Liquid Crystal Phases of Colloidal Platelets and their Use as Nanocomposite Templates

Abstract

This thesis explores the gelation and liquid crystal phase behavior of colloidal dispersions of platelike particles as well as the use of such dispersions for the generation of nanocomposites. We report on the sol-gel, sol-glass and liquid crystal phase transitions of positively charged colloidal gibbsite platelets in water over a wide range of particle concentrations and salt concentrations. The natures of the kinetically-arrested states which enclose the sol region in this system are analyzed. We demonstrate that the hexagonal columnar liquid crystal phase of charged silica coated gibbsite particles can easily be formed by centrifugation. In situ growth of silica can stabilize and fixate the liquid crystal structures such that synthetic opals of gibbsite in silica were produced. Part of the thesis is devoted to systems of sterically stabilized gibbsite platelets. We show that platelets with an average diameter larger than 0.5 micrometer do form strongly iridescent hexagonal columnar phases in tetralin within hours, while particles of the same size in water do not crystallize at all. The undulations of the columns and bending of the particle director in these liquid crystals are studied. Furthermore, we describe the formation of ordered nanostructures of colloidal gibbsite platelets in bulk polymer. First, nematic liquid crystal phases of polymer stabilized colloidal gibbsite platelets were prepared. Then, the solvent was polymerized to achieve a mineral-in-polymer nanocomposite. Finally, systems of platelike model colloids other than gibbsite are discussed. Both charge and polymer stabilized hydrotalcites (layered double hydroxides of magnesium and aluminium) are investigated. It is demonstrated that the formation of nematic phases of charged hydrotalcites takes years, while dispersions of sterically stabilized hydrotalcite particles displayed the isotropic-to-nematic phase transitions within days. The orientation of the hydrotalcite platelets in nematic droplets (tactoids) and at the isotropic-nematic interface are discussed. Similarly, this thesis reports on systems of sterically stabilized, natural hectorite clay particles. In toluene, these lathlike particles display nematic tactoid formation and isotropic-to-nematic liquid crystal phase separation. Nanocomposites of hectorite-in-polymer were produced and analysed in high resolution electron microscopy.