Electrospun Fe₃O₄-PVDF Nanofiber Composite Mats for Cryogenic Magnetic Sensor Applications

Abstract

Magnetically responsive, mechanically stable and highly flexible iron (III) oxide-polyvinylidene fluoride (Fe₃O₄-PVDF) piezoelectric composite fiber mats were fabricated via one step electrospinning method for magnetic sensing at cryogenic temperature. The properties of Fe₃O₄-PVDF composite fiber mats were characterized using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, d₃₃ and magnetization test. The fiber diameter decreased as the concentration of Fe₃O₄ increased. The DSC results suggested a decrease in the crystallinity of the composite fiber mats after adding Fe₃O₄, and the XRD curves identified that the decrease in crystallinity took place in the β crystalline phases of the fibers. FT-IR results further confirmed the reduction of β phases of the composite fiber mats which dropped the piezoelectric response of the fiber mats by 38% for 5% Fe₃O₄-PVDF than PVDF fiber but still 400% higher than PVDF pellets. The magnetization test advocated a superparamagnetic state of the fiber at room temperature but a ferromagnetic behavior at a lower temperature. The coercivity values of the mats suggested a homogeneous dispersion of the Fe₃O₄ nanoparticles into the PVDF matrix. Young’s modulus (E) of the fibers remained the same before and after the magnetization test, indicating the mechanical stability of the fiber in the range of 5 K to 300 K. Its mechanical stability, superparamagnetic behavior at room temperature and ferromagnetic at low temperature could open up its application in spintronic devices at cryogenic temperature and cryogenic power electronic devices.

1. Introduction

Magnetically responsive flexible fabrics are a growing need for shielding electromagnetic interference especially in microelectronic applications to reduce growing electromagnetic pollution, sheaths for portable electronics and stealth weapon systems in military applications [1,2]. These flexible fabrics are also useful for different biomedical applications like magnetic cell separation membranes [3], magnetic resonance imaging contrast agents [4], etc. Their application could also be extended to spintronics and power electronic devices at extremely low temperatures [5,6]. Iron (III) oxide (Fe₃O₄) nanoparticles are a valuable additive because of their electronic, magnetic, optical, and mechanical properties [7]. Composite nanofibers/thin films consisting of magnetic nanoparticles have been under intensive investigation in the last few years [7]. There remains a need to create flexible processable magnetically responsive and mechanically rugged films.

PVDF is a widely used piezoelectric semi-crystalline polymer with five different crystalline phases (α, β, δ, γ and ε) [8,9]. Its piezoelectric properties are directly related to its crystal phases [10]. Nonpolar α and polar β phase are the most common crystalline structures of PVDF where the β phase is responsible for the piezoelectric properties. Thereby, increasing β phase with eliminating α phase content in PVDF was a major area of focus for the last decade. Cold-drawing (stretching) [11], high-pressure quenching [12], and poling (applying a high electric field) of PVDF [13] were such processes to increase β phase. Electrospinning is another optional simple one-step method for fabricating PVDF nanofibers under a high electric field which converts the α phase into the β phase [14,15]. The high voltage that is involved in this process, boosts up the β phase of the fibers [16]. Moreover, electrospun fibers mats are highly flexible and mechanically strong compare to the solvent cast films [17].

Different filler materials have been introduced into the PVDF matrix for different functional properties such as Fe₃O₄, MnFe₂O₄ and MgFe₂O₄ for magnetic properties; graphene oxide, CNT for conductive properties, BaTiO₃, PbTiO₃, KNbO₃ for piezoelectric properties [18]. Researchers prepared Fe₃O₄-PVDF nanocomposites films by solvent casting method. Porous systems have been produced using the phase inversion approach [19,20,21,22]. According to their reports, the magnetic property of Fe₃O₄-PVDF nanocomposites increased with an increase in Fe₃O₄ loading. However, their limitations were attributed to the poor dispersion of Fe₃O₄ nanoparticles and poor processability. The ruggedness of composites has also been limited at low temperature applications such as spintronics and power electronic devices at cryogenic conditions (<123 K) [5,6]. Thermal stresses paired with low temperature brittleness have been associated with this weakness. Iron oxide has proved valuable as an additive to polymers to increase low temperature retention of strength in aerospace applications [23]. For example, a modified diglycidyl ether of bisphenol-A (DGEBA) epoxy resin reinforced with carbon fiber showed excellent mechanical performance at cryogenic temperature after Fe₃O₄/GO nanofillers were added to it [24]. The filler material (Fe₃O₄/GO) helped the epoxy to retain its mechanical performance at cryogenic temperature.

