Polyaniline film expansion ratio

Figure 8.4 Dependency of the expansion ratio on the level of reduction, y, for various kinds of electrolytes in polyaniline films at pH = 0. The definition of y is shown by the inset, and y = 0 is taken as the ES state, /q is the length of film at y = 0. BSA is benzene sulfonic acid.

The electrolytic expansion for the thickness direction in polyaniline cast film [20] shows an extremely large expansion ratio of more than 25% as shown in Figure 8.10, and is comparable to that of natural muscles [6]. A similar result was also obtained in the cast film of poly(o-methoxyaniline) for the thickness direction. The large expansion ratio for the thickness direction is conjectured to relate to the condensation process of the cast film. It may be remarked that the evaporation of NMP solution results in shrinkage only in the thickness direction, but not in the area. Therefore, the cast film has more freedom to expand in the thickness direction than that parallel to the film surface. 

Apart from polyaniline, other condncting polymers that are being studied for electrolytic expansion include polypyrrole [11, 15-17], poly(alkylthiophene) [26] and carbon nanotubes [5]. For example, electrochemically prepared polypyrrole films were used to study the qualitative movement of electrolytic expansion by fabricating a bimorph actuator. The movement of bending and stretching of the actuator was demonstrated in electrolyte solution [15]. Actuators fabricated by electrodeposition on gold-coated polyethylene films were studied [11] for the evaluation of expansion ratio and response time. Also, a microactuator of several tens of microns made from two layers of gold and... 

The fundamentals of electrolytic expansion in polyaniline films have been discussed. Ion insertion and exclusion by electrolytic oxidation and reduction are the primary mechanisms. However, it is also evident that the changes in molecular conformations, arising due to the delocalisation of 7t-electrons and the electrostatic repulsion between the polycations, are other mechanisms operating in a conducting polymer microactuator. By investigating the molecular structure and the higher order structure to optimise the electrolytic expansion, it should be possible to improve the expansion ratio and the force for practical usage. 

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