Strain Sensing Polymer Nanocomposites at Ultralow Graphene Loading Level

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An electrically conductive ultralow percolation threshold of 0.1 wt% graphene was observed in the thermoplastic polyurethane (TPU) nanocomposites. The homogeneously dispersed graphene effectively enhanced the mechanical properties of the TPU significantly at a low graphene loading of 0.2 wt%. These nanocomposites were subjected to cyclic loading to investigate the influences of graphene loading, strain amplitude and strain rate on the strain sensing performances. The two dimensional graphene and flexible TPU matrix were found to endow these nanocomposites with wide range of strain sensitivity (gauge factor ranging from 0.78 for TPU with 0.6 wt% graphene at the strain rate of 0.1 min-1 to 17.7 for TPU with 0.2 wt% graphene at the strain rate of 0.3 min-1) and good sensing stability for different strain patterns. In addition, these nanocomposites demonstrated good recoverability and reproducibility after stabilization by cyclic loading. An analytical model based on tunneling theory was used to simulate the resistance response to strain under different strain rates. This study provides a guideline for the fabrication of graphene based polymer strain sensors. Experimental The graphene/TPU nanocomposites were fabricated by co-coagulation plus compression molding technique1. Briefly, 2.0 g TPU was dissolved in 50 mL DMF at 40 °C by vigorously stirring for 30 min. The required amount of aqueous graphene dispersion was mixed with 15 mL DMF and treated under ultrasonication for 10 min to disperse the graphene homogenously. Subsequently, the TPU/DMF and graphene/DMF were mixed together and sonicated for additional 30 min. The mixture was then added dropwise into 300 mL methanol under strongly stirring to obtain the flocculate of graphene/TPU. The obtained flocculate was filtered, dried at 80 °C under vacuum for 20 h, and hot pressed at 210 °C for 10 min under a pressure of 15 MPa. The thickness of the nanocomposites sample was 0.5 mm. The neat TPU control sample was also fabricated in the same manner for comparison. The obtained nanocomposites were denoted as TPU-xG, where x represents the mass loading of graphene, G is the abbreviation of graphene. For example, TPU-0.2G represents the TPU/graphene nanocomposites containing 0.2 wt% graphene Results and Discussion The SEM and TEM of graphene demonstrated the full exfoliation of graphene after the ultrasonic dispersion in DMF. Significant enhancement of mechanical properties of graphene/TPU nanocomposites was obtained due to the good interfacial interaction between graphene and TPU. The XRD, SEM and TEM of graphene/TPU nanocomposites showed homogeneous dispersion of graphene in TPU matrix2, and an ultralow percolation threshold was obtained. Strain sensing behaviors of the nanocomposites were conducted under different graphene loadings, strain amplitudes and strain rates, good discernment, sensitivity and sensing stability were obtained. All of these are benefited from the two dimensional graphene and good flexibility of TPU3. Conclusion Graphene/TPU nanocomposites with an ultralow percolation threshold of 0.1 wt% graphene were successfully prepared by co-coagulation plus compression molding technique. The H-bonding interaction between graphene sheets and TPU was confirmed from FT-IR. Graphene was distributed in the TPU matrix homogeneously and the mechanical property of the nanocomposites was enhanced significantly. The nanocomposites exhibited good sensitivity and sensing stability for different strain patterns, showing good discernment in strain sensing. Good recoverability and reproducibility were also obtained after stabilization by cyclic loading. The change of the number of conductive pathways and tunneling distance were responsible for the resistance-strain sensing behaviors under different strain rates according to the fitting results. Due to the fascinating strain sensing behaviors, the nanocomposites have great potential for applications as strain sensors to meet various demands in detecting various external environment stimuli4. Acknowledgments The financial supports from National Natural Science Foundation of China (Contract Number: 11572290, U1204507), and the University of Tennessee Knoxville are kindly acknowledged. Figure 1. (a) AFM of graphene and its dispersion in TPU matrix; (b) Stable strain sensing behaviors of graphene/TPU nanocomposites with different graphene loadings after stabilization by cyclic loading.