Light scattering study of vitrification during the polymerization of model epoxy resins

The growth of macromolecules in reactive systems is associated with slowing down of the structural motions in a manner that closely resembles the effect of cooling or compressing glass-forming liquids. Depolarized photon correlation spectroscopy-which probes the molecular dynamics of a material via optical anisotropy fluctuations-has been used to monitor the reaction, at different temperatures, of three epoxy-amine formulations leading to network polymers via step-growth polymerization. The correlation function was fitted by the Kohlrausch-Williams-Watts form, and the parameters characterizing the structural relaxation process in the reactive mixtures were studied as a function of the extent of reaction, a quantity that was accurately measured by calorimetry. The behavior of the relaxation time successfully compares with a recently extended Adam-Gibbs entropy equation, derived from a connection between the reduction in configurations and the increase in number of chemical bonds during step polymerization.