Enhancing the mechanical properties and self ...

European Polymer Journal 105 (2018) 85–94

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Enhancing the mechanical properties and self-healing efficiency of hydroxyethyl cellulose-based conductive hydrogels via supramolecular interactions

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Imtiaz Hussain, Sayed Mir Sayed, Shunli Liu, Olayinka Oderinde, Mengmeng Kang, Fang Yao, ⁎ Guodong Fu School of Chemistry and Chemical Engineering Southeast University, Jiangning District, Nanjing, Jiangsu Province 211189, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Supramolecular interactions Metal-ligand interactions Conductive hydrogel Dynamic interactions Self-healing

Natural polymer based self-healing hydrogels have attracted intense consideration due to their attributable and a wide range of applications. However, to design hydrogels having excellent self-healing efficiency and super mechanical strength is still a big challenge. Herein, we report hydroxyethyl cellulose based self-healing conducting hydrogels with enhanced mechanical properties by the molecular engineering of Fe3+ ions among the functional groups of polyacrylic acid-co-polyacrylamide and hydroxyethyl cellulose chain through supramolecular interactions. The engineered hydrogels exhibit a high mechanical strength with a tensile stress of 3.50 MPa and tensile strain of 1245%, along with compression stress of 32 MPa. These hydrogels also show about 98% selfhealing efficiency as well as exhibit 2.4 × 10−3 S/cm electrical conductive. Moreover, manipulating the various parameters, the mechanical and self-healing efficiency of the prepared hydrogel can be adjusted. This work will encourage researchers to focus on this facile technique for the synthesis of self-healing hydrogel materials with enhanced mechanical properties.

1. Introduction Recently, hydrogels have acquired a great attention in various applications, including actuators [1], alternatives to articular cartilage [2], drug delivery carriers [3], and tissue engineering scaffolds [4]. However, most of the synthetic hydrogels limited their applications due to their mechanical brittleness and feebleness, poor tensile strength, low toughness and a minimum elongation at break point. To get a hydrogel having all the properties simultaneously, such as toughness, high stretchability, and self-healing properties is still a big challenge in the field of hydrogel materials [5]. To overcome these challenges, researchers have extended their studies to double-network hydrogels [6], triple network hydrogels [7], slide-ring hydrogels [8], nanocomposite hydrogels [9], polyampholyte hydrogels [10], tri-block copolymer hydrogels [11], microgel-reinforced hydrogels [12], macro-molecular microsphere composite hydrogels [13], tetra-PEG gels [14], and others. Usually, covalent bonds are formed very commonly in the formation of hydrogel network between the polymer chains, which provide stability and mechanical strength to them [15]. But, due to the irreversible nature and permanent bond breakage, these covalent bonds could not recover or repaired under normal conditions during any damage, which

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Corresponding author. E-mail address: [email protected] (G. Fu).

https://doi.org/10.1016/j.eurpolymj.2018.05.025 Received 10 March 2018; Received in revised form 9 May 2018; Accepted 23 May 2018 Available online 25 May 2018 0014-3057/ © 2018 Published by Elsevier Ltd.

limits their applications on industrial scales. To overcome this shortcoming of covalent bonding hydrogels, the researcher has introduced the supramolecular interactions in the hydrogel materials due to their reversible nature [16–18] and an excellent self-healing ability [19]. Supramolecular interactions, including hydrogen bonds [20,21], hostguest interactions, metal-ligand interactions, and electrostatic interactions [22,23] have been applied for the formation of supramolecular hydrogel materials that can reform their network structure and attain the pristine function of the hydrogels. Although the host-guest interactions can achieve a maximum recovery among all these interactions, it takes a long time to heal completely [24]. Due to the dynamic nature of these coordination interactions, supramolecular hydrogels receiving a great attention that bestow on hydrogel excellent self-healing and shear-thinning properties [25,26]. Usually, dynamic interactions (covalent and non-covalent interactions) are designed to engineer hydrogel materials and such dynamic interactions are responsible for the stability and reversibility within a single network system [27–29]. Metal-ligand interactions have attained a considerable attention among all the non-covalent interactions due to their kinetically labile and thermodynamically stable nature [30,31]. In the designing of selfhealing hydrogels, such interactions are used as a powerful synthetic


