Breakdown of Surface State

Water permeates a flawless polymer film through the amorphous phase via solution-diffusion mechanisms. Therefore, water permeability is inversely proportional to the volume fraction of the crystalline phase (crystallinity). Water molecules are first dissolved into the polymer matrix at the interface the dissolved water molecules diffuse through the polymer according to the chemical potential 

Accordingly, the transport of salt requires larger elementary free volume than does the transport of water molecules. Hydrated ions are much larger than water, and hydrated cation and hydrated anion must move together because of coulumbic attractive force between them. Consequently, salt ions cannot permeate an amphoteric hydrophobic/hydrophilic polymer, of which the hydration value is low, i.e., less than few volume percent, by the solution-diffusion principle. Therefore, salt permeation through a hydrophobic polymer film such as low-density polyethylene (LDPE) and parylene C film should not occur. 

In reality, however, salts dissolved in water find or create paths into a hydrophobic polymer matrix and cause the breakdown of an insulating layer or the corrosion of the substrate metal. In contrast to the diffusion process described above, the process of salt going into polymer matrix could be termed salt intrusion because the breakdown of the surface state does not occur with water that does not contain salts. The salt intrusion starts with the breakdown of the surface state. In a study of the electric insulation characteristics of LDPE film, it was found that salt ions intrude into the polymer matrix by different mechanisms [3,4]. The exact mechanisms for salt intrusion are not known, but the phenomenological salt intrusion found can be summarized as follows, in an effort to explain the nature of salt intrusion that causes the breakdown of the surface state. 

The similar breakdown of the surface state was also observed with parylene C film. Parylene C is a semicrystalline polymer and one of the most effective barriers for gases and vapors according to permeability values. It was thought that such an excellent barrier film would provide superior corrosion protection of a metal when it was deposited on the metal surface. Contrary to the expectation, parylene C film didn t provide good corrosion protection due to the surface state breakdown described above. This conclusion was ascertained by studies using electrochemical impedance spectroscopy (EIS), which is described in Chapter 28. 

The mechanisms involved in the deterioration of insulation characteristics of LDPE are analogous to the principle of water vapor permeation through it. Two distinctively different paths can yield the water vapor permeability at the same value, i.e., many water molecules moving rather slowly or few molecules moving 

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The insulation breakdown under electrical stress occurs in a fatigue mode, i.e., not a gradual deterioration but an abrupt failure, and is correlated to the salt intrusion characteristics of the film, as shown in Figures 24.12 and 24.13. The hydrophobic surface state created by plasma polymerization of (HFE + H2) significantly prolongs the time when the breakdown of surface state by salt intrusion occurs. 

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