Properties of Graphene-Thermoplastic Polyurethane Flexible Conductive Film

: Flexible conductive ﬁlms were prepared via a convenient blending method with thermoplastic polyurethane (TPU) as matrix and nanocrystalline cellulose (NCC) modiﬁed chemically reduced graphene oxide (RGO / NCC) as the conductive ﬁllers. The relationships between the electrical and thermal properties as well as the tensile strength and electrothermal response performance of the composite ﬁlm and the mass content of reduced graphene oxide (RGO) and the initial TPU concentration were systematically investigated. The experimental results show that the resistivity of the composite ﬁlm with the mass content of RGO / NCC of 7 wt% and an initial TPU concentration of 20 wt% is the minimum of 8.1 Ω · mm. However, the thermal conductivity of composite ﬁlm with mass content of RGO / NC C of 5 wt% and the initial TPU concentration of 30 wt% reaches a maximum of 0.3464 W · m − 1 · K − 1 , which is an increase of 56% compared with pure TPU. The tensile strength of the composite ﬁlms with mass contents of RGO of 3 wt% prepared with the initial TPU concentrations of 20 wt% reaches the maximum of 43.2 MPa, which increases by a factor of 1.5 (the tensile strength of the pure TPU is 28.9 MPa). The composite conductive ﬁlm has a fast electrothermal response. Furthermore, superhydrophobic composite conductive ﬁlms were prepared by immersing the composite conductive ﬁlm into ﬂuorinated decyl polyhedral oligomeric silsesquioxane (F-POSS) ethanol solution. The water contact angle of the superhydrophobic composite conductive ﬁlm reaches 158.19 ◦ and the resistivity of the superhydrophobic composite ﬁlm slightly increases and still has good conductivity. ◦ C, respectively, under the operation for input voltage of 20 V, and are 42.4 and 78.3 ◦ C, respectively, under the operation for input voltage of 15 V, respectively. The experimental results indicate that the e ﬃ cient transduction of electrical energy into Joule heating is caused by the good conductivity of the composite ﬁlm. The results, therefore, demonstrate that


Introduction
In recent years, with the development trend of electronic equipment toward the intelligence, miniaturization, and multi-functions, flexible conductive composites have attracted more attention in scientific research and industry and were widely used in the fields of wearable electronic devices, stretchable antennas, electroluminescent devices, flexible displays, energy devices, and more [1][2][3][4][5]. Flexible conductive composites are often used as electrodes for flexible devices, which functionalizes in building connections between different components of the devices. Until now, the flexible conductive composites are typically fabricated by embedding conductive nanomaterial into or onto a surface Graphene oxide (GO) was prepared from flaky graphite using a modified Hummers method [36]. The GO was washed and centrifuged using a water-to-HCl volume ratio of 9:1. The GO mixture was centrifuged again and washed with deionized (DI) water until the pH of the solution was reduced to 6. The GO obtained, therefore, was diluted using DI water to 1 mg·mL −1 . Nano-crystalline cellulose (NCC) was prepared by sulfuric acid hydrolysis from microcrystalline cellulose. Additionally, 100 mL of 50% sulfuric acid was added to a three-mouth flask. Then, 6 g of microcrystalline cellulose were added into sulfuric acid solution and dissolved under electric stirring. After microcrystalline cellulose completely dissolved, the reaction solution was heated to 40 • C and stirring continued for 2 h to obtain an NCC suspension. The NCC suspension was transferred into a 5000 mL beaker and added 1000 mL of DI water. The NCC suspension was washed and centrifuged until the pH of the solution was reduced to 2. The NCC suspension obtained was diluted using DI water to 10 mg·mL −1 .
A total of 100 mL of GO solution and 100 mL of NCC solution were ultrasonic at 400 W and 40 • C for 1 h, respectively. Then, the GO and NCC mixed solution with the mass ratio of 1:1.5 was prepared and dispersed by ultrasound at 400 W and 40 • C for 1 h to obtain the GO/NCC mixed solution. The GO/NCC mixed solution was added into a three-necked flask. Then hydrazine hydrate (weight ratio of hydrazine hydrate to GO is 8:10) and ammonia water (volume ratio of ammonia to hydrazine hydrate is 5:1) were added into the mixed solution under vigorous stirring and heated at 95 • C for 2 h. Lastly, the product was filtered through a 0.2 µm filter to obtain reduced graphene oxide and NCC (RGO/NCC) slurry. The mass content of RGO/NCC in the slurry was measured using a gravimetric method.

