| Diblock copolymers poly (n-butylene oxide-b-ethylene oxide) (BOEO) with a variety of compositions were synthesized and employed as modifiers for epoxy resins. In blends of the diblock copolymers, epoxy resin (DER383), and curing agent (PN), the diblock copolymers form well-defined microstructures that persist upon the curing of the epoxy. Vesicles, worm-like micelles, spherical micelles, as well as mixed and transitional structures associated with these morphologies were identified from single or binary diblock copolymers in the dilute state. When blended with epoxy at high concentrations, these diblock copolymers generated ordered structures.; Addition of block copolymers at low concentrations (e.g. 5 wt%) considerably increased the fracture toughness of the epoxy, as much as nineteen times, with a relatively small decrease in the elastic modulus. This improvement in mechanical performance appears to be sensitive to the associated morphology as well as the concentrations of diblock copolymers. Dramatic toughness was found in the systems containing wormlike micelles, branched worm-like micelles, and small vesicles.; A close examination of the fracture surfaces of these composites indicates that while all the dispersed morphologies played a similar role as inclusions in the particle toughened epoxy, crack deflection toughening contributed to the significantly higher levels of toughness for the worm-like micelle systems. Regardless of the type of inclusions, the optimal toughness is attributed to a compromise between the applied stress fields and the size of ligaments generated in the matrix material. |
