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[Aya Saruwatari](https://orcid.org/0000-0001-7388-4626), [Yuji Kamiyama](https://orcid.org/0000-0001-9483-2112), [Akifumi Kawamura](https://orcid.org/0000-0003-4876-0685), [Takashi Miyata](https://orcid.org/0000-0002-6747-4118), [Ryota Tamate](https://orcid.org/0000-0002-1704-1058), [Takeshi Ueki](https://orcid.org/0000-0001-9317-6280)

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[Straightforward preparation of a tough and stretchable ion gel](https://mdr.nims.go.jp/datasets/6dc5bea2-bea0-49de-9ff3-fa68505f053d)

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Straightforward preparation of a tough and stretchable ion gelThis journal is © The Royal Society of Chemistry 2024 Soft MatterCite this: DOI: 10.1039/d4sm00628cStraightforward preparation of a tough andstretchable ion gel†Aya Saruwatari, ab Yuji Kamiyama, ab Akifumi Kawamura, cdTakashi Miyata, cd Ryota Tamate a and Takeshi Ueki *abIon gels, polymer networks swollen by ionic liquids, are expected tobe applied to wearable devices that are tolerant to repeatedstretching. High strength and excellent stretchability was achieveddue to the numerous physical cross-links with abundant polymerchain entanglements in addition to a small number of immobilechemical cross-links, even though the ion gel was prepared by afacile methodology.Ion gels are a class of gels swollen by ionic liquids (ILs). Because ILsare exclusively composed of ions, they have unique characteristicssuch as high ion conductivity, (electro)chemical stability, ther-mal stability, and low volatility. Owing to these features, ion gelshave attracted attention as electrolyte membranes for batteries,1–3transistors,4,5 and supercapacitors,6 gas absorption/separation mem-branes,7,8 catalyst-supported membranes.9,10 In addition to theseprevious studies, ion gels have recently been used as electrolytemembranes for artificial muscles and wearable devices attached tohuman skin,11–13 especially as strain sensors which measure bodymovements such as finger/wrist/elbow/knee bending, pronouncing,facial expressions, breathing, and pulse rate. Hydrogels are alsocandidates for strain sensors,14–19 but there may be serious problemssuch as the degradation of mechanical properties and the instabilityagainst long-term strains due to high volatility. To fulfil this purpose,many studies have focused on the design of stretchable ion gel. Forexample, Liu et al. applied a double network strategy, which hasbeen widely explored in the research field of hydrogels. The ion gelcomposed of poly(2-acrylamido-2-methyl-1-propanesulfonic acid)(PAMPS) and 1-ethyl-3-methylimidazolium dicyanamide exhibitedan ionic conductivity as high as 1.9 S m�1 with 66.4 wt% IL.20 Theyalso found that the ion gel sensor had excellent stretchability over awide temperature range between�70 and 100 1C. Sun et al. reporteda tough ion gel composed of a poly(urethane–urea) polymer networkand 1,2-dimethyl-3-ethoxyethyl-imidazolium trifluoromethyl-sulfo;nylimide.21 The ion gels showed no degradation in rela-tive change of electrical resistance at 5% strain, even aftersevere cyclic tensile conditions of 10 000 cycles. Beyond therepresentative studies, a number of ion gels for wearabledevices are available, including physically cross-linked ion gelsbased on intermolecular hydrogen bonding,22,23 self-assembledblock copolymers,24 poly(ionic liquid)s,25,26 and so on.27,28However, the preparation of mechanically superior ion gelsgenerally requires complex polymer synthesis techniques,accompanied by advanced knowledge of organic chemistry.Stretchable ion gels that are easily prepared and showingsuppressed residual strain for repeated stretching and stressrelaxation—characteristics that are potentially applicable towearable devices—remain rudimentary.Herein, we describe the preparation and characterization ofa stretchable ion gel with excellent mechanical properties. Theion gel can be produced in a simple one-step procedure. As themonomer and cross-linker, methyl methacrylate (MMA) andethylene glycol dimethacrylate (EGDMA), respectively, werecombined with an extremely small concentration of photoini-tiator ([I] = 0.02 mol% against MMA monomer in feed) andmixed with the IL, 1-ethyl-3-methylimidazolium trifluoro-methylsulfonylimide ([C2mim][TFSI]). This mixture was irra-diated with UV light.We previously reported a new class of ion gels cross-linked solelyby polymer chain entanglements of ultrahigh-molecular weightpolymers that did not undergo flow deformation and were stableover a wide range of temperatures and frequencies (UHMW iongels). UHMW ion gels were prepared by