Preparation of RGO/NCC-TPU Composite Film
According to the experimental formula, a certain amount of TPU masterbatch were dissolved in 500 mL of DMF. A formulated amount of RGO/NCC slurry was added into a flask containing DMF solution under vigorous stirring and ultrasound for 30 min. Then, the formulated TPU solution was added into the above RGO/NCC DMF under vigorous agitation. Afterward, the mixed solution was dispersed by a high-speed shear disperser for 60 min. Lastly, the mixed solution was poured into the Teflon mold and dried at 70 • C until the weight stopped changing. Then, RGO/NCC-TPU composite film were obtained. RGO/NCC-TPU composite film was peeled off for further testing. Figure 1 shows a schematic diagram of the fabrication process of RGO/NCC-TPU-TPU composites film.
Coatings 2020, 10, x FOR PEER REVIEW 4 of 13 added into the above RGO/NCC DMF under vigorous agitation. Afterward, the mixed solution was dispersed by a high-speed shear disperser for 60 min. Lastly, the mixed solution was poured into the Teflon mold and dried at 70 °C until the weight stopped changing. Then, RGO/NCC-TPU composite film were obtained. RGO/NCC-TPU composite film was peeled off for further testing. Figure 1 shows a schematic diagram of the fabrication process of RGO/NCC-TPU-TPU composites film.

Preparation of Superhydrophobic Conductive Composite film of RGO/NCC-TPU
A total of 0.368 mL of PDTOS and 0.1 g of PSS-OVS were added into 1.5 mL of methylene chloride by magnetic stirring for 2 h. Then, 0.0094 g of DMPA as initiator were added into the above mixed solution by magnetic stirring for 1 h. Furthermore, the mixed solution was placed under 250 W of UV high-pressure mercury lamp for 10 min, and white precipitations were formed. The precipitations were purified and centrifuged three times with ethanol and dried in an oven at 80 °C to obtain fluorinated decyl polyhedral oligomeric silsesquioxane (F-POSS). Lastly, F-POSS was dispersed into ethanol solvent using ultrasound to prepare 3 mol•L −1 of F-POSS ethanol dispersion. A superhydrophobic RGO/NCC-TPU conductive composite film was prepared by immersing the RGO/NCC-TPU conductive film into F-POSS ethanol dispersion for 40 min. After rinsing with ethanol, the superhydrophobic conductive composite film of RGO/NCC-TPU was dried in air at room temperature.

Characterization
Differential scanning calorimetry (DSC) analysis was conducted via simultaneous differential thermal analysis (STA449F5, NETZSCH-Gertebau GmbH, Selb, Germany). The scanning electron microscope (SEM, zeiss sigma 500, Carl Zeiss, Oberkochen, Germany) is used to characterize the microstructure of film. The resistance was measured by a four-point probe system (ST2253, Suzhou Jingge Electronics Co., Ltd., Suzhou, Zhejiang, China). The resistance of each sample was each measured at 20 different sites and calculated from the average value of those measurements. The thermal conductivity of the sample was measured by a DRL-III heat flow meter instrument (Xiangtan Xiangyi Instrument Co., Ltd., Xiangtan, Hunan, China), according to the standard American Society of Testing Materials (ASTM) D5470. Tensile strength of the sample was measured by a universal tensile testing machine (UTM500, Shenzhen Sansi Zongheng Technology Co., Ltd., Shenzhen, Guangdong, China). The extension rate was 50 mm/min. Samples were cut into strips of 45 mm × 10 mm × 0.16 mm using a scalpel and then tested. The water contact angle was tested by the contact Angle tester (JCY-3, Shanghai fangrui instrument Co., Ltd., Shanghai, China).

Preparation of Superhydrophobic Conductive Composite film of RGO/NCC-TPU
A total of 0.368 mL of PDTOS and 0.1 g of PSS-OVS were added into 1.5 mL of methylene chloride by magnetic stirring for 2 h. Then, 0.0094 g of DMPA as initiator were added into the above mixed solution by magnetic stirring for 1 h. Furthermore, the mixed solution was placed under 250 W of UV high-pressure mercury lamp for 10 min, and white precipitations were formed. The precipitations were purified and centrifuged three times with ethanol and dried in an oven at 80 • C to obtain fluorinated decyl polyhedral oligomeric silsesquioxane (F-POSS). Lastly, F-POSS was dispersed into ethanol solvent using ultrasound to prepare 3 mol·L −1 of F-POSS ethanol dispersion. A superhydrophobic RGO/NCC-TPU conductive composite film was prepared by immersing the RGO/NCC-TPU conductive film into F-POSS ethanol dispersion for 40 min. After rinsing with ethanol, the superhydrophobic conductive composite film of RGO/NCC-TPU was dried in air at room temperature.

