Thermoset (TS) epoxy resins can be toughened with a thermoplastic (TP) for high-performance applications. The final structure morphology has to be controlled to achieve high mechanical properties and high impact resistance. Four polyethersulfone-modified epoxy resins are considered. They consist of different epoxy monomer structure (TGAP, triglycidyl-p-aminophenol and TGDDM, tetraglycidyl diaminodiphenylmethane) and a fixed amount of thermoplastic, and they are cured with two different amounts of curing agent. A reactioninduced phase separation occurs for all formulations generating morphologies, different in shapes and scales. The aim is to control the final morphology and in particular its dominant length scale. This morphology depends on the phase separation process, from the initiation to its final stage. The initiation relies on the relative miscibility of the components and on the stoichiometry between epoxy and curing agent. The kinetics depends on the viscosity of the systems. The different morphologies are characterized by electron microscopy or neutron scattering. Dynamic mechanical analysis allows confirming the presence of a phase separation even when it is not observable by electron microscopy.Vermicular morphologies with few hundreds nanometer width are obtained for the systems containing the TGAP as epoxy monomer. Systems formulated with TGDDM presents morphologies on much smaller scale of order a few tens of nanometers. We interpret the different sizes of the morphologies as a consequence of a larger viscosity for the TGDDM systems as compared to the TGAP ones rather than by a latter initiation of phase separation. K E Y W O R D S epoxy resins, morphology, polymer blends, toughening

Thermosets are frequently toughened by a high-T g thermoplastic (TP). Blend morphologies obtained by curing induced phase separation with scales of a few hundreds of nanometers are relevant for high-performance applications, but no quantitative description for obtaining these morphologies exists yet. We propose such a quantitative approach for predicting and controlling the final morphology. The key is the degree of curing and the corresponding T g of the blend and both phases when phase separation takes place. It is controlled by the Flory interaction parameter χ of the constituents and their respective T g 's. We show that if phase separation takes place too early during curing, the T g is too low and the morphologies grow to reach sizes of a few micrometers or more. Our study of different systems allows us to propose the relevant range of Flory interaction parameter χ and temperature window T−T g for which the sizes of interest may be obtained. Our work opens the way for devising thermoplastics−thermosets couples with the appropriate affinity and T g 's in order to make blends with tailored morphologies.

No abstract