mixing the monomer(MMA), solvent ([C2mim][TFSI]), and an extremely low concentrationof thermal initiator (2,20-azobis(isobutyronitrile) (AIBN)), and thena Research Center for Macromolecules and Biomaterials,National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044,Japan. E-mail: UEKI.Takeshi@nims.go.jpb Graduate School of Life Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku,Sapporo, Hokkaido 060-0810, Japanc Department of Chemistry and Materials Engineering, Kansai University, 3-3-35,Yamate-cho, Suita, Osaka 564-8680, Japand Organization for Research and Development of Innovative Science and Technology,Kansai University, 3-3-35, Yamate-cho, Suita, Osaka 564-8680, Japan† Electronic supplementary information (ESI) available: Details for materials;preparation procedures of ion gels; measurement procedures; 1H NMR spectrum;observation results of gelation behaviour; and additional characterization resultsof the ion gels. See DOI: https://doi.org/10.1039/d4sm00628cReceived 23rd May 2024,Accepted 10th July 2024DOI: 10.1039/d4sm00628crsc.li/soft-matter-journalSoft MatterCOMMUNICATIONOpen Access Article. Published on 11 July 2024. Downloaded on 7/18/2024 12:28:37 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journalhttps://orcid.org/0000-0001-7388-4626https://orcid.org/0000-0001-9483-2112https://orcid.org/0000-0003-4876-0685https://orcid.org/0000-0002-6747-4118https://orcid.org/0000-0002-1704-1058https://orcid.org/0000-0001-9317-6280http://crossmark.crossref.org/dialog/?doi=10.1039/d4sm00628c&domain=pdf&date_stamp=2024-07-16https://doi.org/10.1039/d4sm00628chttps://rsc.li/soft-matter-journalhttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sm00628chttps://pubs.rsc.org/en/journals/journal/SMSoft Matter This journal is © The Royal Society of Chemistry 2024heating the mixture to initiate free radical polymerization.29,30Because the propagation reaction rate of free radical polymer-ization is significantly enhanced in ILs,31 almost 100% conver-sion of the monomer to UHMW was achieved. The syntheticstrategy presented in this report is based on this procedure.Related to this ion gel preparation, in the research field ofhydrogels, Miyata et al. and Suo et al. independently reportedthat a high monomer concentration and low cross-linker con-tent yielded stretchable hydrogels in which polymer chainentanglements greatly outnumbered chemical cross-links.32–34These pioneering works encouraged us to devise a syntheticstrategy for an ion gel with enough toughness and stretchabil-ity, which would be applied to wearable devices. In addition, weattempted to use photoinitiated polymerization to prepare theion gel instead of thermal initiation because rapid initiationand spatial control are required for processing thin films orflexible patterned shapes. More specifically, instead of the AIBNthermal initiator, an extremely small amount of the 2,2-diethoxyacetophenone (DEAP) photoinitiator, EGDMA as across-linker, MMA monomer, and [C2mim][TFSI] solvent weremixed and then irradiated with UV light (365 nm, 3 mW) tophotoinitiate the polymerization. A transparent ion gel wasobtained (Fig. 1(a)). The transmittance of 10 mm-thick ion gelexceeded 97.5% in the visible light region (400–800 nm)(Fig. 1(b)). The transparency was comparable to or higher thanthat of the previously reported ion gels even with a longeroptical path length (Table S2, ESI†). Furthermore, the ion gelwas more thermally stable compared to the hydrogel preparedwith the same concentration of initiator, cross-linker and molarconcentration of monomer (Fig. S2, ESI†). The hydrogel shrankto 70% of its initial size heated after 60 1C for 30 min andshrank to 80% kept after room temperature for 3 h due tosolvent (water) evaporation. Contrary to this, the ion gel did notchange in size and shape even after thermal aging. The ion gelis expected to maintain the performances longer than hydro-gels owing to high thermal stability.First, the dependence of the mechanical properties on thecross-linker concentration ([CL] = 0.001–1 mol% vs. [MMA]) wasevaluated. The concentrations of the monomer and initiatorwere fixed at 30 wt% and [I] = 0.02 mol% vs. [MMA], respec-tively, where topological polymer chain entanglements wereexpected to act as energy-dissipating components. A stress–strain curve was obtained from the tensile test, as shown inFig. 