Multifunctional Self-healing Ionogels from Supramolecular Assembly

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Multifunctional Self-healing Ionogels from Supramolecular Assembly: Smart Conductive and Remarkable Lubricating Materials Shipeng Chen, Nanxiang Zhang, Baohao Zhang, Bao Zhang, and Jian Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15722 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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ACS Applied Materials & Interfaces

Multifunctional Self-healing Ionogels from Supramolecular Assembly: Smart Conductive and Remarkable Lubricating Materials Shipeng Chen†, Nanxiang Zhang §, Baohao Zhang†, Bao Zhang†,*, and Jian Song†, ‡,*

†School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China; ‡Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China; §School of Material Science and Engineering, Beijing Institude of Technology, Beijing 100081, China Corresponding Authors *E-mail: [email protected] (B. Zhang), [email protected] (J. Song)

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ABSTRACT: Self-healing ionogel is a promising smart material because of its high conductivity and reliable stimuli responsiveness upon mechanical damage. However, self-healing ionogels possessing rapid, complete recovery properties and multi-functionality are still limited. Herein, we designed a new D‑gluconic acetal-based gelator (PB8) bearing a urea group in the alkyl side chain. Interestingly, the balance between hydrophilicity and hydrophobicity of the molecule is achieved. Thus, PB8 could form transparent ionogels because of its excellent affinity to ionic liquids (ILs), which exhibited appropriate mechanical strength, high viscoelasticity, and efficient self-healing properties. The presence of synergistic effects from hydrogen bonding, π−π stacking, and interactions between the urea-containing side chains was responsible for the self-assembly of gelators in ILs, and the self-healing property mainly related to the side chains of PB8. Interestingly, the transparent PB8-IL4 ionogel possessed high conductivity and mechanical strength, moldable and injectable properties, rapid and complete self-healing characteristics (complete recovery within 14 min), which showed excellent performance as a smart ionic conductor. Furthermore, the self-healing PB8-based ionogels with anticorrosion properties are remarkable lubricating material in the steel/steel contact and exhibited excellent lubricating performances. Overall, an efficient PB8-based ionogel with self-healing properties has been developed for potential use both as a smart electrical conductor and a high-performance lubricating material. The unique structure of PB8 bearing a urea group in the side chain is found to be responsible for the multi-functional ionogel formation.

KEYWORDS: rapid self-healing ionogel, high conductivity, excellent mechanical properties, smart ionic conductor, remarkable lubricating material


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INTRODUCTION Ionogels are soft materials that formed by the supramolecular assembly of gelators in ionic liquid (IL).1-2 ILs thus can be trapped in 3D networks developed by gelators so that the free flowing of ionic liquid is inhibited, and functionalization of the gel can be realized attributing to the intrinsic nature of ILs, such as low vapor pressure, nonflammability, high thermal stability, high ionic conductivity, and wide electrochemical window.3-6 In terms of application, ionogels possessing wide electrochemical window and high ionic conductivity are excellent electrolytes for supercapacitor,7 electrochromic device,8-9 ionic conductor,10 lithium-ion batteries2 and so on; besides, in recent years, thixotropic and anticorrosian ionogels were found to be potential high performance semi-solid lubricants,11-14 which is based on the remarkable tribological performance of ILs (the nonflammability, nonvolatility, thermo-oxidative, and high polarity of IL is necessary for a lubricant).15 Compared with pure ILs, ionogels can not only prevent leakage in the practical application, but also show the potential of friction-reduction and anti-wear; moreover, the potentials of ionogels in membrane separation and drug release were also reproted.2 The effective gelators for ILs including inorganic nanoparticles, polymers, and low molecular mass gelators (LMMGs).2 Carbon nanotubes16 and silica nanoparticles17 have been widely used in forming ionogels, and the obtained ionogels usually possess excellent ionic conductivity and

electro-activity,

but

the

low

mechanical

strength

restrict their application range.

