A Facile Soaking Strategy towards Simultaneously Enhanced

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Applications of Polymer, Composite, and Coating Materials

A Facile Soaking Strategy towards Simultaneously Enhanced Conductivity and Toughness of Self-Healing Composite Hydrogels through Constructing Multiple Noncovalent Interactions Shuting Wang, Guoqiang Guo, Xiaoxuan Lu, Shaomin Ji, Guoxin Tan, and Liang Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04999 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

A Facile Soaking Strategy Towards Simultaneously Enhanced Conductivity and Toughness of Self-Healing Composite Hydrogels Through Constructing Multiple Noncovalent Interactions Shuting Wang,# Guoqiang Guo,# Xiaoxuan Lu, Shaomin Ji, Guoxin Tan, Liang Gao* School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, China E-mail: [email protected] # These authors contribute equally to this work. Keywords: Conductive hydrogel, PVA, Tough hydrogel, Noncovalent interactions, Self-healing hydrogel.

Abstract. Tough and stretchable conductive hydrogels are desirable for the emerging field of wearable and implanted electronics. Unfortunately, most existing conductive hydrogels have low mechanical strength. Current strategies to enhance mechanical properties include employing tough host gel matrices or introducing specific interaction between conductive polymer and host gel matrices. However, these strategies often involve additional complicated processes. Here, a simple yet effective soaking treatment is employed to concurrently enhance mechanical and conductive properties, both of which can be facilely tailored by controlling the soaking duration. The significant improvements are correlated with co-occurring mechanism of deswelling and multiple noncovalent interactions. The resulting optimal sample exhibits attractive combination of high water content (75 wt.%), high tensile stress (~2.5 MPa), large elongation (>600%), reasonable conductivity (~25 mS/cm) and fast 1

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self-healing property with the aid of hot water. The potential application of gel as a strain sensor is demonstrated. The applicability of this method is not limited to conductive hydrogels alone but can also be extended to strengthen other functional hydrogels with weak mechanical properties.

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1. Introduction Conductive polymer-based hydrogels (CPHs) have exhibited great potential for various applications. 1-7 CPHs are conventionally prepared by incorporating conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) 8 and others 9-11

into ductile host gel matrices (e.g. polyvinyl alcohol (PVA) as shown in Figure 1).

In addition of conductivity, maintaining good mechanical properties is also important for CPHs so that they can withstand daily elongation/compression cycles. Unfortunately, most reported CPHs have low tensile fracture stresses (e.g. < ~1 MPa) and suffer low elongation at break (e.g. < 500%). 8, 9, 12-14 In some cases, tough/elastic but nonconductive substrates must be adopted to achieve necessary mechanical performances, 15 whereas the substrates occupy a significant amount of weight and volume. Therefore, developing advanced CPHs that combine both excellent mechanical properties and good conductivity is highly desirable but remains a major challenge. The limited mechanical performance of hydrogels can be inherently attributed to the swelling, structural inhomogeneity, and/or poor organization of host matrix gel. 12, 16-18 The swelling drastically lowers the density of polymer chains, while the irregularity causes stress concentration. Both disrupt the interconnection of conductive polymers for efficient charge transportation, leading to low conductivity. The current strategies on preparing tough CPHs concentrate on either (1) adopting strong host gel (e.g. double network (DN) hydrogel) matrices 8, 19-20 or (2) establishing a chemical interaction between conductive polymer and gel matrices. 9 In spite of great progress, both involve relatively complicated processes. Thus, the advantages 3



may be discounted by the multi-stepwise synthesis and production of waste disposal. 21

