MOLECULAR-DYNAMICS IN A LIQUID-CRYSTAL WITH REENTRANT MESOPHASES

It is well known that Liquid crystalline compounds with a cyano terminal group can present-peculiar polymorphisms in particular different types of smectic A mesophases and a reentrant behaviour for both nematic and smectic A mesophases. In this work we study by proton NMR relaxation the influence of these features on the molecular dynamics of the compound 4-cyanobenzoate-4'-octylbenzoyloxyphenyl (DB8CN Sym) in its nematic (N), partial bilayer smectic A (S-Ad), reentrant nematic (N-re) and reentrant smectic A (S-A1) mesophases. Standard and fast field-cycling techniques were used for our spin-lattice relaxation's study over a broad frequency range of 6 decades (200 Hz up to 300 MHz). It was found that the molecular dynamics in the nematic mesophases is rather different from the molecular dynamics in the smectic A mesophases. However, the reentrant aspect present in both nematic and smectic A states is not associated to a major difference on the molecular dynamics of the nematic and reentrant nematic or smectic and reentrant smectic A mesophases. Order director fluctuations and rotations/reorientations are the most important relaxation mechanisms in the nematic mesophases in the lower and higher frequency Limits, respectively, while self-diffusion has a very small contribution to the overall relaxation. As for the smectic A mesophases, self-diffusion and rotations/reorientations are the predominant relaxation mechanisms for frequencies above 20 kHz. The collective motions, which for these mesophases have to be associated with layer undulations with the frequency law T-1 similar to v, are only important to the spin-lattice relaxation on the low part of the frequency spectrum (v < 10 kHz). The inclusion in the relaxation study of a contribution from the cross-relaxation between protons and nitrogen nuclei improves the quality of the 1/T-1 data fits in both kinds of mesophases. The combined study of the molecular dynamics in the N, S-Ad, N-re and S-A1 mesophases of DB8CN Sym reveals that it is necessary to consider one more contribution to the relaxation in the analysis of the 1/T-1 data in the S-Ad and high temperature N mesophases. This contribution is associated with a dynamic process of dissociation and recombination of molecules in groups that could be present in this kind of systems as predicted in the literature to explain the layer thickness detected in the S-?(Ad) mesophases. The characteristic time for this process was estimated.