Understanding liquid crystal order parameters deduced from different vibrations in polarised Raman spectroscopy

ABSTRACT

Polarised Raman Spectroscopy (PRS) has been used to measure order parameters in liquid crystalline materials for decades. However, it is well known that different values of the order parameters are deduced for the same material when different vibrational modes are used in the analysis. This is an undesirable discrepancy that has somewhat hindered the use of the technique. Here we use two Raman active bands namely the phenyl (1606 cm⁻¹) and cyano (2220 cm⁻¹) stretching modes in the nematic phase of 5CB (4-cyano-4ʹ-pentylbiphenyl) as the example to explore the origin of such discrepancy. Two approaches are proposed in the data a nalysis taking either non-axial or non-cylindrical symmetric vibration into consideration. Together with a systematic discussion based on experimental data, we can conclude that whether or not the vibration satisfies the conditions associated with cylindrical symmetry is the correct physical explanation for the discrepancy in the order parameters.

Graphical Abstract

KEYWORDS:

• Nematic
• polarised Raman spectroscopy
• order parameter
• molecular biaxiality

Acknowledgements

We would like to thank Dr J.C.Li for the use of Material Studio programme and technique support in the University of Manchester. We would also like to thank A. E. Hassett and J. T. Barrett, students at the University of Manchester, for their help on data collection. The measurements were undertaken at the University of Manchester with this paper written after both Z. Zhang and H. F. Gleeson moved to their current address.

Disclosure statement

No potential conflict of interest was reported by the authors.