Tuning the Ultimate Strain of Single and Double Network Gels Through Reactive Strand Extension

The stretchability (ability to be elongated) and toughness (capacity to absorb energy before breaking) of polymer network materials, such as elastomers and hydrogels, often determine their utility and lifetime. Direct correlations between the molecular behavior of polymer network components and the physical properties of the network inform the design of materials with enhanced performance, extended lifetime, and minimized waste stream. Here, we report the impact of the fused ring size in bicyclic cyclobutane mechanophores within the strands of polymer network gels. The mechanophores and their polymer strands share the same initial covalent contour length, whereas the capacity for reactive strand extension (RSE) is varied by changing the size of the ring fused to the cyclobutane from 5 to 12 carbon atoms. We observe the first evidence of covalent RSE effects in a single-network gel, and strands with greater RSE lead to gels with greater stretchability and toughness. The same qualitative correlation between molecular and macroscopic extension is also observed in DN hydrogels with mechanophores in the prestretched first network.