CoFe2) composite films

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Studies on the functional properties of free-standing polyvinyl alcohol/ (CoFe2O4/CoFe2) composite films ⁎

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T. Prabhakarana, , R.V. Mangalarajaa, , Juliano C. Denardinb,c, R. Udayabhaskara, K. Varaprasadd, H.D. Mansillae, David Contrerasf,g a Advanced Ceramics and Nanotechnology Laboratory, Department of Materials Engineering, Faculty of Engineering, University of Concepción, Concepción, 4070409, Chile b Department of Physics, University of Santiago and CEDENNA, Santiago, Chile c Departamento de Física, Universida de Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil d Center for Advanced Polymer Research (CIPA), CONICYT Regional, GORE BIO-BIO PRFC0002, Avenida Collao 1202, Concepción, Chile e Department of Organic Chemistry, Faculty of Chemical Sciences, University of Concepción, Concepción, Chile f Renewable Resources Laboratory, Center for Biotechnology, University of Concepción, Concepción, Chile g Department of Analytical and Inorganic Chemistry, Faculty of Chemical Sciences, University of Concepción, Concepción, Chile

A R T I C L E I N F O

A B S T R A C T

Keywords: Functional polymer composites Free-standing films Magnetic properties Optical properties Elastic properties Infrared (IR) spectroscopy Solution casting

In this report, the functional properties of polyvinyl alcohol/(CoFe2O4/CoFe2) composite films fabricated through solution casting method were reported. Highly-magnetic CoFe2O4/CoFe2 composite was obtained through thermal reduction process and the effect of CoFe2O4/CoFe2 loading on the magnetic, optical, thermal and mechanical properties of PVA composite films was evaluated. At room temperature, the optical transparency was decreased from 68 to 38% but the magnetization was enhanced to 42.37 emu/g by an increase of CoFe2O4/ CoFe2 to 15 wt%. Also, the visible-light emission intensity near 400–521 nm was influenced. We demonstrated that the wt% of CoFe2O4/CoFe2 and -OH intramolecular interactions between the polymer-chains played an important role in controlling the functional properties of composites. The performance analysis and comparison of the results revealed that PVA-composite film fabricated with 5 wt% of CoFe2O4/CoFe2 displayed significant magnetization, luminescence intensity, and high tensile-strength compared to others and was found to be a promising candidate for smart devices.

1. Introduction In the present scenario, the development of functional materials is needed and important to find an alternative material which shows high competence to the existing material systems. The combination of different properties such as magnetic and ferroelectric, strain and ferroelectric, strain and magnetic, optical and magnetic, and conductive and transparent are said to be functional or multifunctional properties of materials and are current breakthroughs in the fabrication of smart devices [1–3]. The functional properties of materials are impressive as it can be tunable with one another simultaneously. For example, the magnetization of the material can be switched by an external electric field and the electrical polarization can be switched by an external magnetic field and the phenomenon is said to be a magnetoelectric effect [3]. Similarly, the magneto-optical effect is found in some materials where the optical energy is tunable with external magnetic field and the magnetization by external light signals. The materials which

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display this effect are called magneto-optical materials [4]. The mutual control of optical and magnetic properties is an attractive combination which ultimately reduces the fabrication cost, but they are rarely produced. Because the magnetic materials are weakly transparent to the visible light and optical materials are weakly magnetic or non-magnetic in nature. By combining them in single material result either high magnetization or weak transparency or vice versa. There are several materials such as oxides [5,6], composites [7,8] and ternary mixtures are synthesized and reported for the application of magneto-optical devices [9,10]. Now, the two-phase systems and polymer nanocomposites attract the researchers for the magneto-optical application due to their advantage and simplicity over others. In the composites, the polymer matrix acts as an optically transparent material and magnetic nanoparticles as a magnetic reinforcement, anticipating the interaction between them would give rise to the functional magneto-optical effect. In addition, the polymer nanocomposites are free-standing and light weight with sufficient mechanical strength.

Corresponding authors. E-mail addresses: [email protected] (T. Prabhakaran), [email protected] (R.V. Mangalaraja).

http://dx.doi.org/10.1016/j.mseb.2017.09.024 Received 25 June 2017; Received in revised form 29 August 2017; Accepted 28 September 2017 0921-5107/ © 2017 Elsevier B.V. All rights reserved.

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of the present results with earlier reports given at the end of the discussion emphasized the advantage of the present composites to others.

