Exploring ideality and reality in an archetypal rodlike nematic liquid crystal

ABSTRACT

Numerous theories of molecular ordering in the nematic phase exist that attempt to incorporate the features of weak attractions, excluded volume (aspect ratio), and rigid and non-rigid segments, as well as assess entropic and enthalpic effects. Para-quinquephenyl represents a model rodlike molecule, but study of this liquid crystal presents challenges and surprises given the high temperature of the nematic phase. We present NMR experiments throughout the nematic range on two specifically deuterium-labelled quinquephenyl isotopomers, which we combine with high fidelity molecular dynamics simulations to explore and quantify various aspects of orientational order. The striking agreement between experiment and simulation provides new insights into the role of molecular flexibility and correlations (clustering, director fluctuations) on observed order parameters. We compare several nematic theories with our observations and discuss implications for further theoretical understanding.

GRAPHICAL ABSTRACT

KEYWORDS:

• Deuterium NMR
• molecular dynamics
• Maier-Saupe
• director fluctutations
• molecular flexibility

Acknowledgments

The authors wish to thank Professor Zeev Luz and the wonderful community of scientists he supported and spawned, for expanding our scientific horizons and our understanding of dynamics and ordering in oriented phases. We are also indebted to Professor Demetri Photinos for clarifying discussions. We also thank Dr. Jochem Struppe at Bruker-Biospin for assistance with colecting the SSNMR spectrum on d-PPPPP-d.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported in part by the US National Science Foundation (NSF) under Award [DMR-1810194].