Synthesis of One-Dimensional Titanium Oxide Nanowires for Polyvinylidene Fluoride Membrane Optimization

Abstract

Due to their beneficial characteristics, 1D nanowires have made significant advancements in different realms due to their large surface area, unique surface chemistry, and tunable transport properties. Herein, a comprehensive study of titanium oxide nanowires from titanium (IV) isopropoxide, with the addition of CNTs in the polymeric membrane, has been conducted to form crystals. This method is quite proficient for nanowire synthetization, incorporated with CNTs and polymeric membranes. Carbon nanotubes (CNTs) were used to enhance their properties with polymeric polyvinylidene fluoride PVDF membrane. The concentration ratio of titanium oxide in PVDF membrane was 1, 3, 5, 10, and 15%. The development of titanium oxide nanowires at the nano level shaped, as the emulsion electrospinning created 1D nanowires. The use of an additive makes the membrane more conductive. FT-IR, XRD, EDS, and SEM techniques were used for size, surface geometry, crystalline nature, blend membrane, and thin film composition determination. Thermal Gravimetric Analysis (TGA) analysis shows that weight loss with temperature increases at optimum level.

1. Introduction

Nanomaterials have emerged as an exciting class of materials with a wide range of applications in chemistry, physics, the medical field, and many more areas [1,2,3]. In the last 25 years, scientists have developed nanowires a thousand times thinner than a human hair, which enables new frontiers in medicine, energy, photonics, and computing. Nanowire properties like higher storage and velocity of transmission, the absorption edge of nanoparticles for the blue shift, conductance, increasing mechanical properties, etc. can be easily understood with a theoretical approach and calculation done in a low dimension system [4,5,6,7,8].1D nanostructures are ideal systems as compared to 0D and 2D systems to explain transport phenomena at a nanoscale level [9,10]. With low-dimensional systems, the functional nanostructured arrays for fabrication play a vital role in nanotechnology. The single and multi-walled carbon nano tubes with 1D nanostructures given their versatile and attractive properties for multiple applications [11]. In the field of lithography, many innovative procedures i.e., extreme ultraviolet lithography (EUL) and electron-beam lithography (EBL), limit the cost when the smallest dimension in the circuit is diminished through predetermined masks. Nanowires become an important feature to integrate one dimension (1D) nanostructure, nanocircuit fabrication, magnetic storage information, spintronics, magneto electronics, and magnetic recording in low dimension material synthesis [12]. The surface chemistry and morphology of nanofibers include ZnO, SnO, and TiO₂, as reported [13,14,15,16,17].

Polyvinylidene fluoride (PVDF) possesses semicrystalline thermoplastic and electroactive polymers with piezoelectric and pyroelectric properties. Due to its thermal ability, high elasticity and chemical resistance, PCDF has many applications in chemical, biochemical and electronic industries for supercapacitors, transducers, sensors, and batteries. PVDF membranes have been extensively used in ultrafiltration and microfiltration for separation purposes [18,19]. PVDF is a thermoplastic engineering material with several applications with a combination of flexibility, low weight, low thermal conductivity, high chemical corrosion resistance, and heat resistance [20,21]. Classical CNTs exhibit some of their very unique properties like high flexibility, low mass density, large aspect ratio (>1000) high tensile moduli, and strengths, and these properties made CNTs a very important and ideal reinforcing agent. The hydrophobic property of CNTs becomes an interesting feature for the absorption of long-chain covalent and non-covalent molecules on the surface. The fabrication of ultrafiltration membranes of PVDF/MWCNTs by phase inversion, however, oxidized MWCNTs with -OH and -COOH functionality enhance the architecture and efficiency of ultra-membrane filtration (UF) membrane.

In the present study, a new approach was used for the synthesis of the polymeric membrane of TiO₂, and the effects were evaluated on their properties by using additives for the functionalized carbon nanotubes. The interaction of CNTs/PVDF/TiO₂ nanocomposite enhanced the dielectric property and conductivity with an increase in the concentration of CNTs/TiO₂.

