Anodic shrinking

Otero, T.R, and J. Padilla. 2004. Anodic shrinking and compaction of polypyrrole blend. Electrochemical reduction under conformational relaxation kinetic control. J Electroanal Chem 561 167. 

Fig. 5.20. Simulated electric field and deformation of the beam-shaped gel with one end fixed The gel is driven by 4ch electrodes. The surface of the gel facing the anode shrinks, while that facing the cathode expands. This makes the gel to bend toward the anodic electrodes.

I will first discuss the effect of pH on the deformation of PAANa gel which is placed so as to touch the anode. As shown in Table 1, the deformation observed involves only shrinking at the anode side in each solution. It is independent of the pH of the surrounding solution. The ratio of deformation with and without applied fields depends on the nature of the surrounding solution. PAA gel in aqueous hydrochloric acid shrinks slightly under an electric field. 

The deformation in neutral solutions is interesting. A PAANa gel in water shrinks at the anode side, but swells and then shrinks in an NaCl solution. It is suggested that the salt in the solution plays a key role in the swelling. 

Flory osmotic pressures at the anode and cathode sides have been calculated using Eqs. 3-10. In a system of i cations and j anions, the osmotic pressures are given by a summation of Eqs. 3-10. When dn/dt > 0 in an electric field, the gel swells. It shrinks when dn/dt < 0. When n at the anode side becomes larger than that at the cathode side, the gel bends toward the cathode. 

The shrinking of a PAANa gel touching the anode in an NaCl solution is the same phenomenon as that reported by Hamlen et al. When an electric field is applied, Cl ions concentrate near the anode. Therefore, the pH at the anode decreases with time, and the pH change undoubtedly leads to a change from a PAANa gel to a PAA gel. 

The shrinking of a PAANa gel touching the anode in an NaOH solution has been analyzed by Doi et al. [14], They have calculated the osmotic pressure at the anode side using Eq. 16. At the anode, H+ ions are produced by the electrolysis of water, and this suppresses the dissociation of carboxyl groups near the anode. As a result, n at the anode side decreases very quickly. Thus the gel shrinks when it is kept in contact with the anode. 

There are two explanations for the small shrinkage in hydrochloric acid. One is that the deformation may be caused by the electrostatic forces between the anode and the negatively charged polyions in the gel. The other relates to the screening effect of COO H + by CP which comes in from the D phase. As a large number of CP ions restrain interactions between COO H+, the gel will shrink. 

The deformation of a PAANa gel in water, placed to separate two electrodes, has been studied by Shiga et al. [23]. UV spectra of a PAANa gel at the anode side have been measured with and without an electric field. The absorption of COONa decreases with time in an electric field, while that of COOH increases. The shrinking has been concluded to occur through a change in the chemical structure of PAANa to PAA. 

As the electric current passes through this system, the cathode (negative electrode) grows in thickness while the anode (positive electrode) shrinks. At the cathode, M+ ions are converted to M atoms, which results in growth of the cathode. From this observation, it is clear that the cations are primarily responsible for conductivity, and this is the result of a vacancy type of mechanism. In this case, the positive ion vacancies have higher mobility than do the vacancies that involve negative ions. 

It is well known that polyelectrolyte gels swell, shrink or bend when DC electric current is applied [169]. These properties of gels are applicable for the construction of chemomechanical devices, artificial muscles, energy conversion systems etc. [170]. Osada and co-workers [171] have constructed an eel-like gel actuator on the basis of poly(2-acrylamido-2-methylpropane sulfonic acid) and studied its chemomechanical properties. Polyampholyte gel is bent to the cathode or anode side if it has predominantly negative or positive charges along the macromolecules (Fig. 39) [172]. As seen from Fig. 39, the amplitude of deflection is gradually decreased with the approach to the lEP. This is probably due to... 

From their theory derivations, they concluded that for a cationic gel exposed to an electric field, if the gel comes in contact with the anode, that particular side will undergo shrinking. However, if the gel is sufficiently separated from the anode, die side of the gel near the anode will first swell and then shrink (Doi et al. 1992). As for die cathode side, shrinking of the gel occurs regardless of contact. 

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