Dielectric thermal analysis of polymeric matrices
While the commercial application of polymeric composite materials is increasing, methods of processing composites are becoming more complex and cost prohibitive. Several in situ monitoring techniques have been proposed to permit the development of "smart" processes, which would reduce processing time and cost per part. One proposed technique, dielectric spectroscopy, monitors changes in the electrical properties of polymers during processing. While dielectric instrumentation has become quite sophisticated and commercially available, the relationships between the dielectric properties and the changing chemical and physical states of polymers during processing have not been quantified. This study seeks to elucidate these relationships by considering dielectric spectroscopy as a thermal analysis technique for polymer characterization. The behaviors of both ionic and dipolar species in polymers were considered. Classical thermoanalytical expressions were developed for determining the activation energies of dielectric transition and polymerization from the dielectric properties. Models which combine the contributions of ionic conductivity and dipolar relaxation were developed for relating the extent of reaction to the dielectric properties of thermosetting polymers during both isothermal and nonisothermal reaction. Ionic conduction was described using the Keinle-Race expression; the isofrequency dielectric response was described in terms of the dipolar relaxation time, using the Lane-Seferis-Bachmann equation. Using the principles of kinetic viscoelasticity, the relaxation time was considered as a function of the extent of reaction and an entanglement parameter. An expression relating the entanglement parameter to the parameters in the WLF equation was derived. Methods for removing polarization effects from dielectric cure data were also derived. The developed dielectric data analysis methods were tested using a model epoxy/amine resin for high performance polymeric composites, TGDDM cured with 25 phr DDS hardener. Differential scanning calorimetry (DSC) studies of the model resin were performed for comparison and integration into the dielectric models. The success of this approach demonstrates the usefulness of dielectric thermal analysis techniques and provides further basis for the development of quantitative dielectric techniques for polymeric composite process monitoring.
- Chemical engineering