Preparation, characterization and formation mechanism of thermoplastic polyurethane nanostructures using solution...

Template wetting with polymer solutions has not been well understood up to now, and the formation mechanism of polymer nanostructures with this method is still to be revealed. In the present work, thermoplastic polyurethane (TPU) nanostructures were prepared by wetting cylindrical nanopores in an anodic aluminum oxide (AAO) membrane with TPU solution in dimethylformamide (DMF) followed by solvent evaporation. Both tubular structures and solid structures were simultaneously observed at four different concentrations of TPU solutions (i.e., 5, 7, 10, and 15 wt%) by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Furthermore, a transition from a tubular structure to a solid structure was also found at a higher concentration of TPU solution (15 wt%). It is speculated that the top part of the single nanostructure is tubular, and the lower part is solid. A "capillary wetting-evaporation-deposition" mechanism is given to explain such a phenomenon. According to this mechanism, the higher polymer concentration, the larger amount of solution, and the smaller pore size are believed to be beneficial for the formation of solid structures.