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polyaniline electrospinning nanofibrous mats
Synthetic Metals 219 (2016) 11–19
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Electromechanical deformation sensors based on polyurethane/ polyaniline electrospinning nanofibrous mats Mingwei Tiana,b,c,1,* , Yujiao Wanga,b,1, Lijun Qua,b,c,* , Shifeng Zhua,b,c , Guangting Hanb,c, Xiansheng Zhangd , Quan Zhoue, Minzhi Dua,b , Shuli Chia,b a
College of Textiles and Clothing, Qingdao University, Qingdao, Shandong 266071, China Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong Province, Qingdao University, Qingdao, Shandong 266071, China c Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao, Shandong 266071, China d College of Textiles, Donghua University, Shanghai 201600, China e Central Laboratory, The Affiliated Hospital of Qingdao University, Qingdao, Shandong 266071, China, China b
A R T I C L E I N F O
Article history: Received 17 December 2015 Received in revised form 27 April 2016 Accepted 6 May 2016 Available online xxx Keywords: Polyurethane Polyaniline Electrospinning Electrical conductivity Strain sensitivity
A B S T R A C T
Conducting electrospun nanofibrous mats elicit great interest in a wide range of applications. In this paper, polyurethane/polyaniline (PU/PANi) mats were prepared via the elecrospinning route with a polyurethane/dimethyl formamide spinning solution, and then in situ chemical polymerization of aniline monomers on the surface of PU nanofibers to deposit conductive PANi layer. In details, the morphology and structure of the PU/PANi mats were characterized by SEM, FTIR and the results indicated that the polyaniline stably deposited on the surface of polyurethane nanofiber. The electrical conductivity measurements indicated PU/PANi mats expressed good electrical conductivity (0.43 S/cm) at the optimal polymerization time as 120 min. As expected, PU/PANi mats showed strain sensitivity of (R-R0)/R0 = 0.75 (curvature = 0.4 cm1) and the average strain gauge factor could reach 17.15 with applied stretching deformation (0 110%). Therefore it could be deemed as the recommended nano-materials for flexible wearable device. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Conductive polymers such as polyaniline, polypyrrle and polythiophene attracted tremendous attention due to a wide variety of applications, including anti-static coating [1–3], batteries [4–7], sensors [8–11], electrodes [12,13] and so on. Among these conductive polymers, polyaniline (PANi) might be the most promising because of its good environmental stability, ease of synthesis, thermal stability and an inexpensive synthetic route. However, as an intrinsically conducting polymer (ICP), the widely practical application of PANi is restricted by its limited processability, insolubility, infusibility and poor mechanical properties [8]. In order to overcome these drawbacks, many researchers attempted to introduce PANi with an insulating polymer matrix with high mechanical strength, e.g. rubber [14], textiles [15,16],
* Corresponding authors at: College of Textiles and Clothing, Qingdao University, Qingdao, Shandong 266071, China. E-mail addresses: [email protected] (M. Tian), [email protected] (L. Qu). 1 These authors equally contributed to this work. http://dx.doi.org/10.1016/j.synthmet.2016.05.005 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
plastic [17]. Especially, Polyurethane (PU) elastomer was receiving considerable attention for a large range of transducer and actuator applications. Furthermore, PU exhibited excellent flexibility, good film/fiber forming property and resistance to solvent and it could carry out the energy conversion between mechanical energy and electrical energy [18]. The remarkable composites with PU as elastomer substrate and PANi as electrical filler have attracted considerable attention during the last decade. The PU/PANi composites can be commonly classified into two categories based on the preparing procedure, the first type is surface-modified PU/PANi whose structure is composed by the inner substrate PU substrate and the outside deposited PANi layer, the other type is volume-modified PU/PANi which is blended with PU and PANi under certain weight ratios. Hrehorova et al. [19] pointed out that surface-modified PU/PANi film possessed lower percolation threshold level than volumemodified PU/PANi ones, therefore, surface-modified PU/PANi composite materials can be recommended as an ideal candidate for electromechanical strain sensors. Prabhakar et al. [20] modified PU films with PANi and PANi-AgNp and could render the surface conductive, suggesting potential application in electrochemical
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biosensors. Gu et al. [21] prepared biomimetic artificial nanofibrous bundle using electrospun PU nanofibers deposited with PANi nanoparticles. In smart strain sensor application, Qin et al. [22] prepared elastic, conductive PU/PANi fiber by in-situ chemical oxidative polymerization of PANi on PU fiber surfaces. And the average strain gauge factor of resultant functional fiber could reach to 3 which is 1.5 times higher than normal case. As we all know, nanofiber with high specific surface area, ultrathin and flexible properties could show some advantage performances in strain sensor area. Nanofiber sensor could contact with human skin in a more compact interface owing to its ultrathin and flexible structure. It could also perform isotropic strain response owing to its random nonwoven structure. In addition, more PANi nanoparticles could deposited on the substrate nanofiber owing to its higher specific surface area character. Therefore, depositing PANi nanomaterials onto flexible PU materials was seemed as a reasonable route to produce a novel strain sensors for monitoring minute structural deformations or damage detection. Compared with the structure of conventional macro-conductive PU fiber, electrospun flexible PU nanofibers could provide extraordinarily high surface area/volume ratios, lightweight, flexible and designable multifunctionalities. These outstanding structure properties might be helpful for the performance of PU/PANi strain sensor and the detailed results will be investigated in this paper. Moreover, after deposited with PANi layer, electrospun PU/PANi nancomposite fiber mats had no specific direction sensing and provided multidirectional and multipoint strain sensing, similar to carbon-nanofilm strain sensors [23]. In this paper, PU nanofiber mats were prepared through the elecrospinning process and used as substrate for the preparation of conducting nanocomposites. It was reported that in situ polymerization was a reasonable method for preparing PANi-coated materials because it did not result in the damage of the substrate and enhanced conductive properties [24]. So we prepared PU/PANi fiberous nanocomposite through in situ chemical polymerization of aniline monomers on the surface of the PU nanofibers. And then
