Double-Cross-Linked Polymethacrylimide Aerogels via Supercritical CO ₂ Drying toward Ultrahigh Mechanical Properties and Thermal Insulation

Polymethacrylimide (PMI) aerogels are promising lightweight thermal insulation materials; however, those prepared by freeze-drying often exhibit poor mechanical properties due to structural collapse and the formation of large macropores induced by ice crystal growth. In this study, robust PMI nanoporous structure aerogels featuring methylene-bridged and imide-cross-linked networks were successfully fabricated via a supercritical CO2 drying process combined with the introduction of N,N′-methylenebis(acrylamide) (MBA) as a cross-linking agent. The effects of varying MBA contents on the shrinkage behavior, microstructure, thermal stability, mechanical strength, and thermal conductivity were systematically investigated. Increasing the cross-linker content effectively reduced shrinkage and thermal mass loss while enhancing density, structural integrity, and compressive strength. FTIR and TGA analyses confirmed the formation of methylene and imide cross-linked networks, contributing to improved thermal stability. SEM and BET characterizations revealed a highly porous mesostructure (pore size of ∼15–30 nm) with porosity exceeding 70% and specific surface areas up to 156.81 m2·g–1. The optimized aerogel achieved an ultralow thermal conductivity of 0.0385 W·m–1·K–1 and a high compressive modulus of 112.73 ± 5.67 MPa. This work demonstrates a scalable dual-cross-linking strategy combined with supercritical CO2 drying to fabricate mechanically robust and thermally insulating PMI aerogels, offering great potential for applications in aerospace, transportation, and advanced thermal protection systems.