Multiple Morphologies and Characteristics of “Crew-Cut” Micelle-like Aggregates of Polystyrene-<i>b</i>-poly(acrylic acid) Diblock Copolymers in Aqueous Solutions

TLDR

Polystyrene‑b‑poly(acrylic acid) diblock copolymers with 80–98 wt % insoluble PS block are used to form micelle‑like aggregates in aqueous solution. The study aims to prepare stable nanosize glassy vesicles of these block copolymers that form spontaneously and can be isolated in water for direct electron‑microscopy analysis. This is achieved by preparing crew‑cut micelle‑like aggregates under near‑equilibrium conditions and characterizing them with transmission electron microscopy. Crew‑cut micelle‑like aggregates display a rich spectrum of morphologies—including spheres, cylinders, bilayers, and a novel compound micelle—whose structure varies with PS content and homopolystyrene addition, constituting the first direct observation of such diversity in a low‑molecular‑weight solvent.

Abstract

Crew-cut micelle-like aggregates of various morphologies prepared from polystyrene-b-poly(acrylic acid), PS-b-PAA, diblock copolymers under near-equilibrium conditions, were studied by transmission electron microscopy (TEM). The insoluble block (PS) contents in the copolymers ranged from 80 to 98 wt %. In spherical micelles, the micelle cores, formed by aggregation of the PS blocks, were generally monodisperse. A comparison between star and crew-cut micelles showed that the latter are distinguished by a low density of corona chains on the core surface and a low degree of stretching of the PS blocks in the cores. As the PAA content in block copolymer decreased, the morphology of the aggregates changed progressively from spheres to cylinders, to bilayers (both vesicles and lamellae), and eventually to compound micelles consisting of an assembly of inverted micelles surrounded by a hydrophilic surface. The compound micelles are believed to be a new morphology for block copolymers. The addition of homopolystyrene to the diblocks changed the morphologies from bilayers or cylinders to spheres. The present system provides the first instance in which all these multiple aggregate morphologies have been observed directly in block copolymers in a low molecular weight solvent with changing copolymer composition. It is believed that this is the only way so far to prepare stable nanosize glassy vesicles of block copolymers which form spontaneously and can be isolated in water and studied directly by electron microscopy.