Formation of Hierarchical Structure in Microspheres Fabricated with Microfluidic Method based on Homopolymer Blend

Hierarchically unique morphologies were observed inside microspheres with a diameter of 70-80 µm fabricated with the combination of a microfluidic emulsification and a solvent-evaporation method, which were based on binary polymer blends consisting of poly(styrene) (PS) / poly(methyl methacrylate) (PMMA) and poly [4-(N, N-diphenylamino)styrene] (PDAS) / PMMA. The phase separated structure in composites was dependent on type of hydrophobic polymer, the molecular weight of polymers, and the evaporation rate. The particles prepared from high-molecular weight components exhibited macroscopically phase separated structure (incomplete core-shell). In addition, PS / PMMA composite showed the hierarchical microscopic phase separated structure, spherical PMMA domains dispersed in the PS-rich core in the macroscopic core-shell structure. Low molecular PS / PMMA composite formed macroscopically homogeneous, but microscopically bi-continuous phase separated structure. Low molecular PDAS / PMMA composite also afforded the hierarchical structure, where the macroscopical core-shell structure (pseudo core-shell) existed, and the shell contained dispersed micron-sized PDAS domain in PMMA phase, and the core exhibited the bi-continuous structure comprised both components. This hierarchical structure is considered to be formed due to the kinetical limitation, and this was more simplified and thermodynamically more stable structure (sea-island) by lowering the evaporation rate. It is noteworthy that the uniform sized microspheres with hierarchical structure were fabricated only from homopolymer blends without the assist of block or graft copolymer.