Yielding Behavior of Glassy Amorphous Polymers

The effect of thermal history on yielding behavior of atactic polystyrene, isotactic polystyrene, polycarbonate, and polymethyl methacrylate was examined by correlating the deformed microstructure with measured density changes and compressive stress-strain studies. Electron micrographs of bulk polymers and thin films demonstrate the tendency for well-annealed materials to undergo localized shear deformation at 100–1000-Å interspacings, and density measurements show an over-all density increase of about 0.15% upon plastic deformation. Rapid cooling from the melt decreases both the material density (about−0.04%) and the tendency for plastic strain to localize into narrow bands. Compressive stress-strain studies in which the strain rate, test temperature, and thermal history were systematically varied show a semilogarithmic relationship between nominal strain rate and yield stress. These data were analyzed according to an Eyring-type exponential model where the ``shear activation volume'' and the ``activation energy'' were calculated and compared for the ranges of variables studied.