PROTON CONDUCTING POLYMER ELECTROLYTE BASED ON PVA-PAN

Abstract

Various polymers such as PVA, PVAc, PVP and etc., doped with ammonium salts have been studied for proton conduction. But the study of proton conduction in polymer PAN is scarce. The proton conducting polymer electrolytes composed of polyacrylonitrile (PAN) with ammonium nitrate (NH4NO3) in different molar ratios, have been prepared by solution casting method, using DMF as solvent. The increase in amorphous nature of the polymer electrolytes has been confirmed by X-ray diffraction analysis (XRD). The complex formation between polymer and dissociated salt has been confirmed by Fourier transform infrared spectroscopy (FTIR). The ionic conductivity, dielectric permittivity (e*) and electric modulus (m*) have been calculated from the ac impedance spectroscopy in the frequency range 42HZ-5MHZ. The activation energy of doped PAN polymer electrolyte is calculated using Arrhenius plot. Based on the study of relaxation spectra, it is found that the relaxation time decreases with increase in temperature. INTRODUCTION In recent years, proton conducting polymer electrolytes have attracted considerable attention owing to their application in fuel cells, humidity & gas senors and electrochromic displays.1,2 The main advantages of polymer electrolytes are their good mechanical properties, the ease of fabrication of films of desirable sizes, and their ability to form good electrode – electrolyte contact. Solid polymer electrolytes (SPE), consisting of an ionic conductive polymer matrix and a supporting electrolyte salt, were firstly introduced by Fenton et al.3 The development of polymer system with high ionic conductivity is one of the main objectives in polymer research. A variety of polymers such as Poly (acrylonitrile) (PAN), Poly (vinyl chloride) (PVC) and Poly (methyl methacrylate) (PMMA) has been used as polymer matrices.4-5 Proton conducting polymer electrolytes can be obtained by doping the polymer either with alcohol, amine, amide or amide groups6 or with strong acids or ammonium salts. Ammonium salts have been reported as good donors of proton to the polymer matrix. Literature survey indicates that the synthesis and characterization of ammonium salts doped proton conducting polymer electrolytes based on Poly (acrylonitrile) (PAN) is rare. PAN is one of the most important fibers – forming polymers and has been widely used because of its high strength, abrasion resistance and good insect resistance.7 PAN is used to produce large variety of products including ultra filtration membranes, hollow fibers for reverse osmosis, fibers for textiles, oxidized frame retardant fibers like PANOX and carbon fiber. However the conductivity of PAN is < 10-14 S cm-1 and the static problem restricts its further applications. PAN is usually synthesized using free radical polymerization. Usually they are copolymers of acrylonitrile and methyl acrylate or acrylonitrile and methyl methacrylate. PAN has a melting point of about 3190C, and it also decomposes at this temperature. So PAN membrane is prepared by solution casting technique. In the present work, NH4NO3 doped PAN polymer electrolytes have been prepared and subjected to various characterizations such as XRD, FTIR and Ac impedance spectroscopy. EXPERIMENTAL DETAILS The polymer PAN (Aldrich) of average molecular weight 1, 50, 000 and the salt NH4NO3 were used as the raw materials in this study. Dimethyl formamide (DMF) was used as the solvent. The polymer electrolytes PAN doped with NH4NO3 in different compositions such as 100:0; 95:05; 90:10; 85:15; 80:20 and 75:25 were prepared by solution casting technique. The mixture of PAN and NH4NO3 was stirred continuously with a magnetic stirrer for several hours at temperature 600 C to obtain a homogenous solution. The solution was then poured into propylene petri dishes and allowed to evaporate in vacuum oven at 650C. After 48 hours, free standing transparent films were obtained. X-ray diffraction patterns of the prepared samples were recorded at room temperature on a Philips X’Pert PRO diffractometer using Cukα radiation. FTIR spectra were recorded for the polymer electrolyte films using a SHIMADZU IR Affinity -1 Spectrometer in the range of 400 4000 cm-1 at room temperature. Conductivity measurements were carried out by using a HIOKI 3532 LCZ meter in the frequency range of 42 Hz – 1MHz over the temperature range of 303 – 343 K. Possible interaction between PAN and NH4NO3. The possible interaction between PAN and NH4NO3 has been pictorially represented in the scheme 1 along with the structure. The proton from the salt interacts with the polar groups of the host polymer matrix.