Polymer blend electrolytes, where PEO-PMMA polymer blend is used as polymer host matrix, doped with AgNO3 and plasticized with ethylene carbonate (EC) and Al2O3 as nano-filler were synthesized using the solution cast techniques. The polymer films were characterized by impedance spectroscopy, XRD, DSC, SEM, FT-IR and ionic transport mea-surements. The results indicate an enhancement in conductivity of PEO-PMMA-AgNO3-EC polymer electrolytes. The ionic conductivity of the polymer films is also found to increase with temperature. Electrical properties of polymer films in the framework of dielectric and modulus formalism are studied and discussed
Solid polymer electrolytes have attracted attention since more than three decades due to their practical applications as well as for fundamental knowledge [1-5]. In literature, researchers have worked on many host polymers e.g. poly(ethylene oxide) (PEO), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC) and poly(vinyl acetate) (PVA)etc. [6-10] to optimize electrical conductivity and other properties for their practical application as polymer electrolytes. Among these polymers, PEO is still an active candidate for polymer electrolytes [
Hence, authors have made an attempt to investigate the effect of nano-filler Al2O3 on structural and ionic transport properties of PEO-PMMA-AgNO3-EC (PPAE) polymer electrolyte. The effects of this and other nano-fillers in different systems have been reported previously by many workers [6,7,13,14] but a systematic study of nanofiller Al2O3 to plasticized PEO-PMMA polymer blend has not been observed yet. Therefore, the effect of nanofiller on the microstructural, structural, thermal, electrical and dielectric properties of plasticized polymer film has been undertaken to enhance the understanding of the ion transport in this blended polymer nano-composite.
Commercially available chemicals of PEO (Mw= 3 × 105, Alfa Aesar), PMMA (Mw = 3.5 × 105, Alfa Aesar), AgNO3, EC (Aldrich) were employed as starting materials. PEO and PMMA were weighed in 50:50 weight percentages and the concentration of AgNO3 and EC were kept fixed as 5 wt%. Nano-composite was prepared by adding 4 wt% of nano-filler Al2O3 in PMMA-PEO-AgNO3 polymer electrolytes using acetonitrile as solvent. All the samples were prepared by solution cast method.
PEO and PMMA were separately dissolved in acetonitrile separately and stirred by using a magnetic stirrer. The stirring of PMMA solution was carried out at 323 K for 24 h to dissolve PMMA in acetonitrile. PEO and silver nitrate (AgNO3) were mixed and stirred for 4 - 5 h at room temperature. Both the solutions were mixed along with the desired amount of plasticizer and nano-filler. The obtained mixture was again stirred at room temperature for another 10 h for homogenous mixing. Finally the solution was poured into a Teflon Petri dish and left to evaporate the solvent slowly at ambient temperature. The resulting films were kept in an oven at 313 K for 2 days to ensure the removal of the solvent traces. The dried films were peeled off from the Petri-dish and then were stored in dark desiccators to prevent them any contamination from moisture and/or light.
X-ray diffraction (XRD) measurement was carried out using monochromatic Cu-Kα radiation (=1.5418 Å) in x-ray diffractometer (Brucker NSZ, model D8) in the range of 10˚ - 50˚ at a scan rate of 2˚. The differential scanning calorimetry (DSC) of the prepared polymer films was carried out using SII EXSTAR-6000 equipment in the range from ambient temperature to 363 K with a heating rate of 10 K/min in nitrogen atmosphere. The polymer samples of about 2 - 3 mg were encapsulated in aluminum pans.
The surface morphology of the plasticized polymer complexes were investigated by using scanning electron microscope (JOEL JSM-6380LV) at 20 kV. The polymer films were gold coated under vacuum by electron beam gold palladium source (80% Au, 20% Pd) by JEOL coater (Model JFC-1600) to make them conducting and mounted onto circular aluminum stubs with double side sticky tapes. Fourier transform infrared (FT-IR) spectra of the prepared samples were recorded in the wavenumber range of 400 - 3000 cm–1 using single beam FT-IR 4100 JASCO model by directly mounting in the sample holder in a transmission mode.
X-ray Complex impedance spectroscopy has been employed for electrical measurements using the impedance gain/phase analyzer (SOLARTRON 1260) interfaced to a computer in the frequency range of 10 Hz - 1MHz at different temperatures between 273 and 353 K. For the impedance measurements, the polymer electrolyte films were sandwiched between two silver electrodes of radius 1cm under spring pressure in a temperature controlled furnace. The obtained impedance plots were fitted using Zview2 program (developed by Solartron Analytical).
