Phase behavior of liquid crystals confined to controlled porous glass studied by deuteron NMR

The phase behavior of pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB) liquid crystals confined to controlled porous glass of different characteristic void sizes R is studied using deuteron NMR. In samples with the 5CB liquid crystal the discontinuous isotropic-nematic (I-N) transition becomes gradual for R<0.025μm. The discontinuous I-N phase transition temperature shift ΔTIN scales as ΔTIN∝R^-1.3±0.3. A detailed theoretical analysis of the origins of the temperature shift and the change of the character of the I-N and nematic--smectic-A phase transition is given. The influence of the surface treatment is studied for the 8CB liquid crystal immersed in a R=0.2μm matrix. The results suggest that for the silane-treated sample the anchoring is homeotropic, whereas it is planar for the nontreated surface. In both cases the corresponding anchoring strength is estimated to be larger than 10^-5J/m². The smectic-A ordering seems to be more affected by the confinement in the nontreated sample than the one treated by silane.