Research Leads to Better Understanding of Polymers

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Polymers are huge molecules made up of thousands and millions of atoms bonded together in a repetitive pattern resembling a chain. Many things that are a part of everyday life are made up of these polymers, including tires, bottles, medical devices and airplanes.

To develop new operational materials for various contemporary and future technologies, it is essential to understand what gives polymers their special properties. An international group of scientists have pooled their experimental results and expertise over the years to explain the highly powerful temperature dependence of polymers melts’ viscoelastic properties, a mystery that has not been explained previously.

Fragility index can be used as a parameter to quantify the speed at which a solid transforms to liquid with an increase in temperature. The high fragility of the polymers compared to the small molecules has been documented for many years. The Fragility index of most polymers is about 1.5 times more than that of even the most weak, small molecular liquids. The reason behind this has remained unclear, until now.

A group of researchers from the U.S., China, and Italy were able to discover a more comprehensive picture of the glass transition phenomenon in polymers by combining numerous techniques and tools. They were also able to discover where the difference between the polymers and small molecular liquids occur. The Journal of Chemical Physics from AIP Publishing has published the researchers’ explanation to their findings.

We worked on this problem with our colleagues for a long time and though our paper with the similar title, 'Why many polymers are so fragile?' was published in 2007, we could only formulate the problem, we had no answer. Over the years we accumulated many experimental results obtained by many different techniques (this is why the paper has so many authors) on a model polymer polystyrene to come up with this idea.

Alexei P. Sokolov, Research Scientist at Oak Ridge National Laboratory

Sokolov is also a professor of Chemistry and Physics at the University of Tennessee.

This offered a clear view of numerous polymer specific properties that are needed to understand what was missing. The researchers used polystyrene of various chain lengths to correlate a number of their properties to their fragility. Using this they showed that the correlations work for chains of short lengths, but increasingly fail when the length or the number of repetitive sections or units increases. This finding offers a new perspective on the issue.

The scientists found that the segmental relaxation, also known as the structural relaxation, of polymers offers the relaxation of merely a small portion of the molecule. The full molecular scale relaxation for polymers occurs only on a significantly longer time scale that matches the chain relaxation.

They demonstrate that the correlations characteristic of non-polymeric systems are restored by the analysis of the chain relaxation rather than the relaxation of segments. This provides a new perspective on the issue.

How does this impact the polymers that are a part of daily life?

Our work has broader implications, because similar mechanisms may account for rather high fragility of other complex systems in soft condensed matter. Whether this will help to make better polymers remains to be seen, but it should help in the design of polymers with the desired viscoelastic properties.

Alexei P. Sokolov, Research Scientist at Oak Ridge National Laboratory

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