Soft Matter
View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

PAPER

Cite this: Soft Matter, 2023,
19, 2745

View Journal | View Issue

Toughening of poly(ionic liquid)-based ion gelswith cellulose nanofibers as a sacrificial network†
Takaichi Watanabe,* Emiho Oe,

Yuna Mizutani

and Tsutomu Ono

Ion gels have the potential to be used in a broad range of applications, such as in carbon dioxide
separation membranes and soft electronics. However, their low mechanical strength limits their practicalapplications. In this study, we developed double-network (DN) ion gels composed of TEMPO-oxidized
cellulose nanofibers with hydrophobic groups (TOCNF) and cross-linked poly[1-ethyl-3-vinylimidazoliumbis(trifluoromethanesulfonyl)imide] (PC2im-TFSI) networks. The mechanical strength of the gel increased
as the amount of TOCNF in the gels increased up to 6 wt%. Moreover, the fracture energy of the DN
ion gels with 6 wt% TOCNF was found to be 19 times higher than that of the PC2im-TFSI single network(SN) ion gels. Cyclic stress–strain measurements of the DN gels showed that the loading energy on thegels dissipates owing to the destruction of the physically cross-linked TOCNF network in the gels. The
DN ion gels also exhibited a high decomposition temperature of approximately 400 1C because of thethermal stability of all components. Additionally, the fracture energy of the TOCNF/poly(ionic liquid) (PIL)
Received 27th January 2023,
Accepted 22nd March 2023

DN ion gel was two times higher than that of the silica nanoparticles/PIL DN ion gel developed in our

DOI: 10.1039/d3sm00112a

nanomaterials are more effective than spherical nanomaterials in enhancing the mechanical properties

previous study [Watanabe et al., Soft Matter, 2020, 16, 1572–1581]. This suggests that fiber-shapedof ion gels. These results show that TOCNF can be used to toughen PIL-based ion gels and hence

rsc.li/soft-matter-journal

broaden their applications.

Introduction
Ion gels, which are composed of three-dimensional crosslinked polymer networks swollen with a large amount of ionicliquid (IL), have recently attracted significant attention as a newclass of soft materials because of their unique properties suchas high ionic conductivity,1–5 thermal stability,6,7 and electrochemical stability,8,9 processability,10,11 and handleability.
These gels have potential applications in various fields, including
as solid-state polyelectrolytes,12,13 supercapacitors,14,15 carbon
dioxide separation membranes,16–19 and as sensors andactuators2,20,21 in soft electronics. However, their low mechanical strength hinders their practical use.
Over the past few decades, various strategies have been
employed to improve the mechanical strength of ion gels, suchas the use of homogeneous tetra-arm poly(ethylene glycol)
networks22,23 and self-assembled ABA-type triblock copolymers
with IL-philic middle blocks and IL-phobic cross-linkable end
blocks.24,25 Recently, Tamate et al. developed the fabrication of
Department of Applied Chemistry, Graduate School of Natural Science, Okayama
University, 3-1-1, Tsushima-naka, Kita-ku, Okayama, 700-8530, Japan.
E-mail: wata-t@okayama-u.ac.jp, tono@okayama-u.ac.jp; Tel: +81-86-251-8072,+81-86-251-8083† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d3sm00112a

This journal is © The Royal Society of Chemistry 2023

highly stretchable and self-healing ion gels using ultra-highmolecular weight polymers synthesized in an ionic liquid.26,27
This approach does not use a chemical cross-linker in the gel,thereby achieving relatively high toughness and self-healing.
Hu et al. fabricated tough and stretchable gels through thein situ random copolymerization of two monomers with diﬀerent aﬃnities for ionic liquids;28 in this system, a polymer-richphase with hydrogen bonds contributed to the energy dissipation, while an elastic solvent-rich phase withstood large strains,
leading to gel toughening.
The double network (DN) concept, which has been widelysuccessfully applied in hydrogels,29–31 can also be effective in
improving the toughness of ion gels. These gels comprise twointerpenetrating networks: a rigid and brittle network and a softand stretchable network; these two networks work together todissipate the loading energy and maintain the integrity of the gel,respectively. Examples of DN ion gels include those made ofchemically cross-linked poly(furfuryl methacrylate-co-methyl
methacrylate) and physically cross-linked poly(vinylidene fluoride-co-hexafluoropropylene),32 and those composed of poly(vinyl
alcohol) and poly(vinylpyrrolidone).33 Kamio et al. developed
inorganic/organic DN and micro-double-network (m-DN) ion gelsusing silica nanoparticles and poly(N,N-dimethylacrylamide).34–37
In these gels, the inorganic silica nanoparticles form a partiallyclustered network in the IL via hydrogen bonding and van derSoft Matter, 2023, 19, 2745–2754 | 2745

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Paper

Soft Matter

Scheme 1 Schematic illustration of the preparation of the TOCNF/PIL DN ion gel via photo-induced radical polymerization and subsequent solvent
evaporation.

