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[Ryota Tamate](https://orcid.org/0000-0002-1704-1058), [Yuji Kamiyama](https://orcid.org/0000-0001-9483-2112), Ken Kojio

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[Stiff and Fracture‐Resistant Ion Gels Enabled by Synergetic Physical Entanglement and Hydrogen Bonding](https://mdr.nims.go.jp/datasets/2cc13368-b82e-4607-87ac-cd1c57fd6124)

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Stiff and Fracture‐Resistant Ion Gels Enabled by Synergetic Physical Entanglement and Hydrogen BondingRESEARCH ARTICLEwww.small-journal.comStiff and Fracture-Resistant Ion Gels Enabled by SynergeticPhysical Entanglement and Hydrogen BondingRyota Tamate,* Yuji Kamiyama, and Ken KojioIn this study, ion gels are developed that simultaneously exhibit exceptionalstiffness and fracture resistance through the synergistic effects of physicalentanglements and hydrogen bonding between polymer chains within an ionicliquidmatrix. Through radical copolymerization conducted in an ionic liquid un-der extremely low initiator concentrations, ultrahighmolecular weight polymersin situ with nearly complete monomer conversion are successfully synthesized.This strategy enabled the one-pot synthesis of physically crosslinked polymergels composed of abundant entanglements and hydrogen bonds betweenpolymer chains. Notably, it is demonstrated that the synergy between physicalentanglements arising from ultrahigh molecular weight polymer chainsand noncovalent hydrogen bonding enables the simultaneous enhancementof mechanical properties that typically exhibit trade-off relationships, such asstiffness, toughness, and fracture resistance. Consequently, the synthesized iongels exhibited outstanding mechanical performances, ranking among the bestpreviously reported tough polymer gels, while maintaining a favorable balancebetween ionic conductivity and mechanical strength. These findings under-score the broader significance of the approach, indicating that the integrationof physical entanglements and reversible interactions offers a generalizedpathway to mechanically robust materials across various polymer systems.R. Tamate, Y. KamiyamaResearch Center for Macromolecules & BiomaterialsNational Institute for Materials Science1-2-1 Sengen, Tsukuba 305-0047, JapanE-mail: TAMATE.Ryota@nims.go.jpR. Tamate, K. KojioPRESTOJST7Gobancho, Chiyoda-ku, Tokyo 102-0076, JapanK. KojioInstitute forMaterials Chemistry andEngineeringKyushuUniversityFukuoka 819-0395, JapanK. KojioInternational Institute for Carbon-Neutral EnergyResearch (WPI-I2CNER)KyushuUniversityFukuoka 819-0395, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/smll.202509922© 2025 The Author(s). Small published by Wiley-VCH GmbH. This is anopen access article under the terms of the Creative Commons AttributionLicense, which permits use, distribution and reproduction in anymedium, provided the original work is properly cited.DOI: 10.1002/smll.2025099221. IntroductionPolymer gels are soft materials composedof a 3D polymer network swollen with asolvent. Their versatility stems from theproperties of the solvent, rendering themhighly promising for diverse applications,ranging from biomedical uses to elec-trochemical devices.[1–9] However, a well-known limitation of polymer gels is theirinherently low mechanical strength, whicharises from their high liquid content. Toaddress this issue, various strategies havebeen proposed for developing tough poly-mer gels.[10–13] Representative examples in-clude double-network gels that use sac-rificial bonds and tetra-poly(ethylene gly-col) hydrogels with well-defined networkstructures.[14–18] A variety of studies havealso reported the enhancement of mechan-ical strength in ion gels using ionic liq-uids (ILs) as solvents, which possess uniquephysicochemical properties such as non-volatility, nonflammability, and high ionicconductivity.[19] For example, Hu, Dickey,and co-workers reported that polymerizingtwo monomers with different solubilitiesin the IL led to the formation of an in situ phase-separated struc-ture, enabling the creation of tough and stretchable ion gels.[20]Yan and co-workers demonstrated that employing halometallateILs, consisting of cations and coordinating anions, enabled dy-namic and reversible physical crosslinking with polymer chains,thereby yielding exceptionally tough ion gels.