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PAni composite hydrogels with conductive and magnetic properties
Journal Pre-proof Tough and stretchable Fe3O4/MoS2/PAni composite hydrogels with conductive and magnetic properties Hengfeng Hu, Ximing Zhong, Shuibin Yang, Heqing Fu PII:
S1359-8368(19)34377-X
DOI:
https://doi.org/10.1016/j.compositesb.2019.107623
Reference:
JCOMB 107623
To appear in:
Composites Part B
Received Date: 27 August 2019 Revised Date:
21 November 2019
Accepted Date: 25 November 2019
Please cite this article as: Hu H, Zhong X, Yang S, Fu H, Tough and stretchable Fe3O4/MoS2/PAni composite hydrogels with conductive and magnetic properties, Composites Part B (2019), doi: https:// doi.org/10.1016/j.compositesb.2019.107623. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Tough and Stretchable Fe3O4/MoS2/PAni Composite Hydrogels with Conductive and Magnetic Properties Hengfeng Hu1, Ximing Zhong1, Shuibin Yang * 2, Heqing Fu*1 (1 School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, P.R. China;2 Hubei Key Laboratory for Processing and Application of Catalytic Materials, College of Chemical Engineering, Huanggang 438000, China) *Corresponding author. Tel: +86 020 87114919;Fax: +86 020 87112047. E-mail: [email protected]; [email protected]
ABSTRACT Novel composite hydrogels with conductive and magnetic properties are prepared by introducing ternary composites into a double-network hydrogel. The ternary composites are comprised of Fe3O4, MoS2 and PAni. Fe3O4 possessing high surface area could be served as a carrier. And MoS2 with a special sandwich structure will provide enough interlayer spaces to combine with PAni via electrostatic attraction. The ternary composites are further introduced into a double-network hydrogel. The resultant composite hydrogels have maximum saturation magnetization of 1.28 emu·g−1 and conductivity of 2.052 × 10−3 S·cm−1, respectively. In addition, the thermo-stability and the mechanical property of the composite hydrogels are improved by the incorporation of ternary composites, and the tensile strength of the composite hydrogels is about 140 kPa with a good stretchability (≈170%). Therefore, these flexible and robust composite hydrogels with conductive and magnetic properties have good prospect in the fields of the electronic skin, drug release,
electromagnetic interference shielding and wave-absorbing materials . Keywords: hydrogel; ternary composite; toughness; conductivity; magnetic properties
1. Introduction Hydrogels have been extensively studied owning to their hydrated molecular three-dimensional network structures that could serve as a matrix for the combination of various composites[1-4]. Currently, a convenient approach to enrich multiple functionalities is to introduce composites into hydrogel matrix, and the resultant composite hydrogels possess many features, including robust mechanical, magnetic and electrical properties, which make composite hydrogels have good potential in many fields, such as sensor[5], self-healing materials[6], catalyst[7, 8], drug release[9, 10], adsorbent[11] and so on. The main challenge in this research field is how to incorporate suitable composites into hydrogels so as to provide multiple functionalities without adversely affecting the intrinsic properties of hydrogels. Composites, including carbon nanotubes (CNTs)[12], graphene[13], metal-oxide nanoparticles[14], C-dot[15], and clay nanosheets[16, 17], are combined with hydrogel through covalent and noncovalent bond interactions, and the intermolecular forces such as hydrogen bond, van der Waal’s interactions) are also employed to reinforce the properties in three-dimensional network. As we know, magnetic materials, such as Fe3O4, CoFe2O4 and γ- Fe2O3 are widely used in hydrogel as tissue engineering, soft actuators, cancer therapy, drug delivery,
electromagnetic
interference
shielding[18,19]
and
enzyme
immobilization[20-23]. And such materials with uniform particle size can be easily controlled by adjusting reactant concentration, reaction time, and stirring speed. In the previous research, many methods were used to prepare of magnetic hydrogels, including the grafting method, blending method and in situ precipitation method. Hernandez and Mijangos[24] have incorporated the Fe3O4 into double-network hydrogels comprising alginate and poly(N-isopropylarcylamide) to improve the deswelling rate by in situ precipitation method. To the best of our knowledge, Fe3O4 is an abundantly available and low-cost material with super-paramagnetic and responsive properties, making it a great candidate for practical functional hydrogels. Besides, as one of transition-metal dichalcogenide (TMDC), molybdenum disulfide (MoS2) possesses a typical “sandwich” structure, and it shows good potentials in various fields, such as sensors, lithium-ion batteries, catalyst, supercapacitors, semiconductor materials[25-27]and so on. Although it possesses low electrical conductivity, inherent large surface area can be easily combined with other conductive polymers to improve its electrical conductivity. Owning to low cost and environmental stability, conductive polymers[28-30], including polypyrrole (PPy), polyaniline (PAni) and poly(3,4-ethylenedioxythiophene) (PEDOT), have been paid attention to in recent years and used in hydrogels for different uses[31-33]. It is found that the electrical conductivity of PAni can be readily changed by doping time, the kind of doping acid, and PAni also can be merged into hydrogel networks to increase self-healing or mechanical strength through hydrogen bond or electrostatic attraction[34]. There is no report about ternary conductive and magnetic composite
hydrogels consisting of Fe3O4 particles, MoS2 and PAni, and they may have good potentials in many fields. Herein, we developed a robust composite hydrogel with conductive and magnetic properties by introducing ternary composites into a double-network hydrogel. Typically, Fe3O4 particles were used as a template, and then MoS2 nanosheets were grown on the surface of the magnetic particles via hydrothermal method to obtain precursor composite. Thereafter, PAni was synthesized on the surface of the above precursor through the facile in situ chemical polymerization to obtain the novel ternary composites. The double-network hydrogel was made up of the biocompatible flexible network of acrylamide and guar gum. To the best of our knowledge, guar gum had been paid attention due to its advantages of cost-effectiveness, non-corrosion, eco-friendliness and commercial availability. As a macromoleclar polymer linked by glycosidic bonds, it could be not only used to form a physically crosslinked network to strengthen the hydrogel network, but also used as a thickener and a stabilizer to keep composites evenly dispersed and to prevent them from precipitation. Compared with polyvinyl alcohol, guar gum has a better biodegradability and is suitable to form biodegradable double-network hydrogels. The non-oxidizable water solubility initiator V50 exhibited little negative impact on the electrical conductivity of the composites when the free radical polymerization occurred among these monomers. The ternary composites are embedded into the robust double-network hydrogel and provide both conductive and magnetic properties. Although the single or dual-functional nanocomposite hydrogels had been fabricated
in previous work, they had some drawbacks.Kai Liu[35] had developed a nanocomposite hydrogel with multifunction but the resultant hydrogel was too soft to go through tensile strength test. Meng Hu[36] had synthesized robust and super-stretchable nanocomposite hydrogels, while single function limited their widespread applications. Therefore, we developed a novel ternary composite and incorporated it into a flexible and robust hydrogel system to obtain composite hydrogels with conductive and magnetic properties. They may be used in many fields, such as electronic skin, drug release, electromagnetic interference shielding, and wave-absorbing.
