The effect of crystalline morphology of poly(butylene terephthalate) phases on toughening of poly(butylene terephthalate) epoxy blends

In an effort to investigate the effect of the crystalline morphology of a p oly(butylene terephthalate) (PBT) phase on the toughening of PBT/epoxy blen ds, the blends, having different degrees of perfectness of the PET crystall ine phase, were prepared by blending PET and epoxy at various temperatures ranging from 200 to 240 degrees C. As the blending temperature decreases, t he degree of perfectness of the PET crystalline phase increases as a result of the increase of crystal growth rate. For PBT/epoxy blends, the change i n crystalline morphology induced by processing may be the most important ca use for the dependency of the fracture energy on blending temperatures. It has been found that PET phases with a well-developed Maltese cross are most effective for epoxy toughening. This dependency reveals the occurrence of a phase transformation toughening mechanism. Also, the higher relative enha ncement of fracture energy of a higher molecular weight epoxy system is fur ther indirect evidence for a phase transformation toughening mechanism. Som e other toughening mechanisms observed from the fracture surfaces, such as crack bifurcation, crack bridging, and ductile fracture of PET phases, have been found to also be affected by the blending temperatures. (C) 1999 Kluw er Academic Publishers.