AbstractMechanical properties of polycarbonates (PCs) and elastomer‐modified polycarbonates with various molecular weights (MW) are investigated. Higher MW PCs show slightly lower density, yield stress, and modulus. The ductile–brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of PC MW and with the increase of elastomer content. The elastomer‐modified PC has higher impact strength than does the unmodified counterpart if the failure is in the brittle mode, but has lower impact strength if the failure is in the ductile mode. The critical strain energy release rate (Gc) measured at −30°C decreases with the decrease of PC MW. The extrapolated zero fracture energy was found atMn= 6800 or MFR = 135. TheGcof the elastomer‐modified PC (MFR = 15, 5 elastomer) is about twice that of thee unmodified one. The presence of elastomer in the PC matrix promotes the plane–strain localized shear yielding to greater extents and thus increases the impact strength andGcin a typically brittle fracture. Two separate modes, localized and mass shear yielding, work simultaneously in the elastomer‐toughening mechanism. The plane–strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane–stress mass shear yielding dominates at higher temperatures and ductile failure. For the elastomer‐modified PC (10 elastomer), the estimated extension ratio of the yielding zone of the fractured surface is 2 for the ductile failure and 5 for the brittle crack. A criterion for shifting from brittle to ductile failure based on precrack critical plastic‐z
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