2. Experimental section 2.1. Materials Nano-Fe3O4 (< 200 nm) was obtained from Macklin Biochemical Co. Ltd. (Shanghai, China). Aniline (ANI, 99.5%), acrylamide (AAm, 99%) and N, N’-methylenebis (acrylamide) (MBAA, 99%) were purchased from Tianjin Damao Chemical Reagent Company. (NH4)6Mo7O24·4H2O, thiourea (CN2H4S), Guar, phytic acid (50%, w/w in water), 2, 2’-azobis(2-methyl-propionamidine) dihydrochloride (V50, 99%) and ammonium persulfate (APS) were provided by Adamas Reagent Co. Ltd. All of reagents were of analytical grade, but aniline monomer was deionized under vacuum and stored in a refrigerator, other reagents were used directly without further purification. 2.2. Preparation of Fe3O4/MoS2/PAni Composites
In order to synthesize the Fe3O4/MoS2 intermediate composite, we applied a simple hydrothermal method [27]. 10.0 mL deionized water containing 0.35 g (NH4)6Mo7O24·4H2O and 0.75 g thiourea were stirred continuously until it turned to transparent solution. Afterwards, 20.0 mg Fe3O4 was added into the above mixed solution and then ultrasonicated for about 30 min. The resulting suspension was heated up to 180 ˚C for 10 h in a 25 mL Teflon-lined stainless autoclave. After cooling, the resultant composites were collected by magnetic separation, washed by deionized water and ethyl alcohol for three times, and then dried at 50 ˚C for 6 h under vacuum. The ternary composite was fabricated by using in-situ polymerization. First, 0.2 g Fe3O4/MoS2 composites, 0.489 mL aniline and 1.98 mL phytic acid were mixed with 38.0 mL deionized water and then dispersed by ultrasound to form a homogeneous solution before it was poured into a round-bottom flask. After the solution was treated at 5 ˚C using a cooling bath, ammonium persulfate aqueous solution that prepared by mixing 0.305 g APS with 2.0 mL deionized water was injected dropwise into above solution, and the reaction was continued for 4 h at a speed of 200 r min-1 to obtain the dark green product. Afterwards, the Fe3O4/MoS2/PAni composites were obtained by magnetic separation, sequentially washing by deionized water and ethyl alcohol thrice, and drying under vacuum at 50 ˚C for 6 h. 2.3. Preparation of the composite hydrogels The composite hydrogels were prepared via a simple free radical polymerization.
2.7 g AAm, 30.0 mg Guar, 10.0 mg MBAA, 22.4 mg V50 and quantitative ternary composites (denoted as CH-0, CH-0.5, CH-1.0, CH-1.5, CH-2.0, where the number represents the weight ratio of composites, respectively) were mixed with 10.0 mL deionized water, the above solution was subjected to ultrasound to form a homogeneous solution. Afterwards, the solution was bathed in water at 50 ˚C for 5 h after it was injected into a mold comprised of two glass plates sandwiched by a silicone rubber spacer. 2.4. Characterization of the ternary composites Fourier transform infrared (FTIR) spectra was conducted using a Bruker 550 infrared spectrophotometer over the range of 500−4000 cm−1 with KBr pellet. The structure and morphology of the composite were investigated by transmission electron microscope (JEM 2000F).
The XRD analysis of the composites were characterized
by X-ray diffractometer Bruker D8 Advance that was equipped with a nickel-filtered using Cu Kα radiation (λ = 0.154 nm) and 2θ ranging from 10° to 80° with 2° min−1 scan rate. The morphology and chemical constitution of the intermediate composite and ternary composite were observed by means of a Zeiss Merlin scanning electron microscope with energy dispersive spectroscopy (EDS). Micromeritics Tristar Area and Porosity (Micromeritics Instrument Co. USA) was used to investigate the specific surface area and pore size distribution of the composites and the composite hydrogels at 77 K. 2.5. Characterization of the composite hydrogels All optical photographs were taken by a digital camera (Canon EOS 700D). In
order to investigate the morphology of the composite hydrogels, scanning electron micrograph (SEM) was applied to survey at an accelerating voltage of 5 kV. The pure hydrogel and composite hydrogels were quenched in liquid nitrogen, fractured, and then gold was sputtered on the cross-sectional surface for SEM imaging. The electrical conductivity of the composite hydrogels was measured using an RTS-9 four-probe detector from Guangzhou 4 PROBSTECH at room temperature. Vibrating sample magnetometer (LAKESHORE-7410, USA) was used to characterize the magnetic property of the composites and the composite hydrogels, the magnetic properties of all samples were measured at 300 K. The thermal degradation behavior of the composite hydrogels was characterized by thermogravimetric analysis (STGA449C Netzsch, Germany), all samples were heating from 30°C to 800°C under a nitrogen atmosphere with 10 °C min−1 heating rate or 25 °C to 120°C with 1 °C min−1 heating rate. The swelling ratio of the composite hydrogels was carried out by using a conventional method. Specifically, 0.5 g circular dry sample was immersed into a phosphate-buffered saline (PH = 6.7) at room temperature for 2 days, and the water on composite hydrogels surface was wiped off with a tissue before the weight of swollen composite hydrogels was recorded. The Ksr (swelling ratio) was calculated as follows: =(
−
)/
Where Wd and We are the weight of dry hydrogel and soused hydrogel, respectively. The mechanical properties of the composite hydrogels were measured by tensile
tests using CMT 4204 electronic universal testing machine (MTS Systems Co. Ltd, China) at room temperature. The samples with a thickness of 2.0 mm were cut into dumbbell-shaped samples with 4.0 mm in width and 10.0 mm in length, and the crosshead speed was set at 20 mm min-1. To avoid water evaporation, the surface of the composite hydrogels was coated with silicon oil during storage time. The rheological characterization of the composite hydrogels was measured by MARS III Haake rheometer with frequency sweep ranging from 0.01 to 15 Hz at room temperature.
