Printed thin magnetic films based on diblock copolymer and magnetic

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Printed thin magnetic films based on diblock copolymer and magnetic nanoparticles Senlin Xia, Ezzeldin Metwalli, Matthias Opel, Paul A. Staniec, Eva M. Herzig, and Peter Muller-Buschbaum ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16971 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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ACS Applied Materials & Interfaces

Printed thin magnetic films based on diblock copolymer and magnetic nanoparticles Senlin Xia1, Ezzeldin Metwalli1, Matthias Opel2, Paul A. Staniec3,4, Eva M. Herzig5, Peter Müller-Buschbaum1 * 1

Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien,

James-Franck-Straße 1, 85748 Garching, Germany 2

Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften, Walther-Meissner-Str. 8,

85747 Garching, Germany 3

Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot,

Oxfordshire, OX110DE, GB 4

Tritium Engineering and Science Group (TESG), Culham Centre for Fusion Energy (CCFE),

United Kingdom Atomic Energy Authority (UKAEA), Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, GB 5

Universität Bayreuth, Physikalisches Institut, Herzig Group – Dynamik und Strukturbildung,

Universitätsstr. 30, 95447 Bayreuth, Germany KEYWORDS:

printing,

diblock

copolymer,

magnetic

nanoparticles,

GISAXS,

superparamagnetic behavior

ACS Paragon Plus Environment

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ABSTRACT

Printing techniques have been well established for large-scale production and have developed to be effective in controlling the morphology and thickness of the film. In this work, printing is employed to fabricate magnetic thin films composed of polystyrene coated maghemite nanoparticles (γ-Fe2O3 NPs) and polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer. By applying an external magnetic field during the print coating step, oriented structures with a high content of nanoscale magnetic particles are created. The morphology of the magnetic films and the arrangement of NPs within the polymer matrix are characterized with real and reciprocal space techniques. Due to the applied magnetic field, the magnetic NPs selfassemble into micro-scale sized wires with controlled widths and separation distances, endowing hybrid films with a characteristic magnetic anisotropy. At the nanoscale level, due to the PScoating, the NPs disperse as single particles at low NP concentrations. The NPs self-assemble into nano-sized clusters inside the PS domains when the NP concentration increases. Due to a high loading of uniformly dispersed magnetic NPs across the whole printed film a strong sensitivity to an external magnetic field is achieved. The enhanced superparamagnetic property of the printed films renders them promising candidate materials for future magnetic sensor applications.


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1. INTRODUCTION Nanostructured materials have attracted increasing attention due to their enhanced properties compared with bulk materials. Nanostructured materials containing magnetic nanoparticles (NPs) demonstrate important improvements in many technological areas.1 They are ideal candidate materials for applications requiring superparamagnetism,2 magneto-electrical transport,3 magnetization quantum tunneling4 or electromagnetic wave absorption.5 As a consequence, they will be potentially used in high-density storage,5 magnetic sensors6, resonance imaging,7 ferro-fluids,8 and magnetic refrigeration.9 In many applications the spatial order of the NPs plays an essential role as for example in magnetic sensors and switching. So far, a variety of methods have been proposed for the realization of well-ordered magnetic NP arrangements, among which block copolymers proved to be one of the most promising NP guiding templates.10, 11 Upon micro-phase separation and the self-assembly processes of block copolymers, periodically ordered structures (such as spheres, cylinders and lamellae), can be achieved in the nanometer-scale range.12 Therefore, introducing NPs into such polymer matrices offers the possibility of obtaining NP/polymer hybrid materials with a highly ordered morphology. Generally, the localization of NPs in block copolymer matrices, especially in one particular domain of the block copolymer, is the challenge in the fabrication of these hybrid materials. Parameters such as the NP size and concentration,13 the NP surface chemistry,14 as well as the block copolymer properties,15 need to be considered, when designing the hybrid materials. Previous investigations showed a successful incorporation of NPs in a specified domain of an ordered block copolymer matrix via enthalpic and entropic contributions from the NPs and polymer.16 The grafting of polymeric ligands on the NP surface was demonstrated to enable tailoring of the enthalpic interactions and enhance the localization of NPs in one block of


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the block copolymer.17-19 NPs coated with polystyrene (PS) chains become well distributed inside PS domains of a block copolymer, due to the selectivity and preference to the PS blocks as seen in the literature and our earlier studies.20-22 The preparation of hybrid films with wellordered highly dispersed magnetic NPs has been demonstrated using thin film deposition methods such as spin coating.20-22 While the incorporation and localization of the NPs inside block copolymer matrices and the resulting overall structure of the obtained hybrids are ruled by thermodynamics of the system, the preparation and post-processing methods can also be key factors in influencing the desired morphology of the NP/polymer hybrid materials.23,

