MAA:EA polymer blend filled with different concentrations of Lithium Perchlorate

The pure polymer blend (50PVA:50MAA:EA) electrolyte and Polymer blend (50PVA:50MAA:EA) electrolytes with different concentrations (5, 10 and 15 wt%) of LiClO 4 have been prepared by the [r]

Original Article

Thermal, structural, optical and electrical properties of PVA/MAA:EA

polymer blend filled with different concentrations of Lithium

Perchlorate

T Siddaiaha, Pravakar Ojhaa, N.O Gopala, Ch Ramua,*, H Nagabhushanab

aDepartment of Physics, Vikrama Simhapuri University PG Centre, Kavali, 524201, India bCNR Rao Centre for Advanced Materials Research, Tumkur University, Tumkur, 572103, India

a r t i c l e i n f o

Article history: Received 18 July 2018 Received in revised form 17 November 2018 Accepted 18 November 2018 Available online 27 November 2018 Keywords:

Polymer

Lithium Perchlorate PVA/MAA:EA blend FTIR

TGA Xe ray

SEM and optical energy

a b s t r a c t

Structural, optical, thermal and morphological studies were performed on pure Polyvinyl alcohol/ Methacrylic Acid e Ethyl Acrylate (PVA/MAA:EA (50:50)) blend and PVA/MAA:EA blend filled with different concentrations (5, 10 and 15 wt%) of Lithium Perchlorate (PVA/MAA:EA: LiClO4) prepared by a

solution casting method XRD patterns demonstrated that the peak intensity at 2q¼ 19.5decreased and

the bandwidth increased with increasing the concentration of LiClO4, which implied a decrease in the

degree of crystallinity and hence caused an increase in the amorphous nature UVe Visible analysis revealed that the values of both direct and indirect band gaps were decreased with increasing LiClO4

content in the polymer host This indicated the formation of charge transfer complexes between the polymer blend and thefiller The dTGA curves show three different steps of weight loss This is due to the loss of water adsorbed, the elimination of the side chains, and the decomposition of the main chain For LiClO4filled PVA/MAA:EA, FTIR spectra showed disappearance of some bands with the change in their

intensities as compared to pure PVA/MAA:EAfilm This indicated considerable interaction between the polymer blend and LiClO4filler SEM images of the polymer blend films complexed with LiClO4suggested

the presence of a structural rearrangement of the polymer chains The electrical conductivity of the preparedfilms was measured using the impedance analyzer in the frequency range from Hz to MHz at room temperature It was observed that the conductivity increased with increase of the Liỵion concentration

© 2018 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/)

1 Introduction

Nowadays, the rechargeable battery market is about 27.7 billion dollars and in 2019 it has been assessed to be 54.9, a fast growth is expected in the rechargeable battery market due to the increase in the demands such as laptops, mobile phones, e-books, watches, toys, automotive sectors and transportation[1] Liỵion batteries are dominant in the electronic devices market due to their compact and lightweight, safety, reliability, cost, design, efficiency, being more environmentally friendly, the highest energy density, high average discharge rate (~37 V) and the absence of memory effects among other types of batteries[2e4]

Materials based on the polymers originate from the need for self-standing, leak-free battery systems The three types of polymer materials are single polymer electrolyte (SPE), composite polymer electrolyte (CPE) and polymer blend electrolyte (PBE) In PBE, two different polymers having complementary properties i.e one polymer showing good affinity with the liquid electrolyte and the other showing proper mechanical properties are used The most used polymer blend electrolytes are (PVAe PANI), (PVA e PAN), (PVPe PVA), (PMVEMA e PVA), (PVA e PEI), (PVA e PVdF), (PEO e PVA), (PVA e PAA), (PVA e PEG), (PEO e PMMA), (PVC e PS), (Chitosane PVA), (PVA e NyC), (PEMA e PVdF), (PVA e NaAlg) and its copolymers P(VDFe TrFE), P(VDF e HEP) and P(VDF e CTFE)

There has been considerable interest in polymer blends in recent decades, owing to their better combination of physical properties, biocompatibility and potential applications than those of single components [5,6] It is generally recognized that the properties of the polymer blends are greatly dependent on their