3. Results and discussion 3.1. Design Strategy for the ternary composite hydrogels Insert Figure 1 As demonstrated in Figure 1, the ternary composite hydrogels were fabricated in several steps. First, the initial composites composed of Fe3O4 and MoS2 were synthesized via a hydrothermal method. As we know, the magnetic Fe3O4 was cost-effective, and it possessed high surface area that could serve as a carrier for MoS2 nanosheets to cover via in-suit growth. MoS2 had a unique sandwich structure that could provide enough interlayer spaces for accepting other compounds. Insert Figure 2 Insert Figure 3 Figure 2a1 and Figure 2a2 show the morphology of Fe3O4/MoS2. It was found that the petaloid MoS2 was grown on the surface of Fe3O4 and also provide many absorption sites for aniline monomers. And the aniline monomers were therefore
absorbed on these sites and formed polyaniline via in-situ polymerization in the presence of doped acid and ammonium persulfate. More strikingly, the negative surface of flake-like MoS2 could form electrostatic attraction with the molecular chain of polyaniline to make the composites more stable. As seen in Figure 2b1 and Figure 2b2, the punctate polyaniline was uniformly interspersed on the surface of petaloid MoS2 without agglomeration occurring, and the elemental distribution is demonstrated in Figure 3. Compared with Fe3O4/PAni composites synthesized via in-situ polymerization of PAni on the surface of Fe3O4 [37], MoS2 will serve as a shell to provide a preferable approachability for PAni to settle and also prevent the agglomeration and oxidization of PAni. Furthermore, the combination of MoS2 and PAni exhibited better electrical conductivity (0.32 S cm-1) and stability. From the EDS mapping, it could be found that C, Mo, O, S, N, and Fe were dispersed uniformly, indicating the successful synthesis of Fe3O4/MoS2/PAni ternary composites. The microstructures of the as-prepared composites and the composite hydrogels were further investigated through N2 adsorption-desorption isotherms. As shown in the Table S1, the specific surface area of Fe3O4/MoS2 was larger than that of Fe3O4, which further proved the lamellar MoS2 forming a sandwich structure to increase the specific surface area. Besides, as demonstrated in Figure S1, the hysteresis loops of three particles indicated that they exhibited mesoporous structures with a pore diameter ranging from 3 to 50 nm, and the hysteresis loops of these two composites could be classified into H3 hysteresis effects, indicating that the lamellar MoS2 piled up on the surface of Fe3O4 through layer-by-layer to form slit-like pores. As shown in
Figure S2, due to the embedment of composites in hydrogels, the distribution of pore diameter became denser, and the ternary composites obstructed the pores of hydrogels to act as crosslinking sites to improve the density of cross-linked network. As shown in Figure e1~e3, It was found that the lattice fringes of MoS2 on the surface of Fe3O4, revealed a decent crystal and sandwich structure. Besides, the interlamellar was calculated as 0.677 nm, which was assigned to the (002) plane of MoS2 and consistent with the following XRD results. Furthermore, the as-prepared ternary composites were incorporated into double-network hydrogels to obtain the desirable composite hydrogels with both conductive and magnetic properties. Guar gum (GG) and acrylamide (AM) were selected
for
constructing
the
binary
networked
hydrogel
with
N,N-methylenebisacrylamide (MBAA) as a cross-linking agent. Non-oxidizable initiator was used so that it had little effect on the conductivity of the composites and the structure of GG. It was found that GG with cost-effective, nontoxic, and eco-friendly features not only acted as a thickener to keep the stabilization of composites within the hydrogel but also provided a physically crosslinked network to strengthen the hydrogel network. The chemically crosslinked network was made up of the long chain of polyacrylamide via covalent crosslinking with the help of the cross-linking agent and initiator, and the hydrogen bonds existing in hydrogel also facilitated the enhancement of tough network. In addition, the ternary composites were closely associated with the networks through the hydrogen bonds between amide and aniline groups, and such hydrogen bonds also provided effective pathways for
dissipating mechanical energy under stretching or compressing. Furthermore, it was found that the zeta potential of Fe3O4/MoS2 was -29.4 mV, and the negative zeta potential of Fe3O4/MoS2/PAni went up to −8.2 mV after polyaniline was adsorbed on these sites. And the Fe3O4/MoS2/PAni composites still maintained a negative surface. Therefore, they could be well dispersed in the hydrogels (Figure 2d). The resultant intermolecular forces were expected to enhance the mechanical properties of the resultant composite hydrogels.
3.2. Characterization of the ternary composites Insert Figure 4 Insert Figure 5 Figure 4a shows the FT-IR spectra of Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites, recpectively. For the spectra of Fe3O4/MoS2, the characteristic peak at about 610 cm−1 was assigned to Mo-S vibration, the peaks at 612 cm−1 and 567 cm−1 were attributed to the symmetrical stretching vibrations of Fe-O. Compared with the FT-IR of Fe3O4/MoS2 composites, PAni had two characteristic peaks at 1479 cm−1 and 1559 cm−1 corresponding to the C=C and C=N stretch of quinoid rings and benzenoid structure, recpectively. The peaks located at 799 cm−1 and 1137 cm−1 were related to C-C bending vibration of benzenoid unit and C-H in-plane bending vibration, respectively. The major peaks belonged to PAni could be distinctly inspected in the FT-IR pattern of the ternary composites, which were proved the successful synthesis of PAni via in-suit polymerization. Due to the presence of Fe3O4, the electron cloud density of the polymer macromolecular chain increased, leading to the shift of most
peaks to lower wavenumber compared with pure PAni. The XRD analysis results of Fe3O4, Fe3O4/MoS2 and Fe3O4/MoS2/PAni samples are shown in Figure 4b，The diffraction peaks of the synthesized samples were investigated in a range from 10˚ to 80˚. The obvious characteristic peaks of Fe3O4 at 2θ = 18.3˚, 30.1˚, 35.5˚, 43.1˚, 57.0˚, 62˚, which were consistent with the（111）， （311）， （400）， （511）， （440）planes of the magnetite nanocrystal, respectively. （220）， For Fe3O4/MoS2, a pristine weak diffraction peaks at 32.5˚ corresponded to the （100） reflections of the MoS2, indicating the MoS2 nanosheets were grown on the surface of Fe3O4 nanoparticles. After in-situ polymerization, the peak at 14.2˚ emerged in the XRD patterns, which demonstrated the presence of PAni in the composites. Due to the PAni chains were regularly covered on the outer surface of composites, and whose benzenoid and quinoid rings appeared repeatedly, the crystallinity of the ternary composites was low. The XRD pattern of ternary composites contains the major characteristic peaks of the three materials confirming the successful preparation of ternary composites. Besides, as shown in Figure 5, compared with the composites formed by Fe3O4 and MoS2, the addition of PAni made the diameter of resultant composites increase due to PAni had been formed on the surface of composite
3.3. Characterization of the composite hydrogels Insert Figure 6 The thermal stability of the composite hydrogels with different composite contents was investigated by TGA measurement with different heating rates. As shown in Figure 6a, the degradation curves of the composite hydrogels demonstrated
the same trend, and the gradual weight loss from 30 ˚C to 350 ˚C was assigned to the evaporation of bound water among hydrogel networks, the decomposition of guar gum[38] and the branched chain of PAM[39]. Besides, the decomposition of the hydrogel had an evident thermal decomposition peaks located at around 370 ˚C, and the significant weight loss in the temperature ranging from 370 ˚C to 700 ˚C was attributed to the decomposition of residual guar gum and the framework of hydrogel[40, 41], respectively. When the weight loss reached 70 wt%, the temperature of pure hydrogel and the composite hydrogels with 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt% composite content was 494.1 ˚C, 552.2 ˚C, 552.4 ˚C, 553.1 ˚C and 558.2 ˚C, respectively. And the heat-resistance index (THRI)[42, 43] was 118.9 ˚C , 121.0 ˚C , 122.1 ˚C , 124.8 ˚C , 131.1 ˚C, respectively, indicating that the thermal stability of the composites was improved. This is because the ternary composites served as a physical junction to increase the crosslinking density of the composite hydrogels. To further investigate the thermal stability, TGA measurement was used under a lower heating rate. As shown in Figure 6b, the composite hydrogels exhibited better thermal stability than that of the pure hydrogels, which further proved above conclusion. Insert Figure 7 Water absorbency is an important property for hydrogel and it is closely related to the immersion time. The swelling test of the composite hydrogels was implemented in a phosphate-buffered saline at room temperature, and the swelling trend for each sample is shown in Figure 7a. It was found that all composite hydrogels took about
