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Vol. 8. Issue 1.
Pages 904-913 (January - March 2019)
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Vol. 8. Issue 1.
Pages 904-913 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.04.023
Open Access
Characterization and some physical studies of PVA/PVP filled with MWCNTs
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Hamdy M. Zidana, Elmetwally M. Abdelrazekb,
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emabdelrazek@yahoo.com

Corresponding author.
, Amr M. Abdelghanyc,
Corresponding author
a.m_abdelghany@yahoo.com

Corresponding author.
, Ahmed E. Tarabiahd
a Physics Department, Faculty of Science, Damietta University, New Damietta, 34517, Egypt
b Physics Department, Faculty of Science, Mansoura University, Mansoura, 35516, Egypt
c Spectroscopy Department, Physics Division, National Research Center, 33 Elbehouth St, Dokki, 12311 Cairo, Egypt
d Dental Biomaterials Department, Faculty of Oral and Dental Medicine, Delta University, Gamassa, Egypt
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Tables (2)
Table 1. Band assignment of IR vibrational modes for (a) pure PVA, (b) pure PVP, (c) functionalized-MWCNTs and (d) unfilled and filled PVA/PVP blend with MWCNTs.
Table 2. The optical parameters values of films of PVA/PVP blend filled with MWCNTs.
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Abstract

Pristine films of polyvinyl alcohol (PVA)/polyvinyl pyrrolidone (PVP) polymer blend filled with gradient contents of (MWCNTs) multi-walled carbon nanotubes have been prepared using ordinary casting technique. Fourier transform infrared (FT-IR) revealed the existence of main characteristic peaks corresponding to vibrational groups that characterized the synthesized samples. The interaction between nano-composite components was indicated by variation of main vibrational bands in the spectral range 1500–1750cm−1. X-ray diffraction (XRD) confirms the structural modification in PVA/PVP matrix due to MWCNTs filling. Transmission electron microscopy (TEM (shows the presence of MWCNTs with a diameter between 80 and 30nm and length of about several micrometers. Scanning electron microscopy (SEM) used to approve the homogenous nature of prepared samples. The absorption coefficient spectra show the appearance of two absorption peaks at 290 and 620nm attributed to nπ* and ππ* electronic transitions. The optical energy gap (Eg) have been obtained from the indirect allowed transition. It was found that, Eg decrease with increasing MWCNTs content. Analysis of refractive index n showed a normal dispersion in the wavelength range 866–2500nm, as well as an anomalous dispersion in the wavelength range 190–866nm. The oscillator parameters (oscillator energy and dispersion energy) were calculated. The decrease in optical energy gap and the increase in refractive index due to filling with MWCNTs suppose the possibility of their use in optical devices.

Keywords:
MWCNTs
PVA
PVP
Refractive index
FTIR
UV/vis
Full Text
1Introduction

In the last years, carbon nanotubes (CNTs) have attracted much attention as a result of their excellent optical, electronic, structural, chemical and mechanical properties. It can be used in a wide range of applications, such as nano-electronics [1], chemical sensing [2], and drug delivery [3]. The outer wall of pristine CNTs is, in principle, considered as chemically inert. So pristine CNTs is not always accepted for applications. One of the most favorable methods to get over this obstacle is to functionalize CNTs. Functionalization enhances their features and in turn their application ability [4].

Polymer composites are one of the most attractive near-term means to make use of the features of carbon nanotubes and graphene. Polyvinyl alcohol (PVA) is an essential polymer, due to its physical and chemical features and so it attracted researchers’ attention over the years. This polymer can be a powder, film and fiber forms. It has a semi-crystalline nature arises from the role of OH group and the hydrogen bonds [5]. Due to its low protein adsorption characteristics, biocompatibility and high water solubility, PVA is commonly used in medical devices [6]. Polyvinylpyrrolidone (PVP) has a good stable environment, easy processing, and moderate electric conductivity. It has a wide range of applications such as electrochemical devices (batteries, displays) [7].

