FRACTURE-TOUGHNESS AND FRACTURE MECHANISMS OF POLYBUTYLENE-TEREPHTHALATE POLYCARBONATE IMPACT-MODIFIER BLENDS .3. FRACTURE-TOUGHNESS AND MECHANISMS OF PBT PC BLENDS WITHOUT IMPACT MODIFIERS

A series of polybutylene-terephthalate/polycarbonate (PBT/PC) blends w ith different compositions were prepared using a twin-screw extruder. The morphologies of the blends were revealed by scanning electron micr oscopy (SEM) and transmission electron microscopy (TEM). It was found that a 50/50 PBT/PC blend possessed a bicontinuous structure and the o ther blends had a dispersed phase of either PBT or PC depending on whi ch was the minor component. A relatively strong interface was observed in the blends with 20%, 40% and 50% PBT; but poor interfacial adhesio n was found in the blends with 60% and 80% PBT. The strength of the in terfacial boundary was believed to depend on the composition and blend ing conditions of the individual blend. Fracture experiments showed th at the sharp-notch fracture toughness of PC could be significantly inc reased by mixing with up to 50% PBT without losing its modulus and yie ld stress. The toughening mechanisms involved in the fracture processe s of the blends were studied using both SEM and TEM together with sing le-edge-double-notched-bend (SEDNB) specimens. It was found that in th e toughened blends the growing crazes initiated by the triaxial stress in front of the crack tip were stabilized by the PC domains. The debo nding-cavitation mechanism occurred at the PBT/PC interface, which rel ieved the plane-strain constraint and promoted shear deformation in bo th PBT and PC. This plastic deformation absorbed a tremendous amount o f energy. Crack-interface bridging by the PC domains was clearly verif ied by the TEM study. Thus, the PC domains not only stabilized the gro wing crazes they also bridged crack surfaces after the crack has passe d by. This effect definitely caused a large plastic-damage zone and he nce a high crack resistance. Poor crack resistances of the blends rich in PBT was caused by the poor interfacial adhesion between PBT and PC . In these polymer blends, the growing crazes easily developed into cr acks, which subsequently passed through the weak interface of PBT/PC a nd finally produced fast unstable fracture.