Polymer-based ionogels are usually developed by forming covalent bonds between polymer chains, which exhibit robust mechanical properties.18 However, these ionogels are not able to


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repair themselves upon damage and cannot be remolded after being fabricated. Without self-healing characteristic, the service life of ionogels would be short, and the safety of ionogels in applications could not be guaranteed.19-25 In order to achieve self-healing performance, introducing reversible supramolecular interactions (host-guest recognition,26 hydrogen bonding,27 metal coordination,28-30 π-π stacking,31 electrostatic32 or hydrophobic33 interactions) into gel systems gradually become mainstream as a result of the efficient healing without any external PH, electrical, heat or light stimuli. 23-25, 34 Presently, a few polymer-based self-healing ionogels have been developed based on hydrogen bonding,7,

35

electrostatic,36 and ion-dipole

interactions between gelators,10 respectively. But these self-healing ionogels still suffered from complex preparation process, long self-repair time, difficulties to achieve high mechanical strength and high conductivity simultaneously. In contrast with polymer ionogels, LMMG-based ionogels have many advantages, such as versatilities of gelator structures, excellent miscibility of LMMGs in ILs, the smallest influence on ILs’ intrinsic characteristic related to the low gelator concentration, and rapid stimulus-responsive behavior to PH, temperature, external force, and so forth.9 Since the first report of LMMG-based ionogel by Kimizuka,37 ionogels employing glycolipid derivatives,37 amino acid derivatives,38 cholesteryl derivatives,39 sorbitol derivatives,40 compounds containing the urea group,41 organometallic42 and ionic compounds11, 43 as gelators have

been

developed.

In

particular,

Fang

et

al.

developed

a

cholesterol-based

stimulus-responsive ionogel with good mechanical strength;39 Qi et al. reported a macrocycle-equipped supramolecular ionogel possessing high mechanical strength and rapid self-recovery property.44 However, as a complex solvent system of ILs, it is difficult to determine the interactions between gelator molecules in ILs as well as between gelator molecule and the ions in ILs. Thus,


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to design a multi-functional self-healing ionogel with rapid and complete recovery properties is still challenging. Previously, we demonstrated that a rapid self-healing ionogel with excellent mechanical and electrical properties could be achieved by employing a D ‑ gluconic acetal-based LMMG (PG16).9 The most important characteristics of the PG16-based self-healing ionogel is the high mechanical strength (G’  104) even when the gelator concentration is 2 wt%, and meanwhile, the conductivity is almost equal to that of pure IL. However, the PG16-based ionogel is not transparent and the IL employed is hygroscopic, which would limit its reliability in applications. Herein, further investigation of structure-property correlations of gelator molecule in ILs was performed. It was known that an optimal balance of the hydrophilic and hydrophobic interactions is helpful for a gel to possess self-healing abilities.45 We thus introduced a urea group into the side chain of PG16 to balance the hydrophobic and hydrophilic properties of the molecule, leading to PB8 (as shown in Chart 1). Fortunately, PB8 exhibited better affinity toward almost all the ILs tested here than PG16, and all the ionogels formed by PB8 showed better mechanical strength and self-healing abilities. Thus, a transparent, injectable, and moldable ionogel (PB8-IL4) with excellent self-healing abilities, remarkable electric conductivity and high mechanical strength was developed based on hydrogen bonding, π−π stacking, and interactions between side chains, indicating its potential use as a smart conductive material. Interestingly, the self-healing PB8-based ionogels were also proved to be a remarkable lubricating material. It is believed that the unique structure of PB8 with the amide and urea moieties present is critical to avoid wear of the friction interface. Thus, a multi-functional PB8-based self-healing ionogel has been developed for potential use both as a smart electrical


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conductor and a high-performance lubricating material. The study also shed some light on the future molecular design for the development of multi-functional and high-performance ionogels.

O

OH O

O O

H N

N H

O

O

H N

OH O

O

O

O

N H

O

PG16

PB8

Cl

C16H33

Cl

Cl

Cl

O O OO F F S S N F F F

N

F

N N

IL-1 [BMIM][TFSA]

N

IL-4 [BDMIM][TFSA]

PF6 N

N

IL-3 [BMIM]PF6

IL-2 [BMIM]BF4

O O OO F F S S N F F F F

N

N

BF4

O O OO F F S S N F F F F

N

IL-5 [BMPyr][TFSA]

Chart 1. Chemical stuctures of PB8, PG16 as gelators and ionic liquids as solvents

EXPERIMENTAL SECTION Synthesis. Synthetic routes of PB8 are shown in Scheme S1. The detailed synthetic procedures and characterization data of PB8 are given in the Supporting Information. General Procedures. Gelation test was progressed in a sealed tube (inner diameter: 10mm), the gelators were dissolved in ILs at a certain concentration by heating and the solution was naturally cooled to room temperature. Gelation was confirmed by inverting the tube with no


