Structure and magnetic properties of mechanochemically synthesized UHMWPE/ferrite composites as precursors for electromagnetic shielding materials

In this paper, a study of composite materials based on ultrahigh molecular weight polyethylene (UHMWPE) and magnesium ferrite particles — precursors for electromagnetic shielding materials was carried out. Scanning electron microscopy, X-ray diffraction, Mossbauer and FTIR spectroscopy have been applied to study the influence of structural, morphological and magnetic state of ferrite particles on the composite polymer materials formed by the method of mechanochemical activation in mixtures of ferrites with UHMWPE. It was revealed that synthesis method type showed a significant effect on the size, morphology, crystal structure (inversion parameter) and magnetic properties of ferrite particles. All of these studied parameters determine their functional properties as an independent material, and their properties in the composition of the functional polymer composite material.


Introduction
Composites combining magnetic materials with polymers have found a wide range of applications in sensors, electromagnetic wave absorbers, vibration isolators, magnetic resonance imaging contrast agents as well as magnets [1]. Incorporation of magnetic fillers into polymeric matrices leads to unique properties which can be implemented in new specific purposes. The problem of protecting electronic equipment and human from electromagnetic and ionizing radiation, both artificial and  [2,3], and therefore the search for materials for these purposes requires new approaches and simple solutions. To solve the problem of obtaining light and moldable materials for protection against ionizing and electromagnetic radiation, a number of approaches are currently being proposed, one of which is the use of polymer composite materials [4]. The use of individual absorbing capacities of polymers and modifying particles in composite materials, as well as the comparability of particle sizes used as a component of a composite material and radiation wavelengths, allows the effective length of the radiation path in a composite material to be varied due to the effects of its additional absorption and scattering. Application of ferrite's particles as components of a composite is very attractive due to their well-known electromagnetic properties, which they manifest as an independent material that has shown importance in electronic and biomedical technologies [5,6]. Interest in ferrites as a component of a polymer composite material is due to the possibility of using protection from electromagnetic radiation (radio frequencies from hundreds to thousands MHz) for coatings, including for individual protection of portable devices [7].The properties of ferrites are strongly dependent on the composition, structure, morphology and particle size [8] that is determined by the applied synthesis method. Practical application inevitably faces the problem of uniformity of phase and size distribution requirements, which are still only possible in small quantities and under highly controlled conditions and need the reduction of economic costs. At the same time, achieving a uniform distribution of particles in the polymer, a strong interfacial compound is the most important task in the preparation of composite materials.
We turned to magnesium ferrite, MgFe 2 O 4 , an important member of the spinel family [9] which finds a number of applications from electronics to biotechnology. To emphasize the microstructural aspects of this spinel, its structural formula is written as (Mg 1-z Fe z )[Mg z Fe 2-z ]O 4 where parentheses and square brackets denote cation sites of tetrahedral (A) and octahedral [B] coordination of a cubic spinel structure, respectively. z represents the degree of inversion characterizing the distribution of cations. Ultimate magnesium ferrite (MgFe 2 O 4 ) is an inverse spinel taken to be collinear ferromagnetic, whose degree of inversion depends on the synthesis temperature and cooling rate [8]. Ultrahigh molecular weight polyethylene (UHMWPE) occupies a special place among the polymeric materials used as matrices and binders in composite materials [10]. This is due to the combination of its physicochemical properties, which include mechanical properties, chemical resistance to aggressive environments. The physicochemical properties of composite materials based on UHMWPE will be determined by its atomic, molecular, and supermolecular structure.
One of the most effective methods for composite materials synthesis for the purposes of applying them as precursors for formation of functional components -is mechanochemical activation [10,11] of mixtures of polymer and modifying particles. During mechanical processing of such mixtures in activator machines, a number of effects are realized [12]: an increase in the effective surface of the initially spherical polymer particles of UHMWPE in the process of shear deformation when interacting with the balls and walls of the activator, the change of the molecular structure of UHMWPE due to the rupture of intramolecular bonds and hydrocarbon chains, mechanical doping of UHMWPE particles with filler particles or products of their decomposition when they are ground, mixing UHMWPE particles and a filler with possible formation of encapsulating structures. All of this allows to ensure a distribution of particles in the polymer with formation of almost homogeneous precursor for the effective subsequent formation of functional products, for example, by hot pressing.
In this work, we research the influence of magnesium ferrite particles synthesis method on the structural and morphological characteristics of mechanochemically synthesized composite materials based on ultrahigh molecular weight polyethylene. The studies were carried out using scanning electron microscopy, X-ray diffraction, Mossbauer and FTIR spectroscopy, magnetic measurements.

