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Composites Science and Technology

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

Jie Yuana,b, Zhi-Quan Liua,b,∗

a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, Chinab School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

A R T I C L E I N F O

Keywords:Flexible compositesNano particlesAnisotropyMagnetic propertiesMechanical properties

A B S T R A C T

Magnetic field induced self-assembly is used to successfully fabricate flexible and anisotropic FeCo nanochain-PDMS composite films. The embedded nanochains are controllable in average length (< 200 μm) and width(< 2 μm) through optimizing the filler mass fraction and the magnetic field intensity, and an ultra-high aspectratio of 150 can be achieved. In-situ observations on the growth of a 600 μm long nanochain verify the con-nection and coarsening mechanism. The composite films are anisotropic along the directions parallel or per-pendicular to the alined nanochains or nanobundles. The highest tensile strength in the tension direction per-pendicular to the nanochains is 3.20MPa for the composite film fabricated at 60mT with 1.0 wt% of filler, whichis 85.0% higher than that of the pure PDMS films (1.73MPa). Different tensile models are proposed to discuss thefracture mechanism, concerning the effectiveness of stress transfer between soft matrix and filler. VSM mea-surements indicate that the nanochains are easier to be magnetized in parallel geometry due to the larger dipolarcoupling. The saturated magnetization (Ms) of nanocomposite films with 1.0 wt% FeCo nanocube filler is 0.45emu g−1. Such highly flexible, magnetically responsive anisotropic composite films have potential applicationsespecially for highly stretchable devices.

1. Introduction

Magnetic materials have aroused researcher's attention for manyyears and play a vital role in many fields such as high density datastorage [1–3], microwave absorption [4–6], biotechnology [7,8],micro-actuators [9], drug delivery [10,11] and electro-catalytic activity[12,13]. Traditional magnetic materials include metals, metallic alloysand oxides. As well known, all these magnetic materials are “hard”materials which can not be used for the flexible electronic-magneticdevices. Compared with the magnetic materials, the polymers ownsome great characteristics including flexibility, machinability andtransparency. So, some researchers combine these two kinds of mate-rials to get magnetic polymer composites which possess the advantageof polymer and the magnetic properties introduced by the magneticmaterials. It's worth mentioning that the addition of nanoparticles tothe polymer composites leads to a change of glass transition tempera-ture, the crystallization rate and the degree of crystallinity, which affectthe properties of the composite in an essential way [14]. Using thismethod, the polymer nanocomposites can open innovative possibilitiesfor 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 properties ofmagnetic polymer nanocomposites clear.

Magnetic materials, such as soft magnetic or superparamagneticmaterials can respond to an external magnetic field. This highly tunablebehavior makes these materials highly attractive for many applications[18,19]. In the presence of external magnetic field, the soft magneticmaterials or superparamagnetic nanoparticles can self-assembly in avariety of anisotropic structures such as nanowires, nanochains andnanostrands. For nanofabrication application with responsive mate-rials, assembly is essential. The magnetic fillers usually are Fe2O3 [20],Fe3O4 [21], spherical Co [22,23] and cylindrical Co [23], FeNi nano-spheres [14] and acicular nickel nanocrystallites [24]. Stephan Barci-kowski et al. [14] demonstrated the preparation of transparent andmagnetically anisotropic FeNi nanostrand-PMMA-composites that con-tain nanostrands with aspect ratios of as high as 160. And the researchresults revealed that the dimensions of the nanostrands were controlledby the application time of the magnetic field and the concentration ofnanoparticles. But there is no discussion about the mechanical prop-erties which is vital for the application. Hongyi Yuan et al. [23] in-troduced a method of magnetic flow coating to fabricate magneticpolymer nanocomposites. This magnetic flow coating method allows us

https://doi.org/10.1016/j.compscitech.2018.05.025Received 6 March 2018; Received in revised form 6 May 2018; Accepted 13 May 2018

∗ Corresponding author. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.E-mail addresses: zqliu@imr.ac.cn, zhiquanliu@yahoo.com (Z.-Q. Liu).

