Elsevier

Carbon

Volume 93, November 2015, Pages 878-886
Carbon

Shrinkage induced stretchable micro-wrinkled reduced graphene oxide composite with recoverable conductivity

https://doi.org/10.1016/j.carbon.2015.06.011Get rights and content

Abstract

A novel thermo-mechanical shrinking method is reported to fabricate a three dimensional (3D) stretchable and highly conductive micro-wrinkled reduced graphene oxide (MWrGO) supported on an elastic polydimethylsiloxane (PDMS) substrates. This 3D rGO architecture not only increases the specific area for more electrons to pass through but also bestows stretchability to the conductive pathway. The structural change of micro-wrinkles has been monitored by an in situ straining microscopy. The electrical conductivity of the samples remained fairly constant and stayed above 25 S/m under low deformation (no more than 30% strain) for up to 500 mechanical stretching-release cycles. Additionally, the MWrGO/PDMS composite can be stretched bi-axially because the shrinking process itself is isotropic. This MWrGO based stretchable composite with stable electrical properties and long life span could form a new platform of stretchable electronics.

Introduction

Stretchable conductors have been developed in the past decade for use in growingly diverse applications with new technological breakthroughs. For instance, motion sensors made from deformable conductors can check patients’ breath and heartbeat rates, and monitor the movement of athletes’ joints and muscles [1], [2] when attached to human bodies. Flexible electronics can also be used as foldable displays and stretchable devices [3], [4]. The performance of all these devices relies on the stable conductivity and long life span under frequent deformation [5], [6], [7]. Incorporation of conductive fillers in elastic matrices [8], [9] is the common practice used in conventional processes. The conductive pathway in matrix can hereby only be formed when the volume content of conductive filler exceeds a threshold of the volume fraction according to the predictions of percolation theory [10], [11]. However, the increase in volume content of conductive fillers can stiffen the composite and thus limit the stretchability and life span of stretchable conductors [12]. Therefore, there is an urgent need to develop both materials and conductive structures which can be stretched under strain for high performance stretchable electronics.

Graphene is an ideal candidate for this purpose due to its high electrical conductivity and other superior properties, such as high mechanical properties and flexibility [13], [14], [15], [16], [17]. Graphene is a two dimensional (2D) nanomaterial, which exhibits good flexibility and bendability, but this 2D graphene structure lacks stretchability to satisfy the deformation demanded by extra force [18]. This could limit the application of graphene materials as stretchable conductors. Therefore, developing three dimensional (3D) graphene based composites that have high mechanical strength has been recognised as a powerful approach to forming a stretchable architecture that maintains conductivity under deformation.

Various 3D graphene materials have been developed to improve the stretchability of conductive materials. Porous structures were investigated in this regard, such as graphene/polymer hydrogels [19], [20], [21], foams [22], [23], [24] and networks [25], which were constructed either by coating graphene onto porous polymeric matrices, or by incorporating polymer into 3D graphene foams and networks. Extraordinary mechanical flexibility has been demonstrated by hundreds of compressive or bending tests, but achieving a real stretchability is still quite challenging for these porous graphene structures. Chen et al. [25] reported a conductive carbon nanotube/graphene network which can be stretched up to 80% elongation. Although 100 cycles of stretching and releasing test at 50% strain could be achieved, this is far from the requirement for the application of stretchable conductors. Furthermore, wrinkled or crumpled graphene structures [26], [27], [28], [29], [30], [31] have been recently developed to fabricate stretchable conductors with high stretchability. The elastic substrate was pre-stretched, coated with graphene prepared by chemical vapour deposition (CVD) and relaxed to its original state. The pre-stored strain allows for the conductive pathway to remain intact for deformations within the pre-stretched range. The tensile strain of the wrinkled graphene composite was shown to be as high as 450%, and the resistance from zero strain to 450% strain of the composite increased from 0.8 × 104 Ω to 1.8 × 104 Ω [30]. Although wrinkled structure with high stretchability can be formed by this pre-stretching and releasing method, weak adhesion between graphene and substrate may lead to delaminated buckles and could not tolerate a series of stretching and release cycles [30]. Moreover, the tolerance to fatigue is still not well documented in the area of graphene based stretchable conductors, where the existing studies showed so far that the electrical conductivity can remain stable for only up to 100 stretching-release cycles [13], [25], [30], [32]. Therefore, graphene based stretchable conductors with both continuously electrical conductivity and high fatigue resistance require further development.

Herein, we report a novel approach to fabricating large-area micro-wrinkled reduced graphene oxide (MWrGO)/polydimethylsiloxane (PDMS) composite via a thermo-mechanical shrinking process. Diluted PDMS was permeated into gaps formed by MWrGO to strengthen the wrinkle structure and make the composite more robust. In situ stretching microscopy was employed to explore the structural evolution of MWrGO/PDMS composites, which indicates that an intact conductive pathway plays a key role in maintaining the stable electrical conductivity. Under cyclic testing, the stable conductivity and durability of MWrGO/PDMS composites can be maintained for elongations up to 30% for 500 stretching-release cycles, which is superior to results from existing studies on graphene based stretchable conductors. Since the shrinking of the film is isotropic, the MWrGO film acquires a multi-axial pre-loaded compressive strain that allows the composite films to be isotropically stretched in any direction across the plane.

Section snippets

Materials

Graphite powder (particle size < 45 μm), hydrazine hydrate (80%) and dichloromethane (>99%) were purchased from Sigma Aldrich (United States). Sulphuric acid (98%) (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2) (30%) were ordered from Chem Supply (South Australia). Shrink films (SFs) (PolyShrink™) (the main chemical component is polystyrene) were purchased from Lucky Squirrel (United States). Polydimethylsiloxane (PDMS) (SYLGARD® 184) was ordered from

Preparation and morphology analysis of the MWrGO/PDMS

Fig. 1 illustrates the fabrication route of the MWrGO/PDMS composite films, which consists of three main steps: coating, shrinking and transferring. The synthesis of well-dispersed GO solution was obtained from the low-cost graphite powder and can be produced in a large amount [34]. Firstly, a thin GO film was coated onto the surface of SF (Fig. 1A (1) and (2)). The thickness of GO film measured by profilometer was 1.22 μm (Fig. S1). Because of the hydrophobicity of the SF surface, it was

Conclusions

This study has demonstrated a simple and cost-effective method to construct MWrGO structure supported with a PDMS substrate to form a stretchable and electrically conductive composite. The microscopic and spectroscopic characterisations indicate the wrinkled structure of rGO can be formed after the shrinkage and the electrical conductivity of MWrGO can achieve as high as 25.6 S/m after chemical reduction in hydrazine vapour. The changes of electrical conductivity were measured under different

Acknowledgements

The authors acknowledge Deakin University for providing the PhD scholarships. We thank Dr. David Fox (CSIRO) and Dr. Xiangping Sun (Deakin) for developing the software to automatically measure the cyclic tensile strength and conductivity). We also thank Dr. Jane Dai, and Gayathri Devi Rajmohan for providing the SourceMeter. We thank Dr. Anthony Somers (Burwood Campus, Deakin University, Australia) for the thickness measurement, Dr. Mark Nave and Dr. Andrew Sullivan for discussion about SEM

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