Polycarbonate(PC)is an excellent optical material with high transparency

but it also has high birefringence (Δn0=0.106)which has limited its application in computer-use disk．It has been known that the birefringence of polymer blends may be improved by blending two polymers with opposite birefringence．If polycarbonate is mixed with a kind of polymer that has negative birefringence

for example polystyrene(Δn0=-0.10)

it is possible to get a low birefringence polymer blend．Unfortunately

PC and PS is immiscible so that the blend is opaque due to high light scattering at the interface between them．To get a homogenous blend

a styrence copolymer was introduced．The miscibility of the binary polymer blend polycarbonate／styrene-acrylonitrile coploymer was studied based on Flory-Huggins theory in this paper．The effect of molecular weight and copolymer composition on the phase behavior and the dependence of transparency and birefringence of the blend on the blend composition were studied．A homogenous PC／SAN blend was prepared．It has been proved that the phase behavior of binary blend is determined by the combination of molecular weight and copolymer composititon．The condition of making a homogenous PC／SAN blend with low birefringence and transparency of 70％ at 630 nm was found

if the polymeri-zation degree of PC and SAN(S volume fraction is 60％) is 90 and 30

respectively．All polymers with required copolymer composition and different molecular weight in this work were prepared in our laboratory．A series of polymer blends with different blend ratio were made and their miscibility was judged by Tg measurements using DSC technique

and the birefringence was measured by a polarizing microscope fitted with a Cenamen compensator．