Single crystals of the poly(L-lactide) block and the poly(ethylene glycol) block in poly(L-lactide)-poly(ethylene glycol) diblock copolymer

Single crystals of the poly(L-lactide) (PLLA) block and the poly(ethylene glycol) (PEG) block in poly(L-lactide)-poly(ethylene glycol) diblock copolymer were obtained by melt crystallization. The morphology, structure, and evolution process of the single crystals were investigated using transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and real-time atomic force microscopy (AFM). Two types of crystal morphology were obtained. One was the regular morphology of a single crystal: lozenge-spiral dislocation, lozenge multilayer, and hexagonal (or truncated-lozenge) multilayer, which was layer-by-layer structure. The foregoing crystallization of the PLLA block determined the regular morphology of single crystal. The other type of crystal morphology was that the layer-dendritic crystal, which was affirmed to be the PEG crystal by real-time AFM, formed at the edge of the regular single crystal and grew along some certain directions on the crystal surface of the PLLA block. The forming of layer-dendritic crystal was through the reorganization of metastable phase crystal of the PEG block. The SAED results indicated that the (001) plane of the PLLA crystal was parallel to the (10-4) plane of the PEG crystal and the substrate. The foregoing crystallization of the PLLA block had an effect on the crystal orientation of the PEG block.