High-Performance Double-Network Ion Gels with Fast Thermal

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High-Performance Double-Network Ion Gels with Fast Thermal Healing Capability via Dynamic Covalent Bonds Zhehao Tang, Xiaolin Lyu, Anqi Xiao, Zhihao Shen, and Xinghe Fan Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Chemistry of Materials

High-Performance Double-Network Ion Gels with Fast Thermal Healing Capability via Dynamic Covalent Bonds Zhehao Tang, Xiaolin Lyu, Anqi Xiao, Zhihao Shen* and Xinghe Fan* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ABSTRACT:

A

tough

methacrylate-co-methyl

double-network

(DN)

ion

gel

composed

methacrylate)

(P(FMA-co-MMA))

and

of

chemically

physically

crosslinked

crosslinked

poly(furfuryl

poly(vinylidene

fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) networks with 80 wt% of ionic liquid (IL) was fabricated via a one-pot method. This ion gel exhibits excellent mechanical strength and considerable ionic conductivity, which can be used as a solid gel electrolyte. By adjusting the weight ratio of P(FMA-co-MMA) to P(VDF-co-HFP) and the content of the crosslinker, remarkably robust DN ion gel (failure tensile stress 660 kPa, strain 268%; failure compressive stress 17 MPa, strain 85%) was obtained. The high mechanical strength is attributed to the chemical/physical interpenetrating networks. The rigid chemically crosslinked P(FMA-co-MMA) network dissipates most of the loading energy, and the ductile physically crosslinked P(VDF-co-HFP) network provides stretchability for the whole gel. More importantly, the P(FMA-co-MMA) network is formed by dynamic covalent bonds that can undergo a thermally reversible reaction, giving the gel a unique and effective thermal healing capability. Furthermore, with the high content of IL, the DN ion gel possesses a high ionic conductivity of 3.3 mS cm1 at room temperature, which is higher than those of most solid polymer electrolytes and comparable to commercial organic liquid electrolytes.

better flexibility, but their ionic conductivity is relatively

INTRODUCTION In the past two decades, great progresses have been achieved on smart devices like smart phones, home automation, and wearable devices. Many impressive advances in electronic technology have led to the development of thinner, lighter, and softer smart devices.1 Significantly, flexible electronics such as artificial e-skin,2 flexible display,3 and flexible energy storage devices,4-5 are the next generation of electronic devices and may bring a new

electronic

technology

revolution.

The

key

components of flexible devices are flexible materials. Although

traditional

inorganic

conductors

and

semiconductors can be constructed into structures of wavy patterns, buckles, and others to realize satisfactory stretchability through geometric designs, the complex construction processes limit their applications.6 As an alternative, soft solid polymer electrolytes (SPEs) have

poor.7-9 Researchers are striving to find new materials that combine excellent flexibility and outstanding electrical performance. Ionic liquids (ILs),10-11 mostly composed of inorganic cations and organic anions, maintain the liquid state at room temperature. They have attracted much interest owing

to

their

high

ionic

conductivity,

wide

electrochemical window, and outstanding stability (both electrochemical and thermal). More importantly, ILs have negligible vapor pressures and are non-flammable, which greatly improves the safety in use. Mixing ILs with crosslinked polymers,12-14 low-molecular-weight organic gelators,15-16 or inorganic scaffold,17-18 can produce ion gels with both flexibility and excellent electrical properties. Many new flexible electrical devices containing ion gels have demonstrated their own special properties and applied

ACS Paragon Plus Environment

in

electrochromic

devices,19-20

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrochemiluminescent electrolytes,22

devices,21

solid-state

Page 2 of 12

polytetrafluoroethylene

(PTFE)

mold

instead

of

a

etc. However, the ion gels obtained in early

completely sealed reactor, which is favorable for mass

studies were easily broken attributed to their fragile

production. Third, because of the thermal-reversible

structures, which limits their preparation and applications.

dynamic

Researchers have studied and developed a variety of

containing this network underwent a transition from the

strategies to increase the strength of ion

gels.23-25

covalent

furan-maleimide

bond,

the

gel

static state to the dynamic state with increasing

The construction of a double-network (DN) polymer

temperature and showed the ability of thermal healing.