* Corresponding author

E-mail address:chramu8@gmail.com(Ch Ramu)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.11.004

miscibility and phase behavior When two polymers experience miscibility, a well-ordered microstructure is obtained, which gives the mixture certain unique physical properties according to the formation of the microphase configuration The resulting micro-phase configuration can induce drastic changes in all the proper-ties (Optical, thermal, electrical…etc.) that are different from those of the individual polymers Therefore, the miscibility and phase behavior of polymer blends have been the subject of numerous studies[7]

Polyvinyl alcohol (PVA) has received a great deal of attention due to its high dielectric strength, good charge storage capacity and dopant dependent electrical properties either pure or composite with other materials [8] PVA is well known for its low cost, excellent transparency, flexibility, toughness, nontoxicity, water solubility, biodegradability and gas barrier properties Hence, it is widely used in controlled drug delivery systems, membrane prep-arations, recycling of polymers, packaging textiles and leather in-dustries because of excellent film-forming characteristics The optical uses of PVA are related to the delay, polarization and filtering of light and to photography[9]

Methacrylic Acid e Ethyl Acrylate (MAA:EA) copolymer has been placed at the center amongst the copolymers due to its ad-vantages like easier processability, good environmental stability and transparency It has substantial charge storage capacity and dopant dependent electrical and optical properties Acrylates are a group of easily UV- polymerizable monomers with an unending possibility of polymer chain compositions which can thus be optimized to meet the desired material properties Different types and amounts of selected monomer units, the length and compo-sition of the main polymer chain as well as chemistry of the side chains can be varied and the polymer structure can be further altered by cross-links[10,11]

In order to benefit from the advantages of the polymer and copolymer, in this work, PVA and MAA:EA have been blended at a fixed weight ratio of 50:50 and Lithium has been added to provide the charge carriers Blended host matrices also help to increase ionic conductivity[12] Both PVA and MAA:EA contains electron pairs that can coordinate with a cation from inorganic salt like Liỵto form polymer-salt complexes and hence produce ionic conduction Electron pairs are formed on oxygen atoms of C¼ O and C e O e C groups of PVA and hydrogen atoms in MAA:EA copolymer[13] Experimental

MAA: EA copolymer (1:1) dispersion of 30 percent is a disper-sion in water of a copolymer of Methacrylic Acid and Ethyl Acrylate having an average relative molecular weight of about 250,000 (supplied by Merck Millipore India Ltd.) and high purity PVA of molecular weight 17,000 in form of grains was provided by Merck C Darmstadt, Germany Films of (thickness ~ 150 mm) pure PVA/ MAA:EA and different compositions of LiClO4complexedfilms of (PVA/MAA:EA) were prepared by the solution cast technique in different weight percent ratios (50.0:50.0:0), (47.5:47.5:5), (45:45:10), (42.5:42.5:15) using double distilled water as a solvent Required amounts of PVA, MAA: EA and LiClO4were dissolved in double distilled water and the solutions were stirred magnetically for 10e12 h to obtain a homogeneous solution and then poured into polypropylene dishes and evaporated slowly at room temperature for 48 h to obtain free-standing pure and dopedfilms at the bottom of the dishes The thermal properties of the prepared samples were studied using SEIKO thermal analysis system (TGAe 20) in the presence of nitrogenflow from 40 to 700C, at the heating rate of 10C/min The X-ray diffraction patterns were performed using a Siemens D5000 diffractometer with CuKaradiation (l¼ 1.5406 Å) The preparedfilms were scanned at 2qangles with a step size of

0.02 between and 40 FTIR spectra of the prepared samples were recorded using a PerkineElmer FTIR spectrometer, over a wavenumber range 500e4000 cm1 SEM images of the polymer blend electrolytefilms were characterized by Hitachi (TM e 3000 and He 8100) electron microscope with scanning attachment The optical absorption curves of thefilms were recorded in the range of 200e800 nm at room temperature using JASCO UV e VIS e NIR spectrophotometer (model e V.700) The electrical properties of the polymer electrolytefilms were studied using a Hitester 3532-50 LCR in the frequency range of Hze5 MHz

