Actuator Performance of Dielectric Elastomers Comprising Hydrogenated Carboxylated Acrylonitrile-Butadiene Rubber/Nitrile Group-Modified Titanium Oxide Particles.

We prepared a dielectric elastomer actuator composed of hydrogenated carboxylated acrylonitrile-butadiene rubber (HXNBR)/nitrile group (CN)-modified and non-modified titanium oxide (TiO2) particles with insulation properties. The CN group-containing silane coupling agent was synthesized via a thiol-ene reaction between acrylonitrile and 3-mercaptpropyltrimethoxysilane and immobilized onto the TiO2 particle surface. The HXNBR/CN-modified and non-modified TiO2 particle composite elastomer showed a high relative dielectric constant and generated stress in a low electric field. The relative dielectric constant increased proportionally with the amount of CN-modified TiO2 particles, showing a value of 22 at 100 Hz. As the dielectric constant increased, the volumetric resistivity decreased; however, the dielectric breakdown strength was maintained at 95 V/mm. The generated stress of the composite elastomer increased in proportion to the relative dielectric constant, showing a maximum of 1.9 MPa. The card-house structure of TiO2 particles in the composite elastomer is assumed to suppress the dielectric breakdown in a low electric field. Thus, we demonstrated that an elastomer containing a high dipole group on an insulating particle surface is capable of improving the power performance of soft actuators.

Actuator Performance of a Hydrogenated Carboxylated Acrylonitrile-Butadiene Rubber/Silica-Coated BaTiO3 Dielectric Elastomer.

We synthesized silica-coated barium titanate (BaTiO3) particles with different silica shell thicknesses and evaluated the effect of silica coating on the relative dielectric properties of silica-coated BaTiO3 particles. Furthermore, composite elastomers were prepared using hydrogenated carboxylated acrylonitrile-butadiene rubber (HXNBR) with a high relative dielectric constant (εr) and silica-coated BaTiO3 particles, and their performance as an actuator was evaluated. Both εr and relative dielectric loss of non-coated BaTiO3 particles increased at low frequencies (<200 Hz) associated with ionic conduction. However, εr and relative dielectric loss were reduced for the silica-coated BaTiO3 particles with thick silica shells, indicating that silica coating reduced ion migration. The dielectric breakdown strength increased with the thickness of the silica shell; it increased up to 80 V/µm for HXNBR/silica-coated BaTiO3 particles with 20 nm-thick silica shells. The maximum generated stress, strain, and output energy density of the composite elastomer with HXNBR (with a high relative constant) and silica-coated BaTiO3 were 1.0 MPa, 7.7%, and 19.4 kJ/m3, respectively. In contrast, the values of the same parameters for a reference elastomer (acrylic/BaTiO3; with low εr) were 0.4 MPa, 6.7%, and 6.8 kJ/m3 at the dielectric breakdown strength of 70 V/µm. The results indicated that the elastomers composed of HXNBR and silica-coated BaTiO3 exhibited higher generated stress, strain, and output energy density than elastomers for conventional dielectric actuators.

Relationship between the Relative Dielectric Constant and the Monomer Sequence of Acrylonitrile in Rubber.

Acrylonitrile-butadiene rubbers (NBRs) have a lower glass transition temperature (T g) and a higher dielectric constant than other rubbers. To understand how a low T g and a high dielectric constant are compatible, we focused on the acrylonitrile (AN) monomer sequence in rubber and synthesized random and alternating copolymers to evaluate the effect of the sequence. The AN monomer sequence dependence of the relative dielectric constant was investigated by the C-N stretching vibration of the nitrile group through Fourier transform infrared spectroscopy and internal rotation potential energy measurements around the C-C bond within the nitrile group based on dimer model calculations. The alternating copolymers, including NBR, showed a higher dielectric constant than random copolymers. The alternating copolymer shifted from ∼2243 cm-1 for polyAN to ∼2236 cm-1 for NBRs, while the random copolymer only shifted to ∼2239 cm-1. The peak of the C-N stretching vibration was correlated with the AN sequence. The sequence dependence of the shift can be explained by the C-N bond length calculation. The internal rotation potential energy between gauche and trans of the NBR model was the lowest, indicating that the NBR main chain is flexible and that AN in the main chain rotates easily. Therefore, NBR has a high dielectric constant and a low T g because of the presence of an alternating sequence and the flexibility of the NBR main chain.