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 a crystals (Tm alpha). A flow phase diagram of iPP is constructed in strain rate-temperature space, composing of melt, non-crystalline shish, alpha and alpha & coexistence regions, based on which the kinetic and dynamic competitions among these four phases are discussed. Above Tm alpha imposing strong flow reverses thermodynamic stabilities of the disordered melt and the ordered phases, leading to the occurrence of FIC of beta and alpha crystals as a dynamic phase transition. Either increasing temperature or stain rate favors the competiveness of the metastable beta over the stable a 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.