Facile Assembly of Aligned Magnetic Nanoparticle Chains in Polymer

Feb 27, 2017 - Magnetic nanoparticle chains are found in biosystems, such as in the brain of migratory birds. Inspired by natural assemblies, in a nov...
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Facile Assembly of Aligned Magnetic Nanoparticle Chains in Polymer Nanocomposite Films by Magnetic Flow Coating Hongyi Yuan,† Irina J. Zvonkina,† Abdullah M. Al-Enizi,‡ Ahmed A. Elzatahry,§,∥ Jeffrey Pyun,⊥ and Alamgir Karim*,† †

Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States Chemistry Department, Faculty of Science, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia § Materials Science and Technology Program, College of Arts and Sciences, Qatar University, PO Box 2713, Doha, Qatar ∥ Polymer Materials Research Department, Advanced Technology and New Materials Research Institute, City for Scientific Research and Technology Applications, New Borg El-Arab City, P.O. Box: 21934, Alexandria, Egypt ⊥ Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States ‡

ABSTRACT: Magnetic nanoparticle chains are found in biosystems, such as in the brain of migratory birds. Inspired by natural assemblies, in a novel approach, the facile assembly of magnetically aligned polymer grafted cobalt nanoparticle (MPGNP) chains in thin polymer films was accomplished by using low strength permanent magnets directly during the flow-casting process. Unlike previous studies of MPGNP chain alignment in the high viscosity melt phase, the high mobility of such dispersed MPGNPs during casting by magnetic flow coating of polystyrene (PS) nanocomposite thin films from a dispersion allowed for formation of well-aligned MPGNP chains at the PS film/air interface. Both spherical (symmetric) and cylindrical (asymmetric) MPGNP aligned chains were obtained with distinct properties. The average chain length and width, number of particles per chain, spacing between parallel chains, and chain alignment were quantified using surface probe and electron microscopy, and grazing incidence X-ray. The aligned chains did not randomize when annealed above the film glass temperature, apparently due to the high translational entropic barrier for macroscopic (GISAXS) chain realignment. The Young’s bending modulus of the aligned MPGNP nanocomposite films as revealed by a thin film wrinkling metrology showed that the elastic modulus along the chain axis direction was higher for the film with the cylindrical but not the spherical MPGNP chains. This suggests that PGNP chain flexural properties depend on asymmetry of the local MPGNP unit, much like the persistence length “stiffness” effect of polymer chains. The ferromagnetic nature of the aligned PGMNP chains resulted in film rotation, as well as repulsive and attractive translation under an applied external magnetic field. Such magnetically responsive films can be useful for sensors and other applications. KEYWORDS: polymer nanocomposite films, magnetic nanoparticles, self-assembly, magnetic alignment, thin films

1. INTRODUCTION Magnetic nanomaterials can respond to an external magnetic field, which makes them ideal candidates for compact fieldresponsive and electromagnetic shielding applications. Conventional magnetic materials include metals, metallic alloys, and metal oxides, all of which are “hard materials” that may not be easily compatible for incorporation into materials format suitable for applications such as flexible electronics. On the other hand, polymers possess unique properties including flexibility, processability, and transparency that bring benefits compared to conventional inorganic magnetic materials. The combination of a polymer matrix and magnetic nanoparticles is commonly adopted to make magnetic polymer nanocomposites, which possess the advantages of polymeric materials and the magnetic properties brought by the nanoparticles. Magnetic polymer nanocomposites have become an emerging field and © 2017 American Chemical Society

have attracted great research interest. In addition, magnetic polymer nanocomposite thin films have also found novel applications, such as magnetic storage and EMI shielding. It is, therefore, of notable significance to investigate the fabrication, structure, and properties of magnetic polymer nanocomposite thin films. Conventional magnetic nanoparticles are usually inorganic and generally not expected to have strong interaction with polymers at their interface without surface modification. As a result, dispersions of these nanoparticles in a polymer matrix can be poor, which can be an obstacle in fabrication of uniform and homogeneous nanocomposites. Additionally, there is Received: February 14, 2017 Accepted: February 27, 2017 Published: February 27, 2017 11290

