Ionic liquid monomers with ion pair interaction energies ranging from 73.0 to 101.4 kcal/mol were designed by using the calculated electrostatic potential (ESP) values of the component ions. The ionic liquid monomers were classified as cationic monomers with an anionic counterion and anionic monomers with a cationic counterion. We evaluated the calculated ion pair interaction energy using counterion mobility as an indicator. One component of the ion pair was fixed onto a dielectric elastomer by using alkoxysilane coupling agents, while the counterion remained free to move under the applied voltage. We then measured the relative dielectric constant at 0.01 Hz, which is an indicator of the mobility of the counterion. The results showed a good correlation between the calculated ion pair interaction energy and the relative dielectric constant. The lower the ion pair interaction energy is, the easier the dissociation of the ion pair. From this result, we were able to prove the correlation between the calculated ion pair interaction energy and the mobility of the counterion. Furthermore, classification of other ion pair compounds and polyelectrolyte polymer brushes that follow the anion Hofmeister series based on ion pair interaction energies revealed the correlation between physical properties and the ionic structure. Various ionic compounds with desired physical properties can be designed by using the calculated ion pair interaction energies.

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.

We synthesized rubbers including cyanoethyl groups and evaluated the effect of side-chain structures on dielectric properties, electrical properties at room temperature, and glass-transition temperatures (T g). New monomers including cyanoethyl were synthesized via the thiol–ene reaction and esterification; these included 3-((2,4-dicyanobutyl)­thio)­propyl acrylate (DCEA3M) and 6-((2,4-dicyanobutyl)­thio)­hexyl acrylate (DCEA6M). Copolymers consisting of 2-cyanoethyl acrylate (CEA) and DCEA3M or DCEA6M were synthesized to obtain a rubber with a high dielectric constant. Reference copolymer samples with varying CN wt % consisted of CEA and acrylate monomers were also synthesized. Undesirable gelation occurred during polymerization of monomers including cyanoethyl; however, gelation could be resolved by changing the monomer concentration and reaction time. The copolymers exhibited a T g of 5 °C or below, and the relative dielectric constant at 100 Hz was above 10. pDCEA3M exhibited the highest dielectric constant of 20.3 corresponding to a high CN wt % of 20.6%, and pDCEA6M displayed the lowest T g of −33.2 °C due to the internal plasticization corresponding to the long side chain; however, the dielectric losses of pDCEA3M and pDCEA6M were as high as 3.2 and 7.7, respectively. On the other hand, p­(CEA–DCEA3M) and p­(CEA–DCEA6M) copolymerized with CEA could reduce the dielectric loss (∼2.4) while maintaining high dielectric constant (16.3–18.9) and low T g (−22.9––3.36 °C). The copolymers also exhibited a high dielectric constant at a comparable volume resistivity (1010–1012 Ω·cm) compared to commercial rubbers. Such elastomers with high dielectric constant and low dielectric loss are expected to improve the performance of soft actuators.

We prepared a dielectric elastomer actuator composed of hydrogenated carboxylated acrylonitrile-butadiene rubber (HXNBR)/nitrile group (CN)-modified and non-modified titanium oxide (TiO 2 ) 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 TiO 2 particle surface. The HXNBR/CN-modified and non-modified TiO 2 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 TiO 2 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 TiO 2 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.

We synthesized silica-coated barium titanate (BaTiO 3 ) particles with different silica shell thicknesses and evaluated the effect of silica coating on the relative dielectric properties of silica-coated BaTiO 3 particles. Furthermore, composite elastomers were prepared using hydrogenated carboxylated acrylonitrile−butadiene rubber (HXNBR) with a high relative dielectric constant (ε r ) and silica-coated BaTiO 3 particles, and their performance as an actuator was evaluated. Both ε r and relative dielectric loss of non-coated BaTiO 3 particles increased at low frequencies (<200 Hz) associated with ionic conduction. However, ε r and relative dielectric loss were reduced for the silica-coated BaTiO 3 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 BaTiO 3 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 BaTiO 3 were 1.0 MPa, 7.7%, and 19.4 kJ/m 3 , respectively. In contrast, the values of the same parameters for a reference elastomer (acrylic/BaTiO 3 ; with low ε r ) were 0.4 MPa, 6.7%, and 6.8 kJ/m 3 at the dielectric breakdown strength of 70 V/μm. The results indicated that the elastomers composed of HXNBR and silica-coated BaTiO 3 exhibited higher generated stress, strain, and output energy density than elastomers for conventional dielectric actuators.