Influence of surface treatment on the smectic ordering within porous glass

The influence of the surface treatment on the Sm-A-N phase transition of th e 8CB (octylcyanobiphenyl) liquid crystal confined to controlled pore glass (CPG) matrices is studied. The characteristic linear size of voids in the chosen CPG matrix is 0.2 mu m. The voids' surface was either nontreated or silane treated enforcing tangential or homeotropic anchoring, respectively. In both cases the x-ray measurements reveal a qualitative change of the te mperature dependence of the smectic order-parameter correlation length in c omparison to the bulk sample. In addition, the apparent smectic pretransiti onal ordering is observed for the silane-treated sample. A theoretical desc ription based on the Landau-de Gennes type approach is de,eloped to explain the experimental data. The surface positional anchoring strength of the si lane-treated sample is estimated to be of the order of 10(-4) J/m(2) and at least 100 times weaker for the nontreated case.