3 Results and discussion 3.1 X-ray diffraction analysis

X-ray diffraction analysis provides useful structural information, such as crystal structure, crystal size, strain, preferred orientation and layer thickness of a tested material Researchers used the X-ray diffraction to analyze a wide range of materials such as powders, thinfilms, nanomaterials and solid objects.Fig 1shows the XRD scans of pure PVA/MAA:EA (50:50) blend and PVA/MAA:EA blend filled with various weight fractions (5, 10 and 15 wt%) of Lithium Perchlorate (LiClO4) filler The observed diffractograms exhibit three peaks centered at about 2q¼ 19.5, 1.4and 0.6, which in-dicates the semi-crystalline nature of the PVA/MAA:EA polymer blendfilm[14] Thefirst one has a clear crystalline peak at a scat-tering angle 2q¼ 19.5which corresponds to a (1 0) reflection

[15] The present X-ray pattern revealed no significant changes in the positions of three peaks after complexation withfiller, the in-tensity of the diffraction peaks is further decreased This could be due to the interaction between the blend and filler, leading to decrease in the intermolecular interaction between the blend chains as well as the degree of crystallinity[16] As the Liỵcontent is increased in the polymer, the diffraction peaks become less intense, suggesting a decrease in the degree of crystallinity and a simultaneous increase in the amorphicity of these polymer elec-trolyte systems The decrease in crystallinity with increasing LiClO4 content in the blended sample involves a decrease in the number of

hydrogen bonds formed between PVA and MAA: EA The peak at 2q¼ 19.5has been found to increase in broadness and decrease in intensity as Liỵion content increases and enhances the complex-ation between the polymer blend and LiClO4[17]

Hodge et al.[18]established a relationship between the peak intensity and degree of crystallinity They observed that the in-tensity of the diffraction pattern decreases as the amorphous na-ture increases by the addition of a dopant In the present work, no sharp peaks were observed for higher concentrations of LiClO4salt in the polymer, which indicates the dominant presence of the amorphous phase [19] This amorphous nature leads to greater ionic diffusion and high ionic conductivity, which can be observed in amorphous polymers having aflexible main chain[20] 3.2 Ultraviolet and visible analysis

The optical absorption study can be used to gain the detailed information about the band structure of solid materials In optical absorption phenomena, an electron is excited to a higher energy state from a lower energy state by the absorption of a photon of sufficient energy in the transmitting radiation The electronic transitions can be estimated from the changes in the transmitted radiation The optical absorption coefficient (a) can be evaluated from the absorbance by the following relation[21]

Absorption coefficientaị ẳ 2:303 A

d (1)

where A is the absorbance and d is the thickness of thefilms The absorption coefficient (a) can be useful to determine the optical energy band gap (Eg), which is an important parameter of both organic and inorganic materials The optical energy band gap of the preparedfilms was estimated by using Tauc's relationship

[22],

ðahyÞ ¼ B hy Eg

n

(2) where B is a factor that depends on the electronic transition probability, which can be treated as the constant within the optical frequency range and the index n represents the type of electronic transition, which related to the distribution of the density of the states For direct electronic transitions, n takes the values of 1/2 or 3/2 and for indirect transitions n is equal to or 3, depending on whether they are allowed or forbidden, respectively

When the direct and indirect band gap exist, the absorption coefficient has the following dependence on the energy of the incident photon,

ahy¼ B1

 hy Egd

1=2

(3) ahy¼ B2 hy Egi

2

(4) where Egdand Egiare the direct and indirect band gaps, respec-tively, B1and B2are constants

Figs and 3represent the plots of (ahy)2and (ahy)1/2versus photon energy (hy) for pure and different weight fractions (5, 10 and 15 wt%) of LiClO4doped PVA/MAA:EA polymer blend electro-lytefilms The direct and indirect band gap values are determined by extrapolating the linear part of the curves (ahy)2and (ahy)1/2 versus photon energy (hy) to zero absorption value The evaluated band gap values (both direct and indirect) are reported inTable FromTable 1, it is clear that both direct and indirect energy band gap values decrease with increasing in the LiClO4content in the polymer blend The doped LiClO4salt increases the disorder of the

polymer structure, which results in decreases in the optical band gap value Among all the prepared samples, the 15 wt% LiClO4 doped polymer blend electrolyte shows the minimum direct en-ergy band gap and indirect enen-ergy band gap value Hence it has more semiconducting nature than remaining PVA complexed polymer blend electrolytefilms[23]