Waals interactions, and the loading energy is dissipated by theinternal rupture of the silica nanoparticle network in the gels. In
our previous study, we developed poly(ionic liquid) (PIL)-based
DN ion gels composed of silica nanoparticles and PIL andexamined the effect of the surface functionality of the silica
nanoparticles on the mechanical strength of the ion gels.38 We
found that only pristine silica nanoparticles that contain hydroxide
or silanol groups on the surface formed a partially clusterednetwork in the IL and contributed to the toughening of the iongels. This result suggests that nanomaterials with strong hydrogenbonding formation properties, such as cellulose and cellulose
derivatives, could be effective in toughening ion gels.
In this study, we developed a new type of DN ion gel
composed of physically cross-linked TEMPO-oxidized cellulose
nanofibers with hydrophobic groups (TOCNF)39 and chemicallycross-linked poly[1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide] (PC2im-TFSI, PIL) networks with an ionic liquid(Scheme 1). TOCNF, a cellulose derivative known for its biocompatibility and ability to reinforce soft materials,40–43 was
expected to contribute to the formation of a physically crosslinked network in the IL44 and improve the toughness of the iongels through energy dissipation. To the best of our knowledge,
this is the first study to report on the preparation of DN ion gelsusing TOCNF and PIL. The rheological behavior of TOCNF in
the IL was examined to confirm the formation of a physicallycross-linked TOCNF network. TOCNF/PIL DN ion gels withvarying amounts of TOCNF and PIL single network (SN) ion
gels were prepared, and their mechanical properties were compared with those of silica nanoparticles/PIL DN ion gels, whichwere previously reported by our group.38 These ion gels were also
subjected to cyclic tensile testing to evaluate their degree ofenergy dissipation. The differences in network structure between
PIL SN and TOCNF/PIL DN gels were evaluated by SEM. Furthermore, the thermal stability and ionic conductivity of the ion gels
was evaluated using thermogravimetric analysis and impedance
analyzer, respectively.

Results and discussion
To determine whether TOCNF has the potential to cause
gelation in an ionic liquid, an oscillatory shear measurement
was performed using a TOCNF dispersion in [C4mim][TFSI], as
illustrated in Fig. 1(a). The elastic modulus (G 0 ) of the 3 wt%
TOCNF dispersion in [C4mim][TFSI] is significantly larger thanits viscous modulus (G00 ); moreover G 0 is independent of theFig. 1 (a) Elastic (G 0 ) and viscous (G00 ) moduli as a function of frequency for an IL dispersion with 3 wt% TOCNF. (b) Shear rate dependencies of viscosity
for IL dispersions with diﬀerent concentrations of TOCNF.

2746 | Soft Matter, 2023, 19, 2745–2754

This journal is © The Royal Society of Chemistry 2023

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Soft Matter
frequency across the measured range. This result indicates that
the TOCNF dispersion acts as a soft solid-like material or gel,and that TOCNF is capable of physically cross-linking within[C4mim][TFSI]. To further investigate the gelation of TOCNF
in [C4mim][TFSI], the viscosities of the [C4mim][TFSI] and[C4mim][TFSI] dispersions containing various concentrationsof TOCNF were measured under diﬀerent shear rates, as shownin Fig. 1b. The viscosity of pure [C4mim][TFSI] was independentof the shear rate, which is consistent with previously reported
literature values.45 This observation suggests that pure[C4mim][TFSI] behaves as a Newtonian fluid. In contrast, theviscosity of the TOCNF-containing dispersions decreased withincreasing shear rate, up to 6 wt%. This shear-thinning behavior is probably caused by the rupturing of interfibrous physicalbonds of TOCNF under shear, similar to what has been
observed in dispersions of colloidal particles such as silica
nanoparticles46,47 and polysaccharides.48 When a 6 wt% TOCNF
dispersion was used, shear-thickening was also observed atshear rates between 1 and 10 [s1]. This indicates that TOCNF
forms pseudo clusters at high concentration of TOCNF whenshear is applied. Additionally, the zero-shear viscosity of the
TOCNF-containing dispersions increased with increasing
TOCNF concentration, indicating that the degree of interfibrous interactions and apparent cross-linking density of
TOCNF in the ionic liquid can be enhanced by increasing theTOCNF concentration. However, when the concentration of
TOCNF was increased to over 6 wt%, no significant change inthe zero-shear viscosity was observed. This suggests that an
excess amounts of TOCNF in [C4mim][TFSI] is unlikely tocontribute to network formation. Based on these findings, we
expected that TOCNF may potentially improve the mechanicalstrength of PIL-based ion gels through the principle of energy
dissipation.
PIL-based DN ion gels, comprising TOCNF and cross-linked
PC2im-TFSI, were fabricated by the photo-induced radical polymerization of [C2vim][TFSI] in a mixture containing varying
concentrations of TOCNF, [C4mim][TFSI] (solvent), [(Vim)2C4][TFSI]2 (cross-linker), Igragures 2959 (photoinitiator), and ethyl
acetate (solvent). Subsequently, ethyl acetate was removed
using the solvent evaporation method. PIL SN ion gels werealso prepared using a similar method, but without the inclusion of TOCNF. Fig. 2(a) and (b) show the photographs of a PIL
SN ion gel and a TOCNF/PIL DN ion gel, respectively. Both iongels appear transparent and self-standing, but are yellow incolour owing to the use of a cross-linker in their preparation.
The PIL DN ion gels appear more yellow in color than the PIL
SN ion gels because the TOCNFs scatter light. However, nomacroscopic phase separation of TOCNF in the IL wasobserved, indicating that TOCNF was successfully incorporated
into the ion gels.
To determine the mechanical strength of the TOCNF/PIL DN
ion gels, uniaxial stretching experiments were conducted onion gels containing various amounts of TOCNF (Fig. 3). The
stress–strain curves for ion gels with different amounts of
TOCNF are shown in Fig. 3(a). The results for the PIL SN ion
gel composed of 50 wt% PC2im-TFSI and 50 wt% [C4mim][TFSI],This journal is © The Royal Society of Chemistry 2023

Paper

Fig. 2 Representative photographs of (a) PIL SN and (b) TOCNF/PIL DN
ion gels, containing 50 wt% of [C4mim][TFSI]. The amount of TOCNF in the
DN ion gel was fixed at 6 wt%.

