Kinetics of microstructure rearrangement during annealing of cold‐drawn isotactic polypropylene

Abstract

The microstructure of polypropylene, annealed after cold drawing to an oriented state, was examined using density measurements and small‐ and wide‐angle x‐ray scattering (SAXS and WAXS) techniques. SAXS patterns were obtained after annealing (in an oil bath), and during annealing (of samples annealed in a furnace placed directly in the x‐ray beam). Data, for isothermal annealing, showed an increase in SAXS intensity, in crystal perfection, and in density over the observed time range, but no change in long spacing during the same period. Long spacing, SAXS intensity, and density were all strongly dependent upon annealing temperature, increasing at higher temperatures. Upon annealing the elastic modulus and yield strength dropped below the as‐drawn values in an immeasurably short time, and did not appear to change thereafter during the time range examined. The decrease was more marked for higher‐temperature annealing. The kinetics of the micro‐structural changes are compared qualitatively to the predictions of nucleation and growth theory and of spinodal decomposition. The defects in the microstructure are considered as the “solute molecules” of the spinodal model. The experimental results do not agree neatly with either model. However, a nonequilibrium thermodynamic approach appears to be the more promising. It is suggested that the kinetics of mechanical property change may be due to different rates of migration for different defect species.