Characterization
Differential scanning calorimetry (DSC) analysis was conducted via simultaneous differential thermal analysis (STA449F5, NETZSCH-Gertebau GmbH, Selb, Germany). The scanning electron microscope (SEM, zeiss sigma 500, Carl Zeiss, Oberkochen, Germany) is used to characterize the microstructure of film. The resistance was measured by a four-point probe system (ST2253, Suzhou Jingge Electronics Co., Ltd., Suzhou, Zhejiang, China). The resistance of each sample was each measured at 20 different sites and calculated from the average value of those measurements. The thermal conductivity of the sample was measured by a DRL-III heat flow meter instrument (Xiangtan Xiangyi Instrument Co., Ltd., Xiangtan, Hunan, China), according to the standard American Society of Testing Materials (ASTM) D5470. Tensile strength of the sample was measured by a universal tensile testing machine (UTM500, Shenzhen Sansi Zongheng Technology Co., Ltd., Shenzhen, Guangdong, China). The extension rate was 50 mm/min. Samples were cut into strips of 45 mm × 10 mm × 0.16 mm using a scalpel and then tested. The water contact angle was tested by the contact Angle tester (JCY-3, Shanghai fangrui instrument Co., Ltd., Shanghai, China).

Electrical and Thermal Properties of RGO/NCC-TPU Film
In order to discuss the effect of RGO/NCC on the curing behavior of the TPU, the curing profiles of the pure TPU solution and RGO/NCC-TPU solution containing 1.5 wt% RGO was measured by DSC, as shown in Figure 2. Seen from Figure 2, the curing temperatures of the pure TPU solution and RGO/NCC-TPU solution containing 1.5 wt% RGO are 150 and 160 • C, respectively, which indicated that graphene improve the thermal stability of TPU. The total reaction heat of the pure TPU and RGO/NCC-TPU containing 1.5 wt% RGO is 1.572 mW·mg −1 and 2.852 mW·mg −1 , respectively. It is indicated that RGO/NCC affect the curing behavior of TPU and reduce the cross-linking density.
Coatings 2020, 10, x FOR PEER REVIEW 5 of 13 In order to discuss the effect of RGO/NCC on the curing behavior of the TPU, the curing profiles of the pure TPU solution and RGO/NCC-TPU solution containing 1.5 wt% RGO was measured by DSC, as shown in Figure 2. Seen from Figure 2, the curing temperatures of the pure TPU solution and RGO/NCC-TPU solution containing 1.5 wt% RGO are 150 and 160 °C, respectively, which indicated that graphene improve the thermal stability of TPU. The total reaction heat of the pure TPU and RGO/NCC-TPU containing 1.5 wt% RGO is 1.572 mW•mg −1 and 2.852 mW•mg −1 , respectively. It is indicated that RGO/NCC affect the curing behavior of TPU and reduce the cross-linking density. Few studies have reported the effect of the initial TPU solution concentration on the properties of the composite film. However, the viscosity of TPU solution is determined by the initial TPU solution concentration, which affects the dispersion of RGO in the composite film. We prepared TPU solutions with initial concentrations of 10, 20, and 30 wt%, respectively, and then added RGO/NCC slurries into the TPU solutions with different concentrations, respectively, according to the mass contents of RGO in the composite films of 0.2, 0.4, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0 and 7.0 wt%, respectively. Figure 3a shows resistivities of RGO/NCC-TPU composite films with different mass contents of RGO and the initial TPU concentrations of 10 wt% (curve a), 20 wt% (curve b), and 30 wt% (curve c), respectively, and Figure 3b shows photos of the RGO/NCC-TPU slurry prepared with initial TPU concentrations of 20 and 5 wt% RGO (above) and the corresponding RGO/NCC-TPU composite film (below), respectively. The inset in Figure 3a is local magnification. When the RGO/NCC mass content of it ranges from 0.2-1.5 wt%, the composite films are non-conductive, so there are no data shown in Figure 3a. When the RGO/NCC mass content is 2.0 wt%, the resistivities of RGO/NCC-TPU composite films fabricated with the initial TPU concentrations of 10, 20 and 30 wt%, respectively, are 2509.1, 3109.1 and 3879.3 Ω•mm, respectively. With an increase of the RGO mass content, the resistivity of composite film decreases gradually. When the mass content of RGO/NCC is less than 4%, the lower the initial TPU concentration is, the lower the resistivity of the composite film is. When the RGO mass content reaches 4 wt%, the resistivities of RGO -TPU composite films fabricated with the initial TPU concentrations of 10, 20 and 30 wt%, respectively, are 29.5, 21.8 and 41.6 Ω•mm, respectively. Clearly, the resistivity of the composite film fabricated with the initial TPU concentration of 20 wt% is the lowest. As the mass content of RGO continues to increase, the resistivity of the composite film fabricated with the initial TPU concentration of 20 wt% remains minimal and reaches 8.1 Ω•mm when the mass content of RGO is 7 wt%. However, the resistivity of the composite film fabricated with the initial TPU concentration of 30 wt% increases to 51.4 Ω•mm when the mass content of RGO is 7 wt%. During heat treatment of the sample, as the solvent evaporated, the crosslinking reaction is carried out in the composite film, and the TPU matrix continues to shrink. This makes the RGO sheets more tightly overlapped and stacked. When the RGO mass content in the composite film reaches a certain