37

framework can significantly increase the mechanical

Therefore, this kind of DN ion gel that can be simply

strength of the hydrogels, which was first proposed by

prepared will become a new type of high-strength,

Gong

recyclable and repairable soft material.

et

al.

in

2003.26

Meanwhile,

the

unique

characteristics of various functional networks can also give the resulting gels many outstanding features, such as self-healing,27 shape memory,28 super adhesion,29 and so

EXPERIMENTAL SECTION Materials

on. Based on such a design, many researchers have

Methyl methacrylate (MMA, 98%) was purchased from

introduced DN structures into ion gels and successfully

Beijing Yili Fine Chemicals and purified by neutral

prepared some high-performance DN ion gels.30-36 They

alumina column chromatography to remove the inhibitor.

used the two-step in-situ free-radical polymerization that

Poly(vinylidene

is

with

(P(VDF-co-HFP)) was purchased from Sigma-Aldrich.

low-molecular-weight gelators or inorganic particles to

Bromoethane (99%) and furfuryl alcohol (97%) were

construct

results

purchased from Aladdin. Methacryloyl chloride (97%),

indicate that this strategy can greatly and easily improve

N,N'-(4,4'-diphenylmethane)bismaleimide (BMI, 98%),

the mechanical properties of ion gels.

and

easy

to

operate

or

double-network

to

combine

structures.

These

Herein, we report a new kind of high-performance and thermal-healable

DN

ion

gel

containing

two

interpenetrating networks: the rigid and brittle chemically crosslinked

poly(furfuryl

methacrylate-co-methyl

methacrylate) (P(FMA-co-MMA)) as the first network; the

fluoride-co-hexafluoropropylene)

1-Ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI], 98%) were purchased from Energy Chemical. All reagents were used as received without purification unless otherwise noted.

Synthesis of furan-2-ylmethyl methacrylate (FMA)

soft and ductile physically crosslinked poly(vinylidene

Furfuryl alcohol (1.00 eq, 9.81 g, 0.100 mol) and

fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) as the

triethylamine (1.50 eq, 15.2 g, 0.150 mol) were dissolved in

second network (Figure 1a). We utilized the thermally

DCM (100 mL). Then the mixture was stirred at 0 oC for 30

reversible Diels-Alder (DA) reaction between furan

min. A dry DCM solution of methacryloyl chloride (1.50 eq,

moieties on the side chain and a multi-maleimide

15.6 g, 0.150 mol) was added dropwise slowly. The mixture

crosslinker to construct the chemically crosslinked

was stirred at 0 oC for another 1 h, and then the reaction

network. While P(VDF-co-HFP) is a widely used soft

was allowed to proceed at ambient temperature for 12 h.

material and is able to form gels in many solvents, its high

Afterward, the mixed solution was filtered to remove the

molecular weight and flexible chain structure give it good

trimethylamine salt. The filtrate was washed with 100 mL

stretchability and flexibility. Our design was based on the

of distilled water for three times and then dried with

following considerations. First, P(FMA-co-MMA) has

anhydrous sodium sulfate. The solution was concentrated

There is no macroscopic phase

under a reduced pressure to give the crude product. The

separation between the polymer and the matrix. Second,

purified product was obtained by silica gel column

the monomer can be easily synthesized and the DA

chromatography with n-hexane/ethyl acetate (10:1, v/v) as

reaction is highly resistant to water and oxygen. Therefore,

eluent to give a colorless liquid. Yield: 82%. 1H NMR (400

the preparation of the DN ion gel can be facilely realized

MHz, CDCl3, δ, ppm): 1.95 (t, 3H), 5.14 (s, 2H), 5.58 (t, 1H),

by

6.13 (t, 1H), 6.36 (m, 1H), 6.42 (d, 1H), 7.43 (m, 1H).

good solubility in

a

simple,

ILs.12

one-pot

method

using

a

common


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Figure 1. The P(FMA-co-MMA)/P(VDF-co-HFP)/[EMIM][TFSI] DN ion gel. (a) Schematic illustrations of the composition of the DN structure and its change after stretching; (b) Photograph of a large DN ion gel thin film with a diameter of 7 cm; (c) Photograph of the abbreviation of Peking University made of the DN ion gel; (d) Photograph of a cylindrical DN ion gel with a height of 1 cm and a diameter of 1.3 cm. (Logos in b and c Copyright 2018 Peking University)

Synthesis of P(FMA-co-MMA) P(FMA-co-MMA)

was

addition-fragmentation polymerization.

