Dispersible Shaped Nanoobjects from Bulk Microphase Separation of High Tg Block Copolymers without Chemical Cross-linking

This work presents a facile procedure to fabricate the nanoobjects of block copolymers with lamellar, cylindrical, and spherical morphology via bulk microphase separation and dispersing in selective solvents without chemical cross linking The morphologies of these nanoobjects in solution were stabilized by their cores in a glassy state First the bulk microphase separation of a series of poly(tert butyl acrylate)-block polystyrene (PtBA b PS) block copolymers was conducted to generate the ordered materials with lamellar, cylindrical and spherical morphology with the PS domains as the discontinuous phases as characterized by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) Then by direct dispersing these microphase separated materials into methanol that is a selective solvent of the PtBA segments, the nanoobjects, lamellae, cylinders and spheres, inherited from their bulk structures were obtained As a contrast micellization of these block copolymers directly in methanol all gave spherical micelles Therefore, those nanoobjects of PtBA b-PS via bulk microphase separation were thermodynamic unfavorable in methanol but were fixed by the glassy PS cores Under a refluxing condition of methanol the nanoobjects of lamellae and cylinders were transformed Into spherical micelles of thermodynamic stucture as traced by TEM It was observed that the dispersed lamellae transformed first into nanorods and then into spheres during refluxing, whereas the cylinders gradually became spheres Furthermore the PtBA segments tethered onto the glassy PS cores were hydrolyzed into the poly(acrylic acid) (PAA) segments by treating with trifluoroacetic acid, and as a result, the PAA coated nanoobjects of different morphology dispersed in the basic water were obtained without chemical cross linking