Low frequency relaxation in liquid crystals in relation to structural relaxation in glass-formers

• Published in: Journal of non-crystalline solids 2011-01, Vol.357 (2), p.740-745
• Main Authors: Urban, Stanisław, Roland, C. Michael
• Format: Article
• Language: English
• Subjects: Center of mass; Condensed matter: structure, mechanical and thermal properties; Crystal structure; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Flip-flops; Fluctuation; Glass-formers; High pressure; High-pressure and shock-wave effects in solids and liquids; Liquid crystals; Liquids; Low frequencies; Mechanical and acoustical properties of condensed matter; Molecular dynamic; Physics; Rotational; Specific phase transitions; Structure of solids and liquids; crystallography; Transitions in liquid crystals
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/j.jnoncrysol.2010.05.093

Liquid crystals (LC) are the state of matter intermediate between isotropic liquids and the crystalline state. LC-forming molecules have strongly anisotropic shapes (rod-like in most cases). This leads to an interaction potential that consists of distance-dependent and orientation-dependent parts. Rotational dynamics of LC molecules falls into two frequency regions. Rotations about the short axes are strongly hindered by the potential barrier and thus coupled to fluctuations of the molecular centers of mass. This in turn causes these longitudinal or “flip-flop” motions, characterized by a relatively large relaxation time τ ||, to exhibit considerable temperature, pressure and volume dependences. Experimental relaxation times determined to date for various LC phases (nematic, smectic A, C, and E) for different thermodynamic conditions (isobaric, isothermal and isochoric) are discussed herein, adopting the formulae applied for characterization of the structural relaxation times of glass-formers (GF). This analysis appears fruitful; in particular, the strength parameter characterizing the steepness of the interaction potential can be determined from the relaxation times, and τ || is independent of temperature and pressure along the nematic–isotropic transition line, similar to the behavior of the structural relaxation time along certain transitions in GFs.