FLUORESCENCE SPECTROSCOPY FOR LOCAL VISCOSITY MEASUREMENTS IN POLYACRYLONITRILE (PAN)-BASED POLYMER GEL ELECTROLYTES

A fluorescence probe sensitive to local viscosity was used to study co nduction property of the polymer gel electrolytes, polyacrylonitrile ( PAN)/propylene carbonate (PC) + ethylene carbonate (EC) in 1:3 volume ratio/LiClO4. The conductivity of the gel electrolytes was in the rang e of 10(-2)-10(-3) S cm(-1), which is much higher than values for the conventional polymer electrolytes such as PEO/Li salts. The conductivi ty profile of the polymer gel electrolytes, according to the LiClO4 co ncentration ([LiClO4]/[PC + EC] = 0.05-0.11), looked similar to that o f the liquid electrolytes of the same composition. Also, the Arrhenius activation energies of the gel electrolytes (10-15 kJ mol(-1)) were r ather closer to those (5 kJ mol(-1)) of the liquid electrolytes than t hose for the unplasticized polymer electrolytes (a few tens to hundred s kJ mol(-1)). However, the observed local viscosities of the gel elec trolytes were higher than those of the liquid electrolytes by a factor of 20, while the conductivity was smaller only by a factor of 2-9, su ggesting that the inverse relationship between local viscosity and con ductivity does not hold in this system. FTIR spectra of the gel electr olytes showed that the -C=N groups in PAN interact with Li+ ions, caus ing an increase in the local viscosity, probably due to a contraction of the local free volumes. In short, the conduction property of the ge l electrolytes looked similar to that of the liquid electrolytes in se veral ways. However, the Li+ ion-polar group interactions in the gel e lectrolytes render the inverse relationship invalid and consequently t he observed conductivity and activation energy values are somewhat dif ferent from those for liquid electrolytes of the same composition.