‘Bottlebrush’ Silicone Elastomer Changes Shape and Size when Exposed to Small Electric Field

Electrical response of a circular diaphragm composed of a pure bottlebrush elastomer upon electroactuation with increasing voltage and without any external pre-strain. The numbers indicate the electric field-induced area expansion under constant-volume conditions at room temperature. (Credit: NORTH CAROLINA STATE UNIVERSITY)

A new electroactive polymer material capable of changing size and shape when exposed to a comparatively small electric field has been developed by a multi-institutional research team.

This new material overcomes two longstanding challenges dealing with the use of electroactive polymers to produce new devices, making room for a variety of applications ranging from microrobotics to designer haptic, microfluidic, optic and wearable technologies.

The research was carried out by researchers at North Carolina State University, the University of North Carolina at Chapel Hill, Carnegie Mellon University and the University of Akron

Dielectric elastomers are the most responsive electroactive polymers in terms of achievable strains, but two big hurdles have effectively prevented the smart materials community from using them in commercial devices. First, previous dielectric elastomers required large electric fields in order to trigger actuation, or movement -- on the order of at least 100 kilovolts per millimeter (kV/mm). With our new material, we can see actuation at levels as low as ca. 10 kV/mm.

The second challenge is that, previously, materials had to be pre-strained. This would either mean using a frame to physically strain the material, or adding a second component to the polymer to retain the strain after it was applied. But our material consists of a single component that is specifically designed at the molecular level to inherently possess pre-strain. In other words, we don't need a frame or a second component - our material is ready to be used as soon as it is cross-linked into a specific shape.

Richard J. Spontak, Professor, NC State

A "bottlebrush" silicone elastomer is the new material that has played a vital role in permitting this breakthrough. This new material has been engineered to comprise of these unique properties, and manufacturing it is not a difficult task.

"We are working specifically with bottlebrush polymers, which are prepared by grafting long polymeric side chains to a polymer backbone," says Sergei S. Sheiko, George A. Bush, Jr. Distinguished Professor of Chemistry at UNC and corresponding author of the paper. "The resulting molecules may be viewed as filaments that are thick, yet remain quite flexible, which allows for significant reduction of the materials' rigidity and makes them more stretchable. Furthermore, the mechanical properties can be controlled by varying the bottlebrush architecture - for example, by preparing molecules with different degrees of polymerization of grafted chains and different grafting densities.

"This architectural control of mechanical properties has reduced the limit of stiffness in dry polymer materials by 1,000 times, demonstrated extensibility of up to eight times, and opened up new applications not available to stiffer materials or materials with liquid fractions," Sheiko says. "One of these applications -- their use as free-standing dielectric elastomers - has been demonstrated, which we discuss in this paper."

"We're at the earliest stages of identifying all the potential ways in which we could use this new class of material," Spontak says. "It works better than anticipated, and now we're beginning to consider potential applications."

Advanced Materials features the paper, “Bottlebrush Elastomers: A New Platform for Freestanding Electroactuation.” Lead author of the paper is Mohammad Vatankhah-Varnoosfaderani, a postdoctoral researcher at UNC. Co-authors of the paper include William F. M. Daniel, Alexandr P. Zhushma, Qiaoxi Li and Benjamin J. Morgan of UNC; Daniel P. Armstrong of NC State; Krzysztof Matyjaszewski of Carnegie Mellon; and Andrey V. Dobrynin of the University of Akron. The research received support from the National Science Foundation under grants DMR 1122483, DMR 1407645, DMR 1436201 and DMR 1409710, and from Becton Dickinson Technologies.

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