Dominant Role of Polypropylene Chain Architecture in Differentiating Flame Retardancy and Mechanical Performance

Achieving a synergistic improvement in flame retardancy and mechanical performance remains a persistent challenge in intumescent flame-retardant (IFR) polypropylene (PP) systems. Previous studies have predominantly focused on optimizing flame retardant formulations while largely overlooking the critical role of polymer matrix chain architecture in determining the overall composite performance. In this work, three PP matrices with distinct chain architectures-homopolymer (hPP), random copolymer (rPP), and block copolymer (bPP)-were systematically investigated within an identical IFR formulation. The results reveal a dominant role of chain architecture in differentiating flame retardancy and mechanical performance, which are governed by distinct structural factors, namely melt rheological behavior and phase morphology. Specifically, bPP exhibits superior flame retardancy, as evidenced by a higher limiting oxygen index (LOI) and improved UL 94 rating, which may be associated with its higher melt viscosity and resistance to dripping during combustion. In contrast, rPP shows significantly improved mechanical performance, owing to its more homogeneous phase structure and enhanced chain mobility. These findings demonstrate that flame retardancy and mechanical properties can be effectively tuned through different structural pathways, providing a viable strategy to mitigate the conventional trade-off in IFR systems. This work highlights the importance of polymer chain architecture as a complementary design parameter alongside flame retardant additives for developing high-performance PP composites.