The material time of physical aging - and beyond

Kurzfassung

All glassy materials undergo physical aging, that is the slow evolution of material properties as the system approaches an equilibrium supercooled liquid state. Physical aging is non-linear in temperature history, meaning that the relaxation pathway depends critically on its temperature history. In 1971, Narayanaswamy proposed that aging becomes linear when described in terms of material time rather than standard time. It can be viewed as a material's inner clock whose rate changes as the material ages [1,2]. This idea implies that physical aging is governed by the same mechanisms as equilibrium structural relaxation-but with a time-dependent relaxation rate. We show how material time can be determined experimentally from time-resolved autocorrelation functions obtained by light scattering. For different systems, we demonstrate that the extracted material time renders physical aging linear in temperature history, with stationary and time-reversible thermal fluctuations [3]. Finally, we discuss mechanisms beyond material-time scaling found in metallic glasses using time-resolved XPCS. These systems exhibit compressed-exponential relaxation, whose appearance correlates with off-equilibrium values of the material's density. While the microscopic dynamics do not obey material-time scaling, the evolution of the density shows an asymmetry of approach-as expected for aging obeying material time-suggesting the material-time formalism may still apply at a more global scale.