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.

In this study, composite gelatin-polyaniline (PANI) nanofibers doped with camphorsulfonic acid (CSA) were fabricated by electrospinning and used as substrates to culture C2C12 myoblast cells. We observed enhanced myotube formation on composite gelatin-PANI nanofibers compared to gelatin nanofibers, concomitantly with enhanced myotube maturation. Thus, in myotubes, intracellular organization, colocalization of the dihydropyridine receptor (DHPR) and ryanodine receptor (RyR), expression of genes correlated to the excitation-contraction (E-C) coupling apparatus, calcium transients, and myotube contractibility were increased. Such composite material scaffolds combining topographical and electrically conductive cues may be useful to direct skeletal muscle cell organization and to improve cellular maturation, functionality, and tissue formation.

Local dielectric spectroscopy is performed to study how relaxation dynamics of a polyvinyl-acetate ultrathin film is influenced by inorganic nanoinclusions of a layered silicate (montmorillonite). Dielectric-loss spectra are measured by electrostatic-force microscopy in the frequency-modulation mode in ambient air. Spectral changes in both shape and relaxation time are evidenced across the boundary between pure polymer and montmorillonite sheets. Dielectric-loss imaging is also performed, evidencing spatial variations of dielectric properties near nanostructures with nanometer-scale resolution.(C) 2010 American Vacuum Society. [DOI: 10.1116/1.3368597

The viscoelastic response of inhomogeneous rubbery blends upon interacting with an atomic force microscopy (AFM) cantilever is characterized in both the contact and intermittent contact states. In particular, loss tangent spectra and images are measured in tapping mode AFM at relatively high frequencies (>105 Hz) and in an AFM-based method in the contact state recently developed for the characterization of the viscoelasticity of soft materials in a frequency range of 100–104 Hz. Comparing the measured data to that from a bulk technique reveals that a combination of these two methods can qualitatively characterize the nanoscale viscoelastic behavior of inhomogeneous rubbers over an unprecedented frequency range. However, the loss tangent measured in the tapping mode is overestimated compared to those measured in the contact state and the bulk technique, which is attributed to the existence of the adhesion energy hysteresis during the approach and withdrawal of the tip from the sample in the tapping mode. Such an overestimation becomes less pronounced near the glass transition region of the materials.

The effect of confinement on structural relaxation in ultrathin poly(vinyl acetate) films has been studied by local dielectric spectroscopy. This scanning probe method allows the investigation of dielectric relaxation at nanometer scale of supported ultrathin films having a free surface. Measurements have been performed at ambient pressure and controlled atmosphere on films with decreasing thickness. A deviation of dynamic properties from the bulk behavior, showing up as an increase of the relaxation rate, was observed starting from film thickness of 35 nm, which corresponds to about 3 times the gyration radius of polymer chains. A 2-fold increase of relaxation rate was measured for the thinnest investigated film of 18 nm. Local dielectric spectroscopy is therefore an effective method to elucidate confinement effects on relaxation dynamics in ultrathin polymer films with a free upper surface

The effects of supporting substrate and thermal annealing on plasticization of poly(vinyl acetate) ultrathin films by absorption of ambient moisture have been studied by local dielectric spectroscopy. Upon exposure to ambient moisture, the relaxation rate of the α-process increases to a different extent at variance of film thicknesses and supporting substrates. Namely, on hydrophobic gold substrates the speeding up is slightly reduced by decrease of film thickness down to about 20 nm. Moreover, the increase of the relaxation rate measured on a 21 nm thick film supported on gold is smaller compared to that measured on a film with similar thickness supported on the relatively hydrophilic aluminum. We interpret such results in terms of the presence of an interfacial polymer layer at the substrate surface with different contents of water in dependence of the substrate. Furthermore, plasticization effect in ultrathin films significantly decreases after annealing at high temperature (∼ T g + 60 K) and for times longer than usual time scales of relaxation processes. Such results are consistent with the formation of a high-density irreversibly adsorbed polymer layer at the interface, able to hinder the effect of the substrate.