Recently, soft actuators made of various types of electroactive polymers (EAPs) with electric-field-induced deformation have received considerable attention in soft robotics, biomedical implantation, and aerospace due to light weight, high flexibility, and easy control. Different from the actuators with ferroelectric polymers, dielectric elastomer actuator (DEAs), an electrically stimulus-responsive actuator, show great potential for the large strain and superior flexibility. A large variety of commercial elastomers, including silicone rubbers, polyurethanes, and acrylic elastomers (e.g., VHB^TM 4910 from 3M), have been widely used to assemble DEAs. Among them, acrylic-based elastomers outperform others for composing non-magnetic motors because of their high dielectric constant (~4.4@1 kHz), large area strain (> 380%), and high energy density (3.4 MJ m^-3). However, commercial VHB^TM acrylic elastomers usually require high driving electric field (E) (> 80 MV m^-1) due to their inherently high stiffness (Young’s modulus around 0.2~1.0 MPa), and exhibit slow response speed (bandwidth usually below 10 Hz) and severe viscoelastic creep, and high mechanical loss due to viscoelastic character (tan δ_m~0.5).

Numerous efforts have been made to reduce the driving E and increase the response speed of acrylic-based elastomer. According to the Maxwell stress, the actuation strain (S_z) in the thickness direction is proportional to the dielectric constant of elastomer (ε_r), the square of applied electric field (E) and the reciprocal of Young’s modulus of elastomer (Y), respectively. Thus, the driving E for a certain actuation strain can be reduced by enhancing the actuation sensitivity (the ratio of  ε_r to Y), which is usually achieved by either increasing the ε_r or decreasing the Y of the elastomer. As for the response speed, it can be improved by decreasing the hysteresis of elastomer stemming from an amount of intermolecular interaction. However, most strategies with an effort of contributing to the improvement of one performance (i.e., low modulus, high dielectric constant, low mechanical loss) for acrylic elastomers, may cause the degradation of the others. The design and preparation of high-quality dielectric elastomers remains a challenging problem for the mass applications of DEAs.

Here, we have demonstrated an innovative strategy to improve actuation performance of dielectric elastomers by optimizing crosslinking network. These performances could be mainly attributed to the flexible long chain structure of crosslinking agent and the existence of dissociative chains inside the network. This strategy readily enables elastomers with extraordinary soft (~0.073 MPa), ultra-high toughness (elongation ~2400%), low mechanical loss (tan δ_m=0.21@1 Hz, 20 ℃), satisfactory dielectric properties (ε_r=5.75, tan δ_e=0.0019 @1 kHz), large actuation and fast response (>100 Hz). Large actuation and high energy density under low electric field is promising for the development of soft actuators in low-voltage driving fields. The principle for the selection of elastomer component and strategy employed here provides a different insight for high-performance dielectric elastomers and actuators.

Figure 1 Performance of uniform hybrid polymer network.

The crosslinked network structure of elastomer has a significant influence on the intrinsic mechanical and electrical properties of dielectric elastomer, accordingly affecting its actuation behavior achieved by the conversion of electrical to mechanical energy. Here, we innovatively develop a strategy for the optimization of crosslinked polymer network through modulating the average molecular weight (dimension) of crosslinking agents. When the average molecular weight of crosslinking agents matches that of crosslinking points, our as-synthesized elastomer exhibits desirable mechanical and electrical properties:

• Low modulus yet high toughness. Generally, it is a huge challenge to simultaneously improve the softness and toughness of elastomer and such a paradox is solved in this contribution through the optimization of crosslinked network. The BAC2 sample not only exhibits a low Young’s modulus (0.071 MPa) and a high elongation but also displays a high toughness (6.77 MJ m^-3) and an ultrahigh ultimate strength (32.2 MPa), which, to the best of our knowledge, surpasses that of most advanced dielectric elastomers.
• High actuation strain. Due to low elastic modulus and high dielectric constant, the actuation sensitivity of BAC2 elastomer film can reach up to 78.8, which, consequently, brings in large actuation strain under low electric fields (18.5% at 15 MV m^-1 without pre-strain) and makes non-magnetic motor rotate under 32 MV m^-1.
• Fast response. Owe to suppressed mechanical loss and creep, frequency response of BAC2 is nearly flat in the 1-100 Hz frequency range. As a result, the maximum rotation rate of BAC2 based motor is 0.72 r s^-1, which is 15 times larger than that of VHB^TM 4910 based motor.

For more information, please read our recent publication in Nature Communications:

L.J.Yin, Y. Zhao, J.Zhu, et al.  Soft, tough, and fast polyacrylate dielectric elastomer for non-magnetic motor. Nature Communications (2021), 12, 4517.

 https://doi.org/10.1038/s41467-021-24851-w