[en] Within this work we present Brillouin and ultrasonic investigations performed on the liquid crystals p-methoxybenzylidene p-(n-butylaniline) (MBBA), p-azoxyanisol (PAA), 4-cyano-4-n-alkylbiphenyles (nCB), and 4-n-pentoxybenzylidene-4'-n-octoaniline (5O.8). Special attention has been paid to the evolution of a significant splitting of the two longitudinal polarized acoustic modes within the nematic phase. Angle-resolving Brillouin spectroscopy allowed the determination of the complete stiffness tensors and hence a discussion of the propagation of quasitransverse polarized acoustic modes in the nematic state of liquid crystals. We analyze the behavior of the eigenvectors of the relevant acoustic modes and discuss their significance for the propagation of quasishear modes. For 5O.8 we report an acoustic instability within its nematic state. We confirm the existence of a universal thermal relaxation mechanism of weak activation energy for the nematic state of classical liquid crystals. The observed hypersonic anisotropy is discussed in terms of that relaxation mechanism and is compared with that of glass-forming liquid crystals.
Disciplines :
Materials science & engineering Physics
Identifiers :
UNILU:UL-ARTICLE-2009-107
Author, co-author :
Krüger, Jan-Kristian ; University of Luxembourg > Faculty of Science, Technology and Communication (FSTC) > Physics and Materials Science Research Unit
Grammes, C.
Jiménez, R.
Schreiber, J.
Bohn, K.-P.
Baller, Jörg ; University of Luxembourg > Faculty of Science, Technology and Communication (FSTC) > Physics and Materials Science Research Unit
Fischer, C.
Rogez, D.
Schorr, C.
Alnot, Patrick
Language :
English
Title :
Second-order elasticity of liquid crystals within their nematic state at high frequencies
Publication date :
1995
Journal title :
Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics
ISSN :
1063-651X
Publisher :
American Physical Society, College Park, United States - Maryland