ON THE DEFORMATION MECHANISMS OF ORIENTED PET AND PP FILMS UNDER LOAD

Uniaxially oriented semicrystalline poly(ethylene terephthalate) (PET) and poly(propylene) (PP) films were loaded parallel to draw direction at various temperatures. Changes in the submicroscopical structure of the films under load were examined by small and wide-angle x-ray scat tering (SAXS; WAXS) and birefringence measurements. WAXS measurements reveal a decrease of the initial high orientation of the chains in the crystallites during deformation. Simultaneously, an increase of the b irefringence was detected, indicating an orientation of chains in the amorphous regions. The alteration of the long period reflections in th e SAXS patterns give strong evidence that lamellar stacks with differe nt orientation angles according to load direction are present. Dependi ng on the orientation of stacks, the contribution of lamellar separati on to sample deformation alters, giving rise to different amounts of d ensity changes in the stacks. Absolute intensity measurements of SAXS using a Kratky apparatus reveal that lamellar separation occurs prefer entially below or in the range of the glass-transition temperature at small strain. With increasing strain and temperatures above the glass- transition slip deformation mechanisms become more important. The form ation of microvoids was observed at strain near to elongation at break below or in the range of glass-transition temperature.