Journal of PolymersVolume 2014 (2014), Article ID 846140, 7 pageshttp://dx.doi.org/10.1155/2014/846140

Research Article

Microstructural and Mechanical Studies of PVA Doped with ZnO and WO₃ Composites Films

N. B. Rithin Kumar,¹ Vincent Crasta,² Rajashekar F. Bhajantri,³ and B. M. Praveen⁴

¹Department of Physics, Srinivas School of Engineering, Mukka, Mangalore, Karnataka, India²Department of Physics, St. Joseph Engineering College, Vamanjoor, Mangalore, Karnataka, India³Department of Physics, Mangalore University, Mangalagangotri, Mangalore, Karnataka, India⁴Department of Chemistry, Srinivas School of Engineering, Mukka, Mangalore, Karnataka, India

Received 6 November 2013; Revised 3 December 2013; Accepted 5 December 2013; Published 21 January 2014

Academic Editor: Dirk Kuckling

Copyright © 2014 N. B. Rithin Kumar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Polymer composites of ZnO and WO₃ nanoparticles doped polyvinyl alcohol (PVA) matrix have been prepared using solvent casting method. The microstructural properties of prepared films were studied using FTIR, XRD, SEM, and EDAX techniques. In the doped PVA, many irregular shifts in the FTIR spectra have been observed and these shifts in bands can be understood on the basis of intra/intermolecular hydrogen bonding with the adjacent OH group of PVA. The chemical composition, phase homogeneity, and morphology of the polymer composites of the polymer film were studied using EDAX and SEM. These data indicate that the distribution of nanosized ZnO and WO₃ dopants is uniform and confirm the presence of ZnO and WO₃ in the film. The crystal structure and crystallinity of polymer composites were studied by XRD. It was found that the change in structural repositioning and crystallinity of the composites takes place due to the interaction of dopants and also due to complex formation. The mechanical studies of doped polymer films were carried out using universal testing machine (UTM) at room temperature, indicating that the addition of the ZnO and WO₃ with weight percentage concentration equal to 14% increases the tensile strength and Young’s modulus.

1. Introduction

The doping of nanoscopic organic or inorganic materials into polymeric matrices represents a strategic route to improve the performance of material characteristics like structural, physical, chemical, optical, electrical, and mechanical properties. These nanocomposites are of new class of materials made with nanosized fillers like metals, metal oxides, and so forth. Thus metal-polymer composites can be obtained by in situ and ex situ techniques [1–4]. In the in situ methods, metal particles are generated inside a polymer matrix by decomposition (e.g., thermolysis, photolysis, radiolysis, etc.) or chemical reduction of a metallic precursor dissolved into the polymer. In the ex situ approach, nanoparticles are first produced by soft-chemistry routes and then dispersed into polymeric matrices [5–9]. For the synthesis of metal-polymer composites the ex situ techniques are frequently preferred because of the high quality of the film.

Over the past decades, transition metal oxides (TMOs) have been widely investigated for use in applications related to electronic, optical, and mechanical properties. Among these, TMOs, zinc oxide (ZnO), and tungsten oxide (WO₃) are promising materials for the various above mentioned applications [10–13]. When these TMOs are incorporated in polymers, it improves their electrical, mechanical, and optical properties due to their high interfacial interaction between organic moieties and inorganic nanoparticles. The basic idea of this research is preparation of different concentrations of both zinc oxide (ZnO) and tungsten oxide (WO₃) doped PVA composite films and improving the structural and mechanical property of the PVA composite. The preparation of the nanoparticles doped polymer composite film can be done by many methods, out of which solution casting is the simplest technique.

In the present paper, polyvinyl alcohol (PVA) is chosen as basic polymer because of eco-friendly, nontoxic, water-soluble, degradable nature and the easiness of making thin films with metal oxide. We report the preparation of different concentrations of both zinc oxide (ZnO) and tungsten oxide (WO₃) doped PVA composite films and improve the structural and mechanical property of the PVA composite. The films were characterized with spectroscopic techniques for examining functional group by Fourier transform infrared spectroscopy (FTIR). The crystallinity and morphological properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The mechanical properties were studied using universal testing machine (UTM).

