Using the full Raman depolarisation in the determination of the order parameters in liquid crystal systems

The depolarisation ratio for the Raman-active phenyl stretching mode has been measured over the whole of the mesophase range, and the orientational order parameters deduced, in the uniaxial nematic liquid crystal octylcyanobiphenyl (8CB). Linearly polarised light was incident normally on a homogeneo...

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Veröffentlicht in:The European physical journal. E, Soft matter and biological physics Soft matter and biological physics, 2007-10, Vol.24 (2), p.119-127
Hauptverfasser: SOUTHERN, C. D, GLEESON, H. F
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description The depolarisation ratio for the Raman-active phenyl stretching mode has been measured over the whole of the mesophase range, and the orientational order parameters deduced, in the uniaxial nematic liquid crystal octylcyanobiphenyl (8CB). Linearly polarised light was incident normally on a homogeneously aligned sample and a chi(2) minimisation routine performed on the 360 degrees depolarisation ratio profile. The order parameters (P(200)) and (P(400)), together with the differential polarisability ratio, r , are used as fitting parameters and measured as a function of temperature. Interestingly, we show that the value for r , conventionally measured in the isotropic phase and assumed to remain constant, has a clear temperature dependence, ranging from -0.032 + or - 0.008 in the isotropic phase through to -0.245 + or - 0.015 at the nematic-to-smectic A phase transition. The measured order parameters (P(200)) and (P(400)) varied from 0.35-0.55 + or - 0.02 and 0.180-0.245 + or - 0.02, respectively, across the 8 degrees C wide nematic phase range. The values of both (P(200)) and (P(400)) are in excellent agreement with theory, but it is noteworthy that (P(400)) shows a much better quantitative match than has been reported in previous work. Crucially the temperature dependence of r is shown to be a contributing factor in the low (P(400)) values that have been conventionally reported from Raman scattering measurements. The potential for fitting the entire angular depolarisation ratio distribution in liquid crystalline systems that are described by more order parameters, specifically biaxial materials, is discussed.
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subjects Computer Simulation
Condensed matter: structure, mechanical and thermal properties
Crystallization - methods
Equations of state, phase equilibria, and phase transitions
Exact sciences and technology
Liquid crystals
Liquid Crystals - chemistry
Models, Chemical
Models, Molecular
Molecular Conformation
Nephelometry and Turbidimetry - methods
Phase Transition
Physics
Refractometry - methods
Specific phase transitions
Spectrum Analysis, Raman - methods
Structure of solids and liquids
crystallography
Transitions in liquid crystals
title Using the full Raman depolarisation in the determination of the order parameters in liquid crystal systems
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