CRYSTAL TRANSFORMATION AND MICROPORE FORMATION DURING UNIAXIAL DRAWING OF BETA-FORM POLYPROPYLENE FILM

Polypropylene (PP) films with a high content of beta crystal were prepared at crystallization temperatures (T-cr) of 20 and 110 degrees C and then uniaxially drawn at different temperatures (T-dr) The drawn film prepared at T-cr = 20 degrees C had smaller pore volume and average pore radius than the one prepared at T-cr = 110 degrees C under similar drawing conditions (T-dr = 130 degrees C, draw ratio = 3.5-4.0). Electron microscopy revealed that most of the pores are formed by plastic deformation of the weak boundaries of lamellar blocks in the beta-PP films. During drawing, the unstable beta crystals in the film prepared at T-cr = 20 degrees C were easily deformed to undergo crystal transformation of beta --> alpha or beta --> smectic without much increasing the pore volume of the film. In contrast, the more stable beta crystals in the film prepared at T-cr = 110 degrees C remained in the drawn film, and the micropores were enlarged. These results suggested that crystal transformation and micropore formation in the drawn films can be controlled by changing both T-cr and T-dr The static and dynamic mechanical properties of the beta-PP films were also measured to analyse the structural differences of samples of beta-PP formed at different T-cr.