2. Experimental Details

2.1. Sample Preparation

97 to 100 mol% hydrolyzed PVA compound (Mowiol 10–98 fully hydrolysed grade, Sigma Aldrich, Germany) was procured commercially as a basic polymeric material. The procured PVA has molecular weight  g/mol, degree of polymerization , and bulk density of 0.4–0.6 g cm⁻³. For the doping purpose two metal-oxide nanomaterials, zinc oxide (average size < 50 nm,  g/mol), and tungsten oxide (average size < 100 nm,  g/mol), were also procured. 6 g of PVA is dissolved completely in 100 mL of distilled water under constant stirring at 1000 rpm for one hour while the mixture is heated up to 90°C until the polymer completely dissolved and formed a clear viscous solution. The viscous solution was cooled down to room temperature and stirring was continued using mechanical stirrer at 180 rpm for 3 hrs to ensure the homogeneity of the composition. The obtained PVA solution was divided into six equal parts and each part of viscous solution of prepared PVA was treated by adding mixed fillers PVA/() ZnO () WO₃, where , 1%, 5%, 7.5%, 10%, 14%, and 15% weight concentration and then it was mixed ultrasonically by using probe ultrasonicator for 20 minutes with pulse on time 10 s and off time 10 s. To prepare polymer film of treated polymer, the viscous solution was cast onto a Petri dish and left to dry in air for 3 days at room temperature. The films were peeled off from the casting glass plate and kept in vacuum desiccators for further study. The thickness of the prepared polymer composite film is measured by using dial thickness gauge (Mitutoyo, Japan). Three trails of thickness were measured at different places of each polymer film and average thickness of the film was calculated. Thin films of thickness ranging from 100 to 150 μm were obtained.

2.2. Physical Measurements

IR transmittance studies of the PVA, with and without nanoparticles doped films, were carried out using FTIR spectrophotometer. The infrared spectra were recorded using JASCOFTIR 4100 type A spectrophotometer in the wavenumber range 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. The X-ray diffractograms of the samples were recorded using a Bruker D8 Advance X-ray diffractometer with Ni filtered and Cu Ka radiation of wavelength  Å, with a graphite monochromator. The scanning was taken in the range 10°–120° with a scanning speed 2°/min. The surface morphological properties were investigated using JEOL JSM-6380LA scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX). A LLOYDS universal testing machine (LLOYDS—5 KN, London, UK) was used to measure tensile strength, percent elongation (%), and modulus of elasticity. The tests were carried out according to ASTM D-882 standard test (ASTM, 1992) and calculated using NEXYGEN Plus software. Rectangular shaped samples of films ( mm) were taken for the determination of tensile properties.

3. Results and Discussion

3.1. FTIR Spectra of Pure and Doped PVA Samples

The FTIR spectra of pure and ZnO with WO₃ nanoparticles doped PVA samples were obtained using KBr pelleting method and the results are shown in Figure 1. The FTIR spectrum shows a strong absorption broad band at 3624.55 cm⁻¹ is assigned to O–H stretching vibration of hydroxyl groups of PVA. The band corresponding to C–H asymmetric stretching vibration occurs at 2996.84 and 2920.66 cm⁻¹ whereas C–H symmetric stretching vibration at 2878.24, 2850.27, and 2821.35 cm⁻¹. The sharp band at 1151.29 cm⁻¹ corresponds to C–O stretching of acetyl group present on the PVA back bone. Corresponding bending deformation and wagging of CH₂ vibrations are at 1479.13 cm⁻¹ and C–H wagging at 1221.68 cm⁻¹ [14–16]. In the case of PVA doped ZnO, PVA doped with WO₃, and PVA doped with both ZnO and WO₃, the FTIR spectra show shifts in the corresponding bands with a change in intensities as given in Table 1. The changes in the regularly arranged hydroxyl groups of the polyvinyl alcohol chain are capable of forming chemically more or less stable complex compounds associated with certain substances. This indicates the considerable interaction between PVA doped with ZnO and WO₃. The new bands may be correlated to the defects induced by charge transfer reaction between the polymer chain and the dopant. These observations suggest that the interaction between ZnO and WO₃ ions with acetyl group dominates (as given in Scheme 1). The shift in bending of CH₂ vibrations, respectively, indicates the chemical interactions of Zn and W ions with PVA matrix. The shift in acetyl C–O stretching of PVA also supports the existence of chemical interaction. This modification may be understood by invoking to the intra/intermolecular hydrogen bonding and complex formation of the PVA molecules with ZnO and WO₃ as shown in Scheme 1. It also modifies the wagging, bending, skeletal, and out of plane vibrations. Further, the vibrational peaks found in the range 800–600 cm⁻¹ can be attributed to Zn–O or W–O, which indicates the mixed dopants are complexed with the polymer matrix. Hence, it can be concluded that the OH group of the PVA interact with Zn and W forms a complex. Due to this interaction and complex formation, the frequencies corresponding to other vibrations in PVA such as –CH₂, –CH group, and C–O groups will be affected.

