Reactive strand extension (RSE) has been developed recently as a technique for improving the toughness of polymer networks. In RSE, fused-ring monomers that represent loops along polymer strands mechanochemically open under applied stress, leading to chain extension rather than cleavage, and thereby increasing the network tearing energy. To date, however, RSE has only been demonstrated with cyclic monomers that store a small amount of length each, necessitating their use in high quantities to achieve desirable toughness enhancements. Here we describe a general approach for forming polymer networks with high toughness by taking advantage of inherent loop formation around cyclobutane-based crosslinkers. In such systems, network extensibility is increased when cyclobutane crosslinks mechanically open, converting loops into elastically effective strands. We show that toughness increases with loop defect density in end-linked PEO organogels, investigate the effect of crosslinker substitution on extensibility and toughness, and then extend this approach to industrially relevant vulcanized polybutadiene rubber materials. Importantly, this approach relies only on network topology and crosslinking chemistry and requires only a very small concentration of mechanophore. It can thus be used with a plethora of polymer chemistries for various applications, including toughening of otherwise brittle biodegradable polymer materials.

Turning loops into lemonade: Topological toughening of polymer networks using loops for strand extension

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ACS Fall 2022

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