Elsevier

Chemical Engineering Journal

Volume 352, 15 November 2018, Pages 876-885
Chemical Engineering Journal

Prestrain-free electrostrictive film sandwiched by asymmetric electrodes for out-of-plane actuation

https://doi.org/10.1016/j.cej.2018.07.094Get rights and content

Highlights

  • DEA is constructed to achieve out-of-plane actuation without requiring prestrain.

  • NH2-PDMS elastomer is synthesized and sandwiched by two asymmetric electrodes with distinct mechanical properties.

  • DEA diaphragm takes periodic vibration and exhibits a large flection angle of 26.3°.

  • DEA exhibits well controlled electromechanical performance by driving voltage or frequency.

Abstract

This study presents a novel and facile strategy to construct prestrain-free dielectric elastomer (DE) film with large out-of-plane actuation. An extremely soft elastomer of amino functionalized poly(dimethylsiloxane) (NH2-PDMS) with thickness of ∼446 µm was employed as dielectric matrix, which was sandwiched between two asymmetric PDMS electrodes with distinctly designed mechanical properties. Electromechanical coupling factor (ECF, dielectric constant/elastic modulus) of the DE film increased with amino density coupled into dielectric matrix, providing a strategy to construct a DE actuator (DEA) with large displacement under low electric field. The thin, soft electrode was made of an off-ratio polymer of PDMS doped by conductive graphite microflakes, while the thick, stiff electrode was made of completely cured PDMS elastomer doped by conductive graphite microflakes and Ag nanoparticles. The film thicknesses, Young’s moduli, and surface resistances were 54.1 µm, 9.21 MPa, 1.02 kΩ/cm2 for the soft electrode, and 166.7 µm, 87.5 MPa, 0.13 kΩ/cm2 for the stiff electrode, respectively. The prepared DEA diaphragm exhibited well controlled electromechanical properties by driving voltage or frequency, and a high flection angle of 26.3° at an extremely low electrical field of 8.8 V/μm. Thus, it can be used as intelligent valve flap/pump for microfluidic device and artificial muscle with minimized energy assumption activated by low voltage.

Introduction

Electroactive polymers (EAP) are lightweight materials that can convert electrical energy into mechanical energy [1], [2], [3], [4]. They are considered as promising candidates for electrical motors, robotic actuators, artificial muscle, and dynamic sensors [5], [6], [7], [8]. As an electronic type of EAP, dielectric elastomer actuator (DEA) possesses quick response, large actuation force, and high mechanical energy density. Thus, DEA fabrication and application have become an emerging inter-disciplinary research area that combines chemistry, materials, mechanics, control, nanotechnology, and other techniques [9], [10], [11], [12]. DEA consists of a dielectric elastomer membrane sandwiched between two compliant electrodes [13]. When a voltage is applied between two electrodes, the opposite charges on electrodes generate an electrostatic force (Maxwell stress) to squeeze the dielectric layer, causing its in-plane or out-of-plane actuation depending on design [12], [14], [15].

The common DEA displays the in-plane deformation, where the electrostatic force causes the dielectric elastomer to stretch or contract in plane [16], [17]. Using this actuation mechanism, many layers of DEAs can be stacked into a roll to generate a higher tension or compression along the core axis [18], [19], [20], and therefore the DEA-drived motors have been quickly developed and commercialized [21]. This kind of actuator has been widely used in the well-known arm wrestling match between an EAP-actuated robotic arm and a human, where, the DEA robotic arm produced a blocking force of 0.2 pound and a fast actuation speed of 0.045 in/S [22].

To obtain out-of-plane actuation, a commonly used approach is to prestrain the DEA matrix [23], [24], [25], [26], [27]. This kind of DEA actuators allow a reduction in energy consumption by up to 80% as well as reduction in operation and maintenance costs by using fewer moving parts, which are important issues for pumps, valves, and other applications [26], [27], [28], [29], [30], [31], [32]. An outstanding example is that the DEA with prestrained acrylic membrane fabricated by Pei’s group, exhibited high levels of electromechanical strain (380%), elastic energy density (3.4 J/g), stress (8 MPa), and conversion efficiency (60–90%) [23]. Additionally, Ali group made a DEA pump prestrained by a pull-up spring [26], Zhang’s group made a DEA drived diaphragm preloaded by a 10 g weight [33], both DEAs exhibited desired actuations with out-of-plane deformation. However, the prestrained/preloaded DEA always exhibits stress relaxation that affects the subsequent actuation [34]. In addition, the use of supporting structures, such as bow, rigid-frame, weight, and spring-roll required and generated substantially more space and weight [35].

