A more complete picture of the glass transition phenomenon in polymers has been developed by a team of researchers from the US, Europe and China. [Sokolov and Alba-Simionesco et al. J. Chem. Phys. (2016) DOI: 10.1063/1.4964362]
Polymers are so common in our everyday life that even materials scientists might forget that we only partially understand their behavior in terms of the strong temperature dependence of their structural relaxation and viscoeleastic properties, for instance. Of course, improved understanding would allow us to develop new functional materials.
Alexei Sokolov, of both Oak Ridge National Laboratory and the University of Tennessee and his co-workers have struggled to answer the question as to why many polymers are so "fragile", polymers have a steep temperature dependence of their structural relaxation. The fragility index of a material quantifies how quickly it will undergo the transition from solid to liquid as the temperature rises. It had been known for some time that many polymers have a fragility index around one and a half times greater than that of small molecules. This particular characteristic seemed inexplicable.
"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," Sokolov says. "Over the years we accumulated many experimental results obtained by many different techniques on a model polymer polystyrene to come up with this idea." This study gives the researchers a broad view of many polymer specific properties that they could use to determine what was missing from the bigger picture. With polystyrene molecules of different chain length, they could correlate several properties with fragility index. What emerged is that the correlations fail increasingly as chain length grows, which ultimately helped to solve the puzzle.
"Our analysis revealed that segmental relaxation in polymers cannot be completed on the structural relaxation timescale, polymers remain non-ergodic (not completely relaxed) on this timescale," Sokolov told Materials Today. "This is in strong contrast to small molecules where complete ergodicity is restored on this timescale."
"Whether this will help to make better polymers remains to be seen, but it should help in the design of polymers with desired viscoelastic properties," Sokolov says. He told us that the next step will be to find additional experimental evidence as to whether or not the proposed idea is correct. "Next, we need to develop more rigorous theoretical description of the proposed mechanism," he adds.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".