Modification of epoxy resins for higher fracture toughness without sacrificing their other desired properties is needed in various applications. Amphiphilic block copolymers, containing “epoxy-philic” and “epoxy-phobic” sub-chains, can self-assemble into different structures in uncured epoxies. The resultant nano-structured thermosets exhibit remarkable toughness improvement. Over the last decade, extensive studies have been devoted to exploring the territory of block copolymer modifiers, while the selection of epoxy resins is constrained to several model systems that have limited practical utility.<br/>In this work the microstructure and mechanical properties of a commercially viable block copolymer modified epoxy resin were investigated. The epoxy resin, containing a bio-derived curing agent, is widely used for high-quality coatings. Four poly(ethylene oxide)-<i>b</i>-poly(butylene oxide) (PEO-PBO) diblock copolymers with different molecular weights were synthesized by anionic polymerization, with the same composition (approximately 35% by weight PEO) as a commercially available product. Based on characterization results obtained using TEM, DSC, and SAXS, we engineered sample preparation and curing procedures that led to the dispersion of the PEO-PBO diblocks in the solid product. As the total molecular weight increased from 7 kg/mol to 48kg/mol, the morphology transitioned from polydisperse micelles, to entangled wormlike micelles, to a mixture of spherical and cylindrical micelles, and finally spherical micelles. Time-resolved SAXS experiments were employed during epoxy curing to identify how the morphology evolves with the extent of crosslinking. Modified epoxies containing between 1.5 and 5 weight percent PEO-PBO displayed greatly improved fracture toughness. Maximum toughness enhancement, a 10-fold increase in the strain energy release rate over the unmodified material, was achieved with 5 wt.% loading and a wormlike morphology. Spherical micelles produced a 6-fold improvement. The topography of the fracture surfaces suggests that wormlike micelles are capable of triggering localized plastic deformation in the epoxy matrix, facilitating matrix shear yielding and thereby contributing to a much greater toughness increase. This improvement in toughness has been achieved without sacrificing modulus, glass transition temperature, or transparency of the epoxy product.

Engineering superior toughness in commercially viable block copolymer modified epoxy resin

249th National Meeting (2015)

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