The

The high-performance DN ion gel was prepared by

reversible

through a one-pot method (Figure S3 in Supporting

transfer

(RAFT)

Information). It mainly comprises the following steps. At

(CTA),

first,

synthesized chain

chain

transfer

agent

calculated

amounts

of

P(FMA-co-MMA),

(S)-1-dodecyl-(S)-(α,α-dimethyl-α-aceticacid)-trithiocar

P(VDF-co-HFP), and the crosslinker BMI were dissolved

bonate, was synthesized following a reported procedure.38

in acetone. After mixing with an appropriate amount of IL

CTA

mmol),

([EMIM][TFSI]) and stirring for 4 h, the solution was

2,2'-azobis(2-methylpropionitrile) (AIBN, 0.300 eq, 28.0

poured into a PTFE mold, and acetone was volatilized at

mg, 0.170 mmol), MMA (150 eq, 7.50 g, 75.0 mmol), and

room temperature. Finally, we successfully obtained the

the synthesized FMA (50.0 eq, 4.15 g, 25.0 mmol) were

high-strength DN ion gel by heating the precursor at 70

(1.00

eq,

182

mg,

0.500

dissolved in 20 mL of dioxane and then transferred into a

oC

dry polymerization tube. After being dehydrated and

crosslinking reaction (Scheme 1). Taking advantages of the

deoxygenated by freeze-pump-thaw cycles for three times,

simple preparation process, we were able to fabricate a

the polymerization tube was sealed with gas fire. The

variety of DN ion gels having different shapes with the

reaction was allowed to proceed at 70 oC for 18 h. The tube

mold (parts b, c, and d of Figure 1). The prepared DN ion

was immersed into liquid nitrogen to terminate the

gel shows a smooth and dense surface without pores. We

reaction. The mixture was precipitated by adding into 500

used small-angle X-ray scattering (SAXS) and differential

mL of methanol for three times to obtain a pale yellow

scanning

solid. The precipitate was filtered and dried under

homogeneity of the DN ion gels prepared through

vacuum at 60 oC overnight. The molecular weight (MW)

solution blending. Compared to the P(VDF-co-HFP) SN

and MW dispersity (ĐM) were measured by gel

ion gel, there is no additional enhancement in the

permeation chromatography (GPC, Waters, 2414) with

scattering intensity of the sample when the two polymers

tetrahydrofuran (THF) as eluent (Figure S1 in Supporting

are mixed together (Figure S4 in Supporting Information).

Information). The molar ratio of FMA to MMA was

The DSC thermograms only show a single glass transition

determined by

process in the low temperature region (Figure S5 in

1H

NMR (Figure S2 in Supporting

Information).

Preparation of the DN ion gels

for 24 h to form complete the furan- maleimide DA

calorimetry

(DSC)

to

investigate

the

Supporting Information). Moreover, visible macroscopic phase separation did not occur in the samples without



crosslinkers for over ten days (Figure S6 and S7 in

Page 4 of 12

Table 1. Gelation time of P(FMA-co-MMA) ion gels

Supporting Information). Mn (Da)

FMA:MMA b)

Gelation time (min)

a)

Sample-1

25600

1:1

13

Sample-2

22100

1:3

19

Scheme 1. Furan-maleimide DA crosslinking reaction.

Sample-3

22100

1:5

47

RESULTS AND DISCUSSION

Sample-4

20200

1:7

93

Gelation Time of P(FMA-co-MMA) Ion Gels

Sample-5

20600

1:12

∞

A series of P(FMA-co-MMA) samples were synthesized

a)

Determined by GPC; b) Determined by 1H NMR.

by reversible addition-fragmentation chain transfer (RAFT) polymerization and characterized by GPC and 1H NMR

