In recent years the effects of gamma radiation on polymers have been investigated by a number of researchers [1] -[4] . Gamma ray (γ-ray) irradiation becomes an advanced approach to optimize the physical properties of polymer materials such as structural, thermal, optical, dielectric and electrical properties. The property modification depends upon the irradiation dosage and material characteristics. γ radiation is reported to alter the structural pattern within the polymer matrix through the formation of new functional groups, rearrangements of linking- bonds, and scission and/or cross-linking of chains, etc. [5] -[7] . Among them, the major governing effects observed to alter the structural pattern in polymer films are scissions and cross-linking or joining of chains by formation of bonds [8] [9] . The scissions and cross-links of the polymers are more easily determined due to its nature. Hence, the polymers have been catalogued as to whether they undergo one or the other of the two processes [10] . The radiation not only alters the structure of the polymer, but it can also produce significant changes in the thermal, optical dielectric and electrical properties of irradiated polymer films [11] [12] . When the polymer films are irradiated with γ-rays, the radiation enhances the presence of trapped charges and to create defects in the polymer matrix. Thus the electronic transport in polymer films is highly affected by ionizing gamma radiation.

The physical and chemical properties of unirradiated and γ-ray irradiated blends based on miscible polymers have been extensively investigated [13] -[18] . The structural, optical and electrical properties of the polymers are drastically affected by the presence of another polymer. Polyvinyl Alcohol (PVA) and Polystyrene (PS) are the important polymers and are available in the form of powders, fibers and films. PVA and PS are non-toxic and water soluble polymers which have good film forming and high hydrophilic properties. PVA is a semicrystalline whereas PS is an amorphous in nature. They are a unique pair of polymers and they are chemically different; however, they display compatibility over different composition ranges at room temperature. Interest in such studies has gained significance of potential applications in integrated electronics, phosphors, surface modifications and used industrially for sizing, adhesives and emulsification.

The literature review revealed that very little work had been reported on γ-ray irradiated induced effect on structural, morphology [19] , thermal, photo [20] , dielectric and AC conductivity of PVA and PS polymer films. However, to the best of our knowledge, no reports are available on optical and electrical properties of gamma irradiated PVA_(1-x)PS_x. The detailed structural analysis of unirradiated and γ-ray irradiated PVA, PS films and PVA_0.5PS_0.5 blind film have already been reported elsewhere [21] . Thus, in the present investigation we study the frequency dependence of dielectric properties of gamma irradiated PVA_(1-x)PS_x (x = 0.0, 0.25, 0.50, 0.75 and 1.0) films.

The γ-ray irradiation can modify the dielectric properties of PVA_(1-x)PS_x (x = 0.0, 0.25, 0.50, 0.75 and 1.0) films. Figures 1-3 illustrate the change in dielectric constants (ε_r), dielectric loss (tanδ) and AC conductivity (σ_ac) for the unirradiated and irradiated samples of different doses (1.0 kGy and 1.5 kGy) within the frequency range 50 Hz - 5 MHz at room temperature as well as, at different temperature range from 27˚C - 138˚C at constant 1 kHz frequency.

From Figure 1, it was observed that the dielectric constant increases with the increasing temperature for all the pure and different concentration of PS in PVA_(1-x)PS_x blends. Also the dielectric constant increases with the increase of radiation dosage from 1.0 kGy to 1.5 kGy. For example, at 27˚C (room temperature) and at 1 kHz frequency, the dielectric constant of the pure PVA, PVA_0.75PS_0.25, PVA_0.5PS_0.5, PVA_0.25PS_0.75 and PS (unirradiated films) is 8.0329, 11.6976, 14.6452, 17.1556 and 20.9068, respectively. Whereas for the γ irradiated films (1.5 kGy) it is 12.2268, 16.5231, 21.2954, 25.1157 and 28.3029, respectively. The variation in ε_r for the unirradiated PVA_(1-x)PS_x films showed a maximum value at lower values of frequencies and then decreases rapidly up to 10 kHz for all doses as the frequency increases. Then, it remains almost constant up to 5 MHz. The increasing ε_r value with the γ-ray irradiation dosage is attributed to the formation of few defects sites in the band gaps of PVA_(1-x)PS_x films as a result of occurrence of chain scisson and rearrangements of linking-bonds. These defects taps the charge carriers exist in the band gap of the polymer/polymer blend films. Thus the γ-ray irradiation increases the ability of the PVA_(1-x)PS_x polymer/blend films to store charges. In both the unirradiated and irradiated PVA_(1-x)PS_x films, ε_r decreases with the increasing frequency. The reduction in value of the dielectric constant with the gain in frequency can be excused by the fact that at low frequency, the electric dipoles can follow the electric field, but with an increase in frequency, the electric dipoles can no longer follow the electric field, resulting in a reduction in the value of the dielectric constant [23] . It is recognized that the first appearance of a dielectric into an electric field causes its polarization. Polystyrene is a solid amorphous dielectric material containing no polar groups. In such polymers, the main kind of polarization is electronic polarization caused by deformation of the electronic shells and appearance of induced dipoles. The contribution of the other kinds of polarization like ionic, space charge and orientation is negligibly low because of the absence of free ions and polar molecules in the amorphous PS dielectric material. Hence the addition of PS in PVA_(1-x)PS_x (x = 0.25, 0.50, 0.75) blend films, induced dipoles are arises and enhance the ε_r.

