Effect of carbon nano tube in the structural and physical properties of polyvinyl chloride films

Firstly, TEM and XRD techniques are used to identify CNT. Figure 1 indicates the TEM image for CNT sample at two different magnifications.

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TEM image shows the existence of CNT in the form of walls with outer diameter range 20–30 nm and an inner diameter range 7–10 nm. As shown, long fine tubes are tangled with each other due to the Van der Waals forces between nanotubes.

Figure 2(a) shows the XRD pattern for pure CNT sample. Sharpe and broad characteristic peaks for CNT are observed at 26.1° indexed to (002) plane of graphite carbon, and two other less intense peaks at 42.87° and 44.71° indexed to (100) and (101) planes respectively [26–28]. These peaks are related to an interplanar d-spacing of 3.41 Å, 2.1 Å and 2.025 Å, respectively. The broad of these peaks testifies the little size of the CNT. The CNT crystallite size (D) was calculated using Scherrer's equation [13],

$$\begin{eqnarray}&&D=k\lambda /\beta \,\cos (\theta ),\end{eqnarray} \tag{ 2 }$$

where k is a constant (with a value of 0.9), $\beta$ is the full-width at half maximum (FWHM), λ is the x-ray wavelength, and θ is the diffraction angle. The average crystallite size of CNT is about 5 nm, which is compatible with the result obtained by TEM analysis.

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Figure 2(b) shows the XRD patterns for the pure PVC film and PVC films doped with different content of CNT. The PVC film shows a broad halo in between 15°–30°, indicating the amorphous nature of the PVC film [7, 12]. This halo broadens with the addition of small amounts of CNT into the PVC matrix, and becomes pronounced at higher content of CNT (0.5 wt.%). It may be caused by the orientation of PVC macromolecules relative to nanotubes, as a result of dipole - dipole interaction between the carboxylic groups (-COOH) on the surface of the CNT and C-Cl bonds of the polymer (see FTIR spectra). In addition, hydrogen bonding between COOH groups and Cl atoms in PVC is possible.

Figure 3 shows the SEM images for PVC-CNT films. It is observed that the sonication process destroyed the CNT in the PVC matrix, and the CNT seems like particles with spherical shape. Sonication with high power causes the CNT to be fragmented into smaller pieces, and also causes the formation of carboxylic groups (-COOH) on the surface of the CNT as proved in FTIR analysis (discussed below) [29]. It leads to the orientation of PVC macromolecules relative to nanotubes, as a result of dipole - dipole interaction between these carboxylic groups and polar C-Cl bonds of the polymer. At that, CNTs are distributed uniformly in PVC with little agglomeration, as the CNT can form clusters due to Van der Waals attraction between nanotubes [29].

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Figure 4 depicts the FTIR of absorption bands spectra for the pure PVC and PVC-CNT nanocomposite thin films with different concentrations of CNT (i.e., 0.15, 0.20 0.35, 0.50, and 0.70 wt.%). The FTIR spectrum of pure PVC film shown in figure 2 exhibits the number of pronounced bands at 2972 cm^–1 and 1434 cm⁻¹ corresponding to C−H stretching and the wagging of methylene groups, respectively [6, 30]. In addition, single bands at 1332 cm⁻¹, 1254 cm⁻¹, 1064 cm⁻¹, and 959 cm⁻¹ represent the CH₂ bending, C–H rocking and C–C backbone of PVC chain structure, and trans C–H wagging group of PVC [9, 31]. Moreover, pure PVC FTIR spectrum shows numerous C–Cl stretching modes at 833 cm⁻¹ and 693 cm⁻¹ [9, 31]. In addition, the band at 3660 cm⁻¹ is attributed to the stretching vibration of OH group. The FTIR spectra of PVC–CNT nanocomposite films exhibit the same bands found in the pure PVC film, but with recognizable decrease in their band intensities significantly. This decrease in the band intensities of PVC–CNTs nanocomposites is caused by the high absorption coefficient due doping with CNTs and the reduced content of PVC polymer in the films. Further, the decrease in the bands intensity of C–Cl bands in the PVC–CNTs nanocomposite films may be due to a partially remove of the chlorine and formation of polyene groups (−HC = CH−)_n in the composite film [32]. The obtained results indicate to the complexity of the interaction process resulting from the doping of oxygen-functionalized CNT. This was confirmed by the degradation extent of pure PVC films due to the doping with oxygen-functionalized CNT, which detected by investigating the band intensity at 1772 cm⁻¹ corresponding to carbonyl group.

