Synchrotron X-ray study of liquid crystalline polyoxetanes containing two mesogenic groups connected by a flexible spacer in the side chain

The study of the mesophase behaviour of a family of new side chain liquid crystalline polymers by means of synchrotron X-ray diffraction is reported. These systems contain two bibenzoate rigid units in the side chain that are interconnected through a flexible spacer. A polymerisable oxetane unit is linked to one end of the dimer structure, and a flexible terminal chain is linked to the other end. All the polymers from phases with variable degree of order (from low ordered smectic to crystalline) depending on the chemical constitution of the different segments. The influence of the length, parity, and lateral substitution of the spacers on the transitional parameters and the symmetry of the mesophases that are formed is analysed. Close similarities were found between the phase behaviour of the monomer precursors and the derived polyoxetanes. The arrangement of the mesogens, the flexible groups and the polymeric backbone in the layered structure is discussed with respect to the L/d ratio. Different values were obtained depending on the parity of the central spacer and on the degree of order of the mesophase. Interpenetrated structures, where the flexible groups of different lengths are mixed, seem to be compatible with low ordered smectic phases, but sterically disfavoured when building up highly ordered mesophases.