C-13 NMR INVESTIGATION OF MOLECULAR ORDER IN LIQUID-CRYSTAL POLYSILOXANES

High-resolution variable-temperature13C NMR experiments have been carried out on a series of side-chain liquid-crystal polysiloxanes. The13C chemical shift dependence on temperature observed in the mesophases is a clear indication that the polysiloxanes orient in the magnetic field. Results have been interpreted in terms of molecular order and local dynamics of the different parts of the molecule: spacer, mesogenic core, and terminal group of the side chain. It has been shown that the spacer and the terminal group do not have the same influence on the order parameter associated with the mesogenic core. The apparent variations of the order parameter along the side chain reflect the existence of several motional processes occurring in the oriented mesophase. Among them are the trans-gauche conformational changes in the spacer, the internal rotation of the phenyl rings about their symmetry axis, the motion of the COO plane associated with the β2 transition of polysiloxanes, and the overall rotation of the whole side group about its molecular axis. © 1990, American Chemical Society. All rights reserved.