Electrical and Electrochemical Characterization of Electroconductive PPy-p(HEMA) Composite Hydrogels

Two groups of polymers that have been the focus of widespread research are hydrogels and conducting electroactive polymers (CEPs). 'Intelligent' hydrogels are highly hydrophilic, cross-linked polymers possessing hydration properties that change in response to specific environmental stimuli such as pH, ionic strength, chemical species, magnetic fields, etc. Conducting electroactive polymers such as polypyrrole, polyaniline and polythiophene are highly conjugated, redox-active polymers with electrical and optical properties that change through many orders of magnitude depending upon redox state (doping). We have formed composites of inherently conductive polypyrrole within highly hydrophilic poly(hydroxyethyl methacrylate)-based hydrogels. These materials retain the hydration characteristics of hydrogels as well as the electroactivity and electronic conductivity of CEPs and are thus called 'electroconductive hydrogels'. The electrical and electrochemical properties of these polymer composites have been 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 Ohms, even upon approach to DC, over all frequencies and at all offset potentials suggesting retained electronic (bipolaronic) conductivity within the composite.