twenty hours to reach the equilibrium-swollen state, and the swelling ratio decreased along with the increase of ternary composite content, illustrating that there were hydrogen bonding interactions between the ternary composites and double-network hydrogels. Because the ternary composites acted as a physical cross-linker and enhanced the cross-linking density of the composite hydrogels, it could be explained why the composites had effect on the water absorbency of the resultant composite hydrogels. From Figure 7b and c we found that the volume of the composite hydrogels changed dramatically after being subjected to immersion in the water over twenty hours, and the water remained transparent, indicating there was no release of composites and the composite hydrogels were stable. This result also provided the hydrogen bonding between the composites and the molecular chain of hydrogel. Due to the superabsorbent ability of guar gum acted as a physical crosslinked network, the water absorbency of the binary network hydrogels composed of PAM/GG surpassed other binary network hydrogels such as PAM/PVA[44]. Insert Figure 8 The mechanical properties of the composite hydrogels with different composite contents were tested at room temperature, and the tensile stress-strain curves were shown in Figure 8a. With the increasing of the content of composite hydrogels, the tensile strength increased from 113 kPa to 140 kPa, while the fracture strain decreased from 307% to 170% , and the toughness had a slight decrease (Figure 8b). The novel multifunctional hydrogels were tough and had a better mechanical property than that of other multifunctional hydrogels[35], In addition, the binary network hydrogels
improved the tensile stress compared with the pure hydrogels (with a tensile stress of 88.3 kPa), and the novel binary network hydrogels composed of PAM and GG had a better mechanical property than that of conventional PAM/PVA [45]. The rheological properties of the composite hydrogels containing different contents of ternary composites were also investigated. As shown in Figure S3, it was found that its loss modulus G′′ was lower than the storage moduli G′, indicating that all composite hydrogels presented elastic character. Besides, both G′ and G″ of the composite hydrogels increased with the increasing of the contents of the ternary composites. The hydrogen bonds between the ternary composites and the binary network hydrogels contributed to the hydrogel stiffness. There were two reasons for the improvement of the mechanical properties. First, guar gum (GG) could be considered as a physically crosslinked network to construct binary networks with PAM via covalent crosslinking. And a binary network comprising physical and chemical network was an effective approach to enhance the strength of hydrogel, which had been proved in many reports[45]. Second, PAni existing on the surface of the ternary composites could form some hydrogen bonds with the long chains of PAM network. And the ternary composites could be used as crosslinking sites to improve the density of cross-linked network and also provided pathways for dissipating mechanical energy when it was undergone large deformation. Therefore, the composite hydrogels had good mechanical properties and they could overcome different deformations such as bending, twisting and stretching (Figure 8c, d and e). Insert Figure 9
After Fe3O4/MoS2/PAni composite was inserted, the composite hydrogels obtained magnetic properties. The magnetic hysteresis loops of Fe3O4/MoS2 (a), Fe3O4/MoS2/PAni (b) and CH-2.0 (c) are shown in Figure 8. The saturation magnetization of pure Fe3O4 [46] was 63.8 emu g-1, after coated by MoS2 and PAni, the saturation magnetization decreased to 14.96 and 9.82 emu g-1, respectively. Due to the low content of ternary composites were incorporated into the hydrogels, the composite hydrogels possessed a weak magnetism about 1.28 emu g-1. Magnetic and biocompatible hydrogels showed potentials for drug delivery in external magnetic field. Although the magnetic intensity of composite hydrogels was much lower than that of the pure magnetic composite, it had a typical hysteresis loop and its performance could be easily changed by adjusting the content of ternary composites. Insert Figure 10 The conductivity of the composite hydrogels with different composite contents was measured by a four-point probe. As shown in Figure 10, with the increasing of the content of composites, the conductivity of the composite hydrogels increased. The conductivity of composite hydrogels increased from 1.235×10-3 S cm-1 to 2.052× 10-3 S cm-1, which was much higher than that of other PAni composite hydrogels[47]. And it had a slight decrease after one month of aging owing to oxidation, indicating of the well in vitro stability of the composite hydrogels. This is because, first, the ternary composites had incorporated into hydrogel to form continuous circuits. Second, molybdenum disulfide (MoS2) possessing a large surface area that could be easily coated by polyaniline to reinforce electrical conductivity through synergistic effect
among them. Meanwhile, the polyaniline molecules had been embedded to the interlayer of MoS2 to avoid overoxidation and a far sharper decline in the conductivity during storage time. In addition to storage stability, the composite hydrogels maintained high conductivity even being subjected to stretching, bending, or twisting.
4. Conclusions In summary, flexible and tough composite hydrogels with conductive and magnetic properties were synthesized by incorporating the ternary composites into hydrogel. MoS2 nanosheets were grown on the surface of the magnetic particles via hydrothermal method to obtain the precursor composite, and then polyaniline was coated on the surface of MoS2 via electrostatic attraction. The introduction of the ternary composites in hydrogel not only endowed conductive and magnetic properties, but also provided hydrogen bonds with molecular chain to obtain a tough composite hydrogel. The maximum saturation magnetization and conductivity of the composite hydrogel were 1.28 emu·g−1 and 2.052 × 10−3 S·cm−1, respectively, and the tensile strength reached up to 140 kPa. Therefore, novel ternary composite hydrogels have been prepared and they will have potential applications in many fields, such as electronic
skin,
drug
release,
electromagnetic
interference
shielding,
and
wave-absorbing.
Acknowledgment We appreciate the financial support from the National Natural Science Foundation of China under grant No.21571065.
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Figure Captions Figure 1. (a) Schematic illustration of the fabrication process of the ternary composite hydrogels. (b) Conceptual graphs of composite structures and the hydrogen bonds
among binary networks and composites. Figure 2. (a1,a2) SEM images of Fe3O4/MoS2 composites at different magnifications. (b1,b2) SEM images of Fe3O4/MoS2/PAni composites at different magnifications. Cross-sectional SEM images of the pure binary networked hydrogel (c) and the composite hydrogel (d). TEM image of Fe3O4/MoS2/PAni composites at different magnifications(e1～e3). Figure 3. EDS mapping area of Fe3O4/MoS2/PAni composites, and the elemental signals of N, C, Mo, S, Fe, O. Figure 4. (a) Comparison of FT-IR of Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites. (b) Comparison of XRD spectra of the Fe3O4, Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites. Figure 5. Particle size distribution of Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites. Figure 6. TGA curves of the pure hydrogel and the composite hydrogel with different composite contents under the heating rate of 10 °C min-1 (a) and 1 °C min-1 (b). Figure 7. (a) Swelling ratio of the pure hydrogel and different composite hydrogels as a function of immersion time in phosphate-buffered saline (PH=6.7) at room temperature. (b) The photos of the composite hydrogel immersed in water and (c) the composite hydrogels were immersed in water after 2 days. Figure 8. Tensile stress-strain curves (a) of the hydrogel with different composite hydrogels and corresponding toughness (b). The photos of the composite hydrogel (c) in flexuous state and (d) in twisted state. (e1) Process of uniaxial tensile of the composite hydrogel in original state and (e2) the composite hydrogel in maximum
tensile state. Figure 9. Hysteresis loops of (a) Fe3O4/MoS2, (b) Fe3O4/MoS2/PAni and the composite hydrogels. Figure 10. Electrical conductivity of the composite hydrogels (original state and after one month) with various composite contents(a). The photo of the electrical circuit (b), the composite hydrogels in original state(c) and the composite hydrogel in tensile state(d), direct voltage applied for all electrical circuit was 3.0 V.
