Fabricating Polymer Network Nanocomposites for Recyclabilty with Full Property Recovery and the Sometimes Complex Roles of Polymer-Nanofiller Interfaces

Polymer network nanocomposites cannot be recycled for high-value applications because permanent covalent cross-links prevent melt-state reprocessing. We have developed novel dynamic covalent cross-linking methods that yield recyclabiity of polymer networks with full recovery of cross-link density. In the fabrication of network nanocomposites, filler surface functionality can provide challenges. With addition-type reversible covalent cross-linking, e.g., a controlled radical polymerization method, surface functionality (e.g., hydroxyl groups) has no significant effect on reprocessability. However, with step-growth-type dynamic covalent cross-linking, e.g, a hydroxyurethane cross-link, surface functionality has deleterious effects. With hydroxyl or amine functionality which can participate in hydroxyurethane chemistry, silica nanoparticles speed the reprocessing but modestly reduce cross-link density and property recovery. Silica nanoparticles with superhydrophobic surfaces (absence of functionality) impact reprocessing kinetics but not recovery of cross-link density. We will also discuss how filler surface functionality combined with dynamic covalent cross-linking may yield novel, well-dispersed nanocomposite networks with especially robust properties.