Dually synergetic network hydrogels with integrated mechanical

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Applications of Polymer, Composite, and Coating Materials

Dually synergetic network hydrogels with integrated mechanical stretchability, thermal responsiveness and electrical conductivity for strain sensors and temperature alertors Zhiwen Wang, Hongwei Zhou, Weixing Chen, QiuZhao Li, Bo Yan, Xilang Jin, Aijie Ma, Hanbin Liu, and Weifeng Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02060 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Applied Materials & Interfaces

Dually synergetic network hydrogels with integrated mechanical stretchability, thermal responsiveness and electrical conductivity for strain sensors and temperature alertors Zhiwen Wang, Hongwei Zhou*, Weixing Chen, Qiuzhao Li, Bo Yan, Xilang Jin*, Aijie Ma, Hanbin Liu*, Weifeng Zhao

Zhiwen Wang, Prof. Hongwei Zhou, Prof. Weixing Chen, Qiuzhao Li, Bo Yan, Dr. Xilang Jin, Dr. Aijie Ma, Dr. Weifeng Zhao School of Materials and Chemical Engineering, Xi'an Technological University, Xi'an, 710021, P. R. China. E-mail: [email protected], [email protected] Dr. Hanbin Liu School of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi'an, 710021, P. R. China. E-mail: [email protected]

Keywords: Conductive hydrogel, dually synergetic network, mechanical flexibility, thermal responsiveness, strain sensor

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Abstract: The first example of dually synergetic network hydrogel, which has integrated

mechanical

stretchability,

thermal

responsiveness

and

electrical

conductivity, has been constructed by a versatile and topological co-crosslinking approach. Poly(N-isopropylacrylamide) (PNIPAAm) is introduced as the thermally responsive ingredient and polyaniline (PANI) is selected as the electrically conductive ingredient. PNIPAAm network is crosslinked by double bond end-capped Pluronic F127 (F127DA). PANI network is doped and crosslinked by phytic acid. These two ingredients are further mechanically interlocked. Due to the integrated multiple functionalities, the topologically co-crosslinked hydrogels, as will be mentioned as F-PNIPAAm/PANI hydrogels, can be fabricated into resistive-type strain sensors. The strain sensors can achieve a gauge factor of 3.92, response time of 0.4 s, and sensing stability at least for 350 cycles and can be further applied in monitoring human motions, including motion of two hands, bending of joints, even swallowing and pulse rate. Moreover, F-PNIPAAm/PANI hydrogels are utilized to construct efficient temperature alertors based on the disconnection of circuits induced by the volume shrinkage at high temperature.

1. Introduction Despite the highly integration of electronic circuits according to Moore's law, further development of electronic devices is inevitably limited by the physical limits of electron components and the low flexibility of rigid inorganic electronic materials.1-2 Electronic materials that combine flexible mechanical properties and controllable electrical properties are increasingly required in wearable electronics, soft


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robots and human-machine interfacing facilities.3-6 Electrically conductive hydrogels (ECHs), which have variable structures, mechanical flexibility and controllable electronic properties, have become popular candidate materials for flexible sensors,7-8 tissue engineering,9 supercapacitors10-11 and actuators.12-13 ECHs composed of intrinsically conductive polymers, such as polyaniline (PANI),14 polypyrrole (PPy)15-16 and polythiophene (PTh)17-18 have high electrical conductivity (up to 0.11 S·cm-1) and are regarded as promising alternatives to inorganic electronic materials for fabricating flexible electronics, but their mechanical flexibility was limited by the rigidity of conjugated polymer chains and the mechanical properties are rarely investigated.19-20 Dually synergetic network, which is inspired by the intrinsic “soft and hard” structure of animal dermis,21 provides a clue to fabricate and modulate ECHs with both excellent mechanical performances and controllable conductivity. Similar to the structure of animal dermis, ECHs with dually synergetic network are composed of interlocked rigid conductive chains and flexible chains and can be modularly controlled by the structure of these two ingredients and the interactions between them. By introducing dynamic covalent bonds between polyvinyl alcohol (PVA) and PANI, Ma and his coworkers11 obtained PVA/PANI hydrogels with high tensile strain (250%), strength (5.3 MPa) and excellent electrochemical capacitance (928 F g-1). But preparation of such hydrogels requires unique meta or ortho hydroxyl groups in flexible polymer chains and boronic acid groups in monomers of conductive ingredients, which limits the versatility of this method. In another example, Yu and his



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coworkers[13] demonstrated that the architecture of the conductive polymer phase played an important part in determining the conductivity of hybrid ECHs. While high conductivity (0.8 S m-1) was obtained, the stretchability of the hydrogels is limited by the N,N'-Methylenebisacrylamide (BIS) crosslinked network structure, which are often fragile and cannot withstand large deformations in stretchable electronics22-25. Herein, the first example of dually synergetic network hydrogel, which has integrated

mechanical

stretchability,

thermal

responsiveness

and

electrical

conductivity, has been constructed by a versatile and topological co-crosslinking approach (Figure 1). As a model, PNIPAAm is introduced as the thermally responsive ingredient and PANI is selected as the electrically conductive ingredient. These two ingredients are crosslinked respectively by double bond end-capped Pluronic F127 (F127DA) and phytic acid, and further mechanically interlocked. Hereafter, F127DA crosslinked PNIPAAm hydrogels are mentioned as F-PNIPAAm hydrogels. F127DA is selected as the topological crosslinking agent, because it is amphiphilic and has the ability to self-assemble into micelles with polymerizable double bonds on the surface in its aqueous solution. Double bonds participate polymerization and form chemical crosslinking points in hydrogels, while the high molecular weight F127 chains provide large buffering space between two rigid chemical crosslinking points. Such kind of crosslinking points have been demonstrated to be efficient for highly flexible hydrogels.26-30 Phytic acid is used as both the doping agents and crosslinking agents.14 After doping PANI chains, phytic acid molecules were deprotonated and forms more than one interaction points with protonated PANI chains, which promotes the


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formation of PANI network. Due to the unique topologically co-crosslinked structure, the hydrogels, which will be mentioned as F-PNIPAAm/PANI hydrogels, are mechanically stretchable, thermally responsive and electrically conductive and can be further applied in sensitive strain sensors and temperature alertors.

Figure 1. Illustration of the network structure of F-PNIPAAm/PANI hydrogels. In F127DA, PEO and PPO are poly(ethylene oxide) and poly(propylene oxide), respectively. 2. Experimental Section 2.1. Materials. Pluronic F127 diacrylate (F127DA) was synthesized according to the previous method.26, 30 NIPAAm were purchased from Energy Chemical and purified by recrystallization before use. Aniline (ANI, 99.5%) was purchased from Energy Chemical and distilled under reduced pressure. Phytic acid (50% in water) and α-ketoglutaric acid (98%) were purchased from Energy Chemical. Ammonium persulfate (APS, 98%) was purchased from Chinese Medicine Group Chemical



Reagent Co., Ltd. and recrystallized from its methanol/H2O solution.

2.2. Preparation of F-PNIPAAm hydrogels. An aqueous solution containing 3 mol/L NIPAAm and 0.067 g/mL F127DA was first prepared with the assistance of ultrasound. After the solution was bubbled with N2 for 15 min, 0.5 mol% of

α-ketoglutaric acid with respect to NIPAAm was added and the gelation process was carried out by illumination with UV light (intensity 70 mW/cm2) for 60 min. The F-PNIPAAm hydrogels formed were purified by dialyzing against H2O and ethanol.

2.3 Preparation of F-PNIPAAm/PANI hydrogels. F-PNIPAAm/PANI hydrogels were prepared by an in-situ oxidative polymerization of aniline in the presence of F-PNIPAAm hydrogel. Solutions of aniline (0.06 mol/L) and ammonium persulfate (0.02 mol/L) were cooled to 0~5 oC and then quickly mixed. F-PNIPAAm hydrogel was immediately immersed into the above mixture and kept under ice-water bath for 1 h and then kept at 25 oC for 12 h. The resulting black hydrogel was then immersed into a phytic acid solution (13 wt%) for 12 h to prepare F-PNIPAAm/PANI hydrogel. Unreacted residues and monomers were removed by dialyzing against ethanol and H2O.