3.3 Fourier transform infrared analysis

Infrared spectroscopy has been used to identify interactions in the polymer blends FTIR spectroscopy is very sensitive to the

Fig The plot of (ahy)2vs (hy) of (a) pure (50:50) and different concentrations (b)

5 wt%, (c) 10 wt% and (d) 15 wt% of LiClO4doped PVA/MAA:EA polymer blend

elec-trolytefilms

Fig The plot of (ahy)1/2vs (hy) of (a) pure (50:50) and different concentrations (b)

5 wt%, (c) 10 wt% and (d) 15 wt% of LiClO4doped PVA/MAA:EA polymer blend

formation of hydrogen bonds[24].Fig 4(aed) shows FTIR trans-mittance spectra of PVA/MAA:EA (50:50) blend without and with different concentrations 5, 10 and 15 wt % of LiClO4recorded at room temperature in the region 500e4000 cm1 FTIR trans-mittance band positions and their corresponding assignments are presented inTable 2for all prepared compositefilms

Fig 4(a) shows the FTIR spectrum of pure PVA/MAA:EA blend The bands at about 3135e3531 and 1436 cm1 belong to the stretching and bending vibration of the hydroxyl group, respec-tively[25] The band characterized for the methylene group (CH2) asymmetric stretching vibration occurs at about 2934 cm1 The band at about 1099 cm1corresponds to Ce O stretching of acetyl groups present on the PVA backbone[16,26] The vibrational band at about 1722 cm1is assigned to C¼ O stretching of PVA and MAA:EA copolymer

The FTIR spectra of 5, 10 and 15 wt% of Lithium Perchlorate doped films indicate a clear decrease in the intensity of the bands From

Fig 4, the following changes in the spectral features have been observed for the PVA/MAA:EA (50:50) blend without and with different concentrations of 5, 10 and 15 wt% of LiClO4 A broad and very strong band observed at 3085e3539, 3135e3522 and 3119e3565 cm1for 5, 10 and 15 wt% of LiClO4complexedfilms, respectively, arises from Oe H stretching frequency and is an indi-cation of the presence of hydroxyl groups [24] With increasing dopant concentration the width increases and intensity of these bands is found to decrease compared to the pure PVA/MAA:EA blend The vibrational peaks observed at 1099 and 1436 cm1for pure blend are found to be absent in Liỵ ion doped blend lms The disappearance of bands observed in blendlm with Liỵion doping suggests the co-ordination or complexation of chlorine with the blendfilm[27]

It was found that a small absorption band around 922 cm1is characteristic of the syndiotactic structure of the prepared samples Syndioactivity of PVA/MAA: EA samples induces dense molecular packing in a crystal, as well as stronger intermolecular hydrogen bonds, which in turn, are responsible for the disappearance of molecular motion[28] This band appeared in the spectra of pure blendfilm, but it disappeared in the spectra of the Liỵion doped blend and this may be attributed to the addition of Lithium

3.4 Thermogravimetric analysis

Thermal stability and degradability of the prepared samples were evaluated by thermogravimetric analysis[29] Moreover, the kinetics of the accompanied decomposition relation has been reviewed The sample weight decreases slowly as the reaction be-gins, then decreases rapidly over a comparatively narrow temper-ature range andfinally levels off as the reactants are used up The shape of the curve depends primarily upon the kinetic parameters involved, i.e., order of reaction (n), and activation energy (E) The values of these parameters are important in the estimation of thermal stability TGA is used for studying the samples in the temperature range of 30e600C Based on the TGA curves all the films have shown three stages of weight loss It is evident that the initial weight loss for the pure PVA/MAA:EA blend sample starts from 30 to 130C with a weight loss of 7.8%, which may be due to the loss of entrapped water molecules and moisture The second weight loss occurs at 131e390C, which is attributed to melting temperature (Tm), it is around 344C This weight loss is attributed to the degradation of the unsaturated groups of PVA The third weight loss starts from 391 to 472C with a weight loss of 21%