its electrical properties were measured and the strain sensitivity was discussed. 2. Experimental 2.1. Materials Polyurethane (PU) was purchased from Huakai resin Co., Ltd., China. Aniline monomer (ANI) was commercially available from Tianjin Guangfu technology Co., Ltd., China. Ammonium persulfate (APS) (99.5%), hydrochloric acid (HCl) (37%) was supplied by Tianjin Guangfu technology Co., Ltd., China. Dimethyl formamide (DMF) (99.5%) and alcohol (99.7%) were purchased from Tianjin Fuyu chemical Co., Ltd., China. All the chemicals used were of analytical grade and were used as received. 2.2. Preparation of polyurethane/polyaniline fiberous nanocomposites The prepared process of polyurethane/polyaniline fiberous nanocompsites was shown in Fig. 1. Suitable amount of polyurethane was dissolved in N,N-dimethylformamide (DMF) with a concentration 20 wt.%. The mixture was continuously stirred until a homogeneous solution formed. This solution was fed into a 5 mL syringe with a stainless steel needle and the syringe attached to an electrospinning apparatus. The flow rate of the solution kept constant at 0.1 mL/h. A voltage of 20 kV was applied directly to the spinneret. And the distance between the needle tip and the collector was maintained at 12.5 cm. Polyurethane/polyaniline (PU/PANi) nanofibrous mats were prepared through in situ chemical polymerization of aniline monomers on the surface of the as-obtained PU nanofibers. In the typical procedure, aniline monomers were dissolved in ethanol solution (the volume ratio of aniline to ethanol at 1:4) and PU nanofibrious mats were first immersed in the solution for 2 h. And then PU nanofibers were removed and placed into APS/HCl reactive solution and the concentration of APS and HCl was 30 g/L and 0.5 mol/L respectively. Under this condition, the polymerization
Fig. 1. The schematic process of polyurethane/polyaniline nanofibrous mats.
M. Tian et al. / Synthetic Metals 219 (2016) 11–19
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took place on the fiber surface to form a conducting polymer layer that fully coated the PU nanofibrious mats. After different polymerization time (20, 40, 60, 80, 100, 120, 140, 160 and 180 min), the dark green fibers were washed with water for three times and dried at room temperature. And the sample at 120 min polymerization time performed the best electrical properties; therefore, we chose this sample as the case of the following characterization. 2.3. Characterization The surface of the resulting PU/PANi nanofibrious mats was observed by scanning electron microscopy (SEM, SUPRA 55, Carl Zeiss Jena, Germany). The infrared spectra were recorded on a Fourier transform infrared spectrometer (FT-IR, Nicolet 5700, Thermo Electron, USA) at room temperature. The tensile strength of the resultant PU/PANi nano-mats was conducted by the INSTRON 5500R Universal Tester (Instron Corp., USA) according to ISO 13934-1, the gage length was 10 mm with the tensile speed of 20 mm/min. 2.4. Electrical measurement The conductivity of PU/PANi nanofibrious mats was measured at room temperature (25 C) by a UNI-T UT33D digital multimeter analyzer. In the measuring process, the applied voltage was varied from 10 to 10 V with an automatic increment of 0.5 V. The I/V curve was plotted for PU/PANi nanofibrious mats under different polymerization time and the electrical resistance R was deduced from the slope of the curve. Five times of measurement was carried out to calculate the average value of the electrical resistance. The thickness of fiberous nanocomposites was measured by a coating thickness gauge (EC-770, YUWESE, China). The average value of the electrical conductivity was calculated by the equation as follows:
s¼
1 Rd
where s , R and d are the electrical conductivity (S cm1), surface square resistance (V cm2) and film thickness (cm), respectively. The laundering durability test of the resultant PU/PANi nanofibrous mats was determined following the AATCC Test Method 61-2006 test using a standard color-fastness to washing laundering machine (Model SW-12AII, Wenzhou Darong Textile Instrument Co., Ltd., China). The nano-mats specimen was laundered in a closed canister containing 200 mL aqueous solution of an AATCC standard reference detergent WOB (0.37%, w/w) [25]. Then the electrical conductivity was repeatedly measured after one, two and three times laundering, respectively. 2.5. The stretching and bending sensitivity of flexible PU/PANi nanofibrous mats Stretching and bending motion are two common movements of human body, and therefore, bending and stretching response of sensor are deemed as two important factors for designing wearable electronics, robotic systems and electron skin. Herein, we evaluate the stretching sensing response of PU/PANi nanofibrous mats in our paper. The stretching sensing property of PU/PANi nanofibrious mats was measured at room temperature (25 C) by a UNI-T UT33D digital multimeter analyzer, and the normalized nanofibrous mats were fixed under different deformation with a home-made stretching device. Furthermore, cyclic test of PU/PANi nanofibrous mats was also carried on and the membrane was repeatedly stretched and relaxed for 20 cycles at strain 5%.
Fig. 2. The experimental set-up of the flexible sensor.
