smectic-A Elastomers Research Articles

Electroclinic effect in chiral smectic-A liquid crystal elastomers.

Chiral smectic-A liquid crystal elastomers are rubbery materials composed of a lamellar arrangement of liquid crystalline mesogens. It has been shown experimentally that these materials shear when subjected to an electric field due to the electrically induced tilt of the director. Experiments have also shown that shearing a chiral smectic-A elastomer gives rise to a polarization. Roughly, the shear force tilts the directors which, in turn, induce electric dipoles. This paper builds on previous works and models the electromechanical response of smectic-A elastomers using free energy contributions that are associated with the lamellar structure, the relative tilt between the director and the layer normal, and the coupling between the director and the electric field. To illustrate the merit of the proposed model, two cases are considered-a deformation induced polarization and an electrically induced deformation. The predictions according to these two models qualitatively agree with experimental findings. Finally, a cylinder composed of helical smectic layers is also considered. It is shown that the electromechanical response varies as a function of the helix angle.

Mechanics of Homeotropically Ordered Smectic‐A Elastomers with Global Oblate Chain Conformation

Smectic-A elastomers combine one-dimensional translational order of rod-like segments with the rubber elasticity of a polymer network. In recent years, detailed investigations were carried out on elastomers showing a global prolate chain conformation. In this communication, the first experiments on fluorinated SA elastomers exhibiting a global oblate chain conformation are presented, where the polymer chains are on average compressed along the layer normal of the lamellar phase structure. The mechanical anisotropy is studied by means of thermoelastic experiments and stress-strain measurements. For the first time, the layer compression modulus B of smectic elastomers is directly measured. B is significantly larger as compared to conventional low molar mass liquid crystals and decreases significantly with increasing local disorder introduced by the isotropic crosslinker.

Mechanically induced tilt in smectic-A elastomers

Recently it has been predicted that shear forces can unlock the director and the layer normal in smectic-A elastomers, leading to a smectic-C like tilt analogous to the electroclinic effect of chiral SA* phases. In order to investigate this phenomenon, we present detailed X-ray investigations of SA elastomers under shear, comparing networks of different chemical constitution. Upon shear imposed in the layer planes, an induced SC like tilt is observed, which develops linearly with the applied shear angle. With decreasing layer compression modulus B of the smectic network, the effect becomes smaller, and additional layer rotations are found. For elastomers containing fluorinated mesogens, layer normal and director are rigidly locked, despite their large B modulus, and the phase structure remains unchanged under shear strain. The experimental results provide clear evidence for recent theoretical predictions.

Smectic-Ctilt under shear in smectic-Aelastomers

Stenull and Lubensky [Phys. Rev. E 76, 011706 (2007)] have argued that shear strain and tilt of the director relative to the layer normal are coupled in smectic elastomers and that the imposition of one necessarily leads to the development of the other. This means, in particular, that a smectic-A elastomer subjected to a simple shear will develop smectic-C-like tilt of the director. Recently, Kramer and Finkelmann [e-print arXiv:0708.2024; Phys. Rev. E 78, 021704 (2008)], performed shear experiments on smectic-A elastomers using two different shear geometries. One of the experiments, which implements simple shear, produces clear evidence for the development of smectic-C-like tilt. Here, we generalize a model for smectic elastomers introduced by Adams and Warner [Phys. Rev. E 71, 021708 (2005)] and use it to study the magnitude of SmC-like tilt under shear for the two geometries investigated by Kramer and Finkelmann. Using reasonable estimates of model parameters, we estimate the tilt angle for both geometries, and we compare our estimates to the experimental results. The other shear geometry is problematic since it introduces additional in-plane compressions in a sheetlike sample, thus inducing instabilities that we discuss.

Shear-induced tilt in smectic-Aelastomers

Smectic-A elastomers combine the positional long-range order of mesogenic molecules in one dimension with the rubber elasticity of a polymer network. While the influence of uniaxial mechanical fields on the phase structure has been investigated intensively during recent years, the impact of shear forces on the orientation of mesogens remains unclear. We present x-ray experiments under shear strain, showing an induced macroscopic tilt depending on the applied shear angle and geometry of the setup.

Elasticity of smectic-Aelastomers

We present a fully nonlinear model of the elasticity smectic-A elastomers, and compare our results with a wide range of experimental observations: extreme Poisson ratios, the in-plane modulus, the modulus before and after threshold to layer rotation in response to stretches along the layer normal, the threshold strain, the characteristic, and singular rotation of layers after the threshold. We calculate the x-ray scattering from rotating layers and compare with available data. The model is derived in two ways: from geometrical constraints imposed by layers on a nematic elastomer, and from application of statistical mechanics to a microscopic model of the effect of crosslink points confined in a corrugated potential.

One-dimensional long-range order of smectic-A elastomers with diverse crosslinking density

One-dimensional long-range order of smectic-A elastomers with diverse crosslinking density

One-dimensional long-range order of smectic-A elastomers with diverse crosslinking density

The influence of crosslinking a smectic-A polysiloxane on the Landau-Peierls instability is investigated by X-ray scattering techniques. It is observed that the half value width of reflections at small angles increases with increasing crosslinking density, indicating a decrease of layer correlation. In this system, therefore, the crosslinking leads to a destabilisation of the smectic-A phase contrary to a polymethacrylate system. This fundamental property is obviously influenced by the chemical constitution of the crosslinking agent.