Stanford researchers inserted chain-like molecules of polystyrene – the same material in Styrofoam coffee cups – between layers of nanocomposites to make these materials tougher and more flexible. (Image courtesy of Stanford/Dauskardt Lab.)

Nanocomposites are a special class of materials made out of components smaller than one-thousandth the thickness of a human hair.

They are designed to possess physical properties greater than what can be achieved by their constituent parts, such as greater flexibility, strength and resistance to chemicals or temperature.

A team of materials scientists and engineers at Stanford University have been testing the upper boundaries of lightweight nanocomposite mechanical toughness in a class of materials that have been toughened by the inclusion of polystyrene molecules.

Their research resulted in a model of a previously unknown toughening mechanism they have described as “confinement-induced toughening.”

This new model diverges from the conventional understanding of how composites get their toughness, a quality defined as the ability to resist fracture.

Toughening Up Nanocomposites

The existing understanding was that toughness results from polymer molecules entangling with each other within a composite material.

The Stanford team’s composite, however, disperses the polymer molecules throughout the composite in a way that confines them inside the pores within the material, preventing and limiting the effect of entanglement.

They started with a nanocomposite material possessing a glass-like molecular skeleton, called a matrix. The material is interlaced with billions of nanometer-sized pores, giving it a sponge-like texture.

“This sponge is not soft or pliable like those in your kitchen, however, but very brittle,” said Reinhold Dauskardt, professor of materials science and engineering at Stanford.

Using a method that differs from conventional ways to alter nanocomposites, the team infused long chain-like molecules of polystyrene into the pores of the matrix structure.

“We took these extremely large molecules, many, many times larger than the pores themselves, and confined them in these tiny spaces,” Dauskardt said. “It was quite special. Typically, if you heat these molecules too much they break, but we figured out how to heat them just enough so that they diffuse uniformly into the matrix.”

As a composite with these polystyrene molecules bends, twists and stretches, the long polymers are drawn out and extend from the confines of the pores.

"The molecules act like a special kind of spring – what engineers would call 'entropic springs' – to hold the composite together," Dauskardt said.

He continued, "In our model, the polymer segments bridge across potential fractures, stuck inside the matrix pores to hold the material together. If a crack were to propagate, the confined chains pull out from the pores and, collectively, elongate by large amounts to dissipate energy that would otherwise break the material."

Testing the Limits

Ultimately, however, there are still limits to the toughness of these nanocomposites.

The team determined that the amount of toughening depends on the molecular size of the polymer used in the nanocomposite, and how confined the molecules are inside the pores.

"We've shown that there is a fundamental limit that these molecules eventually reach before they break, which depends upon the strength of the individual molecules themselves," Dauskardt said.

But the ability to test for and know these limits helps scientists and engineers understand exactly how tough a material could possibly be made and why it will achieve a specific level of toughness.

"Once you understand that, there is the potential to work around these limits by controlling the way the molecules interact with the pores and preventing them from breaking," Dauskardt said. "If we can do that, then there is a real possibility of creating colossal toughening in low-density nanocomposites. That would lead to some very promising new materials."

Jet airplane wings could be made lighter with the use of lightweight nanocomposite materials that are designed for toughness and flexibility.

The team sees future applications in the aerospace industry, particularly developing and testing nanocomposite materials to be as lightweight as wood but as strong as steel.

This could greatly impact the design and development of aircraft, enabling wings and panels to be both extremely lightweight while also possessing strength and flexibility.

The team also proposes that these materials could be used for spacecraft, with the ability to withstand high tension and extreme temperatures.

However, since the full limitations of these nanocomposites are not yet known, further research and testing will be necessary to demonstrate their suitability for use in the aerospace industry.

If required levels of strength and flexibility can’t be achieved, these materials may not be suitable for aircraft or spacecraft applications.

But even if these materials aren’t useful in aerospace, there are still potential consumer applications for these nanocomposites much less stressful environments, such as improving computer circuitry, equipment used in athletics, and for use in the automotive and transportation industries.

The full research paper is published in Nature Materials and is available to read here.