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

Abstract

We present a quasielastic neutron scattering (QENS) study of a main-chain liquid crystalline polyester (LCP) containing 4,4′-dihydroxy-α,α′-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 (flD-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.