Microstructure, Viscoelasticity, and Extensional Rheology of Ethylene–Propylene Copolymer Vitrimers

Abstract

Vitrimers of ethylene–propylene (EP) copolymers were synthesized by postreactor melt reactions with a dynamic covalent cross-linker (diacrylate bis(dioxoborolane), DBDA) and dicumyl peroxide (DCP). A novel two-step synthetic route to prepare DBDA is presented. The EP vitrimer melts show the expected chain relaxation delay due to the dynamic network formation. This delay manifests as a significant increase in melt elasticity, viscosity, and extensional strain hardening. However, it was found that the degree of relaxation delay depends not only on the amount of cross-linker added but also on the comonomer ratio (E/P) in the EP copolymer precursor. The E/P ratio also determines the cure kinetics due to the well-known side reactions (i.e., beta scission, branching, and cross-linking) induced by DCP on EP copolymers. AFM and SAXS studies reveal formation of spherical nanodomains during curing. The nonoccurrence of macrophase separation in the cured vitrimers led us to postulate that the nanodomains are made of clustered poly(DBDA) chains grafted onto the EP backbones. The complex topology, resulting from DCP-induced branching, combined with the formation of nanoclusters, possibly acting as nanofillers, has a strong effect on the structure–rheology correlations.