Zhihua Cai

Effect of reagent concentrations used to synthesize polypyrrole on the chemical characteristics and optical and electronic properties of the resulting polymer

Abstract Our objectives were to correlate conditions used to synthesize polypyrrole with the chemical characteristics of the resulting polymer, and then to correlate these chemical characteristics with electronic and optical properties of the polymer. The polypyrrole samples were synthesized from aqueous solutions of Fe 3+ and pyrrole. Samples with differing chemical characteristics were obtained by varying the initial concentrations of Fe 3+ and pyrrole in the polymerization solutions. The initial concentration of Fe 3+ in all polymerization solutions was 50 times higher than the initial concentration of pyrrole in the solution. However, the initial concentrations of Fe 3+ and pyrrole varied from 2.0 M Fe 3+ /0.04 M pyrrole to 0.06 M Fe 3+ /0.0013 M pyrrole. This yielded polypyrrole samples having different chain and conjugation lengths and different concentrations of chemical defect sites. We show that both electronic and optical conductivity are affected by these changes in conjugation length and concentrations of chemical defects.

Electrochemical investigations of electronically conductive polymers: Part VI. Mechanism of the redox reactions for the electronically conductive form of polypyrrol

Abstract A small amplitude current-step method has been developed to study redox reactions of polypyrrole at high doping levels (equilibrium potentials between 0.1 and 0.6 V vs. SCE). The polymer is an electronic conductor in this potential window. It has been proven rigorously that both the oxidation and reduction processes of polypyrrole films are purely capacitive in this potential region. The effect of external electrolyte concentration on the film resistance and capacitance was studied. The results suggest that the film ionic conductivity is determined by the concentration of excess free electrolyte inside the film. Charge balancing anions do not contribute to the ionic conductivity. It was also found that the morphology of polypyrrole films is uniform throughout the film thickness. Finally, we present here a simple heuristic model for redox reactions of conductive polymers.

Template-Synthesis: A Method for Enhancing the Ionic and Electronic Conductivity in Electronically Conductive Polymers

Abstract : Template-synthesis entails the synthesis of an electronically conductive polymer (or other material) within the pores of a microporous membrane. The membranes used have cylindrica pores of equivalent pore diameter; as a result, polymeric fibers are obtained, where the diameters of the fibers are determined by the diameter of the pores in the template membrane. We show in this paper that template-synthesized polypyrrole fibers show faster rates of charge transport, during electrochemical undoping, and dramatically higher electronic conductivitie than conventional forms (e.g. electrochemically-synthesized film) of this polymer.

Electrochemical investigations of electronically conductive polymers VII. Charge transport in lightly doped polypyrrole

Abstract We, and other researchers, have shown that the mechanism and rate of charge transport in polypyrrole films (on electrode surfaces) depend on whether the polymer is in its electronically conductive or electronically insulating state. This paper focuses on the mechanism and rate of charge transport in the electronically insulating state, i.e., we describe results of electrochemical investigations of polypyrrole films which were equilibrated at initial potentials negative of −0.30 V versus SCE. With regard to the mechanism of charge transport, we show that this lightly doped material behaves like a redox polymer. In particular, we show that, like other redox polymers, redox reactions for lightly doped polypyrrole begin at the polymer/electrode interface and propagate to the polymer/solution interface. With regard to the rate of charge transport, we show that apparent diffusion coefficients for polypyrrole synthesized from acetonitrile solutions containing 2% added water are significantly lower than charge transport rates in films synthesized in rigorously dried acetonitrile. Finally, we report exchange current densities associated with the oxidation of lightly doped polypyrrole. These exchange current densities are on the order of 4 mA/cm 2 , indicating relatively facile electron transfer kinetics.

Measuring Conductivities of Highly Conductive Membranes

Abstract : The theoretical and experimental aspects of an electrochemical approach for evaluating the conductivities of highly conductive membranes are described. The methods developed involve either a current or a potential step at a solid state electrochemical cell. It is assumed, and we prove, that only capacitive currents flow during these current or potential step experiments. The theory for these methods is derived from classical electrochemical theory for current and potential steps at RC circuits. We show that if the conductivity of the system under study is not too high (less than ca. 0.2/ohm/m), the classical theory provides accurate conductivity data. However, we also show that if the conductivity of the system is greater than ca. 0.2/ohm/m, conductivities obtained from the classical expressions are inaccurate. The modified theoretical analysis developed here, however, yields very accurate conductivity data for such highly conductive systems. The modifications of the classical theory entails accounting for the non-ideal wave shape of real potential or current step waveforms. Keywords: Conductive membranes, Electrochemical cell, Capacitive currents.