ELECTROCHEMICAL APPLICATIONS OF THE BENDING BEAM METHOD .2. ELECTROSHRINKING AND SLOW RELAXATION IN POLYPYRROLE

Cyclic voltammetry of chloride-doped polypyrrole (PPy(Cl)) in an aqueo us solution of tetraethylammonium chloride and dodecyl benzenesulfonat e-doped polypyrrole (PPy(DBS)) in an aqueous solution of sodium dodecy lbenzenesulfonate is studied. Emphasis is put on the effect of holding the polymers at the negative or positive potential limit on the subse quent cyclic voltammograms. The bending beam method, where an active p olypyrrole layer and a substrate polyethylene layer are bound together to sense volume changes in the active layer, is used to detect the vo lume changes in PPy(CI) and PPy(DBS) during doping and undoping under constant potentials and while the polymers are held at the undoped or doped state. Hysteresis of the first peak on the subsequent cyclic vol tammogram has been observed after holding PPy(CI) at the undoped state and after holding PPy(DBS) at the doped state. Holding PPy(CI) at the doped state or PPy(DBS) at the undoped state does not induce this eff ect. Slow volume shrinking occurs by -0.92% in PPy(CI) held at the und oped state for 10 h and by -0.45% in PPy(DBS) held at the doped state for 10 h. The underlying nature is slow relaxation of the polymer matr ix: the free spaces left in the matrix after the undoping of PPy(CI) d ue to the ejection of dopant anions or after the doping of PPy(DBS) du e to ejection of cations from the matrix are gradually filled in by th e adjacent polypyrrole chain (in doped PPy(DBS) also by the nearby alk yl chains of DBS-). The volume of PPy(Cl) shrinks by -0.20% whereas th at of PPy(DBS) swells by +1.71% during undoping. Re-doping reverses th ese volume changes. Such electroshrinking and electroswelling are indu ced by the transport of charge compensating ions in the course of the redox.