An Electrolyte-Free Conducting Polymer Actuator that Displays

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An Electrolyte-Free Conducting Polymer Actuator that Displays Electrothermal Bending and Flapping Wing Motions under a Magnetic Field Kyungchan Uh, Bora Yoon, Chan Woo Lee, and Jong-Man Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09981 • Publication Date (Web): 30 Dec 2015 Downloaded from http://pubs.acs.org on January 6, 2016

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ACS Applied Materials & Interfaces

An Electrolyte-Free Conducting Polymer Actuator that Displays Electrothermal Bending and Flapping Wing Motions under a Magnetic Field Kyungchan Uh,† Bora Yoon, † Chan Woo Lee*,‡ and Jong-Man Kim*,†, ‡ †

Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea

‡

Institute of Nano Science and Technology, Hanyang University, Seoul 133-791, Korea

KEYWORDS: Polyaniline, Actuator, Conducting polymer, Polydiacetylene, Electroactuation, Microfiber, Wet-spinning

ABSTRACT Electroactive materials that change shape in response to electrical stimulation can serve as actuators. Electroactive actuators of this type have great utility in a variety of technologies including biomimetic artificial muscles, robotics and sensors. Electroactive actuators developed to date often suffer from problems associated with the need to use electrolytes, slow response times, high driving voltages and short cycle lifetimes. Herein, we report an electrolyte-free, single component, polymer electroactive actuator, which has a fast response time, high durability and requires a low driving voltage (< 5 V). The process employed for production of this material involves wet-spinning of a pre-organized camphorsulfonic acid (CSA) doped polyaniline (PANI) gel, which generates long, flexible and conductive (ca. 270 S/cm) microfibers. Reversible bending motions take place upon application of an alternating current (AC) to the PANI polymer. This motion, promoted by a significantly low driving voltage (< 0.5 V) in the presence of an external magnetic field, has a very large swinging

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speed (9000 swings/minute) that lies in the range of those of flies and bees (1000-15000 swings/min), and it is fatigue-resistant (> one million cycles).

INTRODUCTION A great effort has been given to the development of efficient biomimetic actuators that mimic the performance of animal muscles. Typically, mechanical movement of actuators is triggered by various physical (electric current,1-22 light,23-27 magnetic field,28 heat29,30) and chemical (water,31-34 gases,35 organic solvents36-38) inputs.

Among these, an electric field is

the most popular signal stimulant of actuators because of operational simplicity considerations as well as the relative ease in controlling the input electric current. For the most part, actuators developed to date are based on hydrophilic/ionic polymers,8-11 conducting polymers,12 carbon materials (carbon nanotubes (CNT), graphene)1-7 or combinations of these substances.13-22 Owing to a relatively high conductivity, unique redox behavior, low cost, environmental compatibility and chemical stability, polyaniline (PANI) is one of the most technologically promising conducting polymers.39,40 Although applications of this attractive polymer to actuating systems have been investigated,41-45 PANI-based artificial muscles described to date suffer from problems that are similar to those associated with the common electroactive actuators involving the use of electrolytes and slow response times. In the investigation described below, we developed a durable PANI actuator that is electrolyte-free, rapidly responsive and functions at a low voltage. Significantly, the PANI actuator undergoes flapping wing motion in the presence of a magnetic field.

EXPERIMENTAL SECTION


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Materials and instruments. Aniline, ammonium persulfate (APS), camphorsulfonic acid (CSA) were purchased from Sigma-Aldrich. m-Cresol was purchased from Tokyo Chemical Industry. 10,12-Pentacosadiynoic acid (PCDA) and 4-6-heptadecadiynoic acid (HDDA) were purchased from GFS Chemicals, Ohio, USA. Paraffin (Samchun Chemicals), 4aminobenzonitrile (Alfa Aesar), 1-naphthol (Sigma-Aldrich), p-toluenesulfonic acid monohydrate (Sigma-Aldrich), 4-hydroxybenzaldehyde (Sigma-Aldrich), 6-nitropiperonyl alcohol (Alfa Aesar) were purchased from commercial sources. 3-Pentacosa-10,12diynamidobenzoic acid (PCDA-mBzA) was prepared according to the literature method.46 Polyaniline fibers were fabricated using a KDS-210 Syringe pump (KD Scientific). Optical microscopic images were collected with a Olympus BX 51W/DP70. Scanning electron microscope (SEM) images were obtained using a JEOL (JSM-6330F) FE-SEM at an accelerating voltage of 15 kV. Each sample was coated with Pt for 25 s before analysis. Waveform Generator (33500B Series, Agilent) and 2 Channel Isolated Amplifier (33502A, Agilent) were used for application of voltages. Magnetic flux density measurements were performed using TM-701 TESLA meter (KANETEC). Tensile strength and Young’s modulus were recorded using a Universal Testing Machine (WITH LAB Co., LTD.) for tensile strength and a Universal Testing Machine (WITH LAB Co., LTD.) for Young’s modulus, respectively. Raman spectra were obtained using the wide illumination (WAI) scheme (PhAT system, Kaiser Optical Inc., Ann Arbor, MI, USA). XRD data were collected using a D8 Discover (Bruker, Germany). UV/VIS/NIR spectra were obtained using a UV/Vis/NIR

Spectrophotometer

(Lambda

1050,

PerkinElmer).

The

direct-current

conductivity measurements of the films and fibers were performed with the four-line-probe method using Keithley 237 Source-Measure Unit. TGA curve was obtained using a SDT Q600 Simultaneous TGA/DSC analyser (TA Instruments, USA).



