Molecular microstructure in thermotropic liquid crystals studied by nuclear quadrupole resonance spectroscopy

• Autor(a) principal: Pusiol,Daniel J.
• Data de Publicação: 1998
• Outros Autores: Anoardo,Esteban
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Brazilian Journal of Physics
• Texto Completo: http://old.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331998000400004

Nuclear Quadrupole Resonance studies in Nematic and Smectic C thermotropic liquid crystalline mesophases of p-heptyl azoxyanizole are reported. The local molecular microstructure is modeled from NQR spectra. In the Smectic C mesophase, the NQR data is explained by assuming two features: i) biaxiality in the electric field gradients at the sites occupied by 14N nuclei, and ii) the elemental unit cell of the mesophase is composed by two HpAb molecules. These bimolecular units coexist together with single molecules and the relative proportions change with temperature, the quantity of individual molecules and the relative proportions change with temperature, the quantity of individual molecules being predominant as the temperature approaches the Nematic phase transition. At the lowest temperatures in the Nematic mesophase the NQR spectrum behaves similarly to the corresponding at low temperatures in the SmC mesophase; this means that we can propose again the existence of a huge number of bimolecular and biaxial unit cells. At higher temperatures the transition to single molecular units is deduced from NQR spectra. That transition is characterized by the one describing the passage from a partially disordered molecular system -the system is conserving some local anisotropic properties of symmetry from the crystalline solid state- to a partially ordered system that resembles an oriented liquid.