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The absorption and transmission spectra for PVC films doped with various content of CNT are illustrated in figure 5. Figure 5(a) indicates that polymer films have enhanced their optical absorption by the increase in CNT content in the PVC matrix. The observed band at about 280 nm in the absorption spectra is related to π–π* electronic transition [3–5]. Furthermore, it is observed that the absorption edge ranged from 200 to 250 nm is redshifted by the increasing of CNT content in the PVC matrix, and this edge is specified with C–Cl bond [3, 5]. The inset of figure 5(b) shows the transmittance (T) of the PVC film without any additive. The PVC film transmittance is very high ∼90% in the visible region. The transmittance becomes lower with the addition of CNT in the PVC matrix, T ∼ 30% at 0.15 wt.% CNT and T ∼ 1% at 0.5 wt.% CNT (see figure 5(b)). This is related to the high absorption nature of the CNT. Along with this some CNT aggregates are formed at higher concentration, and the absorption edge is shifted toward higher wavelength region. Such result is in good accordance with the data obtained previously as for PVC embedded with NiO [4], CdO [5], SiO₂ [33], graphene oxide [9], and Al₂O₃ [3]. The low transmittance and high absorption films make it suitable for using as thermoplastic solar collector [34].

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The optical absorption spectra of the polymer films have been studied to determine the optical parameter: band gap (E_g ), absorption coefficient (α), Urbach energy (E_U), and to identify the type of electronic transitions in the material. The absorption coefficient of the polymer films can be calculated using the following equation;

$$\begin{eqnarray}&&\alpha =2.303\times A/d\end{eqnarray} \tag{ 3 }$$

where A is the film absorbance and d is the film thickness. The optical band gap can be determined using Tauc's equation [35–37]:

$$\begin{eqnarray}&&(\alpha h\upsilon )={\rm{B}}\,{(h\upsilon -{E}_{g})}^{m}\end{eqnarray} \tag{ 4 }$$

where B is constant, hυ is the energy of the incident photon, E_g is the optical band gap for both possible transition direct and indirect depending on the value of the variable parameter m (m = 1/2 and 2 for allowed direct and allowed indirect transitions, respectively). The direct (E_gd ) and indirect (E_gi ) optical band gap are determined at hυ-axis from the intercept of the extrapolation of the straight portion of the curves to zero absorption [(αhυ)² or (αhυ)^1/2 = 0] as shown in figure 6 (direct state) and figure 7 (indirect state). The determined values are summarized in table 1. It is observed that the direct and indirect band gap decreases from 5.8 to 4.8 eV and from 4.8 to 4.05 eV, respectively, as the CNT content increases in the PVC matrix. This can be attributed to the rise of localized states formed within the band gap as a result of CNT loading into the PVC matrix. This result was confirmed by investigation of the band tail width using Urbach equation [3]. The obtained result of Urbach energy (E_U ) are determined and summarized in table 1. Increasing of E_U with an increase in CNT concentration confirms the increase of the amorphous state in the material and creation of defects in the band structure. Hence, optical band gap decreases as well.

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Figure 8 shows the refractive index (n) and the extinction coefficient (k) as the functions of wavelength for PVC-CNT nanocomposite films. It is obvious that both parameters (n and k) increase with the increase in CNT concentration in PVC matrix. The high refractive index of polymer materials became important parameter for the fields of highly reflective and polymer-optoelectronics applications [38]. The incident photons interact with the electrons of the host material and slow down their velocity, the film reflectivity increases as well due to the density of packing increasing with the increase in CNT concentration in the PVC matrix. Consequently, the refractive index increases as well. Furthermore, the change in the polymer structure contributes strongly in the change of the physical properties of the polymer films. In addition, the increase in the extinction coefficient is related to the increase in the absorption coefficient [39, 40].