Polyvinyl alcohol (PVA) is well known due to its high mechanical property, optical transparency, bio-compatibility, film forming capability, chemical resistance, hydrophilicity and easy processability. Moreover, the unique properties of PVA are owing to the presence of hydroxyl group (–OH) and the strong –OH interaction between intra and inter-molecular polymer chains [11]. In addition, the properties of PVA depend on the molecular weight and degree of crystallinity. Due to their special properties and strong hydrogen bonding with the reinforcements, numerous composites have been synthesized and reported in the different area of interest. PVA composites were synthesized with different metal nanoparticles such as Ag [12] and Au [13] and their optical, thermal and mechanical properties were reported. The different weight percentage of CeO2, ZnO, and CuO nanoparticles were embedded into PVA matrix and the effect of concentration of nanoparticles on the optical and luminescence properties of the nanocomposite films was discussed [14–16]. In addition, the gas sensing efficiency of PVA/In2O3 and linear and non-linear optical properties of spin-coated PVA/Co3O4 nanocomposite films were reported [17,18]. In recent years, the developments of PVA/graphene oxide and PVA/ MWCNT nanocomposites were increased and were mainly focused on the formation mechanism and enhancing the mechanical behavior of the composites [19–21]. The photoluminescence behavior of PVA/ carbon quantum dots composites was reported [11]. In addition, the shape memory behavior in different pH and temperature environments was discussed [11]. We also found several reports that discussed the properties of ferrite based PVA nanocomposite films. The different weight percentage of CoFe2O4 nanoparticles were embedded into PVA matrix and the effect of filler on the magnetic properties was reported by different research groups [22–27]. The similar composite films were made but the thermal and mechanical properties were only discussed [28]. Magnetite nanoparticles were embedded into PVA matrix by ultrasound radiation method and their magnetic and thermal properties of heat-treated composites were elaborated [29]. Similarly, the magnetic properties of PVA/γ-Fe2O3 and PVA/Fe3O4 composite films were described [30–32]. For the fabrication of PVA nanocomposites, the multiferroic BiFeO3 nanoparticles were also used as reinforcement. The ferroelectric and magnetic properties of PVA/BiFeO3 [33] and the magnetoresistance and magnetic properties of PVA/PANI-BiFeO3 nanocomposite films were found [34]. PVA/PANI/Nickel nanocomposites were synthesized using different wt% of NiCl2 and dosage of gamma irradiation. The optical bandgap and electrical conductivity of the composites were influenced by the concentration of metal precursor and dose of gamma irradiation [35]. The magnetic and magneto-optical properties on the Cds:Mn quantum dots in PVA matrix were reported by Fediv et al. [36] where the magnetic susceptibility and magneto-optical Faraday rotation were evaluated. The short review on PVA-based composites conveys that the reports were either focused on the optical or the magnetic properties of the PVA composites in addition to the electrical, thermal and or mechanical properties. To the best of our knowledge, no reports were found in the literature to study both optical and magnetic properties of PVA/ (CoFe2O4/CoFe2) composite films. Hence, the present research article is mainly focused on the fabrication of PVA/(CoFe2O4/CoFe2) composite films through solution casting method and the detailed investigation on the effect of CoFe2O4/CoFe2 content on the functional properties such as magnetic, optical, thermal and mechanical properties of the composites. For the fabrication of polymer composite, PVA is selected as an optically transparent matrix and CoFe2O4/CoFe2 composite is chosen as magnetic reinforcement due to its high magnetization of about 200 emu/g and less coercivity of 50–70 Oe at room temperature [37]. To prove the multifunctionality of the composite, the optical and magnetic properties of PVA composite films are investigated individually at room temperature. The overall performance of the freestanding PVA composite films is compared and suggests the best composition for the magneto-optical devices. Additionally, the comparison

2. Experimental 2.1. Materials The chemicals, cobalt nitrate hexahydrate (Co(NO3)2·6H2O), iron nitrate nonahydrate (Fe(NO3)3·9H2O) and sodium hydroxide (NaOH) pellets were purchased from Sigma-Aldrich (ACS 98%) and were used without further purification. For the composite fabrication, poly (vinyl alcohol) (PVA) (CH2CH (OH))n with a molecular weight (MW) of 85,000–124,000 g/mol, 99+% hydrolyzed fine powders was purchased from Sigma-Aldrich. Double-distilled water was used as a solvent for the synthesis of nanoparticles and the fabrication of composite films. 2.1.1. Synthesis of CoFe2O4/CoFe2 composite Various procedures were reported for the synthesis of CoFe2O4/ CoFe2 (CFO/CF) composite by thermal reduction method [37,38]. The CFO/CF composite was usually synthesized in two steps as follows: Cobalt ferrite nanoparticles were co-precipitated at the reaction temperature of 85 °C for 1 h, where 1:2 M of aqueous cobalt and iron nitrate solutions and 0.84 M of NaOH solution were used. The final black precipitates were thoroughly washed, filtered and dried at 100 °C for 18 h in an oven and grounded. To obtain CFO/CF composite, the synthesized cobalt ferrite nanoparticles were thermally reduced under a hydrogen atmosphere. First, the nitrogen flow was allowed with a flow rate of 10 ml/min for 15 min and then hydrogen flow was released during the heating segments. To achieve the maximum reduction process, the hydrogen flow and temperature were maintained as 10 ml/min and 800 °C for 2 h, respectively. The final product was made into fine powders and was used for the fabrication of PVA/(CFO/CF) composite films. 2.1.2. Fabrication of PVA/(CoFe2O4/CoFe2) composite films Free-standing PVA/(CFO/CF) composite films were fabricated by solution casting method as follows: 1 g of PVA powder was dissolved in 20 ml of double distilled water at 90 °C for 20 min. After ensuring the complete dissolution of PVA, the desired amount of magnetic CFO/CF particles was loaded. The solution was mechanically stirred and continuously sonicated for 30 min to achieve uniform dispersion of CFO/CF particles. The uniform solution thus obtained was casted on wellcleaned glass substrates and dried at ambient condition for 2 days. Later, the dried films were heat-treated at 60 °C for 3 h. The samples were cooled down to room temperature and the films were peeled off from the glass substrates. To study the effect of magnetic filler on various functional properties of PVA/(CFO/CF) composite films, 5 and 15 wt% (wt%) of fillers to polymer ratio were taken. The concentration of the PVA solution was kept constant to maintain the thickness of the film in the range of about 65 ( ± 5) μm. As a lot of magnetic nanoparticles lead to aggregation and non-uniform distribution, we decided to use the concentration of filler ≤ 15 wt%. The fabricated composite films were of free-standing, flexible and transparent as displayed in Fig. 1. For the convenience, the sample codes for polyvinyl alcohol, PVA/(CFO/CF) 5 wt% and PVA/(CFO/CF) 15 wt% are named as PVA, PCF5, and PCF15, respectively. 2.2. Characterizations The crystal structure and phase formation of CoFe2O4/CoFe2 (CFO/ CF) and PVA/(CFO/CF) composite films were found by X-ray diffraction (XRD) analysis. XRD patterns were obtained using Cu-Kα radiation for 2θ value range from 10 to 80° with the step of 0.02° using Model Bruker-axs 104025-0. The morphology and the compositional details of CFO/CF composite were recorded by using a scanning electron microscopy (SEM) model ZEISS EVO MA10 attached with the energy 212

