Poly(m-xylylene adipamide)/LiCl Composite Foams with Excellent Compressive Strength and High Heat Resistance Prepared by the sc-CO₂ Foaming Technique

High-performance polymer foams from engineering plastics are highly desirable in the field of core materials for high-performance composites, especially under high temperature conditions. Here, semicrystalline poly(m-xylylene adipamide) (MXD6-PA) was used to prepare high-performance MXD6-PA/LiCl composite foams by a supercritical carbon dioxide (sc-CO2) foaming technique. LiCl was added to form cross-linking sites via the complexation between LiCl and amide groups of MXD6-PA, which could restrain the cold crystallization process of MXD6-PA melt and mediate the melt behavior of MXD6-PA during the foaming process. As a result, the change in the melt strength of MXD6-PA/LiCl composites remained small during the foaming process; thus, the foamability of the MXD6-PA/LiCl composite was dramatically improved, including a reduced foaming temperature and widened temperature window for the foaming process. The obtained MXD6-PA/LiCl composite foam showed an excellent compressive performance (1.92 MPa, density: ∼115 kg/m3) compared with other engineering plastic foams. Furthermore, the presence of cross-linking sites in the MXD6-PA/LiCl composite increases the glass transition temperature, so the heating resistance performance was dramatically improved. This novel fabrication method presents great potential in the preparation of high-performance foams for meeting the application requirements.