1 second peel! Scientists report new methods for ionogel membrane synthesis

On May 11, 2023, Professor Haipeng Yu of Northeast Forestry University, Professor Dawei Zhao of Shenyang University of Chemical Technology, and Professor Guihua Yu of the University of Texas at Austin published a paper entitled “A general strategy for synthesizing biomacromolecular ionogel membranes via” in the journal Nature Synthesis solvent-induced self-assembly”.

This achievement reports a general solvent-induced gel film peeling strategy, which realizes the rapid, reliable and reproducible preparation of ionogel membranes with adjustable shape, thickness and performance by inducing and regulating the self-assembly behavior of biological macromolecules. The first author of the paper is Zhu Ying, a doctoral candidate at Northeast Forestry University.

Two-dimensional ion gel membrane is a class of flexible functional materials with potential, which are widely used in flexible electronics, intelligent robots and artificial intelligence. The traditional preparation of ionogel membrane involves impregnation, dissolution, paving, humidity control, degassing and other operations, which is time-consuming and laborious, and also causes the thickness, morphology and quantity of gel film to be controllable. At the same time, the spontaneity and randomness of molecular behavior make it extremely difficult and challenging to develop the rapid, reproducible and designable ion of ionogel membranes. Recently, researchers have reported a general solvent-induced gel film peeling strategy inspired by the science of milk heating surfaces to form milk skins. This strategy is suitable for the rapid and controlled development of many biomacromolecules such as cellulose, silk fibroin, chitosan, and guar gum plasma gel membranes. The peeled ion gel film shows unique advantages, such as high ionic conductivity (14.1 mS cm-1), good tensile (strain over 130%), mechanical thermal stability and more than 700 in-situ reducible peels, showing great application prospects in the fields of flexible sensors, electronic skin, intelligent robots, separation and purification.

Figure 1: A solvent-induced peeling gel membrane strategy inspired by the “milk skin effect”.

Molecular self-assembly is the spontaneous aggregation behavior of molecules from disorder to order, however, there are still great challenges in how to guide the molecular assembly behavior to make it directional and orderly aggregation. The science of natural organism structure and daily life provides innovative ideas for researchers to develop new functional materials. For example, when daily heating milk, as the heating time continues, a thin white film will gradually form, and it can be easily torn out of the liquid milk interface, which is the famous “milk skin effect”. The occurrence of this phenomenon is attributed to the controlled assembly behavior induced by protein and fat globule heat. As shown in Figure 1a-d, thermal stimulation gradually transforms the originally curled protein molecules into an extended state, and under the guidance of the dynamic hydrogen bond between fat globules and protein molecules, the protein molecules are controlled to aggregate in an orderly self-assembly manner, and finally form a thin peelable milk film. Like the “milk skin effect” of thermal stimulation, the assembly behavior of molecules can also be effectively regulated by some stimuli (such as light, electricity, solvents, etc.).

The research team passed[Bmim]Cl breaks the hydrogen bond between cellulose molecules to construct a molecular-scale supramolecular viscocolloid, and then uses acetonitrile solvent molecular stimulation to guide the assembly behavior of cellulose molecules, making it directional and designable, and realizing the rapid peeling and forming of gel film under stimulation as short as 1s. It can be imagined that as long as the molecular colloids are rich enough, the unlimited peeling preparation of gel membranes can be realized.

Figure 2: Molecular dynamics simulations to study the directed assembly behavior of cellulose molecules.

In order to further verify and resolve the directional assembly behavior of cellulose molecules under acetonitrile stimulation, the research team conducted effective molecular dynamics simulations of the system (Figure 2). The hydrogen bonding energy (Cel-AN) of cellulose molecules and acetonitrile is 4.5 kcal mol-1, which is much stronger than the 2.3 kcal mol-1 of cellulose and ionic liquid (Cel-IL), indicating that the introduced acetonitrile molecules can easily break the hydrogen bond link between the original cellulose molecules and ionic liquids (Cel-IL). At the same time, acetonitrile molecules with –C≡N bonds will establish a new hydrogen bond pattern with the hydroxyl group (-OH) on the cellulose molecular chain, guiding the assembly behavior of the cellulose molecular chain. Thanks to this unique solvent-guided action, cellulose molecular chains are placed close to each other for orderly self-assembly, forming a peeling membrane with bond energies of up to 34.5 kcal mol-1. In addition, the hydrogen bonding energy of 3.1 kcal mol-1 between AN–IL is higher than that of 2.3 kcal mol-1 of Cel–IL, indicating that acetonitrile molecules can simultaneously form hydrogen bonding systems with IL, acting as a “blocker” to prevent IL from destroying assembled cellulose aggregates, allowing the gel membrane to be removed from cellulose/cellulose/cell[Bmim]Controlled peeling in CL colloid.

Figure 3: A study of the universality of solvent-induced molecularly directed assembly and peeling of gel membranes.

At the same time, the authors used Hansen solubility parameters to screen out other solvent molecules with similar induction behavior to acetonitrile molecules, and successfully established a “database of induced solvent molecules”, such as acetic acid, dichloromethane, glycerol and n-amyl alcohol and other solvent molecules can achieve the controlled assembly behavior of cellulose molecules (Figure 3a, b). In addition, the authors extended the research objects of macromolecular substances to chitosan, silk protein and guar gum, and realized the controlled assembly of many macromolecular chains and the rapid peeling design of gel membranes, which further verified the excellent universality of the solvent-induced self-assembly strategy (Figure 3c).

In short, inspired by the “milk skin effect”, the research team proposed a simple and effective solvent-induced peeling strategy, which can quickly prepare biomacromolecular ion gel membranes by tearing from various renewable biomass, and realize the controllable design, mechanical stretchability, high ionic conductivity and other characteristics of gel thickness and shape. This strategy provides a reference strategy for the rapid and controllable preparation of other two-dimensional polymer membranes or gels, and provides scientific technical guidance for the tear design of future biomolecule-based functional soft materials or devices.

This work was supported by the National Science Foundation for Outstanding Young Scholars (grant number 31925028), the National Natural Science Foundation of China (approval number 32171720), and the Welch Foundation (approval number F-1861). (Source: Science Network)

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