No abstract

We report results of glass transition (T(g)) measurements for polymer thin films using atomic force microscopy (AFM). The AFM mode, shear modulation force microscopy (SMFM), involves measuring the temperature-dependent shear force on a tip modulated parallel to the sample surface. Using this method we have measured the surface T(g) of thin (17-500 nm) polymer films and found that T(g) is independent of film thickness (t>17 nm), strength of substrate interactions, or even presence of substrate.

No abstract

Atomic force microscopy (AFM), neutron reflection (NR) and secondary ion mass spectroscopy (SIMS) are used to examine phase separation in symmetrically segregating thin polymer blend films (≤1000 Å). Phase separation in the film leads to undulations of the liquid−air interface, provided the film is sufficiently thin to suppress surface-directed spinodal decomposition waves. Flattened droplets are formed at a very late stage of phase separation, and the aspect ratio of these droplets can be rationalized by an interfacial free energy minimization argument.

No abstract

This article discusses capillary forces measured by scanning force microscopy ͑SFM͒, which, as recently reported, show a discontinuous behavior at a low relative humidity between 20% and 40% depending on the solid surfaces. A capillary force discontinuity is very interesting in terms of a possible phase change or restructuring transition of bulk water in the interfacial solid-liquid region. Unfortunately, we have found that SFM measurements show an inherent weakness in the determination of the origin of the forces that are obtained during pull-off measurements. This article critically discusses the origin of the adhesive interactions as a function of relative humidity with chemically modified probing surfaces. Our measurements indicate that force discontinuities in pull-off measurements are strongly affected by the inability of the liquid to form capillary necks below a critical threshold in relative humidity. In the course of this article, we will discuss roughness effects on capillary forces and provide a modified capillary force equation for asperity nanocontacts.

We have indented the surface of ice at temperatures between Ϫ1°C and Ϫ17°C with sharp atomic force microscope tips. For a thick viscous interfacial melt layer, a Newtonian treatment of the flow of quasiliquid between the tip and the ice suggests that indentations at different indentation velocities should have the same force/velocity ratio for a given pit depth. This is observed for silicon tips with and without a hydrophobic coating at temperatures between Ϫ1°C and Ϫ10°C implying the presence of a liquid-like layer at the interface between tip and ice. At temperatures below about Ϫ10°C the dependence of force on velocity is weaker, suggesting that plastic flow of the ice dominates. A simple model for viscous flow that incorporates the approximate shape of our tip is used to obtain an estimate of the layer thickness, assuming the layer has the viscosity of supercooled water. The largest layer thicknesses inferred from this model are too thin to be described by continuum mechanics, but the model fits the data well. This suggests that the viscosity of the confined quasiliquid is much greater than that of bulk supercooled water. The hydrophobically coated tip has a significantly thinner layer than the uncoated tip, but the dependence of thickness on temperature is similar. The estimated viscous layer thickness increases with increasing temperature as expected for a quasiliquid premelt layer.

The dissipation mechanism of nanoscale kinetic friction between an atomic force microscopy tip and a surface of amorphous glassy polystyrene has been studied as a function of two parameters: the scanning velocity and the temperature. Superposition of the friction results using the method of reduced variables revealed the dissipative behavior as an activated relaxation process with a potential barrier height of 7.0 kcal/mol, corresponding to the hindered rotation of phenyl groups around the C-C bond with the backbone. The velocity relationship with friction F(v) was found to satisfy simple fluctuation surface potential models with F proportional to const-ln(v) and F proportional to const-ln(v)2/3.