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The new approach predicts behavior in constant evolution of the glass at different temperatures


Make the glasses clearer

Multivalency modeling of a polymer glass to predict its properties dependent on the temperature. Credit: Wenjie Xia / NIST

Not all glass is clear. How do they behave and behave their atoms, in particular, is surprisingly opaque.

The problem is that the glass is an amorphous solid, a class of materials found in the mysterious kingdom between solids and liquids. Glass materials also include polymers or common-use plastics. Although it may seem stable and static, glass atoms are constantly mixed in a frustrated and useless pursuit of balance. This desire behavior has made glass physics almost impossible for researchers.

Now, a multi-agency team, such as Northwestern University, North Dakota State University and the National Institute of Standards and Technology (NIST), has designed an algorithm with the objective of giving a little more clarity to polymeric glasses. The algorithm allows researchers to create thick-grain models to design materials with dynamic properties and predict their changing behaviors continuously. Named "energy renormalisation algorithm", it is the first to predict with precision the mechanical behavior of the glass at different temperatures and could result in the rapid discovery of new materials, designed with optimal properties.

"The current process of discovery of materials may take decades," said Sinan Keten, from the northwest, who co-led the investigation. "Our approach makes scale of the molecular simulations about a thousand times, so that we can design materials more quickly and examine their behavior."

"Although all the glassy materials surround us, scientists are still struggling to understand their properties, as well as their fluidity and diffusion, as temperature or composition vary," said Jack F. Douglas, researcher at NIST, who co-directed the work with Keten. "This lack of understanding is a serious limitation in the rational design of new materials."

The study recently published in the magazine Scientific advances. Wenjie Xia, assistant professor of civil and environmental engineering at North Dakota State University, was the first author of the document.

The strange behavior of Glass comes from the way it is made. It begins with a hot pool of molten material that cools quickly. Although the final material wants to achieve equilibrium in a cooled state, it is highly susceptible to changing temperature. If the material is heated, its mechanical properties can change drastically. This causes researchers to efficiently predict mechanical properties using existing molecular simulation techniques.

"As simple as glass seems to be, it is a very weird material," said Keten, associate professor of mechanical engineering and civil and environmental engineering at the Northwestern McCormick School of Engineering. "He is amorphous and does not have a balance structure, so it constantly evolves through the slow movements of its molecules. And then there is a lot of variation in how it evolves according to the temperature and the molecular characteristics of each glassy material. of time to calculate in molecular simulations. It is only possible to accelerate the calculations if the positions of the molecules can be mapped to simpler structural models. "

The structure of Glass contrasts clearly with a crystalline solid, in which the atoms are arranged in an orderly, predictable and symmetrical manner. "It's easy to map atoms in crystalline materials because they have a repetitive structure," explained Keten. "While in an amorphous material it is difficult to map the structure due to the lack of long-range order."

"Due to the amorphous and disordered nature of the glass, its properties could vary according to the temperature substantially, making the prediction of its physical behavior very difficult," added Xia. "Now, we have found a new way to solve this problem."

To cope with this challenge, Keten, Douglas, Xia and his collaborators designed their algorithm to take into account many ways of moving glass molecules or not depending on the different temperatures over time. The calculation of the position of each atom in the glass would be meticulously slow and tedious (even for a high power algorithm) to calculate. Thus, Keten and his collaborators used "thick-grain modeling", a simplified approach that considers atomic groups more than unique atoms. Its new methodology efficiently creates parameters for the interactions between these thicker particles, so that the model can capture the dramatic decrease in velocity in the molecular movement as the glassy material is cooled down.

"We can not do an atomic simulation by atom even for thick glass films at a nanometric scale because even this would be too large," Keten said. "There are still millions of molecules. Heavy-duty models allow us to study larger systems comparable to the experiments carried out in the laboratory."

So far, Keten and his team have proven their algorithm against three types of polymeric fluids that are already well characterized and very different. In each case, the algorithm predicts with precision the known dynamic properties across a wide range of temperatures.

"Explaining the physics of the glasses has been one of the biggest problems that scientists have not been able to solve," Keten said. "We get to understand their behavior and solve the mystery."

The liquid has a structure, which can be key to understanding the metal glass

More information:
Wenjie Xia et al, Energy Renormalisation for thick-grained polymers that have different segmental structures, Scientific advances (2019). DOI: 10.1126 / sciadv.aav4683

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Northwestern University

The new approach predicts behavior in constant evolution of glass at different temperatures (2019, April 30)
recovered on April 30, 2019

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