Nanoscale Elastic Modulus Variation in Loaded Polymeric Micelle

Article pubs.acs.org/Langmuir

Nanoscale Elastic Modulus Variation in Loaded Polymeric Micelle Reactors Alim Solmaz,†,‡ Taner Aytun,†,§ Julia K. Deuschle,∥ and Cleva W. Ow-Yang*,† †

Materials Science and Engineering Program, Sabanci University, Orhanli, Tuzla, 34956 Istanbul, Turkey Institute for Materials Science, University of Stuttgart, Heisenbergstraße 3, 70569 Stuttgart, Germany

∥

S Supporting Information *

ABSTRACT: Tapping mode atomic force microscopy (TM-AFM) enables mapping of chemical composition at the nanoscale by taking advantage of the variation in phase angle shift arising from an embedded second phase. We demonstrate that phase contrast can be attributed to the variation in elastic modulus during the imaging of zinc acetate (ZnAc)-loaded reverse polystyrene-block-poly(2-vinylpyridine) (PS-bP2VP) diblock co-polymer micelles less than 100 nm in diameter. Three sample configurations were characterized: (i) a 31.6 μm thick polystyrene (PS) support film for eliminating the substrate contribution, (ii) an unfilled PS-b-P2VP micelle supported by the same PS film, and (iii) a ZnAc-loaded PS-b-P2VP micelle supported by the same PS film. Force−indentation (F−I) curves were measured over unloaded micelles on the PS film and over loaded micelles on the PS film, using standard tapping mode probes of three different spring constants, the same cantilevers used for imaging of the samples before and after loading. For calibration of the tip geometry, nanoindentation was performed on the bare PS film. The resulting elastic modulus values extracted by applying the Hertz model were 8.26 ± 3.43 GPa over the loaded micelles and 4.17 ± 1.65 GPa over the unloaded micelles, confirming that phase contrast images of a monolayer of loaded micelles represent maps of the nanoscale chemical and mechanical variation. By calibrating the tip geometry indirectly using a known soft material, we are able to use the same standard tapping mode cantilevers for both imaging and indentation.

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INTRODUCTION Tapping-mode atomic force microscopy (TM-AFM) enables the simultaneous imaging and nanoindentation measurements of systems with sub-micrometer compositional heterogeneities.1−8 The objective of this work is to demonstrate that the variation in elastic behavior as a result of embedded phases can be manifested by a shift in phase angle of the cantilevered tip response to the excitation frequency.9 The complexity of the tip−sample interaction in TM-AFM has been broadly exploited to form images from spatial variations in phase contrast, initially in phase-separated block co-polymer systems and more recently in cells and supramolecular structures.6,10−13 To interpret the relationship between the phase contrast and the structure being analyzed, techniques have also been developed for quantifying energy-dissipative processes that induce shifts in phase.11,14−17 The ability to obtain sub-surface information18 is a compelling development on the instrumentation side,16 as well as on the interpretation of phase contrast, as demonstrated for chitin accumulation in yeast cell walls,19 for sub-micrometer inclusions of glassy polystyrene (PS) in rubbery polybutadiene,20 and for other complex material systems containing an embedded second phase.6,21−23 In the recent decade, the wide use of polymeric micelles as reactor vessels entails loading the micelle cores with inorganic reactants for the synthesis of nanoparticles.9,24−27 For such systems, particularly complex systems that rely on the loaded micelles as an intermediate processing step, it would be © 2012 American Chemical Society

advantageous to be able to characterize the compositional distribution in a hybrid nanocomposite without a plasma etch removal of the polymer using TM-AFM.9,27 In fact, the characterization of biological matter would benefit from mastering the interpretation of chemical and mechanical properties from the TM-AFM images of hybrid composite systems with embedded phases, such as the calcified deposits in vascular cells.28 With oscillation of a cantilevered tip at a specific frequency, TM-AFM can be used to form an image of the sample from topographical variations, as well as from changes in phase angle, with minimal lateral deformation.6,9,10,12 The variations in phase angle from the excitation frequency were shown to produce contrast in images of a monolayer of diblock copolymer reverse micelles loaded with zinc acetate dihydrate (ZnAc) in the micelle core. Although contrast in the phase image was associated with the relative compliancy, the details were as yet not well-understood for relating the local mechanical properties to the underlying compositional variations.9 In this work, we present the results of experiments to relate the compositional variation in such inorganic loaded micelle systems to the elastic modulus variation that gives rise to phase Received: April 30, 2012 Revised: June 12, 2012 Published: June 14, 2012 10592

