Dielectric monitoring

When applying an alternating electric field to a polymer placed between two electrodes, the response is generally attenuated and the output current is out of phase compared with the input voltage. This response stems from the polymer s capacitive component and its conductive or loss component, as represented by a complex dielectric permittivity measured frequencies f, and temperatures T  

The applied electric field perturbs the orientational distribution function of the dipolar molecules. Dielectric relaxation due to classical molecular reorientational motions is a form of pure absorption spectroscopy whose frequency range of interest for materials, including polymers, is between 

Values of s and e are calculated using equations in which the contributions of dipolar effects, ionic displacements, and electrode polarization effects are additive. If conduction effects predominate, i.e., when neither interfacial effects nor dipolar effects are significant, the loss factor is given by (Kranbuehl et al., 1986)  

For a thermosetting polymer, s and e can be recorded during reaction at frequencies between 1 and 10 Hz. The behavior can be very complex because of the changes in the chemical structure, including ions and polar groups. 

Many dielectric studies of epoxy-amine systems have been reported. It appears that the change in dielectric properties during cure may be due to three main contributions  

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Dielectric monitoring has been used to determine the optimum time at which pressure can be applied on adhesives and composites that are very pressure-dependent. This is especially important for films and prepregs that have a high degree of retained solvent. Optimum pressure should not be applied until the solvent has had a chance to escape the adhesive when it is in the liquid state. Pressure is then applied when the solvent has escaped and the viscosity of the adhesive is high enough to prevent squeeze-out from the joint. 

Allen, J. D. In-process dielectric monitoring of polymeric resin cure, p. 270, Proc. 20th SAMPE Symp., 1975... 

Dielectric monitoring can be accomplished in a variety of ways as illustrated in Figure 14. Here, phase angle (phase meter), tan 6 (dielectrometer) and DC resistivity (iongraphing) are plotted as a function of a simple cure cycle, a fixed rate of heating... 

Battisti, A., Skordos, A.A., and Partridge, I.K. 2009b. Dielectric monitoring of carbon nanotube network formation in curing thermosetting nanocomposites. J. Phys. D Appl. Phys. 42 55402. 

Figure 2 Measuring cell used for dielectric monitoring of gas hydrate formation in W/O emulsions. (Adapted from Ref 11.)...

Maistros GM, Partridge IK, Dielectric monitoring of cure in a commercial carbon fibre composite, Composite Sci Technol, 53(4), 355-359, 1995. 

In the preparation of certain laminates (e.g., polyimide) dielectric monitoring is used to determine the stage in the processes at which laminating pressure can be increased so as not to extrude excessive resin out of the laminating stack and yet not have voids and poor interlaminar adhesion because the resin is too far advanced when pressure is finally applied. Dielectric monitoring in this manner is also called dielectrometry or dielectric spectroscopy. 

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