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approach of the self-assembly of nanoparticles is advantageous as it is performed under a relatively low magnetic field of ∼0.3 T. Furthermore, once formed, the aligned nanostructures are thermally stable well above the glass transition temperature of the nanocomposite film in the absence of the field, so that the field response and magnetic response are permanently coded into the film. The magnetic nanoparticle chains are also located at the air/film interface, so that such films are nanoscale roughened at that interface. Interestingly, the elastic modulus of these nanostructured thin films is found to be 2D anisotropic in the film surface plane for the cylindrical nanoparticle aligned chains only, but not for the spherical ones.

strong magnetic attraction between ferromagnetic nanoparticles that can also prevent their uniform dispersion. Recently, a family of functional inorganic magnetic nanoparticles were synthesized and characterized, which were specifically designed to improve dispersion of nanoparticles in polymer matrixes.1−11 Using a chemical modification approach, these nanoparticles were prepared with a core−shell structure, having an inorganic magnetic particle core and a surrounding polymer serving as a shell. For example, Korth et al. successfully synthesized polystyrene (PS)-coated (end-grafted PS chains) cobalt nanoparticles, which was confirmed by high-magnification TEM.10 The polymer shell surrounding the metallic core is beneficial, as it improves interactions between the nanoparticles and polymer matrixes at the interface due to their common organic nature enhancing their chemical affinity. Additionally, it separates individual particles and alleviates their aggregation caused by the magnetic attraction forces. Therefore, these nanoparticles are beneficial for magnetic polymer nanocomposites, and potentially, their uniform dispersion can be controlled by adjusting the chain end-grafted polymer coating layer characteristics. Several pioneering approaches to preparation of encapsulated nanoparticles with tunable and functional properties and nanocomposites with embedded nanoparticles have been demonstrated in recent publications by Lin and co-workers.12−16 Magnetic/plasmonic nanoparticles with a core−shell structure were synthesized using a starlike structure of amphiphilic triblock copolymers as nanoreactors.12 This approach demonstrates a new strategy for the design and synthesis of core−shell nanoparticles with desired characteristics. For instance, encapsulation of ZnFe2O4 nanoparticles in a carbon network matrix was demonstrated.13 Several strategies have been applied to align magnetic nanoparticles by an applied magnetic field.17−21 Suhendi et al.21 reported alignment of single domain core−shell nanoparticles under a magnetic field during spin-coating promoting vertical orientation of the nanoparticles in the polymer matrix. Yoshida and coauthors have demonstrated that magnetization of immobilized nanoparticles is affected by the orientation of their alignment.22 Preparation and alignment of hybrid organic−inorganic magnetic microrods under an external magnetic field was also illustrated.23 Allia et al. produced anisotropic nanocomposites by curing epoxy composition containing aligned under magnetic field titania/iron oxide magnetic nanoparticles.24 While all of the studies presented above produce a versatile set of magnetic nanoparticle alignment and orientation in polymer films, they are not entirely suitable for continuous nanomanufacturing processes such as roll-to-roll (R2R) assembly. In this regard, our present study provides a straightforward, efficient, and scalable R2R compatible method for in situ magnetic alignment of both isotropic and anisotropic nanoparticles during film casting that form chains at the air/ polymer interface of the polymer matrix film structure. Flow coating performed in this study is analogous to doctor blading that is commercially used in roll-to-roll manufacturing of polymer films. Our magnetic flow coating (MFC) method allows us to directly create magnetically functionalized polymer films by the R2R method. The paper discusses the effect of these aligned magnetic nanoparticle chains on the elastic modulus of the thin films, as investigated by the strain induced elastomer buckling instability measurement method (SIEBIMM).25,26 The demonstrated

