writer：H Wang, CL Liu, G Wu, SC Chen, F Song, YZ Wang
keywords：DIBLOCK COPOLYMERS，MICELLAR MORPHOLOGIES，CYLINDRICAL MICELLES，AQUEOUS-SOLUTIONS; OXIDE)，WATER，VESICLES，CORE，MICELLIZATION
source：期刊
specific source：Soft Matter
Issue time：2013年

The morphological evolution and phase transition of a branched crystalline-coil multi-block copolymer, poly(p-dioxanone)-block-poly(ethylene glycol) (PPDOstar-b-PEG), in aqueous solution under heating and cooling were investigated. The changes in size and morphology of the nano-aggregates were monitored by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). A semitransparent and uniform dispersion of nano-aggregates with star anise-like morphology was obtained from PPDOstar-b-PEG at room temperature. The dispersion gradually turned transparent during heating to 80 degrees C because of the melting of the crystallized PPDO blocks. The crystals with low regularity melted first leading to dissociation of the star anise nano-aggregates to flake-like particles. The copolymer formed sphere-like micelles when the temperature was high enough for melting all PPDO crystals. During the cooling run, a hysteresis of phase transition was observed because of the supercooling of crystallization. The morphological evolution of the copolymer micelle suggested that the formation of the star anise-like nano-aggregates was a hierarchical assembly process. A "crystallization induced hierarchical assembly" mechanism was therefore proposed to explain the formation of the star anise-like nano-aggregates. Metastable flake-like nano-particles formed at the initial stage of crystallization of PPDO blocks. The hydrophobic core of the flake was composed of several crystal lamellae or plates piled up in a layer-by-layer fashion. With further crystallization of PPDO blocks, the flakes tended to aggregate because of the variation of the hydrophilic-hydrophobic balance. The active edge of crystalline lamellae in the hydrophobic core of one flake may induce two different growth modes: epitaxial growth with amorphous spherical micelles and interparticle interpenetration crystallization in the amorphous region of other flakes. The branched structure of the nano-particles was therefore formed driven by interparticle interpenetration crystallization and epitaxial crystallization simultaneously.