Designing soft and tough multiple-network elastomers: impact of reversible radical deactivation on filler network architecture and fracture toughness

Polymer networks are crucial for engineering and biomedical applications; however, their excessive brittleness in the absence of viscoelastic dissipation limits their use in applications requiring high temperatures and water concentrations. Multiple-networks, consisting of a stiff and pre-stretched "filler" network phase embedded within a soft and extensible "matrix" network, offer a promising route to overcome this limitation; yet, the relationship between "filler" network architecture and fracture toughness remains unknown. Here, we synthesized three poly(ethyl acrylate) "filler" networks via free radical polymerization (FRP), RAFT, and ATRP, and assessed the interplay between "filler" network architecture, irreversible chain breakage, and fracture toughness by labelling their crosslinks with fluorogenic mechanophores. These networks had similar elastic chain densities but distinct chain length distributions and mesoscopic structures. They were used to prepare multiple-networks, whose structure and mechanical properties were evaluated using mechanical tests, confocal microscopy, reactive Monte Carlo, and coarse-grained molecular dynamics simulations. Our results reveal that "filler" networks synthesized by RAFT and ATRP yield more brittle multiple-networks than those made by FRP, primarily due to the reduced average extensibility of their "filler" network chains. Their narrower chain length distributions in the load-bearing phase promote strain hardening but compromise energy dissipation through molecular friction and chain breakage, as well as fracture toughness. Overall, these findings underscore the need for advanced gelation methods that provide control over elastic chain distributions within networks, as, without such improvements, the use of RAFT and ATRP may result in long curing times, opacity, and a greater tendency to fracture.