The toughening of thermosetting materials has been recently achieved by the incorporation of high performance thermoplastics such as polysulfone and polyetherimide. The fracture behavior of the thermoplastic modified thermosetting materials is dependent upon the morphological features which are mainly determined with the concentration of the thermoplastic modifier. At low concentration of thermoplastic, the thermoplastic will segregate into spherical domains within thermosetting matrix resulting a moderate increase in fracture toughness, whereas at high concentration of thermoplastic, a co-continuous nodular structure is formed, which gives substantial increase in toughness.
However, thermosetting materials with high thermoplastic content often display poor heat and solvent resistance as well as the processing difficulties caused by the high viscosity of the mixture. Thus, it is necessary to develop new methods for toughening thermosetting materials with low thermoplastic content.
In the conventional method for thermoset/thermoplastic semi-IPNs, homogeneous mixture with uniform concentration was preformed before the phase separation occurred. Thus, the final morphology of the conventional semi-IPNs was mainly determined by the composition of the thermoplastic modifier. However, in the novel method using morphology spectrum concept, the concentration profile of thermoplastic component was preformed in thermosetting matrix before the phase separation occurred. The semi-IPN showed the gradient morphology, so-called 'morphology spectrum' which was corresponding to the concentration profile.
In this work, thermoset(epoxy)/thermoplastic(polysulfone) semi-IPNs with morphology spectrum were prepared by controlling the kinetics of dissolution and diffusion of the polysulfone component which was preformed as film or particles or porous membrane in shape, and by controlling the kinetics of the curing reaction of epoxy component.
Semi-IPNs with morphology spectrum displayed improved toughness with the overall thermoplastic content less than 10 wt %. It was found that the fracture toughness enhancement was proportional to the volume fraction of continuous polysulfone-rich phase. Anisotropic semi-IPN prepared by inserting a porous polysulfone membrane showed $K_{IC}$ value of $1.31 MN/m^{1.5}$, which is 130% higher than that of unmodified epoxy resin.