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gravitational flow. The xerogel was prepared through freeze-drying following solvent exchange,46 all the employed xerogels here were prepared from PB8-IL2 because IL2 was soluble in water. A certain ionogel was immersed into water for 5 days and the water was refreshed every 5h. Solvent exchange was conducted at room temperature without stirring, ultrasound, and so on. The xerogel was obtained by the following freeze-drying operation. The following experiments and tests were all conducted at room temperature and the humidity around 50% if there were no special mentions. Rheological studies of ionogels were conducted through a rheometer (Anton Paar Physica MCR 301). The gel was equilibrated under a parallel plate (15mm diameter) with a thickness of 0.5 mm. Dynamic frequency sweep was conducted at a 0.05% strain. Dynamic strain sweep was conducted at a constant frequency (1 Hz). Step-strain measurement started at a constant strain of 0.05%, then, a following strain which was enough to break the gel was applied, and then, a constant strain (0.05%) was applied again. The conductivities of the ionogels were measured by a conductivity meter (DDS-307A, Shanghai INESA Scientific Instrument Co., Ltd., China). The electrode was immersed into the ionogel at a temperature above the Tg, and then it was cooled to 25 °C for gelation. The conductivity at different temperature was determined and the temperature was controlled by a thermostatic heater. The conductivity changes of PB8-IL4 ionogel were monitored by a conductivity meter for several damage and self-repair cycles and upon the mechanical damage, the ionogels were completely broken by stirring. The conductivities of the original PB8-IL4 ionogel and the completely repaired ionogel after damage were recorded and compared. All NMR studies were performed on a Bruker DPX 400 MHz spectrometer. Mass spectra were obtained on a TOF-QII high resolution mass spectrometer. The thermal IL solution of gelator


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was spread on the wall of the glass tube with an inner diameter of 7mm and cooled to room temperature to form the gel, and the original morphologies of the gel samples could be obtained by leaving the glass tube under the polarizing optical microscopy (POM, Eclipse 50iPOL, Nikon); the gel samples were damaged by a glass bar and placed under the POM immediately to monitor its self-healing process. Fourier transform infrared (FTIR) spectroscopy measurements were performed on a FTS 3000 spectrometer with KBr pellets. X-ray diffraction (XRD) diagrams were obtained using Bruker D8-S4 (Cu Kα radiation, λ = 1.546 Å), operating at 40KV and 100mA, the sample was obtained by dropping the thermal IL solution of gelator into the sample slot until the ionogel was formed. Theoretical calculation was realized by the AM1 method using Gaussian 09. The contact angle (CA) measurement of IL droplet placed on xerogel was measured by a JC2000D1 contact angle measuring instrument (POWEREACH, Shanghai, China). The xerogel was pressed into sheet before test. The corrosion properties of ILs and corresponding 2 wt% ionogels were evaluated. AISI 52100 steel were polished with a series of emery papers and immersed in ILs and ionogels. The temperature and humidity were kept at 50 ℃ and 60%, respectively. After 30 days, the steel were taken out and washed with acetone to remove the organic residue. The dried steel were examined by scanning electron microscopy-energy dispersive Spectrometer (SEM-EDS) to obtain the morphologies and elemental surface distribution. SEM images were obtained on a Hitachi S-4800 SEM instrument at 3−5 kV, 10mA. The tribological tests of pure ILs and ionogels were performed using an Optimol SRV-IV friction and wear tester (applied load: 300N; frequency: 25 Hz; stroke: 1mm; duration: 30min; temperature: 25℃). The frictional pairs were composed of the lower stationary disk (AISI 52100


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steel) and the upper running ball (10mm in diameter, GCr15 steel). The corresponding friction coefficient (COF) and electrical contact resistance (ECR) were recorded with a computer connected to the tester. A MicroXAM-800 3D noncontact surface mapping profiler was used to measure the wear volume of the lower disk. SEM and X-ray photoelectron spectroscopy (XPS, ThermoFisher K-Alpha) were used to analyze the morphologies and composition of worn surfaces, respectively. RESULTS AND DISCUSSIONS Investigation of the Ionogel Properties. Five ILs were selected to investigate the gelation behavior of PB8. The critical gelation concentration (CGC) and Gel−sol transition temperature (Tg) values were determined to illustrate the gelation abilities of PB8 in different ILs. The rheological data (detailed data shown in Figure S1, 2) were employed to show the mechanical properties of gels formed by PB8: G’ (storage modulus) values could indicate the mechanical strength, tanδ (G’/G”) values could show the viscoelasticity, and the recovery ratio values were employed to evaluate the self-healing ability. All the data mentioned above are summarized in Table 1 and the corresponding data of PG16 (a gelator reported before)9 are also included as a contrast. As a result, PB8 and PG16 both can gelate all the ILs. For a certain IL, the CGC of PB8 is higher and the Tg is lower than that of PG16, except for IL2. However, at the same concentration (2 wt %), the G’ values of PB8 gels in ILs (except for IL1) are greater than those of PG16 gels, and all the G’ values exceed 104 Pa which is a high level value. The tanδ values of PB8 gels are also higher than those of PG16 gels. Most importantly of all, the recovery ratio values of the PB8-based gels formed in all the ILs are obviously higher than those of PG16-based gels. The recovery ratio values of PB8-IL2, PB8-IL5 and PB8-IL4 ionogels even reach 100%.