Table 1: Irregular shifts in FTIR spectra of pure and doped PVA composites.

Sl. no.
Sample
C–H asymmetric  stretching vibration cm⁻¹
C–H symmetric  stretching vibration cm⁻¹
C–O stretching of  acetyl group cm⁻¹
CH₂ vibrations cm⁻¹

1
Pure PVA
2996.84
2878.24
1151.29
1479.13

2
PVA/15% WO₃
2992.02
2878.24
1150.33
1476.24

3
PVA/1% ZnO 14% WO₃
2950.55
2907.16
1148.4
1451.17

4
PVA/5% ZnO 10% WO₃
2956.34
2868.59
1150.33
1475.28

5
PVA/7.5% ZnO 7.5% WO₃
2953.45
2877.27
1151.29
1482.99

6
PVA/10% ZnO 5% WO₃
2955.38
2872.45
1149.37
1480.1

7
PVA/14% ZnO 1% WO₃
2955.38
2877.27
1151.29
1479.13

8
PVA/15% ZnO
2992.98
2880.17
1149.37
1475.28

Scheme 1

Figure 1: FTIR spectra of PVA/() ZnO () WO₃ with (a) pure PVA, (b)  wt%, (c)  wt%, (d)  wt%, (e)  wt%, (f)  wt%, (g)  wt%, and (h)  wt%.

3.2. XRD Studies of Pure and Doped PVA Samples

The XRD studies in the range ( to 120°) reveal that all the films under study show semicrystalline structure. There is a broad intense peak appearing at scattering angle (), corresponding to “” spacing of 4.57 Å for PVA. The crystalline nature of PVA is due to the strong intermolecular interaction between PVA chains through intermolecular hydrogen bonding [17]. The crystalline peaks observed at , 34.39, 36.93, 48.49, 57.66, 64.23, 67.67, and 69.35 degrees correspond to the hexagonal phase of zinc oxide (ICSD Collection Code: 154490; JCPDS code: 36-1451, 89-7102, and 65-3411) and peaks at , 24.4, 26.5, 33.2, 34.39, 36.22, 47.50, 55.6, 62.3, and 66.9 degrees correspond to tungsten oxide (ICSD Collection Code: 836; JCPDS code: 05-0363 and 33-1387). The peaks at , 23.8, 37.8, 49.75, 55.44, and 62.51 degrees (ICSD Collection Code: 153197) do not belong to PVA crystalline spectra but it may arise from scattering atomic planes of some crystalline patterns of the PVA-ZnO and/or PVA-WO3 complex. The intensity of the diffraction peak and also the size of the crystals in PVA are determined by the number of PVA chain packed together. After complexation with dopants, the intensity of the PVA diffraction peaks is further decreased. This may be due to the interaction between PVA and mixed dopants, which leads to decease in the intermolecular interaction between PVA chains. The diffraction pattern confirms the presence of ZnO and WO3 in the film. The change in intensity of sharp peaks for different values of “” can be attributed to the change in the content of the mixed dopants when added to the PVA matrix.

