Tougher when stressed: Hydrogels are typically weak materials because they contain only a small fraction of polymeric material highly swollen with water. Strength can be increased by adding extra cross-linking or interpenetrating networks with some sacrificial bonds to enhance toughness; however, these properties deteriorate upon stretching and may be slow to recover after relaxation. Liu et al. developed polyethylene glycol hydrogels cross-linked with moderate fractions of polymers that form sliding rings. These allow the chain to orient in parallel when stretched, leading to rapid and reversible strain-induced crystallization and thus much tougher hydrogels. Science , aaz6694, this issue p. 1078

We prepared a mechanically tough, highly transparent elastomer by using a polyrotaxane cross-linker acting as a molecular pulley.

The demand for mechanically robust polymer-based electrolytes is increasing for applications to wearable devices. Young’s modulus and breaking energy are essential parameters for describing the mechanical reliability of electrolytes. The former plays a vital role in suppressing the short circuit during charge-discharge, while the latter indicates crack propagation resistance. However, polymer electrolytes with high Young’s moduli are generally brittle. In this study, a tough slide-ring solid polymer electrolyte (SR-SPE) breaking through this trade-off between stiffness and toughness is designed on the basis of strain-induced crystallization (SIC) and phase separation. SIC makes the material highly tough (breaking energy, 80 to 100 megajoules per cubic meter). Phase separation in the polymer enhanced stiffness (Young’s modulus, 10 to 70 megapascals). The combined effect of phase separation and SIC made SR-SPE tough and stiff, while these mechanisms do not impair ionic conductivity. This SIC strategy could be combined with other toughening mechanisms to design tough polymer gel materials.

Slide-ring (SR) gels are polymer networks with movable cross-links that are prepared by cross-linking polyrotaxane (PR) in which many cyclic molecules are threaded into a linear polymer chain. The elastic modulus E of SR gels shows a unique dependence on cross-linking density: at a high cross-linking density, E decreases with increasing cross-linking density. This tendency is not in agreement with conventional rubber elasticity theory. In order to explain this abnormal dependence, we propose a novel molecular theory for SR gels, which considers the alignment entropy of cyclic molecules on polymer networks. The alignment yields new entropic elasticity of the slidable network in SR gels.