Soft, tough, and fast polyacrylate dielectric elastomer for non-magnetic motor

Electromagnetic motors, while crucial to the Second Industrial Revolution, have limitations in terms of specific power, size, rigidity, and control complexity. These limitations hinder their use in applications requiring lightweight, flexible, and easily controlled soft micromotors. Such micromotors find use in flexible electronics, soft robotics, biomedical implants, and aerospace. Electroactive polymers (EAPs), particularly dielectric elastomer actuators (DEAs), offer potential due to their large strain and flexibility. Various commercial elastomers, including silicone rubbers, polyurethanes, and acrylic elastomers (like VHBTM 4910), have been used in DEAs. Acrylic-based elastomers are particularly attractive for non-magnetic motors because of their high dielectric constant, large area strain, and high energy density. However, commercial acrylic elastomers like VHBTM 4910 suffer from high stiffness, slow response speed, viscoelastic creep, and high mechanical loss, requiring high driving electric fields. Many attempts have been made to improve these properties, such as incorporating high-dielectric-constant fillers or adding plasticizers. However, these methods often compromise other desirable properties. This research aims to address these challenges by designing and synthesizing a novel polyacrylate dielectric elastomer with significantly improved properties through optimization of its crosslinking network.

Previous research has explored various strategies to enhance the performance of acrylic-based dielectric elastomers. The use of fillers to increase the dielectric constant has been investigated, but high filler concentrations often lead to increased stiffness and reduced electrical breakdown strength. The addition of plasticizers to lower the Young's modulus has also been attempted, but this can lead to plasticizer leakage. Efforts to improve response speed by reducing hysteresis often result in a decrease in the dielectric constant. These limitations highlight the need for a holistic approach that simultaneously optimizes multiple properties rather than focusing on individual improvements. This study builds upon the existing literature by proposing a novel approach based on crosslinking network optimization to address the inherent trade-offs between softness, toughness, dielectric properties, and response speed in acrylic-based elastomers.

This study introduces a novel polyacrylate dielectric elastomer synthesized by optimizing the crosslinking network. Instead of conventional small-molecular crosslinkers, a high-molecular-weight urethane acrylate compound was used. This compound contains flexible polyether diol and aliphatic diisocyanate segments. *n*-butyl acrylate (nBA) served as the monomer. The macromolecular crosslinker acts as a lubricant, reducing dipole-dipole interactions and resulting in lower modulus and mechanical loss. Matching the average molecular weight (Mc) between crosslinking points helps to eliminate stress concentration and improve toughness. The presence of uncrosslinked chains enhances the dielectric constant. Several elastomer samples were prepared with different crosslinkers (including small-molecule crosslinkers for comparison). The crosslinking network structure, mechanical properties, dielectric properties, and actuation performance of the resulting elastomers were characterized. The mechanical properties were evaluated using stress-strain curves, including Young's modulus and toughness. Dielectric properties were measured using broadband dielectric spectroscopy, determining the dielectric constant and dissipation factor (tan δ). The actuation performance was assessed using a custom-designed experimental apparatus, including static actuation (area strain and energy density), dynamic response (response time and frequency response), and cyclic actuation (durability). Finally, a rotational motor was fabricated to demonstrate the superior performance of the optimized elastomer. The motor's rotation speed, torque, and power were measured and compared to a motor made with VHBTM 4910.

The optimized polyacrylate dielectric elastomer (BAC2) exhibits significantly improved properties compared to commercial VHBTM 4910 and other synthesized samples. BAC2 possesses a low Young's modulus (~0.073 MPa), high toughness (elongation ~2400%, toughness 6.77 MJ m⁻³), low mechanical loss (tan δm = 0.21@1 Hz, 20 °C), and excellent dielectric properties (εr = 5.75, tan δε = 0.0019 @1 kHz). The high dielectric constant and low Young's modulus contribute to its high actuation sensitivity (78.8, 3.75 times higher than VHBTM 4910). BAC2 demonstrates a large actuation strain (118% @ 70 MV m⁻¹ with pre-strain) and high energy density (0.24 MJ m⁻³ @ 70 MV m⁻¹). The response time is significantly faster than VHBTM 4910 (35.2 s vs. 327.6 s to reach 90% of final strain). The frequency response of BAC2 is flat up to 100 Hz, much higher than VHBTM 4910. A non-magnetic motor fabricated using BAC2 exhibited a 15-fold increase in rotation speed (0.72 r s⁻¹ at 48 MV m⁻¹) compared to a VHBTM 4910-based motor under the same conditions. Further, the output torque and power of the BAC2 based motor are 6 and 18 times higher, respectively, than that of the VHBTM 4910 based motor. The superior performance of BAC2 is attributed to its optimized crosslinking network, the flexible long-chain structure of the crosslinking agent, and the presence of uncrosslinked chains within the network, which enhance the dielectric properties and reduce mechanical loss.

The findings demonstrate the effectiveness of optimizing the crosslinking network to simultaneously enhance the softness, toughness, dielectric properties, and response speed of polyacrylate dielectric elastomers. The improved actuation sensitivity, large actuation strain, high energy density, and fast response speed of BAC2 address the key limitations of existing commercial elastomers, making it a highly promising material for soft actuators. The improved performance of the non-magnetic motor fabricated using BAC2 highlights the potential of this material for applications requiring high speed and efficiency. The strategy used here, focusing on the rational design of the crosslinking network, provides a novel approach for developing high-performance dielectric elastomers with tailored properties.

This study presents a novel polyacrylate dielectric elastomer (BAC2) with superior mechanical, dielectric, and actuation properties compared to existing commercial materials. The optimized crosslinking network results in a soft, tough, and fast elastomer suitable for high-performance soft actuators. The significant improvement in the rotation speed and output power of a non-magnetic motor fabricated with BAC2 demonstrates the practical implications of this work. Future research could explore further optimization of the crosslinking network, investigation of other macromolecular crosslinkers, and the integration of this elastomer into various soft robotic and electromechanical devices.

The current study focuses on a specific type of polyacrylate elastomer and crosslinking agent. The generalizability of these findings to other elastomer systems needs further investigation. The maximum achievable rotation speed of the motor might be limited by the power supply and the experimental setup rather than the inherent properties of the elastomer. Further optimization of the motor design and the use of higher-power amplifiers could potentially lead to even higher rotation speeds.