Considerations in the determination of orientational order parameters from X-ray scattering experiments

ABSTRACT

An assessment of the data processing and analysis methods used to obtain the second- and fourth-rank orientational order parameters of liquid crystals from X-ray scattering experiments has been carried out, using experimental data from four extensively studied alkyl-cyanobiphenyls and calculated data generated from two general types of theoretical orientational distribution function. The application of a background subtraction and two different baseline correction methods to the scattering profiles is assessed, along with three different methods to analyse the processed data. The choice of baseline correction method is shown to have a significant effect: an offset to zero overestimates the order parameters from the experimental and calculated data sets, particularly for lower order parameters arising from broad distributions, whereas an offset to a value estimated from regions of low scattering intensity provides experimental values close to those reported from other experimental techniques. By contrast, the three different analysis methods are shown generally to result in relatively small absolute differences between the order parameters. We outline a straightforward general approach to experimental X-ray scattering data processing and analysis for uniaxial phases that results in order parameters that match well with those reported using other experimental techniques.

Graphical Abstract

KEYWORDS:

• X-ray scattering
• order parameter
• background subtraction
• baseline offset
• nematic
• orientational distribution
• alkyl-cyanobiphenyl

Acknowledgements

We thank Dr Stephen Cowling for experimental assistance and advice. Data from this article are available by request from the University of York Data Catalogue at https://doi.org/10.15124/57d8b69a-c6d8-4a6a-bfae-39814f572860

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by The Engineering and Physical Sciences Research Council (EPSRC) under grant EP/M020584/1 for the development of dyes for liquid crystal applications and the Core Capability grant EP/K039660/1.