Epoxies of systematically varying crosslink density containing 5% by weight of a poly(ethylene oxide)-b-poly(ethylene-alt-propylene) (OP) block copolymer were prepared and characterized. The block copolymer self-assembled to form particles with diameters ranging from 15 to 100 nm. Transmission electron microscopy of the modified epoxies revealed that the block copolymer nanostructure can be altered by changing the epoxy crosslink density. The block copolymer structures displayed a decrease in surface curvature as the crosslink density was reduced.
The strain energy release rate, Gc, of the block copolymer-modified epoxies, which can be related to fracture resistance, increased dramatically with a decrease in the epoxy network crosslink density and plateau at a value 13 times greater than the unmodified material. This trend was observed with both high and low molecular weight OP additives. The toughening behavior is dependent on the block copolymer nanostructure in highly crosslinked system while lightly crosslinked block copolymer-modified epoxies display similar fracture resistances for each block copolymer additive. Scanning electron microscopy of fracture surfaces revealed extensive voiding and plastic deformation near the crack tip of the modified epoxies. Addition of the block copolymer did not appreciably decrease the Young's modulus or glass transition temperature compared to the unmodified material.
Epoxies with varying concentrations of block copolymer additive were prepared and further demonstrated the role of crosslink density on improving fracture resistance. Lightly crosslinked epoxies were able to maintain extraordinary toughness at block copolymer concentrations as low as 1 wt%. Increasing crosslink density decreases the toughening ability of the block copolymer and a higher concentration is required to provide adequate fracture resistance.