Ion conductivity and permselectivity measurements of polypyrrole membranes at variable states of oxidation

Electro-chemo-biomimetics from conducting polymers: fundamentals, materials, properties and devices

Conjugated conducting polymers, intrinsic conducting polymers or conducting polymers are complex and mixed materials; their electroactive fractions follow reversible oxidation/reduction reactions giving reversible volume variations to lodge or expel charge-balance counterions and osmotic-balance solvent molecules. The material content (reactive macromolecules, ions and water) mimics the dense intracellular matrix gel of living cells. Here the electropolymerization mechanism is reviewed highlighting the presence of parallel reactions resulting in electroactive and non-electroactive fractions of the final material. Conducting polymers are classified into nine different material families. Each of those families follows a prevalent reaction-driven exchange of anions or cations during oxidation/reduction (p-doping/p-dedoping or n-doping/n-dedoping). Polyaniline families also follow reaction-driven exchange of protons. The polymer/counterion composition changes for several orders of magnitude in a reversible way with the reversible reaction. The value of each of the different composition-dependent properties of the material also shifts in a reversible way driven by the reaction. Each property mimics another change in functional biological organs. A family of biomimetic devices is being developed based on each biomimetic property. Those electrochemical devices work driven by reactions of the constitutive material, as biological organs do. The simultaneous variation of several composition-dependent properties during the reaction announces an unparalleled technological world of multifunctional devices: several tools working simultaneously in one device. Such properties and devices are driven by electrochemical reactions: they are Faradaic devices and must be characterized by using electrochemical cells and electro-chemical methodologies.

Dual-transmission line modeling of electrochemical processes in polyaniline-cellulose ester composite porous membranes

The charge transport processes in polyaniline (PANI) composite porous membranes have been elaborated in this study using dual-transmission line impedance model conventionally used for macroscopically homogeneous (nanoporous) membranes. Mixed cellulose ester (ME)-PANI porous membranes were prepared using various in situ chemical polymerization techniques including solution- and vapor-phase polymerizations, and two-compartment cell diaphragmatic polymerization. Each technique yielded different PANI deposition site and content in the membranes. As a result, the modeling of electrochemical impedance spectroscopy (EIS) data yielded different model parameters that have been correlated with the PANI content and deposition site (i.e., surface layering versus in-bulk deposition) in the membranes. The modeling results showed that PANI deposition enhanced charge transport by shifting the interfacial transfer mechanism at pore walls from simple double layer charging to the charge transfer involving oxidation of PANI molecular chains deposited at the pore walls of the composite membranes. In addition, in-bulk PANI deposition in the membranes by means of two-compartment cell polymerization showed several orders of magnitude faster charge transport as compared to the membranes where PANI deposited only at the surface. This study shows that pore-controlled diffusion in PANI composite porous membranes can be satisfactorily modeled using dual-transmission line model and correlated with PANI deposition site in the membranes.

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.

Polymerization of Aniline on Polyaniline Membranes

When solutions of aniline hydrochloride and ammonium peroxydisulfate were separated by a semipermeable cellulose membrane, the reactants met at the membrane and produced a polyaniline (PANI) membrane at the interface. The oxidative polymerization of aniline then proceeded in situ on the PANI-cellulose composite membrane. PANI was produced entirely at the monomer side of the membrane; about 80% conversion of aniline to PANI was observed after 24 h. The oxidation of aniline with peroxydisulfate consists in the transfer of electrons from aniline to the oxidant; it is proposed that electrons pass through the PANI membrane, which is conducting, and electroneutrality is maintained by the simultaneous transfer of protons. The reaction between aniline and peroxydisulfate thus takes place without the need for both reactant molecules to be in physical contact. The residual aniline is located only at its original side of the membrane, but the product of ammonium peroxydisulfate conversion, ammonium hydrogen sulfate, was found on both sides of the membrane. Fourier-transform infrared spectroscopy has been used to analyze PANI, the reaction residues and byproducts, and to prove that PANI is protonated with counter-ions of the sulfate type. Using this technique, we have detected only small differences in the molecular structure of PANI prepared with the membrane-separated reactants and in the polymerization when reactants were mixed; also, the molecular weights differed only marginally. The conductivity of both types of PANI was about the same. The repeated polymerization of aniline on the earlier prepared PANI-cellulose membrane yielded similar results, thus confirming the proposed concept of coupled electron- and proton-transfer through the PANI membrane.

The use of conducting polymers in membrane-based separations: a review and recent developments

As a material family, pi-conjugated polymers (also known as intrinsically conductive polymers) elicit the possibility of both exploiting the chemical and physical attributes of the polymer for membrane-based separations and incorporating its electronic and electrochemical properties to enhance the separation figures-of-merit. This review article, although by no means comprehensive, provides a current snapshot of the investigations from many research laboratories in the use of conducting polymers for membrane-based separations. The review focuses primarily on polyaniline, polypyrrole, and substituted-polythiophene and includes applications in gas separations, liquid (and/or vapor) separations, and ion separations. Additionally, we discuss the broad challenges and accomplishments in membrane formation from conducting polymers.