Conflict of interest We all declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted
S1359-8368(19)34377-X
DOI:
https://doi.org/10.1016/j.compositesb.2019.107623
Reference:
JCOMB 107623
To appear in:
Composites Part B
Received Date: 27 August 2019 Revised Date:
21 November 2019
Accepted Date: 25 November 2019
Please cite this article as: Hu H, Zhong X, Yang S, Fu H, Tough and stretchable Fe3O4/MoS2/PAni composite hydrogels with conductive and magnetic properties, Composites Part B (2019), doi: https:// doi.org/10.1016/j.compositesb.2019.107623. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Tough and Stretchable Fe3O4/MoS2/PAni Composite Hydrogels with Conductive and Magnetic Properties Hengfeng Hu1, Ximing Zhong1, Shuibin Yang * 2, Heqing Fu*1 (1 School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, P.R. China;2 Hubei Key Laboratory for Processing and Application of Catalytic Materials, College of Chemical Engineering, Huanggang 438000, China) *Corresponding author. Tel: +86 020 87114919;Fax: +86 020 87112047. E-mail: [email protected]; [email protected]
ABSTRACT Novel composite hydrogels with conductive and magnetic properties are prepared by introducing ternary composites into a double-network hydrogel. The ternary composites are comprised of Fe3O4, MoS2 and PAni. Fe3O4 possessing high surface area could be served as a carrier. And MoS2 with a special sandwich structure will provide enough interlayer spaces to combine with PAni via electrostatic attraction. The ternary composites are further introduced into a double-network hydrogel. The resultant composite hydrogels have maximum saturation magnetization of 1.28 emu·g−1 and conductivity of 2.052 × 10−3 S·cm−1, respectively. In addition, the thermo-stability and the mechanical property of the composite hydrogels are improved by the incorporation of ternary composites, and the tensile strength of the composite hydrogels is about 140 kPa with a good stretchability (≈170%). Therefore, these flexible and robust composite hydrogels with conductive and magnetic properties have good prospect in the fields of the electronic skin, drug release,
electromagnetic interference shielding and wave-absorbing materials . Keywords: hydrogel; ternary composite; toughness; conductivity; magnetic properties
1. Introduction Hydrogels have been extensively studied owning to their hydrated molecular three-dimensional network structures that could serve as a matrix for the combination of various composites[1-4]. Currently, a convenient approach to enrich multiple functionalities is to introduce composites into hydrogel matrix, and the resultant composite hydrogels possess many features, including robust mechanical, magnetic and electrical properties, which make composite hydrogels have good potential in many fields, such as sensor[5], self-healing materials[6], catalyst[7, 8], drug release[9, 10], adsorbent[11] and so on. The main challenge in this research field is how to incorporate suitable composites into hydrogels so as to provide multiple functionalities without adversely affecting the intrinsic properties of hydrogels. Composites, including carbon nanotubes (CNTs)[12], graphene[13], metal-oxide nanoparticles[14], C-dot[15], and clay nanosheets[16, 17], are combined with hydrogel through covalent and noncovalent bond interactions, and the intermolecular forces such as hydrogen bond, van der Waal’s interactions) are also employed to reinforce the properties in three-dimensional network. As we know, magnetic materials, such as Fe3O4, CoFe2O4 and γ- Fe2O3 are widely used in hydrogel as tissue engineering, soft actuators, cancer therapy, drug delivery,
electromagnetic
interference
shielding[18,19]
and
enzyme
immobilization[20-23]. And such materials with uniform particle size can be easily controlled by adjusting reactant concentration, reaction time, and stirring speed. In the previous research, many methods were used to prepare of magnetic hydrogels, including the grafting method, blending method and in situ precipitation method. Hernandez and Mijangos[24] have incorporated the Fe3O4 into double-network hydrogels comprising alginate and poly(N-isopropylarcylamide) to improve the deswelling rate by in situ precipitation method. To the best of our knowledge, Fe3O4 is an abundantly available and low-cost material with super-paramagnetic and responsive properties, making it a great candidate for practical functional hydrogels. Besides, as one of transition-metal dichalcogenide (TMDC), molybdenum disulfide (MoS2) possesses a typical “sandwich” structure, and it shows good potentials in various fields, such as sensors, lithium-ion batteries, catalyst, supercapacitors, semiconductor materials[25-27]and so on. Although it possesses low electrical conductivity, inherent large surface area can be easily combined with other conductive polymers to improve its electrical conductivity. Owning to low cost and environmental stability, conductive polymers[28-30], including polypyrrole (PPy), polyaniline (PAni) and poly(3,4-ethylenedioxythiophene) (PEDOT), have been paid attention to in recent years and used in hydrogels for different uses[31-33]. It is found that the electrical conductivity of PAni can be readily changed by doping time, the kind of doping acid, and PAni also can be merged into hydrogel networks to increase self-healing or mechanical strength through hydrogen bond or electrostatic attraction[34]. There is no report about ternary conductive and magnetic composite
hydrogels consisting of Fe3O4 particles, MoS2 and PAni, and they may have good potentials in many fields. Herein, we developed a robust composite hydrogel with conductive and magnetic properties by introducing ternary composites into a double-network hydrogel. Typically, Fe3O4 particles were used as a template, and then MoS2 nanosheets were grown on the surface of the magnetic particles via hydrothermal method to obtain precursor composite. Thereafter, PAni was synthesized on the surface of the above precursor through the facile in situ chemical polymerization to obtain the novel ternary composites. The double-network hydrogel was made up of the biocompatible flexible network of acrylamide and guar gum. To the best of our knowledge, guar gum had been paid attention due to its advantages of cost-effectiveness, non-corrosion, eco-friendliness and commercial availability. As a macromoleclar polymer linked by glycosidic bonds, it could be not only used to form a physically crosslinked network to strengthen the hydrogel network, but also used as a thickener and a stabilizer to keep composites evenly dispersed and to prevent them from precipitation. Compared with polyvinyl alcohol, guar gum has a better biodegradability and is suitable to form biodegradable double-network hydrogels. The non-oxidizable water solubility initiator V50 exhibited little negative impact on the electrical conductivity of the composites when the free radical polymerization occurred among these monomers. The ternary composites are embedded into the robust double-network hydrogel and provide both conductive and magnetic properties. Although the single or dual-functional nanocomposite hydrogels had been fabricated
in previous work, they had some drawbacks.Kai Liu[35] had developed a nanocomposite hydrogel with multifunction but the resultant hydrogel was too soft to go through tensile strength test. Meng Hu[36] had synthesized robust and super-stretchable nanocomposite hydrogels, while single function limited their widespread applications. Therefore, we developed a novel ternary composite and incorporated it into a flexible and robust hydrogel system to obtain composite hydrogels with conductive and magnetic properties. They may be used in many fields, such as electronic skin, drug release, electromagnetic interference shielding, and wave-absorbing.
