Crystallization kinetics of linear and long-chain branched polypropylene

Isothermal crystallization kinetics of linear polypropylene (PP) and long-chain branched (LCB) PPs were investigated on the basis of the Avrami theory. The Avrami exponents of LCB PPs are smaller than that of linear PP; moreover, they decrease with an increasing LCB level. The crystallization half-time of LCB PP depends more strongly on the crystallization temperature than does that of linear PP. The Lauritzen-Hoffman theory was used to study the effect of LCB on the crystal growth rate of PP. The fold surface free energy of LCB PP is lower than that of linear PP; moreover, it decreases with an increasing LCB level. However, when the LCB level is over a certain value, the fold surface free energy increases again. Furthermore, the crystal structures of linear PP and LCB PPs were studied by wide-angle X-ray diffraction (WAXD); it was observed that linear PP can crystallize in the alpha and beta forms, while LCB PPs have only the alpha crystalline form. Moreover, the relative intensities of different alpha peaks were also influenced by the LCB level.