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Fig. 1. Photographs of flexible PCF5 composite film (Left side), and transparent nature of A. PVA, B. PCF5 and C. PCF15 composite films (Right side).

dispersive X-ray spectroscopy (EDS). SEM of PVA and its composites were captured by using SEM model JEOL JSM 6380. Transmission electron microscopy (TEM) of reduced CFO/CF was obtained by TEM model HITACHI HT7700. IR vibrational spectra for three samples were characterized in the wavenumber range 4000–400 cm−1 using Perkin Elmer spectrophotometer (Spectrum RX1). UV-visible absorption spectra from 190-600 nm were recorded at room temperature using UVvisible spectrophotometer model Shimadzu UV-1800. In addition to the absorption and transparent properties, the luminescence of the composite films was recorded using fluorescence spectrometer model PLPerkin Elmer LS 45 at the excitation wavelength of 330 nm. The magnetic properties of the samples were measured using a vibrating sample magnetometer (5TminiVSM from Cryogenic Ltd.) with a maximum applied field of 7.2 and 40 kOe for room temperature and 5 K (−268 °C), respectively. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) were performed using TA instrument model TGA-Q50 by heating from 30 to 700 °C at the rate of 10 °C/min under nitrogen gas flow (60 ml/min). The melting and crystallization behavior of pure PVA and composites were recorded in the temperature range from 25 to 300 °C by using differential scanning calorimetry (DSC) with a model DSC 822e. Here, the heating rate and nitrogen gas flow of 5 °C/min and 30 ml/min, respectively, were used. In addition to the above characterizations, the mechanical studies on PVA/(CFO/CF) composites were performed at room temperature using Zwick/Roell Z2.5 universal tensile testing machine equipped with a 2.5 kN load. The load was applied until the rapture was taken place.

Fig. 2. X-ray diffraction patterns of CFO/CF, PVA, PCF5 and PCF15 composite films.

[40,41]. The composites, PCF5, and PCF15 also show the peaks that correspond to pure PVA. In addition, the emergence of the peak at 44.7° in PCF5 and PCF15 films confirms the incorporation of CFO/CF phase in PVA matrix. The intensity of diffraction peaks at 44.7° and 65.13° increases, conversely the intensity of PVA diffraction diminishes with increase of CFO/CF loading from 5 to 15 wt%. The decrease in X-ray intensity of PVA with CFO/CF concentration (shown in the inset of Fig. 2) implies that the changes in the structural regularity of the main chains of the polymeric molecules on nanoparticle incorporation. Thus, leads to drop in the degree of crystallinity of polymer [42]. The crystalline peaks of PVA and CFO/CF are differentiated by the symbols as shown in Fig. 2. Fig. 3(a), (b) and (c) show SEM, TEM and EDS details of thermally reduced CFO/CF phase, respectively. SEM and TEM micrographs show that CFO/CF phase has a strong agglomeration of particles. They present aggregates and seem like a larger particle of size more than 100 nm composed of many numbers of small particles of about 8 to 12 nm. However, the TEM image presented here is incompetent for the estimation of the particle size distribution of CFO/CF due to severe agglomeration. This issue is usually observed at high-temperature reduction process of nanoparticles. The atomic weight percentage of Fe, Co and O elements listed in Fig. 3(c) endorses the formation of CoFe2 along with a few fraction of CoFe2O4 phase. The presence of oxide phase in the sample is due to incomplete reduction of CoFe2O4 into CoFe2 phase. However, these minor amounts of oxide phases are not detected in X-ray diffraction pattern discussed earlier. The surface feature and the distribution of CFO/CF phase in PVA matrix are displayed in two different magnifications as shown in Fig. 4. The micrographs of pure PVA, PCF5, and PCF15 prove that the films are non-porous and CFO/CF phase is uniformly distributed all over the PVA matrix. At high magnification, the PCF5 and PCF15 composite films display the aggregation of CFO/CF phase. As CFO/CF filler is high magnetic in nature, the accumulation of particles became unavoidable though the sonication is used for the dispersion of magnetic particles. This is most commonly observed in the synthesis of polymer nanocomposites [43]. However, the micrographs convey that good quality composite films with minor aggregation are achieved. IR vibrational spectra of pure PVA, PCF5, and PCF15 composite films are shown in Fig. 5. The characteristic vibrations of pure PVA are observed at the wavenumbers 3270, 2939, 2909, 1649, 1561, 1413, 1327, 1237, 1142, 1087, 916, 835, 474 and 414 cm−1. The broad vibration at 3270 and low intense peak at 1649 cm−1 are due to the OeH stretching vibrations from intermolecular and intramolecular hydrogen

3. Results and discussion 3.1. Structural studies X-ray diffractogram of CFO/CF composite, pure PVA, and its PVA/ (CFO/CF) composites are shown in Fig. 2. The typical XRD pattern of the reduced sample (CFO/CF) matches with the standard diffraction data JCPDS No.48-1816 and it belongs to the cubic structure of CoFe2 phase. The interplanar spacing (d), the lattice constant (a), the volume of the unit cell (a3), and crystallite size of the reduced CoFe2 phase are estimated to be 2.025 Å, 2.864 Å, 23.491 Å3, and 31.14 nm, respectively and are equal to the values mentioned in the JCPDS No.48-1816. Further, these parameters reveal that cobalt ferrite nanoparticles are thermally reduced into CoFe2 phase. No other phase found in the X-ray diffraction pattern of CFO/CF confirms that the maximum percentage of reduction is achieved. However, the presence of a negligible fraction of oxide phase can be evidenced by the elemental analysis and magnetization value. Pristine PVA exhibits characteristic diffraction at 2θ values, 19.98°, 28.88° and 30.82° which correspond to (1 0 0), (1 1 0) and (1 0 1) planes of semicrystalline PVA, respectively [39]. The small reflections observed in higher angles are due to the crystalline peaks of PVA and the pattern is very similar to the XRD of PVA reported in the literature 213