The bending sensing of the flexible sensor was evaluated with the experimental set-up as shown in Fig. 2. The as-spun 10 10 mm PU/PANi nanofibrious mat was glued on a flexible PVC plate, whose thickness was 300 mm, here, PVC plate is just employed as the supporting substrate to measure the bending response of PU/PANi nanofibrous mats because PU/PANi nanofibrous mats can not be bent without supporting substrate. Two copper electrodes were placed onto the corresponding opposite sides and the distance between two electrodes was set as 10 mm. X direction and Y direction were illustrated as in Fig. 2. To take X direction as an example, the bending sensing properties of the PU/PANi nanofibrious mat was tested under the standard conditions (T = 25 C and RH = 65%). The PU/PANi fiberous nanocomposites were bent at different curvatures and the I/V curves were plotted respectively. Sequentially, the membrane was repeatedly bent and relaxed for 20 cycles at a constant curvature (0.4 cm1). The conductivity change of the sensing nanofiberous membrane in both bent and relaxed states was recorded to observe its strain sensitivity. 3. Results and discussion 3.1. Morphology of PU/PANi nanofibrious mats The morphology of pure PU nanofiber and PU/PANi nanofibrious mat was shown in Fig. 3. It could be seen that pure PU nanofiber exhibited a very smooth surface (Fig. 3a) while the asprepared PU/PANi fiberous nanocomposite was featured with a rough surface due to the formation of PANi layer on its surface (Fig. 3b). By contrast, it was founded that the diameter of PU/PANi nanofiber was obviously larger than PU nanofiber and the increased thickness was about 100 200 nm. In other words, the thickness of the PANi layer was about 100 200 nm. The thickness of electrospun PU nanofibrous mats was evaluated from the cross section SEM image of PU nanomats in Fig. 3(c)-1, its thickness was around at 8 mm which was even thinner than the single cotton fiber (around 20 mm), and therefore, we defined the mats at such thickness level (<10 mm) as the ultrathin mats in our paper. Compared Fig. 3c and d, the PU/PANi fiberous nanocomposite was almost twice as thick as the PU fiberous mat. 3.2. Fourier transfer infrared spectroscopy The FT-IR spectra of PU nanofiber and PU/PANi composite nanofiber were shown in Fig. 4. Two peaks at 2936 cm1 and 2841 cm1 corresponded to asymmetric CH2 stretching and symmetric CH2, respectively [26]. The two bands at 1732 cm1 and 1705 cm1 arose from non-bonded and hydrogen-bonded urethane carbonyl C¼O stretching, respectively [8]. The peak at 1066 cm1 could be attributed to the C O C stretching
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Fig. 3. SEM images under different magnification for the surface of pure PU nanofiber (a)-1, (a)-2, and PU/PANi nanofibrous mats (b)-1, (b)-2, and the cross-section of pure PU nanofiber (c)-1, (c)-2 and; PU/PANi nanofibrous mats (d)-1 and (d)-2.
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Fig. 4. FT-IR spectra of PU nanofiber and PU/PANi fiberous nanocomposites.
vibrations. The peak at 1505 cm1 corresponded to symmetric vibration of N C N in the urethane-aniline because of the reactions of the NCO groups with the NH2 groups of the PANi [27]. A characteristic band of the conductive form of PANi was observed at 1308 cm1, which was due to p-electron delocalization induced in the polymer by protonation [28]. The adsorption peak at 1224 cm1 was assigned to a CN+ stretching vibration in the polaron structure. The adsorption band at 1083 cm1 was ascribed to the aromatic CH in-plane bending. The peak at 817 cm1 associated with an aromatic C H out of plane bending modes. Compared with the spectrum of the pure PU nanofiber, this peak was obviously enhanced, revealing the increasing amount of aromatic C H. As a consequence, the formation of PANi on the surface of PU nanofiber was demonstrated.
3.3. The mechanical properties of PU/PANi nanofibrious mats The representative stress-strain curve of PU/PANi nano-mats is shown in Fig. 5. Sample exhibits an average tensile strength (93 MPa) and excellent elasticity (114%). Such reasonable mechanical properties of PU/PANi nano-mats could be helpful to be applied in wearable electronics, robotic systems and electron skin. 3.4. Influence of polymerization time on the electrical properties of PU/ PANi nanofibrous mats The electrical conductivity of PU/PANi nanofibrous mats under different polymerization time were shown in Fig. 6. The electrical conductivity of PU nanofiber (0 min) was 0 S cm1. During 0 120
Fig. 5. The mechanical stress-strain curve of PU/PANi nano-mats.
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Fig. 6. The I/V curves (a) and electrical conductivity (b) under different polymerization time.
min, the electrical conductivity took on nearly six orders of magnitude increasement with the increasing polymerization time. However, the electrical conductivity exhibited a sustained fall with the increasing polymerization time, during 120 180 min. Similarly, the maximum value of the electrical conductivity (0.43 S/cm) appeared at 120 min. Hence, the optimal polymerization time was 120 min. Polyaniline could undergo three oxidation states: leucoemeraldine, emeraldine and pernigraniline during the polymerization process. Only at emeraldine oxidation state, PANi could perform electrical property, and in our paper, 120 min is the time of emeraldine oxidation state. After this moment, further oxidation a second redox process occurs, which yields a new insulating material, pernigraniline. Therefore, the conductivity of nanofibrous mats decrease for long polymerization time.
3.5. Washing durability In order to investigate the washing durability, three samples (the polymerization time as 100 min, 120 min and 140 min) were chosen to carry out washing durability. The electrical conductivity of PU/PANi nanofibrous mat before and after washing was displayed in and Fig. 7. For the samples after one time washing, it was observed that the electrical conductivity of polymerization 100 min and 140 min exhibited 42.39% and 28.01% decreasing ratio higher than that of 120 min case (17.65%), such result might be caused that two former case possessed more unstable PANi removed with the vigorous washing force and the later case had stable PU and PANi interface with the aid of hydrogen bonds. However, after three times washing, the electrical conductivity
Fig. 7. The electrical conductivity of PU/PANi fiberous nanocomposites before and after washing.
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Fig. 8. Normalized resistance under different stretching strain (a) and cyclic deformation at strain 5% (b).
tended to be stable which meant that the residual PANi has already stably deposited on PU nanofiber surface. Polyaniline is synthesized and deposited on nanofibrous mats surface via in-situ chemical polymerization of aniline. The mechanism of aniline polymerization is subject to the principles of the redox processes. The polymerization of aniline could be ignited once aniline monomer contacts with protonic acid and oxidant, and the duration of such reaction should be lasted for a certain time, in our case 120 min is the optimal time. Before this optimal time, the aniline monomer does not yet completely polymerize into polyaniline, the monomer could be more easily washed out once being laundered. On the contrary, after this optimal time, the high molecular weight polyaniline could be leaded to slow hydrolysis, resulting in low molecular weight polyaniline. During washing procedure, low molecular weight polyaniline could be more unstable and easily washed away. Therefore, at the optimal time 120 min in our case, nanofibrous mats deposited with high molecular weight polyanline performs the highest structural stability.