(i.e., TOCNF concentration of 0 wt%) are also displayed. The
mechanical strength of the TOCNF/PIL DN ion gel is higher
than that of the PIL SN ion gel. Additionally, the mechanicalstrength of the TOCNF/PIL DN ion gels increased with increasing TOCNF concentration up to 6 wt%. These results demonstrate that the incorporation of TOCNF into the PIL-based ion
gel enhances the mechanical properties of the ion gels. However, when the TOCNF content was 7 wt%, the fracture stressdecreased. This could be due to the excess amount of TOCNF in
the IL, which could have potentially hindered the extension of
the PIL network upon stretching. Interestingly, the Young’s
modulus of the DN ion gel increased when the TOCNF concentration was increased up to 4 wt%. However, the Young’s
modulus did not significantly change when the TOCNF concentration was increased above 4 wt%. In contrast, the fracture
stress of the DN gel increased up to 6 wt%. This result differsfrom that of the silica nanoparticle/PIL DN ion gel developed in
our previous study.38 The Young’s modulus of the silica nanoparticle/PIL DN gel monotonically increased with increasing
silica nanoparticle concentration, indicating that the dominantfactor determining the mechanical properties of the TOCNF/
PIL DN ion gels can change when the TOCNF amount is above
4 wt%. The mechanical properties, such as fracture strain, fracture stress, and fracture energy, are summarized in Fig. 3(b–d).
The fracture strain and fracture stress of the TOCNF/PIL DN ion
gel increased up to 5 wt% and 6 wt% TOCNF, respectively, andthen exhibited a declining trend (Fig. 3b and c). Additionally,both the fracture strain and stress of the ion gels containing
TOCNF were significantly higher than those without TOCNF.
These results suggest that TOCNF does not act as a filler, but
acts as a network to enhance both fracture strain and stress of
the ion gel up to approximately 6 wt%. Moreover, the fractureenergy of the DN ion gel, calculated from the area of the stress–
strain curve, increased with increasing TOCNF concentration
up to 6 wt% (Fig. 3d). From these results, we conclude that theincorporation of TOCNF into the PIL-based ion gel toughens
the ion gel, and the optimal concentration of TOCNF in thissystem is 6 wt% relative to the polymer network.
SEM observation was then carried out to investigate diﬀerences in the microstructure of PIL SN and TOCNF/PIL DN ion
gels. Fig. S3 (ESI†) shows the cross-sectional SEM images forlyophilized PIL SN and TOCNF/PIL DN gels after extraction of IL
from the matrix. The microstructure of the PIL SN gel was aSoft Matter, 2023, 19, 2745–2754 | 2747

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Paper

Soft Matter

Fig. 3 Mechanical properties of DN ion gels composed of the TOCNF/PIL composite network (50 wt%) with various amounts of TOCNF and[C4mim][TFSI] (50 wt%), and PIL SN ion gels composed of the PIL network (50 wt%) and [C4mim][TFSI] (50 wt%). (a) Uniaxial tensile stress–strain curves of
the DN with various TOCNF concentrations and SN ion gels, (b) fracture strain, (c) fracture stress, and (d) fracture energy. The dashed lines in each figure(b–d) indicate the value of the SN ion gel.

porous PIL network (Fig. S3a, ESI†), while that of the TOCNF/
PIL DN gel was not only a porous PIL structure but also a
TOCNF fibrous network and the TOCNF fibrous network was
distributed throughout the PIL network (Fig. S3b, ESI†). It
should be noted that these SEM images were taken after
freeze-drying, and thus the domain sizes of the microstructurein the SEM images may diﬀer from the as-prepared ion gels.
However, SEM observation clearly shows that there are clear
diﬀerences in the microstructure of PIL SN and TOCNF/PIL DN
gels and that TOCNFs form a fibrous network in the DN gels.
Judging from the shear thinning behavior of TOCNF in IL(Fig. 1b), the network formation of TOCNF in the DN ion gels
would be due to the formation of physical bonds based onstrong attractive forces such as hydrophobic interaction andhydrogen bonding. To further confirm the physical network
formation of TOCNF in the DN ion gel, the temperaturedependence of G 0 of the DN ion gels was evaluated by oscillatory shear measurement at diﬀerent temperatures (Fig. 4). ThePIL SN ion gel showed a weak frequency dependence and aweak temperature dependence (Fig. 4a). On the other hand, theTOCNF/PIL DN ion gel showed strong frequency and temperature dependence (Fig. 4b). The G 0 decreased with increasing
measurement temperature, indicating that the physical bonding between TOCNFs weakened with increasing temperature.
These results strongly suggest that the physically cross-linked

2748 | Soft Matter, 2023, 19, 2745–2754

TOCNF fibrous network is formed before the PIL network isformed, and the TOCNF network could have physical adsorption to the PIL chains.
To confirm the toughening mechanism of the ion gel byintroducing TOCNF, cyclic tensile stress loading–unloading
experiments were conducted for the TOCNF/PIL DN and PIL
SN ion gels, as shown in Fig. 5a and b, respectively. The TOCNF/
PIL DN ion gel clearly showed a hysteresis loop in the cyclic
stress–strain curves and softening behavior of the ion gel when
the loading was applied (Fig. 5a), whereas the PIL SN ion gel didnot (Fig. 5b). The hysteresis loop observed in the stress–strain
curve of the TOCNF/PIL DN ion gel is derived from the partial
deformation or destruction of the TOCNF network when theloading energy is applied on the gel, during which the PIL
network can be stretched without critical crack formation that
leads to the tearing of the gel. This result strongly implies thatthe toughening mechanism of the TOCNF/PIL DN ion gel isbased on the energy dissipation derived from the disruption of
the physically cross-linked TOCNF network when a large strain
is induced.
To investigate the eﬀect of the IL content on the mechanicalproperties of the TOCNF/PIL DN ion gels, the DN ion gels withvarious [C4mim][TFSI] concentrations (50, 60, 70, 80 wt%) wereprepared in the same manner as described above, and the uniaxial
stretching experiments were performed. The concentrations of theThis journal is © The Royal Society of Chemistry 2023

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Soft Matter

Paper

Fig. 4 Frequency dependence of storage modulus (G 0 ) and loss modulus (G00 ) of (a) PIL SN ion gel and (b) TOCNF/PIL DN ion gel measured at diﬀerent
temperatures. All ion gels had [C4mim][TFSI] as IL. The IL content in the PIL SN and TOCNF/PIL DN ion gels was approximately 50 wt%. The TOCNF
content in the TOCNF/PIL DN ion gels was 6 wt% relative to the PIL network.