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to continuously create magnetically functionalized polymer films byroll to roll. And this method is a kind of the magnetic-field-induced-assembly which is widely used to form nanochains or nanostrands. Itindicates that spherical Co nanoparticles did not strengthen the poly-styrene matrix significantly, regardless of their random distribution oralignment in the polymer matrix. And the results of cylindrical Co na-noparticles with random distribution as well as parallel alignment weresimilar to those of spherical Co nanoparticles. But the elastic modulus ofthe films with the alignment of cylindrical Co nanoparticles perpendi-cular to the buckling direction is 10% higher than that of the neatpolystyrene. It is clear that the morphology and the alined direction ofnanoparticles have effects on the mechanical property of compositefilms, which is vital for the application of the flexible magnetic device.Other researches indicate that suitable tuning of shape and dimensionsof fillers allow us to strongly increase magnetoelectric response of thecomposites [25]. So, the filler's shape is crucial for the composites.

In this article, we choose the perfect FeCo nanocube [26] as filler inthe Polydimethylsiloxane (PDMS) matrix, which is not reported else-where. The flexible, freestanding magnetic nanoparticle compositefilms consisting of alined nanochains or nanobundles are successfullyfabricated using the magnetic-field-induced-assembly method whichhas reported in Ref. [14]. In that paper, the novelty of obtainingcomposites with high aspect ratio nanochains was first demonstrated.Compared with Ref. [14], the effects of filler mass fraction and externalmagnetic field intensity on the length and width of the nanostrands areclarified in detail. And concerning the orientation of the embeddednanochains, the magnetic property and tensile strength along differentdirections were also investigated, which reveals the magnetic and me-chanical anisotropy of the composite films.

2. Experimental section

Materials: Analytical-grade reagents polydimethylsiloxane (PDMS,sylgard184, Dow corning), ethanol, acetone, isopropanol,(Heptadecafluoro-1,1,2,2-tetradecyl) trimethoxysilane and hydrogennitrate were used in all experiments. All the reagents were used as re-ceived without further purification. FeCo nanocube were synthesized asdescribed in our previous study [26], of which the mass weighted sizedistribution(D [4,3]) of the particles is 0.168 μm.

Preparation of FeCo-PDMS nanocomposite films: Si wafer (100)without copper seed layer or glass sheet was used as substrate, whichwere cleaned with acetone, ethanol and doubly distilled water for 3times, respectively. Then the cleaned and dried substrate was immersedin the fluorine solutions for 8 h to obtain a hydrophobic layer on thesurface, and the solutions are made of (Heptadecafluoro-1,1,2,2-tetra-decyl) trimethoxysilane (0.5 ml), isopropanol (50ml) and hydrogennitrate. The PH of the fluorine hydrophobic solution is about 2 whichwas adjusted by the hydrogen nitrate. Next, the substrate should bewashed by ethanol for 5 times to remove the sticky materials on thesurface of the substrate. Then the hydrophobic substrate was dried at80 °C for 5–10min. Fig. 1 shows the schematic diagram for preparationof the magnetic FeCo composite films, the presentation of preparedfilms and its flexible. Initially, mix the PDMS with FeCo nanocubehomogeneously by using a ultrasonic cleaner. Next, the homogeneousmixtures are dropped on the hydrophobic substrate (Si wafer or glasssheet) as shown in Fig. 1(a). Spin-coating (Fig. 1(b)) is the next stepregulated by the pre-defined program. And the thickness of polymerfilms is regulated by the spin-coating speed and the viscosity of thepolymer. For example, the thickness of the film for morphology ob-servation is about 7 μm with the maximum 5000 rpm for 120s using thespin coater (Spin master 51, Chemat). Then put the substrate into theexternal magnetic field which is created by NdFeB rare earth permanentmagnet bar (Fig. 1(c)). Moreover, the specific field intensity was mea-sured by Gauss Meter (ShangHaiHengTong, HT20). And the magneticfield intensity was controlled by the distance of the magnet bar and thenumber of the magnet bar at each side. After a while, the polymer

nanocomposite films with alined nanochains or nanobundles will beobtained due to self-assembly. Then the polymer nanocomposite filmswere solidified in the drying oven at 100 °C for 2 h. While the substratecooled down to the room temperature, it is easy to demould the filmfrom the hydrophobic substrate.

