THERMOTROPIC LIQUID-CRYSTALLINE POLY(ORGANOPHOSPHAZENE)

The liquid crystalline behavior of a poly(organophosphazene) with two 2-(((4-n-butylphenyl)-azo)phenoxy)ethoxy side chains per phosphorus atom has been investigated by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction. On cooling from the isotropic liquid (above 185-degrees-C) a viscous mesophase is first observed which shows the characteristic focal conic texture typical of low molar mass smectic A phases. As the temperature falls below 160-155-degrees-C, a smectic C-like mesophase appears. In these structures, pairs of side chains decorate the backbone at ca. 2.45-angstrom intervals, and this congestion can be relieved in the cis-trans (2(1) helix) conformation when the stacking periodicity becomes 4.9-angstrom. In the smectic phases it is likely that the backbone disorders sufficiently to allow the mesogenic side groups to stack and, aided by the flexible spacer, allow the creation of layers perpendicular to the stacking direction. In the smectic A phase, the layer thickness is ca. 25.5-angstrom, which implies a "bilayer" structure in which the side chains would overlap in an antiparallel interdigitated structure. The smectic A-smectic C transition involves expansion in the plane orthogonal to the polymer chains. The interpenetration of the side groups disappears but the side chains tilt down with respect to the normal to the layer planes to stack closer. Below 150-145-degrees-C the sample crystallizes and in the oriented form can be indexed on an orthorhombic unit cell with alpha = 35.7-angstrom. b = 17.85-angstrom, and c (fiber axis) = 9.85-angstrom. This doubling of the 4.9-angstrom chain repeat has been reported for other polyphosphazene structures and can arise from alternating perturbations of the side chains or a change in the backbone conformation.