A Novel Plasma Treatment Based Solid-State Foaming Process for Producing Skinless Foams

Abstract

Solid‑state foaming (SSF) reliably produces micro‑ and nanocellular polymer structures but consistently forms a dense, unfoamed surface layer (skin) due to CO₂ desorption near free surfaces. This work investigates novel atmospheric‑pressure plasma jet (APPJ) processing as a surface‑treatment technique to overcome this limitation and enable skinless polymer foams. Using localized, rapid, and cyclic surface heating combined with charged‑species interaction, APPJ treatment activates nucleation in the near‑surface region without heating the entire specimen, allowing cellular structures to extend to the surface.Direct plasma foaming of CO₂‑saturated PC, PEI, PEEK, and COC demonstrates that the APPJ process produces skinless foams across both amorphous and semi‑crystalline polymers. Process parameters such as stand‑off distance, flow rate, and number of passes control surface temperatures and foamed‑layer depth. Even under reduced CO₂ availability, either through lower saturation pressures or extended desorption times, plasma foaming continues to yield skinless microcellular structures, highlighting the role of localized high‑temperature surface heating in overcoming near‑surface gas depletion. Mechanistic studies decouple thermal and non‑thermal plasma effects. Etching is negligible compared to typical skin thicknesses, while electron‑rich plasma conditions are essential for uniform surface porosity. Temperature‑matched nozzle experiments show that electrons and reactive species reduce the near‑surface nucleation barrier, whereas heat conduction governs bulk foaming. Complementary unidirectional heating experiments further confirm that surface‑selective thermal gradients can initiate subsurface cell growth but do not replicate the uniform porosity achieved by plasma. Building on these findings, a post-processing technique of selective plasma re-foaming is developed for prefabricated PEI nanofoams. Partial CO₂ resaturation of the dense skin followed by APPJ treatment produces a porous, accessible surface while preserving the core morphology and minimizing warping. Finally, the work extends SSF to epoxy‑based vitrimers, demonstrating low‑temperature CO₂ foaming and catalyst‑dependent control of cellular architecture. Together, these results establish atmospheric‑pressure plasma treatment as a robust, surface‑enabled pathway for producing skinless and, in thin COC films, fully open‑cellular foams. The work provides a mechanistic framework linking localized cyclic heating, plasma–polymer interactions, and subsurface cell growth and broadens the range of polymer systems, including dynamic networks, that can be processed using solid‑state foaming.