Flexible sensor based on polymer nanocomposites reinforced by carbon nanotube foam derivated from cotton

Abstract

Carbon nanotubes (CNTs) attract great interest in developing conducting polymer nanocomposites for various applications. This paper reported the preparation of CNT foam and corresponding nanocomposites using a monomer infusion process, aiming at eliminating the problems arising from the dispersion and possible re-agglomeration of nanofillers in a polymer matrix. Various techniques were employed to study the morphology and properties of the developed materials. The results showed that by using cotton as a template, CNTs were grown on the carbonized cotton fiber, forming foam-like hierarchical structures with excellent hydrophobic and absorption performance. The nanocomposites reinforced by CNT foam exhibited a satisfactory stability and reproducibility on piezoresistivity under the cyclic operations, and had the potential to be used as a sensor for measuring mechanical deformation in flexible electronics.

Introduction

Flexible sensor, an electronic device that is sensitive to the applied strain, has the advantage of ordinary one for excellent monitoring of mechanical force [[1], [2], [3]], at the same time processes a good flexibility and deformation capacity, thus can be widely used in e-skin [4,5], supercapacitors [6,7] and wearable devices [8]. The key element in a flexible sensor is the conducting basement with excellent mechanical performance, which requires a material to maintain a controllably electrical conductance under the mechanical deformation. Generally, conducting basement can be metal membranes [9], metal oxide films [10,11] and carbon-based nanomaterials [[12], [13], [14]]. Polymer nanocomposites filled with carbon nanotubes (CNTs) have attracted much attention to satisfy the material requirements for flexible sensors. For example, Sanli and co-workers [15] fabricated nanocomposite film by dispersing CNTs into an epoxy matrix using a horn sonicator. The obtained material exhibited a piezoresistive effect (A change on the electrical resistance of a material when mechanical strain is applied) at a low CNT concentration of 0.3 wt%, and was considered to be suitable for practical strain sensor. In another study, conducting nanocomposites consisting of ethylene-propylene-diene rubber and CNTs were prepared [16]. The introduction of CNTs significantly enhanced the mechanical and electrical properties of the nanocomposites, and a non-symmetric linear resistance change was observed in the material subjected to the mechanical deformation, proving the potential of CNT-reinforced nanocomposites as a sensing material. Much progress has been made in this field with varying degree of success, however, if the material was effectively used as reinforcement, proper dispersion of CNTs into the polymer matrix had to be guaranteed [17]. In addition, there was a concern that dispersed CNTs had a tendency to re-agglomerate in the matrix during the processing of nanocomposites [18], especially for those using thermosetting and rubbery matrices which generally required a curing process under an elevated temperature. In this context, CNTs with three-dimensional (3-D) structures, such as foam, aerogel, sponge, were developed in recent years to overcome these problems [[19], [20], [21], [22]].

Generally, there were two methods to prepare 3-D CNT materials [22]. The first one was the so-called sol-gel process. The principle of this technique was that by freezing the well-dispersed CNT suspension, a sol precursor was produced. The resulting assembly was dried and subsequently pyrolyzed to convert CNT aerogel. Kim and co-workers [23] dispersed CNTs in water by using a frozen-drying process to prepare CNT aerogel with a low density of 8–10 mg/cm³. When incorporating the material into a silicone rubber, the developed nanocomposites showed stretchable conductive properties. While the prepared CNT aerogel using this technique showed a low density and high surface area, the proper dispersion of CNTs in solvent and morphology control of sample were two challenges for the scale-up production of 3-D CNT material. As an alternative, chemical vapor deposition (CVD) method was adopted to prepare CNT foam in large scale by using metallic foam (Cu, Ni) as a template [24,25]. This method provided a way to solve the dispersion problem of CNTs to some extent, however, it brought negative effects on the properties of CNTs as strong acid was employed to remove metal template after CNT growth, which obviously damaged the walled structure of CNTs, leading to the degraded mechanical and electrical performance of material. Therefore, a convenient and environment-friendly method is highly desirable to prepare CNT foam.

The current study is a part of a larger project in developing materials with hierarchical structures for engineering and environmental applications. In this paper, we reported the preparation of CNT foam using a nature polymer, raw cotton, as a template. The foam showed a hydrophobic property and spontaneous behavior to adsorb organic compounds with excellent structural stability, thus having the possibility to solve the problems associated with the dispersion and secondary agglomeration of CNTs in a polymer matrix. The piezoresistance of CNT foam/polymer nanocomposites was studied, and the feasibility of using such nanocomposites as a flexible sensor was demonstrated.