The average particle size can be calculated using the first approximation of Debye-Scherrer formula [17]: where is the average size of the particles, is the wavelength of X-ray radiation, is the full width at half maximum intensity of the peaks, and is the diffraction angle.

The particle size of ZnO and WO₃ embedded in polymer matrix obtained for different dopants is in the same range of procured nanoparticles for doping with polymer matrixes shown in Table 2. The apparent fluctuation of the particle size of ZnO and WO₃ phase may be attributed to the particles aggregation and complexation of ZnO with WO₃ in PVA polymer matrix.

Table 2: Variation of degree of crystallinity as a function of different concentrations of the ZnO and WO₃ as calculated from the X-ray patterns.

Sl. no.
Sample
Percentage of  crystallinity
Average crystallinity  size (nm)
Average intercrystalline  separation ()

1
Pure PVA
56.86
2.92
5.70

2
PVA/15% WO₃
46.53
24.675
4.77

3
PVA/1% ZnO 14% WO₃
38.72
20.726
4.70

4
PVA/5% ZnO 10% WO₃
35.51
16.45
4.55

5
PVA/7.5% ZnO 7.5% WO₃
32.81
11.048
4.65

6
PVA/10% ZnO 5% WO₃
37.46
15.42
4.63

7
PVA/14% ZnO 1% WO₃
48.02
14.92
4.79

8
PVA/15% ZnO
45.86
30.18
5.57

Crystallinity is defined as the weight fraction of the crystalline portion of a polymer. The physical and mechanical properties of polymers are considerably dependent on this parameter. XRD is most frequently used to measure the crystallinity in the polymers: Table 2 illustrates the variation of degree of crystallinity as a function of different concentrations of the ZnO and WO₃ as calculated from the X-ray patterns (Figure 2) by using (1). It is clear from the table that maximum value for the PVA/() ZnO () WO₃ where is detected. The values of degree of crystallinity of PVA/() ZnO () WO₃ where , 1%, 5%, and 7.5% doped samples decrease and increase for % and 14%. This implies changes in the structural regularity of the main chains of the polymeric molecules on doping.

Figure 2: X-ray diffraction patterns of PVA films.

The average intercrystallite separation () in the amorphous region of the sample was evaluated using the relation [18] where is the wavelength of X-ray radiation and is the diffraction angle.

The calculated distance between the hydroxyl groups of crystalline PVA is 5.7056 Å.

The decrease in the crystallinity separation indicates the weak intermolecular hydrogen bonding, which is responsible for the disappearance of molecular movements and causes less dense molecular packing in the crystal. As a result the crystallinity decreases.

3.3. Mechanical Studies of Pure and Doped PVA Samples

Mechanical properties were carried out to study the tensile properties of nanoparticles doped polymer composites films by using universal testing machine (UTM). The two metallic grips were attached for griping both ends of the test specimen of the film. The lower grip was stationary and the upper grip moved upward with constant rate of extension 25 mm/min keeping constant initial grip separation 100 mm for all samples. An automatic speed controller was attached to keep the speed of the upper grip. The machine was electrically driven. All measurements were carried out at room temperature in air. The tensile strength, Young’s modulus (modulus of elasticity), and percent of elongation could be used to describe how the mechanical properties are related to their chemical structure. Tensile strength was calculated by dividing the maximum load for breaking the film by cross-sectional area and elongation at break is the maximum change in length of a test film before breaking. Then, the modulus of elasticity (Young’s modulus) is calculated by the ratio of stress to strain at the linear portion of the curve. The mechanical properties may be used to assess the doping of nanoparticles on polymer matrix through a comparison of experimental results. The mechanical properties of doped polymer depend on the intermolecular forces, complex formation due to interaction of dopants, chain stiffness, and molecular symmetry of the individual polymer. From Table 3 and Figure 3 it is obvious that PVA/() ZnO () WO₃ where  wt% exhibit high tensile strength. Tensile strength also depends on molecular weight because of the effect of entanglement. The degree of crystallinity and the morphology of the crystalline material also have profound effects on the mechanical behavior of polymers, since these factors can be varied over a wide range. It was found that degree of crystallinity was high for % and crystallinity of a polymer increases with the degree of crystallinity increases because of secondary bonding. This in turn increases the stiffness (elastic modulus) and strength of the polymer. On the other hand, ductility declines as crystallinity increases. It is observed that maximum percentage total elongation at fracture was found for PVA doped with WO₃ and ZnO nanoparticles for concentration  wt%. Elongation at break is the maximum change in length of a test film before breaking. The results indicate that the addition of the ZnO and WO₃ with weight percentage % increases the tensile strength and Young’s modulus.

