Temperature-independent onset of diffusion control during polymerization in a diepoxide–amine mixture by dielectric measurements Original Research Article

Polymerization of a stoichiometric liquid mixture of a diepoxide, ethane-1,2-diamine and cyclohexylamine (molar ratio, 1:0.25: 0.5) at different temperatures has been studied by dielectric measurements in real time. Although ethane-1,2-diamine reacts with the diepoxide to form a crosslinked structure at a different rate than cyclohexylamine reacts to form a linear chain structure, this difference vanished in their ternary mixture, and only one broad distribution of relaxation times was observed. Also the relaxation time, τ, increased with the time, t, as long as polymerization continued. As τ is related to the molecular diffusivity, which determines the polymerization rate, a distribution of τ corresponds to a distribution of reaction rate constant, as in Plonkaʹs description [Annu. Rep. Roy. Soc. Chem. C 95 (1998) 89] of dispersive reaction kinetics in diffusion-controlled reactions. It is proposed that the rate of change of an equilibrium property with the polymerization time, t, can be used to determine the state of the liquid at the onset of diffusion-controlled reaction kinetics, and that this state would be the same for all polymerization temperatures. Plots of the rate of change of dielectric loss with t, (dlnε″/dt)T, against ε″ in the later stage of polymerization, were used to determine the onset of diffusion-control kinetics. It is argued that the diffusion of chemically reacting groups is controlled by the relaxation of their molecular environment. The latter seems to determine the characteristics of both the dielectric relaxation and the polymerization processes.