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Quantitative Measurement of the Mechanical Contribution to Tapping-Mode Atomic Force Microscopy Images of Soft Materials S. Kopp-Marsaudon,† Ph. Lecle`re,‡ F. Dubourg,† R. Lazzaroni,‡ and J. P. Aime´*,† CPMOH Universite´ de Bordeaux I, 351, cours de la Libe´ ration, F-33405 Talence Cedex, France, and Service de Chimie des Mate´ riaux Nouveaux, Centre de Recherche en Science des Mate´ riaux Polyme` res (CRESMAP), Universite´ de Mons-Hainaut, Place du Parc, 20, B-7000 Mons, Belgium Received April 1, 2000. In Final Form: July 10, 2000 In atomic force microscopy, tapping-mode (also called intermittent contact mode) operation is known for its ability to image soft materials without inducing severe damage. For soft materials, the determination of the relative contributions of the topography and the local mechanical properties to the recorded image is of primary importance. In this paper, we report a systematic comparison between images and approachretract curve data. We show that this experimental comparison allows the origin of the contrast that produces the image to be straightforwardly evaluated. The method provides an unambiguous quantitative measurement of the contribution of the local mechanical response to the image. To achieve this goal, experimental results are recorded on a model system, a triblock copolymer, with a nanophase separation between elastomer and glassy domains. In this particular case, we show that most of the contrast in the height and phase images is due to variations of the local mechanical properties.
1. Introduction Local scanning force techniques (such as atomic force microscopy (AFM)) have received considerable interest since their invention a decade ago. One reason for this interest is their ability to produce images of the surface on small length scales, ranging from hundreds of microns down to nanometers. Here we will address the question of the origin of the contrast in the dynamical force probe method applied to soft materials. Two types of operating dynamical modes are possible: Either the oscillating amplitude is fixed and the output signal is the resonance frequency (this is called the noncontact resonant force mode)1 or the oscillation frequency is fixed and the variation of the amplitude and phase are recorded. This mode is commonly named tapping mode (also known as intermittent contact mode)2 and is the one that is considered in this study. Tapping-mode atomic force microscopy (TMAFM) is commonly used because of its ability to probe soft samples, due to the minimization of sample damage during the scans. Moreover, tapping-mode images can be of two different types: in one type, the image is the record of changes of the piezoactuator height necessary to maintain a fixed oscillation amplitude through a feedback loop (the height image); in the other type, the image records changes of the oscillator phase delay relative to the excitation signal. This additional imaging possibility has revealed in many cases a high sensitivity to variations of the local properties. A number of studies have shown the possibility to extract useful information from the tapping-mode images of soft samples, especially with samples showing a particular contrast on a small scale, like blends of hard and soft materials or copolymers.3-5 † ‡
CPMOH Universite´ de Bordeaux I. Universite´ de Mons-Hainaut.
(1) Albrecht, T. R.; Gru¨tter, P.; Horne, D.; Rugard D. J. Appl. Phys. 1991, 69, 668. (2) Zhong, Q.; Inniss, D.; Kjoller, K.; Elings, V. B. Surf. Sci. 1993, 290, L688.
