Experimental Investigation and Modeling of the Dynamic Resistance Response of Carbon Particle‐Filled Polymers

Abstract

An important aspect of soft robotics, next to the different actuation mechanisms, is the monitoring of the robot's position and condition and, thus, the local and global strains via integrated sensors. Currently investigated actuation classes and materials like dielectric elastomer actuators or twisted-coiled polymeric actuators offer large strain potentials, rendering conventional metal or semiconductor strain gauges unsuitable. [3] Apart from their inherent strain limit of less than 5%, the difficult integration into and adhesion to smart materials is preventing their use in novel complex and compact soft robots. [4,5] Additionally, as some of the currently researched actuator mechanisms and principles offer high strain rate actuation, the necessity of not only a quasistatic but also dynamic monitoring and suitable control strategies is conspicuous. One approach to integrate actuators and sensors and to achieve resilient but flexible structures is using fiber-shaped active materials similar to muscles and nerves in biological systems. [6] A popular choice as a sensor material are carbon-based particle-filled highly stretchable elastomers (CPFEs). Such particles can be carbon black (CB), carbon nanotubes (CNT), graphene, or combinations thereof. Until today, there has been a vast amount of studies investigating different polymer materials, fillers, and mixing ratios as well as their applications to monitor soft robots or body posture. [7,8] Additionally, the conducting particles can also be used in order to tailor the electro-thermal response of shape memory polymers. [9] The standard analysis performed to evaluate the different soft sensor systems is a quasistatic tensile test while tracking the resistance. Investigations of dynamic sensor properties with systematically varied strain rates and loading scenarios are scarce. This lack of data also contributes to the poorly understood phenomena occurring during cyclic, dynamic loading of CPFEs. [10] To employ CPFEs as sensors, their linearity, sensitivity, and monotonicity are important features. [11] Unfortunately, the few investigations so far on dynamic mechano-electrical properties show non-monotonic responses, meaning that, apart from the resistance peak at maximum strain, additional peaks occur. [12,13,14] These peaks either appear while releasing or when the imposed strain is fully released and have been called shoulder phenomenon or secondary peaks. The phenomenon appears independently whether the filler material is CB, CNT, or graphene. [11,15,16] To distinguish between the different phenomena described in literature, the nomenclature in Figure 1 Carbon particle-filled polymers are frequently used as stretchable conductors and strain sensors. Many of the proposed resistance-based stretchable strain sensors show non-monotonic strain response, especially under dynamic conditions. This is commonly attributed to the competing destruction and reformation of the conductive network, but the underlying mechanism is still unknown. Therefore, systematic cyclic tens...

Electromechanical Performance of Strain Sensors Based on Viscoelastic Conductive Composite Polymer Fibers

“…The relative change in resistance (Δ R / R 0 ) varies with time for a typical CB/CNTs/TPU composite fiber sample according to Figure b. Indeed, the response signal decays with the occurrence of secondary peaks resulting from the response of conductive fillers to the transverse contraction, which occurs as piezoresistive polymer materials are stretched repetitively . The maximum and minimum values of Δ R / R 0 for each cycle are fitted with eq , which is simply modified from eq .…”

Electrical Stability and Piezoresistive Sensing Performance of High Strain-Range Ultra-Stretchable CNT-Embedded Sensors

“…Upon finger bending, all the sensors showed slight decreases in resistance at peaks. This can be explained by the shoulder phenomenon exhibited by the sensors incorporating carbon-based conductive particles [ 40 ]. This type of shoulder phenomenon is reported to be due to the reconstruction, re-distribution, and simultaneous destruction of the electrically conductive network [ 41 ].…”

Development of an Elastic, Electrically Conductive Coating for TPU Filaments

“…The strain load, on the other hand, leads to the disruption of the conductive paths. This interaction of viscoelasticity and compression of the conductive network as a result of transverse contraction leads to a complex, non-monotonic response of the resistivity signal as a function of strain [ 22 ]. In the first strain cycle, the separation of the conductive paths due to strain and the increase of the CNT packing density due to transverse contraction are superposed.…”

“…Nevertheless, secondary peaks or shoulder phenomena can be seen in each cycle. These are also due to a superposition of the resistivity change resulting from the strain load and the viscoelasticity respectively transverse contraction [ 22 ]. At this point, more in-depth experimental and theoretical work would be required to understand and quantify the influences of transverse contraction and viscoelasticity in more detail in order to gain a deeper understanding of the material behavior of the CNT-filled TPU coating.…”