Spectroscopic, thermal, structural and electrical studies on VO2 ions doped PVA/MAA:EA polymer blend films

It has been reported in literature that studies have been conducted on electrical conductivity and dielectric properties of polymer blends with regard to storage and dissipation of electric and magnetic energies. These materials reveal numerous phenomena in optics, electronics and solid state physics.

Journal of Science: Advanced Materials and Devices (2019) 267e275 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Spectroscopic, thermal, structural and electrical studies on VO2ỵ ions doped PVA/MAA:EA polymer blend films Ojha Pravakar a, T Siddaiah a, P.V.R.K Ramacharyulu b, N.O Gopal a, *, Ch Ramu a, H Nagabhushana c a b c Department of Physics, Vikrama Simhapuri University PG Centre, Kavali, 524201, India Department of Physics, National Dong Hwa University, Hualien, 97401, Taiwan CNR Rao Centre for Advanced Materials Research, Tumkur University, Tumkur, 572103, India a r t i c l e i n f o a b s t r a c t Article history: Received 28 November 2018 Received in revised form 10 March 2019 Accepted 13 March 2019 Available online 19 March 2019 Pure and VO2ỵ ions doped PVA/MAA:EA polymer blend films were prepared by a solution casting method XRD pattern reveals an increase in amorphicity with increase in doping The dTGA study shows an enhancement of thermal stability of the system with increase in dopant concentration The optical absorption spectrum exhibits three bands corresponding to the transitions 2B2g/2A1g, 2B2g/2B1g and B2g/2Eg, characteristic of VO2ỵ ions in octahedral symmetry with tetragonal distortion and reflects that the optical band gap decreases with the increase of mol% of VO2ỵ EPR spectra of all the doped samples show a characteristic eightline hyperne structure of VO2ỵ ions, which arises due to the interaction of unpaired electron with the 51V nucleus The spin-Hamiltonian parameters (g and A) evaluated from the EPR spectra conrm that the vanadyl ions exist as VO2ỵ ions in octahedral co-ordination with a tetragonal compression and have a C4v symmetry The impedance spectroscopic study shows that the addition of VO2ỵ ions into the polymer blend system enhances the ionic conductivity which is explained in terms of an increase in the amorphicity © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: PVA/MAA:EA polymer blend VO2ỵ doping Optical band gap EPR Amorphicity Electrical conductivity Introduction Nowadays polymer blend films have attracted the attention of researchers, scientists and technologists towards the development and modification of innovative materials Polymer blend films are used in a number of technological and scientific applications due to their optical, thermal, mechanical, electrical and magnetic properties In recent years, there has been a considerable interest shown in the preparation and characterization of polymer blend films for their possible use as light stable colour filters [1] as well as solar cells and optical sensors [2] Transition metal ion doped polymer blends are considered as potential materials for both theoretical and experimental research and found to be essential because of their increasing technological applications If the properties of a polymer blend are tuned in the right direction by adding suitable dopants, this polymer blend could be an appropriate candidate for a number of applications [3] and as permanent and transient data * Corresponding author E-mail address: gopalnovsu@gmail.com (N.O Gopal) Peer review under responsibility of Vietnam National University, Hanoi storage material [4] Changes in polymer structure and optimised properties can be obtained by introducing suitable transition metal ions into a polymer chain Polyvinyl alcohol (PVA) is semi-crystalline, water soluble and exhibits a low electrical conductivity PVA has certain physical properties resulting from crystal-amorphous interfacial effects and can be tailored to a certain specific requirement by the addition of suitable dopant material, depending on the chemical nature of the doping substance and the way they interact with the polymer matrix Addition of dopant alters the physical properties of the host matrix to different degrees Applications of the organometallic polymers include high temperature