The experimental data validates the new theory for mo

picture: Prof. Ken Schweizer (top left), Dr. Baicheng Mei (top right), Prof. Chris Evans (bottom left), Grant Sheridan (bottom right)
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Credit: Grainger College of Engineering at the University of Illinois at Urbana-Champaign

After several years of developing theoretical ideas, researchers at the University of Illinois at Urbana-Champaign have validated several new predictions about the fundamental mechanism of the transport of (penetrating) atoms and molecules in chemically molecular and polymeric liquid matrices. complex.

Teacher. Ken Schweizer (top left), Dr Baicheng Mei (top right), Prof. Chris Evans (bottom left), Grant Sheridan (bottom right)

The studyrecently published in Proceedings of the National Academy of Sciences (PNAS) of Professor Ken Schweizer in Materials Science and Engineering (MatSE) and Dr. Baicheng Mei, extended the theory and tested it against a large amount of experimental data. Associate Professor MatSE Chris Evans and graduate student Grant Sheridan collaborated on this research by providing additional experimental measurements.

“We have developed an advanced, state-of-the-art theory to predict how molecules move in complex media, especially in polymeric liquids,” Schweizer said. “The theory summarized important features of chemically complex molecules and the polymeric medium they pass through that control their rate of transport.”

Diffusion of penetrants into polymer matrices has many applications, including membrane separations, barrier coatings, drug delivery, and self-healing. However, controlling the transport rate of penetrants through these matrices used in materials applications is a complicated process. The rate at which molecules move through a material can vary by more than 15 orders of magnitude depending on various characteristics of the molecule and the matrix, including molecule size and temperature.

Once the theory was developed, the team scoured the literature to find as much experimental data as possible for various penetrant-matrix pairs. They were looking for datasets with the widest range of temperatures to test their predictions: from high temperatures where the polymers are rubbery, to low temperatures where the polymers eventually vitrify into a solid glass, and for chemical structure penetrants. very different in various polymers and molecular matrices. A total of 17 datasets were analyzed and Schweizer says “we found very strong evidence for the new ideas in the theory.”

A temperature change of even 30% can change the diffusivity of penetrant molecules by 10 orders of magnitude. When the temperature is lowered, the diffusion becomes increasingly slower, and finally in a qualitatively faster way which depends on the molecular nature of the penetrant and the polymer matrix.

Schweizer explains: “When the molecule is small and/or the temperature is high, the influence of its environment on its movement is simpler and it feels much less resistance. But when the molecule gets big enough, especially under cold enough conditions, it actually has to push off more of the surrounding matter, and it becomes much harder to move. And that completely changes the temperature dependence, in a way that greatly amplifies the differences in transport rates of different molecules that can be exploited to design more selective polymer membranes.

Each pair of penetrant and polymer matrix has its own fingerprint for the speed at which the molecule can move, and small changes in temperature or penetrant size can generate huge changes in the speed of movement of the molecule. Of this result, Schweizer says “it is a spectacular experimental phenomenon.”

After validating this theory, “what emerges is not only an explanation of the existing data, but a way of thinking about the synthesis of new molecules or new polymeric materials with a transport rate adapted according to the temperature. It’s like reverse engineering,” Schweizer concluded.

This research was funded by the Department of Energy Basic energy sciencesthrough the Materials Research Laboratory at UIC.

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Sharon D. Cole