Flexible and self-assembly anisotropic FeCo nanochain-polymer composite films for highly stretchable magnetic device

Accepted Manuscript Flexible and self-assembly anisotropic FeCo nanochain-polymer composite films for highly stretchable magnetic device Jie Yuan, Zhi-Quan Liu PII:

S0266-3538(18)30554-2

DOI:

10.1016/j.compscitech.2018.05.025

Reference:

CSTE 7228

To appear in:

Composites Science and Technology

Received Date: 6 March 2018 Revised Date:

6 May 2018

Accepted Date: 13 May 2018

Please cite this article as: Yuan J, Liu Z-Q, Flexible and self-assembly anisotropic FeCo nanochainpolymer composite films for highly stretchable magnetic device, Composites Science and Technology (2018), doi: 10.1016/j.compscitech.2018.05.025. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Flexible and Self-Assembly Anisotropic FeCo Nanochain-Polymer Composite Films for Highly Stretchable Magnetic Device Jie Yuan,1, 2 Zhi-Quan Liu *1, 2 1

School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

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2

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Abstract Magnetic field induced self-assembly is used to successfully fabricate flexible and

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anisotropic FeCo nanochain-PDMS composite films. The embedded nanochains are controllable in average length (< 200µm) and width (< 2µm) through optimizing the filler

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mass fraction and the magnetic field intensity, and an ultra-high aspect ratio of 150 can be achieved. In-situ observations on the growth of a 600µm long nanochain verify the connection and coarsening mechanism. The composite films are anisotropic along the directions parallel or perpendicular to the alined nanochains or nanobundles. The highest tensile strength in the

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tension direction perpendicular to the nanochains is 3.20MPa for the composite film fabricated at 60mT with 1.0wt% of filler, which is 85.0% higher than that of the pure PDMS films (1.73MPa). Different tensile models are proposed to discuss the fracture mechanism,

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concerning the effectiveness of stress transfer between soft matrix and filler. VSM

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measurements indicate that the nanochains are easier to be magnetized in parallel geometry due to the larger dipolar coupling. The saturated magnetization (Ms) of nanocomposite films with 1.0wt% FeCo nanocube filler is 0.45 emu g-1. Such highly flexible, magnetically responsive anisotropic composite films have potential applications especially for highly stretchable devices. Keywords: flexible composites, nano particles, anisotropy, magnetic properties, mechanical properties *Corresponding author: Zhi-Quan Liu, Professor, Tel: +86-24-83970826, Email: [email protected]

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ACCEPTED MANUSCRIPT 1. Introduction Magnetic materials have aroused researcher’s attention for many years and play a vital role in many fields such as high density data storage[1-3], microwave absorption[4-6], biotechnology[7,8], micro-actuators[9], drug delivery[10,11] and electro-catalytic activity

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[12,13]. Traditional magnetic materials include metals, metallic alloys and oxides. As well known, all these magnetic materials are “hard” materials which can not be used for the flexible electronic-magnetic devices. Compared with the magnetic materials, the polymers

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own some great characteristics including flexibility, machinability and transparency. So, some researchers combine these two kinds of materials to get magnetic polymer composites which

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possess the advantage of polymer and the magnetic properties introduced by the magnetic materials. It’s worth mentioning that the addition of nanoparticles to the polymer composites leads to a change of glass transition temperature, the crystallization rate and the degree of crystallinity, which affect the properties of the composite in an essential way[14]. Using this

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method, the polymer nanocomposites can open innovative possibilities for scientific and industrial applications, such as magnetic storage[1-3] and electro magnetic interference shielding[15-17]. Therefore, it is of great significance to make the fabrication, structure and

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properties of magnetic polymer nanocomposites clear. Magnetic materials, such as soft magnetic or superparamagnetic materials can respond to

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an external magnetic field. This highly tunable behavior makes these materials highly attractive for many applications[18,19]. In the presence of external magnetic field, the soft magnetic materials or superparamagnetic nanoparticles can self-assembly in a variety of anisotropic structures such as nanowires, nanochains and nanostrands. For nanofabrication application with responsive materials, assembly is essential. The magnetic fillers usually are Fe2O3[20], Fe3O4[21], spherical Co[22,23] and cylindrical Co[23], FeNi nanospheres[14] and acicular nickel nanocrystallites[24]. Stephan Barcikowski et. al.[14] demonstrated the

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ACCEPTED MANUSCRIPT preparation of transparent and magnetically anisotropic FeNi nanostrand-PMMA-composites that contain nanostrands with aspect ratios of as high as 160. And the research results revealed that the dimensions of the nanostrands were controlled by the application time of the magnetic field and the concentration of nanoparticles. But there is no discussion about the mechanical