Fig. 5 Cyclic tensile stress–strain curves of (a) TOCNF/PIL DN ion gel and(b) PIL SN ion gel. The DN ion gel is composed of the TOCNF/PIL composite
network (50 wt%) and [C4mim][TFSI] (50 wt%), and the PIL SN ion gel iscomposed of the PIL network (50 wt%) and [C4mim][TFSI] (50 wt%).

cross-linker and TOCNF relative to the polymer component were
fixed at 0.75 mol% and 6 wt%, respectively, based on the resultsof our preliminary uniaxial stretching experiments. As shown

This journal is © The Royal Society of Chemistry 2023

in Fig. 6a, the fracture stress of the DN ion gel decreased as the
IL content increased from 50 to 80 wt%, while the fracture strain
increased. The reduction in the mechanical properties of the DN
ion gels was caused by the decrease in the amount of the PIL
network in the ion gels. Although the DN ion gel with 80 wt% IL
showed the weakest with the fracture stress of approximately
50 kPa, the gel was self-standing, and leakage of the IL from thegel was not observed. Fig. 6b–d show the cyclic tensile stressloading–unloading curves of the DN ion gels with various amountsof IL. The DN ion gels clearly show hysteresis loops in the cyclic
stress–strain curves, regardless of the IL content in the gels. This
result indicates that the TOCNF network can be formed in the ion
gel and it contributes to energy dissipation to toughen the ion gels.
PIL-based ion gels exhibit high thermal stability because all
the components of the ion gel have high thermal stability. To
investigate the thermal stability of the PIL-based DN ion gels,thermogravimetric analysis was conducted for the PIL SN andTOCNF/PIL DN ion gels (Fig. 7). The decomposition temperatures (Td10) of the PIL SN and TOCNF/PIL DN ion gels were
399 1C and 391 1C, respectively. Although the introduction of
TOCNF in the PIL-based ion gel slightly decreased the Td10, thevalue was higher than that of the previously reported ion gels.49
To evaluate the eﬀect of TOCNF on the ionic conductivity of
the gels, the ionic conductivity of PIL SN and 6 wt% TOCNF/PIL
DN ion gels with 50 wt% IL content was investigated by
impedance analyzer. PIL SN ion gel and 6 wt% TOCNF/PIL
DN ion gel showed the same level of ionic conductivity with
1.63  104 and 1.45  104 S cm1 at 20 1C, respectively. This
result indicates that the addition of TOCNF to PIL ion gels
would not aﬀect the conductivity. However, these values wereapproximately one order of magnitude lower than that of thepure [C4mim][TFSI] (2.30  103 S cm1 at 20 1C). This may be
due to the increase in IL viscosity in the gels, resulting in highelectrical resistance. In order to increase the ionic conductivity
for the ion gels with and without TOCNF, the next step will beto properly select IL species with high ionic conductivity andincrease the IL content while maintaining the toughness ofthe gels.

Soft Matter, 2023, 19, 2745–2754 | 2749

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Paper

Soft Matter

Fig. 6 Mechanical properties of the DN ion gels with various amounts of [C4mim][TFSI]. The concentrations of the cross-linker and TOCNF are fixed at
0.75 mol% and 6 wt% relative to polymer concentration, respectively. (a) Uniaxial tensile stress–strain curves of the DN gels with various IL contents, and(b–d) cyclic tensile stress–strain curves of the DN ion gels.

Fig. 7 Thermogravimetric analysis curves of the TOCNF/PIL DN ion gel
and PIL SN ion gel. The IL contents in both gels were 50 wt%.

In our previous study, we have demonstrated that theaddition of fumed silica nanoparticles (silica NPs) into a PILbased ion gel enhanced its mechanical properties through theformation of a DN.38 This toughening mechanism is based onenergy dissipation through the rupture of a partially clusteredsilica NP network upon the application of loading energy; this2750 | Soft Matter, 2023, 19, 2745–2754

mechanism is similar to the toughening mechanism of the
TOCNF/PIL DN ion gel as discussed above. In this study, we
compared the mechanical properties of the TOCNF/PIL DN ion
gel and the silica NP/PIL DN ion gel. Fig. 8a shows the stress–
strain curves of the TOCNF/PIL DN ion gel, silica NP/PIL DN ion
gel, and PIL SN ion gel. The TOCNF/PIL DN ion gel exhibited
approximately two times higher fracture stress than the silica
NP/PIL DN ion gel, while their fracture strain was almost
the same (500%). Therefore, the fracture energy of the
TOCNF/PIL DN ion gel (773  43 kJ m3) was approximately
two times higher than that of the silica NP/PIL DN ion gel(390  53 kJ m3). These results indicate that there are no
significant differences between the stretchability and Young’s
moduli of the TOCNF/PIL and silica NP/PIL DN ion gels, andthat similar cross-linked PIL networks can be formed in both
ion gels. The main difference in the mechanical properties of
the DN ion gels is the increase in stress when the strain
increases. This observation indicates that there may be adifference in the degree of energy dissipation through thedestruction of the physically cross-linked nanomaterial network. To estimate the difference in the degree of energy
dissipation by the nanomaterial networks in the DN ion gels,
we calculated the dissipated energy per cycle in the cyclic stressloading–unloading experiments for the TOCNF/PIL DN ion gelsand silica NP/PIL DN ion gels from the area of each hysteresis

This journal is © The Royal Society of Chemistry 2023

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Soft Matter

Paper

Fig. 8 Comparison of the mechanical properties of the TOCNF/PIL DN ion gel, silica NP/PIL DN ion gel, and PIL SN ion gel. The concentration of thenanomaterial is fixed at 6 wt% relative to the polymer network. (a) Uniaxial tensile stress–strain curves, (b) dissipated energy as a function of cycle number
during the cyclic tensile measurements.