The objective of this paper is to explore the viability of iron oxide-PVDF as electrospun textiles retaining mechanical and magnetic performance in cryogenic conditions. The effect of the addition of iron oxide on the polymer is explored through the investigation of the crystallinity and piezoelectricity of the composite fiber mats after the addition of Fe₃O₄ nanoparticles into the PVDF matrix. Magnetic response and mechanical properties were evaluated at room temperature and cryogenic temperature (5 K) to see the effect of cryogenic temperature on the performance of the fiber mats.

2. Materials and Methods

PVDF was obtained from Arkema (Kynar^® 721, pellet form) with the following properties: density: 1.78 g/cm³, melt flow index (MFI): 10 g/10 min, tensile strength: 54 MPa, melting temperature: 168 °C. Fe₃O₄ was obtained from SkySpring Nanomaterials Inc. (Houston, TX, USA) with an average particle size of 20–30 nm. Acetone and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

2.1. Preparation of Fe₃O₄-PVDF Fiber Mats

Firstly, 18 wt% (w/w) PVDF was dissolved in DMF/acetone (2:1) solution by stirring at 50 °C for 2 h using a magnetic stirrer in a 250 mL beaker at 200 rpm. After all the pellets dissolved, Fe₃O₄ nanoparticles (1 wt%, 3 wt% and 5 wt% (w/w) with respect to polymer) were added into the solution. Then, ultrasonication was performed with Sonic Vibracell II (25% pulse amplitude) for 5 min to disperse the Fe₃O₄ nanoparticles in the polymer solution. Next, 5 mL of this electrospinning solution was taken in a syringe and used for electrospinning. The electrospinning parameters were: voltage 15 kV, flow rate 0.05 mL/h, distance 15 cm, and the needle diameter of 0.038 in. At 400 rpm, a spinning roller with nonstick aluminum foil was capturing the fiber. The average thickness of the fabricated fiber mats was 0.1 mm.

2.2. Nanofiber Mats Characterization

Surface morphology of fiber mats was observed via FEI Nova Nano SEM 230 (Thermo Fisher Scientific, Hillsboro, OR, USA). All the samples were sputter coated to reduce the charge development on their surface. Using a Perkin Elmer DSC6 differential scanning calorimeter, the thermal properties of PVDF and Fe₃O₄-PVDF mats were investigated. The scanning temperature range was 30 °C to 210 °C at a rate of 10 °C per minute. Crystal structures were studied using a Rigaku Ultima III X-ray Diffractometer (Rigaku Americas Corporation, The Woodlands, TX, USA). A Scientific Nicolet 6700 Fourier Transform Infrared (FT-IR) Spectroscopy (Thermo Scientific, Waltham, MA, USA) with an attenuated total reflectance (ATR) detector was used to characterize the functional groups in PVDF and Fe₃O₄-PVDF mats. A d₃₃ meter (APC International, Mackeyville, PA, USA) was used to measure the d₃₃ value of the fiber. The fiber mats were rolled to make them thick enough for d₃₃ testing. The d₃₃ testing was done on 5 different spots of the fiber mats and average values were taken. Magnetic properties of the fiber mats were explored by a DynaCool-14 (Quantum Design, San Diego, CA, USA) Physical Properties Measurement System (PPMS) machine. Like in d₃₃ testing, fiber mats were rolled and fitted into the coffee straws to do the magnetization testing. The weights of the samples used in the magnetization test were (0.1 ± 0.01) g.

3. Results

3.1. Scanning Electron Microscopy (SEM)

Scanning electron microscopy images of the fibers as a function of iron oxide concentration were analyzed using ImageJ software to calculate the diameter of electrospun fibers. The fiber diameter reduced gradually as the concentration of the Fe₃O₄ increased as shown in Figure 1 and Figure 2. The average fiber diameter reduced up to 26% for 5% Fe₃O₄-PVDF fiber. Similar results were obtained from electrospun PLA and PMMA fibers loaded with Fe₃O₄ nanomaterials [25]. At room temperature, Fe₃O₄ is a conductor material and adding the Fe₃O₄ particles into the PVDF solution increased the dielectric constant of the solution. A higher dielectric constant means higher polarity in the solution as well as a higher ability to stabilize the charges in the Taylor cone which leads to a decrease in nanofiber diameters [26]. O and Fe signals in the EDX chart of the fiber mats, shown in Figure 3, confirmed the presence of Fe₃O₄ in PVDF fiber. Au and Pd picks came from the sputter coated thin conductive film over the samples.

