Synthesis, thermal and magnetic behavior of iron oxide-polymer nanocomposites

2.1 Materials

PUD (Sigma-Aldrich; Code: 446084) was used after dehydration at 100°C for 12 h in vacuum oven. PCL (Sigma-Aldrich; Code: 704105) was dried under vacuum at 60°C for 4 h. 4,4′-Diaminodiphenylmethane (DDM), hexamethylene diisocyanate (HDI; Fluka), a catalyst [dibutyltindilaurate (DBTDL; Sigma-Aldrich)], and Fe₂O₃ nanoparticles (Sigma-Aldrich) were received and used without further purification.

2.2 Synthesis of PUD/PCL-Fe₂O₃ nanocomposite

Dehydrated PUD (40 g) was melted at about 90°C in a preheated mixer. Then, 10 g PCL was added to the melt and the resulting mixture was subjected to 415 rpm rotor speed for 30 min to obtain an 80:20 ratio of polymer blend (PUD/PCL). The temperature of the blend was increased to 120°C and 0.5 g Fe₂O₃ nanomaterial was added slowly and carefully, and the blend was stirred vigorously for 45 min. This is followed by sonication for 1 h to disperse the nanoparticles homogeneously throughout the blend. To this mixture, 2 ml DBTDL was added and stirred for 15 min. Then, heating was stopped and 12 ml curing agent, HDI, and DDM were added and stirred. The prepared nanocomposite was transferred to the mold and dried in a vacuum air oven at 100°C for 1 h and kept overnight at room temperature.

2.3 Characterization methods

The prepared nanocomposites were characterized by the following techniques: Fourier transformed infrared (FTIR) spectroscopy recordings were made using a JASCO-FTIR 4100/Japan spectrometer within the spectral range of 4000 to 400 cm⁻¹ and spectral resolution of 4 cm⁻¹ using KBr pellets. X-ray diffraction (XRD) experiments were performed directly on the samples using a PANalytical X’Pert Powder X’Celerator Diffractometer between the measurement ranges of 10° and 80° in 2θ. Thermogravimetry analysis (TGA) was carried out with Perkin Elmer STA 6000 thermoanalyzer instrument. In each case, about 5 mg specimens were heated from 20°C to 750°C using a linear heating rate of 5°C/min under nitrogen flow. Dynamic mechanical analysis (DMA) measurements were conducted using a DMA Q800 V20.6 Build 24 dynamic mechanical analyzer at a frequency of about 1 Hz over the temperature range 10°C–80°C to room temperature. The scanning rate was 3°C/min and the thickness of film specimen for measurement was 3–5 mm. High-resolution scanning electron microscopy (HR-SEM) images of the gold-coated samples were obtained using FEI Quanta FEG 200 microscope. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were obtained using a Philips model CM 200 microscope at the operating voltage of 20–200 kV. The magnetization measurements of all synthesized samples were carried out using a vibrating sample magnetometer (VSM; Lakeshore VSM 7410 model) at room temperature in the magnetic field range of ±15 kG.

3.1 FTIR analysis

The FTIR spectra (Figure 1) of Fe₂O₃ nanoparticle-filled PUD/PCL composite have revealed the characteristic absorption band at 3320, 3330, 3331, 3332, and 3321 cm⁻¹ corresponding to the N-H stretching of urethane bonds (NH-COO) [26]. Bands at 2925, 2924, 2926, and 2928 cm⁻¹ and 2856, 2857, 2858, and 2859 cm⁻¹ [27] correspond to the asymmetric and symmetric stretching vibration of the methylene group of PU and PCL polymers in the blend. Bands at 1691, 1692, and 1693 cm⁻¹ are observed for blend and nanocomposites, which reflect the presence of the urethane carbonyl group [28]. No band is found at about 2255 cm⁻¹ for composites [29], which reveals that the isocyanate of HDI is completely involved in the curing reaction. The absorption peak at about 849 and 864 cm⁻¹ [30] also appears in the nanocomposites alone, but this peak is absent for the pure blend. The presence of these absorption bands is due to the nanoparticle in the pure polymer blend. As there is no change in the position and presence of the absorption bands of the nanoparticles, it is obvious that there is no physical and chemical interaction between the blend and nanoparticles in the composites. Hence, it is very clear that the prepared materials are complete composites.

Figure 1:

FT-IR spectra of (A) pure blend (B) blend+0.5 wt% Fe₂O₃ nanoparticle (C) blend+1 wt% Fe₂O₃ nanoparticle (D) blend+1.5 wt% Fe₂O₃ nanoparticle (E) blend+2 wt% Fe₂O₃ nanoparticle (F) blend+2.5 wt% Fe₂O₃ nanoparticle.

