ZnO nanostructures were synthesized in the reaction temperature of 80Â°C without any extra treatments. (Zn(NO3)2.6H2O) and (NaOH) were adopted as synthesis and the production of ZnO nanostructures occurred relatively in short time The obtained ZnO nanostructures were characterized by X-ray diffraction (XRD) and the atomic force microscope AFM . Carboxy methyleted PVA (CPVA) has been prepared and characterized. (CPVA) were composited with different ZnO nanoparticles concentration .The composites are casted into films. The dielectric constant properties of the films were with hp LCR measured.
Polymer nanocomposites are the subject of increased interest because they combine the features of polymers with those of nanoparticles with small quantities (less than 5% by weight) of nano particles having high aspect ratios (1). Nano particle size inclusions are defined as those that have at least one dimension in the range 1 to 100 nm. (2,3). The structure of the polymer is very important to determines if it is polar or non-polar and this determines many of the dielectric and electrical properties of the polymer. In polar polymer (for example, PMMA, PVC, PA (Nylon), PC) the imbalance of electrons distribution on the molecules are created the dipoles and the presence of an electric field these dipoles will move to align with field. This will create dipole polarization of the material, the movement of the dipoles will take a time element to the movement, which effects the magnitude of the conductivity value.
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Polymers nanocomposites provide advantages over micron-filled polymers because they provide resistance to degradation, improvement in thermo-mechanical properties and no reduction in dielectric strength value (4,5). Nanoparticles have higher surface area-to-volume ratio than in micro particles size. The interfacial area leads to a significant volume fraction of polymer surrounding the particle that is affected by the particle surface and has properties different from the bulk polymer in interaction zone (6). Since this interaction zone is much more extensive for nanocomposites than for microcomposites, it can have significant impact on electrical and mechanical properties (7). The effective permittivity and conductivity of a composite is a complicated function of the physical properties of the individual components, such as, shape and particle size distribution, porosity, volume loading, the interaction between filler and insulating matrix (8-12).
When the measurements of permittivity is performed in the frequency domain from 10Ë‰Â² to 10Â¹ Hz.. At low frequencies when the charge redistribution/reorientation process in polymer nanocomposites material is sufficiently fast compared to the changes in the external field the permittivity is independent of the frequency, when the frequency of the external field approaches the characteristic frequency for the charge redistribution process a strong frequency dependence of the permittivity is seen as a downward step in the real part of the permittivity and a peak in the imaginary part (13). ZnO has received much attention in recent years due to its properties like semiconductor, visible photoluminescence, acoustic wave filters and piezoelectric material, Ferromagnetic properties and its abundance in nature (14-17). It can be synthesized practically into different nano forms (18). In this work ZnO was synthesized by hydrothermal method (19). The formation of ZnO nanoparticles were characterized by x-ray diffraction (XRD) and Atomic Force Microscopy (AFM). Polyvinyl alcohol (PVA) is known polymeric material good chemical stability and hydrophilicity ( 20-21) for which there have been many experiments using PVA for the fabrication of the reverse osmosis (RO) or nanofiltration membrane but there flux and rejection are rarely satisfactory. Mostly , such pure PVA membrane show low flux and low rejection due to the relative high thickness of PVA membrane (22-23) as such or their composite membrane need to ensure adequate mechanical strength, PVA has been chemically modified to its carboxymethylated form using monochloroacitic acid (MCAA) and the product designated as CPVA( 24 ). The electrical properties of CPVA are particularly important in view of the fact that there is practically, as yet, no exploration for the characteristics of CPVE electric properties or any ZnO nano type composite with this polymer.
2. EXPERIMENTAL DETAILS:
2.1. ZnO nanoparticles preparation:
ZnO nanoparticles prepared according to (25), all the reagents used in this work, NaOH and Zn(NO3)2.6H2O, were from sigma Aldrich. In two liter beaker was one liter of 1.0 M NaOH in deionized water prepared and the resulting solution was heated to 80Â°C under constant stirring, After achieving this temperature was added 250 ml of 0.5M Zn (NO3)2.6H2O slowly (dripped for 90 minutes) under continual stirring. In this procedure the reaction temperature was constantly maintained in 80Â°C.The suspension formed with the dropping of 0.5 M Zn(NO3)2.6H2O solution to the alkaline aqueous solution was kept stirred for three hours in the temperature of 80Â°C. The formed ZnO was filtered and washed several times with deionized water. The ZnO product was dried at 80Â°C in oven for several hours. The ZnO nanostructures yield by this method is about 91%.The crystalline structure and morphology of ZnO powder was assessed by XRD. (Shimadzu XRD-6000) was with copper radiation (Cu KÎ±, 1.5406 Å) as incident radiation and with Atomic Force Microscopy (AFM) was studied. To obtain the average crystallite size and micro strain.
