Relaxation of Vitrimers with Kinetically Distinct Mixed Dynamic Bonds

Designing materials with tunable modulus and viscosity is key for applications such as three-dimensional (3D) printing, sound damping, and wearable devices. Vitrimers provide an ideal platform for viscoelastic design because their dynamic, conserved bond exchange allows for control of both cross-link density and exchange kinetics. Here, multiple boronic acid cross-linkers with different functionalities and kinetics were reacted with silicone diols to form poly(dimethylsiloxane) (PDMS) vitrimers. Networks cross-linked with boric acid or two phenyl-substituted boric acids exhibited relaxation times and viscosities within one order of magnitude of each other. Conversely, a cross-linker with nitrogen neighboring groups led to a four order of magnitude acceleration in network relaxation time while still exhibiting a similar modulus to the slower systems. All of these samples demonstrate an increase in moduli with temperature due to entropic elasticity. To understand the effect of more than one dynamic bond on the viscoelastic response, multiple cross-linkers were then combined into a single network and the relaxation spectrum was characterized. The mixed vitrimers exhibit a single relaxation peak, which more closely follows the dynamics of their faster component. These rheological observations are essential for designing complex viscoelastic materials.