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Optical dielectric parameters (dielectric constant ε' and dielectric loss ε'') depend on n and k values according to the following equations,

$$\begin{eqnarray}&&\varepsilon ^{\prime} ={n}^{2}-{k}^{2}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&\varepsilon ^{\prime\prime} =2nk\end{eqnarray} \tag{ 6 }$$

Figure 9 shows the dependences of (ε') and (ε'') on photon energy (hυ) for PVA–CNT films. It is observed that the real and imaginary parts increase by increasing of CNT concentration in PVC films. The same behavior like obtained in the absorption spectra, where the higher values are observed at the UV-region where high photon energy exist. Moreover, both ε' and ε'' are found to be increased with the increase of the CNT concentration. This can be related to the change in the band structure of the polymer films, as more states are created in the forbidden gap with increasing CNT concentration. In addition, the higher value of ε'' (in the UV region) is associated with the polarization effect which decreases gradually with increasing the incident photon energy [41].

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Optical conductivity (σ_opt ) depends on the absorption coefficient and refractive index. It is calculated using the following equation,

$$\begin{eqnarray}&&{\sigma }_{opt}=\displaystyle \frac{n\alpha C}{4\pi }\end{eqnarray} \tag{ 7 }$$

where C is light speed in vacum. Figure 10 shows the variation of the optical conductivity with the incident photon energy for the PVA–CNT nanocomposite films. As shown, the optical conductivity increases as the CNT increases in the PVC matrix. This is caused by the creation of the new states in the forbidden gap, which contributes to the transfer of electrons from one state to another [9]. Hence, the optical band gap decreases and the optical conductivity increase as well.

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Optical dispersion parameters can be determined using single-oscillator fit (Wemple -DiDomenico model), which expressed in the following form [42],

$$\begin{eqnarray}&&{({n}^{2}-1)}^{-1}=\displaystyle \frac{E0}{{E}_{d}}-\left(\displaystyle \frac{1}{{E}_{0}{E}_{d}}\right){(h\upsilon )}^{2},\end{eqnarray} \tag{ 8 }$$

where E₀ and E_d are the oscillator energy and the dispersion energy, respectively. The relation between (n²–1)⁻¹ and (hυ)², taken for the normal dispersion region below the absorption edge region at (hυ < E_g), is represented in figure 11. As shown, the lines intercept the (n²–1)⁻¹ axis at points equal to (E₀/E_d ) and the slope of these lines is equal to (1/E₀ E_d ). The extracted values E₀ and E_d are summarized in table 2.

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The static refractive index (n₀) is the refractive index at hυ = 0. So, equation (7) at this condition can be written as [10],

$$\begin{eqnarray}&&{n}^{2}=1+\displaystyle \frac{{E}_{d}}{{E}_{0}}\end{eqnarray} \tag{ 9 }$$

The extracted values of energies, E_d and E₀, were used to calculate n₀ values. Table 2 shows that n₀ increases with an increase in CNT content in the PVC matrix.

Other important parameters like dipole strength for optical transitions (f) and static optical dielectric constant (εs) are calculated using the following equations [9],

$$\begin{eqnarray}&&f={E}_{0}{E}_{d}\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{\varepsilon }_{{\rm{s}}}={{n}_{0}}^{2}\end{eqnarray} \tag{ 11 }$$

The obtained values of εs and f are summarized in table 2. It is obvious that the increase in CNT content in the PVC matrix enhances these parameters.

The linear optical susceptibility χ⁽¹⁾, the third-order nonlinear optical susceptibility χ⁽³⁾, and the nonlinear refractive index (n₂) can be calculated in accordance to the obtained E₀, E_d and n₀ values using the following equations [3, 43],

$$\begin{eqnarray}&&{\chi }^{\left(1\right)}={E}_{d}/4\pi {E}_{0}\end{eqnarray} \tag{ 12 }$$

$$\begin{eqnarray}&&{\chi }^{\left(3\right)}=6.82\times {10}^{-15}{\left({E}_{d}/{E}_{0}\right)}^{4}\end{eqnarray} \tag{ 13 }$$

$$\begin{eqnarray}&&{n}_{2}=12\pi {\chi }^{\left(3\right)}/{n}_{0}\end{eqnarray} \tag{ 14 }$$

It is obvious that the calculated values of χ⁽¹⁾, χ⁽³⁾ and n₂, summarized in table 2, increase with the increase in CNT content in the PVC matrix. WDD model have been used previously to evaluate the χ⁽¹⁾, χ⁽³⁾ and n₂ parameters for PVC system doped with different oxides like: graphite oxide [9], Al₂O₃ [3], NiO [4], CdO [5], and tungsten oxide [10]. As our results show, the PVC-CNT films are more promising in the production of nonlinear optical devices.