Selected mechanical properties of uniaxial side chain liquid crystalline elastomers

Abstract

This short review gives an overview of selected mechanical properties of uniaxial nematic side chain liquid crystalline elastomers obtained by measurements of the shear modulus performed in the linear regime as a function of temperature and frequency. It focuses on three closely related topics: (a) the determination of the three main shear moduli of the dry elastomers and the influence of the preparation process on these moduli, (b) the evolution of the shear moduli of the dry elastomers when the latter are progressively swollen by a nematic solvent and (c) the influence of a stretching of the dry elastomers on the shear modulus for the situation where the applied sinusoidal shear is parallel to the stretching direction and perpendicular to the initial orientation of the director (reorientation transition). These experiments lead to the following conclusions: (a) the dry elastomers in their un-stretched state are uniaxial rubbers with an internal variable, which are characterized by a small shear anisotropy in the hydrodynamic regime and a Rouse-like frequency behavior in the viscoelastic regime, (b) the elasticity of the network is Gaussian for the elastomers prepared by photo polymerization of a nematic polymer oriented by an electric field, a magnetic field or a surface treatment of the sample-bearing glass slides, and non-Gaussian for the usual twice cross-linked elastomers oriented by a mechanical stretching of the network formed after the first cross-linking step, (c) the concept of soft or semi-soft elasticity does not apply to the un-stretched elastomers, whatever the way they were formed, (d) the soft elastic response expected for the reorientation transition is not observed for an elastomer prepared by UV irradiation.