Langmuir 2008, 24, 14183-14187
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Nanoscale Composition Mapping of Segregation in Micelles with Tapping-Mode Atomic Force Microscopy Taner Aytun, Omer Faruk Mutaf, Osman J. el-Atwani,† and Cleva W. Ow-Yang* Materials Science and Engineering Program, Faculty of Engineering & Natural Sciences, Sabanci UniVersity, Orhanli, Tuzla-Istanbul, 34956, Turkey ReceiVed July 24, 2008. ReVised Manuscript ReceiVed September 24, 2008 Under energy-dissipative cantilevered tip-sample interaction, phase imaging using tapping-mode atomic force microscopy enables compositional mapping of composites containing a harder inorganic phase at the nanometer scale, embedded in a polymer matrix. The contrast in the phase images is shown to be dependent on the variation in the elastic properties of the diblock copolymer reverse micelles loaded with zinc acetate. Tapping conditions are also shown to determine whether the contrast is positive or negative for the harder core of the loaded micelles, based on the competition between attractive and repulsive tip-sample interaction forces. The broader implications are significant for scanning probe microscopy of other soft materials systems containing the segregation of a harder phase.
Introduction The ability to map the composition of composite systems containing nanoscale-sized phase domains embedded in soft materials would serve a compellingly broad range of polymeric and biological applications. Polymeric micelles used as nanoscale reactor vessels for nanoparticle synthesis contain micelle cores loaded with ionic reactants,1,2 while the accumulation of calcium in vascular cells lead to the formation of calcifying deposits on arterial walls.3 In such composite systems, the stiffer material is embedded in a soft matrix, and characterizing the location would enable monitoring the system dynamics. By the use of tapping-mode atomic force microscopy (TMAFM), destructive lateral forces of the vertically oscillating tip are reduced while still enabling the fine nanoscale resolution afforded by the intermittent contact with the sample surface. With a vibrating probe, more degrees of freedom are afforded for studying the probe tip interaction with the sample, via the amplitude and phase angle.4 By adjusting the strength of tapping, the tip-sample interaction could be modulated between strong and weak to obtain different levels of information from the sample surface properties. To date, TM-AFM has been used for the mapping of different phases in polymer blends5-10 and self-assembled monolayers11 at nanometer length scales. Magonov and co-workers12,13 used * To whom correspondence should be addressed. E-mail: cleva@ sabanciuniv.edu. † Current address: College of Engineering, Purdue University, West Lafayette, Indiana, 47905.
(1) El-Atwani, O. M.Sc. Thesis, Sabanci University, 2008. (2) Foerster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195–217. (3) Bobryshev, Y. V.; Lord, R. S. A. Tissue Cell 1998, 30, 383–388. (4) Garcia, R.; Gomez, C. J.; Martinez, N. F.; Patil, S.; Dietz, C.; Magerle, R. Phys. ReV. Lett. 2006, 97, 016103-016106. (5) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H.-J.; Whangbo, M.-H. Langmuir 1997, 13, 3807–3812. (6) Bar, G.; Thomann, Y.; Whangbo, M.-H. Langmuir 1998, 14, 1219–1226. (7) Thomann, Y.; Cantow, H.-J.; Bar, G.; Whangbo, M.-H. Appl. Phys. A: Mater. Sci. Process. 1998, 66. (8) Raghavan, D.; Gu, X.; Nugyen, T.; van Landingham, M.; Karim, A. Macromolecules 2000, 33, 2573–2583. (9) Kietzke, T.; Neher, D.; Landfester, K.; Montenegro, R.; Guenter, R.; Scherf, U. Nat. Mater. 2003, 2, 408–414. (10) Bar, G.; Brandsch, R.; Whangbo, M.-H. Langmuir 1998, 14, 7343–7347. (11) Brandsch, R.; Bar, G.; Whangbo, M.-H. Langmuir 1997, 13, 6349–6353. (12) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. Lett. 1997, 375, L385-L391. (13) Magonov, S. N.; Elings, V.; Papkov, V. Polymer 1997, 38, 297–307.