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Fabrication of conductive polyaniline (PANI) microfibers. A typical procedure for the preparation of a PANI fiber is as follows. High molecular weight and conductive PANI was prepared according to the literature method.47 The PANI powder (700 mg) was mixed with camphorsulfonic acid (CSA) (840 mg, 3.62 mmol) by grinding in a crucible. The mixture was dissolved in m-cresol (51.3 mL) using a homogenizer and a viscous gel was obtained. The CSA-doped PANI gel was transferred to a syringe and spun into a cold EtOH solution using a syringe pump (0.2 mL/min). The long polymer fiber collected was dried and stretched in air. The diameter of the spun fiber was adjusted by using different needle sizes. A four probe method was employed to measure conductivity of the fiber.

Electroactuation of PANI fiber. A CSA-doped PANI fiber (diameter: 64 µm, length: 2.2 mm) was suspended on a glass substrate with silver electrodes attached at the ends. Actuation of the fiber promoted by varying the input voltage was monitored using an optical microscope. The surface temperature of the polymer actuator was measured using an indirect method because precise recording of the temperature of micron-sized fiber was very difficult. Thus, materials having different melting points were coated on the PANI fiber and the melting temperature was monitored using an optical microscope by increasing the input voltage.

Surface temperature of PANI fiber. A direct measurement of surface temperature of a PANI fiber upon application of current was found to be very difficult. We have used materials that have different melting points and the PANI fiber was coated with these materials. Melting temperature of the materials was recorded by varying the input voltage. The following materials were used for temperature measurement: paraffin (mp: 50-54 oC), 10,12-pentacosadiynoic acid (mp: 62-63 oC), 4-6-heptadecadiynoic acid (mp: 77-79 oC), 44 ACS Paragon Plus Environment

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aminobenzonitrile (mp: 83-87 oC), 1-naphthol (mp: 95-96 oC), p-toluenesulfonic acid monohydrate (mp: 103-106 oC), 4-hydroxybenzaldehyde (mp: 114 oC), 6-nitropiperonyl alcohol (mp: 121-125 oC).

Electrothermochromism of polydiacetylene (PDA)-coated PANI fiber. A PANI fiber was

coated

with

diacetylene

monomer

PCDA-mBzA

(CH3(CH2)11C≡C–

C≡C(CH2)8CONHPhCO2H) by dipping the PANI fiber in a THF solution containing the diacetylene monomer. After drying in air, the diacetylene-coated PANI fiber was irradiated with 254 nm UV light (1 mW/cm2) for 30 s using a laboratory hand-held UV lamp to induce photopolymerization of the diacetylene monomer. Formation of blue colored polydiacetylene (PDA) on the surface of the PANI fiber was observed by naked eye. Current was applied to the PDA-coated fiber and colorimetric transition was monitored.

Flapping wing actuator. A wing shape PANI fiber (diameter: 89 µm, length: 52 mm) was placed 3 mm over a disk magnet. Flapping wing motion was induced by applying an input AC voltage of 0.5-5 V. A four wing flapping motion actuator was also prepared.

RESULTS AND DISCUSSION The designed electromechanical actuators are comprised of micron-sized PANI fibers that are fabricated using a wet-spinning method. High molecular weight, nanofibrous PANI was prepared by self-stabilized dispersion polymerization in a manner that has been described elsewhere (Supporting Information, Figure S1a).47 A viscous and spinnable PANI gel was generated by grinding a nanofibrous PANI powder with camphorsulfonic acid (CSA), followed by dissolving the mixture in m-cresol using a homogenizer (Figure 1a, Supporting 5 ACS Paragon Plus Environment


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Information, Figure S1b). The resultant CSA-doped PANI gel, whose conductivity was found to be ca. 275 S/cm, was placed into a syringe equipped with a syringe pump and spun axially into a cold EtOH solution (Figure 1b). The wet-spinning process affords long (>1 m) and flexible CSA-doped PANI fibers (Figure 1c) (see also Supporting Information, Figure S2) with diameters in the range of 5-200 µm depending on the size of the syringe needle. For instance, a 77 µm diameter PANI fiber is obtained when a 25G syringe needle is used while the diameter of the fiber increases to 193 µm when a 19G needle is employed (Supporting Information, Figure S3). In this way, highly flexible PANI fibers are produced in straight (Figure. 1d), two-fly (Figure. 1e) as well as knotted (Figure. 1f) and spring (Figure, 1g) forms. The Young’s modulus and tensile strength of the CSA doped and stretched PANI fibers were found to be ca. 280 MPa and ca. 35 MPa, respectively. The conductivity of the dried and stretched fiber is ca. 270 S/cm, a large value that is a consequence of efficient π-π stacking in the PANI fiber as evidenced by XRD data (Supporting Information, Figure S4). The diffraction patterns displayed in the XRD data are nearly identical to those reported earlier for highly conductive PANI.39 The d-spacing values for the peaks at 15° (5.6 Ǻ), 20° (4.2 Ǻ) and 26° (3.5 Ǻ) are assigned to the periodicity parallel and perpendicular arrangements of the polymer chains. The respective d-spacing values of 5.6 Ǻ and 4.2 Ǻ correspond to the side-by-side interaction distance between chains and face-to-face π-π stacking distance between phenyl rings in PANI. The significantly high d-spacing value of 3.5 Ǻ is attributed to the distance between nitrogen atoms coordinated with CSA anions. Thus, the increase intensity of the peak at 26° implies improved π-π interchain stacking. It should be noted that the conductivity decreases significantly to ca. 100 S/cm when the spun fibers are placed in the coagulant solution for a long time period (> 2 h) owing to loss of the CSA dopant from PANI to the coagulant solution. Thus, the use of short dipping times 6 ACS Paragon Plus Environment

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An Electrolyte-Free Conducting Polymer Actuator that Displays

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