Tuning the Relaxation Spectra of Vitrimers via Crosslinker Chemistry and Mixing

To design materials for vibrational damping applications, tuning the rheological relaxation spectra is key. Controlling the placement, breadth, and height of spectrum peaks determines which waves are dampened. Vitrimers, dynamic polymer networks with associative covalent bonds, provide an ideal platform for chemical and mechanical tunability. Here, four boronic acid crosslinkers of varying functionality and structure were reacted with silicone diols to form PDMS vitrimers. Three networks experienced relaxation times within one order of magnitude, with aromatic groups leading to faster times due to destabilization of the boronic ester. In contrast, a crosslinker with nitrogen neighboring groups led to a four order of magnitude drop in network relaxation time. The modulus at one temperature remains nearly constant regardless of various exchange kinetics. These samples, like all vitrimers, also experience modulus increase with temperature due to entropic elasticity. Effects of mixing multiple crosslinkers were also examined. When relaxation times are within an order of magnitude, the faster of two crosslinkers controls the relaxation time; when modes are further apart, a blending of dynamics occurs. These viscoelastic patterns and mixing rules are critical for damping applications.