[21]In recent years, careful design of the topological structure ofthe polymer network has emerged as an effective strategy for en-hancing the mechanical robustness of polymer gels. A pioneer-ing example is the slide-ring gel developed by Ito et al., whichuses polyrotaxane-basedmovable crosslinks.[22–24] Recently, stud-ies have developed high-performance polymer gels by incorpo-rating polymer chain entanglements, a ubiquitous feature ofpolymers, into the gel structure. Notably, the Miyata and Suogroups have independently reported the development of hydro-gels with abundant entanglements by optimizing the use ofchemical crosslinkers and physical entanglements within the hy-drogel networks.[25,26]Inspired by such entanglement-based strategies, we previouslydeveloped ultrahigh molecular weight (UHMW) ion gels, whichare physical gels composed solely of entangled UHMW poly-mers formed in situ via radical polymerization within ILs underextremely low initiator concentrations.[27] Despite the absenceSmall 2025, e09922 © 2025 The Author(s). Small published by Wiley-VCH GmbHe09922 (1 of 9)http://www.small-journal.commailto:TAMATE.Ryota@nims.go.jphttps://doi.org/10.1002/smll.202509922http://creativecommons.org/licenses/by/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fsmll.202509922&domain=pdf&date_stamp=2025-10-07www.advancedsciencenews.com www.small-journal.comFigure 1. a) Schematic of an ion gel composed of hydrogen bonding and polymer chain entanglements. b) Polymerization scheme for ion gel.c) Relationship between monomer-to-initiator molar ratio and number-averaged molecular weight (Mn) of in situ formed P(EA-r-MMAm) copolymersin [C2mim][TFSI].of chemical crosslinkers, the UHMW ion gels exhibit high me-chanical stability and strength. Their network, formed exclusivelythrough reversible chain entanglements, imparts unique proper-ties, such as recyclability via thermal reprocessing and rapid self-healing at room temperature via re-entanglement of the network.Recently, several research groups have reported novel syntheticapproaches for producing UHMW polymers across various poly-mer systems, further advancing this field.[28–32]Despite the outstanding recyclability and self-healing capabili-ties ofUHMWion gels, which rely solely on polymer chain entan-glements, their mechanical strength is relatively low comparedto state-of-the-art tough polymer gels. In contrast, commerciallyadopted UHMW-based polymers, such as UHMW polyethyleneand natural rubber, exhibit remarkably high mechanical proper-ties. UHMW polyethylene features a structure in which foldedcrystalline domains of polyethylene chains are interconnected byamorphous regions.[33–35] For natural rubber, although its tough-ening mechanisms are not fully understood, strain-induced crys-tallization (SIC) plays a key role in its exceptional durability.[36–38]A common feature of these materials is the coexistenceof highly entangled UHMW polymer chains and rigid nan-odomains. Based on this insight, we designed a new type ofpolymer gel featuring dense entanglements of UHMW poly-mer chains and rigid nanodomains formed by noncovalent hy-drogen bonding, achieved through in situ radical copolymeriza-tion within an IL (Figure 1a). During gel synthesis, with a fixedmonomer/IL composition, the molecular weight of the resultingcopolymer can be controlled by adjusting the amount of radi-cal initiator. Notably, we demonstrated that combining hydrogenbonding with UHMW polymer chain entanglement enables thesimultaneous enhancement of physical properties that have tra-ditionally been considered mutually exclusive, such as stiffnessand fracture resistance, or mechanical strength and ionic con-ductivity. The synergistic toughening strategy based on physicalentanglements and noncovalent interactions described here doesnot depend on any specific chemical structure, offering a ver-satile framework for designing next-generation tough polymermaterials.2. Results and DiscussionWe discovered that a physical gel based on non-covalenthydrogen bonding can be formed via radical copolymer-ization of ethyl acrylate (EA) and N-methylmethacrylamide(MMAm) in an aprotic IL, 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide ([C2mim][TFSI]), initiatedunder UV irradiation using 2-hydroxy-2-methylpropiophenone(HMPP) as a photoinitiator (Figure 1b). Based on our previousfinding,[27,39,40] in the present study, we successfully synthesizedhydrogen-bonded ion gels across a wide range of molecularweights, up to the UHMW region (>106 g mol−1), by controllingthe monomer-to-initiator ratio through variation of HMPPconcentration during UV-induced copolymerization of EA andMMAm in [C2mim][TFSI] (Figure 1c). The gel permeationchromatography (GPC) traces of polymers extracted from thesynthesized ion gels are shown in Figure S1 (Supporting Infor-mation). Notably, this copolymerization system results in highmonomer conversion even at extremely low initiator concen-trations by simply prolonging the polymerization time, therebyenabling the one-pot synthesis of UHMW polymers. 