2. Materials and Methods

2.1. Materials and Methods

Titanium (IV) isopropoxide 97%, N,N-dimethylformamide anhydrous 99.8% DMF, acetic acid, and polyvinyl pyrolidone PVP (C₆H₉NO)_n with a molar mass of 8,000,000 g·mol⁻¹ were purchased from Sigma-Aldrich, St. Louis, Missouri, United States. polyvinylidene fluoride PVDF and carbon nano tubes were purchased from Sun Nanotechnology.

2.2. Preparation of TiO₂ Nanowires

Highly porous TiO₂ nanowires were prepared by a two-step process. About 1.6 mL of titanium isopropoxide was dissolved in 6 mL N,N-dimethylformamide anhydrous and stirred till the solution became clear and labeled as solution 1.1 g Polyvinyl pyrolidone (PVP) was dissolved in 10 mL anhydrous dimethylformamide and 5 mL ethanol mixture and stirred for 2 h and was labeled as solution 2. Both solutions 1 and 2 were mixed to get a viscous gel-like sample. About 2 mL acetic acid were added dropwise until the solution became clear and ready for the electrospinning process. As with a typical electrospinning process, the spinneret had an inner diameter of 0.6 mm. Grounded aluminum strips (2 cm in width) with parallel gaps of about 1 cm were used as the collectors. A distance of 15 cm and a direct current voltage of 13 kV were maintained between the tips of the spinneret and the collector. During the electrospinning process, the temperature was maintained at 30 °C. After electrospinning, a portion of the obtained TiO₂ nanowires was calcined at 450 °C at a rate of 2 °C per min for 4 h.

2.3. Pre and Post Calcination Coupling PVDF Membrane with TiO₂ Nanowires

For the preparation of films, PVDF membranes were coupled with both calcined and non-calcined TiO₂ nanowires. Different mass percent ratios of TiO₂ nanowires (1, 3, 5, 10, and 15%) and PVDF were prepared in DMF solvent. About 1.5 g PVDF were mixed with non-calcined TiO₂ nanowires as fillers in 7 mL DMF solvent and the mixture was stirred vigorously until homogeneous. This homogenous mixture was refluxed for 10 h before the film casting inversion method was applied. The obtained samples were dried at 80 °C and denoted as PVDF/XTO_n, where X denotes different mass percent ratios of PVDF and ‘n’ in the subscript stands for the non-calcined TiO₂ nanowires. The same procedure was followed for the coupling of PVDF with calcined TiO₂ nanowires and the obtained samples were denoted as PVDF/XTO_c, where X denotes different mass percent ratios of PVDF and ‘c’ in the subscript stands for the calcined TiO₂ nanowires.

2.4. Coupling CNT Functionalized PVDF with TiO₂ Nanowires

For the preparation of films, CNT functionalized PVDF was coupled with both calcined and non-calcined TiO₂ nanowires. Different mass percent ratios of TiO₂ nanowires (1, 3, 5, 10, and 15%), functionalized CNTs (-COOH), and PVDF were prepared in DMF solvent. About 1.5 g PVDF were mixed with functionalized CNTs and non-calcined TiO₂ nanowires as fillers in 10 mL DMF solvent and the mixture was stirred vigorously until homogenous. The obtained mixture was refluxed for 10 h before the film casting inversion method was applied. The obtained samples were dried at 80 °C and denoted as PVDF/CNT/XTO_n, where X denotes different mass percent ratios of PVDF and ‘n’ in the sub-script stands for the non-calcined TiO₂ nanowires. The same procedure was followed for the coupling of PVDF/CNT with calcined TiO₂ nanowires and the obtained samples were denoted as PVDF/CNT/XTO_c, where X denotes different mass percent ratios of PVDF and ‘c’ in the sub-script stands for the calcined TiO₂ nanowires. The experimental detail has been listed in

Table 1. Composition of PVDF/TiO₂/MWCNTs nanocomposite with (Wt. %).