value, the RGO in the composite film overlaps and stacks with each other, and the conductive networks are established in the film. As such, the composite film exhibits conductivity [28−32]. As Few studies have reported the effect of the initial TPU solution concentration on the properties of the composite film. However, the viscosity of TPU solution is determined by the initial TPU solution concentration, which affects the dispersion of RGO in the composite film. We prepared TPU solutions with initial concentrations of 10, 20, and 30 wt%, respectively, and then added RGO/NCC slurries into the TPU solutions with different concentrations, respectively, according to the mass contents of RGO in the composite films of 0.2, 0.4, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0 and 7.0 wt%, respectively. Figure 3a shows resistivities of RGO/NCC-TPU composite films with different mass contents of RGO and the initial TPU concentrations of 10 wt% (curve a), 20 wt% (curve b), and 30 wt% (curve c), respectively, and Figure 3b shows photos of the RGO/NCC-TPU slurry prepared with initial TPU concentrations of 20 and 5 wt% RGO (above) and the corresponding RGO/NCC-TPU composite film (below), respectively. The inset in Figure 3a is local magnification. When the RGO/NCC mass content of it ranges from 0.2-1.5 wt%, the composite films are non-conductive, so there are no data shown in Figure 3a. When the RGO/NCC mass content is 2.0 wt%, the resistivities of RGO/NCC-TPU composite films fabricated with the initial TPU concentrations of 10, 20 and 30 wt%, respectively, are 2509.1, 3109.1 and 3879.3 Ω·mm, respectively. With an increase of the RGO mass content, the resistivity of composite film decreases gradually. When the mass content of RGO/NCC is less than 4%, the lower the initial TPU concentration is, the lower the resistivity of the composite film is. When the RGO mass content reaches 4 wt%, the resistivities of RGO -TPU composite films fabricated with the initial TPU concentrations of 10, 20 and 30 wt%, respectively, are 29.5, 21.8 and 41.6 Ω·mm, respectively. Clearly, the resistivity of the composite film fabricated with the initial TPU concentration of 20 wt% is the lowest. As the mass content of RGO continues to increase, the resistivity of the composite film fabricated with the initial TPU concentration of 20 wt% remains minimal and reaches 8.1 Ω·mm when the mass content of RGO is 7 wt%. However, the resistivity of the composite film fabricated with the initial TPU concentration of 30 wt% increases to 51.4 Ω·mm when the mass content of RGO is 7 wt%. During heat treatment of the sample, as the solvent evaporated, the crosslinking reaction is carried out in the composite film, and the TPU matrix continues to shrink. This makes the RGO sheets more tightly overlapped and stacked. When the RGO mass content in the composite film reaches a certain value, the RGO in the composite film overlaps and stacks with each other, and the conductive networks are established in the film. As such, the composite film exhibits conductivity [28][29][30][31][32]. As the RGO mass content increases, more and more conductive networks in the film are formed and the conductivity of the composite film gradually increases. However, the excessive RGO in the slurry tends to form large agglomerations, which results in poor dispersion [21]. Typically, graphene is prone to aggregate and precipitate irreversibly in a variety of matrices due to its insolubility, van der Waals forces, and π-π stacking between RGO sheets [21]. Currently, the dispersion methods of RGO mainly includes physical dispersion, covalent bonding, and noncovalent bonding. In our experiments, a stirring treatment was utilized to disperse the RGO/NCC in the TPU solution. Although the stirring dispersion owns a simple operation, the dispersion rate of the RGO is low. Especially at a high mass content of RGO, the formation of aggregates of RGO is inevitable [21]. In addition, the lower the initial TPU concentration is, the lower the TPU solution viscosity is, which is not conducive to forming the compact stacking and overlapping structure of RGO sheets in the composite film. The higher the initial TPU concentration is, the greater the TPU viscosity is, which results in the difficulty of dispersion of RGO in the TPU solution and the formation of aggregates of RGO. We added commercial graphene sheets as conductive fillers into TPU solution and, when the mass content of graphene exceeded 5 wt%, the mixture could not be stirred. However, in this case, the mass content of RGO can reach 7 wt% because we used NCC to modify RGO. RGO was added into TPU in the form of slurry, which improved the dispersion of RGO in TPU solution.