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

Abstract

We report the formation of cylindrical micelles, sheet-like micelles, tubular micelles, as well as polymer vesicles by a new series of amphiphilic linear-dendritic block-copolymers (BCs). The BCs, noted as PEGm-AZOn, are composed of poly(ethylene glycol) (PEG) chains of different molecular weights as hydrophilic blocks and the first four generations of azobenzene-containing dendrons based on 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) as hydrophobic blocks (m represents the degree of polymerization of PEG, and n is the number of azobenzene units at the periphery of dendron). The polymeric aggregates were formed by adding water to solutions of the BCs in dioxane. The micellar dispersions in water were finally obtained by removing dioxane via dialysis against water. The morphology of the micellar self-assemblies was studied by transmission electron microscopy (TEM), cryo-electron microscopy (cryo-TEM), and atomic force microscopy (AFM). A generation-dependent aggregation behavior was found for the series of BCs PEG45-AZOn. Core-shell structured nanofibers with an inner diameter of 8 nm were observed for the copolymer PEG45-AZO2 (hydrophilic/hydrophobic weight ratio equal to 67/33). Lyotropic liquid crystalline behavior was detected for the aqueous solution of the nanofibers. The coexistence of sheet-like aggregates and tubular micelles was detected for the copolymer PEG45-AZO8 in which the number of cyanoazobenzene units is increased to 8 (hydrophilic/hydrophobic weight ratio equal to 33/67). The tubular micelles could be intermediates in the sheet-like aggregate-to-vesicle transition. Polymer vesicles (polymersomes) with a diameter in the range 300-800 nm were observed for the copolymer PEG45-AZO16 (hydrophilic/hydrophobic weight ratio equal to 20/80). The membrane of the sheet-like aggregates, tubular micelles, and polymersomes was shown to have a bilayer structure, as revealed by cryo-TEM. UV illumination of the aqueous polymersome dispersion induced the formation of wrinkles in the vesicle membrane, thus showing that this type of polymeric aggregate is photoresponsive.

Air–water interfacial behavior of linear-dendritic block copolymers containing PEG and azobenzene chromophores

“…A previous study has demonstrated that the size and morphology of the vesicular structures depends on their hydrophobic/hydrophilic ratio [18]. In the case of PEG-AZO2, long nanofibers have been observed, whereas for PEG-AZO4 sheet-like micelles have been reported.…”

“…The aggregation behavior of PEGAZOn depends on the generation number of the dendritic block; i.e., the hydrophilic/hydrophobic ratio of the LDBC (see Chart 1). Moreover, the photo-responsiveness of the polymeric vesicles was also shown [18].…”

Stimuli-Responsive Polymersomes

“…We have recently reported photo-responsive polymersomes (del Barrio et al 2010) base on amphiphilic PEG-b-azodendron block copolymer with a hydrophobic block based on the fourth generation of aliphatic polyester dendrons functionalized at the periphery with cyanoazobenzene mesogens (PEG45-AZO16). The polymersomes in water were prepared by nanoprecipitation with dioxane/water co-solvents followed by extensive dialyse against water.…”

Synthesis of side-on liquid crystalline diblock copolymers through macromolecular azo coupling reaction

“…The isotropic blocks are highly immiscible in the liquid crystal (LC) phase, the systems are easily segregated to form microphase-separated morphology [6][7][8][9]. The competition between the LC order and phase-separated confinement can result in a rich variety of morphologies and nanostructures like spheres, cylinders, lamellae phase domains with an increase in volume ratio of the LC blocks [8][9][10][11][12][13][14][15]. The amphiphilic liquid crystalline block copolymers (LCBCs) can also self-assemble in selected solvents to form various polymeric aggregates such as spherical micelles, vesicles and nanotubes, which can be used in drug delivery systems, nanoreactors and others [14][15][16][17].…”