José Manuel Muñoz Rodríguez

Artificial muscles from bilayer structures

Abstract A different electrochemical behaviour of polypyrrole in different solvents is observed. Differences are attributed to the presence of interactions between solvent and polymer. These interactions change when the polymer is oxidized or reduced. The high polarity of water promotes an intense interaction with polarons and bipolarons. During oxidation counterions and water molecules penetrate in the polymer promoting an important swelling and molecular strain. Counterions and water molecules are expelled from the polymer during reduction promoting its contraction. The effect is lighter when different solvents are used. A bilayer structure polypyrrole-flexible and inactive polymer was constructed to transform molecular strain in macroscopic movements. An artificial muscle (electrochemomechanical device) was constructed. Electrical currents trigger oxidation or reduction (chemical) processes developing a mechanical movement. The movement can be stopped or inverted by stopping or inverting the current flow.

Parallel kinetic studies of the electrogeneration of conducting polymers: mixed materials, composition and properties control

Abstract The parallel obtention of microgravimetric, electrochemical, electrical and charge-storage empirical kinetics for polypyrole and polythiophene electrogeneration from different systems are analysed, together with other parallel electrochemical and chemical studies. Each system formed by a monomer, a solvent, a salt and an electrode material develops, at least, simultaneous electrochemical, chemical and degradation (passivation) processes during the electrogeneration. The final result is a polymer film formed by a mixed material: an electroactive part and passive lakes. The properties of the film depend on the material composition, which is related to the chemical and electrical conditions of synthesis. Models of interfacial reactions are developed for each system and tailored films are optimised from the kinetic and mechanistic models. The electrogeneration of conducting polymers is concluded to be a fast method, through a complex (and flexible) mechanism to obtain mixed and tailored materials.

CONFORMATIONAL RELAXATION DURING POLYPYRROLE OXIDATION: FROM EXPERIMENT TO THEORY

Abstract The closure of the polymeric entanglement is proposed to occur during the expulsion of small anions from an oxidized polypyrrole film during reduction. The compactness of the polymeric structure rises by polarization at more cathodic potentials. So increasing anodic overpotentials are required to promote conformational relaxation, allowing the penetration of counterions into the polymer matrix, as was observed experimentally from thick (15 μm) polypyrrole films in 0.1 M LiClO 4 /acetonitrile solutions. The relaxation controlled oxidation is not uniform: it is initiated on different points on the polymer-electrolyte interface, as was confirmed by chronoamperometry. The developed electrochemical relaxation model, including this like-nucleation behaviour of the relaxation process, is able to simulate experimental chronoamperograms. A good agreement between theoretical and experimental results was obtained.

Electrochemomechanical and Electrochemopositioning Devices: Artificial Muscles

A bilayer (3×1 cm) formed by an electroactive material (polypyrrole) and an adherent, non conducting and flexible polymeric film works like a muscle. Polypyrrole swells and contract during oxidation or reduction. The molecular movement promotes asymmetric strains on the flexible layer which bends. When one of the ends is fixed the free end describes an angular movement of more than 180 degrees. The position depends on the applied potential (electropositioning). The movement rate can be controlled through the intensity of the current flow, the movement can be stopped at any point by stop of the current flow and reversed from any point by current inversion. Bilayers able to trail 200 times their weight along 180 degrees are described. Macroscopic models based on asymmetric strains through the flexible films and microscopic models based on molecular movements during oxidation and reduction are proposed.

Polythiophene electrogeneration on a rotating disk electrode. The influence of water on polymerization and polymer properties

Abstract Polythiophene was electrogenerated from acetonitrile solutions using a platinum rotating disk electrode by consecutive potential sweeps, and potential steps. When the rotation rate increases in a 0.1 M thiophene solution, thinner and more passive films were obtained. An increase of the monomeric concentration from 0.1 M to 0.25 M promotes thick polymer layers at high rotation rates again. A competitive electrodic discharge between monomer and water was confirmed for water contents between 0.04% and 1.24% (by weight). The competitive water discharge is responsible for the polymer passivation, as was confirmed by polymer generation at constant potential in different water contents and by voltammetry of active polythiophene electrodes in the background solution with different water contents. At low monomer concentrations the water flow arriving at the electrode increases at increasing rotation rates developing thinner and more passive layers. The strong acidification close to the electrode promoted by the current flow seems to be the origin of a simultaneous chemical polymerization on blocked electrodes. The extensive polymerization at high rotation rates indicates that polymerization takes place through species grafted to the electrode surface. A model of interfacial reactions according with experimental results is proposed.

