Impact of Excess Amine Cross-linker on Polystyrene Vitrimer Rheology

Vitrimers are polymer networks cross-linked by associative dynamic covalent bonds, which achieve bond exchange through an addition-elimination mechanism. This allows the network to fluctuate its topology while still maintaining connectivity. Understanding the rheology of these thermo-rheologically complex materials has garnered significant interest, as their viscoelasticity regulates their processing and recycling efficiency. This work studies the rheological properties of polystyrene vitrimers with excess diamine cross-linker. Polystyrene vitrimers with imine cross-links were formed via hydrolysis between 1,6-hexanediamine and pendant aldehyde group with amine-to-aldehyde molar ratio (r) values ranging between 0.8 and 2.4. Through a combination of variable temperature small amplitude oscillatory shear (SAOS), stress relaxation, and creep compliance measurements, the effect of r on the rheological properties of polystyrene vitrimers was evaluated. SAOS showed two distinct relaxation modes: high-frequency relaxations relating to the backbone dynamics and low-frequency relaxations corresponding to the associative dynamic bond exchange. As r increased, the G’-G” crossover shifted to higher frequency. Activation energies estimated from SAOS, stress relaxation, and creep were larger than the nominal activation energy for the imine cross-link exchange.