Speed and strain of polypyrrole actuators: dependence on cation hydration number

Abstract

Polypyrrole films have been characterized by simultaneous cyclic voltammetry driven force-displacement measurements. The aim was to clarify the role of cations in the electrolyte on the speed of response and on the strain of the film. The strain as a function of actuation frequency was studied in alkali metal chloride aqueous electrolytes. The intention was to test the hypothesis of the division of the inserted H 2 O molecules into categories: a smaller number strongly bound to the cation (corresponding to the inner solvation shell) and a larger number entering the film after the insertion of the cations because of forces related to osmotic pressure difference. The two processes have very different time constants: The solvated H 2 O molecules are associated directly with the cations, and are therefore inserted in a faster process, whereas the second type enters the film much more slowly. At higher frequencies, the strain depends almost exclusively on insertion of strongly solvated cations and therefore depends on the hydration number of the cations: Li + (hydration number~5.4) gives more strain than Na + (~4.4) and much more than Cs + (~0) as predicted by the model.

The effect of film thickness on polypyrrole actuation assessed using novel non-contact strain measurements

“…We term this "the steady state" of the actuating film. The greatest increase in the irreversible expansion occurs during the first reduction scan and is believed to be the result of both ion and solvent movements 4,24 We later show that the reversible and irreversible expansion is dependent upon both the film thickness and cycling conditions, although it is also likely to be dependent upon the materials properties of the film as determined by different synthesis conditions such as the modulus of elasticity. Metrics obtained during the stable state are more representative of the long term behaviour and final dimensions of the actuating film and as such are more relevant to those engaged in the design and construction of polymer actuators than metrics determined earlier on in the sequence e.g.…”

“…1 or eq. 2) and osmotic swelling 3,4,7,24 . Figure 6 also reveals a rapid increase in the apparent film thickness (spikes) on switching the potential from -1 V to 0 V for the 10 μm films.…”

Wireless Electrochemical Actuation of Conducting Polymers

“…Communications analog to what has been observed for conventional electrochemical actuation. [14,15] This indicates that ion insertion is very efficient at the rough bottom side due to its higher porosity.S ince assynthesized polymer is in its doped oxidized form, the polymer extremity facing the negative feeder electrode (blue), which should undergo an oxidation reaction, does not show any bending in both cases,b ecause it cannot be oxidized further. Solely water oxidation will occur on this side to compensate the reduction at the negatively polarized extremity.…”

“…[13] Theb igger the hydration shell, the more pronounced will be the expansion and vice versa. Hence,inthe case of Li + the hydration number is larger ( % 5.4) than for Na + ( % 4.4), [14] which allows understanding the more efficient actuation in the first case,…”

Electronic Control of Cell Cultures Using Conjugated Polymer Surfaces

“…[64] The valence of the mobile ions is also important, with less expected volume expansion for di-or trivalent ions. [34] In addition, both osmotic forces [66,67] and migration of ions in the electric field [56] has been suggested to influence the volume changes. Electrochemical control of volume changes has been studied mainly for polypyrrole films.…”