Hierarchical Dynamics in a Transient Polymer Network Cross-Linked by Orthogonal Dynamic Bonds

Dynamic bonds have been widely incorporated into polymeric materials for achieving self-healing, recycling, and stimuli-responsive properties. As a result, these polymeric chains often exhibit diverse relaxation behaviors due to the interplay of the kinetics of transient bond exchange/association/dissociation and relaxation of polymer chains themselves, which lead to diverse and unusual viscoelastic and dynamic behaviors. Herein, using a combined strategy of rheology and solid-state NMR spectroscopy, we investigate the hierarchical dynamics of a transient polymer network cross-linked by orthogonal dynamic bonds, i.e., quadruple hydrogen-bonding interactions between 2-ureido-4[1H]-pyrimidinone (UPy) dimers and the dynamic covalent boronic ester bonds. The rheology experiments can provide dynamics information on the time scale beyond milliseconds. A comparison of the frequency-dependent linear viscoelasticity and infrared spectra among different samples suggests that the incorporation of dynamic covalent bonds can enhance the formation of hydrogen bonds between UPy dimers. The characteristic relaxation time in rheology also gives an indication of the time required for the material to flow, which is decreased by dynamic covalent bonds (i.e., enhanced chain relaxation, with respect to the permanently cross-linked polymers) but increased by the strong quadruple hydrogen bonds (i.e., impeded chain relaxation, with respect to the polymers cross-linked by the boronic ester bonds). However, on the time scale around milliseconds, the incorporation of dynamic bonds (both hydrogen bonds and boronic ester bonds) resulted in severe topological constraints, leading to reduced segmental relaxation as revealed by proton multiple-quantum (MQ) NMR spectroscopy. The heterogeneous structural changes of the anisotropic network induced by the incorporation of dynamic bonds were revealed by the residual dipolar coupling (D-res) distribution obtained via numerical simulations on the normalized double-quantum (DQ) curves. The molecular motions on the time scale of around microseconds were further probed by temperature-dependent T-1 experiments, which indicated that the incorporation of dynamic covalent cross-linkage imposed stronger restrictions on the molecular mobility than the permanent cross-linkages when the network was also physically cross-linked by the hydrogen bonds. The correlations among dynamics on different time scales were discussed in detail, while the diverse viscoelastic behaviors can be understood in terms of heterogeneous structural changes, both of which could provide insights into achieving comprehensive enhancement of mechanical properties as well as self-healing/recyding efficiency with the incorporation of orthogonal dynamic bonds.