PVA/PVP interactions have been described in many papers because of the unique properties of the resulting blend, which combine the characteristics of both polymers [7,8]. Using modifiers would change the features of PVA/PVP blend in many aspects [9]. However, it is important to take into consideration the original idea of the application when choosing the suitable filler.

The present study aims at presenting new composite films from PVA/PVP blend filled with different contents of MWCNTs with a simple method of preparation and study its physical properties. A systematic investigation of structural, and optical properties of this system are discussed by different tools and techniques.

2Experimental work

In the present work, PVA (supplied by S.D. Fine-Chem Ltd. Mumbai, India – M.W. 14,000) and PVP (supplied by SISCO Research Laboratory Pvt. Ltd. Mumbai, India – M.W. 40,000) were used as a blend matrix material for a filler from multi-walled carbon nanotubes which was developed by chemical vapor deposition (CVD).

Functionalized MWCNTs (NTX10) were obtained from Nanothinx is added to distilled water and sonicated for 15min. Equal amounts of PVA and PVP (50/50) by weight percent were mixed with that solution and stirred at 70°C till a proper viscous solution is formed. To prepare the filled polymeric films, the viscous solution was poured onto cleaned glass Petri dishes and dried in the oven at 60°C for 2 days to confirm the removal the solvent traces. When drying, the films were peeled from Petri dishes and kept in vacuum desiccators till use. The filler contents of MWCNTs in the PVA/PVP blend are 0, 0.1, 0.2, 0.5, 1, 3, 5%Wt. The thickness of the prepared samples between 0.005–0.01cm.

FT-IR first used in the characterization of the presence of functional groups on the surface of MWCNTs and the efficiency of functionalization on it. A fine powder prepared MWCNTs was blended with KBr in the ratio of 1:100 for quantitative analysis and the weighted mixtures were subjected to a load of 5t/cm2 in a sample cell i.e. to get clear homogenous discs. Then, the FT-IR absorption spectra were immediately measured after preparing the discs to avoid moisture attack. FT-IR is used also for studying the internal structure of the pure polymeric blend and intermolecular interaction between the polymeric blend and the filler. FT-IR absorption spectra were done using the single beam Fourier transform-infrared spectrometer (Nicolet iS10, USA). FTIR spectra of the samples were produced in the spectral range of 4000–400cm−1.

The properties of the crystalline phases that were formed in the samples before and after filling were analyzed by X-ray diffraction (XRD) to determine the structural changes. X-ray diffraction scans were measured at room temperature by using PANalytical X’Pert PRO XRD system and CuKα radiation (where, λ=1.540A and the tube operated at 30kV). The measurements were done for the Bragg's angle (2θ) in the range of 5°–80°.

Transmission electron microscope (TEM), (JEOL-JEM-2100, Japan) was used to study the shape and size of the functionalized MWCNTs. Scanning electron micrograph was used to study surface morphology of the samples, SEM model (Quanta 250 FEG FEI Company, Netherlands) was used, with accelerating voltage 30kV. The surface of the samples was coated with 3.5nm layer of gold to decrease the sample charging effects as a result of the electron beam.

The spectrophotometric method was utilized to identify the optical parameters and structural changes in PVA/PVP blend films filled with various contents of MWCNTs. Both the reflectance R(λ) and the transmittance T(λ) spectra of samples were measured at normal light incidence in the wavelength range of 190–2500nm using a double beam spectrophotometer JASCO model (V-570-UV-VIS-NIR). The reflectance and transmittance spectrum were measured at a wavelength range of 190–2500nm. A deuterium discharge tube (190–350nm) was used in the UV region and a tungsten-iodine lamp (340–2500nm) was used in the region of VIS/NIR as a source of light.

3Results and discussion3.1Fourier transform infrared analysis (FT-IR)

Fig. 1 shows FT-IR spectra of functionalized MWCNTs, pure PVA, pure PVP, unfilled and filled PVP/PVA blend with different contents of MWCNTs. The spectra showed characteristic bands of bending and stretching vibrations of the functional groups formed in the prepared films. FT-IR absorption band positions and their assignments of the prepared samples are listed in Table 1.

