A novel polyetherimide was prepared and used to improve the toughness of bi
smaleimide resin composed of bis(4-maleimidediphenyl) methane and O,O ' -di
allyl bisphenol A. The morphologies of the modified resins change from sphe
rical particles to an inverted phase structure, depending on the modifier's
content based on the scanning electronic microscopy results. Dynamic mecha
nical analysis is also used to characterize morphologies of the modified re
sins. The phase-separation process of the modified system is traced by time
-resolved light scattering. The change in the light-scattering profile with
curing time showed that the phase separation mechanism depended on the mod
ifier concentration. Phase separation took place via the spinodal decomposi
tion mechanism in the PEI 15-phr- and 20-phr-modified system. The fracture
energy (G(IC)) increased with PEI content in the modified system; in the PE
I 15-phr-modified system, the G(IC) value was three times greater than that
of the unmodified BMI resin. (C) 2001 John Wiley & Sons, Inc.