We investigated the stress-strain behavior of low molecular weight polycarbonate for optical disc grade (OD-PC) under carbon dioxide (CO 2 ) at various pressures, and compared the results with that under ambient pressure at various temperatures. Elongation at break decreased sharply with increased CO 2 pressure at around 2 MPa, while the elastic modulus decreased gradually up to 6 MPa. These results indicate that the tensile property changed from ductile to brittle with increased CO 2 pressure, although the molecular motion is accelerated due to the plasticization effect of CO 2 . Such ductile-to-brittle transition is similar to that observed under elevated temperatures caused by chain disentanglement due to accelerated molecular motion. Although the changes of tensile properties were similar, the craze structure obtained by the brittle behavior was different, i.e., a filamented-craze structure was obtained under high-pressure CO 2 , while a lace-like one was obtained under elevated temperatures. POLYM. ENG. SCI., 58:683-690, 2018.

Abstract:We found that a filament-shaped porous structure with periodic distance was obtained in polycarbonate for optical disk grade (OD-PC) film by stretching under compressed carbon dioxide (CO 2 ). The evolution of the characteristic porous structure was investigated by in situ observation during the stretching under compressed CO 2 and the optical microscopic observation of the stretched specimen. The voids were obtained under high CO 2 pressure as in the case of elevated temperature, suggesting that the evolution of the voids was caused by crazing due to chain disentanglement by accelerated molecular motion owing to the plasticization effect of CO 2 . The filament-shaped voids were initiated at around the yielding point and increased continuously by nucleation in the matrix around the surface of the pre-existing voids. The shape of the voids did not change to an ellipsoidal one during stretching due to suppression of the craze opening by the hydrostatic pressure effect. The stretching of the CO 2 -absorbed depressurized OD-PC revealed that the initiation of the voids was not only caused by the plasticization effect, but the hydrostatic pressure effect was also required.