Effect of stress state and polymer morphology on environmental stress cracking in polycarbonate

Environmental stress crazing or cracking (ESC), a long studied phenomenon, is the brittle failure of glassy thermoplastics, which are normally ductile, under the synergistic action of stress and certain surface active agents. This work involves a study of a polycarbonate-oleic acid system under two novel in-depth conditions: multiaxial stress states and changes in the polymer morphology. Initial uniaxial creep tests showed that the formation of cracks rather than crazes is observed. Multiaxial testing is done using blister tests where the polycarbonate film is stressed using a pressurizing medium to form a blister that is in a biaxial state of stress. Changes in the polymer morphology are induced by orientation of the polymer film. On samples exposed to stress and surface active agents, the stress component that is perpendicular to the direction in which the crazes/cracks form appears to influence the crack patterns. The polymer orientation has a significant influence. The orientation not only induces crack formation in a direction parallel to the orientation (regardless of the direction of the major principal stress), but it also reduces the stress required for crack formation for all stress states. Any attempt at modeling the phenomenon of environmental stress cracking therefore needs to take these effects into account. (C) 2003 Wiley Periodicals, Inc.