3.2 XRD analysis

The XRD patterns of Fe₂O₃-filled PUD/PCL nanocomposites and Fe₂O₃ nanoparticles alone are shown in Figure 2A–G. The diffracted intensities were recorded from 10°C to 80°C. Figure 2 shows sharp crystalline peaks for 2θ at 30.272°, 35.684°, 43.34°, 53.852°, 57.4°, and 63.011° and the planes are identified as (220), (311), (400), (422), (511), and (440), respectively, for nanoparticle alone and polymer nanocomposites [31]. These values correspond to the diffraction of γ-Fe₂O₃ (JCPDS 25-1402), indicating that nanoparticles are single phase with tetragonal structure [32], [33]. These values match the various Fe₂O₃-filled nanocomposites prepared, which reveal that Fe₂O₃ nanoparticles exist in the same size as the prepared composites.

Figure 2:

XRD pattern of (A) pure blend (B) blend+0.5 wt% Fe₂O₃ nanoparticle (C) blend+1 wt% Fe₂O₃ nanoparticle (D) blend+1.5 wt% Fe₂O₃ nanoparticle (E) blend+2 wt% Fe₂O₃ nanoparticle (F) blend+2.5 wt% Fe₂O₃ nanoparticle (G) Fe₂O₃ nanoparticle.

3.3 TGA analysis

The thermal analysis and the thermal stability of the nanocomposites and pure PUD/PCL were carried out from room temperature to 800°C and are shown in Figure 3. The thermogram obviously depicts the three to five weight loss during the course of the experiment for the pure blend and nanocomposites. The first weight loss temperature at about 117°C and 132°C is attributed to the evaporation of moisture present in the material. From the literature, the PU thermal degradation starts at about 230°C, corresponding to the degradation of hard segments formed by urethane [34], [35]. However, these nanocomposites have weight loss individually at 266°C, 273°C, 274°C, and 284°C, which is attributed to the degradation of hard segments. This increase in degradation temperature is due to the interaction of PU hard segment with the increasing amount of nanomaterial and with PCL. The thermogram also reveals a weight loss at about 316°C, 332°C, 341°C, 343°C, and 354°C for different nanocomposites; this is due to the degradation temperature for PCL in the blend. These values are almost comparable to the reported pure PCL degradation temperature [36]. The weight loss temperature at about 451°C–458°C is due to the complete decomposition of blend and various nanocomposites prepared.

Figure 3:

TG-DTG thermograms of (A) pure blend (B) blend+0.5 wt% Fe₂O₃ nanoparticle (C) blend+1 wt% Fe₂O₃ nanoparticle (D) blend+1.5 wt% Fe₂O₃ nanoparticle (E) blend+2 wt% Fe₂O₃ nanoparticle (F) blend+2.5 wt% Fe₂O₃ nanoparticle.

3.4 DMA

DMA is used to investigate the dynamic mechanical properties of the pure PUD/PCL blend and Fe₂O₃ nanoparticle-filled polymer nanocomposites. Figure 4A–C represents the storage modulus (E^I), loss modulus (E^II), and damping factor (tan δ) of the blend and nanocomposites as a function of temperature at 1 Hz. Figure 4A shows that 2.5 wt% Fe₂O₃ nanocomposites have higher storage modulus than the other nanocomposites and pure blend. The storage modulus at higher temperature drops due to the loss in stiffness of the blend. The E^I for pure blend at 28.1°C is 169.7 MPa; however, as the concentration of nanoparticles increases in the blend, the storage modulus increases to 165.2, 220.8, 388.47, 695, and 870.7 MPa, respectively. This increase in storage modulus is attributed to the reinforcement of metallic nanoparticles in the blend [37].

Figure 4:

Storage modulus (A), loss modulus (B) and Tanδ (C) of polymer nanocomposites (a) blend (b) blend +0.5 wt% Fe₂O₃ nanoparticle (c) blend +1 wt% Fe₂O₃ nanoparticle (d) blend +1.5 wt% Fe₂O₃ nanoparticle (e) blend +2 wt% Fe₂O₃ nanoparticle (f) blend +2.5 wt% Fe₂O₃ nanoparticle.

Figure 4B shows the variation of loss modulus for pure PUD/PCL blend and for various nanocomposites. The maximum loss modulus value of a material corresponds to its glass transition temperature (T_g) value. When compared to the pure blend, E^II shows an increasing trend with the addition of nanoparticles to the blend. From the figure, the loss modulus values of the nanocomposites are higher than that of the pure blend, but after the T_g the E^II values are comparable, reflecting the characteristic resemblance of the nanocomposites and blend in the rubbery region [38]. The obtained (T_g) from loss modulus is given in Table 1.