2(a). The toughness (Fig. 2(b)), calculated from the areaunder the stress–strain curve, exhibits a maximum with respectto [CL]. Toughness increases as the [CL] increases from 0.001 to0.01 mol%, but after passing through maximum at 1 MJ m�3 at0.01 mol%, it decreases with an increase in the [CL] from0.01 to 1 mol%. In other words, the toughness exhibits aFig. 1 (a) A photograph, (b) a transmittance spectrum, and (c) a schematicillustration of the ion gel used in this study. The white bold lines representPMMA with a small amount of cross-linker (red spheres). The orange andblue spheres represent the [C2mim]+, and [TFSI]�, respectively.Fig. 2 (a) Tensile stress–strain curves of various ion gels with varyingcross-linker concentrations and an initiator content of 0.02 mol%. Tough-ness (b) and Young’s modulus (c) of the ion gels in relation to cross-linkerand initiator contents of 0.02 (circle), 0.04 (triangle), 0.06 (square), and1 mol% (diamond). (c) The primary axis plots Young’s modulus (Eexp), asdetermined from the stress–strain curve (a), while the secondary axis is thetheoretical value of the Young’s modulus (Etheo), calculated based on thecross-linker concentration at I = 0.02 mol%.Communication Soft MatterOpen Access Article. Published on 11 July 2024. Downloaded on 7/18/2024 12:28:37 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sm00628cThis journal is © The Royal Society of Chemistry 2024 Soft Matternonlinear response to the concentration of the cross-linkerwhen the concentrations of the monomer and initiator remainunchanged. As discussed later, the maximum toughness wasdue to the synergistic effect of a large number of physical cross-links based on polymer chain entanglements and a smallnumber of covalent chemical cross-links. Fig. 2(b) also plotsthe toughness when the initiator concentration was increasedto 0.04 mol% and 0.06 mol%, but the toughness values aresignificantly lower than those obtained at 0.02 mol%. This isbecause the polymer chain entanglements that contribute totoughness significantly decrease as the initiator concentrationincreases. By contrast, the Young’s modulus (Eexp) increasedslightly as the cross-linker concentration increased (Fig. 2(c)).Thus, in contrast to the conventional trend of gels becomingharder yet more brittle as [CL] increases, the ion gel developedin this study hardens and toughens at [CL] = 0.001–0.01 mol%when the monomer concentration is fixed as 30 wt% and anextremely low concentration of photo-initiator (0.02 mol% vs.monomer) is used.We speculated that physical cross-links based on polymerchain entanglements inside the gel were the cause of suchnonlinear behaviour and roughly investigated the effect ofphysical cross-links by calculating the Young’s modulus.Assuming that the chemical cross-links are uniformly dispersedin the polymer network and that all of them contribute to thestress, the theoretical value of the Young’s modulus (Etheo) wascalculated from ntheo35,36 based on the affine network model(eqn (1) and (2)).ntheo = Cf/2 = nCL f/2 V (1)where C is concentration of cross-linker, f is the cross-linker’sfunctionality ( f = 4 for EGDMA), nCL is molar quantity of cross-linker in the feed solution and V is the volume of the feedsolution.Etheo = 3ntheokT (2)where k is Boltzmann constant and T is temperature. Theexperimental Young’s modulus (Eexp) was determined fromslope of stress–strain curve in the 0–20% strain range. For[CL] r 0.1 mol%, the experimental Young’s moduli (Eexp)clearly exceeded the theoretical Young’s moduli (Etheo)(Fig. 2(c)) because Etheo only considered chemical cross-links;it did not account for physical cross-links. The contribution ofphysical cross-links was then evaluated from the ratio of thewhole cross-linking density that contributed to Eexp(nexp) to thechemical cross-linking density (ntheo), where nexp was calculatedfrom Eexp (eqn (3)) and ntheo was calculated from the concen-tration of cross-linkers (eqn (1)).nexp = Eexp/3kT (3)A nexp/ntheo ratio of 41 indicates the existence of physical cross-links based on polymer chain entanglements as well aschemical cross-links, while a nexp/ntheo ratio of o1 implies thatnot all the incorporated chemical cross-linkers actually cross-link the polymer chains. The contribution of physical cross-links against the total Young’s modulus tended to linearlyincrease as the cross-linker concentration decreased, and for[CL] r 0.1 mol%, nexp/ntheo 4 1 indicates that physical cross-links exist in addition to chemical cross-links (Fig. 3). Inter-preting the toughness and Young’s modulus (Fig. 2(b) and (c))from these estimations, in the region of nexp/ntheo 4 10, wherethe contribution of physical cross-links is more than 9-fold thatof chemical cross-links, the ion gels became stiffer and tougheras the cross-linker concentration increased. This is due to thepresence of numerous mobile physical cross-links derived frompolymer chain entanglements that suppress stress