Rigid-flexible dynamic polymers based on borate bonds

Strain-dependent materials, such as non-Newtonian fluids and solid-liquid dynamic polymers, are soft in the normal state and exhibit significant stiffness only at high strain rates. No material has been reported to date to maintain hardness under normal conditions (>0.17% s(-1)) and exhibit flexibility under slow load. Herein, this unique mechanical property is realized in polymer networks constructed by dense dynamic borate bonds and rigid benzene/azine ring structures. The polymer exists in a "rigid solid" state under normal conditions and in a "flexible solid" state under extremely slow strain. This unique mechanical property is derived from the slow relaxation behavior of the borate bonds in the rigid networks. The tensile strength of the rigid-flexible polymers could reach up to 3.9 MPa and Young's modulus was about 85 MPa, which far exceeds those of previously reported strain-dependent materials. In addition, the polymer exhibits some special features (including self-healing, room-temperature welding, and shape editing). The special dynamic material is considered to have great application prospects in the field of adaptive equipment.