The formation of γ-crystal in long-chain branched polypropylene under supercritical carbon dioxide
2008
Liao, Ruogu | Yu, Wei | Zhou, Chixing | Yu, Fengyuan | Tian, Jinghua
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 γ-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 γ-crystal were investigated. The results showed that the influence of LCB on the formation of γ-crystal was obvious when PP was crystallized in CO₂. The content of γ-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 γ-crystal easier. However, CO₂ was the key factor in the formation of γ-crystal, and the influence of CO₂ on γ-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 γ-crystal. It was found that the content of γ-crystal increased almost linearly with CO₂ pressure (CO₂ content), and the contribution of CO₂ to γ-crystal increased with pressure, while that of LCB increased with temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 441-451, 2008
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