phase imaging under moderate tapping force conditions to reveal a strong correlation between local mechanical stiffness and shift in phase angle in microlayered polyethylene, which consisted of alternating high and low density polyethylene layers, as well as in poly(diethylsiloxane) (PDES) consisting of a mixture of amorphous and mesomorphic phases. In both types of samples, all phases were exposed at the surface and therefore directly imaged. Another investigation directly at the surface of microscale phase separated PPO/PES blends was performed by Bar et al. in which the tip oscillation amplitude and tapping strength were systematically varied to elucidate the relative contributions of attractive and repulsive forces on tip-sample interactions.5 To characterize film morphology, irrespective of film roughness, McLean and Sauer demonstrated that the comparison of height and phase imaging, under large drive amplitude and moderate tapping conditions, could be used to distinguish between phases of different crystallinity, in addition to amorphous phases of different local softness.14 Phase contrast variation with hydrophilicity was also investigated on patterned self-assembled monolayers on polycrystalline gold.11 In this study, the phase contrast was merely used to deduce the tip-sample interaction resulting from the applied tapping parameters, while the height image contrast provided the morphological information. In general, when the oscillating tip interacts with sample, the energy dissipated in the interaction would engender a phase angle shift that would vary between regions of different elastic properties as well as hydrophilicity and adhesion.4,15,16 Clearly, the characteristics of the oscillating tip being used for probing are just as important as the sample properties in the images derived.5 Therefore, the contrast in the topographical and the phase images strongly depends on both physical properties of material and the AFM probing parameters, and analysis of these phase images under different tapping conditions could elucidate composition heterogeneity. A careful selection of the probing conditions would then enable interpretation of the underlying material properties. A model system for demonstrating the power of phase contrast imaging by TM-AFM is a monolayer of reverse diblock copolymer micelles loaded with an inorganic precursor. The use (14) McLean, R. S.; Sauer, B. B. Macromolecules 1997, 30, 8314–8317. (15) Garcia, R.; Magerle, R.; Perez, R. Nat. Mater. 2007, 6, 405–411. (16) Tamayo, J.; Garcia, R. Appl. Phys. Lett. 1997, 71, 2394–2396.
10.1021/la802384x CCC: $40.75 2008 American Chemical Society Published on Web 11/21/2008
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of soft materials to template the synthesis of metal and semiconductor nanoparticles has become a subject of intense research activity in recent years.2 Because these systems contain domains with a spatial variation in chemical, structural, and mechanical properties, they are well-suited for characterization by AFM, particularly through investigating the phase angle variation in TM. Previous work using AFM to investigate micellar systems involved using topographical, phase, and amplitude images to investigate geometrical factors, contours that could not be observed by transmission electron microscopy.17 Topographical and amplitude imaging were leveraged to reveal the fusion of micelles, through their increase in diameter and shape change,18 the 2-D arrangement of micelles in monolayer films,17 and the periodicity in the 2-D arrays of metal nanoparticles that had formed in the cores of such micelles.19,20 It should be noted that images of the metal nanoparticle arrays were obtained after plasma etch removal of the polymeric micelles. When diblock copolymer micelles are used as nanoscale reactor vessels1,21 for the synthesis of inorganic nanoparticles, characterization of the micelles at each step aids in developing a controlled synthesis reaction inside the micelles. In this study, we are presenting the interpretation of the contrast in TM-AFM phase images of polystyrene-b-poly2vinlpyridine (PS-b-P2VP) reverse micelles, loaded with zinc acetate dihydrate (ZnAc) in the micelle core. The introduction of this cation reactant source would be the first step in inducing a controlled precipitation of ZnO nanoparticles in the reverse micelles. Thermodynamics of the metal salt-loaded micelles in toluene suggests that the ionic Zn complexes would segregate to the micelle core and coordinate with the pyridine units (on the P2VP blocks) in the micelle core. The variation in elastic properties across the entire micelle structure would induce variations in the phase angle during TMAFM analysis.4,22 To enable characterization of the elastic property variation in such a PS-b-P2VP reverse micelle system, it is important to know the elastic and viscoelastic response to the vibrating probe. A study of PS-b-P4VP diblock copolymer reverse micelles had revealed that the mechanical properties were determined by the unimer structure, i.e., the relative symmetry and molecular weight of the constituent blocks.23 For micelles composed of PS627-bP2VP721 unimer aggregates, the relative stability of these spherical micelles when packed in a monolayer had been established.1,17 In particular, the PS block of 627 PS units (75,000 g/mol) would determine the mechanical properties of the micelles. For PS blocks of molecular weight >40400 g/mol, the surface is in the glassy state at 293 K, and the surface dynamic storage modulus and surface loss tangent are effectively the same as that of bulk PS (Tg ≈ 100 °C).24
