Elsevier

Sensors and Actuators B: Chemical

Volume 216, September 2015, Pages 24-32
Sensors and Actuators B: Chemical

Electrolyte and solvent effects in PPy/DBS linear actuators

https://doi.org/10.1016/j.snb.2015.03.098Get rights and content

Highlights

  • PPy/DBS actuation is established in organic and ‘ueous electrolytes.

  • Changing of actuation direction was achieved during cycling.

  • PPy/DBS can be modified from cation-driven to anion-driven actuation.

Abstract

An important question with conducting polymer linear actuators concerns the processes that occur once the electrolyte is changed. To address this question, a series of electro-chemo-mechanical deformation (ECMD) measurements on electrochemically prepared free-standing films of polypyrrole doped with dodecylbenzenesulfonate (DBS) were investigated under isometric (constant force) conditions in an organic solvent- and in aqueous based-electrolytes during cyclic voltammetric and square wave potential step experiments. The same film was actuated first in propylene carbonate electrolyte (0.1 M TBACF3SO3), where cation-driven actuation (12% strain) was observed. The film was then exposed to an aqueous electrolyte (0.2 M TMACl) where the actuation changed to anion-driven (7% strain). The same film was then returned to the propylene carbonate electrolyte, where anion-driven actuation (10% strain) was observed rather than the original cation-driven actuation. Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and scanning ionic conductance microscopy were applied to evaluate changes in morphology and electrochemical properties before and after the actuation cycles to examine changes in the actuation mode. The mechanism and reasons for the observed behaviour are proposed.

Introduction

The properties of conducting polymer linear actuators have been examined over the past 20 years [1], [2], [3]. Polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) free-standing films were shown to actuate owing to anion, cation and mixed ion fluxes under reversible redox cycles [4], [5], [6]. Linear strains for PPy/LiTFSI [7] and PPy/TBACF3SO3 [8] in the range of 12–30% were reported, but also high strain fatigues in most films after continuous cycling [9]. In the case of PEDOT/TBACF3SO3 linear actuators, it was shown that the polymerization process, and in particular the polymerization potential, influences which ion dominates the actuation properties [10]. With potentiostatic polymerization (E = 1.1 V) mainly cation-driven actuation was found in both organic and aqueous electrolytes. To explain this effect the spherical form of the CF3SO3 anion was taken in account, and suggested to be responsible for the CF3SO3 being immobile within the PEDOT network [4]. PPy/DBS actuators have been considered as cation-driven actuators due to the immobile DBS ion incorporated during electropolymerization and leading to volume increases during reduction [11]. Recent research showed a mixed ion influence in the linear actuation properties of PPy/DBS-PVdF trilayer actuators [12]. Fuchiwaki et al. recently investigated PPy/para-phenolsulfonic acid free standing films glued with a tape forming a bilayer artificial muscle. The ion exchange properties were studied in different electrolytes, where anion movement was observed in the case of NaPF6 with two times higher bending in comparison with NaCl, where dual ion movement was observed (Na+ insertion upon reduction and Cl insertion upon oxidation) [13].

To have movement of only one ion type (anions or cations with solvent molecules) at anodic or cathodic potentials to dominate the actuation has been a major goal in the development of CP actuators. Synthesis temperature, electrolyte, polymerization conditions (i.e. galvanostatic, potentiostatic and potentiodynamic) and different solvents [14], [15], [16] have been investigated. However, the reasons why on occasions anion, cation or mixed-ion actuation occurs are not fully understood. A model that fits most of the charging/discharging properties of CPs, explaining anion and cation effects, is the electrochemically stimulated conformational relaxation (ESCR) model [17], [18], [19]. According to the model, ions are inserted in the CP film during redox reactions causing volume changes. During oxidation the polymer structure opens via conformational changes of the chains and ion channels are formed [18].

In this research PPy was polymerized at low temperature in a DBS solution and subsequently actuated in an organic (TBACF3SO3, PC) and an aqueous (TMACl) electrolyte under ECMD measurements. The goal was to evaluate what changes in actuation occur when the electrolyte and solvent changes. The PPy properties were studied before and after actuation using Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). An in-house built characterization system, developed in the Polymer Electronics Research Centre laboratory, based on scanning ion conductance microscopy (SICM) [20], [21], was applied to evaluate how the electronic properties at the film surface change before and after actuation in different electrolytes.

Section snippets

Materials

Sodium dodecylbenzenesulfonate (NaDBS, 99%), propylene carbonate (PC, 99%), tetrabutylammonium trifluoromethanesulfonate (TBACF3SO3, 99%), tetramethylammonium chloride (TMACl, 99%), ethylene glycol (EG, 99.8%) were obtained from Sigma–Aldrich and used as supplied.

