Multilayer microcellular structures by steam-assisted one-step supercritical CO₂ foaming of PMMA

In this work, we introduce a one-step steam-assisted supercritical CO₂ foaming process to create multilayer PMMA foams with tunable pore structures. The method operates entirely above the polymer’s effective glass transition temperature (125 °C), allowing saturation and foaming to take place simultaneously in a matter of minutes. By adding subcritical water before saturation, the system triggers a steam explosion during depressurization, leading to much faster pressure drops (up to 40 % faster), improved nucleation, and a notable reduction in structural defects. As a result, foams with more uniform cells, finer pore sizes (2.3 µm), and lower densities (91 kg / m³; +10 X expansion) are obtained even at moderate pressures. A particularly interesting outcome is the formation of multilayer architectures: polymer pellets with different levels of CO₂ uptake fuse naturally into foams with distinct porosities across layers. This opens new opportunities for designing multifunctional materials, where different layers could be tailored for specific mechanical, thermal, or acoustic roles. The creation of multilayer is mostly attributed by the combination of one-step foaming above the Tg of the polymer together with a pellet sudden ejection from the autoclave while foaming and freezing the structure. Overall, the steam-assisted approach offers a scalable and energy-efficient pathway to produce polymer foams with customized microstructures and properties.