2. EXPERIMENTAL SECTION Polystyrene with a weight-average molecular weight (Mw) of 192 000 g/mol was obtained from Sigma-Aldrich Inc. Cobalt ferromagnetic nanoparticles in both spherical and cylindrical geometries with attached end-grafted polystyrene (PS) chains were synthesized as described in ref 10, and we refer to these as “magnetic polymer grafted nanoparticles (MPGNPs)”. The polystyrene coated cobalt magnetic nanoparticles were prepared by a thermolysis process of dicobaltoctacarbonyl (Co2CO8) performed in the presence of polymer surfactants with end functional benzylamine or dioctylphosphine groups. The prepared MPGNPs were either spherical with an average diameter of 27 nm or cylindrical with an average length of 101 nm and an average diameter of 31 nm. Thus, the diameter of the spherical nanoparticles was relatively similar (within 14%) to the diameter of the cylindrical nanoparticles. For both types of the nanoparticles, a PS of Mw of 5000 g/mol was used for the shell with an average thickness of 2 nm. All materials were used as received without further purification. Si(100) wafers (Silicon Quest International Inc.) were used as casting substrates, which were cleaned with methanol and deionized water and exposed to UV light treatment at a PSD Series Digital UV Ozone System (Novascan Inc.) for 30 min. PS was dissolved in the laboratory grade toluene (BDH Chemicals Inc.) to prepare 2 wt % solutions. Different weight fractions of MPGNP (2, 5, and 10 wt % with respect to the polymer) were subsequently added to the solutions, respectively, and then ultrasonicated in a water bath for 2 h using a B2500A-MTH Ultrasonics Cleaner (VWR) to facilitate nanoparticle dispersion. Thin film samples with randomly distributed or aligned MPGNP were prepared by flow coating.27 The thickness of the applied films was ∼100 nm, exceeding the dimensions of the incorporated nanoparticles by ∼2−3×. A pair of cylindrical permanent magnets were used to provide the external magnetic field for nanoparticle alignment during flow coating. Magnets of opposing polarities of diameter ∼ 1 cm were placed with their equatorial plane in line with the surface plane of the flow coating substrate mounted on a translation stage (Figure 1). The height of the magnets was adjusted so that the magnetic fields of both magnets

Figure 1. Experimental setup for flow coating of thin nanocomposite films under magnetic field providing self-assembly of nanoparticles during the film-casting process. 11291

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Figure 2. Optical micrographs of polystyrene thin films containing spherical Co(s) nanoparticles (10 wt%) (PS+10%Co (s)) randomly distributed as cast (a), aligned as cast (b), aligned and annealed (c); and cylindrical Co(c) nanoparticles (PS+10%Co (c)) randomly distributed as cast (d), aligned as cast (e), and aligned and annealed (f). Scale bars represent 10 μm. crossed symmetrically across the flow-coated polymer nanoparticle dispersed liquid films above and below the film plane in the x-direction of Figure 1. We believe there is lower resistance for the nanoparticles to align and organize themselves at the film surface rather than the film interior that is more viscous. Presumably the polymer/air interface has a slightly higher magnetic field strength due to the permeability of the polymer in its casting solution state as well that aids the surface alignment of the MPGNP chains. The magnetic alignment of cobalt nanoparticles was performed simultaneously with the application of the films solution casting process termed magnetic flow coating (MFC), providing an in situ magnetic alignment (Figure 1). The magnetic field strength was measured by a magnetometer to be about 0.3 T at the center, a relatively weak field strength compared to what would be required to align the same magnetic nanoparticles in equivalent melt polymer films due to the increased film viscosity. Also, a much longer time would be required for such alignment to occur due to low diffusional rates of such relatively bulky nanoparticles in melt films. The morphology of the nanocomposite thin films was examined using an Olympus BX41 optical microscope and a DI-Veeco Nanoscope V atomic force microscope. Orientation of Co nanoparticles in the polymer matrix was additionally observed by a JEM 1200XII transmission electron microscope and using grazing incidence small-angle X-ray scattering (GISAXS) measurements at Brookhaven National Laboratory. Polydimethylsiloxane (Sylgard 182, Dow Corning Inc.) was used as an elastomeric substrate for the strain induced elastomer buckling instability measurement method (SIEBIMM) as described in ref 25. The weight ratio of the oligomer to the cross-linking agent was 19:1, and the mixture was heated at 120 °C for 2 h to facilitate cross-linking. The cross-linked PDMS films were cut into 75 mm × 25 mm × 1 mm strips, and the elastic modulus of each strip was measured by a TA.XT Plus Texture Analyzer (Texture Technologies Inc.) before use. For investigation of the mechanical reinforcement, the prepared nanocomposite thin films were transferred from Si substrates to PDMS substrates using a water-wetting film delamination technique.28 Samples with the magnetically aligned nanoparticles were placed on PDMS substrates either parallel or perpendicular to the direction of the magnetic alignment, providing different orientations with respect to the film buckling. The PDMS strips with the films were then strained by up to 3% along the PDMS strip length direction, thus applying a compressive stress to the films in the width direction,