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morphological changes because of the loading of the micelles at room temperature and a relative humidity of 30%. AFM images were processed using the WSxM software package.34 Before using each tip for a set of measurements, the spring constant of the cantilever was determined from the free oscillation frequency via the autotuning procedure of the AFM controller. This method was used because we are primarily interested in the relative differences in the tip−sample interaction. In the final preparation step, the deflection sensitivity of the tip was calibrated on a silicon wafer. Deflection sensitivity displays the change in piezo displacement corresponding to that of cantilever deflection. For a stiff sample, such as silicon, the slope should be equal to 1. Prior to indentation by the cantilevered tip, the sample surfaces were imaged in survey scans, to identify the target locations, such as those of the micelles. Afterward, imaging was performed on the same region, to confirm the location of the indentation target, e.g., roughly centered on a micelle. To understand the influence of embedded ZnAc on the mechanical properties of the loaded micelles, measurements for each distinct sample configuration were performed using the force−indent mode of AFM: (i) a 31.6 μm thick PS film on a mica substrate (type 1), (ii) empty diblock co-polymer micelles on the same PS film coating a mica substrate (type 2), and (iii) ZnAc-loaded micelle monolayer on a PS film-coated mica substrate (type 3). A curve of piezo displacement versus cantilever deflection was recorded and converted to a force versus indentation (F−I) curve by applying Hooke’s law, using the predetermined cantilever spring constant. To extract the effective elastic modulus from the loading F−I curve, the Hertz model was applied29 to the initial region of loading, where contributions from viscoelastic and adhesive forces are insignificant and deformation is predominantly elastic. The shape of the tip was determined by fitting Sneddons’ modifications for different tip geometries to the type 1 sample, the 31.6 μm thick PS film on mica, for which the elastic modulus was determined independently by standard nanoindentation.7 The best fit came from assuming a tip of parabolic shape (eq 1)

image contrast. With indentation and comparison of loaded micelle monolayers to unloaded micelle monolayers, force− indentation (F−I) curves could be used to elucidate the composition−compliancy-phase contrast relationship. Toward this end, AFM force−displacement measurements were used to determine the elastic modulus values,23,29−32 which were demonstrated to be consistent with values obtained from correlated studies using instrumented nanoindentation. Standard tapping mode probes were used for both scanning the surface to obtain the image and for the F−I measurements.

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EXPERIMENTAL SECTION

Three types of samples were characterized, as summarized in Figure 1. The first (type 1) was a 31.6 μm thick film of PS [molecular weight

Figure 1. Series of schematics showing the AFM cantilever tip indenting (a) 31.6 μm thick PS film on a mica substrate (type 1 sample), (b) empty polymeric micelle on a PS film-coated mica substrate (type 2 sample), and (c) ZnAc-loaded polymeric micelle on a PS-coated mica substrate (type 3 sample).

Fparaboloid =

(MW) = 48 000; polydispersity index (PDI) = 1.16]. The polymer solution [0.2 g of PS in 2 mL of dichloromethane (DCM) with a 0.1 g/mL weight per volume ratio] was deposited onto a mica substrate by spin coating. The second sample (type 2) consisted of a monolayer of reverse polystyrene-block-poly(2-vinylpyridine) [PS(721)-b-P2VP(627), Polymer Source, Inc.] diblock co-polymer micelles, deposited on a 31.6 μm thick PS film on mica. The micelles were prepared by dissolving 25 mg of PS(721)-b-P2VP(627) diblock co-polymer in 5 mL of toluene. To load the core of such micelles with precursor for nanoparticle synthesis, 18 mg of ZnAc was added after the formation of stable micelles and the solution was vigorously stirred for 3 days. The segregation of ZnAc to the micelle core was verified with dynamic light scattering (Zetasizer NanoZS, Malvern Instruments, Malvern, U.K.) (see the Supporting Information). The average size of the micelles was increased upon loading by ZnAc. In addition, the bimodal distribution of micelles became a single peak, as expected because of the enhanced stability of the micelle structure, after coordination between the core blocks with the metal salt.33 To avoid the substrate influence on the measured elastic modulus values, the polymeric micelles were deposited onto PS buffer films, by spin coating 20 μL of solution onto the dried PS thin films for 10 s at 1500 rpm. Prior to AFM characterization, the samples were baked for 1 day at 55 °C to ensure complete evaporation of solvent molecules from the thin film. Two types of such samples were prepared. Type 2 consisted of a monolayer of unfilled micelles on a layer of PS supported by mica, whereas type 3 consisted of micelles loaded with ZnAc. AFM characterization was performed using a Nanoscope III atomic force microscope (Digital Instruments, Santa Barbara, CA) using the tapping mode with single-beam silicon cantilevers (Olympus OMCLAC160TS-W2). The tips had a nominal radius of 8 nm, and the spring constants of the cantilevers were 40.85, 41.4, and 39.36 N/m for sets 1, 2, and 3 data, respectively. Other scan parameters used were the following: scan rate for indentation of 1 Hz, ramp size of 30 nm, and threshold value of 2.5 nm. For tapping mode imaging, the tapping mode with single-beam silicon cantilevers was used to investigate the

