Positron lifetime spectroscopy characterization of thermal history effects on polycarbonate

The effect of a short-term anneal above T_g on the free volume cavity size and concentration and on the fracture toughness of polycarbonate is examined. The positron annihilation lifetime (PAL) technique is used to measure the change in free volume concentration and cavity size during isothermal relaxation experiments at 10, 20 and 30 ° C. An activation energy of 16.5 kJ mol^-1 is calculated for the relaxation of the annealed polycarbonate, compared to 12.3 kJ mol^-1 for the unannealed material. The fracture toughness and brittle fracture morphology of compact tension specimens are unchanged by the anneal. The similarity in the PAL parameters and physical properties between the unannealed polycarbonate and the material annealed above T_g suggests that the short-term anneal does not appreciably alter the structural state of glassy polycarbonate.