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properties which is vital for the application. Hongyi Yuan et. al.[23] introduced a method of magnetic flow coating to fabricate magnetic polymer nanocomposites. This magnetic flow coating method allows us to continuously create magnetically functionalized polymer films

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by roll to roll. And this method is a kind of the magnetic-field-induced-assembly which is widely used to form nanochains or nanostrands. It indicates that spherical Co nanoparticles

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did not strengthen the polystyrene matrix significantly, regardless of their random distribution or alignment in the polymer matrix. And the results of cylindrical Co nanoparticles with random distribution as well as parallel alignment were similar to those of spherical Co nanoparticles. But the elastic modulus of the films with the alignment of cylindrical Co

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nanoparticles perpendicular to the buckling direction is 10% higher than that of the neat polystyrene. It is clear that the morphology and the alined direction of nanoparticles have effects on the mechanical property of composite films, which is vital for the application of the

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flexible magnetic device. Other researches indicate that suitable tuning of shape and dimensions of fillers allow us to strongly increase magnetoelectric response of the composites

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[25]. So, the filler’s shape is crucial for the composites. In this article, we choose the perfect FeCo nanocube[26] as filler in the Polydimethylsiloxane (PDMS) matrix, which is not reported elsewhere. The flexible, freestanding magnetic nanoparticle composite films consisting of alined nanochains or nanobundles are successfully fabricated using the magnetic-field-induced-assembly method which has reported in the Ref. [14]. In that paper, the novelty of obtaining composites with high aspect ratio nanochains was first demonstrated. Compared with the Ref. [14], the effects

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ACCEPTED MANUSCRIPT of filler mass fraction and external magnetic field intensity on the length and width of the nanostrands are clarified in detail. And concerning the orientation of the embedded nanochains, the magnetic property and tensile strength along different directions were also investigated, which reveals the magnetic and mechanical anisotropy of the composite films.

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2. Experimental Section

Materials: Analytical-grade reagents polydimethylsiloxane (PDMS, sylgard184, Dow corning), ethanol, acetone, isopropanol, (Heptadecafluoro-1,1,2,2-tetradecyl) trimethoxysilane

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and hydrogen nitrate were used in all experiments. All the reagents were used as received without further purification. FeCo nanocube were synthesized as described in our previous

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study[26], of which the mass weighted size distribution(D[4,3]) of the particles is 0.168µm. Preparation of FeCo-PDMS nanocomposite films: Si wafer (100) without copper seed layer or glass sheet was used as substrate, which were cleaned with acetone, ethanol and doubly distilled water for 3 times, respectively. Then the cleaned and dried substrate was

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immersed in the fluorine solutions for 8h to obtain a hydrophobic layer on the surface, and the solutions are made of (Heptadecafluoro-1,1,2,2-tetradecyl) trimethoxysilane (0.5ml), isopropanol (50ml) and hydrogen nitrate. The PH of the fluorine hydrophobic solution is

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about 2 which was adjusted by the hydrogen nitrate. Next, the substrate should be washed by

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ethanol for 5 times to remove the sticky materials on the surface of the substrate. Then the hydrophobic substrate was dried at 80°C for 5-10min. Figure 1 shows the schematic diagram for preparation of the magnetic FeCo composite films, the presentation of prepared films and its flexible. Initially, mix the PDMS with FeCo nanocube homogeneously by using a ultrasonic cleaner. Next, the homogeneous mixtures are dropped on the hydrophobic substrate (Si wafer or glass sheet) as shown in Figure 1(a). Spin-coating (Figure 1(b)) is the next step regulated by the pre-defined program. And the thickness of polymer films is regulated by the spin-coating speed and the viscosity of the polymer. For example, the thickness of the film for

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ACCEPTED MANUSCRIPT morphology observation is about 7µm with the maximum 5000rpm for 120s using the spin coater (Spin master 51, Chemat). Then put the substrate into the external magnetic field which is created by NdFeB rare earth permanent magnet bar (Figure 1(c)). Moreover, the specific field intensity was measured by Gauss Meter (ShangHaiHengTong, HT20). And the magnetic

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field intensity was controlled by the distance of the magnet bar and the number of the magnet bar at each side. After a while, the polymer nanocomposite films with alined nanochains or nanobundles will be obtained due to self-assembly. Then the polymer nanocomposite films

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were solidified in the drying oven at 100°C for 2h. While the substrate cooled down to the

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room temperature, it is easy to demould the film from the hydrophobic substrate.

