Extensional Flow-Induced Dynamic Phase Transitions in Isotactic Polypropylene

With a combination of fast extension rheometer and in situ synchrotron radiation ultra-fast small- and wide-angle X-ray scattering, flow-induced crystallization (FIC) of isotactic polypropylene (iPP) is studied at temperatures below and above the melting point of α crystals (Tmα). A flow phase diagram of iPP is constructed in strain rate–temperature space, composing of melt, non-crystalline shish, α and α&β coexistence regions, based on which the kinetic and dynamic competitions among these four phases are discussed. Above T_mα, imposing strong flow reverses thermodynamic stabilities of the disordered melt and the ordered phases, leading to the occurrence of FIC of β and α crystals as a dynamic phase transition. Either increasing temperature or stain rate favors the competiveness of the metastable β over the stable α crystals, which is attributed to kinetic rate rather than thermodynamic stability. The violent competitions among four phases near the boundary of crystal-melt may frustrate crystallization and result in the non-crystalline shish winning out.

A phase diagram based on different flow induced structures of isotactic polypropylene regulated by strain rate and temperature is constructed. β crystal is generated simultaneously with α crystal showing the enhancement of nucleation ability compared to α crystal. The generation of non-crystalline shish is attributed to frustration effect of violent competitions among all involved four phases.

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