A unimorph nanocomposite dielectric elastomer for large-scale actuation

Dielectric elastomer actuators (DEAs) can undergo large, reversible in-plane deformation. In a new report now published in Science Advances, Junhong Pu and a team of scientists in soft materials research and polymer science at the University of California, Los Angeles, U.S., and Sichuan University, China, introduced an electrophoretic process to concentrate boron nitride nanosheet dispersion in a dielectric elastomer precursor solution onto a selected electrode surface. The team obtained a unimorph nanocomposite dielectric elastomer abbreviated UNDE with a seamless bilayer structure containing 13 times the modulus difference. The team could actuate the UNDE construct to large bending curvatures with enhanced durability compared to conventional nanocomposite dielectric elastomers. They arranged multiple UNDE units in a simple electrophoretic concentration process using patterned electrode areas; then, by using the actuator, they developed a high-speed lens motor with variable focal length to form a two-lens optical system.

Unimorph nanocomposite dielectric elastomers (UNDE)

Dielectric elastomers are a class of electroactive polymers that can transduce electromechanical energy via an electrostatic stress mechanism, in response to an applied voltage. The materials are characterized by their large strain and high energy density and have attracted tremendous interest in the past decade for applications as artificial muscles and soft robotics. Acrylic elastomers are of interest due to the largest actuation strain that they exhibit, and the pre-stretch procedure involved during fabrication. Materials scientists aim to avoid the pre-stretch process with acrylic elastomers by introducing a second interpenetrating polymer network and chemical modification to achieve large actuation strains without pre-stretch. In this work, Pu et al. introduced an electrophoretic approach followed by in situ crosslinking to fabricate an interface-free unimorph nanocomposite dielectric elastomer made of locally concentrated boron nitride nanosheets (BNNS). The team used the electrophoretic process to produce multiple functional unimorph units in a disk-shaped monolithic DEA film through customized electrode patterning. They varied the actuation strain with applied voltage without materials degradation and the compact actuator produced large linear actuation for use as a direct-drive lens motor, for optical zoom systems.

Developing and characterizing a unimorph nanocomposite dielectric elastomer (UNDE) film via electrophoretic concentration

Pu et al. developed the UNDE (Unimorph nanocomposite dielectric elastomer) films with highly concentrated BNNS (boron nitride nanosheets) on one surface via electrophoresis. The boron nitride nanosheets are commonly used as a dielectric filler to enhance dielectric strength and can be dispersed in a dielectric elastomer to form a colloidal suspension. The team injected the dispersion between two parallel electrodes with a direct current field applied between them. Since the BNNS was negatively charged, the materials were attracted to the surface of the positive electrode. The team cured the precursor via ultraviolet exposure and formed a continuous bilayer structure. They imaged the process using a light beam passed through the cuvette chamber during electrophoresis. After developing the UNDE structure, Pu et al. used scanning electron microscopy images to observe the characteristics of conventional nanocomposite dielectric elastomers as a control material, and the UNDE with 3 percent BNNS in its composition. The team noted smaller bending curvatures for the UNDE film with BNNS concentrated on the top layer, compared to the bottom layer. The work indicated higher stiffness for BNNS-concentrated layers when compared to those with depleted concentrations. The scientists explored the optimized materials to obtain binding actuation by applying a high voltage across the UNDE film. After applying an electric field, the two layers of the construct experienced uniform compressive strains under cycles of bending and recovery.

Actuation of UNDE (unimorph nanocomposite dielectric elastomer) actuators

The research team analyzed the binding actuation of 3 percent weight UNDE, shaped in a trapezoid and noted how the dielectric elastomer actuators functioned in a unidirectional binding manner relative to the applied electric fields across its thickness. For instance, at a field intensity of 28 MV/m, the team obtained a binding curvature of 4.4 cm^-1 to create an almost closed-loop structure. They noted the specific dependence of binding curvature on the electric field intensity where UNDE with higher boron nitride nanosheet concentrations required higher electric field strength to achieve the same bending curvature, on account of increased stiffness. The team fitted the actuation and recovery processes with an exponential response and credited the fast response of the bending dielectric elastomer actuators to the direct energy conversion from electricity to mechanical work. The binding curvature indicated a trade-off between large bending curvature and high operation frequency. The interface-free nature between the passive boron nitride nanosheets and the active dielectric elastomer layer offered the UNDE actuators nondestructive binding performance after 180 degrees folding.

Applications of the dielectric elastomer actuators

The team adopted the UNDE fabrication process to form multiple individually accessible unimorphs within a monolithic film. Pu et al. used disk-shaped dielectric elastomer actuators as a self-contained motor lens to directly reposition an optical element and change the focal length of a compact and adaptive zoom lens system across a broad range. Using the two-lens zoom system, they increased the distance between the two lenses to decrease the distance from the system's focal length and facilitate the projection of objects from far to close working distances, onto the same plane. Compared to tunable liquid lens technology, the linear actuator-driven optical zoom system achieved greater focal length tuning capability, desirable for endoscopes, smartphone cameras, virtual reality and machine vision.

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Outlook

In this way, Junhong Pu and colleagues developed a new method to implement unimorph configurations in a monolithic dielectric elastomer film through electrophoresis. The method facilitated the concentration of boron nitride nanosheet (BNNS) nanofillers in monomers to form an interface-free unimorph nanocomposite dielectric elastomer (UNDE). The team created several UNDE units during the study by designating BNNS concentrations to surface areas. The resulting linear dielectric elastomer actuators can be optimized as promising materials for artificial robotic vision due to their customizable and scalable nature.