Self-Assembly of Linear−Dendritic Diblock Copolymers: From Nanofibers to Polymersomes

摘要

Abstract: We report the formation of cylindrical micelles, sheet-like micelles, tubular micelles, as well asnpolymer vesicles by a new series of amphiphilic linear dendritic block-copolymers (BCs). The BCs, notednas PEGm-AZOn, are composed of poly(ethylene glycol) (PEG) chains of different molecular weights asnhydrophilic blocks and the first four generations of azobenzene-containing dendrons based on 2,2-nbis(hydroxymethyl)propionic acid (bis-MPA) as hydrophobic blocks (m represents the degree of polymer-nization of PEG, and n is the number of azobenzene units at the periphery of dendron). The polymericnaggregates were formed by adding water to solutions of the BCs in dioxane. The micellar dispersions innwater were finally obtained by removing dioxane via dialysis against water. The morphology of the micellarnself-assemblies was studied by transmission electron microscopy (TEM), cryo-electron microscopy (cryo-nTEM), and atomic force microscopy (AFM). A generation-dependent aggregation behavior was found fornthe series of BCs PEG45-AZOn. Core shell structured nanofibers with an inner diameter of 8 nm werenobserved for the copolymer PEG45-AZO2 (hydrophilic/hydrophobic weight ratio equal to 67/33). Lyotropicnliquid crystalline behavior was detected for the aqueous solution of the nanofibers. The coexistence ofnsheet-like aggregates and tubular micelles was detected for the copolymer PEG45-AZO8 in which thennumber of cyanoazobenzene units is increased to 8 (hydrophilic/hydrophobic weight ratio equal to 33/67).nThe tubular micelles could be intermediates in the sheet-like aggregate-to-vesicle transition. Polymer vesiclesn(polymersomes) with a diameter in the range 300 800 nm were observed for the copolymer PEG45-nAZO16 (hydrophilic/hydrophobic weight ratio equal to 20/80). The membrane of the sheet-like aggregates,ntubular micelles, and polymersomes was shown to have a bilayer structure, as revealed by cryo-TEM. UVnillumination of the aqueous polymersome dispersion induced the formation of wrinkles in the vesiclenmembrane, thus showing that this type of polymeric aggregate is photoresponsive.