According to the actuation expression (Eq. (1)), dielectric constant (ε) and Young’s modulus (Y) of elastomer matrix are two potent parameters to influence the electro-mechanical response [4], [36], [37], [38]. Generally, driven by the Maxwell force, two electrodes will push the elastomer matrix equally, and their stiffness inevitably constrains the elastomer matrix to expand or shrink. Thus, the mechanical properties of the electrodes affect the electro-mechanical response. The majority of current electrode materials are acrylates [25], silicone [13], urethanes [39], and similar stretchable polymers [13]. Additionally, two electrodes are made of same component and their deformations are equivalent. Therefore, contribution of electrode stiffness has been always overlooked, and not involved into Eq. (1).Sz=-εrε0E2/Y=-E2kwhere Sz is the thickness strain, ε0 and εr are the permittivity of free space and the relative permittivity of the elastomer matrix, respectively, Y is the Young’s modulus, E is the applied electrical field, and k is electromechanical coupling factor (ε/Y).

To study the influence of electrode stiffness on electromechanical performance, we made a DEA by two asymmetric electrodes with distinct mechanical properties (Fig. 1). One electrode was made of a soft and thin PDMS membrane (Sylgard 184, mass ratio of polymer: cross-linker is 20:1) doped by conductive graphite microflakes. Such off-ratio elastomer had a low modulus, and high stretchability [40]. The other coupling electrode was made of a stiff and thick PDMS membrane (Sylgard 186, polymer: cross-linker, 10:1) doped by both graphite microflakes and Ag nanoparticles. This electrode had a high modulus, which can be used to control flection direction of DEA. The dielectric matrix was made of an amino-PDMS (NH2-PDMS), resulting from the condensation reaction between the commercial OH-terminated PDMS and silane coupling agent of (3-aminopropyl) triethoxysilane (APTES). To enhance ECF, NH2-PDMS was synthesized by using overdosed APTES content. Because more amino groups were introduced into the elastomer, the resultant elastomer was expected to exhibit an intrinsically higher dielectric constant due to the high polarity of the amide fragments [2]. Meanwhile, the excess oxyethyl groups in APTES could not crosslink with the OH groups in OH-PDMS, producing an off-ratio elastomer with a very low modulus. Therefore, the yielded NH2-PDMS elastomer with high amino density would exhibit a large ECF and deformation according to Eq. (1).

Under actuation of a pulsed electrical field, the deformations of two electrodes driven by the Maxwell force would be different due to distinct mechanical properties. Thus, the DEA takes asymmetric actuation and bends to soft electrode side. The electrodes and elastomer materials are made by similar PDMS species, so that their interface connections are very strong and the electrodes do not detach from the elastomer surface. Moreover, the DEA has a concise structure and no prestrain-supporting device or additional mechanical guiding component is required in the device. The resultant DEA diaphragm exhibited good actuation stability under extremely low electrical field of 8.8 V/μm and reliability during long-term actuation cycling. We therefore present a facile strategy to fabricate a novel, reliable electrostriction actuator which enables large out-of-plane actuation, using a simple, low-cost fabrication method without requiring any prestrain and bias mechanism. The resultant DEA can be used as intelligent valve flap/pump for microfluidic device and novel artificial muscle with minimized energy assumption activated by low voltage.

Section snippets

Materials

Two PDMS products of Sylgard 184 & 186 were obtained from Dow Corning (Midland, MI, USA). OH-terminated silicone elastomer (OH-PDMS, Mn = ∼4200), 3-aminopropyl triethoxysilane (APTES), (3-mercaptopropyl) methdimethoxysilane (MPTMS), dibutyltin dilaurate (DBTDL), conductive graphite microflakes (Gr, ∼325 mesh), and heptane were obtained from Sigma-Aldrich. Ag nano particles (Ag NPs) with varying diameters of 200–600 nm was obtained from Alfa-Aesar.

Fabrication of the soft electrode films

According to the supplier (Dow Corning), Sylgard

Composition characterizations

The synthesis of NH2-PDMS was monitored by ATR-FTIR spectra as shown in Fig. 4a. Two characterization peaks of O–H vibration were observed at 3699 and 3328 cm−1 in the spectrum of OH-PDMS. The peak at 3699 cm−1 was small and sharp, resulting from the dissociative O–H group. By contrast, the peak at 3328 cm−1 was very broad due to formation of H-bond (Fig. S2). Other PDMS characterization vibrations were discussed below: the peaks at 1083 and 1005 cm−1 were attributed to the stretching vibration

Conclusion

This paper reported a facile strategy to fabricate DEA with out-of-plane actuation under a low activated voltage. The produced DEA consisted of a soft dielectric elastomer matrix of NH2-PDMS sandwiched between a thin, soft electrode Gr-PDMS184-40 and a thick, stiff electrode Gr/Ag-PDMS186-40. The NH2-PDMS elastomer with Young’s modulus of 0.78 MPa was produced by condensation reaction between the commercial OH-terminated PDMS precursor and APTES. The high ECF of NH2-PDMS elastomer was achieved

Acknowledgments

This work was supported by Yangtze River Scholar Innovation Team Development Plan (IRT1187), National Natural Science Foundation in China (no. U1704149, 21471046, 51705473), and Henan Province University Science and Technology Innovation Talent (16HASTIT048).

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