Tetra-PEG Network Containing Ionic Liquid Synthesized via Michael Addition Reaction and Its Application to Polymer Actuator

Ion gels consisting of poly(ethylene glycol) (PEG) network and ionic liquids were synthesized via Michael addition reaction using tetra-arm PEG (tetra-PEG) precursors with amino groups and maleimide groups at the chain ends. The use of the addition reaction to synthesize the tetra-PEG networks ensures that any byproducts, which may influence the electrochemical properties of the obtained gel, are not released in the reaction system. Fourier transform infrared (FT-IR) spectra, gel fraction, and rheological measurements indicated the progress of the addition reaction. A polymer network started to be formed after 2 h when the two tetra-PEG precursors were mixed in an ionic liquid at polymer concentrations above overlap concentration (= 7.2 wt %). From tensile test, the elastic modulus of the ion gel was estimated to be lower than that of conventional hydrogel, indicating some flaws in the network. Compared with the theoretical elastic modulus for tetra-PEG network, the reaction efficiency of the tetra-PEG ion gel (10 wt %) using the Michael addition reaction was ca. 80%, which was lower than that of conventional hydrogels using condensation reaction (ca. 90%). However, a Mooney–Rivlin plot of the ion gel indicates that the polymer network has few loop chains and entanglements and relatively homogeneous structure. The fracture energy of the tetra-PEG ion gels (10 wt %) was more than 30 times higher than that of a 30 wt % PMMA ion gel prepared by conventional free radical polymerization. The improved strength of the tetra-PEG ion gel was caused by relatively few structural defects. Polymer actuators were fabricated using the tetra-PEG ion gel as an electrolyte layer by sandwiching the gel between two carbon electrodes. The tetra-PEG ion gel actuators showed greater durability than a PMMA ion gel actuator.