Fig. 1.

FT-IR absorbance spectra of (a) functionalized MWCNTs, (b) pure PVA, pure PVP, unfilled PVA/PVP blend and (c) filled PVA/PVP blend with MWCNTs.

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Table 1.

Band assignment of IR vibrational modes for (a) pure PVA, (b) pure PVP, (c) functionalized-MWCNTs and (d) unfilled and filled PVA/PVP blend with MWCNTs.

(a) Pure PVA
Wavelength (cm)−1  Band assignment  Ref. 
3475  OH stretching  [12,13] 
2933  CH2 asymmetric stretching  [13,17] 
1713  CO stretching  [13,14] 
1658  CC stretching  [13] 
1432  Symmetric bending of CH2  [13,18–20] 
1332  (CH+OH) bending  [13,19] 
1247  CH wagging  [13] 
1096  CO stretching  [13,14] 
921  CH2 rocking  [13] 
844  CC stretching vibrations  [13,18] 
651  (OH) wagging  [13] 
(b) Pure PVP
Wavelength (cm−1Band assignment  Ref. 
3455  OH stretching  [17] 
2955  CH2 asymmetric stretching  [21] 
1660  CO stretching  [9,17] 
1500  characteristic vibration of CN (pyridine ring)  [21] 
1460  CH bending of CH2 and/or OH bending  [21] 
1435  CH2 scissoring vibrations  [21] 
1374  CH2 bending  [9] 
1288  CH2 twisting or wagging  [21] 
1224  CC stretching and/or CH2 deformation  [9] 
846  CH2 bending  [22] 
733  CH2 rocking  [9] 
(c) Functionalized-MWCNTs
Wavelength (cm−1Band assignment  Ref. 
3425  OH stretching  [10] 
2922 & 2855  CH stretching  [10] 
1705  CO stretching  [23] 
1630  CO stretching  [23] 
1575  CC stretching  [11] 
1175  CO stretching  [11] 
1045  CO stretching  [24] 
(d) Unfilled and filled PVA/PVP blend with MWCNTs
Wavelength (cm−1Band assignment  Ref. 
3380  OH stretching  [15] 
2933  CH2 asymmetric stretching deriving from PVA  [13,17] 
1660  CO stretching deriving from PVP  [9,17] 
1432  Symmetric bending of CH2 deriving from PVA  [13,18–20] 
1374  CH2 bending deriving from PVP  [9] 
1247  CH wagging deriving from PVA  [13] 
844  CC stretching vibrations deriving from PVA  [13,18] 

For functionalized MWCNTs the broad peak at 3425cm−1 is due to (OH) stretching vibration of hydroxyl groups and the peak at 1705cm−1 represent the stretching vibration of CO from the carboxylic groups (COOH) [10]. The band at 2922cm−1 and 2855cm−1 were assigned to CH stretching vibration of methylene produced at the defect sites of the acid-oxidized MWCNTs surface [10]. The peaks at 1630, 1575 and 1175cm−1 have been ascribed to the CO, CC and CO stretching mode respectively [11]. These groups are considered to be located at defect locales on MWCNTs side wall surfaces.

For pure PVA the broadband at about 3475cm−1 is apportioned to the stretching vibration of the hydroxyl group (OH) of PVA [12,13]. The band matching to CH2 asymmetric stretching vibration appears at about 2933cm−1. The peaks at 1713 and 1658cm−1 have been ascribed to the CO and CC stretching mode respectively [13,14]. The absorption peak at 1432cm−1 has been assigned to symmetric bending of CH2. The band at about 1096cm−1 corresponds to CO stretching of carbonyl groups present in the PVA backbone. The CC stretching vibrations of the moderate absorption planar zigzag carbon backbone is observed at 844cm−1. The peak at 651cm−1 is apportioned the wagging mode of (OH) groups, while the peak at 921cm−1 assigned to CH2 rocking and the peak at 1332cm−1 correspond to (CH+OH) bending [13].

