Linear viscoelasticity of polybutadiene vitrimers

Vitrimers are covalently cross-linked polymer networks that are both insoluble in good solvent but processable at high temperatures. These paradoxical traits are enabled by their cross-links, which engage in dynamic associative exchange reactions. Altering the chemical structure of the vitrimer cross-linker, backbone, or both modifies flow and mechanical properties. Recently, Ricarte and Shanbhag used the inhomogeneous Rouse model (IHR) to elucidate structure-viscoelasticity relationships for vitrimer melts. Their findings suggested that the interplay between chain friction and cross-link exchange controls flow under small-strain conditions. In this work, we experimentally investigate the rheological behavior of polybutadiene (PB) vitrimers which bear cross-links that undergo dioxaborolane metathesis. PB vitrimers were formed by cross-linking commercial PB (M_n = 4.3 kg/mol and Ð = 1.06) with bis-boronic esters via a thiol-ene click reaction. Cross-link density was varied from 1 to 12 mol%. Linear viscoelasticity was evaluated using small-amplitude oscillatory shear (SAOS), stress relaxation, and creep and recovery measurements. For SAOS, the elastic modulus remains constant while the viscous modulus increases as angular frequency decreases. For stress relaxation and creep, terminal relaxation is not observed over a period of 8 hrs, even at temperatures of 160 °C. The rheological data are compared to the IHR model to gauge the impact of XL mobility and temperature on vitrimer relaxation.