For filler composite systems used in strain sensor applications, piezoresistive effect, strain hysteresis, and repeatability are critical factors, which have to be clearly evaluated and understood. To investigate the effects of the aspect ratio and content of a multi-walled carbon nanotube (MWCNT) on the strain sensor properties of the composite, MWCNT/Polydimethylsiloxane (PDMS) composites with varying filler contents and aspect ratios were fabricated. In order to uniformly disperse MWCNTs on the polymer matrix, we used a three-roll milling method to generate high shear force for de-bundling MWCNTs. Mechanical and electrical properties of the MWCNT composites were evaluated for each case. In addition, through the cyclic stretching test, we optimized the strain-sensing properties of the MWCNT composites by considering their piezoresistive effects and strain hysteresis.

Conducting polymer composites consisting of carbon nanotubes (CNTs) as a conductive filler and polydimethylsiloxane (PDMS) as a polymer matrix were fabricated to investigate their capacitive and piezoresistive effects as pressure sensors. The pressure-sensing behavior and mechanism of the composites were compared in terms of basic configuration with a parallel plate structure. Various sensing experiments, such as sensitivity, repeatability, hysteresis, and temperature dependence according to the working principle, were conducted with varying filler contents. The hysteresis and repeatability of the pressure-sensing properties were investigated using cyclic tensile tests. In addition, a temperature test was performed at selected temperatures to monitor the change in the resistance/capacitance.

Polymer composites containing conductive fillers that utilize the piezoresistive effect can be employed in flexible pressure sensors. Depending on the filler used, different characteristics of a pressure sensor such as repeatability, sensitivity, and hysteresis can be determined. To confirm the variation of the pressure sensing tendency in accordance with the dimensions of the filler, carbon black (CB) and carbon nanotubes (CNTs) were used as representative 0-dimension and 1-dimension conductive fillers, respectively. The piezoresistive effect was exploited to analyze the process of resistance change according to pressure using CB/PDMS (polydimethylsiloxane) and CNT/PDMS composites. The electrical characteristics observed for each filler were confirmed to be in accordance with its content. The pressure sensitivity of each composite was optimized, and the pressure-sensing mechanism that explains the difference in sensitivity is presented. Through repeated compression experiments, the hysteresis and repeatability of the pressure-sensing properties were examined.

The key characteristics of bending sensors are piezoresistive effect, hysteresis, and durability. In this study, to investigate the influence of the aspect ratio and contents of multi-walled nanotubes (MWNTs) on the properties of bending sensors, MWNT/polydimethylsiloxane (PDMS) composites were fabricated with various aspect ratios and filler contents. The MWNTs were uniformly dispersed in the composites using the three-roll milling method. By increasing the bending angle gradually, the sensitivity of each composite was analyzed. Furthermore, discontinuous cyclic bending tests were conducted to investigate the piezoresistive effect and hysteresis. In addition, stable repeatability of the composites was confirmed through continuous cyclic bending tests. As a result, optimal aspect ratios and filler contents have been presented for application in bending sensors of MWNT composites.

To manufacture composites with required properties, it is extremely important to select an appropriate filler. Carbon-based nano materials such as carbon black, graphene and carbon nanotube (CNT) have been extensively investigated as reinforcing and conducting fillers. Because of their high aspect ratio coupled with superior physical properties, 1-dimensional CNTs are ideal as filler materials to impart electrical conductivity to insulating polymers, while enhancing mechanical strength. In this study, we investigated the piezo-resistive and mechanical properties of composites consisting of two types of CNT classified as multi-walled CNT (MWNT) or single-walled CNT (SWNT) depending on the number of walls. Since MWNT and SWNT have different physical properties such as specific surface area and aspect ratio, this can affect the composite’s performance. To more effectively evaluate the effect of MWNTs and SWNTs in composites, we used thermoplastic polyurethane (TPU) as a matrix, which is an insulating stretchable elastomer. Morphological and mechanical/electrical characterizations were conducted to determine differences in the MWNT and SWNT composites. In addition, we conducted dynamic strain sensing tests on each type of CNT composites to compare the sensitivity as a strain sensor. Differences in piezo-resistive behaviors were attributed to the loss of electrical contact points during stretching. These results can serve as a useful design guideline for the wider use of CNT composites.