1H-NMRanalysis of the ion gels after synthesis confirms that nearly allmonomers are consumed, with monomer conversion exceedingSmall 2025, e09922 © 2025 The Author(s). Small published by Wiley-VCH GmbHe09922 (2 of 9) 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202509922 by National Institute For, Wiley Online Library on [23/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 2. a) Uniaxial tensile tests of P(EA-r-MMAm)/[C2mim][TFSI] ion gels with different molecular weights. b,c) Relationship between fracture strainand stress b), and Young’s modulus and toughness c) for P(EA-r-MMAm)/[C2mim][TFSI] ion gels. d) Strain-induced opacification observed in P(EA-r-MMAm)/[C2mim][TFSI] ion gel with Mn = 1205 kDa. e,f) Temperature sweep measurements for P(EA-r-MMAm)/[C2mim][TFSI] ion gels with Mn =79 and 1205 kDa, respectively. g) Viscoelastic master curves of P(EA-r-MMAm)/[C2mim][TFSI] ion gels obtained using the tTS principle. Referencetemperature is 50 °C. h–j) Stress–strain curves h), Young’s modulus i), and FTIR spectra in amide I region j) for P(EA-r-MMAm)/[C2mim][TFSI] andP(EA-r-MMAm)/[C2OHmim][TFSI] ion gels.99% even in the samples containing UHMW polymers (FigureS2, Supporting Information). Details on the effect of UV poly-merization time on monomer conversion are provided in theSupporting Information (Figure S3, Supporting Information).Table S1 (Supporting Information) summarizes the GPC char-acterization results of the hydrogen-bonded P(EA-r-MMAm)copolymers forming the ion gels. In addition, thermal analysiswas performed using differential scanning calorimetry (DSC)(Figure S4, Supporting Information). The results revealed twopeaks in the derivative heat flow, indicative of glass transitiontemperatures, at ≈−61 and 14 °C. Similar dual DSC derivativeheat flow peaks have been reported by Lodge et al. for misci-ble PMMA/[C2mim][TFSI] blend systems.[41] These peaks areconsidered to originate from the effective local concentrationassociated with polymer connectivity at the length scale relevantto glass transition dynamics, as proposed by Lodge and McLeishfor miscible polymer blend systems.[42] In the present ternarysystem consisting of a copolymer and an ionic liquid, the situ-ation is more complex; however, these peaks are presumed toarise from effective local concentrations of polymer-rich andionic-liquid-rich domains.Figure 2a shows the results of uniaxial tensile tests forP(EA-r-MMAm)/[C2mim][TFSI] ion gels with different molec-ular weights. Notably, an increase in the molecular weight ofthe P(EA-r-MMAm) copolymer simultaneously enhances me-chanical properties that are typically considered to exhibita trade-off relationship, such as fracture stress–strain andYoung’s modulus–toughness (Figure 2b,c). In particular, theUHMW ion gel with a number-average molecular weight (Mn)of 1205 kDa exhibited exceptional mechanical performance,Small 2025, e09922 © 2025 The Author(s). Small published by Wiley-VCH GmbHe09922 (3 of 9) 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202509922 by National Institute For, Wiley Online Library on [23/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comwith Young’s modulus >80 MPa, fracture stress >20 MPa,fracture strain >500%, and toughness >50 MJ m−3. Theseresults suggest that the synergy between physical entangle-ments and non-covalent hydrogen bonding enables simultane-ous enhancement of otherwise conflicting mechanical proper-ties. For comparison, a UHMW ion gel composed of neutralUHMWpoly(methyl methacrylate) (PMMA) and [C2mim][TFSI],which lacks hydrogen bonding, at the same polymer fractionwas considered. The gel exhibited significantly lower mechan-ical strength (Young’s modulus of 0.5 MPa, fracture stress of1.3 MPa, and toughness of 2.9 MJ m−3). Uniaxial stress-straincurves and the resultant toughness of the ion gel composed ofP(EA-r-MMAm)/[C2mim][TFSI] with UHMW polymers (Mn =1205 kDa), featuring both physical entanglements and hydrogenbonding, are compared with the hydrogen-bonded ion gel com-posed of P(EA-r-MMAm)/[C2mim][TFSI] with lower-molecular-weight