Smectic polymer micellar aggregates with temperature-controlled morphologies

The morphological control of polymer micellar aggregates is a very important issue in applications such as drug delivery and material science. We report the temperature-controlled formation of nanotubes, nanofibers, ellipsoidal and faceted vesicles, and spherical aggregates by nanoprecipitation of amphiphilic diblock copolymers in dioxane/water mixture. The copolymers used are composed of a cholesterol-based smectic liquid crystal core-forming block and a PEG hydrophilic block. The morphology of the micellar self-assemblies was studied by transmission electron microscopy (TEM), cryo-electron microscopy (cryo-TEM) and atomic force microscopy (AFM). In all these aggregates smectic organization is clearly present in the hydrophobic cores. We propose a smectic "liquid crystallization"-driven self-assembly process for the formation of nanofibers and nanotubes on the basis of small angle X-ray scattering (SAXS) studies during the nanoprecipitation. The temperature dependence of the morphology (from T = 5-55 degrees C) is explained by the free energy consideration. The different aggregates finally dispersed in water after the removal of dioxane are thermally stable at temperature <= 55 degrees C and can be preserved for years at room temperature without structural change.