concen-tration as well as a small number of covalent chemical cross-links. Interestingly, though nexp/ntheo decreased linearly with anincrease of cross-linker content for the hydrogel consisting ofacrylamide (AAm) as a matrix polymer cross-linked with N,N0-methylenebisacrylamide (BIS),32 in this study a shallower slopewas exhibited at [CL] Z 0.5 mol%. The reactivity of propagatingradicals is relatively higher for BIS than for AAm, resultingin the formation of highly cross-linked clusters inside hydro-gel,36–38 whereas the less likely formation of clusters for MMAand EGDMA led to relatively close theoretical values at highcross-linker concentration.If polymer chain entanglements are significantly correlatedwith the mechanical properties of the gel, particularly in thelower region of the chemical cross-linking density, the mole-cular weight of the polymer chain appears to directly contributeto the probability of polymer chain entanglement inside thepolymer network. Note that the molecular weight in this dis-cussion indicates the degree of monomer linkage of the linearpolymer, assuming that no chemical cross-links are introduced,rather than the degree of polymerization (molecular weight)between two cross-links along the polymer chain. In the free-radical polymerization of linear polymers, the molecular weightof the polymer produced is inversely proportional to the root ofthe initiator concentration in the feed if no cross-linker isadded to the polymerization system. For this reason, weexplored the dependence of the toughness of the ion gels onthe initiator concentration ([I] = 0.02–2 mol%) fixing [MMA]and [CL] at 30 wt% and 0.01 mol% vs. [MMA], respectively,exhibiting maximum toughness (Fig. 2(b)). We also preparedion gels with [CL] = 0 and 1 mol% vs. [MMA] as a reference. ForFig. 3 Relationship between the cross-linker content in the pre-gelsolution and the ratio of experimental to theoretical cross-linking densities(nexp/ntheo) of the ion gels.Soft Matter CommunicationOpen Access Article. Published on 11 July 2024. Downloaded on 7/18/2024 12:28:37 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sm00628cSoft Matter This journal is © The Royal Society of Chemistry 2024[CL] = 1 mol%, in the case of the typical cross-linker concentra-tions for the preparation of polymer gels, ion gels possessingsufficient mechanical strength for handling were obtained atall initiator concentrations ([I] = 0.02–2 mol%). However, whena cross-linker concentration is reduced to 1/100th of the typicalinitiator concentration ([CL] = 0.01 mol%), no gelation occurs at[I] Z 0.2 mol%, although the polymerization reaction appearedto proceed (Table S1, ESI†). Although self-standing gels wereobtained at low initiator concentrations of [I] = 0.04 and0.06 mol%, the toughness and Young’s modulus was less than[I] = 0.02 mol%, indicating poor polymer chain entanglement(Fig. 2(b) and (c) and Fig. S4, ESI†). To clarify the effect of themolecular weight of the polymers synthesized in the IL, themolecular weight of the polymers inside the gels was evaluatedby changing [I] from 0.02 to 0.04 and 0.06 mol% without addingthe cross-linker ([CL] = 0 mol%) (Table 1). At [I] = 0.02 mol%,the molecular weight of poly(methyl methacrylate) (PMMA)exceeded 1000 kDa, whereas at [I] = 0.04 and 0.06 mol%, themolecular weight was less than 1000 kDa, reducing the Young’smodulus and toughness. To obtain ion gels with high moduliand toughness, it is important to induce abundant polymerchain entanglements, which can be cross-links and dissipateenergy, by lowering the amount of initiator. We have previouslyreported that in the ion gels formed by UHMW polymers(Mn Z 1000 kDa) as a matrix, in the absence of cross-linkers,the abundant polymer chain entanglements act as physicalcross-links, resulting in stable elastic gels without flow defor-mation in a wide range of temperature and frequency regions.Although a self-standing ion gel was obtained in the absenceof UHMW polymers (Mn = 175 kDa), flow deformation wasobserved at high temperatures and in the low-frequencyrange.29 The effect of molecular weight was also observed inthe ion gels loaded with a small amount of chemical cross-linkers. Note that we have already observed that the monomerconversion is significantly lowered when the initiator concen-tration in the feed is less than 0.02 mol%.29 This suggests thatthe remaining low-Tg component of the unreacted monomerinside the resultant gel acts as a plasticizer, which reduces therobustness of the materials. Therefore, the amount of initiatorshould be reduced, but the conversion should be kept nearly100%, making the residual monomer concentration