Vitrification during the isothermal cure of thermosets. Part I. An investigation using TOPEM, a new temperature modulated technique

The process of vitrification that occurs during the isothermal cure of a cross-linking system at temperatures below T-g infinity, the glass transition temperature of the fully cured resin, has been studied by TOPEM, a new temperature modulated DSC (TMDSC) technique based upon the use of stochastic temperature pulses. A comparison is made between TOPEM and another TMDSC technique, and some advantages of TOPEM are considered. The TOPEM technique is used to show that the mobility factor is not always a reliable approach to predicting the cure rate during vitrification, in view of its frequency dependence. Also, the dependence of the apparent vitrification time on frequency is examined. There appears to be a non-linear relationship between the apparent vitrification time and log(frequency), which is further discussed in the second part of this series.