Interfacial stabilizing and reinforcing effects of silsesquioxane-based hybrid Janus molecules in extrusion supercritical CO2 foaming of polypropylene

•Supercritical-CO2 foaming of polypropylene (PP)/heptaisobutyl open-cage silsesquioxane (HOS) nanocomposites.•HOS, a low-cost hybrid Janus molecule, act as a surfactant during the foaming of PP/HOS nanocomposite.•HOS content at cell surface is higher than that in cell walls; stabilises growing cells and reinforces solidified cells.•Expansion ratio and compressive strength of PP/HOS-1 wt% foam are 38% and 20% higher than that of PP foam, respectively.•With ≤1 wt% HOS, it also slightly improves extensional viscosity and strength of the melt at low strain rates.