Abstract
Molecular order and dynamics of the CB-C9-CB liquid crystalline dimer exhibiting the nematic (N) and the twist bend nematic (Ntb) phases were investigated by proton NMR spectroscopy, using fields of 0.78 T and 7.04 T, and relaxometry. The first relaxometry experiments for a very wide Larmor frequency domain (8 kHz-300 MHz) on this system, using a combination of standard and fast field cycling NMR techniques, were performed. The spectroscopy results in the Ntb phase allowed us to probe the local molecular orientation relative to the Ntb helix axis. The relaxation data were analyzed considering order director fluctuations (ODF), molecular self-diffusion (SD) and local molecular rotations/reorientations (R) relaxation mechanisms. Global fits of theoretical relaxation models, as a function of temperature and Larmor frequency, for the phases under investigation, allowed for the determination of rotational correlation times, diffusion coefficients, viscoelastic parameters, correlation lengths and activation energies (in the case of thermally activated mechanisms). A clear difference between the structures of the N and Ntb phases was detected from the results of proton spin-lattice relaxation through distinct temperature and frequency dependencies' signatures of the collective modes. Significant pre-transitional effects were observed at the N-Ntb phase transition both from relaxometry and spectroscopy data. The experimental results correlate to data and models for comparable liquid crystalline systems.
Highlights
An alternative hypothesis for the molecular packing in the low temperature nematic phase exhibited by these dimer systems was presented in the literature 11
3.1 Proton NMR Spectroscopy in the N and Nematic Twist Bend phase (Ntb) phases The molecular organization in the Ntb phase is still being discussed and NMR spectroscopy can bring a relevant insight into the problem
Relaxometry results in the isotropic and nematic phases of CBC9CB are similar to those obtained in 5CB, reinforcing the central role that the individual monomers play in these phases
Summary
An alternative hypothesis for the molecular packing in the low temperature nematic phase exhibited by these dimer systems was presented in the literature 11. Proton NMR relaxation is an experimental technique useful for the study of molecular dynamics, especially when the results are obtained over large Larmor frequency domains, revealing the corresponding wide range of molecular movements timescale. Besides molecular rotations/reorientations and self-diffusion that are sensitive to the local molecular arrangements, the detection of collective movements, typical of liquid crystalline systems, are specially useful to the mesophases structural characterization, since this mechanism is mostly determined by the long range molecular disposition 14. In the work presented T1 proton NMR relaxation data were obtained, for the compound CBC9CB, as a function of the Larmor frequency, from 8 kHz to 300 MHz, and of temperature in the isotropic liquid and nematic phases exhibited by the compound. This work was carried out aiming to contribute to the interesting discussions on the physical properties of the new found nematic phase and its molecular organization
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call