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SYNOPSISTwo commercial core-shell rubbers were used as impact modifiers for polycarbonate (PC) .Specimens with a single semicircular edge notch were stretched uniaxially in order to study the prefracture damage evolution of blends under a triaxial tensile stress state. The irreversible deformation of modified PC included a cavitation mechanism in addition to the three shear modes of unmodified PC. A t the macroscopic level, the cavitation condition could be described by a mean stress concept. The corresponding critical volume strain for cavitation in PC blends was determined to be independent of rubber content but differed for the two impact modifiers. The critical volume strain for cavitation was used as an index of cavitation resistance for the impact modifiers. The effect of rubber content and temperature on Izod impact strength of the PC blends was also reported. From the relationship between the cavitation resistance and the Izod impact strength, it was proposed that impact modifiers with a higher cavitation resistance impart better toughness to blends with PC.

SYNOPSISThe relationship of blend morphology to deformation mechanisms and notched Izod impact strength was studied with three butadiene-based impact modifiers for polycarbonate ( P C ) .The impact modifiers were a linear polybutadiene ( P B ) , a styrene-butadiene-styrene block copolymer (SBS) , and a structured latex particle having a P B core and methyl methacrylate/ styrene shell (MBS). The particle-size distribution in the blends was determined from transmission electron micrographs ( T E M ) . Fractographic analysis combined with TEM examination of thin sections from impacted specimens provided insight into the failure mechanisms. Good impact was achieved with PC/MBS blends when cavitation of the coreshell particles relieved triaxiality and enabled the matrix to fracture by the plane stress ductile tearing mode that is characteristic of thin PC. The best impact properties were obtained with PC /SBS blends when the modifier was dispersed as aggregates of small particles. Cavitation at the weak internal boundaries relieved triaxiality, but subsequent coalescence of cavitated particles during ductile drawing of the matrix created critical size voids and the resulting secondary cracks reduced the toughness of the blend. In general, P B did not significantly enhance the impact strength of PC. 0 1994 John Wiley & Sons, Inc. INTRODUCTIONPolycarbonate ( PC ) is an engineering thermoplastic with excellent clarity, high heat-deflection temperature, and toughness in thin sections. However, its notched impact properties become poorer as the thickness increases. This notch sensitivity is due to the change in stress state at the notch from plane stress to plane strain and the resulting change in failure mechanism from shearing to crazing. The notch sensitivity can be reduced by blending a small amount of an elastomer with PC. Cavitation of the elastomer relieves the triaxiality at the notch and permits the matrix to deform by ductile shearing. The effectiveness of the elastomer in this function depends on various factors; the ones of particular interest in this study are the chemical architecture of the elastomer and the dispersion of the elastomer in the matrix.' * To whom correspondence should be addressed. Linear polybutadiene ( P B ) alone is not commonly used for toughening ductile matrices. Without cross-linking, it has poor strength, and being difficult to disperse, the morphology of its blends is difficult to control. Instead, PB is incorporated as a component into copolymers or terpolymers of various architectures such as styrene-butadiene-styrene block copolymer (SBS) and structured latex particles with methyl methacrylate/styrene (MBS 1.Styrenic block copolymers with high elastomer content possess a phase-separated morphology in which polystyrene (PS) domains function as physical cross-links for the continuous PB phase. These copolymers are quite attractive as impact modifiers because they significantly improve the toughness at relatively low Nevertheless, elastomer dispersion depends on the processing condit...

SYNOPSISDeformation of polycarbonate (PC) impact-modified with a core-shell rubber (MBS) was examined a t the microscale and nanoscale. The stress-whitened zone (SWZ) that formed ahead of a semicircular notch was sectioned and examined in an optical microscope and transmission electron microscope. At the microscale, the texture of the SWZ consisted of fine shear lines that formed when cavitation of the rubber particles relieved triaxiality and enabled the PC matrix in the SWZ to deform in shear. Examination of thin sections from the SWZ in the transmission electron microscope revealed nanoscale deformation of the rubber particles. When the particle concentration was low (2%), only random cavitation of rubber particles was observed. At higher particle concentrations (5 and lo%), cooperative cavitation produced linear arrays of cavitated particles. The matrix ligaments between cavitated particles were strong enough that they did not fracture; higher strains were accommodated by particle cavitation and matrix extension in the regions separating the arrays. The cavitated arrays were also observed in the damage zone that accompanied the fracture surface of specimens impacted a t -2OOC. Cooperative cavitation may have implications for the impact strength of blends with higher concentrations of rubber particles. The possibility that particle-particle interactions facilitate cavitation and promote matrix shear deformation is especially relevant to low-temperature impact strength. 0 1995 JohnWiley & Sons, Inc.