Contact electrification and surface composition

We have studied the charge transfer to polypropylene films containing varying concentrations of amide (donor) and fluorine (acceptor) groups grafted onto the polymer backbone. Pure polypropylene charges very weakly but the addition of amide causes the charging to become strongly positive, and fluorine causes it to become strongly negative. The magnitude of charge density is similar in the two cases but is small compared to the density of donor or acceptor groups. Moreover, the charge transfer is independent of donor or acceptor density over a wide range of composition, indicating that charge transfer is limited by a feedback mechanism (such as back-tunnelling) during the separation of the metal and polymer surfaces. When gradually increasing amounts of fluorine or chlorine are added to a nylon surface (so that donors and acceptors are simultaneously present) the magnitude of the charge transfer is in general similar to that found for polypropylene with amide or fluorine groups, but depends on the relative concentrations of donors and acceptors. We show that this observation is also compatible with feedback mechanisms such as back-tunnelling. However, the precise nature of the feedback mechanism has not been ascertained.