Experimental Details Reverse micelles were synthesized and loaded as described in previous work.1 Briefly, a relatively symmetric diblock copolymer, (17) Spatz, J. P.; Roescher, A.; Sheiko, S.; Krausch, G.; Moeller, M. AdV. Mater. 1995, 7, 731–735. (18) Spatz, J. P.; Moessmer, S.; Moeller, M. Chem.-Eur. J. 1996, 2, 1552– 1555. (19) Spatz, J. P.; Moessmer, S.; Hartmann, C.; Moeller, M. Langmuir 2000, 16, 407–415. (20) Kaestle, G.; Boyen, H.-G.; Weigl, F.; Lengl, G.; Herzog, T.; Ziemann, P.; Riethmueller, S.; Mayer, O.; Hartmann, C.; Spatz, J. P.; Moeller, M.; Ozawa, M.; Barnhardt, F.; Garnier, M. G.; Oelhafen, P. AdV. Funct. Mater. 2003, 13, 853–861. (21) Moeller, M.; Kuenstle, H.; Kunz, M. Synth. Met. 1991, 41-43, 1159– 1162. (22) Martinez, N. F.; Garcia, R. Nanotechnology 2006, 17, S167-S172. (23) Buitenhuis, J.; Foerster, S. J. Chem. Phys. 1997, 107, 262–272. (24) Tanaka, K.; Taura, A.; Ge, S. R.; Takahara, A.; Kajiyama, T. Macromolecules 1996, 29, 3040–3042.
Aytun et al. Scheme 1. Cross-Section Schematic View of the Zn-Loaded Reverse Diblock Copolymer Micelle
composed of a PS block (721 styrene units) and a P2VP block (627 vinyl pyridine units) (Polymer Source, Inc.), was dissolved in toluene (w/v ) 0.5%) for 10 h under vigorous stirring at room temperature. Because toluene is a highly selective solvent for the PS blocks only, phase separation would produce aggregates of the unimers assembled into a core of collapsed P2VP blocks forming a globule, while the PS blocks were stretched out into the solvent.25 The micelles that had assembled were subsequently loaded with zinc acetate, using a loading ratio of 0.5 (zinc per pyridine unit). FTIR (Bruker, Saint Paul, MN, USA) spectra revealed a broadening of the characteristic vibration resonance of pyridine at 1590 to 1600-1620 cm-1, verifying that the zinc did indeed form the expected complex with pyridine in the micelle core. To estimate the hydrodynamic diameter of the micelles before and after loading, dynamic light scattering (DLS) measurements were performed with a Zetasizer NanoZS (Malvern Instruments, Malvern, UK). The detector was fixed at the scattering angle of 173° (the Noninvasive Backscattered optical detection technology maximizes the sample volume analyzed, enabling study of a broader range of particle sizes and concentrations), and a He-Ne laser was used as a light source with λ ) 632 nm. Before characterization, the solution was centrifuged for 30 min at 5000 rpm to separate the unincorporated ZnAc, and samples for analysis were prepared from the optically clear solution in midsection of the tube. Upon loading with 0.3 Zn/VP units, the average hydrodynamic diameter of the micelles had increased from approximately 63 to 78 nm. For TM-AFM analysis, close-packed monolayers of PS627-bP2VP721 diblock copolymer reverse micelles loaded with ZnAc in toluene were prepared by dip-coating and were then characterized as shown in Scheme 1, after allowing the solvent to evaporate. By use of a KSV Sigma 700 Tensiometer (KSV Instruments Ltd.), monolayers of micelles were prepared by dipping a freshly cleaved mica substrate into the micellar solution, with a velocity of 40 mm/ min and pulled from the solution under a constant velocity of 15 mm/min. Drying was conducted in a closed box to maintain cleanliness. The monolayer sample preparation conditions were optimized in a separate study, in which the stability of the micelle structures were found to be consistent with the DLS measurements and prior work in the literature.1,17 AFM characterization was performed using a Nanoscope III Atomic Force Microscope (Digital Instruments, Santa Barbara, CA USA) using the tapping mode with single-beam silicon cantilevers (spring constant ) 23 N/m, drive frequency ) 255 kHz) to investigate the morphological changes due to loading of the micelles at room temperature and a relative humidity of 30%. Other scan parameters used in all images presented in this work were a scan rate of 2.0, an integral gain of 1.26, and a proportional gain of 5.18. Tapping probe conditions were varied between soft and hard tapping to optimize image contrast through the drive amplitude, A0, and set point ratio, rsp, values. AFM images were processed using the WSxM software package.26 (25) Foerster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys. 1996, 104, 9956–9968. (26) Horcas, I.; Fernandez, R.; Gomez-Roriguez, J. M. C., J.; Gomez-Herrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 78, 013705.
