SAXS ANALYSIS OF THE TRANSITION INDUCED BY ANNEALING QUENCHED ISOTACTIC POLYPROPYLENE

Abstract

The morphologic and structural behavior of isotactic polypropylene (i-PP), quenched in a mixture of dry ice and acetone (−78°C), cold-stretched, and then annealed with fixed ends at 70°C, 80°C, 110°C, 140°C, and 150°C, was analyzed by small-angle X-ray scattering (SAXS). The study of the transition induced by annealing, from the quenched form to the well-crystallized one, was carried out through the analysis of the changes that occurred in the macrolattice parameters. The SAXS patterns showed, for all the samples, a meridional interference peak, indicating a coherent macroreticular sequence in the fiber axis direction. The analysis of the meridional spots was performed two dimensionally; that is, the intensity profiles both along the meridian and normal to it were analyzed according to paracrystal theory. The best fit between the calculated and observed intensities of these profiles is evidence of larger paracrystalline fluctuations in the equatorial direction with respect to the meridional one. The equatorial intensity profiles are continuous-like; from the iterative Guinier-Warren analysis of these profiles, three Gaussian components resulted for all the samples except the one annealed at 150°C, for which only two Gaussian components were found. The morphology of the i-PP samples studied here was interpreted on the basis of ultrafibrils, partially isolated and partially packed into two different kinds of aggregates, with the larger one disappearing at the higher annealing temperature. The more remarkable changes occurring in the transition progress concerned the macroreticular parameters in the fiber axis direction. In the temperature range 80°C–110°C, a discontinuity appeared in the changes of the parameters that we interpret in terms of a latent disorder released at this energy level.