J. Xu et al., A morphological investigation of thermosets toughened with novel thermoplastics. I. Bismaleimide modified with hyperbranched polyester, J APPL POLY, 72(8), 1999, pp. 1065-1076
The morphology of a bismaleimide (BMI) toughened with a thermoplastic hyper
branched aliphatic polyester (HBP) was studied by scanning electron microsc
opy (SEM). The effect of thermoplastic architecture, molecular weight, and
end group on the size and arrangement of the dispersed phase was investigat
ed and compared with the thermoset fracture toughness. SEM micrographs show
ed that higher molecular weight HBP formed roughly spherical dispersed doma
ins of up to similar to 60 mu m, which contained BMI inclusions. Lower mole
cular weight HBP formed spherical dispersed thermoplastic domains, with dia
meters up to similar to 10 mu m with no BMI inclusions. A low molecular wei
ght linear polyester with a repeat unit structure, which was similar to tha
t of the HBP, was prepared and used as a control. Within error, BMI toughen
ed with the linear control yielded the same fracture toughness as the best
values obtained with HBP-modified BMI, but the morphology differed. The lin
ear polyester phase separated into particles with a larger average diameter
and also possessed some phase-inverted regions. End group effects were stu
died by modifying the hydroxy-terminated HBP to unreactive nitrophenyl, phe
nyl, and acetyl end groups. The nitrophenyl-terminated HBP did not phase se
parate from the thermoset, whereas the nonpolar phenyl- and acetyl-terminat
ed HBP phase separated to form small (less than or equal to 1 mu m and simi
lar to 2 mu m, respectively) spherical domains. Some comparisons were made
to other results with HBP thermoplastics in BMI and epoxy thermosets. (C) 1
999 John Wiley & Sons, Inc.