J. Arias-Pardilla

Sensing and Tactile Artificial Muscles from Reactive Materials

Films of conducting polymers can be oxidized and reduced in a reversible way. Any intermediate oxidation state determines an electrochemical equilibrium. Chemical or physical variables acting on the film may modify the equilibrium potential, so that the film acts as a sensor of the variable. The working potential of polypyrrole/DBSA (Dodecylbenzenesulfonic acid) films, oxidized or reduced under constant currents, changes as a function of the working conditions: electrolyte concentration, temperature or mechanical stress. During oxidation, the reactive material is a sensor of the ambient, the consumed electrical energy being the sensing magnitude. Devices based on any of the electrochemical properties of conducting polymers must act simultaneously as sensors of the working conditions. Artificial muscles, as electrochemical actuators constituted by reactive materials, respond to the ambient conditions during actuation. In this way, they can be used as actuators, sensing the surrounding conditions during actuation. Actuating and sensing signals are simultaneously included by the same two connecting wires.

Reduction and Oxidation Doping Kinetics of an Electropolymerized Donor-Acceptor Low-Bandgap Conjugated Copolymer

The electrochemical synthesis of a new dithienylcyclopentadienone-derivative/3-methylthiopene copolymer was performed by cyclic voltammetry. The obtained material shows redox processes very close to those from the pristine DTCPD. A new redox process at -1.24 V, with a large anodic shift (0.51 V) related to the poly(3-methylthiophene) reduction, indicates the existence of a copolymer with a strong influence of the neighboring (n-doped) DTCPD comonomer. The new copolymer is electrochemically n-doped at more cathodic potentials than -750 mV and p-doped at more anodic potentials than 250 mV, with a bandgap of 1.0 eV. The cations entrance in the film from the solution during n-doping and anions entrance during p-doping for charge balance was checked by QCM. The reduction of the DTCPD part suffers a partial trapping of the negative charges that can be reoxidized only at high overpotentials (>1 V related to the reduction potentials). After polarization of the material at any potential inside the band gap, subsequent p- or n-doping reactions performed by potential steps start by nucleation-relaxation kinetic control, followed by anodic or cathodic, respectively, chronoamperometric maxima. At the maxima, both reactions were checked to occur under chemical kinetic control, allowing the determination of the reaction orders for p- and n-doping processes.

Synthesis, electropolymerization and characterization of a cross-linked PEDOT derivative

The synthesis of a novel electropolymerizable monomer, 2,2′,3,3′-tetrahydro-2,2′-bithieno[3,4-b][1,4]dioxine (THBTD), based on two 3,4-ethylenedioxythiophene (EDOT) moieties connected through the ethylenedioxy bridge is reported. The new monomer paves the way for the development of cross-linked networks based on the EDOT moiety. Polymer films were electrogenerated from monomeric solutions by consecutive potential sweeps or by flow of constant anodic currents and the polymer structure was studied using FTIR. The relationship between the mass of the electrogenerated polymer (after reduction) and the polymerization charge gives the productivity of the consumed charge (mg C−1) while the charge stored and delivered by the films was determined by cyclic voltammetry getting the specific charge (C g−1). Simultaneously with the electroinitiated polymerization a chemical polymerization occurs around the electrode by monomer protonation giving protonated and no electroactive polymer chains. This chemical polymerization was followed in solution by UV-Vis spectroscopy using different conditions. Productivities and specific charges change with the conditions of synthesis in opposite directions. The voltammetric control presents a main redox couple (0.54/0.50 V) and a strong reduction process at −2.89 V that only can be reoxidized at more anodic potentials than 0.3 V as usual for charge trapping effects. EQCM studies indicate a remarkable difference between PEDOT and poly(THBTD). While poly(THBTD) shows a predominantly reversible anionic exchange during oxidation from the neutral state, the parent pristine PEDOT presents a mixed anionic and cationic exchange. Electrochromic color changes from an intense blue color of the oxidized film to a clear orange color in the reduced films as observed by in situUV-Vis spectroscopy. Interestingly, the electrochromic changes in poly(THBTD) are opposed to those of PEDOT. Both thiophene derivative polymers present a similar thermal degradation at 305 and 314 °C, respectively.