coating, bio sensors, strong electrodes for batteries, etc [5] Among the copolymers, Methacrylic Acid- Ethyl Acrylate (MAA: EA) copolymer is a potential material having a good charge capacity and dopant dependent electrical and optical properties Bajaj et al [6] studied the thermal behaviour of MAA:EA copolymers Madhava Kumar et al [7] studied the interaction between MAA:EA copolymer and vanadyl (VO2ỵ) dopant Polyvinyl alcohol/Methacrylic acid-ethyl acrylate (PVA/ MAA:EA) polymer blend has drawn a special attention amongst the polymer blends because of its good environmental stability, easy https://doi.org/10.1016/j.jsamd.2019.03.004 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 268 O Pravakar et al / Journal of Science: Advanced Materials and Devices (2019) 267e275 processing and transparency There are many methods to improve the ionic conductivity, optical properties, mechanical and electrical properties of polymer films, viz polymer blending, addition of plasticizer, doping of ionic liquid, filler and mixed salt systems Siddaiah et al [8] studied the thermal, structural, optical and morphological properties of PVA/MAA:EA polymer blend films and reported that 50:50 concentration of polymer and copolymer blend shows optimal properties These authors also studied the thermal, structural, optical and electrical properties of PVA/MAA:EA polymer blend filled with lithium Perchlorate [9] Among transition metal ions, VO2ỵ has been extensively used as a probe to study the symmetry of crystalline electric field [10] Vanadium oxides are known to be promising optical switching materials due to their electro-chromic properties [10,11] The vanadyl ion (VO2ỵ) is the most stable cation among a few molecular paramagnetic transition metal ions and is used extensively as an impurity probe for EPR studies It has been reported in literature that studies have been conducted on electrical conductivity and dielectric properties of polymer blends with regard to storage and dissipation of electric and magnetic energies These materials reveal numerous phenomena in optics, electronics and solid state physics In order to store electric charges, dielectrics are used whereas insulators are used to block flow of electric charges To the best of our knowledge, no work has been carried out on PVA/MAA:EA polymer blend system doped with vanadyl ions Therefore, this present work studies the thermal, optical, electron paramagnetic resonance, structural, morphological and electrical properties of the VO2ỵ ions doped PVA/MAA:EA(50:50) polymer blend films Experimental 2.1 Preparation of the material Pure and VO2ỵ ions (1.0, 2.0, 3.0, 4.0 and 5.0 mol %) doped PVA/ MAA:EA polymer blend films were prepared at room temperature by a solution casting method The polyvinylalcohol (PVA) from Merck-Germany with molecular weight 14000 and Methacrylic Acid - Ethyl Acrylate (MAA:EA) copolymer (1:1) dispersion 30 percent having a mean relative molecular weight of about 250000 (Purchased from Merck Millipore India Ltd.,) are water soluble polymers PVA polymer blended with MAA:EA copolymer in the concentration of 50:50 wt % was prepared at room temperature by using distilled water Five millilitres of PVA solution was added to five millilitres of MAA:EA solution and then the solution was stirred magnetically for 10e12 h to get a homogeneous mixture The desired concentrations of VOSO4 solution (1.0, 2.0, 3.0, 4.0 and 5.0 mol %) were prepared at room temperature by using distilled water Then, different concentrations of (1.0, 2.0, 3.0, 4.0 and 5.0 mol %) dopant solution was added to the mixture solution The resulting solution was magnetically stirred for 3e4 h in order to get a homogeneous mixture and then cast onto polypropylene dishes The solution was slowly evaporated at RT to get the doped polymer blend films measurements were carried out using a Siemens D5000 diffractometer with Cu Ka radiation (l ¼ 1.5406 Å) The films were scanned at 2q angles between 10 and 40 with a step size of 0.02 Morphological studies were conducted using a scanning electron microscope (SEM) JEOL JSM (Model- 840A) EPR spectra were recorded at room temperature on a Bruker EMX spectrometer equipped with a Bruker TE102 cavity operating at X-band frequency The impedance measurements were conducted using a computer