Molecular dynamics of a main-chain liquid crystalline polyester below the crystalline to nematic phase transition

We present a quasielastic neutron scattering (QENS) study of a main-chain liquid crystalline polyester (LCP) containing 4,4'dihydroxy-alpha,alpha'-dimethylbenzalazine as the mesogenic unit. The elastic neutron scattering intensity was monitored in the temperature range 2-400 K, below the crystalline to nematic phase transition (T-m = 423 K). Two selectively deuteriated LCPs were investigated: (a) a polymer deuteriated in the mesogenic units (mD-LCP) and (b) a polymer deuteriated in the flexible spacers (FID-LCP). This made it possible to distinguish the dynamics of the rigid mesogens from the molecular motion of the flexible spacers. A single process was detected in mD-LCP which reflects the motion of the alkyl chains whereas flD-LCP shows an additional low-temperature process which was attributed to reorientational motion of the methyl groups in the mesogens. The slow processes in mD-LCP and flD-LCP are similar, suggesting strong coupling between the dynamics of the flexible chains and the mesogens.