Characterization of gamma-irradiated polymethyl methacrylate by means of mechanical properties and positron annihilation lifetime spectroscopy

An experimental investigation was undertaken to assess the effect of gamma irradiation on the "structural" state and the corresponding inelastic defor mation behavior of polymethyl methacrylate (PMMA). Uniaxial and constant st rain rate compression tests were conducted over a range of strain rates at room temperature on glassy polymer specimens of PMMA subjected to different gamma-irradiation doses. Measurements of positron annihilation lifetime sp ectroscopy were performed on samples before the compression test in an atte mpt to correlate free volume content with yield stress. A kinetic law of pl astic flow considering a local shear transformation in a small volume Omega is used to represent the observed mechanical behavior. The density and siz e of the clusters are found for the material subjected to different irradia tion dose. Positron annihilation lifetime spectroscopy was also used to est imate the size and number density of free volume sites in the polymer speci mens. The dependence of both magnitudes with the irradiation dose follows t hose of the plastic cluster of volume Omega. Both results provide experimen tal evidence that the density and the size of high free volume sites can be used as an internal state variable for characterizing the mechanical state of glassy polymers. [S0163-1829(99)05029-8].