controlled phase sensitive multimeter (PSM 1700) in the frequency range of Hze5 MHz at room temperature Results and discussion 3.1 XRD studies X-ray diffraction analysis gives valuable information on the crystal structure of the polymer blend films The X-ray diffraction patterns of pure and VO2ỵ ions doped PVA/MAA:EA polymer blend  lms in the range of 2q ¼ 5e35 are shown in Fig In Fig 1, the appearance of two peaks at 2W z 11 and 27 indicates the amorphous nature of the samples and the broad peak at 2W z 19 indicates semi-crystalline nature of the polymer blend film [12] It is clear from Fig that a decrease is observed in the peak intensity without any significant change in its position at 2W z 19 with the increase in the dopant concentration It may be attributed to the strong intermolecular interaction between polymer blend and VO2ỵ ions, which implies a change in the degree of crystallization combined with increase in the amorphous regions [13] This increase in the amorphicity causes a reduction in the energy barrier and an increase in segmental motion of the polymer blend due to VO2ỵ doping leading to a continual rise in electrical conductivity [14,15] 3.2 Thermogravimetric studies In thermogravimetric analysis, a substance decomposes in the presence of heat due to breaking of the bonds within the molecules The sample loses its weight slowly as the reaction begins, then rapidly over a comparatively narrow temperature range and finally levels off as the reactants become spent The rst derivatives of thermogravimetric analysis (dTGA) curves for VO2ỵ ions doped PVA/MAA:EA polymer blend samples with a heating rate of 10  C/min in the temperature range from room temperature to 600  C are shown in Fig It is clear from this 2.2 Characterization In order to investigate the properties of the transition metal ion doped polymer blend films, thermogravimetric analysis (TGA) was done using SEIKO thermal analysis system (TGA e 20) in the presence of nitrogen flow from 40 to 600  C, at the heating rate of 10  C/min UV-Vis absorption spectra of pure and doped films were recorded in the range 200e900 nm at room temperature using JASCO UV-Vis spectrometer (Model- V.700) X-ray diffraction Fig XRD Spectra of (a) pure and (b) mol%, (c) mol%, (d) mol%, (e) mol% and (f) mol% VO2ỵ ions doped PVA/MAA:EA polymer blend films O Pravakar et al / Journal of Science: Advanced Materials and Devices (2019) 267e275 figure that the recorded dTGA curves of VO2ỵ ions doped PVA/ MAA:EA polymer blend samples show multi step decomposition mechanisms It is also clear from Fig that all tested samples show weight losses for the first decomposition stage and more significant weight losses for the second decomposition stage The lower values of weight loss in the first stage may be attributed to the presence of a thermal process due to moisture evaporation of adsorbed water molecules from samples and also may be due to splitting or volatilization of small molecules and/or polymers in which case the weight loss is at around 80  C [16e18] The latter decomposition stage in dTGA curves covers a wider temperature range (180  C e 400  C), which includes the melting points, as physical transitions, and the degradation temperatures of polymers [19] Therefore, the higher values of weight loss in the second decomposition stage indicate the existence of a chemical degradation process resulting from bond scission (carbonecarbon bonds) in the polymeric backbone The lower temperature loss (i.e., the first stage) may be due to the breaking of the ester linkages, and the second corresponds to the breaking of intermolecular hydrogen bonding in the polymer blend system The third stage in the temperature range 400  C e 600  C in the thermograms shows the complete dissociation of polymeric system, which may corresponds to the degradation of the whole VO2ỵ ions doped PVA/MAA:EA polymer blend [10] It is observed from the thermograms, that there is a gradual decrease in weight loss occurring in different temperature regions as well as in the peak decomposition temperature of all samples with increase in dopant concentration Weight losses at different stages and peak decomposition temperatures of all samples are mentioned in Table The inclusion of VO2ỵ ions perturbs the van der Waals interaction between