Polymer Crystallization Induced by Biaxial Stretching During Film Manufacture

Abstract

In the manufacture of semicrystalline polymer films, orientation is commonly introduced. This orientation may be uniaxial, unbalanced biaxial, or balanced biaxial. Machine-direction and transverse stretching may be concurrent or sequential; each orienting process is characterized by a trajectory on the biaxial stress plane and the biaxial extension plane. The presence of uniaxial or biaxial tensile stress strongly affects the process of polymer crystallization, influencing the crystal-amorphous equilibrium, crystallization kinetics, and the resulting polymer morphology. Post-solidification alterations in morphology can be imposed by drawing or heat-setting under biaxial tension, below the crystalline melting point. The machine-direction and transverse properties of a semicrystalline polymer film depend strongly on the crystalline morphology, and hence on the processing history. The dependence of film properties on processing conditions are well recognized and widely exploited; but the morphology of biaxially oriented films and the structure-property relationships involved are only partly understood.