loop cycle, as shown in Fig. 8b. In both DN ion gels, the amountof energy dissipated increased with increasing number ofcycles. Moreover, the TOCNF/PIL DN ion gel dissipated moreenergy per cycle than the silica NP/PIL DN ion gel when thesame amount of strain was applied to the gels. This may haveoccurred due to the high network density of TOCNF. We assumed
that fiber-shaped TOCNFs (aspect ratio 4 30) interact more easily
with each other than sphere-shaped silica NPs (aspect ratio B 1),
leading to the larger apparent cross-linking density of the TOCNF
network compared to the silica NP network. When a loadingenergy is applied, the TOCNF network in the ion gel can be
destroyed more easily than silica NP network, while maintaining
the PIL network without breakage. This increases the energy
dissipation and fracture stress. This indicates that fiber-shaped
nanomaterials are more effective for toughening the ion gels as
sacrificial networks than sphere-shaped nanomaterials. However,the various interactions among nanomaterials, ILs, and PIL
networks need to be quantitatively analyzed in future studies.
The toughening of ion gels is a major topic in the field andhas therefore received considerable attention over the last few
decades. The most common approach to toughening ion gels isby analogy with hydrogels. For example, Fujii et al. developed iongels with conductivity as high as pure IL by using much lower
concentrations of a homogeneous tetra-PEG network (3–6 wt%).22
In addition, Tang et al. developed DN ion gels consisting of achemically cross-linked polymer network and a physically crosslinked polymer network, which exhibited high toughness, selfhealing ability and relatively high ionic conductivity.32 Kamio
et al. proposed DN ion gels composed of a physically crosslinked silica nanoparticle (nanomaterial) network and a chemically cross-linked polymer network,34 and recently developed
tough ion gels by incorporating zeolitic imidazole framework-8(ZIF-8) nanoparticles into a poly(N,N-dimethylacrylamide)-based
polymer network. ZIF-8 nanoparticles serve as multifunctional
reversible crosslinkers, imparting toughness and self-healing
properties to the gels.50 In this study, we reported TOCNF/PIL
DN ion gels, which belong to the nanomaterial/polymer DN ion
gel series and can be easily prepared by adding nanomaterial toThis journal is © The Royal Society of Chemistry 2023

the sol solution. These gels exhibited excellent thermal stability
and higher toughness than silica nanoparticle/PIL DN ion gels
previously reported by our group.38 Although it is difficult tomake a fair comparison of the properties of ion gels such as
toughness, ionic conductivity, and CO2 separation due to their
sensitivity to compositional variables such as IL species, IL
content and viscoelastic properties, our results show that introducing fiber-shaped nanomaterial into ion gels as the first
network can be an effective method of toughening as it induces
large energy dissipation. Depending on the task-specific application of the gels, further fine-tuning of their chemical composition may be required.

Conclusions
We demonstrated that the incorporation of TOCNF in a PILbased ion gel enhances its mechanical properties through theformation of a DN. Uniaxial stretching experiments showedthat the fracture energy of the TOCNF/PIL DN ion gel was
19 times higher than that of the PIL SN ion gel (without
TOCNF). Moreover, the fracture energy increased as the TOCNF
concentration in the ion gel increased up to 6 wt%. Cyclic stressloading–unloading experiments of the TOCNF/PIL DN ion gel
clearly showed a hysteresis loop, indicating that the toughening
mechanism involves energy dissipation. The physically crosslinked TOCNF network can be preferentially destroyed when aloading energy is applied on the gel, during which the chemically cross-linked PIL network is stretched without breaking thegel. This toughening mechanism is similar to that of the silica
NP/PIL DN ion gel, which was developed in our previous study.
However, the toughness of the TOCNF/PIL DN ion gel was twotimes larger than that of the silica NP/PIL DN ion gel. This may
be due to the fiber-shaped structure of the TOCNFs, which caneasily interact with each other and dissipate more energy thanspherical silica NPs. These findings suggest that the incorporation of TOCNF into the PIL-based ion gels can effectively improvetheir mechanical properties. Consequently, these tough ion gels

Soft Matter, 2023, 19, 2745–2754 | 2751

View Article Online

Paper

Experimental section

24 h to remove any residual salt. The product yield of [(Vim)2C4][TFSI]2 was 4.2 g (69.5%), as shown in the 1H NMR spectrum in
Fig. S2b (ESI†). The successful anion exchange from Br to TFSI was
confirmed by peak shifting from a0 (9.46 ppm, 2H in Fig. S2b,
ESI†) to a (9.40 ppm, 2H in Fig. S2a, ESI†).

Materials

Preparation of TOCNF/PIL DN ion gels

A 7.8 wt% dispersion of TEMPO-oxidized cellulose nanofiberwith hydrophobic groups (TOCNF) in ethyl acetate was kindly
provided by Nippon Paper Industries Co., Ltd. The diameter andlength of the TOCNF were 32.5  5.8 nm and larger than 1 mm,
respectively. 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][TFSI]) and 1-ethyl-3-vinylimidazoliumbis(trifluoromethanesulfonyl)imide ([C2vim][TFSI]) were purchased from FUJIFILM Wako Pure Chemical Corp. (Osaka, Japan)
and used as received. In addition, 1-vinylimidazole and 1,4dibromobutane were also purchased from FUJIFILM Wako
Pure Chemical Corp. (Osaka, Japan) and used without further
purification. Ethanol, diethyl ether, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were purchased from FUJIFILM Wako Pure Chemical Corp. (Osaka, Japan) and used asreceived. Furthermore, 2-Hydroxy-4 0 -(2-hydroxyethoxy)-2-methylpropiophenone (Cibas IRGACUREs 2959), which was purchasedfrom Ciba Specialty Chemicals (Switzerland), was used as aphotoinitiator to synthesize the PC2im-TFSI network in an IL.
The deionized water used in all experiments was produced using
an Elix UV purification system (Millipore, Japan). The chemical
structures of [C4mim][TFSI], [C2vim][TFSI], Cibas IRGACUREs
2959, [(Vim)2C4]Br2, and [(Vim)2C4][Tf2N]2 are shown in Fig. S1(ESI†).