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Table 1. Gelation abilities of PB8, PG16 in ionic liquids and the rheological data of corresponding ionogelsa, b CGC (w/v %, ILs

Recovery ratio

Tg (℃)

mg/ 100μL)

G’ (×104 Pa)

Tanδ (G’/G”) (%)

PB8

PG16

PB8

PG16

PB8

PG16

PB8

PG16

PB8

PG16

IL1

0.53

0.40

58.9

70.3

6.72

13.20

0.154

0.110

69.9

37.9

IL2

0.45

0.70

59.3

58.2

1.90

0.05

0.190

0.105

100.0

27.8

IL3

0.40

0.30

59.1

83.1

1.69

0.19

0.249

0.150

58.8

15.8

IL4

1.10

1.00

54.4

79.7

2.22

1.00

0.188

0.099

100.0

26.9

IL5

0.80

0.50

57.5

71.5

1.54

0.91

0.193

0.176

100.0

66.4

aCritical

gelation concentrations (CGCs) indicate the minimum amount of gelator required to immobilize 1 mL IL. Gel−sol transition temperature (Tg) was determined by the “falling ball method” described in literature.47 G’ is storage modulus, G” is loss modulus, recovery ratio is the recovery ratio of G’ after the first cycle in the step-strain measurement of oscillatory rheological study (Figure S1, 2); bAll the Tg, G’, tanδ and recovery ratio values were determined with ionogels at the concentration of 2% (w/v). The temperature dependence of the ionic conductivities of IL4 and PB8-IL4 ionogel was studied (Figure 1). The relationship between ion conductivity and 1000/T showed good linearity for both IL4 and PB8-IL4 ionogel in spite of a change at the Tg of ionogel, suggesting that the temperature dependence of the conductivity followed the classical Arrhenius equation. Generally, the ionic conductivity of ionogel shown in Figure 1 was slightly lower than that of IL4, and it unusual increased as the temperature exceeded the Tg. The reason is that a small number of IL molecules may interact with gelator molecules, reducing the efficiency of ion conduction. But the interaction between IL and gelator molecules would be weakened when the


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temperature exceeded the Tg and the efficiency of ion conduction could recover to some extent. It could be concluded that the IL nature would not be changed in ionogel, and most of the cations and anions could move freely in the 3D network of ionogel so that the ionogel could possess high conductivities.

log(S/cm)

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


-1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0

IL4 only 2% PB8-IL4

Sol Gel

2.8

2.9

3.0 3.1 3.2 1000/T (1/K)

3.3

Figure 1. Arrhenius plots for the ionic conductivity of IL4 and 2 wt% PB8-IL4 ionogel. From the CGC results, it could be concluded that PB8 molecules possess better affinity with ILs than PG16, which might be related to the presence of urea group in the side chain of PB8. In fact, PB8-IL1, PB8-IL4 and PB8-IL5 ionogels are gels with excellent transparency, but all the PG16 ionogels are opaque (Figure S3). Moreover, the rheological data indicate that all the PB8 ionogels exhibit much better mechanical properties than PG16 gels generally. Greater G’ and tanδ values of PB8 ionogels suggest the better mechanical strength and better viscoelasticity, respectively, and the higher recovery ratio values of PB8 ionogels, especially the complete recovery of PB8-IL2, PB8-IL4 and PB8-IL5 ionogels demonstrate the prominent self-healing abilities of PB8 ionogels. Overall, the excellent characters including transparent properties, high mechanical strength, remarkable self-healing abilities, good viscoelasticity, and high conductivity of the ionogels formed by PB8 provided a potential for PB8-based ionogels to be used as multifunctional self -healing materials.


