Atomic force microscopy (AFM) is one of the most popular techniques used for surface characterization. In order to map surface topography on the nanometer scale, the interaction forces between sample and tip can be used. Moreover, with appropriate accessories, the AFM can detect material properties based on interaction forces between the tip and sample. Hence, WITec’s Mercury 100 AFM incorporates the Digital Pulsed Force Mode (DPFM) to facilitate imaging of all surface properties that can be obtained from force distance and pulsed force curves. Temperature-dependent studies on such properties are important for many technical applications.
Imaging Phase Separations in Polymer Blends
The integration of DPFM AFM with a sample stage of variable temperature helps in imaging phase separations in polymer blends as a virtue of the glass transition temperature. The DPFM assesses and stores the complete force distance cycle, which enables a comprehensive analysis of the tip-sample interactions. In these studies, the material properties of PS-PMMA (Polystyrol-Polymethylmetacrylate) blend as well as their variations with temperature (25°C and 140°C, Scan range: 5x5 ìm; adhesion and cantilever oscillation damping images restructured from pulsed force curves) were measured.
Surface Topography of Polymer Blends
Figure 1. Topography images at 25°C (left) and at 140°C (right).
In Figure 1, the left topography image at 25°C is analogous to the one at 140°C (right). Nonetheless, the total height increases at 140°C signifying a swelling of the polymer film at increased temperature. Scan range at 140°C (right) is 5x5 ìm, z = 30nm and scan range at 25°C (left) is 5x5 ìm, z = 20nm.
Analysis of Adhesion Images
Large differences can be observed in the adhesion image. The adhesion variation at increasing temperature denotes that the holes in the topographical picture correlate to PS, while the higher domains correspond to PMMA.
Damping of the Vantilever Free Oscillations
The oscillations were recorded immediately following detachment from contact with the sample, clearly demonstrating the material contrast at increased temperatures. At room temperature, a thin water layer occurs on the sample covering material contrast. At 140°C temperature, the material contrast can be clearly seen between PMMA and PS, since the water layer is no longer present.
Analysis of Thin Paraffin Films
Figure 2. Topography images at the following scan range: 10x10 ìm, 25°-90°C: z = 35nm; 130°C: z = 20nm.
For analysis of thin paraffin films, the films were deposited onto a silicon substrate. The sample stage of variable temperature enabled temperature-controlled measurements up to 200°C. Together with the topography, the paraffin’s adhesion was assessed at increasing temperatures with the DPFM. Figure 2 shows the topography images at the following scan range: 10x10 ìm, 25°-90°C: z = 35nm; 130°C: z = 20nm.
At room temperature, two types of crystalline phases exist, with large crystals divided by finer structures. The adhesion image displays mainly topographical features because of larger interaction between tip and sample surfaces. Upon heating, the fine structures disappear at 100°C. The images obtained at 120°C display the presence of the large structures. They show lower adhesion (dark area in adhesion image) in contrast to the area where the fine structure was present previously. The melting of the large crystals can be observed at 130°C in the topographical image. The crystals can still be identified, although the adhesion displays a nearly uniform picture.
The Mercury 100 AFM integrates the DPFM to allow imaging of all surface properties that can be obtained from force distance and pulsed force curves. Studies on such properties are important in various technical applications. WiTec develops high-performance instrumentation for both scientific and industrial applications focused on novel solutions for optical and scanning probe microscopy.
This information has been sourced, reviewed and adapted from materials provided by WITec GmbH.
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