thereby generating the film buckling patterns. The wavelengths of the buckling patterns were measured using image processing software ImageJ (National Institutes of Health).

3. RESULTS AND DISCUSSION Nanoparticle chain alignment by an external magnetic field has been reported previously for bulk magnetic polymer nanocomposites in the melt state, which typically required a strong magnetic field of the order of a few Tesla.17−20 The demonstrated MFC method of alignment of nanoparticles in this study, however, can be applied to thin polymer nanocomposite films using a weak magnetic field below 1 T that is highly effective and technologically convenient. The maximal coercive force applied to a nanoparticle at a gradient magnetic field leading to the nanoparticle alignment in the direction of the magnetic field can be estimated according to the following equation Fmax = m·∂B∂m / = Vm·M ·ΔB

(1)

where F is magnetic force on a nanoparticle, m is magnetic dipole, ΔB is field induction gradient, Vm is volume of the nanoparticle, and M is volume magnetization. This leads to formation of nanoparticle chains under such applied magnetic field, given suitable mobility for the nanoparticles to align. The induced self-assembly conditions are provided by a high mobility of the polymer grafted cobalt nanoparticles (spheres and cylinders) in a homopolymer solution during flow coating. The macroscopic morphology of the nanocomposite thin films containing either randomly distributed or aligned MPGNPs was analyzed using optical microscopy, while their nanoscopic surface distribution and detailed structure formation including aggregation were imaged by AFM, TEM, and GISAXS. In the following figures, “s” refers to spherical nanoparticles and “c” to cylindrical. It follows clearly from Figure 2a,d that both spherical and cylindrical Co nanoparticles were uniformly distributed in the polymer matrix prior to application of the magnetic field. The polystyrene shell surrounding each Co nanoparticle facilitates favorable inter11292

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Figure 3. AFM 2D images of PS+10%Co thin films containing spherical Co nanoparticles aligned as cast (a) and after annealing (b); and cylindrical Co nanoparticles aligned as cast (c) and after annealing (d).

Figure 4. AFM 3D images of PS+10%Co thin films containing spherical Co nanoparticles aligned as cast (a) and after annealing (b); and cylindrical Co nanoparticles aligned as cast (c) and after annealing (d). The maximal height of the nanoparticle structures in the Z-direction is 79.1 nm (a), 92.6 nm (b), 47.7 nm (c), and 23.9 nm (d).

average chain length of 20 μm and an average interchain distance of 12 μm for the thin films containing spherical nanoparticles. Given the average diameter of 27 nm for these nanoparticles, the average number of nanoparticles per chain line in the longitudinal direction is estimated as 750 (20 μm/27 nm). Thus, the chains are formed by aggregates of nanoparticles with relatively high collective mass that can stabilize them from translational and rotational motion once formed. In order to investigate the effect of the thermal annealing on the stability of the formed nanostructures, the flow-coated thin film samples were then annealed for 24 h under vacuum at 160

actions between the nanoparticles and the polymer chains, and to prevent aggregation of the nanoparticles. It is found that, under the magnetic field, both types of the nanoparticles followed the magnetic field direction, and self-assembled into chains, as shown in Figure 2b,e. As will be shown later, these “chains” can consist of individual or several parallel lines of MPGNPs bundled together. For the chains shown in the optical images of Figure 2, an average width of each chain is about 1 μm. Therefore, an average number of nanoparticles per chain in the width direction is estimated as 37 (1 μm/27 nm). Calculations from the digitized optical micrographs give an 11293

