In situ phase separation design principle realizes high mechanical properties of ionic liquid gels

  Ionic liquid gel is a soft material with a spatial network structure, with excellent ionic conductivity, non-volatility, thermochemical stability, and wide electrochemical window, which can be used in flexible electronics, such as wearable electronic devices , energy storage devices and drives, etc.
  However, most of the existing ionic liquid gels have poor mechanical properties, and the strength, toughness and modulus are very low. The ionic liquid gels developed for this purpose have the disadvantages of complex material system, cumbersome preparation process and limited performance improvement. , resulting in the material not being widely used.
  Recently, the team of Professor Jian Hu from the School of Aeronautics and Astronautics of Xi’an Jiaotong University, in cooperation with the team of Professor Michael Dickey from the School of Chemical and Biomolecular Engineering of North Carolina State University, reported for the first time a “universal design principle for in situ phase separation”, which combines a one-step method Two polymers with different solubility form a “bicontinuous phase-separated structure” in situ in ionic liquids, thereby achieving a simultaneous order of magnitude improvement in strength, toughness, and modulus, and a combination of “self-healing, self-healing, and shape.” Novel ionic liquid gels with functions such as memory, anti-swelling, and 3D printability.
  One of the paper review experts, Professor Zhao Xuanhe of the Department of Mechanical Engineering of the Massachusetts Institute of Technology, believes: “These transparent ionic liquid gels have very tough mechanical properties, and the biggest highlight is that they are simple to manufacture and easy to use.
  ” Preparation of high-strength and stretchable ionic liquid gels by phase separation method.
  It is reported that the paper was successfully accepted by “Nature Materials” after only one round of review, and the three reviewers all said, “Using ordinary raw materials and a simple one-step copolymerization preparation method, we obtained a very outstanding mechanical property. Ionic liquid gels. The proposed design principles for in situ phase separation are versatile and are expected to be of broad interest to researchers in the fields of soft electronics and soft machines.”

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  In addition, Professor Kim Dae-hyun from the School of Chemical and Biological Engineering, Seoul National University, South Korea, wrote a News&Views review article on this research in the same period of “Nature·Materials”. He said, “The researchers have proposed a simple method to synthesize high-strength ionic liquid gels, which is undoubtedly a major contribution to the field of soft materials. Moreover, the combination of high-strength ionic liquid gels and other nanomaterials is possible. It provides new opportunities for many applications such as soft electronics, and the potential uses of these high-strength soft materials are unlimited.”
  It is understood that the team initially only wanted to optimize the mechanical properties of a common ionic liquid gel material system in the early stage. After systematic experimental design, the researchers obtained an ionic liquid gel with greatly improved mechanical properties. At the time, they simply attributed the improved mechanical properties to an increase in the density of internal hydrogen-bonded crosslinks.
  Afterwards, Meixiang Wang, who went to North Carolina State University for postdoctoral research, hoped to continue in-depth research on this topic, and both Hu Jian and Mike Dickey expressed strong interest in cooperation. During the research, Wang Meixiang used SEM and AFM-IR to take clear pictures of the internal microstructure of the ionic liquid gel.
  Next, the researchers realized that the anomalous mechanical properties of this random copolymer ionic liquid gel may originate from its bicontinuous phase-separated structure generated in situ. Hu Jian said that he has been thinking about the principle of strengthening and toughening of ionic liquid copolymer gels for more than a year, but he has never been able to justify the explanation of various experimental phenomena.

Schematic diagram of the microstructure of the ionic liquid gel

  It was not until I saw the microstructure photos of the ionic liquid gel that I suddenly realized, and then overturned the previous network structure assumptions, condensed the bicontinuous phase separation structure of soft and hard interpenetrating, and finally successfully revealed the high mechanical properties of the ionic liquid copolymer gel. and versatility of the working mechanism.
  In addition, during the experiment, the team keenly found that subtle changes in the mole fraction (x) of the copolymerized components have a great impact on the optical and mechanical properties of the ionic liquid copolymerized gel material. Through many detailed experimental iterations, they determined that when x=0.8125, the gel material will undergo obvious mutation from structure to performance, which provides a breakthrough for revealing the abnormal mechanical properties of the material from the perspective of microstructure.

?Self-healing, self-healing and shape memory functions of ionic liquid gels

  Finally, the researchers successfully developed an ionic liquid gel with a strength of 12.6MPa, a toughness of 24kj/m2, a modulus of 46.5MPa, and a stretchability of 600%, which also integrated a variety of practical functions .
  In terms of the application of this achievement, Hu Jian said that combined with the unique advantages of ionic liquids, this achievement may have broad prospects in the fields of sports equipment, automobile anti-collision, explosion-proof and bullet-proof, and related fields of soft electronics and soft machines.

Photographs, demonstration of mechanical properties and SEM images of ionic liquid gels

  In addition, the design principle of in situ phase separation has generality and can be extended to other soft material systems of hydrogels and elastomers. Just like the development history of hydrogels, the solution of mechanical properties of ionic liquid gels will help to expand their applications in various high mechanical loading scenarios, such as polyelectrolyte separators for solid-state batteries, sensors and actuators in extreme environments, etc. . In the future, the team plans to carry out further work on these research directions.
  Hu Jian said that the research work lasted for 3 years, and the core contribution of in situ phase separation was not an advance design, but a certain contingency, which often happens in the field of basic research. The extremely simple preparation method and universal design principle may be the endogenous source of power for the long-term impact of this research work.