Rethinking the Analysis of the Linear Viscoelastic Behavior of an Epoxy Polymer near and above the Glass Transition

In this communication, we propose a different approach for analyzing linear viscoelastic relaxation data that is more faithful to the underlying physics and naturally 2 accommodates the thermorheological complexity that is observed in glass-forming polymers. Specifically, the linear viscoelastic behavior was evaluated for a diglycidyl ether of bisphenol-A epoxy cured with 4,4'-methylenedianaline with a glass transition temperature (T-g) of 101.5 degrees C. The dynamic storage and loss moduli were measured from 10(-2) to 10(1.7) Hz for 19 temperatures between 90 and 180 degrees C. The experimental window was extended by two orders of magnitude using stress relaxation experiments for temperatures between 90 and 112.5 degrees C. The G' and G '' responses for this single-phase polymer are thermorheologically complex, thus precluding the construction of master curves via time-temperature superposition. The traditional method of determining the relaxation spectrum implicitly assumes a constant spectral density where the spectral strength changes with the relaxation time. An alternative approach presented herein is to assume that individual spectral contributions have a constant strength where the spectral density changes. This alternative approach is in better agreement with the physics of dielectric relaxation and readily accounts for thermorheological complexity. Using this new approach, a relaxation map of how the individual relaxation times change with temperature has been developed, which is the only relaxation information that can be rationally extracted from viscoelastic isotherms. The relaxation map for the bisphenol-A epoxy shows a smooth transition between the high temperature alpha+ process, the main a transition, and the excess wing, where none of the relaxation regions exhibit Arrhenian behavior.