Polyelectrolyte Gels PG

As early as the end of the forties, studies about water-swollen polymer gels converting chemical energy into mechanical work were reported [184-186]. Reversible contractions and dilatations, due to reversible ionizations of suitable groups (for example polycarboxilic (-COOH) groups), are obtained by alternating addition of alkalis and acids. Katchalsky denoted such transformations as mechano-chemical reactions. 

Water swollen hydrogels are generally amorphous without any particularly ordered structure at molecular level. For many years, polymer gels have been studied for the development of low-voltage soft actuators [187-193]. As an example, they can be used to construct thermo-responsive diaphragms capable of automatically opening and closing a valve [194]. They can also show shape memory effects. For instance, a thermal activation of a shape memory gel is shown in Fig. 6.96. 

If a water-swollen cross-linked polyelectrolyte gel is inserted between a pair of planar electrodes and a voltage difference is applied, the material can undergo anisotropic contractions and concomitant fluid exudations [197,198], Electrically induced contractions of the gel are caused by transport of hydrated ions and water in the network (electrokinetic phenomena). In fact, when an outer electric field is applied across a gel, both macro- and micro-ions are subjected to electrical forces in opposite directions. However, macro-ions are typically in a stationary phase, being chemically fixed to the polymer network, while counter ions are mobile and are capable of migrating along the electric field, dragging water molecules with them. 

Actuators with fiber configuration have also been demonstrated. They can be particularly interesting because a small thickness permits reduction of the response time. Modifications of the pH of aqueous media around the fibers (e. g. by electrolysis) are frequently used to induce their dimensional changes [202]. 

Active PG fibers can be obtained from PAN fibers by means of a heating at 220 °C in atmospheric pressure for 5 hours, and then saponification with boiling in 1M (molar) NaOH for 30 min, following the process reported in [203], The procedure transforms the original PAN fibers into swollen fibers of amphoteric amino-carboxylic polyelectroljde gels. An example of preparation of PG samples is also reported on the following web site http //ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-recipe-UA.htm. 

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