Electro-Mechanical Response of Nematic Elastomers: an Introduction
We review in these lecture notes some of our recent work on modeling the response of nematic elastomers to applied mechanical loads and/or to electric fields, both in the static and in the dynamic regime. Our aim is to compare theoretical results based on
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Introduction
In these lecture notes we focus on the electro-mechanical behavior of one specific material: nematic elastomers. It is a new material, so our understanding of it is still incomplete. Among its distinguishing features are large spontaneous deformations, actuation by many different means including electric fields, and mechanical compliance. This makes it suitable for fast soft actuators and, in particular, for new applications such as artificial muscles, which are currently of great technological interest. The reader is referred to the monograph by Warner and Terentjev (2003) for a detailed account of the chemistry and physics of nematic elastomers, and for an extensive list of references. The mechanism for electro-mechanical coupling is the anisotropy of dielectric constants, as it is typical for liquid crystals. Nematic Liquid Crystal
R. W. Ogden et al. (eds.), Mechanics and Electrodynamics of Magneto- and Electro-elastic Materials, © CISM, Udine 2011
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A. DeSimone
Displays (LCDs), which represent one of the biggest market arenas for technological devices based on electro-mechanical coupling, exploit precisely this mechanism. Indeed, a localized applied voltage is able to change the local orientation of nematic molecules, which in turn results in a change of optical properties: the material can change from being transparent to opaque when sandwiched between crossed polarizers, giving rise to a very reliable optical micro-shutter. Individual pixels of LCDs are realized in this fashion. We notice that the mechanism for electro-mechanical coupling based on dielectric anisotropy is different from those based on either permanent or induced polarization, which occur in ferroelectric and piezoelectric materials, respectively. Indeed, nematic elastomers are neither ferroelectric nor piezoelectric. Nematic elastomers provide a counterpart in the world of rubbery solids to nematic liquid crystals. Thanks to the coupling with nematic degrees of freedom, their entropic elasticity can be activated by temperature changes (similarly to what happens in shape-memory alloys, SMAs), electric fields (like in electro-active polymers, EAPs), or by irradiation with UV light. The lessons one can learn by studying this fascinating model material may provide very useful insight on the behavior of many other interesting systems.
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Molecular Structure and Macroscopic Response
Nematic elastomers consist of cross-linked networks of polymeric chains containing nematic mesogens. The three main chemical constituents of this assembly are a polymer backbone, nematic mesogens, and cross-linkers. The polymer backbone results from the repeat of monomers containing tetra-valent atoms, such as Carbon (C) or Silicon (Si), that are able to form long and flexible chains. In these geometries, two bonds are used to construct a connected chain, while two more bonds are free and available for attachment of side units (see Figure 1). Nematic mesogens are rigid rod-like molecules containing benzenic rings. They are responsible for the establishm
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