Processing and modeling of conductive thermoplastic/carbon nanotube films for strain sensing
Introduction
Carbon nanotubes (CNTs) have received a large amount of attention due to their remarkable mechanical, electrical, and thermal properties [1], [2], [3], [4], [5]. Due to their capability to change electronic properties when subjected to strains, nanotubes have been considered a potential candidate for strain sensors [6]. It has been demonstrated that nanotubes embedded in a polymer matrix can be used as strain sensors at a nanoscale level by observing the Raman band shift, as the load is transferred from the matrix to the nanotubes [7]. However, this method is not practical in field applications due to the difficulties associated with the implementation of the measuring equipment. A more common technique for measuring strains on the surface of a structure is the use of resistance-type strain gages. Commercially available, constantan- or nickel–chromium-alloy-based strain gages offer wide static, dynamic, and temperature ranges. However, these gages lack versatility and flexibility, as they can only measure strains at specific locations (where they are bonded) and directions (along the grid). In addition, they exhibit relatively low and narrow range of gage factor (2.0–3.2) [8]. (Gage factor is the measure of sensitivity of a resistance strain gage, proportional to the change in resistance when subjected to a strain change.)
CNTs are effective fillers for fabricating electrically conductive polymer compounds. To date, CNT composites of various polymer systems have been shown to reach percolation threshold at only a few volume or weight percents of CNTs, markedly lower than those of other common conductive fillers, such as metal particles or carbon black [9], [10], [11], [12], [13], [14], [15], [16]. Evidently, this is because the high aspect ratio and nanoscale size allow CNTs to form conductive networks much more efficiently [16], [17], [18], [19], [20]. A possible consequence of such efficiency, however, is that the conductive network may lack robustness. That is, the network configuration may easily be influenced by mechanical disturbances, such as stress or shear. Because the conductive network dictates electrical properties, conductive behavior of CNT composites can be expected to change, even when subjected to appreciably small mechanical loadings. Park et al. [21], for instance, noted an increase in electrical resistance of epoxy/multi-walled carbon nanotube (MWNT) films under tensile stresses.
This paper reports our findings on the change in resistivity of polymer/CNT films due to tensile strains. Electrically conductive polymer films, offering such capabilities as electrostatic discharge (BSD) protection and electromagnetic interference (EMI) shielding, have immense potential applications in electronics, automotive, and aerospace industries [2]. In many of such applications, for example, BSD-protected coating for spacecraft, it is common that the conductive films are affected by strain-inducing, mechanical or aerodynamic loads. An understanding of the strain–resistivity relationship, therefore, is instrumental in ensuring satisfactory conductive performance of the films. Additionally, knowledge of conductive characteristics of the films against stress/strain can open the door to newer material applications, such as strain gage or multifunctional conductive coating with strain-sensing capability.
The polymer/CNT films used in this study were fabricated via two routes. The first is a simple melt-based process, where neat MWNTs and polymer powder were dry-blended and subsequently hot-pressed multiple times to produce samples. This method was employed because of its simplicity, which can readily be scaled up in production. The second route was a solution- or solvent-based method, which is commonly used to fabricate polymer/CNT composites. MWNTs and polymer pellets were dissolved in a solvent and mixed using ultrasound. MWNT/polymer composite obtained from casting the homogenized solution then was hot-pressed to make film samples. This method is expected to produce polymer/MWNT with better nanotube dispersion, and thus higher electrical conductivity.
MWNTs were selected for the study because of their increasing availability and low cost compared to single-walled nanotubes (SWNTs). More importantly, unlike SWNTs, MWNTs are always conductive and have a relatively high conductivity, 1.85 × 103 S/m, compared to other nano- or micro-fillers such as carbon black [22]. This paper reports the effects of tensile strains on electrical resistance in PMMA films filled with different weight fractions of MWNTs. In particular, the development of PMMA/MWNT films with tunable sensitivity is highlighted. A semi-empirical model that captures the relationship between CNT volume fraction and sensitivity is proposed.
Section snippets
Materials and sample preparation
MWNTs having a purity rating of 95% were obtained from Aldrich (St. Louis, MO). The MWNTs were 0.5–500 μm in length, 5–10 nm in ID, and 60–100 nm in OD. Molding-grade PMMA compound (Acrylite S10/8N) was available from Cyro (Rockaway, NJ). The polymer had a density of 1.19 g/cc and a flow index of 3.4 g/10 min.
Preliminary experiments
Resistance measurements of the films under no-load condition were performed to evaluate the resistivity ranges of the films and the time dependence of the resistance measurements. The former was performed to screen out films with surface resistivity greater than 108 Ω/square, the upper sensitivity limit of the resistance measuring apparatus. The latter was performed to assess the possible time decay of measured resistance values due to CNT capacitive behavior. Such phenomenon has been reported
Conclusions
In this study, the electrical resistance of PMMA/MWNT composite films subjected to tensile strains was measured, and the potential applications of the films as strain sensors with a broad range of tunable sensitivity were investigated. The surface resistivity of the films was observed to increase with increasing tensile strain. This is due to the reduction in conductive network density and increase in inter-tube distances induced by applied strains. Evidently, electrical resistance is less
Acknowledgements
The authors gratefully acknowledge the financial supports from the High-Performance Materials Institute (HPMI). The authors also thank Drs. Bing Jiang and Hsin-Yuan Miao for their kind assistance with the micro-tensile tester and electrical resistivity measurement system. The sample preparation efforts by Mr. Juan Typaldos and Miss Amanda Chu are also acknowledged.
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