“Confinement-Induced Toughening” May Mean Stronger Materials in Aerospace Engineering.
Stanford researchers inserted chain-like molecules of polystyrene – the same material in Styrofoam coffee cups – between layers of nano-composites to make these materials tougher and more flexible.
Nano-composites 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 in aerospace engineering applications.
A team of materials scientists and engineers at Stanford University have been testing the upper boundaries of lightweight nano-composite 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 in aerospace engineering.
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 nano-composite material possessing a glass-like molecular skeleton, called a matrix. The material is interlaced with billions of nano-meter-sized pores, giving it a sponge-like texture.
“This sponge is not soft or pliable like those in your kitchen, however, but very brittle. Using a method that differs from conventional ways to alter nano-composites, the team infused long chain-like molecules of polystyrene into the pores of the matrix structure.
“ The extremely large molecules many times larger than the pores themselves, and confined them in these tiny spaces. “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.
“In this 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.”
Ultimately, however, there are still limits to the toughness of these nano-composites.
The team of researchers determined that the amount of toughening depends on the molecular size of the polymer used in the nano-composite, and how confined the molecules are inside the pores.
“They’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.
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 in aerospace engineering.
The team sees future applications in the aerospace engineering, particularly developing and testing nano-composite materials to be as lightweight as wood but as strong as steel.
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