On-Demand Cross-Linkable Bottlebrush Polymers for Voltage-Driven Artificial Muscles

Dielectric elastomer actuators (DEAs) generate motion resembling natural muscles in reliability, adaptability, elongation, and frequency of operation. They are highly attractive in implantable soft robots or artificial organs. However, many applications of such devices are hindered by the high driving voltage required for operation, which exceeds the safety threshold for the human body. Although the driving voltage can be reduced by decreasing the thickness and the elastic modulus, soft materials are prone to electromechanical instability (EMI), which causes dielectric breakdown. The elastomers made by cross-linking bottlebrush polymers are promising for achieving DEAs that suppress EMI. In previous work, they were chemically cross-linked using an in situ free-radical UV-induced polymerization, which is oxygen-sensitive and does not allow the formation of thin films. Therefore, the respective actuators were operated at voltages above 4000 V. Herein, macromonomers that can be polymerized by ring-opening metathesis polymerization and subsequently cross-linked via a UV-induced thiol–ene click reaction are developed. They allow us to fast cross-link defect-free thin films with a thickness below 100 μm. The dielectric films give up to 12% lateral actuation at 1000 V and survive more than 10,000 cycles at frequencies up to 10 Hz. The easy and efficient preparation approach of the defect-free thin films under air provides easy accessibility to bottlebrush polymeric materials for future research. Additionally, the desired properties, actuation under low voltage, and long lifetime revealed the potential of the developed materials in soft robotic implantable devices. Furthermore, the C–C double bonds in the polymer backbone allow for chemical modification with polar groups and increase the materials’ dielectric permittivity to a value of 5.5, which is the highest value of dielectric permittivity for a cross-linked bottlebrush polymer


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The tensile tests were performed using a Zwick Z010 tensile test machine with a cross-head speed of 50 mm min −1 . Tensile test specimens with a gauge width of 2 mm and a gauge length of 18 mm were prepared by die-cutting. The strain was determined using a longitudinal strain extensometer. The curves were averaged from different independent experiments. The Young's modulus at 10% strain was determined from the slope of the stress−strain curve using a linear fit to the data points from 0 to 10% strain, while for the Young's moduli at 50 and 100%, a linear fit to the data points from 40 to 50% and from 90 to 100% strain, respectively, was used.
Dynamic mechanical analysis was carried out on a RSA 3 DMA from TA Instruments. Stripes of 10 mm × 20 mm were measured under a dynamic load of 2 g, at 2% strain in the frequency range of 0.1−10 Hz at 25 °C. The mechanical loss factor tan (δ) is given as the fraction of imaginary and real storage modulus at 2% strain.
Dielectric measurements were performed in the frequency range from 1 to 10 6 Hz using a Novocontrol Alpha-A frequency analyzer. The VRMS (root mean square voltage) of the probing ac electric signal applied to the samples was 1 V. The permittivity ɛ' was determined from the capacitance C = ɛ'ɛ 0 A/d, where A is the electrode area, d is the thickness of the film, and ε 0 is the vacuum permittivity. The thickness of the film was measured by a micrometer gauge with an uncertainty of ±5 μm. The samples were prepared by placing the film between two stainless steel discs with a diameter of 20 mm. Before measurement, the samples were annealed at 80 °C in a vacuum oven.
Electromechanical tests were performed using circular membrane actuators at ambient temperature and humidity. Before crosslinking the bottlebrush polymer, a substrate was made by putting and casting PVA with a thickness of 400 μm on a glass substrate. After PVA solidified in 1 hour, the mixture of bottlebrush polymer was cast on the PVA substrate and cross-linked. Then the films together with the PVA substrate were fixed between two circular rigid frames with an inner diameter of 25 mm. To remove the PVA substrate, the fixed film and circular rigid frames were put into de-ionized warm water with 60 °C. The water was constantly changed every hour. After about 6 h, the fixed film and circular rigid frames were put into vacuum oven with 60 °C to remove residual solvent and water. Circular electrodes (8 mm diameter) of carbon black powder was applied to each side of the film. A FUG HCL-35-12,500 high voltage source served as a power supply for actuator tests. We gradually increase the voltage by 100 V and up to 1700 V. The actuation strain was measured optically as the extension of the diameter of the electrode area via a digital camera, using an edge detection tool of a LabView program to detect the boundary between the black electrode area and the transparent film.
3 Figure S1. 1 H NMR spectra of mono-hydride and mono-hydroxyl terminated polydimethylsiloxane (top) and 13 C NMR spectrum of alcohol terminated PDMS (bottom) in CDCl 3 .    11 Figure S8. IR spectra of the materials made from bottlebrush polymer mix-P (a), and exo-P (b). Figure S9. Stress-strain curves of materials mix-E n (a to c), exo-E n (d to g), and exo-ME n ' (h-i).    and 45 mm was applied from the top, and then the entire ensemble was fixed with two clamps. The clamps created a uniform pressure on the actuator. During the measurement, the power supplier measured the voltage and a camera recorded a video of the actuator. Afterward, the actuation analysis 16 was processed by software Photoshop to measure the parameters according to the scale bar taken from the thickness of the white rigid frame. Then by software CorelDraw, the determination of the dome's geometry was accomplished using the side-profile image, which was captured by Photoshop.
Thereafter, by simulation of software Autodesk fusion 360, the actuation surfaces before and after actuation were constructed and surface areas were calculated. The original film thickness was 84 μm, therefore, considering the corresponding thickness, the breakdown field of this airbag actuator was 21.6 V/ μm.
The extent of polar group modification was determined by the ratio between integration from the vinylene group protons' signal (a) and -C(O)O-CH 2 -group's signal (g), which equaled to 16% ( Figure   S15). Figure S15. 1 H (top) and 13 C (middle) NMR spectra of polar group modified bottlebrush polymer exo-MP in CDCl 3 , and stacked 1 H NMR spectra of polar group, exo-P, and exo-MP in CDCl 3 (bottom).