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 13 conductivity of the composite film gradually increases. However, the excessive RGO in the slurry tends to form large agglomerations, which results in poor dispersion [21]. Typically, graphene is prone to aggregate and precipitate irreversibly in a variety of matrices due to its insolubility, van der Waals forces, and π-π stacking between RGO sheets [21]. Currently, the dispersion methods of RGO mainly includes physical dispersion, covalent bonding, and noncovalent bonding. In our experiments, a stirring treatment was utilized to disperse the RGO/NCC in the TPU solution. Although the stirring dispersion owns a simple operation, the dispersion rate of the RGO is low. Especially at a high mass content of RGO, the formation of aggregates of RGO is inevitable [21]. In addition, the lower the initial TPU concentration is, the lower the TPU solution viscosity is, which is not conducive to forming the compact stacking and overlapping structure of RGO sheets in the composite film. The higher the initial TPU concentration is, the greater the TPU viscosity is, which results in the difficulty of dispersion of RGO in the TPU solution and the formation of aggregates of RGO. We added commercial graphene sheets as conductive fillers into TPU solution and, when the mass content of graphene exceeded 5 wt%, the mixture could not be stirred. However, in this case, the mass content of RGO can reach 7 wt% because we used NCC to modify RGO. RGO was added into TPU in the form of slurry, which improved the dispersion of RGO in TPU solution.  Figure 4 shows SEM images of surface morphology of the composite films prepared with the initial TPU concentration of 20 wt% and the mass content of RGO of 0.4, 1.5, 3.0, 5.0, and 7.0 wt%, respectively. It is clearly that the RGO/NCC are buried in the TPU matrices and exhibit grainy and rough characterization. With the increase of the mass content of RGO, the density of stacked RGO sheets increased, which is advantageous for forming the conductive pathways. When the mass content of RGO is 7.0 wt%, the clear cracks can be observed in the sample. This may be due to the higher content of graphene, which leads to greater internal stress in the composite film. Figure 5 shows SEM images of the cross-section morphology of the composite films prepared with the initial TPU concentration of 20 wt%. The mass content of RGO of 1.5, 3.0 and 5.0 wt%, respectively. In the cross-section view surface of composite films, it clearly shows a smooth surface coated with TPU in the low mass content of RGO (Figure 5a,b). The RGO sheets dispersed and overlapped in the TPU matrices can be observed in the high mass content of RGO (Figure 5c). It indicates a good compatibility between the RGO/NCC and TPU matrices, which makes closer contacts between them, promotes forming some conductive pathways, and builds conductive networks. However, when the mass content of RGO increases to 5.0 wt%, RGO sheets are easy to form agglomerates in the polymer samples.  Figure 4 shows SEM images of surface morphology of the composite films prepared with the initial TPU concentration of 20 wt% and the mass content of RGO of 0.4, 1.5, 3.0, 5.0, and 7.0 wt%, respectively. It is clearly that the RGO/NCC are buried in the TPU matrices and exhibit grainy and rough characterization. With the increase of the mass content of RGO, the density of stacked RGO sheets increased, which is advantageous for forming the conductive pathways. When the mass content of RGO is 7.0 wt%, the clear cracks can be observed in the sample. This may be due to the higher content of graphene, which leads to greater internal stress in the composite film.  In the cross-section view surface of composite films, it clearly shows a smooth surface coated with TPU in the low mass content of RGO (Figure 5a,b). The RGO sheets dispersed and overlapped in the TPU matrices can be observed in the high mass content of RGO (Figure 5c). It indicates a good compatibility between the RGO/NCC and TPU matrices, which makes closer contacts between them, promotes forming some conductive pathways, and builds conductive networks. However, when the mass content of RGO increases to 5.0 wt%, RGO sheets are easy to form agglomerates in the polymer samples.  In order to reveal the effect of RGO on thermal conductivity of the composite film, we measured the thermal conductivity of the composite films with different mass contents of RGO and different initial TPU concentrations, as shown in Figure 6. The thermal conductivity of the pure TPU is about 0.2213 W•m −1 •K −1 and the thermal conductivity of the composite film first increases and then decreases In order to reveal the effect of RGO on thermal conductivity of the composite film, we measured the thermal conductivity of the composite films with different mass contents of RGO and different initial TPU concentrations, as shown in Figure 6. The thermal conductivity of the pure TPU is about 0.2213 W·m −1 ·K −1 and the thermal conductivity of the composite film first increases and then decreases with the growth of the mass content of RGO in composite film. When the mass content of RGO reaches 5 wt%, the thermal conductivity of composite film fabricated with the initial TPU concentration of 30 wt% reaches a maximum of 0.3464 W·m −1 ·K −1 , which increases by 56% when compared with that of pure TPU. When the mass content of RGO is less than 5 wt%, the thermal conductivity of the composite film with the same RGO mass content increases with the initial TPU concentration. However, when the RGO mass content is 7 wt%, the thermal conductivity of the composite film fabricated d with the initial TPU concentration of 30 wt% is the smallest. Many interfaces are produced when RGO sheets are added into TPU due to a very high specific surface area of RGO sheets [36]. These interfaces result in phonon scattering and introduce interfacial thermal resistance. RGO is a highly thermal