For PVP, the peaks monitored at 3455cm−1 is assigned to (OH) stretching, the peaks at 1435cm−1 and 846cm−1 corresponding to the CH2 scissoring vibrations and CH2 bending respectively. The peaks at 1660cm−1 and 1288cm−1 are apportioned to CO stretching and CN stretching [9].

For PVA/PVP blend the hydroxyl groups of PVA at 3475cm−1 and PVP at 3455cm−1 shifts to lower wavenumber 3380cm−1 due to the formation of hydrogen linking between CO of PVP and OH of PVA [15].

The spectra of filled PVA/PVP blend with various contents of MWCNTs shows that the peak at 3380cm−1 of (OH) stretching decreased in the intensity and shifts to a lower wavenumber with increasing MWCNTs content. Also, the CO peak (at 1660cm−1) decreased in intensity and became broader with increasing MWCNTs content in the blend compared with the pure blend. These confirm that the interaction between the PVA/PVP blend and the MWCNTs. The effect of MWCNTs on some characterizing IR bands can be taken as a measure of structural changes of PVA/PVP blend due to the filling. It is known that the wavenumber shifts of IR bands evidence the change in the potential energy distribution along the polymeric chain due to the filling. It is noticed that the double bond segments in the present system are considered as suitable sites for polarons and/or bipolarons which may act as hopping sites for the charge carriers [16].

3.2X-ray diffraction (XRD)

Fig. 2 explains the XRD of pure MWCNTs, pure PVA, pure PVP, unfilled and filled PVA/PVP blend with MWCNTs. The diffraction pattern of unfilled PVA shows a diffraction peak at 2θ=19.4°. This band can be apportioned to the partially crystalline nature of PVA polymer molecules. The pure PVP scan shows very broad diffraction peaks around 2θ=22 and 11° which confirm the amorphous nature of the prepared polymer film. X-ray diffraction pattern of MWCNTs shows also intense peak at 2θ=25.9° matching to the (002) reflection and small diffraction peaks at the angles 2θ=42, 53 and 77° which indexed to the (100), (004) and (110) reflections [25].

Fig. 2.

X-ray diffraction pattern for (a) MWCNTs, (b) pure PVA, pure PVP, unfilled PVA/PVP blend and (c) filled PVA/PVP blend with MWCNTs.

(0.41MB).

For unfilled PVP/PVA blend, it is observed that the peak at 2θ=19.4° is the only exist and its intensity decreased and became broader. This indicates that the addition of PVP to PVA increases its amorphous fraction and the PVA/PVP blend has semi-crystalline nature. The appearance of one peak only for PVP/PVA blend may indicate the good compatibility between the blend components.

After filling with MWCNTs the peak intensity at 2θ=19.4° decreased more, this resorts to the interactions between the blend and the filler which leads to a decrease in the intermolecular interaction between the blend chains and as well as the crystalline degree [17]. This amorphous nature is responsible for greater ionic diffusivity which led to high ionic conductivity [8]. This behavior implies that a structural modification in PVP/PVA matrix due to MWCNTs filling and confirm the results of FT-IR study (Section 3.1).

The disappearance of the MWCNTs characteristic peaks after filling may be attributed to the good dispersion of the filler in the polymer matrix [26]. Also, may be due to the presence of MWCNTs in small contents.

3.3Transmission electron microscopy (TEM)

Fig. 3 show TEM of pure functionalized MWCNTs with some flaws in the carbon-carbon bonding associated with the formation of carboxylic acid groups on its surface [27]. Also, the figure shows that the diameter of MWCNTs is between 80 and 30nm and length of about several micrometers.

Fig. 3.

TEM for functionalized MWCNTs.