The TOCNF/PILs DN ion gels were prepared via the photopolymerization of an ionic monomer ([C2vim][TFSI]) in the presenceof an ionic liquid ([C4mim][TFSI]), ethyl acetate, and TOCNF/
ethyl acetate dispersion. Ethyl acetate was then removed by
vacuum drying at 40 1C. The typical experimental procedure forthe preparation of the TOCNF/PIL DN ion gel is described asfollows. First, a 4.7 wt% TOCNF dispersion in ethyl acetate wasdried at 40 1C under reduced pressure for 1 hour to prepare a
15.1 wt% TOCNF dispersion in ethyl acetate. The 15.1 wt%
TOCNF dispersion (1.013 g) was then mixed with [C4mim][TFSI](3.5 g) using a vortex mixer for 1 h to obtain a homogeneous
TOCNF dispersion, followed by drying at 40 1C under reducedpressure for 1 h to remove excess ethyl acetate. Next,[C2vim][TFSI] (3.0 g), [(Vim)2C4][TFSI]2 (0.021 g), and Irgacure2959 (0.0016 g) were added, and mixed using a vortex mixer,
and stirred to prepare the precursor solution. The precursor
solution was injected into a mold composed of two glass plateswith an FEP film and a PTFE spacer with 1 mm thickness andwas irradiated with 365 nm UV light for 13 h (1820 mW cm2).
The product was dried in vacuo at 40 1C for more than 3 h. The
IL content after drying was fixed at 50 wt%.

can be applied in carbon dioxide separation membranes and soft
electronics such as polyelectrolytes, sensors, and actuators.

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Soft Matter

Synthesis of 1,4-bis(3-vinylimidazolium-1-yl) butanebis(trifluoromethanesulfonyl)imide [(Vim)2C4][TFSI]2 as across-linking reagent containing ionic liquid groups
A cross-linking reagent, [(Vim)2C4][TFSI]2, containing ionicliquid groups, was synthesized by reacting 1-vinylimidazole with
1,4-dibromobutane, followed by an anion exchange reaction with
LiTFSI (as shown in Scheme S1, ESI†) In this process,1-vinylimidazole (0.10 mol, 9.1 mL) was mixed with ethanol(7.5 mL) in a round-bottom flask (150 mL) that had been dried
and purged with argon. Subsequently, 1,4-dibromobutane(0.055 mol, 6.8 mL) was added dropwise to the flask and the
mixture was stirred at 70 1C under an argon atmosphere for
24 h. The reaction mixture was then precipitated in ethyl acetate(200 mL) three times to remove any unreacted reagents. The
purified product was dried at 50 1C under reduced pressure
overnight, yielding 20.1 g of [(Vim)2C4]Br2 in 99% yield, as
shown in the proton nuclear magnetic resonance (1H NMR)
spectrum in Fig. S2a (ESI†). The anion exchange reaction of[(Vim)2C4]Br2 with LiTFSI was then performed by mixing 60 mLof an aqueous solution containing [(Vim)2C4]Br2 (7.5 mmol, 3.03 g)with 60 mL of an aqueous solution containing LiTFSI (16.5 mmol,
4.74 g). The resultant mixture was stirred at room temperature for
24 h under an argon atmosphere to exchange the Br2 counter
anion with the [TFSI]2 anions. The resulting [(Vim)2C4][TFSI]2 waswashed three times with pure water (100 mL) and freeze dried for2752 | Soft Matter, 2023, 19, 2745–2754

Mechanical property measurement of ion gels
The mechanical properties of the ion gels were evaluated using
an automatic recording universal testing instrument (EZ Test
EZ-SX 500 N, Shimadzu Co., Japan) at 25 1C. To conduct thestretching test, dumbbell-shaped specimens with lengths,
widths, and thicknesses of 35, 2, and 1.0 mm, respectively, wereused. The fabricated sample was attached to the instrument at
15 mm between the jigs. The uniaxial stretching test was
conducted by stretching the sample at a constant strain rateof 50 mm min1. The stress–strain curves were recorded automatically until the specimens were broken. The fracture stress,
fracture strain and fracture energy of the ion gels were obtained
as the mean values of the measurements in triplicate for eachsample. In the cyclic tensile test, the loading and unloading
operations were performed, and the stress–strain curves wererecorded until the sample broke, while gradually increasing thestretching strain in intervals of 0.3.
Rheological measurement of TOCNF/[C4mim][TFSI] dispersion
A TOCNF dispersion in ethyl acetate (4.71 wt%) was dried invacuo at 40 1C for 1 h to remove the excess ethyl acetate, whichaﬀorded a 14.7 wt% TOCNF dispersion in ethyl acetate. The
TOCNF dispersion was then mixed with [C4mim][TFSI] using avortex mixer for 1 h to obtain a homogeneous TOCNF/[C4mim][TFSI] dispersion. The dynamic frequency sweep andviscosity measurements of the TOCNF/[C4mim][TFSI] dispersion were performed using a rheometer (MCR302, Anton Paar,

