Electron Spin Resonance studies of order and dynamics in complex liquid crystal systems

Electron Spin Resonance (ESR) technique has proven to be a powerful mean for investigating liquid crystals and model membranes from a molecular point of view [1]. Liquid crystals containing a suspension of silica particles of nanometric size (aerosil) present fascinating fundamental and applicative aspects. From the fundamental point of view, the interest in these “filled nematic” systems stems from the intricacies of the effect of a random perturbation on ordered phases. These complex systems also present technological interest in view of scattering displays exhibiting the so-called “memory effect” [2]. In this study we report on a comparison between the random disorder effects of an hydrophobic (R812) or an hydrophilic (A380) aerosil on the order and dynamics of the 3b-DOXYL-5a-cholestane spin probe in the liquid crystal 4-n-pentyl-40-cyanobiphenyl (5CB) using the ESR technique. Increasing the aerosil concentration up to 10 wt% the probe order parameter is depressed with a larger effect for the hydrophilic aerosil. The dynamics in the isotropic phase is of Arrhenius type and essentially concentration and aerosil-type independent. Deviations from this behavior are observed in the nematic even at 1 wt% of hydrophilic aerosil and are well represented by a Vogel–Fulcher–Tammann type law, suggesting that a glass-like state can be induced by a very small amount of random disorder. [1] L.J. Berliner (Ed.), Spin Labeling, Theory and Applications, Academic Press, New York, 1976. [2] Kreuzer M., Eidenschink R., in Liquid Crystals in Complex Geometries Formed by Polymer and Porous Networks, edited by Crawford G.P., Zumer S., chap. 3, 307-324, Taylor & Francis, London, 1996.