Abstract
High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-Å-diam silica spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of silica density The excess specific heat peaks associated with the nematic-isotropic and the nematic–smectic-A transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as is increased. Two distinct regimes of -dependent behaviors are observed with a crossover between them at For lower silica densities, sharp second-order peaks are observed at the transitions, characterized by effective critical exponents that decrease monotonically with from the pure 8CB value toward the three-dimensional XY value, and two closely spaced but distinct first-order features are observed at the transition. For higher silica densities, both the and the transitions exhibit a single rounded peak, shifting in temperature and decreasing in total enthalpy in a manner similar to that observed in 8CB+aerogel systems. Small angle x-ray scattering data are qualitatively aerogel-like and yield temperature-independent mass-fractal dimensionalities for aerosil aggregates that differ for samples with silica densities above and below the crossover density.
- Received 13 May 1998
DOI:https://doi.org/10.1103/PhysRevE.58.5966
©1998 American Physical Society