Pressure tuning of mass and charge transport in redox-active polymeric networks

The combination of electrochemical techniques and high hydrostatic pressures has allowed a novel way of probing and tuning electron transport in redox polymeric networks, as presented here. Electrochemical studies of quaternized poly(4-vinylpyridine) (QPVP) films placed on electrode surfaces, loaded with ferricyanide ions and exposed to 0.1 M KNO3, were carried out at pressures ranging from ambient to 8 kbar (1 kbar approximate to 1000 atm). At atmospheric pressure, where the films have a gel-type structure, the electrochemical responses show the expected thin-layer behavior for the electron-diffusional process. The application of pressure causes a gradual change leading to a process that, on the same time scale, is governed by semiinfinite linear diffusion. These results reveal that the propagation of charge in these systems is drastically restricted with compression. Permeation experiments show that the transport of mass is restricted under high pressures also. Our data provide evidence that compression alters the microstructure of the film, probably through dehydration, and causes the loss of fluidity and a concomitant decline in the rate of mass and electron transport. We estimate that the electron diffusion coefficient D experiences a decrease of at least 2 orders of magnitude at 8 kbar, from a value of similar to 10(-7) cm(2)/s at ambient pressure. The relation between electron-transport rate and film fluidity is in agreement with previous studies from our laboratory on the QPVP/Fe(CN)(6)(3-/4-)/KNO3 system and yields further evidence for a mechanism of electron transport that is mainly controlled by the physical motion of the redox centers. From our pressure data we infer positive volumes of activation Delta v double dagger for the diffusional process (between 10 and 20 mL/mol at 8 kbar). We also find that the reduction of ferricyanide incorporated in the film involves a volume decrease (volume of reaction Delta v degrees approximate to -30 mL/mol), as it does in solution.