In recent years, nanocomposites based on various nano-scale carbon fillers, such as carbon nanotubes (CNTs), are increasingly being thought of as a realistic alternative to conventional smart materials, largely due to their superior electrical properties. Great interest has been generated in building highly sensitive strain sensors with these new nanocomposites. This article reviews the recent significant developments in the field of highly sensitive strain sensors made from CNT/polymer nanocomposites. We focus on the following two topics: electrical conductivity and piezoresistivity of CNT/polymer nanocomposites, and the relationship between them by considering the internal conductive network formed by CNTs, tunneling effect, aspect ratio and piezoresistivity of CNTs themselves, etc. Many recent experimental, theoretical and numerical studies in this field are described in detail to uncover the working mechanisms of this new type of strain sensors and to demonstrate some possible key factors for improving the sensor sensitivity.

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We improved the piezoelectric property of poly(vinylidene fluoride) (PVDF) by employing graphene. The reduced graphene oxide (rGO)–PVDF nanocomposites were prepared by a solution casting method and the rGO contents ranged from 0.0 wt% to 0.2 wt%. To induce the piezoelectric β-phase crystal structure, the nanocomposite films were drawn in a ratio of 4–5 and polarized by a step-wise poling method. To evaluate the piezoelectric property, the output voltages of the rGO–PVDF nanocomposite films were measured through extensive experimental vibration tests. The experimental results show that the rGO–PVDF nanocomposite film with 0.05 wt% rGO loading possesses the highest output voltage compared with other loadings, which is around 293% of that of the pure PVDF film. Moreover, it can be found that with the increase of the rGO content from 0 wt% to 0.2 wt%, the output voltage tends to have a peak at 0.05 wt%. The main reason for this phenomenon is that a more β-crystalline phase can be formed at those rGO loadings, as confirmed by XRD and FT-IR spectrum analyses.

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The interfacial mechanical properties between graphene (GR) and a polymer matrix play a key role in load transfer capability for GR/polymer nanocomposites.

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A temperature sensor was fabricated from a polymer nanocomposite with multi-walled carbon nanotube (MWCNT) as nanofiller (i.e., MWCNT/epoxy). The electrical resistance and temperature coefficient of resistance (TCR) of the temperature sensor were characterized experimentally. The effects of temperature (within the range 333-373 K) and MWCNT content (within the range 1-5 wt%) were investigated thoroughly. It was found that the resistance increases with increasing temperature and decreasing MWCNT content. However, the resistance change ratio related to the TCR increases with increasing temperature and MWCNT content. The highest value of TCR (0.021 K(-1)), which was observed in the case of 5 wt% MWCNT, is much higher than those of traditional metals and MWCNT-based temperature sensors. Moreover, the corresponding numerical simulation-conducted to explain the above temperature-dependent piezoresistivity of the nanocomposite temperature sensor-indicated the key role of a temperature-dependent tunneling effect.