3.2. Differential Scanning Calorimetry (DSC)

The thermal behavior and crystallinity of the fiber mats were studied using the DSC. The results are presented in Figure 4a, where PVDF and all composite electrospun fiber mats showed a similar melting temperature (T_m) at ~160 °C, indicating no disruption in the PVDF thermal transitions. Furthermore, the melting enthalpy of the nanofiber mats was extracted from DSC curves and used to calculate the degree of crystallinity of the fiber mats based on Equation (1):

$$X_c=\frac{∆H}{∆H^0}\times 100$$

(1)

where X_c is the degree of crystallinity of the fiber mats; ∆H⁰ is the melting enthalpy of 100% crystalline PVDF (105 J/g), and ΔH is the melting enthalpy of Fe₃O₄-PVDF meshes. ΔH was corrected by multiplying it with the mass fraction of PVDF in the composite mats. The results, presented in

Table 1. Enthalpy and percentage crystallinity values obtained from DSC curves.

Sample	Tm (°C)	∆H (J/g)	Corrected ∆H	Xc (%)
PVDF Fiber	160.12	26.40	26.40	25.14
1% Fe3O4-PVDF Fiber	160.46	22.91	22.68	21.81
3% Fe3O4-PVDF Fiber	160.35	15.07	14.62	14.35
5% Fe3O4-PVDF Fiber	160.18	14.52	13.79	13.82

, indicated a reduction in crystallinity and enthalpy as the magnetite content increases. For 5% Fe₃O₄-PVDF, the crystallinity of the fiber mats decreased by up to 55%. Previous experiments on Fe₃O₄-PVDF films yielded similar findings [14,27]. This may be linked to the partial inhibition effect of Fe₃O₄ addition on polymer crystal formation, similar to inorganic fillers like Al₂O₃, TiO₂, c-LiAlO₂, and Sm₂O₃ decreased the crystalline phase of PEO-based polymer electrolyte systems [28,29,30,31,32].

3.3. X-ray Diffraction (XRD)

The X-ray diffraction (XRD) patterns of PVDF and Fe₃O₄-PVDF nanocomposite fiber mats exhibited strong diffraction peak for all samples at 2θ = 20.5° corresponding to β(110) and β(200) planes, as shown Figure 4b. Weaker diffraction peaks at 36° assigned to β(201) planes were also present [33]. These results indicated that electrospun nanofiber membranes contain mainly β-phase crystal structure of PVDF. Due to the high electric voltage (15 kV) used during the electrospinning method, the random electric dipoles in the PVDF solution aligned, resulting in the forming of the β phase crystal structure. Thereby, the piezoelectric property increased. Also, with increasing loading of the Fe₃O₄ in the polymer matrix, the intensity of the peak at 20.5° decreased (7155, 5634, 4684 and 3401 a.u. for PVDF, 1%, 3% and 5% Fe₃O₄-PVDF respectively). So, the addition of the Fe₃O₄ into PVDF did not promote the crystallization of the PVDF in the β phase. This has been ascribed to inhibition of crystallization by the inorganic particles during the solidification process, which decreases the volume fraction of the crystalline phase in the fiber as discussed in the DSC section [34].

3.4. Fourier-Transform Infrared Spectroscopy (FTIR)

To confirm the XRD results, the authors compared the FTIR spectra of electrospun PVDF nanofiber and Fe₃O₄-PVDF nanofiber mats (Figure 5a). The PVDF pellet exhibited strong peaks at 615 cm⁻¹ (CF₂ bending and skeletal bending), 762 cm⁻¹ (CF₂ bending), 795 cm⁻¹ (CF₂ rocking), and 976 cm⁻¹ (CH out-of-plane deformation) which were all recognized as α phase and relatively weak peaks at 840 cm⁻¹ (CH₂ rocking), 877 cm⁻¹ (CF₂ rocking), 1273 cm⁻¹ (CF out-of-plane deformation) and 1402 cm⁻¹ (CH₂ scissoring) were considered as β phase [35,36,37]. Following electrospinning, the α peaks became weaker, while the β crystalline peaks grew stronger. Additionally, the percentage change in β phase can be determined using the following equation (Equation (2)) [35]:

$$F\left(\beta \right)=\frac{X_{\beta }}{X_{\alpha }+X_{\beta }}\times 100=\frac{A_{\beta }}{1.26A_{\alpha }+A_{\beta }}\times 100$$