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aqueous solution of different concentrations with different concentration of metal ions and monomers AA and AAm, we designed stable hydrogels through supramolecular complexation within minutes. The facile association of Fe3+ ions with the carboxyl group of AA and hydroxy groups in HEC chains promoted the formation of gel-network. The designed hydrogel shows an excellent self-healing, high mechanical strength, moldability, free-standing, and conductive properties.

strategy that can tune their reversibility by adjusting the combination of respective metal ion and ligand. Metal-ligand not only boosts the selfhealing efficiency of the hydrogel materials but can also endow some special functions to the materials, depending on the metal ion used in the synthetic process. For example, Wei et al. reported hydrogels with autonomous self-healing properties having acrylic acid side chain that is ionically coordinated with ferric ions and such dynamic ionic interactions are responsible for the self-healing of the reported hydrogels [32]. Some groups have reported the ionic coordination interactions between Fe3+-catechol complexes that have an excellent self-healing ability after damaging [33]. Xin et al. also reported quaternized chitosan-g-polyaniline based hydrogels with injectable, conductive, and self-healing properties with the application in dressing for cutaneous wound healing and antioxidant agent [34]. Researchers have synthesized polysaccharide/PAAm hybrid hydrogels with ionically/covalently cross-linked double-network structure. Although they possess a good strength and toughness but have slow autonomous self-healing ability due to the slow diffusion of chains [35,36]. Thus, to prepare hydrogels having properties of autonomous self-healing as well as robust mechanical properties, it is required to design double network associated with non-covalent interactions. Inspired by natural materials, Qun Xu and coworkers reported selfhealing, tough, and conductive hydrogels, which were fabricated by functionalized-boron nitride to build a 3D hierarchical f-BNNS/clay/ PNIPAM ternary network by introducing non-covalent sacrificial bonds [37]. Chen et al. developed a DN agar based hydrogel that exhibited high extensibility and energy dissipation, having a tensile strength of 267 kPa and very low self-healing efficiency (≈40%) at room temperature after 24 h [35]. Qian et al. reported a novel DN hydrogel, physically cross-linked Agar/hydrophobically associated polyacrylamide with excellent mechanical properties, high toughness, and rapid recovery [35]. Recently, Hussain et al. reported HEC based hydrogels which showed high mechanical tensile strength (1.35 MPa), highly stretchability (1660%), high compression fracture tensile stress of 28 MPa along with excellent autonomous self-healing (87%) without any external healing agent [38]. On the flip side, hydrogels are usually non-conductive in nature that bound their applications, especially in modulated cell functions for excitable and non-excitable cells [39]. Therefore, researchers are trying to design conductive hydrogels that would provide an electrical cue to living cells by using an extracellular matrix, such as environment, which enable the unexplored role of electrical energy in modulating cellular response [40]. Conductive hydrogels may be developed by mixing them with conductive elements or conductively inherent polymers [41,42]. On one hand, conductive hydrogels are formed by doping them with a conductive substrate, like nanowires, metallic nanoparticles, carbon nanotubes or graphene within the hydrogel matrix [43–47] and on the other hand, some conductively inherent polymers, including polyaniline, poly(pyrrole), etc., are infused or grafted onto the hydrogel matrix, e.g., PEG and PAAm [48–50]. Recently, Zexing et al. presented β-cyclodextrin-based hydrogels with excellent selfhealing, flexible, and an elastic mechanical properties as well as high conductivity [51]. Guo and coworkers reported chitosan-graft-aniline tetramer based self-healing hydrogels possessing about 10−3 S/cm conductivity and antibacterial activity [52]. Most of the supramolecular hydrogels reported in the literature are usually formed via tedious and time-consuming methods, including enzyme triggers [53], repeat heat/cool cycles [54], but the designed biopolymer HEC hydrogel can be formed readily by simple mixing the HEC, FeCl3·6H2O solutions with a calculated amount of monomers AA and AAm in the presence of APS as initiator at room temperature. The Fe3+ ions then cross-link with the functional groups (eOH & eOe) of biopolymer and P(AA-co-AAm) chains, leading to the formation of hydrogels in a very short time. We reported a new type of hydrogel cross-linked by Fe3+ ions as metal ions and HEC as a biopolymer in the presence of AA and AAm monomers. By mixing the HEC biopolymer

