Conducting polymer actuators bending structures

FIGURE 16.9 Beam actuator with conductive polymer electrodes, (a) Structure for bending actuator and (b) deflection of beam. 

The most diffused actuating configuration, in which these materials are used, is represented by the so-called unimorph bilayer bender. This kind of actuator consists of a film of active material coupled to a passive supporting layer. The bilayer structure is operated within an electrochemical cell, having a liquid electrolyte in which the device is immersed. The active polymeric layer of the actuator works as one electrode of the cell, while a counter electrode and a third reference electrode are separately immersed in the electrolyte. One end of the bilayer is constrained, while the other is free. The potential difference applied between the electrodes causes red-ox reactions of the conducting polymer. Since the CP and the passive layers are mechanically interlocked, when the polymer swells/shrinks the passive layer, which can not modify its dimensions, transforms the CP linear displacement into a bending movement of the structure [238-242]. Very similar is the bimorph structure. In this case the passive layer is substituted by a second CP film and they work in opposition of phase. 

The intercalation of dopants to conducting polymer chains leads to an increase in volume of up to 30 % [8], This property is used in actuators (polymer-based artificial muscles). Bilayer structure of polypyrrole-based anode and cathode is a simple model. At anode, p-doping of polymer occurs to swell, while the other side shrinks because of the expulsion of counterions. This volume changes promote a bend of the layers. The change of poles cancels the volume changes and gives rise to the movement in the opposite direction. 

Fig. 2 Schematic structure of bending actuators, (a) bilayer and (b) trilayer actuators operating in liquid electrolyte, (c) trilayer and (d) pseudo-trilayer actuators containing their own electrolyte and operating in open-air. fVB working electrode, CE counter-electrode. Ref reference electrode. ECP electronic conducting polymer, PVDF Polyvinylidenefluoride. IPN interpenetrating polymer network...

Polymer composite actuators with a bilayer structure of different coefficients of thermal expansion (CTE) can generate bending displacement. When one side of the film is introduced with conductive fillers, the actuator can be electrothermally driven. PDMS/CNT composite was reported to be driven by the thermal expansion of the PDMS with CNTs as... 

The electrodes bonded to the top and bottom surfaces of the polymer film become part of the structure as shown in Figure 16.7. For example, the electrodes are commonly thin strips of gold (for good conductivity) that are much stiffer than the polymer material. Figure 16.7a and b shows the structure of a piezoelectric stress gauge and a bending actuator. 

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