Scheme 1. (A) Schematic Illustration of Thermal Stabilization and Carbonization Processes of PP Fibers from Surgical Masks; (B) Crosslinking Mechanism of PP Is Initiated through a Sulfonation Step Which Is Followed by Olefination and Subsequent Addition/Rearrangement; PP Chains Can Then Crosslink, Followed by Ring Closure and Degradation of Functional Groups at Elevated Temperatures.