The conformational preference of dynamic crosslinker modulates the ‘closed-loop’ circular economy in polypropylene vitrimer

Abstract

A key challenge in achieving a circular economy within the plastics industry lies in the upcycling and revalorization of post-consumer recycled (PCR) thermoplastic polyolefins (TPOs). PCR polyolefins typically exhibit inferior mechanical properties compared to virgin materials, limiting their use to low-value applications. Covalent adaptable networks (CANs), a class of dynamic crosslinked structures, present a promising solution by combining the strengths of thermosets and thermoplastics. However, the conformational behavior of crosslinkers is often overlooked, with the performance of the vitrimer attributed solely to crosslinking during synthesis or processing. Our research highlights that the spatial arrangement and flexibility of crosslinkers significantly influence the network formation (gelling) process, as demonstrated through classical molecular dynamics (MD) and density functional theory (DFT) simulations. DFT simulations provided the most stable structure at room temperature by optimizing molecular coordinates, while MD simulations revealed detailed insights into the structural dynamics and oscillatory behavior of CANs during high-temperature extrusion. These findings suggest that the conformational preferences of crosslinkers should be considered a critical design parameter for developing polypropylene (PP) vitrimers. In this study, maleic anhydride (MA) was used as a reactive agent to enhance crosslinking, with styrene included as a grafting enhancer to lower the surface energy of MA-grafted PCR PP. MA was also evaluated independently as a grafting agent. By integrating dynamic crosslinkers with equivalent chain lengths, the resulting vitrimer enabled the direct upcycling of PCR PP into high-performance materials, advancing the potential for sustainable plastic reuse.