Morphological studies of polymeric systems with liquid crystalline order

Abstract

This thesis addresses the structure, properties, and processing of various liquid crystal polymer systems. Morphologies of microphase separated side chain liquid crystal block copolymers are determined, both in the bulk and when confined to thin films. We show for the first time how these materials could potentially be used as a novel kind of surface stabilized liquid crystal display device. Magnetic field induced structures in a thermotropic main chain liquid crystal homopolymer were also examined. Defects in orienting fields are of interest because liquid crystal displays must be defect free over large areas. A study of the interaction of defects in an applied field yields information towards optimally processing these materials in order to minimize or completely eliminate these defects. The basic building block of the systems investigated is a liquid crystal mesogen. Liquid crystal mesogens are easily oriented by electric, magnetic, and flow fields and interact with polarized light such that they are desirable materials for display devices. Additionally, these systems are all polymeric, lending the complexity of the molecules' long-chain nature to the material properties, and the resulting processing challenges and mechanical advantages inherent to polymers. Polarized light microscopy (PLM), high resolution scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), small angle X-ray scattering (SAXS), and wide angle X-ray scattering (WAXS) are the tools employed. Two-dimensional Ndel inversion walls formed in a nematic monodomain during magnetic realignment of a highly elastically anisotropic thermotropic liquid crystal polyether are examined via PLM and AFM.

(cont.) he elastic anisotropy is defined as [epsilon] [approximately equal to] (kll 1 - k33)/(kll + k33), where kll is the Frank elastic constant for splay distortions and k33 is the Frank elastic constant for bend distortions. For this polymer, [epsilon] [approximately equal to] +0.5; kll - 3k33. Following PLM examination, the detailed director patterns of these walls are imaged at high-resolution by AFM through a lamellar decoration technique, employed after walls have been fixed by quenching while under field. Walls form as closed loops comprised of a continuous inversion wall with antiparallel director alignment of the interior region with respect to that of the exterior. The energy of N6el walls is theoretically evaluated as a function of elastic anisotropy. 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 walls in this polyether as measured by AFM yields an effective elastic constant of 1.6 x 10-6 dynes at 160⁰C. Inversion wall dynamics may be followed through time-under-field experiments. Loops coalesce, shrink, and smooth their curvature, occasionally splitting into a partial loop 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 expected wall energetics based on the elastic anisotropy of the polymer. The bulk microdomain morphologies of eleven different polymers, seven with chiral mesogens based on biphenyl benzoate as pendent ...