Time-dependence of the electromechanical bending actuation observed in ionic-electroactive polymers

Abstract

The characteristics of the electromechanical response observed in an ionic-electroactive polymer (i-EAP) are represented by the time (t) dependence of its bending actuation (y). The electromechanical response of a typical i-EAP -poly(ethylene oxide) (PEO) doped with lithium perchlorate (LP) -is studied. The shortcomings of all existing models describing the electromechanical response obtained in i-EAPs are discussed. A more reasonable model: y ¼ y max e À=t is introduced to characterize this time dependence for all i-EAPs. The advantages and correctness of this model are confirmed using results obtained in PEO-LP actuators with different LP contents and at different temperatures. The applicability and universality of this model are validated using the reported results obtained from two different i-EAPs: one is Flemion and the other is polypyrrole actuators.Keywords: i-EAP; PEO; electromechanical response; model.Electroactive polymers (EAPs), also named as artificial muscles, are a newly developed type of polymeric materials exhibiting giant electromechanical response.1,2 EAPs are lightweight, produce large actuation performance, show good fracture tolerance, and can be made into almost any shape. 1 Therefore, a great deal of attention has been given to the EAPs over the past two decades for their ability to generate large electromechanical actuation under an applied electric field without the need for any moving parts, external motors or servos.2 This biomimetic functionality fosters rich possible applications in robotics, the medical field, microfluidics, aerospace applications, etc.2 The EAPs studied can be classified into two categories: electronic EAP (e-EAP) and ionic EAP (i-EAP). 1 The electromechanical response observed in e-EAPs can originate from an electrostrictive effect, 3 the Maxwell-stress (i.e., electrostatic force) effect, 4,5 and the re-orientation of dipoles.6 The e-EAPs are usually good electrical insulators that can stand with a high electric field. They exhibit a fast electromechanical response with a linear strain and a response time from s to ns and their strain response is well defined by the electric field applied on it. Therefore, the electromechanical response of an e-EAP is usually characterized by the relationship between the electric induced strain and the electric field applied on. 6,7 For the e-EAPs to exhibit a high strain, a high electric field (> 100 MV/m) is usually required. 6,7 Regarding i-EAPs which include ionic polymer-metal composites (IPMCs), 8 conducting polymers, 9 hydrogels, 10 etc., when an electric field is applied on it, the ions in the polymer are forced to move so that the distribution of the ions changes with time, 1,8-10 which leads to the accumulation of cations and anions onto different electrodes. The size disparity between the two types of mobile ions results in a differential expansion/shrinkage across the polymer matrix. 1,[8][9][10][11] Therefore, a bending actuation as the electromechanical response is induced in a single layer of an i-EAP film as show...

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