Abstract
The mechanism behind the steep slowing down of molecular motions upon approaching the glass
transition remains a great puzzle. Most of the theories relate this mechanism to the cooperativity in
molecular motion. In this work, we estimate the length scale of molecular cooperativity for many
glass-forming systems from the collective vibrations the so-called boson peak. The obtained
values agree well with the dynamic heterogeneity length scale estimated using four-dimensional
NMR. We demonstrate that directly correlates to the dependence of the structural relaxation on
volume. This dependence presents only one part of the mechanism of slowing down the structural
relaxation. Our analysis reveals that another part, the purely thermal variation in the structural
relaxation at constant volume, does not have a direct correlation with molecular cooperativity.
These results call for a conceptually new approach to the analysis of the mechanism of the glass
transition and to the role of molecular cooperativity.