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58.3 to 50.7% with an increase of CFO/CF filler from 0 to 15 wt% and it is clearly evidenced from the inset shown in Fig. 5. The absorption band at 1141 cm−1 becomes weaker with further increase of filler concentration corroborating the low degree of crystallinity. The possible interaction mechanism and the reason for losing the crystalline nature of PVA are explained as follows. Fig. 6 shows the schematic representation of the possible interaction mechanism between PVA and CFO/CF magnetic filler. From the Fig. 6(a), it is realized that pure PVA has strong intramolecular interactions between the polymer chains, where the hydroxyl group, OeH of the polymer chain is electrostatically interacting with OeH group of another polymer chain. The presence of strong intramolecular interaction is soundly supported by the broad and strong absorption band observed at ∼3270 cm−1 of pure PVA as shown in Fig. 5. The dispersion of CFO/CF phase in PVA molecular chains is shown in Fig. 6(b). The schematic representation is shown in Fig. 6(c) conveys that the incorporation of CFO/CF filler into PVA matrix abruptly reduces the intramolecular interaction between OeH groups of the polymer chains and it is said to be hydrogen bond barrier effect. Moreover, the interaction is further diminished with the increase of CFO/CF filler. The addition of CFO/CF filler cut off the hydrogen bonds between the polymer molecules as it is observed in graphene/graphene oxide nanosheets in PVA matrix [19]. And it is clearly evidenced by the sudden drop in the intensity of vibrations that correspond to the functional groups of PVA as shown in Fig. 5. However, due to the presence of a fractional amount of CoFe2O4 on CoFe2 phase (as confirmed from EDS), it is expected that some electrostatic interactions are formed between CFO/CF phase and hydroxyl group (OeH) of PVA matrix as represented in Fig. 6(c). The interaction becomes weaker due to the less number of oxygen ions available for bonding. The existence of a weak interaction between the oxide phase and hydroxyl groups and the dissociation of hydrogen bond among the hydroxyl groups of PVA are confirmed from the small shift in the absorption bands of OeH groups from 3270 to 3265 cm−1, and 1649 to 1646 cm−1. The weak interaction between them does not favor the crystallization rather it inhibits the crystallization of PVA that results in a decrease in the degree of crystallinity as it is observed and reported in various PVA nanocomposites [17,49]. Fig. 3. (a) SEM, (b) TEM, and (c) elemental composition of CFO/CF phase.

3.2. Magnetic studies on CFO/CF and PVA/(CFO/CF) composites

bonds of PVA [44–46]. The wavenumbers observed at 2939 and 2909 cm−1 are attributed to CH2 anti-symmetric and symmetric stretching vibrations, respectively from the alkyl groups of PVA [45]. Moreover, a sharp and low intense peak at 1413 cm−1 is ascribed to CH2 stretching and the peaks at 916 and 835 cm−1are due to CH2 rocking vibrations of PVA. The CH-OH bending and CeH wagging vibrations of PVA are observed at 1327 and 1237 cm−1, respectively and they are weak. A low intense and weak C]C vibration of PVA is observed at 1561 cm−1. Lastly, CeOeC bond, CeO stretching, CeO wagging and CeO bending vibrations of PVA are observed at 1142, 1087, 474 and 414 cm−1, respectively [45]. The composite films, PCF5, and PCF15 also exhibit the vibrations that correspond to pure PVA and no other extra peak is found in the spectra. However, the strength of functional group vibrations is strongly influenced by CFO/CF magnetic filler and the intensity of vibration decline with the increase of CFO/CF concentration. In addition to that, a small shift in the absorption band (3270–3265 cm−1, 2909–2917 cm−1, 1649–1646 cm−1, 1413–1411 cm−1, and 835– 830 cm−1) is observed. These behaviors indirectly say that the interaction between PVA and CFO/CF filler leads to the weakening of the functional group vibrations of PVA. The degree of crystallinity of PVA and its composites is calculated by using the equation β = Ic /(Ia + Ic ) , Where, Ic and Ia are the intensity of absorption bands at 1141 cm−1 of the crystalline phase and 1087 cm−1 of the amorphous phase of PVA, respectively [47,48]. As the intensity of absorption band at 1087 cm−1 is independent of the degree of crystallinity, it is considered as the reference for the comparison and calculation. The degree of crystallinity decreases from

The magnetic properties of CFO/CF, PCF5, and PCF15 composites are measured at room temperature as shown in Fig. 7(a) and (b). The MH loop of PCF5 and PCF15 films at 5 K (−268 °C) is shown in Fig. 7(c). As the M-H curves of CFO/CF and PVA composite films are not saturated at the applied magnetic field, the saturation magnetization of the samples is obtained by normalizing the magnetic response with its weight and then extrapolating the high field region of M versus 1/H to H = 0 [50] as shown in the inset of Fig. 7(a) and (b). The saturation magnetization (Ms ), remanence magnetization (Mr ) and coercivity (Hc ) of CFO/CF phase are 176.6 emu/g, 4.25 emu/g and 60.41 Oe, respectively. High magnetization and low coercivity of reduced CFO/CF phase are due to the presence of high percentage of soft magnetic CoFe2 phase and low fraction of hard magnetic CoFe2O4 phase, respectively. From the magnetization value (58.6 emu/g) of CoFe2O4 reported for the sample calcinated at 800 °C for 2 h [51] and the present sample CFO/CF of 176.6 emu/g, the percentage of reduced phase are estimated approximately. It gives that, the present reduction process results CoFe2O4/CoFe2 composites composed of approximately 67% of CoFe2 and 33% of CoFe2O4 phases. With the aim to convert the non-magnetic PVA matrix into a functional magnetic polymer composite, the reduced CFO/CF is incorporated into plain PVA matrix. The M-H hysteresis loops of PCF5 and PCF15 display soft ferrimagnetic nature at room temperature and 5 K (−268 °C). The characteristic curves of polymer composites are very similar to the M-H curve of CFO/CF filler as shown in Fig. 7(b) and convey that the non-magnetic PVA matrix is positively converted into a 214

