Thermosets are covalently cross-linked networks that cannot be recycled by melt processing because the percolated network prevents melt-state flow. Although much research has focused on making networks melt-reprocessable or malleable via cross-links that are reversible or that undergo exchange reactions, very little success has been reported in achieving full property recovery after reprocessing associated with cross-link density. Here, we describe results of two studies in which we achieve full recovery of cross-link density as determined by rubbery plateau modulus. The first exploits nitroxide-mediated polymerization (and its reversible radical capping/uncapping step) in a novel way to achieve dynamically cross-linked polybutadiene and polyisoprene both without any filler and with substantial levels of carbon black and other fillers as models for rubber tires. With reprocessing conditions comparable to those used in commercial tire molding operations, results show full recovery of cross-link density after multiple reprocessing steps for both polymer networks and appropriately designed network composites. The second demonstrates that, under appropriate catalyzed reprocessing conditions promoting reversible and exchange reactions, polyhydroxyurethane (PHU) networks achieve full recovery of cross-link density after multiple reprocessing steps. Such PHUs are potential substitutes for isocyanate-based polyurethane networks. We will also describe a study aimed at addressing an "Achille's heel" of vitrimers, that being their propensity to creep, which results from the presence of exchange reactions above the vitrification temperature. We have a developed a novel approach that substantially reduces creep in vitirimers and using a modification of Flory-Stockmayer theory have quantified conditions under which this approach is expected to work.

Dynamic polymer networks and network composites: Reprocessing leading to full property recovery associated with cross-link density for models of rubber tires and non-isocyanate polyurethane networks and a quantitative approach to suppress creep in vitrimers

256th National Meeting (2018)

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