Unraveled
Composite adhesives like epoxy resins are excellent tools for joining and filling materials such as wood, metal and concrete. There’s just one problem: once a composite sets, it’s there forever.
Now, researchers have developed a simple polymer that serves as a strong and stable filler but can later be dissolved. It works like a tangled ball of yarn that, when pulled, unravels into separate fibers.
The research team — led by Ting Xu, professor of materials science and engineering, of chemistry and a faculty senior scientist at Berkeley Lab — pioneered a way to engineer a composite that bonds like epoxy but can be disassembled on demand. Rather than relying on irreversible chemical bonds, they used fully reversible “pseudo bonds” between the polymer chains.
To solidify the new material, the researchers attached polystyrene chains to silica particles, creating what Xu dubbed “hairy particles.” These nano-sized particles self-assemble into a crystal-like structure, leaving spaces between each unit for the hairy polymers to fill. But because they are confined to such tiny spaces, the polymer chains have less freedom to move, limiting how entangled they can become.
The researchers then found that the polystyrene chains loosened up when they were squeezed. And by adjusting parameters like the polystyrene chain size, as well as precisely where and how many chains were affixed to each facet of the silica particle, Xu’s team was able to tweak the structure’s response. These parameters ultimately provided the key to engineering the entanglement-based pseudo bonds.
With no chemical bonds to break, the newly engineered composite can be returned to its component particles with just a drop of solvent and some stirring. Then it’s ready for reprocessing — making a smarter, more sustainable way to engineer composites.
Learn more: A new way to engineer composite materials (Berkeley Lab)