The

(Figure S1 and S2 in Supporting Information). Results

dumb-bell-shaped testing sample with a length of 4 mm

show that polymers with different molar ratios of FMA to

at the narrowest position. Then the tensile stress–strain

MMA have number-averaged molecular weights (Mn’s) of

curves were obtained by stretching the sample at a rate of

about 20000 Da with unimodal distributions. Meanwhile,

5 mm min1. As shown in Figure 2a, with increasing degree

the polymers synthesized are random copolymers, as

of crosslinking, the ion gel film will be tougher, and the

demonstrated by DSC results (Figure S8 in Supporting

elastic modulus (represented by the slope of the curve)

Information). Detailed data are shown in Table S1. Before

will improve because of the enhancement of the rigid

the preparation of the DN ion gel, we need to confirm

structure. However, the highly crosslinked network will

that a stable ion gel can be formed through the DA

weaken the chain entanglement of the second network,

reaction by using this kind of polymer containing the

and it reduces the stretchability of the gel. Meanwhile,

furan moiety. The polymer and the crosslinker were

adding insufficient amount of the crosslinking agent

dissolved in an appropriate amount of ionic liquid (with

cannot form a complete skeleton structure. Therefore, it is

constant

important to choose an appropriate crosslinker content.

concentrations

of

the

polymer

and

the

sample

film

was

cut

into

a

2

cm

long,

crosslinker). After being heated up to 70 oC for a while, all of the mixture solutions gelled except the polymer with an FMA:MMA molar ratio of 1:12, and the gelation time decreases greatly with increasing content of the furan moiety owing to the increasing number of reaction sites in the solution (Table 1). We chose the polymer with an FMA:MMA molar ratio of 1:3 to prepare the DN ion gels because it can form a gel in just 20 min and the cost is lower than sample 1 due to the less content of FMA. The DN ion gel samples prepared in this work were all based on Sample-3.

Mechanical Properties and Toughening Mechanisms The strategy of constructing the double-network can greatly improve the mechanical strength of the gel. We maintained the IL mass fraction of 80% and investigated the effect of the composition of polymer network on the mechanical properties. The film tensile tests were conducted with a universal testing system (5567, Instron).

Figure 2. Comparison of tensile stress–strain curves of DN ion gels with different crosslinker loadings (a) and different mass ratios of the two networks (b).

The P(FMA-co-MMA)/P(VDF-co-HFP) mass ratio (r) is also a key parameter. A series of DN ion gels with different r values were prepared. The results of the tensile tests indicate that the DN ion gel will be more rigid with increasing content of P(VDF-co-HFP) owing to the higher density of the crystalline region of the PVDF segments (Figure 2b). However, it is not beneficial to the formation


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of interpenetrating structures when any component is

Like other DN ion gels and DN hydrogels, the high

excessive. It is worth noting that the mechanical

mechanical strength can be mainly attributed to the two

properties of the ion gel prepared can be controlled by

interpenetrating

networks.39-40 P(FMA-co-MMA) was

adjusting the amount of each component of the gel. For example, to meet the demands of flexible electrical devices for relatively high toughness and stretchability, the

degree

of

crosslinking

and

the

content

of

P(VDF-co-HFP) can be appropriately reduced. Such an adjustability indicates that the gel is a designable material. Finally, the optimal composition of the toughest DN ion gel

was

determined

to

be

P(FMA-co-MMA):P(VDF-co-HFP):BMI = 100:100:5 (mass ratio). The toughest DN ion gel has excellent stretchability with the tensile fracture strain of 268% (parts a and b of Figure 3 and Video VSI1 in Supporting Information), which is about 10 times higher than their individual single-network (SN) counterparts and is similar to those of other DN ion gels that have been reported in the literature such as the inorganic/organic DN ion gel (about 300% strain).31 The failure tensile stress of the DN ion gel prepared is approximately 660 kPa, which is substantially higher than those of most DN ion gels reported in the literature.30-32

It

can

be

attributed

to

the

high

entanglement of the P(VDF-co-HFP) chains. Figure S9

Figure 3. Mechanical properties of the DN ion gels. (a) Tensile stress–strain curves of DN and respective SN ion gels; (b) Photographs of the dumb-bell-shaped DN ion gel film before and after stretching; (c) Compressive stress–strain curves of DN and respective SN ion gels; (d). Photographs of a cylindrical DN ion gel before, during, and after compression.

shows the cyclic tensile behaviors of the DN and SN ion

crosslinked by the reaction between furan and maleimide.

gels.

chemically

Owing to the low molecular weight (MW), the SN

crosslinked network into the P(VDF-co-HFP) SN ion gel

P(FMA-co-MMA) ion gel shows a poor mechanical

could resist the deformation during the tensile test. The

strength (parts a and c of Figure 3) and is easily broken

DN ion gel could easily recover to its original length at a

(Figure S10b in Supporting Information). However, this

relatively small loading.

