Two crossover regions in the dynamics of glass forming epoxy resins

Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol-A. In spite of their rather simple molecular structure, the dynamics of these systems is characterized by two well-separated crossover regions where the relaxation times of main transition and the two secondary relaxations β and γ approach each other. The main transition has three parts: The a process at high temperature, the $a′$ process between the two crossover regions, and the α process at low temperatures. Both the γ-crossover region [around a temperature $Tc(γ)∼(1.4–1.5)Tg$ and a relaxation time $τc(γ)≈10−10 s$] and the β-crossover region [around $Tc(β)∼(1.1–1.2)Tg$ and $τc(β)≈10−6 s$] could be studied within the experimentally accessible frequency–temperature window. Different typical crossover properties are observed in the two regions. The γ-crossover region is characterized by onset of the $(a′,α)$ process, with a relaxation time about one decade greater than that of the quasicontinuous $(a,γ)$ trace. The β-crossover region is characterized, besides splitting of main andβ relaxation times, by a change in the temperature dependence of the main-relaxation time as reflected by a bend in the Stickel plot of the continuous $(a′,α)$ trace, the separation of individual temperature dependences of different transport properties such as impurity-ions diffusion coefficient and viscosity, and a temperature-dependent main relaxation time that starts to be in accordance (at lower temperatures) with the Adam–Gibbs model. The cooperativity of the main process between the γ and β crossover seems to be small. Below the β crossover, cooperativity increases up to values of order $Nα∼100$ near $Tg,$ and configurational entropy seems to correlate with the main relaxation time.