Micrometrically controlled surface modification of Teflon (R) by redox catalysis: Electrochemical coupling between Teflon (R) and a gold band ultramicroelectrode

Carbon-fluorine bonds of Teflon(R) (polytetrafluoroethylene, PTFE) carl be reduced eIectrochemicalIy with the purpose of modifying its adhesive and we tting surface properties by micrometrically controlled surface carbonizatio n of the material. This can be performed adequately by redox catalysis prov ided that the redox mediator couple has a sufficiently negative reduction p otential. The process a investigated kinetically with benzonitrile as the m ediator and a gold-band ultramicroelectrode mounted adjacent to a PTFE bloc k, though separated from it by an insulating micrometric mylar gap. For mod erate fluxes of reduced mediator, the whole device behaves as a generator-c ollector double-band assembly with a constant: current amplification factor . This is maintained over long periods of time, during which the carbonized PTFE zones extends over distances that are much wider than the slowly expa nding cylindrical diffusion layer generated at the gold-microband electrode , This establishes that the overall redox catalysis proceeds through electr onic conduction in the n-doped carbonized material. Thus, carbonization pro gresses at the external edge of the freshly carbonized surface in a diffusi on-like fashion (dependence on the square root of time), while the redox-me diator oxidized Corm is regenerated at the carbonized PTFE edge facing to t he gold ultramicroelectrode, so that the overall rate of carbonization is c ontrolled by solution diffusion only. For larger fluxes of mediator, the he terogeneous rate of reduction and doping of PTFE becomes limiting, and the situation is more complex. A conceptually simple model is developed which p redicts and explains all the main dynamic features of the system under thes e circumstances and allows the de termination of the heterogeneous rate con stant of carbon-fluorine bonds at the interface between the carbonized zone and the fresh PTFE. This model can be further refined to account for the e ffect of ohmic drop inside the carbonized zone on the heterogeneous reducti on rate constants and henceforth gives an extremely satisfactory quantitati ve agreement with the experimental data.