Towards a better understanding of the crystallization and melting behaviors of high-density polyethylene samples prepared from quasi-isothermal and stretching oriented localized melts

Abstract The structural evolution and temperature dependence of crystal thickness in high-density polyethylene (HDPE) generated from quasi-isothermal and stress-induced localized melts were studied by employing small-angle X-ray scattering and differential scanning calorimetry techniques. By analyzing the relationship between the reciprocal lamellar thickness and crystallization temperature, two well-defined crystallization lines with similar slopes were obtained, implying an organization of molten chain segments into crystallites passing through a mesomorphic phase. However, two different equilibrium crystallization temperatures were yielded. Furthermore, two recrystallization lines were ascertained for both stretched and isothermal crystallized HDPE samples. These phenomena are mainly caused by the differences between the thermodynamic parameters of polymer chains and crystal stabilities. Specifically, HDPE upon stretching processed much thinner crystalline lamellae, while the sample during isothermal crystallization exhibited better stability due to the high internal mobility of the polymeric chain segments and global flexible surroundings. Moreover, neither melting line coincided with their crystallization lines, which confirmed that these two processes are irreversible.