Invited Lecture. Molecular order and motion in nematogens from pulsed dynamic N.M.R. A comparative study of model compounds and parent liquid-crystal polymers

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Abstract
Model compounds of semiflexible liquid-crystal polymers (LCPs), specifically deuteriated at various positions of the mesogenic units and aliphatic chains, have been studied by multipulse dynamic N.M.R. techniques. Analysis of the various experiments, employing a density-matrix treatment based on the stochastic Liouville equation, provides new information about the dynamic organization of the different systems. The results, referring to monomers and dimers, are discussed in relation to the properties of the parent LCPs. The pronounced increase in orientational order from the monomers to the dimers can be rationalized by an intramolecular order transfer via highly extended spacers, in agreement with observations for the parent LCPs. A strong dependence of the nematic order on the parity of the spacer (even-odd effect) supports this concept. However, long-range orientational order of both model compounds is completely lost upon crystallization. Thus, despite the fact that dimers already achieve unusually high order parameters in the nematic phase, many of the exceptional properties of LCPs are restricted to systems with higher molecular weights. Molecular motions in the model compounds occur within an extremely broad dynamic range, extending from 10−12 s (internal reorientation) in the fast-rotational to 10−3 s (director order fluctuations) in the ultraslow-motion regime. With respect to these dynamic properties, dimers behave like conventional monomeric liquid crystals, exhibiting much faster motions than the polymers.