Some electrical properties of PVA:PEG/MnCl2 thin film composites

PVA:PEG/MnCl2 composites have been prepared by adding (MnCl2) to the mixture of the poly vinyl alcohol (PVA) and poly ethylene glycol (PEG) with different weight percentages (0, 2, 4, 6, 8 and 10) wt.% by using casting method. The type of charge carriers, concentration (nH) and Hall mobility (μH) have been estimated from Hall measurements and show that the films of all concentration have a negative Hall coefficient. In D.C measurement increase temperature leads to decrease the electrical resistance. The D.C conductivity of the composites increases with the increasing of the concentration of additive particles and temperature. The activation energy decreases for all composites with increasing the concentration of the additive particles. The A.C conductivity increases with increasing of the frequency and the concentration of MnCl2 particles. The A.C electrical properties show that the dielectric constant and dielectric loss of the composites decrease with increasing of the frequency.


Introduction
Polymeric materials were given a great interest, such as the ease of fabrication, low cost, light weight, ease of chemical modification and excellent insulation or good conduction properties, in many industrial application owing to their desirable characteristics and properties which made them favorable compared to other commercial materials [1,2]. Considerable efforts focused on an applied research in the field of polymer composites to turn these materials into useful products for electronic industry. This is mainly because they possess interesting properties which can be utilized to develop a lot of related potentials.
Recently, many reports have appeared in literature dealing with the effects of the filler concentration, frequency of the applied field and temperature on the physical properties of the conductive polymer composite such as impedance, dielectric behavior and electrical conduction [3][4][5].
Many studies showed that physical properties of polymers clearly depended on many factors concerning their preparation methods and chemical structure [6]. The thermal stabilities of crystalline PEG-MnCl 2 system depend on the salt molar ratio, the PEG molecular weight, the choice of the solvent and the concentration and the thermal history. The melting temperatures also, depend on the nature of the complexion salt s [7].
The poly vinyl alcohol (PVA) is a polymer with exceptional properties such as water solubility, biodegradability, biocompatibility, non-toxicity, and non-carcinogenetic that possesses the capability to form hydrogels by chemical or physical methods [8]. PVA has high enough tensile strength and satisfactory flexibility. The PEG polymer has a wide range of application including the use as pharmaceutical recipients, food additives and plasticizers [9]. Studies were centered on the enhancement of its ionic conductivity with the aim of developing the material to have the promising electrical application [9,10]. In the present study, the electrical properties of PVA:PEG/MnCl 2 composites is investigated as a function of applied frequency and concentration to use this polymeric materials in many industrial applications owing to their desirable characteristics and properties which made them favorable compared to other commercial materials.

Experimental part
The materials used in the present work were poly-vinyl alcohol (PVA) and Poly-ethylene glycol (PEG) with addition of different concentrations from (MnCl 2 ). All the polymer composite films were prepared by casting from solution. Equal weights (one gram of PVA and PEG) with ratio (50:50) wt.% was first dissolved in the common solvent (30 ml of distilled water) and then both polymers were homogeneously mixed using a magnetic stirrer at the constant temperature 325K for 12 hours. Complete homogeneous solution was casted on teflon petri-dishes and left until the evaporation of the solvent. The resulting PVA:PEG films were dried up to four days at room temperature in the desiccators to remove the traces of solvent, and then lifted out of the petri-dish for further analysis. On the other hand, MnCl 2 was dissolved in double distilled water in the same condition. The resulting solution of MnCl 2 particles were added to the polymer blend PVA:PEG solution with mass fraction (0, 2, 4, 6, 8 and 10) wt.%. mixed using a magnetic stirrer at the constant temperature at 373 K for 12 hours until a homogenous solution is obtained. The resulting solution was then cast to glass slides and leave over it for about 72 hr. After drying, the films were kept in vacuum desiccators until use. The thicknesses of films were in ranging from 0.15 to 0.2 μm using micrometer. The type of charge carriers, concentration (n H ) and Hall mobility (µ H ), have been estimated from Hall measurements.

