Shedding light on the viscoelasticity of conductive polymer composites
Conductive rubber is widely used in the field of pressure sensors. In the past, DC driving was used to obtain a resistive response to pressure, but the method has substantial disadvantages, such as time and temperature drift. We have found that these drawbacks can be eliminated or mitigated by AC driving.1–3 Because resistive viscoelasticity affects the repeatability, linearity, and hysteresis (i.e., plastic strain) of pressure sensors based on these kinds of composites, we would like to better understand the effects of AC driving.
Measures such as cycling load and adding coupling agents during preparation are acknowledged to improve the repeatability of pressure sensors. But as yet there is no method for simultaneously enhancing the time and temperature stability of sensor resistance and piezoresistive repeatability. Time and temperature drift make it difficult to design and adjust a sensor's conditioning circuit. Consequently, we have conducted a series of experiments to study the piezoresistivity of AC-driven composites. Here we report findings from an experimental (transient testing4) investigation of the resistive viscoelasticity of a silicone-rubber/carbon black composite, including creep, relaxation, and recovery.
We measured the resistance response of a composite film subjected to rapid application of constant stress and strain, and rapid removal of a constant load. Step stress was achieved by allowing a 4kg load to fall freely along a vertical orbit from a height of 30mm. Figure 1 shows the property of DC-driven resistive creep, which appears to be composed of two different segments. The first (0~0.35ms) can be explained microscopically: when the deformation is small, the conductive mechanism is compliant with tunneling conductance. Based on conductive5 and Voigt-Kelvin models4 derived elsewhere, resistance creep can be calculated as ε1=−14.54, ε2=11.73, γs0=1.786, τ1=1.537ms, and τ2=1.249ms (see Equation 1):
The explanation for the second segment (0.36~0.86ms) of resistive creep is that when a deformation reaches a critical point, destruction of the conductive network may play a major role.6 According to the conducting-network-destruction model6 and the previously mentioned Voigt-Kelvin model,4 the strain of resistance can be calculated as ε1=−14.54, ε2=11.73, a=0.05026, τ1=−1.143ms, and τ2=0.13ms, where a is an exponential constant (see Equation 2).
We examined the property of resistive recovery driven by an electrical field in the frequency range from DC to 1MHz (see Figure 2). All the plots have two parts: steep at the front followed by a slowing down. When the force is removed, recovery occurs abruptly. Many current paths destroyed in the loading process are reconstructed simultaneously, resulting in a steep decrease in impedance. Thereafter, the composite goes into slow creep. A decrease of separation between the conductive particles ensues, the tunneling current rises, and impedance subsides.
Figure 3 shows the property of resistive relaxation driven by an electrical field in the frequency range from DC to 1MHz. Although relaxation speed generally becomes slower as driving frequency increases, it is much slower under DC driving than under AC. Moreover, under DC excitation, resistance only decreases 0.5% 200s after compression. In contrast, resistance under 1MHz—the slowest relaxation in the AC frequency range we measured—decreases 7.05% in the same period.
In summary, the creep property of resistance exhibits two different forms: a short period of slow creep resulting from the change of separation between conductive particles and a relatively long period of fast creep due to the destruction of current paths. As frequency increases, resistance recovery time becomes shorter and the relaxation speed of resistance slower. Consequently, some of these composite materials can be improved when driven with different frequencies, depending on the specific application. As a next step, we plan to investigate the influence of driving frequency on properties of pressure sensors such as repeatability, linearity, and hysteresis.
- http://tekscan.com Tekscan Inc. force sensor. Accessed 4 August 2010.
- http://www.interlinkelectronics.com Interlink Electronics Inc. force-sensing resistors. Accessed 4 August 2010.
- S.-L. Wang, P. Wang and T.-H. Ding, Piezoresistivity of silicone-rubber/carbon black composites excited by AC electrical field, J. Appl. Polym. Sci. 113, pp. 337-341, 2009.
- R. F. Joel, Polymer Science and Technology, Prentice Hall, Englewood Cliffs, NJ, 1995.
- X. W. Zhang, Y. Pan, Q. Zheng and X. S. Yi, Time dependence of piezoresistance for conductor-filled polymer composites, J. Polym. Sci., Part B: Polym. Phys. 38, pp. 2739-2749, 2000.
- M. Knite, V. Teteris, A. Kiploka and J. Kaupuzs, Polyisoprene-carbon black nanocomposites as tensile strain and pressure sensor materials, Sens. Actuators, A 110, pp. 142-149, 2004.