Electroconductive hydrogels: Electrical and electrochemical properties of polypyrrole-poly(HEMA) composites

Composites of inherently conductive polypyrrole (PPy) within highly hydrophilic poly(2-hydroxyethyl methacrylate)-based hydrogels (p(HEMA)) have been fabricated and their electrochemical properties investigated. The electrochemical characteristics observed by cyclic voltammetry suggest less facile reduction of PPy within the composite hydrogel compared to electropolymerized PPy, as shown by the shift in the reduction peak potential from - 472 mV for electropolymerized polypyrrole to - 636 mV for the electroconductive composite gel. The network impedance magnitude for the electroconductive hydrogel remains quite low, ca. 100 Ω, even upon approach to DC, over all frequencies and at all offset potentials suggesting retained electronic (bipolaronic) conductivity within the composite. In contrast, sustained application of + 0.7 V (vs. Ag/AgCl, 3 M Cl^-) for typically 100 min. (conditioning) to reduce the background amperometric current to < 1.0 μA, resulted in complete loss of electroactivity. Nyquist plots suggest that sustained application of such a modest potential to the composite hydrogel results in impedance characteristics that resembles p(HEMA) without evidence of the conducting polymer component. PPy composite gels supported a larger ferrocene monocarboxylate diffusivity (D_appt = 7.97 × 10^-5 cm² s^-1) compared to electropolymerized PPy (D_appt = 5.56 × 10^-5 cm ² s^-1), however a marked reduction in diffusivity (D _appt = 1.01 × 10^-5 cm² s^-1) was observed with the conditioned hydrogel composite. Cyclic voltammograms in buffer containing H₂O₂ showed an absence of redox peaks for electrodes coated with PPy-containing membranes, suggesting possible chemical oxidation of polypyrrole by the oxidant