Sustainable Polymethacrylimide Foams via Methacrylic Acid/Acrylamide Chemistry in a Water-in-Water Emulsion Template

Abstract This study proposes a sustainable and green method for preparing polymethacrylimide (PMI) foam. The core innovations are twofold. First, inexpensive and relatively safe acrylamide (AM) is used to replace the highly toxic and costly methacrylonitrile (MAN) required in the conventional process. Second, a water-in-water (W/W) emulsion templating strategy is employed to produce PMI foam with a controllable structure. This method eliminates the need for post-treatment steps such as ethanol washing, which are typically required in previously reported emulsion templating routes, and also overcomes the limitations of traditional chemical foaming methods, including the use of chemical blowing agents and the associated poor structural controllability. In this work, prepolymer foams were first prepared via W/W emulsion templating using methacrylic acid (MAA) and AM as the primary monomers and N,N′-methylenebisacrylamide (MBAM) as the crosslinking agent, followed by thermal imidization to convert them into PMI foams. The resulting foams exhibit densities ranging from 0.12 to 0.20 g·cm–3, compressive strengths of 1.50–6.23 MPa—comparable to those of conventional closed-cell foams at equivalent densities—along with excellent thermal stability and structural integrity under compression. These properties position the developed PMI foam as a promising high-performance core material for advanced composite applications. Overall, this approach significantly improves production safety, reduces material costs, and lowers the environmental footprint, offering a viable and more sustainable alternative to conventional PMI foam manufacturing routes.