Dynamic Mechanical Spectrum of an Epoxy Polymer

Dynamic mechanical experiments were performed on an epoxy thermoset with a Tg=101°C over a wide temperature range from -150°C to 260°C. Time-temperature superposition (TTS) is only observed at temperatures above Tg+100°C where the network relaxation is the dominant relaxation process. At lower temperatures, both above and below Tg, TTS is clearly violated. The traditional representation of the relaxation spectrum as a sum of several broad empirical functions each with its own independent temperature dependence that are often designated as α-, β-, etc. processes cannot describe the set of isotherms. Using a procedure that includes Tikhonov regularization with adaptive meshing, a relaxation spectrum consisting of discrete processes emerges. It is shown that determination of the spectrum is robust, resulting in a continuous spectrum at high temperatures and discrete peaks at lower temperatures. Each relaxation process has its own temperature dependence, which becomes stronger as Tg is approached. Below Tg the relaxation time of each process is affected differently by aging. The implications of a discrete spectrum of relaxation times vs. the more commonly assumed continuous distribution of relaxation processes for a theory of the amorphous state will be discussed.