Ion Clusters Boost Extensional Flow-Induced Crystallization in Ionomers

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