2. Experimental section 2.1. Materials Nano-Fe3O4 (< 200 nm) was obtained from Macklin Biochemical Co. Ltd. (Shanghai, China). Aniline (ANI, 99.5%), acrylamide (AAm, 99%) and N, N’-methylenebis (acrylamide) (MBAA, 99%) were purchased from Tianjin Damao Chemical Reagent Company. (NH4)6Mo7O24·4H2O, thiourea (CN2H4S), Guar, phytic acid (50%, w/w in water), 2, 2’-azobis(2-methyl-propionamidine) dihydrochloride (V50, 99%) and ammonium persulfate (APS) were provided by Adamas Reagent Co. Ltd. All of reagents were of analytical grade, but aniline monomer was deionized under vacuum and stored in a refrigerator, other reagents were used directly without further purification. 2.2. Preparation of Fe3O4/MoS2/PAni Composites
In order to synthesize the Fe3O4/MoS2 intermediate composite, we applied a simple hydrothermal method [27]. 10.0 mL deionized water containing 0.35 g (NH4)6Mo7O24·4H2O and 0.75 g thiourea were stirred continuously until it turned to transparent solution. Afterwards, 20.0 mg Fe3O4 was added into the above mixed solution and then ultrasonicated for about 30 min. The resulting suspension was heated up to 180 ˚C for 10 h in a 25 mL Teflon-lined stainless autoclave. After cooling, the resultant composites were collected by magnetic separation, washed by deionized water and ethyl alcohol for three times, and then dried at 50 ˚C for 6 h under vacuum. The ternary composite was fabricated by using in-situ polymerization. First, 0.2 g Fe3O4/MoS2 composites, 0.489 mL aniline and 1.98 mL phytic acid were mixed with 38.0 mL deionized water and then dispersed by ultrasound to form a homogeneous solution before it was poured into a round-bottom flask. After the solution was treated at 5 ˚C using a cooling bath, ammonium persulfate aqueous solution that prepared by mixing 0.305 g APS with 2.0 mL deionized water was injected dropwise into above solution, and the reaction was continued for 4 h at a speed of 200 r min-1 to obtain the dark green product. Afterwards, the Fe3O4/MoS2/PAni composites were obtained by magnetic separation, sequentially washing by deionized water and ethyl alcohol thrice, and drying under vacuum at 50 ˚C for 6 h. 2.3. Preparation of the composite hydrogels The composite hydrogels were prepared via a simple free radical polymerization.
2.7 g AAm, 30.0 mg Guar, 10.0 mg MBAA, 22.4 mg V50 and quantitative ternary composites (denoted as CH-0, CH-0.5, CH-1.0, CH-1.5, CH-2.0, where the number represents the weight ratio of composites, respectively) were mixed with 10.0 mL deionized water, the above solution was subjected to ultrasound to form a homogeneous solution. Afterwards, the solution was bathed in water at 50 ˚C for 5 h after it was injected into a mold comprised of two glass plates sandwiched by a silicone rubber spacer. 2.4. Characterization of the ternary composites Fourier transform infrared (FTIR) spectra was conducted using a Bruker 550 infrared spectrophotometer over the range of 500−4000 cm−1 with KBr pellet. The structure and morphology of the composite were investigated by transmission electron microscope (JEM 2000F).
The XRD analysis of the composites were characterized
by X-ray diffractometer Bruker D8 Advance that was equipped with a nickel-filtered using Cu Kα radiation (λ = 0.154 nm) and 2θ ranging from 10° to 80° with 2° min−1 scan rate. The morphology and chemical constitution of the intermediate composite and ternary composite were observed by means of a Zeiss Merlin scanning electron microscope with energy dispersive spectroscopy (EDS). Micromeritics Tristar Area and Porosity (Micromeritics Instrument Co. USA) was used to investigate the specific surface area and pore size distribution of the composites and the composite hydrogels at 77 K. 2.5. Characterization of the composite hydrogels All optical photographs were taken by a digital camera (Canon EOS 700D). In
order to investigate the morphology of the composite hydrogels, scanning electron micrograph (SEM) was applied to survey at an accelerating voltage of 5 kV. The pure hydrogel and composite hydrogels were quenched in liquid nitrogen, fractured, and then gold was sputtered on the cross-sectional surface for SEM imaging. The electrical conductivity of the composite hydrogels was measured using an RTS-9 four-probe detector from Guangzhou 4 PROBSTECH at room temperature. Vibrating sample magnetometer (LAKESHORE-7410, USA) was used to characterize the magnetic property of the composites and the composite hydrogels, the magnetic properties of all samples were measured at 300 K. The thermal degradation behavior of the composite hydrogels was characterized by thermogravimetric analysis (STGA449C Netzsch, Germany), all samples were heating from 30°C to 800°C under a nitrogen atmosphere with 10 °C min−1 heating rate or 25 °C to 120°C with 1 °C min−1 heating rate. The swelling ratio of the composite hydrogels was carried out by using a conventional method. Specifically, 0.5 g circular dry sample was immersed into a phosphate-buffered saline (PH = 6.7) at room temperature for 2 days, and the water on composite hydrogels surface was wiped off with a tissue before the weight of swollen composite hydrogels was recorded. The Ksr (swelling ratio) was calculated as follows: =(
−
)/
Where Wd and We are the weight of dry hydrogel and soused hydrogel, respectively. The mechanical properties of the composite hydrogels were measured by tensile
tests using CMT 4204 electronic universal testing machine (MTS Systems Co. Ltd, China) at room temperature. The samples with a thickness of 2.0 mm were cut into dumbbell-shaped samples with 4.0 mm in width and 10.0 mm in length, and the crosshead speed was set at 20 mm min-1. To avoid water evaporation, the surface of the composite hydrogels was coated with silicon oil during storage time. The rheological characterization of the composite hydrogels was measured by MARS III Haake rheometer with frequency sweep ranging from 0.01 to 15 Hz at room temperature.
