Electron Spin Resonance Studies of Order and Dynamics in a Nematic Liquid Crystal Containing a Dispersed Hydrophobic Aerosil

Abstract

We have studied a system composed of a hydrophobic aerosil (R812) dispersed in the liquid crystal 4-noctyl-4′-cyanobiphenyl (8CB) using the spin probe electron spin resonance (ESR) technique, and in particular, we have determined, for different aerosil concentrations, the temperature dependence of the orientational order parameter, 〈P 2 〉, and the rotational diffusion coefficient, D ⊥ , of the probe 5-doxyl stearic acid in the ordered and isotropic phases of the system. We have found that increasing the silica concentration up to 10 wt % does not significantly change the transition temperatures of the system. The probe order parameter is instead depressed, and we found that the β exponent of an empirical Haller-type equation, used to fit its temperature dependence, changes roughly linearly with the aerosil concentration. The concentration effect on the probe dynamics is relatively small in the isotropic phase, where the D ⊥ temperature dependence is well fitted for all of the systems with an Arrhenius-type equation. In the nematic phase, the dynamical behavior is more complex: we found that, while local probe motion is still rather fast even when the macroscopic behavior is gellike, the temperature dependence of D ⊥ is still of Arrhenius-type up to 3 wt % aerosil concentration but it becomes of Vogel-Fulcher-Tammann-type for the 10 wt % R812 system.

Chapter 2. Computational approaches for simulating motional EPR spectra

“…It has also found successful applications to EPR spectral simulations of partially ordered soft matter systems with doped spin probes such as liquid crystals. [51][52][53][54] For instance, Zanonni and co-workers have employed the SLE method for the fitting of EPR spectra of n-cyanobiphenyl (nCB) nematic liquid crystals doped with the cholestane (CLS) and methyl-5-DOXYL-stearate (5DS) spin probes. 28,53,55 This has allowed the authors to extract rotational diffusion coefficients of molecules along different directions and also to investigate the effects of the dispersed nanometric size silica nanoparticles on the director distribution of the mesogens as well as molecular behaviour in polymer-dispersed LC (PDLC) composites.…”

“…[51][52][53][54] For instance, Zanonni and co-workers have employed the SLE method for the fitting of EPR spectra of n-cyanobiphenyl (nCB) nematic liquid crystals doped with the cholestane (CLS) and methyl-5-DOXYL-stearate (5DS) spin probes. 28,53,55 This has allowed the authors to extract rotational diffusion coefficients of molecules along different directions and also to investigate the effects of the dispersed nanometric size silica nanoparticles on the director distribution of the mesogens as well as molecular behaviour in polymer-dispersed LC (PDLC) composites. More recently, Polimeno and co-workers have studied nematic liquid crystalline environments by performing SLE simulations of the EPR spectra of the prototypical nitroxide probes 4-hexadecanoyloxy-2,2,6,6-tetramethylpiperidine-1-oxy and pmethylthiophenylnitronyl nitroxide in isotropic and nematic phases of nematic liquid crystals.…”

EPR spectroscopy and molecular dynamics modelling: a combined approach to study liquid crystals

“…Over the years, Claudio Zannoni's group has produced eminent contributions to the study of various LC phenomena by EPR. Examples of applications include the study of polyliquid crystalline states [4], determination of the helix pitch and handedness of induced cholesteric mesophases, CONTACT Vasily S. Oganesyan v.oganesyan@uea.ac.uk including the ones induced by a chiral probe [5,6], analysis of the effects of dispersed hydrophobic aerosol particles and solutes such as azobenzene on the order and dynamics of nematic LCs [7,8] and determination of director configuration in the twist-bend nematic phase [9] as well as applications to lyotropic LCs [10,11]. Figure 1(a) shows two nitroxide spin probes commonly used in LC studies, namely 3-DOXYL-5-cholestane (CSL) and 5-DOXYL stearic acid (5DSA) alongside two nematic 4-cyano-4-n-pentylbiphenyl (nCB) LC mesogens.…”

Surface and disorder effects in aerosil dispersed liquid crystals

“…The properties of the phase transitions and the dynamics and structure of these dispersions have been studied by e.g. theoreticians [4][5][6], calorimetry [7,8], NMR [9], X-ray scattering [10][11][12], light scattering [13,14], computer simulations [15], and ESR [16]. Given the large number of papers in this field, the selection in this introduction should be considered indicative.…”