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3.4Scanning electron microscope (SEM)

We studied surface morphology by SEM (Fig. 4) which show that the MWCNTs are uniformly dispersed in the polymer blend up to 1%Wt. which indicate that the blend made a strong interaction with the MWCNTs and so a homogeneous composite film. At relatively high MWCNTs contents (3% and 5%Wt.), the adhesion between MWCNTs and the blend decrease, and so MWCNTs aggregates appear in the polymer blend. These aggregations occurred by the high content of MWCNTs may restrain the enhancement of the mechanical properties of the composites.

Fig. 4.

SEM for PVA/PVP blend filled with different contents of MWCNTs.

(0.83MB).
3.5Optical properties3.5.1Reflectance and transmittance spectra

To study the optical properties of the present system, the spectral distribution of both reflectance R(λ) and transmittance T(λ) was analyzed. Fig. 5 represents the spectral distribution of R(λ) and T(λ) for PVA/PVP blend filled with different levels of MWCNTs at room temperature and normal incidence light in the wavelength range 190–2500nm.

Fig. 5.

Spectral distribution of (a) transmittance T(λ) and (b) reflectance R(λ) for PVA/PVP blend filled with different levels of MWCNTs.

(0.2MB).

Samples show a strong absorption in the wavelength range 190–866nm (T+R)<1. This inequality is owing to the presence of absorbing color centers induced as a result of presence the MWCNTs in the polymeric matrix. On the other hand in wavelength λ>866nm, the films become transparent and no light is spread or absorbed, i.e. (T+R)1.

3.5.2Dispersion characteristics

It is believed that the optical features of various materials can be fully characterized by the complex refractive index n*, where

Generally, the real part n is connected to the dispersion; while the imaginary part (extinction coefficient) k presents a measure of dissipation rate of the electromagnetic wave in the dielectric medium.

The real part of refractive index n can be calculated from the measured values of R(λ) and T(λ) from the following equations:

where d represents the thickness of the sample. Fig. 6 explains the spectral distribution of refractive index n(λ) in the wavelength range 190–2500nm for films of PVA/PVP blend that is filled with various levels of MWCNTs. From this figure, we found that n(λ) value show normal dispersion behavior at the wavelength (λ>866nm), and anomalous dispersion at (λ<866nm). Such behavior has been reported for many organic compounds [28]. In the anomalous dispersion region, there are peaks owing to the quick increase in the absorption mechanism in the fundamental absorbing edge or as a result of the presence of absorbing color centers produced as a result of presence MWCNTs in the polymeric matrix. This behavior can be illustrated by a multi-oscillator model [29].

Fig. 6.

Spectral dispersion of refractive index n(λ) for PVA/PVP blend filled with different contents of MWCNTs.

(0.08MB).

The dispersion curves of the refractive index n(λ) in the normal dispersion region (λ>866nm) for the studied system can be represented in Fig. 7. It is found that the values of n decrease with increasing wavelength and reach to nearly constant value at the very long wavelength. Also, it is found that n increases with increasing MWCNTs content [30]. This increase in refractive index of PVA/PVP blend after embedding MWCNTs may be due to the structural modification in polymeric matrix [30].

Fig. 7.

The dispersion curves of the refractive index n(λ) in the normal dispersion region λ>866 for the studied system.

(0.07MB).

The complex dielectric constant (permittivity) ɛ* is related to the complex refractive index n* by the following equation:

Using Eq. (1) the real part ɛr and imaginary part ɛi of this description can be calculated by;

The dispersion of real and imaginary parts of dielectric constants can be represented as shown in Fig. 8 for films of pure PVA/PVP blend filled with different contents of MWCNTs. From this figure, it is noticed that the behavior of ɛr was the same as refractive index due to the smaller value of k2 compared to n2, whereas ɛi basically relies on the values of k, that is connected to the variety of absorption coefficient. Also, it is seen that the maximum values of the ɛr and ɛi were reached in the low wavelength region (absorption region) and the values of ɛr is higher than that of ɛi.

Fig. 8.

Dispersion of (a) real parts and (b) imaginary part of dielectric constants for PVA/PVP blend filled with different contents of MWCNTs.

