High melt strength polypropylene by ionic modification: Preparation, rheological properties and foaming behaviors

- A facile process to introduce ionic interactions to polypropylene.
- Physical cross-links from the ionic domains improves PP melt strength.
- Model extensional rheology by modified Winder-Mours constitutive equation.
- Improves PP foamability by optimizing polymer rheology and polymer-CO2 interaction.

In this paper we report a facile process to introduce ionic interactions to polypropylene (PP) based on Zn-neutralization and amine modification reactions of maleic anhydride grafted PP (PP-g-MAH) for the preparation of high melt strength PP (HMSPP). A series of PP ionomers with different structures and ionic properties were synthesized via the choice of the pendant groups, and their effects on the rheological properties were investigated. Increased melt viscosity and elasticity resulting from the ionic interaction and aggregation of the ionic domains were confirmed by both shear and extensional rheological measurements. The rheological behaviors of the PP ionomers share features similar to those of critical gels. Moreover, foaming behaviors of the PP ionomers were investigated by using CO2 as the foaming agent and compared to that of the original PP-g-MAH. All ionomers show significantly improved foamability. Of the group of PP ionomers, the amine-modified PP ionomers (Zn-AM-ionomer) shows the best foamability due to the combined effects of optimal rheological properties and the underlying favorable interactions with CO2. These findings provide new insights into the design of high melt strength polypropylenes for foaming application assisted with CO2.

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