Nematic ordering in highly restrictive Vycor glass

Using high resolution calorimetry and deuterium nuclear magnetic resonance we report a systematic study of alkylcyanobiphenyl (nCB) liquid crystals confined to the three-dimensionally interconnected random networks of pores of Vycor glass. Composed of nearly pure ${\mathrm{SiO}}_2$, Vycor has pores with mean diameter and chord length of ∼70 and ∼300 Å, respectively. We studied effects due to the severe and random confinement on the orientational order of the weak first order nematic to isotropic phase transition. For pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB), the nematic to isotropic (NI) transition is replaced by a continuous evolution of local microscopic orientational order extending from 15 K below to 30 K above the bulk NI transition. A modified Landau–de Gennes theory is presented to successfully model the orientational order within this system. From the results of the NMR analysis and with no adjustable parameters, we determine the excess heat capacity due to the orientational order and find it to be in qualitative agreement with the experiment. © 1996 The American Physical Society.