Material Characterization: The mass weighted size distribution ofthe used FeCo particles was measured by Malvern Mastersizer 2000particle size analyzer. The morphology of the polymer composite filmsand tensile crack surface were analyzed by Laser Scanning DigitalMicroscope (Olympus LEXT OLS4100). Films for the morphology ob-servation is 20mm in length, 20mm in width and 7 μm in thickness.And films containing alined nanochains or nanobundles on Si substratewere cut together into 3×3mm pieces for TEM samples. Thin planesamples for TEM observation were prepared by precision ion polishingsystem with liquid nitrogen cooling (Gatan, Model 691). The micro-cosmic morphology of the nanochains or nanobundles was observed byTEM (JEOL, JEM-2100). The self-assembly process of the nanostrandswas observed by the body type optical microscope (Olympus BX51).The viscosity of the pure PDMS and the polymer mixture with differentmass fraction of FeCo filler were measured by rotational viscometer(NDJ-5S). The No. 4 rotor is chose for the measurement and the rotatespeed is 60r/min. In order to obtain the average value, 10 viscosityvalues of each sample were recorded by every 15 s. The magneticproperties were investigated through a vibration sample magnetometer(Lakeshore VSM 7407). For hysteresis loop measurement, samples wereprepared by cutting the films into a small block of which the length andwidth are 3× 3mm. The stretch strength of pure PDMS and FeCo-PDMS films was measured with a high-precision mechanical testingsystem (Hounsfield H5k-S materials tester). The samples for tensiletesting is 20mm in length, 10mm in width and 1.3 ± 0.2mm inthickness. The sample size was measured using a standard caliper.

3. Results and discussion

3.1. Morphology of composite films with different mass fraction of magneticfillers

The polymer nanocomposite films are prepared by spinning themixture consisting of PDMS and FeCo cubic nanoparticles as shown inFig. 1. The as-prepared film and its excellent flexibility are shown inFig. 1(e) and (f) respectively. In this study, the magnetic-field-induced-assembly method can be applied to the thin nanocomposite films usinga weak external magnetic field below 50mT. This method is more ef-fective and convenient to obtain nanocomposite films with alined na-nochains or nanobundles. Fig. 2 shows the morphology of the polymerfilms with three different mass fractions of FeCo filler. Without externalmagnetic field, the optical morphology of the composite films con-taining 0.05 wt%, 1.0 wt% and 5.0 wt% magnetic fillers are shown inFig. 2(a)–(c), respectively. It is obvious that the distribution density ofthe filler in composite film increase with more fillers. When the externalmagnetic field of 45mT was applied, the alined nanostrands in thecomposite films with 0.05 wt%, 1.0 wt% and 5.0 wt% filler can be ob-served as shown in Fig. 2(d)–(f), respectively. While the mass fraction is0.05%, the length and width of nanostrands are small. As the massfraction of the FeCo cube nanoparticles increase, larger nanostrands canbe observed as shown in Fig. 2(e) and (f). Fig. 2(g)–(i) show the brightfield image of the single nanostrand with filler mass fractions of 0.05%,1.0% and 5.0% respectively. Compared with the narrow nanostrand inthe film of 0.05% mass fraction (Fig. 2(g)), wide nanostrand consistingof many nanocubes can be obtained while the mass fraction increases to5%, whose width is larger than 1 μm as shown in Fig. 2(i). This phe-nomenon results from the large amount of nanocubes in the film, whichcontributes to the assembly of the nanochains. It is clear that the massfraction of the filler dramatically affects the morphology of the na-nochains or nanobundles under the specific external magnetic field[27,28]. And it is worth mentioning that the mean viscosity of the pure

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PDMS is 2700mPa.s which was measured by rotational viscometer. Theaverage viscosity of the polymer with 0.05 wt%, 1.0 wt% and 5.0 wt%magnetic FeCo fillers are 2590 mPa.s, 2682 mPa.s and 2672 mPa.s,respectively, which is similar to the pure one. So, the difference of theviscosity which might affect the morphology of the nanochains or na-nostrands can be ignored.

