The effect of the lithium trifluoromethanesulfonate (LiCF3SO3) addition on the structure and properties for binary blends of poly(methyl methacrylate) (PMMA) and a random copolymer composed of vinyl butyral and vinyl alcohol, that is denoted as PVB in this paper, was evaluated. Although PMMA and PVB are immiscible, the LiCF3SO3 addition enhances the compatibility/miscibility by the ion-dipole interaction between lithium cation and both polymer chains. Only single peak ascribed to the glass-to-rubber transition is detected in the tensile loss modulus curve for PMMA/PVB/LiCF3SO3 (70/30/20), although there are distinct double peaks for PMMA/PVB (70/30). This result demonstrates that the salt, LiCF3SO3, acts as a compatibilizer. Furthermore, the salt addition enhances the storage modulus in the glassy region and the heat resistance due to the high-temperature-shift of glass transition temperature. The blend containing LiCF3SO3 is highly transparent and shows a significantly small value of the stress-optical coefficient in the glassy state, which are required for various optical films used for display, such as polarizer protective film. (C) 2018 Elsevier Ltd. All rights reserved.