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

Abstract

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 (Rc), effective hard-sphere radius (Rhs), and shell thickness (Ts) 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 KIC and strain energy release rate GIC) of the epoxy blends. KIC and GIC were found to increase with increasing Rc and Rhs but decrease with Ts. The blend containing SSEBS-c-PCL with least PCL, i.e., 2.4 wt %, shows nanostructure of the largest Rc and Rhs, and smallest Ts, displaying highest KIC and GIC. 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.