4 E*R1/2δ 3/2 3

(1)

where the load force is represented by F, the tip radius is represented by R, the indentation depth is represented by δ, and the effective elastic modulus is represented by E*, which is the slope of the line extracted from the fit equation. When the cantilevered tip−sample system is modeled as two springs in series (eq 2), the compliance of the sample can be obtained in terms of the measured effective modulus

(1 − vs 2)+ (1 − vi 2) 1 = E* Es Ei

(2)

where Poisson’s ratio and the elastic modulus of the sample are vs and Es, respectively, while those of the cantilevered tip are vi and Ei. Because the cantilevered tips were much stiffer than the compliant polymeric samples investigated, Ei ≫ Es, the second term on the right side becomes negligibly small, and Young’s modulus of the sample can be determined from the effective modulus and a known Poisson’s ratio.4 For PS, a value of 0.35 was used for Poisson’s ratio.34

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RESULTS AND DISCUSSION For each sample, 8−10 displacement−deflection curves were obtained and analyzed to determine the elastic modulus value of the samples. It should be emphasized that the number of curves obtainable for statistical averaging was limited by the propensity of the tip to deform. It was necessary to perform measurements using the same tip for which the properties were determined by calibration on the bare silicon. In this experiment, the applied maximum force was maintained constant around 100 nN and an indentation depth of 2−10 nm was targeted. Because the tip radius is difficult to determine accurately in the course of the indentation curve measurements, it was 10593

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extrapolated from the measurements performed on a PS film with a known elastic modulus (i.e., sample type 1), to facilitate subsequent measurements on the other more complex samples. Therefore, the elastic modulus was initially characterized using instrumented (nano)indentation in a NanoIndenter SA 2 (MTS NanoInstruments, Oak Ridge, TN) in continuous stiffness measurement mode for a 31.6 μm thick PS film on mica. The modulus was determined to be 3.39 ± 0.21 GPa, averaged over 25 measurements, by applying the Oliver−Pharr analysis, i.e., fitting the F−I curve with a power law function.35 To avoid viscoelastic displacement and to obtain the unrelaxed elastic response, dynamic indentation was used with an indenter frequency of 75 Hz.36 Poisson’s ratio of 0.35 was used for PS. The indentation depth for 25 measurements was 500 nm. The average elastic modulus values are summarized in Figure 2 for the standard nanoindentation measurement and three sets

performed over the loaded micelle, such that the mechanical response of the loaded core was being analyzed. Figure 4 shows the comparison of the force versus indentation depth (F−I) curves of the samples at a maximum applied force of 100 nN and reveals that the ZnAc-loaded sample region deforms less elastically compared to the region devoid of ZnAc, because of the higher stiffness in the loaded micelle core. Upon closer examination, it can be seen that the onset of the yield, i.e., the yield point, in the loading curves corresponds to an indentation depth of 2 nm for the type 3 sample, whereas it is 3 and 4.7 nm for the type 2 and type 1 samples, respectively. The rest of the indentation is plastic deformation. Deep indentations were avoided during the measurements, to maintain the tip quality and ensure reproducibility of the results. Therefore, indentation was performed with a maximum depth of 10 nm, of which depths up to 4−5 nm constitute the elastic regime. Previously reported studies have shown that to avoid contribution from the substrate to the measured properties, the indentation must be shallower than 10−15% of the total film thickness.3,5 In the case of the smooth PS films used in this study, the thickness of 31.6 μm was sufficient. The observed increase in elastic modulus above the ZnAcfilled micelle core is consistent with the sub-surface information obtained in other nanomechanical studies. In the direct-current (DC) force−volume measurements of a co-polymer film of PSb-polybutadiene (PB), which contained regions of sub-micrometer-sized inclusions of glassy PS droplets embedded in the rubbery PB, an enhancement of the average elastic modulus was reported.20 When approach−retract curves were analyzed, not only was the contrast in elastic modulus proven between the surface-exposed thermoplastic and elastomeric phases but more importantly the mixed mechanical response because of a region of embedded thermoplastic phase could be quantified.6 Finally, Touhami et al. demonstrated the detection of chitin accumulation in the walls of yeast cells, demonstrating the potential impact of further development of TM-AFM techniques toward sub-surface nanomechanical characterization.19 It should be noted that the use of standard nanoindentation for comparison to the TM-AFM nanoindentation in this study was justifiable for the PS that was analyzed. For PS with a MW larger than 40 400, the surface elastic behavior and Tg of the film are indistinguishable from that of bulk PS. The surface layer thickness was defined to be 2Rg, where Rg is the radius of gyration and falls within a depth of 2.7 nm from the surface.18 Because the indentation depths that were used for the results summarized in Figure 3 were around 5 nm, the nanoindentation data obtained from AFM and the dedicated instrument are comparable for the PS used in the samples. For the PS homo-polymer film, the MW was 48 000, whereas PS in the diblock co-polymer micelles had a MW of 75 000 (MW of the entire unimer was 147 500). Therefore, the elastic modulus values determined in both sets of experiments were not influenced by variation in Tg between surface and bulk values.