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Transparent, Moldable, Injectable and Self-healing Ionogels. The PB8-IL4 ionogel was further investigated because of its excellent comprehensive performance: transparency, non-hygroscopicity (see Figure S4), high mechanical strength (G’ = 1.54×104 Pa at 2 wt %), complete self-recovery, high ionic conductivity and remarkable viscoelasticity (tanδ = 0.193). Step-strain measurement of oscillatory rheology was employed to investigate the self-healing property of 2 wt % PB8-IL4 ionogel (Figure 2a); continuous step changes of 0.05 and 50% strain at the same frequency of 1 Hz were performed to the gel. Interestingly, rapid and complete recovery of the storage modulus were observed following strain-induced damage; moreover, the rate and extent of recovery were almost unchanged over three cycles of breaking and self-repairing (Figure S5), indicating the reversible and durable properties of the ionogel. The continuous flow experiment in Figure 2b indicated that the PB8-IL4 ionogel possessed good shear-thinning properties upon increasing shear rate from 0.1 to 10 s-1, whereas the viscosity of the ionogel could almost remain in the immediate following reverse cycle, and the slight deviation might be related to the inadequate recovery time. The electrical properties were also able to stay at the same level after several damage-healing cycles as shown in Figure 2c; the ionic conductivity of the ionogel hardly changed even after 10 damage-healing cycles. More interestingly, the PB8-IL4 ionogel exhibited remarkable transparent, self-standing, and load-bearing properties (Figure 2d, e, and f). In Figure 2g, four pieces of ionogel could merge into one integrated block within minutes by connecting them together, which illustrate the self-healing properties sufficiently. Furthermore, injectable and moldable properties of the ionogel were also revealed (Figure 2h and i); the ionogel could be injected through a common syringe and it could be processed into truncated-cone-type through heating–cooling method and injection respectively. All the above illustrations were based on the high mechanical strength,


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remarkable viscoelasticity and self-healing abilities of this ionogel, which combined with its high conductivity, make the PB8-IL4 ionogel an ideal multifunctional material for a wide range of potential applications.

Figure 2. (a) Step-strain measurements of PB8-IL4 ionogel (2%, w/v) over 3 cycles (with alternating strain of 0.05 and 50% with 1 Hz at 25 °C); (b) viscosity of PB8-IL4 ionogel (2%, w/v) at increasing shear rate followed by reverse decreasing shear rate; (c) conductivities of PB8-IL4 ionogel (2%, w/v) at 25 °C after different cycles of damage-healing; (d) transparency comparison of 2 wt% PB8-IL4 ionogel (left) and pure IL4 (right), the thickness is 2mm; (e) free-standing gel column of PB8-IL4 ionogel (3%, w/v); (f) load-bearing capacity of 3 wt% PB8-IL4 ionogel (the weight is 100g), (g) self-healing process of PB8-IL4 ionogel (3%, w/v), (h) injection of PB8-IL4 ionogel (2%, w/v) through narrow and wide needles, (i) moldable property of

PB8-IL4 ionogel (3%, w/v), the truncated-cone-type gels are processed through heating–

cooling method (upper) and injection (lower). All the colored gels are doped with rhodamine B.


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Figure 3. POM images of original PB8-IL4 ionogels at 2% (w/v) (a, a’), POM images of repaired PB8-IL4 ionogels (self-repaired at room temprature for 15 minutes, a period enough for the PB8-IL4 iongel to restore to the original state) (b, b’). Insights into the Mechanism of Self-assembly and Self-Heal.

Morphology examination is

helpful for the investigation of the assembly model of a gel system in micron and even nanometer scale. As shown in Figure 3 a, a’, a reticular network with many micropores was observed from the POM images of the original PB8-IL4 ionogel. This characteristic porous structure will facilitate the smooth dispering of ions in the gel and guarantee the high conductivity of PB8-IL4 ionogel. We also characterized the morphology of the ionogel that experienced the damage-repair process. As shown in Figure 3 d, d’, the morphology of repaired ionogel could be observed, which was still a reticular network similar to the original PB8-IL4 ionogel. The maintenance of the morphology before and after self-repair is the premise that the conductivity can stay at a same level before and after the damage-repair process is the premise that the conductivity can stay at the same level. These results revealed the fundamental reason


