Shear mechanical properties of main chain liquid crystalline elastomers

The behavior of the complex shear modulus G=G'+iG '' of a main chain liquid crystalline elastomer film oriented with the director in the plane of the film is investigated for the first time by shearing the film in a direction perpendicular or parallel to the director. The film exhibits a small mechanical anisotropy around the N-I transition, which disappears slightly above T-NI. A hydrodynamic softening of both G'(perpendicular to) and G'(parallel to) (symbols parallel to and perpendicular to refer to the direction of the director parallel, respectively perpendicular, to the shear displacement) is observed in the isotropic phase near T-NI which is ascribed to the reorientation of the SmC domains revealed by X-rays. These SmC domains are frozen-in the temperature range investigated. They relax with a characteristic time which is deduced from the frequency dependence of the mechanical response of the film. It is shown that the time-temperature superposition method does not apply and that there is no indication in favor of soft or semi-soft elasticity. The influence of a static compression induced either mechanically on the film or by the shape change of the film as a function of temperature is also investigated. Data on the polydomain analogue complement this study.