2. Materials and methods 2.1. Materials Hydroxyethyl cellulose (CELLOSICE™ QP 4400H) was purchased from the DOW Chemical Company. Acrylamide (Am) was purchased from Sinopharm Chemical Reagent Co. Ltd and Acrylic acid (AA, ≥99.5%) was purchased from Shanghai Macklin Biochemical Co. Ltd., while ammonium persulfate (APS) and ferric chloride hexahydrate were purchased from Aladdin Co. Ltd. (Shanghai, China). For the purification of AA basic alumina oxide column was used. Other reagents were used without further purification and the solutions were prepared in deionized water. 2.2. Preparation of HEC/P(AA-co-AAm)-Fe3 + Hydrogels Hydroxyethyl cellulose based biopolymer hydrogel was synthesized through free radical polymerization in the presence of APS as initiator at room temperature within 20 min with a little modification as reported in the literature [38]. Briefly known amount of HEC was dissolved in deionized (DI) water to form various concentrations (1, 2, 3, 4 & 5%) of HEC solutions at 60 °C under continuous stirring for 3 h and then slowly cooled to room temperature and marked as solution A. After that various amount of Am was dissolved in AA at different ratio of Am to AA (1:2, 1:1, and 2:1) and marked them as solutions B. 5.0 mL of solution A was added to various volumes of solution B under continuous stirring for 10 min. After that, this mixture was charged with various concentrations (0.01, 0.05, 0.1 and 0.2 M) of FeCl3·6H2O aqueous solution and stirred for 10 min to form a homogenous mixture. The resulting mixture was purged with nitrogen gas, then 1 mL of APS containing 100 mg/mL was added and stirred for further five minutes. After transferring the homogeneous mixture to cylindrical glass vials, the mixture was sonicated for 2 min and the process of polymerization was carried out at room temperature. After 20 min, hydrogels were formed and removed from the glass vials safely, which were immersed in the DI water for 24 h to remove the unreacted monomers and excess metal ions. 3. Results and discussion Hydroxyethyl cellulose-based self-healing conducting hydrogels were prepared by the molecular engineering of poly(acrylic acid-coacrylamide) with hydroxyethyl cellulose chain through ionic crosslinker Fe3+ and hydrogen bond interactions. The engineered hydrogels exhibit high mechanical strength, self-healing ability, and conductive properties. We prepared hydrogels with different concentration of AA, Am, Fe3+, and HEC. The monomers (AA: Am) concentration ratio by mass was standardized to be 1:1, while the concentration of HEC and Fe3+ were optimized at 5% and 0.10 M, respectively. The developed hydrogel is symbolically represented by HECx/P(AA-co-AAm) y:z-Fe3+ 0.10, where x represents the percent by weight of HEC and y:z shows the ratio of AA to Am by mass, while 0.10 shows the molar concentration of the FeCl3·6H2O solution. Scheme 1 describes the formation of hydrogels by supramolecular complexation between the native HEC biopolymer and the P(AA-co-AAm) chains with ionic crosslinker Fe3+. The gelation was formed by mixing the precursor solutions via magnetic stirring at room temperature and keep for few minutes at ambient temperature for control and safe polymerization of AA and AAm. During the process of 86


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Scheme 1. Schematic representation of the formation HEC/P(AA-co-AAm)-Fe3+ hydrogels.

fingerprint regions at 1016–1187, 811–881, and 562 cm−1 also suggest the complexation of eOH groups of HEC with metal ions (O-Fe3+) through coordination interactions [56].

polymerization, the oxygen of ether on the HEC chain and the carboxyl groups on PAA participated in the coordination interactions with the ionic crosslinker Fe3+ ions, while the eOH groups on HEC chain formed hydrogen bonds with the eOH and eNH groups on P(AA-coAAm) polymer chain.