the PVA/MAA:EA chains This weakened interaction between the polymer chains results in a decrease in decomposition temperature This indicates that the thermal stability of a PVA/MAA:EA polymer blend increases by doping it with VO2ỵ ions 3.3 UV visible studies UV-Vis absorption spectroscopy is a technique wherein the absorption of electromagnetic waves is measured as a function of frequency or wavelength Optical absorption induces an interaction between VO2ỵ ions and the polymer blend, which is reected as a variation in the absorption spectra An absorption spectrum is a fingerprint of a molecule or polymer material UV-Vis absorptions of pure and VO2ỵ ions doped PVA/MAA:EA polymer blend films are shown in Fig The absorption spectrum shows strong absorption bands in the range of 200 nme900 nm It is clear from Fig that an absorption peak can be noticed at 212 nm for the pure film which Fig dTGA thermograms of pure (a) and different concentrations 1.0 mol% (b), 2.0 mol% (c), 3.0 mol% (d), 4.0 mol% (e) and 5.0 mol% (f) of VO2ỵ doped PVA/MAA: EA polymer blend films 269 Table The decomposition steps and percentage weight loss for pure and different concentrations (1.0, 2.0, 3.0, 4.0 and 5.0 mol%) of VO2ỵ ions doped PVA/MAA: EA polymer blend films Sample Regions of decomposition 50-50 (pure) 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1.0 mol% 2.0 mol% 3.0 mol% 4.0 mol% 5.0 mol%  Temperature ( C) Weight loss (%) Start End TP Partial Total 31 272 388 40 268 403 32 251 390 38 245 394 35 219 405 35 163 396 131 388 482 146 402 504 140 336 507 121 340 501 136 300 501 135 273 500 77 343 423 85 308 422 83 300 433 74 293 433 84 256 425 217 227 430 5.4 41.0 35.2 5.1 39.9 34.9 4.9 38.2 34.2 4.8 37.4 34.3 4.8 36.1 34.2 4.8 29.1 34.2 81.6 79.9 77.3 76.5 75.1 68.1 may be due to the formation of charge transfer complexes While doping of VO2ỵ ions into PVA/MAA:EA polymer blend, a new absorption peak is introduced at 304 nm in addition to the peak at 212 nm This new peak may be due to the interaction of dopant and polymer blend and attributed to the formation of a charge transfer complex [20] The amount of absorbance at each peak position increases with dopant concentration The shift in the absorption edge is due to a decreased band gap which improves the samples semi conducting behaviour The optical absorption spectrum of VO2ỵ doped PVA/MAA:EA polymer blend film also consists of three absorption bands centred at around 375, 606 and 810 nm as shown in the inset of Fig The d-orbital energy level ordering of the vanadyl complex has been given by Ballhausen and Grayin terms of the molecular orbitals [21] In a C4v symmetry environment, the ground state is an orbital singlet and the d-electron is in the nonbonding (2B2g) type dxy orbital These three absorption bands were assigned to ded transitions 2B2g/2A1g, 2B2g/2B1g and B2g/2Eg respectively [22] The absorption coefficient a can be directly determined from the spectra by a ¼ 2:303ÃðA=dÞ (1) Fig Ultra Violet absorption edge plots of (a) pure and (b) mol%, (c) mol%, (d) mol%, (e) mol%, (f) mol% VO2ỵ ions doped PVA/MAA:EA polymer blend films 270 O Pravakar et al / Journal of Science: Advanced Materials and Devices (2019) 267e275 where A is the absorbance and d is the thickness of the film The variation in the absorption coefficient with incident photon energy for pure as well as doped PVA/MAA:EA polymer blend films (Fig 4) shows that the absorption edge for pure PVA/MAA:EA lies at 5.11 eV while for the doped films, the values were found to decrease from 4.89 eV to 4.58 eV (Table 2) In case an indirect band gap exists, the absorption coefficient has the following dependence on the energy of the incident photon: [23,24] À ahy ¼ c hy À Eg Ám (2) where Eg is the optical band gap, and c is the band tailing parameter related to a constant Two types of optical transitions can take place in case of semiconductor materials In both types of optical transitions, the photon interacts with the electron in the valence band and raises it to the conduction band There is no lattice interaction in the direct transition The phonon interacts with the lattice in an indirect transition In Eq m decides the transition: for m ¼ 1/2 the transition is directly allowed while for m ¼ it is indirectly allowed After applying all values of