Doping-induced volume changes in poly(3-octylthiophene) solids and gels

The bending beam method (BBM), a sensitive way to detect and make use of volume change in electrically conducting polymers, is used to study poly(3-octylthiophene) (POT) during electrochemical redox (doping/undoping). Films of POT prepared from soluble, chemically polymerized POT, from gels prepared by electropolymerization, and from gels prepared by chemical polymerization of alkylthiophene and difunctional alkyldithiophene, all expand during doping, and then reversibly contract upon undoping, in organic electrolytes. The volume change is, at least partly, due to the counter-anion transport in the conducting polymer. Overoxidation of the POT makes the film shrink after initial swelling. The chemical structure of the film (the density and character of crosslinks) determines the volume change. The soluble POT solid (linear POT) and electropolymerized POT gel (hence, highly crosslinked) have volume changes around 1–2%. The lightly crosslinked, chemically copolymerized POT gel shows a 9% doping-induced volume swelling, indicating the desired electroelastic property.