Figure 1. Schematic diagram for preparation of the magnetic FeCo-PDMS composite films. (a)mixture deposition, (b) spin coating, (c) magnetic field induced self-assembly, (d) film formation, (e) as-prepared film and (f) its flexibility. Material Characterization: The mass weighted size distribution of the used FeCo particles was measured by Malvern Mastersizer 2000 particle size analyzer. The morphology of the polymer composite films and tensile crack surface were analyzed by Laser Scanning Digital -5-

ACCEPTED MANUSCRIPT Microscope (Olympus LEXT OLS4100). Films for the morphology observation is 20mm in length, 20mm in width and 7µm in thickness. And films containing alined nanochains or nanobundles on Si substrate were cut together into 3×3 mm pieces for TEM samples. Thin plane samples for TEM observation were prepared by precision ion polishing system with

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liquid nitrogen cooling (Gatan, Model 691). The microcosmic morphology of the nanochains or nanobundles was observed by TEM (JEOL, JEM-2100). The self-assembly process of the nanostrands was observed by the body type optical microscope (Olympus BX51). The

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viscosity of the pure PDMS and the polymer mixture with different mass fraction of FeCo filler were measured by rotational viscometer(NDJ-5S). The No. 4 rotor is chose for the

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measurement and the rotate speed is 60r/min. In order to obtain the average value, 10 viscosity values of each sample were recorded by every 15 seconds. The magnetic properties were investigated through a vibration sample magnetometer (Lakeshore VSM 7407). For hysteresis loop measurement, samples were prepared by cutting the films into a small block of

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which the length and width are 3×3mm. The stretch strength of pure PDMS and FeCo-PDMS films was measured with a high-precision mechanical testing system (Hounsfield H5k-S materials tester). The samples for tensile testing is 20mm in length, 10mm in width and

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1.3±0.2mm in thickness. The sample size was measured using a standard caliper.

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ACCEPTED MANUSCRIPT 3. Results and Discussion

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3.1 Morphology of Composite Films with Different Mass Fraction of Magnetic Fillers

Figure 2. The optical morphologies of the homogeneous nanoparticle composite films without external magnetic field at the mass fraction of (a) 0.05%, (b) 1.0% and (c) 5.0%, as well as those of alined nanochain composite films fabricated under the magnetic field intensity of 45mT at the mass fraction of (d) 0.05%, (e) 1.0% and (f) 5.0%, together with the

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corresponding bright field TEM images of the alined nanochains at the mass fraction of (g) 0.05%, (h) 1.0% and (i) 5.0%.

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The polymer nanocomposite films are prepared by spinning the mixture consisting of PDMS and FeCo cubic nanoparticles as shown in Figure 1. The as-prepared film and its

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excellent flexibility are shown in Figure 1(e) and Figure 1(f) respectively. In this study, the magnetic-field-induced-assembly method can be applied to the thin nanocomposite films using a weak external magnetic field below 50mT. This method is more effective and convenient to obtain nanocomposite films with alined nanochains or nanobundles. Figure 2 shows the morphology of the polymer films with three different mass fractions of FeCo filler. Without external magnetic field, the optical morphology of the composite films containing 0.05 wt%, 1.0 wt% and 5.0 wt% magnetic fillers are shown in Figure 2(a)-(c), respectively. It is obvious that the distribution density of the filler in composite film increase with more -7-

ACCEPTED MANUSCRIPT fillers. When the external magnetic field of 45mT was applied, the alined nanostrands in the composite films with 0.05 wt%, 1.0 wt% and 5.0 wt% filler can be observed as shown in Figure 2(d)-(f), respectively. While the mass fraction is 0.05%, the length and width of nanostrands are small. As the mass fraction of the FeCo cube nanoparticles increase, larger

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nanostrands can be observed as shown in Figure 2(e) and 2(f). Figure 2(g)-(i) show the bright field image of the single nanostrand with filler mass fractions of 0.05%, 1.0% and 5.0% respectively. Compared with the narrow nanostrand in the film of 0.05 % mass fraction

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(Figure 2g), wide nanostrand consisting of many nanocubes can be obtained while the mass fraction increases to 5%, whose width is larger than 1µm as shown in Figure 2(i). This

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phenomenon results from the large amount of nanocubes in the film, which contributes to the assembly of the nanochains. It is clear that the mass fraction of the filler dramatically affects the morphology of the nanochains or nanobundles under the specific external magnetic field[27,28]. And it is worth mentioning that the mean viscosity of the pure PDMS is 2700

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mPa.s which was measured by rotational viscometer. The average viscosity of the polymer with 0.05 wt%, 1.0 wt% and 5.0 wt% magnetic FeCo fillers are 2590mPa.s, 2682mPa.s and 2672mPa.s, respectively, which is similar to the pure one. So, the difference of the viscosity

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which might affect the morphology of the nanochains or nanostrands can be ignored.