m for the samples, m ¼ (indirect transition) is found most suitable to calculate the band gap The indirect band gaps were obtained from the plots of (ahy)1/2 vs hy (Fig 5) For the pure PVA/MAA:EA polymer blend film, the indirect band gap lies at 5.09 eV, while for doped films the values vary from 4.94 eV to 4.51 eV (Table 2) From Table 2, it is clear that there is a decrease in the band edge and in the indirect band gap values with an increase in dopant concentration The red shift of the optical band gap on doping may be explained on the basis of the fact that incorporation of a small amount of dopants forms charge transfer complexes in the host lattice The band edge and indirect band gap values shifting to lower energies on doping with VO2ỵ ions are due to interband transitions [25] 3.4 Electron paramagnetic resonance studies EPR studies reveal the magnetic properties and spin dynamics, and provide information regarding magnetic behaviour of the materials and the associated interparticle dipolar interactions and super exchange interactions [26e30] No EPR signal was observed in the spectra of the pure PVA/MAA:EA polymer blend film (Fig 6) indicating that no paramagnetic impurities were present in the starting materials At doping various amounts of VO2ỵ ions to the PVA/MAA:EA polymer blend, the EPR spectra of all the investigated Fig a vs hy plots of (a) pure and (b) mol%, (c) mol%, (d) mol%, (e) mol%, (f) mol% VO2ỵ ions doped PVA/MAA:EA polymer blend films [30e32] The values of the spin-Hamiltonian parameters observed in the present work are gjj ¼ 1.9339, g⊥ ¼ 1.9854, Ajj ¼ 199.55 G and A⊥ ¼ 66.93 G, which agree with the above conditions From this observation, it is suggested that the paramagnetic V4ỵ ions in the framework exist as vanadyl ions (VO2ỵ) in the octahedral environment of oxygen with tetragonal distortion With the the increase in concentration of VO2ỵ ions from to mol%, the intensity of the EPR spectrum also increases, which is due to the exchange of dipolar interaction of VO2ỵ [33] The EPR spectra of VO2ỵ ions doped in PVA/MAA:EA polymer blend films show a remarkable concentration dependence as shown in Fig 3.4.1 Calculation of the molecular orbital coefficient (b2*2), the Fermi contact term (k) and the dipolar hyperfine coupling parameter (P) By correlating EPR and optical absorption data, the molecular orbital coefficient (b2*2), the Fermi contact term (k) and the dipolar hyperfine coupling parameter (P) have been calculated for vanadyl ion from the following formulae given by Kivelson and Lee: [34] Ã2 Ajj ¼ Aj jj À P b2 k j (3) where i h   *2 Aj jj ẳ P b2 4=7ị ỵ gjj 2:0023 þ 3=7ðg⊥ À 2:0023Þ þ Djj þ 3=7D⊥ i h j *2 A ẳ P b2 2=7ị ỵ 11=14g 2:0023ị ỵ 11=14 D samples at room temperature exhibit resonance signals as shown in Fig Generally, the coordination of vanadium is octahedral with tetragonal distortions [29] The observed resonance signals are due to the hyperfine interaction of an unpaired electron (S ¼ 1/2) with a 51 V nucleus whose nuclear spin is 7/2 and which is present in abundance (99.76%) From Fig 6, it is clear that the intensity of the EPR signal increases with increasing dopant concentration from mol% to mol%, indicating a very large concentration of VO2ỵ ions in the polymeric matrix An octahedral site symmetry with a tetragonal compression would give values of gjj < g⊥ < ge and Ajj > A⊥ Ã2 A⊥ ¼ A⊥ À P b2 k (4) Here P ¼ 2gbbN < rÀ3 > ¼ 0:0128 cmÀ1 (5) is the dipolar hyperfine coupling parameter and k is a dimensionless Fermi contact interaction parameter which represents the amount of unpaired electron density at the vanadium nucleus Djj ¼ À0.0125 and D⊥ ¼ À0.0015 are small corrections [35] b2*2 represents a measure of the degree of p-bonding with the equatorial ligands and can be taken as unity for vanadium [36], k is the Fermi contact parameter which is related to the unpaired electron O Pravakar et al / Journal of Science: Advanced Materials and Devices (2019) 267e275 Table Absorption edge and indirect band gap values of pure and VO2ỵ ions doped PVA/ MAA:EA polymer blend lms Concentration mol% VO2ỵ: PVA/MAA:EA Absorption edge (eV) Indirect band gap energy (eV) Pure PVA/MAA:EA 5.11 4.91 4.85 4.74 4.65 4.58 5.09 4.94 4.87 4.76 4.68 4.51 271 Symphonia) The paramagnetic susceptibility (c) of the sample can be