Micro-Structure Design Of Carbon Based Dielectric Elastomer Composites And Their Electro-Mechnical Properties

Dielectric elastomers (DEs), as an attractive branch of electro-active polymers (EAPs), produce deformation and Maxwell stress by applying an electric field and thus convert electrical energy into mechanical energy. DEs have good performance such as large active strain, fast response, high energy density, flexibility, and lightweight. Thus, DEs have drew much attention in the past twenty years and find applications in various devices, such as artificial muscle, micro-robot, and prosthetic device. Meeting the requirement of lightweight, flexibility and miniaturization for the design of future device, dielectric elastomer actuator (DEA) is becoming a new developing direction for microdriving technique. However, DEs can only be actuated by a high electric field (50-100 kV-mm-1) due to the low dielectric constant (?'). which reduces use safety, lowers design flexibility, and limits application area. Thus, the preparation of DEs with high ?' and large actuated strain at low electric field is top priority for DEAs to develop into a reliable and applicable microdriving technique. In the other hand, the present fundamental mechanisms of DEs are based on ideal elastomers, which not consider the influence of dielectric loss and viscosity loss. The correlation research between the primary properties of DEs and electro-mechanical properties is far from satisfaction. In this research, we prepared DE composites filled with nano carbon fillers (graphene and carbon nanotube), which could increase ?' effectively based on percolation theory. But composites'dielectric loss (tan?) and modulus also increase greatly, limits electro-mechanical properties. Tn addition, the surface properties, dispersion and interfacial adhesion of fillers are important for dielectric properties. In our research, we studied the influence of surface properties of fillers, dispersion and interfacial adhesion as well as inorganic-organic synergism on dielectric, mechnical and electro-mechanical properties. The relationship between the primary properties (including modulus, viscoelasticity and dielectric properties) and the electro-mechanical properties of DEs was also studied to guide the development and practical application of DEs. Creative work and results include the followings:(1) Thermally reduced graphene oxide (TRG)/thermoplastic polyurethanes (TPU) DE with high ?',low tan? and greatly improved actuated strain at low electric field was prepared by partial reduction of graphene oxide and the disruption of hydrogen bonds in TPU. The results showed that a good dispersion and alignment of TRG in the TPU matrix was obtained because of the assembly of hydrogen bonds between TRG and TPU. The ?' at 1000 Hz was sharply increased from 7 for pure TPU to 1875 for the composite with 2 vol.% of TRG because of the partial restoration of ?-? electronic structure of TRG and the increased dipole polarizability of TPU caused by the disruption of hydrogen bonds of TPU chains. The tan? remained low (0.43 at 1000 Hz for 2 vol.% TRG/TPU composite) ascribed to the coating of TPU on TRG, as the coating can avoid direct connect of TRG with one another and suppress leakage current. Despite of the increase in elastic modulus with the increase in the content of TRG, the great increase in ?'lead to the great increase in electromechanical sensitivity(?). As a result, a 106 times increase in ? at 1000 Hz was achieved by adding 2.0 vol.% of TRG Moreover, the influence of tan? and hysteresis loss on actuated strain was studied. Losses lowered energy efficiency, but the effective ? increased 20 times by adding 2.0 vol.% of TRG, and thus the actuated strain increased 17 times at low electric field (250 V-mm-1).(2) TPU/polyethylene glycol (PEG)/reduced graphene oxide (rGO) DE with high ?', low elastic modulus and greatly improved actuated strain at low electric field was prepared by inorganic-organic synergism of rGO and PEG. The results showed that a good dispersion of rGO and a dispersion at the molecular scale of PEG in the TPU matrix was obtained because of the assembly of hydrogen bonds between rGO/PEG and TPU. The e'increases with the increasing content of PEG and rGO because of the partial restoration of ?