There has been a demand for the use of transparent polymer materials based on glassy polymers as alternatives to inorganic glass. However, the brittleness of these polymer materials is preventing the expansion of practical use. The addition of metal salts to PMMA increased the amount of moisture due to the water absorbency of the salts and brought the brittle-to-ductile transition to the mechanical properties. Previous research has also reported that the mechanical properties of PMMA and moisture-absorbed samples were different depending on the anion species of the lithium salts. However, it was still unknown that the differences of mechanical properties of PMMA among the cation species of the metal salts. Therefore, we have investigated the differences in the hygroscopicity and the modification effects on the mechanical properties especially on the water-absorbed samples depending on the cationic species of the metal salts. We found that the slope of the dependencies of the Young's modulus and maximum stress in the uniaxial tensile properties on the water contents is proportional to the valence of the cations of the metal salts.
