Orientational relaxation in a discotic liquid crystal

TL;DR

This study uses molecular dynamics simulations to analyze how discotic liquid crystals relax orientationally near phase transitions, revealing different relaxation behaviors linked to the nature of the phase boundary.

Contribution

It provides a detailed comparison of orientational relaxation near I-N and I-C phase boundaries in discotic liquid crystals, highlighting the role of long-range correlations.

Findings

Power law decay in orientational relaxation near I-N transition

Absence of power law decay near I-C transition

Growth of orientational pair distribution functions near I-N boundary

Abstract

We investigate orientational relaxation of a model discotic liquid crystal, consists of disc-like molecules, by molecular dynamics simulations along two isobars starting from the high temperature isotropic phase. The two isobars have been so chosen that (A) the phase sequence isotropic (I)-nematic (N)-columnar (C) appears upon cooling along one of them and (B) the sequence isotropic (I)-columnar (C) along the other. While the orientational relaxation in the isotropic phase near the I-N phase transition in system (A) shows a power law decay at short to intermediate times, such power law relaxation is not observed in the isotropic phase near the I-C phase boundary in system (B). In order to understand this difference (the existence or the absence of the power law decay), we calculated the the growth of the orientational pair distribution functions (OPDF) near the I-N phase boundary and also near the I-C phase boundary. We find that OPDF shows a marked growth in long range correlation as the I-N phase boundary is approached in the I-N-C system (A), but such a growth is absent in the I-C system, which appears to be consistent with the result that I-N phase transition in the former is weakly first order while the the I-C phase transition in the later is not weak. As the system settles into the nematic phase, the decay of the single-particle second-rank orientational OTCF follows a pattern that is similar to what is observed with calamitic liquid crystals and supercooled molecular liquids.