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Fig. 4. SEM micrographs of pure PVA and PVA/(CFO/CF) composite films.

Fig. 6. (a) PVA, (b) CFO/CF dispersion in PVA matrix and (c) Possible interaction mechanism between filler and PVA matrix.

respectively. The magnetization of PCF15 composite is increased about 133.7% of the PCF5 with an increase of CFO/CF content. It clearly states that the magnetization of the composites is strongly dependent on the content of magnetic CFO/CF filler. Moreover, the increasing trend of magnetization further supports the uniform distribution of CFO/CF composite into PVA matrix as observed in SEM micrographs. At low temperature, the magnetization of the composites saturated and exhibit ferrimagnetic nature with coercive field as shown in Fig. 7(c). Ms , Mr and Hc values of PCF5 are 20.48 emu/g, 1.149 emu/g and 130 Oe, respectively and for PCF15 the values are 46.4 emu/g, 2.363 emu/g

Fig. 5. IR vibrational spectra of pure PVA and PVA/(CFO/CF) composite films.

soft ferrimagnetic composite. At room temperature, Ms , Mr and Hc of PCF5 composite are 18.13 emu/g, 0.334 emu/g and 81.07 Oe, respectively. The magnetic properties of PCF15 composite are enhanced with increase of CFO/CF filler from 5 to 15 wt% and the values are 42.37 emu/g, 1.398 emu/g and 69.06 Oe for Ms , Mr and Hc , 215

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increase the filler content from 5 to 15 wt%, it reaches the point called the percolation threshold. At the concentration of 15 wt%, CFO/CF particles interact with neighboring magnetic particles due to agglomeration and create the percolation path. In addition, the interaction between the magnetic moments of neighboring CFO/CF particles are strong enough to align themselves along the field direction, thereby reducing the pinning effect imposed by a non-magnetic PVA matrix. Xiao et al. explained similar trend for Ni0.5Zn0.5Fe2O4/BaTiO3 composite with varying ferrite content [53]. In addition to the significant magnetic properties proved in PVA/(CFO/CF) composite films, we are also interested and discussed the optical absorption and emission performance in the following sections.

3.3. Optical properties of PVA/(CFO/CF) composite films Fig. 8(a) shows the UV-visible absorbance spectra of pure PVA and PVA composite films. It displays that the absorbance of films decreases in the wavelength range from 190 to 250 nm and becomes stable at higher wavelengths. This indirectly says that all the films are transparent to the entire visible wavelength above 250 nm. In addition, the absorbance of the PVA films increases whereas; the transparency of 68% of pure PVA is reduced to 64 and 38% with an increase of CFO/CF filler from 5 to 15 wt%, respectively. Usually, PVA polymer molecules exhibit prominent absorbance band at 193–195 nm refers to the most favorable syndiotactic conformation of PVA and the wavelength of 277–270 nm is due to the existence of atactic or isotactic PVA polymer conformations [54]. PVA and its composites show weak and broad absorbance in the wavelength range of 250–300 nm as shown in the inset of Fig. 8(a). The wavelength band at the maximum absorbance edge is shifted from 277 nm to a lower wavelength of 271 nm confirms the tuning of optical band gap of PVA with an increase of CFO/CF filler. Furthermore, it approves that the effect of filler dominates over the possible small variation in thickness of the composite films. The optical band gap of the polymer nanocomposite films is estimated by using Tauc’s relation [55]. A plot of (αhv )2 versus photon 1 energy (hv ) and (αhv ) 2 versus photon energy (hv ) are evaluated for direct and indirect transitions, respectively as shown in Fig. 8(b) to (d). The direct band gap of PVA of about 6.35 eV enhanced to 6.41 eV and the indirect band gap of PVA of 6.12 eV reduced to 6.03 eV with increase of CFO/CF filler. The variation of optical band gap infers that the addition of CFO/CF filler modifies the electronic structure of PVA molecules. In addition, the structural disorder of PVA increases thereby the degree of crystallinity of the polymer decreases with CFO/CF filler may also be the reason for the changes in the optical band gap [56]. The direct band gap of PVA is close to the value of 6.28 eV [57] and higher than the value of 4.96 eV reported for pure PVA [58]. Furthermore, they reported that the optical band gap of pure PVA is reduced to 3.18 eV and 4.78 eV with 12 wt% of CuO and 6.5 × 10−4 g of Ag nanoparticles, respectively [57,58]. However, in the present case, the optical band gap of PVA is not significantly reduced even at 15 wt% of the filler content. This further gives the information that the tuning of optical band gap is not only depends on the particle size and concentration of the filler but also on the nature of filler. In addition to the optical absorbance and band gap performance with filler concentration, the photoluminescence (PL) spectra of PVA, PCF5 and PCF15 films are recorded at room temperature as shown in Fig. 9. Pure PVA and PVA-based composite films show broad emission band from 362-546, 369–530 and 370–521 nm for PVA, PCF5, and PCF15, respectively, where the maximum emission intensity is observed near blue light as shown in Fig. 9. The maximum emission intensity in PL spectra is shifted towards red region compared to the maximum absorption observed in UV-visible spectra. This may due to Stokes’ shift [59]. The optical band gap estimated from Tauc’s plot ∼ 6 eV corresponds to π → π∗ electronic transition of aromatic CeC band of PVA [54]. However, the broad emission observed for PVA

Fig. 7. M-H loops of (a) CoFe2O4/CoFe2 composite at room temperature, PVA/(CFO/CF) composites (b) at room temperature and (c) at 5K (−268 °C).

and 120 Oe, respectively. Ms and Mr values are higher than the values obtained at room temperature. The values of Ms and Mr increase with CFO/CF filler at room temperature and low temperature, whereas Hc of the composite decreases. The coercivity of PCF5 and PCF15 composites is higher than CFO/CF phase at room temperature as well as at low temperature. The extra anisotropy induced by a non-magnetic PVA matrix and dipolar interactions may give rise to the coercivity of the composites [52] compared to CFO/CF phase. The decrease of Hc with an increase of CFO/CF content is explained as follows: When we 216

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Fig. 8. (a) UV-Visible absorption spectra, direct and indirect band gap of pure (b) PVA, (c) PCF5 and (d) PCF15 composite films.

bridging of OeH radicals of PVA that leads to decrease in luminescence intensity. The emission of visible light is blocked to some extent at 15 wt% of CFO/CF content due to less number OeH intramolecular chain interaction compared to pure PVA and PCF5 films. These results reveal that the luminescence intensity of the PVA composites could be enhanced only when the OeH groups are dominant. 3.4. Thermal behavior of PVA and PVA/(CFO/CF) composite films In this section, the effect of CFO/CF filler on thermal stability, melting endotherm and the degree of crystallinity of PVA and PVA composite films are discussed. Fig. 10(a) and (b) show the TG-DTG curves and DSC endotherms, respectively for pure PVA and composite films. As shown in Fig. 10(a) the thermal degradation of pure PVA follows three different stages in the entire temperature scan and it is clearly notable in the derivative curves (DTG). The first stage of degradation is observed in the temperature range from Ton , 41 to Toff , 191 °C, where 4.5 wt% of loss is estimated at the maximum temperature (Tmax ) of 120 °C and is due to the removal of absorbed water molecules [63]. In the second stage, the weight loss of about 37.4 wt% is observed in the temperature range from 206 to 348 °C due to the degradation of OeH side chains of PVA [63]. The third stage of weight loss is observed from 393 to 546 °C, nearly 73.1 wt% of loss is observed due to the thermal degradation of CeC backbone or carbonation [64]. Approximately 10.9 wt% of PVA residue is found at the end of the temperature scan (700 °C), which indicates the presence of few crystalline phases of pure PVA. The PCF5 and PCF15 composite films also show three stages of degradation behavior that corresponds to the H2O, degradation of OeH, and CeC backbone as observed in pure PVA. The details of degradation and the percentage of weight loss are summarized in Table 1. In the first and second stages, the TGA curve of the composite films

Fig. 9. PL spectra of PVA, PCF5 and PCF15 composite films.

and PVA composites correspond to the n ← π electronic transition of OeH groups of isotactic, syndiotactic and atactic conformations of PVA [54,60]. Such a broad PL spectrum is commonly observed in PVA based composite films [43,60,61]. About 25 to 58% of luminescence intensity of pure PVA is suppressed and eventually shifted to lower wavelength with increase of filler. The visible light emission in pure PVA is only due to the presence of the resonance electronic excitation with the vibronic levels of OeH stretching [62]. As it is evidenced from the FTIR spectra shown in Fig. 5, the incorporation of CFO/CF filler into PVA host matrix and the interaction between them decrease the intra and inter-chain 217

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Fig. 11. Force versus strain variation of PVA, PCF5 and PCF15 films.

polymer. The degree of crystallinity of the polymer is estimated using ΔH the following equation X c (%) = ( ΔH × 100) , where ΔH100 and ΔH are 100 the enthalpy of 138.6 J/g of 100% crystalline pure PVA and enthalpy of the testing sample, respectively [21], and the data are summarized in the inset of Fig. 10(b). The crystallinity of pure PVA is 39.3% and it is apparent that about 29.3% of PVA crystallinity is reduced to 27.8% with an increase of CFO/CF content. The values are not close to the degree of crystallinity estimated from FTIR spectra however, it is reasonably supported by the trend observed. 3.5. Mechanical properties of PVA and composite films Fig. 10. (a) Thermal degradation and (b) Melting endotherm of PVA and PVA/(CFO/CF) composites films.

The mechanical properties of PVA and composite films are characterized to illustrate the effect of CFO/CF content on the tensile strength behavior. The force versus strain curves for PVA and PVA composite films are shown in Fig. 11. The tensile strength, strain at ultimate strength, tear strength and strain at break are evaluated from the graphs and are summarized in Table 2. As it seen from Fig. 11 and Table 2 that, pure PVA follows the elastic region until 6.8% of strain and later falls under plastic deformation. It has the maximum tensile strength of 89.4 MPa at 6.8% and the tear strength of 18.0 MPa at 68.4% of strain. The tensile strength of pure PVA presented in this report is higher than the values, 77.4 MPa [20], 38 MPa [65], and 50 MPa [66] reported for pure PVA. From Fig. 11, it is understood that the strain versus force curves of the composite films varies linearly until it reaches 3.5 and 2.2% of strain at ultimate yield strength for PCF5, and PCF15, respectively and becomes plastic until it breaks. The reinforcement of CFO/CF ultimately reduces the tensile strength of pure PVA approximately from 22.4 to 72.3% (69.4 to 24.8 MPa) with an increase of filler from 5 to 15 wt% rather than enhancing the mechanical properties. In addition, strength at break and strain of the composite films also decreases. However, the

shows faster degradation compared to pure PVA. However, the third degradation is observed at slightly high temperature. Above 350 °C, TGA curves of the composite films exhibit low weight loss compared to pure PVA, which is evident from DTG curve and wt% listed in Table 1. By comparing the weight loss of PVA, PCF5 and PCF15 films, the weight percentage of filler into PVA matrix is estimated to be 6.3 and 12.2 wt% for PCF5 and PCF15 films, respectively, which is close to weight percentage used for film fabrication. The DSC heating curves of pure PVA and composite films are displayed in Fig. 10(b). The temperature of samples is heated from 25 to 300 °C at a heating rate of 5 °C/min. It is clear from the Fig. 10(b) that all the films exhibit single endothermic peak and the melting temperature slowly decreases from 233.5 to 232.8 °C with the increase of CFO/CF content into PVA matrix. This result indicates that the great changes in the intramolecular interaction between the neighboring polymer chains and insignificant intermolecular interaction between filler and matrix do not significantly affect the melting behavior of Table 1 Thermal degradation of PVA and PVA/(CFO/CF) composite films at various stages. Samples

Stage 1

PVA PCF5 PCF15

Residue wt%@ 700 °C

Thermal degradation temperature at various stages Stage 3

Stage 2

Ton °C

Toff °C

Tmax (°C) and loss in wt%

Ton °C

Toff °C

Tmax (°C)and loss in wt%

Ton °C

Toff °C

Tmax (°C)and loss in wt%

49 47 38

191 161 158

120 (4.47%) 105 (3.55%) 100 (3.16%)

206 198 196

348 319 308

270 (37.4%) 260 (34.43%) 260 (36.65%)

393 381 382

546 540 544

450 (73.12%) 455 (70.56%) 450 (64.14%)

218

10.87 17.21−10.87 = 6.34 23.10−10.87 = 12.23

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most of the reports were deal only with magnetic properties. Hence, it is decided to compare its magnetization value with the literature reports. The room temperature Ms values are extracted and are plotted versus weight percentage of filler content as shown in Fig. 13. The magnetization of the composites varies almost linearly with ferrite content as shown in Fig. 13. It reveals that composites synthesized at a different range of ferrite content from 0 to 95 wt%, where the maximum magnetization of 46 and 51 emu/g was reported for PVA/Fe3O4 composite with 33 and 43 vol% of Fe3O4 content, respectively [32]. This value is higher than the Ms value of 42.37 emu/g of the present (PCF15) composite fabricated with 15 wt% of CFO/CF filler. Ferromagnetic PVA/ CoFe2O4 composite synthesized with 80 wt% of ferrite content showed Ms of 37.8 emu/g [22] which is close to the Ms value of PCF15 composite. The PVA/Fe3O4 composite with a high weight percentage of (95 wt %) Fe(CO)5 content displayed Ms value of 35 emu/g [29]. PVA/(CFO/ CF) composite (PCF5) with 5 wt% of CF/CFO filler exhibits the magnetization close to the value of 20 emu/g of PVA/Fe3O4 [32] with 11 wt % of filler. The magnetization of PVA/Fe3O4 nanofibers [30] and PVA/ γ-Fe2O3 composite [31] with 15 and 1.5 wt% of filler, respectively are comparable and it is close to 6.5 emu/g. In the series, the magnetic behavior of PVA/sPS-CoFe2O4 [23], PVA/CoFe2O4 [24], PVA/Fe3O4 [30], PVA/Fe3O4 (3 wt%) [32], PVA/BiFeO3 [33], and PVA-PANI/ BiFeO3 [34] composites within 0 to15 wt% of ferrite contents fall below the value of 18.13 emu/g of the PCF5 film. In addition to these composites, PVA with CoCl2, NiCl2, and FeCl3 salts at 10 and 20 wt% were reported, in which PVA/NiCl2 exhibited the Ms value of 13.06 emu/g [67]. Furthermore, the rare earth elements Gd3+, Ho3+, Cr3+, and Ti3+ were doped at PVA matrix showed comparatively very weak magnetism [68,69]. It concludes that PVA composites synthesized with the present compositions exhibit appreciable magnetization compared to others even at a low weight percentage of filler content. Overall, the different magnetization and magnetic behavior observed in the composites are attributed to the nature of ferrite filler, the concentration of the filler (wt%), particle size, morphology and also on the interaction between the filler and polymer matrix. When comparing the optical properties of the composites, UV and blue light emissions near 311 and 434 nm were reported for PVA/rare earth composites [68,69] which are close to the emission observed in the present composite. As far as the mechanical properties concern, PVA/BiFeO3 composite showed the tensile strength of 40 and 21 MPa for 0.04 and 5 wt% of BiFeO3, respectively [33] and are less than 69.4 and 24.8 MPa for 5 and 15 wt% of CFO/CF filler, respectively of the present composite. The detailed analysis of the present composites undoubtedly reveals that the PVA composite fabricated with different weight percentage of CFO/CF content is multifunctional smart material and it could be utilized for different applications. It is also suggested that either one should have to use the same matrix or highly transparent matrix with appropriate fraction of CFO/ CF composite for the fabrication of the magneto-optical devices.

Table 2 Mechanical properties of PVA and PVA/(CFO/CF) composite films. Mechanical properties

PVA

PCF5

PCF15

Tensile strength, (MPa) Strain at ultimate strength (%) Tear strength, (MPa) Strain at break, (%)

89.4 6.8 18.0 68.4

69.4 3.5 22.1 18.4

24.8 2.2 4.9 14.9

tensile strength of the PCF5 film is comparable to that for 1 wt% of graphene oxide in PVA matrix [20]. The decrease in tensile strength of pure PVA with an increase of filler content is due to the decrease in intramolecular interaction between the polymer chains and also by the degree of crystallinity supported from FTIR and DSC analyses. The above results clearly evidence that the films fabricated in the present work are flexible and it can withstand an appreciable amount of load during the measurements based on the application but within the declared tensile limit.

3.6. Performance analysis and comparison of the results With the aim of studying the performance of pure PVA and PVAbased composite films on various functional properties, the value of saturation magnetization (emu/g), optical transparency (%), photoluminescence emission intensity (%), degree of crystallinity (%) from DSC, and tensile strength (MPa) for flexibility are considered for PVA, PCF5 and PCF15 films. For better understanding, the values are compared using a pie chart. In this protocol, the properties of the samples are compared together and the maximum and minimum performance is represented by a value in percentage (%) as shown in Fig. 12(a) to (e). For example, it finds the total magnetization of three samples and ratio of magnetization of the individual sample to the sum multiplied by 100 is represented in the pie chart. Similarly, other properties of the samples are also compared and given in percentage (%). The percentage mentioned in Fig. 12 is only for the evaluation of the best sample. It is very clear from the pie chart that the functional properties of pure PVA are superior to PCF5 and PCF15 composite films other than the magnetic performance. It exhibits significant transparency (40%), visible light emission (46.28%), the degree of crystallinity (41.11%) and mechanical properties (48.69%). When we look at the performance of the PCF5 film, the development of magnetization (29.97%) in addition to the functional properties of PVA proves the multi-functionality of the composite. However, the optical (34.34% of emission) and mechanical (37.8%) properties of the PCF5 film are lower than that of pure PVA. The magnetization of the PCF15 film is enhanced (70.03%) when compared to PVA and PCF5 films, but the important optical (19.38%) and mechanical (13.51%) properties are reduced. The optical, magnetic and mechanical properties of the composite film’s concern, the overall performance of the film prepared with 5 wt% of CFO/CF content is better compared to PVA and PCF15 films and is said to be functional composite. Also, the present analysis suggests that by carefully varying the weight percentage of CFO/CF reinforcement from 5 to < 15 wt% even better multifunctional composites with tunable properties could be achieved. In addition to the performance analysis, we made a comparison with earlier reports to show the importance of the present composites. A large number of different kinds of PVA based composites were synthesized and reported for various applications. However, PVA composites synthesized with different concentration of ferrite content and the investigations on the magnetic (saturation magnetization (Ms ), emu/g), optical (PL emission, nm) and mechanical properties (flexibility, MPa) are preferred for the comparison. The following articles [22–24,29–34,67–69] are figured out based on PVA/ferrite composite and are used for the analysis. It is clear from the reports that the important functional properties are not available for the comparison, as

4. Summary and conclusions Free-standing and flexible polyvinyl alcohol/(CoFe2O4/CoFe2) composite films fabricated by solution casting method exhibited multifunctional properties at room temperature. The thermally reduced CoFe2O4/CoFe2 sample was used as filler and it showed a cubic structure with average crystallite size of 31.14 nm, non-uniform morphology, and soft ferrimagnetic nature with a saturation magnetization of 176.6 emu/g. The composition of CoFe2O4/CoFe2 was estimated to be 67 and 33% of soft CoFe2 and hard CoFe2O4 phases, respectively. The effect of 5 and 15 wt% of filler content on various functional properties of PVA composite films were elaborated. The reinforcement and uniform distribution of CoFe2O4/CoFe2 filler into PVA matrix was confirmed through X-ray diffraction and electron microscopic analyses. The composite films displayed non-porous nature even at 15 wt% of filler content. The vibrational analysis concluded that the incorporation 219

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Fig. 12. Performance of PVA and composite films on various functional properties, (a) Magnetization, (b) Optical transparency, (c) PL emission intensity, (d) Degree of crystallinity, (e) Tensile strength.

temperature and low temperature. It showed the saturation magnetization of 18.13 and 42.37 emu/g for 5 and 15 wt% of CFO/CF content, respectively, at room temperature. The optical band gap and visible light emission near 362–546 nm of PVA were also influenced and decreased with CFO/CF content. The thermal degradation of the composite was shifted to lower temperature compared to pure PVA showed the decrease in melting temperature of PVA. In addition, the mechanical properties of the composites resulted that the tensile strength of pure PVA was reduced from 89.4 to 24.8 MPa with 15 wt% of CFO/CF content. In general, the results conveyed that CFO/CF content mainly affected the intramolecular interaction between the PVA molecules and enhancing such kind of interaction was more important to retain the optical, thermal and mechanical properties of pure PVA. Based on the performance analysis and comparison it was suggested that PVA composite films fabricated with CFO/CF content were significant compared to earlier reports. Furthermore, the PVA/(CFO/CF) composite with 5 wt % of filler presented considerable magnetic, transparency, PL emission, thermal and mechanical properties compared to pure PVA and PCF15 films and most suitable for the fabrication of functional devices. Fig. 13. Comparison of the magnetization with earlier reports.

Acknowledgments of CFO/CF content did greatly influenced the functional groups of PVA and it significantly reduced the intramolecular -OH interaction among the polymer chains. The decrease in the degree of crystallinity with filler was evidenced from the decrease in the intensity of the crystalline vibration at 1141 cm−1 and the trend was further supported by DSC results. Both PCF5 and PCF15 films showed ferrimagnetism at room

The authors greatly acknowledge FONDECYT Postdoctoral Research Project No.: 3160170, FONDECYT Project No.:1140195, and CONICYT BASAL CEDENNA FB0807, Government of Chile for the financial support.

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