GRAFT INTERPENETRATING POLYMER NETWORKS OF POLYURETHANE AND EPOXY .2. TOUGHENING MECHANISM

Graft interpenetrating polymer networks (graft‐IPNs) of polyurethane (PU) and the diglycidyl ether of bisphenol A (epoxy) were prepared by first grafting excess PU prepolymer to the epoxy and then simultaneously polymerizing the PU prepolymer and epoxy. The fracture properties, at high shear rate (e.g., impact) and low shear rate (e.g., pseudostatic tensile fracture energy measurement) of these graft‐IPNs exhibit opposite behavior. Although dispersed rubber particles can enhance the Izod impact strength, toughening of the matrix of graft‐IPNs was found to be the main contribution. In contrast, it was found that a heterogeneous morphology with suitably dispersed rubber domains of appropriate size as well as the toughness of the matrix are requirements for effectively increasing the fracture energy at low shear rate. A reinitiating crack in the plastic matrix is proposed as the main toughening mechanism and can be invoked to interpret the fracture behavior at high and low shear rates of the graft‐IPNs. Copyright © 1990 John Wiley & Sons, Inc.