3.6. The stretching and bending sensitivity of flexible PU/PANi nanofibrous mats Fig. 8 shows the normalized resistance (R-R0)/R0 as the function of different stretching strain (a) and cyclic test at strain 5% (b) for PU/PANi nanofibrous mats, where R is the resistance under deformation, and R0 is the resistance of the original nano-mats. From Fig. 8(a), the resistance curve significantly increases with the applied strain deformation (0 110%), and the curve can be divided into two regions, resistance of nano-mats first perform linearly growth with increasing of strain (0–80%) and turn to a faster growth at (80–110%). The average strain gauge factor at the first region, defined as (R-R0)/R0/e (e is deformation rate), is calculated to be 17.15, which is more than 8 times higher than that of a traditional polymer strain gauge [29]. Fig. 8(b) presents the response of normalized resistance (R-R0)/R0 value of PU/PANi nanofibrous mats of cyclic deformation with a maximum applied strain of 5%. The normalized resistance of nano-mats perform an increasing response in the loading period while decreasing
Fig. 9. The normalized resistance of PU/PANi nanofibrous mats sensor at different curvatures.
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Fig. 10. Cyclic test for fiberous nanocomposites based bending strain at the curvature of 0.4 cm1: (a) the X direction and (b) the Y direction.
response in the unloading period, showing a reversible response, which well corresponded to the applied cyclic deformation. The normalized resistance curve with different bending curvatures was illustrated in Fig. 9. Especially, the strain sensitivity was defined as (R-R0)/R0, where R was the resistance in bent state of a certain curvature and R0 was the original resistance, that was, the resistance in relaxed state. From Fig. 9, the resistance sharply increased with big bending curvatures, and the strain sensitivity (R-R0)/R0 increased with the increasing curvature and this change approximated to a linear function. When the PU/PANi nanofiberous mat is bent, PU substrate might response faster deformation than the deposited PANi nanoparticles owing to its dramatic elasticity and leading to increased electrode distance, decreased thickness of PU/PANi mat and decreased electrode length. Such architectural change might result in increasing electrical resistance with increasing bending curvature according to Electrical Resistivity
Equation. Furthermore, the electrical resistance change should also be attributed to the rigid nature of PANi molecular chains covering PU nanofibrous mat surface. The outer conductive layer might be disrupted when the deformation is applied to the PU/PANi nanofibrous mats and accordingly, the (R-R0)/R0 value shows increasing tendency. The typical curves of conductivity change with strain of the PU/PANi nanofibrous mats during 20 cycles of bending and relaxation measurements were exhibited in Fig. 10. Compared Fig. 10(a) with (b), it was found that the variation rule of electric resistance in the X direction was the same as the Y direction, indicating that PU/PANi nanofibrous mats were an isotropic elasticity. This was attributed to the fact that PU nanofiber nonwoven substrate was isotropic. Choosing the X direction as an example, the strain sensitivity was investigated during 20 cycles of bending and relaxation. At a constant strain (curvature = 0.4 cm1),
Fig. 11. The resistance change with strain.
M. Tian et al. / Synthetic Metals 219 (2016) 11–19
PU/PANi mats showed highly strain sensitivity of (R-R0)/R0 = 0.75, the stress was relatively stable so the resistance also rose to a relatively stable level. When the deformation recovered, the resistance basically returned to the original. With the cyclic deformation continues, slight increase of the bottom and top values could be detected. We presumed it was induced by the irreversible deformation by the destruction of PANi nanoparticles and the distance between them, resulting in the break-up of conductive networks. Furthermore, the micro-structure of PU nanofibrous mats and the alignment between nanofibers might be changed by residual strain under cyclic deformation which also affected the stability of normalized resistance (R-R0)/R0 value of PU/PANi nanofibrous mats. The response time served as an important parameter to assess the sensitivity of the sensor. The response process of resistance with strain was expressed in Fig. 11. Once the strain loaded (0 s), the resistance value produced an immediate increase without time lag, indicating a sensitive response. Sequentially the resistance diminished gradually until reaching a stable value and this process took about 2 s. The stress relaxation should be taken into account as the reason of the decreased resistance. When the strain disappeared (10 s), the resistance started to decline until getting back to the original resistance but this process (about 10 s) was relatively slow. Because PU core possessed the excellent elastic recovery capacity and PANi layer expressed the inferior one. 4. Conclusion Electrospun polyurethane (core)/polyaniline (skin) nanofibrous mat was fabricated by in situ chemical polymerization of aniline monomers on the surface of electrospun PU nanofiber substrate. The thickness of PANi/PU mats was around at 8 mm which was even thinner than the single cotton fiber (around 20 mm), and it exhibited an average tensile strength (93 MPa) and excellent elasticity (114%). During polymerization process, 120 min is the time of emeraldine oxidation state and the resultant PU/PANi nanofibrous mats manifested remarkable electrical conductivity (0.43 S/cm). During washing procedure, low molecular weight polyaniline could be more unstable and easily washed away. For the stretching measurement case, the average strain gauge factor is calculated to be 17.15, which is more than 8 times higher than that of a traditional polymer strain gauge. For the bending measurement case, PU/PANi nanomats showed highly strain sensitivity of (R-R0)/R0 = 0.75 (curvature = 0.4 cm1) with cyclically repeatable sensor responses. Compared with the conventional conductive fiber, the PU/PANi conductive nanofiber possessed higher specific surface area, higher porosity and better flexibility so it was expected to be applied in wearable electronics, robotic systems and electron skin. Conflict of interest None. Acknowledgments Financial support of this work was provided by Natural Science Foundation of China via grant Nos. 51273097 and 51306095, China Postdoctoral Science Foundation via grant Nos. 2014M561887 and 2015T80697, Qingdao Postdoctoral Application Research Funded Project and Qingdao Application Basic Research Funded Project (14-2-4-1-jch, 15-9-1-41-JCH). References [1] A.J. Heeger, Semiconducting and metallic polymers: the fourth generation of polymeric materials, Synth. Met. 125 (2001) 23–42.
