Recently, Professor Zheng Zhikun's team from the School of Chemistry at Sun Yat-sen University successfully prepared a highly tough, elastic, and mechanically strong woven grain boundary polymer membrane. They reported a method that utilizes sacrificial small molecule structure-directing agents to guide the formation of woven grain boundary structures between adjacent crystalline domains. The related findings were published in "Nature."
Synthesis Schematic of Porous Membrane with Interwoven Grain Boundaries. Image provided by Sun Yat-sen University.
"This work establishes a solid foundation for the application of crystalline materials in flexible devices and separation membranes," said Zheng Zhikun, the sole corresponding author of the paper, to China Science Daily. The research has introduced a method to enhance the mechanical strength and toughness of crystalline materials by constructing interwoven grain boundaries, potentially expanding the applications of crystal membranes in separation, optoelectronics, flexible devices, and other fields.
"Our starting point was to create a more efficient, reliable, and durable separation membrane," Zheng Zhikun explained. In Zheng Zhikun's laboratory, we observed an elastic crystal membrane attached to a silicon wafer. Except for the imperfections caused by tweezers, its presence is nearly invisible to the naked eye. However, it took Zheng Zhikun's team a full five years to perfect such a membrane.
"In theory, membrane separation costs only a tenth of conventional separation costs," Zheng Zhikun stated. Traditional separation membranes made of organic polymers dissolve or swell when immersed in oil, while membranes made of fully crystalline materials do not.
Grain boundaries are defect structures within crystals. Typically, natural and synthetic crystalline materials consist of multiple single crystal domains connected together, with numerous grain boundaries limiting the material's mechanical stability. This effect is particularly severe in two-dimensional crystals composed of single atomic layers, where a linear grain boundary can cause the fracture of the two-dimensional crystal film. Furthermore, similar to how wood is either rigid and prone to breaking or flexible and unable to bear weight, the mechanical strength and toughness of two-dimensional crystals often constrain each other.
Professor Zheng Zhikun guided doctoral student Yang Yonghang in conducting experiments. Image provided by Sun Yat-sen University.
After extensive observation and experimentation, the team led by Zheng Zhikun came up with a typical structure in polymer materials - the braided structure. Similar to how a sweater is woven from yarn, some polymer materials can also intertwine and intersect during polymerization, thus possessing strong flexibility. "This structure generally does not exist in crystals, but in order to obtain this flexible property, perhaps this structure can be transferred to crystals," said Zheng Zhikun.
Experiments have shown that this novel crystal boundary structure - the braided crystal boundary connection forming polymer film has the characteristics of high toughness, high elasticity, and high mechanical strength, with compressive performance close to aluminum alloys and gold. When the material is subjected to force and fractures, the crack does not propagate and does not affect the mechanical properties of the film near the crack.
Guided by this idea, the team led by Zheng Zhikun added sacrificial directing agents when preparing two-dimensional crystal polymers, using linear polymers as "shuttles" to weave the two-dimensional polymers together through their spontaneous winding and interweaving characteristics, forming braided crystal boundaries. Once the crystal boundaries are formed, the linear polymers will automatically depart during the crystallization process due to repulsion.
Further experiments have shown that this novel crystal boundary structure - the braided crystal boundary connection forming polymer film has the characteristics of high toughness, high elasticity, and high mechanical strength, with compressive performance close to aluminum alloys and gold. When the material is subjected to force and fractures, the crack does not propagate and does not affect the mechanical properties of the film near the crack.
Zheng Zhikun stated that this lays the foundation for the application of two-dimensional crystal materials in flexible devices and separation membranes. Flexible materials can be used to produce flexible displays, flexible batteries, flexible sensors, etc.; membrane separation technology is widely used in fields such as chemical engineering, environmental protection, and bioengineering. Compared to conventional membrane separation, fully crystalline polymer films are expected to separate substances with higher purity more efficiently.
Related paper information: https://doi.org/10.1038/s41586-024-07505-x
