A.C conductivity and dielectric properties of (PVA/ PEO) blends doped with MWCNTs

A.C electrical conductivity and dielectric properties for poly(vinyl alcohol) (PVA) /poly (ethylene oxide) (PEO) blends undopedand doped with multi-walled carbon nanotube (MWCNTs) withdifferent concentrations (1, and 3 wt %) in the frequency range(25x103 - 5x106 Hz) were investigated. Samples of (PVA/PEO)blends undoped and doped with MWCNTs were prepared usingcasting technique. The electrical conductivity measurements showedthat σA.C is frequency dependent and obey the relation σA.C =Aωs forundoped and doped blends with 1% MWCNTs, while it is frequencyindependent with increases of MWCNTs content to 3%. Theexponent s showed proceeding increase with the increase of PEOratio (≥50%) for undoped blends samples, while s value for dopedblends exhibits to change in different manner, i.e. s increases andreach maximum value at 50/50 PVA/PEO, then decreases forresidual doped blends samples with 1% MWCNTs on the other handthe exponent s decrease and reach minimum value at 50/50PVA/PEO for samples doped with 3% MWCNTs, then return toincrease. The results explained in different terms.

were interested in polymer/CNT composites and so continuous studies are needed to improve their physical properties and their applications. These studies show that the polymer/CNT composites are better than composites filled with metallic particles consisting electrical and thermal transfer mechanisms, even a small amount of CNT added to the composites will enhance these properties [2]. Polyvinyl alcohol (PVA) polymers have especially been given a great deal of attention. These polymers are useful in broad applications due to their excellent chemical resistance, physical properties, and biodegradability. The binding characteristics of PVA offer excellent adhesion to porous and water-absorbent surfaces. To further improve the electrical properties of PVA, many researchers have reported the synthesis of PVA/CNT nanocomposites [3]. The PEO polymer has a wide range of application including the use as pharmaceutical recipients, food additives and plasticizers. However, much progress was made in the electrical conduction in polyethylene (PEO). Previous studies were centered on the enhancement of its ionic conductivity with the aim of developing the material to have the promising electrical application [4]. The present work is focused on the study of the electrical conductivity and dielectric properties for (PVA-PEO) /MWCNTs blends nanocomposites, to show the effect of MWCNTs concentrations on these properties.

Experimental part
Polyethylene oxide (PEO) of molecular weight (8x10 6 ) were purchased from (Sigma-Aldrich USA), Polyvinyl alcohol (PVA) with an average molecular weight of (160,000) supplied by (HIMEDIA, India) and MWCNTs from (Sigma-Aldrich USA) with diameter between 5-15 nm. All polymer blends and blends composite films were prepared using casting technique. The polyvinyl alcohol (PVA) and Polyethylene oxide (PEO) was dissolved in deionize water using magnetic stirrer and heating throughout mixing process to get homogeneous solution. To prepare doped blends, weight percentages of MWCNTs are (1, and 3) wt.% were added to the blends solutions and mixed for 30 minute using ultrasonic homogenizer to get more homogenous solution, then the solutions of undoped blends was transferred to clean glass Petri dish of (10 cm) in diameter placed on plate form. The dried film was then removed easily using tweezers clamp. Samples used for electrical measurements were shaped as films with thicknesses values approximately (0.17mm). The electrical properties (AC dielectric and AC conductivity) of (PVA/PEO) + MWCNTs in the frequency range (25x10 3 -5x10 6 ) Hz were investigated using Hewlett Packard model (HP4274A & HP4275A). The samples of (PVA/PEO)/ MWCNTs nanocomposites were placed in the holder. AC dielectric and Ac conductivity were calculated as a function of frequency. The AC conductivity σ a.c (ω), dielectric constant(ε r ) and dielectric loss (ε i ) of the prepared (PVA/PEO)/MWCNT composites were calculated from the following relations [5].
where ε 0 : is the permittivity of free space = 8.85x10 -14 F/cm, t, A thickness and surface area of the sample respectively. R: resistance of the composite, εr : dielectric constant. A.C electrical conductivity σ a.c (ω) is measured by the following equation where ω is the angular frequency (=2 f), σ tot (ω) is the measured total electrical conductivity, σ d.c (ω) is the DC conductivity which depends strongly on temperature, it dominates at low frequencies and high temperatures, Whereas the σ a.c (ω), which has a weaker temperature dependence than σ d.c and dominates at high frequency and low temperature. The relation for the frequency dependence AC conductivity is given by: A is a constant, and (s) is a function of temperature which is determined from the slope of a plot ln σ d.c (ω) versus ln(ω) [5], then the value of s can be calculated from; Results and discussions 1. A.C electrical conductivity AC electrical conductivity of undoped and doped (PVA-PEO) nanocomposites samples with different concentration were studied as a function of frequency, as shown in Fig. 1. The increase of AC with frequency referred that is obey the relation (5). The increase of a.c conductivity with frequency indicate that charge carriers are transported by hopping through defect sites along the chains [6]. The 1% and 3%wt MWCNT nanocomposites show a slight increase in the conductivity compared to that of the pure sample, but still the samples at this wt% remains an insulator. The reason of such behavior can be attributed to the tunneling conduction mechanisms (TCM). This behavior may be explained as: at the surface of MWCNTs the carboxylic groups decrease the tunneling current making the tunneling difficult occurs leading to a slightly increasing of the conductivity. Also it can be noticed that at high content of MWCNT i.e. 3% the nanocomposites become independent of frequency which indicates the electron type of the charge transport. So it can be seen that the change in the electrical conductivity depends on the amount of MWCNT in the nanocomposites. At small amount, the conductivity of the nanocomposites increases with increasing frequency while at higher amount the conductivity shows a direct current and a non-dielectric behavior. This agrees with the result mentioned by other workers [7,8]. On the other hand it is evident that addition of PEO to PVA increases the conductivity of the latter i.e. increases the charge carriers The exponential factor (s) were obtained from the plotting of Ln (σ(w)) versus Ln(w), the values of (s) were listed in Tables 1-3. It is clear that (s) values exceeded unity for undoped blends samples which indicate that the conductivity is pure A.C. The results showed that s value decreases i.e. less than unity with addition of nano filler to blends samples which confirmed the hopping mechanism. It is obvious that s value increases with the increase of PEO ratio in undoped samples. To explain our results we suggests small polaron (SP) model while s value increases with the increase of PEO ratio and reach maximum value then decreases for residual doped blends samples at 1%MWCNT, thus the convenient models are small polaron (SP) and Correlated Barrier Hopping (CBH). The results declared that at 3% MWCNT doped blends samples s value decrease and reach minimum value but then return to inc suitab and C Small when occur the co of lo