3.2. UV measurements 3.1. FTIR analysis

UV–vis absorption spectra of the hydrogels were recorded in water by using a UV-2600 UV–vis spectrophotometer Shimadzu. The absorption peaks for HEC, HEC/PAA-Fe3+, and HEC/P(AA-co-AAm)-Fe3+ at different positions reflect the formation of coordination bonds within the hydrogel network, as depicted in Fig. 1B.

The bond formation was characterized by comparative analysis of FTIR spectra of the substrates and the prepared hydrogels. The FTIR spectra of HEC, HEC-PAA, and HEC/P(AA-co-AAm)-Fe3+ hydrogels are displayed in Fig. 1A. The absorption bands for the stretching vibrations of OeH and NeH in all samples could be observed at 3452 cm−1. Compared to the HEC/P(AA-co-AAm)-Fe3+ hydrogel, the absorption bands for HEC and HEC-PAA samples at 3452 cm−1 is broadened due to a large number of hydroxyl groups on HEC and PAA chains, which becomes narrow for the HEC/P(AA-co-AAm)-Fe3+ hydrogel, suggesting the formation of ionic coordination which reduce the number of eOH of the carboxyl groups [55]. The peaks at 2865 and 2923 cm−1 disappeared in the HEC/P(AA-co-AAm) and HEC/P(AA-coAAm)-Fe3+ hydrogels samples, suggesting the formation of hydrogen bonds among the eOH groups of HEC and PAA. The peaks at 1710 cm−1 in PAA-Fe3+ (Fig. S1A) and at 1730 cm−1 in HEC-PAA hydrogel samples (Fig. S1B) attributed to the C]O stretching vibration, which disappeared in the spectrum of HEC/P(AA-co-AAm)-Fe3+ hydrogel. This is because of the reorganization of the carboxyl groups with the Fe3+ ions and the corresponding peaks overlap with the amide I band at 1625 cm−1. Similarly, the disappearing and narrow peaks in the

3.3. Structural morphology The surface and cross-sectional morphology of the prepared hydrogel was characterized by using scanning electron microscopy (SEM) (INSPECT F50). The morphology of the hydrogels was observed at different scales, as shown in Fig. 2. SEM analysis of the surface morphology shows that the hydrogel has tight, dense, rough, and wavy surface, which indicate its high toughness and dense cross-linking, as shown in Fig. 2A. The cross-sectional images (Fig. 2B). also confirm a wrinkled, rough, lamellar morphology, which may arise from the incorporation of metal ions. This comparatively rough surface is convenient for increasing the surface area and will facilitate the diffusion of ions and polymer chains movement, which enhances the self-healing ability of hydrogels.

Fig. 1. (A) FTIR spectra of HEC, HEC-PAA, and HEC/P(AA-co-AAm)-Fe3+ Hydrogels, (B) UV–vis spectra of HEC, HEC/PAA-Fe3+ and HEC/P(AA-co-AAm)-Fe3+ hydrogels. 87


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Fig. 2. SEM image of hydrogels showing (A) surface morphology and (B) cross-sectional morphology.

structure via supramolecular interactions. The larger number of hydroxyl groups in HEC can form intramolecular hydrogen bonds or competitively form hydrogen bonds with water and PAA chain. The reason for the increase in compression stress is that the higher concentration of HEC in the hydrogels results in more and strong supramolecular interactions within the hydrogel network, which lead to form a dense network. However, higher HEC concentration produces a brittle network, which decreases the compression strength of the network. The strength of mechanical properties was also performed by compressing the cylindrical hydrogel with a 3 Kg stainless steel block and it was interesting to find that after removing the load, the hydrogel recovered its original shape instantaneously and this process is summarized in Fig. 3D–F. and supplementary video S1.