Sample	PVDF wt%	MWCNTswt%	Uncalcined TiO2 wt%	Sample	PVDF wt%	MWCNTs wt%	Calcined TiO2 wt%
PVDF/TiO2 (1)	99	0	1	PVDF/TiO2 (1)	99	0	1
PVDF/TiO2 (3)	97	0	3	PVDF/TiO2 (3)	97	0	3
PVDF/TiO2 (5)	95	0	5	PVDF/TiO2 (5)	95	0	5
PVDF/TiO2 (10)	90	0	10	PVDF/TiO2 (10)	90	0	10
PVDF/TiO2 (15)	85	0	15	PVDF/TiO2 (15)	85	0	15
PVDF/TiO2/MWCNTs (1)	99	0.5	0.5	PVDF/TiO2/MWCNTs (1)	99	0.5	0.5
PVDF/TiO2/MWCNTs (3)	97	1.5	1.5	PVDF/TiO2/MWCNTs (3)	97	1.5	1.5
PVDF/TiO2/MWCNTs (5)	95	2.5	2.5	PVDF/TiO2/MWCNTs (5)	95	2.5	2.5
PVDF/TiO2/MWCNTs (10)	90	5	5	PVDF/TiO2/MWCNTs (10)	90	5	5
PVDF/TiO2/MWCNTs (15)	85	7.5	7.5	PVDF/TiO2/MWCNTs (15)	85	7.5	7.5
Sample PVP wt%	TiO2 wt%
10	90

.

2.5. Characterization

X-ray diffractometric analysis were recorded on a STOE Germany THETA-THETA diffractometer to study the crystal structure and atomic spacing. The anode material was made of Cu, the scan step time was 0.4000 s, and the receiving slit size was 0.3000 mm at 25 °C. The IR spectra in the range of 4000–400 cm⁻¹ were measured using the FTIR spectrometer of Thermo Fisher Scientific Waltham, MA, USA, (Spectrum 100 series). Scattering electron microscopic images were recorded on JEOL JSM–6490A. Electron diffraction spectroscopy was performed on JEOL model JED-2300 analysis station. Tensile strength and elongation at break were measured using an MTS testing machine with an initial speed of 100 mm/min. The average of five measurements along with their standard deviation was taken.

3. Results

3.1. FTIR Analysis

The FTIR spectra of TiO₂ nanowires are shown in Figure 1a–e. The band positioned around 487 cm⁻¹ is due to the Ti-O-Ti stretching vibration of the tetragonal TiO₂. The figure shows that the peaks at 874 and 1100 cm⁻¹ are attributed to the middle and strong stretching vibrations of Ti-O-Ti, respectively. The strong absorption peaks at around 3400 and 1630 cm⁻¹ are assigned to O-H stretching and H-O-H bending vibrations of physically adsorbed water molecules [22]. The FTIR results of PVDF/XTO_n, PVDF/XTO_c, PVDF/CNT/XTO_n, and PVDF/CNT/XTO_c that show characteristic peaks at 1375, 1176, and 973 cm⁻¹ are respectively assigned to C-H deformation, C-F stretching and Ti-OH stretching [15,16,17]. The FTIR spectra reveal the spectra of TiO₂ nanowires in Figure 1a, whereas the FTIR spectrum in Figure 1b,c shows the effect of TiO₂/PVDF 1%, 3%, 5%, 10%, and 15% before and after calcination, respectively. The nanowires of TiO₂/PVDF composite show the absorbance on wave number (cm⁻¹). As the concentration of composite increases, the absorbance of the materials becomes variable due to the increase in the concentration of the PVDF polymer. The band position between 400 to 1100 cm⁻¹ expressed the stretching vibration of TiO₂ and 1400 to 3500 cm⁻¹ shows the organic peaks of the PVDF polymer [23] whereas after calcination the organic peaks were removed. Moreover, Figure 1d,e reveals the FTIR spectrum of TiO₂/CNTs/PVDF 1%, 3%, 5%, 10% and 15% before and after calcination, respectively. The absorbance peaks were determined with the increase in the concentration of the PVDF polymer. The absorbance peak from 1600 to 3500 cm⁻¹ shows the organic peaks of the thin sheet, whereas the one from 400 to 1100 cm⁻¹ indicates that the TiO₂ stretching vibration and CNTs peaks were observed. After calcination, organic and CNTs peaks were removed as indicated in the spectrum.

