Toughening of Soft Materials by Low-hysteretic Dissipations

Toughening of soft materials has been long attributed to hysteretic dissipations such as Mullins effect and viscoelasticity, which still suffer from fatigue fracture under multiple cycles of mechanical loads due to the shakedown of hysteretic dissipations. Burgeoning interests are being focused on toughening of soft materials by low-hysteretic dissipations, enabling highly elastic, resilient, and fatigue-resistant properties under cyclic mechanical loads. This talk will discuss one low-hysteretic dissipation mechanism, strain-induced crystallization (SIC), a molecular phenomenon prevalently strengthening and toughening in elastomers. The reported SIC in common elastomers made of randomly crosslinked polymer networks, such as natural rubber, is typically below 20%. We recently discovered a new class of ideal-network elastomers made of end-linked ideal polymer networks, which can achieve a SIC of up to 50%. The ideal-network elastomers exhibit a high work to rupture of 6.3-24.6 MJ m^-3 and fracture energy of 4.2-4.5 kJ m^-2, while maintaining low bulk hysteresis of 0.05. We further investigate the impact of dangling-chain defects on SIC in such ideal-network elastomers, which show two competing effects: (i) the effective increase of polymer chain length and stretchability due to the presence of defects, and (ii) the nonuniform deformation and local stress concentration due to the random distribution of defects that leads to reduced stretchability and SIC. This work will not only elucidate the low-hysteresis toughening mechanism adopted in load-bearing biological tissues but also facilitate healthcare and sustainability challenges associated with robustness and longevity of soft materials.