Micellar structure and mechanical properties of block copolymer-modified epoxies

Amphiphilic block copolymers provide a unique means for toughening epoxy re sins because they can self-assemble into different inclusion shapes before epoxy curing. The two examples reported here are spherical micelles and ves icles, which form in blends containing epoxy and symmetric or asymmetric po ly(ethylene oxide)-poly-(ethylene-alt-propylene) (PEO-PEP) block copolymer with PEO volume fractions of 0.5 and 0.26, respectively. The vesicles and s pherical micelles were characterized by transmission electron microscopy an d small-angle X-ray scattering (SAXS), respectively. SAXS data from the sph erical micelles were fit to the Percus-Yevick model for a liquid-like packi ng of spheres with hard-core interactions. Mechanical properties of spheric al-micelle-modified and vesicle-modified epoxies in the dilute limit are co mpared. The glass-transition temperature and Young's (storage) modulus were tested with dynamic mechanical spectroscopy, and compact-tension experimen ts were performed to determine the critical plane-strain energy release rat e for fracture. Vesicles were most effective in improving the epoxy fractur e resistance. (C) 2001 John Wiley & Sons, Inc.*.