Tuning Hydrogel Mechanics by Kinetically Dependent Cross-Linking

Free radical polymerization is an extensively used method to form a hydrogel network, in which the spatial inhomogeneity can be manipulated by kinetic control. However, it is still a challenge to direct mechanical properties by tuning the kinetics of free radical polymerization. Herein, kinetically dependent cross-linking is used to directly connect with the mechanical properties of hydrogels by tuning the reactivity of the macro-cross-linkers. F127 (PEO99-PPO65-PEO99) diallyl ether (F127DE) macro-cross-linker with low reactivity was first synthesized, and it can induce inner-micelles cross-linking due to its kinetic characteristic that its incorporation in the primary chain is slow at an early stage while rapid at a late stage of copolymerization with acrylamide (AAm) monomer. Thus, the highly cross-linked agglomerations can be effectively controlled in the well-dispersed micelle cross-linking, leading to an internally cross-linked micelle that is far stronger than a micelle formed only by weak supramolecular interaction. Compared to the weak and brittle hydrogel based on F127 diacrylate (F127DA) macro-cross-linker with high reactivity, the hydrogel based on F127DE exhibits a homogeneous network and outstanding strength with a fracture stress of 0.8 MPa and a fracture strain of 1600%. This novel and facile strategy can provide new insights into the utilization of cross-linking kinetics to improve the mechanical property of hydrogels.