Effect of phase separation on rheological properties during the isothermal curing of epoxy toughened with thermoplastic polymer

Rheological behavior of thermoset/thermoplastic blends of epoxy/poly(ether sulfone) (PES) was monitored during the curing of epoxy resin. During isothermal curing of the mixture, a fluctuation in viscosity before the abrupt viscosity increase was observed. This fluctuation is believed to be due to the phase separation of PES from the matrix epoxy resin during curing and confirmed by small-angle light scattering (SALS) results. The viscosity behavior is classified-into three regions: the initial low viscosity of the mixture gradually increases with time during the early stage of curing and then it reaches a local maximum. After that, the viscosity slightly decreases because of a decrease of the PES concentration in an epoxy-rich medium upon phase separation and finally increases abruptly because of the extensive network formation of the epoxy matrix. The effects of the domain viscoelasticity and the change of the PES composition in the medium due to the phase separation on rheological properties of the blends are considered, verifying that the change of the PES composition in the medium upon phase separation is known to be the key to interpret the observed viscosity fluctuation. We also observed that the initially negligible storage modulus abruptly increases upon phase separation because of the elastic contribution of high molecular weight PES domains.