3.4. Mechanical strength measurements The typical mechanical properties of the HEC/P(AA-co-AAm)-Fe3+ hydrogels were studied by measuring their tensile and compression strength. The tensile strength was measured on samples with the dimensions of 35 × 4 × 3 mm and compression strength was recorded on samples in the cylindrical forms with a diameter of 15 cm and a height of 10 mm. The effect of HEC concertation was systematically studied on the mechanical strength of HEC/P(AA-co-AAm)-Fe3+ hydrogel, which is given below. 3.4.1. Compression Analysis Cylindrical hydrogels samples with different concentrations of HEC (1, 2, 3, 4 & 5%) were subjected to evaluate their compression strength and the typical compression stress-strain curves are given in Fig. 3A. Hydrogel with 1% HEC concentration shows a compression stress of 23 MPa, while its compression strain is 90%. With the increase in the HEC concentration up to 4%, there is an increase in both the compression stress and fracture strain percent, while further increase shows a decrease in both the compression stress and strain. With 4% HEC concentration, the prepared hydrogels show the highest compression stress of 32 MPa and a fracture strain of ∼100%. Further 1% increase in the HEC concentration drops the compression stress to ∼28 MPa and fracture strain to 95%. The comparative compression fracture stress and strain bar graphs are given in Fig. 3B and C, respectively. The compression stress of all the hydrogels samples with different HEC concentrations increased slightly with the increasing deformation and all the five samples showed remarkably similar curves deposited at the different concentrations of HEC. This indicates that these composite hydrogels have good elasticity and ductile property because of the hydrogen bonds and electrostatic interactions. In addition to the fact that the content of HEC in this study is in the range of 1–5 wt%, the larger number of hydroxyl groups result in assembly of the three-level

3.4.2. Tensile strength analysis The tensile strength of the developed hydrogels was recorded by using tensile test machine on the rectangular shape samples and the cross-head speed was fixed at 60 mm/min. The typical tensile stressstrain curves for hydrogels with different concentrations of HEC are given in Fig. 4A. It is clear from Fig. 4A, B and C, that there is a trend of first increase and then decrease in both the tensile and fracture stress and strain with an increase in the HEC concentration. Hydrogels with 1% HEC concentration shows the minimum tensile strength of all the hydrogel samples with a higher concentration of HEC, a fracture tensile stress of 1.25 MPa, and a breaking strain of 680%. The increase of 1% HEC concentration causes about 32% strength in its tensile stress and 38.23% increase in the tensile breaking strain. Further increase in the HEC concentration up to 3% results in an increase of 64% and 53% in the tensile stress and strain, respectively. At 4% HEC concentration, the hydrogels show maximum tensile strength, which is 187.5% increase in stress and 83.09% in the breaking strain as compared to the hydrogel sample with 1% HEC concentration. While the further increase in the HEC concentration (5% HEC) results in a decrease in the tensile stress 88


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Fig. 3. (A) Compression stress-strain curve of HEC/P(AA-co-AAm)-Fe3+ hydrogel, (B and C) compressions fracture stress and strain, and (D-E) images of original, compressed, and after removing the load from the hydrogel, respectively.

Fig. 4. (A) Tensile stress and strain curves of HEC/P(AA-co-AAm)-Fe3+ hydrogels, (B) toughness of hydrogels, (C and D) tensile fracture stress and strain of hydrogels. 89


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G′ value of ∼9000–10000 Pa and G″ of 2500–3500 Pa. However, tanδ = G″/G′ below 0.4 shows a week dependency on the frequency change.

(7.14%) as well as in the fracture strain (34.94%) as compared to the hydrogel sample with 4% HEC concentration (Fig. 4A). Therefore, 4% HEC concentration is the optimum concentration for the development of hydrogels with maximum mechanical strength. The reason for the first increase in the tensile strength is due to the increased in a number of hydrogen bond formations among the eOH groups of HEC with the functional groups of P(AA-co-AAm). As the HEC concentration increases, the number of the hydrogen bonds formation also increases, and the higher number of the hydrogen bonds acts as sacrificial bonds, which are responsible to dissipate a large amount of energy when the hydrogel undergoes a deformation. The decrease in the mechanical strength at higher concentration of HEC is due to the formation of heterogeneous hydrogels and the formation of homogenous hydrogels is difficult due to the high viscosity of HEC solution at high concentration. In this case, the number of hydrogen and metal-ligand bonds decreases which in turn decrease the mechanical strength of the hydrogel. Similarly, the HEC concentration also affects the toughness of the hydrogels in the same manner as the tensile strength. Hydrogels with 1% HEC concentration shows the minimum toughness of 5.35 MJ m−3, while a further increase of 1% in the HEC concentration results about 86.73% increase in the toughness of hydrogel. Increasing the concentration of HEC from 1% to 4% results in 433.40% increase in the toughness of hydrogel. While further 1% increase beyond 4% shows a decrease of 39.15% in its toughness, and this whole phenomenon is given in Fig. 4B. Here again, the reason is the formation of heterogeneous hydrogel due to the high viscosity of HEC, which decreases the formation of sacrificial bonds within the hydrogel networks.

