Pull-off force measurement

Fig. 13. Measurement of surface energies of PS and PMMA. It can be seen that there was a finite adhesion hysteresis. At a given load, the contact radius during loading was less than the contact radius during unloading. From the unloading data, we get yi>s = 45 1 mJ/nr, and yi),viMA = 53 1 mj/m . These number are in good agreement with the values of surface energies determined from the pull-off force measured using the SFA.

Thus, the different surface free energies ji can, in principle, be experimentally determined based on AFM pull-off force measurements if the surface free energy of the tip is varied in a controlled manner. These equations form the basis for the so-called chemical force microscopy (CFM) approach and allow one to discriminate between different materials [7]. 

Fig. 4.8 (a) Histograms of pull-off force values obtained with an unmodified Si3N4 tip on untreated and oxyfluorinated iPP films in ethanol. The total surface free energy y of the polymer film is shown, (b) Mean values of pull-off force measured with COOH-terminated tips on modified polyolefin surfaces (iPP, isotactic polypropylene LDPE, low-density polyethylene) in ethanol (top) and with OH-terminated tips on oxyfluorinated iPP in water (pH 3.8, bottom) as a function of cos 0 (contact angle measured with water). (Reprinted in part/adapted with permission from [26, 27]. Copyright 1998, 2000, American Chemical Society.)... 

First Polymer Series. The first series of polymers was investigated, in order to limit the interaction between an AFM probe and polymer surfaces to only the dispersion (London) component of the Van der Waals force. Adhesional forces were measured between a S10,t probe and a set of nonpolar polymers that provided a range of refractive indexes (as measured) polystyrene (1.582), isotactic polypropylene (1.501), poly(vinylidene fluoride) (1.407), and poly(tetrafluoroethylene-co-hexafluoropropylene) (1.348). The histograms of the pull-off forces, measured with a SiOx probe, are shown in Figure 2 and tabulated with the calculated values for adhesion energy in Table 1. 

Second Polymer Series. Our second series of experiments involved measurements of the pull-off forces in PFD between AFM probes and the surfaces of polymer films with varying degrees ofhydrophobicity/hydrophilicity PS, PAN, PMMA, and PAA. The histograms of the distributions of pull-off forces measured between AFM probes and polymer surfaces in PFD (Figures 4—6) clearly indicate two trends one for the nonpolar tips (virtually identical results were obtained for both the gold and the polystyrene... 

Figure 2. Histograms of pull-off forces measured between a SiO, probe and PS, i-PP, PVDF, and FEP surfaces in perfluo-rodecalin.

Figure 13 (a) Representation of the modified tip and surface to measure CTC interactions, (b) Pull-off force measurements in different... 

Thus by contact angle measurements using three different liquids (L), of which two must be polar, with known y Y and y values, the ys", Ys and ys of any solid (S) can, in principle, be determined. The value of yl must be known or determined independently [108]. The apolar component of the surface tension of solids (yj" ) can be determined by contact angle measurements using strictly apolar liquids for which yL = y These surface tension components can be related to experimentally determined pull-off forces between chemically modified AFM tips and an oxyfluorinated isotactic polypropylene surface in CFM approaches [110]. It was observed that the pull-off force measured with carboxylic acid tips in ethanol depended hnearly on the basic term of the surface tension (y,") on the modified polymer surface. 

A second example for CFM imaging is shown in Fig. 16. Here a hydrophobic - CH3 terminated tip is used to differentiate between hydrophilic and hydrophobic regions in a microcontact printed SAM in friction and adhesion (pull-off) force measurements in water [163]. 

Figure 2.10a shows pull-off forces measured on the same pattern withont snr-face scanning between each measnrement (i.e., a no-scan pull-off force). Figure 2.10b shows the friction and pull-off forces measured for the arrays as a function of groove depth. The measured friction force was averaged over a scan area of about 2 to 4 pm. The puU-off forces in this lignre were measured before and after each friction measurement at the same scan area, and were then averaged (i.e., a scan pull-off force). 

Figure 2.11 shows the friction and pull-off forces measured for various spacings and widths of periodic grooves using the patterns shown in fig. 2.2. The abscissa represents the ratio of mound width to total width of monnd and groove, which was determined from the milling conditions. Both forces increase as this mound ratio increases. 

Figure 2.14b shows the friction force measured on platinum patterns and the averaged scan pull-off force measured before and after each friction measurement for the same scanned area. In fig. 2.14b, both the friction and pull-off forces decreased as the asperity height increased, but not as rapidly as the forces with silicon groove depth (fig. 2.10b). The difference in contact conditions, as mentioned previously, possibly caused the differences in rates of decrease in the friction and pull-off forces. 

Figure 2.16a shows the no-scan pull-off force on the sputtered platinum patterns (fig. 2.15). Figure 2.16b shows the friction and the scanned pull-off force measured on the same specimen. In fig. 2.16b, both the friction and pull-off forces clearly decreased as the asperity height increased. The superficial layer was probably removed by the sputtering process by FIB, because the fluctuation of the forces significantly decreased. In fig. 2.16a, the pull-off forces measured on the pattern of 6.6-nm asperity height showed wide scatter compared with the others because of the lower uniformity of the asperity shape. 

FIGURE 2.16 Friction and pull-off forces measured on platinum asperity arrays after sputtering. (a) Relation between pull-off force aud asperity height and (b) effect of asperity height on the friction and pull-off forces. 

FIGURE 2.17 Relation between friction force and pull-off force measured on a platinum asperity array as shown in fig. 2.15. Friction and pull-off forces are extracted from fig. 2.16. The maximum friction force of 100 nN (shown as O) was measured at the asperity height of 0 nm in fig. 2.6, which means the friction force was measured on a bare silicon surface without... 

Figure 2.25 shows the pull-off force measured for the asperity arrays covered with two kinds of LB films. The curvature radius on the jc-axis was shown in table 2.3. The data for each plate with CHCCnHjjCOOHl-LB or CFCH(C6F,3C H22COOH)-LB film were fitted with a line passing through the origin. The pull-off force decreased with smaller curvature radius and was roughly proportional to the curvature radius. The pull-off force on the CH-LB film was about l/5th of the pull-off force on the CFCH-LB film for the same curvature radius. 

Figure 2.26 shows the pull-off forces on two kinds of asperity arrays as a function of the relative humidity. The average curvature radius of each asperity array was 150 and 440 nm for the CH-LB film and 95 and 370 nm for the CFCH-LB film. Each plot in fig. 2.26 is the average of 256 pull-off force measurements. The error bar shows the standard deviation for each data point. The average pull-off force clearly increased with higher relative humidity for the asperity array of 370-nm radius with the CFCH-LB film... 

FIGURE2.30 Friction force vs. pull-off force measured on asperity arrays on silicon plates coated with CH(C H35COOH)-LB and CFCH(C F,3CiiH22COOH)-LB films. Measured data were fitted with a line that passes through the origin. The gradient of each approximated line is shown in the parentheses in the inset box. 

The change in surface composition could be measured accurately because the adhesive force between ester (the reactant) and alkyl-terminated surfaces is large. Average pull-off forces measured between neat ester-SAMs and methyl-terminated tips were found to be 9 2 nN, whereas neat hydroxyl-terminated SAMs and methyl-terminated tips show an average pull-off force of 0.4 0.3 nN. 

Figure 5 Histograms of pull-ofF forces measured between a SiOx probe (top) and gold probe (bottom) with PS, PAN, PMMA, and PAA surfaces in perfluorodecalin. Reproduced from ref. (10) 1998 American Chemical Society.

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