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3.2 Control of Nanochain’s Length and Width by Different Magnetic Field Intensity

Figure 3. The optical morphologies of self-assembly nanochains at (a) low and (b) high

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magnifications, and the corresponding bright field image of (c) a big nanobundles and (d) a single alined nanochain, together with (e) an ideal layout model for single nanochain with ferromagnetic FeCo nanocubes.(magnetic field intensity - 140mT, mass fraction – 1.0wt%).

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Besides the mass fraction of the magnetic filler, external magnetic field is another important factor which affect the distribution of the self-assembly nanostructures. Taking the

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film containing 1.0 wt% magnetic filler for example, without external magnetic field the FeCo nanoparticles disperse uniformly in the film as shown in Figure 2(b), which is different from the magnetic orientated one in Figure 2(e). Figure 3 shows the characteristic morphology of the self-assembly nanostrands, which are fabricated with 1.0 wt% FeCo nanoparticles filler at an external magnetic field intensity of 140mT. The low and high magnification optical microscopy images of the film are shown in Figure 3(a) and 3(b), respectively. Due to the increase of the external magnetic field intensity, the length and width of the nanostrands in Figure 3(b) are larger than the alined nanostrands at 45mT in Figure 2(e) -9-

ACCEPTED MANUSCRIPT and 2(h). The bright field image of the nanobundles confirms the effect of the magnetic field as shown in Figure 3(c). It is no doubt that the nanobundles are broader than the one in Figure 2(h) where the magnetic field is 45mT. Besides the nanobundles, single nanochain is also observed as shown in Figure 3(d). It is interesting to find that FeCo nanocubes connect each

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other by face to face to form a single nanochain. But the nanochains with nanocube are not perfectly straight or parallel to the magnetic field in the polymer substrate. The ideal layout model for single nanochain with the ferromagnetic FeCo nanocube is shown in Figure 3(e).

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All the nanocubes are along the direction of easy magnetization [100] which is perpendicular to the surface facets. The discrepancy between practical morphology and ideal layout model

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interaction with other nanochains.

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may cause by the viscosity of the polymer, the shrinkage of solidification process and the

Figure 4. In-situ growth of nanochains in composite films after applying magnetic field of 140mT for (a)90s, (b)120s, (c)180s, (d)240s, (e)840s, (f)960s and (g)1140s. Arrows indicate the docking positions of initially separated chains, which form a single chain later by connection-coarsening mechanism.(mass fraction – 1.0wt%). In order to understand the formation of the alined nanochains in the polymer nanocomposite films, the in-situ observation of the process of self-assembly nanochains under the external magnetic field is investigated. Figure 4(a)-4(g) show the optical morphology - 10 -

ACCEPTED MANUSCRIPT images of the films after applying magnetic field (140mT) for 90s, 120s, 180s, 240s, 840s, 960s and 1140s, respectively. It is clear that many smallish nanochains are formed within 90s due to the self-assembly process as shown in Figure 4(a). Arrows (A and B) indicate the docking positions of the two initially separated chains, which will form a single chain later.

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As time prolongs, the nanochain at position A gradually approaches the apex of another two nanochains. At 960s, the left apex of the nanochain (position A) connects to another nanochain as shown in Figure 4(f). Then the totally connection with other nanochains have

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been observed at 1140s (Figure 4(g)). This self-assembly mechanism is named as connection type, which can contribute to the length extension. There is another mechanism of coarsening

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type as shown at position B. The nanochain attaches to another chain completely as shown in Figure 4(a) and (d). So, this coarsening mechanism can enlarge the width of the nanochains effectively. The formation mechanism of such nanochains are also reported elsewhere, which is simply described as connection and coarsening mechanism[29]. In essence, the nanochain

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formation is considered being resulting from the composition of the magnetic attraction, the

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surface energy and the entropic contribution of small nanowires[30].