3.2. EDS Analysis

The existence and composition of the synthesized TiO₂ nanowires were also confirmed by the energy dispersive x-ray spectroscopy and the recorded EDX spectrum is given in Figure 2 which shows the peaks of titanium and oxygen on the left-hand side while the specific area from where the EDX spectrum was obtained is given. It is clear from the EDX spectrum that titanium and oxygen are the only elements present in the spectrum and no other peaks have been detected confirming that no impurities are present and the formation of TiO₂ has successfully taken place. The elemental composition by weight percent (Wt%) and atomic percent (At%) is given in

Table 2. Elemental composition of TiO₂ nanowires by weight% and Atomic%.

Element	Weight%	Atomic%
O K	33.46	60.09
Ti K	66.54	39.91
Totals	100.00

.

3.3. SEM Analysis

The scanning electron microscopy of the resulting TiO₂ nanowires was done to study the morphology of the obtained nanowires before and after calcination. The SEM micrographs of the as-spun nanowires (pre-calcined nanowires) are given in Figure 3a–g at low and high magnifications, respectively. The morphology of the as-spun nanowires looks very smooth and compact at this stage. The SEM micrographs of the TiO₂ nanowires obtained after calcination are given in Figure 3c,d captured at two different magnifications, and Figure 3e–g shows the overall as-spun nanowires at high and low magnifications. The morphology of the calcined nanowires looks a bit rough, and the size has shrunk due to the removal of the solvent and polymer content. The SEM images of TiO₂ nanowires with wire sizes range from 500 nm to 5 µm. The wire size is regular in structure with aligned characteristics of 1D nanostructures and confirms the geometry of TiO₂ from XRD. The obtained nanowires are several micrometers in length and have a diameter between 150–250 nm.

Figure 4 shows the SEM images of XPVDF/TO_n which are smooth and porous in morphology. It is clear that for the SEM images in the range of 10–50 the morphology becomes more and smoother as the amount of PVDF increases. The distribution of the as-spun TiO₂ nanowires in the PVDF films/membranes was revealed by the scanning electron micrographs. Figure 4a–e shows the cross-sectional SEM of TiO₂/PVDF nanocomposite films obtained at different magnifications. In Figure 4a,b, some white spots are visible in the given SEM which are from the TiO₂ nanowires. Figure 4c–e gives a closer look at the composite films showing a very homogenous distribution of the TiO₂ nanowires across the PVDF matrix without any agglomeration.

The scanning electron micrographs (SEM) obtained for the calcined TiO₂/PVDF nanocomposite films are shown in Figure 5a–e. These SEM provides a cross-sectional view of the microstructure of the TiO₂/PVDF composite membranes showing the chemistry and distribution of the nanofillers of TiO₂ nanowires in the PVDF matrix. The cross-sectional view at low resolution shows the breakage of the surface and some voids. At higher resolution, the one-dimensional TiO₂ nanowires have aligned themselves in a longitudinal way with the PVDF matrix and show a very uniform distribution in the PVDF matrix. Figure 5c–e shows SEM images of XPVDF/TO_c samples and the morphology is relatively more porous compared to non-calcined samples. The TiO₂ nanowires are found scattered on the surface of the PVDF. Since calcination increases the crystallinity of the samples, more visible TiO₂ nanowires are seen scattered on the surface of the PVDF membrane. The whole morphology of the images is in the range of 5–50 µm.

The surface morphology of TiO₂/f-MWCNTs/PVDF nanocomposite films in which the f-MWCNTs were also studied with the SEM to look for the surface study and appearance of the nanofillers in the PVDF matrix. Figure 6a–e shows the SEM of the TiO₂/f-MWCNTs/PVDF which were obtained from the fracture cross-section of these films. It can be seen that the added nanofillers of both TiO₂ and f-MWCNTs are uniformly embedded in the PVDF matrix and show a compact surface. Some white nanoparticles attached to the PVDF matrix can also be seen. Conversely, Figure 5c–e shows white indications that may lead toward the head of 1D TiO₂ nanowires whose tails are dipped and dispersed vertically in the PVDF matrix as the nanowires possess a high aspect ratio. Figure 6a–e shows the SEM images of pre-calcined XPVDF/CNT/TO_n and XPVDF/CNT/TO_c samples with more high porosity and more visible TiO₂ nanowires of a relatively small range between 1–10 µm.