This journal is © The Royal Society of Chemistry 2023

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Soft Matter
Japan). The viscosity dependence of the shear of the TOCNF/[C4mim][TFSI] dispersion as a sample was evaluated using arheometer with a parallel-plate geometry (PP50; diameter =50 mm, gap = 0.3 mm) at 25 1C. A similar measurement was
conducted using [C4mim][TFSI] as the control. After preshearing the samples at 1000 s1 for 60 s, the change in theviscosity of the solution was measured as a function of theshear rate from 1000 to 0.01 s1. Dynamic frequency sweep
measurements of the TOCNF/[C4mim][TFSI] dispersion were
performed with a parallel-plate geometry (PP25; diameter =25 mm, gap = 0.3 mm). The measurements were performedin the angular frequency range of 0.6–628 rad s1 at 20 1C andstrain level of 0.1%, respectively. The temperature dependenceof the viscoelastic behavior of the gels was measured with aparallel-plate geometry (PP25; diameter = 25 mm, gap =0.1 mm) in the angular frequency range of 0.1–1000 rad s1
at different temperatures ranging from 20 to 50 1C in 5 1C
increments, with a strain level of 1%.
Thermal gravimetric (TG) measurements of the samples
TG measurements of the samples were conducted using open
Al pans on a thermogravimetry/diﬀerential thermal analyzer(DTG-60, Shimadzu, Japan) from 25 to 600 1C at a heating rateof 10 1C min1 under a nitrogen atmosphere. The weight ofeach sample was fixed at 5 mg. The DN ion gel samples were
heated from room temperature to 700 1C at a heating rate of
10 1C min1 under a nitrogen atmosphere.
SEM observation of the samples
The morphology of the samples was observed with a fieldemission scanning electron microscope (SEM, S-4700, HitachiLtd., Japan) at a voltage intensity of 2 kV. Prior to observation, asmall piece of ion gel was washed three times with acetone toremove IL and then immersed in a large amount of tert-butyl
alcohol. The sample dispersion was then freeze-dried overnight. The lyophilized sample was cut with a blade and thensputter-coated (E-1030 Ion-Sputter, Hitachi Ltd., Japan) with
Pd/Pt for 1 min to reduce the sample charge. The cut surface
was observed with SEM.
Ionic conductivity measurements of the samples
The ionic conductivity of the gel films was measured by the
impedance analyzer using LCR meter (IM3536, Hioki, Japan) in
the frequency range 4 Hz –10 MHz. The ionic conductivity(s [mS cm1]) was then calculated as s = l/(Ra), where l is the
sample thickness [cm], a is the surface area of the sample [cm2],
and R is the bulk resistance determined from the real part ofthe impedance, Z 0 , in the high-frequency plateau [O].

Author contributions
T. W., E. O., Y. M., and T. O. designed the experiments. T. W., E.
O. and Y. M. performed the experiments and analyzed the data.
T. W., E. O., Y. M., and T. O. interpreted the results and wrote
the manuscript. T. W. and T. O designed the research.

This journal is © The Royal Society of Chemistry 2023

Paper

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
This work was partly supported by the JSPS KAKENHI (grant
numbers: JP20KK0325 and JP21H04629) and the New Energy
and Industrial Technology Development Organization (NEDO)(grant number: JPNP20004). We also thank Nippon Paper
Industries Co., Ltd. (Japan) for supplying the TOCNF dispersion
in ethyl acetate.

References
1 S. Zhang, K. H. Lee, J. Sun, C. D. Frisbie and T. P. Lodge,
Macromolecules, 2011, 44, 8981–8989.
2 Y. J. Son, J. W. Bae, H. J. Lee, S. Bae, S. Baik, K.-Y. Chun and
C.-S. Han, J. Mater. Chem. A, 2020, 8, 6013–6021.
3 K. Shahzadi, W. Xiong, M. Shekh, F. J. Stadler and Z.-C. Yan,ACS Appl. Mater. Interfaces, 2020, 12, 49050–49060.
4 B. Tang, S. P. White, C. D. Frisbie and T. P. Lodge, Macromolecules, 2015, 48, 4942–4950.
5 T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto,T. Takigawa, N. Ishii and T. Aida, Science, 2003, 300, 2072–2074.
6 Y. Wang, Y. Chen, J. Gao, H. G. Yoon, L. Jin, M. Forsyth,T. J. Dingemans and L. A. Madsen, Adv. Mater., 2016, 28,
2571–2578.
7 M. Que, Y. Tong, G. Wei, K. Yuan, J. Wei, Y. Jiang, H. Zhu
and Y. Chen, J. Mater. Chem. A, 2016, 4, 14132–14140.
8 B. Huang, Y. Zhang, M. Que, Y. Xiao, Y. Jiang, K. Yuan and
Y. Chen, RSC Adv., 2017, 7, 54391–54398.
9 T. Ueki, R. Usui, Y. Kitazawa, T. P. Lodge and M. Watanabe,
Macromolecules, 2015, 48, 5928–5933.
10 Y. Isano, H. Fujita, K. Murakami, S. Ni, Y. Kurotaki,
T. Takano, Y. Isoda, R. Matsuda, F. Nakamura, Y. Nishitai,
N. Ochirkhuyag, K. Inoue, H. Kawakami, Y. Okubo, K. Ueno,
T. Fujie and H. Ota, Adv. Mater. Technol., 2022, 7, 2200209.
11 B. Tang, D. K. Schneiderman, F. Zare Bidoky, C. D. Frisbie
and T. P. Lodge, ACS Macro Lett., 2017, 6, 1083–1088.
12 L. Yu, L. Yu, Q. Liu, T. Meng, S. Wang and X. Hu, Adv. Funct.
Mater., 2022, 32, 2110653.
13 F. Chen, C. Guo, H. Zhou, M. W. Shahzad, T. X. Liu,
S. Oleksandr, J. Sun, S. Dai and B. B. Xu, Small, 2022,
18, e2106352.
14 X. Yang, F. Zhang, L. Zhang, T. Zhang, Y. Huang and
Y. Chen, Adv. Funct. Mater., 2013, 23, 3353–3360.
15 D. Lee, Y. Song, Y. Song, S. J. Oh, U. H. Choi and J. Kim, Adv.
Funct. Mater., 2022, 32, 2109907.
16 J. Zhang, E. Kamio, A. Matsuoka, K. Nakagawa, T. Yoshioka
and H. Matsuyama, Ind. Eng. Chem. Res., 2021, 60,
12640–12649.
17 A. R. Nabais, L. A. Neves and L. C. Tomé, ACS Appl. Polym.
Mater., 2022, 4, 3098–3119.
18 M. G. Cowan, D. L. Gin and R. D. Noble, Acc. Chem. Res.,
2016, 49, 724–732.

