The rigid rod-like 4,4'-bis(6-hydroxyhexyloxv)biphenyl (BHHBP) units were distributed in either the epoxy or polyurethane to become SR-epoxy and PU (with or without BHHBP) polymer matrices. The interpenetrating polymer networks (IPNs) of PU (with or without BHHBP) and SR-epoxy were synthesized through simultaneous polymerization, and connected each other via the grafting reaction between the -NCO groups of the PU polymer network and the -OH groups on the side chains of R-epoxy network. The thermal and mechanical characteristics, compatibilities, and morphologies of these PU (with or without BHHBP)/SR-epoxy graft-IPNs were investigated. The polyether-type PU(PPG series)/SR-epoxy graft-IPNs exhibited two-phased morphologies (i.e., phase separation occurred), and higher fracture energies (G(IC)). Whereas the polyester-type PU(PBA series)/SRepoxy graft-IPNs were homogeneous (no phase separation), and exhibited higher tensile and Izod impact strengths. (c) 2007 Wiley Periodicals, Inc.