The surface morphology of the nanocomposite films prepared by solution casting of as-spun TiO₂/f-MWCNTs/PVDF was studied by the SEM to see the distribution of the nanofillers in the polymer matrix. Figure 7a–e show the post-calcinated cross-section SEM micrographs of the nanocomposite at a low and high resolution where one can see the dispersion of MWCNTs and TiO₂ nanowires in the PVDF matrix with morphology. However, due to the one-dimensionality and very thin size of MWCNTs, they have covered the whole surface of the PVDF matrix. It is the good dispersibility and directionality of the nanofillers which play a role in the alignment of the PVDF structure which results in phase transformation (from α to β) on one hand, and on the other hand it provides good thermal stability to the matrix when heated. In Figure 7c–e the standalone nanowires of the as-spun TiO₂ nanowires can also be seen in the given micrographs, which also strengthen the thermal stability of the matrix and provide additional stability.

3.4. XRD Analysis

Figure 8, matches the XRD pattern of TiO₂ nanowires named Anatase (PDF reference code 01-075-1537). The X-ray diffractions clear the crystallinity of the compound gives unit cell indication and illustrate the TiO₂ anatase phase pattern. The h, k, and l values of the crystal are (112) and (200) peaks, respectively [23]. The diffraction pattern TiO₂ exhibits at 25° and 45°, indicating the strong diffraction pattern of anatase phase TiO₂.

In Figure 9a, matches the XRD pattern of TiO₂ nanowires name as Anatase (PDF reference code 01-075-1537). The X-ray diffractions clear the crystallinity of the compound gives unit cell indication and illustrate the TiO₂ anatase phase pattern. The h, k, and l value of the crystal is (101) and (200) peaks, respectively [23]. The diffraction pattern TiO₂ is exhibited at 25° and 45°, indicating the strong diffraction pattern of anatase phase TiO₂. The series (1%, 3%, 5%, 10%,15%) reveals the crystallinity of TiO₂ structure with diffraction pattern and exhibits an angle 2θ = 25° and 2θ = 45° with h, k, and l values (101) and (200) respectively.

Figure 9b matches the XRD pattern of TiO₂ nanowires named as Anatase (PDF reference code 01-075-1537). The X-ray diffractions clear the crystallinity of the compound gives unit cell indication and illustrate the TiO₂ anatase phase pattern. The h, k, and l value of the crystal are (101) and (200) peaks, respectively [24]. The diffraction pattern TiO₂ is exhibited at 25° and 45°, indicating the strong diffraction pattern of anatase phase TiO₂. The series (1%, 3%, 5%, 10%,15%) reveals that the crystallinity of the TiO₂ structure with diffraction pattern exhibits an angle 2θ = 25° and 2θ = 45° with h, k, and l values of (101) and (200), respectively.

3.5. Thermal and Mechanical Stability

The typical thermogravimetric (TG) curve for non-calcined TiO₂ nanowires illustrated in Figure 10a shows three steps. The first step of approximately 7.54% mass loss is found in the range of 20–110 °C. The second substantial mass loss of approximately 20.36% appears between 110 and 370 °C and can be attributed to the removal of water molecules and organic material as well. The third mass loss of approximately 13.33% appears between 370 to 450 °C, indicating the removal of the PVP organic template to form nanowires. There are weak exothermic peaks observed and they are related to the slow transformation of the anatase phase of TiO₂ to the rutile phase at high heating.

The typical thermogravimetric (TG) curve shown in Figure 10b shows three steps. The first step is approximately 6.54% mass loss from 350 to 410 °C. The second substantial mass loss is approximately 22.63% between 410 and 480 °C and can be allocated for water molecules removal from the hydroxyls on titanium atoms and organic material as well. For the he temperature ranges from 480 to 550 °C, there is a mass loss of approximately 10.12%, which indicates the removal of the PVDF organic template to form nanowires. From this, the organic template can be removed from fibrous material to form nanowires upon calcination at 550 °C. There are weak exothermic peaks that are observed in the differential scanning calorimetry (DSC) measurement. It reveals the slow transformation of the anatase phase of TiO₂ to the rutile phase as with the pattern involved with the series (1%, 3%, 5%, 10%, and 15%).