(2)

where, $F\left(\beta \right)$ represents the β phase percentage of PVDF, $A_{\alpha }\mathrm{and} A_{\beta }$ are their absorption bands at 763 and 840 cm⁻¹.

F(β) of the PVDF fiber increased significantly by 68% than PVDF pellets due to the electrospinning method as shown in

Table 2. Percentage of beta phase F(β) and piezoelectric coefficient (d₃₃)_.

Sample	F(β)	d33 (pC/N)
PVDF Pellet	48.45 ± 3.43	5 ± 3
PVDF Fiber	81.16 ± 1.81	32 ± 1.73
1% Fe3O4-PVDF	73.39 ± 1.24	28 ± 1
3% Fe3O4-PVDF	71.69 ± 2.16	26 ± 1
5% Fe3O4-PVDF	65.26 ± 3.39	20 ± 2.65

. With the addition of Fe₃O₄, the β phase decreased slightly compared to the β phase of pure PVDF fiber due to the partial inhibition effect as described in the DSC result discussion section and it kept decreasing as the loading of Fe₃O₄ increasing. The 5% Fe₃O₄-PVDF had the lowest F(β) compare to all other electrospun fiber mats but still that value is higher than PVDF pellet by 35%. So, the 5% Fe₃O₄-PVDF should have higher piezoelectric properties than the PVDF pellets which is discussed in d₃₃ section.

3.5. Piezoelectric Coefficient (d₃₃) Test

The piezoelectric coefficient (d₃₃) of a material specifies the intensity of its piezoelectric effect. The greater the d₃₃ value, the more effective the piezoelectric effect [38]. Figure 5b represents the piezoelectric coefficient d₃₃ of the PVDF fiber mats as a function of Fe₃O₄ mass fraction. The d₃₃ of PVDF mats was 32 pC/N and it dropped gradually to 20 pC/N as Fe₃O₄ increased to a 5% mass ratio. This result was in an agreement with FTIR data. The piezoelectric coefficient d₃₃ is directly related to the β phase of the fiber mats. The β phase of the fiber decreased after adding Fe₃O₄ as recorded from FTIR data, which led to the decrease in the d₃₃ value. It was also notable that the d₃₃ of PVDF pellet was 5 pC/N as recorded in Table 2, whereas the lowest d₃₃ value of electrospun fiber mat (5% Fe₃O₄-PVDF) was 20 pC/N; four times higher than PVDF pellets. So, adding Fe₃O₄ might reduce the piezoelectric property of the fiber mats but it has still four times higher piezoelectricity than the PVDF pellet.

3.6. Magnetization Test

M–H measurements were done at 300 and 5 K with the magnetic field varying from −2 T to 2 T (1 T = 10,000 Oe; 1 Oe = 79.577 Am⁻¹) as presented in Figure 6. At 300 K (Figure 6a,b), the samples showed zero coercivity, indicating that the nanofiber mats were in the superparamagnetic state. However, samples showed ferromagnetic behavior at 5 K with a coercivity of ~370 Oe for all the samples as shown in Figure 6c,d. Superparamagnetic materials can fluctuate randomly by thermal fluctuation at high enough temperatures just as an atom spin in paramagnetic materials. When the temperature is low, this thermal energy decreases and the magnetic moments are blocked [39]. The temperature below this happens known as blocking temperature which is 290 K as shown in Figure 7. Below the blocking temperature, there is some net alignment of the particle spins, while above it, the spins are in random directions which caused the hysteresis loops to appear at 5 K and increased the magnetic saturation value as shown in Figure 6d and

Table 3. Magnetic saturation values of composite fiber mats at 300 K and 5 K.

