Dielectric elastomers polyurethanes

Since the early stages of their development, a wide range of rubbers has been used as dielectric elastomers. The most commonly used elastomers belong to the silicone, polyurethane, and acrylic families. Other elastomers, such as polyisoprenes, fluorinated elastomers,... 

Dielectric elastomers Silicone elastomers Acrylic elastomers Polyurethane elastomers [101... 

Requirements for elastomers used as dielectric elastomers (DEs) are many and are eurrently not all met by one single elastomer. For dielectric elastomers, several good candidates exist, each with their particular strength. In the following sections, the four major types of elastomers are discussed in more detail, namely, silicones, acrylates, polyurethanes, and thermoplastic elastomer (TPE) copolymers. 

In any case, polyurethane dielectric elastomers have continued to be studied in the last decade, particularly with regard to the possibility of increasing their actuation performance. It is well known that both dielectric and mechanical properties are key parameters governing the electromechanical response of any dielectric elastomer, which can be in principle improved by an increase of the dielectric constant and by a decrease of the elastic modulus. In order to increase the dielectric permittivity of a polymer elastomeric matrix, various methods are available (Carpi et al. 2008), such as making composites or blends with highly polarizable phases. Table 1 constitutes a non-exhaustive list of works fi-om the literature, mostly relying on such methods for improving the performance of polyurethane dielectric elastomers. The studies are classified in terms of system complexity and component materials. 

Table 1 Some examples of modified polyurethane-based dielectric elastomers selected from the literature...

The presence of hard and soft domains in segmented polyurethanes also has been confirmed by experimental results using pulsed NMR and low-frequency dielectric measurements. Assink (55) recently has shown that the nuclear-magnetic, free-induction decay of these thermoplastic elastomers consists of a fast Gaussian component attributable to the glassy hard domains and a slow exponential component associated with the rubbery domains. Furthermore, the NMR technique also can be used to determine the relative amounts of material in each domain. 

The polyurethane elastomer A and the nylon B (Fig. 36) cannot be used under high voltage stress or in some electronic applications because of high dielectric losses. Other materials in the same class might be more suitable. The polycarbonate G is more suitable where low dielectric loss is needed than Mylar E over a wide range of frequency and temperature. 

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