Dependence of polypyrrole bilayer deflection upon polymerization potential
Introduction
Polypyrrole (PPy) has become one of the most widely studied conducting polymers in recent years. Many potential applications are based on the biocompatible and actuation properties of PPy and involve various biomaterial [1] and micro-fabrication [2] applications (micro-robots, micro-valves [3], [4], [5], micro-pump [6] and biosensors [7]).
The deposition of a conducting polymer such as PPy on a non-conductive layer (e.g. a polyethylene terephthalate foil of 12 μm in thickness, covered on one side with 80 nm of Pt) leads to the bending of a bilayer upon charging and discharging. One of the main interests is in controlling and reproducing the bending of the bilayer to make it applicable to industrial products. To guarantee proper functioning of these actuators it is necessary to optimize the properties of such devices, which depend on the conditions of polymerization. Thus, the positional control of PPy bilayers upon charging and discharging has been studied for different concentrations of electrolyte [7], different PPy thicknesses on a gold bilayer [8] and upon changing the pH value during polymerization [9]. To investigate the influence of the polymerization potential on the actuation during charging and discharging PPy bilayers at polymerization potential between 0.85 and 1.5 V are prepared and the bending beam method (BBM) [10], [11] is applied with different electroanalytical methods.
The effect of polymerization potential upon actuation behaviour has already been studied by Smela et al. [12] using a PPy/Au/polyimide laminate in aqueous NaClO4 and Kiefer et al. [13], [14] using PEDOT bilayer actuation (PC/TBAPF6) and PEDOT free standing films (PC/TBACF3SO3) which were prepared at different formation potentials.
In order to analyze the ion transport in the polymeric films, ECQM measurements [15], [16] were carried out. Cyclic voltammetry experiments correlated with the frequency change were performed and the participation of different ion species upon oxidation and reduction was determined. The different methods were combined with cyclic voltammetric measurements in order to compare the PPy bilayer deflection with mass transport.
In order to understand the charging/discharging of conducting polymers and their influence on actuation relevant models are discussed in the literature. Otero et al. [17], [18], [19], [20] have designed the ESCR model (electrochemical stimulated conformational relaxation). They assume that anions are inserted in the film during oxidation and expulsed during reduction causing volume changes. During oxidation the polymer structure opens via conformational changes of the chains and ion channels are formed. The polaron/bipolaron model describes the charging of polymer chains [21] with three redox steps starting from neutral to the polaron state, from the polaron to the bipolaron state and from the bipolaron to the metallic state at different potentials.
The model of “reversible σ-dimerization”, which has been developed by Heinze [22], [23], is based on the observation that during polymerization “σ-dimers” are formed. These result from the intermolecular coupling of charged conjugated oligomers via the formation of α,α′- or β,β′-bonds between different chains, forming two sp3-centers with localized charges. Normally, during oligomerisation, protons are eliminated and a neutral fully conjugated system is formed.
The goal in this work was to investigate the influence of the polymerization potential of the PPy generation to the actuation properties of the PPy bilayers and to find an explanation for anion and cation participation during the reversible redox process.
Section snippets
Chemicals
All chemicals were of analytical grade. Pyrrole was obtained from Aldrich. Propylene carbonate (PC, 99%) and tetrabutylammonium hexafluorophosphate (TBAPF6, 99.9%) were obtained from Fluka and used without further purification.
Electrochemical polymerization and bending beam measurements
Cantilever strips were prepared by sputtering a 12 μm thick poly(ethylene terephthalate) (PET) foil with a 80 nm thickness platinum layer. The Pt-covered foil was cut into pieces 15 mm long and 5 mm wide. PET/Pt strips as the working electrode were produced with a standard
BBM measurements
The deflection of the PPy bilayer over a certain potential range was measured using screenshots (Fig. 1a and b) of constant time intervals combined with chronoamperometric (Fig. 1c) measurements. PPy bilayers deflection polymerized at different polymerization potentials are presented in Fig. 1d. To test the reproducibility of the deflection more than three PPy bilayers were produced for each polymerization potential.
The movement of the PPy bilayer was recorded using a CCD camera and individual
Discussion
In the case of conducting polymers, the actuation process is based on the charging and discharging of the electro active material. Both sequences lead to the movement of ions and solvent in and out of the films and consequently volume changes take place which generate the actuation process.
The experiments show that when the applied potential for actuation is outside the range of −0.25 and 0.25 V, deflection occurs for both reduction and oxidation of the PPy film (Fig. 2). They also show that the
Conclusions
The actuation behaviour of PPy bilayers produced at different polymerization potentials (0.85–1.5 V) was tested using different electrochemical methods (chronoamperometry and cyclic voltammetry), which showed an effect of polymerization potential upon actuation. To ensure that the amount of polymer deposited at different polymerization potentials was the same, the charge, related to the surface area of the polymer. For repeat experiments for each PPy bilayer (at least in triplicate) the
Acknowledgements
The research was supported from DFG 428 Macromolecular networks (Freiburg, Germany) and partly by the European Union through the European Social Fund (MTT76).
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