Influence of Cross-linker Nucleophilicity and Diffusion on Polystyrene Vitrimer Dynamics

Vitrimers are polymer networks whose dynamic covalent cross-links facilitate network topology rearrangements without network dissociation. This structural adaptability gives rise to unique viscoelastic properties critical for material processing and recycling. This work elucidates the interplay between nucleophilicity and cross-linker mobility on network dynamics in a polystyrene (PS) vitrimer system. A series of PS vitrimers featuring dynamic imine cross-links were synthesized by reacting diamine cross-linkers with pendant aldehyde functionalities on the polymer backbone. The rheological properties were investigated through a combination of small amplitude oscillatory shear (SAOS) and creep compliance measurements. The viscoelastic spectra showed two distinct relaxation mechanisms: a high-frequency fast process corresponding to segmental polymer motion and a low-frequency slow process related to the associative imine exchange. Rheological activation energies for the slow relaxation regime (E_a,rh) were obtained by performing Arrhenius analyses on the shift factors obtained from time-temperature superposition of both SAOS and creep data. The measured E_a,rh – independent of the characterization technique – directly scaled with both the cross-linker pKa  and activation energy for diffusion. This relationship indicates that both cross-linker nucleophilicity and mobility significantly impact the vitrimer slow relaxations.