T_g corresponds to the temperature at the peak of tan δ curves. Moreover, it is known to estimate the damping behavior of the material. The tan δ values of the pure blend and various nanocomposites are given in Table 1 and illustrated in Figure 4C. When the nanoparticle concentration to the blend increases, the movement of molecular chains at the interface decreases and a reduction in damping factor occurs. The incorporation of Fe₂O₃ nanoparticles to the blend weakens the interfacial bonding interaction between the polymer chains, which results in minimum tan δ peak [39]. The maximum tan δ value corresponds to the T_g of the material and then the composite moves to the rubbery region. In this region, the molecular segments are reasonably free to move and result in the low tan δ value. Hence, it is clear from the investigation that nanoparticle-incorporated polymer blend has better stability than the pure polymer blend.

3.5 TEM and HR-SEM analyses

The surface morphology of Fe₂O₃ nanoparticle-filled polymer composites investigated by TEM and HR-SEM is shown in Figure 5A–E. The TEM bright-field image with corresponding SAED patterns of the Fe₂O₃ nanoparticle-filled polymer blend is shown in Figure 5A–E. All the micrographs showed very high dispersion of Fe₂O₃ nanoparticles within the blend. The average particle size of the TEM image is measured to be ~50 nm [40], which is again a confirmation for the XRD investigation. The SAED pattern (inset) of the samples shows clear diffraction spots, which indicates the high crystallinity of Fe₂O₃ nanoparticles inside the blend.

Figure 5:

HR-SEM & TEM (inset SAED) images of (A) blend +0.5 wt% Fe₂O₃ nanoparticle (B) blend +1 wt% Fe₂O₃ nanoparticle (C) blend +1.5 wt% Fe₂O₃ nanoparticle (D) blend +2 wt% Fe₂O₃ nanoparticle (E) blend +2.5 wt% Fe₂O₃ nanoparticle.

The HR-SEM images reveal that PUD and PCL are extremely miscible with each other and therefore form a uniform blend. For the preparation of the composite, Fe₂O₃ nanoparticles of size <50 nm was used. From the picture, it is observed that nanoparticles are present as granules with small and big spherical particles. The particles are found to exist as single, double, and triple and rarely as quadruple. This clearly indicates that when the concentration of nanoparticles to the hybrid increases the size of the particle also rises. The drying of nanocomposites through heating also aids particle agglomeration in magnetic nanoparticle systems [41]. However, most of the nanoparticles exist as single and are found to be in the nanoscale; hence, the prepared materials are nanocomposites only as supported by XRD analysis as well.

3.6 VSM analysis

The magnetic characteristics of Fe₂O₃-filled PUD/PCL blend of various compositions are obtained by measuring the magnetization with respect to the applied field from −15,000 to +15,000 Oe at room temperature (Table 2). Figure 6 shows the hysteresis curves obtained for the prepared composites. Coercivity (Hc) is an important physical parameter to differentiate hard (>100 Oe) and soft (<100 Oe) magnetic materials [42]. From the graph, it is observed that the coercivity of pure blend is 60.91 Oe and therefore termed as a soft material. When Fe₂O₃ nanoparticle concentrations are increased in the blend, their coercivity values increase from 382.0 to 2920.2 Oe. This indicates that Fe₂O₃ nanoparticles are observed to be magnetically harder after dispersion in the polymer matrix [43]. The magnetic remanence for the blend is 12×10⁻⁶ emu/g, whereas for the prepared nanocomposites the values substantially increase to 46×10⁻⁶ emu/g. Therefore, a greater magnetic moment is left behind even after the removal of external magnetic field for composites, which will help to bind the nanoparticles with the blend. The magnetization saturation (Ms) of the composites has significantly changed from the blend, which has improved the magnetic properties of the composites. This will also assist the magnetic detectability of the composites [44]. The SQR values from 0.5 wt% nanoparticle-loaded blend is found to be 0.0126, which can be attributed to the superparamagnetic character [45], [46]. As the nanoparticle loading is increased in the blend, SQR values tend to increase, which will deviate from the superparamagnetic behavior.

Figure 6:

VSM for (A) pure blend (B) blend+0.5 wt% Fe₂O₃ nanoparticle (C) blend+1 wt% Fe₂O₃ nanoparticle (D) blend+1.5 wt% Fe₂O₃ nanoparticle (E) blend+2 wt% Fe₂O₃ nanoparticle (F) blend+2.5 wt% Fe₂O₃ nanoparticle.