Biaxial stretching of PET films: A molecular description

Biaxially oriented poly(ethylene terephthalate) films were prepared under w ell defined stretching conditions in order to mimic the three stages of the industrial inverse drawing process. Molecular orientation has been charact erized through X-ray diffraction and infrared dichroism. The main orientati on mechanisms in the constant-speed drawing of an amorphous film as well as in the constant-force transverse drawing of monodrawn samples are describe d. It is shown that relaxation phenomena dominate the orientation of an amo rphous sample. Reorientation along the second drawing direction involves ro tation of crystalline blocks along the draw direction and further crystalli ne growth. The high-temperature heating stage leads to an almost four-fold increase in the size of the crystallites. The orientation of the amorphous phase is controlled by the mechanisms occurring during crystallization (rel axation followed by extension).