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Figure 1. AFM height (a) and phase (b) images of zinc acetate loaded PS-b-P2VP diblock copolymer micelles with A0 ) 40.5 nm, rsp ) 0.2. The image in (c) is a contour plot representation of the phase image, which facilitates visualization of the phase contrast of the micelle features.
Discussion
Figure 2. The plot of phase shift vs setpoint amplitude ratio rsp on zinc acetate loaded PS-b-P2VP diblock copolymer micelles with A0 ) 40.5 nm.
Results The topographical (topo) and phase images shown in Figure 1 were generated using A0 of 40.5 nm and rsp of 0.2, i.e., when probed under large drive amplitude and hard tapping conditions. The spherical shape of the micelles could be discerned in the topo image, while a higher contrast could consistently be observed in the phase image (part b) corresponding to the center of each spherical micelle. In fact, this is particularly evident when the phase image is viewed in the contour plot representation (part c). To understand the relationship between phase image contrast and tapping strength, i.e., the rsp values, the phase shift was plotted as a function of rsp for tapping above the core of the micelle and compared with tapping above the corona-only part of micelles, using a drive amplitude of A0 ) 40.5 nm (see Figure 2). When a moderate tapping strength (rsp ) 0.4) was used, the resulting images were as shown in Figure 3. For comparison, a monolayer of empty, PS-b-P2VP reverse micelles was also characterized using a large drive amplitude and hard tapping strength (A0 ) 40.5 nm, rsp ) 0.2). The relatively flat contrast phase image is shown in Figure 4b along with the corresponding topo image in Figure 4a. Next, to investigate the dependence of phase contrast on drive amplitude, the variation of phase shift with rsp was compared for a larger and a smaller drive amplitude, A0 ) 40.5 and 20.25 nm (Figures 2 and 5, respectively). An inversion in the contrast was observed when a small drive amplitude was used in the moderate tapping strength regime. The phase image formed under these tapping conditions did indeed show darker contrast, when tapping above the core, in addition to a correspondingly brighter contrast above the corona-only region (Figure 6).
The topographical images of ZnAc-loaded PS-b-P2VP diblock copolymer micelles did not provide distinguishable information about the micelle core. However, changes in the phase angle were induced by the presence of inorganic material in the core and led to heightened contrast at the core. By variation of the parameters, tip oscillation, or “drive”, amplitude (A0), the set point amplitude (Asp), and their ratio Asp:A0, which defines the set point ratio, rsp, the contrast between the core and corona parts of micelle was enhanced or reversed, as a consequence of spatially correlated variation in tip-sample interaction. The topo and phase images shown Figure 1 were formed using A0 ) 40.5 nm and rsp ) 0.2. When rsp ) 0.2, at hard tapping, the tip-sample interaction is higher, because the tip is forced to vibrate at a lower amplitude than the free oscillation amplitude. In the phase image, bright spots could be seen at the center of virtually all micelles, while the corona region is consistently darker. The higher brightness indicates a larger phase-angle shift during tapping. Although the tip was interacting with PS across the entire diameter of each micelle, the change in the phase angle had resulted from PS with an underlying accumulation of the stiffer inorganic matter. The dependence of phase angle shift on tip-sample interaction forces could be described as in eq 112
k π Qσ ∆φ0 ) φf - φi ) tan-1 ≈ 2 Qσ k
( )
(1)
Here, ∆φ0 represents phase angle shift between the freely oscillating tip, φf, and interacting tip, φi, k the spring constant, Q the quality factor, and σ the sum of all the force derivatives for all forces Fi acting on a cantilever, as shown in eq 2
σ)
∑ ∂Fi ⁄ ∂Z
(2)
i
The approximation could be made in eq 1 when σ is very small compared to k. The resulting equation suggests the following for the relationship between σ and ∆φ0: if the overall acting force on the tip is repulsive, the phase shift would be positiVe, whereas if the force is attractive, the phase shift would be negatiVe. Under hard tapping conditions, the tip-sample interaction is dominated by repulsive forces, and therefore the shift in phase angle should be positive, and reflected by brightness scaling with sample rigidity in the phase image. Both types of forces could be present during the tip-sample interaction. The dominant one depends primarily on 2 tip-vibration parameterssA0 and the relative strength of tip impact on the sample, which is given by rspsand on the material stiffness.