3.2. Control of Nanochain's length and width by different magnetic fieldintensity

Besides the mass fraction of the magnetic filler, external magneticfield is another important factor which affect the distribution of the self-

assembly nanostructures. Taking the film containing 1.0 wt% magneticfiller for example, without external magnetic field the FeCo nano-particles disperse uniformly in the film as shown in Fig. 2(b), which isdifferent from the magnetic orientated one in Fig. 2(e). Fig. 3 shows thecharacteristic morphology of the self-assembly nanostrands, which arefabricated with 1.0 wt% FeCo nanoparticles filler at an external mag-netic field intensity of 140mT. The low and high magnification opticalmicroscopy images of the film are shown in Fig. 3(a) and (b), respec-tively. Due to the increase of the external magnetic field intensity, thelength and width of the nanostrands in Fig. 3(b) are larger than thealined nanostrands at 45mT in Fig. 2(e) and (h). The bright field imageof the nanobundles confirms the effect of the magnetic field as shown in

Fig. 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.

Fig. 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 45 mT at the mass fraction of (d) 0.05%, (e) 1.0% and (f)5.0%, together with the 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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Fig. 3(c). It is no doubt that the nanobundles are broader than the onein Fig. 2(h) where the magnetic field is 45mT. Besides the nanobundles,single nanochain is also observed as shown in Fig. 3(d). It is interestingto find that FeCo nanocubes connect each other by face to face to form asingle nanochain. But the nanochains with nanocube are not perfectlystraight or parallel to the magnetic field in the polymer substrate. Theideal layout model for single nanochain with the ferromagnetic FeConanocube is shown in Fig. 3(e). All the nanocubes are along the di-rection of easy magnetization [100] which is perpendicular to thesurface facets. The discrepancy between practical morphology and ideallayout model may cause by the viscosity of the polymer, the shrinkageof solidification process and the interaction with other nanochains.

In order to understand the formation of the alined nanochains in thepolymer nanocomposite films, the in-situ observation of the process ofself-assembly nanochains under the external magnetic field is in-vestigated. Fig. 4(a)–(g) show the optical morphology images of thefilms 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 asshown in Fig. 4(a). Arrows (A and B) indicate the docking positions ofthe two initially separated chains, which will form a single chain later.As time prolongs, the nanochain at position A gradually approaches theapex of another two nanochains. At 960s, the left apex of the nanochain(position A) connects to another nanochain as shown in Fig. 4(f). Thenthe totally connection with other nanochains have been observed at1140s (Fig. 4(g)). This self-assembly mechanism is named as connectiontype, which can contribute to the length extension. There is anothermechanism of coarsening type as shown at position B. The nanochainattaches to another chain completely as shown in Fig. 4(a) and (d). So,this coarsening mechanism can enlarge the width of the nanochainseffectively. The formation mechanism of such nanochains are also re-ported elsewhere, which is simply described as connection and coar-sening mechanism [29]. In essence, the nanochain formation is con-sidered being resulting from the composition of the magnetic attraction,the surface energy and the entropic contribution of small nanowires[30].