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2.2 Preparation of carboxy methyleted PVA (CPVA):
Condensation of Polyvinyl Alcohol PVA (MW 72000 g) with Monochloroacitic Acid (MCAA) from Aldrich .PVA was dissolved in the desired amount of aqueous potassium hydroxide solution of desired concentration and heated in a water bath. The calculated amount of the MCAA was then added (1:2:: OH:MCAA) and the reaction mixture was stirred at 65Â°C for 3 h. At the end of the reaction the mixture was acidified with 0.1N hydrochloric acid. The product was precipitated with methanol. It was then dissolved in distilled water and re-precipitated from the solution using methanol as non-solvent. The process was repeated till the polymer became free of chloride ions (24).
2.2. CPVA with ZnO nanocomposites film fabrication:
Three grams CPVA , was dissolved completely in 120 ml distilled water under constant stirring for one hour while the mixture was heated up till 50Â° C then the mixture was left to cool down to (24Â°C) and the stirring was carried out to ensure a homogenous composition.
The obtained CPVA solution was divided in six equaled parts and each part mixed with various concentrations of ZnO nanoparticles (0.0%, 0.0008%, 0.004%, 0.008%, 0.018%, 0.038%) were ultrasonically for 20 min. mixed. To cast the film, the mixture for each ZnO nanoparticles concentration was poured in a casting glass plate 12x6 cm and let it dry at room temperature for 140 hours. At the expiry of this time, the films were ready which were peeled off the casting glass plate
2.3 Dielectric constant measurements:
The above fabricated films were cut into 2x1.5 cm pieces to fit a homemade silver electrode for characterization by measuring dielectric properties using Precision LCR meter HP 4274 A connected with HP 4275 A and with Test Fixture HP 16047 A at frequency range 10Â² Hz to 10âµ Hz . The dielectric parameter as a function of frequency is described by the complex permittivity.
Æ* (Ï‰) = Æ'(Ï‰) - Æâ€³ (Ï‰) â€¦â€¦â€¦. (1)
Where the real part Æ' and imaginary part Æ" are the components for the energy storage and energy loss, respectively, in each cycle of the electric field. The measured capacitance, C was used to calculate the dielectric constant, â„‡Â´ using the following expression.
Where d is the thickness between the two electrodes, A is the area of the electrodes, â„‡âˆ˜ is permittivity of the free space, â„‡âˆ˜ = 8.85x 10Ë‰Â¹Â²/N.mÂ² and (Ï‰) is the angular frequency; Ï‰ = 2 Æ’, Æ’ is applied frequency, where d is sample thickness and A is surface area of the sample. Whereas for dielectric loss, Æ"(Ï‰) and tanÎ´ is tangent delta (27):
Æ" (Ï‰) =Æ'(Ï‰). tanÎ´ (Ï‰) â€¦â€¦â€¦ (3)
The electric modulus is the reciprocal of the permittivity in complex form (28) was found using eq. (5):
Where MÂ´ and M" are the real and imaginary part of dielectric modulus and it was calculated by Eq. (6and 7):
3. RESULTS AND DISCUSSION
3.1. Atomic Force Microscope
The Atomic Force Microscope (AFM) figure (1: a-b) represent ZnO nanoparticles. The particle size histogram was performed and shown as in Figure (1: a and1: b) the particles which are to a large extent well-separated from one another throughout the field of the micrograph. Their typical diameter was less than (41.7 nm).
Figure 1A: Represent the AFM 2-D image with maximum high (40 nm) of the nano ZnO particles
Figure 1B: Represent the AFM 3-D image with maximum high (40.7 nm) of the nano ZnO particles
3.2. X- Ray Diffraction (XRD).
The XRD spectra of ZnO nanoparticles are shown in Fig. 3, a series of characteristic peaks: 2.8112(100), 2.5996(002), 2.4702(101), 1.9092(102), 1.6239(110), 1.4763(103), 1.4060(200), 1.3777(112) and 1.3590(201) are observed, and they are in accordance with the ZnO (International Center for Diffraction Data, JCPDS 5-0664). No peaks of impurity are observed, suggesting that the high purity ZnO was obtained. In addition, the peak is widened implying that the particle size is very small according to the Debye-Scherrer formula:
Where K is the Scherrer constant taken as 0.94, Î» the X-ray wavelength (CuKÎ± = 0.15406 nm), B the peak width of half-maximum, and Î¸ is the Bragg diffraction angle. The average crystallite size D is 41Â±1 nm calculated using the Debye-Scherrer formula.
Figure 3: XRD pattern of ZnO nanoparticles powder.