Fig shows the dTGA curves of pure PVA/MAA:EA (50:50) blend and PVA/MAA:EA blendfilled with various weight fractions of LiClO4filler As shown inFig 5(bed), all the doped films show three stages of degradation The initial weight loss for all the doped samples starts from 30 to 150C with the weight loss of 7e9%, which may be due to the evaporation of the residual solvent, moisture and impurities due to Chlorine compound (LiClO4) Beyond thefirst stage, the samples have a drastic weight loss in the temperature range 151e380C with the weight loss of 30e60%, which is assigned to melting temperature (Tm) The melting tem-perature of all the LiClO4dopedfilms exhibits decreasing nature as LiClO4content increases in the polymer matrix This result in the increase in the amorphous nature of the polymer blend electrolyte films The melting temperatures and decomposition temperatures of pure and LiClO4doped polymer blend electrolytefilms are pre-sented inTable The second weight loss is ascribed to dehydro-chlorination of LiClO4 which leads to the volatilization of monomers and oligomers The degradation of MAA:EA and the elimination of the unsaturated functional group of PVA are the contributions forfinal weight loss, which starts from 381 to 470C. In conclusion, among all the LiClO4 complexed polymer blend electrolytefilms, 15 wt% of LiClO4dopedfilm shows more amor-phous nature than and 10 wt%

The activation energy (E) for the main thermal decomposition for TGA measurements of the present samples, which depend on

Table

Direct band and indirect band gap values for pure and LiClO4doped PVA/MAA:EA (50:50) polymer blend electrolytefilms

PVA/MAA:EA: LiClO4(wt%) sample composition Direct band gap (eV) Indirect band gap (eV)

50: 50: 5.04 4.36

47.5: 47.5: 4.90 3.91

45: 45: 10 4.68 3.82

42.5: 42.5: 15 4.04 3.66

the residual mass, can be calculated in general using the integral equation of Coats and Redfern[30]

log 

log1 aị T2



ẳ logDRE 

12RTE 

2:304RE (5)

where T is the absolute temperature, E is the activation energy in J/ mol, R is the universal gas constant (8.3136 J/mol K) andais the fractional weight loss at that particular temperature is calculated as

a¼wi wt

wi wf

(6) where wiis the initial weight, wtis the weight at a given temper-ature and wfis thefinal weight of the sample For n ¼ 1, Eq.(1) reduce to

log 

log1 aị T2

 ẳ logDR

E 

12RT E



 0:434 E

RT (7)

By plotting logẵlog1 aị=T2 against 1000/T for each sample, we obtain straight lines Then, the apparent activation energies are

determined from the slope of these lines using the following equation

E¼ 2:303R  slope (8)

Values of the apparent activation energy (E) of the samples are noted inTable

3.5 Scanning electron microscopy

A scanning electron microscope creates images of a sample by scanning it with a focused electron beam An electron beam in-teracts with electrons in a sample that generates various signals that can be detected, and that contains information about the morphology and composition of the sample surface The electrons in the beam interact with the sample, producing various signals that can be used to obtain information about the surface topog-raphy, morphology and composition that makes it invaluable in a variety of science and industry applications[31]

SEM is also used to study compatibility between different components of polymer electrolytes by detecting phase separations and interfaces[32,33] Compatibility between the polymer mixture and inorganic additives has a great influence on the thermal, me-chanical and ionic properties of polymer electrolytes

Fig 6shows the SEM images of pure PVA, Pure MAA:EA and PVA/MAA:EA (50:50) blend without and with different concen-trations 5, 10 and 15 wt% of Lithium Perchlorate It can be seen from

Fig 6(a) that pure MAA:EA copolymer film shows the smooth surface, suggesting that MAA and EA molecules may disperse in the soft e segment phase with little influence on the microphase separation and mixing of the hard and soft segments[34] The SEM image of pure PVAfilm has no characteristics attributable to any crystalline morphology Hence the semicrystallinity of PVA dis-cussed earlier is likely to be submicroscopic in nature[35]

Fig 6(c) shows the SEM images of PVA/MAA:EA (50:50) polymer blend electrolyte The lateral branches correlate poorly with the length of the trunk of the dendrites The growth of a dendrite-like form, which is a collection of branched aggregate clusters has begun and lead to the formation of a condensed aggregated form of dendrites This suggests the presence of structural reorganizations of polymer chains [36] Fig 6(def) shows the SEM images of different concentrations (5, 10 and 15 wt%) of LiClO4doped polymer blendfilms The morphology was uniform with different degrees of roughness The increase in the degrees of roughness with increased