Reversible electrochemical reactions in conducting polymers: A molecular approach to artificial muscles

Electrical currents trigger oxidation or reduction reactions in conducting polymers. Changes in volume associated with these redox processes can be transformed into macroscopic movements of more than 180° by the construction of a bilayer: polypyrrole-flexible and inactive polymer (artificial muscle). The effects of the applied potential, the nature of the solvent and the electrolyte concentration on the angular movement of the free end of the bilayer were analysed. The movement accelerates with increasing anodic (or cathodic, when the movement is reversed) overpotentials, with increasing electrolyte concentration or by using more polar solvents, leading to the conclusion that the movement is linked to electrochemically driven exchange of hydrated counterions between the solution and the conducting polymer. Geometrical considerations give a simple equation for both the microscopic and macroscopic changes of volume associated to the penetration of counterions during oxidation, which is able to explain the experimental behaviour.

Kinetics of polypyrrole and polythiophene electrogeneration followed by microgravimetry

Abstract The kinetics of polypyrrole and polythiophene electrogeneration were followed by microgravimetry. The generation of polypyrrole from water solutions depends of [ClO 4 − ] 0.8 and [pyrrole] 0.5 . When acetonitrile solutions were used, both reactions orders were 0.5. The empirical kinetics of the polythiophene generation was dependent of [ClO 4 − ] and [thiophene]. The water content influence on the kinetics followed in a solution of pyrrole in acetonitrile. The different behaviour of both monomers, using a rotating disk electrode, and when the temperature of polymerization was modified, along with the great influence of the electrolyte on the kinetics, lead us towards a complex electrodic mechanism. A general model of interfacial reactions is proposed, able to explain the experimental results and to suggest new hypothesis of work.

Solid state transitions during oxidation/reduction processes in conducting polymers. Electrochemical detection and study

Abstract Polypyrrole and polythiophene electrogenerated films were studied in the background electrolyte by potential steps. The kinetic of the oxidation/reduction processes were followed. Both processes have an answer in current density similar to a nucleation process. Neverthless, an amorphous structure was kept before and after oxidation. Those like nucleation processes were controled by the conditions of generation and the polymer degradation state. They were interpreted as a transition in amorphous phase, controled by the opening of the molecular structure allowing the counterions to penetrate. The movement in one point promotes a chain movement arround, growing along the surface in a sigmoidal shape and promoting an increase in conductivity. The oxidized structure is amorphous, like the reduced one, including counterions. The inverse process (counterions expulsion) is easy and the correlative nucleation like a faster process.

Polypyrrole electrogeneration at different potentials in acetonitrile and acetonitrile/water solutions

Abstract A comparative study of polypyrrole electrogeneration on platinum, by polarization at different potentials, in acetonitrile and acetonitrile + 2% of water content, was followed, through microgravimetric determination of the polymer grafted onto the electrode. The presence of water favours the polymerization at each potential. At high anodic overpotentials the polymer production decreases, when the potential rises, in both media. The simultaneous presence of three processes: electrochemical polymerization, homogeneous acid-catalyzed polymerization and water discharge, and the variation of its relative rate with the water content and the potential of polymerization is supported by experimental results. The proposed variation of these relative rates along the polarization times at high anodic overpotentials, allows to postulate the presence of a bilayer on these electrogenerated films.

Optimization of the electrical conductivity of polypyrrole films electrogenerated on aluminium electrodes

Abstract Polypyrrole films were electrogenerated by square-wave potentials, as homogeneous, adherent smooth layers, from NtBu p -CH 3 C 6 H 4 SO 3 in acetonitrile solutions on aluminium electrodes. The conductivities of the obtained films range from 10 to 350 S cm −1 as a function of electrical and chemical variables of synthesis. The observed effects of the preparation conditions on the film conductivity are explained in terms of the polymerization mechanism, in order to prepare high-quality films. The evolution of the conductivity of these polymers as a function of their oxidation states has also been investigated. The conductivity changes from 10 −6 to 3×10 2 S cm −1 when the polymer passes from a neutral state to an oxidized state, by variation of the electrochemical potential from −800 to 200 mV (versus SCE).