Sample	PVDF		1% Fe3O4-PVDF		3% Fe3O4-PVDF		5% Fe3O4-PVDF
	emu	emu/g	emu	emu/g	emu	emu/g	emu	emu/g
300 K	0	0	0.15	1.28	0.28	3.29	0.44	5.11
5 K	0	0	0.17	1.45	0.33	3.88	0.51	5.93

[40]. Previous magnetization measurements also indicated that the Fe₃O₄ particles are superparamagnetic at room temperature and ferromagnetic at low temperatures [41]. So, the composites made of Fe₃O₄ particles show similar characteristics.

On the other hand, coercivity measures the resistance of a ferromagnetic material to become demagnetized. A similar coercivity for all the samples in our case indicated that the coercivity is independent of Fe₃O₄ nanoparticles loading, and a homogeneous distribution of nanoparticles in the polymer matrix [42]. PVDF fiber mats did not show any magnetic response at all on both the temperatures but the Fe₃O₄ nanoparticles loaded samples did. With the increase of Fe₃O₄ nanoparticles, the magnetic saturation value of the composites also increased (1.28, 3.29, 5.11 emu/g for 1%, 3% and 5% Fe₃O₄-PVDF respectively) which suggests that ferromagnetic behavior observed in the composites arose from the magnetic Fe₃O₄ nanoparticles.

The magnetic moment vs. temperature (M–T) graph as shown in Figure 7, the separation of zero-field-cooled (ZFC) and field-cooled (FC) curves occur at large temperatures (blocking temperature T_b~290 K) indicating thermal irreversibility of the samples below room temperature. In a superparamagnetic state around 5 K, when the samples were cooled at zero applied field, the magnetic moments were randomly oriented leading to the total magnetization to be around zero as shown in Figure 7. When the temperature started to increase, magnetic moments began to fluctuate, and the presence of an externally applied field aligned them that led to the increase in the magnetization. Similarly, in the case of the field-cooled sample, the magnetic moments were aligned at a higher applied field resulting in higher total magnetization at 5 K starting to decrease with heating. These observations are in agreement with previously reported studies on Fe₃O₄ nanoparticles incorporation in other materials, including ceramic matrices [43].

3.7. Dynamic Mechanical Analyzer (DMA)

To see the stability of the fiber mats, DMA was done for all the samples before and after the magnetization test. Since the magnetization test of fiber mats was done at cryogenic temperature (5 K), it was important to determine their mechanical stability after the test. Dynamic strain sweep was done for all the samples as shown in Figure 8. The Young’s modulus (E) calculated from the stress–strain curves are listed in

Table 4. Modulus of elasticity (E) before and after magnetization test.

Sample	PVDF	1% Fe3O4-PVDF	3% Fe3O4-PVDF	5% Fe3O4-PVDF
E Before magnetization test (MPa)	0.11 ± 0.02	0.30 ± 0.03	0.62 ± 0.03	0.71 ± 0.01
E After magnetization test (MPa)	0.09 ± 0.01	0.29 ± 0.02	0.62± 0.04	0.71 ± 0.03

. The retention of modulus following cryogenic temperature exposure in composite samples over the PVDF which shows a slight decrease indicates that Fe₃O₄ helped the PVDF matrix to retain its mechanical property after being exposed to cryogenic temperature.

4. Conclusions

Flexible and mechanically stable PVDF and Fe₃O₄-PVDF composite fiber mats were successfully prepared from one step electrospinning method. A decrease in the fiber diameter was revealed from SEM images as the mass fraction of Fe₃O₄ increased. DSC data suggested that the crystallinity of the composite fiber mats also decreased as a function of Fe₃O₄ concentration in the PVDF matrix. XRD curves identified the reduction in β crystalline phases of the fiber, causing the decrease detected in DSC results. Due to the reduction in the β crystalline phases, the piezoelectric response of the fiber mats dropped by 38% for the maximum Fe₃O₄ loading but that is still 4 times higher than PVDF pellets. The magnetization test advocated a superparamagnetic state of the fiber at room temperature but a ferromagnetic behavior at a lower temperature. The coercivity values of the mats suggested a homogeneous dispersion of the Fe₃O₄ nanoparticles into the PVDF matrix. Moreover, stable and reusable fibers mats were obtained from this process which was confirmed from DMA analysis. The overall results suggest the potential of using Fe₃O₄-PVDF fiber mats as a magnetically responsive textile in spintronics and power electronic devices at cryogenic conditions.