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Largely Enhanced Stretching Sensitivity of Polyurethane/ CNTs Nanocomposites via Incorporation of Cellulose Nanofiber Shuman Xu, Wenjin Yu, Mengfan Jing, Rui Huang, Qin Zhang, and Qiang Fu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11783 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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The Journal of Physical Chemistry

Largely Enhanced Stretching Sensitivity of Polyurethane/CNTs Nanocomposites via Incorporation of Cellulose Nanofiber Shuman Xu, Wenjin Yu, Mengfan Jing, Rui Huang, Qin Zhang and Qiang Fu∗ College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China.

∗

Corresponding author. E-mail: [email protected] (Q. Fu), Tel./Fax: +86 28 8546

1795.

1

ACS Paragon Plus Environment


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ABSTRACT Stretchable sensors have drawn a great deal of attention due to its importance and necessity in hi-tech areas. However, it’s difficult to obtain sensors with high sensitivity companied high tenacity. Taking advantage of very large aspect ratio and amphiphilicity of nano-fibrillated cellulose (NFC), in this study, we fabricated polyurethane (TPU) / multiwall carbon nanotubes (CNTs) nanocomposites with excellent dispersion using NFC as stabilizer. Then the mechanical and electrical properties, particularly the stretching sensitivity of the prepared TPU/NFC@CNTs nanocomposites were investigated. It was found that the prepared TPU/NFC@CNTs has much better mechanical properties and electrical conductivity compared with that of TPU/CNTs composites. More importantly, a linear change of electrical conductivity as function of stretching is observed for at least strains up to 300% and a very high sensitivity whose gauge factor close to 50 could be achieved. The excellent stretching sensitivity could be attributed to the unique role of NFC: 1.assisting the dispersion of CNTs, 2. enhancing interaction between NFC and TPU matrix as due to its amphiphilicity, and 3. increasing the overall aspect ratio of CNTs via connecting many tiny CNTs bundle together along its long axis.

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1 Introduction The development of stretchable sensors have drawn extensive attention from the research community mainly owing to their wide range of application and easy fabrication methods.1,

2

Especially the strain sensors, which change the electrical

characteristics (capacitance or resistance) during suffering mechanical deformations, have various potential applications such as for movement sensor,3, 4 structural health monitoring,5, 6 mass measurements,7 wearable electronics8, 9 and so on. Compared with the traditional rigid conductors such as metal and ceramic, which is dense and finite deformation, stretchable conductive polymers have the advantages of being deformable and facile processing, which has potential to provide exciting opportunities, particularly in the smart textile and wearable electronics. Massive research has been reported in fabricating flexible conducting composites by doping the soft polymer with conductive fillers.10-15 Carbon nanotubes (CNTs) have been considered as one of the most suitable conductive fillers for the preparation of stretchable sensors due to its unique physical properties, including mechanical, thermal, and electrical properties.15-19 Several factors have been demonstrated to affect the strain sensitivity of the stretchable strain sensors, such as the aspect ratio of conductive fillers, distribution of conductive fillers in the polymer matrix and the affinity between fillers and matrix. For example, Dang et al.20 reported that higher sensitivity of silicon rubber/CNTs nanocomposites was obtained with CNTs with higher aspect ratio. Lin et al.21 observed that the sensitivity of thermal plastic polyuria (TPU)/CNTs composite strongly depended on the interaction between the filler and matrix, stronger interaction induced higher sensitivity. And Shang et al.13 and Murugaraj et al.22 demonstrated that homogenously dispersed conductive fillers is responsible for the high sensitivity of conductive nanocomposites. To quantitative characterize the sensitivity of stretchable sensors, gauge factor has been proposed, which is a criterion for evaluating the sensitivity of strain sensors and defined as G = (R –R0) / R0ε. Conventional metal strain sensors are unable to operate at high strain levels (more than 5%) and with a low GF. Shin et al.23 fabricated a kind of conductive 3



