Ion gels, containing a polymer network and an ionic liquid as their solvent, are deemed one of the most promising soft materials for stretchable electronics, which may supersede incumbent rigid wearable devices in the future. Developing tough ion gels is indispensable for applications, including, but not limited to, conformal wearables or soft actuators. One of the mainstream strategies for materializing tough gels is introducing a sacrificial polymer network. Therein, noteworthy work includes that of double-network (DN) hydrogels [1], and successful fabrication of tough DN or self-healing ion gels followed. However, the very nature of the “sacrificial” network in those gels deteriorates their instantaneous mechanical reversibility. Aiming to overcome this shortcoming, we turned to strain-induced crystallization (SIC, Fig. 1) in gels. Although SIC itself is a long-renowned reinforcement mechanism within vulcanized natural rubber or certain types of polymers, it proved equally valid in polymer gels, which was found in slide-ring (SR) hydrogels of a high polymer concentration and a high molecular weight between cross-links [2]. We implemented this SIC strategy for fabricating tough ion gels. In this presentation, we will overview scattering patterns of in-situ wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) obtained from the ion gels being stretched, and discuss what is inferred about the structural deformation of the polymer network therein. We will also discuss the paths to designing an ion gel of better mechanical performance, from the results of the complementary scattering experiments.

We fabricated SR ion gels, in which polyrotaxane (PR) supramolecules possess dynamic crosslinking points via their ring-shaped molecules. The PR was comprised of the axial component, polyethylene glycol (PEG), and the annular ones, (2-hydroxypropyl)-α-cyclodextrin (hpCD). We used an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C2mim][NTf2]), as their solvent. Since [C2mim][NTf2] has an X-ray scattering length density close to that of hpCD, the SAXS patterns reflected the contrast derived from PEG, suppressing the one from hpCD. On the other hand, no contrast matching was realized in the gels for in-situ SANS between the three constituents: deuterated [C2mim][NTf2]-$d_8$, PEG, and hpCD. SANS measurements were performed at SANS-U, JRR-3.

At a large strain ($\lambda =$10, $\lambda$ represents the extension ratio), as shown in Fig. 2, scattering spots corresponding to the planar zigzag crystalline structure of PEG were confirmed in the WAXS patterns, and a vertical streak perpendicular to the stretching direction was identified in SAXS, which we concluded as an indication of highly oriented PEG. The SANS patterns suggests that hpCDs form aggregations and that the aggregates are deformed by stretching [3].

References

• [1] J.P. Gong, Y. Katsuyama, T. Kurokawa, Y. Osada., Adv. Mater., 15, 1155 (2003).
• [2] C. Liu, N. Morimoto, L. Jiang, S. Kawahara, T. Noritomi, H. Yokoyama, K. Mayumi, and K. Ito., Science, 372, 1078 (2021).
• [3] T. Enoki, K. Hashimoto, T. Oda, K. Ito, and K. Mayumi, Macromolecules, 57, 11498 (2024).

Authors

• K. Mayumi, T. Enoki, and K. Hashimoto^a
• ^aGifu University