Influence of pre-shearing on the crystallization of an impact-resistant polypropylene copolymer

Impact-resistant polypropylene copolymer (IPC) samples with various pre-shear histories were prepared by a Brabender Rheometer. The influence of pre-shearing on crystallization of IPC, including isothermal and nonisothermal crystallization behaviors was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The results of nonisothermal crystallization showed that compared with as-received IPC, the temperatures referred to the peak of crystallization exotherm, Tp, were prominently elevated for pre-sheared IPC. In isothermal crystallization experiment, combining Avrami Method and Hoffman–Lauritzen Model, it was found that the half-time of crystallization (t1/2) of pre-sheared IPC was greatly shortened and the calculated fold surface free energy (σe) within the isothermal temperatures investigated showed a large reduction. Morphological development during isothermal crystallization observed by POM clearly confirmed that the enhancement in crystallization is mainly due to the fast formation of nuclei during crystallization. Besides, using successive self-nucleation and annealing (SSA) thermal fractionation technique, changes in chain structures induced by pre-shearing were obtained. The relaxation behavior of pre-sheared IPC was also evaluated and it was found that the shear-induced enhancement in crystallization could be relaxed to various extents under annealing conditions.