3.3. Dielectric constant
The dielectric properties of materials are mainly determined by their polarizabilities at a given frequency. For multicomponent systems, when free charge carriers migrate through the material, space charges build up at the interfaces of the constituents owing to the mismatch of the conductivities and dielectric constants of the materials at the interfaces (26). This is called interfacial polarization. The interfacial polarization in polymers having structural inhomogeneities (e.g., nanoparticles) can be identified by low-frequency dielectric measurement based on Maxwell-Wagner-Sillar's(26). and the changes in the permittivity values as a function of frequency are attributed to dielectric relaxations. These are more pronounced at low frequencies due to micro-Brownian motion of the whole chain (segmental movement). Nevertheless, these changes are also affected by the interfacial polarization process known as Maxwell-Wagner-Sillar, which exists in heterogeneous dielectric materials and is produced by the traveling of charge carriers (27). In order to study the effect of different frequencies on different filler concentrations with dependence of relaxation processes, effective permittivity was used Figure (4and 5) shows the real and imaginary part of permittivity respectively obtained through Equations (1-3) and the electrical modulus was used. Figure (6and 7) shows the real and imaginary parts of the electrical modulus respectively obtained through Equations (4-6) (28) as a function of frequency.
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It can be seen from Figures 4 and 5 that the effective permittivity is increased for all polymer composites with decreasing frequency. Permittivity is a frequency dependent parameter in the (CPVA) polymer systems. The (CPVA) system component of permittivity is governed by the number of orientable dipoles present in the system and their ability to orient under an applied electric field (29, 30). Usually, the molecular groups which are attached perpendicular to the longitudinal polymer chain contribute to the dielectric relaxation mechanisms. At lower frequencies of applied voltage, all the free dipolar functional groups in the (CPVA) chain can orient themselves resulting in a higher permittivity value at these frequencies. As the electric field frequency increases, the bigger dipolar groups find it difficult to orient at the same pace as the alternating field, so the contributions of these dipolar groups to the permittivity goes on reducing resulting in a continuously decreasing permittivity of the (CPVA) system at higher frequencies. Similarly, the inherent permittivities in ZnO nanoparticles also decrease with increasing frequencies of the applied field (31, 32). This combined decreasing effect of the permittivity for both (CPVA) and the filler particles result in a decrease in the effective permittivity of the (CPVA) composites also when the frequency of the applied field increases. ZnO displays strong ionic polarization due to Zn2+ and O2- ions and therefore has a high value of static permittivity (32). Therefore, in the range of frequencies under study, ZnO dielectric behaviors should have an influence on the resultant dielectric behaviors of (CPVA) composite.
In figure:4 the real permittivity slope variations with respect to frequency can be considered to be very minimal since the nanocomposite permittivity slope is almost the same as that of pure (CPVA) in frequency rang more than 3,5x10Â³ Hz, but at frequencies less than 3.5x10Â³ Hz, there is a noticeable change in the permittivity slope, this observation of the steepness in the permittivity slope at frequencies lower than 3.5x10Â³ Hz is due to the influence of ZnO filler nanoparticles.
Figure 4: Variations of real permittivity with respect to frequency of polymer at different concentration ZnO nanoparticles composites.
Figure 5: Variations of imaginary permittivity with respect to frequency of polymer at different concentration ZnO nanoparticles composites.
In Figure 6 it can be seen that MÂ´ values increased with frequency. Nevertheless, figure (7) peaks in MÂ´Â´ values were developed at the same frequency range, indicating the appearance of a relaxation process. The maximum of MÂ´Â´ increased when ZnO nanoparticles concentration amount increased (the frequency at the maximum of the peak of M" show the (Ï‰Â´) the relaxation frequency). Relaxations peaks were displaced to higher frequencies, since relaxation processes were influenced by the interfacial polarization effect which generated electric charge accumulation around the ZnO nanoparticles and the displacement of peak as the particle content increased and this is identify with work of Tsangaris G, et.al (33).
Figure 6: Variations of real electrical modulus of polymer at different concentration of ZnO nanoparticles composite.
Figure 7: Variations of imaginary electrical modulus of polymer at different concentration of ZnO nanoparticles composite.
In this study, dielectric behavior of the polar (CPVA) /ZnO nanocomposite films has been investigated. The results show that the dopant composition has great influence on the magnitude of dielectric properties. The results also show that the composite polymer films have both electric and electronic properties. The composite polymer films exhibit the combination of intrinsic dielectric anisotrophy as a result of the competition of free charges and electronic polarization corresponded to CPVA matrix. Relaxation times become shorter as the composition of ZnO nanoparticles concentration is increased indicates high availability of free charges.