Specific interaction between host gel and conductive polymer reinforces the

dimensional stability of CPHs. An intriguing work by Ma’s group demonstrated a type of ultra-tough polyvinyl alcohol (PVA)-based CPHs. 9 Despite ultrahigh tensile strength (5 MPa) and conductivity (0.1 S/cm), the synthesis of this gel relied on chemical reactions for constructing (quasi)covalent bonding. Very recently, Goding et. al. successfully manipulated the electrochemical growth of PEDOT within a taurine modified PVA-based hydrogel matrix.22 The presence of nagatively charged taurine groups on PVA can effectively guide the growth of CP throughout the PVA-based hydrogel matrix, leading to uniform interpenetration of PVA and PEDOT networks. The higly interpenated network demonstrated improved electronic properties and microscopic stiffness, but the macroscopic tensile strain-stress behavior was not studied. Notably, soft supporting tissue possesses organized structure as guided by rich noncovalent interactions and controllable swelling/deswelling in physiological environments. 23-24 Inspired by these co-occurring mechanisms, here we developed a type of PVA-based tough CPHs by simply soaking a preformed PVA-PEDOT hydrogel in sulfosuccinic acid (SA) aqueous solution. In contrast to these above-mentioned pathways to enhance the mechanical strength of CPHs, our soaking strategy is facile, straightforward and robust. The combination of SA/PVA/PEDOT was specially selected for enhancing both mechanical and conductivity properties because of (1) its strong ionic strength, which can cause deswelling of PVA; 25 (2) 4


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enriched ionic functional groups (two -COOH and one -SO3- groups), which enable multiple interaction to guide the assembly of PVA and PEDOT. As schematically illustrated in Figure 1, deswelling of PVA-PEDOT increased the crystal domains of the PVA host matrix upon contact with SA. Subsequently, the embedded SA promoted the integration of rigid PEDOT in PVA matrix due to “bridging” and doping effect, viz., that the carboxylic groups of SA and hydroxyl groups of PVA form hydrogen bonding, while the electrostatic/dipole interaction was being established between the sulfonated groups of SA and the positive charged PEDOT backbone.26 This strategy demonstrated spectacular improvements on both mechanical and conductive performances, which can be conveniently tailored by simply controlling the soaking process. Our optimal sample showed good toughness (~7.8 MJ/m3), tensile strength (~2.5 MPa), large elongation (>600%) and reasonable conductivity (~12-25 mS/cm). In addition, our hydrogel showed fast and efficient self-healing properties with the aid of hot water. Within 15 min, over 90% toughness and conductivity could be restored from the damaged samples. By combining the features of high toughness, reasonable conductivity, and rapid self-healing, the hydrogel promises capability as an application as a strain sensor to monitor a body’s motion.

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Figure 1. Schematics of the preparation of GEL-n, their interaction mechanisms and major contributions to the enhanced performance. For the sake of clarity, H-bonding between PVA chains, PVA−water, and SA−water in the hydrogel are not depicted.

2.1 Synthesis The preparation of our CPHs and their interaction mechanisms are briefly illustrated in Figure 1. (ESI for detailed synthetic procedure). It generally included two steps. First the initial PVA-PEDOT hydrogel was prepared by using conventional polymerization (Figure S1) and subsequent freezing/thawing cycles (Figure1). Secondly, the pre-formed PVA-PEDOT gels were soaked in SA aqueous solution for various durations to produce tough CPHs denoted as GEL-n, where n stands for the soaking time in the unit of minutes. Note that the optimal mechanical performance was achieved when the pH of SA solution was adjusted to 4 (Figure S2). Like polysaccharides and guar gum, 27-28 the abundant hydroxyl groups on PVA can serve as a stabilizer to disperse hydrophobic EDOT. To simultaneously maintain high 6


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conductivity and uniformity in microstructure, the optimized molar fraction of EDOT in PVA was fixed at 0.1 (Figure S3). The soaking treatment resulted in deswelling. As show in Figure S4 and Figure 2A-2B (inset pictures), after soaking in SA for 30 min, the volume of the gel decreased by 55% and water content reduced from 87 wt.% to 75 wt.%(Figure S4). The soaking treatment results in significant improvement in the uniformity and compactness of hydrogel, as confirmed by the cross-section image of scanning electron microscopy (SEM) (Figure 2A-2B). SA penetrated into PVA-PEDOT matrix during soaking. Quantitative analysis suggests the molar ratio of SA and PVA (based on -OH group) in GEL-30 is about 0.19:1 (Figure S5). Therefore, the molar ratio of SA and PEDOT (based on EDOT monomer) in GEL-30 can be estimated to be around 1.9:1. We also prepare a CPH by in-situ addition of SA into solution of PVA-PEDOT mixtures. The molar ratio of SA to PVA is also controlled to be 0.19:1. Adding SA into PVA-PEDOT leads to the formation of a highly viscous semifluid, whose storage modulus was higher than loss modulus (Figure S6). This phenomenon may imply the existence of interaction among PVA, SA and PEDOT. Freeze-thaw cycles can convert this kind of semifluid into a free-standing hydrogel (denoted PVA-PEDOT/SA), but the formed hydrogels were full of pinholes with diameter of hundreds of nanometers (Figure S7A).

