New ways to engineer composites: polymer design combines strength and reversibility

Composite adhesives, such as epoxy resins, are great tools for bonding and filling materials such as wood, metal, concrete. But there is one problem. Once the composite is set, it will be there forever. Now there’s a better way. Researchers have developed a simple polymer that acts as a powerful, stable filler that can later dissolve. It functions like a ball of tangled threads. This releases into separate fibers when pulled.

A new study, led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), outlines how to engineer pseudo-bonds of materials. Instead of forming chemical bonds that make epoxy and other composites extremely robust, the chains of the molecules are intertwined in a completely reversible way. This study is published in Journal Advanced Materials.

“This is a new way to solidify materials. We have paved a new path to composites that don’t suit traditional methods,” said Ting Xu, a senior faculty scientist at Berkeley Lab and one of the study’s lead authors.

Traditionally, there are two ways to make polymeric materials strong and tough. First, adding a setter creates a crosslinked network of polymer molecules that are combined by permanent chemical bonds. Second, increasing the length of the polymer molecular chains will make them more intertwined and not fall apart.

The latter Xu proposed, but offers reversible design possibilities. She was able to compare the concept to folded proteins that interact without chemical bonds to create robust structures in nature, and then deploy to its components.

Xu, along with her colleagues in the Materials Science department at Berkeley Lab, wanted to build on this concept, starting with a collection of simple polystyrene chains, entwined them into a sturdy, stable structure and return them to their starting point. “Suppose you have a ball of thread. It’s confusion. You can’t solve it,” Xu said. “But if you play with the thread, you might be able to trick it into solving it.”

With this in mind, researchers tie polystyrene chains to hundreds of nanometers of silica particles to create what Xu calls “hairy particles.” By forming nanocomposites, these hairy particles are self-assembled into crystal-like structures, providing different spaces between each unit for filling with hairy polymers. The space available to each polystyrene chain was dependent on its location within the structure, which determined how entangled it was with its neighbors.

By confining polymer chains in these small spaces of different shapes, Xu reduced the freedom that clusters of polystyrene chains could move. Or, after all, how it doesn’t entangle: due to a particular arrangement, the response to compression was that certain clusters of polystyrene chains loosened depending on the force applied.

“How entangled particles are determined by their response to external forces,” said Xu, a professor at the College of Engineering and Chemistry in Berkeley, California. By adjusting the size of the polystyrene chains and precisely adjusting the number and exact location of the chains stuck to each facet of the silica particles, she was able to adjust how the structure reacts to dissipate external stresses. Ultimately, these parameters provided the key to entanglement-based “pseudo-joining.”

Microscopic studies have revealed that some chains became trapped and stiffened, while others eventually unleashed, dissipating external stress. The result was a strong, sturdy thin film material that was firmly tied by the pseudo-bondage of entangled polystyrene chains. Adding small amounts of polystyrene chains themselves to the nanoparticle assembly resulted in an additional 50% increase in final loading properties.

“We were really excited to be able to use nanoconfeeding to manipulate the amorphous polymer tissue,” Xu said. Until now, amorphous polymers are often randomly intertwined, but proteins fold well. Variations in the arrangement of polystyrene chains hit the sweet spot. Sweet Spots can be used to engineer composites in a smart way. Additionally, drops of solvent were added to dissolve the agitated nanocomposite, and the component particles were suspended. There were no broken chemical bonds and the material could be reprocessed.

According to Xu, Berkeley Lab research can easily be extended to other polymers and fillers. Polystyrene is one of the most common polymers, and silica is an inexpensive nanoparticle. Nevertheless, Xu hypothesizes that the results apply to other composites as well. She imagines the future with particles with other optical or magnetic properties, for example, to create composite materials for optoelectronic devices. “You can just adjust how the polymer is distributed and have both strength and toughness,” Xu said.