Exclusion effect of carbon dioxide on the crystallization of polypropylene

Abstract

Abstract We investigated the crystallization growth of isotactic polypropylene under carbon dioxide (CO 2 ) at various CO 2 pressures and temperatures by in situ observation with a digital high‐fidelity microscope and a specially designed high‐pressure visualized cell. The fibrils within the spherulite were distorted and branched by crystallization under CO 2 at pressures higher than 2 MPa, and this suggested the exclusion of CO 2 from the growth front of the fibrils. The spherulite growth rate ( G ) at 140 °C increased with the CO 2 pressure, attained a maximum value around 0.3 MPa, and then decreased. Above 6 MPa, it became slower than that under air at the ambient pressure. An analysis of the crystallization kinetics by the Hoffman–Lauritzen theory revealed that the pressure dependence of G could be ascribed to the change in the transportation rate of crystallizable molecules (β g ) with pressure; that is, β g increased and then decreased with pressure. The increase in β g at a low pressure was caused by the plasticizing effect of CO 2 , whereas the decrease in β g at a high pressure was due to the exclusion of CO 2 from the crystal growth front. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1565–1572, 2004