polymers (Mn = 79 kDa), and the UHMW ion gel com-posed of PMMA/[C2mim][TFSI] (Mn = 1605 kDa) lacking hydro-gen bonding (Figure S5a,b, Supporting Information). From theseresults, it is evident that, compared with ion gels relying on onlyone of these interactions, the synergistic combination of physi-cal entanglements of UHMW polymers and hydrogen bondingenables a substantial simultaneous enhancement of mechanicalproperties. During tensile deformation, the initially transparentP(EA-r-MMAm)/[C2mim][TFSI] ion gel withMn = 1205 kDa be-comes visibly opaque upon stretching (Figure 2d). This opaci-fication was reversible, where the gel became transparent oncethe stress was released. Therefore, in contrast to the crazing of-ten observed in plastics, the reversible opacification suggests thatstrain-induced structural heterogeneity, likely nanophase separa-tion, is dynamically formed during stretching deformation.Figure 2e,f, and Figure S6 (Supporting Information) showthe temperature dependence of storage modulus (G′), lossmodulus (G″) and loss tangent (tan 𝛿 = G″/G′) for P(EA-r-MMAm)/[C2mim][TFSI] ion gels with different molecularweights, Mns of 79 and 1205 kDa. In both samples, the tan 𝛿peak, which is indicative of the glass transition temperature,was observed at ≈40 °C and was presumably owing to the re-laxation of hydrogen bonds. At lower temperatures, no signifi-cant differences in linear viscoelasticity are observed; however,as the temperature increases, the low molecular weight sam-ple (Mn = 79 kDa) exhibits a rapid decrease in G′ and G″,and a crossover of G′ and G″ is observed (Figure 2e), suggest-ing enhanced chain mobility owing to thermal dissociation ofhydrogen bonds, resulting in liquid-like behavior. In contrast,the UHMW sample does not show a crossover of G′ and G″even at 150 °C, indicating that it retains solid-like integritiesat higher temperatures (Figure 2f). This finding was likely at-tributed to the high entanglement density, whichmaintains phys-ical crosslinks even when hydrogen bonds are thermally weak-ened. Figure 2g shows the viscoelastic master curves of G′ andG″ constructed from frequency sweep measurements at varioustemperatures using the time–temperature superposition (tTS)principle. Differences in low-frequency (i.e., long-timescale) vis-coelastic behavior were clearly observed depending on themolec-ular weight. Although the low molecular weight sample exhib-ited a G′–G″ crossover and terminal relaxation behavior, theUHMW sample did not exhibit any crossover within the mea-sured frequency range, suggesting solid-like behavior over ex-tremely long timescales. These results demonstrated that despitebeing a physical gel with a characteristic relaxation time, theP(EA-r-MMAm)/[C2mim][TFSI] ion gel exhibited excellent shapestability. For comparison, Figures S7 and S8 (Supporting Infor-mation) show temperature sweepmeasurements and tTSmastercurves for the PMMA/[C2mim][TFSI] system, which lacks hydro-gen bonding. Although the PMMA/[C2mim][TFSI] ion gel withMn = 1626 kDa also exhibits high thermal stability and a wideplateau region owing to its abundant chain entanglements, thetemperature dependence of the shift factor aT used in tTS super-position differs significantly from that of the P(EA-r-MMAm) sys-tem (Figure S9, Supporting Information). The P(EA-r-MMAm)system exhibited considerably higher temperature sensitivity ofaT than the PMMA system. The activation energy calculatedfrom the Arrhenius plot was Ea = 196 kJ mol−1 for P(EA-r-MMAm)/[C2mim][TFSI] compared to Ea = 106 kJ mol−1 forPMMA/[C2mim][TFSI]. Notably, molecular weight had minimalinfluence on aT in either system, suggesting that the high activa-tion energy in the P(EA-r-MMAm) system was attributed to theinterpolymer hydrogen bonding.To further investigate the role of hydrogen bonding, weperformed a comparative experiment using 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide([C2OHmim][TFSI]), an IL with a hydroxyl group capable offorming hydrogen bonds, instead of [C2mim][TFSI]. The tensileproperties of the resulting P(EA-r-MMAm)/[C2OHmim][TFSI]ion gel are shown in Figure 2h. Compared to that of the gelformed in [C2mim][TFSI], themechanical strength wasmarkedlyreduced. Most notably, the Young’s modulus decreased from≈83 MPa to below 10 MPa (Figure 2i). FTIR spectra exhibit ashift in the amide I peak corresponding to the C═O stretchingof the amide group in MMAm when the solvent is replacedwith [C2OHmim][TFSI] (Figure 2j). This shift suggests thatthe