화학공학소재연구정보센터
Macromolecules, Vol.34, No.19, 6658-6669, 2001
Morphologies and energies of Neel inversion wall defects in a liquid crystal polyether
Two-dimensional Neel inversion walls formed in a nematic monodomain during magnetic realignment of a highly elastically anisotropic thermotropic liquid crystal polyether (epsilon approximate to +0.5; k(11);, 3k(33)) 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 Neel walls is theoretically evaluated as a function of elastic anisotropy, and it is shown that, in the extreme of epsilon = +1.0, indicating easy bend distortion, a Neel bend wall is 58% lower in energy than a Neel splay wall. Correspondingly, for easy splay distortion, i.e., epsilon = -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 x 10(-6) dyn at 160 degreesC. 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.