New Material Solves Three Problems
The plastic known as acrylic or plexiglass, found in eyeglasses, aquariums, and many other products, shares a problem with similar materials: Increasing its strength (resistance to deformation) usually makes it more brittle. Attempts to reduce brittleness or increase strength also typically raise the viscosity of the molten material, or the “melt,” which makes processing more difficult. Now a team of chemists has shown that adding nanoparticles to the material can increase both strength and resistance to fracture while also decreasing the melt viscosity [1]. The team hopes that the discovery will lead to easier manufacturing of a wide class of improved polymer materials.
Polymer scientists have long faced this fundamental difficulty—referred to as the strength-toughness-processibility “trilemma”—says chemist Hu-Jun Qian of Jilin University in China. Strength is the amount of stress a material can withstand before it starts to deform, while toughness reflects how much energy the material can absorb before it fractures—a measure of its ability to deform without snapping. So a strong material can carry high loads but may fail suddenly once pushed past its limit. A tough material, by contrast, can bend, stretch, or yield, absorbing energy and delaying fracture—like the shell of a football helmet or a car that crumples on impact to protect the passengers.
Some researchers have managed to improve strength and toughness by adding nanocrystals formed from organic molecules. Within the crystals, the molecules form rigid networks that include many nanometer-sized pores. When blended into a polymer, these crystals act like microscopic sieves. When polymer chains move, they are forced to pass through or align with the pores, which restricts chain mobility. This increased rigidity of the polymers also allows local stress to be transmitted more widely throughout the polymer network, which reduces the likelihood that cracks form from concentrated stress. However, this technique generally increases the viscosity of the melt, making processing harder.
Qian and colleagues’ new study was inspired by a discovery two decades ago: Adding nanoparticles to a polymer can decrease the viscosity if the nanoparticles aren’t rigid but have deformable surfaces [2]. In 2019, using simulations and experiments, Qian and others explained why this trick works: The long chains of the polymer, they showed, can partially penetrate and slide along the rugged, flexible surfaces of soft nanoparticles [3]. In a flowing melt, the deformability allows the particles to act like a lubricant that helps nearby polymer chain segments to disentangle more rapidly, reducing the melt viscosity.
Armed with this insight, the researchers have now gone one step further to show how to exploit this effect to improve the class of materials called polymer glasses, which includes plexiglass. In experiments, they used a poly(ethyl methacrylate) polymer, into which they added nanoparticles that were made from balled-up single-chain polymers. They then compared the properties of two such mixed polymers, one with the nanoparticles simply blended in, and the second produced by chemically bonding (cross-linking) the nanoparticles with the background polymers. Using standard measurement techniques, they found that both procedures increased strength and toughness. However, the team was surprised that the blended polymer mix also reduced the viscosity of the melt.
Further experiments and molecular-dynamics simulations revealed additional unusual behaviors in the blended polymer. In a polymer glass under stress, microscopic fibril–void networks can form as a precursor to the moment of fracture—a process called crazing. This process generally weakens the material and leads to an abrupt brittle fracture. But in the new blended nanocomposite, simulations showed that the nanoparticles move around during stretching and form stabilizing crossties between fibrils. The result is a slower, smoother, and more uniform deformation that lets the plastic absorb far more energy before failing. “This redistribution seems to allow the nanoparticles to adapt to local stress fields, effectively delaying crazing and stabilizing the system and making it tougher,” Qian says.
The results are significant and surprising, says polymer expert Wei Jiang of the Chinese Academy of Sciences’ Changchun Institute of Applied Chemistry. “In the future, this will be very important for designing and preparing polymer materials with balanced rigidity and toughness and also good processability, which is one of the most challenging topics in polymer physics.”
–Mark Buchanan
Mark Buchanan is a freelance science writer who splits his time between Abergavenny, UK, and Notre Dame de Courson, France.
References
- L. Zhang et al., “Single-chain nanoparticles break the strength-toughness-processability trilemma in polymer glasses,” Phys. Rev. Lett. 135, 118101 (2025).
- A. Tuteja et al., “Effect of ideal, organic nanoparticles on the flow properties of linear polymers: Non-Einstein-like behavior,” Macromolecules 38, 8000 (2005).
- T. Chen et al., “An unexpected N-dependence in the viscosity reduction in all-polymer nanocomposite,” Nat. Commun. 10, 5552 (2019).