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Figure 5. TEM images of PS+10%Co thin films containing spherical Co nanoparticles randomly distributed (a); aligned single chains (low magnification) (b); aligned chain line bundles (high magnification) (c); and cylindrical Co nanoparticles randomly distributed (d); aligned single chains (low magnification) (e); and aligned chain line bundles (high magnification) (f). Low- and high-magnification images were taken at different regions, which also illustrate the range in diversity of the chain bundles.

°C, which is ∼60 °C higher than the glass transition temperature (Tg) of PS, giving the polymer matrix chains and presumably the individual polymer grafted nanoparticles sufficient mobility and time to achieve their equilibrium state. However, even after such extended annealing conditions above the Tg, the nanoparticle chains were found to be stable in their linear alignment state, as shown in Figure 2c,f. The orientation of the chains remained constant following the initial magnetic field direction. Such behavior is important, as these magnetic polymer nanocomposites may be used in applications involving elevated temperatures such as high temperature sensors. The prepared nanocomposite thin films were additionally examined using AFM. Figure 3 shows the AFM images of the thin film surface containing nanoparticle chains, as cast and after thermal annealing. It is found that an average width of the chains remained constant at approximately 1 μm before and after annealing. In other words, annealed thin films maintained the structure of as cast chains even after thermal annealing, consistent with the optical microscopy observations. A possible reason for this may be due to the nanoparticle magnetization. Figure 4 demonstrates a 3D nanoscopic view of the AFM images of the prepared Co nanocomposite thin films, which show that Co nanoparticles are affixed together and protrude from the film surface. The height of the Co nano-structures above the film surfaces was found to be of a few tens of nanometers, which is in the range of 1−2 diameters of the nanoparticles. The large ∼1 cm diameter of macroscopic pole dimensions of the two permanent magnets held on each side of the flow coating glass slides at a midplane level ideally creates an aligning magnetic field zone equally above and below the cast film surface. However, given that the MPGNPs are all present at the top film surface, it is possible that a slightly stronger field strength exists at the air/film interface causing the particles to partition to the air/film interface as well. Additionally, it is

anticipated that there is lower self-assembly resistance for the nanoparticles to align and organize themselves by protruding from the film surface rather than the film interior, which is more viscous. Thus, under film drying conditions, aided presumably by the fact that the film interior also has a slightly lower magnetic field strength due to the magnetic permittivity of the polymer solution, the MPGNP chains form at the film surface. At a few locations, larger aggregates of Co nanoparticles are formed as can be observed in the AFM images of Figure 3. Our previous studies have demonstrated the effect of the magnetic field strength and the average nanoparticle agglomeration area and morphology when organized at an oil−water liquid−liquid interface by a method termed “Fossilized Liquid Assembly (FLA)”.6,29−31 By removal of one (water) liquid phase, and embedding the particles in the other (oil) phase by its polymerization with UV light, such nanostructures protruding from the air/film surface were observed under an applied magnetic field. The shape and the morphology of such magnetic field induced agglomerate structures can also decrease the total Gibbs energy of the system at the liquid−liquid interface. Stronger particle−particle interactions are possible at the air/film interface rather than film interior due to enhanced magnetic interactions that contribute to this phenomenon as well. We can consider some advantages of the aligned nanoparticle chains that could be important in load-bearing applications for ultrathin films. An analogy would be the underlying veins of plant leaves that protrude below the leaf surface in order to better support the overall weight of the structure by providing reinforcement on a periodic interval. Protruding magnetic nanoparticle chains aligned at the air/film surface with periodic spacings may well have similar mechanical reinforcement performance (in flipped state). On the other hand, magnetic sensor films may benefit from protruding MPGNP at the film/ 11294

