Molecular orientation in liquid crystal elastomers

Free-standing and confined anisotropic liquid crystal elastomer films of various thicknesses have been prepared using mesogens with laterally affixed polymerizable side chains. Textures have been studied by microscopy, whereas molecular orienation of the mesogen groups has been investigated by polarized FTIR and Raman spectroscopies considering an uniaxial orientation of their long molecular axis with respect to the rubbing direction. Attenuated total reflectance (ATR) experiments have clearly shown a higher orientation of the mesogen groups near the film surface due to the anchoring effect of the rubbing layer. Inside the polymer, we have shown from transmittance experiments that the mesogens move away from the rubbing direction with a non cylindrical symmetry. In addition, a distribution function with a maximum at 48 degrees from the rubbing direction has been calculated from the < P-2 > and < P-4 > order parameters obtained by polarized Raman spectra. The FTIR study of partially deuterated polymer films has shown that the backbone orientation along the rubbing direction is good inside the polymer whereas it is poor near the surface where the mesogens are well oriented. All these results suggest that the backbone orientation is constrained by the mesogen one. The nonhomogeneous mesogen orientation near the surface of the film is also featured by disclination lines observed by microscopic observations. Finally, experiments performed at different temperatures evidence a cylindrical symmetry of the mesogens above the nematic-isotropic transition temperature, a residual mesogen orientation in the isotropic phase and a mesogen orientation memory in the nematic phase.