conductive channel in the composite film. When the mass content of RGO is below the percolation threshold, RGO cannot connect together to form a thermal conduction pathway. In this case, the interfacial thermal resistance of RGO and TPU is the main factor determining the thermal conductivity of the composite film. While the mass content of RGO is above the percolation threshold, the heat in the composite film mainly transfers through the thermal conduction pathways due to the high thermal conductivity of RGO, and the thermal conductivity of the composite film increases significantly. However, when the mass content of RGO is too high, RGO sheets tend to form a large number of aggregates, which makes it impossible to form effective thermal channels, so that the thermal conductivity decreases [21,[37][38][39]. with the growth of the mass content of RGO in composite film. When the mass content of RGO reaches 5 wt%, the thermal conductivity of composite film fabricated with the initial TPU concentration of 30 wt% reaches a maximum of 0.3464 W•m −1 •K −1 , which increases by 56% when compared with that of pure TPU. When the mass content of RGO is less than 5 wt%, the thermal conductivity of the composite film with the same RGO mass content increases with the initial TPU concentration. However, when the RGO mass content is 7 wt%, the thermal conductivity of the composite film fabricated d with the initial TPU concentration of 30 wt% is the smallest. Many interfaces are produced when RGO sheets are added into TPU due to a very high specific surface area of RGO sheets [36]. These interfaces result in phonon scattering and introduce interfacial thermal resistance. RGO is a highly thermal conductive channel in the composite film. When the mass content of RGO is below the percolation threshold, RGO cannot connect together to form a thermal conduction pathway. In this case, the interfacial thermal resistance of RGO and TPU is the main factor determining the thermal conductivity of the composite film. While the mass content of RGO is above the percolation threshold, the heat in the composite film mainly transfers through the thermal conduction pathways due to the high thermal conductivity of RGO, and the thermal conductivity of the composite film increases significantly. However, when the mass content of RGO is too high, RGO sheets tend to form a large number of aggregates, which makes it impossible to form effective thermal channels, so that the thermal conductivity decreases [21,37−39].    Figure 7 are samples. Graphene is often regarded as excellent reinforcing fillers of the composites due to outstanding mechanical performance. In this case, we can see that the tensile strength of the composite films first decreases (0.2 wt% RGO), then increases (≥ 0.4 wt% RGO), reaches the maximum until the RGO mass content is 3 wt%, and then decreases. The tensile strength of the composite films with mass contents of RGO of 3 wt% and the initial TPU concentrations of 20 wt% is the maximum of 43.2 Mpa, which increases by a factor of 1.5 (the tensile strength of the pure TPU is 28.9 MPa). There is no clear trend between the tensile strength and the initial TPU concentration of the sample when the RGO mass content in the composite film is less than 3 wt%, except that the tensile strength of the sample fabricated with the initial TPU concentration of 20 wt% is relatively high. The experimental results reveal that the mechanical property of TPU film can be improved by adding the appropriate amount of RGO. When the mass content of RGO is relatively low, the dispersion of RGO in the film is non-uniform and RGO in the film becomes the stress concentration point. This process results in a decrease in the mechanical property of the film. With the increase of the mass content of RGO, the dispersion uniformity of RGO in the film is also enhanced, which improves the TPU mechanical property. However, when the RGO mass content in the film is too high, the RGO aggregates degrade the mechanical property of the film [21]. In recent years, the electrothermal response performance of conductive films has also aroused the interest of researchers due to the potential application of conductive film as a flexible electrothermal heater [40−43]. In this study, we investigated the electrothermal response performance of the RGO/NCC-TPU conductive film by applying direct current (DC) voltage-stabilized power in a laboratory environment. The RGO/NCC-TPU conductive film was made through two clips coated with copper foil that contacted the film edges as electrodes. Figure 8 shows a plot of temperature versus time for the RGO/NCC-TPU conductive films with resistivity of 29.5 and 79.5 Ω•mm, respectively, under the operation for input voltage of 20 ( Figure 8a) and 15 V (Figure 8b), respectively. The experimental results reveal that the temperature increases linearly in 10 and 20 s for the films of 79.5 and 29.5 Ω•mm, respectively, and, thereafter, the temperature increases slowly with time until it reaches a steady state temperature, which confirms the fast electrothermal response performance. The state temperatures of the films of 79.5 and 29.5 Ω•mm are 54.5 and 120.7 °C, respectively, under the operation for input voltage of 20 V, and are 42.4 and 78.3 °C, respectively, under the operation for input voltage of 15 