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composite sensor based on polyurethane filled with CNTs which can strain up to 300% but with a low GF ranging from 0.34 to 1.07. Yamada et al.24 designed stretchable sensor based on CNT/poly (dimethyl siloxane) (PDMS) with a GF ranging from 0.06 in 40% strain to 0.82 in 200% stain. In general, sensors with high sensitivity can only sustain limited strains and highly stains exhibit lower sensitivities.25, 26 Therefore, stretchable sensors with high sensitivity while maintaining high strain levels get more and more attention from the researchers, especially in high – tech application area. To obtain high performance stretchable conductive CNTs nanocomposites，and further improve the value of gauge factor, several fatal problems still need to be solved. The first challenge is to obtain a homogeneous dispersed polymer/CNTs composites. CNTs prefer assembling together to individual dispersed due to the strong Van der Waals interaction. Chemical functionalization of CNTs has been used extensively in order to improve dispersion.27 However, chemical functionalization increases the CNTs price and disrupts the electronic network of the nanotubes. The second challenge is to improve the interaction between CNTs and polar polymer matrix. Strain sensors change their electrical characteristics during suffering mechanical deformations, if the interaction between CNTs and polymer matrix is strong, the stress applied on polymer matrix could be transferred to the CNTs conductive network more easily. Again this usually needs surface modification via chemical functionalization. The third challenge is to increase the aspect ratio of CNTs, since a small deformation will cause big change of conductivity if the aspect ratio of CNTs is large. This needs to prepare CNTs with very long size, which is difficult and expensive. Nano-fibrillated fibers (NFC), which are derived from natural cellulose fibers through TEMPO-mediated oxidation, have attracted wide attention because of their unique properties: high aspect ratio, good aqueous stability, high specific strength and stiffness, and most importantly, environmental sustainability.28-30 Few research have been carried out to use NFC as a stabilizer for the dispersion of nanomaterials such as CNTs,31-33 graphene,34-36 BN,37 CB,

38

and so on. The use of NFC as a stabilizer has 4


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considerable advantages compared to other stabilizers employed in the previous reports. Firstly, NFC assisted nanomaterials can be dispersed in water and organic solvents effectively, and even gained high dispersibility (up to 10mg/ml in water, the best aqueous dispersibility ever reported for RGO35). Secondly, unlike small molecular surfactants or polymeric dispersants, theoretical and experimental results show that NFC is mechanically stiff with an elastic modulus ~145GPa in the axial direction. Thirdly, there are numerous oxygen – containing functional groups on the surface of NFC contributing to the good affinity with the polar polymer. In a word, NFC is an ideal dispersant of CNTs which can well disperse CNTs as well as maintain the physical structure of CNTs. Thus in this work, we explore the possibility to use NFC as stabilizer for the preparation of thermal-plastic polyurethane (TPU)/CNTs nanocomposites with excellent stretching sensitivity. TPU is picked as matrix attributing to its high tenacity, easy processing and wide range of applications. There are three purposes to use NFC as stabilizer: increasing the dispersion of CNTs, enhancing the interaction between CNTs and PU matrix for better stress transfer, and achieving overall large aspect ratio of CNTs via connecting many tiny CNTs bundle together along its long axis.

2 Experimental section 2.1 Materials. Thermos-plastic polyurethane (TPU, Irogran PS455-203), provided by Huntsman Corp, was used as matrix in the composites. Micro-fibrillate cellulose (MFC, solid content, 25%) (Celish KY100-S, Daicel Chemical Industries, Ltd, Japan) was suffered several homogenization procedures from wood pulp. TEMPO (2,2,6,6-Tetramethyl-1-piperidinyloxy), which is used as catalyst together with Sodium bromide (NaBr) in the oxidation process of MFC, was offered by Sigma-Aldrich. CNTs produced by Nanocyl S.A. (Belgium), were used as conductive filler in the conductive polymer composites (CPCs). Other reagents used in relevant experiments were purchased from Chengdu Kelong Chemical Reagent Company

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(China) and used as received without further purification. 2.2 Preparation of nano-fibrillated cellulose (NFC) and NFC@CNTs hybrid fillers. NFC is fabricated using MFC as the original materials through TEMPOmediate oxidation.28 10g MFC was pre-dispersed in 1000ml deionized water containing dissolved 0.16g TEMPO and 1g NaBr. Then 12wt.% NaClO solution was added in the MFC suspension to activate the TEMPO-mediate oxidation (10mmol per gram of cellulose). The oxidation process is under the condition of pH = 10-10.5, 0.5M NaOH was used to guarantee that until the color of this system become milk white and pH doesn’t decrease. The product was thoroughly washed with deionized water by filtration. Then the NFC was obtained via centrifugation (8000rpm/min, 15min), subsequently, transferred into DMF via solvent exchange method. For the NFC@CNTs hybrids, 0.09g of CNTs was added into a well-dispersed solution of NFC (containing 0.09g of NFC) under magnetic stirring, then the mixture was pre-dispersed 10min using high-shear homogenizer at 7000~8000 rpm/min, subsequently sonicated for 30min in ice bath to obtain a uniform NFC@CNTs nanohybrid suspension. 2.3 Preparation of TPU/NFC@CNTs nanocomposite films. The operation procedure for TPU/NFC@CNTs nanocomposites was as follows: TPU (3g) was dissolved in 50ml DMF by magnetic stirring for 1h at 60℃. After complete dissolution of TPU, the NFC@CNTs hybrids were gradually dropped into the TPU solution and carefully stirred for 24h at room temperature. After subsequently sonicated for 30 min, the homogeneous TPU/NFC@CNTs suspension was obtained and then poured it into an aluminum dish and slowly evaporated at 80℃ for film formation. A series of TPU/NFC@CNTs composite films with 3wt.% CNTs and various NFC loadings: 1, 2, 3, 4 and 5 wt.% were similarly prepared. As for reference samples, neat TPU and TPU/CNTs were also prepared using the same method. The prepared films were tailored into uniform samples with length of 50mm and width of 5mm for the subsequent tests. 2.4 Characterization. The prepared NFC@CNTs nanohybrids or neat CNTs and 6


