A morphological investigation of thermosets toughened with novel thermoplastics. I. Bismaleimide modified with hyperbranched polyester

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.