Highly stretchable and self-healing polymer gels formed solely by physical entanglements of ultrahigh–molecular weight (UHMW) polymers were fabricated through a facile one-step process. Radical polymerization of vinyl monomers in ionic liquids under very low initiator concentration conditions produced UHMW polymers of more than 10 6 g/mol with nearly 100% yield, resulting in the formation of physically entangled transparent polymer gels. The UHMW gels showed excellent properties, such as high stretchability, high ionic conductivity, and recyclability. Furthermore, the UHMW gel exhibited room temperature self-healing ability without any external stimuli. The tensile experiments and molecular dynamics simulations indicate that the nonequilibrium state of the fractured surfaces and microscopic interactions between the polymer chains and solvents play a vital role in the self-healing ability. This study provides a physical approach for fabricating stretchable and self-healing polymer gels based on UHMW polymers.

The gelation mechanism of tetra-arm poly(ethylene glycol) (TetraPEG) prepolymers via a Michael addition reaction was investigated from the viewpoint of chemical reaction kinetics. The polymer network was formed by mixing two different TetraPEGs functionalized with maleimide and thiol terminal groups (TetraPEG-MA and TetraPEG-SH) in aqueous solutions, and the gelation rate was strongly dependent on the solution pH. We found that the gelation reaction can be a second-order reaction when the acid-base equilibrium of the terminal SH groups (-SH ⇆ -S + H) was taken into account, resulting in a quantitative estimation of the rate constant (k) in the current polymer solution system. Based on the k value, the network connectivity (p), which corresponds to efficiency at the linking point, was evaluated to be p > 95% at the end of the reaction; thus, the resulting TetraPEG hydrogels have a homogeneous polymer network without network defects. We used the TetraPEG network as a polymer matrix in a lithium-ion battery gel electrolyte: dried TetraPEG gels were swollen with ionic liquid-based electrolytes containing Li salts to prepare TetraPEG ion gel electrolytes. Swelling behaviors of the TetraPEG network were characterized from the swelling rate and the equilibrium swelling ratio, and we found that these swelling behaviors were significantly affected by the Li-ion component. We concluded that an intermolecular interaction between Li-ions and the polymer (Li-ion coordination with the O atoms within the PEG chains) plays a key role in the fundamental physical properties of the gel electrolyte.

Li ion coordination-induced conformational change of poly(ethylene glycol) in ionic liquids.

Extremely tough and stretchable gel electrolytes, which can be prepared by leveraging the strong interpolymer hydrogen bonding in concentrated lithium (Li)-salt electrolytes, are reported. These electrolytes can be realized by optimizing the competitive hydrogen-bonding interactions between polymer chains, solvent molecules, Li cations, and counteranions. Free polar solvent molecules, which typically impede interpolymer hydrogen bonding, are scarce in concentrated electrolytes; this feature can be exploited to prepare hydrogen-bonded gel electrolytes with unprecedented toughness. In contrast, free solvent molecules are abundant in electrolytes with typical concentrations, yielding considerably weaker gel electrolytes. The tough gel electrolyte can be used an artificial protective layer for Li-metal anodes, as it considerably enhances the cycling stability of a Li symmetric cell through uniform Li deposition/dissolution. Additionally, employing the gel electrolyte as the protecting layer significantly improves the cycling performance of the Li||LiNi 0.6 Co 0.2 Mn 0.2 O 2 full cell.