The dynamics of melts — or concentrated solutions of linear polymers — is reasonably well understood in terms of 'reptation': snake-like motion of the polymer chains through the spaghetti structure. But systems of ring polymers, lacking free ends, behave completely differently. By means of computer simulations, Davide Michieletto and Matthew Turner have now uncovered a fascinating property of ring polymers: their ability to form a glass, not via temperature or density changes, but from topological constraints alone.
Michieletto and Turner modelled ring polymers as coils of beads numbering up to 2,048, and first made the observation that a system of rings extensively interpenetrates. The authors then investigated what happens when — starting from an equilibrium configuration — a randomly chosen fraction of the polymers is pinned. Spatially and temporarily freezing certain rings makes the others subject to inter-coil, non-crossability constraints. Remarkably, the ensuing inter-ring 'threadings' can result in kinetically arrested states — in other words, glasses. The more beads on each coil, the smaller the fraction of pinned rings needed to create a glassy state — suggesting that a ring-polymer glass could be realized by optically trapping just a few polymers.