3.5. Self-healing experiment For self-healing experiment, the hydrogel samples with different concentrations of HEC (%) were prepared in the rectangular shape and then each sample was cut in the middle with a knife. After that, the two cut pieces were brought together along their cut interfaces and put them in a petri dish. The petri dish was sealed with a polyethylene bag to eliminate the water evaporation from the hydrogel samples and the healing process was carried out at room temperature for 24 h without any external agent. The tensile tests of the healed samples (H) were recorded with the same instrument used for the original samples (O) at the same cross-head speed. The self-healing efficiency in stress, strain, and toughness was calculated by comparing the stress, strain, and toughness of the healed hydrogels with the original one. The percent self-healing efficiency in the stress was calculated by Stress(H)/ Stress(O) × 100 and the percent strain healing efficiency by Strain(H)/ Strain(O) × 100. Toughness was calculated by the area under the stressstrain curve of each hydrogel in MJ m−3. The typical stress-strain curves for original (O) and healed (H) samples are depicted in Fig. 6A and the whole summary is given in Table 1. Hydrogel with 1% HEC concentration shows about 92.64% healing efficiency (H.E.) in stress and 88.23H.E. in tensile strain within 24 h healing time. The excellent healing ability is attributed to the diffusible metal ions and polymer chains along the cut surfaces, which reform the sacrificial dynamic noncovalent bonds. When the HEC concentration was increased to 4%, the hydrogels exhibited excellent healing efficiency (H.E.) both in the tensile stress and tensile strain by achieving 97.14% H.E. stress and 94.78% in H.E. strain. At 5% HEC concentration, there is a decrease in the H.E. stress and strain, which is 93.84 and 93.2, respectively. The H.E. in toughness also shows the same trends, with 1% HEC concentration, the hydrogels exhibit about 81.93% H.E. in toughness, which reached to 90.51%, when the HEC concentration was increased to 4%. Beyond 4%, it decreases to 86.25% due to the formation of fewer hydrogen bonds in a heterogeneous hydrogel network and this whole phenomenon is given in Fig. 6B. The higher H.E. in stress, strain, and toughness at 4% HEC concentration is due to the reformation of maximum sacrificial hydrogen and dynamic non-covalent interactions. The Fig. 6C, D and E shows the comparison of stress, strain, and toughness of original and healed hydrogels samples, which shows that hydrogel with 4% HEC concentration recovers the maximum strength. Thus, 4% HEC concentration is the optimum concentration of the strength as well as for the self-healing efficiency of the HEC/P(AA-co-AAm)-Fe3+ hydrogels. These results indicate that cutting the hydrogel with a knife causes the rupturing and destroying the supramolecular interactions within the hydrogel network. After contacting the cut surfaces, the

3.4.3. Rheological measurements A strain amplitude sweep measurements were performed to analyze the storage modulus G′ and the loss modulus G″ of the hydrogel as a function of the oscillatory strain amplitude and Fig. 5 presents the rheology properties of hydrogels. The storage modulus (G′) was approximately 10200 Pa and the loss modulus (G″) was about 1500 Pa. Both G′ and G″ values are independent of the frequency, demonstrating that the hydrogel was stably formed. Fig. 5A shows the typical storage (G′) and loss (G″) moduli versus oscillatory strain amplitude, “εo”. As shown in Fig. 5A, at εo < 100%, the hydrogel displayed elastic features, as both the G′ and G″ move constantly and in this rage they do not depend on angular frequency “ω” and it is clear that G′ > G″ when εo ∼ < 100%. At εo > 100%, the value of G′ decrease while the value of G″ increase but does not approach each other. This indicates that the hydrogel is too tough and do not collapse during the large deformation stress. In addition, a frequency sweep measurement was also recorded to further confirm the gel-like behavior of the prepared hydrogel. As indicated in Fig. 5B, the storage modulus G′ is larger than the loss modulus G″ under all the frequency ranges, which are gradually and almost parallelly increases with the increase in frequency in the range of 0.1–0.15 Hz, and reach to