PPy/DBS electropolymerization

PPy/DBS was deposited galvanostatically (0.2 mA cm−2) on stainless steel working electrodes (surface area 5.4 cm2) in a three electrode cell (with a platinum sheet counter electrode and a Ag/AgCl wire reference electrode, 0.52 V vs. SHE),

Polymerization of PPyDBS on stainless steel

The potential–time curve, Fig. 1, records the PPy/DBS deposition under galvanostatic conditions with an initial drop in potential at the beginning of the polymerization, followed by an increase in the potential from 0.6 V to 0.72 V. The low value of the potential indicates uniform deposition of PPy on the stainless steel substrate [24].

ECMD measurements.

Cyclic voltammetric measurements for PPy/DBS free-standing films were performed in parallel with ECMD measurements to evaluate the linear actuation of the film

Conclusions

PPy doped with DBS free standing films are expected to show mainly cation-driven actuation due to the DBS anions immobilized during the polymerization process. Nevertheless a rare phenomenon was observed where the progressive ion mobility changed, matching the size and mobility of the anions and cations, seen during square wave potential step experiments (300 steps), and the direction of mainly cation-driven actuation changed to mainly anion-driven actuation during cycling. The anion driven

Acknowledgements

This research was supported by the European Union through the European Social Fund (MTT76), the European Scientific Network for Artificial Muscles (ESNAM, ECOST-STSM-MP1003) and partly through the New Zealand Foundation for Science and Technology (New Economy Research Fund, contract no. UOAX0408).

Nihan Aydemir was born in Turkey and currently doing her PhD work at University of Auckland under supervision of Prof. J. Travas-Sejdic and Prof. David Williams. She has received her bachelor's degree in chemistry and master's degree in polymer science and technology at Istanbul Technical University. Her research interests cover nano/micro fabrication of conducting polymers, label free electrochemical biosensors and developing highly localized surface characterization techniques with using

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    Nihan Aydemir was born in Turkey and currently doing her PhD work at University of Auckland under supervision of Prof. J. Travas-Sejdic and Prof. David Williams. She has received her bachelor's degree in chemistry and master's degree in polymer science and technology at Istanbul Technical University. Her research interests cover nano/micro fabrication of conducting polymers, label free electrochemical biosensors and developing highly localized surface characterization techniques with using Scanning Ion Conductance Microscope (SICM).

    Paul Kilmartin is a Professor in the School of Chemical Sciences at the University of Auckland. He completed a PhD in the electrochemistry of polyanilines at the same University in 1997 and joined the academic staff soon after. His research interests have diversified to include the characterization antioxidants in beverages such as wines, and projects on conducting polymers within the Polymer Electronics Research Centre, including the development of electrochemical sensors, antioxidant packaging and work with actuators.

    Jadranka Travas-Sejdic is a Professor in Macromolecular Sciences at the School of Chemical Sciences at the University of Auckland (New Zealand). Her current research is in the field of advanced polymeric materials for health, bioelectronics and polymer electronic devices with projects including development of novel gene sensors, conducting polymers-based switchable surfaces, biointerfaces and actuators. She is a Fellow of the International Union of Pure and Applied Chemistry and the New Zealand Institute of Chemistry, and a Councilor of the Pacific Polymer Federation.

    Arko Kesküla is a PhD student at University of Tartu (IMS lab) with main focus on polymeric ionic liquids as membranes for electrochemical actuators under supervision of Dr. A.-L. Peikolainen and Prof. Uno Mäeorg. He received his master degree 2012 at University of Tartu in material science.

    Anna-Liisa Peikolainen obtained her PhD degree in 2011 in chemistry at Tallinn University of Technology. Her main research topic was nanoporous organic aerogels. Organic aerogel being one of the sources of high surface area carbon led her to field of carbon based actuators. Since 2012 she is a Material Research Scientist at University of Tartu at Intelligent Materials and Systems Lab focusing on carbon–polymer composites and carbon based actuators.

    James Parcell enrolled 2001 his bachelor studies in physics at Otago University (New Zealand) and received his master in Mechatronics at University Auckland 2012. Now he is developing software for scanning, weighing, dimensioning systems for freight, food, chemical and concrete industries based in Brisbane.

    Madis Harjo finished his bachelor's degree in Tallinn University of Technology in 2012. Starting from 2012 he is doing his master thesis in University of Tartu (IMS lab) under supervision of Dr. Martin Järvekülg and Prof. Rudolf Kiefer. His main works includes cell activation with glucose-gelatine fiber mat with embedded actuators.

    Alvo Aabloo received his PhD in solid state physics in 1994 from University of Tartu. Since 2007 he is a Professor in the Institute of Science and Technology, University of Tartu and the Head of Intelligent Materials and Systems Laboratory. His current research interests include atomistic and multi scale modeling of polymer electrolytes and their interfaces, radiation damage studies, electromechanically active polymer transducers based on ionic electroactive polymers and their applications in robotics and micro devices.

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