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NFC suspension were diluted for suspension observation. Digital photos of a desired amount of NFC, neat CNT and NFC@CNTs nanohybrids suspension were taken after standing for a month. The ultraviolet-visible spectra (UV-vis, UV-1800PC spectrophotometer) on the suspensions (the concentration of the solutions were 0.3 wt.%) was carried out at room temperature and the data was collected from 200 to 800 nm.. Transmission electron microscopy (TEM, JEOL JEM-100CX, Japan) was carried out to observe the morphology of the NFC, CNTs and NFC@CNTs nanohybrids suspension. Diluted NFC@CNTs nanohybrids suspension was directly dropped on a copper grid for observation. Scanning electron microscopy (SEM, JEOL JSM-5900LV) was carried out to examine the cross-section morphology of nanocomposite films. Surfaces cryo-fractured using liquid nitrogen and the accelerateing voltage was 20Kv. Dynamic mechanical analysis (DMA Q800, TA Instruments) was carried out to investigate the dynamic mechanical properties of the TPU/NFC@CNT nanocomposite films in tension mode at a frequency of 1Hz and a heating rate of 3℃/min over the temperature range from -80℃ to 80℃. Typical stress-strain curves was recorded on an Instron 5567 universal testing machine. The gauge length was 20 mm and the crosshead speed was 50mm min-1. Five specimens were measured for each kind of samples to achieve an average value. For the strain sensing test, sample strips were stretched in a SANS CMT4000 universal testing machine and resistance was measured with Keithley 6487 picoammeter, simultaneously. The resistance measurement set up and tensile test machine are both interfaced with a computer as illustrated in Fig S1 to record the resistivity-strain dependence of these samples. A constant rate of 7 mm/min was used for the resistance-strain measurement. Cyclic stretching and recovery was also conducted to investigate the dynamic resistance-strain behavior. The program of cyclic deformation includes stretching to 100% strain and withdrawing to the initial length. Raman spectra (JY HR800) were carried out on a micro-Raman spectrometer. The excitation laser is 532 nm and the beam size is 1µm.

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3 Results and discussion 3.1 The enhanced dispersion of CNTs in solution by adding NFC. The solubility of NFC and pristine CNTs in solution was first investigated. As shown in Fig. 1a, the obtained NFC solution is transparent and stable in DMF due to the introduction of charged carboxyl groups via TEMPO-oxidation. For pristine CNTs, they all precipitated at the bottom, leading to the upper solution vacant after standing for 24h. However, by adding NFC into the suspension of CNTs/DMF (NFC@CNTs, within the relative weight ratio 1/1), the obtained hybrid suspension is homogenous and remains stable for quite a long time. To further investigate the effect of NFC on the dispersion state of CNTs, the UV-vis spectra of the three suspensions was carried out and the result is shown in Fig. 1b. The characteristic absorption peak of CNTs appears between 200-300 nm in the UV-vis region. As shown in Fig. 1b, it is clear that the curves of NFC and pristine CNTs suspension do not show any characteristic absorption peaks in the UV-vis region, indicating there is no individual CNTs suspended in the solution. However, the plot of NFC@CNTs suspension exhibits an obvious absorption peaks around 267.3 nm, revealing aboundant individual CNTs were obtained by adding NFC. The UV-vis results further demonstrate the effect of NFC on the dispersion of CNTs. In addition, Zeta potential measurement was also carried out to value the quality of NFC@CNTs hybrid suspension. A higher Zeta potential represents a more stable dispersity of the suspension. The Zeta potential of the quite stable NFC suspension is -58.7 mV. For the pristine CNTs, whose Zeta potential is just -1.64 mV nearly neutral, exhibiting highly unstable. With the addition of NFC, however, the Zeta potential of CNTs suspension is decreased from -1.64 mV to -49.3 mV, suggesting much enhanced dispersity and stability of CNTs as induced by the addition of NFC. To investigate the morphology of the NFC, CNTs and NFC@CNTs in microscopic state, TEM was carried out and the results are shown in Fig. 1c-e. One can see NFC is uniformly dispersed in the horizon with approximate dimeters of 2-5 nm and lengths of 3-5µm (Fig. 1c). For the pristine CNTs suspension, however, exhibits a huge entanglement morphology instead of individual tubes (Fig. 8


