Extensional rheology and flow-induced crystal alignment in polypropylene ionomers

The nonlinear response to the uniaxial extension of a series of isotactic polypropylene (iPP) ionomers is studied by melt rheology and ex situ small and wide-angle x-ray scattering measurements. These ionomers bear iPP backbones decorated with pendant aluminum carboxylate groups. Minuscule amounts of ion groups (<0.1 mol. %) are sufficient to produce remarkably high extensional strain hardening ratios of up to 200 and maximum stretch ratios (before breakage) of up to 50. Small and wide-angle scattering measurements from an iPP ionomer sample quenched during an extensional flow reveal monotonic correlations between Hencky strain, crystallinity, and crystal alignment. These results indicate a direct correlation between extensional stress and the chain alignment in the ionomer melt. Intriguingly, the ion clusters in the ionomer show no alignment induced by extensional deformation, suggesting undetermined rearrangements involving cluster dissociation and reassociation that occur during the flow. Slow stress relaxation, after flow cessation, was measured in the ionomers, in sharp contrast to much faster relaxation typically observed in the iPP homopolymer. Stress relaxation is not concomitant with a decrease in crystal alignment, which indicates that chain recoiling is not the stress relaxation mechanism in iPP ionomers.