The tubular film extrusion of atactic polystyrene is described. The in-plane and out-of-plane birefringence of the polystyrene film was determined and compared with both the kinematics (drawdown, blow-up ratio) and applied tensions and bubble pressures. The data can be correlated by comparing the birefringences with the stresses acting in the film at the position of vitrification. The data compare quantitatively with Oda, White, and Clark's correlation developed for the birefringence of samples vitrified during shear and uniaxial extensional flow. The experimental results are interpreted in terms of White and Spruiell's biaxial orientation factors.
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Choi et al. (1980) studied this question.