AbstractMelt viscosity and flow birefringence of bisphenol A‐type polycarbonate were measured and analyzed by the application of rubber‐like photoelastic theory. The melt viscosity in the Newtonian flow region increased with the molecular weight to the power of 3.4. In polycarbonate, the shear stress of the Newtonian flow region was to 106dyn/cm2, whereas in PMMA it was at most 3 = 105dyn/cm2.The flow birefringence δn has a linear relation with shear stressS, that is δn = 5.7 × 10−10S. The principal polarization difference of flow unit α1– α2was 1.62 × 10−23cm3, which was obtained by the application of the rubber‐like elastic theory. In PMMA, it was 3.9 = 10−25cm3; about 1/40 of that was polycarbonate. The anisotropy of polarizability of the flow unit of polycarbonate was also about 40 times larger than that of PMMA. So the anisotropy reflected the large flow birefringence o
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