Electroactive polymers (EAPs) refer to a broad range of relatively soft materials that change size and/or shape upon application of an electrical stimulus. Of these, dielectric elastomers (DEs) generated from either chemically- or physically-crosslinked polymer networks afford the highest levels of electroactuation strain, thereby making this class of EAPs the leading technology for artificial-muscle applications. While mechanically prestraining elastic networks remarkably enhances DEs electroactuation, external prestrain protocols severely limit both actuator performance and device implementation due to gradual DE stress relaxation and the presence of a cumbersome load frame. These drawbacks have persisted with surprisingly minimal advances in the actuation of single-component elastomers since the dawn of the “pre-strain era” introduced by Pelrine et al. (Science, 2000). In this work, we present a bottom-up, molecular-based strategy for the design of prestrain-free (freestanding) DEs derived from covalently-crosslinked bottlebrush polymers. This architecture, wherein design factors such as crosslink density, graft density and graft length can all be independently controlled, yields inherently strained polymer networks that can be readily adapted to a variety of chemistries. To validate the use of these molecularly-tunable materials as DEs, we have synthesized a series of bottlebrush silicone elastomers in as-cast shapes. Examination of these materials reveals that they undergo giant electroactuation strains (>300%) at relatively low fields (<10 V/m), thereby outperforming all commercial DEs to date and opening new opportunities in responsive soft-material technologies (e.g., robotics). The molecular design approach to controlling (electro)mechanical developed here is independent of chemistry and permits access to an unprecedented range of actuation properties from elastomeric materials with traditionally modest electroactuation performance (e.g., polydimethylsiloxane, PDMS). Experimental results obtained here compare favorably with theoretical predictions and demonstrate that the unique behavior of these materials is a direct consequence of the molecular architecture.
Mohammad Vatankhah-Varnosfaderani, William F. M. Daniel, Alexandr P. Zhushma, Qiaoxi Li, Benjamin J. Morgan, Krzysztof Matyjaszewski, Daniel P. Armstrong, Andrey V. Dobrynin, Sergei S. Sheyko, and Richard J. Spontak, "Bottlebrush elastomers: a promising molecular engineering route to tunable, prestrain-free dielectric elastomers (Conference Presentation)," Proc. SPIE 10163, Electroactive Polymer Actuators and Devices (EAPAD) 2017, 1016323 (Presented at SPIE Smart Structures and Materials + Nondestructive Evaluation and Health Monitoring: March 29, 2017; Published: 10 May 2017); https://doi.org/10.1117/12.2261913.5427376516001.
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