Ion Clusters\nBoost Extensional Flow-Induced Crystallization\nin Ionomers

The effects of strain and temperature on the extensional\nflow-induced\ncrystallization of an isotactic polypropylene (iPP) and an iPP ionomer\n(iPP-ion) were investigated by using a novel hybrid instrument that\ncombines extensional rheometry with in situ Raman spectroscopy. The\nisothermal crystallization (after a pulse of extensional flow) of\nthe iPP-ion is much faster than that of the iPP. This is attributed\nto the delay in chain relaxation imposed by the ion clusters in the\nionomers. Ex situ (small- and wide-angle) X-ray scattering measurements\nand polarized optical microscopy reveal that the enhancement in crystallization\nkinetics is concomitant with the development of highly aligned shish-kebab\ncrystalline morphologies in the iPP-ion, which are not observed in\nthe iPP. Increasing the temperature has the expected effect of decreasing\nthe crystallization rate. Increasing the mechanical work (or strain)\nhas three main effects: (1) mildly reducing the crystallization activation\nenergy, (2) boosting the crystallization rate, and (3) enhancing the\ncrystal orientation.