For the purpose of controllable fabrication, the effect of magneticfield intensity on the length and width of nanochains is investigated indetail. The length of the nanochains was calculated by the optical mi-croscopy. Due to the resolution limits of the optical microscope, TEMwas used to count the width of the alined nanostructure. In order to getan accurate value, as many of nanochains or nanobundles as possiblewere calculated (more than 50 nanochains). Fig. 5 shows the evolutionof the length, width and aspect ratio of the nanochains as a function ofmagnetic field intensity. The mean length reaches to the maximum of180 μm with a filler factor of 1.0 wt% in an external magnetic field(140mT). Before the saturation point of the length is reached, an ap-proximate linear increase is observed with the increase of the externalmagnetic field intensity as shown in Fig. 5(a). Then the mean length ofthe self-assembly nanostructures decreases and levels off while themagnetic fields vary from 160mT to 200mT. The change of the widthof the nanochains is similar to the length which is shown in Fig. 5(a).But the maximum value of width slightly fall behind the saturationpoint of the length. And then the width of the nanochains also varieslittle with the increase of the magnetic field. Fig. 5(b) shows the aspectratio of nanochains or nanostrands at different external magnetic fieldintensity. According to the data distribution, the effect of magnetic field

Fig. 3. The optical morphologies of self-assembly nanochainsat (a) low and (b) high magnifications, and the corre-sponding bright field image of (c) a big nanobundles and (d)a single alined nanochain, together with (e) an ideal layoutmodel for single nanochain with ferromagnetic FeCo nano-cubes.(magnetic field intensity - 140mT, mass fraction –1.0 wt%).

Fig. 4. In-situ growth of nanochains in composite films after applying magneticfield 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.(massfraction – 1.0 wt%).

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can be divided into three zones: low magnetic field region (Ι), mediummagnetic field region (ΙΙ) and high magnetic field region (ΙΙΙ). At lowmagnetic field region, the length and width increase synchronously,because the magnetic field is so weak that no direction of nanochaincan grow abnormally. With the increase of the magnetic field, all thedirections grew quickly, especially the length of the nanochains in-creases faster than the width at the medium magnetic field region (ΙΙ) as

shown in Fig. 5(b), which can also be confirmed by Fig. 5(a). The aspectratio of nanochain at region(ΙΙ) is about 130, and the maximum aspectratio is close to 150. In this region (II), the proper magnetic field leadsto the quick growth of the length. That is to say, the connection me-chanism dominates the growth of nanochain at this situation. At thehigh magnetic field region (ΙΙΙ), the length of the alined nanochainsdecreased obviously at 160mT, because the linear chains will tend toaggregate in order to reduce the total energy which have been reportedby computational results [31]. And the high magnetic field is vital,which contributes to overcome the potential energy barrier of longchains. In this way, one can obtain nanobundles with laterally ag-gregated chains to get a lower energy by a higher external magneticfield(160mT). So the width of the nanochains will increase, while thelength will decrease. While the magnetic field ranges from 180mT to200mT, the aspect ratio tends to be stable owing to the competitivegrowth of the length and width at high magnetic field.

According to the above investigations, the mass ratio of filler andthe external magnetic field intensity have obvious effects on the growthof nanochains. Without the magnetic field as shown in Fig. 2(a)–(c),particles randomly aggregate as a result of Brownian motion and di-polar interaction between the particles. Once the magnetic field wasapplied, these particles can orient along the magnetic field direction toform nanochains. With the increase of the mass fraction of the magneticfillers, the morphology of the self-assembly nanoparticles changed fromthe arrayed nanostructure (Fig. 2(d), 0.05 wt%), nanochains (Fig. 2(e),1.0 wt%) to nanobundles with laterally aggregated chains (Fig. 2(f),5.0 wt%) at the external magnetic field of 45mT. The growth of na-nochain follows the connection-coarsening mechanism as shown inFig. 4. Because of the lower potential energy of a particle located at theend or tip of the chains [14], the length of nanochains grew with theconnection mechanism. When there is a higher external field intensity,the potential energy barrier of long chains can be overcome, so thecoarsening took place obviously.