hydroxyl group of [C2OHmim] cations formed hydrogenbonds with the carbonyl groups of MMAm, competing with theinterpolymer hydrogen bonds. Similarly, a shift in the amideII peak, corresponding to C─N─H, is also observed (FigureS10, Supporting Information). These results indicate that in[C2OHmim][TFSI], competitive hydrogen bonding betweenthe solvent and polymer chains disrupts interpolymer hydro-gen bonding, resulting in a significant reduction in stiffness.In summary, these comparisons highlight the critical role ofinterpolymer hydrogen bonding in determining the mechan-ical performance of P(EA-r-MMAm)/[C2mim][TFSI] ion gels,indicating that the synergistic effects of dense physical entangle-ments and noncovalent bonding give rise to their outstandingmechanical properties.To investigate the structural evolution of ion gels during ten-sile deformation, we performed in situ small-angle X-ray scat-tering (SAXS) and wide-angle X-ray scattering (WAXS) measure-ments during uniaxial stretching.[43] Figures 3a,b, and S11 (Sup-porting Information) show the two-dimensional SAXS patternsof P(EA-r-MMAm)/[C2mim][TFSI] ion gels with three differentmolecular weights upon stretching. In the unstretched state,none of the samples exhibited any discernible scattering peaks,suggesting the absence of phase-separated structures larger thanthe nanometer scale typically observable in the small-angle re-gion. In contrast, upon stretching, scattering peaks appearedalong the stretching direction as the strain increased, resultingSmall 2025, e09922 © 2025 The Author(s). Small published by Wiley-VCH GmbHe09922 (4 of 9) 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202509922 by National Institute For, Wiley Online Library on [23/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 3. a,b) 2D SAXS patterns of P(EA-r-MMAm)/[C2mim][TFSI] ion gels before stretching and at 300% strain. c,d) 1D SAXS profiles along thestretching direction. e,f) 1D SAXS profiles along the perpendicular direction to stretching. a,c,e) Mn = 1205 kDa. b,d,f) Mn = 79 kDa. g,h) Azimuthalplots of 2D WAXS patterns in the q range of 7.8–10.5 nm−1. g) Mn = 1205 kDa. h) Mn = 79 kDa. i) Schematic of emergence of nanophase-separatedstructures upon stretching and formation of bundled structures at high stretching ratio.in an “abnormal butterfly pattern.” These abnormal butterfly pat-terns have also been reported during the elongation of chem-ically crosslinked hydrogels and crystalline polymers, as wellas polymer solutions under shear deformation.[44–47] This ob-servation indicates that strain promotes structural heterogeneitywithin the gel,[44] leading to the formation of hydrogen-bond-richnanodomains that are immiscible with the IL [C2mim][TFSI].Notably, in the UHMW sample, the scattering peaks appear atlower strains compared to the lower molecular weight samples,as shown in Figure S11 (Supporting Information). Figure 3c,dshows the 1D SAXS profiles along the stretching direction forthe 79 and 1205 kDa samples. In both ion gels, a scattering peakappears at ≈q = 0.25 nm−1 as the strain increases. The UHMWsample exhibited this peak starting from 50% strain, whereas thelow-molecular-weight sample required up to 200% strain for thepeak to emerge. This finding suggests that strain-induced inho-mogeneity is more readily promoted in the UHMW ion gel. Thisbehavior might be attributed to the structural relaxation in low-molecular-weight samples owing to rapid disentanglement uponstretching. In contrast, in the UHMW sample, the dense entan-glements effectively “freeze” the structure on the timescale of thetensile test. Consequently, even at low strains, nanodomain for-mation is evident as the UHMW polymer chains bridge thesedomains. Similarly, the 219 kDa sample does not exhibit anypeak up to a strain of 200% (Figure S12, Supporting Informa-tion), further supporting that UHMWpolymers significantly pro-mote the development of strain-induced nanophase separation.Notably, only in the UHMW gel did a thin streak appear perpen-dicular to the stretching direction under high strains, as shownin the 2D SAXS images. This streak peak is considered to be as-sociated with the formation of bundled structures. For example,Kojio et al. reported that in polyurethane and polythiourethaneelastomers composed of hard and soft domains, a streak peakwas observed in the absence of chemical cross-linkers, whichwas attributed to the formation of nanofibrils resulting from therearrangement of hard domains.