-? electronic structure of TRG and the increased dipole polarizability of TPU caused by the disruption of hydrogen bonds of TPU chains. Meanwhile, the elastic modulus decreased greatly because of the plasticization of PEG A 4 times decrease in modulus was achieved for the composite with 30 phr (parts per hundreds of rubber) of PEG and 1.5 phr of rGO (denoted by TPU-30-1.5) compared with pure TPU. As a result, the effective ? at 1000 Hz increased 7 times and the actuated strain increased 5 times at a low electric field (5 kV·mm-1) for TPU-30-1.5.(3) The preparation of TRG and rGO is complicated, which not only introduces toxic or corrosive chemicals but also results in numerous structural defects and unstable performance. DE composites with high performance were prepared by aligned carbon nanotubes (CNTs) stabilized liquid phase exfoliated graphene hybrid. The liquid phase exfoliation (LPE) method, as a cheap, easily scalable, and eco-friendly method, was used to produce defect-free, unoxidized graphene. A surfactant-free technique was used to concentrate the graphene dispersion through stabilization by CNTs without functionalization to increase the solution concentration and thus experimental feasibility. A special kind of aligned CNT bundles, which can be well dissociated into single CNT in N-methyl-pyrrolidone by sonication, was used and fabricate Gr-CNT hybrid by ?-? interaction. A redispersed stable Gr-CNT dispersion at a concentration of 2 mg·mL-1 was used for preparing Gr-CNT/TPU dielectric composite. The conductivity of Gr-CNT hybrid is 1900±5S·m-1. The results show that the addition of 3.0 vol.% of hybrid improves the dielectric and electro-mechanical properties of the TPU greatly. As a result,? at 1000 Hz increased 10 times and actuated strain increased 3.4 times at a low electric field (7.5 kV-mm-1). The breakdown strengths of 0.25 vol.% and 1.0 vol.% Gr-CNT/TPU composites are much higher than that of the pure TPU because of the more homogeneous dispersion of electric charge caused by reduced number of loose TPU chains and the disruption of the phase separation of TPU as a result of the strong interfacial interaction between the Gr-CNT hybrid and TPU. As a result, the maximum actuated strain increases greatly from 4.8% for the pure TPU to 7% for the composite with 0.25 vol.% of the hybrid. Meanwhile, the energy density increases from 18 kJ·L-1 for the pure TPU to 25 kJ·L-1 and 48 kJ·L-1 for the composites with 0.25 and 1.0 vol.% of hybrid, respectively.(4) We prepared hydrogenated butadiene-acrylonitrile (HNBR) elastomer composites with random orientation of CNTs and aligned CNTs, denoted by random composites and aligned composites, respectively, by using a simple mechanical blending method. The influence of CNT alignment and filler content on dielectric properties was studied. The relationship between the microstructure and dielectric properties was qualitatively analyzed by using the percolation theory and intercluster polarization model. The percolation threshold of aligned and random composites is 0.65 and 0.7 vol.%, respectively. A 3D CNT network in random composites and 2D CNT network in aligned composites were obtained as filler content higher than percolation threshold. Interestingly, at CNT contents of 1-2.5 vol.%, the tan? of the aligned composites increases slightly and the ?' of aligned composites increases largely with increasing content of CNTs, whereas both the tan? and the ?' of the random composites increase largely with increasing content of CNTs. As a result, a high ?'(5000 at 1000 Hz) and a low tan? (0.42 at 1000 Hz) were obtained in the aligned composite with a CNT content of 2.5 vol.%, whereas a high ?' and a high tan? were obtained in the random composites. The mechanism is that the CNTs are well aligned in the HNBR matrix and are isolated by the HNBR matrix, forming a large number of CNT-HNBR-CNT microcapacitors in the aligned composites, resulting in a high ?'. Meanwhile, there are less conductive paths in the aligned composites than in the random composites because of the formation of 2D networks, resulting in a lower conductivity and thus lower tan?. Importantly, a higher ? were obtained in the random composites, but the higher tan? and hysteresis loss of the random composites lowers the energy efficiency and effective?. As a result, a higher effective ? and actuated strain was obtained in the aligned composites.