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Figure 3. AFM height (a) and phase (b) images of zinc acetate loaded PS-b-P2VP diblock copolymer micelles with A0 ) 40.5 nm and rsp ) 0.4. The image in (c) is a contour plot representation of the phase image, which facilitates visualization of the phase contrast of the micelle features.
Figure 4. AFM topo (a) and phase (b) images of “empty” PS-b-P2VP diblock copolymer micelles with A0 ) 40.5 nm and rsp ) 0.2. (c) The contour plot representation of the phase image in part b facilitates visualizing the compositional variation in the micelle monolayers.
Figure 5. The plot of phase shift vs set point amplitude ratio rsp on zinc acetate loaded PS-b-P2VP diblock copolymer micelles with A0 ) 20.25 nm.
Because the tip is intermittently in contact with the sample, rather than in permanent contact, in tapping mode, time-averaged values influencing the stiffness of the sample could dominate the overall force derivatives, σ, which would be the tip-sample contact radius, a, and the effective modulus of the sample, E*, related as shown in eq 312
σ ≈ 〈S〉 ) 〈a〉E* Q Q ∆φ0 ≈ 〈S〉 ) ε〈a〉E * k k
()
()
(3) (4)
Here, S is the stiffness and a number between 1.9 and 2.4. The brackets indicate time-averaged values. Equation 4 is particularly important for interpreting the contrast in phase images. For example in Figure 1, the core part of the micelles appeared brighter, because the phase shift is positive,
generated by the stiffer inorganic-containing core under the diblock copolymer micelle corona. However, under low driving amplitudes, the cantilever is positioned close to the sample, and attractive adhesion forces dominate the tip-sample interaction under light tapping. The time-averaged contact area (via 〈a〉) increases with compliancy. A large phase shift would be induced when tapping softer materials, compared to harder ones. However, hard tapping engenders repulsive indentation forces, which scale with the effective elastic modulus. Thus, increasing the tapping strength induces repulsive forces that would mitigate the attractive ones. In brief, the phase shift, as the tapping strength is swept from harder (i.e., smaller rsp values) to lighter, follows the trend of a force-distance curve. Magonov et al. had proposed that probing a softer material would lead to a larger time-averaged tip-sample contact area, which would be highly sensitive to changes in the elastic properties under large A0 and small rsp conditions or all small A0 ones. This would lead to a greater phase shift from probing a softer material, rather than from a harder one. In the diblock copolymer micelles, the material underneath the PS-P2VP micelle corona that contained the Zn complex might not introduce changes in the energy dissipation of the tip interacting with the polymeric corona. However, the higher effective elastic modulus from the core underneath the corona, due to the increased local stiffness stemming from the segregated inorganic species, contributes to increased phase shift, and therefore higher brightness in the phase image.10 Garcia and co-workers had demonstrated that phase shifts could be related to elastic modulus variations, only if the kinetic energy of oscillating cantilever was dissipated by tip-sample interaction forces, i.e., under large drive amplitude
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Figure 6. AFM topo (a) and phase (b) images of zinc acetate loaded PS-b-P2VP diblock copolymer micelles with A0 ) 20.25 nm and rsp ) 0.6. (c) The contour plot representation of the phase image in part b facilitates visualizing the compositional variation in the micelle monolayers.
