Ion Movement and Conducting-Polymer Electrochemistry

Quartz crystal microbalance studies have shown that the movement of the solvent molecules associated with ions can be considerable. Using PPy prepared in sodium dodecyl sulfate, a mix of both cation- and anion-driven processes was seen when cycled in NaCl, and the mass changes involved indicated that four water molecules moved per Cl and 15 water molecules per Na [11]. The role of solvent water molecules has also been examined for PPy in dodecyl benzene sulfonate (DBS), a very widely studied system, where the insertion of cations accounted for only 20% of the mass change upon polymer reduction, indicating that four water molecules were brought into the film with each Na+ [12]. As the electrolyte concentration was changed from 0.1 M to 6 M, the total inserted mass became smaller and the mechanism moved from pure cation transport to an equal amount of anion transport [13]. These results were said to support an osmotic expansion model, whereby the difference in osmotic pressure between the electrolyte and polymer bulk (greater with more dilute electrolyte solutions) drives solvent movement. 

Alongside swelling and deswelling effects, the role of conformational changes in the polymer backbone have been recognized as being equally important for certain systems (see below). 

The first actuators described for conducting polymers in the early 1990s were bilayer or bending-beam stmctures in which a polymer film on a thin conductor would bend to well 

Trilayer PPy actuators have also been constructed in which PPy is electrodeposited on either side of a gold electrode using the large DBS ion on one side (cation-driven PPy) and a smaller benzenesulfonate on the other (anion-driven PPy), which bent quickly in a cooperative manner as the trilayer was oxidized or reduced in 1 M LiC104 [26]. A problem with any design in which the conducting-polymer remains attached to an inert metal 

---