Plasticized polymer electrolytes are of growing importance in solid state electrochemistry due to their potential application such as polymer based electrolyte in primary and secondary high-energy-density batteries. Polymer complexes consisting of polymer, plasticizer, and alkali metal salts have been one of the most favorable systems for practical application. Although these systems have been shown to have high ionic conductivities suitable for the electrochemical use at ambient temperature, the fundamentals underlying the plasticized polymer electrolyte have not been fully understood.
The PMMA based polymer electrolytes were prepared by blending of various kinds of plasticizers with $LiCF_3SO_3$ and PMMA. The plasticizers used in this study were chosen to investigate the effects of dielectric constants and those of molecular geometry on the conductivity behavior. The molecular geometry were varied with the linear and the ring shape of ether derivatives. The effects of dielectric constants were investigated by comparision of the ionic conductivities or EC of PC having the high dielectric constants with those of low dielectric constant plasticizers, ether derivatives, such as 1,4-dioxane, 15- or 18-crown ether, and PEGDME.
The ionic distribution and interaction between the components of polymer electrolyte were comprehensively studied by FT-IR spectroscopic techniques. It was found that the free ion fraction, indicating the number of charge carrier, rapidly increased and the approached to a certain value with the increase of dielectric constant of plasticizers. For the plasticizers having the dielectric constants higher than 5, the dielectric constants have little effect on the distribution of ionic states within the polymer electrolytes. In the consideration of molecular geometry for the ether derivatives, the ring shape molecules, crown ethers were found to have a better solvating power than the linear ether, PEGDME. It was interpreted in terms of the entropy change during the salt-solvation process of plasticizers. It was also considered as a shielding effects of crown ethers to prevent the carbonyl group of PMMA from interacing with cations.
The ionic conductivities of polymer electrolytes were significantly affected by the amount of the added plasticizers. A certain critical amount of plasticizers was necessary for ionic conduction because the mobility of charge carrier could be accomplished by the segmental motion of the solid polymer electrolyte. It was generally found that ionic conductivities of the polymer electrolyte increased significantly with the addition of plasticizer. When the amount of added plasticizer exceeded a certain value, the increasing rate became slow down. The behavior of ionic conduction of the polymer electrolytes as a function of plasticizer content was discussed by the size of charge carriers and dimension of free volume through which the charge carrier moves.
