Tough and Rapidly Recoverable Polymer Gels: Self-Reinforcement by Strain-Induced Crystallization

Hydrogels, cross-linked polymer networks containing water, are expected to be applied for biomaterials such as artificial cartilage, ligaments, and prosthetic joints due to their high water content and biocompatibility. For the applications as the biomaterials, the mechanical toughness and instant recoverability are needed because artificial ligaments and prosthetic joints should survive under repeated high stress at a high frequency (ex. 1 Hz). Most tough hydrogels are reinforced by introducing sacrificial structures that can dissipate input energy [1]. However, since the sacrificial damages cannot recover instantly, the toughness of these gels drops substantially during consecutive cyclic loadings. In this presentation, we propose a damageless reinforcement strategy to realize tough and instantly recoverable hydrogels utilizing strain-induced crystallization (SIC) [2].

We used slide-ring (SR) gel containing movable figure-of-eight cross-links that work as pulleys to eliminate stress heterogeneities during deformation. SR hydrogel was prepared from polyrotaxane in which only 2 % of its polyethylene glycol (PEG) axis was threaded with cyclodextrin rings. The SR gel can survive at high stress (>5 MPa) and high strain (> 1000%). Also, the mechanical hysteresis under repeated cycle is quite small, and the instant recovery of extension energy between two consecutive cycles is 95 %. From in-situ wide-angle X-ray scattering (WAXS) experiments on the SR gel under repeated tensile deformation, we found that crystalline of PEG in the gels forms at a large strain and destructs quickly when applied stress is reduced. The reversible strain-induced crystallization yields the high toughness and instant recoverability of the SR gel.

[1] J. P. Gong, Soft Matter, 6, 2583 (2010).

[2] C. Liu, N. Morimoto, L. Jiang, S. Kawahara, T. Noritomi, H. Yokoyama, K. Mayumi, K. Ito, Science 372, 1078 (2021).