Abstract
In this paper we study the macroscopic behavior of nematic side-chain liquid single crystal elastomers exposed to an external electric or magnetic field. For this purpose we use the framework of a continuum model. The geometries investigated comprise the bend and the twist geometry known from the classical Frederiks transition in low molecular weight liquid crystals. For the bend geometry we find a laterally homogeneous and a two-dimensional undulatory instability, which may compete at onset. In the case of the twist geometry three instabilities can occur at onset, two of which are two dimensional and clearly show undulations. As a major result we propose how the values of the twist coefficient K(2) and the values of the material parameters D(1) and D(2) connected to relative rotations between the director field and the polymer network can be determined from experimental observations. In addition, we explain why a twist experiment is probably the most suitable set-up in order to measure the material parameter D(1).
Highlights
The synthesis of nematic side-chain liquid crystal elastomers was first reported in 1981 [1]
In this paper we study the macroscopic behavior of nematic side-chain liquid single crystal elastomers exposed to an external electric or magnetic field
As a major result we propose how the values of the twist coefficient K2 and the values of the material parameters D1 and D2 connected to relative rotations between the director field and the polymer network can be determined from experimental observations
Summary
The synthesis of nematic side-chain liquid crystal elastomers was first reported in 1981 [1]. The director field is uniformly oriented over the whole sample in its ground state These materials have to be generated by special ways of synthesis: during the final crosslinking step the mesogenic units must on average be oriented in a certain direction by the influence of an external field, which can be mechanical, electric, magnetic, or imposed by the boundaries of the system [3,4]. This special direction gets locked in or “frozen in” [5], so that no spontaneous breaking of rotational symmetry occurs at the “transition” to the nematic state. Some details of the analysis are presented in several appendices
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