Enhanced Compressive Properties of Lightweight PMIFoams via Bis-Olefinic Cross-Linking in Suspension Polymerization

To address the limitations of low cross-linking density and inadequate mechanical performance in poly(methacrylimide) (PMI) foams derived from thermally expandable precursor beads, three bis-olefinic cross-linkers were used to synthesize cross-linked PMI precursor beads during the suspension polymerization stage in this study, followed by the fabrication of lightweight PMI foams with enhanced compressive strength via in-mold foaming. The results demonstrate that the formation of cross-linked networks during suspension polymerization markedly improves the melt strength of the precursor beads. An optimal cross-linking level (1.0 mol %) refines the cellular structure, suppresses cell coalescence, and consequently improves the overall compressive performance of the foams. In contrast, excessive cross-linking (≥1.5 mol %) reduces melt plasticity, leading to poor interfacial adhesion between beads and subsequent deterioration in mechanical properties. Among the three cross-linkers, MBA exhibits the best overall performance, attributed to its favorable compatibility with the PMI matrix and balanced melt rigidity. The MBA-cross-linked PMI foams achieve compressive strengths of 0.79, 1.28, and 2.41 MPa at densities of 52, 75, and 110 kg/m3.