(0.19MB).

The obtained data of refractive index n in the non-absorbing region would also be analyzed according to single oscillator model. The carrier concentration to the effective mass ratio (N/m*), the lattice dielectric constant ɛL, the single oscillator energy Eo, the dispersion energy Ed, and the dielectric constant at infinity ɛ, which depicts the contribution of the free carriers and the lattice vibration modes of the presentation can be obtained using the following procedure: The real part of the dielectric constant ɛr related to the wavelength of the incident light in the non-absorbing region by the following relation [28]:

where e is the charge of the electron, ɛ0 is the permittivity of free space and c is the velocity of light. Fig. 9 explains the link between ɛr and λ2 in the non-absorbing region for studied films. The value of ɛL can be acquired from the intersection of inferring of the straight line with the ɛr axis, the value of N/m* can be calculated through the slope of the straight line.

Fig. 9.

Relation between ɛr and λ2 in the non-absorbing region for studied films for PVA/PVP blend filled with different contents of MWCNTs.

(0.08MB).

Wemple and Didomenico [31] checked the refractive index data below the absorption edge using a single oscillator equation to show energy parameters Ed and Eo which assess the average strength of the interband optical transformation in accordance with the following equation:

where is the energy of the incident light. By plotting (n21)−1 vs. ()2 (see Fig. 10) the values of parameters Ed and Eo can be obtained from the slope (=(EoEd)−1) and the intersection of the vertical axis (=(Eo/Ed)). The values of ɛ can be taken from the intersection of the straight line with the Y-axis when it equals ((n)21)−1 and ɛ=(n)2. The values of ɛ, ɛL, N/m*, Ed, and Eo are listed in Table 2.

Fig. 10.

Relation between (n21)−1 vs. E2 for PVA/PVP blend filled with different contents of MWCNTs.

(0.06MB).
Table 2.

The optical parameters values of films of PVA/PVP blend filled with MWCNTs.

MWCNTs content  Ed (eV)  Eo (eV)  ɛ  ɛL  N/m* (kg−1m−3Eg (eV)  ΔE (eV) 
0.0%  9.31  4.82  2.93  3.05  3.2E+47  5.06  0.62 
0.1%  12.76  6.47  2.97  3.02  3.8E+46  5.02  0.82 
0.2%  13.91  6.66  3.09  3.13  9.3E+45  5.01  0.51 
0.5%  15.13  6.86  3.20  3.25  2.7E+45  4.96  0.47 
1.0%  10.33  4.67  3.21  3.33  2.0E+47  4.93  0.43 
3.0%  12.26  5.23  3.35  3.46  2.3E+47  4.87  0.47 
5.0%  19.48  7.97  3.44  3.53  3.0E+47  4.46  0.60 

It is evident that, ɛL>ɛ for all compounds under study. This trend was ascribed to the contribution of a small concentration of free carriers [28].

3.5.3Absorption characteristics and energy gap determination

The value of absorption coefficient α can be calculated at different values of wavelength (λ) from the measured values of T(λ) and R(λ) using Eq. (2) and the following equation:

Fig. 11 shows the absorption coefficient spectra for pure PVA/PVP blend and PVA/PVP blend filled with different contents of MWCNTs. This figure demonstrates an absorption peak at 198nm, which ascribed to the carbonyl groups (CO and/or CC) along the polymeric chain [32]. Also, for samples with filler content ≥1%Wt. the spectral data illustrates two absorption peaks at 290 and 620nm, ascribed to electronic transition nπ* and ππ* respectively. The peak position was found to be independent on filling level, while its intensity increases with increasing the filling level. This increase can be considered as an evidence for the incorporation of MWCNTs into the polymeric matrix. From the inspection of Fig. 11, it is noticed that all spectral data show one absorption edge shifted gradually toward longer wavelength side with increasing MWCNTs content. This shift in the absorption edge can be correlated to the change in the optical energy gap.

Fig. 11.

