Comparative study of Structural, Optical and Dielectric properties of [(PVP:PVA)-Cr3+] and [(PVP:PVA)-La3+] Composite films

Polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) polymer blend composite films doped with lanthanum (La) and chromium (Cr) ion were prepared via solution casting method. The x-ray diffraction (XRD) technique was used to examine the structural characteristics. UV-Visible photospectroscope was employed to measure optical properties and dielectric properties was measured using LCR meter. XRD results showed the semicrystalline behaviour of prepared films. Cr ion doped polymer blend composite film had an optical band gap of 4.17 eV, whereas La ion doped films had a band gap of 4.769 eV. From the comparative study of the (PVP:PVA)-Cr3+ and (PVP:PVA)-La3+ composite polymer films it was observed that La3+ doped film had lower dielectric constant. The ac conductivity of the (PVP:PVA)-Cr3+ film is found to be higher than the (PVP:PVA)-La3+ film.


Introduction
The incorporation of fillers into the polymer matrix results in better material characteristics, including structural, physical, chemical, optical, electrical, and mechanical properties.Polyvinyl alcohol (PVA) is being used by many researchers due to its biocompatibility, nontoxicity, water solubility, thermostability, high tensile strength, and optical transmission.Moreover, it is economical and both mechanically and chemically robust.Several non-biocompatible polymers, such as polyethylene, polypropylene, poly (vinyl chloride), and many more, are being replaced with PVA [1,2].Because of its high solubility in water, ease of processing, and biocompatibility, PVA is a formidable material for manufacturing electronic and optoelectronics devices and has been used in bioengineering and several other sectors [3][4][5].It has tremendous dielectric strength and a remarkable capacity to store charges.[3].PVA has good film-forming capabilities, making it suitable for application in nonlinear optical equipment and optical sensors.As a result, a lot of researchers are concentrating on altering its properties using different methods for particular applications, such as adding fillers to the polymer matrix to adjust the band gap.Fillers can therefore be added to PVA to customize its electrical and optical properties [6,7].PVP is also a water-soluble polymer with high glass transition temperature.PVA and PVP are both polymers that mix well with each other.The -CO groups of PVP and the -OH groups of PVA interact through hydrogen bonds to form the (PVA:PVP) polymer blend, which is widely used to make solid polymer composites that are light, flexible, and safe [8][9][10].
To change the characteristics of polymers, many earlier researchers introduced metallic ions into the matrix.Ion incorporation into the polymer matrix may have an impact on the polymer chain, which is a phenomenon known as adsorption.The metal-polymer composites are predicted to produce innovative and advantageous functions for optical and optoelectronic applications [6,7].
In the current study, we fabricate (PVA:PVP) blend composites with small amount of lanthanum and chromium ions using a basic, inexpensive, and ecofriendly solution-casting approach.Doping a small weight percent of ions ensure better compatibility and integration between the dopants and the polymer matrix.It minimizes potential chemical or physical incompatibilities that may arise from a higher concentration of ions, allowing for improved interfacial interactions and overall material performance as well reduces the aggregation of ions within the polymer matrix [11].By using X-ray diffraction (XRD), UV-Vis-IR spectroscopy, and a dielectric measurement system, we study the structural, optical, and dielectric properties of these polymer blend composites.

Materials
Polyvinylpyrrolidone (PVP) powder and polyvinyl alcohol (PVA) powder was provided by Merck, India.Lanthanum nitrate hexahydrate, and chromium (III) nitrate nonahydrate was purchased from Sigma Aldrich and distilled water were used as a solvent.chromium (III) nitrate nonahydrate

Preparation of PVA:PVP polymer blend composite films doped with La and Cr ions
In this work, the solution casting method was used for the preparation of PVA:PVP polymer blend composite films doped with La and Cr ions.Firstly, PVA and PVP polymers were dissolved along with different doping materials in distilled water in fixed proportion.Precursor's ratio was fixed to 10 wt%.PVA and PVP were blended in an equal weight (4 wt%) and dissolved in distilled water.Then solution was stirred for 4 hr.at 60 °C using magnetic stirrer till the solution become homogenous and transparent followed by addition of 2 wt.% lanthanum nitrate hydrate Furthermore, solution was stirred well for 2 h to obtain homogenous casting solution.Same process was followed to prepare Cr ion doped casting solution.

Characterizations
The structural properties of polymer blend composites were studied using a Proto AXRD benchtop diffractometer with Cu-Kα radiation source of wavelength λ=1.54 Å.Double beam ultraviolet-visible (UV-Vis) spectrophotometers-model elite (BioEra) was employed to study the optical properties.Dielectric properties were obtained using LCR meter-IM3536 (HIOKI).

XRD analysis
XRD spectra of the polymer composite films (PVP:PVA) -Cr 3+ and (PVP:PVA) -La 3+ are shown in Figure 1.The Figure shows that a slightly wide hump is present in the spectra at about 2θ=21.39° and 2θ=22.05°(PVP:PVA)-Cr³⁺ and (PVP:PVA)-La³⁺ films respectively.This peak corresponds to (101) reflection plane of PVA polymer which is in good agreement with several published studies [5][6][7] and supports the idea that C=O groups of PVP polymer strongly interacted with the OH groups of the PVA polymer and form a hybrid film with both amorphous and crystalline nature.After adding lanthanum and chromium salts to the PVA:PVP polymer blend, the additives are crystalline, however, the XRD patterns don't exhibit any additional distinctive peaks, which shows that the salt has been completely dissociated and homogenously complexed in the polymer matrix through linking the carbonyl group of the polymer chains with the La 3+ and Cr 3+ ions.Structural characteristics, including the inter-planar spacing (d), the size of the crystallites (D), and the average crystallite separation (R) were determined using Bragg's angle and FWHM of principal peaks obtained in the XRD pattern.The relationships stated below [10,12]  where n = 1 correspond to the first-order diffraction, λ is 1.5406 Å, the wavelength of the X-ray and B is FWHM.Table 1 listed these structural parameters.