Nevertheless, questions remain about an accurate description of the physical origin of the tapping-mode image contrast.6-11 In many cases, the height images are considered to display topographic information, but it must be kept in mind that the local mechanical properties of the samples may also contribute to contrast in the height image. For the phase image, using the dominant repulsive regime11 and disregarding the nonlinear deformation of the resonance peak, the phase shifts are related to the local mechanical properties. At this point, it is worth it to mention that, to keep a well-defined oscillating behavior of the tip, the perturbation to the oscillator due to the contact with the surface is chosen to be small; in other words, the reduction of the free amplitude (the set point) is only a few percent. This method has two advantages: From an experimental point of view this allows us to identify immediately hard and soft domains, the bright parts of the image corresponding to hard domains.5 From a theoretical point of view, this allows us to use simple (3) Magonov, S. N.; Elings, V.; Wangbo, M. H. Surf. Sci. 1997, 389, 201. (4) Stocker, W.; Beckmann, J.; Stadler, R.; Rabe, J. P. Macromolecules 1996, 29, 7502. (5) Lecle`re, Ph.; Lazzaroni, R.; Bre´das, J. L.; Yu, J. M.; Dubois, Ph.; Je´roˆme, R. Langmuir 1996, 12, 4317. Lecle`re, Ph.; Moineau, G.; Minet, M.; Dubois, Ph.; Je´roˆme, R.; Bre´das, J. L.; Lazzaroni, R. Langmuir 1999, 15, 3915. (6) Tamayo, J.; Garcia, R. Appl. Phys. Lett. 1998, 73, 2926. Tamayo, J.; Garcia, R. Appl. Phys. Lett. 1997, 71, 2394. (7) Michel, D. Ph.D. thesis, Universite´ Bordeaux I, 1997, no. 1812. (8) Wang, L. Appl. Phys. Lett. 1998, 71, 2394. (9) Haugstadt, G. D.; Hammerschmidt, J. A.; Gladfelter W. L. In Microstructures and Tribology of Polymer Surfaces; ACS: Boston, 1998. Haugstad, G.; Jones, J. Ultramicroscopy 1999, 76, 79. (10) Anczykowsky, B.; Kru¨ger, D.; Fuchs, H. Phys. Rev. B 1996, 53, 15485. Bar, G.; Thomman, Y.; Brandsch, R.; Cantow, H. J.; Whangbo, M. H. Langmuir 1997, 13, 3807. Brandsch, R.; Bar, G.; Whangbo, M. H. Langmuir 1997, 13, 6349. Cleveland, J. P.; Anczykowski, B.; Schmid, A. E.; Elings, V. B. Appl. Phys. Lett. 1998, 72, 2613. Pickering, J. P.; Vancso, G. J. Polym. Bull. 1998, 40, 549. Anczykowsky, B.; Gotsmann, B.; Fuchs, H.; Cleveland, J. P.; Elings, V. B. Appl. Surf. Sci. 1999, 140, 376. (11) Nony, L.; Boisgard, R.; Aime´, J. P. J. Chem. Phys. 1999, 111, 1615.
10.1021/la0005098 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/30/2000
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Chart 1
approximations providing an analytical solution able to fit the experimental data.11,16 It must be noted that techniques such as the pulsed-force mode could also be used to discriminate between the different contributions, that is, topographic and mechanical, to the image.17 In this paper, we propose a straightforward and easy experimental method to evaluate the contribution of the local mechanical properties to the image contrast. Our approach is based on the reconstruction of height or phase image sections via a rapid analysis of approach-retract curves recorded on the image section lines. We will illustrate the proposed method with height and phase images of symmetric triblock copolymer materials, known as thermoplastic elastomers. The central block of these copolymers consists of a rubbery sequence (i.e., the glass transition temperature (Tg) is lower than room temperature), which is the major component of the copolymer. The outer two blocks are thermoplastics (i.e., Tg is well above room temperature). Because the components are incompatible, the material phase-separates on the microscopic scale, into pure rubbery and glassy domains. Below the Tg of the thermoplastic component, the thermoplastic microdomains are rigid and act as physical cross-links for the elastomeric matrix, such that the material behaves as a vulcanized rubber. Above the Tg of the thermoplastic microdomains, the copolymer flows and the material can be processed by traditional techniques. Thermoplastic elastomers are therefore spontaneously and thermoreversibly cross-linked rubbers. The phase separation leads to microdomains with very different (i.e., glassy versus elastomer) properties. This type of material thus constitutes a model system that may exhibit a regular, periodic array of domains with different local mechanical properties, thus inducing different local oscillator responses. 