Inhomogeneities in epoxy resin networks

Inhomogeneities in the crosslink density of polymer networks have been proven, e.g., by electron microscopic studies, thermomechanical measurements, swelling experiments, light scattering and mechanical investigations. Using the differential scanning calorimetry (DSC)‐method, a new effect arising from inhomogeneities could be found by annealing studies of amorphous polymers carried out well below Tg. At those temperatures, the annealing of common epoxy resin systems causes a phase separation of the higher and lower cross‐linked regions. The results are a splitting of the glass transition into one part occurring at the initial transition temperature, while another part shifts towards significantly lower temperatures. This phenomenon may be used to obtain some information about the extent of the regions of different crosslinking density which, consequently, influence the physical properties of the material. The effect is obviously not restricted to crosslinked polymers, since it could also be confirmed for thermoplastic resins like poly(methyl methacrylate) or polystyrene. With increasing annealing temperatures, the morphology of the polymer is gradually changed to a state where a pronounced short‐range orientation, depending on the chemical structure of the segments, takes place. This state produces an endothermic peak in the DSC diagram superimposed on the glass transition.