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NMR study of PF₆̄ doped polypyrrole : a potential candidate for artificial muscles Tso, Chien-Hsin


Conducting polymer actuators, such as polypyrrole, are promising materials for novel applications because they are more muscle-like and potentially less expensive than common motors. A variety of NMR techniques are employed to examine the ion content and dynamics of PF-₆, doped polypyrrole films in different oxidation states. The ion content decreases linearly with decreasing electrochemical potential, directly confirming the ion insertion mechanism of polymer actuation. With known ion content and deposition current, a doping level (dopant ion/pyrrole) of 0.26 was determined for the as-grown film. A T₁ relaxation study reveals that the rotational correlation time of PF-₆ ions in the oxidized film (10.76 ps) is similar to that in the solvent (8.08 ps), suggesting the ions are located inside solvent pockets rather than at stable sites in the polymer matrix. 1D Nuclear Overhauser Effect (NOE) difference experiments confirm the solvated ion environment in oxidized films, but reveal a drastic decrease in the NOE enhancement factor in reduced films, implying that polypyrrole undergoes a significant structural change when reduced. This change leads to a much less solvated environment as experienced by the dopant ions in the reduced state. Translational motion of the PF-₆ ions in the oxidized films at two orientations is probed via self-diffusion coefficient measurements made using pulsed-field gradient (PFG) NMR. The D values obtained at different diffusion times range from 3 x 10-⁸̄ cm²/s to 5 x 10-⁹ cm²/s. The echo attenuation obtained by PFG NMR experiments not only gives direct information on ionic diffusion but also serves as a potential tool to explore the pore morphology of doped polypyrrole films.

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