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Figure 5. The effect of magnetic field intensity on (a) the mean length and width as well as (b)

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the aspect ratio of nanochains or nanobundles. (mass fraction – 1.0wt%) For the purpose of controllable fabrication, the effect of magnetic field intensity on the length and width of nanochains is investigated in detail. The length of the nanochains was

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calculated by the optical microscopy. Due to the resolution limits of the optical microscope, TEM was used to count the width of the alined nanostructure. In order to get an accurate

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value, as many of nanochains or nanobundles as possible were calculated (more than 50 nanochains). Figure 5 shows the evolution of the length, width and aspect ratio of the nanochains as a function of magnetic field intensity. The mean length reaches to the maximum of 180µm with a filler factor of 1.0 wt% in an external magnetic field (140mT). Before the saturation point of the length is reached, an approximate linear increase is observed with the increase of the external magnetic field intensity as shown in Figure 5(a). Then the mean length of the self-assembly nanostructures decreases and levels off while the magnetic fields vary from 160mT to 200mT. The change of the width of the nanochains is similar to the - 12 -

ACCEPTED MANUSCRIPT length which is shown in Figure 5(a). But the maximum value of width slightly fall behind the saturation point of the length. And then the width of the nanochains also varies little with the increase of the magnetic field. Figure 5(b) shows the aspect ratio of nanochains or nanostrands at different external magnetic field intensity. According to the data distribution,

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the effect of magnetic field can be divided into three zones: low magnetic field region (Ι), medium magnetic field region (ΙΙ) and high magnetic field region (ΙΙΙ). At low magnetic field region, the length and width increase synchronously, because the magnetic field is so weak

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that no direction of nanochain can grow abnormally. With the increase of the magnetic field, all the directions grew quickly, especially the length of the nanochains increases faster than

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the width at the medium magnetic field region (ΙΙ) as shown in Figure 5(b), which can also be confirmed by Figure 5(a). The aspect ratio of nanochain at region(ΙΙ) is about 130, and the maximum aspect ratio is close to 150. In this region (II), the proper magnetic field leads to the quick growth of the length. That is to say, the connection mechanism dominates the growth of

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nanochain at this situation. At the high magnetic field region (ΙΙΙ), the length of the alined nanochains decreased obviously at 160mT, because the linear chains will tend to aggregate in

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order to reduce the total energy which have been reported by computational results[31]. And the high magnetic field is vital, which contributes to overcome the potential energy barrier of

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long chains. In this way, one can obtain nanobundles with laterally aggregated chains to get a lower energy by a higher external magnetic field(160mT). So the width of the nanochains will increase, while the length will decrease. While the magnetic field ranges from 180mT to 200mT, the aspect ratio tends to be stable owing to the competitive growth of the length and width at high magnetic field. According to the above investigations, the mass ratio of filler and the external magnetic field intensity have obvious effects on the growth of nanochains. Without the magnetic field as shown in Figure 2(a)-(c), particles randomly aggregate as a result of Brownian motion and

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ACCEPTED MANUSCRIPT dipolar interaction between the particles. Once the magnetic field was applied, these particles can orient along the magnetic field direction to form nanochains. With the increase of the mass fraction of the magnetic fillers, the morphology of the self-assembly nanoparticles changed from the arrayed nanostructure (Figure 2(d), 0.05wt%), nanochains (Figure 2(e),

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1.0wt%) to nanobundles with laterally aggregated chains (Figure 2(f), 5.0wt%) at the external magnetic field of 45 mT. The growth of nanochain follows the connection-coarsening mechanism as shown in Figure 4. Because of the lower potential energy of a particle located

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at the end or tip of the chains[14], the length of nanochains grew with the connection mechanism. When there is a higher external field intensity, the potential energy barrier of

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long chains can be overcome, so the coarsening took place obviously.

3.3 Tensile Strength and Fracture Mechanism of the FeCo-PDMS Nanocomposite Films Considering the application potential of this polymer film, it is vital to study the mechanical properties of this anisotropic FeCo-PDMS films. Figure 6 shows the typical

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stress-strain curves and tensile strength of the pure PDMS and FeCo-PDMS composite films with 1.0 wt% filler loading. It is obvious that the filler of FeCo nanocubes improves the tensile strength of the PDMS matrix at all conditions regardless of the application of magnetic

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field. However, for the anisotropic film fabricated under magnetic field, the tensile strength of the film at a tensile direction perpendicular to the alined nanochains is higher than that at a

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tensile direction parallel to the alined nanochains. And the tensile strength of films with dispersive FeCo cube nanoparticles (without magnetic field) is also higher than that of anisotropic film when tensile direction is horizontal to the nanochain. As shown in Figure 6(b), the tensile strength of the pure PDMS is only 1.73MPa, and the tensile strength of the sample with dispersive FeCo nanocube is 2.43MPa. The tensile strength along the alined or assembly direction (2.20MPa) is 27.2% higher than that of pure PDMS (1.73MPa), while the tensile strength perpendicular to the assembly direction (3.20MPa) is even 85.0% higher than

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ACCEPTED MANUSCRIPT that of pure PDMS (1.73MPa). The tensile strength of the pure PDMS and FeCo-PDMS composite films with 0.05 wt% or 5.0wt% filler loading have been shown in Figure 7(a) and 7(b). It is clear that the enhanced trend of the tensile strength is similar to the composite films with 1.0wt% filler loading. And the stress-strain curves of the composite films with 0.05wt%

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or 5wt% FeCo filler are displayed at the Supporting Information(SI-1).

