OPTICAL DISK SUBSTRATE

PURPOSE:To obtain an optical disk substrate ideal for use as a high-density information recording medium by using a specific copolymer resin as an essential component. CONSTITUTION:This disk substrate consists essentially of the copolymer resin which has the molecular structure randomly arranged with 5 kinds of repeating units; a methyl methacrylate unit (A), an arom. vinyl compd. unit (B), an unsatd. fatty acid monomer unit (C), a 6-membered ring acid anhydride unit expressed by formula I (D), and a 6-membered ring imide unit expressed by formula II (E) and satisfies the numerical conditions a+b+c+d+e=100, 95>=a>=40, 56>=b>=1, 96>=a+b>=50, 10>=c>=1, 50>=d+e>=3 where the contents of the respective units A-E are respectively designated as a-ewt.%. The arom. vinyl compd. used here is preferably alpha-methyl styrene, p-methyl styrene, or a mixture composed thereof and the unsatd. fatty acid monomer is preferably acrylic acid, methacrylic acid or a mixture composed thereof.