Cynthia Buenviaje

Critical phenomena of water bridges in nanoasperity contacts

This article discusses capillary forces measured by scanning force microscopy (SFM), 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.

Glass and Structural Transitions Measured at Polymer Surfaces on the Nanoscale

This paper reviews our recent progress in determining the surface glass transition temperature, Tg, of free and substrate confined amorphous polymer films. We will introduce novel instrumental approaches and discuss surface and bulk concepts of Tg. The Tg of surfaces will be compared to the bulk, and we will discuss the effect of interfacial interactions (confinements), surface energy, disentanglement, adhesion forces, viscosity and structural changes on the glass transition. Measurements have been conducted with scanning force microscopy in two different shear modes: dynamic friction force mode and locally static shear modulation mode. The applicability of these two nano-contact modes to Tg will be discussed.

Glass transitions of thin polymeric films: Speed and load dependence in lateral force microscopy

The glass transition of thin polymeric films can be profitably studied using lateral force microscopy (LFM) if the system is calibrated regarding operational parameters, in particular the applied load and the scanning velocity. We have established that these two parameters significantly influence the occurrence of an apparent glass transition. In particular, we have found that the local pressure, applied by the LFM tip, is insufficient to generate a hydrostatic pressure effect causing an increase in the apparent transition temperature. In fact, at a constant scan velocity and for increased load, the apparent transition temperature decreases towards the actual bulk value. Further discussions in this article are based on viscoelastic theories. Critical time scales that are characteristic for sliding are compared to polymer relaxation times, and provide an estimate of the viscosity temperature dependence.

Glass transition measurements on heterogeneous surfaces

Abstract The local glass transition temperature ( T g ) of heterogeneous polymer surfaces has been successfully determined by lateral force microscopy (LFM) on the submicron–nanometer scale. This paper addresses interfacial confinement effects in thin films of blends and emulsions from polystyrene (PS), poly(butylmethacrylate) (PBMA) and poly(methylmethacrylate) (PMMA). The paper addresses the difficulty of thermal rheological experiments with a microscopic scanning technique, and introduces an in-situ calibration method.

Nanorheological approach for characterization of electroluminescent polymer thin films

Shear-modulated scanning force microscopy (SM-SFM) is introduced as a nanorheological characterization method for the determination of optoelectronic properties of semiconducting polymer thin films (<100 nm). In this letter, the photoluminescence quantum efficiency of poly(p-phenylenevinylene) (PPV) was directly correlated to the glass transition property by SM-SFM. Conformational changes and chain packing were discussed as a function of the conversion temperature of the soluble PPV precursor. Compared to the bulk, very low glass transition temperature values in the range of 65 to 85 °C were found, which imply an increased molecular mobility in thin films of conjugated polymers.

Thermal transition measurements of polymer thin films by modulated nanoindentation

A modulated nanoindentation technique has been developed for thermal transition analysis of polymer thin films. The procedure was applied to glass transition measurements of poly-t-butylacrylate, with results that compare well with shear-modulated force microscopy and differential scanning calorimetry. The advantages of the modulated indentation technique are twofold: (i) it provides in situ experimental access to material properties in nanoscopically confined geometries, and (ii) it incorporates well-defined indenter geometries, which provide a means for quantitative thermorheological analysis that is not available with conventional scanning probe microscopy. The experimental procedure and critical test parameters are detailed.

Investigations of heterogeneous ultrathin blends using lateral force microscopy

In order to study the glass transition of thin film polymer blends, high spatial resolution and temperature sensitivity is needed. In this paper, we emphasize the importance of the calibration of scanning parameters such as load and speed when measuring the glass transition temperature of polymers using lateral force microscopy. Once calibrated, this method is ideal for investigations of heterogeneous samples such as blends and co-polymers. We present an analysis technique for lateral force imaging using a fast and stable cooling/heating stage. This approach involves mapping the friction forces over a certain area and identifying regions of different frictional properties. The difference in the average friction force can then be plotted as a function of temperature. The friction force is expected to vary around the glass transition. Therefore, the glass transition temperature can be defined as the temperature at which the difference in the average friction force undergoes a slope change. We present investigations of blends using polystyrene mixed with poly(butylmethacrylate). The transition temperatures obtained are in good agreement with the bulk values of corresponding homopolymeric films.