ESR study of atmospheric pressure plasma jet irradiated aramid fibers

Electron spin resonance (ESR) spectroscopy is used to investigate the free radicals trapped in aramid (Kevlar 49) fibers by an atmospheric pressure plasma jet treatment. The effects of various plasma operational conditions and fiber moisture content on free radical products are evaluated. It demonstrates that both free radical formation reaction and coupling reaction occur and compete with each other on the fiber surface during the plasma treatment and the final free radical outcomes are determined by the competition results, depending on treatment conditions. Fiber moisture content not only has a tremendous influence on the concentration of the trapped free radicals but also affect their evolution. Radicals trapped in moistened fibers show poorer stability than those trapped in dry fibers during storage in nitrogen. However, when the plasma treated samples are stored in a dry environment allowing oxygen access to the fiber surface, free radicals trapped in dry fibers decay rapidly while those in moistened fibers increase within the chosen storage period.