Calorimetric and small angle x-ray scattering study of phase transitions in octylcyanobiphenyl-aerosil dispersions
- Department of Chemistry and Center for Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)
- Manual Juan Jr. Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
- Center for Microengineered Materials Laboratory, Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131 (United States)
High-resolution calorimetric studies have been made of the liquid crystal phase transitions for several dispersions of 70-{Angstrom}-diam silica spheres (aerosil) in octylcyanobiphenyl (8CB) as a function of silica density {rho}{sub S}. The excess specific heat peaks associated with the nematic-isotropic (N-I) and the nematic{endash}smectic-{ital A} (N-SmA) transitions both exhibit shifts to lower temperatures, decreases in the specific heat maximum values, and decreases in the transition enthalpies as {rho}{sub S} is increased. Two distinct regimes of {rho}{sub S}-dependent behaviors are observed with a crossover between them at {rho}{sub S}{congruent}0.1thinspgthinspcm{sup {minus}3}. For lower silica densities, sharp second-order C{sub p} peaks are observed at the N-SmA transitions, characterized by effective critical exponents that decrease monotonically with {rho}{sub S} from the pure 8CB value toward the three-dimensional {ital XY} value, and two closely spaced but distinct first-order C{sub p} features are observed at the N-I transition. For higher silica densities, both the N-SmA and the N-I transitions exhibit a single rounded C{sub p} 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. {copyright} {ital 1998} {ital The American Physical Society}
- OSTI ID:
- 664701
- Journal Information:
- Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, Journal Name: Physical Review. E, Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics Journal Issue: 5 Vol. 58; ISSN 1063-651X; ISSN PLEEE8
- Country of Publication:
- United States
- Language:
- English
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