Table 3: Tensile properties of pure and doped PVA films.

Sample
Tensile strength (MPa)
Stiffness (kN/m)
Young’s modulus (MPa)
Percentage total  elongation at fracture

Pure PVA
2.901
5.3490
45.330
92.53

PVA/15% WO₃
5.175
3.3851
58.872
90.309

PVA/1% ZnO 14% WO₃
11.375
5.1376
70.864
263.99

PVA/5% ZnO 10% WO₃
10.979
6.1740
100.390
150.45

PVA/7.5% ZnO 7.5% WO₃
6.964
5.8880
90.585
50.304

PVA/10% ZnO 5% WO₃
11.971
7.0552
176.380
60.279

PVA/14% ZnO 1% WO₃
13.806
6.4073
183.067
64.952

PVA/15% ZnO
12.517
9.8261
196.523
195.74

Figure 3: Stress-strain curves of PVA/() ZnO () WO₃ with (a) pure PVA, (b)  wt%, (c)  wt%, (d)  wt%, (e)  wt%, (f)  wt%, (g)  wt%, and (h)  wt%.

3.4. SEM and EDAX Studies

Scanning electron microscopy has been used to study the compatibility between various components of the polymer composites through the detection of phase separations and interfaces. It has great influence on the physical properties of the polymer composite. Figures 4(a) and 4(b) show the SEM photograph of nanocomposite polymer composite. The films exhibit uniform density of grain distribution at surface morphology. The surface morphology of the PVA/() ZnO () WO₃ polymer composite films shows many aggregates or chunks randomly distributed on the top surface. The results indicate that the nanosized ZnO and WO₃ particles tended to form aggregates and dispersed into the PVA polymer matrix.

Figure 4: SEM images of PVA/() ZnO () WO₃ where (a) and (b) weight polymer composites.

The chemical composition and homogeneity of PVA doped with ZnO and WO₃ polymer composite film is studied by an analytical technique called energy-dispersive analysis of X-rays (EDAX). Polymer composites films were sputter covered by gold particles to oppose the artifacts produced above the surface because of charging. Figure 5 indicates that the distribution of nanosized ZnO and WO₃ dopants is uniform and confirms the presence of ZnO and WO₃ in the film. The Percentage distribution of nanosized ZnO and WO₃ dopants in the film is given in Table 4.

Table 4: Percentage distribution of nanosized ZnO and WO₃ dopants in the film.

ZAF method standardless quantitative analysis

Fitting Coefficient: 0.3711

Element
(keV)
Mass%
Error%
At%
K

C K
0.277
49.13
0.13
66.23
37.6617

O K
0.525
30.54
0.44
30.91
28.7637

Zn K
8.630
6.69
1.21
1.66
12.3502

W M
1.774
13.64
0.49
1.20
21.2211

Total

100.00

100.00

Figure 5: Distribution of nanosized ZnO and WO₃ dopants in the film.

4. Conclusions

We have studied the effect of ZnO and WO₃ nanoparticles on the semicrystalline polymer PVA matrix allowed us to draw the following conclusions.(i)The FTIR study shows that the Zn and W ions of the dopant interacts with the OH groups of PVA and form a complex via intra/intermolecular hydrogen bonding.(ii)The XRD results show that, due to the interaction of dopant and hence the complex formation, the structural repositioning takes place, which changes the crystallinity.(iii)The SEM and EDAX data indicates that the distribution of nanosized ZnO and WO₃ dopants is uniform and confirms the presence of ZnO and WO₃ in the film.(iv)The mechanical studies indicate that the addition of the ZnO and WO₃ with weight percentage concentration % increases the tensile strength and Young’s modulus.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this article.

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