[48,49] Figure 3e shows the 1DSAXS profiles in the direction perpendicular to stretching for the1205 kDa sample. With increasing strain, the scattering inten-sity increases, suggesting a higher density of scatterers. In con-trast, despite the increase in scattering intensity along the stretch-ing direction, the lower-molecular-weight samples (Mns= 79 and219 kDa) exhibit almost no scattering intensity in the perpendic-ular direction (Figure 3f; Figure S13, Supporting Information).While the detailed origin of this streak remains unclear, we spec-ulate that the strain-induced nanodomains align to form bundledstructures perpendicular to the stretching direction.In the 2D WAXS measurements during stretching, no dis-tinct spot peaks were observed. However, the UHMW sam-ple exhibited relatively stronger scattering intensity perpendic-ular to the stretching direction (q = 90°) compared to theSmall 2025, e09922 © 2025 The Author(s). Small published by Wiley-VCH GmbHe09922 (5 of 9) 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202509922 by National Institute For, Wiley Online Library on [23/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comlower molecular weight samples (Figure S14, Supporting Infor-mation). Recently, reported high-strength and highly resilientgels exhibiting SIC, spot peaks corresponding to polymer chaincrystallization, have been observed in WAXS patterns understretching.[23,50–52] In the present ion gels, the absence of suchspot peaks indicated that polymer crystallization was not ob-served. However, azimuthal plots of the 2D WAXS imagesreveal peaks in the perpendicular direction at high strains,which are absent in low molecular-weight ion gels (Figure 3g,h;Figure S15, Supporting Information). Such molecular orienta-tion has also been reported in polymer systems withmicrophase-separated polymer systems, such as thermoplastic block copoly-mer elastomers.[53] These SAXS and WAXS results suggest thatthe P(EA-r-MMAm)/[C2mim][TFSI] ion gels withMn = 1205 kDaformed nanophase-separated structures even at low strain owingto strain-induced heterogeneity, where the hydrogen-bond-richnanodomains were connected byUHMWpolymers. This findingis consistent with the presence of hydrogen bonding observedin the FTIR spectra (Figure 2j; Figure S8, Supporting Informa-tion). At higher strains, these domains are further stretched intobundle-like structures. Simultaneously, owing to suppressed en-tanglement relaxation, weak molecular orientation of stretchedUHMWpolymer chains is observed at high strain (Figure 3i). Re-cently, polymer gels designed to actively control phase-separatedstructures to achieve mechanical functionalities have attractedincreasing attention.[20,54–57] There are also pioneering reportson polymer gels that exhibit deformation-induced phase sepa-ration under stretching.[58,59] In this study, we report that theunique strain-induced phase separation observed in the P(EA-r-MMAm)/[C2mim][TFSI] ion gel withMn = 1205 kDa might con-tribute to the mechanical strength. We are currently conductingfurther investigations into the detailed structure–property rela-tionships.To further evaluate the fracture resistance of the P(EA-r-MMAm)/[C2mim][TFSI] ion gels, we conducted single-edgecrack tests (Figure 4a; Figure S16, Supporting Information).[60,61]Notably, in the P(EA-r-MMAm)/[C2mim][TFSI] ion gel withMn =1205 kDa, crack blunting is observed upon stretching the crackedsamples, and the fracture energy increases with an increase inmolecular weight (Figure 4b,c). Lake and Thomas formulatedthe fracture energy of vulcanized rubbers by considering a mi-croscopic picture of polymers.[62] They assumed that, in order torupture polymer chains at a crack tip, the same degree of defor-mationmust be applied to allmonomer units in the vicinity of thecrack. According to the Lake–Thomas model, the fracture energyΓ can be estimated as Γ∼𝜈LNU, where 𝜈 is the number of net-work chains per unit volume, L is the displacement length at thecrack tip,N is the degree of polymerization between cross-linkingpoints, and U is the energy required to rupture a monomer unit.If the cross-linked polymer chains are assumed to behave asGaussian chains, L scales as N1/2, while N scales as 𝜈−1. Further-more, by incorporating the fact that the Young’smodulusE is pro-portional to the network density 𝜈, according to rubber elasticitytheory, the relationship Γ∼E−1/2 is obtained. In other words, thefracture energy and Young’smodulus can be regarded as being ina trade-off relationship. Sakai and co-workers verified the valid-ity of the Lake–Thomas model for chemically cross-linked hydro-gels through experiments using tetra-branched poly(ethylene gly-col) hydrogels with uniform networks.