negligible.To determine monomer conversion, 1H-NMR was measured forthe ion gels without cross-linker instead of the cross-linked iongels that are not soluble in deuterated solvent, and the conver-sion was found to be almost 100% (Table 1).Unlike chemical gels, which are formed by covalent bondswith a permanent lifetime, physically cross-linked gels usuallyexhibit a reduction in stress when strain is applied, owing tothe short lifetime of the cross-links, that is, stress relaxation.For example, UHMW ion gel29,30 exhibits excellent mechanicalproperties over a wide range of frequencies and temperatures,but the stress relaxation originating from the loop-out of thepolymer chain can be a serious drawback in the realization ofmaterial applications such as strain sensors, which areintended to be subjected to strain for a long period of time.We expected that our optimized ion gel would exhibit less stressrelaxation because it not only possesses physical cross-linksbased on polymer chain entanglements but also a smallamount of immobile covalent chemical cross-links. Thus, weattempted to compare the stress relaxation behaviour of thetoughest ion gel ([CL] = 0.01 mol%, [I] = 0.02 mol%) withan ion gel prepared without cross-linker ([CL] = 0 mol%,[I] = 0.02 mol%) (Fig. 4). Although the initial stresses werecomparable as well as these samples exhibited similar stressesat the initial strain (Fig. S3, ESI†), the relaxation time of the iongel incorporating chemical cross-linkers was more than 10times longer than that of the ion gel without cross-linkers,indicating that the introduction of a small amount of cross-linker (one chemical cross-linker per 1000 kDa of MMA)improved the long-term durability under constant strain.In addition, to evaluate the capability of recovery fromrepetitive strain, an important mechanical property for wear-able devices such as strain sensors and flexible batteries, theion gels were subjected to 80 successive loading–unloadingcycles at a strain of 0–300% with 10 min intervals betweencycles (Fig. 5). In this study, we compared the toughest ion gel([CL] = 0.01 mol%, [I] = 0.02 mol%) with the ion gel preparedwithout cross-linker ([CL] = 0 mol%, [I] = 0.02 mol%). Hysteresiswas observed for both ion gels, which was attributed to energydissipation owing to the relaxation of polymer chain entangle-ments. At [CL] = 0 mol%, the slope of the stress–strain curve inthe low-strain region decreased, and the residual strain gradu-ally accumulated after the cycle. After 80 cycles, the residualstrain finally reached E20%. By contrast, the ion gel loadedwith [CL] = 0.01 mol% showed almost no slope change, and thefinal residual strain was less than only 10%. Given that theTable 1 Characterization of PMMA polymerized at [CL] = 0 mol%[I] [mol%] Mn [g mol�1]a Mw/Mna Conversionb [%]0.02 1.11 � 106 1.61 99.90.04 5.33 � 105 2.48 99.90.06 2.37 � 105 3.16 99.9a Estimated from GPC (eluent: DMF, standard: PMMA, detector: RID).b Calculated from 1H-NMR (Fig. S1, ESI).Fig. 4 Stress relaxation tests at g = 10% for ion gels with a cross-linkercontent of 0.01 mol% (blue plot) and 0 mol% (black plot). In both samples,the initiator content was 0.02 mol%.Communication Soft MatterOpen Access Article. Published on 11 July 2024. Downloaded on 7/18/2024 12:28:37 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sm00628cThis journal is © The Royal Society of Chemistry 2024 Soft Matterresidual strain indicates disentanglement of the polymerchains, the introduction of a small amount of cross-linkersuppressed the disentanglement of the polymer chains,resulting in a significant improvement in the cyclic strainresistance of the ion gels. When physically cross-linked gelsformed by non-covalent bonds such as ionic bonds arestretched, the initial bonds break and temporary bondsare formed with other partners. After unloading, the tem-porary bonds are repetitively formed and broken to returnto the initial bonds by the driving force of rubberelasticity.39,40 In present system, physical cross-linking isbased on polymer chain entanglement, however; similar‘‘exchangeable cross-linking’’ must play important role on mechan-ical property of material. The introduction of chemical cross-linkers,which are permanent bonds, is expected to inhibit the formation oftemporary bonds, resulting in a higher recovery to the initial state.To further investigate the kinetics of disentanglement and entangle-ment for the ion gels with and without cross-linkers, we are planningto measure the hysteresis and residual strain of the ion geldepending on tensile speed, on deformation range and onwaiting time.In general, there is a trade-off