The role of boundary layer capacitance at blocking electrodes in the interpretation of dielectric cure data in adhesives

The use of dielectric measurement techniques for monitoring the cure of adhesives and matrix resins used in composite materials is well known. Either parallel-plate electrodes or recently introduced dielectric microsensors can be used. During a typical cure, the quantitative results obtained from dielectric measurements early in cure, where the resin is a viscous liquid, have been shown to depend on the presence or absence of blocking layers at one or both electrodes. This paper reports a quantitative evaluation of the effects of blocking layers on the interpretation of such dielectric data. The approach follows that used for dilute electrolytes, which is a reasonable model for the liquid resins, in which ion conduction will always be present to some degree. It is shown, first, that blocking layers can be modeled as capacitances in series with the bulk adhesive; second, that when the dielectric loss factor of the material is high (which is typically true early in cure), the apparent dielectric response is dominated by the charging and discharging of boundary layer capacitance through the bulk resistance of the sample; and, third, that this phenomenon leads to behavior that is similar to a Debye model for dipole orientation, but with an apparent permittivity (dielectric constant) that is inversely proportional to the boundary layer thickness.