Glass-like behavior at molecular level induced in a nematic by a dispersion of aerosil nanoparticles

We report on a comparison between the random disorder effects of an hydrophobic (R812) or an hydrophilic (A38O) aerosil on the order and dynamics of the 3/3-DOXYL-5α-cholestane spin probe in the liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) using the ESR technique. Increasing the aerosil concentration up to 3 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 con¬centration and aerosil-type independent. Deviations from this behavior are observed in the nematic even at 0.3 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.