The formation of γ-crystal in long-chain branched polypropylene under supercritical carbon dioxide

The crystallization behavior of long-chain branched (LCB) polypropylene (PP) in the supercritical carbon dioxide (scCO(2)) atmosphere was investigated to show the influences of LCB and CO2 on the formation of gamma-crystal. The crystallization experiments were performed in CO2 atmosphere with the pressure from 1.3 to 10.4 MPa and temperature between 90 and 130 degrees C. The effects of LCB level, CO2 pressure, and crystallization temperature on the content of gamma-crystal were investigated. The results showed that the influence of LCB on the formation of gamma-crystal was obvious when PP was crystallized in CO2. The content of gamma-crystal increased with LCB level and reached a maximum of 88.2%. It could be explained that, as LCB increased the chainfolding energy of PP molecular chain and hindered it from folding back into crystal lamella, which made the formation of gamma-crystal easier. However, CO2 was the key factor in the formation of gamma-crystal, and the influence of CO2 on T-crystal was much significant than that of LCB. It was believed that the increase of free volume after dissolving Of CO2 in PP was helpful in the formation of gamma-crystal. It was found that the content of gamma-crystal increased almost linearly with CO2 pressure (CO2 content), and the contribution of CO2 to gamma-crystal increased with pressure, while that of LCB increased with temperature. (C) 2008 Wiley Periodicals, Inc.