Analysis of zone-drawing process in preoriented polypropylene

Preoriented isotactic polypropylene was used to clarify the molecular process in zone-drawing with which the necking part is confined in a thin heating zone during uniaxial drawing. The process was analyzed on the basis of rate process, the mechanical properties of zone-drawn samples, and the superstructural change during zone-drawing. The values of activation energy for deformation, ΔH^*, and the activation volume, ΔV^*, were affected by the deformation mechanisms preferentially taking place during the zone-drawing. The attainable maximum modulus of zone-drawn sample at θ = 45° was larger than those at θ = 0° and 90°. The highest strength was also obtained at θ = 45°. The values of modulus and strength strongly depended on both the orientation function of the crystal c-axis and the orientation function of amorphous chains. In the region of a very high zone-drawing rate, in which microcracks are preferentially formed, both modulus and strength decreased, whereas they increased with increasing the zone-drawing rate below this region, giving the optimum condition for achieving the maxima in the modulus and strength.