Specimen preparation
Composite samples were obtained by the method of joint mechanical activation of ultrahigh-molecular powder polyethylene (UHMWPE) (Ticona GmbH (Germany), particle size 100-160 m with a molecular mass of 4·10 6 g/mol) with magnesium ferrite particles MgFe 2 O 4 . The composition of the mixture for the formation of a composite material was selected as follows: 85 wt.% UHMWPE -15 wt. % MgFe 2 O 4 particles. Ferrite particles for using as composite component were obtained by four different methods: solid-phase ceramic synthesis (S1), reverse co-precipitation (S2), sol-gel method (S3) and mechanosynthesis (S4).
Solid-phase synthesis (S1) included mixing stoichiometric compositions of powdered oxides (MgO and Fe 2 O 3 ) for an hour, followed by calcining the mixture at 1100°C for one hour in air.
Then the mixture was ground in a spherical planetary mill (the rotational speed of the drum was 600 rpm).
In the method of reverse co-precipitation (S2), solutions of Mg(NO 3 ) 2 ·6H 2 O and Fe(NO 3 ) 2 ·9H 2 O in distilled water were used as reagents. 6 mol of NaOH solution were added to precipitate, followed by lowering the pH to neutral with distilled water. Then followed the calcination of the powder at a temperature of 1100°C. Sol-gel synthesis (S3) was carried out using magnesium nitrate Mg(NO 3 ) 2 , 0.02 mol and 0.03 mol iron nitrate Fe(NO 3 ) 2 with addition of citric acid in the calculation of the molar masses of metal nitrates to citric acid 2:1. Thermal destruction of the resulting gel was carried out at 400°C for 1 hour. The final calcination of the powder was carried out at a temperature of 1100°C.
Mechanochemical synthesis (S4) was carried out from high-purity MgO and Fe 2 O 3 oxides in an AGO-2 ball planetary mill, for 2 hours in air, followed by calcinationat 1100° . The mechanochemical synthesis of UHMWPE/S1, UHMWPE/S2, UHMWPE/S3, UHMWPE/S4 composites was carried out in an AGO-2 activator for 2 min in an argon atmosphere. The volume of the drum was 250 cm 3 , the diameter of the steel balls -5 mm, the loading of the balls -200 g, the weight of the sample to be processed was 10 g, the speed of drum rotation around a common axis was 1000 rpm.

Experimental methods
For collection of scanning electron microscopy (SEM) data, specimens' powders were dusted on a carbon duct tape. Sputtering of Au particles (~200 nm) has been performed to avoid charging decrease sample damage due to electron beam and also to increase resolution. Images were recorded using FEI Quanta 3D FEG microscope with secondary electrons signalin high vacuum mode, which made it possible to obtain images in secondary electrons using an Everhart-Thornley detector (ETD) with high resolution. Analysis of the obtained images was carried out using the IMAJEI software.
X-ray diffraction data (XRD) was obtained using PANalytical Empyrean powder diffractometer equipped with a PIXcel3D detector (Bragg-Brentano geometry, Cu K -radiation, Ni filter). HighScore (PAnalytical) and ICDD PDF2 database was used for phase identification.
Mössbauer study (MS) of structure and magnetic state of samples was carried out at 300 K with 57 Co(Rh) source using MS1104Em spectrometer. Analysis of the spectra has been performed with SpectrRelax software [13]. All spectra were referenced to -Fe at room temperature.
Infra-red spectroscopy (FTIR) studies were performed using a Nicolet iS10 FTIR spectrometer (Thermo Scientific, USA) by the method of impaired total internal reflection on a diamond crystal in the frequency range of 4000-400 cm -1 . Spectra analysis was performed using average function of apodization of Norton-Bearer and Merz method for phase correction.
Magnetic properties (magnetic hysteresis loops and their parameters) were measured at room temperature on a VSM-3900 vibrating magnetometer (Lake Shore, USA) in fields up to 2 T.

