Viscoelastic properties of PDMS vitrimers over a 200C window

We have prepared vitrimers of poly(dimethyl siloxane) with tunable crosslink density of dynamic boronic ester bonds to investigate the viscoelastic properties of dynamic networks with extremely low T_g. Both frequency sweeps and stress relaxation tests show the anticipated Arrhenius behavior of relaxation time with inverse temperature. Frequency domain data show an increase in relaxation times with increased crosslink density at a fixed temperature. Time-temperature superposition of the frequency sweeps demonstrates that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, consistent with a preserved network topology. As stress relaxation times decrease, the rubbery plateau modulus increases, which indicates decoupling dynamics from mechanics. Relaxation times deduced from both frequency sweeps and stress relaxation data were compared, and all give the same activation energy within error. For data taken below 40°C, stress relaxation tests show the appearance of a second Arrhenius regime with lower activation energy, which emphasizes the importance of taking measurements over a broad window.