calculated from the EPR data using the formula: [36,37] c¼ N g b JJ ỵ 1ị kB T (6) where N is the number of spins per Kg, J is the total angular momentum quantum number, b is the Bohr magnetron, kB is the Boltzmann constant, T is the absolute temperature and g ẳ (gjj ỵ g)/3 is taken from EPR data Since the number of spins (N) varies directly with the paramagnetic susceptibility (c) of the sample, they show similar trend as shown in Fig The number of spins (N) and the paramagnetic susceptibility (c) increase negligibly with the increase in dopant concentration from to and mol %, increase rapidly from to mol% and increase slowly from to mol% (Fig 7) An almost linear variation of the number of spins (N) and the paramagnetic susceptibility (c) is observed within the 2e4 mol% dopant concentration This has been attributed to the ligand eld uctuations in the VO2ỵ ion vicinity and to spin espin interactions The number of spins and the susceptibility data are presented in Table 3.5 Morphological studies Fig (ahy)1/2 vs hy plots of (a) pure and (b) mol%, (c) mol%, (d) mol%, (e) mol%, (f) mol% VO2ỵ ions doped PVA/MAA:EA polymer blend lms density at the vanadium nucleus, P ( ¼ 2gbbN) represents the dipoleedipole interaction of the electronic and nuclear moments and the other symbols have their usual meaning Correlating EPR and optical data, Eqs (3) and (4) can be solved to get b2*2(¼0.78) and k (¼ 0.71) The parameter k indicates an extreme sensitivity to deformations of the electron orbitals of the central vanadium ion The value of k (0.71) indicates a contribution to the hyperfine constant by the unpaired s-electron and also probably a contribution from spin polarization Morphological studies of the samples are done using scanning electron microscopy (SEM) in order to investigate the compatibility between various components of polymer films through the detection of phase separation and interfaces [38,39] The compatibility between the polymer host matrix and the dopants has a great impact on the properties (mechanical, thermal, ionic conductivity) of the polymer films Also topography studies of the samples give important information regarding the growth mechanism, shape and size of the particles Scanning electron microscope (SEM) 3.4.2 Calculation of the number of spins participating in the resonance and paramagnetic susceptibility (c) The number of spins participating in the resonance has been calculated by double integrating the EPR spectrum (using Bruker Fig Variation of the number of spins (N) participating in the resonance and paramagnetic susceptibility (c) of (a) mol%, (b) mol%, (c) mol%, (d) mol% and (e) mol% VO2ỵ ions doped PVA/MAA:EA polymer blend films Table EPR characteristics of EPR spectra of (a) mol%, (b) mol%, (c) mol%, (d) mol% and (e) mol% of VO2ỵ ions doped PVA/MAA:EA polymer blend films at room temperature Fig EPR spectra of (a) mol%, (b) mol%, (c) mol%, (d) mol% and (e) mol% VO2ỵ ions doped PVA/MAA:EA polymer blend lms Sample Concentration Number of spins (N) (108spins KgÀ1) Paramagnetic Susceptibility (c) (10À8m3 kgÀ1) 1.0 2.0 3.0 4.0 5.0 18.79 18.94 32.79 44.95 47.4 77.06 17.29 234.53 441.01 490.4 mol% mol% mol% mol% mol% 272 O Pravakar et al / Journal of Science: Advanced Materials and Devices (2019) 267e275 Fig SEM images of (a) pure and (b) mol%, (c) mol%, (d) mol%, (e) mol%, (f) mol% of VO2ỵ ions doped PVA/MAA:EA polymer blend lms O Pravakar et al / Journal of Science: Advanced Materials and Devices (2019) 267e275 273 images of pure and VO2ỵ ions doped PVA/MAA:EA polymer blend films are shown in Fig Fig 8(a) clearly shows a smooth and uniform surface morphology of the pure PVA/MAA:EA polymer blend films This smooth morphology confirms the completely amorphous nature of PVA/MAA:EA polymer blend Adding VO2ỵ to the PVA/MAA:EA polymer blend, the surface morphology of pure PVA/MAA:EA polymer blend films changes from smoother to rougher It is clear from Fig that, with the increase in dopant concentration, the number of rumples as shown in the inset of Fig 8(f) increases This increase in rumples count indicates the semi-crystalline nature of the polymer blend caused by the addition of VO2ỵ ions It may be benecial for conductivity of the polymer blend film [40] 3.6 Electrical studies 3.6.1 AC electrical conductivity studies AC impedance