Directional electromechanical properties of PEDOT/PSS films containing aligned electrospun nanofibers

Polymer Journal (2011) 43, 849–854 & The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/11 $32.00 www.nature.com/pj ORIGINAL ARTICLE Directional electromechanical properties of PEDOT/PSS films containing aligned electrospun nanofibers Jian Zhou, Tadashi Fukawa and Mutsumi Kimura We demonstrate directional electromechanical properties of poly(3,4-ethylenedioxythiophene)/poly(4-styrene sulfonate) (PEDOT/PSS) composite films containing aligned poly(vinyl pyrrolidone)/poly(methyl methacrylate) nanofiber assemblies. The aligned nanofiber assemblies showed anisotropic wettability based on the high alignment degree of the nanofibers. The PEDOT/PSS composite film containing aligned nanofibers displayed an anisotropic actuation response when a voltage was applied in air. The orientation of the embedded nanofibers within the PEDOT/PSS matrix leads to the control of actuation direction because of the difference of anisotropic mechanical properties in the composite films. Polymer Journal (2011) 43, 849–854; doi:10.1038/pj.2011.62; published online 27 July 2011 Keywords: anisotropic actuation; conductive polymer; electrospinning; mechanical properties; wetting INTRODUCTION Living things have acquired special anisotropic functions based on directional orientations of micro- and nanostructures during their evolution. Anisotropic structures in nature make possible directional motion, adhesion, friction or wetting.1–4 These natural structures provide important insights for constructing artificial materials. Thus, bio-inspired advanced materials have been developed to mimic natural anisotropic structures.5–9 Long, fibrous structures, such as collagens and celluloses, are a major component of the extracellular matrix that supports most tissues by maintaining their strength and elasticity.10 Moreover, composite materials with incorporated highly dense fibrous assemblies have been widely developed, and these materials show significantly different physical and chemical properties, which remain separate and distinct at the macroscopic or microscopic scale within the finished structure.11,12 Fiber orientation within natural and artificial composites is a key factor in determining final properties. Several anisotropic functions of aligned nanofiber assemblies have been reported in the literature. Vertically aligned multiwalled carbon nanotube assemblies show strong adhesion to vertical surfaces, similar to gecko legs.13 Anisotropic hydrophobic properties have been demonstrated by aligned nanofiber assemblies.14 In this study, we focus on control of electromechanical properties of conjugated polymer films by using aligned electrospun nanofiber assemblies. Electrospinning is a simple method for producing nanofibers and nanofibrous non-woven mats for various applications. It involves discharging a polymer solution in air from a nozzle under high voltage and producing nanofibers by exploiting electrostatic repulsion of the polymer solution.15–17 Furthermore, the fiber orientation can be controlled by using modified fiber collectors.18 Aligned electrospun nanofibers have potential applications in structural reinforcement, tissue engineering and blood vessel engineering, which often require well-aligned and highly ordered structures.16,17 There are only a few reports in the literature on anisotropic actuation based on anisotropic structures. Anisotropic actuation of polypyrrole films was achieved by orientation of polymer backbones through directional stretching.19 Uniaxial orientation of nafion, and the subsequent formation of electrode layers to create ionic polymer–metal composites, can also yield anisotropic actuation on electrical stimulation.20 As the application of fiber-reinforced composite becomes more widespread, there is a desire to add functionality not only simple enhancement of mechanical properties but also development of smart materials. Bent21 reported that the active fibers composites, which incorporated unidirectional piezoelectric transducer fibers into hybrid matrix, produce a highly conformable and anisotropic actuator material. We also reported anisotropic actuation of poly (3, 4-ethylenedioxythiophene)/poly(4-styrene sulfonate) (PEDOT/ PSS) coated papers, for which the actuation stress depended on the relative orientation of paper fibers and the loading direction of the coating.22 When the aligned nanofiber assemblies were embedded within the conductive polymer matrix, the actuation properties followed the direction of fiber orientation. In this study, we selected aligned electrospun nanofibers for anisotropic actuation of conductive polymers. We fabricated aligned poly(vinyl pyrrolidone)/poly(methyl methacrylate) (PVP/PMMA) nanofiber assemblies up to several centimeters in length to realize anisotropic motion of PEDOT/PSS Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda, Japan Correspondence: Professor M Kimura, Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan. E-mail: Received 8 April 2011; revised 24 May 2011; accepted 30 May 2011; published online 27 July 2011 Directional electromechanical properties of PEDOT/PSS films J Zhou et al 850 films. The aligned nanofiber assemblies displayed anisotropic wettability because of the high alignment degree of the nanofibers. We further demonstrated anisotropic electromechanical actuation of PEDOT/PSS film by using the aligned PVP/PMMA nanofibers. The composites with incorporated aligned nanofiber assemblies offer new opportunities to achieve directional electromechanical actuation in soft actuators. EXPERIMENTAL PROCEDURE Materials Poly(vinylpyrrolidone) (PVP, Nacalai Tesque, Tokyo, Japan, Mw 1 000 000 g mol1), poly(methyl methacrylate) (PMMA, Wako Chemicals, Tokyo, Japan, Mw 100 000 g mol1), ethylene glycol (EG, Wako Chemicals), N,N-dimethylformamide (Wako Chemicals), tetra-n-butylammonium bromide (Wako Chemicals) and fuchsine (Wako Chemicals) were used as received. Poly(vinyl alcohol) (Mw 140 000 g mol1) was provided by Kuraray Co., Ltd (Tokyo, Japan). Fabrication of nanofiber assemblies by electrospinning Nanofiber assemblies were fabricated using a NANON-01A electrospinning machine with a drum collector (MECC Co., Ltd, Fukuoka, Japan). PVP/PMMA nanofiber assemblies were produced from 250 mg ml1 PVP and PMMA in a N,N-dimethylformamide solution containing 5 wt% tetra-n-butylammonium bromide (relative to PMMA).22,23 The mixed solutions for the PVP/PMMA composite nanofibers were prepared by dissolving PVP and PMMA (weight ratio PVP: PMMA¼10:90) in N,N-dimethylformamide. Tetra-n-butylammonium bromide was used as an organic salt to increase the electrical conductivity of the solution.24 In the electrospinning process, the prepared solutions were placed in 6 ml disposable plastic syringes and injected through a stainless steel needle (18 gauge, orifice diameter 1 (...truncated)