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arrangement when forming multiple aligned lines or bundles. High GISAXS contrast arises from the fact that the nanoparticles are arranged in contact at the film−air interface. To further demonstrate the magnetic properties of the nanocomposite thin films, the films prepared on silicon wafer substrates were immersed in water to float-up at the water−air interface in a beaker and subjected to external magnetic field using a single permanent magnet (Figure 7). When the films

air interface due to sensitivity and spatial resolution of detection capability. The film nanostructure and the orientation of the Co nanoparticle chains were additionally demonstrated by TEM, as shown in Figure 5. Without alignment (no applied magnetic field during flow coating), nanoparticles are randomly distributed in the PS matrixes (Figure 5a,d), which agrees well with the images of the optical microscopy (Figure 2). With the magnetic alignment, the nanoparticles tend to self-assemble by interacting with each other at their interface and to form lines along the direction of the initial external magnetic field (Figure 5b,e). At certain locations, bundles formed by several chains of nanoparticles aligned parallel to each other are present (Figure 5c,f), which formed the ∼0.5−1 μm wide chains observed using optical microscopy (Figure 2). Comparing the TEM images of the films containing the spherical and the cylindrical nanoparticles, one can conclude that the spherical ones appear to demonstrate a better dispersion of the nanoparticles in the polymer matrix and can be identified individually (Figure 5). On the other hand, cylindrical nanoparticles are found to form larger agglomerates within the film structure. A possible reason for that can be related to a difference in the aspect ratio and particle size of the spherical and cylindrical nanoparticles. In order to explore the orientation and structure of the selfassembled magnetic nanoparticle chains, the nanostructured films were additionally examined using grazing incidence smallangle X-ray scattering (GISAXS). At varying angles of incidence, GISAXS probes the x−y in-plane alignment ordering properties at varying depths in the film. For incident X-rays above the critical angle of the film, GISAXS probes the entire film thickness, whereas, below the critical angle of the film, it probes the near air/film surface structure. Figure 6 examines the scattering from the aligned nanoparticles chains structure, with the X-ray beam aligned parallel

Figure 7. Rotation of the ferromagnetic polymer nanocomposite thin films immersed in water under external magnetic field of different polarities, directed by the north pole (a) and south pole (b). Given the transparency of the floating nanocomposite film, its film boundaries are marked in blue for identification, and the relative magnetization condition shown in red and white color is maintained relative to the schematic of Figure 1.

were placed closer to the north pole of the magnet, they rotated from their initial position in a clockwise direction in order to align with the magnetic field (Figure 7 a). The response of the film to rotation and alignment with the external field was completed in 8s, a relatively short amount of time. Undoubtedly, the low viscosity of the supporting fluid (water ∼ 1 cP) allows for the fast response. In contrast, when the south pole of the permanent magnet was placed nearby, the films rotated in a counterclockwise direction, so that the opposite side of the film aligned close to the south pole (Figure 7b). Thus, the north and the south poles of the magnetic nanocomposite films enabled film rotation to align with the magnetic field created by the magnets. Considering the relevance of the mechanical performance of nanocomposite materials, it is of interest to study the effect of the magnetic alignment on the elastic modulus of PS/Co thin

Figure 6. GISAXS patterns of the self-assembled nanoparticles within thin nanocomposite films in-depth (a and c) and at the film surface (b and d).

(ϕ = 0°) and perpendicular (ϕ = 90°) to the chain axis. The images were performed at the grazing incidence angle below (Figure 6a,c) and above (Figure 6b,d) the critical angle. The scattering along the chain alignment direction is broad, indicative of varied intrachain correlation within chain lines in a bundle and in-plane undulations within single chain alignment. In contrast, for ϕ = 90°, the Inter-nanoparticle correlations along a line can be observed, serving as evidence that the self-assembled nanoparticles are in a compact 11295

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Figure 8. Buckling patterns of PS+10%Co nanocomposite thin films with a random (a and d), parallel (b and e), and perpendicular (c and f) to the buckling orientation of the spherical and cylindrical nanoparticles, respectively. Arrows indicate orientation of the aligned chains of nanoparticles. Scale bars represent 20 μm.