spectroscopy is an important characterization technique to study ionic conductivity of doped polymer blend lms Impedance plots of pure and VO2ỵ ions doped PVA/MAA:EA polymer blend films in the frequency range of Hz e MHz at room temperatures are shown in Fig It is observed that the idealized impedance plane plots of Z‫ ׀׀‬as a function of Z‫׀‬, i.e ColeeCole plots of the films (Z‫ ׀‬and Z‫ ׀׀‬denote the real and imaginary parts of the complex impedance Z*), contain a semi circular arc and an inclined spike, both of which are characteristic behaviours of ionic conductivity of solids with blocking electrodes [41] The semicircle indicates the parallel combination of bulk resistance (due to the migration of ions) and bulk capacitance (due to the immobile polymer chains) Hence the frequency response of the given sample could be represented by an equivalent circuit consisting of a parallel combination of the circuit elements R (resistance) and C (capacitance) The depression of the presence of semi circle with increasing dopant concentration reveals the non-Debye nature of the sample [42] due to the potential well for each site, through which the ion transport occurs The inclined spike represents the formation of a double layer capacitance at the electrodeeelectrolyte interface due to the migration of ions at low frequency The capacitance values (in the PF range) represent the bulk response of the sample [43] The inclination of the spike at an angle less than 90  C to the real axis is due to the roughness of the electrodeeelectrolyte interface [44,45] The ionic conductivity of pure and VO2ỵ doped PVA/MAA:EA polymer blend lms is calculated from the relation; Fig 10 Variation of conductivity (s) as a function of frequency of pure and VO2ỵ doped PVA/MAA:EA polymer blend films at room temperature s ¼ l=R A b (7) where l, is the thickness of the film, A, the area of the film and Rb, the bulk resistance of the film material which is obtained from the intercept on the real axis at the high frequency end of the Nyquist plot of complex impedance [46] Fig 10 shows the variation of the AC electrical conductivity of the doped samples with frequency at room temperature Fig 10 shows the existence of mobile charge carriers, which can be transported by hopping through defect sites along polymer blend Fig 11 Variation of conductivity (s) as a function of VO2ỵ ions dopant concentration in PVA/MAA:EA polymer blend films at room temperature Table Conductivity values of pure and VO2ỵ ions doped PVA/MAA:EA polymer blend films at room temperature Fig Room-temperature Nyquist impedance plots of pure and VO2ỵ doped PVA/ MAA:EA polymer blend lms Concentration mol% of VO2ỵ:PVA/MAA:EA Conductivity at 303 K (S cmÀ1) Pure 2.46 Â 10À8 3.68 Â 10À8 5.09 Â 10À8 6.8 Â 10À8 8.5 Â 10À8 1.45 Â 10À7 274 O Pravakar et al / Journal of Science: Advanced Materials and Devices (2019) 267e275 Fig 12 Variation of dielectric constant (a) and dielectric loss (b) as a function of frequency of pure and VO2ỵ doped PVA/MAA:EA polymer blend lms at room temperature chains [47] Also, there is an enhancement in the ionic conductivity of polymer blend lms by adding VO2ỵ ions, which is due to the increase in mobile charge carriers and the charge carrier mobility enhancement, as well as to the improvement of amorphicity This can be explained by the general conductivity relation.s ¼ ni qi mi where ni is the number of charge carriers, qi is the charge of mobile charge carrier and mi is the mobility of charge carriers According to this relation, the improvement in the ionic conductivity of polymer blend films can be achieved by increasing ni and mi whereasqi is the same for all charge carriers in the polymer blend system 3.6.2 Dependence of conductivity on concentration The variation of conductivity (s) with VO2ỵ concentration at room temperature is shown in Fig 11 From this figure, it can be noticed that the conductivity of pure film is about 8.25 Â 10À9 ScmÀ1 at room temperature and increases to 1.14 Â 10À7 ScmÀ1 for mol% of VO2ỵ ions The increase in conductivity with increase in VO2ỵ concentration is attributed to a reduction in crystallinity of polymer blend films and also to an increase in number of mobile charge carriers An enhancement in the amorphous nature of VO2ỵ doped PVA/MAA:EA polymer