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To date, several techniques have been

explored for hybrid film fabrication, including spin coating, 19 solution casting, 18 spray coating 25 and dip coating. 26 In addition, film deposition via printing is very attractive, since it is a low-cost technique, enables large-scale processing, is simple to scale-up and compatible with many different types of substrates.27 Moreover, it is material efficient, because all the ink supplied to the print head is printed with no loss. The thickness, morphology, and surface topography of printed hybrid films can be controlled by the print parameters during the film fabrication. In research, the use of printing for film deposition is gaining strong interest for a variety of different applications. For example Chen et al. printed the first all-polymer filter circuit using a soluble conducting polymer mixture.28 Their investigation illustrates the advantages of using printing in comparison with more traditional film deposition methods.28 Pierre et al. have demonstrated the fabrication of fully printed organic thin-film transistors.27 The printed devices exhibited a high reliability, a low variability and an ideal electrical behavior.26 However, reports involving printing techniques so far have mainly focused on homopolymers or polymer blends with applications in organic electronics26-31 or photovoltaics.32-35 Examples on printing block


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copolymer films are still rare. In particular, to achieve block copolymer based hybrid materials with NPs, the used inks need to be optimized for the print process. The viscosity and volatility of the ink can significantly influence the film quality and an in-depth understanding will be highly beneficial. Since printing is also a solvent-evaporation based processing method, the investigation of the drying kinetics is necessary. Recently, for example, Palumbiny et al. and Pröller et al. used synchrotron radiation based in-situ measurements to study the structure formation of printed films for use in organic photovoltaics.32, 35 Due to the uniaxial deposition method in combination with the solvent evaporation step, the print technique can be combined with the use of an external field such as a magnetic field, which is very difficult to be realized with spin coating or spray deposition. For example, Kokkinis et al. recently employed a rotating external magnetic field to control the orientation of magnetized platelets in their 3D printing platform.36 Also, Yao et al. demonstrated that hybrid film systems of a block copolymer matrix and magnetic NPs show a superparamagnetic behavior and a remarkable shape anisotropy when combing solution casting with a magnetic field guiding the NPs.37 In principle, magnetic anisotropy linked with sample geometries is subject of many studies, as it can be used in device.34 Such samples are typically prepared with lithography techniques.38 The field-directed assembly of magnetic NPs can provide an economical, simple and reliable technique to realize magnetic anisotropy, when using print techniques to deposit the hybrid film of block copolymer and magnetic NPs in combination with magnetic field guidance of the NPs. In the present work, we use a printing technique to fabricate hybrid magnetic films composed of maghemite (γ-Fe2O3) NPs and polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer. The films are deposited on solid supports with slot die coating being a


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method compatible with roll-to-roll printing. The maghemite NPs are coated with PS to achieve a preferred localization and dispersion of NPs inside the PS blocks of the block copolymer. Furthermore, an external magnetic field is applied during the printing step. The combination of printing and magnetic field exposure ensures the production of continuous hybrid films with an oriented anisotropic magnetic nanostructure. Under the external magnetic field, the magnetic NPs assemble into elongated clusters or wires aligning along the magnetic field direction. Due to the use of small sized NPs, a superparamagnetic behavior of the printed hybrid film is achieved which is probed. Compared with films fabricated via solution casting, the printed films exhibit a higher magnetic susceptibility due to the higher density of nanostructures containing dispersed single NPs or nanosized clusters.37 The high sensitivity to external magnetic fields renders the printed films as interesting potentials candidates in magnetic sensing applications. Combining an external magnetic field with the printing technique enabled the preparation of homogeneous hybrid films with highly oriented mesoscopic structures. Given the possibilities of up-scaling the printing of the magnetic sensors is of particular interest for real-world applications.

2. EXPERIMENTAL SECTION 2.1 Printing Setup The printer was specially designed based on the principle of slot die coating for the use in polymer thin layer deposition. The printer consisted of three main parts, which were the print head, a linear stage and a syringe pump. A shim mask (solution guide mask) and a meniscus guide mask were incorporated in the print head. Thus, parameters such as printing width could be easily varied by applying different masks. The linear stage with tunable height and syringe


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pump system were connected through USB interfaces to a computer. By running the corresponding program, full control over the printing speed was achieved. Hybrid films with different thickness were obtained by adjusting the substrate-print head distance, the solution concentration and the stage-moving speed. To guarantee a safe working environment as well as a stable atmosphere during the whole printing process, a transparent chamber is applied to accommodate the printer. Figure S1 shows a sketch of the printing set-up.