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Electromechanical deformation sensors based on polyurethane/ polyaniline electrospinning nanofibrous mats Mingwei Tiana,b,c,1,* , Yujiao Wanga,b,1, Lijun Qua,b,c,* , Shifeng Zhua,b,c , Guangting Hanb,c, Xiansheng Zhangd , Quan Zhoue, Minzhi Dua,b , Shuli Chia,b a
College of Textiles and Clothing, Qingdao University, Qingdao, Shandong 266071, China Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong Province, Qingdao University, Qingdao, Shandong 266071, China c Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao, Shandong 266071, China d College of Textiles, Donghua University, Shanghai 201600, China e Central Laboratory, The Affiliated Hospital of Qingdao University, Qingdao, Shandong 266071, China, China b
A R T I C L E I N F O
Article history: Received 17 December 2015 Received in revised form 27 April 2016 Accepted 6 May 2016 Available online xxx Keywords: Polyurethane Polyaniline Electrospinning Electrical conductivity Strain sensitivity
A B S T R A C T
Conducting electrospun nanofibrous mats elicit great interest in a wide range of applications. In this paper, polyurethane/polyaniline (PU/PANi) mats were prepared via the elecrospinning route with a polyurethane/dimethyl formamide spinning solution, and then in situ chemical polymerization of aniline monomers on the surface of PU nanofibers to deposit conductive PANi layer. In details, the morphology and structure of the PU/PANi mats were characterized by SEM, FTIR and the results indicated that the polyaniline stably deposited on the surface of polyurethane nanofiber. The electrical conductivity measurements indicated PU/PANi mats expressed good electrical conductivity (0.43 S/cm) at the optimal polymerization time as 120 min. As expected, PU/PANi mats showed strain sensitivity of (R-R0)/R0 = 0.75 (curvature = 0.4 cm1) and the average strain gauge factor could reach 17.15 with applied stretching deformation (0 110%). Therefore it could be deemed as the recommended nano-materials for flexible wearable device. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Conductive polymers such as polyaniline, polypyrrle and polythiophene attracted tremendous attention due to a wide variety of applications, including anti-static coating [1–3], batteries [4–7], sensors [8–11], electrodes [12,13] and so on. Among these conductive polymers, polyaniline (PANi) might be the most promising because of its good environmental stability, ease of synthesis, thermal stability and an inexpensive synthetic route. However, as an intrinsically conducting polymer (ICP), the widely practical application of PANi is restricted by its limited processability, insolubility, infusibility and poor mechanical properties [8]. In order to overcome these drawbacks, many researchers attempted to introduce PANi with an insulating polymer matrix with high mechanical strength, e.g. rubber [14], textiles [15,16],
* Corresponding authors at: College of Textiles and Clothing, Qingdao University, Qingdao, Shandong 266071, China. E-mail addresses: [email protected] (M. Tian), [email protected] (L. Qu). 1 These authors equally contributed to this work. http://dx.doi.org/10.1016/j.synthmet.2016.05.005 0379-6779/ã 2016 Elsevier B.V. All rights reserved.
plastic [17]. Especially, Polyurethane (PU) elastomer was receiving considerable attention for a large range of transducer and actuator applications. Furthermore, PU exhibited excellent flexibility, good film/fiber forming property and resistance to solvent and it could carry out the energy conversion between mechanical energy and electrical energy [18]. The remarkable composites with PU as elastomer substrate and PANi as electrical filler have attracted considerable attention during the last decade. The PU/PANi composites can be commonly classified into two categories based on the preparing procedure, the first type is surface-modified PU/PANi whose structure is composed by the inner substrate PU substrate and the outside deposited PANi layer, the other type is volume-modified PU/PANi which is blended with PU and PANi under certain weight ratios. Hrehorova et al. [19] pointed out that surface-modified PU/PANi film possessed lower percolation threshold level than volumemodified PU/PANi ones, therefore, surface-modified PU/PANi composite materials can be recommended as an ideal candidate for electromechanical strain sensors. Prabhakar et al. [20] modified PU films with PANi and PANi-AgNp and could render the surface conductive, suggesting potential application in electrochemical
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biosensors. Gu et al. [21] prepared biomimetic artificial nanofibrous bundle using electrospun PU nanofibers deposited with PANi nanoparticles. In smart strain sensor application, Qin et al. [22] prepared elastic, conductive PU/PANi fiber by in-situ chemical oxidative polymerization of PANi on PU fiber surfaces. And the average strain gauge factor of resultant functional fiber could reach to 3 which is 1.5 times higher than normal case. As we all know, nanofiber with high specific surface area, ultrathin and flexible properties could show some advantage performances in strain sensor area. Nanofiber sensor could contact with human skin in a more compact interface owing to its ultrathin and flexible structure. It could also perform isotropic strain response owing to its random nonwoven structure. In addition, more PANi nanoparticles could deposited on the substrate nanofiber owing to its higher specific surface area character. Therefore, depositing PANi nanomaterials onto flexible PU materials was seemed as a reasonable route to produce a novel strain sensors for monitoring minute structural deformations or damage detection. Compared with the structure of conventional macro-conductive PU fiber, electrospun flexible PU nanofibers could provide extraordinarily high surface area/volume ratios, lightweight, flexible and designable multifunctionalities. These outstanding structure properties might be helpful for the performance of PU/PANi strain sensor and the detailed results will be investigated in this paper. Moreover, after deposited with PANi layer, electrospun PU/PANi nancomposite fiber mats had no specific direction sensing and provided multidirectional and multipoint strain sensing, similar to carbon-nanofilm strain sensors [23]. In this paper, PU nanofiber mats were prepared through the elecrospinning process and used as substrate for the preparation of conducting nanocomposites. It was reported that in situ polymerization was a reasonable method for preparing PANi-coated materials because it did not result in the damage of the substrate and enhanced conductive properties [24]. So we prepared PU/PANi fiberous nanocomposite through in situ chemical polymerization of aniline monomers on the surface of the PU nanofibers. And then