blend lms can also be concluded from the SEM analysis that shows an increase in rumples count with the addition of VO2ỵ ions A polymer chain in the amorphous phase is more flexible It results in an increase in segmental motion of the polymer, which facilitates higher ionic mobility [48] The conductivity data of pure and VO2ỵ doped PVA/MAA:EA polymer blend lms at room temperature are presented in Table 3.6.3 Dielectric studies The dielectric property reflects the amount of charge that can be stored by a material and can be used as an indicator to prove that the increase in conductivity is due to an increase in the charge carriers or free mobile ions In case the dielectric values of the material increase, the amount of charge stored by the material will also increase The dielectric response is generally described by the complex permittivity ε* ¼ ε0 Àiε00 , where the real ε0 and imaginary ε00 components represent the storage and loss of energy in each cycle of the applied electric field Fig 12(a) and (b) show that high values of the dielectric constants in the low frequency region can be attributed to charge accumulation at the electrodeeelectrolyte interface At higher frequencies the periodic reversal of the electric field occurs so fast that there is no excess ion diffusion in the direction of the field This indicates that the electrode polarization and space charge effects are prevalent The decrease of dielectric constants with increasing frequency is mainly attributed to the mis-match of interfacial polarization of composites to external electric fields at high frequencies [49] It is also evident that the dielectric constant is a function of dopant concentration as shown in Fig 12 The real component ε0 , which represents storage of energy during each cycle of the applied electric field, increases with filler loading and is attributed to the fractional increase in charges due to the addition of VO2ỵ ions in the pure PV/MAA:EA polymer blend whereas the imaginary component ε00 , which represents the loss of energy in each cycle of the applied electric field, decreases with an increase in filler loading and is attributed to the reduction of charge transport due to the building up of space charge near the electrode/electrolyte interface resulting in high conductivity [50] Conclusion The pure and VO2ỵ doped PVA/MAA:EA polymer blend lms have been successfully synthesized using solution casting method A structural analysis shows an increase in amorphicity of the doped polymer blend films The dTGA study shows an enhancement of thermal stability of the system with increase in dopant concentration The optical absorption spectrum exhibits three bands corresponding to the transitions 2B2g/2A1g, 2B2g/2B1g and B2g/2Eg, characteristic of VO2ỵ ions in octahedral symmetry with tetragonal distortion and reveals that band gap values shift towards longer wavelength on VO2ỵ ions doping which is due to interband transitions The EPR results show that gjj (1.9339) < g⊥(1.9854) A⊥ (66.93) G for all VO2ỵ ions doped polymer blend lms which conrm that VO2ỵ ions exist in the polymer blend as VO2ỵ in octahedral coordination with tetragonal compression The conductivity study shows that addition of VO2ỵ ions to the polymer blend system enhances the ionic conductivity which is attributed to the increase in amorphicity Acknowledgements Authors are thankful to Professor Shyue-Chu Ke, Department of Physics, National Dong Hwa University, Hualien, Taiwan and Dr E Bhoje Gowd, Department of Materials and Minerals, National Institute of Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram, Kerala, India for providing 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(2.0023) and Ajj (199.55) G > A (66.93) G for all VO2? ?? ions doped polymer blend lms which conrm that VO2? ?? ions exist in the polymer blend as VO2? ?? in octahedral coordination with tetragonal compression... along polymer blend Fig 11 Variation of conductivity (s) as a function of VO2? ?? ions dopant concentration in PVA/MAA:EA polymer blend lms at room temperature Table Conductivity values of pure and. .. of the VO2? ?? ions doped PVA/MAA:EA( 50:50) polymer blend films Experimental 2.1 Preparation of the material Pure and VO2? ?? ions (1.0, 2.0, 3.0, 4.0 and 5.0 mol %) doped PVA/ MAA:EA polymer blend films