Microstructure and annealing behaviour of cold-drawn isotactic polypropylene

Abstract

Isotactic polypropylene tensile bars were cold-drawn at room temperature and subsequently annealed for various times at temperatures ranging from 50 to 155° C. The material was examined at room temperature in the as-drawn state at several stages of annealing. SAXS, density and mechanical loss data were obtained. Furthermore, thin films were cast. These films were drawn at −196° C and subsequently examined in the electron microscope at −120° C and at higher annealing temperatures. SAXS results for lower temperature annealing showed increases in the intensity of the small-angle Bragg hump with no change in position. High temperature annealing produced a very large intensity increase. In no case did the density of the material show a large increase. Electron microscopy indicated a microstructureless material in the as-drawn state. Annealing at low temperatures produced a fibrous morphology with no observable density modulation in the draw direction. High temperature annealing produced a lamellar microstructure with “normal”, stepwise density modulation. Dynamical mechanical loss curves exhibited no or little β relaxation, except after high temperature annealing. On the basis of these observations, a microstructural model is proposed. The model suggests a very highly defective crystal or paracrystal in the as-drawn state. Low temperature annealing promotes a fibrillar, fringed micellar morphology, in which “crystalline” and “amorphous” regions are not clearly delineated. At higher annealing temperatures, a lamellar, two-phase microstructure is produced.