Fracture surface work measurements on glassy polymers by a cleavage technique. I. Effects of temperature

It has recently been postulated that viscous flow or plastic deformation occurs near the tip of an advancing crack in glassy amorphous polymers such as Plexiglas, and this results in molecular orientation of the fracture surface. The evidence for this has been the appearance of colors on fresh fracture surfaces of Plexiglas and the extremely high measured surface work which is 1000 times greater than the theoretical surface energy. The surface energy of a solid is defined as the amount of energy required to create unit area of surface in the material by separating two planes each of half a unit area in extent, and the measured surface work of a polymer may also include any irreversible work which occurred during the fracture process. In the present investigation a cleavage test has been empolyed to measure the surface work of glassy polymers and to measure the effects of temperature on the surface work. A reproducible procedure has been developed to measure the surface work of cracks which propagate continuously and for cracks which propagate in a stick-slip fashion. The measured surface work of Plexiglas II is 1.2 × 102 erg/cm.2, and the value for a typical polystyrene is 4 × 105 erg/cm.2, compared to a theoretical estimate of 450 erg/cm.2. The surface work varies with temperature over the range of −40 to 80°C. The surface work of Plexiglas II and polystyrene decrease with increasing temperature. Colors have been seen on the Plexiglas fracture surfaces over the entire temperature range, and the fracture surface changes from rough to mirror-smooth at elevated temperatures. The inherent flaw size of the polymers, from Griffith's equation, also varies with temperature and is thought to be related to the crazing behavior of the polymer. At 23°C. the flaw size for polystyrene is 0.05 in. and for Plexiglas II, 0.002 in.