Thermotropic biaxial nematic order parameters and phase transitions deduced by Raman scattering

¹  School of Physics and Astronomy, The University of Manchester - Manchester, M13 9PL, UK, EU
²  Department of Chemistry, The University of York, Heslington - York, YO10 5DD, UK, EU

Corresponding author: helen.gleeson@manchester.ac.uk

Received: 27 December 2007
Accepted: 17 April 2008

Abstract

Raman Scattering was used to investigate biaxiality in the nematic phase formed by the bent-core material, C5-Ph-ODBP-Ph-OC12. Linearly polarised light was normally incident on a homogeneously aligned sample, and the depolarisation ratio was measured over a 360° rotation of the incident polarisation for the Raman-active phenyl stretching mode. By modeling the bent-core structure and fitting to the depolarisation data, both the uniaxial ( P₂₀₀ and P₄₀₀ ) and biaxial ( P₂₂₀ , P₄₂₀ and P₄₄₀ ) order parameters, are deduced. We show unequivocally the presence of a uniaxial to biaxial nematic phase transition approximately 30°C above the underlying smectic phase. Further, we report the temperature evolution of the biaxial and uniaxial order parameters, which increase in magnitude continuously with reducing temperature, reaching values of 0.1, -0.15 and -0.18 for P₂₂₀ , P₄₂₀ and P₄₄₀ , respectively.