3.3. Tensile strength and fracture mechanism of the FeCo-PDMSnanocomposite films

Considering the application potential of this polymer film, it is vitalto study the mechanical properties of this anisotropic FeCo-PDMS films.Fig. 6 shows the typical stress-strain curves and tensile strength of thepure PDMS and FeCo-PDMS composite films with 1.0 wt% fillerloading. It is obvious that the filler of FeCo nanocubes improves thetensile strength of the PDMS matrix at all conditions regardless of theapplication of magnetic field. However, for the anisotropic film

Fig. 5. The effect of magnetic field intensity on (a) the mean length and widthas well as (b) the aspect ratio of nanochains or nanobundles. (mass fraction –1.0 wt%).

Fig. 6. (a) Typical stress-strain curves of pure PDMS and FeCo-PDMS composite films with 1.0 wt% FeCo ferromagnetic nanoparticles loading. (b) The tensilestrength of pure PDMS and FeCo-PDMS composite films with or without external magnetic field. (magnetic field intensity - 60mT, mass fraction – 1.0 wt%).

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fabricated under magnetic field, the tensile strength of the film at atensile direction perpendicular to the alined nanochains is higher thanthat at a tensile direction parallel to the alined nanochains. And thetensile strength of films with dispersive FeCo cube nanoparticles(without magnetic field) is also higher than that of anisotropic filmwhen tensile direction is horizontal to the nanochain. As shown inFig. 6(b), the tensile strength of the pure PDMS is only 1.73MPa, andthe tensile strength of the sample with dispersive FeCo nanocube is2.43MPa. The tensile strength along the alined or assembly direction(2.20MPa) is 27.2% higher than that of pure PDMS (1.73MPa), whilethe tensile strength perpendicular to the assembly direction (3.20MPa)is even 85.0% higher than that of pure PDMS (1.73MPa). The tensilestrength of the pure PDMS and FeCo-PDMS composite films with0.05 wt% or 5.0 wt% filler loading have been shown in Fig. 7(a) and(b). It is clear that the enhanced trend of the tensile strength is similarto the composite films with 1.0 wt% filler loading. And the stress-straincurves of the composite films with 0.05 wt% or 5.0 wt% FeCo filler aredisplayed at the Supporting Information(SI-1).

Fig. 8 shows the cross-section morphology of tensile fracture and theillustration of crack propagation model (planar graph, the observationlocation is shown in the inset) for four kinds of composite films(FeCo-1.0 wt%). The stretch fracture morphology of the pure PDMS is shownin Fig. 8(a), in which the surface of the fracture is clean and onlyfracture pattern can be observed. As for the pure PDMS films, crack willfirstly appear at uneven place after the tensile force increases to someextent. Then the cracks propagate up to the fracture of films. Theschematics of crack formation in pure PDMS are shown in Fig. 8(b). Andthe white arrow at the bottom-left indicates the tensile direction. Thecross-section morphology of the composite films (1.0 wt%) withoutmagnetic field is shown in Fig. 8(c). It is easy to find that there are someparticle clusters on the surface of the fracture. Some typical clusters aremarked by the black circle. It is obvious that the composite filmswithout external magnetic field will have some glomerate nano-particles, although the mixture is carefully made with the polymer andfillers. Besides the aggregated filler particles, such inhomogeneities alsoinclude holes around aggregated particles which can not be completelywetted by the matrix [32]. All of these inhomogeneities can cause thestress concentration, which favors rapid crack propagation. So, thecrack will generate around the nanoparticle cluster. The fracture modelof the film fabricated without magnetic field is shown in Fig. 8(d), inwhich the crack is apt to generate and extend around the nanoparticlescluster. The fracture morphology of the nanochain film at a tensile di-rection parallel to the chain axis is shown in Fig. 8(e). Some puny pitsmarked with A1, A2 and A3 can be observed at the surface of thefracture, which should be formed by the breakage of the nanochains.

