Electrochemically tunable mechanical behavior of bulk nanoporous gold/polypyrrole

Abstract

This thesis explores the viscoelastic modulus and actuation behavior of an electroactuator based on nanoporous gold coated with polypyrrole film (NPG/PPy). The experiments were performed using in situ dynamic mechanical analysis under electrochemical control. Novel mechanical responses of NPG/PPy hybrids have been revealed. First, during the potential sweep, a reversible change in effective elastic modulus of the NPG/PPy hybrid is observed, characterized by alternating stiffening-softening features. This non-monotonous modulus response of NPG/PPy hybrids is different from the linear modulus behavior as a function of electrode potential of non-coated NPG. Moreover, the amplitude of the modulus variations increases with PPy thickness, showing a 7-fold increase for the thickest PPy film (∼ 40 nm) coated NPG as compared to non-coated NPG. It has been suggested that the modulus behavior strongly correlates with the processes in PPy matrix during doping and dedoping of the polymer. The competition between the enhanced intermolecular bonding due to the charged polymer chains-anion interactions and softening of PPy caused by the incorporated solvent has been suggested to lead to the non-monotonous modulus behavior. Second, the actuation strains of NPG/PPy electroactuator under both potential jumps and potential sweeps have been found to be invariant under increasing applied load. The actuation strain of NPG/PPy hybrids comprises of two parts: The free stroke induced by the swelling of PPy which is independent of load and the strain caused by the stiffness variations which is load-dependent. The findings of the modulus behavior mentioned above help to distinguish the strain contributions of these two factors to the total strain. These experiments allow evaluating a contribution of the electrochemically modulated elastic modulus of NPG/PPy to the actuation strain. Depending on the predeformation of the material as well as on the oxidation or reduction state of PPy, this contribution adds an expansion or contraction to the actuation strain. Yet, in the range of the applied loads tested, the actual performance is not affected by the load, demonstrating an enhanced load-bearing efficiency of NPG/PPy electroactuators in contrast to non-coated NPG.