Pulsed-force mode, interaction forces

The modulation frequency is typically in the range from 100 Hz to 3 kHz, and thus much lower than the resonance frequencies of the cantilever and the scanner. This enables better control of the forces exerted on the sample. The z-mod-ulation amplitude can be varied between 10 nm and 1 pm to ensure that that the tip is retracted from the surface. Shear forces are reduced permitting investigation of soft samples because of the small duration of the tip-surface contact, between 10 3 and 10 4 s. Pulse force mode SFM has been used to map adhesion of heterogeneous polymers in dependence of temperature and molecular weight as well as map electrostatic double-layer interactions [158-160]. 

The Pulsed Force Mode allows investigations of the tip-sample interactions on a more quantitative level. For PFM measurements, the Dimension 3100 SFM was equipped with a Pulsed Force Mode box (WITec Instruments, Ubn, Germany). For adhesive force measurements, a standard micro-fabricated silicon cantilever with aluminum reflex coating (Olympus, AC240TS) with spring constants of about 2 N/m was used. To choose cantilevers with similar spring constants, their resonance frequencies were measured [13]. All measurements are performed under ambient conditions using the same set point of 0.5 V. 

To simulate the toner-silica interactions, PS/PMMA blends were used as a model for polystyrene/acrylic toners. Silicon tips modified either with HMDS or with PDMS were applied to model surface-treated silica particles. The Pulsed Force Mode images of PS/PMMA films displayed... 

For pulsed force mode imaging of the PP sample, a modulation frequency in the range of 800 Hz is selected. The cantilever oscillations are adjusted such that the adhesive interactions between tip and surface are overcome, as monitored using an oscilloscope. The modulation must clearly show the snap-off of the tip. Next the four markers are set as shown in Fig. 3.22. 

Variable temperature Scanning Force Microscopy of mixed polystyrene (2000 - 100000 g/mol) and poly (methylmethacrylate) (100000 g/mol) thin films was used to probe mechanical properties such as surface stiffness and pull-off forces. Adhesion data can be explained by the molecular properties of the constituents. The adhesion of Polystyrene samples was measured by force distance curves and using the Pulsed Force Mode. It can be shown that surface tension is not the dominant part of the tip-surface interaction, but the mechani cal properties of the material will influence the measured adhesive force. Wetting of the tip by polymer molecules at higher temperatures due to increasing mobility is one possible model. 

SFM also enables us to measure specific interaction forces between a small silicon tip and the surface. The pull-off forces between the tip and the surface estimated from Force vs Distance Curves (FDC) can be correlated to the adhesive interactions between tip and surface [9]. Recording of FDCs line-by-line allows us to image surface topography and adhesive surface properties simultaneously [10]. This technique has some disadvantages, like the requirement for a large amount of data acquisition and analysis, which have been alleviated by the invention of the Pulsed Force Mode (PFM). The PFM simplifies and accelerates the measurements of adhesive properties with high lateral resolution [11, 12]. 

To analyze the density-dependent vibrational lifetime data displayed in Fig. 3, it is necessary to separate the contributions to Ti from intramolecular and intermolecular vibrational relaxation. The intermolecular component of the lifetime arises from the influence of the fluctuating forces produced by the solvent on the CO stretching mode. This contribution is density dependent and is determined by the details of the solute-solvent interactions. The intramolecular relaxation is density independent and occurs even at zero density through the interaction of the state initially prepared by the IR excitation pulse and the other internal modes of the molecule. Figure 5 shows the extrapolation of six density-dependent curves (Fig. 3 three solvents, each at two temperatures) to zero density. The spread in the extrapolations comes from making a linear extrapolation using only the lowest density data, which have the largest error bars. From the extrapolations, the zero density lifetime is —1.1 ns. To improve on this value, measurements were made of the vibrational relaxation at zero solvent density. 

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