Table 1. Elastic Modulus for PS/Co Nanocomposite Thin Films Calculated Using SIEBIMM materials

elastic modulus (GPa)

PS+10%Co(s), spherical, all orientations

PS+10%Co(c), cylindrical, random

PS+10%Co(s), cylindrical, parallel

PS+10%Co(c), cylindrical, perpendicular

3.28 ± 0.17

3.24 ± 0.23

3.26 ± 0.20

3.61 ± 0.31

Figure 9. Schematic demonstration of the effect of the particle shape on the films compression characteristics at a constant applied compression force for the films containing spherical nanoparticles at the initial state (a) and after the film compression (b) and cylindrical nanoparticles at the initial state (c) and after the film compression (d). Borders are darkened to highlight film edges.

films. Figure 8 demonstrates the buckling patterns of the films on stretched PDMS for each system using the SIEBIMM method. For the samples with magnetically aligned nanoparticles, orientation of the alignment with respect to the SIEBIMM compressive strain direction was taken into account due to the expectedly anisotropic behavior of the elastic modulus. In this study, the nanocomposite thin films with aligned Co chains were placed on the surface of PDMS either parallel or perpendicular to the buckling direction, as marked in Figure 8 (b,c,e,f). Arrows in the images indicate the orientation of the

aligned Co chains. The chains oriented orthogonal to the bucking direction can be clearly observed as dark lines resembling cracks in the film. However, the lines parallel to the buckling could not be clearly observed (Figure 8b,e), presumably due to relatively shallow variations in height compared to the buckling height. As a general observation from the optical micrographs, incorporation of Co nanoparticles in the PS matrix did not interfere with the buckling pattern formation and the chain lengths are several times the buckling wavelength when aligned 11296

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ACS Applied Materials & Interfaces across the wrinkles, hence enabling valid use of SIEBIMM for determining the Young’s elastic modulus evaluation. As shown in Table 1, spherical Co(s) nanoparticles did not strengthen significantly the PS matrix, regardless of their distribution/alignment in the polymer matrix. The modulus was calculated, and the average value was found to be 3.28 GPa, which is virtually the same as the elastic modulus of neat PS films (3.27 GPa). It is known that anisotropic nanoparticles can provide a higher reinforcement to polymer matrixes.17 In the case of spherical Co nanoparticles, despite the formation of the chains, our results demonstrate that such isotropic nanoparticles do not have a strengthening ability at comparable nanofiller contents. We believe this is due to a lower flexural modulus of isotropic nanoparticles linked together compared to anisotropic nanoparticles that are linked end-on (Figure 9). Future simulation studies may probe this interesting observation in more detail. The SIEBIMM analysis was further applied to PS+10%Co(c) thin films containing either randomly distributed or magnetically aligned cylindrical nanoparticles. The results for the samples with randomly distributed as well as parallel aligned cylindrical nanoparticles were similar to those containing the spherical nanoparticles. Their elastic modulus was determined as 3.24 and 3.26 GPa, respectively, demonstrating no increase of the modulus over neat PS. However, the modulus of the films with alignment of the cylindrical nanoparticles perpendicular to the buckling direction is 3.61 GPa, which is about 10% higher than that for the neat PS. Additionally, the effect of the aspect ratio of the filler is relevant in the explanation of the reinforcement and enhancement of the mechanical properties of the nanocomposite films. According to the Halpin−Tsai theory, with increasing the aspect ratio, the ratio of the storage modulus of the composite film to the storage modulus of the matrix polymer increases, as demonstrated by the following equation Ec /Ep = (1 + 2·A r ·μ·Φr )/(1 − μ·Φr )

Figure 10. Elastic modulus for PS/Co(c) nanocomposite thin films as a function of weight ratio of Co nanoparticles.

