AbstractUniaxially and biaxially oriented polystyrene films were prepared in a biaxial stretching machine. The state of orientation was determined by birefringence measurements and represented in terms of a set of biaxial orientation factors ƒ jB. The data are plotted on an isosceles triangular diagram whose coordinates represent the orientation factors ƒ 1B, ƒ 2B. Tensile force–elongation curves were obtained on the films in the machine stretching direction (MSD) and at various angles to the MSD in the plane of the film. The isotropic film is brittle, while uniaxial films yield in the machine direction but are brittle in the transverse direction. The biaxially oriented films exhibit ductile yielding in all directions in the plane o
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