Inhibition of plasticizing effect in nanostructured epoxy thermosets toughened with SEBS-g-PEG copolymers

Block copolymers are widely applied to toughen epoxy thermosets but usually imparts plasticizing effect. Herein, SEBS-g-PEG copolymers (BCPs) were synthesized by grafting short and miscible branches of polyethylene glycol (PEG-350) on polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene (SEBS) and were applied to toughen epoxy thermosets. The effect of BCPs loading level on epoxy thermosets' properties were studied. Surprisingly, both the toughness and glass transition temperature (Tg) of epoxy composites were increased with increase in BCPs loading level in the range of 4–15 wt%. Morphology analysis revealed that the immiscible poly (ethylene-butylene) blocks (PEB) formed a soft spherical micro-domains surrounded by rigid PS/PEG/epoxy composites. The epoxy miscible PEG, grafted with the PS, created a homogeneous dispersion of the BCP particles in the epoxy matrix and the PS phases were expanded into the epoxy phases. As BCPs loading level increased, the particle density increased but the particle size decreased, subsequently the cross-linking density increased among the rubbery particles, which restricted the movement of segments in the neighboring chains. The fracture toughness of modified epoxy exhibited significant increment by 87% without compromising its Tg.