Striking Effect of PbPU Multiblock Copolymers on the Morphology Evolution and Performance of PP/TPU Blends before and after the sc-CO2-Foaming Process

In this work, polypropylene/thermoplastic polyurethane (PP/TPU) blends compatibilized by the multiblock copolymer of PP and TPU (PbPU) were used as a model of polymer blends to study the morphology evolution of the blends before and after foaming. Scanning electron microscopy (SEM) with a secondary electron detector (SEM-SE2) was applied to observe the morphology of PP/TPU blends stained by ruthenium tetroxide (RuO4). For the unfoamed blends, the addition of PbPU greatly improved the dispersion degree of PP/TPU blends and promoted the mechanical properties of the blends. Compared to the binary PP/TPU blends, the compatibilized blends presented a better performance to form a cellular structure during the sc-CO2 (supercritical carbon dioxide)-foaming process. It was found that the presence of PbPU dramatically increased the deformation ability of dispersed phases in the cell wall of the foamed blends as it was subjected to three-dimensional stress during the foaming process. At the same time, the deformation degree at different positions in the cell wall was different. Compared to the TPU foam, the compatibilized PP/TPU blend foams showed a higher expansion ratio and a smaller shrinkage under the same foaming conditions.