Results and discussion 1-Hall effect
The films of all concentration have a negative Hall coefficient (n-type charge carriers). Table 1

2-D.C conductivity
The variation of D.C electrical conductivity of PVA:PEG as a function of (MnCl 2 ) particles concentrations is shown in Fig. 1 From this figure, the electrical conductivity increases with the increase of the concentration of (MnCl 2 ) particles. The increase in D.C electrical conductivity of composite is a result to the rearrangement of MnCl 2 particles throughout the polymer matrix ]11]. The electrical conductivity could be increased as a result of increasing of electronic charge carriers which can be increased due to increasing filler content [12]. The relation between ln(σ) and the inverse absolute temperature for (PVA:PEG/MnCl 2 ) composites shown in Fig. 2 (A and B). The activation energy was calculated by using equation [13]: where: σ: is electrical conductivity at temperature T. σ 0 : is electrical conductivity at absolute zero temperature. K B : is Boltzmann constant. E act : is Activation Energy. The high values existence for activation energy in state pure polymer.

% MnCl 2
When adding MnCl 2 particles, the values of the activation energy decreases for all (PVA:PEG/MnCl 2 ) composites as a result of the impact of space charge. The addition of MnCl 2 creates local energy levels in the forbidden energy gap which act as traps for charge carriers, which move by hopping among these levels [4]. When increasing MnCl 2 particles concentrations, the activation energy decreases as a result of the increase of local centers, as shown in Table 2.  Fig. 3A shows the variation of the dielectric constant of (PVA, PEG and PVA:PEG) composites with frequency.  This figure shows that the dielectric constant of PVA:PEG/MnCl 2 % decreases when increasing the frequency. This is attributed to decrease of space charge polarization with respect to the total polarization. The space charge polarization becomes more contributing type of polarization at low frequencies, and less contributing with the increase of frequency [14]. The other types of polarizations appear at subsequent frequencies. The ionic polarization reacts slightly to the variation in the field frequencies compared with electronic polarization; this is because the mass of ion is greater than that of the electron. The electrons respond even to the high frequencies of the field vibrations. The low mass of electron makes the electronic polarization was the only type of polarization at higher frequencies, this makes the dielectric constant approximately constant for all samples at high frequencies [15]. Fig. 4A show the dielectric loss of PVA, PEG and PVA:PEG as a function of frequency. The effect of adding MnCl 2 particles on the dielectric loss at room temperature (30 0 C) is shown in Fig.4B. It is clear from the figures that dielectric loss decreases with frequency. The larger value of dielectric loss at low frequency could be due to the mobile charges within the polymer backbone. This is attributed to the decrease of the space charge polarization contribution when increasing the frequency [16].

4-The A.C electrical properties of the Composite
From the behavior of the dielectric constant ( 1 ) and the dielectric loss ( 2 ), one can observe a strong frequency dependence especially at low frequencies, which reflects the behavior of the polar materials. It is clearly seen that both  1 and  2 , increase with salt concentration and decrease with the frequency of the electric field. They have a high value at low frequencies and a low value at high frequencies. These results suggest that polar entities of the composite are effectively operating under the electric field. This behavior can be understood as follows: at low frequencies, the time interval required for the molecular dipoles of the polymer to response to the applied electric field is sufficient. This enables these dipoles to follow the oscillating field, i.e., the orientation polarization is high, which leads to enhance the dielectric constant values. While at high frequencies, the time interval needed for the dipoles to response to the applied electric field is insufficient. Hence, the dipoles are unable to follow the rapid alternation of the oscillating field, i.e., the orientation polarization drops down greatly and lead to very small value of  1 and  2 at high frequencies [17] which is similar to the behaviour for polar polymer and materials. This dielectric behaviour explains the increasing in the AC conductivity at high concentration [18]. The variation of the conductivity for PVA, PEG and PVA:PEG composites with frequency is shown in Fig. 5A.  The conductivity is increasing when increasing the frequency for all different concentration of MnCl 2 nanoparticles for PVA:PEG/MnCl 2 composites.
These figures show that A.C electrical conductivity increases with the increase of frequency. This is attributed to the space charge polarization that occurs at low frequencies, and also to the motion of charge carriers by hopping process. The increasing of the conductivity is small at high frequencies; this is attributed to the electronic polarization and the charge carriers which travel by hopping process [19].

Conclusion
The electrical conductivity, dielectric behavior of PVA:PEG/MnCl 2 composite were studied as a function of MnCl 2 particles concentration and the applied frequency. Frequency and M nCl 2 concentration affect on the electrical and dialectical behavior of the composite and explained on the basis of the interfacial (space charge) polarization, dipolar polarization and on the decrease of the hindrance of the polymer matrix. Activation energy for all composites decreased with increasing of concentration. Dielectric constant and dielectric loss decrease with increasing of frequency and increase with increasing of concentration, while the A.C electrical conductivity (σ a.c ) increased with increasing of frequency and MnCl 2 concentration due to enhancement of ionic conduction in the membrane bulk for all samples.