Experimental results and discussion
We synthesized ferrite particles for applying them as a component of UHMWPE composites by different methods that determine the structural and morphological characteristics of particles. These characteristics include particle shape and size distribution, as well as the purity of the phase composition, stoichiometry and cation distribution. Particle size distributions analysis derived from the SEM images (figure1) given for each sample in the diagrams showed quantitatively that the smallest particles with the narrowest particles distribution were obtained by sol-gel method S3 and co-precipitation S2. Average particles size D ~ 0.3 and 0.4 m correspondingly. Ferrite particles S1, S4 have a faceting and well-formed structure. Their average sizes reach 2.2 and 5.3 m, correspondingly (Table 1). Small particles of impurities were found in sample S1. X-ray phase analysis of the synthesized ferrite particles is shown in figure 2. It was established that the samples obtained by methods S2 and S3 turned out to be single-phase.All samples of the series as a main phase contained the cubic phase of magnesium ferrite MgFe 2 O 4 (Fd3m # 88-1943) with a deviation from the tabular value of the lattice parameter of no more than 1%.  It is known that for the stoichiometric composition of MgFe 2 O 4 , the lattice parameter is in the range a = 8.38-8.40 Å and depends on the degree of ordering, which is achieved in specific heat treatment modes. Calculated grain sizes for main phase of ferrite for all compositions do not exceed 2700 Å. Analysis of the intensity ratio for the I(220)/I(400) and I(111)/I(400) reflections sensitive to the cation distribution on the X-ray diffraction pattern showed a corresponding change due to the variation from stoichiometry in the direction to iron oxide. In ferrite particles synthesized by the solid-state method (S1), confirming the results of SEM on the presence of an impurity, an admixture of the hematite phase Fe 2 O 3 (R3c, # 89-0599) was detected in an amount of 20%. Sample S4 contains MgO phase residues (Fm3m, # 87-0653) (5%).
The Mossbauer spectra of synthesized ferrite particles measured at 300 K are shown in figure 2 (right panel). Spectra analysis allowed to resolve at first exact phase composition and their magnetic state (to identify purity of synthesized ferrite particles), reveal size effects if present, extract distributions of atoms in different sub-lattices of the main MgFe 2 O 4 phase, and also find the degree of inversion (z). Supporting XRD data the main phase of ferrite particles was spinel MgFe 2 O4, which spectra profile fitting in a simple model consist of (A) and [B] subspectra (blue and green painted on figure 2). The magnetic hyperfine fields in MgFe 2 O 4 , clearly depend on the distribution of Mg(2+) and Fe(3+) among the A-and B-sites. As seen from the spectra of particles obtained by different synthesis methods, a considerable overlap of the (A) and [B] subspectra due to the similar values of the hyperfine fields is observed. Applying an algorithm restoration of hyperfine field's distribution function P(H) for (A) and [B] sites independently an asymmetric broadening of the [B]-site lines indicating the presence of several subpatterns arising from the different possible nearest-neighbor [14] has been revealed. (A)-site lines usually do not show any structure that indicates the presence of a narrower hyperfine field's distribution [15].

