Helix-coil fluctuations show glasslike behavior in gelatin gel networks

There is a lot that remains opaque about glasslike materials and the mechanisms leading to their rigidity in some conditions. Recent observations of glasslike stress relaxation in permanently linked gelatin gel networks raise the question of the origins of the particular phenomenon. Researchers building upon this earlier work report in The Journal of Chemical Physics that, unlike in structural glasses, the behavior of these gelatin networks does not result from cooperative effects generated by long-range elastic couplings, but only from helix content fluctuations when approaching the temperature at which the polymer undergoes helix-coil conformation transition.

The authors considered the relaxation dynamics of two gelatin gel models: The first was a full network simulation with fixed topology, and the second consisted of independent, uncoupled strands. They found that the dynamical responses of the network and the uncoupled models were almost identical, indicating the irrelevance of elastic coupling effects in the relaxation of the systems. This suggests that the relaxation is controlled by helix-coil transition effects in a set of independent and identical strands.

A new set of experiments identical to the prior work tested this model, but with a system that was permitted to reach equilibrium before stress application. The researchers found that their experimental observations were in very close agreement with the predictions of their model system.

They conclude that the slow dynamics of gelatin gel stress relaxation is an indication of helix content fluctuations as the helix-coil transition approaches, and that the observed glasslike behavior occurs just above the transition temperature. They also note that while such stress relaxation behavior is essentially identical to that observed in some organic glasses, the physical mechanisms are quite different, highlighting the diverse origins of glasslike dynamics.

Source: “Glass-like stress relaxation of a permanent gelatin network as a signature of pretransitional helix-coil fluctuations,” by Christiane Caroli, Olivier Ronsin, and Anaël Lemaître, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5003212 .