Viscoelastic properties of epoxy resins derived from creep and relaxation tests at different temperatures

Summary

Experimental data have been obtained on the creep compliance and the relaxation modulus of two epoxy resins in their transition regions. These tests were combined with birefringence measurements to follow the changes in bond orientation of the polymers as the strain or stress varies. Experimental data on the stress- and strain-optical coefficients are presented for both creep and relaxation experiments. It was proved that the validity of the time-temperature superposition principle can be extended to these two coefficients characterizing the optical viscoelastic properties of the two epoxy resins.

Master curves ofF (t/k) versus log (t/k) were constructed, whereF (t/k) represents in turn one of the quantities studied namely creep compliance, relaxation modulus and stress- or strain-optical coefficients. The creep and relaxation time factorsk, corresponding to the creep and relaxation curves at the various temperatures, are plotted against the inverse of the absolute temperatureT. The approximate equality of the time factors for mechanical and separately for optical creep and relaxation was established from these plots. This equality proves that creepD-functions and relaxationE-functions are the reciprocals of one another at any time and that the variation of birefringence with time follows the same law in the respective creep and relaxation tests.

Zusammenfassung

Es wurden experimentelle Werte der Kriech-Compliance und der Relaxationsmoduln zweier Epoxydharze in ihren Transformationsbereichen festgestellt. Diese Prüfungen wurden mit Doppelbrechungsmessungen kombiniert, um die Änderung der Orientierung in den Polymeren bei Änderung von Spannung und Verformung zu ermitteln. Die experimentellen Ergebnisse für die optischen Koeffizienten sowohl bei verschiedenen Zug- wie Verformungswerten wurden sowohl für das Kriechen wie für die Relaxatíon untersucht. Damit ließ sich prüfen, inwieweit die Zeit-Temperatur-Superposition auf diese beiden Koeffizienten angewendet werden kann, die die optischen visko-elastischen Eigenschaften der zwei Epoxydharze charakterisieren.