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that the conductivity of PB8-IL4 ionogel could be maintained before and after the damage-repair process To investigate the driving forces for the self-assembly of PB8 molecules, FT-IR and 1H NMR experiments were performed. As shown in Figure 4A, in a solution state, the stretching vibration bands of O-H and N-H overlapped at 3487 cm-1, the stretching vibration bands of CH2 were observed at 2933, 2857 cm-1, and the characteristic carbonyl stretching vibration band appeared at 1669 cm-1. But in the xerogel state, these peaks shifted to 3346, 2925, 2851 and 1645 cm-1 respectively, which indicated that the O-H, N-H and C=O might be involved in the hydrogen bonding, and Van der Waals forces between the neighboring alkyl chains in gel. Variable-temperature 1H NMR experiment of PB8 in [D6]DMSO was performed in the presence of IL5 (Figure 4B). Pyrrolidine cation was employed rather than imidazolium in this 1H NMR experiment because the signal of the amide proton in PB8 overlapped with those of imidazolium cation. At 30℃, the signals of protons on benzene ring appeared at 7.92, 7.65 and 7.57 ppm, the N-H signals of amide and urea group at 7.45 and 5.69 ppm respectively, and the only O-H signal at 5.06 ppm. Concurrently, all the above proton signals moved upfield with the increasing of the temperature, indicating the protons of the hydroxyl, amide and urea group have participated in the hydrogen bonding and the benzene ring was involved in π-π stacking. Therefore, the driving forces for the self-assembly of PB8 molecules could be a synergy of hydrogen bonding interactions, π-π stacking and interactions between side chains.


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Figure 4. (A) FT-IR spectra of (a) DMF solution of PB8, (b) xerogel of PB8 ionogel. (B) Changes in 1H NMR chemical shift of N-H, O-H and protons on the benzene ring of 5mg PB8 in IL5/[D6]DMSO at ratios (v/v) of 40:60 as temperature increases from 30 to 70 ℃. (C) XRD spectra of ionogel PB8-IL4 (2%, w/v). (D) Self-assembly mechanism of PB8 gelators in ionic liquid and the proposed self-repairing mechanism upon mechanical damage; (a) theoretical optimized structure of PB8 and (b) simulated assembling form of PB8 molecules by H-bond and π-π interactions provided by the Gaussian 09 AM1 method.


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X-ray diffractions (XRD) of PB8-IL4 ionogel could further provide structural information of the self-assembly of PB8 molecules (Figure 4C). Four reflection peaks corresponding to d-spacing of 3.80, 2.87, 1.43 and 0.45 nm were observed, which verify the lamellar structure with a periodicity of 3.80 nm in the fibers of the gel. Moreover, theoretical calculation was employed to optimize the geometry and simulate the molecular structure in the supramolecular system of the ionogel (Figure 4D, a, b). The optimized horizontal length of PB8 molecule was 2.87 nm (Figure 4D, a), which perfectly matched with the reflection peak (2.87 nm) in XRD. Meanwhile, in the simulated assembly pattern (Figure 4D, b), the 1D assembly of PB8 molecules was formed through intermolecular hydrogen bond and π-π interaction. It was shown that the distance between neighboring side chains was 1.42nm, the distance between neighboring benzene rings 0.43 nm, and the distance between lamellas was simulated as 3.5 nm in Figure S6, which were extremely in accord with the XRD experimental results (1.43, 0.45 and 3.8nm). Therefore, based on the combined analysis of the experiment and theoretical calculation results, it was proposed that assembly of PB8 molecules via intermolecular hydrogen bonding between hydroxyl and amide groups and π-π interaction formed 1D fibers which resulted in the fiber bundles via the hydrogen bonding interactions and Van der Waals forces between the side chains as shown in Figure 4D. The fiber bundles then intertwined and formed the 3D framework in the ionogel. The self-healing of a LMMG-based gel is mainly attributed to the abilities of the fibers in a gel to entangle again after removing mechanical damage.48 Weiss et al. previously proposed that upon application of destructive strain, the interaction at or near junction zones of fibers would be weakened or even disrupted preferentially, but the inner structure of individual fiber remained.49 After removing the external stimulus, the fibers will spontaneously intertwine again through the


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supramolecular interactions between the functional groups on the surface of the fibers, and the rate of the recovery will depend on the diffusion rates of the fibers. We thus believe that the outstanding self-healing ability of PB8-based ionogel is also related to the effective diffusion of aggregates in the system after mechanical damage. This conclusion would be further supported by comparing the related properties and experimental data of PG16 and PB8 ionogels. IL2’s contact angles on xerogel surfaces of PB8 and PG16 were measured to examine the affinity of PB8 and PG16 aggregates with IL2. In Figure S7, the IL2’s contact angles on PB8 and PG16 xerogels were 65.41o and 93.66o, respectively. The contact angle values illustrate that the aggregates formed by PB8 have a better affinity with IL, resulting in the better diffusibility of the PB8 aggregates in IL. The self-assembly modes of PB8 and PG16 are basically the same (assembly of molecules occurs via intermolecular hydrogen bonding between hydroxyl and amide groups and π-π interaction leading to 1D aggregate; 3D aggregates are formed through the interwining of the side chains between 1D aggregates, which have been well discussed in this work and our previous work 9). Based on this assembly mechanism, the affinity of the surface of the aggregates, which was assumed to be covered by the side chain of the gelators, with IL will determine the aggregate diffusibility in ILs. The side chain of PB8 consists of carbon units and the urea moiety, which possesses good balance between hydrophilicity and hydrophobicity, resulting in the better affinity of the side chain of PB8 with ILs, compared with the side chain of PG16 bearing only carbon units. Correspondingly, the aggregates formed by PB8 would have the better affinity with ILs. Thus, the aggregates of PB8 formed by the mechanical forces can more effectively diffuse in IL compared with those of PG16, and thus re-interwine via the interactions between the side chains on the surface of aggregates. Therefore, PB8 ionogels demonstrate better self-healing abilities than PG16 ionogels.


