Thickness strain response

Table 16.1 The thickness strain response and corresponding electromechanical performance of different materials at room temperature...

Many electrostrictive polymers have been developed in the last decade [1] and these newly developed electrostrictive polymers exhibit a high electric-field-induced strain, as shown in Figiure 16.5a, where the maximum thickness strain response of the polymer at different fields is given. A typical relationship between the strain response and the electric field observed in these polymers is shown in Figure 16.5b. All these electrostrictive polymers are polar polymers that contain polar units in the polymer chain. The electrostrictive strain response reflects the change in these polar units due to an electric field. 

These materials have shown thickness strains on the order of 5% with fast response times [18]. Representative results are shown in Rg. 1.5. As seen in the figure, the required fields are quite high as in most electronic EAPs. Strains tend to show a peak and decrease for stresses above and below the peak value. Reported values for irradiated P(VDF-TrFE) show peak strains at 20 MPa, dropping to 50% of the maximum value above 40 MPa and below 5 MPa [112]. Elastic moduli in the range of 0.3-1.2 GPa have been reported with energy densities around 1 MJ m [7]. In order to compete as artificial muscles, strain values will have to be improved. 

A thermoplastic composite pipe produced by the tape winding process consists of two building blocks the mandrel (wall thickness t , internal radius r ) and the load-bearing composite tape (thickness f ) wound under a winding angle a with respect to the axial direction of the pipe and restraining the mandrel (Figure 2). The extruded mandrel is based on one constituent only, i.e. a viscoelastic polymer of the stress/strain response described by ct = f A, B, C, e). The three parameters are relatable to the initial stiffness and the coordinates of the yield point. The... 

MPa can be estimated. One expects that both critical yield tensions and strains increase linearly with the bilayer thickness. Nonlinear responses and memory effects emerge with increasing molecular weight, indicating the onset of chain entanglements at higher molecular weight (151). 

For an isotropic polymer, the thickness ( 3) and traverse (xi) strain responses due to the Maxwell effect are ... 

Figure 16.5 (a) The maximum thickness strain at different electric fields for three newly developed electrostrictive polymers, (b) The relationship between the strain response and electric field for a typical electrostrictive polymer... 

The microlenses of Holmes [17] were bistate types. Chandra et al. utilized a similar mechanism to fabricate a single component, strain-responsive microlens array capable of continuous focus tuning [19]. Figure 6.22 shows the fabrication of the array. A flat PDMS sheet 0.5 mm thick was prepared first. The sheet was clamped at four edges (Figure 6.22a) and then stretched to 20% strain in both planar directions simultaneously (Figure 6.22b). 

The films are epitaxial in the sense that the lattice constant is intermediate between those of copper and nickel. As indicated above, that modulated strain is probably responsible for the increased hardness. Other authors (5) have tried to explain similar effects by stating that the layers were specifically oriented. Our example (6) demonstrates that these considerations must be reexamined since it was possible to achieve the effect in a crystalline multilayer deposited on an amorphous nickel-phosphorus underlayer. It appears that layer thickness is the important parameter here. 

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