Thermosets are cross-linked networks that cannot be recycled by melt processing because the percolated network prevents the polymer from undergoing flow in the melt state. Significant, recent research has focused on making networks melt-reprocessable or malleable via cross-links that are reversible as a function of temperature or that can undergo exchange reactions (e.g., trans-esterification). While progress has been made, relatively little success has been reported after reprocessing in achieving full property recovery associated with cross-link density. Here, we describe results of two studies in which we achieved full property recovery associated with cross-link density. The first exploits nitroxide-mediated polymerization to achieve cross-linked polybutadiene and polyisoprene (with and without substantial carbon black filler) as models for rubber tires. The rubbery state modulus reflects cross-link density, and the modulus was reproduced within error for multiple recycling steps. (See Jin et al., Adv. Mater. 2016, 28, 6746.) The second demonstrates that, under appropriate catalyzed reprocessing conditions promoting reversible and exchange reactions, polyhydroxyurethane (PHU) networks can achieve full property recovery associated with cross-link density after multiple reprocessing steps. Such PHUs are potential substitutes for polyurethane networks that cannot achieve effective property recovery. We also describe work aimed at suppressing creep in networks that are reprocessable due solely to exchange reactions, a class of materials coined as "vitrimers". Because of the presence of exchange at temperatures exceeding the vitrification temperature, vitrimers can undergo creep when subjected to load. We have developed a novel approach to reduce creep in vitrimers and have quantified conditions under which our approach can be used.

Polymer networks with reversible and/or exchangable cross-links: Complete property recovery associated with cross-link density leading to fully reprocessable, recyclable networks and an approach to suppress creep in vitrimers

255th National Meeting (2018)

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