β-relaxation has long been attributed to localized motion of constituent molecules or atoms confined to isolated regions in glasses. However, direct experimental evidence to support this spatially heterogeneous scenario is still missing. Here we report the evolution of nanoscale structural heterogeneity in a metallic glass during β-relaxation by utilizing amplitude-modulation dynamic atomic force microscopy. The successive degeneration of heterogeneity during β-relaxation can be well described by the Kohlrausch–Williams–Watts equation. The characteristic relaxation time and activation energy of the heterogeneity evolution are in accord with those of excess enthalpy release by β-relaxation. Our study correlates β-relaxation with nanoscale spatial heterogeneity and provides direct evidence on the structural origins of β-relaxation in metallic glasses.

We review nano-palpation atomic force microscopy, which offers quantitative mechanical property mapping especially for soft materials. The method measures force-deformation curves on the surfaces of soft materials. The emphasis is placed on how both Hertzian and Derjaguin-Muller-Toporov contact mechanics fail to reproduce the experimental curves and, alternatively, how the Johnson-Kendall-Roberts model does. We also describe the force-volume technique for obtaining a two-dimensional map of mechanical properties, such as the elastic modulus and adhesive energy, based on the above-mentioned analysis. Finally, we conclude with several counterpart measurements, which describe the viscoelastic nature of soft materials, and give examples, including vulcanized isoprene rubber and the current status of ISO standardization.

The influence of interfacial interactions and annealing time on dynamics of the α-relaxation in ultrathin poly(vinyl acetate) films deposited on different substrates has been studied using local dielectric spectroscopy at ambient pressure and controlled humidity. After annealing at 323 K for about 3 days, for polymer films supported on gold and aluminum substrates, an increase of the relaxation rate with decreasing film thickness below 30−35 nm was observed, whereas for films deposited on silicon substrates a thickness-independent dynamics was found for films as thin as 12 nm. The difference in size effect on dynamics of the films could reasonably be related to the difference in interfacial energy between polymer films and substrates, even though a criterion simply based on interfacial energy cannot be used to explain all the results. In fact, further annealing at a higher temperature evidenced an annealing-dependent dynamics in films prepared on aluminum substrates consistent with the presence of long-living metastable states at the polymer/substrate interface. The lifetime of such metastable states seems related to the nature of the substrate as well as to the molecular weight of the polymer.