Figure 6. (a) Typical stress-strain curves of pure PDMS and FeCo-PDMS composite films with 1.0wt% FeCo ferromagnetic nanoparticles loading. (b) The tensile strength of pure

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PDMS and FeCo-PDMS composite films with or without external magnetic field. (magnetic

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field intensity - 60mT, mass fraction – 1.0wt%).

Figure 7. The tensile strength of pure PDMS and FeCo-PDMS composite films with or without external magnetic field. (magnetic field intensity - 60mT, mass fraction – (a) 0.05wt%, (b) 5.0wt%). Figure 8 shows the cross-section morphology of tensile fracture and the illustration of crack propagation model (planar graph, the observation location is shown in the inset) for four - 15 -

ACCEPTED MANUSCRIPT kinds of composite films(FeCo-1.0wt%). The stretch fracture morphology of the pure PDMS is shown in Figure 8(a), in which the surface of the fracture is clean and only fracture pattern can be observed. As for the pure PDMS films, crack will firstly appear at uneven place after the tensile force increases to some extent. Then the cracks propagate up to the fracture of

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films. The schematics of crack formation in pure PDMS are shown in Figure 8(b). And the white arrow at the bottom-left indicates the tensile direction. The cross-section morphology of the composite films (1.0 wt%) without magnetic field is shown in Figure 8(c). It is easy to

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find that there are some particle clusters on the surface of the fracture. Some typical clusters are marked by the black circle. It is obvious that the composite films without external

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magnetic field will have some glomerate nanoparticles, although the mixture is carefully made with the polymer and fillers. Besides the aggregated filler particles, such inhomogeneities also include holes around aggregated particles which can not be completely wetted by the matrix[32]. All of these inhomogeneities can cause the stress concentration,

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which favors rapid crack propagation. So, the crack will generate around the nanoparticle cluster. The fracture model of the film fabricated without magnetic field is shown in Figure 8(d), in which the crack is apt to generate and extend around the nanoparticles cluster. The

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fracture morphology of the nanochain film at a tensile direction parallel to the chain axis is shown in Figure 8(e). Some puny pits marked with A1, A2 and A3 can be observed at the

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surface of the fracture, which should be formed by the breakage of the nanochains. And the nanoparticles which originally are a part of the nanochains are located in the pits. The pits are surrounded by the flexible polymer. It is easy to deduce that the crack will produce at the middle part or the tip of the nanochains in this film, because of the inconsistent deformation between the fillers and polymer matrix. What’s more, for the nanochain which is horizontal to the tensile direction, the nanocubes are poorly boned. So, the fracture will occur as shown in Figure 8(f). As for the films in which the alined nanochains are vertical to the tensile direction,

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ACCEPTED MANUSCRIPT some exposed nanochain can be observed at the surface of the cross-section fracture as indicated by the black arrows of B1, B2 and B3 in Figure 8(g). It is obvious that the B1, B2 and B3 distribute in a straight line which might be a part of one nanochain. Thus, the cracks will prolong along the major axis of the nanochains as shown in Figure 8(h). And this

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situation also results from the inconsistent deformation of the FeCo nanochains and the

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polymer matrix.

Figure 8. The cross-section fracture morphology (left column) and crack propagation model (right column) for (a) and (b) pure PDMS films (the inset shows the two observation location(cross-section and planar graph) of the fracture diagram), (c) and (d) FeCo-PDMS films without external magnetic field, (e) and (f) FeCo-PDMS films in which the alined

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ACCEPTED MANUSCRIPT nanochains are parallel to the tensile direction, (g) and (h) FeCo-PDMS films in which the alined nanochains are vertical to the tensile direction. White arrows indicate the tensile direction. (magnetic field intensity - 60mT, mass fraction – 1.0wt%). According to the experimental observations and deduced models in Figure 8, the fracture

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mechanism of four different films can be discussed. As for nano-particulate composites, the strength mainly relies on the effectiveness of the stress transfer between matrix and fillers, despite of the particle size and the particle loading[27,33]. Previous studies indicate that

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addition of particles leads to an increase in strength, and smaller particles give better reinforcement[27,32,33]. Because smaller particles have a higher total surface area for a given

