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

Abstract

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 depres- surization, 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 multi- functional 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.