Suppressing of γ-crystal formation in metallocene-based isotactic polypropylene during isothermal crystallization under shear flow.

The effect of shear flow on isothermal crystallization behavior of γ-crystals in metallocene-based isotactic polypropylene melt was investigated by in situ synchrotron wide-angle X-ray diffraction (WAXD). In the sample under weak shear (at strain of 300% for 30 s duration), simultaneous evolution of α- and γ-crystals occurred, and the final fraction of γ-crystals (fγ) was 0.66, which was identical to the undeformed sample (PP-Static). In this scenario, α-crystals probably served as effective seeds for nucleation of γ-crystals. In the samples under strong shear (at strain of 500% for 30 s duration or long-time continuous shear at strains of 100% and 500%), the sequential emergence of α- and γ-crystals was observed. In this case, molten polymer chains were probably constrained by the surrounding crystals after intense short-time shear and/or maintained their extended chain conformation after long-time shear. These oriented chains had little chance to form the γ-crystals directly, behaving very differently from the relaxed chains. Under strong shear fields, the emergence of γ-crystals was delayed or inhibited, whereas the fγ value was also decreased rapidly. A simple model for the possible pathway of γ-crystal formation in the strong shear environment was proposed.