Investigation of viscoelastic behavior over wide temperature range in PDMS vitrimers

We have prepared vitrimers of poly(dimethyl siloxane) containing dynamic boronic ester bonds to investigate the viscoelastic properties of dynamic networks with extremely low T_g. The stress relaxation times and frequency dependent behavior are probed over a > 200 °C window and show the anticipated Arrhenius behavior at temperatures ranging from 180 to 40°C. The relaxation times here are independent of network strand molecular weight, which varies from 550-11,000 g/mol. We consider this temperature range to be a reaction-limited regime far above T_g, where bond exchange kinetics dominate macroscopic relaxation. Below 40°C, non-Arrhenius behavior emerges, and the apparent activation energy decreases. Time-temperature-superposition of frequency sweeps shows that the flow regime is thermorheologically simple, while the modulus of the plateau regime increases with increasing temperature, which is consistent with a preserved network architecture. As stress relaxation times speed up by orders of magnitude, the rubbery plateau modulus increases, which leads to a strategy for decoupling dynamics from mechanics. Our studies point to the importance of measuring vitrimer properties over a broad temperature window to understand the full range of response in this class of dynamic network.