Morphological evolution of complex coacervate core micelles revealed by iPAINT microscopy

Structural characterization of polymeric micelles is important to understand the relation between micellar structure and properties for application in e.g. drug delivery, catalysis, and sensing. In this work we demonstrate the potential of iPAINT super-resolution microscopy for the imaging of complex coacervate core micelles (C3Ms). C3Ms are formed out of the flexible cationic diblock copolymer poly(N-methyl- 2-vinyl pyridinium chloride)-b-poly(ethylene oxide) (P2MVP(29)-b-PEO204) and a stiff anionic polyfluorene derivative (PF22). We use iPAINT to 'stain' the C3Ms in a simple, non-covalent fashion to image the morphology of individual micelles. Spherical architectures are detected at low concentrations (< 0.07 mM), while more elongated, non-spherical structures with axial ratios u <= 2.3 are adopted at higher concentrations (> 0.14 mM). The observed ellipsoid-like morphology is unusual and furthermore unexpected for C3Ms of copolymers with a considerably longer corona-forming hydrophilic block than the core-forming ionic block. We attribute the observed morphological transition from spherical to ellipsoidal micelles as concentration rises to the stiffness of the conjugated polyfluorene. (C) 2016 Elsevier Ltd. All rights reserved.