Thermotropic biaxial nematic order parameters and phase transitions deduced by Raman scatteringC. D. Southern1, P. D. Brimicombe1, S. D. Siemianowski1, S. Jaradat1, N. Roberts1, V. Görtz2, J. W. Goodby2 and H. F. Gleeson1
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
received 27 December 2007; accepted in final form 17 April 2008; published June 2008
published online 26 May 2008
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 30C 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.
61.30.-v - Liquid crystals.
61.30.Gd - Orientational order of liquid crystals; electric and magnetic field effects on order.
78.30.-j - Infrared and Raman spectra.
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