SN ion gel can maintain its origin shape and is difficult to

It

indicates

that

adding

another

Moreover, the cylindrical gel also has excellent

deform because of its chemical crosslinking bonds.

compressive properties. The compressive tests were

P(VDF-co-HFP) has excellent tensile strength.41-42 When it

conducted with a universal testing system (UTM4104,

is mixed with ILs, part of the short PVDF segments will

Kasontest). The compressive stress–strain curves were

crystallize, with the crystals acting as physical crosslinking

obtained by compressing a cylindrical sample with a

points to form a physical gel.43-44 Although such a gel

height of 1 cm and a diameter of 1.3 cm at a rate of 5 mm

shows good mechanical properties at a high polymer

min1.

The result shows that the gel obtained has the

concentration, the maximum elongation decreases rapidly

failure compressive stress of more than 17 MPa at 85%

when the content of IL is above 40 wt%.43 More

strain (parts c and d of Figure 3), which is comparable to

importantly, the

that of other high-performance ion gels (7.7 MPa at 92%

“sponge”, which means that it cannot hold the IL well. For

strain),30 and it is not easily deformed (Figure S10a in

example, the masses of a cylindrical sample before and

Supporting Information).

after compression are 1.6 g and 0.9 g, respectively. Most of

P(VDF-co-HFP)

SN ion gel is like a

the IL was pushed out, and the gel was deformed severely (Figure S10c in Supporting Information).



Page 6 of 12

Combining the two networks can overcome these

process to complete the repair. Each of the samples with

shortcomings. These two networks can be dispersed

different sizes has a remarkable healing speed. The

evenly in the DN ion gel prepared through the sol-gel

cracked film sample healed itself in just 10 s (Figure 4 and

transition. The rigid P(FMA-co-MMA) network acts as a

Video VSI2 in Supporting Information). The entire

support structure, while the ductile P(VDF-co-HFP)

thermal healing process of the DN ion gel film was clearly

polymer chains are intertwined with each other. When

observed through an optical microscope (Olympus BX-51,

this tough gel is stretched by the applied force, the brittle

Instec).

chemically

crosslinked

network

ruptures

first

and

dissipates most of the energy. The soft physically crosslinked network serves as “hidden length” to maintain the integrity of the broken first network.39 It is like the “fences and ropes”: a pile of fences are easy to be separated; after intertwined with a bundle of ropes, they can be extremely strong.

Thermal healing capability Because of the excellent thermal stability of the IL, the 5% weight loss (thermal decomposition) temperature of the DN ion gel obtained is above 350 oC under nitrogen or air (Figure S11 in Supporting Information). The IL is also nonflammable. Therefore, the DN ion gel can improve the safety and extend the operating temperature effectively when used in flexible electronics. Benefiting from the excellent thermal stability, some unique properties of the The P(VDF-co-HFP) SN ion gel is a physical gel and can undergo a sol-gel transition owing to its thermal reversibility in that the PVDF crystalline regions melt when being heated and re-form upon cooling.43 For P(FMA-co-MMA) SN ion gels, increasing temperature will cause the cleavage of the introduced furan-maleimide dynamic bonds, and part of the chemically crosslinked network will break. Consequently, this SN ion gel will transform from a static state into a dynamic state. Such kind of crosslinked system is widely used in thermal materials

owing

to

Photographs of the cut film which was healed after being heated at 100 oC for 20 s; (b) Optical micrographs of the healing process at 100 oC for 10 s.