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1d). For the TEM image of NFC@CNTs suspension, it is clear that the CNTs and NFC confirm a complex network and without any obvious agglomeration (Fig. 1e). What’s important, CNTs still maintained high aspect ratio after emulsion followed by sonication in the present of NFC (the arrowed light – colored fiber). Therefore, the results mentioned above demonstrate NFC is an environmentally friendly, nontoxic, and steady dispersant of CNTs compared with small molecules, polymeric materials and other surfactants.

Fig.1 Digital picture of NFC，CNTs and NFC@CNTs nanohybrids suspension after standing for a month (a); UV-vis absorption spectra of NFC, CNTs and NFC@CNTs suspension (b); TEM images of NFC (c, scale bar: 500nm), CNTs (d, scale bar: 50nm) and NFC@CNTs nanohybrids (e, scale bar: 200nm) aqueous suspension.

3.2 The mechanical and electrical properties of TPU/NFC@CNTs nanocomposites. The aforementioned results demonstrated that NFC can effectively stabilize CNTs in solution system, laying a solid foundation for achieving homogenous dispersed CNTs in polymer matrix. It is well known that homogenous dispersion and strong affinity of nanofillers in polymer matrix play important roles in the mechanical properties of composites. Fig. 2a shows the typical stress-strain curves of TPU/NFC@CNTs nanocomposite films with 3wt.% CNTs and different loadings of NFC. Compared to the TPU/CNTs samples, the TPU/NFC@CNTs samples show significantly increase in associated with increasing strain. The introduction of NFC as 9



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compatilizer

of

CNTs

significantly

increases

the

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tensile

strength

and

elongation-at-break, and the results are summarized in Tab. 1. It is clear that the tensile strength increases from 20.8 MPa for TPU/3CNTs to 24.6 MPa for TPU/3CNT@1NFC and further to 35.2 MPa for TPU/3CNT@3NFC, where a 70% improvement has been achieved. The values of elongation-at-break for the TPU/3CNT@NFC nanocomposites increase due to the introduction of NFC, reaching the highest value of 1500% for TPU/3CNT@2NFC, and then experience a decrease when the NFC content is higher than 3 wt%. High strains is a very significant property for stretchable sensors, because it provides the possibility to detect the sensitivity in a greater range of deformation. The improved mechanical properties could be attribute to the better dispersion of hybrid fillers as well as good interface action between NFC and TPU matrix, and between NFC and CNTs. It is interesting to find that when the NFC: CNTs is 1:1 , the sample of TPU/3CNT@3NFC gained the best mechanical properties, high tensile strength as well as high elongation-at-break, a well balance between strength and stiffness can be achieved. For the preparation of stretchable sensors, what we concerned more is the electrical properties of the samples, because higher conductivity is better for us to monitor and record the outputted electrical signal. As shown in Fig. 2b, it is obvious that the conductivity of TPU/NFC@CNTs samples increased with the increasing of NFC loadings. For instance, the conductivity increases from 0.0012 Sm-1 for TPU/CNTs samples to 0.0313 Sm-1 for TPU/NFC@CNTs samples with 1wt.% NFC loading, and further to 0.28 Sm-1 for TPU/NFC@CNTs samples with 3wt.% NFC loading, where a more than 2 order of magnitudes improvement has been achieved, attributing to better dispersion of CNTs in TPU matrix. The result is better corresponded with the mechanical properties, which confirm that the ratio of NFC: CNTs is 1:1 can well balance the mechanical properties with high electrical conductivity. Thus, 3wt.%

appears to be the optimal loading of NFC.

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Fig. 2 Typical stress – strain curves (a) and Electrical conductivity (b) of TPU/NFC@CNTs with 3wt.% CNTs and different NFC loadings. Tab. 1. Mechanical properties of neat TPU and the TPU/NFC@CNTs with 3wt.% CNTs and different NFC loadings.

