BIREFRINGENCE - AN IMPORTANT PROPERTY OF PLASTIC SUBSTRATES FOR MAGNETOOPTICAL STORAGE DISKS

Birefringence is an important substrate property for optical data storage media. The basic definitions of birefringence and its relation to the chemical nature of polycarbonate are outlined. The polarizability of the monomer unit, expressed in the rheo‐optical constant and the degree of remaining polymer chain orientation, determines the level of birefringence in polycarbonate disk substrates. Based on this theory, the theory of blending materials with positive and negative rheo‐optical constants Is developed, and the current work with polycarbonate/polystyrene blends is reviewed. The biggest disadvantage of the reviewed system Js that only binary systems with low LCST (lower critical solution temperature) (<240°C) can be formulated. Finally, we report on successful development work of a single‐phase blend of a modified polycarbonate with a special styrene‐acrylonitrile copolymer. The experimental results are compared with the theoretical expectations, and the implications on the injection molding of disk substrates are discussed. Copyright © 1990 Society of Plastics Engineers