Anchoring and droplet deformation in polymer dispersed liquid crystals: NMR study in an electric field

Abstract

We report a deuterium NMR study of nematic droplets, formed in a polymer dispersed liquid crystal (PDLC). An electric field with variable strength and direction orthogonal to the NMR magnetic field is applied to this liquid crystal, confined to submicrometer quasispherical cavities with planar surface anchoring. A broad transition from predominantly magnetic field aligned droplets to the electric field aligned limit is observed. The Frank-Oseen free energy with an additional surface energy term is used to find nematic director fields, free energies, and finally simulated NMR spectra of structures in different magnetic and electric fields. We are particularly interested in the influence of surface anchoring strength and nonspherical cavity shape on nematic structures and NMR spectra. Cavities are modeled by randomly oriented rotationally symmetric ellipsoids with a Gaussian distribution of elongations measuring droplet deformation. Simulated NMR spectra describe the experimental data very well and yield for the surface anchoring strength $(1\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ J/m${}^{2}$ and for the width of the deformation distribution $(15\ifmmode\pm\else\textpm\fi{}1)%$. The agreement with the scanning electron microscopy results indicates that deformations are the main cause of the random orientation of bipolar structures in these PDLC's. Using the known dielectric anisotropy of our nematic liquid crystal we found the electric field in the droplets to be 60% of the undisturbed electric field.