Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication

Abstract

Proton field-cycling nuclear magnetic resonance relaxometry is used to study the spin-lattice relaxation dispersion of selected standard smectic A liquid crystals at different temperatures. Relaxation features at both, in the presence and absence of a monochromatic ultrasonic field are considered. We show that the laboratory-frame spin-lattice relaxation time is mainly governed by translational diffusion. Order director fluctuations (ODF) are less important while rotational diffusion seems to be only relevant near the clearing point. Our study suggests that sonication enhances the ODF contribution in the SmA mesophase. Within the framework of the approach we have outlined, different features associated with the ODF mechanism can be investigated.

On the role of collective and local molecular fluctuations in the relaxation of proton intrapair dipolar order in nematic 5CB

“…It is worth mentioning that, similar to other thermotropic nematics, 24,25,36 the expression for T 1D calculated with the Redfield high-temperature relaxation theory underestimates the effects of the OFD in 5CB. 50 According to the results of this work, the nature of the smectic collective dynamics and rotational diffusion could be adequately probed also by experiments of T 1D . 35,36 In summary, the following main conclusions about intrapair dipolar spin-lattice relaxation at megahertz frequencies are found, which confirm the potentiality of the experiments of proton dipolar order relaxation as a relevant NMR technique in the field of complex systems.…”

Application of field-cycling NMR relaxometry to the study of ultrasound-induced effects in the molecular dynamics and order of mesomorphic materials

“…(10). This refined model successfully explained the observed T 1 dispersions in the smectic A phase [53]. The frequency dependence of those contributions due to individual relaxation mechanisms and those associated to ODF cannot be directly discriminated in the dispersion.…”

“…This result validates the use of the nematic-like sound-director interaction energy in the smectic A phase, as was previously proposed in Refs. [52,53]. Like in the nematic case, the interaction depends on the acoustic susceptibility ξ s (now corresponding to a smectic phase).…”

“…For comparison, theoretical and experimental curves are presented. For further details see Refs [52,53]…”