Theoretical mechanism behind the higher efficiency of O than OH radicals in polypropylene surface modification: A molecular dynamics study

O and OH radicals are the most important reactive oxygen species (ROS) in the plasma treatment of polymer surfaces. In our previous studies, we found that the modification efficiency of polypropylene (PP) surface by O radicals was approximately four times higher than that by OH radicals. This observation contrasts with the well-established fact that the chemical reactivity of O radicals with saturated hydrocarbons (CnH2(n+1)) is 50–60 times lower than that of OH radicals. In this study, molecular dynamics (MD) simulations with a reactive force field (ReaxFF) were used to explain this contradiction. The results showed that both O and OH radicals penetrated into the bulk of PP, namely physical adsorption occurred. The surface penetration depth of O radicals was greater than that of OH radicals. Compared to the case of OH radicals, alkoxy radicals (RO·) are more readily formed on the interactions of the PP surface with O radicals. Furthermore, the β-scission (splitting the C–C bonds) of alkoxy radicals can be accelerated by the physically adsorbed O radicals, leading to earlier breakage of PP chains. The improved efficacy of surface modification of PP upon exposure to O radicals, in contrast to OH radicals, can be attributed to the distinctions observed in the above three crucial processes.