Young’

Tensile

Elongation

Strength

strength

at break

(MPa)

(MPa)

(%)

neat TPU

3.90 ± 0.7

19.1 ± 0.5

1520 ± 30

TPU/3CNTs

6.23 ± 3.2

20.2 ± 3.2

789 ± 12

TPU/3CNTs@1NFC

7.47 ± 2.7

24.6 ± 6.2

843 ± 27

TPU/3CNTs@2NFC

9.03 ± 1.9

31.1 ± 5.6

1050 ± 23

TPU/3CNTs@3NFC

11.92 ± 2.1

35.2 ± 2.7

986± 14

TPU/3CNTs@4NFC

13.41 ± 2.4

31.7 ± 3.5

861± 11

TPU/3CNTs@5NFC

15.48 ± 3.5

26.6 ± 4.1

660 ± 17

TPU/3NFC

4.76 ± 1.2

23.4 ± 2.3

1430 ± 34

Sample

3.3 The stretching sensitivity of TPU/NFC@CNTs nanocomposites. The resistance – strain dependence was examined to evaluate the potential of the composites as strain sensors. There were four kinds of TPU/NFC@CNTs samples with 3wt.% CNTs and different NFC loadings. As is well known, conductive network constructed when the content of conductive filler is higher than the percolation threshold. Otherwise, there are few conductive paths between conductive fillers 11



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resulting in the CPCs exhibiting ultralow conductivity. TPU/NFC@CNTs nanocomposites exhibit the conductive percolation threshold is about 1.8wt.% (Fig. S2). Based on this, we prepared PVA/NFC@CNTs samples containing 3wt.% CNTs, in which the conductive network has been conformed. In this way, the electrical performance can be well detected for the characterization of the stretchable sensing. Specimens were stretched at a content rate while resistance was recorded simultaneously. Fig. 3a shows the strain – sensing behavior of TPU/NFC@CNTs composites under uniaxial strain. It is noted that the resistivity of all samples increases gradually with increasing strain, which is correspond with previous reports.2, 21, 39 The plot of TPU/CNTs is placid，indicating that the resistivity varies a little as the strain increases. When NFC is added in combination with CNTs into the composite, an obvious increase in the slope can be observed for TPU/NFC@CNTs composites. For the sample of TPU/NFC@CNTs composite with 3wt.% NFC, the steepest slope is obtained, which is meaning the strain – sensing behavior of this sample is much better than that of TPU/CNTs. Dynamic strain – sensing behavior was also carried out to evaluate the stability of the strain sensor. As is shown in Fig. 3b, successive tensile test were performed on TPU/CNTs and TPU/NFC@CNTs with 3wt.% CNTs and the resistance (R) is plotted against time. 20 cycles were explored, during which higher sensitivity was observed in the TPU/NFC@CNTs samples. It is worth noting that there is a declining trend in the resistance of the samples after the first cycle. The essence is that the deformation of the sample strips can’t recover completely after experienced the first stretchable cycle which is caused by strain-induced crystallization. As shown in Fig. 3c, there is a quite difference between the first cycle and the subsequent cycles on the stress-strain behavior. This phenomenon is quite common in the reported literature.40, 41 As for strain sensors, during the first cycle, both part A and B could induce the variation of the electrical conductivity of sample stripes. However, the subsequent cycles exhibit the strain sensitive behavior only in part B. So the rangeability of resistance become smaller as the deformation of the samples become more limited. Despite all this, it 12


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exhibits really stable after several cycles, which is still required for the application of strain sensor.

Fig. 3 Resistivity as a function of tensile strain for TPU/NFC@CNTs nanocomposites (a); Plot of the relative resistance change under repetitive stretch – release cycles of 100% (b). Stress – strain curving of TPU/NFC@CNTs composite during cyclic stretching/releasing process (100% strain magnitude, 20 cycles) (c). Gauge factor (GF) of the samples at 200% strain (d).

As we all known, the sensitivity can be quantitative characterized by gauge factor (GF), which is defined as △R/(εR0), where △R is the change from zero – strain resistance (R0) due to an applied strain. Sensitivity increases with the increase in magnitude of GF. As can be seen from Fig. 3d, the value of G increases from 10.1 for TPU/CNTs to 18.3 for TPU/CNTs@NFC-1wt% and further to 49.1 for TPU/CNTs@NFC-3wt%, where a approximate 4-fold improvement has been obtained. As a comparison, the reported GFs in the literature are arranged in Tab. 2. One can see the GFs of stretchable sensors based on CPC varied from 0.06 to 60, approximately. Compared with the reported works, the GF of our TPU/NFC@CNTs samples is much higher than most of them which may demand tortuous procedures. Tab. 2. GFs reported in the literature. 13



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Materials

Range of GF

Ref.