2.2 Sample Preparation From Polymer Source Inc. the diblock copolymer polystyrene-block-poly(methyl methacrylate), denoted (PS-b-PMMA), was purchased with a number average molecular weight of 89 kg/mol and a polydispersity of 1.12. The weight fraction of PS block was 0.5. The diblock copolymer was used without any further purification. A lamella morphology was expected for the bare PSb-PMMA bulk samples on the basis of the theoretical phase diagram of diblock copolymers.39 PS coated maghemite NPs (γ-Fe2O3) were prepared by the oxidation of magnetite, followed by further treatment with α-lithium polystyrenesulfonate (LPSS), purchased from Polymer Standards Service. Details of the NPs synthesis protocols are described elsewhere.37 The mean diameter of the NPs was 10 nm with a log-normal size distribution (width 0.2). The weight ratio of NPs in toluene solution was 2 wt %.41 The size and chemical property of the NPs were examined by Mössbauer spectroscopy, X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). Details can be found in our previous article.37 Pre-cleaned silicon substrates (Si 100, n-type, Silchem) were used following a cleaning procedure as described in the literature.40 Toluene was used as solvent. At a fixed PS-b-PMMA


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concentration of 10 mg/mL the NP concentration was varied. The studied concentrations were 0.05, 0.2, 0.5, 1, 2, and 5 wt %. Hybrid films of PS-b-PMMA and magnetic NPs were prepared by printing. During printing of the solution containing copolymer and NPs the Si substrates were positioned horizontally. An external magnetic field was oriented vertically to the print direction in the plane of the sample. The permanent magnets used for this setup were purchased from Magnets4you GmbH. Previous work showed that highly oriented NP wires can be achieved at moderate magnetic field strength, rather than at higher ones.37 Thus, the strength of the applied magnetic field was fixed to 149 G at the center of sample. After printing, samples were kept in the printer for 30 min to make sure that the solvent was completely evaporated. Next, all samples were annealed in the same magnetic field at 120 °C in N2 for 3 days. All investigated samples were prepared via the same printing procedure.

2.3 Characterization Techniques

2.3.1. Surface Techniques The sample surfaces were probed with optical microscopy (OM), atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements as explained in the Supporting Information. 2.3.2. Profilometry Film thicknesses were measured with profilometry as explained in the Supporting Information. 2.2.3. Grazing Incidence Small-Angle X-ray Scattering (GISAXS) At the I22 beamline of the Diamond Light Source (Oxfordshire, United Kingdom) GISAXS measurements were performed, using the new GI-SAXS end station developed by Staniec et al. The samples were mounted on a sample-stage positioned with a bespoke hexapod (Symetrie). A


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custom linear stage atop the hexapod allowed for the aligned sample to be translated in the X-ray beam. This allowed fresh positions along the sample to be used for alignment and measured at a range of angles, to avoid the issue of beam damage. The exposure time for each image was 0.1 second. The synchrotron radiation wavelength (λ) was set to 0.1 nm (corresponding to 12.4 keV), the incident angle was αi = 0.35° and the scattering data were collected with a 2D detector (Pilatus P3-2M with 1475 × 1679 pixels, pixel size 172 × 172 µm2) at a sample−detector distance of 4.65 m. Three in-vacuum tungsten beamstops were employed (4 mm diameter for direct and reflected beams, and a 3 mm wide linear rod for the intense vertical specular reflection). To minimize the background parasitic scattering, the sample area was flooded with helium. GISAXS measurements were done in two sample orientations: The NP wires, which were observed on top of the hybrid copolymer films, were oriented perpendicular or parallel with respect to the X-ray beam, as determined from the original mounting direction with respect to the magnetic field. The horizontal line cuts of the 2D GISAXS data obtained from the measurements with the X-ray beam being parallel or perpendicular to the NP wires, probed lateral structures perpendicular or parallel to the NP wires, respectively. 2.2.4. Magnetic Property Measurements (MPM) Direct current (DC) magnetization measurements were done with a superconducting quantum interference device (SQUID) magnetometer (MPMS XL-7, Quantum Design). A range of external magnetic fields was applied as a function of temperature during the measurements. External magnetic fields were varied from −700 to 700 mT in the film plane. Temperatures were set to 2, 5, 20, 50, 100, and 200 K. In order to investigate the shape anisotropy, the samples were measured in two orthogonal directions. The NP wires were oriented either perpendicular or


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parallel to the magnetic field. The diamagnetic contribution originating from the silicon substrate was subtracted.