Table

Assignments of the FTIR characterization bands of pure and LiClO4doped PVA/MAA:EA polymer blend electrolytefilms

Pure PVA/MAA:EA (50:50) PVA/MAA:EA: LiClO4sample

Wavenumber (cm1) Assignment Wavenumber (cm1) Assignment

847 CH2rocking 839 C¼ C stretching

922 Ce O stretching 922 Ce O stretching

1099 Ce O stretching 1722 C¼ O stretching

1436 Oe H and C e H bending 2925 Ce H stretching of methylene group 1722 C¼ O stretching 3085e3565 Oe H stretching

2934 CH2Asymmetric stretching e e

3135e3531 Oe H stretching of alcohols and phenols e e

Fig dTGA thermograms of (a) pure (50:50) and different concentrations (b) wt%, (c) 10 wt% and (d) 15 wt% of LiClO4doped PVA/MAA:EA polymer blend electrolyte

films

Table

Melting and decomposition temperatures of pure PVA/MAA:EA and PVA/MAA:EA: LiClO4composite polymer blend electrolytefilms

PVA/MAA:EA: LiClO4(wt%) Sample Melting temperature (Tm) (C) Decomposition temperature (Td) (C)

50:50:0 344 424

47.5:47.5:5 317 418

45:45:10 311 413

Liỵion concentration indicates that the crystalline salt was broken into small pieces and mixed into the polymer blend[11] The SEM of the blendfilm filled with wt% LiClO4depicts a small spherulite shape, scattered and distributed randomly in an isolated form throughout thefilm With an increase in the concentration of the filler upto 10 wt%, the number of the spherulites considerably in-creases This leads to the increase in the degree of roughness and indicates the segregation of dopant in the host matrix As the dopant concentration increases, the size of the spherulites in-creases leading to the formation of isolated spherulites distributed all over the volume of thefilm[37]

3.6 Electrical conductivity

The ionic conductivity of the polymer electrolytes mainly de-pends on the actual concentration of conducting charge carriers and their mobility Fig shows the typical impedance plots of (PVA/MAA:EA) polymer electrolyte doped with various concen-trations of LiClO4at ambient temperature in the frequency range of Hze5 MHz FromFig 7, one observed a semicircle in the high-frequency region and a tilted straight line in the low-high-frequency region The presence of a semicircle indicates the non-Debye character of the sample[38], since the potential well of each site through which the ion transfer takes place is not equal It is widely accepted that the semicircle is due to the bulk resis-tance and capaciresis-tance of the electric double layer formed at the

electrode/electrode interface [39] This offers an increasing impedance against the ion transfer with decreasing the frequency The linear region in the low-frequencies is due to the influence

Table

Activation energy of pure PVA/MAA:EA and PVA/MAA:EA: LiClO4Composite polymer blend electrolytefilms

PVA/MAA:EA: LiClO4(wt%) Sample Activation energy (E) (KJ/mol) Activation energy (E) (eV)

50:50:0 88.45 1.46

47.5:47.5:5 84.23 1.39

45:45:10 82.78 1.37

42.5:42.5:15 77.34 1.28

Fig SEM images of (a) pure MAA:EA, (b) pure PVA, (c) pure (50:50) and different concentrations (d) wt%, (e) 10 wt% and (f) 15 wt% of LiClO4doped PVA/MAA:EA polymer blend

electrolytefilms

Fig Impedance plots (Cole-Cole plots) for (a) pure (50:50) and different concen-trations (b) wt%, (c) 10 wt% and (d) 15 wt% of LiClO4doped PVA/MAA:EA polymer

of blocking electrodes With a higher concentration of dopant, the lines slightly approach the imaginary axis, which indicates the establishment of a better contact with the electrode The ionic conductivity of the solid polymer blend electrolytes was calculated using the following equation

s¼ t=RbA (9)

where t is the thickness of the polymer blend electrolyte, A is the area of the blocking electrode and Rb is the bulk resistance of polymer blend electrolyte It is evident from the cole-cole plots at different dopant concentrations that the intercept of the semicircle (bulk resistance) on the real axis tends towards lower values with increasing the dopant concentration which indicates that the conductivity increases with dopant concentration

