1H NMR study of molecular order and dynamics in CBC9CB Liquid Crystal

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 0.78T and 7.04T, and relaxometry. The first relaxometry experiments for a very wide frequency domain (8kHz -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 diffusions (SD), and local molecular rotations/reorientations (R) relaxation mechanisms. Global fits of theoretical relaxation models, as a function of temperature and Lamor frequency, for the phases under investigation, allow 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 structure of the N and the Ntb phases was detected from the results of spin-lattice relaxation through distinct temperature and frequency dependencies’ signatures of collective modes. Significant pre-transitional effects were observed in the N-Ntb phase transition both from relaxometry and spectroscopy data. The experimental results correlate to data and experiments for comparable liquid crystalline systems.