Control of Hierarchical Relaxation in Acrylate Vitrimers with Kinetically Distinct Dynamic Bonds

The rheological relaxation characteristics of a polymer network will determine its macroscopic viscoelastic properties and potential applications. When a polymer network exhibits peaks in the tan delta at a certain frequency, incoming sound or vibration waves at similar frequencies can be dampened by the polymer. Design of the peak placement and width can control which vibrations will be dampened. Some degree of control of the relaxation spectrum has been demonstrated with metal-ion hydrogels. However, a robust understanding of how the polymer chemistry, architecture, and crosslinking lead to relaxation peaks and their associated width of frequency is not yet available. By selecting a crosslinked acrylate vitrimer with a high density of dynamic bonds and through mixing dynamic crosslinkers with bond lifetime differences of orders of magnitude, multiple peaks were observed in the network relaxation spectrum. Oscillatory shear rheology shows a network with one crosslinker exhibits Maxwell behavior typical of a rubbery network, but networks with mixed fast and slow crosslinkers demonstrate a second maximum in the loss modulus which is associated with the faster crosslinker. The multimodal behavior was tunable via the percentage of two dynamic bonds and temperature of the system.