3. RESULTS AND DISCUSSION By employing printing we have achieved control over the thickness and morphology of the prepared hybrid films out of maghemite nanoparticles (γ-Fe2O3 NPs) and polystyrene-blockpoly(methyl methacrylate) (PS-b-PMMA) diblock copolymer. Generally, the dry film thickness can be calculated via34 =

where d is the dry layer thickness in cm, c is the solid concentration of the ink in g cm-3, f is the ink flow rate in cm3 min-1, ρ is the density of the dried ink materials in g cm-3, S is the printing speed in cm min-1, and w is the printing width in cm. A polymer concentration of 10 mg/ml is selected, in order to enable a sufficiently high mobility of the NPs in the ink to respond to the external magnetic field. By printing the ink on pre-cleaned Si substrates, hybrid films with a thickness of 1.3 µm were prepared.

3.1 Surface structures Previous investigations41,

42

indicate that the solute concentration is higher at the liquid-air

interface than that near solid/liquid interface due to the fast solvent evaporation during the printing. Thus, under an external magnetic field applied during printing of the hybrid films, an


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excess concentration of NPs will be initially formed at the air-liquid interface, creating ordered mesoscale NP wires on the surface which successively grow deeper into the film. Assemblies of NPs wires with different dimensions are observed for various NPs concentrations as shown in Figure 1. In Figure 1, the light yellow parts represent the polymer matrix, whereas the dark parts show the aggregated NPs assembled into wires which have aligned parallel to the external magnetic field and perpendicular to the movement of the print head (See Figure S1).

Figure 1. Optical microscopy images of the hybrid films at different NP concentrations of (a) 0.05, (b) 0.2, (c) 0.5, (d) 1, (e) 2 and (f) 5 wt%. Samples are prepared under a constant external applied magnetic field (149 G) along the y axis. The analysis is limited to an image size of 30 × 30 µm2, and neighboring distances are only analyzed for the wires which extend along the long-axis beyond this area. The top right number show the root-mean-square roughness values (Rrms, unit: nm) of the wire-free areas for each sample.


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The average width of wires and mean separation distance between two neighboring wires is presented in Figure 2 as function of the NP concentration. These data are extracted from OM images with the help of software Image J. At lower NP concentrations, the NPs assemble into elongated clusters and short narrow wires which exhibit a broad distribution in lengths. No wires elongating over the complete measured film surface are observed (Figure 1a). Short wires are densely packed, but oriented parallel to each other to form characteristic domains of short wire bundles. The applied magnetic field controls the assembly of NP wires. All isolated wires align parallel to the magnetic field. The angular dependence of the dipolar forces between randomly oriented wires prevents aggregation in a random way. In contrast, wires form head-to-tail chains along the magnetic field due to inter-wire interactions.43 Provided the case where two wires are aligned along the magnetic field collinearly, the attractive force between two neighboring short wires can be calculated as follows43 1 1 1 1 = − − − + + + + +

where Qm is the magnetic charge at one end of the wire, L1 and L2 are the lengths of two NP wires respectively, and r is the end-to-end separation in the y axis direction. When increasing the NP loading, the separation distance r becomes smaller, leading to higher attractive force f (r). Therefore short wires become long extending over several hundred micrometers. In addition, wires also grow in width with increasing NP concentration. As the number of NP wires increases, the separation distance between neighboring wires (in the x axis direction) decreases due to the limited sample area. At a NP concentration of 1 wt % the separation distance reaches a minimum. The narrower and shorter wires tend to be located in between two larger wires (Figure 1c and 1d), where the local magnetostatic energy is the lowest. As the NP concentration is increased further, the number of NP wires decreases, while the width starts to increase


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significantly, together with the wire-to-wire separation distance, to ensure the minimum energy of the whole system.43

Figure 2. The average width of NP wires (purple triangles) and mean separation distance between two neighboring NP wires are extracted from OM images and plotted as a function of NP concentration. At 0.05 wt % NP concentration the inter-wire distance is poorly defined (open symbol). Two dashed lines serve as a guide to eyes.