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Moreover, the self-healing process of PB8-IL4 ionogel was monitored by POM. In Figure S8, it could be observed that the initial gel networks were broken into smaller aggregates (2 minutes after removing mechanical damage). The small aggregates gradually gathered and reassembled into big aggregates as shown in Figure S8. The image of the repaired ionogel is similar to the original one after leaving still for 15 minutes upon removing the mechanical damage. Thus, a feasible self-repairing mechanism of PB8 ionogel was proposed (Figure 4D). Upon the mechanical destruction, the hydrogen bonding interactions (via the urea group) and Van der Waals forces (via the alkyl chain) between the side chains at or near intersections of fibers were weakened and broken, and then disassembly of 3D framework reduced the strength of the sample. After removing the mechanical destruction forces, the fiber-like aggregates spontaneously diffused and encountered, and intertwined again through the side chains on the surface of the fibers (reforming the hydrogen bonds and Van der Waals forces). Overall, the remarkable self-healing behavior of PB8-based ionogels is related to the good affinity of PB8 aggregates with ILs due to the presence of the urea moiety in the PB8 side chain. Demonstrated Application of PB8-IL11 Ionogel as Smart Conductive Material. As an ionogel, the most prominent function arises from its high conductivity, and thus, PB8-IL4 ionogel can potentially be a good conductive material. Moreover, as a remarkable self-healing material of PB8-IL4 ionogel, it has the potential to be developed for use as smart conductive materials. As shown in Figure 5, the PB8-IL4 ionogel was used as an ionic conductor to connect an electrical circuit where a bulb was on with a DC voltage. The bulb would be turned off when the gel was cut into two pieces, and turned on again when two gel pieces were reconnected. Notably, the repaired ionogel could not only keep its conductivity but also support itself when hung in the air (Figure 5d) , indicating that both the conductive function and the mechanical


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properties of the PB8-IL4 ionogel remain unchanged after a damage-healing process.

Figure 5. Demonstration of the smart conductive application of PB8-IL4 ionogel: (a) An unconnected circuit where a bulb was not lit under DC voltage; (b) the bulb was lit when the circuit was bridged by PB8-IL4 ionogel; (c) the bulb went dark when the gel was cut into two pieces; (d) the bulb was lit again when two pieces gel were reconnected, and the two pieces gel autonomically merged into one block which could support itself when hung in the air. Demonstrated Application of PB8-based Ionogel as Remarkable Lubricating Material.


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The PB8-based self-healing ionogels can also be a promising lubricating material. The shear-thinning and self-healing of PB8-based ionogels are convenient for processing and application.50-52 Under practical conditions, ionogels would undergo phase transfer from gel to solution uopn external shear strength and participate in lubrication, and the solution would recover to semi-solid state via self-healing so as to prevent IL from creeping and leaking. Besides, in corrosion test (Figure S9-11), PB8-based ionogels are more stable than pure ILs, which attributes to the solid-like character of ionogel and the anticorrosion amide and urea groups in PB8.11, 53-55 The anticorrosion nature is important for a lubricant to avoid wear of the friction interface.15 The lubricating properties of PB8-IL2, PB8-IL4 ionogels and the corresponding ILs for the steel/steel contact were evaluated by SRV-IV tester (Figure 6). Compared with pure ILs, PB8-IL2 (Figure 6a, c) and PB8-IL4 (Figure 6b, d) ionogels not only have a short running-in periods and low friction coefficients (COF), but also have small wear volumes (see Table S1 for detailed data). When the concentration was 2 wt %, the best lubricating properties of PB8-IL2, PB8-IL4 ionogels are realized. Compared with pure ILs, the COF and wear volume values related to 2 wt % PB8-IL4 ionogel decreased 7.7% and 57.6% respectively, the COF and wear volume values related to 2 wt % PB8-IL4 ionogel even decreased 26.0% and 93.9%, respectively. The COF and wear volume values of ionogels were compared with those of some common traditional lubricants and their gels50 (Table S2). It was shown that the ionogels exhibited better lubricating and antiwear properties.


