Abstract
By in situ X-ray photon correlation spectroscopy and small-angle X-ray scattering we quantify the evolution of dynamics and structure during the formation of a cross-linked polymer gel. A fast, non-ergodic relaxation stemming from localized motions of polymer clusters indicates a stiffening network on the nano-scale as a signature of the gelation. A second, much slower relaxation that restores ergodicity is due to the dynamics of the gel network. As the gelation proceeds the reinforcement of the cross-linked network is accompanied by a slowing-down of this second relaxation that also changes line shape during the reaction. Spatial anisotropy develops in the network dynamics but without any signs of dynamical heterogeneity. This distinguishes the observed dynamics from that of typical jammed systems where dynamical heterogeneity is a hallmark.