3. Results and discussion 3.1. Design Strategy for the ternary composite hydrogels Insert Figure 1 As demonstrated in Figure 1, the ternary composite hydrogels were fabricated in several steps. First, the initial composites composed of Fe3O4 and MoS2 were synthesized via a hydrothermal method. As we know, the magnetic Fe3O4 was cost-effective, and it possessed high surface area that could serve as a carrier for MoS2 nanosheets to cover via in-suit growth. MoS2 had a unique sandwich structure that could provide enough interlayer spaces for accepting other compounds. Insert Figure 2 Insert Figure 3 Figure 2a1 and Figure 2a2 show the morphology of Fe3O4/MoS2. It was found that the petaloid MoS2 was grown on the surface of Fe3O4 and also provide many absorption sites for aniline monomers. And the aniline monomers were therefore
absorbed on these sites and formed polyaniline via in-situ polymerization in the presence of doped acid and ammonium persulfate. More strikingly, the negative surface of flake-like MoS2 could form electrostatic attraction with the molecular chain of polyaniline to make the composites more stable. As seen in Figure 2b1 and Figure 2b2, the punctate polyaniline was uniformly interspersed on the surface of petaloid MoS2 without agglomeration occurring, and the elemental distribution is demonstrated in Figure 3. Compared with Fe3O4/PAni composites synthesized via in-situ polymerization of PAni on the surface of Fe3O4 [37], MoS2 will serve as a shell to provide a preferable approachability for PAni to settle and also prevent the agglomeration and oxidization of PAni. Furthermore, the combination of MoS2 and PAni exhibited better electrical conductivity (0.32 S cm-1) and stability. From the EDS mapping, it could be found that C, Mo, O, S, N, and Fe were dispersed uniformly, indicating the successful synthesis of Fe3O4/MoS2/PAni ternary composites. The microstructures of the as-prepared composites and the composite hydrogels were further investigated through N2 adsorption-desorption isotherms. As shown in the Table S1, the specific surface area of Fe3O4/MoS2 was larger than that of Fe3O4, which further proved the lamellar MoS2 forming a sandwich structure to increase the specific surface area. Besides, as demonstrated in Figure S1, the hysteresis loops of three particles indicated that they exhibited mesoporous structures with a pore diameter ranging from 3 to 50 nm, and the hysteresis loops of these two composites could be classified into H3 hysteresis effects, indicating that the lamellar MoS2 piled up on the surface of Fe3O4 through layer-by-layer to form slit-like pores. As shown in
Figure S2, due to the embedment of composites in hydrogels, the distribution of pore diameter became denser, and the ternary composites obstructed the pores of hydrogels to act as crosslinking sites to improve the density of cross-linked network. As shown in Figure e1~e3, It was found that the lattice fringes of MoS2 on the surface of Fe3O4, revealed a decent crystal and sandwich structure. Besides, the interlamellar was calculated as 0.677 nm, which was assigned to the (002) plane of MoS2 and consistent with the following XRD results. Furthermore, the as-prepared ternary composites were incorporated into double-network hydrogels to obtain the desirable composite hydrogels with both conductive and magnetic properties. Guar gum (GG) and acrylamide (AM) were selected
for
constructing
the
binary
networked
hydrogel
with
N,N-methylenebisacrylamide (MBAA) as a cross-linking agent. Non-oxidizable initiator was used so that it had little effect on the conductivity of the composites and the structure of GG. It was found that GG with cost-effective, nontoxic, and eco-friendly features not only acted as a thickener to keep the stabilization of composites within the hydrogel but also provided a physically crosslinked network to strengthen the hydrogel network. The chemically crosslinked network was made up of the long chain of polyacrylamide via covalent crosslinking with the help of the cross-linking agent and initiator, and the hydrogen bonds existing in hydrogel also facilitated the enhancement of tough network. In addition, the ternary composites were closely associated with the networks through the hydrogen bonds between amide and aniline groups, and such hydrogen bonds also provided effective pathways for
dissipating mechanical energy under stretching or compressing. Furthermore, it was found that the zeta potential of Fe3O4/MoS2 was -29.4 mV, and the negative zeta potential of Fe3O4/MoS2/PAni went up to −8.2 mV after polyaniline was adsorbed on these sites. And the Fe3O4/MoS2/PAni composites still maintained a negative surface. Therefore, they could be well dispersed in the hydrogels (Figure 2d). The resultant intermolecular forces were expected to enhance the mechanical properties of the resultant composite hydrogels.
3.2. Characterization of the ternary composites Insert Figure 4 Insert Figure 5 Figure 4a shows the FT-IR spectra of Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites, recpectively. For the spectra of Fe3O4/MoS2, the characteristic peak at about 610 cm−1 was assigned to Mo-S vibration, the peaks at 612 cm−1 and 567 cm−1 were attributed to the symmetrical stretching vibrations of Fe-O. Compared with the FT-IR of Fe3O4/MoS2 composites, PAni had two characteristic peaks at 1479 cm−1 and 1559 cm−1 corresponding to the C=C and C=N stretch of quinoid rings and benzenoid structure, recpectively. The peaks located at 799 cm−1 and 1137 cm−1 were related to C-C bending vibration of benzenoid unit and C-H in-plane bending vibration, respectively. The major peaks belonged to PAni could be distinctly inspected in the FT-IR pattern of the ternary composites, which were proved the successful synthesis of PAni via in-suit polymerization. Due to the presence of Fe3O4, the electron cloud density of the polymer macromolecular chain increased, leading to the shift of most
peaks to lower wavenumber compared with pure PAni. The XRD analysis results of Fe3O4, Fe3O4/MoS2 and Fe3O4/MoS2/PAni samples are shown in Figure 4b，The diffraction peaks of the synthesized samples were investigated in a range from 10˚ to 80˚. The obvious characteristic peaks of Fe3O4 at 2θ = 18.3˚, 30.1˚, 35.5˚, 43.1˚, 57.0˚, 62˚, which were consistent with the（111）， （311）， （400）， （511）， （440）planes of the magnetite nanocrystal, respectively. （220）， For Fe3O4/MoS2, a pristine weak diffraction peaks at 32.5˚ corresponded to the （100） reflections of the MoS2, indicating the MoS2 nanosheets were grown on the surface of Fe3O4 nanoparticles. After in-situ polymerization, the peak at 14.2˚ emerged in the XRD patterns, which demonstrated the presence of PAni in the composites. Due to the PAni chains were regularly covered on the outer surface of composites, and whose benzenoid and quinoid rings appeared repeatedly, the crystallinity of the ternary composites was low. The XRD pattern of ternary composites contains the major characteristic peaks of the three materials confirming the successful preparation of ternary composites. Besides, as shown in Figure 5, compared with the composites formed by Fe3O4 and MoS2, the addition of PAni made the diameter of resultant composites increase due to PAni had been formed on the surface of composite
3.3. Characterization of the composite hydrogels Insert Figure 6 The thermal stability of the composite hydrogels with different composite contents was investigated by TGA measurement with different heating rates. As shown in Figure 6a, the degradation curves of the composite hydrogels demonstrated
the same trend, and the gradual weight loss from 30 ˚C to 350 ˚C was assigned to the evaporation of bound water among hydrogel networks, the decomposition of guar gum[38] and the branched chain of PAM[39]. Besides, the decomposition of the hydrogel had an evident thermal decomposition peaks located at around 370 ˚C, and the significant weight loss in the temperature ranging from 370 ˚C to 700 ˚C was attributed to the decomposition of residual guar gum and the framework of hydrogel[40, 41], respectively. When the weight loss reached 70 wt%, the temperature of pure hydrogel and the composite hydrogels with 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt% composite content was 494.1 ˚C, 552.2 ˚C, 552.4 ˚C, 553.1 ˚C and 558.2 ˚C, respectively. And the heat-resistance index (THRI)[42, 43] was 118.9 ˚C , 121.0 ˚C , 122.1 ˚C , 124.8 ˚C , 131.1 ˚C, respectively, indicating that the thermal stability of the composites was improved. This is because the ternary composites served as a physical junction to increase the crosslinking density of the composite hydrogels. To further investigate the thermal stability, TGA measurement was used under a lower heating rate. As shown in Figure 6b, the composite hydrogels exhibited better thermal stability than that of the pure hydrogels, which further proved above conclusion. Insert Figure 7 Water absorbency is an important property for hydrogel and it is closely related to the immersion time. The swelling test of the composite hydrogels was implemented in a phosphate-buffered saline at room temperature, and the swelling trend for each sample is shown in Figure 7a. It was found that all composite hydrogels took about
