Title

Analysis of diffusional limitations on the cure of epoxy and cyanate ester resins

Date of Award

8-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

George C. Martin

Subject Categories

Biomedical Engineering and Bioengineering

Abstract

Diffusion phenomena during thermoset cure, including the molecular diffusion processes, the diffusion-controlled kinetics, and the effect of diffusion on the structural evolution, have been studied to elucidate the structure-property-processing relationships in these materials. A diepoxide-diamine resin and a zinc-catalyzed dicyanate ester resin were chosen as the model systems.

By considering the impurity ions as probes of the molecular diffusion, a scaling relation was established to correlate the diffusivity with the conductivity. Based on this relation and the free-volume relation between diffusion and dipole relaxation, a modeling strategy has been developed for estimating the average diffusion coefficient of the polymers. The diffusivities in both the epoxy and the cyanate resins were determined and that in the epoxy resin was found to agree with the prediction of the free-volume model.

A diffusion-controlled kinetic model was developed using the Rabinowitch model to account for the diffusional limitations on the cure kinetics. This model accurately predicts the experimental conversion profiles for the two systems over the entire range of cure studied.

A simplified model was formulated for analyzing the diffusional limitations on the epoxy network growth and its predictions showed disappearance of the solubles prior to full cure. To study the effect of diffusion on the cyanate gelation, a novel recursive model has been developed. The modeling results indicate that the diffusion-induced unequal reactivities have a significant effect on the gelation; the transition of the weight-average molecular weight from low to infinity is extended and the gelation is delayed to a higher conversion.

The dielectric relaxation behavior of the epoxy resin during isothermal cure was also studied and was found to be phenomenologically similar to those of unreactive materials. This observation led to the formulation of a single-frequency approach, an efficient way to acquire dipole relaxation data.

Diffusional limitations always exist in the final stages of cure when the properties of the resins attain their end values. The applications of the overall modeling approach to the accurate control of the ultimate properties and the complete characterization of the processabilities of thermosetting resins have also been discussed

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