Graphene-polymer coating for the realization of strain sensors

Abstract

In this work we present a novel route to produce a graphene-based film on a polymer substrate. A transparent graphite colloidal suspension was applied to a slat of poly(methyl methacrylate) (PMMA). The good adhesion to the PMMA surface, combined with the shear stress, allows a uniform and continuous spreading of the graphite nanocrystals, resulting in a very uniform graphene multilayer coating on the substrate surface. The fabrication process is simple and yields thin coatings characterized by high optical transparency and large electrical piezoresitivity. Such properties envisage potential applications of this polymer-supported coating for use in strain sensing. The electrical and mechanical properties of these PMMA/graphene coatings were characterized by bending tests. The electrical transport was investigated as a function of the applied stress. The structural and strain properties of the polymer composite material were studied under stress by infrared thermography and micro-Raman spectroscopy.

Keywords: IR thermography; graphene; graphite; micro-Raman spectroscopy; strain sensor.

Figure 1

(a) SEM images of the slat section of the graphene/PMMA interface; (b) SEM image of the graphene surface at high magnification.

Figure 2

(a) Sketch of the setup used for bending tests; F represents the force produced by the screw S used to apply the bending stress and f is the vertical displacement of the sample surface from the horizontal position. (b) Schematic view of the geometry of the sample under test. The sample dimensions are: 150 × 20 × 3 mm.

Figure 3

Sketch of the setup used for the thermographic analysis of the PMMA/nanocomposite sample.

Figure 4

Sketch of the top view of the sample; A1, A2 are the (10 × 10 mm) areas on the sample monitored by the infrared camera during the bending tests. The positions of the voltage (V⁺, V⁻) and current (I⁺, I⁻) probes are indicated.

Figure 5

Comparison of the IRT temperature changes. The measurements were performed on various composite materials.

Figure 6

Electrical current variation due to the mechanical stress as measured on a PMMA/graphene sample.

Figure 7

Time dependence of the electrical current for PMMA/graphene. A constant voltage bias V = 5 V is applied to the sample while the applied force varies cyclically between unload (F = 0 N) and load (F = 6.9 N).

Figure 8

Schematic view of strain induced degenerate vibrational modes E_2g shown on the left. The uniaxial strain is directed along the x axis. The graphene lattice is turned with respect to the x axis at angle φ_S. On the right, the Raman spectrum of the PMMA/graphene sample is shown as measured (a) and after subtraction of PMMA Raman components (b).

Figure 9

Dependence of the normalized electric resistance variation ΔR/R₀ on strain ε. The bars indicate the relative measurement error estimated at 1% and 8% for ε and ΔR/R₀, respectively. The solid line shows the best fit to the experimental data as calculated from the theoretical prediction given by Equation 3 with τ = 53 ± 11.