An NMR study of PF6−PF6− ions in polypyrrole

A variety of NMR techniques are employed to examine the abundance, dynamics, and translational diffusion of PF6−PF6− ions in doped polypyrrole films in various oxidation states. NMR is employed as a non-invasive alternative to elemental analysis for quantifying ion content, which is found to decrease linearly with decreasing electrochemical potential, directly confirming the ion intercalation mechanism of polymer actuation, and demonstrating the capacitive nature of the polypyrrole film/electrolyte system. With known ion content, sample mass, and deposition current, the relative amounts of pyrrole, ions, and solvent in the films can be determined. A T1T1 relaxation study along with 1D nuclear Overhauser effect (NOE) difference experiments reveal that the rotational correlation time and solvent accessibility of PF6−PF6− ions in the oxidized films are similar to those in the solvent, indicating the ions experience a solvated environment, and do not sit at stable sites in the polymer matrix. A drastic decrease of the NOE enhancement, along with changes in relaxation behavior and electrical conductivity in reduced films implies that polypyrrole undergoes a significant structural change when reduced. This change leads to a much less solvated ion environment, and may be responsible for the expansion of films sometimes observed upon reduction. Translational motion of the PF6−PF6− ions in the oxidized films is probed via diffusion measurements made using pulsed-field gradient NMR. The diffusion coefficients found (∼5×10−9∼5×10−9 cm2/s) are an order of magnitude smaller than those typically extracted from impedance spectroscopy measurements; this is explained in terms of field driven ion migration in the impedance spectroscopy case.