Liquid crystalline behavior of octylcyanobiphenyl confined to submicron-size randomly connected porous glasses

Abstract

We report a study of the orientational order via deuterium NMR and high resolution ac calorimetry on octylcyanobiphenyl (8CB) confined to randomly oriented and interconnected macroporous (1000 \AA{} mean pore size) and microporous (100 \AA{} mean pore size) glasses. In the macroporous glass, the nematic to isotropic $(N\ensuremath{-}I)$ phase transition retains the bulklike weakly first order nature. The nematic phase is characterized by an NMR powder pattern spectra, as expected for a director distribution with no preferred spatial direction, and it coexists with an isotropiclike component. The orientational order is slightly suppressed from the bulk's and there is no sudden increase in orientational order due to the onset of the smectic phase suggesting a weaker coupling between nematic and smectic order parameters. Further, due to the confining size, the smectic-$A$ to nematic transition is considerably suppressed and broadened. In the microporous glass, the orientational order behavior is reminiscent of that in Vycor glass. No phase transitions are detected, rather, there is a continuous evolution of local orientational order. Interporous and intraporous interactions are important.