Permeant molecular sieving with electrochemically prepared 6-nm films of poly(phenylene oxide)

Ultrathin films (5-7 nm) of poly(phenylene oxide) produced by electrochemical polymerization of tetramethylammonium phenoxide onto Pt and Au electrodes from acetonitrile solutions exhibit molecular size-selective permeabilities to solution redox species that can be controlled by details of the film deposition process. The poly(phenylene oxide) films are stable in both organic and aqueous environments; permeabilities in water are at least 200 times less than in acetonitrile. Cross-linking of the poly(phenylene oxide) films is indicated at polymerization potentials greater than the peak oxidation of the phenoxide monomer (+0.4 V vs SSCE) and upon exposure to electrogenerated mediator/oxidants such as ferricinium. Extended electrochemical polymerization yields poly(phenylene oxide) films with permeabilities of 6 × 10-13 cm2/s to 0.1 M [Fe(CN)6]-3 in aqueous solution; this corresponds to the passage of only 5 molecules/s across each molecule-sized (ca. 100 Å2) section of film surface. Double-layer capacitances in both aqueous and acetonitrile solutions are decreased by only 3-10-fold at the poly(phenylene oxide)-coated electrodes, indicating that the polymer film does not appear to bind to the metal in a manner that blocks formation of an ionic space charge at the metal surface.