Lionomers versus Zn-Ionomer: A new route for the preparation of microcellular foams

Abstract

In this work, LIonomers based on maleic anhydride-grafted polypropylene (PPgMA) are successfully designed. These LIonomers are developed from the addition of two imidazolium-based ionic liquids (ImILs) to PPgMA: one unreactive ImIL, i.e. 1-ethyl-3-methylimidazolium diethyl phosphate (E+DEP), and one reactive ImIL, i.e. 1-aminoethyl-3-methylimidazolium bromide (AE+Br). Thus, the effect of the ImIL nature on LIonomers properties was highlighted and those ones compared to conventional Zn-Ionomer. Firstly, NMR and FTIR spectroscopies have revealed the existence of ionic-polar/ionic interactions and/or the creation of chemical bonds depending on the chemical nature of ImILs. These various interactions in polymer blends lead to the different multiscale structurations, i.e. a dual assembling of ionic species (including multiplets and large clusters) and homogeneous morphologies on the blends with E+DEP and AE+Br compared to the phase separation of ions occurring for conventional Zn-Ionomer. The rheological properties have highlighted the presence of such interactions between maleic anhydride groups and cations or anions for the imidazolium IL-based LIonomers, similar to the interactions of Zn2+ with anions pairs within Zn-Ionomers. Thanks to the existence of maleic anhydride/ImIL interactions and the generation of β-crystalline form as well as the various ionic-rich phases, an outstanding compromise between stiffness and stretchability was achieved from LIonomers. In addition, considering supercritical carbon dioxide (scCO2) as physical foaming agent, the foamability of PPgMA has been considerably improved thanks to the ionic networks formed by ionic liquids compared to Zn2+ acetate addition. Finally, the foam morphology and the compressive behavior were investigated as a function of the cation/anion combination of ILs or Zn2+Ac addition.