Tough inorganic/organic composite network gels consisting of a partially developed silicaparticle network and a large amount of an ionic liquid, named micro-double-network (μ-DN) ion gel, are fabricated via two methods. One is a one-pot/one-step process conducted using a simultaneous network formation via sol-gel reaction of tetraethyl orthosilicate and free radical polymerization of N,N-dimethylacrylamide in an ionic liquid. When the network formation rates of the inorganic and organic

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... are almost the same, the μ-DN structure is formed. The second method is simpler and involved the use of silica nanoparticles as the starting material. By controlling the dispersion state of the silica nanoparticles in an ionic liquid, the μ-DN structure is formed. In the both μ-DN ion gels, silica nanoparticles partially aggregate and form network-like clusters. When a large deformation is induced in the μ-DN ion gels, the silica particle clusters 2 rupture and dissipate the loaded energy. The fracture stress and Young's modulus of the μ-DN ion gel increase as the size of the silica nanoparticles decreases. The increment in the mechanical strength would have been caused by the increase in the total van der Waals attraction forces and the total number of hydrogen bonding in the silica particle networks. INTRODUCTION A gel containing a large amount of an ionic liquid, termed ion gel, is a material that combines the unique properties of ionic liquids and the quasi-solid properties of gels. Ion gels can be prepared by forming a three-dimensional network structure in an ionic liquid. Various methods such as the use of a polymer blend with an ionic liquid, in situ polymerization of a vinyl monomer in an ionic liquid, sol-gel processing, and addition of a low-molecular-weight and oligomeric gelator in an ionic liquid have been proposed as methods for preparing ion gels. 1 Such ion gels are expected to be applied in electrochemical devices, actuators, gas separation membranes, etc. owing to the specific properties of ionic liquids such as non-volatility, nonflammability, and high thermal, chemical, and electrochemical stabilities. 2-10 However, the majority of ion gels are limited in practical applicability owing to their low mechanical strength. 7 For instance, in the case of ion gels formed by the radical polymerization of vinyl monomers, as three-dimensional polymer networks formed by radical polymerization contain various heterogeneities, 11-12 the ion gels are easily ruptured by a stress concentration in short crosslinking distances of polymer chains. 7 Therefore, in order to ensure sufficient mechanical strength for practical applications, conventional ion gels require a large network content. Increasing the network content in a gel not only reduces its ionic liquid content but also increases the amount of network that acts as the diffusion resistance, thus causing a significant decrease in ion and solute diffusivities. As a result, the performance of the ion gels, such as ionic conductivity and gas permeability, is limited. The trade-off relationship between mechanical strength and intra-gel diffusivity is the most serious drawback of ion gels. In recent years, in order to overcome this drawback, ion gels having a high mechanical strength have been developed. 8, 13-17 However, some of them were prepared using specific chemicals such as triblock copolymer with ionic liquid moiety and tetra-armed polyethylene glycols, 13-14 and the others were prepared via complicated multistep preparation processes. 8, [15] [16] [17] In our previous work, we developed high-strength ion gels with specific inorganic/organic composite networks 18 using the toughening mechanism of well-known double-network (DN) hydrogels. [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] The inorganic/organic composite ion gels could be prepared with tetraethyl orthosilicate (TEOS) and N,N-dimethylacrylamide (DMAAm) via simple one-pot/one-step processing. In addition, by controlling the formation order of the inorganic silica particle network and organic poly(N,N-dimethylacrylamide) (PDMAAm) network, we can selectively prepare two types of high-strength ion gels with different network structures; one is a DN ion gel with an interpenetrating composite network composed of a highly developed silica particle network and PDMAAm network, and the other is a μ-DN ion gel having a composite network comprising inorganic silica nanoparticles and a PDMAAm network. Although the mechanical properties of the DN and μ-DN ion gels are completely different, both of them exhibit an extremely high compressive fracture stress of more than 25 MPa at 80 wt% of ionic liquid content. Therefore, it can be said that the toughening of an ion gel has been successfully achieved by forming specific inorganic/organic DN and μ-DN structures in an ionic liquid. However, there still remains some challenges in preparing DN ion gels that can be used in practical applications: (1) the network formation is time consuming and (2) the volatile monomer