Instabilities in nematic elastomers in external electric and magnetic fields

Abstract

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 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. In addition, we explain why a twist experiment is probably the most suitable set-up in order to measure the material parameter D1.PACS. 61.30.Vx Polymer liquid crystals -61.30.Dk Continuum models and theories of liquid crystal structure -61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

On the nonlinear stress-strain behavior of nematic elastomers – materials of two coupled preferred directions

“…Later on, relative rotations have been introduced as macroscopic variables into a hydrodynamic description of nematic SCLSCEs by two of the present authors 18 . The concept of relative rotations has since turned out to be very useful in the macroscopic characterization of the materials 11,12,[19][20][21] . Recently, we have extended the expression of relative rotations to the nonlinear regime, so that now also the nonlinear behavior of nematic SCLSCEs can be modeled in this spirit 22 .…”

Bridging from particle to macroscopic scales in uniaxial magnetic gels

“…director rotations can occur relatively to a crosslinked polymer matrix. [59][60][61][62][63][64][65][66][67] Later, relative rotations were also included in a macroscopic characterization of active media for the case in which a dynamic preferred direction can rotate relatively to a surrounding gel matrix. 68 We now have all the variables at hand to reproduce the expression of the generalized energy density F .…”

“…1 for further illustration. The variables of relative rotations and their consequences were initially introduced and afterwards intensively studied in the context of nematic elastomers, where likewise nematic director rotations can occur relatively to a crosslinked polymer matrix [59][60][61][62][63][64][65][66][67]. Later relative rotations were also included in a macroscopic characterization of active media for the case in which a dynamic preferred direction can rotate relatively to a surrounding gel matrix [68].…”

Bio-Inspired Soft Robotics: Tunable Photo-Actuation Behavior of Azo Chromophore Containing Liquid Crystalline Elastomers

“…When the crosslinking was done in the nematic phase, the state of order was fixed within the crosslinking zones, leading to 'frozen-in' order. This can be achieved by exposing the sample to the applied mechanical stress perpendicular to the original direction of the monodomain [11][12][13]. The synthetic strategy proposed by Küpfer and Finkelmann for the preparation of monodomain nematic LCEs was sketched in Figure 1 with chemical structures used.…”

“…mechanical stress perpendicular to the original direction of the monodomain [11][12][13]. The synthetic strategy proposed by Küpfer and Finkelmann for the preparation of monodomain nematic LCEs was sketched in Figure 1 with chemical structures used.…”