2.The dielectric constants
The dielectric constant components were evaluated by measuring equivalent parallel capacitance C P and dissipation factor or equivalent parallel resistance R P of the sample. Fig. 2 shows the variation of real dielectric constant ε r as a function of frequency ω for undoped (PVA-PEO) blends, doped (PVA-PEO) blends while Fig. 3 illustrates the variation of imaginary part of dielectric constant ε i as a function of frequency ω for undoped (PVA-PEO) blends and doped (PVA-PEO) blends. The results show that the values of ε r tends to decrease with the increase of PEO weight fraction but then return to increase with further addition of PEO, although the value of ε r of pure samples are exceeded those of blend one, while it decreases with increasing frequency to reach a lower values at high frequency which represent the onset frequency. This result can be explained by the fact that the electrode blocking layers is dominated mechanism at low frequency region [10]. Thus the dielectric behavior is affected by electrode polarization. At high frequency the dielectric signal is not affected by electrode polarization .The tren blen valu PVA blen  When the MWCNT distributed in the polymer matrix to form nanocomposites, it creates a lot of interfaces a large dominate of nomadic electron could provide with large πorbital of the MWCNT. The interface polarization can take place when electrons oriented under electric field [2]. Further increase of CNT concentration will increase the number of interfaces but increase above a certain value will lead to the contact between the MWCNT leading to the decrease of interfaces (percolation threshold), which will resulting in the decrease of the dielectric constant. Also it can be noticed from Fig. 3 that ε r showed proceeding increase of with increase of weight fraction of filler which reflect the increase of conductivity. Maximum value of ε i obtained for doped blends, for 50/50 PVA/PEO while 75/25 PVA/PEO declare maximum value for blends doped with 1 and 3% MWCNT respectively. Fig. 4 shows the variation of capacitance (C) as a function of frequency (ω) for (PVA/PEO) blends, at Fig. 4(a) The results show that the values of C tends to decrease with the increase of PEO content and reach minimum value for 100%PEO, also the capacitance decreases with the increase of the frequency reach a lower values at high frequency. Maximum value of capacitance obtained at 100% PVA for all doped and undoped samples. It is seen that the capacitance decreases in the low frequency range and attains a constant value in the high frequency range, it is the usual behavior observed in many dielectric films. The decrease of capacitance with increasing frequency is attributed to the increasing inability of the dipoles to orient themselves in a rapidly varying applied field [13].
The observed increase in the dielectric constant as seen from Fig. 3 which is related to the increase in the electrical conductivity is take place as a result of incorporation concentration of MWCNT nano particles especially at high concentration in the PVA /PEO blend matrix was reported as inter phase between particles, polymer matrix, and composite morphology [14].

Conclusion
The following conclusions can be made from this work: A.C conductivity is frequency dependence for undoped and doped blends with low ratio of MWCNT while it is frequency independence for doped blends with high ration of MWCNT. Increase of MWCNT content in blends leads to increase real and imaginary dielectric constants. The exponent s show maximum value for undoped blends and minimum value for doped blends with high MWCNT concentration.