PEO, PEO-PMMA-AgNO3-EC (PPAE) and PEO-PMMAAgNO3-EC-Al2O3 (PPAEA) polymer films. The diffraction peaks of pure PEO are observed at 19.36˚ and 23.52˚. These diffraction peaks are observed to shift towards lower angle side with small broadening with decreased intensity. The incorporation of plasticizer and nano-filler disturb the crystalline region and increase the amorphous phase of PEO. The blended polymer electrolyte undergoes significant structural reorganization when EC and Al2O3 are added and an increase in the amorphous phase is observed to be dominant. This amorphous nature may lead to higher ionic conductivity which is generally observed in amorphous polymer electrolytes with flexible backbone [
To get information about the temperatures of the different phase transitions due to rearrangement of polymer chains on heating, DSC measurements have been carried out on the polymer films. The typical DSC plots of the PPAE and PPAEA polymer films are shown in
FTIR is a useful technique to characterize the organic, inorganic and composite materials [
In PPAEA polymer electrolyte film in which the nanofiller is added, the peaks of IR spectrum is found to be quite broadened as compared to that of PPEA polymer electrolyte. In addition to this, the vibration bands in the wave number 500 - 1480 cm–1 range are found absent. The vibration peaks at 1725, 1812 and 1962 cm–1 remain at same position but with decreased peak intensity. Hence, the above IR analysis confirms that the nano-filler Al2O3 helps in enhancing the amorphous nature with flexible polymer chains.
Impedance spectroscopy is a useful tool to investigate the conduction mechanism, the mobility and participation of polymer chains in carrier generation processes.
temperature. It is observed that the impedance plots are the depressed semicircles which indicate the relaxation of ions as non-Debye in nature [
The ionic conductivity is calculated from the bulk resistance and knowing dimensions of polymer films. The conductivity enhancement is observed when the nanofiller is added in the polymer complex below the melting temperature as well as above this point. Nano-filler favors additional transient sites [
To find the transference number of ionic and electronic transfer, Wagner’s polarization technique was used [
Figures 7(a) and (b) show the frequency dependence of the real and imaginary parts, e’ and e” of the dielectric permittivity, respectively for with filler and filler free polymer films. The incorporation of nano-filler in PPAE polymer electrolyte increases e’ and e” values are over
the entire frequency and temperature range. Inset of Figures 7(a) and (b) also shows that there is a significant increase in e’ and e” values due to incorporation of nanofiller even above Tm. The dielectric constant or real part of dielectric e’, is observed to decrease with frequency which is a characteristic of disordered solids. e’ and e” rise sharply towards low frequencies attributing to the free charge build up at the interface between the polymer film and the electrode (electrode polarization effects) which masks the other relaxation processes. As the temperature increases, the real and imaginary parts of dielectric constant are observed to increase (insets of Figures 7(a) and (b)). Similar behavior has also been observed for other materials [12,25] and this can be understood as the system is assumed to be formed of molecular dipoles which remain frozen at low temperatures. With the rise
in temperature, these dipoles become more thermally activated, having more rotational freedom leading to the increase in dielectric constant. A further analysis of the dielectric behavior would be more successfully achieved by using the formulation of dielectric modulus, which suppresses the effects of the electrode polarization.
Modulus formalism masks the low frequency dispersion due to the electrode polarization effects as observed in e’ and e” values [13,23]. To overcome these effects and to confirm the ionic conduction relaxation processes, modulus formalism is generally used in materials. Figures 8(a) and (b) represent the real (M’) and imaginary (M”) parts of modulus M’, respectively for with nanofiller and without filler polymer electrolytes. The reduction in M’ values for polymer film with nano-filler indicates increase in charge carriers i.e. Ag+-ions as well as polymer segmental motion. While M” peak is found to
shift higher frequency side (
Plasticized solid polymer electrolytes with alumina nanofiller are synthesized. The XRD, DSC, SEM and FTIR analysis indicate reduction of crystallinity phase due to addition of nano-filler. FTIR studies confirmed that the complexation occurs between host polymer blend, plasticizer and salt. The incorporation of nano-filler Al2O3 has led to significantly enhance the ionic conduction as observed from the ionic transference number measurements leading to high ionic conductivity. The observed higher ionic conductivity in polymer film with nano-filler Al2O3 is due to the creation of more conducting pathways resulting in increased chain flexibility, carrier concentration as well as amorphousity. The ionic conductivity of polymer films is observed to increase with temperature with a transition near Tm. Analysis of dielectric permittivity and electric modulus behavior reflects a non-Debye type relaxation and also distribution of relaxation time. The relaxation time is found in good correlation with the conductivity results.
P. Sharma thankfully acknowledges the financial support by UGC, New Delhi, India for RFSMS fellowship.