Similarly, the typical thermogravimetric (TG) curve in Figure 10c shows three steps. The first step shows an approximately 5.58% mass loss from 350 to 410 °C. The second substantial mass loss of approximately 23.26% between 410 and 470 °C can be allocated to the removal of water molecules. The temperature ranges from 470 to 550 °C, and there is a mass loss of approximately 12.44%, which indicates the removal PVDF organic template to form nanowires. This indicates the collection of nanowires of prescribed material of TiO₂ calcinated at 550 °C. There are weak exothermic peaks that are observed in the differential scanning calorimetry (DSC) measurement. It reveals the slow transformation of the anatase phase of TiO₂ to the rutile phase as with the pattern involved with the series (1%, 3%, 5%, 10%, and 15%).

Mechanical attributes are vital in industrial membrane applications for long-term performance stability. As a result, data on membrane tensile strength and elongation ratio at break were determined.

Table 3. Mechanical stability of PVDF/TiO₂ nanocomposites.

Sample ID	Tensile Strength	Elongation at Break
TA	1.57 ± 3.9	35.71 ± 3.7
TA-1	1.61 ± 4.3	44.31 ± 3.2
TA-2	1.70 ± 3.1	45.81 ± 3.2
TA-3	1.97 ± 2.7	50.04 ± 2.5
TA-4	2.04 ± 2.2	53.55 ± 2.6
TA-5	2.19 ± 2.3	56.12 ± 2.1

shows the results of the membranes’ mechanical strength tests. The mechanical strength of membranes was improved as TiO₂ nanowire content levels were increased. The tensile strength of the membrane increased from 1.57 MPa to 2.19 MPa with the addition of TiO₂ nanowires from 1 wt.% to 15 wt.%, while the elongation ratio increased from 36.91% to 59.01 wt.%. The tensile strength increased from 1.57 MPa to 2.19 MPa with the addition of TiO₂ nanoparticles from 1 to 15 wt.% percent, while the elongation ratio increased from 35.71% to 56.12%. This suggested that inserting TiO₂ nanowires could join polymeric chains while also increasing their stiffness and elasticity. As a result, more energy was required to break the link between TiO₂ nanowires and PVDF, and the hybrid membrane mechanical strength and elongation ratio both improved.

3.6. Dielectric Constant

The dielectric permittivity of PVDF/f-MWCNTs/TiO₂ was measured in the range of 1.0 × 10¹–1 × 10⁷ Hz at room temperature. The results reveal that dielectric permittivity has increased appreciably. The interface of reinforcement of functionalized MWCNTs and the matrix of PVDF with the addition of TiO₂ have strong interfacial interaction. There are two factors involved in the enhancement of dielectric permittivity of MWCNTs/TiO₂/PVDF. One is the combined effect of reinforcement, especially with loading content, and the other is the polymeric chain. The resistance of the charge carrier at the interface with the result of Maxwell-Wagner Sillar effect (MWS). Whereas MWS effect established the collection of charge at interface point when current flows acroases. The material possesses different relaxation time (τ), equation as follows (τ = $\epsilon /\sigma$), ε and σ expressed the dielectric permittivity and conductivity of the respective materials within the composite. The relaxation time of PVDF is very high as compared to MWCNT [24].

In composite material with the addition TiO₂ nanowires to C-MWCNTs/PVDF, the MWS effect is strengthened by the electrophilic fluorine (F) of PVDF. The F attracts the delocalized electron of MWCNTs at the interface led to an established donor-acceptor complex between the interface of PVDF and MWCNTs with TiO₂. The large surface area of MWCNTs plays a vital role in the enhancement of the MWS effect during formation of a nanoscale structure with the insulating interface of PVDF.