UV-Vis analysis
Figure 2 shows the optical absorption spectra (PVP: PVA) -Cr³⁺ and (PVP: PVA) -La³⁺ polymer blend composite films.It is observed that La³⁺ doped film showed absorption peak at 233nm and 299nm in UV region while Cr 3+ doped film showed absorption peak at 409nm and 575nm in visible region.
The band gap energy of polymer blend composite films were determined using the Tauc plot.Optical band gap was determined using Tauc equation.This equation can be written as: Where α is the absorption coefficient, h is Planck's constant, ν is the photon energy, A is the proportionality constant which is 1 for an amorphous material, Eg is the band gap energy and γ determines the nature of the electronic transition, γ = 2 is a direct allowed transition and γ=1/2 is indirect in the event of an allowed transition.(PVP:PVA) -Cr³⁺ composite film has a band gap of about 4.17 eV and increases slightly with doping of La 3+ ions in the polymer chains to 4.769 eV.This study confirmed that La 3+ and Cr 3+ ions doping can form charge transfer complexes between the polymer and the dopant ions.In these complexes, electrons can transfer between the polymer and the dopant, leading to a redistribution of electronic states and altering the optical band gap [13].Additionally, Doping ions can induce changes in the electronic structure of the polymer.This can involve modifying the density of states, band dispersion, or the position of energy levels, all of which can influence the optical band gap

Dielectric measurement
The dielectric constant (ε′) and dielectric loss (ε″) of the (PVP:PVA) -Cr³⁺ and (PVP: PVA) -La³⁺ composite polymer films are shown in figure 4(a) and 4(b), respectively where dielectric constant (ε′) represent energy storage and dielectric loss (ε″) represents energy loss of AC external electric field.It may be considered that the dielectric structure is made of low resistive grains separated by poor conducting thin grain boundaries.When electric field is applied then there is a localized accumulation of charges, which give rise to the interfacial polarization [15].It was observed from figure 4(a) that the dielectric constant was high at low frequencies.As the frequency increases, dielectric constant decreases.This is caused by the space charge polarization or interfacial polarization being weakened [15].From the comparative study of the (PVP: PVA) -Cr³⁺ and (PVP: PVA) -La³⁺ composite polymer films it was observed that (PVP: PVA) -La³⁺ had lower dielectric constant.This could be attributed to the fact that heavier ions generally exhibit reduced mobility.Their decreased speed and limited capacity to align in the presence of an electric field lead to a lesser impact on the dielectric constant.Figures 4(a) and 4(b) make it abundantly evident that the dielectric constant (ε′) versus frequency (f) shows the impact of Cr 3+ and La 3+ ions incorporation on the characteristics of the prepared polymer composite films.It may be considered that as La 3+ and Cr 3+ ions are present in the PVA/PVP composite polymer films, decreasing electrical polarization caused by diminishing directional polarization (dipole response) and ionic polarization (elongation and contraction of bonds with ions) [16].

Figure 5.
Frequency versus AC conductivity plot of composite polymer films at room temperature Figure 5 shows the Frequency versus AC conductivity (σAC) plot of composite polymer films.It is observed from figures that conductivity is increasing with increasing frequency.This may be because an increase in applied field frequency increased electron mobility, which in turn increased conductivity.Moreover, the loss factor predominates at higher frequencies, causing the conductivity to rise to considerably larger values for higher frequency ranges.This figure also shows that σAC is higher for (PVP:PVA) -Cr³⁺ film compared to (PVP:PVA) -La³⁺ film.The mass of La 3+ ions is more as compared to Cr³⁺ ions.Therefore, the mobility of La 3+ ions in polymers chains is limited.Thus, the heavy mass of La ions compared to Cr ions causes a decrease in AC electrical conductivity.

Conclusion
Solution casting method was used to synthesize the composite films (PVP: PVA) -Cr³⁺ and (PVP: PVA) -La³⁺.XRD confirmed the semi-crystalline and miscibility of the two polymers.The band gap of the (PVP: PVA) -Cr³⁺ composite film is approximately 4.17 eV, which is significantly increased by doping La³⁺ ions into the composite chains to reach 4.769 eV.The influence of Cr³⁺ and La³⁺ ions content on the characteristics of the prepared composite films is represented by the dielectric parameters versus frequency (f).By incorporating La³⁺ and Cr³⁺ ions into the PVP:PVP composite films, dielectric constant and dielectric loss decreased with increasing frequency.When comparing (PVP:PVA) -Cr³⁺ film to (PVP: PVA) -La³⁺ film, the ac conductivity is shown to be higher for Cr 3+ doped film.

Acknowledgement
Financial support from the MP Council of Science and Technology, Bhopal, is gratefully acknowledged.