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 the 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 &mgr;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 temperature dependence of the smectic order-parameter correlation length in comparison to the bulk sample. In addition, the apparent smectic pretransitional ordering is observed for the silane-treated sample. A theoretical description based on the Landau-de Gennes type approach is developed to explain the experimental data. The surface positional anchoring strength of the silane-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.