Rheology and shear orientation of a nematic liquid crystalline side-group polymer with laterally attached mesogenic units

The shear orientation of a nematic side-group liquid crystalline polymer (SG-LCP) with side-on fixed mesogenic units was investigated by means of rheo-birefringence and rheo-small angle light scattering. Rheological properties of the polymer depend on molecular weight. Macroscopically aligned samples were obtained in creep experiments, but the orientation was destroyed by oscillatory shear flow. The shear modulus of a low molar mass sample in the nematic and isotropic phase superposed to a common master curve. The low-frequency modulus of a high molar mass sample, however, decreased at the phase transition, and time-temperature superposition was not possible. Preshearing led to a further decrease of the modulus. Rheo-birefringence measurements revealed a clear influence of molar mass on both the final value and the orientation state after cessation of shear. Rheo-small angle light scattering also showed a strong influence of molecular weight. In polarizing microscopy, a banded texture was observed with a high molar mass sample. The polymer showed a pronounced pretransitional behavior. This regime slightly above the phase transition revealed a strong decoupling between the polymer backbone and the mesogens. Different relaxation times were found for creep recovery and birefringence, respectively.