its electrical properties were measured and the strain sensitivity was discussed. 2. Experimental 2.1. Materials Polyurethane (PU) was purchased from Huakai resin Co., Ltd., China. Aniline monomer (ANI) was commercially available from Tianjin Guangfu technology Co., Ltd., China. Ammonium persulfate (APS) (99.5%), hydrochloric acid (HCl) (37%) was supplied by Tianjin Guangfu technology Co., Ltd., China. Dimethyl formamide (DMF) (99.5%) and alcohol (99.7%) were purchased from Tianjin Fuyu chemical Co., Ltd., China. All the chemicals used were of analytical grade and were used as received. 2.2. Preparation of polyurethane/polyaniline fiberous nanocomposites The prepared process of polyurethane/polyaniline fiberous nanocompsites was shown in Fig. 1. Suitable amount of polyurethane was dissolved in N,N-dimethylformamide (DMF) with a concentration 20 wt.%. The mixture was continuously stirred until a homogeneous solution formed. This solution was fed into a 5 mL syringe with a stainless steel needle and the syringe attached to an electrospinning apparatus. The flow rate of the solution kept constant at 0.1 mL/h. A voltage of 20 kV was applied directly to the spinneret. And the distance between the needle tip and the collector was maintained at 12.5 cm. Polyurethane/polyaniline (PU/PANi) nanofibrous mats were prepared through in situ chemical polymerization of aniline monomers on the surface of the as-obtained PU nanofibers. In the typical procedure, aniline monomers were dissolved in ethanol solution (the volume ratio of aniline to ethanol at 1:4) and PU nanofibrious mats were first immersed in the solution for 2 h. And then PU nanofibers were removed and placed into APS/HCl reactive solution and the concentration of APS and HCl was 30 g/L and 0.5 mol/L respectively. Under this condition, the polymerization
Fig. 1. The schematic process of polyurethane/polyaniline nanofibrous mats.
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took place on the fiber surface to form a conducting polymer layer that fully coated the PU nanofibrious mats. After different polymerization time (20, 40, 60, 80, 100, 120, 140, 160 and 180 min), the dark green fibers were washed with water for three times and dried at room temperature. And the sample at 120 min polymerization time performed the best electrical properties; therefore, we chose this sample as the case of the following characterization. 2.3. Characterization The surface of the resulting PU/PANi nanofibrious mats was observed by scanning electron microscopy (SEM, SUPRA 55, Carl Zeiss Jena, Germany). The infrared spectra were recorded on a Fourier transform infrared spectrometer (FT-IR, Nicolet 5700, Thermo Electron, USA) at room temperature. The tensile strength of the resultant PU/PANi nano-mats was conducted by the INSTRON 5500R Universal Tester (Instron Corp., USA) according to ISO 13934-1, the gage length was 10 mm with the tensile speed of 20 mm/min. 2.4. Electrical measurement The conductivity of PU/PANi nanofibrious mats was measured at room temperature (25 C) by a UNI-T UT33D digital multimeter analyzer. In the measuring process, the applied voltage was varied from 10 to 10 V with an automatic increment of 0.5 V. The I/V curve was plotted for PU/PANi nanofibrious mats under different polymerization time and the electrical resistance R was deduced from the slope of the curve. Five times of measurement was carried out to calculate the average value of the electrical resistance. The thickness of fiberous nanocomposites was measured by a coating thickness gauge (EC-770, YUWESE, China). The average value of the electrical conductivity was calculated by the equation as follows:
s¼
1 Rd
where s , R and d are the electrical conductivity (S cm1), surface square resistance (V cm2) and film thickness (cm), respectively. The laundering durability test of the resultant PU/PANi nanofibrous mats was determined following the AATCC Test Method 61-2006 test using a standard color-fastness to washing laundering machine (Model SW-12AII, Wenzhou Darong Textile Instrument Co., Ltd., China). The nano-mats specimen was laundered in a closed canister containing 200 mL aqueous solution of an AATCC standard reference detergent WOB (0.37%, w/w) [25]. Then the electrical conductivity was repeatedly measured after one, two and three times laundering, respectively. 2.5. The stretching and bending sensitivity of flexible PU/PANi nanofibrous mats Stretching and bending motion are two common movements of human body, and therefore, bending and stretching response of sensor are deemed as two important factors for designing wearable electronics, robotic systems and electron skin. Herein, we evaluate the stretching sensing response of PU/PANi nanofibrous mats in our paper. The stretching sensing property of PU/PANi nanofibrious mats was measured at room temperature (25 C) by a UNI-T UT33D digital multimeter analyzer, and the normalized nanofibrous mats were fixed under different deformation with a home-made stretching device. Furthermore, cyclic test of PU/PANi nanofibrous mats was also carried on and the membrane was repeatedly stretched and relaxed for 20 cycles at strain 5%.
Fig. 2. The experimental set-up of the flexible sensor.