PDMS/CNTs

0.06-0.82

24

PU/CNTs

0.34-1.07

23

Polyimide/CNTs

3-6

42

TPU/graphene

0.78-17.7

43

Epoxy/graphene

12.8-24

44

Epoxy/CNT-graphene

~10

45

Graphene films

~37

46

Epoxy/graphene

~56.7

30

Chewing gum/CNTs

12-25

2

Natural rubber/CNTs

~43.5

39

Our work

10.1-49.5

--

Scanning electron microscopy (SEM) is carried out to investigate the latent mechanism on the stretchable sensitivity. As shown in Fig. 4a and 4b, an isotropic nanohybrids conductive network structure is observed in the as – prepared samples. Because of their aspect ratio and fibrous shape, nanohybrids filled composites demonstrate entangled state. After stretching, an oriented nanohybrids conductive network structure is observed (Fig. 4c and 4d). The nanohybrids bundles are aligned in the drawing direction. As expected, the orientation degree of TPU/NFC@CNTs is more pronounced than that of TPU/CNTs samples. It is attribute to the good interaction between NFC and TPU matrix as well as the hydrophobic interaction and entanglement between NFC and CNTs, which has significant effect on the transferring stress sheer applied on TPU matrix to the nanohybrids network, resulting in the higher orient degree. All in all, it is confirmed that the deformation of the nanohybrids conductive network and the filler orientation in TPU/NFC@CNTs is greater than those in TPU/CNTs.

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Fig. 4 SEM morphological study of CNTs in TPU matrix. SEM images of as prepared TPU/CNTs (a) and TPU/NFC@CNTs (b) with 3 wt.% CNT. SEM images of samples at 100% strain: TPU/CNTs (c) and TPU/NFC@CNTs (d), the direction of double sided arrow is the strain direction. (The scale bar for a, b, c, d is 3µm, for the else is 1µm.)

To verify the aligned morphology of the CNTs observed above, polarized Raman spectroscopy (The mechanism is stated in supporting information.) was carried out to study the orientation status of these CNTs (Fig. 5). In the as–prepared TPU/CNTs and TPU/NFC@CNTs samples, f are 0.012 and 0.027, respectively, representing the isotropic state of CNTs. Under 200% strain, the TPU/NFC@CNTs (f = 0.6943) exhibits a higher orientation factor than TPU/CNTs (f = 0.4566). This is in agreement with above strain – sensing behavior and morphological study. Higher strain sensitivity is obtained for TPU/NFC@CNTs should attributed to the less entangled dispersion status of CNTs as well as the improved interfacial interaction between nanohybrids allows more efficient stress transfer. This leads to higher strain sensitivity observed above.

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Fig. 5 Polarized Raman spectra of 4 samples corresponding to Fig. 4.

3.4 Mechanism of TPU/NFC@CNTs nanocomposites as strain sensors. As discussed in the Introduction, homogenously dispersion and high aspect ratio of CNTs, strong interfacial interaction between filler and matrix might play an important role on the strain-sensing behavior of CPCs. To affirm the effect of NFC on the mentioned three aspects, relevant characterizations were carried out. In order to intuitively evaluate the effect on the dispersion state of CNTs in the TPU matrix by adding NFC, the cryo-fractured surfaces of TPU/NFC@CNTs nanocomposite films with 3wt.% CNTs and 0,1,2,3,4,5wt.% NFC were characterized by SEM. As shown in Fig. 6, one can see there are large scale agglomerations of CNTs in TPU/CNTs sample (Fig. 6a), this is because the strong Van der Waals interaction between the CNTs and high aspect ratio resulting in forming agglomerates or bundles instead of individual tubes. With addition of NFC, the scale of agglomeration become smaller (Fig. 6b, 6c), when the content of NFC was 3 wt.% (Fig. 6d), there is no obvious agglomeration in the range of view. And Fig. 6e and Fig.6f also exhibit good dispersion state as well as in Fig. 6d. Good dispersion of nanofillers in CPCs play important roles in the sensitivity of stretchable sensors, which is beneficial for the more perfect conductive network embedded in the matrix.

