Cooperativity and the freezing of molecular motion at the glass transition

Abstract: The slowing down of molecular dynamics when approaching the glass transition generally proceeds much stronger than expected for thermally activated motions. This strange phenomenon can be formally ascribed to a temperature-dependent activation energy E(T). In the present work, via measurements of the third-order nonlinear dielectric susceptibility, we deduce the increase of the number of correlated molecules Ncorr when approaching the glass transition and find a surprisingly simple correlation of E(T) and Ncorr(T). This provides strong evidence that the non-canonical temperature development of glassy dynamics is caused by a temperature-dependent energy barrier arising from the cooperative motion of ever larger numbers of molecules at low temperatures.