2.2 Enhancement in Mechanical Strength and Conductivity

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Figure 2. (A-B) Changes on morphology of cross section and volume shrinkage of PVA-PEDOT upon SA treatment; GEL-30 presents extraordinary mechanical properties: GEL-30 can (C) lift up an autoclave of ~2 kg; (D) bear twisting, knotting, (E) large elongation (~6 times) (F) works as conductive connection to light a LED.

GEL-30 gel exhibited sufficient mechanical and electronic conduction properties. For example, a piece of GEL-30 with a thickness of ~1.16 mm and a width ~7.28 mm, could lift up a 2-kilogram autoclave, clearly demonstrating its adequate mechanical strength (Figure 2C, Video S1). It could reversibly withstand substantial deformations such as twisting, knotting (Figure 2D), large elongation (>6 times,

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Figure 2E, Video S2). Figure 2F shows a demo circuit in which GEL-30 could serve as a conductive connection to light an LED device.

Figure 3. Enhancement on mechanical strength and conductivity upon SA soaking. (A) Typical tensile curves, (B) elastic modulus and toughness (C)I-V curves and (D) conductivity of PVA-PEDOT/SA, PVA-PEDOT and GEL-30; the tunable (E) strain/stress and (F) conductivity of GEL-30.

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Figure 3 exhibits that SA treatment drastically enhanced mechanical and conductive properties. Figures 3A and 3B compares tensile curves, toughness, and elastic modulus between PVA-PEDOT, PVA-PEDOT/SA and GEL-30. PVA-PEDOT hydrogels had tensile stress of ~0.42 MPa at the fracture strain of ~270% (Figure 3A). PVA-PEDOT/SA prepared by in-situ addition of SA was very soft, with elastic modulus of about 0.21±0.03 kPa and an elongation of 700%±45%. The low stiffness of PVA-PEDOT/SA could be related to the pinholes in the bulk phase (Figure S7A). The microscopic region of PVA-PEDOT/SA away from the pinholes had very similar morphology to that of GEL-30, exhibiting much more homogeneous networks (Figure S7B). The intrinsic fracture stress of PVA-PEDOT/SA could be much higher than the measured value. GEL-30 reached stress of ~2.4±0.12 MPa, which was about 5 times higher than PVA-PEDOT, and the corresponding ruptured strain of 627±68%, about 2 times longer than PVA-PEDOT, but slightly lower than that of PVA-PEDOT/SA. The toughness of GEL-30 was 7.8±0.4 MJ m−3, which was over one order of magnitude higher than that of PVA-PEDOT (0.68±0.2 MJ m−3) and PVA-PEDOT/SA (0.07±0.01 MJ m−3). The elastic modulus increased by more than 7 times from 0.14±0.01 MPa for PVA-PEDOT to 1.0±0.15 MPa for GEL-30. (Figure 3A and 3C). Despite the excellent mechanical properties, GEL-30 also had high water content of over 75 wt.% (Figure S4). SA treatment comprehensively enhanced all the major mechanical properties. This feature highlighted the advantages of our strategy over those which only focus on enhancing fracture stress or elastic modulus at the expense of elongation.14, 29 10