The bending sensing of the flexible sensor was evaluated with the experimental set-up as shown in Fig. 2. The as-spun 10 10 mm PU/PANi nanofibrious mat was glued on a flexible PVC plate, whose thickness was 300 mm, here, PVC plate is just employed as the supporting substrate to measure the bending response of PU/PANi nanofibrous mats because PU/PANi nanofibrous mats can not be bent without supporting substrate. Two copper electrodes were placed onto the corresponding opposite sides and the distance between two electrodes was set as 10 mm. X direction and Y direction were illustrated as in Fig. 2. To take X direction as an example, the bending sensing properties of the PU/PANi nanofibrious mat was tested under the standard conditions (T = 25 C and RH = 65%). The PU/PANi fiberous nanocomposites were bent at different curvatures and the I/V curves were plotted respectively. Sequentially, the membrane was repeatedly bent and relaxed for 20 cycles at a constant curvature (0.4 cm1). The conductivity change of the sensing nanofiberous membrane in both bent and relaxed states was recorded to observe its strain sensitivity. 3. Results and discussion 3.1. Morphology of PU/PANi nanofibrious mats The morphology of pure PU nanofiber and PU/PANi nanofibrious mat was shown in Fig. 3. It could be seen that pure PU nanofiber exhibited a very smooth surface (Fig. 3a) while the asprepared PU/PANi fiberous nanocomposite was featured with a rough surface due to the formation of PANi layer on its surface (Fig. 3b). By contrast, it was founded that the diameter of PU/PANi nanofiber was obviously larger than PU nanofiber and the increased thickness was about 100 200 nm. In other words, the thickness of the PANi layer was about 100 200 nm. The thickness of electrospun PU nanofibrous mats was evaluated from the cross section SEM image of PU nanomats in Fig. 3(c)-1, its thickness was around at 8 mm which was even thinner than the single cotton fiber (around 20 mm), and therefore, we defined the mats at such thickness level (<10 mm) as the ultrathin mats in our paper. Compared Fig. 3c and d, the PU/PANi fiberous nanocomposite was almost twice as thick as the PU fiberous mat. 3.2. Fourier transfer infrared spectroscopy The FT-IR spectra of PU nanofiber and PU/PANi composite nanofiber were shown in Fig. 4. Two peaks at 2936 cm1 and 2841 cm1 corresponded to asymmetric CH2 stretching and symmetric CH2, respectively [26]. The two bands at 1732 cm1 and 1705 cm1 arose from non-bonded and hydrogen-bonded urethane carbonyl C¼O stretching, respectively [8]. The peak at 1066 cm1 could be attributed to the C O C stretching
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Fig. 3. SEM images under different magnification for the surface of pure PU nanofiber (a)-1, (a)-2, and PU/PANi nanofibrous mats (b)-1, (b)-2, and the cross-section of pure PU nanofiber (c)-1, (c)-2 and; PU/PANi nanofibrous mats (d)-1 and (d)-2.
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Fig. 4. FT-IR spectra of PU nanofiber and PU/PANi fiberous nanocomposites.
vibrations. The peak at 1505 cm1 corresponded to symmetric vibration of N C N in the urethane-aniline because of the reactions of the NCO groups with the NH2 groups of the PANi [27]. A characteristic band of the conductive form of PANi was observed at 1308 cm1, which was due to p-electron delocalization induced in the polymer by protonation [28]. The adsorption peak at 1224 cm1 was assigned to a CN+ stretching vibration in the polaron structure. The adsorption band at 1083 cm1 was ascribed to the aromatic CH in-plane bending. The peak at 817 cm1 associated with an aromatic C H out of plane bending modes. Compared with the spectrum of the pure PU nanofiber, this peak was obviously enhanced, revealing the increasing amount of aromatic C H. As a consequence, the formation of PANi on the surface of PU nanofiber was demonstrated.
3.3. The mechanical properties of PU/PANi nanofibrious mats The representative stress-strain curve of PU/PANi nano-mats is shown in Fig. 5. Sample exhibits an average tensile strength (93 MPa) and excellent elasticity (114%). Such reasonable mechanical properties of PU/PANi nano-mats could be helpful to be applied in wearable electronics, robotic systems and electron skin. 3.4. Influence of polymerization time on the electrical properties of PU/ PANi nanofibrous mats The electrical conductivity of PU/PANi nanofibrous mats under different polymerization time were shown in Fig. 6. The electrical conductivity of PU nanofiber (0 min) was 0 S cm1. During 0 120
Fig. 5. The mechanical stress-strain curve of PU/PANi nano-mats.
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Fig. 6. The I/V curves (a) and electrical conductivity (b) under different polymerization time.
min, the electrical conductivity took on nearly six orders of magnitude increasement with the increasing polymerization time. However, the electrical conductivity exhibited a sustained fall with the increasing polymerization time, during 120 180 min. Similarly, the maximum value of the electrical conductivity (0.43 S/cm) appeared at 120 min. Hence, the optimal polymerization time was 120 min. Polyaniline could undergo three oxidation states: leucoemeraldine, emeraldine and pernigraniline during the polymerization process. Only at emeraldine oxidation state, PANi could perform electrical property, and in our paper, 120 min is the time of emeraldine oxidation state. After this moment, further oxidation a second redox process occurs, which yields a new insulating material, pernigraniline. Therefore, the conductivity of nanofibrous mats decrease for long polymerization time.
3.5. Washing durability In order to investigate the washing durability, three samples (the polymerization time as 100 min, 120 min and 140 min) were chosen to carry out washing durability. The electrical conductivity of PU/PANi nanofibrous mat before and after washing was displayed in and Fig. 7. For the samples after one time washing, it was observed that the electrical conductivity of polymerization 100 min and 140 min exhibited 42.39% and 28.01% decreasing ratio higher than that of 120 min case (17.65%), such result might be caused that two former case possessed more unstable PANi removed with the vigorous washing force and the later case had stable PU and PANi interface with the aid of hydrogen bonds. However, after three times washing, the electrical conductivity
Fig. 7. The electrical conductivity of PU/PANi fiberous nanocomposites before and after washing.
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Fig. 8. Normalized resistance under different stretching strain (a) and cyclic deformation at strain 5% (b).
tended to be stable which meant that the residual PANi has already stably deposited on PU nanofiber surface. Polyaniline is synthesized and deposited on nanofibrous mats surface via in-situ chemical polymerization of aniline. The mechanism of aniline polymerization is subject to the principles of the redox processes. The polymerization of aniline could be ignited once aniline monomer contacts with protonic acid and oxidant, and the duration of such reaction should be lasted for a certain time, in our case 120 min is the optimal time. Before this optimal time, the aniline monomer does not yet completely polymerize into polyaniline, the monomer could be more easily washed out once being laundered. On the contrary, after this optimal time, the high molecular weight polyaniline could be leaded to slow hydrolysis, resulting in low molecular weight polyaniline. During washing procedure, low molecular weight polyaniline could be more unstable and easily washed away. Therefore, at the optimal time 120 min in our case, nanofibrous mats deposited with high molecular weight polyanline performs the highest structural stability.
