Experts Combine Substances To Make Weird, Hard, And Elastic Matter

Smash-proof smartphone screens could be one application of a novel hybrid material that merges multiple incongruous properties in a single substance. The material simultaneously possesses the hardness and strength of a ceramic, the deformable elasticity of rubber, and the re-mouldability of a plastic.

“This combination of properties in a single material is so unique that its potential uses are limited mainly by our imagination,” says LIU Zhaoming, professor in the Department of chemistry at Zhejiang University, in Hangzhou, China. Liu co-led the research.

Traditionally, soft and stretchy organic polymers create materials with elastic and plastic properties, while inorganic materials are more likely to form hard ceramics. Previous attempts to combine organic and inorganic compounds in hybrid materials have been limited by their contrasting chemical natures, Liu explains.

Inorganic compounds are typically held together by ionic bonds, rarely existing in pure molecular forms. Instead, they tend to form solid crystals or dissociate into a solution of free ions in water. Think sodium chloride or table salt. Organic compounds, in contrast, are typically held together by covalent bonds, and readily exist in molecular forms. That difference makes it challenging to combine organic and inorganic compounds at the molecular scale.

In 20191, Liu and his Zhejiang University colleagues, including Professor TANG Ruikang, found a potential way around this problem. “We found we could use small organic molecules as ‘capping agents’ to stabilise calcium carbonate, an inorganic ionic compound, in molecular form,” he says.

In their latest work, the team used this approach to make a new organic-inorganic hybrid molecule2, using an acid-base reaction to connect the calcium carbonate oligomers to an organic molecule, thioctic acid. When these molecules were processed under pressure at 120°C, the organic and inorganic parts of the molecule reacted with neighbouring molecules, cross-linking to form a solid with highly integrated organic and inorganic regions.

The hybrid material’s remarkable properties were not immediately obvious, Liu says. “The first time we made this material, we thought it was just like a hard plastic,” he says. The surprises started when they pressed a sharp point into the material using a technique called nano-indentation used for investigating mechanical properties of solid materials at nanoscales.

“The hybrid material was very hard, just a like ceramic,” Liu says. “But when we made an indent, then retracted the point, the material pushed back, recovering its shape like rubber.”

Hardness and elasticity were not the hybrid material’s only paradoxical properties. The team found that under high temperature and pressure, it could be remoulded, which means it can be recycled for a new use. “We named the material an ‘elastic-ceramic-plastic’, because it combines all three characteristics,” Liu says.

When it comes to real-world applications of this material, Liu says one possibility is a smartphone screen that is hard and strong, but not brittle. “With its combination of hardness and fracture resilience, it could also be used for bone implants or other medical materials,” he suggests.

The elastic-ceramic-plastic might be just the first of a whole new family of materials, Liu adds. “The fundamental innovation of this work is that we fused organic and inorganic chemistry at the molecular scale,” he says. “With this method, we could make many hybrid molecules, perhaps combining other paradoxical properties.”