王灝同學論文在Soft Matter發表
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). 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 80oC because of the melting of the crystallized
PPDO blocks. The crystals
with low regularity melted first leading to a dissociation of the star anise
nano aggregates to flake-like particles. The copolymer formed sphere-like
micelles when temperature was high enough for melting all PPDO crystals.
During the cooling run, 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 layer by layer. With further crystallization of PPDO blocks, the flakes tended to aggregate because of the variation
of 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.