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In addition of enhanced mechanical performance, SA treatment also drastically enhances the conductivity. The conductivity was calculated according to the I-V curves from four-probe method (Figure 3C and setup in Figure S8), which was more likely related to the electronic rather than ionic conductivity.30 As shown in Figure 3D, the conductivities of GEL-30 was estimated to be 14.9±2.9 mS cm-1, lower than that of PVA-PEDOT/SA (~26-22 mS/cm), but 3 to 4 times higher than that of PVA-PEDOT (~4±0.8 mS cm-1). The mechanical and conductive properties could be readily tuned by adjusting the immersion duration (Figure 3E-3F). Reducing the soaking time resulted in decay in comprehensive mechanical properties (Figure 3E, Figure S9). For example, soaking 5 min in SA produced hydrogel with typical fracture stress of pKa of -COOH group on SA. Clearly, in FITR spectra, the resulting GEL-30(pH=7 and pH=5) sample showed an identical peak position of -OH absorption to that of the neat PVA (Figures 4B-i and S13), suggesting no existence of hydrogen bonding when the pH of soaking solution > 3~4. Furthermore, dehydrating PVA-PEDOT to 70% water content cannot match the superior mechanical strength as that of GEL-30 (Figure S14). These results clearly indicated that both the increment of crystal domain and the formation of hydrogen bonding within GEL-n contribute to enhanced dimensional stability. Considering that PVA-PEDOT/SA without deswelling exhibited much higher conductivity than that of PVA-PEDOT, we propose that the improved conductivity of GEL-30 is due to the doping effect. 36 Figures 4C-i and -ii compares the doping state by using Raman spectroscopy and doped structures between PVA-PEDOT and GEL-30 gels, respectively. No noteworthy peaks existed for PVA. The spectrum of 14


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GEL-30 showed a broad band centered at 1427 cm-1(symmetric Cα=Cβ stretching), 36-37

suggesting the doped state of PEDOT.26 PVA-PEDOT gel showed a slightly less

broad band centered at 1431 cm-1. GEL-30 displayed a high Cβ-Cβ stretching band at 1368 cm-1. indicating the presence of long benzoidic PEDOT chains.26 In contrast, PVA-PEDOT without soaking treatment showed much less intense bands at 1368 cm-1, indicating a low doping level of PEDOT.36 Therefore, the penetrated SA effectively altered the conjugation of PEDOT. Note the hydrogel only composing of PVA and SA at a molar ratio of 1:0.19 showed extremely low electronic conductivity (2 mS cm-1, Figure S15), therefore the increase in conductivity of GEL-30 is highly unlike to be caused by the incorporation of SA. Neither deswelling nor incorporation of ions notably contributed to the measured conductivity. The state of SA in GEL-n also influenced the conductivity. As shown in Figure 3F, GEL-5 shows optimal conductivity. The molar ratio between SA and PEODT in GEL-5 was only around 1, much less than that in GEL-30 (Figure S5). The presence of excess of SA in PEDOT may not necessarily increase the doping level of the complex PEDOT (Figure S16),26 as this excess SA may instead cover the surface of the PEDOT and lower its conductivity. A similar phenomenon conventionally occurred to PSS-PEDOT systems, wherein excess insulating PSS often formed a thin layer on PEDOT to deteriorate conductivity. 38-40 This may explain the tendency in conductivity along with soaking time. Overall, soaking treatment in SA contributed to both strengthening and doping. However, their optimal status may not be synchronized. High conductivity required 15



an appropriate amount of SA to allow for a high doping level, but without excessive SA coverage on PEDOT. Meanwhile, for mechanical properties, the appropriate extension of PEDOT and deswelling of PVA was critical to achieve the balance between ductile and brittle networks. In addtion, we should disclose that, similar to many other hydrogels which suffer dramatic weakening in aqueous environment,41 the as-fabricated GEL-30 exhibited notable decreases in mechanical properties when being immersed in DI water (Figure S17). However, the conductivity of GEL-30 barely changes along with the soaking time in water. A detailed explanation on this decay in mechanical strength is out of the scope of this initial study, but we could simply attribute this to the elimination of hydrogen bonding between PVA/PVA and PVA/SA due to the competition of water.

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Figure 5. Organization of PEDOT within GEL-30 matrix. Surface AFM images (in the peak force quantitively analysis mode (PFQNM)) of young modulus mapping of the freeze-dried (A) PVA-PEDOT and (B) GEL-30.

