STUDIES ON SOLID POLYMER ELECTROLYTES BASED ON POLY(VINYL CHLORIDE)

Abstract

Solid polymer electrolytes have been proven to be prospective candidates for advanced electrochemical applications on basis of their characteristics such as flexibility, viscoelasticity and ionic conductivity. The ionic conductivity of solid polymer electrolytes is very low at ambient temperature. Several attempts have been carried out to improve ionic conductivity of solid polymer electrolyte systems. The ionic conductivity of these solid polymer electrolytes can be improved by incorporation of plasticizers, which impart the necessary salt solvating power and ionic mobility. Solid polymer electrolyte system based on PVC containing ethylene carbonate and propylene carbonate as plasticizers have also found applications in lithium secondary batteries. PVC shows immiscibility with plasticizer thereby acts as mechanical stiffener in solid polymer electrolyte system. In the present work solid polymer electrolyte systems based on PVC containing alkali metal salts (LiClO4, LiClO3, LiBF4, Li2SO4, NaClO4), plasticizers(EC, PC), inorganic metallic oxides (ZnO, TiO2, Al2O3, ZrO2) and PVC blended with PMMA and PEO were prepared and examined as solid polymer electrolyte with improved desired properties. The prepared polyelectrolytes were characterized by conductivity, Thermogravimetric, Differential Thermogravimetric Analysis, X-ray diffraction, Scanning electron microscopy and viscosity methods. The conductance studies of different polymer electrolyte systems showed that conductivity values exhibit enhancement with increase in concentration of salts which may be due to the development of amorphous regions in the polymer matrix which facilitates the mobility of ions through the polymer matrix. The increase in ionic conductivity with salt may be attributed to the increase in the number of ions of salts. The effects of different plasticizers on the behavior of polymer electrolytes of different compositions at various temperatures were also observed. The addition of PC was found more effective as compared to EC. The ionic conductivity increased with rise in temperature for all different polymer electrolyte systems containing various content of PVC, salts and plasticizers. The studies of incorporation of various inorganic fillers showed increase in ionic conductivity with increase in concentration of filler up to certain limits of filler concentration beyond which ionic conductivity decreased. At higher concentration of fillers, the formation of crystallites may be responsible for the decrease in ionic conductivity of the polymer electrolyte systems. The activation energies of different polymer electrolyte systems of various compositions at various temperatures were calculated and found that Ea value showed decrease with rise in temperature. Similarly Ea also decreased by addition of salts or plasticizers thereby increasing the ionic conductivity of the polymer electrolyte systems. The effects of addition of salts, plasticizers and polymer on the mechanical properties of polymer electrolytes were also studied. It was found that mechanical strength of polymer electrolytes deteriorated with increase in concentration of salts. The effect of EC and PC as plasticizers on the mechanical strength were studied and found that Young’s modulus and stress at peak values decreased by incorporation of low molecular weight plasticizers while elongation at peak values revealed that elongation decreased by addition of salt. The variation of modulus and tensile strength of PVC-PMMA blend polymer electrolyte system regarding concentration of PMMA was also studied and it was found that Young’s modulus and tensile strength decreased with increase in concentration of PMMA up to 15 wt% concentration of PMMA beyond which both these parameters showed increase. The influence of silica exhibits improvement in mechanical properties. SEM studies of various PVC based polymer electrolyte systems showed that by incorporation of inorganic fillers, the surface becomes rougher as compared to the films without any filler. The effects of salts on the SEM micrographs were also studied. SEM micrographs showed that surface morphology of pure PVC to be similar to rigid and glassy surfaces while blends of PVC with PMMA showed two phase morphology without any sharp boundary between these two phases.