Micro-crosslinking of phosphaphenanthrene/siloxane molecule initiate aggregation flame retardant and toughening enhancement effects on its polycarbonate composite

The precise construction of molecular functional systems is an important way to develop high-performance flame retardant polymer composites with excellent comprehensive mechanical properties. A micro-crosslinked macromolecule (PMD) with group aggregation features has been synthesized from vinyl monomers (MD) bearing phosphaphenanthrene and cyclotetrasiloxane groups. The micro-crosslinking polymerization of MD results in superior structural stability and charring capability. PMD not only endows the polycarbonate (PC) composite with excellent comprehensive flame retardant performance, but also maintains the flexural and tensile properties of the PC matrix, as well as an enhanced anti-impact performance. More importantly, when compared with congeneric low molecular weight contrast, PMD exhibits much higher efficiency in suppressing the combustibility of the PC composite. A toughening effect on the matrix was only observed in the PMD/PCs upon increasing the additive content that did not appear in MVCD/PCs, which fractured more easily. 5 %PMD/PC passed the UL94 V-0 rating with a LOI value of 36.3%, and achieved 31.4% higher impact strength, 46.4% lower pk-HRR, 26.7% longer time to pk-HRR, 57.1% lower FGI, 57.7% higher FPI, and 25.6% lower peak of CO production rate when compared with those of the initial sample. The toughening and flame retardant mechanisms of PMD were also revealed upon analyzing the resultant micro-/macro- morphology and chemical components of the char residue, thermal decomposition volatiles, and pyrolytic fragments. These provide a beneficial molecular-structure construction reference for developing high-performance polymer composites with excellent comprehensive performance in both of flame retardancy and mechanical properties.