Fig. 5. Rheological measurement of hydrogels, (A) storage and loss moduli as a function of strain amplitude sweep test and (B) frequency sweep test of the hydrogels. 90


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Fig. 6. Comparative graphs of (A) tensile stress-strain curves of original and healed hydrogel samples with different concentration of HEC, (B) percent healing efficiency in stress, strain, and toughness, (C and D) fracture stress and strain of original and healed hydrogels, respectively, and (E) toughness of original and healed samples. Table 1 The comparative mechanical strength of original and healed hydrogel samples along with the percent growth rate in stress, strain and toughness of the hydrogel samples with a different concentration of HEC. HEC Conc. (%)

1% 2% 3% 4% 5%

Fracture stress(MPa)

Fracture strain (%)

Toughness (MJ m−3)

O

H

O

H

O

H

1.25 1.65 2.00 3.50 3.25

1.15 1.44 1.80 3.4 3.05

680 940 1040 1245 810

600 780 900 1180 760

5.35 9.99 13.31 28.53 17.37

4.46 7.34 10.38 25.83 14.98

% G.R. H.E. in stress

% G.R. H.E. in strain

% G.R. H.E. in toughness

91.31 85.42 88.89 97.05 93.44

86.67 79.49 84.44 94.51 93.42

80.04 63.89 71.77 89.55 84.05

healing by reconstructing the hydrogen bonding supramolecular interactions between the two cut pieces, which results in the regaining of mechanical strength within the hydrogel. Thus, the formation of interchain supramolecular interactions among HEC, P(AA-co-AAm) and Fe3+ ions at the cut interface of the hydrogel results in the repairing and recovery of hydrogel along with inducing the mechanical strength to the hydrogel network.

supramolecular interactions are reformed by the process of diffusion of ions and polymer side chain. With the passage of time the number of the new supramolecular interactions among various groups within hydrogel increases that strengthen the hydrogel network as the pristine one.

3.6. Macroscopic self-healing test We also performed another healing experiment by cutting the rectangular hydrogel sample with a knife in middle and one piece was dyed with rhodamine B for their better visualization. Then the two pieces were brought together at their cut surfaces and kept it in a polyethylene bags for some time to heal at room temperature. After some time, the sample was taken out from the bag and it was interesting to see that the sample was healed with enough mechanical strength that it can withstand without any fracture (Fig. 7A and B), and can be bent in U-shape or semicircular shape, as depicted in Fig. 7C–E. The ability of self-healing is attributed to the dynamic supramolecular interactions, including electrostatic coordination and hydrogen bonding interactions within the prepared hydrogel, and these supramolecular interactions are responsible for the self-healing of the prepared hydrogels. The self-healing of the designed hydrogel results from the diffusion of Fe3+ ions along the cut interfaces and reforming of the supramolecular interactions at their points of contact. The dynamic nature of polymer chain also plays a critical role in the process of self-

3.7. Re-processability/Re-shapeability To find the reprocess-ability of the synthesized hydrogel, we dyed one-rod shape hydrogel sample with rhodamine B and other with methylene blue for their better visualization and confirmation. These two dyed samples were further cut into small pieces and put them in an irregular way on a glass slide with alternate colors, and this whole process is shown in Fig. 8A. We pressed these pieces between two glass slides for 1 h and after removal of the glass plates, it was very interesting to see that these pieces were healed together into a formed a new film. The diffusion of colors at the cut interfaces shows the formation of new supramolecular interactions among the different pieces of hydrogel sample. The new reformed hydrogel film was so strong that it can withstand without any visible fracture, as shown in Fig. 8B. The Fig. 8C. collectively shows that the film is so strong that it acts as a beam which can be used as a horizontal support. The Fig. 8D. indicates that the reprocessed hydrogel film can be switched in any shape like U, 91


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Fig. 7. (A) rectangular shape healed hydrogel samples dyed with rhodamine B, (B) withstanding of the healed hydrogel, (C-E) bending at various angles of the healed hydrogels.