relationship between theionic conductivity guaranteed by the amount of IL and themechanical properties of the gels.41,42 Sun et al. reported acyclic strain-tolerant ion gel containing 50 wt% IL as a solvent.The ion gel showed E20% residual strain after 10 cycles with a200% loading strain.21 Song et al. also reported that an ion gelswollen by 40 wt% ILs showed no residual strain after 50 cycleswith a 200% loading strain.25 In this study, we prepared anexcellent expansion-strain-tolerant ion gel swelling up to70 wt% using ILs as solvents. Compared to the previouslyreported ion gels containing nearly 70 wt% IL (Table S2, ESI†),the ion gel in this study showed low residual strain, consideringthat the tensile fatigue tests were conducted under relativelysevere conditions with a 300% strain. We expected that the gelwould exhibit both good electrochemical properties and dur-ability against cyclic strain.For demonstration as strain sensors, the ion gel ([CL] =0.01 mol%, [I] = 0.02 mol%) sandwiched between copper foils atboth ends were manually stretched while the current wasmeasured at a constant voltage. The relative resistanceresponded reversibly within five cycles and increased as thestrain was increased to 10%, 25%, and 50% (Fig. S5, ESI†),indicating that the ion gel has the ability to sense differentmovements of the human body. To investigate the stabilityagainst more repetitive stretching, the ion gel was subjected to100 and 50 consecutive loading–unloading cycles at 10 and 50%strain, respectively (Fig. S6, ESI†). The changes of the relativeresistance were stable throughout the overall cycles. The ion geldetected different mechanical strains with reversible and stablesignal output.ConclusionsIn this paper, we report an ion gel consisting of a [C2mim][TFSI]solvent and MMA as a matrix polymer cross-linked with a smallamount of EGDMA. The ion gel was obtained by a facile one-step procedure and exhibited a high toughness of up toE1 MJ m�3 and excellent recovery capability against loading–unloading cycles when the composition of the pre-gel solutionwas appropriately optimized. In contrast to the conventionalmechanical behaviour of gels, which become stiff and brittle asthe [CL] increases, the Young’s modulus and toughnessof our ion gels change non-linearly as a function of [CL] at0.001–0.01 mol% [CL]. The comparison between Etheo and Eexpat a [CL] lower than 0.1 mol% indicates that the numerousphysical cross-links derived from the polymer chain entangle-ments and the small amount of permanent chemical cross-links synergistically contribute to the toughness and stiffness ofthe gel.The initiator concentration ([I]) in the pre-gel solutionalso plays an important role in maintaining the mechanicalproperties of the resultant ion gel. As the toughness and self-standing capability decreased when [I] was significantlyincreased, an increase in the degree of monomer linkage(i.e., increasing the molecular weight of the polymer toUHMW when the chemical cross-links were ignored) wasfound to induce polymer chain entanglements and ensurethat the ion gel was mechanically strong. Furthermore,Fig. 5 Cyclic stress–strain curves at 300% strain at a tensile rate of10 cm min�1 for the ion gels with cross-linker contents of (a) 0.01 mol%and (b) 0 mol% at the same initiator content of 0.02 mol%. Each cycle wasseparated by a waiting time of 600 s. The inset shows magnified images.Soft Matter CommunicationOpen Access Article. Published on 11 July 2024. Downloaded on 7/18/2024 12:28:37 AM.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/https://doi.org/10.1039/d4sm00628cSoft Matter This journal is © The Royal Society of Chemistry 2024because unreacted MMA monomers accidentally plasticizeand reduce the Tg of the ion gel, [I] should be controlled tothe extent that conversion reaches E100%. The ion gel withthe highest toughness, despite the low number of chemicalcross-linkers (one chemical cross-linker per every 10 000repeating units of MMA), demonstrated a 10-fold longerrelaxation time and less than half its the residual strain after80 successive loading–unloading cycles than the ion gelwithout a cross-linker, indicating an improvement in long-term durability.This study would underpin the strategic design of iongels useful for wearable electronics, soft robotics, and otherapplications owing to its simple material preparation pro-cedure, high strength, and high resilience to repetitivestrains.Author contributionsInvestigation: A. S. supervision: T. U. writing – original draft:A. S. and T. U. writing – review and editing: Y. K., A. K., T. M.,R. T., and T. 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