First, we investigate the thermal healing capability of the DN ion gel films. After being cut, the film sample was

gel at high temperatures can be studied.

healing

Figure 4. Thermal healing process of the DN ion gel film. (a)

its

controllable

bonding-breaking process.45-47 Because of the thermally induced de-crosslinking and re-crosslinking processes, the

attached to each other and heated to 100 oC. Under this condition, the gels can enter into a dynamic state in which the polymer chains have a higher mobility. The gels will interpenetrate with each other to eliminate the interface, and the network will re-form during the cooling process to complete the repair. Each of the samples with different sizes has a remarkable healing speed. The cracked film sample healed itself in just 10 s (Figure 4 and Video VSI2 in Supporting Information). The entire thermal healing process of the DN ion gel film was clearly observed through an optical microscope (Olympus BX-51, Instec). To prove that dynamic covalent bond plays an important role in the thermal healing process. Rotational

DN ion gel has the ability to heal the defects. First, we investigate the thermal healing capability of the DN ion gel films. After being cut, the film sample was attached to each other and heated to 100 oC. Under this condition, the gels can enter into a dynamic state in which the polymer chains have a higher mobility. The gels will interpenetrate with each other to eliminate the interface, and the network will re-form during the cooling

rheometer was used to investigate the thermo-rheological behaviors of the DN ion gels prepared. A plate rotor with a diameter of 2.5 cm was used to test the temperature dependencies of G and G at a heating rate of 2 oC min1 and a frequency of 1 Hz. As shown in Figure 5a, the storage

modulus

G

decreases

temperature is increased from 100


dramatically oC

to 120

oC,

when and G

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starts to be lower than the loss modulus G at 110 oC. This

completely disappeared (Figure 6a). For the strip samples,

indicates that the DN ion gel undergoes a sol-gel

the healed gel could be bent freely and withstand a

transition in this temperature range. To further prove that

certain strain (Figure 6b). The cut gels were dyed in

the transition is attributed to the de-crosslinking of the

different colors for easy observation of the healing process.

dynamic network, the temperature dependencies of G

For the healed gel, different colors permeate each other at

and G of the P(FMA-co-MMA) SN ion gels and the

the interfaces, proving that the DN ion gel does enter a

PMMA SN ion gels , as a contrast, were investigated. The

dynamic state at high temperatures.

results show that the P(FMA-co-MMA) gel exhibits the sol-gel thermal transition behavior (Figure 5b), while the PMMA gel remains in the gelled state on heating, and the crack still existed after 150 min (Figure 5c). The transition of the DN ion gel was also studied by differential scanning calorimetry (DSC). There are two main endothermic processes in the thermogram (Figure 5d). According to the literature on the P(VDF-co-HFP) SN ion gel, the peak at around 80 oC can be attributed to the melting of the PVDF crystal.43 The peak originating from the reverse DA reaction is at 110 oC, consistent with rheological results.

Figure 6. Thermal healing capability of the DN ion gel. (a) Three fan-shaped DN ion gels, which were dyed into three colors, fusing together into a cylindrical at 100 oC for 10 min, with the healed cylindrical sample without cracks; (b) Three dyed DN ion gel strips fusing together at 100 oC for 10 min, with the healed strip sample being able to be bent and hold a weight of 50 g. Figure 5. Thermal properties of the DN ion gel in the heating process. (a, b, c) Temperature dependencies of shear storage modulus G and shear loss modulus G of the DN ion gel (a), P(FMA-co-MMA) (b) and PMMA (c) SN ion gels. (d) DSC thermogram of the DN ion gel at a heating rate of 20 °C min1 under a nitrogen atmosphere.

The above results demonstrate that the DN ion gel containing a dynamic crosslinking network possesses attractive thermal healing capability and can be served as a “smart” material. For example, using this repairable gel as a gel polymer electrolyte should be able to help improve the service life of the products. More importantly,

In the same way, we also tested the thermal healing

because of the reparability, the gel is recyclable, which is

capability of bulk samples. Because of poorer heat

also more environmentally friendly.

conduction and larger areas to repair, the cylindrical and

Ionic conductivity of the DN ion gels

strip samples needed more time to realize complete

healing. However, the results were satisfactory. There was no significant difference in the macroscopic appearance of the samples before and after the healing, and the cracks

ILs have excellent electrical performance, and ion gels are widely used in organic electrical devices. We also conducted a preliminary study on the electrical properties



