Metal-polymer interface with oxides

In experiments covering a larger potential region, from the oxidized state until the complete neutral state, a new resonance circuit was found not described by the transmission line model. A new model was suggested by Pickup et al., which was used and modified later by Rammelt and Plieth et This model is corroborated by the duplex film structure (Figure 11.9). A compact layer on the metal/polymer interface with neutral state properties in the neutral state and double-layer properties in the oxidized state describes the compact polymer film the transmission fine model represents the porous part (Figure 11.17). 

The applied electrode potential generates an electrical double layer at the metal/polymer interface, with excess ionic charge accumulating in the polymer phase near the interface (we assume that the polymer is neither oxidized nor reduced in the potential range Ei-Et). According to the concepts governing the distribution of... 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

In an early HREELS study of Cr deposition onto polyimide (2b.81. bonding interactions of the Cr atom affecting the carbonyl stretching vibrations were clearly evident. In a further attempt to gain more details on the chemistry developing at the metal-polymer interface, another preliminary set of spectra was recently collected during the metallization of a polyimide film deposited directly onto a silicon wafer (with its native oxide) (Fig. 7). 

Electrochemical reactions that lead to a degradation of the metal-polymer interface are influenced by the following properties the electron transfer properties at the interface, the redox properties of the oxide between the metal and the polymer and the chemical stability of the interface with respect to those species, which are formed during the electron transfer reaction. 

Fig. 6.1 A schematic picture of a polymer film electrode. In an electrochemical experiment the electron transfer occurs at the metal polymer interface that initiates the electron propagation through the film via an electron exchange reaction between redox couples A and B or electronic conduction through the polymer backbone. (When the polymer reacts with an oxidant or reductant added to the solution, the electron transfer starts at the polymerjsolution interface.) Ion-exchange processes take place at the polymer solution interface in the simplest case counterions enter the film and compensate for the excess charge of the polymer. Neutral (solvent) molecules (O) may also be incorporated into the film (resulting in swelling) or may leave the polymer layer...

The delamination rate was determined with a scanning Kelvin probe. A small amount of fine sodium chloride was introduced into a circular deepening in the middle of the polished and ethanol-cleaned iron sample. After the sample was introduced into the plasma reactor, it was cleaned and activated in one step by an oxygen plasma, leading to a carbon-free and highly oxidized iron surface. In the next step an ultrathin plasma polymer of hexamethyldisilane was deposited on the cleaned substrate, leading to a well-defined metal-polymer interface. The thickness of the deposited polymer was controlled by the in situ measurement of the resonance frequency of the quartz crystal and was about 5 nm, so that the film thickness is in the range of the escape depth of the photoelectrons. 

We consider the polymer to have a significant ion concentration. Thus an electrical double layer will form at the Me/poly interface, leading to capacitive charging current (mechanism a). Second, in dynamic measurements the current associated with the oxidation/reduc-tion of the polymer redox sites P /P (mechanism b) is often very large. Third, it should be noted that the reaction 0/R may proceed as a regular electrochemical charge transfer reaction at the metal/polymer interface. This was found to be the main mechanism in several systems, e.g., H2/H on polypyrrole-coated metal electrodes [235.246]. 

When the polymer flhn is oxidized, its electronic conductivity can exceed the ionic conductivity due to mobile counterions. Then, the film behaves as a porous metal with pores of limited diameter and depth. This can be represented by an equivalent circuit via modified Randles circuits such as those shown in Figure 8.4. One Warburg element, representative of linear finite restricted diffusion of dopants across the film, is also included. The model circuit includes a charge transfer resistance, associated with the electrode/fllm interface, and a constant phase element representing the charge accumulation that forms the interfacial double... 

---