twenty hours to reach the equilibrium-swollen state, and the swelling ratio decreased along with the increase of ternary composite content, illustrating that there were hydrogen bonding interactions between the ternary composites and double-network hydrogels. Because the ternary composites acted as a physical cross-linker and enhanced the cross-linking density of the composite hydrogels, it could be explained why the composites had effect on the water absorbency of the resultant composite hydrogels. From Figure 7b and c we found that the volume of the composite hydrogels changed dramatically after being subjected to immersion in the water over twenty hours, and the water remained transparent, indicating there was no release of composites and the composite hydrogels were stable. This result also provided the hydrogen bonding between the composites and the molecular chain of hydrogel. Due to the superabsorbent ability of guar gum acted as a physical crosslinked network, the water absorbency of the binary network hydrogels composed of PAM/GG surpassed other binary network hydrogels such as PAM/PVA[44]. Insert Figure 8 The mechanical properties of the composite hydrogels with different composite contents were tested at room temperature, and the tensile stress-strain curves were shown in Figure 8a. With the increasing of the content of composite hydrogels, the tensile strength increased from 113 kPa to 140 kPa, while the fracture strain decreased from 307% to 170% , and the toughness had a slight decrease (Figure 8b). The novel multifunctional hydrogels were tough and had a better mechanical property than that of other multifunctional hydrogels[35], In addition, the binary network hydrogels
improved the tensile stress compared with the pure hydrogels (with a tensile stress of 88.3 kPa), and the novel binary network hydrogels composed of PAM and GG had a better mechanical property than that of conventional PAM/PVA [45]. The rheological properties of the composite hydrogels containing different contents of ternary composites were also investigated. As shown in Figure S3, it was found that its loss modulus G′′ was lower than the storage moduli G′, indicating that all composite hydrogels presented elastic character. Besides, both G′ and G″ of the composite hydrogels increased with the increasing of the contents of the ternary composites. The hydrogen bonds between the ternary composites and the binary network hydrogels contributed to the hydrogel stiffness. There were two reasons for the improvement of the mechanical properties. First, guar gum (GG) could be considered as a physically crosslinked network to construct binary networks with PAM via covalent crosslinking. And a binary network comprising physical and chemical network was an effective approach to enhance the strength of hydrogel, which had been proved in many reports[45]. Second, PAni existing on the surface of the ternary composites could form some hydrogen bonds with the long chains of PAM network. And the ternary composites could be used as crosslinking sites to improve the density of cross-linked network and also provided pathways for dissipating mechanical energy when it was undergone large deformation. Therefore, the composite hydrogels had good mechanical properties and they could overcome different deformations such as bending, twisting and stretching (Figure 8c, d and e). Insert Figure 9
After Fe3O4/MoS2/PAni composite was inserted, the composite hydrogels obtained magnetic properties. The magnetic hysteresis loops of Fe3O4/MoS2 (a), Fe3O4/MoS2/PAni (b) and CH-2.0 (c) are shown in Figure 8. The saturation magnetization of pure Fe3O4 [46] was 63.8 emu g-1, after coated by MoS2 and PAni, the saturation magnetization decreased to 14.96 and 9.82 emu g-1, respectively. Due to the low content of ternary composites were incorporated into the hydrogels, the composite hydrogels possessed a weak magnetism about 1.28 emu g-1. Magnetic and biocompatible hydrogels showed potentials for drug delivery in external magnetic field. Although the magnetic intensity of composite hydrogels was much lower than that of the pure magnetic composite, it had a typical hysteresis loop and its performance could be easily changed by adjusting the content of ternary composites. Insert Figure 10 The conductivity of the composite hydrogels with different composite contents was measured by a four-point probe. As shown in Figure 10, with the increasing of the content of composites, the conductivity of the composite hydrogels increased. The conductivity of composite hydrogels increased from 1.235×10-3 S cm-1 to 2.052× 10-3 S cm-1, which was much higher than that of other PAni composite hydrogels[47]. And it had a slight decrease after one month of aging owing to oxidation, indicating of the well in vitro stability of the composite hydrogels. This is because, first, the ternary composites had incorporated into hydrogel to form continuous circuits. Second, molybdenum disulfide (MoS2) possessing a large surface area that could be easily coated by polyaniline to reinforce electrical conductivity through synergistic effect
among them. Meanwhile, the polyaniline molecules had been embedded to the interlayer of MoS2 to avoid overoxidation and a far sharper decline in the conductivity during storage time. In addition to storage stability, the composite hydrogels maintained high conductivity even being subjected to stretching, bending, or twisting.
4. Conclusions In summary, flexible and tough composite hydrogels with conductive and magnetic properties were synthesized by incorporating the ternary composites into hydrogel. MoS2 nanosheets were grown on the surface of the magnetic particles via hydrothermal method to obtain the precursor composite, and then polyaniline was coated on the surface of MoS2 via electrostatic attraction. The introduction of the ternary composites in hydrogel not only endowed conductive and magnetic properties, but also provided hydrogen bonds with molecular chain to obtain a tough composite hydrogel. The maximum saturation magnetization and conductivity of the composite hydrogel were 1.28 emu·g−1 and 2.052 × 10−3 S·cm−1, respectively, and the tensile strength reached up to 140 kPa. Therefore, novel ternary composite hydrogels have been prepared and they will have potential applications in many fields, such as electronic
skin,
drug
release,
electromagnetic
interference
shielding,
and
wave-absorbing.
Acknowledgment We appreciate the financial support from the National Natural Science Foundation of China under grant No.21571065.
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Figure Captions Figure 1. (a) Schematic illustration of the fabrication process of the ternary composite hydrogels. (b) Conceptual graphs of composite structures and the hydrogen bonds
among binary networks and composites. Figure 2. (a1,a2) SEM images of Fe3O4/MoS2 composites at different magnifications. (b1,b2) SEM images of Fe3O4/MoS2/PAni composites at different magnifications. Cross-sectional SEM images of the pure binary networked hydrogel (c) and the composite hydrogel (d). TEM image of Fe3O4/MoS2/PAni composites at different magnifications(e1～e3). Figure 3. EDS mapping area of Fe3O4/MoS2/PAni composites, and the elemental signals of N, C, Mo, S, Fe, O. Figure 4. (a) Comparison of FT-IR of Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites. (b) Comparison of XRD spectra of the Fe3O4, Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites. Figure 5. Particle size distribution of Fe3O4/MoS2 and Fe3O4/MoS2/PAni composites. Figure 6. TGA curves of the pure hydrogel and the composite hydrogel with different composite contents under the heating rate of 10 °C min-1 (a) and 1 °C min-1 (b). Figure 7. (a) Swelling ratio of the pure hydrogel and different composite hydrogels as a function of immersion time in phosphate-buffered saline (PH=6.7) at room temperature. (b) The photos of the composite hydrogel immersed in water and (c) the composite hydrogels were immersed in water after 2 days. Figure 8. Tensile stress-strain curves (a) of the hydrogel with different composite hydrogels and corresponding toughness (b). The photos of the composite hydrogel (c) in flexuous state and (d) in twisted state. (e1) Process of uniaxial tensile of the composite hydrogel in original state and (e2) the composite hydrogel in maximum
tensile state. Figure 9. Hysteresis loops of (a) Fe3O4/MoS2, (b) Fe3O4/MoS2/PAni and the composite hydrogels. Figure 10. Electrical conductivity of the composite hydrogels (original state and after one month) with various composite contents(a). The photo of the electrical circuit (b), the composite hydrogels in original state(c) and the composite hydrogel in tensile state(d), direct voltage applied for all electrical circuit was 3.0 V.
Conflict of interest We all declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted