Materials

Polyvinyl butyral S-Lec (PVB) with an average molecular weight of 53,000 g/mol was used in this work, it was supplied by Sekisui, Japan. As precursor salt of Fe⁺², ferrous chloride tetrahydrate (FeCl₂•4H₂O, 99%) was used, it was supplied by Sigma-Aldrich. Sodium hydroxide (NaOH, 97%) supplied by Fermont was used to obtain alkaline conditions. Tetrahydrofuran (THF) supplied by Fisher Scientific was used as solvent. Finally, the hydrogen peroxide (H₂O₂, 3%) used was supplied by Zuum (Mexico City, Mexico). All materials were used as-received for the synthesis of the PMHM.

Synthesis of the polymeric magnetic hybrid material (PMHM)

The process to obtain the PMHM was performed in two stages. In the first one, it was synthesized a precursor film hybrid material, Fe(II)-PVB. In the second stage, the precursor material was used to synthesize iron oxide nanoparticles into PVB matrix. Both stages are described in this section.

Stage 1: 1.6 g of PVB was dissolved into 20 mL of THF, and then the PVB-THF obtained was stirred for 60 minutes (solution 1) at 40°C. On the other hand, a solution of 15 mL 0.268 M FeCl₂•4H₂O using THF as solvent was prepared (solution 2). Solutions 1 and 2 were mixed and stirred for 60 minutes. With the mixture ready, the precursor material is prepared by casting, at room temperature for 24 h, in order to eliminate the solvent by a natural convection process. After that, precursor film hybrid material (Fe(II)-PVB) was obtained. The thickness of the resulting film was measured using a Mitutoyo micrometer, the result obtained was ~20 µm.

Stage 2: In this experimental part, the precursor film hybrid material was submerged in an aqueous solution of 3.3 M NaOH at 50°C, obtaining a rapid change in color, from yellow to black. After that, 30 mL of H₂O₂ was added to the ‘‘black material’’ observing a color change from dark to brownish. Finally, the brownish material was washed several times with deionized water (18 MΩ-cm) to eliminate the residual products (NaOH and Cl^-). After that the PMHM was obtained.

Characterization of precursor material and PMHM

Both the precursor film hybrid material (Fe(II)-PVB) and the PMHM were studied by X-ray diffraction (XRD) to determine their crystallinity, by Fourier transform infrared spectroscopy (FTIR) to determine the vibration modes of chemical groups of the polymer matrix, by high-resolution transmission electron microscopy (HRTEM) to determine the structure and morphology of PMHM, by vibrating sample magnetometry (VSM) to determine the magnetic behavior of PMHM, and by magnetorheology analysis (MRA) to determine the effect of external magnetic field on the viscoelastic properties of PMHM.

The XRD measurements were carried out using a diffractometer Bruker D8 Advance, over the range from 20 to 80 degrees (2𝜃); three samples (PVB, Fe(II)-PVB and PMHM) were pulverized to perform X-ray diffraction analysis. The FTIR spectra were obtained using a FTIR spectrometer, IRAffinity Shimadzu, in transmittance mode, scanning 30 times at 2 cm^-1 resolution from 400 to 4000 cm^-1. For this case, three samples (PVB, Fe(II)-PVB and PMHM) were used as thin films.

The structure and morphology of PMHM was studied by HRTEM. The analysis of transmission electron microscopy was done in a Titan FEI. The specimen was prepared by dispersing the PMHM powder in water using an ultrasonic bath and placing an aliquot of the dispersion onto a cooper TEM grid.

Magnetic properties were measured by VSM at room and low temperature (300 K and 4.2 K, respectively) using a quantum design SQUID-VSM magnetometer. A PMHM-specimen in the form of thin film was used for its analysis.

Finally, the effect of iron oxide nanoparticles on the rheology behavior of PMHM was analyzed by MRA. A film with a thickness of 20 μm was submitted to an oscillatory rheometry test between two parallel plates using an Anton Paar Rheometer MCR301 equipped with a magnetorheological device MRD 70-1T. The oscillating test was developed at room temperature (25°C) applying a constant magnetic field (~11 kOe) and applying a shear strain amplitude of 0.1% in a frequency interval from 0.01 to 100 Hz. The storage modulus (G′) and the loss factor (tan 𝛿) were registered.

Characterization of the precursor film hybrid material (Fe(II)-PVB)

The microstructure of the precursor film hybrid material (Fe(II)-PVB) was studied by XRD and FTIR analysis. Figure 2a shows the X-ray pattern obtained for the PVB-sample. A broad band is observed at low 2𝜃 values (lower than 50°), which is a typical result for amorphous polymers. Figure 2b shows the diffraction pattern obtained for the Fe(II)-PVB sample. This diagram shows a clear peak that correspond to a peak diffraction associated to iron hydroxides. This peak displayed at 27° is related with goethite phase (α-FeOOH), (^{Luna et al., 2013}; ^{Marinho, 2014}), which was formed because of the tendency of the THF to form peroxides (^{Clark, 2001}; ^{Kumar and Singh, 2014}) that produce oxidation process of Fe²⁺. This diagram (Figure 2b) does not show peaks associated to the FeCl₂•4H₂O (see Figure 2c). Matching between Figures 2a, 2b and 2c suggest that FeCl₂•4H₂O was partially dissolved and partially oxidized into PVB matrix.

Figure 2 X-ray patterns for a) PVB-sample, and for b) precursor hybrid material Fe(II)-PVB, c) FeCl₂•4H₂O 

With respect to the FTIR analysis, the comparison between the FTIR-spectra for PVB and Fe(II)-PVB specimens, allows us to identify if the vibration modes of the chemical groups of the PVB are modified or not due to the presence of Fe²⁺ into the Fe(II)-PVB. Figure 3 shows the FTIR spectrum of a PVB sample, typical bands corresponding to PVB were identified; the broad band at 3486 cm^-1 corresponds to the OH stretching modes; the band at 1739 cm^-1 was attributed to the C=O stretching vibration of the acetate groups, the bands at 2871 cm^-1 and 2956 cm^-1 corresponded to symmetrical and asymmetrical stretching modes of CH₃ and CH₂, respectively; an absorption band corresponding to the C-O-C group of the cyclic acetal at 1130 cm^-1 also is observed, finally another absorption band associated to group C-OH also is observed at 994 cm^-1. These results are in according with results obtained by ^{El-Din et al. (1995)}.

Figure 3 FTIR spectrum of PVB 

Figure 4 shows a matching between the FTIR spectra of PVB, Fe(II)-PVB and the PMHM. Matching between PVB and Fe(II)-PVB samples shown that the vibration modes of chemical groups CH₂, CH₃, C=O, C-OH and C-O-C do not show significant shifts. This behavior is a consequence that precursor salt ions do not interact with these chemical groups in a major way. On the contrary, the vibration modes corresponding to OH groups are affected when the precursor salt (solution 2) is added to PVB (solution 1) to obtain the precursor film hybrid material (Fe(II)-PVB); consequently, bands associated to those chemical groups have a shift towards lower wave numbers. This effect corroborates that OH-groups form with Fe²⁺ on the one hand iron hydroxides (α-FeOOH), (^{Marinho, 2014}) and on the other hand Fe²⁺-ions are coordinated with OH groups of the PVB-matrix (^{Luna et al., 2013}). Table 1 resume the results obtained from FTIR analysis for the three samples studied.

Figure 4 Matching between FTIR-spectrum of PVB, Fe(II)-PVB and PMHM 

Results obtained by XRD and FTIR analysis corroborate that FeCl₂•4H₂O was partially dissolved and partially oxidized into the PVB matrix. Figures 4a and 4c corresponding to PVB and PMHM samples are matched in next section.

Characterization of the PMHM

After chemical treatment (stage 2) of the Fe(II)-PVB with H₂O₂ under alkaline conditions, its change in color was from black to brownish, this fact suggests the formation of iron-oxide nanoparticles embedded into the PVB matrix. To corroborate this fact, ‘‘brownish material’’ was studied by XRD analysis. Figure 5a shows the diffraction pattern obtained. A peak (*) associated to goethite phase is observed, and another diffraction peaks are identified at 30.26°, 35.34°, 39.37°, 53.82°, 56.27°, and 62.54° that correspond to the (220), (311), (320), (422), (511) and (440) plane reflections of maghemite (Figure 5b) or magnetite (Figure 5c) crystalline structures (^{Luna et al., 2013}; ^{Jiang et al., 2014}; ^{Wul et al., 2015b}; ^{ICDD, 2000a} and ^b).

Figure 5 a) Diffraction peaks corresponding to PMHM, b) maghemite and c) magnetite standards 

On the other hand, the interaction between iron oxide nanoparticles and PVB-matrix was studied by FTIR analysis. Figure 4 corresponds to the matching between FTIR spectra of PVB and PMHM samples. With the exception of the vibration modes of OH groups, practically all the other bands corresponding to PVB do not suffer any shift; this means that the synthesized iron oxide nanoparticles interact with the PVB through its OH groups (see Table 1). This fact is very important because it is through the interface between the iron oxide nanoparticles and the polymer matrix that the effect of the body force of an applied magnetic field must be transmitted from nanoparticles to the polymer matrix, generating a surface force that induces the deformation into the PMHM (^{Aowlad et al., 2012}; ^{Philippova et al., 2011}).

The size and morphology of iron oxide nanoparticles into PMHM were studied by HRTEM analysis. The HRTEM images (Figure 6a) show a higher number of iron oxide nanoparticles with sphere-like morphology. Regions corresponding to these nanoparticles were analyzed by energy dispersed spectroscopy (EDAX), and results reveals an elemental composition of Fe 5.15 atm.%, C 69.15 atm.%, O 24.43 atm.% and Na 1.26 atm.%. The average particle size of the iron oxide nanoparticles was measured from different HRTEM-images using image analysis (Digital Micrograph Software of Gatan), and the frequency histogram shows an average size of 5 nm (Figure 6b). These results suggest that the magnetic behavior of the nanoparticles obtained must be superparamagnetic, because according to the literature, the number of magnetic domains decreases until there is a single domain when the characteristic size of material is below some critical size, in consequence small particles at nanometer scale possess a single magnetic domain, consisting of groups of spins all pointing in the same direction and acting cooperatively, and the magnetic behavior produced for these nanoparticles are labeled as superparamagnetic (^{Zhang et al., 2015b}; ^{Teja and Koh, 2009}; ^{Sommertune et al., 2015}). This phenomena was corroborated by magnetization measurements and the result obtained are shown in Figure 8.

Figure 6 a) HRTEM-image of PMHM, b) particle size distribution 

Figure 7 a) and b) HRTEM images in two different regions 

Figure 8 Magnetization curve of PMHM at 300 K 

The HRTEM images of the iron-oxide nanoparticles are shown in Figures 7a and 7b. The well-defined lattice fringes correspond to crystallographic planes of iron oxide nanoparticles and they are identified in both figures.

From these HRTEM images, and using a Gatan Microscopy Suite Software (Version 2.0.1), the interplanar distances of 0.37, 0.29, and 0.23 nm were computed. These computed values correspond to interplanar distances of (210), (220), and (320) planes (^{Jiang et al., 2014}; ^{ICDD, 2000a} and ^b). These crystallographic planes are of maghemite crystalline structure, which is consistent with the XRD analysis. The interplanar distances are very close to the computed values, shown in Table 2.

The magnetic behavior of the PMHM was studied by magnetization measurements. Figure 8 shows the magnetization curve at room temperature (300 K). The saturation magnetization value MS→ is higher than 0.2 emu/cm^-3 (0.638 emu/g Fe₂O₃) whereas both, remnant magnetization, Mr→ and coercitivity, Hc→, are undetectable. These last results corroborate the superparamagnetic behavior of the PMHM, because for a superparamagnetic behavior their magnetization curves not display a hysteresis loop (^{Teja and Koh, 2009}; ^{Philippova et al., 2011}; ^{Sommertune et al., 2015}).

On the other hand, Figure 9 presents the magnetization curve at 4.2 K, and both coercive field and remnant magnetization values are different to zero. It can be seen that the corresponding values for the different magnetic parameters are 0.65 emu/cm³ (2.075 emu/g Fe₂O₃) for saturation magnetization, MS→, 1.81 kOe for coercive field, Hc→, and 0.05 emu/cm³ (0.16 emu/g Fe₂O₃) for remanence value, Mr→ (inset Figure 9). Magnetization curves of Figure 9 correspond to a ferromagnetic behavior, and it is well known that it appears when the magnetic analysis is performed below the blocking temperature, T _B (^{Philippova et al., 2011}; ^{Sommertune et al., 2015}; ^{Kucheryavy et al., 2013}).

Figure 9 Magnetization curve of PMHM at 4.2K 

To estimate T _B , for the PMHM sample, field-cooled (FC) and zero-field cooled (ZFC) analyses (^{Luna et al., 2013}; ^{Joy et al., 1998}) were performed at a magnetic field of 100 Oe (Figure 10). When the PMHM sample is cooled at zero magnetic field (ZFC), Figure 10 shows that the total magnetization is small, but not zero (it is 43% of the maximum), as the magnetic particles are not fully random. When temperature increases, the nanoparticle magnetic moment is oriented with the external field increasing the total magnetization until it reaches a maximum at 25 K which is the value of the T _B . At this temperature, the thermal energy becomes comparable to the energy gained by aligning the nanoparticle magnetic vector in the weak field. At this point, the transition from ferromagnetic to superparamagnetic behavior is observed (^{Joy et al., 1998}; ^{Torres et al., 2014}). When all nanoparticles are at the superparamagnetic relaxation state, above T _B , their magnetization follows Curie’s law decreasing with increasing temperature. In the case of FC, magnetization monotonically increases as the temperature decreases because the nanoparticles are cooled from room temperature under a magnetic field and the magnetization direction of all the nanoparticles is frozen in the field direction (^{Joy et al., 1998}). The magnetization shows the maximum at 4.2 K in the FC process (Figure 10). These result shown that iron oxides nanoparticles embedded into PVB matrix, respond with a superparamagnetic behavior to a magnetic field stimulus, and the superparamagnetic response must be transmitted to polymer matrix trough the interface nanoparticle-polymer, where OH groups plays and important roll.

Figure 10 ZFC and FC magnetization curves at 100 Oe 

Finally, to test the effect of an external magnetic field onto viscoelastic properties of the PMHM, magnetorheological behavior was analyzed, with and without external magnetic field. For the case of the test without external magnetic field, a film of PMHM with a thickness of 20 𝜇m was tested with a sinusoidal strain stimulus, and the response, a sinusoidal stress, was registered for several frequencies from 0.1 to 100 Hz. For the case of the test with external magnetic field, a constant magnetic field of 11 kOe was applied and using the previous protocol. For the two analysis (with and without external field), from the applied stimulus and the registered responses, the complex modulus as a function of frequency was computed, G* = G′ + iG′′. The real part, G′, is associated to stored energy and represents the elastic behavior of the PMHM. On the other hand, the imaginary part, G′′, is related to dissipated energy a represents the viscous behavior of the PMHM. From G′ and G′′ data, the relationship between the stored energy and dissipated energy also known as loss factor was computed, this parameter is represented by tan δ = G´´/G´ (^{Reyes et al., 2004}; ^{Hebert et al., 2008}). For polymeric material, typically, G′ and tan 𝛿 are used to determine the viscoelastic behavior of the samples studied. Typical rheological curves of G′ and tan 𝛿 were obtained (Figures 11 and 12) for the PMHM samples, in both cases when frequency increases, G′ also increases and on the other hand tan 𝛿 decreases. However, when the external magnetic field (11 kOe) is applied, rheological curves are modified. Figure 11 show the effect of 11 kOe of external magnetic field on to G′. A global increases of G′ is observed when magnetic field is applied. In other words, its capacity to store mechanical energy increases when the external magnetic field is applied. This is presumably due to the interactions between iron oxide nanoparticles and the polymer matrix (PVB). Finally, Figure 12 shows that tan 𝛿 do not increases in a very important way, this means that the capacity of the PMHM to dissipate energy (associate to G′′) increases in a similar way to increasing the capacity to store energy (associate to G′).

Figure 11 Magnetorheology measurements of PMHM, G′ vs frequency 

Figure 12 Magnetorheology measurements of PMHM, tan 𝛿 vs frequency 

These latest results, Figures 11 and 12, show that the viscoelastic properties of the PMHM can be modified under action of a magnetic external field. So, it can be considered as a good alternative to carry out functions of actuators or sensors in electronic or mechatronic devices.