Crystallization‐Directed Anisotropic Electroactuation in Selectively Solvated Olefinic Thermoplastic Elastomers: A Thermal and (Electro)Mechanical Property Study

Abstract

Dielectric elastomers (DEs), a class of soft electroactive polymers that change size upon exposure to an external electric field, constitute an increasingly important class of stimuli-responsive polymers due primarily to their large actuation strains, facile and low-cost fabrication, scalability, and mechanical robustness. Unless purposefully constrained, most DEs exhibit isotropic actuation wherein size changes are the same in all actuation directions. Previous studies of DEs containing oriented, stiff fibers have demonstrated, however, that anisotropic actuation along a designated direction is more electromechanically efficient since this design eliminates energy expended in nonessential directions. To identify an alternative, supramolecularlevel route to anisotropic electroactuation, we investigate the thermal and mechanical properties of novel thermoplastic elastomer gels composed of a selectively solvated olefinic block copolymer that not only microphaseseparates but also crystallizes upon cooling from the solution state. While these materials possess remarkable mechanical attributes (e.g., giant strains in excess of 4000%), their ability to be strain-conditioned enables huge anisotropic actuation levels, measured to be greater than 30 from the ratio of orthogonal actuation strains. This work establishes that crystallizationinduced anisotropic actuation can be achieved with these DEs.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201803467.reasons account for this sudden spike in technological interest: (1) development of flexible, stretchable, and otherwise compliant electrode technologies, and (2) discovery of dielectric materials possessing the appropriate electrical and elastomeric properties required to attain large actuation strains. The seminal paper by Pelrine et al. [6] is largely responsible for spurring contemporary DE studies by introducing the use of carbon grease as a compliant electrode in conjunction with a new chemically crosslinked dielectric material, a commercial acrylic adhesive (VHB 4905/4910), that is capable of achieving actuation strains greater than 100% (on an area basis). Since this initial investigation of VHB, the DE community has considered surprisingly few new competitive materials even though VHB remains somewhat proprietary and it is only manufactured in a limited number of thicknesses. Nevertheless, because of its performance, availability and relatively low cost, VHB has dominated the scientific literature as the gold standard to which the performance of other materials is frequently compared. [7][8][9][10] Other examples of chemically crosslinked elastomers that have shown promise as DEs for use in, e.g., (micro)robotics, [11] biomedical devices, [12] and stretchable electronics [13] include silicone elastomers, [14,15] VHB modified [16] with a secondary acrylic network to produce an elastomeric "binetwork," custom-designed acrylic elastomers, [17] and bottlebrush silicone elastomers. [18] An importan...

Morphological Studies of Solution‐Crystallized Thermoplastic Elastomers with Polyethylene Endblocks and a Random‐Copolymer Midblock

“…Block‐selective hydrogenation greatly increases the interblock thermodynamic incompatibility and significantly elevates the order–disorder transition temperature. [ 19,20 ] One approach to circumvent processing limitations associated with this family of TPEs is to substitute crystallizable hard blocks for the vitrifiable blocks to improve drug delivery, [ 21 ] catalysis, [ 22 ] optoelectronics, [ 23 ] electroactive media, [ 24 ] and hybrid materials. [ 25 ] Some crystallizable TPEs possess precise triblock copolymer architectures and well‐defined morphologies due to their synthesis via living anionic polymerization, [ 26,27 ] while others are more accurately described as randomly coupled multiblock copolymers.…”

Intrinsically Anisotropic Dielectric Elastomer Fiber Actuators

“…Fabricating intrinsically anisotropic DEs is an emerging topic. − A simplified theoretical analysis was conducted here to quantitatively illustrate how the linear axial strain increases with the orthogonal modulus ratio (the modulus in radial direction divided by that in axial direction, Y 1 / Y 2 ) of an anisotropic DE. Previous work has proved that the electro-actuated deformation of a rolled DEA can be approximated as that of a planar DEA .…”

“…In very recent works, intrinsically anisotropic aligned crystalline polyolefin organogels and liquid-crystal elastomers , were reported to realize a directional output in a planar DEA. However, it remains uncertain whether these materials can be used to fabricate rolled DEAs, since the lack of self-adhesiveness may lead to the potential slip between layers of the rolled structure.…”

Dielectric Polymer with Designable Large Motion under Low Electric Field

“…[11][12][13] In addition, the inplane actuation needs to be converted into out-of-plane motions via an external rigid frame/film, [17] embedded rigid fibers, [18] printed interdigitated electrodes, [19] or built-in mechanical heterogeneity. [20][21][22] This demands complex device fabrications and limits the actuation modes.We report here a strategy to design a dielectric polymer actuator that can be directly and repeatedly programmed/ reprogrammed to undergo large and designable out-of-plane motions under significantly reduced e-fields (2-10 V µm −1 ). The low-driving e-field and out-of-plane actuation arise from a unique space charge mechanism different from conventional DE.…”

“…[11][12][13] In addition, the inplane actuation needs to be converted into out-of-plane motions via an external rigid frame/film, [17] embedded rigid fibers, [18] printed interdigitated electrodes, [19] or built-in mechanical heterogeneity. [20][21][22] This demands complex device fabrications and limits the actuation modes.…”