Segmental Dynamics and Density Fluctuations in Polymer Networks during Chemical Vitrification

Abstract

Molecular dynamics of network-forming reactive polymers were examined as a function of the advancement of chemical reaction as the materials structure undergoes a transition from liquid to amorphous solid. The accompanying changes in the segmental relaxation time (α process) are a signature of the materials state (within the liquid to amorphous solid spectrum of physical properties). Both broad-band dielectric relaxation spectroscopy (DRS), which probes the α process via dipolar reorientational mobility, and dynamic light scattering (DLS), which probes the α process via density fluctuations, were used to monitor the system under reaction conditions in the reaction bath (to our knowledge, the first study of its kind). An excellent agreement was found between DRS and DLS for the α process characteristic parameters: relaxation time and KWW, the stretched exponential parameter (characterizing the relaxation breadth). That dipole dynamics exhibit the same α relaxation characteristics as time dependent density fluctuations, suggests that the rotational motion observed in a DRS measurement is directly controlled by the behavior of the density domains. It was concluded that the broadening of the α process is due to a general phenomenon: the micro/nanoscale heterogeneous nature of glass formers. The results of the study are discussed in terms of cooperative and local relaxation modes in the growing network.