V, respectively. The experimental results indicate that the efficient transduction of electrical energy into Joule heating is caused by the good conductivity of the composite film. The results, therefore, demonstrate that the composite film is suitable for applications in the field of the fast temperature switching with low input voltages. It can be seen from the above experimental results that the electrothermal response of the RGO/NCC-TPU conductive film is similar to that of the flexible transparent conductive AgNWs film [44]. However, under the same conductivity conditions, the thermal efficiency is low among flexible transparent conductive AgNWs film [44]. The reason may be that the Joule heating energy is consumed when the RGO transfers heat energy through the TPU matrices. In recent years, the electrothermal response performance of conductive films has also aroused the interest of researchers due to the potential application of conductive film as a flexible electro-thermal heater [40][41][42][43]. In this study, we investigated the electrothermal response performance of the RGO/NCC-TPU conductive film by applying direct current (DC) voltage-stabilized power in a laboratory environment. The RGO/NCC-TPU conductive film was made through two clips coated with copper foil that contacted the film edges as electrodes. Figure 8 shows a plot of temperature versus time for the RGO/NCC-TPU conductive films with resistivity of 29.5 and 79.5 Ω·mm, respectively, under the operation for input voltage of 20 ( Figure 8a) and 15 V (Figure 8b), respectively. The experimental results reveal that the temperature increases linearly in 10 and 20 s for the films of 79.5 and 29.5 Ω·mm, respectively, and, thereafter, the temperature increases slowly with time until it reaches a steady state temperature, which confirms the fast electrothermal response performance. The state temperatures of the films of 79.5 and 29.5 Ω·mm are 54.5 and 120.7 • C, respectively, under the operation for input voltage of 20 V, and are 42.4 and 78.3 • C, respectively, under the operation for input voltage of 15 V, respectively. The experimental results indicate that the efficient transduction of electrical energy into Joule heating is caused by the good conductivity of the composite film. The results, therefore, demonstrate that the composite film is suitable for applications in the field of the fast temperature switching with low input voltages. It can be seen from the above experimental results that the electrothermal response of the RGO/NCC-TPU conductive film is similar to that of the flexible transparent conductive AgNWs film [44]. However, under the same conductivity conditions, the thermal efficiency is low among flexible transparent conductive AgNWs film [44]. The reason may be that the Joule heating energy is consumed when the RGO transfers heat energy through the TPU matrices.
The experimental results reveal that the temperature increases linearly in 10 and 20 s for the films of 79.5 and 29.5 Ω•mm, respectively, and, thereafter, the temperature increases slowly with time until it reaches a steady state temperature, which confirms the fast electrothermal response performance. The state temperatures of the films of 79.5 and 29.5 Ω•mm are 54.5 and 120.7 °C, respectively, under the operation for input voltage of 20 V, and are 42.4 and 78.3 °C, respectively, under the operation for input voltage of 15 V, respectively. The experimental results indicate that the efficient transduction of electrical energy into Joule heating is caused by the good conductivity of the composite film. The results, therefore, demonstrate that the composite film is suitable for applications in the field of the fast temperature switching with low input voltages. It can be seen from the above experimental results that the electrothermal response of the RGO/NCC-TPU conductive film is similar to that of the flexible transparent conductive AgNWs film [44]. However, under the same conductivity conditions, the thermal efficiency is low among flexible transparent conductive AgNWs film [44]. The reason may be that the Joule heating energy is consumed when the RGO transfers heat energy through the TPU matrices.

Superhydrophobic Conductive Composite Film of RGO/NCC-TPU
Due to the characteristics of TPU material, the RGO/NCC-TPU conductive film is susceptible to environmental humidity. We considered fabricating the superhydrophobic layer on the surface of the conductive film to improve the humidity effect of materials. Figure 9 shows the water contact angle (CA) of the RGO/NCC-TPU conductive films with different mass contents of RGO. The insets are photos of the water contact angle of the pure TPU and composite film with 4 wt% of RGO. The water contact angle of the pure TPU is about 124.90 • and increases to 155.82 • after adding 2 wt% of RGO into composite film, which displays a superhydrophobic characteristic. With the increase of the mass content of RGO in the film, the water contact angle of the composite film slightly increases and is 157.19 • when the mass content of RGO in the composite film is 4 wt%. The hydrophobic F-POSS molecules deposits not only on the composite conductive film surface, but also penetrates deeply into the composite film. Therefore, the hydrophobicity of the conductive composite film is greatly improved.