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particle loading, this contributes to a more efficient stress transfer mechanism[34,35]. So, the tensile strength of all the films with magnetic nanocube filler is higher than the pure PDMS, owing to the effect of the FeCo nanoparticles. The tensile strength of the composite films without external magnetic field is slightly higher than the alined nanochains at a parallel

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tensile direction as shown in Figure 6(b). The reason might be that the alined nanochains weaken the potentiation between the filler and matrix, because there is large area of blank zones among the nanochains. However, the potentiation of the composite film with disperse

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nanoparticles is holistic. What’s more, The stress can not be alleviated with deformation in the width of the nanochains, so it is easy to generate micro-cracks in the joints where the hard

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FeCo nanoparticles are not close. The orientation between alined nanochains and the tensile direction also has a profound effect on the film strength. While the tensile direction is vertical to alined nanochains, the maximum tensile strength of film will be obtained because of the effectiveness of stress transfer between matrix and filler. The crack will prolong along the major axis of the nanochains. In this situation, the interfacial adhesion between particle and matrix significantly affects the strength of particulate composites. The stress can be alleviated through the deformation of the soft matrix, so it is not easy to generate micro-cracks.

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composite films in flexible devices. And the fracture morphology of the composite films with 0.05wt% or 5.0wt% magnetic filler was also observed as shown in the Supporting Information(SI-2). The fracture way is similar to the composite film with 1.0wt% FeCo filler.

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So, it is no doubt that these films also conform to the crack propagation model which discussed in the above.

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3.4 Magnetic Properties of the FeCo-PDMS Nanocomposite Films

Figure 9. (a) The sketch of FeCo-PDMS films with alined nanochains or nanobundles, the two red frame are two pieces with different nanochains orientation. (b) and (c) are the rotation of the ferromagnetic polymer nanocomposite thin films immersed in water under external magnetic field. As the magnetic FeCo nanoparticles were used as filler, the fabricated nanochain composite film has anisotropic magnetic properties. Using the solidified composite film, its response to an external magnetic field is shown in Figure 9. Two pieces of film were cut from - 19 -

ACCEPTED MANUSCRIPT the FeCo-PDMS film consisting of horizontally aligned nanochains as indicated by the red squares in Figure 9(a). One piece’s length is parallel to the embedded nanochains, while the other is perpendicular to the axis of the nanochain. Both films were immersed in water to float-up at the water-air interface in a glass petri dish. Figure 9(b) and 9(c) show the response

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of these two differently-orientated pieces to the external magnetic field. The initial statuses (0s) are shown in Figure 9(b) and Figure 9(c) respectively, and the earth permanent bar is put at the left side of the glass petri dish. Independent on the shape of the composite pieces, both

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pieces finally stopped at the position where the nanochain in the film is parallel to the external magnetic field. The response of the films to rotation and alignment with the external magnetic

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field was completed in 5s or 8s, a relatively short amount of time.

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Figure 10. (a) The room temperature hysteresis loop measured at two perpendicular directions of the films without external magnetic field. (b) the room temperature hysteresis

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loop of parallel and perpendicular directions to the nanochains(magnetic field intensity 60mT, mass fraction – 1.0wt%). Inset is the enlarge view of the hysteresis loop.

Figure 10 shows the in-plane, room temperature hysteresis loop for the polymer composite films. Without magnetic field during fabrication, the nanocomposite films with 1.0 wt% FeCo nanoparticle filler have similar magnetic properties along A direction and B direction as shown in Figure 10(a). And the A and B are the two perpendicular directions which are corresponding to the length and width of small block test specimen. Due to the very

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ACCEPTED MANUSCRIPT low mass fraction of magnetic filler in the film, the saturated magnetization (Ms) of the composite films is only 0.45 emu g-1, although the Ms of the FeCo nanocube filler is as high as 220 emu g-1[26]. However, the films with alined nanochains are anisotropic as shown in Figure 10(b). It is clear that the films with nanochains have different magnetization behavior

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for parallel(//) and perpendicular(⊥) geometries to the nanochain, which proves that the nanochains are easier to be magnetized in parallel geometry due to the larger dipolar coupling. And the inset shows the details about the hysteresis loop as shown in Figure 10(a) and 10(b).

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After the composite films with 1.0wt% filler were stretched, the magnetic properties are stay the same and magnetic anisotropy is similar with the original one, which are shown in the

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Supporting Information(SI-3). The magnetic properties of the original or stretched composite films with 0.05wt% and 5.0wt% filler are also displayed in the Supporting Information SI-4 and SI-5, respectively. It is clear that the magnetic properties of composite films are steady which is important for the application.

