Polymer Crystallization Induced by Biaxial Stretching During Film Manufacture

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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.

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