2. Experimental Methodology 2.1. Sample Preparation. The chemical structure of the triblock copolymers discussed here is shown in Chart 1 (R ) isooctyl). Two identical thermoplastic sequences of poly(methyl methacrylate), PMMA, are attached to a central sequence of low Tg poly(isooctylacrylate). Spherical, cylindrical, and lamellar morphologies have been observed for block copolymers of increasing PMMA content.13 Here we have chosen a highly regular lamellar morphology in order to obtain a model contrast variation for the images, as shown in Figure 1. Thin copolymer films are obtained by casting on freshly cleaved muscovite mica substrates from dilute solutions in toluene (2 mg/mL). The solvent is evaporated very slowly (12) Aime´, J. P.; Michel, D.; Boisgard, R.; Nony, L. Phys. Rev. B 1999, 59, 1829. Kopp-Marsaudon, S.; Nony, L.; Michel, D.; Aime´, J. P. In Microstructure and Microtribology of Polymer Surfaces; ACS Symposium Series 741; Tsukruk, V. V., Wahl, K. J., Eds.; American Chemical Society: Washington, DC, 1999; Chapter 8. (13) Rasmont, A.; Lecle`re, Ph.; Doneux, C.; Lambin, G.; Tong, J. D.; Je´roˆme, R.; Bre´das, J. L.; Lazzaroni, R. Colloids Surf., B: Biointerfaces, in press. (14) “TESP-NCL-W”, Nanosensors, Veeco Instruments SNC, France. (15) Boisgard, R.; Michel, D.; Aime´, J. P. Surf. Sci. 1998, 401, 199. (16) Dubourg, F.; Kopp-Marsaudon, S.; Aime´, J. P. In preparation. Kopp-Marsaudon, S.; Lecle`re, Ph.; Dubourg, F.; Lazzaroni, R.; Aime´, J. P. In preparation. (17) Burnham, N. A.; Kulik, A. J.; Gremaud, G.; Briggs, G. A. D. Phys. Rev. Lett. 1995, 74, 5092.
Figure 1. Tapping-mode height (a) and phase (b) images of a poly(methyl methacrylate)-poly(isooctylacrylate)-poly(methyl methacrylate) triblock copolymer with 28.6 PMMA wt %. The images reveal a regular phase separation between glassy and elastomer domains, with lamellae oriented perpendicular to the surface and a period of about 27 nm.
at room temperature in a solvent-saturated atmosphere. The final film thickness is approximately 500 nm, as determined by profilometry. The films are studied with TMAFM after annealing at 140 °C in high vacuum for 24 h. 2.2. Tapping-Mode Measurements. The experiments are performed in a glovebox (with typical O2 and H2O concentrations as low as a few ppm) with a Nanoscope IIIa from Digital Instruments (Santa Barbara, CA) operating in tapping mode at room temperature. In the present experiments the main interest of the use of a glovebox is to keep experimental conditions stable over a week. A commercial silicon tip-cantilever is used,14 with a stiffness of about 40 N‚m-1, a measured resonance frequency of 180 066 Hz, and a quality factor of 470. Two types of tapping-mode images are simultaneously
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Figure 2. Sketch of an approach-retract curve. (a) Approachretract experiment: the sample is moved toward the tip and then retracted by the piezoactuator movement in the vertical Z direction. (b) Sketches of the approach part of the approachretract curves when performed at a frequency slightly below resonance: amplitude A and phase φ variations versus vertical displacement Z. The analysis of the curves is as follows: at the set point value, the amplitude Ai, the corresponding piezoactuator vertical location Zi is recorded and then the corresponding phase φi.
