Physico-chemical aspects of dielectric and thermodynamic changes during high-temperature polymerization and their technical use
When polymerization temperature is high, almost 100% polymerization occurs in a relatively short time. The ultimate product is either a high-viscosity fluid or an elastomer: the former when linear-chains form and the latter when a network structure forms. During polymerization at such temperatures, the liquid's configurational entropy still decreases on macromolecular growth, but most of this decrease is compensated by the high thermal energy. The amount of heat released and the change in the dielectric spectra during the step-addition polymerization of cyclohexylamine–diglycidyl ether of bisphenol-A and hexamethylene-1,6-diamine–diglycidyl ether of 1,4 butanediol have been studied in real time both isothermally and during heating, by means of an equipment designed for the purpose. These show that two competing effects determine the molecular dynamics during polymerization: (i) decrease in the configurational entropy on macromolecular growth, and (ii) increase in the configurational entropy on increasing the temperature. It is proposed that a liquid may be polymerized at such a temperature where its chosen dielectric properties reach a time-invariant value prescribed for the polymer shaping procedures. Thus a single step of thermal treatment for polymer production and shaping processes may become sufficient. The temperature for polymerization may be determined from the knowledge of the dielectric properties of the polymerized state. The chemical physics involved in this procedure has been described formally in terms of both the molecular dynamics and configurational entropy.