properties, which is analogous to fiber-reinforced nanocomposites. Alternatively, it suggests that chain flexural properties of the MPGNP depend on asymmetry of the local MPGNP unit, much like the persistence length “stiffness” effect of polymer chains. In addition to the results demonstrated in this article, we would like to mention that the research can mimic the magnetic aspects of the structure of the brain of birds.32,33 As previously investigated, migratory birds orient themselves in space relative to the direction of the Earth’s magnetic field, which is possible due to the special structure of the bird brain containing similar aligned magnetic nanoparticles. Taking this interesting phenomena created by nature into account, we were encouraged to mimic this fascinating structure of the biological systems to create anisotropic nanocomposite thin films with self-assembled aligned chains of magnetic nanoparticles acting as permanent magnets.

4. CONCLUSIONS In this research, a facile route for fabrication of magnetic polymer nanocomposite thin films by magnetic flow coating (MFC), aligned under a low (∼0.3 T) external magnetic field, nanoparticles in a polystyrene matrix was introduced. Optical microscopy and AFM images demonstrate bundles of chains of Co nanoparticles, which were found to be stable containing after 24 h of annealing above the glass transition temperature of PS. More aggressive annealing methods such as directional zone annealing will be tested in future studies. TEM and GISAXS images confirmed the formation of chains of individual Co nanoparticles physically attached to each other. The chains of spherical nanoparticles did not affect significantly the strength of the PS film matrix, presumably due to the lower flexural modulus of chains with nanoparticles of isotropic geometry. On the other hand, the elastic modulus of nanocomposite thin films containing cylindrical nanoparticles was found to be dependent on the alignment of the Co nanoparticles. Cylindrical nanoparticles enhanced the modulus of filled PS films by up to 10 wt % when aligned perpendicular to the buckling compression direction. By creating a mechanical anisotropy of the films, one can tune the mechanical performance of the thin polymer nanocomposite films similar to that of fiber reinforced films, thus enhancing their efficiency and broadening possibilities for their potential applications. Finally, the films were magnetically responsive and able to rotate when free-standing under an external magnetic field, demonstrating that the nanoparticle chains act as permanent magnets with predefined north−south magnetization. Such magnetic nanoparticle chains are found in migratory birds that

(2)

where Ec and Ep are storage modulus values for the composite and for the polymer, respectively, Ar is the aspect ratio, Φr is volume fraction, and μ is a geometrical factor The effect of reinforcement of thin films by nanoparticles was expected to be the highest for a higher content of nanoparticles. On the basis of the obtained results of an increased modulus for the thin films containing 10 wt % cylindrical nanoparticles aligned perpendicular to buckling, the amount of the nanoparticles was reduced to 2 and 5 wt % and the elastic modulus investigated as a function of the ratio of Co nanoparticles for the thin films with the same aligned nanostructure. All samples were placed on PDMS slabs in the direction providing a perpendicular orientation of the chains to the buckling. Results are shown in Figure 10. The findings demonstrate that the elastic modulus of PS/Co(c) thin films increases with increasing contents of the Co nanofiller (Figure 10) but approaches a plateau at 10 wt %. By increasing the ratio of the nanoparticles in the system, a higher reinforcement was achieved due to the mechanical properties provided by the endon chained nanoparticles. On the basis of the findings of this study, one can state that enhancement of the mechanical strength of thin polymer films can be achieved by incorporation of cylindrical magnetic nanoparticles aligned under a magnetic field perpendicular to the applied compression direction. The alignment of the nanoparticles provides a 2D anisotropy of the mechanical 11297

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ACS Applied Materials & Interfaces

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sense direction based on alignment with the Earth’s magnetic field, providing for comparison studies in the future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation via grant DMR-1411046 and the W.M. Keck Foundation. J.P. would like to acknowledge DMR-1307192 for the funding. We thank Prof. Bryan Vogt and Changhuai Ye for help with the PDMS strain stage and Dr. Bojie Wang for TEM imaging. The authors extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this Prolific Research group (PRG-1436-14).

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REFERENCES

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DOI: 10.1021/acsami.7b02186 ACS Appl. Mater. Interfaces 2017, 9, 11290−11298