Stabilization of positive charge on FEP electret films modified with titanium-tetrachloride vapor: Formation of a two-dimensional nanodielectric?

It has been shown recently that the stability of positively corona-charged polytetrafluoroethylene (PTFE) electrets is considerably enhanced as a result of a surface treatment with titanium-tetrachloride (TiCl₄) vapor. In the present study, we investigate the influence of a gas-phase treatment with TiCl₄ on the electret properties of tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films. FEP-film samples were treated with titanium-tetrachloride vapor by means of a nanotechnological chemical process that is based on molecular-layer deposition. Subsequently, the samples were poled in a positive corona discharge at room temperature. A very significant improvement of the surface-charge stability has been observed on the modified FEP films. The phenomenon is not restricted to the thermal stability of the surface charge, but is also found in long-term charge-decay measurements. Typical relaxation times for positive charge under isothermal conditions are at least one order of magnitude higher after the surface modification. Thermally-stimulated-decay data indicate that the half-value temperatures of the vapor-treated samples have been enhanced to almost 200°C. Thus, the thermal stability of positive charge in modified FEP films may reach values that were so far only obtained on negatively charged FEP electrets. The results are discussed in connection with the formation of deep traps on the chemically modified polymer surfaces and in the context of two-dimensional surface-layer nanodielectrics.