The dielectric permittivity measured at frequency of 1 × 10² Hz with a 1%, 3% and 5% C-MWCNT/TiO₂ are 3.66 × 10⁵, 3.10 × 10⁶, and 4.17 × 10⁶, respectively. The dielectric permittivity response of PVDF was observed to decrease within the frequency range between 10^1–10³Hz. The dielectric permittivity value is enhanced with the addition of TiO₂ and reached its optimum level as the quantity increases. Therefore, upon concentration that lies within the vicinity of percolation threshold. In this reinforcement concentration there is strong interfacial interaction between functionalized MWCNTs, TiO_2, and PVDF along with an enhanced MWS effect. The concentration of dielectric permittivity was further enhanced by an increase in interfacial interaction due to an increase in distance between the insulating PVDF and the MWCNTs [25,26]. This can also be expressed by the attributed interfacial polarization between the polymeric matrix and the high β-phase of the polymer with the interaction of nanowires and functionalized MWCNTs [27]. This sequenced increase in the dielectric permittivity in the lower frequency range was based on the mini-capacitor principle. Alongside, the dielectric permittivity of PVDF was observed to decrease sharply with the frequency loaded up to 2.5 × 10² Hz. In the frequency range of 10³–10⁶ Hz has become similar to all reinforcement loading. This decrease in dielectric permittivity in higher frequency may be accredited to a decrease in the polarization of functionalized MWCNTs.

Figure 11 shows the dielectric loss of PVDF/C-MWCNTs nanocomposites with frequency ranges around 10⁶ Hz for all loading concentrations. These low values of dielectric loss of PVDF/C-MWCNTs possess low energy loss and thus find practical applications in dielectric materials [27].

3.7. AC Electrical Conductivity

The AC conductivities of the nanocomposite material with polymeric matrix PVDF/C-MWCNTs/TiO₂ 1%, 3%, and 5% have been measured in the range of 1 × 10¹ Hz to 1 × 10⁷ Hz at room temperature, as shown in Figure 12. The conductivity of the materials is independent of the frequency in the case of PVDF/C-MWCNTs/TiO₂. The samples are showing the independent relation of conductivity with frequency even at the lowest adopted loading (1%). Compared with pure PVDF [28], the independency of conductivity upon different frequencies indicates that the effective conductive network was achieved at 1–5% wt. of the percolation threshold (f_c) was attained or loading 1% wt. in PVDF/C-MWCNTs/TiO₂ nanocomposites. The conductivity of materials at frequency of 1 × 10² Hz has reached 6.01 × 10⁻⁵, 5.1 × 10^−4, and 6.9 × 10⁻⁴ S/cm for loading samples of 1%, 3%, and 5% wt., respectively. However, the electrical conductivity of PVDF has been observed to increase with the addition of functionalized MWCNTs and titanium oxide as in the increased concentration from 1% to 5% wt. Thus, the conductivity of nanocomposite material has increased linearly with the increasing concentration of filler due to the improvement in their dispersion in PVDF.

This mechanism related the conductivity of charges that was proceeded by tunneling of electrons through insulating gaps of PVDF within nanocomposite [29,30,31,32]. This phenomenon was based on the formation of a two-dimensional (2D) network of functionalized MWCNTs in the vicinity of threshold frequency f_c instead of three-dimensional (3D) conductive networks. The movement of electrons is carried out through matrix via the rule given in the equation:

$${\log }_{10}\sigma \infty f_{f-MWCNTs}^{-1/3}$$

(1)

Here σ is the conductivity and f is the fraction of functionalized MWCNTs and TiO₂ in nanocomposites. The log₁₀σ was plotted against the fraction of functionalized substrate as shown in figure. The marly linear relationship between the log₁₀σ and functionalized substrate valued the conductivity through tunneling mechanism, where electrons hop between the conduction zones but CNTs are not contacted physically. The concentration of C-MWCNT/TiO₂ increases, and the formation of the 3D network become prominent and directly contacts it, and thus conductivity increases appreciably. The linear relationship of conductive PVDF with loading material C-MWCNT/TiO₂ was further confirmed through the relationship of the conductivity with reinforcement fraction at 1 × 10² Hz, as shown in Figure 13.

