X-Ray Diffraction by Thermotropic Main-Chain Polymers Having Side Groups. Part A. Diffraction Theory

The diffraction of x rays by parallel extended chain molecules is examined first by considering the nematic packing of rigid-rod-like chains. Typically, such arrays produce disk-shaped intensity distributions about 00 points along the meridian in reciprocal space and hk0 reflections along the equator corresponding to a hexagonal net. The introduction of periodic flexible spacers tends to diminish the long-range order along the chains so that only one or two 00 reflections remain. Concurrently, the packing of the parallel chains becomes less regular, thereby broadening and weakening the equatorial reflections while permitting increased interchain interactions. This can lead to short-range ordering at right angles to the chains as well as smectic layering of the mesogenic units. The latter resembles small-molecule smectic ordering while the former is unique to mesomorphic polymer chains and is characterized by the appearance of off-axis intensity maxima in reciprocal space. Keywords X Ray diffraction, Liquid crystalline polymers.