Healable Ionoelastomer Designed from Polymeric Ionic Liquid and Vitrimer Chemistry

Abstract

The growing demand for all-solid flexible, stretchable, and wearable devices has boosted the need for liquid-free and stretchable ionoelastomers. These ionic conducting materials are subjected to repeated deformations during functioning, making them susceptible to damage. Thus, imparting cross-linked materials with healing ability seems particularly promising to improve their durability. Here, a polymeric ionic liquid (PIL) bearing allyl functional groups was synthesized based on the quaternization of N-allylimidazole with a copolymer rubber of poly(epichlorohydrin) and poly(ethylene oxide) (PEO). The resulting PIL was then cross-linked with dynamic boronic ester cross-linkers 2,2'-(1,4-Phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane] (BDB) through thiol-ene "click" photoaddition. PEO dangling chains were additionally introduced for acting as free volume enhancers. The properties of the resulting all-solid PIL networks were investigated by tuning dynamic cross-linkers and dangling chain contents. Adjusting the cross-linker and dangling chain quantities yielded soft (0.2 MPa), stretchable (300%), and highly conducting ionoelastomers (1.6 × 10^-5 S·cm^-1 at 30 °C). The associative exchange reaction between BDB endowed these materials with vitrimer properties such as healing and recyclability. The recycled materials were able to retain their original mechanical properties and ionic conductivity. These healable PIL networks display a great potential for applications requiring solid electrolytes with high ionic conductivity, healing ability, and reprocessability.

Scheme 1. (a) Illustration of the Dithiol Containing Boronic Ester Cross-Linker BDB and the Synthetic Route to Obtain (i) Dynamic PIL-BDB Networks and (ii) PIL-BDB-PEGSH Dynamic Networks with Dangling Chains; (b) Synthetic Route of PIL Allyl; and (c) Network Rearrangement of Ionoelastomers via Boronic Ester Exchange Reaction

Figure 1

(a) Healing tests at 120 °C during 2 h of PIL-BDB samples and their control samples PIL-DT by stacking two pieces together; (b) DMA curves of PIL-BDB series samples; (c) healing tests of PIL-BDB25-PEGSH25 and PIL-BDB25-PEGSH50 samples; and (d) DMA curves of PIL-BDB-PEGSH series samples.

Figure 2

Evolution of T_g (a,d), Young’s modulus (b,e), and elongation at break (c,f) as a function of BDB for the PIL-BDB and PEGSH content for PIL-BDB25-PEGSH. Insert: photographs of stretched ionoelastomer vitrimers (PIL-BDB25 and PIL-BDB25-PEGSH25). Stress relaxation behaviors of PIL-based ionoelastomer vitrimers.

Figure 3

(a) Comparison of the stress relaxation curves of PIL-BDB25 with PIL-DT25 samples at 30 and 60 °C; stress relaxation curves of (b) PIL-BDB25, (c) PIL-BDB25-PEGSH25, and (d) PIL-BDB25-PEGSH50 at different temperatures; and Arrhenius plots of relaxation times vs temperature of (e) PIL-BDB and (f) PIL-BDB25-PEGSH vitrimer materials.

Figure 4

(a) Ionic conductivity behaviors of PIL-BDB series samples at different temperatures compared with pristine PIL allyl linear polymer and (b) ionic conductivity behaviors of PIL-BDB25-PEGSH series samples with dangling chains at different temperatures compared with PIL allyl polymer and PIL-BDB25 sample. Solid lines: calculation according to the VTF equation.

Figure 5

(a) Recycling tests of PIL-BDB-PEGSH series samples; (b) comparison of the ionic conductivity behavior of pristine and recycled PIL-BDB-PEGSH samples at different temperatures; (c) DMA curves of the PIL-BDB25-PEGSH25 sample after recycling compared with its pristine sample; (d) DMA curves of the PIL-BDB25-PEGSH50 sample after recycling compared with its pristine sample; (e) ionic conductivities at room temperature of PIL-BDB-PEGSH samples after each recycling cycle; and (f) Young’s modulus of PIL-BDB-PEGSH samples measured after each recycling cycle.