On the confinement of liquid crystals in molecular sieves: dielectric measurements

The molecular dynamics of 4-octyl-4'-cyanobiphenyl and 4-pentyl-4'-cyanobiphenyl, confined to the mesopores with diameters less than 25 Angstrom of the molecular sieve AlMCN-41, was investigated over a wide temperature range using broadband dielectric spectroscopy. As expected, the phase transitions of the bulk liquid crystals cannot be detected for the confined molecules. A new, slow relaxational process occurs over the whole temperature range due to the motions of the molecules confined in the pores; the temperature dependence of the characteristic frequency obeys a Vogel-Fulcher-Tamman (VFT) law associated with a glassy state. The orientational order within these small pores is mostly dominated by the local environment. (C) 1999 Elsevier Science B.V. All rights reserved.