Electrically Conductive TPU Nanofibrous Composite with High Stretchability for Flexible Strain Sensor
Highly stretchable and electrically conductive thermoplastic polyurethane (TPU) nanofibrous composite based on electrospinning for flexible strain sensor and stretchable conductor has been fabricated via in situ polymerization of polyaniline (PANI) on TPU nanofibrous membrane. The PANI/TPU membrane-based sensor could detect a strain from 0 to 160% with fast response and excellent stability. Meanwhile, the TPU composite has good stability and durability. Besides, the composite could be adapted to various non-flat working environments and could maintain opportune conductivity at different operating temperatures. This work provides an easy operating and low-cost method to fabricate highly stretchable and electrically conductive nanofibrous membrane, which could be applied to detect quick and tiny human actions.
KeywordsElectrospinning Conductive nanofibrous composite Strain sensor
- DI water
Nanofibrous membranes have stimulated enormous attention for their outstanding chemical and physical performance, such as high specific surface area, high porosity, elasticity in surface functions, and outstanding mechanical performance. These excellent properties make the polymer nanofibrous membrane a potential material in many fields such as tissue template [1, 2, 3, 4], protective clothing application , drug deliver [6, 7, 8], and electronic devices [9, 10]. And these applications usually require highly stretchable devices which could be applied to irregularly shaped objects. There are a lot of approaches to get a nanofibrous membrane, such as template synthesis [11, 12], ultrasonic irradiation synthesis , nanoprinting , and electrospinning . Among these methods, electrospinning is a simple, low-cost, and convenient method to fabricate nonwoven membranes, and it is portable to generate nanofibrous membrane in lab. The electrospun micro/nanofibers display a variety of outstanding properties, such as large surface area, high length/diameter ratio, flexible surface functionality, and superior mechanical performance.
To acquire electrical conductivity, conducting polymers and carbon series semiconductor materials are often used as functional elements in the fabrication of membrane. Polyaniline (PANI) is a kind of conductive polymer with high conductivity and is easily to be polymerized. However, the strong polarity, which induces high conductivity, leads to poor elasticity of PANI . Thermoplastic polyurethane (TPU), as one of the high-elasticity materials, is characterized with high elasticity, low-temperature flexibility, and abrasion resistance . The combination of TPU and PANI can make up the disadvantage of PANI, and the strong polarity of PANI makes efforts to combination. Besides, the TPU membrane obtained by electrospinning is of high elasticity, high stretchability, low cost, and light weight. In situ polymerization exhibits a good way to combine TPU membrane and PANI together. As for flexible strain sensor and stretchable conductor, which could be applied in wearable electronic devices, elasticity and conductivity are essential, so we choose TPU and PANI as raw materials to fabricate nanofibrous composites. In this paper, highly stretchable and electrically conductive TPU nanofibrous membrane based on electrospinning for flexible strain sensor and stretchable conductor has been fabricated via post-processing strategies. The PANI/TPU composite sensor could sustain a maximum tension of 165%, and the conductivity of our strain sensor can be calculated to be about 7.5 × 10−3 S cm−1. Meanwhile, the composite displays good stability and durability. Moreover, the composite could be applied to various non-flat working environments and could maintain almost good conductivity at different operating temperatures. This work provides a facile operating and low-cost method to fabricate highly stretchable and electrically conductive nanofibrous membranes, which have potential applications in flexible strain sensors and stretchable conductors for wearable devices.
Preparation of PANI/TPU Nanofibrous Membrane
There were three steps to prepare PANI/TPU membrane. The first step was to obtain TPU nanofibrous membrane via electrospinning. 2.4 g TPU was dissolved in 8.8 g N,N-dimethylformaminde (DMF) and 8.8 g tetrahydrofuran (THF) to prepare a precursor solution and then stirring the mixture thoroughly for 5 h until it turned into a homogeneous solution. The electrospinning process was carried out with a spinning distance (between the needle and collector) about 10~12 cm, a high voltage (supplied by a high-voltage DC power, DW-P303-1ACFO, Tianjin Dongwen) about 12 kV, and a feeding rate of the solution (maintained by a syringe pump, LSP01-1A, Baoding Longer Precision Pump Co., China) about 15 μl min−1. Moreover, to get a uniform thickness nanofibrous membrane, a roller was used as a collector. Compared with the traditional collector like aluminum foil, the thickness of the membrane was more uniform from the edge to the middle. After getting the TPU membrane, the next step was the polymerization of PANI. Firstly, 4.6 g ammonium persulfate (APS, Mw = 228.20) was added into 50 ml deionized (DI) water to make up the solution A and 1.875 g aniline (Mw = 93.13) and 2.54 g sulfosalicylic acid (SSA, Mw = 254.22) were dissolved into 50 ml DI water to obtain solution B. After stirring for 30 min at room temperature, the TPU membrane(10 cm × 10 cm) was submerged into solution B, and then, solution A was slowly added to B to ensure the intensive mixing. After standing in refrigerator for 12 h at 275 K, the membrane was taken out from the final solution and washed with DI water. With the polymerization reaction of aniline, the color of the mixture changed from canary yellow to deep green and the membrane changed from white to deep green. Finally, the PANI/TPU nanofibrous membrane was obtained after drying for 48 h at room temperature.
