Importance of Broad Temperature Windows and Multiple Rheological Approaches for Probing Viscoelasticity and Entropic Elasticity in Vitrimers

Model polydimethylsiloxane telechelic vitrimers with dynamic boronic ester bonds were synthesized to investigate the viscoelastic properties of dynamic networks with extremely low Tg via multiple rheological approaches. Frequency sweeps and stress relaxation tests, conducted at more than 120 °C above Tg, show the anticipated Arrhenius behavior of relaxation time with inverse temperature and give the same activation energy for a fixed molecular weight, obtained using a variety of analysis methods. Time-temperature superposition demonstrates that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, consistent with a conserved network topology and associative bond exchange. As relaxation times decrease, the rubbery plateau modulus increases, indicating a decoupling of terminal dynamics from mechanics. Below 40 °C, a second Arrhenius regime with lower activation energy emerges, which is attributed to a transition from relaxation dominated by reaction exchange kinetics to relaxation dictated by local polymer dynamics. Our work points to the importance of assessing a broad temperature window and using multiple approaches in probing vitrimers and dynamic networks.