The Effect of Crosslink Density and Complex Fields of Stresses on the Network Chain Mobility of Model Epoxy Resins

The viscoelastic properties of model epoxy resins were investigated using a tensile stress relaxation technique. Simultaneously with the applied tensile strain, the specimens were subjected to torsional oscillations in a specially designed instrument. The crosslinking density of the resins was controlled by the length of the aliphatic chain of the diamine hardener. Smooth master curves were obtained by Computed Aided Superposition (CAS) using a software written especially for this work. The shape of the master curves changed significantly as a function of the frequency of torsional oscillations. Based on previously published data, as well as on pertinent information from literature, it was concluded that the variations in the shape of the master curves reflect corresponding changes in the spectrum of relaxation times of the investigated resins. It is suggested that the observed phenomena are caused by physical processes, such as short-range orientation and physical aging, which lead to densification of the network and a corresponding increase in its stiffness.