On a more locale scale, the topography of the assembled NPs and the nanostructure of the diblock copolymer are also examined using AFM and SEM (Figure S2). While NP wires are composed of aggregated NPs, (Figure S2c and 2d), homogeneously dispersed NPs are additionally found inside the PS domains of the diblock copolymer film forming a lamellar morphology (Figure S2b, 2e, 2f). In general, the self-assembly of magnetic NPs is complex due to numerous competing effects. Magnetic dipole-dipole interactions,43 electronic polarization interactions,44 the coupling of each particle’s dipole moment to the applied magnetic field,45 thermal kinetics,46 the media in which the NPs are aligned, and the ambient conditions during alignment,47 are of importance among other factors.48 Presently, the authors are not aware of a rigorous theory or models accounting for all these effects and describing the NP self-assembly in block copolymer matrices. In a


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simplified view, the process guiding individual NPs into large NP wires can be divided into three steps: (1) At low NP concentrations, the NP can be accommodated by the diblock copolymer matrix and the driving force from microphase separation dominates the system, leading to the incorporation of functionalized NPs into the PS domains. (2) With the increase of NP concentration, the initially dispersed single NPs aggregate into clusters, which can no longer be incorporated inside the PS domains. (3) The NPs within each clusters have a uniaxial anisotropy, therefore, tend to align lengthwise parallel to the applied magnetic field. Thus, NP wires form in the hybrid films. Additional information concerning NP wires formation can be found in our previous studies.37, 49, 50

3.2 Buried morphology To study the inner nanostructure with a high statistical relevance, all films are investigated with GISAXS at an incident angle above the critical angle of the films. Details of the GISAXS technique are described elsewhere.51, 52 GISAXS measurements are performed in two sample orientations. Either the NP wires are oriented perpendicular or parallel with respect to the X-ray beam. The 2D GISAXS scattering patterns of samples measured in these two directions are shown in Figure S3. The 2D GISAXS scattering patterns change significantly with the incorporation of NPs. At high NP concentrations, the scattering features differ in both measuring directions due to the large NP wires present inside the films. For a quantitative analysis, horizontal line cuts at the Yoneda peak position of the polymer from the 2D GISAXS data are investigated and both orientations are compared.


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Figure 3. Horizontal line cuts (qy) of the 2D GISAXS data (symbols) of the hybrid films containing different NP concentrations (0.2, 0.5, 1, 2, and 5 wt % from bottom to top): To probe the inner structure perpendicular or parallel to the NP wires, the X-ray beam is oriented (a) parallel or (b) perpendicular to the NP wires, respectively. The curves are shown together with the fits (solid lines) and are shifted for clarity along the y axis. Arrows in the profiles serve as a guide to eye.

3.2.1 Lateral structures perpendicular to the NP wires Horizontal line cuts are plotted together with the corresponding fits to the data in Figure 3. With the X-ray beam oriented parallel to the NP wires the inner lateral structure, which is perpendicular to NP wires, can be probed (Figure 3a). Three main features are observed and marked as I, II and III in Figure 3a. In the region of low qy values, the intensity contribution can be modelled with a structure factor of nanostructures created by the diblock copolymer. In the fit, this structure is well modeled with a Lorentzian distribution of sizes and the value of the PS


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interdomain distance (D) can be obtained. The corresponding positions are marked as I in Figure 3a. At lower NP concentrations (≤ 0.5 wt %) D increases smoothly (Figure 4) and is slightly larger than that of the pure diblock polymer, which is 44.8 nm.53 Since the NPs are coated with PS chains, the favorable enthalpic interaction can guide the NPs to be localized inside the PS domains, resulting in the swelling of the PS domains. Such swelling causes the interdomain distance D to increases. At higher NP concentrations (> 0.5 wt %), position I progressively shifts to much lower qy values indicating an increase of D values (Figure 4). Such behavior indicates the appearance of ill-defined structures inside the films, meaning that the long-range ordered diblock copolymer structure is perturbed by the overload with NPs.37 In the high qy value region of the scattering profile, broad shoulder-like peaks (denoted II and III) appear. They resemble the intensity contribution from the form factor of single NPs or NP clusters, respectively. Thus, these scattering features reveal information about how NPs exist in the diblock copolymer matrix at different NP concetrations. In case of low NP concentrations, individual NPs are dispersed well inside the PS domain, but aggregate into clusters with the size of around 25 nm as the concentration increases. Because the position of contribution Ⅲ does not change with NP loading, it can be concluded that the nanosized NP clusters exist also at high NP concentrations (> 1 wt %).


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Figure 4. Interdomain distance (D) obtained from analysis of the GISAXS data plotted as a function of NP concentration. Black up-triangles and blue down-triangles show the interdomain distance of buried structure perpendicular or parallel to the NP wires, respectively. Lines serve as guides to the eye.