Rel.Intensity
Rel.Intensity Rel.Intensity S1 Rel.Intensity The value of the average Heff in the distribution of the fields for sample S2 and S3 has a smaller value comparing to the bulk values which is associated with the size effect [9]. The degree of inversion is in agreement with previously published data showing that synthesized MgFe 2 O 4 particles have a partly inverse spinel structure. The inversity degree z was deduced from the quantitative information from P(H) B P(H) A distributions shown on figure 2 (right panel) with green and blue colored components. The calculated degree of inversion was found to be z = 0.96 (S1), 0.91 (S2), 0.81(S3) and 0.99(S4) correspondingly. The more perfect ferrite particle structure in our samples which is faceted on SEM images ( figure 1a, d) is reflected in the Mössbauer spectra by an explicit separation of the components ( figure 2 (right panel a, d), while smaller particles (S2, S3) show a distribution of hyperfine fields that arises as from the dimensionality effect as well as from effects of the distributions variables over the sub-lattices. Fe 2 O 3 phase (H eff = 515 kOe, = 0.38 mm/s, = 0,21 /s, S (22%) [20]) additionally to the main spinel phase has been determined for S1 sample. SEM images of UHMWPE/ferrite composites showed that during mechanochemical activation of initially spherical UHMWPE its shape changes to elongated, lamellar, and even scaly ones with increasing effective contact surface. All this can lead to a change in the internal structure of the polymer itself [16] and formation of composite polymer particles with ferrite particles inside ( figure  3). The morphology of the polymer structure of the composite structure is layered; there are fragments of a sandwiched shape (figure 3b). The sizes of composite particles reach 500 m in the UHMWPE/S1 (figure 3a), 150-200 m in the UHMWPE/S2, UHMWPE/S3 and UHMWPE/S4. Moreover, the analysis of the particle sizes of ferrites inside the polymer composite particle shows a decrease in their average size by almost 10 times compared with the initial state. X-ray phase analysis of all the studied composites ( figure 4, left panel) showed the absence of diffraction maxima of any new phases that could indicate the interaction of UHMWPE with ferrite particles. The diffraction patterns contain broadened structural maxima of UHMWPE and ferrite particles, therefore, during mechanosynthesis of composites, which was carried out in the interaction mode (plastic component is a brittle component), the grain sizes of the crystalline phase of ferrites are reduced and the polymer structure is modified with a change in its crystallinity. XRD line profile analysis reveals that the average grain size of the ferrite phase sharp decrease after mechanical activation in mixture with UHMWPE. This fact is consistent with a decrease in particle size inside the composite particle from SEM image. At the same time UHMWPE/S2, UHMWPE/S3 where the smallest ferrite particles with narrow particles sizes distribution were used, demonstrate that the grain size in the ferrite particles becomes close to the size of the particles themselves.
The room-temperature Mössbauer spectra of the composite samples ( figure 4, right panel) show the different profile from the figure 2: demonstrating the superposition of ferrite magnetically splitted component and a central doublet part. This seems as a result of mechanochemical interaction of ferrite particles with UHMWPE, but it also may be a consequence of the destruction of ferrite particles during mechanical activation. According to Mössbauer results, the magnetic part of the spectra The presence of -Fe2O3 in UHMWPE/S1 remains after composite synthesis as additional sextet subspectra with corresponding parameters were present(orange colored component figure 4 (right panel).The main difference in the spectra that appeared after mechanical activation of ferrite particles with UHMWPE was the appearance of a central doublet component in all samples (light green colored component). Mossbauer parameters of double thave values (isomer shift = 0.15 mm/s and quadruple splitting = 0.46 mm/s) that correspond to Fe(3+) species in octahedral coordination and may be the sign of superparamagnetic state of the ferrite particles due to mechanochemically influenced reducing their sizes to the nanometer range (~10-15 nm) [17] as well as the result of mechanical activation in the vial with steel balls. The other reason is bond formation by mechanically induced interaction with radicals resulting from the destruction of polymer molecules under intensive mechanical activation. To test the possible sign of particle-to polymer interaction we applied FTIR-spectroscopy (figure 5) as one of the informative method to estimate the presence and the amount of double bonds and of oxidation products.
The FTIR spectrum of the initial UHMWPE as known [19] in common should contain a specific vibration characteristics of the molecules in the amorphous-crystalline state with absorption bands (2913 cm -1 and 2846 cm -1 ), responsible for the symmetric and asymmetric stretching vibrations C-H, (1472 cm -1 and 1463 cm -1 ), responsible for the deformation vibrations C-H, and bands corresponding to pendulum oscillations of CH 2 -groups (730 cm -1 and 720 cm -1 ) -the trans segments only in the crystalline phase. The 720 cm -1 band is complex and corresponds to the pendulum oscillations of the CH 2 groups in the amorphous-crystalline regions. All these modes are observed on the spectra of composites without band shift. The difference in the intensity of the bands from different samples is due to the particle size and the amount of UHMWPE in the particles. Spectra consist of absorption bands supporting the presence of  Magnesium oxide MgFe 2 O 4 is a ferrimagnet with spinel structure [8,9]. Figure 6a shows the field dependences (hysteresis loops) of the magnetization (magnetic moment) M(H) at room temperature for samples S1-S4. These curves demonstrate the dependence on the particle size and their phase purity (figure 6a). In sample S1, where iron oxide Fe 2 O 3 (~20%) is present in the phase composition, which has an order of magnitude lower magnetization, the saturation magnetization is much smaller than the rest of the samples. coercive force Hc) shows that upon mechanical activation of ferrite particles with UHMWPE, the values of the coercive force increase and the values of saturation magnetization decrease, which occurs due to a decrease in the percentage of ferrite particles in the mixture, as well as reducing their size as a result of mechanical activation.

Summary
In this work, the structural and magnetic properties of MgFe 2 O 4 ferrite particles and UHMPWE/ferrite composites containing them were investigated. Our results correlate the effects of synthesis method affecting structure-morphological parameters of particles with spinel structure on their magnetic properties and polymer composites comprising them. Synthesis parameters showed a significant effect on the size, morphology, crystal structure (inversion parameter) and magnetic properties. All of thesestudied parameters determine their functional properties as an independent material, and their properties in the composition of the functional composite material. Among all the synthesis methods, a highly pure MgFe 2 O 4 phase with the morphology of tiny particles was revealed by the sol-gel method in controlled certain molar CA/MN ratio. Our results have demonstrated the possibility of simple mechanochemical formation of polymer based composites with magnetic characteristics controlled by synthesis method, structure and properties of ferrite particles in creating precursors for construction EMI shielding materials.