MOLECULAR-DYNAMICS IN A LIQUID-CRYSTAL WITH REENTRANT MESOPHASES

It is well known that Liquid crystalline compounds with a cyano termin al group can present-peculiar polymorphisms in particular different ty pes of smectic A mesophases and a reentrant behaviour for both nematic and smectic A mesophases. In this work we study by proton NMR relaxat ion the influence of these features on the molecular dynamics of the c ompound 4-cyanobenzoate-4'-octylbenzoyloxyphenyl (DB8CN Sym) in its ne matic (N), partial bilayer smectic A (S-Ad), reentrant nematic (N-re) and reentrant smectic A (S-A1) mesophases. Standard and fast field-cyc ling 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 f ound that the molecular dynamics in the nematic mesophases is rather d ifferent from the molecular dynamics in the smectic A mesophases. Howe ver, 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 m esophases. Order director fluctuations and rotations/reorientations ar e the most important relaxation mechanisms in the nematic mesophases i n the lower and higher frequency Limits, respectively, while self-diff usion 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 associ ated with layer undulations with the frequency law T-1 similar to v, a re only important to the spin-lattice relaxation on the low part of th e frequency spectrum (v < 10 kHz). The inclusion in the relaxation stu dy of a contribution from the cross-relaxation between protons and nit rogen 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 necess ary to consider one more contribution to the relaxation in the analysi s of the 1/T-1 data in the S-Ad and high temperature N mesophases. Thi s contribution is associated with a dynamic process of dissociation an d recombination of molecules in groups that could be present in this k ind of systems as predicted in the literature to explain the layer thi ckness detected in the S-?(Ad) mesophases. The characteristic time for this process was estimated.