ESR studies of order and dynamics in a nematic liquid crystal containing a dispersed hydrophobic aerosil

We have studied a system composed of an hydrophobic aerosil (R812) dispersed in the liquid crystal 4-n-octyl-4’-cyanobiphenyl (8CB) using the spin probe ESR technique and, in particular, we have determined, for different aerosil concentrations, the temperature dependence of the orientational order parameter 〈P2〉 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 the systems with an Arrheniustype 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 gel-like, 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.