Fig 8shows the conductivity values of PVA/MAA:EAỵ LiClO4 complexes as a function of salt concentrations at ambient tem-perature in the frequency range of Hze5 MHz FromFig 8, it is

observed that the ionic conductivity increases with increasing LiClO4 content in the polymer blend electrolyte The maximum conductivity of 7.35  108 S/cm was obtained for the PVA/ MAA:EAỵ LiClO4(42.5:42.5:15) system The conductivity values of different complexes at room temperature are summarized in

Table The highest ionic conductivity at 15 wt% salt doping is attributed to the highest electrolyte uptake Notes that, the con-ductivity is related to the number of Liỵion charge carriers (ni) and their mobility (mi) [40] The coordination interaction of oxygen atoms of MAA:EA with Liỵcations of LiClO4salt results in an in-crease in mobile charge carriers and reduction in the crystallinity of the PVA/MAA:EA mixture This is responsible for the increase of ionic conductivity These interactions have also been observed by FTIR, XRD and SEM analysis

Fig 9shows the plots of the logarithm of conductivity as a function of the logarithm of frequency for all the samples at room temperature The spectrum consists of two distinguishable regions within the measured frequency range The low-frequency region describing electrode-electrode interfacial phenomena is ascribed to the space charge polarization at the blocking electrodes[41] The high-frequency region corresponding to the bulk relaxation phe-nomena disappears gradually for the electrolytes, having the salt concentration up to 15 wt% may be due to the increase in jump frequency of charge carriers The conductivity of polymer electro-lytes initially increases due to the increment of charge carriers being introduced into the complex As the salt concentration in-creases, the number of carrier ions of the complex increases upto a particular limit of the salt concentration Above this concentration there is a stronger ion-ion interaction which possibly prevents the polymer backbone's segmental motion and then causes a lowering of the dc conductivity[42]

4 Conclusion

The pure polymer blend (50PVA:50MAA:EA) electrolyte and Polymer blend (50PVA:50MAA:EA) electrolytes with different concentrations (5, 10 and 15 wt%) of LiClO4have been prepared by the solution casting technique using the double distilled water X-ray diffraction patterns show a decrease in the degree of crystalli-zation and cause an increase in the amorphous region The FTIR spectra show the position shifts as well as the intensity changes This indicates the considerable interaction between the polymer blend and LiClO4 The dTGA curve shows three different stages of weight loss, which are due to the loss of the adsorbed water, the decomposition of the side chain and the main chain The melting and decomposition temperatures exhibit a decreasing trend with increasing LiClO4content in the polymer blend, which indicates the increase in amorphous nature SEM images of the Liỵion doped lms revealed the presence of the structural rearrangement of polymer chains UVe Visible analysis revealed that the value of the optical band gap decreases as the LiClO4content increases in the polymer blend This indicates the formation of charge transfer complexes between the polymer blend and the filler The maximum conductivity of 7.35 108S/cm was obtained for the system PVA/MAA:EAỵ LiClO4(42.5:42.5:15) at room temperature

Fig Conductivity vs salt concentration plots of (PVA/MAA:EAỵ LiClO4) polymer

blend electrolyte system at room temperature

Table

The conductivity values of Pure and LiClO4doped (PVA/MAA:EA) Polymer blend

electrolytefilms at room temperature

PVA/MAA:EA: LiClO4sample composition (wt%) Conductivity at 303 K (S/cm)

50.0: 50.0: 2.46 108

47.5: 47.5: 3.86 108

45.0: 45.0: 10 4.39 108

42.5: 42.5: 15 7.35 108

Fig Frequency dependent conductivity at room temperature for (a) pure (50:50) and different concentrations (b) wt%, (c) 10 wt% and (d) 15 wt% of LiClO4doped PVA/

From all the characterization results, the 15 wt% of LiClO4doped polymer blend electrolyte system exhibits the better semi-conducting nature which is more suitable for fabricating solid-state batteries and other electrochemical devices

Acknowledgments

The authors thank E Bhoje Gowd, Senior Scientist, Department of Materials and Minerals, National Institute of Interdisciplinary Science and Technology (NIIST), Thiruvanthapuram, Kerala, for his constant encouragement and active cooperation to carry out the work

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