And the nanoparticles which originally are a part of the nanochains arelocated in the pits. The pits are surrounded by the flexible polymer. It iseasy to deduce that the crack will produce at the middle part or the tipof the nanochains in this film, because of the inconsistent deformationbetween the fillers and polymer matrix. What's more, for the nanochainwhich is horizontal to the tensile direction, the nanocubes are poorlyboned. So, the fracture will occur as shown in Fig. 8(f). As for the filmsin which the alined nanochains are vertical to the tensile direction,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 inFig. 8(g). It is obvious that the B1, B2 and B3 distribute in a straight linewhich might be a part of one nanochain. Thus, the cracks will prolongalong the major axis of the nanochains as shown in Fig. 8(h). And thissituation also results from the inconsistent deformation of the FeConanochains and the polymer matrix.

According to the experimental observations and deduced models inFig. 8, the fracture mechanism of four different films can be discussed.As for nano-particulate composites, the strength mainly relies on theeffectiveness of the stress transfer between matrix and fillers, despite ofthe particle size and the particle loading [27,33]. Previous studies in-dicate that addition of particles leads to an increase in strength, andsmaller particles give better reinforcement [27,32,33]. Because smallerparticles have a higher total surface area for a given 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 ishigher than the pure PDMS, owing to the effect of the FeCo nano-particles. The tensile strength of the composite films without externalmagnetic field is slightly higher than the alined nanochains at a paralleltensile direction as shown in Fig. 6(b). The reason might be that thealined nanochains weaken the potentiation between the filler and ma-trix, because there is large area of blank zones among the nanochains.However, the potentiation of the composite film with disperse nano-particles is holistic. What's more, The stress can not be alleviated withdeformation in the width of the nanochains, so it is easy to generatemicro-cracks in the joints where the hard FeCo nanoparticles are notclose. The orientation between alined nanochains and the tensile di-rection also has a profound effect on the film strength. While the tensiledirection is vertical to alined nanochains, the maximum tensile strengthof film will be obtained because of the effectiveness of stress transferbetween matrix and filler. The crack will prolong along the major axisof the nanochains. In this situation, the interfacial adhesion betweenparticle and matrix significantly affects the strength of particulatecomposites. The stress can be alleviated through the deformation of thesoft matrix, so it is not easy to generate micro-cracks. Therefore, in thistensile experiment, it is necessary to continue to increase the load in

Fig. 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) 0.05 wt%, (b) 5.0 wt%).

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order to get micro-crack and trigger the plastic fracture. Hence the besttensile strength will be achieved. It is important to clarify the po-tentiation of the magnetic fillers and these excellent tensile properties,which are beneficial to the application of self-assembly anisotropiccomposite films in flexible devices. And the fracture morphology of thecomposite films with 0.05 wt% or 5.0 wt% magnetic filler was alsoobserved as shown in the Supporting Information(SI-2). The fractureway is similar to the composite film with 1.0 wt% FeCo filler. So, it is nodoubt that these films also conform to the crack propagation modelwhich discussed in the above.

3.4. Magnetic properties of the FeCo-PDMS nanocomposite films

As the magnetic FeCo nanoparticles were used as filler, the fabri-cated nanochain composite film has anisotropic magnetic properties.Using the solidified composite film, its response to an external magneticfield is shown in Fig. 9. Two pieces of film were cut from the FeCo-PDMS film consisting of horizontally aligned nanochains as indicatedby the red squares in Fig. 9(a). One piece's length is parallel to theembedded nanochains, while the other is perpendicular to the axis ofthe nanochain. Both films were immersed in water to float-up at thewater-air interface in a glass petri dish. Fig. 9(b) and (c) show the re-sponse of these two differently-orientated pieces to the external mag-netic field. The initial statuses (0s) are shown in Fig. 9(b) and (c)

respectively, and the earth permanent bar is put at the left side of theglass petri dish. Independent on the shape of the composite pieces, bothpieces finally stopped at the position where the nanochain in the film isparallel to the external magnetic field. The response of the films torotation and alignment with the external magnetic field was completedin 5s or 8s, a relatively short amount of time.

