the two types of block copolymer modified epoxies are discussed.