(639h) Adhesion Strength of Block Copolymer Toughened Epoxies

Epoxy coatings and structural adhesives have seen widespread use in bonding and coating applications for a variety of industries, ranging from the consumer electronics to aerospace industries. Epoxies exhibit relatively high strength and modulus, stemming from their cross-linked microstructure. However, this microstructure also leads to intrinsically low fracture toughness, affecting both bulk and adhesive performance. To address this, prior studies have shown that the addition of less than 5 wt% of block copolymers to epoxy resins can significantly increase the bulk fracture toughness of these cured epoxies. However, it was still unclear whether this enhancement in bulk toughness also translated to improvements in the adhesive properties of modified epoxy systems. Here, we evaluate the influence of self-assembled block copolymer nanostructures on the adhesion strength of epoxy thermosets.

Poly(ethylene-alt-propylene)-b-poly(ethylene oxide) diblock copolymer modifiers were mixed into epoxy resins, forming well dispersed nanoscale structures. The Poly(ethylene oxide) relative weight fraction within the block copolymers was varied to access two different types of well dispersed self-assembled morphologies: spherical micelles and bilayer vesicles. Several mechanical tests were performed to characterize the behavior of these modified epoxy adhesives. Tensile tests showed no reduction in elastic modulus for both spherical micelle and vesicle modified epoxies. The modified epoxies also exhibited significant improvements in bulk fracture toughness for both morphologies. Single-lap-joint shear tests showed a significant (~46%) increase in the adhesion strength when spherical micelle modified epoxies were tested on surface roughened aluminum. In contrast, vesicle-forming modifiers were detrimental to adhesion strength, with a 28% reduction. Scanning electron microscopy of the failure surfaces was used to gain insight on the possible mechanisms for the adhesion behavior.