Molecular Origins of Flow in Polystyrene Vitrimers

Vitrimers are polymer networks with dynamic covalent bonds that enable topological rearrangements, imparting unique viscoelastic properties critical for processing and recycling. This work investigates the molecular parameters governing the macroscopic flow of polystyrene (PS) vitrimers constructed with dynamic imine cross-links. Three distinct PS-imine networks were synthesized to systematically study the effects of excess amines, nucleophilicity, cross-linker mobility, and cross-linker length on network dynamics. 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 mode corresponding to segmental polymer motion and a low-frequency slow mode attributed to the associative imine exchange. While excess free amines altered the plateau modulus and SAOS cross-over frequency, the temperature dependence of the relaxation modes remained unaffected. The rheological activation energy of the slow relaxation mode (E_a,rh) directly scaled with cross-linker pKa  and diffusion activation energy, indicating the importance of the cross-linker nucleophilicity and mobility within the vitrimer matrix. Furthermore, increasing the cross-linker length resulted in a significant decrease in E_a,rh. These findings provide fundamental insights into the structure-property relationships governing the flow of vitrimers.