Glass transitions in crosslinked epoxy networks

Abstract

Epoxy resins of DGEBA type were thermally cured with diaminodiphenylmethane as crosslinking agent, and then analysed by Differential Scanning calorimetry (DSC) at various heating rates in order to determine the glass transition temperatureT _g of the final networks. First it was shown that during cyclingT _g is shifted towards higher values up to a maximum or$T_{g_\infty }$. Such a change is attributed to an increasing extent of cure which develops during the thermal analysis, and also to relaxation processes thermally activated inside the polymeric matrix. Then the dependence of$T_{g_\infty }$ on the heating rateq imposed by the DSC apparatus was presented forq changing from 0.1 to10‡C min⁻¹. At heating rates exceeding 3‡C min⁻¹ only the classical temperatureT _g was detected, but at smallerq values, an additional endothermic transition was revealed, located at higher temperature and linked to a physical aging-like phenomenon, which takes place at low heating rates. The plot of$T_{g_\infty }$ against logq is divided into two quasi-linear parts on each side ofq=3‡C min⁻¹. In conclusions, an equation was given to describe the$T_{g_\infty }$ vs. logq function.