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

Abstract

The crystallization behavior of long-chain branched (LCB) polypropylene (PP) in the supercritical carbon dioxide (scCO2) atmosphere was investigated to show the influences of LCB and CO2 on the formation of y-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 °C. The effects of LCB level, CO2 pressure, and crystallization temperature on the content of y-crystal were investigated. The results showed that the influence of LCB on the formation of y-crystal was obvious when PP was crystallized in CO2. The content of y-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 y-crystal easier. However, CO2 was the key factor in the formation of y-crystal, and the influence of CO2 on y-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 y-crystal. It was found that the content of y-crystal increased almost linearly with CO2 pressure (CO2 content), and the contribution of CO2 to y-crystal increased with pressure, while that of LCB increased with temperature.