recorded. Recording the vertical displacements of the piezoactuator necessary to maintain the oscillator amplitude at a fixed amplitude (the “setpoint” root mean square value) gives the height image. The phase images are formed by the oscillator phase values for those sampletip positions. All the sections extracted from the images are recorded “as is” without any filter or image treatment. Height and phase images of the area of interest (typically a few hundred nanometers) are recorded at the 90° scanning direction, at a given setpoint and a fixed frequency. The amplitude far from the surface is Afree ) 55 nm; the working amplitude (the setpoint value) is A ) 49 nm. The driving frequency is chosen such that the phase delay far from the surface is φfree ≈ -42°. The important point here is to test the repulsive regime, which can be distinguished by a phase delay value always above -90°.6-11 The precision we want to achieve here is better than 1 nm. For this reason, we perform approach-retract curves; a series of approach-retract curves are recorded for the same drive amplitude and frequency on a line along the Y axis (usually the image central line) every 5 nm. When recording approach-retract curves, the sample is moved up and down (in the Z direction) at a fixed X,Y location on the surface (see Figure 2). The amplitude and phase are recorded as a function of the vertical displacement of the piezoactuator holding the sample. Typical experimental conditions are 0.5 Hz for the vertical movement frequency and 20 nm for the vertical extension. The experiments are performed at an oscillator frequency slightly below its resonance, as this makes the sample
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surface location for the approach-retract curves easier. Previous modeling of the nonlinear behavior of the oscillating cantilever in an attractive field has shown that, at a frequency slightly below resonance, the amplitude and phase exhibit a bifurcation from one state to another (when attractive interaction becomes dominant). This bifurcation happens very close to the surface (typically at 1 nm or less).11,15 The distance at which the oscillator bifurcates from the free amplitude to a higher one (hereafter called Dbif) is connected to the strength of the attractive interaction. To measure and correct the possible drift of the sample holder, height and phase images are then again recorded at the same sample location and the same amplitude conditions after the series of approach-retract curves has been recorded. 2.3. Data Analysis. For the tapping-mode image reconstruction, the approach-retract curves are recorded and then analyzed as indicated in Figure 2b. The first step is to adjust the Dbif, that is, the distance at which the oscillator bifurcates, at the same vertical position value for all the approach-retract curves. Such an adjustment assumes a similar tip-sample attractive interaction for all domains, as discussed in section 4. The Dbif adjustment allows us to extract selectively the mechanical part that participates to the contrast between the different domains; therefore, it provides a quantitative measurement of the mechanical contribution to the contrast. From the amplitude curve, one then checks the vertical position Zi corresponding to the image fixed amplitude Ai (the setpoint). Then, from the phase curve, one obtains the corresponding phase noted φi. This operation is repeated for all the Y positions: Zi and φi values are thus recorded for all Y positions for which the approach-retract curves are measured. One thus obtains plots of the piezoactuator heights (Zi) and phases (φi) versus Y sample locations, which are then compared with the corresponding sections of the tapping-mode height and phase images. To compare those data from the approach-retract curves with the actual images, sections of the images are performed, taking into account the correction for drift. These height and phase image sections are then compared to the reconstructed ones obtained from the approachretract curves. 3. Experimental Results 3.1. Height and Phase Tapping-Mode Images and Approach-Retract Curves. Figure 3 represents the area chosen for the image reconstruction, with lamellae oriented nearly perpendicular to the scan direction and a clearly-marked contrast between the domains in both height and phase images. Approach-retract curves recorded on the two spots noted in Figure 3b are represented in Figure 4, giving the amplitude (Figure 4a) and phase variations (Figure 4b). The oscillator responses are clearly different for the two areas. The signature of the glassy domain is represented with gray symbols whereas the elastomer domain corresponds to the black symbols. The difference in behavior can be attributed to different interactions between the tip and the elastomer and glassy domains. The harder domain (glassy) response has a larger slope than the softer one.11,12 Without any topographic consideration, the amplitude variations on the two domains show that, for a bifurcation distance set at the same vertical location at a given setpoint, the piezoactuator has to move the sample closer to the tip for the elastomer domain than for the glassy one. Consequently, the net result will be an apparent height higher for the glassy domains. For example, with the experimental conditions chosen here, that is, a working amplitude of 49 nm (the setpoint value) for a free amplitude Af ) 55 nm, the sample holder has to move upward of an additional value of ∆ ) 2.7 nm (see Figure 4a) when intermittent contact situations occur on the elastomer