The absorption coefficient spectra in the UV/vis. region for films of PVA/PVP blend filled with different contents of MWCNTs.

(0.08MB).

The electronic transition type and the value of optical energy gap are necessary parameters and can be calculated through the analysis of the spectral dependence of the absorption coefficient near absorption edge using the following equation.

the parameter A is a constant which depends on the electronic transition probability, is the energy of the incident photons, Eg is the value of optical energy gap between the valence band and the conduction band. The power r is characterizing the kind of transition process in the k-space. Specifically, with r=1/2 or 2 for permitted direct and indirect transitions and 3/2 or 3 for forbidding direct and indirect transition respectively.

The collected data showed the best fit for indirect allowed transition when plotting (αhν)1/2 vs. as illustrated in Fig. 12. The values of optical band gap are decided by extrapolating the straight parts of these relations to the axis and listed in Table 2. It is evident that the energy gap decreased with the increase of MWCNTs content. The variation of the calculated value of energy gap may reflect the role of MWCNTs in changing the electronic structure of the polymeric matrix as a result of the appearance of various polaronic and defect levels. The density of localized states N(E) was proved to be proportional to the concentration of such defects and in turn, to MWCNTs contents. Increasing MWCNTs content may result in the localized states of various color centers to extending in the mobility gap. This overlap may prove the decrease in the energy gap when the MWCNTs content is increased in the PVA/PVP blend. The same was recently noticed on the spectral analysis of PMMA films deduced with metal chloride [33] and UV-illuminated PC films [34].

Fig. 12.

The plots of (αhν)1/2 vs. () for films PVA/PVP blend filled with different contents of MWCNTs.

(0.11MB).

The absorption spectra showed an extending tail for lower energies below the edge. This could be because of the transformation from the localized states in the valence band tail that was formed due to the extrinsic origins resulting from defects or impurities, to extended states in the conduction band. The absorption coefficient α is illustrated by the Urbach formula [35].

where αo is a firm and ΔE is the energy that is shown as the width of the tail of localized states in the prohibited band gap. The origin of ΔE is considered as thermal vibrations in the lattice [19]. The logarithm of the absorption coefficient α was designed as a function of the photon energy () for PVA/PVP filled with the various content of MWCNTs Fig. 13. The Urbach energy values (ΔE) were counted by taking the mutual of the slopes of the linear portion in the lower photon energy region of these curves. The values of (ΔE) for the studied system are estimated and recorded in Table 2. For the pure blend, the value of (ΔE) is 0.62eV and that of filled samples lies between 0.43 and 0.83eV.

Fig. 13.

Logarithm of the absorption coefficient α as a function of the photon energy () for PVA/PVP blend filled with different content of MWCNTs.

(0.1MB).
4Conclusion

The PVA/PVP blend filled with gradient levels of MWCNTs films were prepared by ordinary casting technique. FTIR analysis showed the formation of the intermolecular interaction between the pure blend and MWCNTs. The assigned double bond in the IR spectra suggested the presence of polarons and/or bipolarons in the polymeric matrix. The X-ray analysis showed that no significant peaks characterizing MWCNTs were detected in the nanocomposite. Also, showed the semi-crystalline nature of the studied system. TEM images confirm the presence of MWCNTs with a diameter between 80 and 30nm and length of about several micrometers. SEM images approved the homogenous nature without aggregations of prepared samples up to 1%Wt. of MWCNTS. The analysis of refractive index illustrated a normal dispersion in the wavelength range 866–2500nm as well as an anomalous dispersion in the wavelength range 190–866nm. In normal dispersion the refractive index of the blend increase with increasing MWCNTs content. The single oscillator model showed values of ɛ in the range 2.93–3.44 and ɛL in the range 3.05–3.53. In general, ɛL was found to be lower than ɛ for all the compounds which indicate the existence of free carriers. The optical transition is found to be indirect allowed and the energy gap decreases with the increase of MWCNTs content.

Conflicts of interest

There is no conflict of interest.

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