Figure 2. Elastic modulus values as determined by force−indentation depth (F−I) measurements and compared to the value obtained by nanoindentation on the reference PS film. Measurements for three different sample configurations using cantilevers of three different stiffnesses: PS film on a mica substrate (type 1; ◇), empty micelles/ PS/mica (type 2; light gray □), and ZnAc-loaded micelles/PS/mica (type 3; dark gray ▲). The “reference PS” data is that obtained from standard nanoindentation measurements.

of measurements (each set necessitated a fresh cantilever tip with a spring constant of 40.85, 41.4, and 39.36 N/m) for samples of type 1 (PS/mica), type 2 (empty micelles/PS/ mica), and type 3 (ZnAc-loaded micelles/PS/mica), respectively. There is no discernible difference between the elastic modulus values of type 1 samples and those from nanoindentation, because the AFM tip radius was extracted from the algorithm used to obtain a PS with an elastic modulus consistent with that obtained from standard nanoindentation. Although there is an increase in the elastic modulus between the empty reverse PS-b-P2VP micelles on a PS buffer layer (type 2) and the PS film (type 1), the error bars overlap in all three sets. Finally, there is a clear trend of increased elastic modulus, when the core of the micelles contains ZnAc. The wider spread in modulus values can be attributed to the fact that the tip may not be indenting directly above the center of the loaded micelle core in each measurement. Moreover, within each set of measurements, there is no overlap between the elastic modulus band of the loaded micelles and the empty micelles. Topographical imaging performed before and after (Figure 3) indentation ensured that the measurement was

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CONCLUSION The purpose of the study was to understand how loaded ZnAc nanoparticles change the mechanical properties of the polymeric micelle system. Previous work by this research group demonstrated that composition of micelle monolayers could be mapped via the phase contrast in TM-AFM. The 10594

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Figure 3. (a) AFM height image of monolayer of ZnAc-loaded PS-b-P2VP micelles spin-coated on PS-coated mica. (b) Three-dimensional (3D) zoomed-in AFM height image illustrating the shape of the plastically deformed micelle after indentation. (c) Height profile of micelle 1 (indented) shown in panel a. (d) Height profile of micelle 2 (not indented) shown in panel a.

angle that is manifested in TM-AFM phase contrast imaging of a ZnAc-loaded polymeric micelle monolayer. Of more general interest to the soft matter community, by imaging and measuring the elastic modulus with the same cantilever, we are able to overcome a key challenge in AFM nanoindentation measurements, simply by calibrating the tip geometry initially on a common soft material.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the PS synthesis and film preparation and polymeric micelle synthesis and scanning electron microscopy (SEM) image of a representative AFM tip. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. Representative curves for three types of samples: type 1 (PS/mica; light gray line), type 2 (micelle/PS/mica; solid black line), and type 3 (ZnAc/micelle/PS/mica; dark gray line with cross marker). The arrows indicate the end of the elastic loading regime.

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AUTHOR INFORMATION

Corresponding Author

presence of ZnAc in the polymeric micelle core was suggested to have increased the elastic modulus and was attributed to inducing larger shifts in the phase angle. In this study, it was shown that relative differences in elastic modulus values can be distinguished for three different sample configurations: a bare PS film, the PS film supporting a layer of polymeric micelles, and the PS film supporting a layer of ZnAc-loaded micelles. These results thus show quantitatively that differences in mechanical behavior as a result of variations in material composition at the nanoscale give rise to the shift in phase

*Telephone: +90-216-483-9592. Fax: +90-216-483-9550. Email: [email protected]. Present Addresses ‡

Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, Post Office Box 217, 7500 AE, Enschede, The Netherlands. § Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, United States. 10595

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Sebnem Inceoğlu from Istanbul Technical University for PS homo-polymer synthesis and gel permeation chromatography (GPC) characterization. Funding for this work is acknowledged from The Scientific and Technological ̇ AK) through the project Research Council of Turkey (TÜ BIT ̇ EB with Grant 106T657. Taner Aytun acknowledges a BID ̈ ̇ scholarship from TUBITAK.

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