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Figure 3. Tapping-mode images (500 nm × 500 nm) of the area selected for the profile reconstruction. The fixed image amplitude was 49 nm for a free amplitude of 55 nm with a frequency of 179873 Hz. (a) Height image (the gray scale is 10 nm). The bright areas correspond to the upper parts of the image; the dark areas correspond to the lower parts of the image. (b) Phase image (the gray scale is 5°). The two white arrows indicate the two areas, corresponding to two sample different domains, where the approach-retract curves shown in Figure 4 are recorded. The white line corresponds to the section used for the comparison with the section built from the approachretract curves (see part 3.2). domain, compared to the intermittent contact situation on the glassy domain. Due to the dominant repulsive regime, the recorded difference is uniquely related to change of the slopes of the curves and thus in turn to changes of the local mechanical properties. This change partly corresponds to the change in the indentation depth, as sketched in Figure 4c. The different piezoactuator heights on the glassy and elastomer domains also imply differences in the phase values of about 1.5°, as shown in Figure 4b, with a larger phase shift for the elastomer domain. All the approach-retract curves are analyzed following the same procedure, giving a section plot of the piezoactuator heights and oscillator phases. 3.2. Comparison between Reconstructed Sections and Image Sections. The next step is to compare the height and phase image sections with sections built from the approachretract curve data. As mentioned in the methodological section, piezoactuator drift may affect the reconstructed section. In our case, the images taken before and after the approach-retract
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Figure 4. Experimental approach curves recorded on the two spots shown in Figure 3b. The gray results (corresponding to the white areas in Figure 3a or b) correspond to the harder, glassy domain whereas the black ones (black domains of Figure 3a or b) correspond to the softer, elastomer domain. (a) Amplitude A variation with the piezoactuator vertical Z position during the approach. (b) Phase φ variation with the piezoactuator vertical Z position during the approach. Both amplitude and phase variations are different for the two domains due to different local interactions with the tip resulting in a mechanical contrast. The fixed image amplitude of 49 nm corresponds to a vertical piezoactuator movement between the two domains of ∆Zi ) Zg - Ze ≈ 2.4 nm, which implies a phase difference of about ∆φ ) φg - φe ≈ 1.5° for those two particular X,Y locations on the sample. (c) Illustration of the indentation depth difference. curve recording do show a piezoactuator drift of 55 nm in the X direction (toward the bottom of the page) and of 16 nm in the Y horizontal direction (to the right), as shown in Figure 3b. For the reconstructed sections, a continuous drift with time along this particular direction is considered. Different close sections are chosen on the image from the starting point of the approachretract experiments toward the direction of the drift, by considering also the imposed ceramic displacement. They are then compared to the section reconstructed from the approachretract curve analysis; the one with the best correspondence is chosen, as represented by the line in Figure 3b.
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Figure 5. Comparison of the image sections with the profiles built from the approach-retract curve data. (a) Comparison between the height data. The two black arrows correspond to the data extracted from the curves of Figure 4. (b) Comparison between the phase data. The approach-retract curves have been recorded along the line shown in Figure 3b. The correspondence between the two sets of data is very good, indicating a major mechanical contribution to the height and phase images. The resulting image sections are displayed in Figure 5a (height) and b (phase), together with the data from the approach-retract curve analysis. The correspondence between the two sets of data (tapping height or phase images and approach-retract analysis) appears to be very good. This agreement means that, for those copolymers, the contrast in the height image is related to different oscillator responses on the glassy and elastomer domains, with no discernible topographic contribution to the contrast. Therefore, the contrast is mostly due to changes in the sample local mechanical properties.