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Figure 6. Evolution of the friction coefficients with time for IL2 and ionogels formed in IL2 at different concentrations (a), IL4 and ionogels formed in IL4 at different concentrations (b) (applied load: 300N; frequency: 25 Hz; stroke: 1mm; duration: 30min; temperature: 25℃); wear volumes of steel disks lubricated by IL2 and ionogels formed in IL2 at different concentrations (c), IL4 and ionogels formed in IL4 at different concentrations (d) (determined by 3D noncontact optical surface mapping profiler). To further understand the friction-reduction and anti-wear mechanism by using ionogels, subsequent experiments related to PB8-IL2 ionogel were carried out. SEM images and 3D optical microscopy images (Figure S12) indicate that the wear scar lubricated by IL2 exhibites severe adhension and scratches, while that by PB8-IL2 ionogel is smooth and slight, which may


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be attributed to the anticorrosion properties of PB8-based ionogels preventing corrosive wear. The X-ray photoelectron spectroscopy (XPS) spectra of the worn surfaces lubricated by IL2 and 2 wt% PB8-IL2 ionogel are shown in Figure S13, and the corresponding analysis is provided. The results indicate that complicated tribochemical reactions occured during friction. In high resolution XPS spectra (Figure S13b-e), the positions of the peaks obtained from pure IL and ionogel are different, especially for F1s, indicating different tribochemical reactions. Furthermore, the evolutions of electrical contact resistance (ECR)51 with time for IL2 and different concentrations of PB8-IL2 ionogels as lubricants for steel/steel contact are shown in Figure S13f. With the increasing of the PB8 concentration, the ECR value increases, illustrating that the PB8 molecules could adsorb on the steel substrate and form stable film during sliding friction process which caused the increase of the contact resistance. Thus, the working principles of PB8-ionogel for the excellent friction-reduction and anti-wear performance could be proposed. During the friction process, the PB8 molecules gradually adsorb on the steel surface via polar groups such as hydroxyl, amide and urea groups.13, 50-51 A protective film will form on the steel surface which can lead to a good friction reduction and anti-wear performance. Furthermore, the addition of PB8 into the IL complicates the tribochemical reactions because of the existence of anticorrosian groups (amide and urea groups) in PB8. The diverse tribochemical products may form a more effective protective film leading to a better lubricating performance.13 CONCLUSIONS We demonstrated that rapid and complete self-healing could be realized in supramolecular ionogels by tuning the structure of gelators to achieve optimal balance of hydrophilic and hydrophobic interactions. The obtained PB8 gelator exhibited good


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affinity with ILs and it could form ionogels in all the ILs examined here. All the PB8-based ionogels exhibited excellent mechanical properties and the PB8-IL4 ionogel was developed into a transparent, moldable, injectable, and self-healing material, which exhibited remarkable performance in self-healing electrical circuit and the steel/steel contact lubricating. A synergy of hydrogen bonding interactions, π-π stacking and interactions between side chains is responsible for the self-assembly of PB8 molecules, and the rapid self-repair ability of the ionogel was attributed to the good affinity of PB8 aggregates with ILs. In summary, a gelator possessing an appropriate balance of hydrophilic and hydrophobic moieties is important for developing supramolecular ionogels. Moreover, the multifunctional self-healing PB8-IL4 ionogel developed here has great potential for applications in intelligent electronic devices and industrial lubrication. Further studies on the practical applications of the multifunctional self-healing ionogels are still in progress. ASSOCIATED CONTENT

Supporting Information. Detailed synthetic methods and characterizations of PB8, detailed rheological data, photos of PB8 and PG16 ionogels, exhibition of the hydrophobicity of the PB8-IL4 ionogel, overlayed zoom of the recovery of PB8-IL4 ionogel after each cycle, corrosion tests of AISI 52100 steel immersed in pure ILs and ionogels, detail data ralated to lubricating properties, SEM and 3D optical microscopy images of wear scars, XPS spectra of the worn surfaces and contact


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resistance test of steel/steel contacts during friction. AUTHOR INFORMATION

Corresponding Author E-mail: [email protected], [email protected]

ACKNOWLEDGMENTS

This work was supported by the National Nature Science Foundation of China (Grant No. 21676185 and 21476164) and Tianjin science and technology innovation platform program (No. 14TXGCCX00017)

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Multifunctional Self-healing Ionogels from Supramolecular Assembly

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