Morphologies and Energies of Néel Inversion Wall Defects in a Liquid Crystal Polyether

Abstract

Two-dimensional Néel inversion walls formed in a nematic monodomain during magnetic realignment of a highly elastically anisotropic thermotropic liquid crystal polyether (ε ≈ +0.5; k11 ≈ 3k33) are examined via polarized light microscopy (PLM). The detailed director patterns of these walls are then imaged at high resolution by atomic force microscopy (AFM) through a lamellar decoration technique, employed after the walls have been fixed by quenching while under a field. Walls form as closed loops composed of a continuous inversion wall with antiparallel director alignment of the interior region with respect to that of the exterior. The energy of Néel walls is theoretically evaluated as a function of elastic anisotropy, and it is shown that, in the extreme of ε = +1.0, indicating easy bend distortion, a Néel bend wall is 58% lower in energy than a Néel splay wall. Correspondingly, for easy splay distortion, i.e., ε = −1.0, the situation reverses. In our experimental system, the energy of a splay wall is 17% higher than that of a bend wall. The variation of the characteristic width of the walls in this polyether as measured by AFM yields an effective elastic constant of 1.6 × 10-6 dyn at 160 °C. Inversion wall dynamics can be followed through time-under-field experiments. Loops coalesce, shrink, and smooth their curvature, occasionally splitting into partial loops terminated by two opposite-strength 1/2 disclinations. The observed more rapid shrinkage of splay-distortion-rich wall segments as compared to that of bend-rich wall segments corresponds to the wall energetics expected on the basis of the elastic anisotropy of the polymer.