4. Discussion Four assumptions are made to extract the mechanical part of the image contrast from the analysis of the approach-retract curves: (i) First of all, one has to verify that the experimental conditions are similar for approach-retract curves and images. In both experimental situations, the tipcantilever system must be in its equilibrium state at each step, with only a weak influence of the transient regime. For a resonance frequency of 1.8 × 105 Hz and a quality factor Q ) 470, the characteristic relaxation time of the oscillator is about 1 ms. Typical images were recorded at 0.5 Hz frequency per line for a scan size between 500 and 300 nm with 512 data points. Thus, the oscillator spends 1 ms for each step, corresponding to a spatial length of 1-2 nm. In addition, the lateral displacement from one step to the next one represents only a slight perturbation to the oscillator, and the oscillator spends more than 10 ms on a given domain. Therefore, the adiabatic requirement is fulfilled when recording the image. Concerning the approach-retract curves, it was shown previously12 that the characteristic time is given by the period, which is about 1 µs. Therefore, except at the bifurcation spot,
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which is a nonadiabatic transition, the data points of the approach-retract curves correspond to stationary states of the oscillator. (ii) The second point is to ensure that, for the approachretract curves, the additional vertical velocity of the sample holder does not alter significantly the dissipation process. Modeling the dissipation in intermittent contact situations leads to an additional force of the form βsx˘ ,16 where βs is the sample friction coefficient. In our experimental situations, an intermittent contact situation happens for a residence time τres which is 10% of the oscillation period. The average additional dissipation per cycle is proportional to the residence time and the average velocity of the tip when in contact with the sample, which scales as x˘ ≈ Aω02πτres/T. Thus, with A ) 50 nm, one gets an average velocity of 10-2 m/s, which has to be compared to the additional velocity z˘ ) νZscan ) 2 × 10-8 m/s and thus z˘ , x˘ . Therefore, the additional contribution of the vertical displacement is negligible, so that the dissipation processes are nearly identical for the two measurements. (iii) The surface tensions of the two polymers are assumed to be close enough such that the distances at which the oscillator exhibits the amplitude jump are nearly the same whatever the domain investigated. In fact, the semiempirical estimation of the corresponding surface energy gives a slightly higher value for PMMA (42.7 mJ/ cm2) compared to that for poly(isooctylacrylate) (34.3 mJ/ cm2).18 For the chemical species present in our system, this implies that their Hamaker constants (assuming that the tip-sample interaction is given by the van der Waals dispersive term) have similar values. Using the equations given in refs 11 and 15, it can be shown that a small change in the strength of the attractive interaction, that is, an increase (decrease) of 20-30%, does not change significantly the tip-sample distance at which the bifurcation occurs. Therefore, setting the bifurcation distance at the same location is justified, and it allows us to properly determine the extent of the pure mechanical contribution. In addition, the amplitudes chosen in the present work are large enough so that the attractive regime only has a minor contribution. (iv) The effect of the growth of a nanoprotuberance12 is made negligible by choosing a suitable magnitude of the amplitude that minimizes the attractive interaction between the tip and the sample. The last two points define the experimental conditions for which a dominant repulsive regime is obtained. The direct comparison between the data of the two modes gives a simple route to understand the origin of the contrast in the image. Further development requires a theoretical description of the approach-retract curves, as those curves contain all the physics of the interaction between the sample and the tip. Preliminary developments extending the analytical model given in ref 11 indicate that the mechanical contrast is related to the sample local stiffness (and therefore its Young’s modulus) and dissipation processes due to the local viscosity.16 In ref 11 a criterion is proposed to determine the lowest value of the Young’s modulus for which the slope, that gives the rate of variation of the oscillation amplitude versus the oscillator-sample distance, is equal to one. The criterion is
QGφ . 10kc w slope )
∂A ≈1 ∂Z
(18) Van Krevelen, D. W. Properties of Polymers, 3rd ed.; Elsevier: Amsterdam, 1990; p 790.
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Finally, we also performed a few noncontact AFM resonant experiments on the same sample. This technique is known to be sensitive only to the attractive interaction and the actual topography. We have observed no topographic contribution to those images, which confirms the results obtained from the approach-retract curve analysis.