[63] In contrast, Mayumi,Ito, and colleagues demonstrated that slide-ring gels with mov-able crosslinking points along the polymer chains can enhanceYoung’s modulus without sacrificing fracture energy by increas-ing the crosslink density.[64] This finding was attributed to theslippage of crosslinking points at the crack tip, enabling poly-mer chains to be pulled out from the crosslinking points. Sim-ilarly, Suo et al. have shown that hydrogels with rich entangle-ments can achieve high fracture energy and stiffness.[26] Remark-ably, in the P(EA-r-MMAm)/[C2mim][TFSI] ion gels, increasingthe molecular weight leads to simultaneous enhancement ofYoung’s modulus and fracture energy, overcoming the trade-off(Figure 4d). One possible hypothesis is that the movable entan-glement crosslinking points slide along the polymer backbonenear the crack tip, resulting in chain pullout and crack blunt-ing (Figure 4e). Furthermore, the nanophase separation observedunder stretching in the SAXS/WAXS measurements may alsooccur in the highly strained region near the crack tip, poten-tially contributing to suppression of crack propagation (FigureS17, Supporting Information). Elucidating these mechanismsin detail remains a subject for future study. The balance be-tween Young’s modulus and fracture energy achieved here isamong the highest reported for tough polymer gels, includingnot only ion gels but also hydrogels (Figure 4f; Table S2, Sup-porting Information).[20,21,23,26,59,65–73] However, the mechanicalproperties of the ion gels are likely to be timescale-dependentduring mechanical testing, as they rely on physical crosslinkingvia entanglements and hydrogen bonding, both reversible physi-cal interactions. This consideration also applies to various toughpolymer gels listed in Table S2 (Supporting Information); varia-tions in fracture energy quantification methods and other experi-mental conditions also likely contribute to the differences. There-fore, Figure 4f and Table S2 (Supporting Information) shouldnot be interpreted as definitive rankings based on absolutevalues.In addition, the present gel incorporates an IL as the sol-vent, providing high environmental stability due to its non-volatility and nonflammability, along with excellent ionic con-ductivity (Figure S18, Supporting Information). The P(EA-r-MMAm)/[C2mim][TFSI] ion gel with Mn = 1205 kDa exhibiteda high ionic conductivity of 1.4 × 10−4 S cm−1 at 25 °C. Ingeneral, increasing polymer concentration in ion gels enhancesmechanical properties but reduces ionic conductivity, indicatinga trade-off relationship. Therefore, achieving high mechanicalstrength and ionic conductivity remains a critical challenge inion-conductive polymer materials. In this system, decreasing thepolymer content leads to reduced mechanical strength in tensiletests and low resistance in impedance measurements—i.e., en-hanced ionic conductivity (Figure 4g,h). Nevertheless, when plot-ting the ionic conductivity and fracture stress of ion gels with dif-ferent polymer concentrations together with previously reportedhigh-performance ion gels, the present system clearly demon-strates exceptional simultaneous performance in ionic conduc-tivity and mechanical properties (Figure 4i; Table S3, Support-ing Information).[27,55,67,74–86] Although measurement conditionsvary and prevent direct comparison of absolute values, theseresults strongly indicate that this design concept enables thefabrication of polymer gels that combine high ionic conduc-tivity and mechanical robustness at an outstanding level. No-tably, when comparing this system with a composite of neutralSmall 2025, e09922 © 2025 The Author(s). Small published by Wiley-VCH GmbHe09922 (6 of 9) 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202509922 by National Institute For, Wiley Online Library on [23/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 4. a) Uniaxial tensile tests of cracked samples of P(EA-r-MMAm)/[C2mim][TFSI] ion gels with different molecular weights. b) Molecular weightdependence of fracture energy for P(EA-r-MMAm)/[C2mim][TFSI] ion gels. c) Photographs of the tensile test of a cracked P(EA-r-MMAm)/[C2mim][TFSI]ion gel with Mn = 1205 kDa. d) Relationship between Young’s modulus and fracture energy. Dashed line represents Lake–Thomas rule (Γ ∼ E−1/2).e) Schematic of hypothesized crack tip blunting mechanism via slippage of entangled polymer chains. f) Ashby plot of Young’s modulus versus fractureenergy comparing previously reported tough hydrogels/ion gels (black circles) and present P(EA-r-MMAm)/[C2mim][TFSI] ion gel withMn = 1205 kDa(red circle). g,h) Uniaxial tensile tests at room temperature g) and impedance measurements at 25 °C h) of P(EA-r-MMAm)/[C2mim][TFSI] ion gelswith different polymer concentrations. i) Ashby plot of fracture stress versus ionic conductivity for P(EA-r-MMAm)/[C2mim][TFSI] ion gels with varyingpolymer concentrations (red circles) and previously reported ion gels (black circles).polymer PMMA and [C2mim][TFSI], the PMMA/[C2mim][TFSI]system shows a considerably poorer balance between strengthand ionic conductivity across different polymer concentrations,suggesting that nanophase-separated structures may decouplemechanical strength and ionic conductivity. (Figure S19, Sup-porting Information). This finding further supports the effec-tiveness of the present concept in developing polymer gelsthat achieve high ionic conductivity and mechanical strength.As an example of potential applications of the ion gels, wealso examined their use in strain sensing. A 30 vol% P(EA-r-MMAm)/[C2mim][TFSI] ion gel sheet was sandwiched betweenelectrodes, and repeated loading–unloading cycles of strain wereapplied. The resistance change increased with increasing appliedstrain (Figure S20a, Supporting Information). Furthermore, un-der repeated application of 50% strain, changes in resistancecould be reproducibly recorded for over 100 cycles (Figure S20b,Supporting Information). These results demonstrate that the iongels, which combine mechanical durability with high ionic con-ductivity, are promising candidates for strain sensors and otherflexible electrochemical devices.Small 2025, e09922 © 2025 The Author(s). Small published by Wiley-VCH GmbHe09922 (7 of 9) 16136829, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202509922 by National Institute For, Wiley Online Library on [23/10/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.com3. ConclusionIn this study, we developed ion gels that simultaneously exhib-ited high stiffness and fracture resistance by leveraging the syn-ergistic effects of physical chain entanglements and hydrogenbonding in ILs. By conducting radical copolymerization underextremely low initiator concentrations in ILs, we achieved in situpolymerization of hydrogen-bonded UHMWpolymers with highmonomer conversion. This approach enabled the one-pot syn-thesis of ion gels containing abundant physical entanglementsand hydrogen bondswithin the IL. Notably, we demonstrated thatthe synergistic effect between physical entanglements formed byUHMWpolymers and hydrogen bonding enabled the simultane-ous enhancement of mechanical properties that typically exhibittrade-off relationships, such as Young’smodulus, toughness, andfracture energy. Consequently, the developed ion gels exhibitedmechanical performance ranking among the highest reported fortough polymer gels.Our findings also highlight a broadly applicable strategy fordesigning high-performance polymeric materials by combiningphysical entanglements with noncovalent interactions. In addi-tion, this strategy offers the advantage of reusability, as it avoidsthe use of chemical crosslinkers. Furthermore, the developedion gels also achieved an outstanding balance between ionicconductivity and mechanical strength. In recent years, materi-als that can simultaneously exhibit mechanical strengths andionic conductivity have gained attention for applications such asflexible/wearable electronics,[87–89] and high-energy-density sec-ondary batteries where electrodes undergo large deformationsduring electrochemical reactions.[90–92] Therefore, the presention gels are expected to be highly useful for next-generation elec-trochemical device applications.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was financially supported by JSPS KAKENHI (23K26409 to R.T.),JST PRESTO program (JPMJPR2196 to R.T.), and Green Technologies ofExcellence (GteX) Program (JPMJGX23S3 to R.T.).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available in the sup-plementary material of this article.Keywordsentanglements, fracture resistance, hydrogen bonds, ion gels, ultrahighmolecular weight polymersReceived: August 15, 2025Revised: September 29, 2025Published online:[1] S. 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