The final nanofibrous membrane was characterized by an optical microscope (Olympus BX51), a scanning electron microscope (SEM, DB235 FEI), and a Fourier transform infrared spectroscope (FTIR, Thermo Scientific Nicolet iN10). The strain-stress curves of the twisted fibers were obtained by a dynamical mechanical analyzer (Q-800, TA Scientific). Electrical properties were tested by a Keithley 6485 high-resistance meter system at room temperature and a physical property measurement system (PPMS, Quantum Design).
Results and Discussion
Characterizations of Nanofibrous Membrane
Stretchability and Sensitivity Test
Only these properties are not enough. A good strain sensor should be equipped with good stability and durability which implies the sensor can work for a long time without any significant regression after different elastic deformations. To measure the stability, we investigated the response-recovery curve under a fixed strain of 30.7%, and the result is shown in Fig. 6b. Here, the current decreases with tensile strain and the current almost recovered to the initial value. And then, the curve could repeat the same circle under 30.7% mechanical pressure which suggests that our sensor had a good repeatability. In practical applications, durability is an important parameter . To access the endurance of the sensor, we investigated the output signals under 100 times cycling stretching and placed it for 24 h at room temperature. The results are shown in Fig. 6c. Curve A represents the original I-V characteristic of the sensor without any stretch, and curve B is the I-V characteristic of the sensor which was stretched for 100 times and put for 24 h. The function mechanism of the conductivity response may be due to the rupture and falling off of the PANI cluster or separation of PANI particles which makes the conductivity decrease. Figure 6d shows that the I-V characteristic after 1000 times of bending almost has no change compared with the initial value. The results indicate that the sensor is characterized with good durability.
Application in Finger Bending-Release Detection
The sensor has sensitivity and good stretchability, and Fig. 10 indicates the PANI/TPU nanofibrous membrane could be used as a flexible conductor which has the potential to be applied to flexible screen and may be attached to clothing to detect human health .
In summary, we fabricate highly stretchable nanofibrous PANI/TPU strain sensor via electrospinning. The sensor based on PANI/TPU nanofibrous membrane can detect and withstand a strain from 0 to 165% with fast response and excellent stability. In addition to high stretchability, it shows good qualities in the durability and stability under different ambient environments. Moreover, because of the fast and repeatable response to tensile force and finger motions, the simple device could be applied to detect quick and tiny human actions. Meanwhile, thanks to the high conductivity, it could be used as flexible conductors for electronic components. This work provides a facile method to fabricate highly stretchable and conductive nanofibrous membrane with characteristics of fast dynamic motion-sensing abilities, high stability, and cheap fabrication.
Thanks to the help of Xiao-Xiong Wang, Xiao-Xiao He, Guang-Di Nie, Jun Zhang, Bin Zhang, Wen-Zhe Guo, and Yun-Ze Long.
This work was supported by the National Natural Science Foundation of China (51673103 and 51373082), the Taishan Scholars Program of Shandong Province, China (ts20120528), the Key Research and Development Plan of Shandong Province, China (2016GGX102011), and the Postdoctoral Scientific Research Foundation of Qingdao.
Availability of Data and Materials
The dataset(s) supporting the conclusions of this article is (are) available in the [repository name] repository [unique persistent identifier and hyperlink to dataset(s) in http:// format.
YZL developed the concept and designed the experiments. TL, XXW, and XXH performed the experiments. GDN, JZ, BZ, and WZG contributed to the data analysis. All authors wrote and revised the paper. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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