3.2.2 Lateral structures parallel to the NP wires Figure 3b shows the horizontal line cuts of the 2D GISAXS data and corresponding fits obtained with the X-ray beam oriented perpendicular to the NP wires. With this measurement geometry, the inner lateral structure parallel to the NP wires can be investigated. Compared to Figure 3a, Figure 3b shows the same scattering characteristics in the high qy region regardless of the NP concentration. Thus, NPs and NP clusters exist well dispersed inside the PS domains with no preferential orientation. However, the diblock copolymer nanostructure is affected by the different orientation of the magnetic field (position I in Figure 3b). The interdomain distance D increases less markedly (Figure 4, blue triangles) with NP concentration. While it increases at lower NP concentrations, it remains almost constant at a high NP loading. The comparison between the GISAXS results obtained with perpendicular measurement orientations shows the influence of the magnetic field on block copolymer nanostructures. At lower NP concentrations,


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single NPs are well dispersed inside the PS domains and the swelling of the PS blocks leads to an increase in domain distance in both directions (parallel and perpendicular). At higher NP concentrations, NPs aggregate into nanoscale clusters, which are still accommodated inside the PS domains. Under the influence of the external magnetic field, these clusters tend to align along the direction of the magnetic field. Thus, the magnetic field causes an orientation of the NPs and induces a shape anisotropy, resulting in a different elongation of the block copolymer nanostructure in both orientations. Initiated during the printing, such anisotropic orientation is further enhanced by the annealing under the external magnetic field. A simplified sketch in Figure S4 illustrates the evolution of film morphology with increasing NP concentration.

3.3 Magnetic properties The magnetization of the hybrid films is measured with a superconducting quantum interference device (SQUID). A static magnetic field is applied either parallel or perpendicular to the direction of the NP wires during the hysteresis measurements. To characterize the magnetic properties of the hybrid films, the influences of temperature, NP concentration and the alignment of applied magnetic field with respect to the NP wires are investigated. In addition, the magnetic susceptibility is studied and compared with results of our previous study to enable a judgement of the influence of the film morphology induced by the diblock copolymer in combination with the applied preparation process.37

3.3.1 Influence of temperature


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According to literature54, 55, γ-Fe2O3 is ferromagnetic in bulk and at a critical size of NPs the bulk magnetic characteristics vanish and a superparamagnetic behavior is found. As shown in Figure 5a, at 200 K no hysteresis is detected and remanence (Mr) and coercivity (Hc) are almost zero. This superparamagnetic behavior is consistent with the nanoscale size of the NPs. The NPs have a very small volume. As a consequence, the magnetization can spontaneously reverse direction driven by thermal fluctuations, which are sufficient to overcome the anisotropy energy barrier. As the temperature is lowered, the magnetization increases and a hysteresis loop appears (Figure 5a). At low temperatures (2-100 K), i.e. below the blocking temperature, the magnetic moments of the nanoparticles remain “frozen” or “blocked” in the direction of the applied external field. Based on the theory by Néel and Brown the blocking temperature is 55 =

∆ ln

where τm is the measurement time, τ0 ≈ 10-10 s is the inverse attempt frequency, is the Bolzmann constant, and ∆ represents the energy barrier the magnetization flip has to overcome by thermal energy. Thus, a net remanence magnetization Mr remains which very slowly decays as compared to the time scale of the measurement. Only with a coercive magnetic field the net magnetization can be reduced to zero,. With increasing temperature the remanence is reduced as seen in Figure 5b.


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Figure 5. Selected sample with 5 wt % nanoparticles: (a) Influence of temperature on the magnetization M measured as a function of the external magnetic field B with a parallel orientation to the NP wires and (b) temperature dependence of the measured remanence.

To address the blocking temperature (TB) of the printed magnetic films, one can focus on the thermal activated processes.56 From the magnetization loops, the coercive fields was extracted with the corresponding temperatures and fitted using the model57

= 0.48

2$ (1 − ) * % &'

where Hc is the coercivity, Ms is the saturation magnetization and b is the exponent as 0.5 in case of oriented nanoparticles. From the fitting (Figure 6a), a blocking temperature of TB = 182 ±12 K


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is obtained, which is consistent with the non-hysteresis magnetization behavior observed at 200 K in Figure 5a. For a better understanding the thermal dependence on the magnetization process, zero field-cooled (ZFC) and field-cooled (FC) protocols were applied. Figure 6b displays the ZFC/FC curves, in which two important features can be observed. In the ZFC curve, a broad peak appears at ~ 200 K, which indicates the blocking temperature. At ~ 230 K ZFC and FC curves diverge, which gives the irreversibility temperature (Tirr) where the magnetization goes from a blocked superparamagnetic state to an isotropic one. Interestingly a crossover is present around the Tirr. This could be due to an antiferromagnetic contribution which is highly ordered in FC (no signal produced) but disordered in ZFC (positive signal produced). Moreover, ZFC/FC allows determining the distribution of TB. According to the literature, the TB distribution function can be expressed by58 (&+ − &,+ * &' = .ln / 0 − 11 3$ where MS is the saturation magnetization, τ0 ≈ 10-9 s, and f (TB) is the blocking temperature distribution function, assuming that both MS and K dependencies with temperature can be neglected. The inset in Figure 6b shows the derivative of the (FC-ZFC) curve. This curve presents a function with a maximum that is considered the maximum value (TB) of the blocking temperature distribution f (TB), which suggests a particle size distribution.


