Surface molecular anchoring in microconfined liquid crystals near the nematic–smectic-<i>A</i>transition

Abstract

The effect of the diverging bend elastic constant, ${\mathit{K}}_{33}$, on the nematic director field of 4'-octyl-cyanobiphenyl (8CB) in submicrometer cylindrical cavities in a polycarbonate matrix, as the nematic--smetic-A phase transition is approached, is investigated with deuteron nuclear magnetic resonance ${(}^{2}$H-NMR). The $^{2}\mathrm{NMR}$ spectra and the use of Frank elastic theory enable us to monitor the actual surface anchoring angle profile, determine the density of point defects in the escaped structure, and estimate the anchoring strength to be ${\mathit{W}}_{\mathrm{\ensuremath{\theta}}}$=7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ J/${\mathrm{m}}^{2}$, which is temperature independent. The large value of ${\mathit{W}}_{\mathrm{\ensuremath{\theta}}}$ and the lack of evidence for pretransitional smectic layering in our NMR spectra show that although the orientational anchoring is strong, the polycarbonate surface is too rough to induce substantial surface smectic order in the nematic phase. The saddle-splay elastic constant ${\mathit{K}}_{24}$ for 8CB is estimated to be larger than the elastic constant ${\mathit{K}}_{11}$.