Page 8 of 12

of the DN ion gels prepared. The content of IL in the DN

more rigid. With strengthening of the networks, the ionic

ion gel is as high as 80 wt%, leading to a very high ionic

conductivity decreases due to the restricted transport of

conductivity,

free ions. However, the DN ion gel can still maintain a

comparable

to

[EMIM][TFSI], which is 11 mS

that

cm1

at 25

of

the

oC.48

neat

The ionic

high ionic

conductivity. These

excellent

electrical

conductivity of the ion gel was measured by using electrochemical

impedance

spectroscopy

(EIS)

measurements. First, the film sample was cut into a circle with a diameter of 1 cm to fit the circular electrode. Then an

electrochemical

workstation

(CHI

660E,

CH

Instruments) was utilized to obtain the typical “Nyquist” plot (Figure S12 in Supporting Information) with an AC potential amplitude of 10 mV and a frequency range of 10 Hz–100 kHz. The intercept with the x-axis represents the resistance of the ion gel. The ionic conductivity is calculated by the following equation: 𝐿

(1)

𝜎 = 𝑅×𝑆

where L and S represent the thickness and area of the film, and R is the resistance. The DN ion gel exhibits attractive ionic conductivity of around 1 mS cm1 in a wide temperature range (Figure 7a), and the conductivity shows no sharp decrease even at temperatures below 18 oC

at which the IL [EMIM][TFSI] starts to freeze.48 The

DN scaffold plays an important role at low temperatures to weaken the ionic interaction between [EMIM]+ and

Figure 7. Conductivity of the DN ion gel. (a) Ionic conductivity of the DN ion gel from 40 oC to 80 oC; (b) VTF fitting of the conductivity data; Conductivities of samples with (c) different mass ratios of the two networks and (d) different crosslinker loadings.

[TFSI] and help keep the IL in the liquid state. Moreover,

properties enable it to be potentially useful in energy

the temperature dependence of conductivity can be fitted

storage materials, electrochemical sensors, flexible devices,

quite well with the Vogel−Tammann−Fulcher (VTF)

and other fields.

equation (2) (Figure 7b).

𝜎 = 𝜎0 × exp

(

― 𝐸a 𝑅(𝑇 ― 𝑇0)

)

CONCULSIONS (2)

In conclusion, we have successfully prepared a new type of stretchable DN ion gel with excellent mechanical

where σ0 is the conductivity at infinite temperature, Ea is

strength, high ionic conductivity, and outstanding

the activation energy, and T0 is the temperature at which

thermal stability. This new DN ion gel is composed of a

molecular motion is frozen. Parameters of the VTF fitting

chemically crosslinked P(FMA-co-MMA) network and a

are provided in Table S2 of the Supporting Information.

physically crosslinked P(VDF-co-HFP) network and can

The gel with the best mechanical strength (with a

be prepared through a one-pot method under mild and

crosslinker loading of 5 wt% and a ratio of the two

general

networks at 1:1) has a conductivity of 3.3 mS cm1 at room

preparation process. More importantly, the gel has the

temperature. Thus, it has the potential to be used in

capability of fast thermal healing attributed to the

flexible electronics because of its good flexibility and

dynamic covalent crosslinks based on thermal-reversible

conductivity even in harsh conditions.

DA reactions of the first network, which will help improve

conditions,

which

greatly

simplifies

the

The composition of the ion gel affects the electrical

the stability and reparability of electronic devices

properties slightly (parts c and d of Figure 7). Increasing

containing this DN ion gel. Furthermore, the strategy for

the content of the P(FMA-co-MMA) network and

designing

improving the crosslinking degree can make the ion gels

functionalized polymer networks will promote the

and


preparing

smart

ion

gels

with

Page 9 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


development of “smart” soft materials. We can design new

Amaratunga, G. A. J.; Milne, W. I., Flexible Electronics: The Next

types of ion gels with multiple stimuli-responsiveness

Ubiquitous Platform. Proc. IEEE 2012, 100, 1486-1517.

such as responses to force, electricity, and pH by changing

(2) Chortos, A.; Bao, Z., Skin-inspired electronic devices. Mater.

the segment sequence, side-chain group, and topological

Today. 2014, 17, 321-331.

