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Temperature dependent morphological evolution and the formation mechanism of anisotropic nano-aggregates from a crystalline-coil block copolymer of poly(p-dioxanone) and poly(ethylene glycol)
Temperature dependent morphological evolution and the formation mechanism of anisotropic nano-aggregates from a crystalline-coil block copolymer of poly(p-dioxanone) and poly(ethylene glycol)
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 andcooling were investigated. The changes in size and morphology of the nano-aggregates weremonitored by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic forcemicroscopy (AFM). A semitransparent and uniform dispersion of nano-aggregates with star anise-likemorphology was obtained from PPDOstar-b-PEG at room temperature. The dispersion gradually turnedtransparent during heating to 80 C because of the melting of the crystallized PPDO blocks. The crystalswith low regularity melted first leading to dissociation of the star anise nano-aggregates to flake-likeparticles. The copolymer formed sphere-like micelles when the temperature was high enough formelting all PPDO crystals. During the cooling run, a hysteresis of phase transition was observed becauseof the supercooling of crystallization. The morphological evolution of the copolymer micelle suggestedthat 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 theformation of the star anise-like nano-aggregates. Metastable flake-like nano-particles formed at theinitial stage of crystallization of PPDO blocks. The hydrophobic core of the flake was composed ofseveral crystal lamellae or plates piled up in a layer-by-layer fashion. With further crystallization of PPDOblocks, 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 differentgrowth modes: epitaxial growth with amorphous spherical micelles and interparticle interpenetrationcrystallization in the amorphous region of other flakes. The branched structure of the nano-particleswas therefore formed driven by interparticle interpenetration crystallization and epitaxial crystallizationsimultaneously.
