Evolution of the isotropic to nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions

TL;DR

This study investigates how aerosil dispersions affect the isotropic to nematic phase transition in the liquid crystal octyloxycyanobiphenyl (8OCB), revealing liquid crystal specific effects and disorder influences through calorimetry.

Contribution

It provides new insights into the disorder effects on 8OCB phase transitions, distinguishing liquid crystal specific behaviors from general quenched disorder effects observed in similar systems.

Findings

Double heat capacity feature at transition independent of aerosil

Crossover between low and high aerosil density behaviors

Differences in transition shifts and heat capacity suppression between 8OCB and 8CB

Abstract

High-resolution ac-calorimetry has been carried out on dispersions of aerosils in the liquid crystal octyloxycyanobiphenyl (8OCB) as a function of aerosil concentration and temperature spanning the crystal to isotropic phases. The liquid-crystal 8OCB is elastically stiffer than the previously well studied octylcyanobiphenyl (8CB)+aerosil system and so, general quenched random disorder effects and liquid-crystal specific effects can be distinguished. A double heat capacity feature is observed at the isotropic to nematic phase transition with an aerosil independent overlap of the heat capacity wings far from the transition and having a non-monotonic variation of the transition temperature. A crossover between low and high aerosil density behavior is observed for 8OCB+aerosil. These features are generally consistent with those on the 8CB+aerosil system. Differences between these two systems in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover aerosil density between the two regimes of behavior indicate a liquid crystal specific effect. The low aerosil density regime is apparently more orientationally disordered than the high aerosil density regime, which is more translationally disordered. An interpretation of these results based on a temperature dependent disorder strength is discussed. Finally, a detailed thermal hysteresis study has found that crystallization of a well homogenized sample perturbs and increases the disorder for low aerosil density samples but does not influence high density samples.