Network Toughening Strategies: Interplay of Mechanophore Reactivity, Polymer Chemistry and Network Topology

Abstract

When polymer materials fracture, chemical bonds break, revealing a unique molecular perspective on network fracture mechanism. This view highlights critical molecular details of network fracture – such as strand extension and bond scission – that extend beyond conventional macroscopic engineering approaches. This dissertation aims to establish correlations between the microscopic force-coupled reactivity and the macroscopic mechanical properties of polymer networks. Advancements in polymer mechanochemistry have enabled the mechanical studies of delicate mechanophores in single chain polymers through pulsed ultrasound and single molecule force spectroscopy. By strategically incorporating these mechanochemically labile units as crosslinks in side-crosslinked networks, their mechanochemical reactivity has been shown to dictate crack propagation pathways and consequently enhance materials toughness. This dissertation primarily explores three distinct crosslinking strategies for toughening polymer networks: metal-ligand crosslinking, facile carbosilane analogs and remendable Diels-Alder adducts. Our findings demonstrate that mechanically weak linkers can effectively reinforce polymer materials across various polymeric systems. We characterized the mechanochemical reactivities of these crosslinking candidates and demonstrated their effectiveness in strengthening bulk materials. Furthermore, we examined the interplay between covalent reactivity and polymer intermolecular interactions, in the context of two effects: entanglements and viscous dissipation. These insights provide a framework for designing more resilient polymer materials with tailored mechanical properties.