Influence of thermal treatments, molecular weight, and molecular weight distribution on the crystallization of beta-isotactic polypropylene

Six samples of isotactic polypropylene were examined to study the influence of the thermal treatments and the molecular weights and their distribution on the β-crystallization of the polymer. The highest amount of the β-phase was obtained by isothermal crystallization and in correspondence of high average molecular weights and wide molecular weight distributions. Small-angle X-ray scattering pointed out that a partial β-crystallization seems not to influence the lamellar morphology parameters. Differential scanning calorimetry measurements, at different heating rates, allowed us to confirm that the multiple melting endotherms behavior of the β-phase is to be correlated to a melting–recrystallization–melting mechanism.