Dielectric Analysis of Chemically, Thermally and Mechanically Induced Glass-Transition

A comparative study is reported on the vitrification of an organic system, when the glass transition is approached through different paths: cooling, compression and polymerization. In particular, the phenomenology of these three vitrification processes applied to a common epoxy glass-former, the diglycidyl-ether of bisphenol-A (DGEBA), has been described by measuring the changes occurring in some relevant dielectric parameters, such as time and strength of the various relaxation processes, when the glassy state is approached by any of these ways. On the chemical side, two different polymerization reactions, leading to a linear (DGEBA-butylamine) and a crosslinked (DGEBA-etylenediamine) molecular structure, respectively, have been analyzed. Thus, the characteristics of rotational and translational diffusion when the glassy state is approached by the three different ways were compared and discussed in order to highlight the role played by temperature, pressure and molecular structure on the dynamics of the system. A phenomenological description of the behaviour of the dielectric response shows the existence of both strong similarities and differences between the change of some dielectric parameters during a polymerization reaction and during the temperature or pressure changes. A unified picture of our observation has been tempted in the light of the Adam-Gibbs theory