Linear Viscoelasticity of Polystyrene Vitrimers: Segmental Motions and The Slow Arrhenius Process

Vitrimers are polymer networks connected by associative cross-links – covalent linkages that maintain network connectivity but exchange through reversible chemical reactions. Associative cross-links significantly change the dynamics of the molten polymer. This study focuses on the linear viscoelasticity of polystyrene vitrimers (PS-v) bearing imine cross-links. PS-v samples were prepared by condensation between precursor copolymers with pendant aldehydes and 1,6-hexanediamine cross-linker. The number average molecular weights of the precursors were 6 and 8 kDa, and the amine-to-aldehyde molar ratio (r) ranged between 0.8 and 2.4. The glass transition temperature exhibited a non-monotonic relationship with r. The linear viscoelasticity of PS-v was evaluated using a combination of small amplitude oscillatory shear (SAOS), stress relaxation, and creep and recovery. Time-temperature superposition analyses indicated two distinct relaxation regimes: (I) fast high frequency dynamics with a Williams-Landel-Ferry temperature dependence and (II) slow low frequency dynamics with Arrhenius behavior. The fast regime represented the segmental relaxations of the vitrimer backbone. The slow regime was described as a Slow Arrhenius Process (SAP), in which the long time dynamics have a temperature-independent rheological activation energy. For all PS-v samples in this study, the observed SAP had a much weaker temperature dependence than expected from sticky Rouse model predictions. Increasing r altered the plateau modulus and SAOS cross-over frequency but did not affect the temperature dependences of the segmental motions or SAP. To describe the origin of the SAP, three hypotheses are proposed: cross-linker diffusion, polymer matrix effects, and local elasticity fluctuations.