Correlation between structural relaxation and distribution of particle clusters in glass-forming epoxy-amine mixtures undergoing step polymerization

The evolution of the structural relaxation during the step polymerization process of four different formulations of an epoxy-amine mixture-diglycidyl ether of Bisphenol A with diethylenetriaminehas been studied by means of broadband dielectric spectroscopy. Step polymerization progressively turns the liquid into a glass and offers an efficient means to study the effect of the formation of clusters of bonded particles on the structural dynamics. Specifically, we investigate how changes in the distribution of particle clusters reflect in the slowdown and broadening of the relaxation process. We find that the average cluster size diverges as the system freezes at the glass transition and relates to the structural relaxation time in a manner formally similar to that predicted for the size of the cooperatively rearranging regions within the Adam-Gibbs model for glass-forming liquids. This result confirms the one previously obtained by photon correlation spectroscopy on the same systems and indicates it is independent of the experimental technique. Moreover, we observe that the low-frequency broadening of the relaxation function on approaching the glass transition is connected to the increasing polydispersity of the system. We quantify this polydispersity by the variance sigma of the cluster size distribution or by the steepness a of the distribution tail, and we find, over a wide range of these parameters, that the low-frequency power-law exponent m of the relaxation function is linear vs log a and log sigma.