Glass transition: 100 years on


Foreground: Molecular dynamics simulation of polymers in disordered spherical globules. Background: Lattice model simulation of mobility near a free surface in a glassy material. Copyright Nicholas Tito 2017. All rights reserved.

Because of their complex macromolecular architecture, polymers do not crystallize easily. Polymers in solid form are thus commonly present in the form of glasses. In a recent review in Reports on Progress in Physics, Simone Napolitano, Emmanouil Glynos and Nicholas B Tito discuss this transition of polymers from a liquid to a glassy state. Below the authors explain more about what their article is about.

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass—a solid lacking long-range order.

Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers.

We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye–Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature.

We end the review by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

The review is free to read until the end of February 2017.

About the authors

Simone Napolitano received his PhD in Polymer Physics from KULeuven in 2007 and went on to complete postdoctoral work at the Research Foundation Flanders (FWO). In 2011, he joined the Université Libre de Bruxelles. There he served the Faculty of Science as associate professor and leads the Laboratory of Polymers and Soft Matter Dynamics. His research focuses on the molecular origin of the glass transition and the correlation between structure and dynamics in polymers and small molecules under nanoscopic confinement. His group is currently working on the physics of irreversible adsorption and on non-equilibrium phenomena in confined soft matter.

Nicholas Tito received his PhD in Chemistry from Dartmouth College in 2013. He then pursued post-doctoral work at the University of Cambridge, and is currently doing research at the Eindhoven University of Technology, The Netherlands. Nicholas uses molecular theory and simulation to discover microscopic design rules for new materials, often inspired by biology. His current topics of interest include polymer networks and glasses, liquid crystalline materials, and multivalent targeting strategies.

Emmanouil Glynos received his PhD in 2007 in polymer physics at the University of Edinburgh. He then did his postdoctoral research at the University of Michigan and in 2013, he was appointed as a research investigator at the University of Michigan. Since 2015, he has been a research scientist at the Institute of Electronic Structure and Laser at the Foundation for Research and Technology in Greece. His research focuses on the role of interfaces, confinement and chain architecture to structural and dynamical properties of polymers and on the structure-property relationship of polymer nanostructured materials for organic electronics and solid polymer electrolytes.

This work is licensed under a Creative Commons Attribution 3.0 Unported License. The image was prepared by Nichols Tito. Copyright Nicholas Tito 2017. All rights reserved.

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