2.4. Assembling of strain sensors utilizing F-PNIPAAm/PANI hydrogels. To obtain the strain sensors, F-PNIPAAm/PANI hydrogel (water content 83%) was first cut into desired shape and the water on the surface was blowed with N2. Two individual copper electrodes were attached onto two sides of the hydrogel by conductive silver pastes. The above mentioned system was then encapsulated with liquid silicone rubber containing 8 wt% curing agent. The curing process was conducted at room


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temperature for 10 h and the thickness of a single encapsulation layer was about 1 mm. For

large

deformation

detection,

the

length,

width,

and

thickness

of

F-PNIPAAm/PANI hydrogels between two electrodes are 15 mm, 5 mm and 2 mm, respectively.

For

pulse

detection,

the

length,

width,

and

thickness

of

F-PNIPAAm/PANI hydrogel are 10 mm, 5 mm and 1 mm, respectively.

2.5. Characterization of F-PNIPAAm/PANI hydrogels, strain sensors and temperature alertors. FT-IR spectra were recorded by an IR spectrophotometer (Nicolet MX-1E) in the scanning range of 4000~400 cm-1. The samples were obtained by a KBr disc technique. Fracture surfaces morphology of freeze-dried hydrogels was observed on a FEI Quanta 400F scanning electron microscope (SEM) at an acceleration of 20 kV. Tensile stress-strain curves were carried out on a computer controlled electronic universal testing machine at a rate of 20 mm/min. Dynamic mechanical property analysis of the hydrogels was carried out on a TA rheometer (DHR 2) equipped with a Peltier thermostatic system. The resistance of the F-PNIPAAm/PANI hydrogels and temperature alertor systems was measured by a digital electric bridge (LCR, UC2858A). The current signal during motion monitoring was recorded on a PARSTAT 4000+ Potentiostat Galvanostat EIS analyzer system at a constant voltage of 3.0 V. 3. Results and discussion 3.1 Structure and mechanical properties of F-PNIPAAm/PANI hydrogels F-PNIPAAm hydrogel has highly porous structures (Figure 2a) and exhibit high swelling ratio (Figure S1, Supporting Information), which enables the efficient



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absorbing ability to aniline and oxidative agents into hydrogel network and induces efficient in-situ polymerization of ANI monomers. Macroscopically, colorless F-PNIPAAm hydrogels turned into black F-PNIPAAm/PANI hydrogels after introduction of PANI. Together with the FTIR spectra (Figure S2, Supporting Information) in which characteristic absorption peaks of PANI chains appear, it can be deduced that F-PNIPAAm/PANI hydrid hydrogels were successfully prepared. Compared

with

F-PNIPAAm

hydrogels,

the

typical

porous

structure

of

F-PNIPAAm/PANI hydrogels was maintained but the average pore size and content of pores were lowered (Figure 2a). This may be induced by both the filling of PANI chains into the pores of F-PNIPAAm network and the mechanical interlocking of PNIPAAm and PANI ingredients.

Figure

2.

Fracture

surfaces

morphology

and

mechanical

flexibility

of

F-PNIPAAm/PANI hydrogels. (a) Typical SEM images of fracture surface of F-PNIPAAm hydrogels and F-PNIPAAm/PANI hydrogels. (b) Photographs of an F-PNIPAAm/PANI hydrogel strip under different deformations, including stretching,


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twisting, and stretching and twisting. (c) A typical tensile stress-strain curve of F-PNIPAAm/PANI hydrogel. (d) Strain-controlled dynamic frequency sweep test of storage moduli (G') and loss moduli (G'') for F-PNIPAAm/PANI hydrogels with different feed concentrations of aniline. The temperature was 20 oC and the strains were fixed at 1%. Despite the existence of rigid PANI network, flexible F-PNIPAAm/PANI hydrogels can still be obtained by varying the feed concentrations of aniline. When the feed concentration of aniline was fixed lower than 0.06 mol/L, F-PNIPAAm/PANI hydrogels could withstand large macroscopical deformations without visible damage. As shown in Figure 2b, an F-PNIPAAm/PANI hydrogel strip could be safely stretched to at least 230% of its original length and quickly recovered (

Dually synergetic network hydrogels with integrated mechanical

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