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Figure 6. (a) Coercivity versus temperature fitting for a selected sample with 5 wt % nanoparticles. (b) ZFC, FC and (FC-ZFC) curves for sample with 5 wt % nanoparticles. Inset shows the derivative of the MFC-MZFC curve.

3.3.2 Influence of NP concentration The effect of NP concentration on the magnetic properties of hybrid films is shown in Figure 7a. The magnetization data are measured at T = 2 K with the applied magnetic field parallel to the NP wires. For a quantitative analysis, the saturation magnetization Ms, remanence Mr, and relative remanence Mr / Ms at 2 K are plotted as a function of the NP concentration (see Figure 7b). For comparison, data measured at 20 K and 100 K are also included. Within experimental errors, Mr and Ms increase linearly with increasing NP concentration, and show the similar growing tendency at all temperatures. As a consequence, the relative remanence Mr / Ms remains


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constant irrespective of the NP concentration. More NPs or high temperatures can offer more free flips of magnetic moments or thermal activation energy, respectively. Therefore, the NP concentration dependent magnetization confirms the superparamagnetic behavior of the hybrid films.55

Figure 7. (a) Influence of NP concentration on the magnetization M measured as a function of the external magnetic field B at 2 K. (b) Nanoparticle concentration dependence of the saturation magnetization Ms, remanence Mr, and relative remanence Mr / Ms, at 2 K (black solid line), 20 K (red dashed line), 100 K (blue dashed-dotted line).

3.3.3 Influence of orientation The magnetization curves of the selected sample with 2 wt% NPs are measured at 2 K (below the blocking temperature) with different orientations of the NP wires with respect to the magnetic


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field. The magnetization data show a clear influence of the applied field direction (Figure 8a). An obvious magnetic anisotropy is observed for the measured sample. The remanence and coercivity are higher in the parallel versus perpendicular directions (Figure 8b). This phenomenon can be explained from the anisotropic behavior of individual particle with a uniaxial anisotropy.59-63 When the magnetization of such a particle is measured in a direction which is perpendicular to the easy axis, a hysteresis curve is obtained that exhibits reduced remanence and coercivity compared to those obtained when the field is oriented along the easy axis (i.e., in the parallel direction). The saturation magnetizations show very small differences when the same sample is measured in the two different directions. A potential explanation might be that below a critical temperature, the NPs show a ferromagnetic behavior. Therefore, all magnetic moments orient along the easy axis. When measured with a magnetic field vertical to the easy axis, some frozen moments may not flip along the field direction due to the lack of sufficient thermal energy at low temperatures. Thus, slightly smaller Ms is observed in the vertical orientation measurement.


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Figure 8. (a) Influence of orientation on the magnetization data and (b) temperature dependence of the saturation magnetization, coercivity and remanence measured at 2 K. Perpendicular (blue dashed-dotted line) and parallel (black solid line) external magnetic field orientations are compared. The NP concentration of the selected sample is 2 wt %.

3.3.4 Influence of film morphology The magnetic susceptibility indicates the degree of magnetization of a material in response to an external applied magnetic field, which in turn has implications for practical applications. The calculation of initial magnetic susceptibility χinitial can be obtained from the virgin magnetization curve based on the following equation 64 & → ;

23435367 = lim


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where M is the magnitude of internal magnetization and B is the strength of external magnetic field. For superparamagnetic materials, the internal magnetization is oriented in the same direction as the external magnetic field. Thus, for a given external magnetic field, the stronger the internal magnetization is, the higher the susceptibility will be. Increasing NP concentration and temperature contribute to a higher magnetic susceptibility in the studied hybrid films (Figure 5a and Figure 7a). In order to figure out the influence of the film morphology on the susceptibility of the hybrid system, a comparison is made between the present results on lamellar PS domains containing NPs and findings from our previous work in which spherical PS domains were studied.37 The different morphologies resulted from the use of different block copolymers, with different block ratios, in combination with different film preparation methods (printing versus solution casting). At T > ΤΒ, the magnetization curves show no hysteresis behavior as their corresponding virgin curves. Therefore, the initial magnetic susceptibility χinitial can be extracted. At T < ΤΒ, to make a comparison with previous investigation, the susceptibility (χHc) at coercivity position, where the highest value is achieved, is calculated using a linear fit at the zero-remanence area. The obtained susceptibility (χHc and χinitial) values are plotted as a function of temperature and NP concentration, respectively (see Figure 9). It turns out that in case of lamellar domains the susceptibility shows a more pronounced increase with temperature as compared with the spherical domains (Figure 9a). Increasing NP concentration also has a stronger influence on the susceptibility of lamellar domains. These differences in susceptibility originate from the differences in the structures of the hybrid films when changing the block ratio as well as the film preparation method.


