A description of chemical and diffusion control in isothermal kinetics of cure kinetics

During isothermal polymerization reaction, a thermosetting resin vitrifies if the reaction temperature is lower than the maximum glass transition temperature of the fully reacted material. Due to the vitrification process, the kinetics become diffusion-controlled. The kinetics of such reactions can be described using a diffusion control function. The actual reaction rate can be expressed as a product of the reaction rate of the chemically controlled reaction and the diffusion function. It is shown that the chemically controlled kinetics can be evaluated using isoconversional methods (model-free kinetics) from DSC heating experiments at sufficiently high heating rates. For the diffusion control function, a new phenomenological expression is introduced which is independent from the reaction temperature and requires only one parameter. This parameter can be interpreted as the width of the glass transition. The presented approach can be used to predict the kinetics of complex reactions including the change from chemically controlled to diffusion-controlled kinetics, on the basis of a small number of experiments.