Novel liquid crystalline structures of a chiral side chain polymer and its phase transitions

Abstract

Two novel smectic bilayer structures have been identified in an enantiomerically enriched chiral side chain polymer containing the highly dipolar nitrile group at stereocentres. The structures were characterized by electron diffraction, electron microscopy, and X-ray diffraction. In both phases each smectic layer has a bilayer structure with backbones and spacers confined in a thin disordered region between two sublayers of mesogenic segments. One of the structures which we denote as CrE∗ has the unusual feature of having its side chains arranged parallel to the layer normal in spite of its enantiomeric bias and twisted nature. In the second structure side chains are tilted by 34.8° with respect to the layer normal and we denote this phase as CrH∗_c In both structures each sublayer contains three different orientations of orthorhombic (CrE) or monoclinic (CrH∗_c) lattices which are related to one another by rotations of ± 60° about the c-axis. In both the CrH∗_c and the CrE∗ phases, lattices in each sublayer are regularly rotated about the c-axis by 5.9° relative to those in the adjacent sublayer. The observation of a chiral CrH phase is uncommon and in this specific case the structure is unique since the rotation between adjacent layers occurs about the sidechain axis (c-axis) (CrH∗_c) and not about the layer normal (c-axis) (CrH∗_c). We believe the system undergoes a change in molecular organization from CrH∗_c to CrE∗ as a result of a chemical reaction which joins a fraction of the stereocentres through covalent bonds. With increasing temperature the CrE∗ structure was found to transform to a special orthorhombic untwisted smectic phase in which a = 31/2b, denoted here as CrE_h. The structure then transforms to a hexatic S_B phase and finally to a S_A phase at yet higher temperatures.