16


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Fig. 6 SEM images of the TPU/NFC@CNTs with 3wt.% CNTs and different NFC loadings: 0 (a), 1 (b), 2 (c), 3 (d), 4 (e) and 5wt.% (f). (The scale bar in the images is 5µm)

The interfacial interaction between conductive network and polymer matrix is a key point influencing the sensitivity of CPCs, stronger interaction leads to higher sensitivity. The conductive network can be destructed more easily undergoing strain exhibiting different resistivity. The influence of NFC on the interfacial interaction between hybrid filler and polymer matrix was featured by the increase of glass transition temperature (Tg) of the nanocomposite. The dependence of tanδ on temperature for neat TPU and its nanocomposites is shown in Fig.7. The relaxation of the amorphous soft segment domains, i.e. the Tg of PU soft segment is -29.5, -27.6, -24.2 and -23.7 ℃ for neat TPU, TPU/CNTs, TPU/NFC and TPU/NFC@CNTs, respectively. The increase of Tg for TPU/NFC is ascribed to the fact that the NFC is strongly associate with the TPU matrix, resulting in decrease of the degree of freedom for the soft segment in TPU matrix. Compared to TPU/CNTs, the Tg of TPU/NFC@CNTs is increased approximately 4 ℃, which is due to the addition of NFC. Hence, the cooperation of NFC with TPU/CNTs nanocomposites can be well demonstrated to improve the interfacial interaction between CNTs conductive network and TPU matrix resulting in the increasing sensitivity of nanocomposite. To find out its cause, Olivier et.al has reported that there exist short-range hydrophobic interactions between carbon nanotubes (CNTs) and the hydrophobic crystalline faces ((200) planes) of cellulose, which confirmed NFC is a kind of amphipathicity 17



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material.31

Fig. 7 Tanδ of the neat TPU and its TPU/NFC@CNTs nanocomposite with 3wt.% CNTs and different NFC loadings.

Literature has reported higher aspect ratio of conductive fillers results in higher sensitivity of stretchable sensors. Fig.8 shows the TEM image of CNT together with NFC. It is observed that CNTs tend to locate along the NFC (the arrowed light – colored fiber) and exhibit a knot morphology, where NFC is like a rope stringing the CNTs together. Then its overall aspect ratio largely can be considered to increase by the end to end connecting of CNTs. Fig. 8b shows the illustration how the NFC increased the overall aspect ratio of CNTs, which increased from 50 to 150, approximately. Therefore, the addition of NFC could increase the aspect ratio of CNTs, in a sense that a small change of strain will gain increased sensitivity of stretchable sensors.

Fig. 8 TEM image of nanohybrid fillers (a); Schematic illustration of nanohybrid fillers (b).

To gain a clear understanding of network structure during strain sensing, a schematic representation is proposed in Fig.9. Isotropic conductive network often 18


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demonstrate significant increase in resistivity under strain due to strain induced orientation of conductive networks as shown in Fig. 4. Such orientation could in turn result in loss in local contact between networks, leading to decrease of electrical conductivity, exhibiting strain sensitivity, consequently. The difference among these two samples, TPU/CNTs and TPU/NFC@CNTs, is the initial dispersed status of CNTs and varied interfacial interaction between TPU matrix and nanohybrid fillers under stretching. This leads to quite different sensitivity. For the TPU/CNTs sample, CNTs exhibit entangled bundles (Fig. 9a) and less oriented degree during stretching (Fig. 9b) due to the feeble interaction with TPU matrix. The TPU/NFC@CNTs sample, however, gained a homogenous disperse state of CNTs (Fig. 9c) resulting in the optimal mechanical and electrical properties. More importantly, the improved interfacial interaction by using NFC provides better load – transfer across polymer – CNTs matrix and larger driving force for the alignment of conductive network than that in TPU/ CNTs. Therefore, the CNTs bundles are more oriented in TPU/NFC@CNTs (Fig. 9d), and higher sensitivity is observed under the same strain.

Fig. 9 Schematic illustration for mechanism of improved sensitivity of TPU/NFC@CNTs nanocomposites: as – prepared TPU/CNTs (a) and TPU/NFC@CNTs (c); TPU/CNTs (b) and TPU/NFC@CNTs (d) at 200% strain.

4

Conclusions To conclude, NFC-assisted TPU/CNTs free-standing films were fabricated via a

facile approach in solution. It was demonstrated that NFC can significantly enhance 19



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the suspension properties of CNTs bundles by directing the arrangement of CNTs bundles along NFC, yielding nodular NFC@CNTs nanohybrids with excellent suspension stability and high aspect ratio. Moreover, the prepared TPU/NFC@CNTs gained better mechanical properties as well as electrical conductivity compared with TPU/CNTs. What’s more, TPU/CNTs strain sensors with NFC in it exhibit a sensitive electrical response in a large range of tensile strain (100%) and reversible resistance under repetitive stretch – release cycles. The excellent properties of the composites were attribute to the critical role of NFC: assisting the dispersion of CNTs assembling conductive structure in the TPU matrix; substantially enhancing the interaction between fillers and matrix due to the amphipathicity property of NFC; increasing the overall aspect ratio of CNTs forming a knot morphology. All the mentioned three aspects give rise to the increasing of strain sensitivity of the composite. Because of the unique combination of high sensitivity and mechanical properties, the prepared TPU/NFC@CNTs strain sensors may find broad applications in stretchable electronics, skin sensors, wearable communication devices and energy storage.

