Fast Field-Cycling NMR Relaxometry Study of Chiral and Nonchiral Nematic Liquid Crystals

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• 作者：P.J. Sebasti茫o ; A. Gradi拧ek ; L. F. V. Pinto ; T. Apih ; M. H. Godinho ; M. Vilfan
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：December 15, 2011
• 年：2011
• 卷：115
• 期：49
• 页码：14348-14358
• 全文大小：1157K
• 年卷期：v.115,no.49(December 15, 2011)
• ISSN：1520-5207

文摘

We present a proton NMR relaxometry study of the molecular dynamics in three liquid crystalline systems: 4鈥?n-pentyl-4-cyanobiphenyl (5CB), (S)-4鈥?(3-methylpentyl)-4-cyanobiphenyl (5CB*), and a 12% weight mixture of 5CB* in 5CB. The proton spin鈥搇attice relaxation time (T₁) was measured as a function of temperature and Larmor frequency in the isotropic, nematic, chiral nematic (N*), and smectic A phases of these liquid crystalline systems. A unified relaxation model was used to analyze the molecular dynamics, considering local molecular rotations/reorientations, translational self-diffusion, and collective motions as the relaxation mechanisms that contribute most effectively to the T_tack">1^tack">鈥? relaxation. Additionally, in the chiral nematic phase a fourth relaxation mechanism associated with the rotations induced by the translational diffusion along the helical axis (RMTD) was included in the model. All experimental results were consistently analyzed taking into account the physical parameters known for 5CB. The global analysis of the experimental results shows that the RMTDs are associated with the pitch value measured for the N* phases and that its contribution to the T_tack">1^tack">鈥? dispersion is observed at low frequencies. The T_tack">1^tack">鈥? dispersion in the smectic A phase of 5CB* is strongly dominated by the layer undulations relaxation mechanism over a broad frequency range from the low kilohertz regime to tens of megahertz. It was the first time such behavior was observed in a low molecular weight liquid crystalline system.