ExxonMobil Technology & Engineering
The ionic groups attached to the chain backbone ionomers aggregate into nanometer-size clusters, which act as physical crosslinks. The resulting physical network produces substantial increase in both melt strength and extensional strain hardening (SH), which are two desirable rheological properties for industrial processing operations (e.g., thermoforming, blow film molding and foaming). In this presentation I will discuss a novel one-pot synthetic route to produce high melt strength isotactic polypropylene (iPP) ionomers bearing aluminum carboxylate groups [Macromolecules, 55, 284 (2022)]. Next, I will discuss the linear and non-linear viscoelastic response of the iPP ionomers as well as the origin of the remarkable improvements in melt elasticity, viscosity, shear-thinning and extensional strain. These studies were based on ex-situ X-ray scattering measurements on melt-stretched samples which allowed us to elucidate the mechanisms of chain stretching and concomitant SH behavior [J. Rheol. 66, 657 (2022)]. Lastly, I will present an extensional rheo-Raman study where we found that the crystallization is significantly accelerated after a pulse of extensional flow is applied on the PP ionomers at a temperature above the quiescent crystallization temperature, producing highly oriented crystal structures. Such extensional flow-induced crystallization effect also operates in the iPP homopolymer, albeit at a much lower level.