Electropolymerization of naphthaleneamidinemonoimide-modified poly(thiophene)

The new polymer presents a p-doping process with anion exchange and its electrochemical reduction with cation exchange during potential cycling. Stored specific charges of 38 mAh g(-1) for the polymer reduction and 13 mAh g(-1) for its oxidation make the material very promising for fast charge/discharge batteries or specialised supercapacitors in which the material is also required as the anode.

Biomimetic Sensing – Actuators Based on Conducting Polymers

Thirty years after the discovery of conducting polymers (CP) and their electrochemical behavior (Chiang et al. 1977; Shirakawa et al. 1977), CP have been proven to be useful for applications in a variety of areas such as batteries and supercapacitors, photovoltaics, memory devices, light emitting diodes, artificial muscles (actuators), biomedical devices, biosensors, corrosion protection etc., thanks to their unique physical and electrochemical properties. Their physical characteristics are similar to those of commodity polymers; but unlike commodity polymers, CP can be subjected to oxidation and reduction just as in the case of metals and inorganic semiconductors. These reactions yield a complex material combination: polymer-ions-solvent. In the field of actuators, conductive polymers have been used with great success. During its oxidation/reduction the volume of the CP film changes. The volume variation can be used to perform a linear movement or an angular movement. The first CP actuators date back to 1992 (Baughman & Shacklette 1991; Otero et al. 1992a; Pei & Inganas 1992b). Since then, much progress has been achieved in materials generation and design and characterization of devices. Different configurations have been designed and studied, and different models were proposed to understand the properties and to improve the performance of those devices. In addition, due to the electrochemical nature of the actuation, any variable acting on the reaction will be detected and quantified by the actuator response; it means that working under constant current, the potential evolution will detect the change. This is a unique device: artificial muscles based on conducting polymers are also sensors, while working, of the environmental variables (Otero 2009). They are sensors of temperature, electrolyte concentration and the current flowing through them. Since they are capable of detecting an obstacle along its route and moving it to performing a work, they possess tactile properties (Otero & Cortes 2003). These unique features, coupled with the easy construction of actuators in many different sizes, from macro to microscopic devices, constitute a technological challenge. Some multinational companies, like Bayer, became involved in this new technology after acquisition of startup companies. The European Union is supporting the European Scientific

Simultaneous Smart Actuating-Sensing Devices Based on Conducting Polymers

Towards the end of 1970s new artificial organic materials, conducting polymers (CP), were discovered (Chiang et al., 1977; Inzelt, 2011; Shirakawa et al., 1977). Since then, most of the scientists working on CP became interested by the fact that their conductivity can shift, in a reversible way, over several orders of magnitude by oxidation/reduction (also called doping/dedoping) processes. The availability of these new organic semiconductors has opened up possibilities to rebuilt electronics and microelectronics producing flexible devices (Guo et al., 2010; Klauk, 2006; Perepichka & Perepichka, 2009; So, 2010).

Electrochemical Kinetics in Dense, Reactive and Wet Gels. Biomimicking Reactions and Devices

Both p-doping and n-doping processes kinetics were studied by potential steps using different polymers from initial states having different conformational compaction degree. At the chronoamperometric maxima exist, in both cases, a chemical kinetic control. The kinetic coefficient, k, and the reaction order related to the active sites in the polymer chains, β, decrease for increasing packed conformations of the initial state. Rising packed conformations are obtained at rising cathodic potentials for p-doping polymers, or inside the band-gap for both p- and n-doping polymers. These materials can be used as sensors: their electrochemical responses change with the electrolyte concentration or temperature.