Superhydrophobic Conductive Composite Film of RGO/NCC-TPU
Due to the characteristics of TPU material, the RGO/NCC-TPU conductive film is susceptible to environmental humidity. We considered fabricating the superhydrophobic layer on the surface of the conductive film to improve the humidity effect of materials. Figure 9 shows the water contact angle (CA) of the RGO/NCC-TPU conductive films with different mass contents of RGO. The insets are photos of the water contact angle of the pure TPU and composite film with 4 wt% of RGO. The water contact angle of the pure TPU is about 124.90° and increases to 155.82° after adding 2 wt% of RGO into composite film, which displays a superhydrophobic characteristic. With the increase of the mass content of RGO in the film, the water contact angle of the composite film slightly increases and is 157.19° when the mass content of RGO in the composite film is 4 wt%. The hydrophobic F-POSS molecules deposits not only on the composite conductive film surface, but also penetrates deeply into the composite film. Therefore, the hydrophobicity of the conductive composite film is greatly improved. To demonstrate the effect of the superhydrophobic treatment on the conductivity of the RGO/NCC-TPU conductive film, we measured the resistivity of the composite film after superhydrophobic treatment, as shown in Figure 10. The inset is a photo of a light emitting diode (LED) device on the superhydrophobic conductive film. It is clear that the resistivity of the composite film after superhydrophobic treatment gradually increases with the mass content of RGO in the To demonstrate the effect of the superhydrophobic treatment on the conductivity of the RGO/NCC-TPU conductive film, we measured the resistivity of the composite film after superhydrophobic treatment, as shown in Figure 10. The inset is a photo of a light emitting diode (LED) device on the superhydrophobic conductive film. It is clear that the resistivity of the composite film after superhydrophobic treatment gradually increases with the mass content of RGO in the composite film. When the mass contents of RGO are 2 and 7 wt%, the resistivity of the superhydrophobic composite film increases by 1.6% and 22.6%, respectively. With the increase of the RGO mass content in the composite film, the conductivity of the composite film is affected by the F-POSS molecule as a non-conductive layer. To demonstrate the applicability of the superhydrophobic conductive composite film, we fabricated a LED device on the conductive film, as shown in the inset of Figure 10. A 0.5 W LED lamp fixed on the surface of the conductive film with superhydrophobic treatment is lighted, which indicates that the superhydrophobic conductive composite film still has good conductivity.

Conclusions
RGO/NCC-TPU flexible conductive composite films were prepared via a convenient blending method and their electrical and thermal properties and tensile strength and electrothermal response performance were investigated. The experimental results reveal that the electrical and thermal properties of the composite films are closely related to the initial concentration of TPU and the mass content of RGO in the composite film. The resistivity of the composite film with the mass content of RGO/NCC of 7 wt% and the initial TPU concentration of 20 wt% is the minimum of 8.1 Ω•mm. However, the thermal conductivity of the composite film with mass content of RGO/NCC of 5 wt% and the initial TPU concentration of 30 wt% reaches a maximum of 0.3464 W•m −1 •K −1 , which increases by 56% when compared with pure TPU. The tensile strength of the composite films with mass contents of RGO of 3 wt% and the initial TPU concentrations of 20 wt% reaches the maximum of 43.2 MPa, which increases by a factor of 1.5 (the tensile strength of the pure TPU is 28.9 MPa). The composite film has a fast electrothermal response, and the state temperatures of the films of 79.

Conclusions
RGO/NCC-TPU flexible conductive composite films were prepared via a convenient blending method and their electrical and thermal properties and tensile strength and electrothermal response performance were investigated. The experimental results reveal that the electrical and thermal properties of the composite films are closely related to the initial concentration of TPU and the mass content of RGO in the composite film. The resistivity of the composite film with the mass content of RGO/NCC of 7 wt% and the initial TPU concentration of 20 wt% is the minimum of 8.1 Ω·mm. However, the thermal conductivity of the composite film with mass content of RGO/NCC of 5 wt% and the initial TPU concentration of 30 wt% reaches a maximum of 0.3464 W·m −1 ·K −1 , which increases by 56% when compared with pure TPU. The tensile strength of the composite films with mass contents of RGO of 3 wt% and the initial TPU concentrations of 20 wt% reaches the maximum of 43.2 MPa, which increases by a factor of 1.5 (the tensile strength of the pure TPU is 28.9 MPa). The composite film has a fast electrothermal response, and the state temperatures of the films of 79.5 and 29.5 Ω·mm are 54.5 and 120.7 • C, respectively, under the operation for input voltage of 20 V, and are 42.4 and 78.3 • C, respectively, under the operation for input voltage of 15 V. The superhydrophobic composite conductive films were prepared by immersing the composite conductive film into F-POSS ethanol solution. The water contact angle of the superhydrophobic conductive composite film reaches 158.19 and the resistivity of the superhydrophobic composite film slightly increases and still has good conductivity.