These above-mentioned results suggest the critical role of SA in altering the structure of PVA-PEDOT. The PVA host matrix becomes negatively charged due to the SA attachment through hydrogen bonding, while the PEDOT chains carry positive charges. To reveal whether the charge interaction can promote the organization of conductive polymers as reported in literature,42-43 we compared the surface morphology of freeze dried PVA-PEDOT and GEL-30 by using atomic force microscopy (AFM) in the peak force quantitively analysis mode (PFQNM). PEDOT with conjugated polymeric backbone should be more rigid than PVA with linear polymeric backbone, and thus generate obvious contrast on AFM signal under this mode: The bright phase corresponded to PEDOT while the dark phase referred to PVA. We, therefore, can reveal the difference on the distribution of PEDOT domain within PVA matrix with or without the presence of SA. Generally, conjugated PEDOT chains self-assemble into lamellae via π−π stacking. These self-assembled lamellae of PEDOT in PVA-PEDOT were irregularly aggregated (Figure 5A) as evidenced by the large and random domain of bright phase in the young modulus images. In contrast to that of PVA-PEDOT, the topography image of GEL-30 shows an assembly of long grain with more obvious roughness on the surface (Figure S18 for topology images). This could be due to the formation of PEDOT grains.44 The young modulus images evidently suggest that SA treatment leads to the formation of an interlaced structure constructed by belt-shape PEDOT with a length of hundreds of 17



nanometers (Figure 5B). This clearly demonstrates the SA treatment benefits the organization of PEDOT within gel matrix. In principle, the long belts-shaped PEDOT can interlace to reinforce the dimensional stability and provide percolation structure.42, 44

Unfortunately, it must be emphasized that such organization is limited to a short

range (Figure 1), because the intrinsic conductivity of PEDOT itself is much higher (e.g. 3500 S/cm) than that of GEL-n.45

2.4 Self-Healing Properties

Figure 6. Study on the self-healing properties. (A) the self-healing process with the aid of hot water; (B) optical microscopic images of the self-healing process to show the disappearance of the boundary between the two gels; (C) mechanical stretching the healed GEL-30 (yellow arrow indicates the wield interface); (D) stress-strain curves (E) self-healing efficiency of healed GEL-30 upon dipping the cutting surface in hot water for 2 min and 5 min. The self-healing efficiency is calculated by the by 18


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the formula η=Ah/Ao×100, where Ah and Ao are the fracture stress or conductivity of the healed and the original samples, respectively. The self-healing property is desirable for conductive hydrogels.9, 11, 46-47 The self-healing process can be automatous or non-automatous.48 Despite of the rich non-covalent interaction in GEL-n hydrogels, automatic self-healing between two cut surfaces was not observed. It is likely that the high stiffness inhibited the mutual diffusion of interface,48 which is indispensable for automatous self-healing.49 Since there was no covalent interaction, the GEL-30 can be softened and eventually dissolved in water at elevated temperature (Figure S19). Thanks to the dynamic nature of non-covalent interactions and the hot water softening effect, the cut surfaces of GEL-30 could be healed with the aid of hot water. The softening temperature should be controlled to between 60 to 90 oC, beyond which the interface of the cut GEL-30 would be either insufficiently softened or undergo gel-to-sol transition (Figure S20). Therefore, to achieve optimal self-healing efficiency and kinetics, we chose 80 oC water to conduct the softening.

The healing process is illustrated in Figure 6A: First, the cut surface of GEL-30 was dipped into hot water at 80 oC for 5 min. Then, the softened surface was covered with a PE film and brought together to contact at 20 oC. The softened hydrogel matrix possesses sufficient mobility so that the gap was finally filled due to mutual diffusion to these two gel phases. The interfaces fused with each other and, subsequently, stiffened during a contact course of 10 min (Figure 6B). The newly recovered hydrogel could be stretched over several times without fracture from the welded 19



