Defect-free network formation and swelling behavior in ionic liquid-based electrolytes of tetra-arm polymers synthesized using a Michael addition reaction

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_gel) in the current polymer solution system. Based on the k_gel 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.