structure. These functional ion gels will have a wide range

(3) Rogers, J. A.; Bao, Z.; Baldwin, K.; Dodabalapur, A.; Crone, B.;

of applications in solid gel electrolytes, electrochemical

Raju, V. R.; Kuck, V.; Katz, H.; Amundson, K.; Ewing, J.; Drzaic,

sensors, actuators, etc, and broaden the area of flexible

P., Paper-like electronic displays: large-area rubber-stamped

electronic.

plastic

sheets

of

electronics

and

microencapsulated

electrophoretic inks. Proc. Natl. Acad. Sci. U. S. A. 2001, 98,

ASSOCIATED CONTENT

4835-40.

Supporting Information

(4) Zhao, Y.-f.; Zou, W.-j.; Li, H.; Lu, K.; Yan, W.; Wei, Z.-x.,

This Supporting Information is available free of charge via

Large-area, flexible polymer solar cell based on silver nanowires

the Internet at http://pubs.acs.org. Videos of tensile test and

as transparent electrode by roll-to-roll printing. Chin. J. Polym.

thermal

Sci. 2016, 35, 261-268.

healing

process.

Characterization

of

P(FMA-co-MMA); cyclic tensile behaviors; photographs of

(5) Yao, B.; Zhang, J.; Kou, T.; Song, Y.; Liu, T.; Li, Y., Paper-Based

compressive test; surface topography; thermal stability;

Electrodes for Flexible Energy Storage Devices. Adv. Sci. 2017, 4,

“Nyquist” plot.

1700107. (6) Kim, D. H.; Xiao, J.; Song, J.; Huang, Y.; Rogers, J. A.,

AUTHOR INFORMATION

Stretchable, curvilinear electronics based on inorganic materials. Adv. Mater. 2010, 22, 2108-24.

Corresponding Author

(7) Zhou, W.; Wang, S.; Li, Y.; Xin, S.; Manthiram, A.;

*Zhihao Shen. E-mail: [email protected]

Goodenough, J. B., Plating a Dendrite-Free Lithium Anode with a

*Xinghe Fan. E-mail: [email protected]

Polymer/Ceramic/Polymer Sandwich Electrolyte. J. Am. Chem.

ORCID

Soc. 2016, 138, 9385-8.

Zhehao Tang: 0000-0002-1685-8664

(8) Ping, J.; Pan, H.; Hou, P. P.; Zhang, M. Y.; Wang, X.; Wang, C.;

Xiaolin Lyu: 0000-0003-4221-6806

Chen, J.; Wu, D.; Shen, Z.; Fan, X. H., Solid Polymer Electrolytes

Anqi Xiao: 0000-0002-6479-6381

with Excellent High-Temperature Properties Based on Brush

Zhihao Shen: 0000-0003-2858-555X

Block Copolymers Having Rigid Side Chains. ACS Appl. Mater.

Xinghe Fan: 0000-0002-0585-2558

Interfaces 2017, 9, 6130-6137.

Notes

(9) Lyu, Y.-F.; Zhang, Z.-J.; Liu, C.; Geng, Z.; Gao, L.-C.; Chen, Q.,

The authors declare no competing financial interest.

Random binary brush architecture enhances both ionic conductivity and mechanical strength at room temperature. Chin.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grants 51473005 and 51725301).

DN, double-network; SN, single-network; IL, ionic liquid; RAFT, reversible addition-fragmentation chain transfer; DSC, scanning

(10) Plechkova, N. V.; Seddon, K. R., Applications of ionic liquids in the chemical industry. Chem. Soc. Rev. 2008, 37, 123-50. (11) Bonhôte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram,

ABBREVIATIONS

differential

J. Polym. Sci. 2017, 36, 78-84.

calorimetry;

EIS,

electrochemical

impedance spectroscopy

K.;

Grätzel,

M.,

Hydrophobic,

Highly

Conductive

Ambient-Temperature Molten Salts†. Inorg. Chem. 1996, 35, 1168-1178. (12) Susan, M. A.; Kaneko, T.; Noda, A.; Watanabe, M., Ion gels prepared by in situ radical polymerization of vinyl monomers in

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High-Performance Double-Network Ion Gels with Fast Thermal

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