Changes in Molecular Dynamics during the Bulk Polymerization of an Epoxide/Diamine Mixture Containing Inert Diluents as Studied Using Dielectric Relaxation Spectroscopy

Abstract

The evolution of the dielectric α relaxation and ionic conductivity with time tr during polymerization at 60 °C of mixtures of the diepoxide DGEBA, the diamine PACM, and the diluent Decalin (DEC) or di-n-butyl phthalate (DBP) has been studied using real-time dielectric relaxation spectroscopy (DRS). Data are presented as permittivity ε(ω,tr) and impedance Z(ω,tr). For cdil ≤ 20% (w/w), the main effect of diluent was to displace the α process to higher frequencies and, therefore, to increase the time required for glass formation over that for the unplasticized system. At higher values of cdil, the behavior of ε(ω,tr) for DGEBA/PACM/DBP systems was more complex as the properties became stationary at long times, showing that an elastomer rather than a glass was formed. The results are discussed in relation to the “floor temperature” TF for reaction introduced previously. The same data are presented as Z(ω,tr), which emphasizes changes in the ac ionic conductivity with time. As for the ε data, qualitative differences are observed for the behavior of Z(ω,tr) at low and high values of cdil. It is demonstrated that changes in molecular mobility with time for all systems can be judged by the behavior of ε(ω,tr) but not by that of Z(ω,tr) for these systems. The Fuoss−Kirkwood and KWW relaxation functions are used to express the half-width Δ log t of the plot of ε‘ ‘ vs log tr at fixed measuring frequencies for a reactive system. The changes in Δ log t with f and composition for the systems that became glasses are explained in terms of changes with time of (i) the width of the relaxation function and (ii) [Δ log <τα>/Δ log tr], where <τα> is the average relaxation time. The behavior of <τα(tr)> with time during reaction is considered in terms of the Angell fragility index m of the process. It is shown that (i) m increases during reaction for a given mixture and (ii) m, at fixed frequency, decreases as cdil is increased.