Deciphering Low-Temperature Dielectric Relaxation of a Series of Amorphous Polymers

In contrast to the crystalline solid and gaseous phases, the physics behind the behavior of liquids and amorphous solids remains a significant challenge. Current descriptions of relaxation behavior in these materials are highly empirical, either utilizing fitting functions with tenuous physical significance, or fitting to a spectrum of Debye relaxation processes, where typical procedures implicitly make the unphysical assumption that the spectral density with respect to the characteristic times is constant and unvarying with temperature. Recently, we have found that the relaxation behavior of a moderately cross-linked epoxy resin is well described by a relaxation spectrum where the spectral strengths of individual components are constant while the spectral density is non-uniform and varies with temperature. The sub-Tg γ-relaxation is well described by a constant strength spectrum spanning over roughly 12 orders of magnitude in the frequency domain. In this work we use this newly developed approach to study the γ-relaxation of a series of epoxy resins with differing crosslink densities. From this, we develop maps showing the temperature dependence of the relaxation spectra, with which we aim to pinpoint the effect of crosslinking on the relaxation behavior of these materials.