Toughening Epoxy Thermosets with Block Ionomer Complexes: A Nanostructure–Mechanical Property Correlation

Block ionomer complexes based on sulfonated polystyrene- block -poly(ethylene- ran -butylene)- block -polystyrene (SSEBS) and a tertiary amine-terminated poly(ε-caprolactone), denoted as SSEBS- c -PCL, were used to toughen epoxy resin. Well-dispersed spherical microdomains, consisting of a poly(ethylene- ran -butylene) core surrounded by a sulfonated polystyrene shell, were revealed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) in the cured epoxy blends with 10 wt % SSEBS- c -PCL of various compositions. Structural parameters, core radius ( R c ), effective hard-sphere radius ( R hs ), and shell thickness ( T s ) were obtained by fitting the SAXS data with a core–shell model and, for the first time, correlated with the fracture toughness (critical stress intensity factor K IC and strain energy release rate G IC ) of the epoxy blends. K IC and G IC were found to increase with increasing R c and R hs but decrease with T s . The blend containing SSEBS- c -PCL with least PCL, i.e., 2.4 wt %, shows nanostructure of the largest R c and R hs, and smallest T s, displaying highest K IC and G IC . Examination of the fracture surfaces indicates that the increased toughness arises from interfacial debonding of spherical microdomains and plastic expansion of resultant nanovoids, followed by small-scale matrix shear deformation. The correlations between nanostructure parameters and fracture toughness have provided a fundamental understanding of nanostructure toughening of thermosets via an innovative strategy based on block ionomer complexes.