Impact resistance and secondary transitions

Instrumented Charpy impact measurements were made on polycarbonate from liquid nitrogen temperature to room temperature. The polymer had a transition from brittle fracture to ductile failure at −130°C. Scanning electron micrographs of the fracture surfaces did not correlate with the secondary transition or test temperature. A Fourier analysis of the impact pulse covers a wide range of frequencies, but the dominant frequencies at room temperature are below 200 Hz. Time–temperature superposition shows that the secondary transition occurs over a broad frequency range centered at 7 MHz at room temperature. Impact strength and the secondary transition (at impact frequency) both have a maximum value around −75°C.