INFLUENCE OF MOLECULAR-WEIGHT AND MOLECULAR-WEIGHT DISTRIBUTION ON CRYSTALLIZATION AND THERMAL-BEHAVIOR OF ISOTACTIC POLYPROPYLENE

Differential Scanning Calorimetry (D.S.C.) and Optical Microscopy have been used in order to assess the influence of molecular weight and molecular weight distribution (MWD) on thermal behavior and crystallization process of isotactic polypropylene (iPP) samples, all characterized by very high isotacticity index (98 wt%). The values of the equilibrium melting temperature (T-m) and surface free energy of folding (sigma(e)) of iPP lamellar crystals have been determined according to the kinetic theory of polymer crystallization. Such thermodynamic parameters were found to increase with increasing iPP molecular weight, for a given iPP MWD and to decrease with increasing iPP MWD, for a given iPP molecular weight. Also the values of the radial growth rate of iPP spherulites and nucleation density were found to be affected noticeably by molecular weight and MWD of iPP. For a given undercooling, Delta T, G decreases strongly with increasing iPP molecular weight. On the other hand faster nucleation was shown by iPP samples with comparatively lower molecular weight and broader MWD.