Fragile Glass Formation and Non-Arrhenius Upturns in Ethylene Vitrimers Revealed by Dielectric Spectroscopy

Vitrimers, dynamic networks with bonds that exchange without breaking, are an emerging class of reprocessable and recyclable polymers. The dynamics in such materials are complex and span from a single bond exchange or alpha relaxation event up to bulk flow. Most prior work has focused on investigations of stress relaxation times or creep experiments, but little has been pursued to investigate more local dynamics over a wide range of temperatures. A series of precise ethylene vitrimers are synthesized with four to seven carbons between dynamic bonds, and broadband dielectric spectroscopy is used to probe the segmental dynamics. Three distinct modes are identified in the dielectric spectra─an alpha process, beta process, and a normal mode assigned to strand motion in the network between dynamic bonds. The last mode corresponds within error to the rheological crossover time, indicating that this process is responsible for bulk flow at higher temperatures. At lower temperatures, approaching the glass transition causes a positive deviation of the crossover time from Arrhenius behavior in the networks at roughly the same distance above T_g. Finally, we analyze our networks in the context of a previously developed theory for bond dissociation in associating polymers and find evidence that the non-Arrhenius behavior reflects strong decoupling of the bond exchange barrier crossing event with the segmental or alpha relaxation. This implies the bond exchange event that conserves dynamic cross-link density experiences a local frictional resistance due to the surrounding polymer matrix that is smaller and much less temperature dependent than the primary structural relaxation process, and to a larger degree than observed in most associating copolymer melts where physical bond breaking is a dissociative process.