Fabrication of PDA@SiO₂@rGO/PDMS dielectric elastomer composites with good electromechanical properties

Highlights

• Adding PDA@SiO₂@GO to PDMS, the leakage loss of conductive flow was reduced and its breakdown strength was improved.

• PDA@SiO₂@rGO/PDMS-5% shows a large actuated strain of 14.23% at a low electric field of 33.19 kV/mm without any pre-strain.

• We developed a new strategy to prepare high-performance PDMS dielectric elastomer composites with large actuated strain.

Abstract

Since dielectric elastomers (DEs) can undergo muscle-like actuation under external electric fields, they show promising applications in fields such as actuators, generators, and biomimetic robots. However, traditional DEs require strong electric fields and pre-stretching to deliver useful deformation, limiting their practical applications. Herein, a novel method for preparing polydimethylsiloxane (PDMS) composites with high actuation under low electric field without pre-stretching is reported. In this method, polydopamine (PDA) is used to modify sandwich-structured silicon dioxide@graphite oxide hybrids (PDA@SiO₂@GO). The inclusion of the SiO₂ layer improves the dielectric constant and reduces the dielectric loss of the composite, while the PDA shell improves the dispersibility of the filler. The PDA@SiO₂@rGO/PDMS composite containing 5 wt% PDA@SiO₂@GO exhibits a large actuated strain of 14.23% under an electric field of 33.19 kV/mm without pre-stretching; this value is 465% larger than that of pure PDMS and superior to those of most reported DEs composites. The results provide a promising new route to prepare high-performance DEs.

Introduction

Dielectric elastomers (DEs) are a new kind of smart material that can produce large strain under the action of an electric field, thereby converting electric energy into mechanical energy [[1], [2], [3]]. DEs have shown promising applications in many fields, including actuators, generators, and biomimetic robots. Under the influence of external voltage, DEs compress in the thickness dimension and expand horizontally; the film then returns to its original shape after the electric field is removed [4]. At low deformation (<10%), the thickness strain of a DE film S_z can be predicted by S_z =  − P/Y =  − ε_rε₀E²/Y, where P is the Maxwell's stress on the DE film, ε₀ is the permittivity of free space (8.854 × 10⁻¹² F/m), ε_r is the relative permittivity, E is the electric field applied to the DE film, and Y is the elastic modulus of the DE film [5]. Therefore, the dielectric constant, elastic modulus, and field strength are the factors that directly affect the DE. Due to the low dielectric constant of the DEs, a high driving voltage is required to obtain a large actuated strain. In addition to creating challenges related to instrumentation/equipment, this high voltage also presents a significant safety hazard, seriously restricting the application of DEs in many fields. Therefore, numerous studies have focused on developing DEs with large actuated strain under low electric field [6].

In recent years, the DE driving voltage has been reduced by pre-stretching, reducing the elastic modulus, and increasing the dielectric constant. The actuated strain of DE can be effectively improved by pre-stretching, particularly for the VHB 4905 and VHB 4910 films produced by 3 M Company [7,8]. However, the application of pre-stretching requires supporting structures and auxiliary devices that weigh much more than the films themselves, which is not conducive to the practical application of DEs [9]. Therefore, to increase the actuated strain of DEs, the two other strategies are more commonly pursued: reducing the elastic modulus and increasing the dielectric constant.

Among matrix materials for DEs, polydimethylsiloxane (PDMS) is advantageous because of its relatively low elastic modulus, high speed of response, excellent thermal stability, and good biocompatibility [[10], [11], [12], [13], [14]]. However, the ε_r of PDMS is relatively low (~2.5 at 1 kHz), limiting its use as DE material [[15], [16], [17]]. Two strategies have been developed to improve the dielectric constant of PDMS in recent years [18,19]. One such method is blending PDMS with a highly polarizable ceramic such as lead magnesium niobate (PMN) [20], barium titanate (BaTiO₃) [[21], [22], [23], [24]], titanium dioxide (TiO₂) [25], and calcium copper titanate [26,27]. While this method can increase the permittivity of PDMS with a high content of filler, in most cases, the elastic modulus is also increased significantly, and the mechanical properties are negatively affected [28]. To overcome this shortcoming, plasticizers have been added to PDMS to reduce the elastic modulus of the DE. For example, Zhao et al. [21] introduced dimethyl sulfoxide (DMSO) to BaTiO₃/PDMS composites, resulting in a 350% increase in the actuation strain of the BaTiO₃/PDMS composite when the DMSO content was 100 phr. However, the addition of plasticizers decreases the breakdown strength of the material. Furthermore, the plasticizers can volatilize and migrate during the usage of the DE, thereby limiting its application.

The other method to improve the ε_r of PDMS is the addition of conductive fillers into the matrix to form conductive filler/polymer composites, such as metallic particles/PDMS, carbon nanotubes/PDMS [29], and graphene sheets/PDMS [[30], [31], [32]]. These conductive filler/polymer composites often exhibit a high dielectric constant at a low load of conductive particles based on the penetration effect; thus, the modulus is not significantly increased. For instance, Tian et al. [33] prepared thermally expanded graphene nanoplates and added them into PDMS elastomer. The resulting composite exhibited a higher actuated strain than pure PDMS for its high dielectric constant. However, conductive filler/polymer composite elastomers often exhibit high dielectric loss and are easily broken down by electricity when the filler content approaches the percolation threshold, thereby limiting their application.

In this study, to address the disadvantages of conductive filler/polymer composite elastomers, a new polydopamine/silicon dioxide/graphite oxide nanohybrid (PDA@SiO₂@GO) was designed through interfacial engineering. As shown in Fig. 1, the GO sheet was modified with SiO₂ to reduce dielectric loss and improve the breakdown strength; the dense SiO₂ layer grafted on the surface of GO can effectively prevent direct contact with the GO sheet and thus suppress the leakage current. Furthermore, SiO₂@GO was modified with PDA to improve the dispersion of the filler in the PDMS matrix. Upon introducing 5 wt% of PDA@SiO₂@GO into the PDMS elastomer, a large actuated strain of 14.23% at a relatively low electric field of 33.19 kV/mm without any pre-stretching was obtained. This actuated strain is 465% larger than that of pure PDMS. The results provide a simple and effective new strategy to prepare high-performance DEs.