High-performance graphene-based carbon nanofiller/polymer composites for piezoresistive sensor applications

Abstract

Poly(vinylidene fluoride) (PVDF) composites with different carbonaceous nanofillers, prepared by solution casting, were studied their chemical, mechanical, electrical and electro-mechanical properties evaluated. Few-layer graphene (FLG) nanoplatelets (G-NPL), graphene oxide (GO) and reduced graphene oxide (rGO) and single-walled carbon nanohorns (SWCNH)) were found to have a strong influence in the overall properties of the composites prepared with up to 5 wt% nanofiller contents. The mechanical strain of carbonaceous nanofillers/PVDF composites decreases from 15% to near 5% of maximum strain. The electrical percolation threshold depends on the nanofiller type, being below 1 wt% for rGO and near 2 wt% for the remaining nanofillers. The electrical conductivity shows a maximum increase of nine orders of magnitude, from σ ≈ 5 × 10⁻¹¹ S/m of pure PVDF to σ ≈ 1 × 10⁻² S/m for rGO/PVDF composites with 5 wt% nanofillers. The conduction mechanism being related to hopping between the carbonaceous nanofillers for concentrations higher than the percolation threshold.

Furthermore, the composites show electro-mechanical properties, except for G-NPL materials, with rGO/PVDF composites with 5 wt% nanofiller content showing higher Gauge factor (GF) values, reaching GF≈ 11 for deformations between 0.5 and 2 mm in 4-point bending experiments. These results demonstrate the suitability of the composites for strain sensing applications.

Introduction

The study of polymer composites containing conductive or insulating materials is an area of increasing interest [1]. In particular, carbon nanofiller/polymer nanocomposites have been intensively studied due to their application potential, as they allow to tailor the mechanical and electrical properties of the composite by varying carbon nanofiller concentration, aspect ratio and dispersion [2].

Models have been developed that can predict, to some extent, the effect of adding conductive fillers to a dielectric matrix on the electrical and dielectric properties [3]. The effective mean-field medium concept is the foundation for most of the empirical models. The main drawback of these models is that they fail to predict the composite electric behavior near the percolation threshold, defined as the critical point where the physical properties show singularities and scaling behavior [4], [5].

In the past years, a new set of emerging nanomaterials, such as nanocarbonaceous, nanodiamonds and functional nanocomposites with novel structure-dependent functional properties have been developed, being interesting both for fundamental research and advanced applications [6], [7]. Among the most interesting functional properties of those materials the electro-mechanical ones stands out, as they allow the development of sensors and actuators, with applications in consumer electronics and portable devices, among others [6].

In this context, piezoresistive nanofillers/polymer composites are being strongly investigated due to their exceptional mechanical, electrical and electro-mechanical properties. Particularly, the focus has been on the intrinsic properties of the carbonaceous nanofillers, such as electrical properties and high aspect ratio [8], [9]. The most studied carbon nanostructures have been carbon nanofibers (CNF) [10], carbon nanotubes (CNT) [11] and, in recent years, graphene [11], [12]. More recently, novel carbonaceous nanomaterials such as single-walled carbon nanohorns (SWCNH) have been reported with distict geometries [10], [13], intrinsic mechanical (tensile strength) and electrical properties (electrical conductivity), being quite different from the remaining few-layer graphene materials with different chemical treatments [10] as reduction or oxidation.

Typically, the carbonaceous nanofillers are used for improving mechanical properties and electrical or ionic conductivity of polymer matrices with low electrical percolation thresholds [8], [14], the electro-mechanical sensitivity is becoming particularly targeted for study due to their superior response when compared to the strain gages commercial sensors [8], [15]. The overall properties of carbon nanoallotropes/polymer composites strongly depend on polymer type and processing method, nanofiller type, degree of aggregation and orientation [16]. This dependence is particularly relevant for the percolation threshold concentration, being the parameter most critical in determining electrical properties and, therefore, the functionality of the composites [17], [18].

The percolation threshold concentration is particularly relevant for the development of composites, as they typically show the largest electro-mechanical response-variation of the electrical resistivity when a strainis applied to the material, at concentrations around the percolation threshold [19]. In the case of carbon nanofiller/polymer composites, this effect is mainly attributed (in addition of the geometrical effect) to variations of the conductive network with strain due to loss of contact between the fillers, tunneling or hopping effects in neighboring fillers and/or conductivity variations due to the strain of the different carbonaceous, for example [17], [20].

Several strategies and materials have been used for the development of electro-mechanical polymer composites as sensors to detect several external stimulus, including strain, pressure, temperature or light [21]. The electrical resistance variation with external stimulus enables a large range of applications to the piezoresistive composites as strain/pressure sensors in artificial intelligence, electronic devices and industrial production [22]. Typical metal strain gages and silicon piezoresistive sensors present interesting properties but their mechanical properties limit their use [15]. Stretchable or high flexible piezoresistive sensors are fabricated using polymers as matrix and conductive fillers as reinforcement materials [23]. Conductive carbonaceous nanofillers such as graphite, carbon black (CB), graphene, CNF and CNT have been widely employed [18], [24], or combination thereof CNT are the most widely used due to their intrinsic electrical properties and geometry, as high aspect ratio, with higher piezoresistive sensitivity [22], although the difficulty to disperse and the high production cost limit their widespread use [22]. In order to increase the overall properties of the composites, better dispersion of carbon nanofillers in the polymer matrix is essential.

Several preparation methods have been developed to manufacture of nanofillers/polymer composites, such as in-situ polymerization, extrusion, solution blending, and electrospinning [24], [25]. Solution blending is an excellent method to prepare inks to print by inkjet, screen and spray print, or other methods as roll-to-roll [15], [26]. These methods allow printing these materials at a large scale [15], [26]. The sensitivity of electro-mechanical properties of carbon nanofillers/PVDF composites depends on the processing method, type of carbon nanofillers and content in composites and may range between 1 to near 200 [27], [28].

For the production of flexible or stretchable sensors, elastomers and thermoplastic polymers are the most widely used as matrices [2]. Within the thermoplastics, poly(vinylidene fluoride) (PVDF) and its copolymers have suitable electroactive properties to produce sensors and actuators. PVDF is a semicrystalline polymer with five possible crystalline phases, the most technologically relevant and investigated being the non-polar α-PVDF and the polar β-PVDF, which are the most electrically active phases. Furthermore, PVDF presents good mechanical and chemical properties, weather resistance, and excellent properties associated to their polar crystalline forms [29].

In this sense, the incorporation of novel carbonaceous nanofillers, such as single wall carbon nanohorns, FLG nanoplatelets and treated graphene (oxided and reduced), into PVDF matrices in order to tune and optimize the electro-mechanical behavior and response of the composites is a major challenge that constitutes the focus of this work. The overall properties of the composites will be assessed in view of their applicability as electro-mechanical sensors for bending strain detecting. These materials represent a promising alternative to strain gauge sensors due to their higher GF and simple processing and integration into devices.