Deuteron nuclear-resonance studies on liquid-crystalline polymers. Collective motions, director dynamics, and viscoelastic properties

Nuclear magnetic resonance (NMR) spectroscopy was used to investigate the molecular dynamics of specifically deuterated main-chain side-chain liquid crystalline polymer systems. Special emphasis was put on the dynamics in the nematic phase. Analysis of Carr-Purcell-Meiboom-Gill (CPMG) experiments showed, that the transverse deuterium-NMR relaxation of the polymers in this phase is dominated by slow collective motions. The dispersion law observed for the CPMG relaxation times is characteristic for nematic director fluctuations. To characterize the dynamics of the nematic director in a strong magnetic field, the time development of the deuterium-NMR spectra of the polymer systems after a step rotation of the probe is monitored. Analysis of the line shapes of the time dependent deuterium-NMR spectra shows that the reorientation after a step rotation by 90 occurs through a spatially periodic transient perturbation of the director field. This analysis yields information about the time development of the director orientation which can be analyzed based on Leslie-Ericksen theory to obtain detailed information about the viscoelastic behaviour of the investigated polymers: four out of five independent viscosities of the nematic phase, as well as the ratio of splay and bend elastic constants can be determined. If one inserts these results into the analysis of the CPMG experiments, one obtains - with the help of a newly developed slow motional model absolute values for the anisotropic elastic constants. Thus, one can determine for the first time an almost complete set of viscoelastic parameters for thermotropic main chain polymers. These results are of great importance for technological application of the polymers. In addition they can be used to confirm postulated scaling laws for the viscoelastic parameters of these systems. It can be shown that the particularly large splay elasticity corresponds well with the theoretical predictions by de Gennes. (Orig.)