Soft Matter, 2023, 19, 2745–2754 | 2753

View Article Online

Open Access Article. Published on 23 March 2023. Downloaded on 9/26/2024 1:40:09 AM.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Paper
19 E. Kamio, M. Minakata, Y. Iida, T. Yasui, A. Matsuoka and
H. Matsuyama, Polym. J., 2021, 53, 137–147.
20 C. Wang, W. Dong, P. Li, Y. Wang, H. Tu, S. Tan, Y. Wu and
M. Watanabe, ACS Appl. Mater. Interfaces, 2020, 12,42202–42209.
21 S. Imaizumi, H. Kokubo and M. Watanabe, Macromolecules,
2012, 45, 401–409.
22 K. Fujii, H. Asai, T. Ueki, T. Sakai, S. Imaizumi, U.-I. Chung,
M. Watanabe and M. Shibayama, Soft Matter, 2012, 8,
1756–1759.
23 H. Asai, K. Fujii, T. Ueki, T. Sakai, U.-I. Chung, M. Watanabe,
Y.-S. Han, T.-H. Kim and M. Shibayama, Macromolecules,
2012, 45, 3902–3909.
24 Y. Gu, S. Zhang, L. Martinetti, K. H. Lee, L. D. Mcintosh,
C. D. Frisbie and T. P. Lodge, J. Am. Chem. Soc., 2013, 135,
9652–9655.
25 R. Tamate, K. Hashimoto, T. Horii, M. Hirasawa, X. Li,
M. Shibayama and M. Watanabe, Adv. Mater., 2018, 30,
1–7.
26 Y. Kamiyama, R. Tamate, T. Hiroi, S. Samitsu, K. Fujii andT. Ueki, Sci. Adv., 2022, 8, eadd0226.
27 Y. Kamiyama, R. Tamate, K. Fujii and T. Ueki, Soft Matter,
2022, 18, 8582–8590.
28 M. Wang, P. Zhang, M. Shamsi, J. L. Thelen, W. Qian,
V. K. Truong, J. Ma, J. Hu and M. D. Dickey, Nat. Mater.,
2022, 21, 359–365.
29 J. P. Gong, Y. Katsuyama, T. Kurokawa and Y. Osada, Adv.
Mater., 2003, 15, 1155–1158.
30 J. P. Gong, Soft Matter, 2010, 6, 2583–2590.
31 T. Yasui, Y. Zheng, T. Nakajima, E. Kamio, H. Matsuyama
and J. P. Gong, Macromolecules, 2022, 55, 9547–9557.
32 Z. Tang, X. Lyu, A. Xiao, Z. Shen and X. Fan, Chem. Mater.,
2018, 30, 7752–7759.
33 D. Weng, F. Xu, X. Li, S. Li, Y. Li and J. Sun, ACS Appl. Mater.
Interfaces, 2020, 12, 57477–57485.

2754 | Soft Matter, 2023, 19, 2745–2754

Soft Matter
34 E. Kamio, T. Yasui, Y. Iida, J. P. Gong and H. Matsuyama,Adv. Mater., 2017, 29, 1704118.
35 T. Yasui, E. Kamio and H. Matsuyama, RSC Adv., 2020, 10,14451–14457.
36 T. Yasui, E. Kamio and H. Matsuyama, RSC Adv., 2019, 9,11870–11876.
37 E. Kamio, M. Kinoshita, T. Yasui, T. P. Lodge and
H. Matsuyama, Macromolecules, 2020, 53, 8529–8538.
38 T. Watanabe, R. Takahashi and T. Ono, Soft Matter, 2020,
16, 1572–1581.
39 A. Isogai, T. Saito and H. Fukuzumi, Nanoscale, 2011, 3, 71–85.
40 F. Lossada, D. Hoenders, J. Guo, D. Jiao and A. Walther, Acc.
Chem. Res., 2020, 53, 2622–2635.
41 P. Xi, L. Wu, F. Quan, Y. Xia, K. Fang and Y. Jiang, ACS Appl.
Mater. Interfaces, 2022, 14, 24787–24797.
42 Y. Jiao, K. Lu, Y. Lu, Y. Yue, X. Xu, H. Xiao, J. Li and J. Han,
Cellulose, 2021, 28, 4295–4311.
43 H. Takeno, H. Inoguchi and W.-C. Hsieh, Mater. Today
Commun., 2022, 31, 103495.
44 C. Hopson, M. M. Villar-Chavero, J. C. Domı́nguez, M. V.
Alonso, M. Oliet and F. Rodriguez, Carbohydr. Polym., 2021,
274, 118663.
45 M. Vranes, S. Dozic, V. Djeric and S. Gadzuric, J. Chem. Eng.
Data, 2012, 57, 1072–1077.
46 K. Ueno, K. Hata, T. Katakabe, M. Kondoh and M. Watanabe,
J. Phys. Chem. B, 2008, 112, 9013–9019.
47 Y. S. Pek, A. C. A. Wan, A. Shekaran, L. Zhuo and J. Y. Ying,
Nat. Nanotechnol., 2008, 3, 671–675.
48 M. Ognyanov, C. Remoroza, H. A. Schols, Y. N. Georgiev,
N. T. Petkova and M. Krystyjan, Carbohydr. Polym., 2020,
229, 115549.
49 M. A. B. H. Susan, T. Kaneko, A. Noda and M. Watanabe,
J. Am. Chem. Soc., 2005, 127, 4976–4983.
50 E. Kamio, M. Minakata, H. Nakamura, A. Matsuoka and
H. Matsuyama, Soft Matter, 2022, 18, 4725–4736.

This journal is © The Royal Society of Chemistry 2023