3.6. The stretching and bending sensitivity of flexible PU/PANi nanofibrous mats Fig. 8 shows the normalized resistance (R-R0)/R0 as the function of different stretching strain (a) and cyclic test at strain 5% (b) for PU/PANi nanofibrous mats, where R is the resistance under deformation, and R0 is the resistance of the original nano-mats. From Fig. 8(a), the resistance curve significantly increases with the applied strain deformation (0 110%), and the curve can be divided into two regions, resistance of nano-mats first perform linearly growth with increasing of strain (0–80%) and turn to a faster growth at (80–110%). The average strain gauge factor at the first region, defined as (R-R0)/R0/e (e is deformation rate), is calculated to be 17.15, which is more than 8 times higher than that of a traditional polymer strain gauge [29]. Fig. 8(b) presents the response of normalized resistance (R-R0)/R0 value of PU/PANi nanofibrous mats of cyclic deformation with a maximum applied strain of 5%. The normalized resistance of nano-mats perform an increasing response in the loading period while decreasing
Fig. 9. The normalized resistance of PU/PANi nanofibrous mats sensor at different curvatures.
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Fig. 10. Cyclic test for fiberous nanocomposites based bending strain at the curvature of 0.4 cm1: (a) the X direction and (b) the Y direction.
response in the unloading period, showing a reversible response, which well corresponded to the applied cyclic deformation. The normalized resistance curve with different bending curvatures was illustrated in Fig. 9. Especially, the strain sensitivity was defined as (R-R0)/R0, where R was the resistance in bent state of a certain curvature and R0 was the original resistance, that was, the resistance in relaxed state. From Fig. 9, the resistance sharply increased with big bending curvatures, and the strain sensitivity (R-R0)/R0 increased with the increasing curvature and this change approximated to a linear function. When the PU/PANi nanofiberous mat is bent, PU substrate might response faster deformation than the deposited PANi nanoparticles owing to its dramatic elasticity and leading to increased electrode distance, decreased thickness of PU/PANi mat and decreased electrode length. Such architectural change might result in increasing electrical resistance with increasing bending curvature according to Electrical Resistivity
Equation. Furthermore, the electrical resistance change should also be attributed to the rigid nature of PANi molecular chains covering PU nanofibrous mat surface. The outer conductive layer might be disrupted when the deformation is applied to the PU/PANi nanofibrous mats and accordingly, the (R-R0)/R0 value shows increasing tendency. The typical curves of conductivity change with strain of the PU/PANi nanofibrous mats during 20 cycles of bending and relaxation measurements were exhibited in Fig. 10. Compared Fig. 10(a) with (b), it was found that the variation rule of electric resistance in the X direction was the same as the Y direction, indicating that PU/PANi nanofibrous mats were an isotropic elasticity. This was attributed to the fact that PU nanofiber nonwoven substrate was isotropic. Choosing the X direction as an example, the strain sensitivity was investigated during 20 cycles of bending and relaxation. At a constant strain (curvature = 0.4 cm1),
Fig. 11. The resistance change with strain.
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PU/PANi mats showed highly strain sensitivity of (R-R0)/R0 = 0.75, the stress was relatively stable so the resistance also rose to a relatively stable level. When the deformation recovered, the resistance basically returned to the original. With the cyclic deformation continues, slight increase of the bottom and top values could be detected. We presumed it was induced by the irreversible deformation by the destruction of PANi nanoparticles and the distance between them, resulting in the break-up of conductive networks. Furthermore, the micro-structure of PU nanofibrous mats and the alignment between nanofibers might be changed by residual strain under cyclic deformation which also affected the stability of normalized resistance (R-R0)/R0 value of PU/PANi nanofibrous mats. The response time served as an important parameter to assess the sensitivity of the sensor. The response process of resistance with strain was expressed in Fig. 11. Once the strain loaded (0 s), the resistance value produced an immediate increase without time lag, indicating a sensitive response. Sequentially the resistance diminished gradually until reaching a stable value and this process took about 2 s. The stress relaxation should be taken into account as the reason of the decreased resistance. When the strain disappeared (10 s), the resistance started to decline until getting back to the original resistance but this process (about 10 s) was relatively slow. Because PU core possessed the excellent elastic recovery capacity and PANi layer expressed the inferior one. 4. Conclusion Electrospun polyurethane (core)/polyaniline (skin) nanofibrous mat was fabricated by in situ chemical polymerization of aniline monomers on the surface of electrospun PU nanofiber substrate. The thickness of PANi/PU mats was around at 8 mm which was even thinner than the single cotton fiber (around 20 mm), and it exhibited an average tensile strength (93 MPa) and excellent elasticity (114%). During polymerization process, 120 min is the time of emeraldine oxidation state and the resultant PU/PANi nanofibrous mats manifested remarkable electrical conductivity (0.43 S/cm). During washing procedure, low molecular weight polyaniline could be more unstable and easily washed away. For the stretching measurement case, the average strain gauge factor is calculated to be 17.15, which is more than 8 times higher than that of a traditional polymer strain gauge. For the bending measurement case, PU/PANi nanomats showed highly strain sensitivity of (R-R0)/R0 = 0.75 (curvature = 0.4 cm1) with cyclically repeatable sensor responses. Compared with the conventional conductive fiber, the PU/PANi conductive nanofiber possessed higher specific surface area, higher porosity and better flexibility so it was expected to be applied in wearable electronics, robotic systems and electron skin. Conflict of interest None. Acknowledgments Financial support of this work was provided by Natural Science Foundation of China via grant Nos. 51273097 and 51306095, China Postdoctoral Science Foundation via grant Nos. 2014M561887 and 2015T80697, Qingdao Postdoctoral Application Research Funded Project and Qingdao Application Basic Research Funded Project (14-2-4-1-jch, 15-9-1-41-JCH). References [1] A.J. Heeger, Semiconducting and metallic polymers: the fourth generation of polymeric materials, Synth. Met. 125 (2001) 23–42.
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