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Figure 9. Comparing hybrid films containing 1 wt % NPs having a lamellar (red triangles) and spherical (blue spheres) domain structure. (a) Temperature dependence of the magnetic susceptibility (χHc) calculated at the zeroremanence position for an external magnetic field parallel to NP wires. (b) NP concentration dependence of the initial magnetic susceptibility (χinitial) at 200 K with the applied magnetic field parallel (upper) or perpendicular (lower) to the NP wires. All lines in graphs are served as a guide to eyes.

In both investigations hybrid samples have been made out of the same type of magnetic NPs and diblock copolymer (PS-b-PMMA), but the weight fraction of the PS block differs. In the previous work it was 0.15 so that inside solution-cast films PS spheres (diameter around 23 nm) were dispersed in the PMMA matrix with large interdomain distances (about 90 nm for pure polymer). In the present study a significantly higher weight fraction of the PS block (0.5) results in shorter interdomain distances (about 45 nm for pure polymer) and a lamellar structure (size


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around 21 nm). According to the GISAXS data, single NPs are dispersed inside the PS domains at low NP concentrations (< 1 wt %). In this case, the amount of single NPs, which is far below the critical accommodation capacity of the PS domains, is equal in both samples. Thus, within experimental errors, the susceptibilities are almost the same and increase slightly at lower NP concentrations for both samples (Figure 9b). With increasing NP concentration, the critical capacity of the PS domains is reached. Above that concentration the NPs aggregate into nanosized clusters and large aggregates are observed in the whole film matrix. For spherical PS domains this critical capacity for embedding NPs is reached at lower concentrations. Thus, lamellar films can incorporate more magnetic NPs resulting in a stronger increase in susceptibility with NP concentration (Figure 9b). Therefore, increasing the PS domain fraction turns out to be an effective way to enhance the magnetic properties of the hybrid films. Moreover, lamellar domains are less sensitive to morphology changes induced by the forces induced by the magnetic NPs The spherical film morphology turned out to transform into less well ordered structures at increasing NP concentration.

4. CONCLUSION By employing printing using the principle of slot die coating, thin magnetic hybrid films are successfully

prepared

based

on

a

lamellar

PS-b-PMMA

diblock

copolymer

and

superparamagnetic γ-Fe2O3 NPs. Combining an external magnetic field with the printing technique enabled the preparation of homogeneous hybrid films with highly oriented mesoscopic structures. Due to the applied external magnetic field, NPs self-assemble into micro scale sized NP wires in the polymer matrices with controlled widths and separation distances. As a


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consequence, the resulting hybrid films show a magnetic anisotropy. At the nanoscale, the NPs coated with PS are selectively incorporated and dispersed inside the PS domains at a low NP loading. As the NP concentration increases, NPs assembled into nanosized cluster without losing their characteristic superparamagnetic property. Due to the high PS weight fraction of the employed diblock copolymer, a high density of magnetic nanostructures containing dispersed single magnetic NPs or nanosized NP clusters inside the PS domains is achieved. Thus, the printed hybrid films exhibit a higher magnetic susceptibility and stronger sensitivity to an external magnetic field as compared to earlier studies using a diblock copolymer with smaller PS block length. Such improved magnetic property can be especially useful in the field of sensor applications.

ASSOCIATED CONTENT Supporting Information. Experimental details, printing setup, SEM and AFM images, 2D GISAXS data and morphology evolution sketch

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by funding from the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS), the Excellence Cluster “Nanosystems Initiative Munich” (NIM) and the Center for NanoScience (CeNS). S. X. acknowledges the China Scholarship Council (CSC). E.M.H. acknowledges support by the Bavarian State Ministry of Education, Science and the Arts via the project “Energy Valley Bavaria”. We are grateful for DLS for granting in-house beamtime during the commissioning of the GISAXS end station. We thank Professor Alexander Holleitner and Peter Weiser for the chance to carry out SEM measurements.

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Table of Contents (TOC).


35

Printed thin magnetic films based on diblock copolymer and magnetic

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