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4. Conclusion

In summary, flexible and anisotropic FeCo nanochain-polymer composite films were successfully fabricated by magnetic-field-induced-assembly, and its mechanical and magnetic

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properties were investigated in detail. The filler of FeCo nanocubes alined along the easy

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magnetization direction of [100] to form nanochain in the PDMS matrix. The morphologies and dimensions of the nanochains can be controlled by the nanoparticle filler loading (0.05∼5.0%) and the external magnetic field intensity (15∼220mT). With the increase of the external magnetic field intensity, both the length (50∼180µm) and width (0.6∼1.3µm) of the nanochains have an approximate linear increase firstly, and then level off for saturation. The highest aspect ratio of 150 can be achieved at filler mass fraction of 1.0wt% under magnetic field of 140mT. In-situ observations verified the connection-coarsening growth mechanism of the nanochains or nanobundles.

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ACCEPTED MANUSCRIPT Along the directions parallel to or perpendicular to the nanochain axis, the FeCo composite films are anisotropic. The tensile strength along the direction of the nanochains (2.20MPa) is 27.2% higher than that of the pure PDMS (1.73MPa), while that perpendicular to the nanochains is the highest (3.20MPa) when the mass fraction of the filler is 1.0wt%. Different

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tensile models are proposed to discuss the fracture mechanisms of different composite films. When the tensile direction is vertical to the nanochain, the stress can be successfully alleviated through the deformation of the soft matrix between nanochains, which enables the

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highest tensile strength of the films. According to the magnetic measurements, the nanochains are easier to be magnetized in parallel geometry due to the larger dipolar coupling. By

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controlling fabrication, one can tune the mechanical performance and magnetic anisotropy of the thin polymer nanocomposite films synchronously, which enhances their potential applications especially for highly stretchable devices. Acknowledgement

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This work was partially supported by the National Key R&D Program of China (Grant No. 2017YFB0305501) and the Hundred Talents Program of the Chinese Academy of

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Sciences.

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ACCEPTED MANUSCRIPT Supporting Information Flexible and Self-Assembly Anisotropic FeCo Nanochain-Polymer Composite Films for Highly Stretchable Magnetic Device Jie Yuan,1, 2 Zhi-Quan Liu *1, 2

2

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

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1

School of Materials Science and Engineering, University of Science and Technology of

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China, Shenyang 110016, China

SI-1 Typical stress-strain curves of pure PDMS and FeCo-PDMS composite films with (a)

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0.05wt% and (b) 5.0wt% FeCo ferromagnetic nanoparticles loading.

SI-2 The cross-section fracture morphology for (a) FeCo-PDMS films(FeCo-0.05wt%) without external magnetic field, (b) FeCo-PDMS films(FeCo-0.05wt%) in which the alined nanochains are parallel to the tensile direction, (c) FeCo-PDMS films(FeCo-0.05wt%) in

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ACCEPTED MANUSCRIPT which the alined nanochains are vertical to the tensile direction. The cross-section fracture morphology for (d) FeCo-PDMS films(FeCo-5.0wt%) without external magnetic field, (e) FeCo-PDMS films(FeCo-5.0wt%) in which the alined nanochains are parallel to the tensile direction, (f) FeCo-PDMS films(FeCo-5.0wt%) in which the alined nanochains are vertical to the tensile direction. And the A (SI-2(c)) and B (SI-2(f)) point to the position of the

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nanochains.

SI-3 (a) The room temperature hysteresis loop measured at two perpendicular directions of

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the stretched films without external magnetic field(And the A and B are the two perpendicular directions which are corresponding to the length and width of small block test specimen.). (b) the room temperature hysteresis loop (after stretched) of parallel and perpendicular directions to the nanochains (magnetic field intensity - 60mT, mass fraction – 1.0wt%). Inset is the

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enlarge view of the hysteresis loop.

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SI-4 The room temperature hysteresis loop measured at two perpendicular directions of the (a)

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original films or (c) stretched films without external magnetic field(And the A and B are the two perpendicular directions which are corresponding to the length and width of small block test specimen.); the room temperature hysteresis loop of parallel and perpendicular directions to the nanochains((b)-original films, (d)-stretched films). (magnetic field intensity - 60mT,

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mass fraction – 0.05wt%). Inset is the enlarge view of the hysteresis loop.

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SI-5 The room temperature hysteresis loop measured at two perpendicular directions of the (a)

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original films or (c) stretched films without external magnetic field(And the A and B are the two perpendicular directions which are corresponding to the length and width of small block test specimen.); the room temperature hysteresis loop of parallel and perpendicular directions to the nanochains((b)-original films, (d)-stretched films). (magnetic field intensity - 60mT,

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mass fraction –5.0wt%). Inset is the enlarge view of the hysteresis loop.

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