and moderate tapping strength conditions.27 The exact nature of all the energy dissipating mechanisms is difficult to identify, often being a complex mixture of some or all of the following: adhesion forces, such as capillary forces and hydrophilicity, viscoelasticity, etc.4 However, in this particular system being analyzed, any adhesion forces present above the corona-only region would be present above the core region as well, since the PS blocks form the surface of the micelles. By use of energydissipative tapping conditions, the dominant difference in the tip-sample interaction would stem from the elastic properties of the micelle core, hence the bright cores in Figure 1. In contrast, the empty micelles in Figure 4 lack the increased stiffness in the micelle core, and thus did not show a change in contrast above the core. Under large drive amplitude and moderate tapping conditions, the phase shift was positive above the core, as well as the coronaonly regions (see Figure 2). The image contrast in this operating regime stemmed from the larger phase shift over the stiffer micelle core. The contrast of the image was a maximum at rsp ) 0.4 (Figure 3). When rsp exceeds 0.4, the phase shift above the coronaonly region was almost always negative. The negative phase shift indicated the dominance of the attractive forces between the tip and the corona-only part of the micelle. However, above the core, the phase shift remained positive, as the rsp was changed, suggesting that the repulsive forces were more dominant when tapping above the core. The large drive amplitude was necessary to generate such a level of contrast, in order to produce the repulsive indentation force that dominated the cantilever behavior. The corresponding phase image of empty, unloaded reverse diblock copolymer micelles (Figure 4) revealed a minimal variation in contrast brightness, because no phase shift was detected above the core and corona of the micelle. To understand the dependence of phase contrast for different values of A0, the variation of phase shift with rsp is compared in Figures 2 and 5 for a larger and a smaller drive amplitude, A0 ) 40.5 and 20.25 nm, respectively. Under both hard- and lighttapping regimes, the main difference was an inversion in the contrast, when using the smaller drive amplitude and moderate tapping strength. As can be seen in Figure 6, during imaging of the sample under low drive amplitude and moderate tapping conditions, the core + corona region of the micelle appeared darker due to a negative phase shift, while the corona-only region appeared brighter due to a positive phase shift. This contrast reversal suggested that under low drive amplitude and moderate strength of tapping, less energy was dissipated in the tip interaction with the core-containing region, relative to tapping with a large drive amplitude and a moderate strength. In such a case, the local (27) Garcia, R.; Tamayo, J.; San Paulo, A. Surf. Interface Anal. 1999, 27, 312–316.
stiffness, represented by E* in eq 4 would have weaker impact on the phase shift. Moreover, under relatively low drive amplitudes, such as at A0 ) 20.25 nm, attractive forces dominated the tip-sample interaction, effectively “holding” the tip close to the surface. The much lower indentation force engendered only weak elastic interaction with the Zn-containing core. Now the phase shift had become more strongly dependent on the second variable in eq 4, i.e., the time-averaged contact area, which correlates to attractive forces. Consequently, the longer tip contact time in the more compliant corona produced a larger phase shift, in contrast to the smaller phase shift above the micelle core. The contrast in the phase image now appeared inverted from that in Figure 1, i.e., with darker core regions, because ∆φ was determined predominantly by 〈a〉, and weakly from the relative compliancy, E*, at best. Therefore, under hard-tapping conditions (i.e., low rsp), a tip oscillating with low amplitude would have spent more average time in contact with the compliant overlying corona material over all parts of the micelles. Whereas when the strength was decreased (i.e., rsp increased in value) from hard to moderate, the tip would have more time during an oscillation cycle to disengage from the micelle, the indentation forces would increase their opposing contribution to the summation of force derivatives. These repulsive forces, which are more sensitive to changes in the local stiffness, would serve as a counterweight to the attractive forces that are present throughout the low drive amplitude conditions. The more compliant regions would perturb the tip-sample interactions more under moderate tapping, thus giving rise to the negative phase shift when tapping above the core in the moderate rsp regime in Figure 5. A similar reversal in contrast was also reported in the work of Bar et al.5 and Magonov et al.13
Conclusions By selecting tapping conditions to achieve inelastic tip-sample interactions, i.e., with a large drive amplitude and moderate tapping strength, the compositional variation in a nanocomposite could be mapped in the phase image, due to the corresponding variation in elastic properties. This was demonstrated in the phase images of close-packed monolayers of PS-b-P2VP diblock copolymer reverse micelles containing zinc acetate dihydrate that had segregated into the micelle core. The interpretation of the contrast in the phase images has powerful implications, not only for the characterization of micelles used as nanoreactor vessels but also for locating a stiffer phase embedded in biological matter, such as inorganic nanoparticles or calcified phases. Acknowledgment. The authors acknowledge funding for this project from the TUBITAK Basic Sciences Division, Project No. 106T657. O.E. acknowledges the Yousef Jameel Foundation for financial support. LA802384X