Fig. 8. (A) rectangular and cut pieces of the dyed hydrogels, (B) healed film can withstand after 10 min, (C) healed film acting as a beam and horizontal support, (D) switching of healed hydrogel film in various shapes. 92


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Fig. 9. (A) LED illumination of pristine hydrogels, (B) switching off the LED by cutting the hydrogel, (C) conductivity and LED illumination of the healed sample, and (D) electrical conductivity of original and healed hydrogel.

shown in Fig. 9D. The original hydrogel sample shows about 2.4 × 10−3 S/cm. It was interesting to note that the hydrogel healed for 2 min showed the conductivity about 1.8 × 10−4 S/cm and reached to the maximum value as the original hydrogel sample (about 2.4 × 10−3 S/cm) within 2 h of healing time.

semicircular, S, Z, and in any desired shape. Thus, Fig. 8 displays that the HEC/P(AA-co-AAm)-Fe3+ hydrogel could reshape and reprocess with considerable mechanical strength. Herein, the supramolecular interactions are also the key factor that results in the healing of the cut pieces and gives them a mechanical strength. They are also having a great role in the switching of the molecular shapes, which give a flexible nature to the healed hydrogels. Therefore, we can it this in a various shape memory, healable supercapacitors, and many industrials applications.

4. Conclusion We fabricated the biopolymer-based supramolecular hydrogel by ultrafast complexation among HEC, P(AA-co-AAm), and Fe3+ ions with relatively excellent mechanical properties and self-healing capability. The formation of the developed hydrogel is a facile and instantaneous process, which make the most suitable procedure for the hydrogenation. The engineered hydrogel exhibited workable and multifunctional properties, including reshapeability, reprocess-ability, shape-persistent, conductivity, self-healing, and free-standing. We hope biopolymers hydrogels with these unique properties make them very useful in designing of advanced soft materials, gel systems, and find their practical usages in various fields, such as antibacterial gel membranes, logic gates, smart devices, healable electronic devices, and sensors.

3.8. Conductivity test To investigate the electrical conductivity of HEC/P(AA-co-AAm)Fe3+ hydrogel, we performed a macroscopic experiment by using a battery-powered complete circuit with a light-emitting diode (LED) indicator to demonstrate visually the conductivity of pristine and selfhealed hydrogel samples. Fig. 9A shows a complete circuit with a rectangular shape hydrogel sample and lighting the LED indicator confirms the conductivity of the synthesized hydrogel. After breaking the circuit by cutting the hydrogel sample with the knife, causes the switching of the LED, as shown in Fig. 9B. However, bringing the two cut pieces together at their cut interfaces results in a strong illumination of the LED bulb, which indicates the complete recovery of the cut hydrogel with conductive properties same as for original sample, as shown in Fig. 9C and this whole process are also shown in the supplementary video S3. This test described that the dynamic supramolecular interactions within the hydrogel network play a key role in the conductivity of the hydrogel network. As we cut the hydrogels, the supramolecular interactions were dismissed, which resulted in the switching of the LED bulb. On the other hand, combining the cut pieces together caused the reforming of the supramolecular interactions, which resulted in the switching on of the LED bulb by reconstructing the circuit. The electrical conductivity was measured by four-point-probe in S/cm and the conductivity of original and healed samples at different time intervals is

Acknowledgments This work was supported by National Natural Science Foundation of China under the Grant 21274020 and 21304019. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.eurpolymj.2018.05.025. References [1] M.K. Shin, G.M. Spinks, S.R. Shin, S.I. Kim, S.J. Kim, Nanocomposite hydrogel with

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