interface (Figure 6C), indicating a strongly healed interface. In Figure 6D and 6E, quantitative analysis of the temporal dependence of the healing indicates an increase in weld-line strength and connection along with softening duration. Upon 5 min softening, the fracture stress and conductivity of healed GEL-30 could reach ~2 MPa and 14 mS/cm, corresponding to the self-healing efficiency of ~83% and 99%, respectively. The self-healing of GEL-30 stemmed from the interplay of interface fusion and the healing reaction. The fusion of interface was caused by the re-entanglement/remodeling of polymer chains, while healing reaction involved the re-formation of hydrogen bonding between the -OH of PVA themselves or crystallites, but not between the -OH of PVA and the -COOH of SA (Figure S21). The entire self-healing process took no more than 15 min. This is comparable or even relatively faster than that of many state-of-the-art self-healing hydrogels.50-51 The rapid self-healing GEL-30 may offer promise of being a material platform for wearable strain sensors.

2.5 GEL-30 as a Strain Sensor

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Figure 7. Study on the GEL-30-based strain-sensor. (A) Variation of normalized resistances as a function of flexion angle from 0° to 120°, the measuring method and the definition of flexion angle is schematically present in the cartoon above the figure; The inset equation is expressed in the form of y=Aθ2+Bθ+C, where y is the resistance changes and θ is the flexion; (B) Relative resistance changes versus time for the bending and release of the index finger, the above photographs show the sensor is fixed on a finger. R and R0 stand for the resistance before and after bending, respectively.

Lastly, a preliminary study was conducted to demonstrate the potential application of GEL-30 as a strain sensor to monitor bending of a finger knuckle in real time. We note that mechanical strength of GEL-30 showed instant recovery from the deformation of 20% elongation (Figure S22). Given the fact that the average fracture strain of ligaments is among 11%-44% with average value of ~27%,52 20% deformation may suffice the daily bending of body joints, such as a finger knuckle. Larger elongation results in the slow recovery due to the energy dissipation as the breaking of weak non-covalent binding in GEL-30.53-54 Figure 7A displays the resistance change as a function of flexion angle for the GEL-30-based strain sensor. As shown in the cartoon, GEL-30 was mounted on a 21



flexible substrate (rubber rod). When the substrate was flexed, the expanded outer curvature elongated the GEL-30 hydrogel, therefore, the resistance increased with increasing flexion angle. The resistance increases to >160% of its original value with the bending angle increasing from 0° to 120°. This sensitivity towards angle changes is higher than several other advanced CPHs-based strain sensors.15,47 Figure 7B shows the motion detection for the index finger. We checked the response behaviors of the GEL-30-based strain sensor when the fingers were repeatedly bent between 0 to ~70o at a working frequency of 0.1 Hz. The resistances significantly changed by responding to the motion of the finger rapidly and repeatedly.

3. Conclusions In conclusion, we report a simple yet effective soaking strategy to construct a type of conductive hydrogels by controlling the deswelling and post-formation of multiple noncovalent interaction. The resulted hydrogel possesses high tensile strength, toughness, conductivity and efficient healing properties with the aid of hot water. All these properties can be facilely tailored by simply controlling the soaking time. These improved properties are correlated to the alternation on structure upon soaking treatment. The deswelling leads to the enhanced crystallinity of ductile host matrix, while the embedded organic salt invokes multiple noncovalent interaction, contributing to the microscopic integrity and regularity. In the future work, the stability of GEL-n gels under environment loads should be further improved through designing robust multiple interactions. 22


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Supporting Information Supporting Information is available from the ACS or from the author.

Acknowledgements L. Gao is grateful for financial support from NSFC (Grant No. 51603046) and Guangzhou Science and Technology Program (201804010243). S. Ji thanks Guangdong Province Universities and Colleges Young Pearl River Scholar Funded Scheme (2016)

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Table of Content

A Facile Soaking Strategy towards Simultaneously Enhanced Conductivity and Toughness of Self-Healing Composite Hydrogels through Constructing Multiple Noncovalent Interactions

A rapid, simple, robust and cost-effective strategy is developed to convert weak and poor conductive hydrogel to tough and highly conductive hydrogel. The concept we prove here, that is deswelling and multiple noncovalent interaction work in tandem to enhance the comprehensive properties of hydrogels, benefits the whole field of soft and wet polymeric materials.

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A Facile Soaking Strategy towards Simultaneously Enhanced

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