Figure 6. Comparison of approach curves recorded on the same sample with two tips: a large one and the small one used previously. The slope of the amplitude variation with Z is approximately one for the large tip and smaller than one for the small tip.
where Q, G, φ, and kc are the quality factor, the Young’s modulus of the sample, the diameter of the contact area between the tip and the sample and the cantilever stiffness, respectively. The slope is the variation of the amplitude A as a function of the vertical displacement Z of the sample holder. A value of the slope equal to one means that the oscillator feels the surface as a hard one. As a consequence, following the methodology given in this paper, the analysis of approach-retract curves with a slope equal to one whatever the domain investigated would give a perfectly flat section. With the parameters Q ) 470 and kc ) 40 N‚m-1, a small tip giving a contact area of diameter φ ) 1 nm gives a slope of one for Young’s moduli larger than 1 GPa. This would allow us to easily distinguish between the two types of domains studied here. The same limit falls to 10 MPa for a large tip with a contact area of diameter 100 nm. In that case, both types of domain would appear as hard and the height contrast would vanish. A consequence is that the same copolymer, studied with a larger tip, may exhibit no height mechanical contrast at all. Experimentally, such a situation was encountered with another cantilever-tip system. The dominant attractive regime was observed up to a value of the amplitude of 50 nm, indicating a large size of the tip apex. Consequently, the repulsive regime was obtained only for much larger amplitudes, say 80 nm. The approach-retract curves performed on the sample give a slope of one whatever the X,Y location (Figure 6), and accordingly, a flat surface was observed in the height image while minor variations were observed on the phase image. For samples with a low modulus, the method proposed here is useful to establish whether there is a major topographic contribution to the height image: if the reconstruction of the section with the approach-retract curves is not satisfactory, the height discrepancy corresponds to the actual topography.
5. Conclusion In this paper, we present an experimental method to quantitatively record the mechanical and the topographic contributions producing tapping-mode images. The method associates image sections with analysis of multiple approach-retract curves recorded on the same section. The experimental procedure is particularly appropriate for samples that exhibit simple heterogeneous local mechanical responses. In particular, blends or copolymers with softer and harder components are suitable systems. A block copolymer exhibiting a highly regular lamellar nanophase separation between glassy and elastomer domains was used as a model example. The method applied to that system indicates a major contribution of the mechanical properties to the contrast of the image. It is therefore possible to evaluate experimentally the mechanical and topographic contributions to the tappingmode image contrast. To achieve such a goal, a number of experimental conditions are required. The tip must be small enough; typically the dominant attractive regime must be achieved for an amplitude as small as 15 nm. The oscillator must adiabatically scan the sample. Growth of nanoprotuberances has to be prevented, and the change in the strength of the attractive interaction between the two domains must be minimized. The last two points can be easily achieved by choosing a suitable oscillation amplitude such that the oscillator works in the dominant repulsive regime. Nevertheless, while this simple procedure is helpful in describing the physical origin of the contrast, it does not bring information about the relative influence of the reactive and dissipative behavior of the sample. To get an accurate description of the mechanical properties requires further developments. In particular, one has to address the difficult question of the modeling of the local dissipation processes during the intermittent contact situation and their influence on the oscillation behavior. Such an analysis is currently under investigation. Acknowledgment. The authors wish to express their thanks to the CERM (University of Lie`ge, Belgium) for the copolymer synthesis. Research in Bordeaux is supported by the Conseil Re´gional d’Aquitaine. Research in Mons is partially supported by the Belgian Federal Government Office of Science Policy (SSTC) “Poˆ le d’Attraction Interuniversitaire en Chimie Supramole´ culaire et Catalyze Supramole´ culaire” (PAI 4/11), the European Commission and the Government of the Re´gion Wallonne (Project NOMAPOL-Objectif 1-Hainaut), and the Belgian National Fund for Scientific Research FNRS/ FRFC. R.L. is Maıˆtre de Recherches du Fonds National de la Recherche Scientifique (FNRSsBelgium). LA0005098