The thermal conductivity network has been analyzed for the composite material of PVDF/CNTs/TiO₂ to establish the highest thermal conductivity of the material as shown in Figure 13. However, the filler content of composite material with TiO₂ (1%, 3%, 5%, 10%, and 15%) showed conductivity ranges of between 0.6–2 W/mK, which is increased with the increase in concentration. As the filler content increased the thermal conductivity of the material also increased. The material PVDF/TiO₂ nanowires with a percentage composite 1% to 3% is 0.756 W/mK and 0.896 W/mK respectively while 5% and above have three times greater thermal conductivity. The filler content of PVDF/CNTs/TiO₂ as percentage composite by weight (1%, 3%, 5%, 10%, and 15%) increased their thermal conductivity. The PVDF/CNTs/TiO₂ have a thermal conductivity of 0.932 W/mK, 1.001 W/mK, for 1% and 3% by weight, respectively, whereas 5% and above have five times increased thermal conductivity. While comparing the thermal conductivity of PVDF/TiO₂ nanowires with PVDF/CNTs/TiO₂ change were observed as shown in figure. The result opts to be the encouraging conductive path to develop nanowires of TiO_2. The electrical conductivity of the composite increases i.e., many orders higher, near the percolation threshold transition point. However, a steep thermal change was not observed. The overall thermal conductivity of the polymeric material is still low, unlike the electrical conductivity of the pure material of metal oxide. In the formulation of the tunneling electron barrier across the polymer, the main carrier of thermal conductivity are photons that do not have a tunneling effect. Hence, the scattering phenomenon results in a large interface thermal resistance and hinders photon transmission. However, at the same loading of PVDF/TiO₂ nanowires with PVDF/CNTs/TiO₂ there is a thermal conductivity increase while adding 1, 3, 5, 10, and 15 wt. % of titanium oxide nanowires that correspond to 0.756, 0.896, 1, 1.467 and 2 W/mK, respectively.

Generally, comprehending the dispersion of conductive fillers in a matrix is fruitful to construct a thermal conductivity network, which enhances the thermal conductive properties of the polymeric matrix. TiO₂ nanowires show a uniform intermediate coated layer to establish the elastic modulus of thin polymeric films of anatase ~185 GPa [33]. The high elastic modulus TiO₂ increases the modulus mismatch between titanium oxide nanowires and polymer [34,35] and expressed the thermal conductivity is higher for uncoated composites as evident. The lower elastic modulus between loading material and polymeric matrix can increase the phonon spectrum overlap and improves the phonon transmission efficiency. Here, PVDF/TiO₂ nanowires with PVDF/CNTs/TiO₂ composite reveal the highest thermal conductivity. The nanowires of TiO₂ can act as a “dispersion agent”, to improve dispersion in the polymeric matrix, resulting in an apparent improvement in thermal conductivity of the polymeric composite. Recently, it was observed that composite Cu nanowires and titanium oxide nanowires with epoxy revealed a thermal conductivity of 1.124 W/mK with 2.0 percent volume [35]. There are many test methods to check the thermal conductivity of the polymeric composite material, such as the hot wire method and the laser flash method. The result measured by applying these two methods shows some noteworthy differences [36]. Therefore, these two methods are not comparable, as the result shows noteworthy differences. Notably, at relatively low filler content, the hot wire method is used for the best results.

4. Conclusions

The synthesis of TiO₂ nanowires via the emulsion electrospinning method plays a significant role in nanotechnology. The addition of CNTs in the membrane accomplishes very attractive features. For analysis, we used FTIR, SEM, EDX, XRD, and TGA techniques to determine the morphology, crystalline geometry, and thermal stability. To check variation, we catered concentration of nanowires in blend membrane which bring variation in membrane with nanowires. SEM analysis revealed nanowire size as well blend with polymeric membrane up to 20 nm for which magnification of the instrument ranges to 50,000 to 1500. The FTIR outcome indicated that the composition of the nanowires of titanium oxide ranges between 600 to 1100 cm⁻¹. From an application point of view, the polymeric membrane with wires is used in many ways, such as in electrochemical sensing, catalysis and photocatalysis, photovoltaics, UV blocking, smart packing, and functional filler in textiles, paints, paper, and cosmetics, due to their special electrical, optical properties and dielectric applications that are of significant importance.