The variation of the dielectric loss (tanδ) of PVA_(1-x)PS_x films (unirradiated and irradiated) as a function of

temperature at constant 1 kHz frequency and as a function of frequency at constant 27˚C is shown in Figure 2. From the Figure 2, it reveals that the tanδ increases with increasing temperature whereas decreases with increasing

frequency. Also, as the concentration of PS increases in PVA_(1-x)PS_x blends the tan δ decreases in both tempera- tures and frequencies. However, the tan δ increases with increasing irradiation dosage when compared to

unirradiated films.

The variation of AC conductivity (σ_ac) with the temperature at 1 kHz frequency and with frequency at room temperature for pure PVA/PS and blended (PVA_(1-x)PS_x) films of unirradiated and irradiated is represented in Figure 3. From the figure, it is clear that the σ_ac increases as the temperature, frequency and irradiation dose increases whereas decreases with the increasing concentration of PS from x = 0.0 to x = 1.0. It can be noticed that the σ_ac for the PVA_(1-x)PS_x films have a minimum value of low frequency range up to 10 kHz and then increases rapidly while increasing frequency. The σ_ac of the polymer film increases with frequency which is the common characteristic of disordered materials. Furthermore, almost all the prepared PVA_(1-x)PS_x films show similar behaviour up to 10 kHz, which is typical for hopping conduction. In this range there is not much change in AC conductivity with frequency. However, σ_ac slightly increases with increasing frequency within this frequency range. Thereafter an increase in frequency leads to rapid increase in AC conductivity.

When the films are irradiated by γ-rays, ions and free radicals are formed and partially trapped in the bulk of the material [24] . Thus the conductivity of the irradiated film showed higher conductivity than that of unirradiated film for both temperature and frequency dependent conditions. Similar behavior was observed for all the samples (x = 0.25, 0.50, 0.75 and 1). The electrical conductivity of the polymer material depends on the presence of free ions connected chemically with macromolecules. The molecular chain of the polymer does not participate in the transfer of electrical charge [25] . The increase in AC conductivity with temperature for the prepared PVA_(1-x)PS_x films may be accounted for the liberation of charge carriers through the amorphous region. Also, it can be observed that the increase in AC conductivity with increase in irradiation dose is ascribed to the degradation of the polymer chains in PVA_(1-x)PS_x films. The irradiation dosage (1.5 kGy) increases the σ_ac by more than an order of magnitude for all the samples considered in the present study.

PVA_(1-x)PS_x polymer blends were successfully prepared by using simple solution cast method. The changes in the electrical properties of the pure and polymer blend films after exposing to different dosage (1.0 and 1.5 kGy) of γ-ray irradiation have been investigated. The variation in ε_r for the unirradiated PVA_(1-x)PS_x films showed a maximum value at lower values of frequencies and then decreased rapidly up to 10 kHz for all doses as the frequency increased. The irradiation dosage (1.5 kGy) increased the σ_ac by more than an order of magnitude for all the films.

We authors wish to express thanks to Dr. H. M. Somashekarappa and Swaroop. CAART, Mangalagangotri. Mangalore University, Mangalore, Karnataka State.

K.Prabha,H. S.Jayanna, (2015) Study the Frequency Dependence of Dielectric Properties of Gamma Irradiated PVA_(1-x)PS_x Polymer Blends. Open Journal of Polymer Chemistry,05,47-54. doi: 10.4236/ojpchem.2015.54006