Associated content Supporting Information. This material includes one figure of the set-up for strain-sensing measurement and mechanism of polar-Raman spectroscopy.

Author information Corresponding Author E-mail: [email protected] (Q. Fu), Tel./Fax: +86 28 8546 1795. Notes The authors declare no competing financial interest.

Acknowledgment 20


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This work was supported by the National Natural Science Foundation of China (grant no. 51421061 and 21404075).

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Figures and Tables

Fig. 1 Digital picture of NFC，CNTs and NFC@CNTs nanohybrids suspension after standing for a month (a); UV-vis absorption spectra of NFC, CNTs and NFC@CNTs suspension (b); TEM images of NFC (c, scale bar: 500nm), CNTs (d, scale bar: 50nm) and NFC@CNTs nanohybrids (e, scale bar: 200nm) aqueous suspension.

Fig. 2 Typical stress – strain curves (a) and Electrical conductivity (b) of TPU/NFC@CNTs with 3wt.% CNTs and different NFC loadings.

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Fig. 3 Resistivity as a function of tensile strain for TPU/NFC@CNTs nanocomposites (a); Plot of the relative resistance change under repetitive stretch – release cycles of 100% (b). Stress – strain curving of TPU/NFC@CNTs composite during cyclic stretching/releasing process (100% strain magnitude, 20 cycles) (c).Gauge factor (GF) of the samples at 200% strain (d).

Fig. 4 SEM morphological study of CNTs in TPU matrix. SEM images of as prepared TPU/CNTs (a) and TPU/NFC@CNTs (b) with 3 wt.% CNT. SEM images of samples at 100% strain:

TPU/CNTs (c)

and TPU/NFC@CNTs (d), the direction of double sided arrow is the strain direction. (The scale bar for a, b, c, d is 3µm, for the else is 1µm.)

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Fig. 5 Polarized Raman spectra of 4 samples corresponding to Fig. 4.

Fig. 6 SEM images of the TPU/NFC@CNTs with 3wt.% CNTs and different NFC loadings: 0 (a), 1 (b), 2 (c), 3 (d), 4 (e) and 5wt.% (f). (The scale bar in the images is 5µm)

Fig. 7 Tanδ of the neat TPU and its TPU/NFC@CNTs nanocomposite with 3wt.% CNTs and different NFC loadings.

27



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Fig. 8 TEM image of nanohybrid fillers (a); Schematic

illustration of nanohybrid fillers (b).

Fig. 9 Schematic illustration for mechanism of improved sensitivity of TPU/NFC@CNTs nanocomposites: as – prepared TPU/CNTs (a) and TPU/NFC@CNTs (c); TPU/CNTs (b) and TPU/NFC@CNTs (d) at 200% strain.

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Tab. 1. Mechanical properties of neat TPU and the TPU/NFC@CNTs with 3wt.% CNTs and different NFC loadings.

Sample

Young’

Tensile

Elongation

Strength

strength

at break

(MPa)

(MPa)

(%)

neat TPU

3.90 ± 0.7

19.1 ± 0.5

1520 ± 30

TPU/3CNTs

6.23 ± 3.2

20.2 ± 3.2

789 ± 12

TPU/3CNTs@1NFC

7.47 ± 2.7

24.6 ± 6.2

843 ± 27

TPU/3CNTs@2NFC

9.03 ± 1.9

31.1 ± 5.6

1050 ± 23

TPU/3CNTs@3NFC

11.92 ± 2.1

35.2 ± 2.7

986± 14

TPU/3CNTs@4NFC

13.41 ± 2.4

31.7 ± 3.5

861± 11

TPU/3CNTs@5NFC

15.48 ± 3.5

26.6 ± 4.1

660 ± 17

TPU/3NFC

4.76 ± 1.2

23.4 ± 2.3

1430 ± 34

Tab. 2. GFs reported in the literature. Materials

Range of GF

Ref.

PDMS/CNTs

0.06-0.82

24

PU/CNTs

0.34-1.07

23

Polyimide/CNTs

3-6

42

TPU/graphene

0.78-17.7

43

Epoxy/graphene

12.8-24

44

Epoxy/CNT-graphene

~10

45

Graphene films

~37

46

Epoxy/graphene

~56.7

30

Chewing gum/CNTs

12-25

2

Natural rubber/CNTs

~43.5

39

Our work

10.1-49.5

--

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TOC of Graphic

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