Fig. 10 shows the in-plane, room temperature hysteresis loop for thepolymer composite films. Without magnetic field during fabrication,the nanocomposite films with 1.0 wt% FeCo nanoparticle filler havesimilar magnetic properties along A direction and B direction as shownin Fig. 10(a). And the A and B are the two perpendicular directionswhich are corresponding to the length and width of small block testspecimen. Due to the very low mass fraction of magnetic filler in thefilm, the saturated magnetization (Ms) of the composite films is only0.45 emu g−1, although the Ms of the FeCo nanocube filler is as high as220 emu g−1 [26]. However, the films with alined nanochains areanisotropic as shown in Fig. 10(b). It is clear that the films with na-nochains have different magnetization behavior for parallel(//) andperpendicular(⊥) geometries to the nanochain, which proves that thenanochains are easier to be magnetized in parallel geometry due to thelarger dipolar coupling. And the inset shows the details about thehysteresis loop as shown in Fig. 10(a) and (b). After the composite filmswith 1.0 wt% filler were stretched, the magnetic properties are stay thesame and magnetic anisotropy is similar with the original one, which

Fig. 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 twoobservation location(cross-section and planar graph) of the fracture diagram), (c) and (d) FeCo-PDMS films without external magnetic field, (e) and (f) FeCo-PDMSfilms in which the alined nanochains are parallel to the tensile direction, (g) and (h) FeCo-PDMS films in which the alined nanochains are vertical to the tensiledirection. White arrows indicate the tensile direction. (magnetic field intensity - 60mT, mass fraction – 1.0 wt%).

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

4. Conclusion

In summary, flexible and anisotropic FeCo nanochain-polymercomposite films were successfully fabricated by magnetic-field-induced-assembly, and its mechanical and magnetic properties were in-vestigated in detail. The filler of FeCo nanocubes alined along the easymagnetization direction of [100] to form nanochain in the PDMS ma-trix. The morphologies and dimensions of the nanochains can be con-trolled by the nanoparticle filler loading (0.05–5.0%) and the externalmagnetic field intensity (15–220mT). With the increase of the externalmagnetic field intensity, both the length (50–180 μm) and width(0.6–1.3 μm) of the nanochains have an approximate linear increasefirstly, and then level off for saturation. The highest aspect ratio of 150

can be achieved at filler mass fraction of 1.0 wt% under magnetic fieldof 140mT. In-situ observations verified the connection-coarseninggrowth mechanism of the nanochains or nanobundles.

Along the directions parallel to or perpendicular to the nanochainaxis, the FeCo composite films are anisotropic. The tensile strengthalong the direction of the nanochains (2.20MPa) is 27.2% higher thanthat of the pure PDMS (1.73MPa), while that perpendicular to thenanochains is the highest (3.20MPa) when the mass fraction of thefiller is 1.0 wt%. Different tensile models are proposed to discuss thefracture mechanisms of different composite films. When the tensiledirection is vertical to the nanochain, the stress can be successfullyalleviated through the deformation of the soft matrix between na-nochains, which enables the highest tensile strength of the films.According to the magnetic measurements, the nanochains are easier tobe magnetized in parallel geometry due to the larger dipolar coupling.By controlling fabrication, one can tune the mechanical performanceand magnetic anisotropy of the thin polymer nanocomposite filmssynchronously, which enhances their potential applications especiallyfor highly stretchable devices.

Fig. 9. (a) The sketch of FeCo-PDMS films withalined nanochains or nanobundles, the two red frameare two pieces with different nanochains orientation.(b) and (c) are the rotation of the ferromagneticpolymer nanocomposite thin films immersed inwater under external magnetic field. (For inter-pretation of the references to colour in this figurelegend, the reader is referred to the Web version ofthis article.)

Fig. 10. (a) The room temperature hysteresis loop measured at two perpendicular directions of the films without external magnetic field. (b) the room temperaturehysteresis loop of parallel and perpendicular directions to the nanochains(magnetic field intensity - 60mT, mass fraction – 1.0 wt%). Inset is the enlarge view of thehysteresis loop.

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Acknowledgement

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

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.compscitech.2018.05.025.

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